Na-ion batteries are currently an emerging and low-cost energy storage technology, which have attracted enormous attention and research due to its promising potentiality for large-scale energy storage applications. As the key electrode materials for Na-ion batteries, non-graphite carbonaceous materials have been regarded as the best choice for practical application due to its high sodium storage activity, low-cost and non-toxicity. According to the current research, graphite materials are not suitable to be anode materials of Na-ion batteries for practical application due to its low sodium storage capacity in carbonate electrolytes. Hard carbons have a high capacity of ~300 mAh·g-1 with low sodium storage potential and thus are suitable for practical applications. Soft carbons have a sodium storage capacity about 200 mAh·g-1 with sodium storage potential below 1 V vs. Na+/Na. Soft carbons usually exhibit excellent rate performances and thus are suitable to be used as anode materials for power Na-ion batteries. Reduced graphene oxide (rGO) has a sodium storage capacity of about 220 mAh·g-1 and excellent rate performances. A high sodium storage capacity can be obtained by doping heteroatoms and introducing defect sites in rGO. However, the low material density, high sodium storage potential and large irreversible capacity of rGO will restrict its practical application. Porous carbons have high capacities of 300-450 mAh·g-1 with excellent rate performances because their developed porous structure can provide more defects as the active sites for sodium storage and shorten the diffusion path of Na+ to improve rate performances. Carbon nanowires/fibers have good flexibility due to their unique one-dimensional feature and stable sodium storage reversible capacity with good rate performance. These materials have advantages to be flexible electrodes for sodium-based flexible energy storage devices. By introducing N, S and other heteroatoms, heteroatom-doped carbons have more active sites for sodium storage and thus achieve higher sodium storage capacity. In summary, carbon materials with low graphitization degree are important development directions for anode materials of low cost Na-ion batteries. New carbon materials with unique microstructure and morphology have higher sodium storage capacity and rate capability, so they can be used as high power anode materials for sodium storage. Considering many factors, such as cycle life, energy density, power density and manufacturing cost, of practical application, hard carbon anodes is currently the best choice for practical application of Na-ion batteries. In the future, improving SEI stability, increasing Coulombic efficiency and improving electrical conductivity of hard carbon are urgent problems to be solved for practical application. Herein, the recent progress of carbonaceous anode materials is reviewed. The sodium storage mechanism and characteristics of carbon materials are summarized and discussed. Furthermore, the relationship between micro-structures and electrochemical performances, and remained problems of carbon anodes are discussed. This review will promote the development and understanding of carbon anode materials for sodium storage.
Flexible electronic devices have attracted immense attention in recent years. Conventional electronics that are predominantly fabricated with rigid metallic materials demonstrate poor flexibility. Compared to traditional electronic devices, flexible electronic devices with better flexibility can adapt to different working environments. Consequently, they fit perfectly with different systems with minimal rejections. However, such flexible electronic devices need to achieve good extensibility and flexibility without compromising on their electronic properties. Therefore, new challenges and requirements arise while fabricating conductive materials. Manufacturing of flexible metal electrodes for flexible electronic devices include strategies such as reducing the thickness of the electrodes and designing electrodes with unique structures. However, these technologies are complex and expensive. Carbon nanotube (CNT) films exhibit good flexibility, excellent conductivity, good chemical and thermal stability, as well as good optical transparency, making them ideal candidates for flexible electronics. Therefore, the preparation and application of CNT films for the development of next generation flexible electronics have been extensively studied. In this review, we summarize the recent advances in the preparation of CNT films and their application in flexible electronic devices. Initially, the two main kinds of preparation methods for CNT films—dry and wet methods—are introduced. The dry methods for CNT film preparation include the membrane extraction method based on a vertical array of CNTs and the floating catalytic chemical vapor deposition method. Moreover, the wet methods predominantly discussed include vacuum filtration method, impregnation method, electrodeposition method, self-assembly method, and spraying method. Subsequently, the latest research advancements in assembly techniques, their performance and applications in various flexible electronics are discussed. This review primarily introduces the application of CNT films in the fields of flexible sensors, flexible energy devices, flexible transistors, and flexible display screens. The fundamentals of typical flexible sensors, such as strain sensors, pressure sensors, gas sensors, temperature sensors, and humidity sensors are presented. Besides, flexible lithium-ion batteries, flexible nanogenerators, and flexible thermoelectric devices based on CNT films are also investigated. Moreover, other flexible electronic devices, such as flexible transparent conductive film, flexible transistor, and flexible photodetector, based on CNT films are briefly described. Finally, advanced flexible electronics based on CNT films are summarized. The challenges and future prospects of these films are also discussed.
Supercapacitors have been widely used in various fields because of their high power density, long cycle life, and cost-effectiveness. Plant-based porous carbon continues to be the most suitable alternative for manufacturing the commercial electrode materials of supercapacitors because of its good electrochemical performance, simple preparation process, high availability, and low cost. Although plant-based porous carbon prepared using physical activation has been widely used in commercial supercapacitors, its performance is severely restricted because of its low value of specific surface area and highly microporous structure. With a view to achieving high values of specific gravimetric/volumetric capacitances and outstanding rate performance in supercapacitors, this review summarizes the recently developed methods for preparing plant-based ultrahigh specific surface area porous carbon materials, mesoporous carbon materials, hierarchical porous carbon materials, and nitrogen-doped porous carbon materials. The factors affecting the electrochemical performance of plant-based porous carbon are also discussed. We also summarize some novel strategies to improve the volumetric electrochemical performance of plant-based porous carbon materials, such as preparing dense and porous carbon materials, performing heteroatom doping, and combining the carbon with pseudocapacitive materials (conductive polymers or metal oxides). Finally, the challenges and perspectives of using plant-based porous carbon in supercapacitors are also proposed. In brief, when used as the electrode material for supercapacitors, the ultrahigh surface area porous carbon prepared by KOH activation shows high value of specific capacitance at low current densities. However, the tortuous and deep micropores in the plant-based porous carbon result in its sluggish ion-transport kinetics and high value of equivalent series resistance, which, in turn, result in poor rate performance. To improve the rate performance, tremendous efforts have been made to introduce mesopores in the carbon as ion-transport channels. However, this strategy usually involves the coalescence of a large number of micropores, resulting in the reduced surface area as well as energy storage ability of the carbon. Hence, many researchers have utilized the inherent porous structure and inorganic templates of plants to prepare hierarchical porous carbon both with high specific surface area and high mesopore volume for use in devices with high capacitance and power. In addition to altering the surface area and pore structure of the carbon, doping with nitrogen is another promising approach to enhance the capacitance and electronic conductivity of the plant-based porous carbon. Surface nitrogen can be introduced by the direct carbonization/activation of nitrogen-rich plant precursors or by the reaction of the carbon with nitrogen-containing reagents. Porous carbon with large specific area and with developed mesoporous structure may exhibit superior gravimetric capacitance but inferior volumetric capacitance because of the trade-off between its well-developed microporous structure and packing density. To improve the volume performance, some methods, such as preparing dense and porous carbon with reasonably porous structure, using heteroatom-doped carbon, and incorporating the carbon with pseudocapacitive materials, have been developed. Although the electrochemical performance of plant-based porous carbon has been significantly improved using the aforementioned methods, yet issues such as the lack of green methods and low-cost activation methods to prepare large surface area porous carbon, the design and controlled modulation of carbon micro-structures, the influence of heteroatom doping on pseudocapacitance, and weak interaction between pseudocapacitive components and plant-based porous carbon still need to be resolved. We hope that this review may provide the necessary background and ideas to develop more effective preparation methods for high-performance plant-based porous carbon.
The National Natural Science Foundation of China (NSFC) is a vital government agency supporting basic research and people to create knowledge and meet major national needs, where a rigorous and objective merit-based peer review mechanism is the key to funding the most promising research proposals. This invited comment overviews some recent attempts aimed at bettering the academic evaluation environment at the Department of Chemical Science in 2019, through measures such as grouped panel committee meetings, standardized panel committee meeting procedures, and review process refinement to improve the project review at panel committee meeting levels.
Owing to their high power density, excellent rate performance, and good cycle performance, supercapacitors are widely used for energy storage applications. Two-dimensional (2D) layered materials are structural compounds having a layered host structure with a layer thickness of nanometers and a lateral dimension of several micrometers. Their layer spacing can be controlled by changing the interaction between the layers. 2D layered materials can be delaminated into nanosheets. Exfoliated nanosheet materials provide a new strategy for improving the performance of supercapacitors. Unlike bulk layered materials, the exfoliated nanosheets not only provide a unique nano-scale reaction space for electrochemical reactions, but also offer the possibility for improving the specific capacitance and storage rate of the supercapacitor. However, in 2D layered materials, the ion or electron transport in the vertical direction is obstructed, despite their fast ion and electron transport in the horizontal direction. As a result, the occurrence of electrode reactions, and hence the realization of rapid storage become difficult. It is detrimental to the power and energy densities and rapid energy storage of supercapacitors. Therefore, it is imperative to develop novel electrode materials in order to fabricate supercapacitors showinghigh energy densities at high power densities.
Porous 2D layered electrode materials offer two major advantages. First, the porous structure can alleviate the problems caused by the stacking of nanosheets during the assembly process. Second, the porous structure can effectively promote the electrolyte penetration of the electrode, which alleviates the volume changes of the electrode material during the charging-discharging process and releases the structural strain. Hence, such materials facilitate ion or electron transport, thus increasing the specific capacitance of the supercapacitor. Over the past few years, 2D nanosheet holeization has evolved as a promising approach to improve the energy density of supercapacitors at high power densities. Various porous 2D layered supercapacitor electrode materials have been developed. This paper reviews the exfoliation of 2D layered materials, nanosheet holeization strategy, and the application of assembled porous layered electrode materials in supercapacitors. In this paper, we have reviewed the exfoliation of 2D layered materials with different electric properties and the performance of 2D nanosheets. Different methods used for the preparation of holey 2D nanosheets have also been discussed. We prepared holey MnO2 nanosheets and reduced graphite oxide via redox holeization mechanism, and 2D porous nanomaterials were also prepared by using suitable templates such as hard or self-sustaining templates. These holey 2D nanosheets were used to prepare porous 2D layered electrode materials such as holey graphene/manganese dioxide composite fibers and holey graphene/polypyrrole hybrid aerogels. The capacitance of these electrode materials was investigated systematically. Finally, the prospects for the development of porous 2D layered electrode materials such as the optimization of theirrate performance, flexibility, and energy density were discussed. Novel holeization methods should be developed in order to prepare metal oxide nanosheets with controllable hole sizes. In addition, other 2D materials such as MXene should be explored.
Carbon materials can offer various micro- and nanostructures as well as bulk and surface functionalities; hence, they remain the most popular for manufacturing supercapacitors. This article critically reviews recent developments in the preparation of carbon materials from new precursors for supercapacitors. Typical examples are activated carbon (AC) and graphene, which can be prepared from various conventional and new precursors such as biomass, polymers, graphite oxide, CH4, and even CO2 via innovative processes to achieve low-cost and/or high specific capacitance. Specifically, when producing AC from natural biomasses or synthetic polymers, either new, spent, or waste, popular activation agents, such as KOH and ZnCl2, are often used to process the ACs derived from these new precursors while the respective activation mechanisms always attract interest. The traditional two-step calcination process at high temperatures is widely employed to achieve high performance, with or without retaining the morphology of the precursors. The three-step calcination, including a post-vacuum treatment, is also the preferred choice in many cases, but it can increase the cost per capacity (kWh∙g−1). More recently, one-step molecular activation promises a better and more economical approach to the commercial application of AC, although further increase of the yield is necessary. In addition to activation, graphitization, N doping, and template control can further improve ACs in terms of the charging and discharging rates, or pseudocapacitance, or both. Considerations are also given to material structure design, and carbon regeneration during activation. Metal-organic frameworks, which were initially used as templates, have been found to be good direct carbon precursors. Various graphene structures, including powders, films, aerogels, foams, and fibers, can be produced from graphite oxide, CO2, and CH4. Similar to AC, graphene can possess micropores by activation. Self-propagating high-temperature synthesis and molten salt processing are newly-reported methods for fabrication of mesoporous graphene. Macroporous graphene hydrogels can be produced by hydrothermal treatment of graphite oxide suspension, which can also be transferred into films. Hierarchically porous structures can be achieved by H2O2 etching or ZnCl2 activation of the macroporous graphene precursor. Sponges as templates combined with KOH activation are applied to create both micro- and macropores in graphene foams. Graphene can grow on fibers and textiles by electrodeposition, dip-coating, or filtration, which can be woven into clothes with a large area or thick loading, illuminating the potential application in flexible and wearable supercapacitors. The key obstacles in AC and graphene production are high cost, low yield, low packing density, and low working potential range. Most Carbon materials derived from new precursors work very well with aqueous electrolytes. Charge storage occurs not only in the electric double layer (i.e., the "carbon | electrolyte" interface), but also via redox activity in association with the bulk and surface functionalities, and the resulting partial delocalization of valence electrons. The analysis of the capacitive electrode has shown a design defect that prevents the working voltage of a symmetrical supercapacitor from reaching the full potential window of the carbon material. This defect can be avoided in AC-based supercapacitors with unequal electrode capacitances, leading to higher cell voltages and hence higher specific energy than their symmetrical counterparts. There are also emerging ways to raise the energy capacity of AC supercapacitors, such as the use of redox electrolytes to enable the Nernstian charge storage mechanism, and of the three dimensional printing method for a desirable electrode structure. All these developments are promising carbon materials from various precursors of new and waste sources for a more affordable and sustainable supercapacitor technology.
Supercapacitors have attracted considerable attention as new-generation energy storage devices because of their high charge-discharge rate, ultralong lifetime, and high power density. However, the performance of supercapacitors is severely restricted by either the low intrinsic capacitance of porous carbons or the poor conductivity and sluggish electrochemical kinetics of pseudocapacitive components. Therefore, high-performance electrode materials integrated with high gravimetric and volumetric capacitances, high rate capability, and superb cycling stability are urgently needed. As emerging carbon nanomaterials, carbon-/graphene quantum dots (CDs/GQDs) have uniquely small particle sizes, abundant edge sites, and various functional groups, thus endowing them with great potential for developing high-performance electrode materials for supercapacitors. With the purpose of identifying the application advantages and critical problems of CDs/GQDs for supercapacitor electrodes, this review summarizes the development of CDs/GQDs, quantum dots (QDs)/conductive carbon, QDs/metal oxides, QDs/conductive polymer composites, and QD-derived carbon materials in recent years. In each section of this paper, we introduce the typical and updated studies from corresponding fields in terms of novel preparation routes, the crucial roles of CDs/GQDs in composite materials, and the electrochemical performance of electrode materials and assembled devices. Finally, the advantages and limitations of CD/GQD electrode materials are described, and the future development of QD-based materials is discussed. In general, the previous studies have shown that when directly used as electrode materials for micro-supercapacitors, CDs/GQDs performed at an ultrahigh charge-discharge rate of up to 1000 V∙s−1. However, because of the discontinuous conductive network and good dispersibility in electrolytes, they are not suitable for use in common devices because of poor cycle stability. One of the most promising means for fully realizing active ion storage sites for portable devices is to strongly anchor CDs/GQDs onto conductive carbon scaffolds such as activated carbon, graphene nanosheets, and carbon nanofibers. QDs simultaneously improve the capacitive and rate performance owing to the active sites and improved surface wettability of the composite materials. To further improve the capacitance and cycle stability of electrode materials, different types of QDs/pseudocapacitive material composites (metal oxides or conductive polymers) have been developed. QDs have been shown to increase entire conductivity, accelerate ion transport, and depress the volume expansion of metal oxides and conductive polymers. Benefiting from their small-sized structure and outstanding reactivity, CDs/GQDs can also be used as emerging precursors for constructing advanced carbon electrode materials. Several forward-looking studies have shown that QDs can be converted to carbon nanosheets, heteroatom-doped carbon, and dense and porous carbon blocks with high gravimetric/volumetric capacitances and remarkable rate performance for fast and compact energy storage. Although related fields have rapidly developed in recent years, several critical problems including those related to green and effective methods to prepare CDs/GQDs, construction of entire conductive networks without active sites being sacrificed, the synergistic effect, and the underlying mechanism of QDs in composite materials still must be solved. We hope that this review can provide inspiration and references for further investigation in this promising field.
With the continuous miniaturization and integration of electronic and optoelectronic nanodevices, Moore's Law faces huge challenges from the demands of devices with multifunctional and high-performance characteristics. With several recent reports of the successful synthesis of nanomaterials such as nanoparticles, quantum dots, nanowires, and two-dimensional layered materials, the utilization of such materials for the fabrication of electronic and optoelectronic nanodevices has demonstrated potential for realizing multifunctional and high-performance nanodevices in the future. In particular, owing to their excellent electrical, thermal, mechanical, and optical properties, atomically two-dimensional layered materials have emerged as the most promising materials for nanodevices to solve the bottleneck problems of traditional silicon-based devices. Two-dimensional semiconductor materials have been widely applied in many aspects of functional modules, including pn junctions, field effect transistors, rectifiers, photodetectors, and even solar cells. To provide a strong foundation for the development of high-performance and multifunctional nanodevices in the future, this review summarizes the recent advances in electronic and optoelectronic nanodevices based on novel two-dimensional semiconductor materials. We begin the review with a brief introduction of existing two-dimensional materials, including graphene, transition-metal dichalcogenide materials, black phosphorus, hexagonal boron nitride, and van der Waals heterostructures. The atom structure features, electronic and optical properties, and major applications in devices are discussed. The semiconductor materials are suitable for device channels, while graphene and hexagonal boron nitride can be used as electrodes, encapsulating materials, and components of van der Waals heterostructures for channel of field effect transistors. Next, we mainly discuss the advances in electronic and optoelectronic nanodevices based on transition-metal dichalcogenide materials, black phosphorus, and van der Waals heterostructures. In the context of electronic nanodevices, we introduce field effect transistors and other important functional devices, such as sensors, memristors, and integrated circuits. The mobility, on-off ratio, rectification ratio, and other properties of electronic devices are mentioned. In addition, we describe the potential applications of optoelectronic nanodevices for photodetectors, lasers, light-emitting diodes, photovoltaic devices, and so on. The metrics of devices performance such as responsivity, response time, and spectrum response range are compared. Finally, we summarize and compare the advantages and disadvantages of nanodevices based on different materials. Manufacturing comprehensive and high-performance nanodevices will be a promising direction in the future. In addition, the methods for improving the performance of devices are classified. This review will serve as an important reference for the development of future multifunctional and high-performance nanodevices.
The discoveries about the functions of biomolecular liquid-liquid phase separation in cell have been increased rapidly in the past decade. Condensates produced by phase separation play key roles in many cellular curial events. These biological functions are based on the physicochemical properties of phase separation. This review discusses the recent progress in understanding the physical and chemical mechanisms of biological liquid-liquid phase separation. (1) We summarized the basic properties and experimental characterization methods of phase separation droplets, including the morphology, fusion, and wetting, along with the dynamic properties of molecules in droplets, which are usually described by diffusion coefficients or viscosity and permeability. (2) We discussed the conditions affecting the liquid-liquid phase separation of biological molecules, including concentration, temperature, ionic strength, pH, and crowding effects. A database for liquid-liquid phase separation, LLPSDB, was introduced, and three types of nucleic acid concentration effects on the phase separation of protein molecules are discussed. These effects depend on the relative interaction strengths of protein-nucleic acid and protein-protein interactions. The major driving force of phase separation is multivalent interactions, and molecular flexibility is necessary for the dynamic properties. We summarized the diverse sources of multivalence, including multiple tandem repetitive domains, regular oligomerization, low-complexity domains (usually intrinsically disordered with repeat motifs for binding), and nucleic acid molecules via the main chain phosphates or repeat sequences. (3) We reviewed the statistical thermodynamics theories for describing the macromolecular liquid-liquid separation, including the Flory-Huggins theory, Overbeek-Voorn correction, random phase approximation method, and field theory simulation method. We discussed the experimental and simulation methods for studying the physiochemical mechanism of liquid-liquid phase separation. Model systems with simplified sequences for experimental studies were listed, including systems for studying the effects of charge properties, residue types, sequence length, and other properties. Molecular simulation methods can provide detailed information regarding the liquid-liquid phase separation process. We introduced two coarse-grain methods, the slab molecular dynamic simulation and Monte Carlo simulation using the lattice model. (4) The physiochemical properties of liquid-liquid phase separation govern the diverse functions of reversible phase transitions in a cell. We collected and analyzed important cases of biomolecular phase separation in cell activities. These biological functions were classified into five categories, including enrichment, sequestration, biological switching cooperation, localization, and mechanical force generation. We linked these functions with the physiochemical properties of liquid-liquid phase separation. To understand the specific phase-separation processes in biological activities, three types of related molecules must be studied: scaffold molecules mainly contributing to aggregate formation, recruited functional client molecules, and molecules that regulate the formation and disassembly of aggregates. We reviewed four regulation methods for the phase separation process, including changing the charge distribution by post-translational modification, changing the molecular concentration by gene expression or degradation regulation, changing the oligomerization state, and changing the cell solution environment (such as pH). Designing compounds for phase separation regulation has attracted significant attention for treating related diseases. Methods for discovering molecules that can regulate post-translational modifications or inhibit interactions in the droplets are emerging. The recently discovered phase separation phenomena and molecules in living organisms represent only the tip of the iceberg. In the future, it will be necessary to systematically examine liquid-liquid phase separation events and related molecules in all phases of biological processes.
The photocatalytic hydrogen evolution reaction (PHER) has gained much attention as a promising strategy for the generation of clean energy. As opposed to conventional hydrogen evolution strategies (steam methane reforming, electrocatalytic hydrogen evolution, etc.), the PHER is an environmentally friendly and sustainable method for converting solar energy into H2 energy. However, the PHER remains unsuitable for industrial applications because of efficiency losses in three critical steps: light absorption, carrier separation, and surface reaction. In the past four decades, the processes responsible for these efficiency losses have been extensively studied. First, light absorption is the principal factor deciding the performance of most photocatalysts, and it is closely related to band-gap structure of photocatalysts. However, most of the existing photocatalysts have a wide bandgap, indicating a narrow light absorption range, which restricts the photocatalytic efficiency. Therefore, searching for novel semiconductors with a narrow bandgap and broadening the light absorption range of known photocatalysts is an important research direction. Second, only the photogenerated electrons and holes that migrate to the photocatalyst surface can participate in the reaction with H2O, whereas most of the photogenerated electrons and holes readily recombine with one another in the bulk phase of the photocatalysts. Hence, tremendous effort has been undertaken to shorten the charge transfer distance and enhance the electric conductivity of photocatalysts for improving the separation and transfer efficiency of photogenerated carriers. Third, the surface redox reaction is also an important process. Because water oxidation is a four-electron process, sluggish O2 evolution is the bottleneck in photocatalytic water splitting. The unreacted holes can easily recombine with electrons. Sacrificial agents are widely used in most catalytic systems to suppress charge carrier recombination by scavenging the photogenerated holes. Moreover, the low H2 evolution efficiency of most photocatalysts has encouraged researchers to introduce highly active sites on the photocatalyst surface. Based on the abovementioned three steps, multifarious strategies have been applied to modulate the physicochemical properties of semiconductor photocatalysts with the aim of improving the light absorption efficiency, suppressing carrier recombination, and accelerating the kinetics of surface reactions. The strategies include defect generation, localized surface plasmon resonance (LSPR), element doping, heterojunction fabrication, and cocatalyst loading. An in-depth study of these strategies provides guidance for the design of efficient photocatalysts. In this review, we focus on the mechanism and application of these strategies for optimizing light absorption, carrier separation and transport, and surface reactions. Furthermore, we provide a critical view on the promising trends toward the construction of advanced catalysts for H2 evolution.
The world is currently facing a series of energy-related problems and challenges. In response, scientists are committed to seeking green high-performance energy storage devices to meet the demands of long-term, sustainable, and innovative development in the future. As a new type of green energy storage device, the supercapacitor has the advantages of high power density, high theoretical specific capacitance, fast charge and discharge speed, long cycle life, high safety, environmental friendliness, and economy to help people cope with the energy crisis. In addition, energy storage devices including Li-ion batteries and supercapacitors are being transformed from heavy, rigid, and bulky devices into light, flexible, and small units to fulfill the needs of the next generation. Among these energy storage systems, the electrode material is an important factor affecting the performance of supercapacitors. In recent years, supercapacitors based on manganese dioxide have been widely studied owing to their high theoretical specific capacitance, good chemical stability, and environmental friendliness. At the same time, a variety of two-dimensional materials are also used as supercapacitor electrode materials after graphene. Two-dimensional structural features play an important role in improving the energy density of electric double-layer capacitors and improving the pseudocapacitance of capacitors. To achieve high specific capacitance and high rate of performance, combining manganese dioxide with two-dimensional materials is a promising option. In this paper, we systematically introduce the application of composites that combine two-dimensional materials represented by graphene and manganese dioxide in supercapacitors, and considers the electrochemical properties of these composites. However, there is still a long way to go in order to fabricate a suitable hierarchical structure consisting of two-dimensional materials and manganese dioxide. For example, a suitable two-dimensional material must be chosen and combined with manganese dioxide to form composites that possess excellent electrochemical properties. In addition, the fabrication methods for these composites are a principal factor that affects their performance. Thus, there are reasons for us to strongly believe that if these key issues are resolved, the properties of these composites consisting of manganese dioxide and two-dimensional materials will make great progress. Overall, this paper only points out some general directions for these kinds of composites in the future, such as principles for choosing the two-dimensional materials to combine with manganese dioxide, and the composite methods which have been reported previously. We are pleased that other researchers are being inspired by our work, and we are looking forward to seeing better studies in this field.
The increased demand for high-performance supercapacitors has fueled the development of electrode materials. As an important part of supercapacitors, the electrochemical performance of the supercapacitor is directly affected by the specific surface area, conductivity, electrochemical activity, and stability of electrode materials. In the traditional manufacturing method, a binder must be added to powdered electrode materials to enhance their combination with the current collector, which could lead to morphology damage, pore blockage, and reduced conductivity of active materials that will adversely affect their electrochemical behavior. Thus, research on binder-free electrode materials has attracted significant interest. Recently, electrospun nanofibers have been widely used as supercapacitor electrode materials because of their advantages such as large specific surface area, high porosity, and easy preparation. The attainable continuity and flexibility endow electrospun nanofiber membranes outstanding performance among large numbers of binder-free materials. This review considers recent studies on electrospun nanofiber-based binder-free electrode materials for supercapacitors, including carbon nanofibers, carbon-based composite nanofibers, conductive polymer-based composite nanofibers, and metal oxide nanofibers. These studies demonstrate that pore structure construction, activation treatment, and nitrogen doping can improve the specific surface area, electrochemical activity, wettability, and graphitization degree of carbon nanofibers to enhance their electrochemical properties. Moreover, combining carbon nanofibers with metal oxides, metal sulfides, metal carbides, and conductive polymers by methods such as blending, chemical deposition, electrochemical deposition, etc., can improve their capacitance, rate performance, and cycling stabilities, which complement the advantages of different materials and proves that the performance of multicomponent materials is better than that of single-component materials. In particular, conductive polymers based on composite nanofibers and metal oxide nanofibers can be used as binder-free materials by electrospinning technology, but their dependence on other substances as well as fragile fiber membrane limit their widespread application. Therefore, in order to ensure the continuity or flexibility of fiber membranes, carbon-based composite nanofibers with multicomponent and hierarchical structure could potentially be used/constructed as binder-free electrode materials. Combinations with new types of electrode materials such as metal-organic frameworks (MOF), covalent organic frameworks (COF), MXenes, metal nitride, metal phosphide, etc., and the preparation of materials with novel structures have also been attempted. In order to realize the practical application of eletrospun nanofiber-based binder-free electrode materials, more attention should be given to improving their mechanical properties, production efficiency, and research on the application of flexible devices. We hope that this review can broaden ideas for improving the development and application of electrospun nanofiber-based binder-free electrode materials for high-performance supercapacitors.
Carbon dioxide is the most common compound. As a potential source of carbon, it can be used to prepare a variety of high value-added chemicals, such as carbon monoxide, methane, methanol, and formic acid. The traditional method of thermal catalytic conversion of CO2 requires high energy consumption and harsh reaction conditions. Therefore, the efficient conversion of CO2 to value-added chemicals under mild conditions has long been an area of great interest in the field of catalysis. Photocatalysis usually takes place under mild reaction conditions and is environmentally friendly. However, pure photocatalytic reactions generally have a limited solar energy utilization efficiency and low separation efficiency of photogenerated charge carriers. In view of the above problems, the introduction of electrocatalysis on the basis of photocatalysis can improve the charge separation efficiency. At a lower overpotential, multi-electrons and protons can be transferred to CO2, thus improving the catalytic reaction efficiency. Photoelectrochemical catalysis combines the advantages of photocatalysis and electrocatalysis to improve the efficiency of the catalytic reduction of CO2, offering a new method for the clean utilization of CO2. According to the principle of photocatalysis, the absorption capacity of a semiconductor is governed by its band structure. Optimization of the band structure is a major strategy to enhance the absorptivity of photocatalysts. In addition, the loading of light-absorbent materials on photocatalysts is an effective way to enhance the photocatalytic absorption of a photocatalytic system. During photoelectrocatalytic CO2 reduction, numerous photogenerated charge carriers recombine in bulk and on the surface of the catalyst, greatly reducing the efficiency of the catalytic reaction. Therefore, increasing the separation efficiency of charge carriers is an important means to improve the photoelectrocatalytic efficiency. In photoelectrocatalytic CO2 reduction, heterojunction construction and electric field formation often lead to the efficient separation of charge carriers. The interfacial reaction is a crucial step in the photoelectrocatalytic process. After generation, the photogenerated charge carriers need to migrate to the surface of the catalyst to participate in the redox reaction. In photoelectrocatalytic CO2 reduction, electrons participate in the reduction of CO2, while holes participate in the oxidation of water. Studies show that acceleration of the interfacial reaction process is of paramount importance for improving the efficiency of the photoelectrocatalytic reduction of CO2. This review summarizes the basic enhancement strategies of photoelectrocatalytic CO2 reduction from three aspects: light absorption, charge separation, and surface reaction, based on the basic mechanism of the reduction. The future prospects and research areas are also proposed.
Project proposal review is the core of science foundation management. This article systematically overviews existing key issues in panel committee meetings in final reviews of scientific research grants, which are becoming an increasing concern in the science community. Moreover, it reassesses the importance and functionality of panel committee meetings, and accordingly provides suggestions and measures based on the "verification-rectification-and-selection" principle for further installation and improvement of the panel committee meeting mechanism.
Sodium batteries have drawn increasing attention from multiple researchers owing to the abundant reserves and low cost of sodium resources. However, traditional sodium batteries based on organic solvent electrolyte systems have safety risks. Thus, the utilization of solid electrolyte materials instead of organic electrolytes could effectively resolve safety issues and ensure the safe performance of the battery. Solid sodium-ion battery is a promising energy storage device. The sodium ion solid-state electrolytes mainly includes Na-β-Al2O3, Na super ionic conductor (NASICON), sulfide, polymer, and borohydride. Inorganic solid electrolytes have the advantage of ionic conductivity compared with polymer solid electrolyte. This paper summarizes the research progress on three common inorganic sodium ion solid electrolytes: Na-β-Al2O3, NASICON, and sulfide. Research efforts have mainly focused on increasing ionic conductivity and interface stability. Na-β-Al2O3 has been successfully commercialized in high-temperature Na-S and ZEBRA batteries with molten electrodes. Pure β″-Al2O3 is difficult to prepare owing to its low thermodynamic stability. The synthesized β″-Al2O3 based on traditional solid-state reaction generally contains impurities such as β-Al2O3 and NaAlO2 (around the boundaries). Further improvements are required to develop favorable methods for fabricating pure β″-Al2O3 with high production yield, low cost, and well-controlled microstructure. NASICON, one of the most promising ionic conductors for solid sodium-ion batteries, has attracted considerable attention for its high ionic conductivity at room temperature. The general method to enhance ionic conductivity is to increase the bottleneck size by introducing proper substituents. However, the substitution of synthetic elements could result in different optimal calcination temperatures, which would lead to a change in the density of ceramic sintering. β″-Al2O3 and NASICON have higher ionic conductivity at room temperature but cannot achieve good performance in the field of high power densities and long-term cycling owing to the poor interface contact with electrode materials. Because the high polarizability and large ionic radius of sulfur atoms weaken the interaction between skeleton and sodium ions, sulfide solid electrolytes often provide higher ionic conductivity at room temperature than analogous oxides. At the same time, sulfide solid electrolytes can be easily pressed into a mold at room temperature. However, sulfide electrolytes have low chemical stability in air because of hydrolysis by water molecules with the generation of H2S gas, which should be handled in inert gas atmosphere. In conclusion, this review discusses the recent progress in different aspects of ionic conductivity and interface stability.
With the ongoing depletion of fossil fuels, the exploration of sustainable energy resources and advanced energy technologies is necessary and the development of clean and sustainable energy storage devices has become an important topic worldwide. In this regard, rechargeable batteries and supercapacitors (SCs) are currently considered to be promising electrochemical energy storage systems for widespread applications in electronic devices, electric vehicles, and smart-grid energy storage stations. Batteries typically exhibit high energy densities but are limited by their low power density and relatively poor cycling performance. In contrast, SCs exhibit high power density, stable cyclability, and good safety, but the energy densities of SCs are generally inferior to those of batteries, which hinders their widespread application. A reliable approach to addressing this issue is to fabricate hybrid supercapacitors (HSCs) composed of battery-type and capacitive electrodes. This device configuration enables the direct integration of the high energy densities of batteries and high power densities of SCs, making HSCs a promising class of energy storage devices. However, the mismatch of capacity and rate performance between the battery-type and capacitive electrodes hinders the widespread applications of HSCs. A key challenge for the development of high-performance HSCs is to optimize the balance between both electrodes. Recently, tremendous efforts have been focused on the search for suitable electrodes and considerable progress has been achieved. Nevertheless, in traditional electrodes, binders are commonly used to combine individual active materials with conductive additives. Unfortunately, these binders are generally electrochemically inactive and insulating, reducing the overall specific capacity/capacitance and deteriorating the charge/mass transport. Recently, binder-free nanoarray electrodes have provided a promising opportunity for designing effective HSCs owing to the merits of their direct electron transport pathway, short ion diffusion length, and ordered-structure-enabled abundant reaction sites. This review briefly addresses the energy storage mechanism of HSCs and the advantages of array electrodes, and subsequently reviews the recent advances in emerging HSCs developed by our group. The performance-electrode structure relationship is discussed from the perspective of devices featuring different electrolytes, including organic, aqueous neutral and aqueous alkaline electrolytes. Moreover, some solutions are put forward to solve the existing issues of HSCs, and the potential applications of array electrode-based HSCs in flexible/wearable electronics are envisioned. Finally, the challenges and future development trends of HSCs are proposed.
Converting solar light into chemical energy is currently a hot topic for addressing the worldwide energy and environmental crises. However, the utilization of solar energy greatly suffers from its low energy flow density and discontinuous space-time distribution, which are essential for a reasonable energy conversion strategy toward effective storage and utilization. To this end, photocatalytic water splitting is a promising method for utilizing solar light to produce environmentally friendly hydrogen energy; yet, the efficiency needs to be improved. Generally, such processes can be divided into three elementary steps: light absorption, charge separation and migration, and surface redox reaction. The overall performance is determined by the cumulative efficiencies of the above three steps. The construction of cocatalysts is among the extensive efforts taken to improve the solar conversion efficiency. First, the cocatalysts possess higher work function than the semiconductors, and the photogenerated electrons migrate from semiconductor to cocatalysts, thereby promoting the charge separation. Second, cocatalysts usually lower the activation energy and provide abundant surface reactive sites. Particularly, the addition of cocatalysts can remarkably accelerate the four-electron transfer O2 evolution kinetics, which usually requires much higher overpotential and is often considered as the bottleneck for water splitting. Third, cocatalysts can timely remove the photogenerated charges from the surface of the semiconductor and subsequently inhibit the photocorrosion and improve the stability of the photocatalysts. Moreover, the cocatalysts also retard the backward recombination of H2 and O2. In general, cocatalysts for water splitting can be classified into three categories: H2 evolution cocatalysts, O2 evolution cocatalysts, and dual cocatalysts. The H2 evolution cocatalysts mainly contain noble metals such as Pt, Au, and other transition metals such as Co, Ni, and Cu and their phosphides or sulfides, which are capable of trapping electrons and promoting proton reduction. The O2 evolution cocatalysts are often noble metal oxides and transition metal (hydro)oxides and corresponding phosphates, which are always efficient in adsorbing and dissociating water molecules. To realize the overall water splitting, H2 evolution cocatalysts and O2 evolution cocatalysts are often integrated on one photocatalyst, which results in the so-called dual cocatalyst system. Furthermore, the performance of cocatalysts can be improved by modulating the loading amount, morphology, particle size, etc. In addition, composites such as Pt/Ni(OH)2 cocatalyst can not only provide both H2 and O2 evolution sites but also accelerate the intrinsic surface redox kinetics by promoting H2O activation, thus being much more active than the conventional dual cocatalyst system. This review summarizes the important role and design principle of cocatalysts in photocatalytic systems. The construction and functional mechanism of H2 evolution cocatalyst, O2 evolution cocatalyst, and dual cocatalysts in overall water splitting photocatalysts are discussed in detail, and the design strategy of new cocatalysts toward water activation is proposed.
The rapid development of electronic products has increased the demand for safe, low-cost, and high-performance energy storage devices. Lithium-ion batteries have been commercialized owing to their high energy density. However, the limited lithium resources and their uneven distribution have triggered the search for alternative energy storage systems. In this context, rechargeable aqueous zinc-ion batteries have gained immense attention owing to their low cost and environmental friendliness. Nevertheless, it is highly challenging to develop zinc-ion battery cathode materials with both high capacity and long cycle life. Hence, in this study, we prepared three-dimensional porous activated carbon (3DAC) with high specific surface area by using ethylenediaminetetraacetic acid (EDTA) tetrasodium salt hydrate as the raw material. We developed a zinc-ion hybrid capacitor (ZIHC) in 1 mol∙L−1 ZnSO4 using 3DAC as the cathode and a zinc foil as the anode. The ZIHC stored charge by the reversible deposition/dissolution of Zn2+ on the zinc anode and rapid reversible adsorption/desorption of ions on the 3DAC cathode. Owing to the large specific surface area and highly porous structure of the 3DAC cathode, the assembled ZIHC exhibited excellent electrochemical performance. It worked well over the voltage range of 0.1–1.7 V, providing a high specific capacitance of 213 mAh·g−1 at the current density of 0.5 A·g−1 (the highest value reported till date). The ZIHC showed specific capacities of 182, 160, 139, 130, 127, 122, and 116 mAh·g−1 at the current densities of 1, 2, 4, 6, 8, 10, and 20 A·g−1, respectively. Meanwhile, it exhibited the highest energy density of 164 Wh·kg−1 (at a power density of 390 W·kg−1) and still delivered the highest power density of 9.3 kW·kg−1 with a high energy density of 74 Wh·kg−1. In addition, our ZIHC also exhibited excellent cycling stability. After 20000 cycles at 10 A·g−1, it retained 90% of its initial capacity and exhibited high Coulombic efficiency (≈100%). In order to investigate the causes of capacity decay, we examined the cycled zinc foil by scanning electron microscopy and X-ray diffraction. The results showed that a large number of Zn4SO4(OH)6⋅3H2O disordered dendrites were formed on the surface of the zinc foil. These dendrites inhibited the reversible deposition/dissolution of zinc ions, resulting in the capacity decay of the ZIHC during the cycling process. This study will be helpful for developing next-generation high-performance energy storage devices.
Since Fujishima and Honda demonstrated the photoelectrochemical water splitting on TiO2 photoanode and Pt counter electrode, photocatalysis has been considered as one of the most promising technologies for solving both the problems of environmental pollution and energy shortage. This process can effectively use solar energy, the most abundant energy resource on the earth, to drive various catalytic reactions, such as water splitting, CO2 reduction, organic pollutant degradation, and organic synthesis, for energy generation and environmental purification. Except for the various metal-based semiconductors, such as metal oxides, metal sulfides, and metal oxynitrides, developed for photocatalysis, graphitic carbon nitride (g-C3N4) has attracted significant attention in the recent years because of its earth abundancy, non-toxicity, good stability, and relatively narrow band gap (2.7 eV) for visible light response. However, g-C3N4 suffers from insufficient absorption of visible light in the solar spectrum and rapid recombination of photogenerated electrons and holes, thus resulting in low photocatalytic activity. Until now, various strategies have been developed to enhance the photocatalytic activity of g-C3N4, including element doping, nanostructure and heterostructure design, and co-catalyst decoration. Among these methods, element doping has been found to be very effective for adjusting the unique electronic and molecular structures of g-C3N4, which could significantly expand the range of photoresponse under visible light and improve the charge separation. Especially, non-metal doping has been well investigated frequently to improve the photocatalytic activity of g-C3N4. The non-metal dopants commonly used for the doping of g-C3N4 include oxygen (O), phosphorus (P), sulfur (S), boron (B), and halogen (F, Cl, Br, I) and also carbon (C) and nitrogen (N) (for self-doping), as they are easily accessible and can be introduced into the g-C3N4 framework through different physical and chemical synthetic methods. In this review article, the structural and optical properties of g-C3N4 is introduced first, followed by a brief introduction to the modification of g-C3N4 as photocatalysts. Then, the progress in the non-metal doped g-C3N4 with improved photocatalytic activity is reviewed in detail, with the photocatalytic mechanisms presented for easy understanding of the fundamentals of photocatalysis and for guiding in the design of novel g-C3N4 photocatalysts. Finally, the prospects of the modification of g-C3N4 for further advances in photocatalysis is presented.
Lithium-ion batteries have been widely used in portable electronic devices and electric vehicles because of their high energy density and long cycle life. Sodium-ion batteries have broad application prospects in the areas of large-scale electrochemical energy storage systems and low-speed electric vehicles because of their abundant raw materials, low resource cost, safety, and environmental friendliness. However, the development of sodium-ion batteries has been hindered by the low reversibility, sluggish ion diffusion, and large volume variations of the host materials. Suitable electrode materials with decent electrochemical performance must be primarily explored for the successful use of sodium-ion batteries. Since the electrochemical potential and specific capacities of cathode materials have a major impact on the energy densities of sodium-ion batteries, the development of cathode materials is critical. To date, various Na-insertable frameworks have been proposed, and some cathode materials have been reported to deliver reversible capacities approaching their theoretical values. Among them, transition metal oxides show a high reversible capacity and high working potential, but most of them still possess problems such as irreversible phase transition, air instability, and insufficient battery performance. Another type is the Prussian blue analogs. These materials exhibit a favorable operating voltage, cycling stability, and rate capability; however, the main obstacles to their practical application are the control of lattice defects, thermal instability, and low tap density. Polyanionic phosphates are the most promising cathode materials for sodium-ion batteries and have great research value and application prospects because of their stable framework structure, suitable operating voltage, and fast ion diffusion channels. However, their inherent defects, such as poor electronic conductivity and low theoretical energy density, considerably limit their practical applications. Researchers have conducted modification studies through bulk structure adjustment and micro-nano structural control with the goal of improving the performance of phosphate cathode materials and promoting the research and development of sodium-ion energy storage systems. This study reviews the recent advances in phosphate cathode materials for sodium-ion batteries, including orthophosphates, pyrophosphates, fluorophosphates, and mixed phosphate compounds. In this study, the intrinsic relationships among material composition, structure, and electrochemical properties are identified through analyses of the crystal structures, sodium storage mechanisms, and modification strategies of phosphate materials, thereby providing a reference for the continuous modification of polyanion phosphate cathode materials and exploration of high-voltage phosphate cathode materials. Some directions for future research and possible strategies for building advanced sodium-ion batteries are also proposed.
Lithium-ion batteries (LIBs) are widely used in cellphones, laptops, and electric cars owing to their high energy density and long operational lifetime. However, their further deployment in large-scale energy storage systems is restricted by the uneven distribution of lithium resources (~0.0017% (mass fraction, w) in the Earth's crust). Therefore, alternative energy storage systems composed of abundant elements are of urgent need. Recently, sodium-ion batteries (SIBs) have attracted significant attention and are considered to be a potential alternative for next-generation batteries owing to abundant sodium resources (~2.64% (w) of the Earth's crust), suitable potential (−2.71 V), and low cost. SIBs are similar to LIBs in terms of their physical and electrochemical properties. Previous studies have mainly focused on SIB storage materials, including hard carbon, alloys, and hexacyanoferrate, while the safety of SIBs remains largely unexplored. Similar to LIBs, the current electrolytes used in SIBs are mainly composed of flammable organic carbonate solvents (or ether solvents), sodium salts, and functional additives, which pose possible safety issues. Moreover, the chemical activity of sodium is much higher than that of lithium, leading to a higher risk of fire, thermal runaway, and explosion. To overcome this problem, herein we propose a fluorinated non-flammable electrolyte composed of 0.9 mol∙L−1 NaPF6 (sodium hexafluorophosphate) in an intermixture of di-(2, 2, 2 trifluoroethyl) carbonate (TFEC) and fluoroethylene carbonate (FEC) in a 7 : 3 ratio by volume. Its physical and electrochemical properties were studied by ionic conductivity, direct ignition, cyclic voltammetry, and charge/discharge measurements, demonstrating excellent flame-retarding ability and outstanding compatibility with sodium electrodes. The electrochemical tests showed that the Prussian blue cathode retained a capacity of 84 mAh∙g−1 over 50 cycles in the prepared electrolyte, in contrast to the rapid capacity degradation in a flammable conventional carbonate electrolyte (74 mAh∙g−1 with 57% capacity retention after 50 cycles). To test the practical application of the proposed electrolyte, a hard carbon anode was used and exhibited exceptional performance in this system. The enhancement mechanism was further verified by Fourier transform infrared (FTIR), X-ray diffraction (XRD), and scanning emission microscopy (SEM) investigations. Polycarbonate on the surface of the cathode played an important role for the studied electrolyte system. The polycarbonate may originate from FEC decomposition, which can enhance the ionic conductivity of the solid electrolyte interface (SEI) layer and reduce impedance. Hence, we believe that this proposed electrolyte may provide new opportunities for the design of robust and safe SIBs for next-generation applications.
All-solid-state sodium ion batteries (ASIBs) are important for future large-scale energy storage applications. ASIBs have come to occupy an important position in research on advanced secondary batteries in recent years owing to their advantages of abundance in resources, low cost, long lifetime, and high safety. As the key to the success of ASIBs, solid-state electrolytes such as polymers, oxide ceramics, and sulfide glass-ceramics have always attracted immense interest. Chalcogenide electrolytes for ASIBs have high room-temperature conductivity, high elastic modulus, and can be easily pressed into a mold at room temperature; hence, they are the research focus in ASIBs. This paper summarized recent studies on the structure and properties of chalcogenide electrolytes for ASIBs. These studies demonstrate the relationship between the phase structure and ionic conductivity of sulfide-based electrolytes and selenide-based electrolytes. Besides, arguments that the sodium vacancy in the crystal structure dominates ionic conduction, and creating a sodium vacancy via cation substitution is the principal strategy to increasing ionic conduction, are discussed. Further, the intrinsic chemical stability and interface stability between the electrode and electrolyte are highlighted. Based on the soft and hard acid and base theory, some studies adopted various anion/cation ion substitution strategies to improve the chemical stability of chalcogenide electrolytes in humid air. Particularly, the inconsistency in the electrochemical stability window of a representative chalcogenide electrode, Na3PS4, as measured by a semi-blocking electrode and calculated by first-principles, is compared. Additionally, to develop all-solid-state Na-S and Na-O2 batteries with high capacity, the nonnegligible interface instability of the sulfide electrode against the sodium metal anode and feasible solutions are summarized. Next, the research progress on ASIBs using chalcogenide electrolytes is reviewed. Chalcogenide electrolytes are restricted by the electrochemical stability window and chemical compatibility with electrode materials; hence, they are expected to only be applicable to ASIBs using sulfur, sulfide, and organic matter as the cathode and Na-Sn alloy as the anode. However, these ASIBs have long cycling life (> 500 cycles), illustrating their potential applications in large-scale energy storage power stations. Finally, we comprehensively evaluate the ionic conductivity, stability against humid air, stability of the interface, electrochemical stability window, and ease of preparation of typical chalcogenide electrolytes, including Na3PS4, Na3PSe4, Na3SbS4, Na3SbSe4, Na10SnPS12, and Na11Sn2PS12. Moreover, we highlight the challenges and propose possible solutions toward the development of chalcogenide electrolytes in future. Advanced technologies in fine synthesis, in situ characterization, and surface/interface modification are essential to overcome existing challenges and promote the development of chalcogenide-electrolyte-based ASIBs.
Nuclear magnetic resonance (NMR) is an effective and widely adapted technique that can be used for medical diagnosis and chemical analysis. However, its application has been limited by low sensitivity originating from the extremely low polarization of nuclear spins that follow a typical Boltzmann distribution. In principal, it is possible to break this Boltzmann distribution using different physical or chemical mechanisms to generate hyperpolarization and increase NMR sensitivity by several orders of magnitude. The crucial point of hyperpolarization is to transfer the polarization from highly polarized systems to nuclear spins. For example, the unpaired electrons in organic radicals or other systems exhibit much higher polarization than that of nuclear spins (~660 times higher than 1H) under the same magnetic field. The high polarization of electrons at thermal equilibrium can be transferred to nuclear spins via microwave irradiation and hyperfine coupling. This hyperpolarization method is called dynamic nuclear polarization (DNP) and has been successfully and widely applied for the evaluation of the protein structure and the examination of nanomaterial surface chemistry. Electron spins can also be hyperpolarized using circularly polarized light (CPL) or nonpolarized light in some systems, and this polarization can be transferred to nuclear spins as well. These hyperpolarization methods are referred to as optical pumping (OP) and optical nuclear polarization (ONP), respectively. A common application of OP is the production of hyperpolarized noble gases, including hyperpolarized xenon-129, which can be used in magnetic resonance imaging of lungs or evaluation of porous structures. For ONP, the nitrogen-vacancy center in diamond is the most promising system that has demonstrated the ability to track the precession of a single spin. In addition, electrons can be polarized by certain chemical reactions as used in chemically induced dynamic nuclear polarization (CIDNP). CIDNP can be used to study the active sites of proteins and identify low-concentration intermediates that are formed during chemical processes. In addition to electrons, hydrogen molecules with unique spin state, i.e., parahydrogen, can be converted to hyperpolarized NMR signals via hydrogen addition reactions, which is known as parahydrogen induced polarization (PHIP). PHIP was originally used to understand the mechanisms of hydrogenation processes, but has recently become a promising hyperpolarization technique via the protocols of signal amplification by reversible exchange (SABRE). Herein, the basic mechanisms and potential applications of DNP, OP, CIDNP, and PHIP techniques are reviewed. These emerging hyperpolarization techniques have the potential to push the limits of NMR beyond current conceptions.
Sodium-ion batteries (SIBs) are promising candidates to replace lithium-ion batteries (LIBs) to meet the emergent requirements of various commercial applications. SIBs and LIBs are similar in many aspects, including their reduction potentials, approximate energy densities, and ionic semidiameters. Analogously, safety issues, including liquid leakage, high flammability, and explosiveness limit the usage of SIBs. All-solid-state batteries have the potential to solve the aforementioned problems. However, polycarbonates as promising solid electrolytes have been rarely exploited in all-solid-state SIBs. In addition, organic electrode materials, including non-conjugated redox polymers, carbonyl compounds, organosulfur compounds, and layered compounds, have been intensively investigated as part of various energy storage systems owing to their low cost, environmental friendliness, high energy density, and structural diversity. Nevertheless, the dissolution of small organic compounds in organic-liquid electrolytes has hindered its further applications. Fortunately, the utilization of solid polymer electrolytes combined with organic electrode materials is a promising method to prevent dissolution into the electrolyte and improve the cycling performance of SIBs. Thus, we proposed the utilization of a poly(propylene carbonate) (PPC)-based solid polymer electrolyte and cellulose nonwoven with a 3, 4, 9, 10-perylene-tetracarboxylicacid-dianhydride (PTCDA) cathode in an all-solid-state sodium battery (ASSS). The solid electrolyte significantly enhanced the safety of the SIB and was successfully synthesized via a facile method. The morphology of the as-prepared solid electrolyte was examined by electron scanning microscopy (SEM). Furthermore, the electrochemical performances of the PTCDA/Na battery with organic-liquid and solid electrolytes at room temperature were compared. The SEM results demonstrated that the solid polymer electrolyte and sodium bis(fluorosulfonyl)imide (NaFSI) were evenly distributed inside the pores of the nonwoven cellulose. The ionic conductivity of the composite solid polymer electrolyte (CSPE) at room temperature was 3.01 × 10-5 S·cm-1, suggesting that the CSPE was a promising candidate for commercial applications. In addition, the ASSS showed significantly improved cycling performance at a current density of 50 mAh·g-1 with a high capacity retention of 99.1%, whereas the discharge capacity of the liquid PTCDA/Na battery was only 24.6mAh·g-1 after 50 cycles. This indicated that the cycling performance of the PTCDA cathode in the SIB was largely improved by preventing the dissolution of the PTCDA cathode material in the electrolyte. Electrochemical impedance spectroscopy results demonstrated that the CSPE was compatible with the organic cathode electrode.
Because of their high energy density and long cycle life, lithium-ion batteries (LIBs) have dominated the portable electronics market for over 20 years. However, with the increasing demand for large-scale energy storage systems for grid applications, the price of Li resources has increased owing to the low abundance of Li in Earth's crust and non-uniform distribution on the planet. Because Na has similar physical and chemical properties as Li and is an abundant natural resource, room-temperature sodium-ion batteries (SIBs) are expected to be among the most promising next-generation large grid energy storage devices. It is known that the cathode, anode, separator and electrolyte materials are the main components of batteries. Among these, Na-containing cathode materials are of critical importance. As a cathode material for SIBs, polyanion-type compounds have become a hot research topic owing to their versatile structural frameworks, high thermal stabilities, high ambient stabilities even in the charging state, small volume changes, tunable operating voltage by tuning the chemical environment of the polyanions, and high operating voltages owing to the inductive effects of the polyanionic groups (PO43−, SO42−, SiO44−, etc.). In particular, for Earth's abundant resources and inherent stability, polyanion-based compounds are suitable for large-scale stationary energy storage. Taking grid balancing into account, batteries with fast charge rates are in demand, which requires cathodes having high rate capability. However, despite the presence of ion diffusion channels in polyanion compounds, the electronic transport channels are blocked owing to the separation of the metal polyhedral and the strong electronegativity of the anions, leading to poor electron conductivity, which largely limits the rate capability of polyanion compounds. Therefore, it is crucial to understand the inherent limitation of the kinetics in terms of the structural aspects and to determine strategies for improving the rate capability. This review discusses the intrinsic reasons for the factors impacting ion diffusion based on the different structures of polyanion-type cathodes. From the perspectives of surface modification and morphology, strategies for enhancing the transport of sodium ions and electrons at the surface and interface are summarized and discussed. Then, from the standpoint of the hierarchical structures of materials to the design of a structural framework, which have been rarely reported, this review proposes schemes that intrinsically enhance the rate capability of polyanion compounds and provides a perspective on developments that can further improve the rate capability of cathode materials. This review provides suggestions for designing and optimizing high-rate polyanion-type and other kinds of cathodes from both academic and practical viewpoints.
Organic solar cells (OSCs) are a promising next-generation photovoltaic technology that can be used to harvest clean and renewable solar energy. OSCs are typically composed of donor:acceptor blends as photo-active materials. Compared to the conventional inorganic silicon solar cells, OSCs are suitable for large-scale production using roll-to-roll technology, promising low-cost and the potential to avoid environmental pollution. The last few years have witnessed the rapid development of OSCs. The power conversion efficiencies (PCEs) of OSCs have surpassed ~14%–16%, benefiting from the design of novel materials, optimization of blend morphology, and deep understanding of the charge generation mechanism. Currently, the most widely used processing solvents for preparing high-efficient OSCs are chlorinated or aromatic solvents including chlorobenzene, dichlorobenzene, and chloroform, which are highly detrimental to the environment and human health, and may not be utilized for future in industry. Thus, replacing these highly toxic solvents with environmentally friendly alternatives called "green solvents" is an important topic in OSC research. Herein, poly[(2, 6-(4, 8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1, 2-b:4, 5-b′]dithiophene)-co-(1, 3-di(5-thiophene-2-yl)-5, 7-bis(2-ethylhexyl)benzo[1, 2-c:4, 5-c′]dithiophene-4, 8-dione)] (PBDB-T) was used as a reference material to design and synthesize a novel conjugated polymer (PBDB-DT) by extending the alkyl side chains and enlarging the conjugated side groups. The thermal stability of the polymer donor was examined via thermogravimetric analysis, showing that the polymers exhibit very good stability at > 400 ℃. Importantly, PBDB-DT exhibits good solubility in low-toxic solvent tetrahydrofuran (THF) due to its longer alkyl side chains, and shows a strong aggregation effect in THF due to the larger conjugated side groups. A favorable PCE of 10.2% was achieved for the THF-processed PBDB-DT:IT-M based OSC device. In contrast, PBDB-T has limited solubility in THF. The solar cell device based on PBDB-T:IT-M delivered a moderate PCE of 6.41%. The investigation of blend morphology via atomic force microscope suggested that the PBDB-DT:IT-M has a smooth surface, which is favorable for charge generation and transport. These results demonstrate that molecular optimization is a promising strategy to modulate the solubility and achieve high efficiency for organic photovoltaic materials processed using green solvents.
Methane is a promising energy source with vast reserves, and is considered one of the promising alternatives to nonrenewable petroleum resources because it can be converted into valuable hydrocarbon feedstocks and hydrogen through appropriate reactions. Recently, the conversion of CH4 into other high-value-added products has received increasing attention because of their sustainability for energy and the environment. However, methane has a tetrahedral geometry with four equivalent C―H bonds due to the sp3 hybridization of the central carbon atom, with a C―H bond length of 0.1087 nm and an H-C―H bond angle of 109.5°. The absence of a dipole moment and the small polarizability (2.84 × 10−40 C2·m2·J−1) imply that methane requires a high local electric field for polarization and for nucleophilic or electrophilic attack. Nevertheless, it is believed that an effective method to activate CH4 would be available, so that not only methanol, formaldehyde, and ethylene but also other industrially valuable raw materials can be obtained. On the other hand, the conversion of this combustible gas into the corresponding liquid fossil fuel proceeds via secondary chemical conversion, and it can greatly reduce transportation costs. From the economic viewpoint, this can still provide considerable benefits. Homogeneous catalysts have been reported to catalyze methane, but most of them operate at high pressures (2–7 MPa), or in strongly acidic media and at high temperatures (up to 500 K). Heterogeneous catalysts reported in the literature are also active only at high temperatures. Therefore, finding an efficient method to active methane has become a hot research topic. Photocatalysis technology is recognized as the optimal solution for the conversion of CH4 since solar energy is by far the largest exploitable resource of energy. In the past years, much effort has been undertaken for the conversion of CH4 under light at low temperature. In this regard, several photocatalysts, including silica-alumina-titania, silica-supported oxides, and ceria- and zeolite-based materials, have been developed. In photocatalytic methane conversion, the C―H bond can be selectively activated by adjusting the wavelength and intensity of the incident light and the oxidation capacity of the photocatalysts, thereby avoiding the formation of byproducts. This review summarizes a series of photocatalytic direct methane conversion systems developed in recent years, including methane oxidation and coupling processes. The effects of the catalyst composition and structure, oxidant, and electron transfer on the activation of the C―H bond of methane are detailed. Finally, future perspectives and challenges for the photocatalytic conversion of methane are discussed.
Sodium has the advantages of being an abundant resource and having a low cost; thus, sodium ion batteries are considered as one of the best candidates for replacing lithium ion batteries in the future. However, the radius of the sodium ion is larger than that of the lithium ion, and the de-intercalation of the sodium ion will seriously damage the crystal structure of most electrode materials during the charging and discharging process, which considerably limits its charge-discharge specific capacity, cycle performance, and rate performance. However, finding appropriate electrode materials is one of the difficulties in fabricating high-performance sodium ion batteries. Among the many candidate materials, vanadate materials can improve the stability of material structures by introducing cations to increase the coordination numbers of vanadium, thus improving the electrochemical performance of sodium ion batteries. In this paper, an in situ phase separation method to fabricate V2O5/Fe2V4O13 nanocomposite materials is reported. First, we synthesized hydrated crystalline Fe5V15O39(OH)9·9H2O nanomaterials using a water bath heating method; then, we in situ constructed two-phase nanocomposite V2O5 and Fe2V4O13 from the single phase by further high-temperature treatment. The morphology, composition, and structure of the electrode materials were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy(FTIR), as well as other methods. The V2O5/Fe2V4O13 nanocomposite materials were found to have a more stable structure, higher initial discharge capacity (342 mAh·g-1 at a current density of 0.1 A·g-1), longer cycle life, and better rate performance than V2O5 nanowires. Therefore, this research on V2O5/Fe2V4O13 nanocomposite materials has broadened ability to develop new high-performance anode materials for sodium ion batteries.
The rapid economic development has necessitated increased environmental protection and tremendous efforts have been devoted to designing and preparing environmentally friendly energy storage and conversion devices, including batteries, supercapacitors (SCs), and solar cells. As useful energy storage and conversion devices, SCs have received significant attention due to their high power density, long cycle stability, and rapid charging rate. The performance of SCs largely depends on the electrode materials, which can be composed of metal oxides, conducting polymers, carbon materials, and their composites. Carbon materials, including carbon nanotubes, carbon fibers, porous carbons, template carbons, and graphene-like carbon nanosheets (GCNSs), have attracted significant attention owing to their tailorable pore size and excellent physiochemical stability. GCNSs are considered to be outstanding carbon materials for use in high-performance SC because of their large specific surface area and high electrical conductivity. To date, many carbon precursors have been used to synthesize GCNSs for SCs such as coal, biomass, and chemical by-products. In particular, cheap and abundant chemical by-products for the synthesis of carbon materials can reduce preparation costs and environmental pollution as well as achieve high value-added chemicals. Coal tar, a by-product of the coal coking process, is rich in aromatic polycyclic hydrocarbon molecules, which can be polymerized, carbonized, and activated to synthesize GCNSs for SCs. In addition, MgO particles can be used as templates due to their stable properties and low-cost compared with other metal oxide templates (e.g. Fe2O3, NiO, or CuO), imparting space confinement and structure guidance during the preparation of GCNSs. Herein, we report a facile method for the preparation of interconnected graphene-like nanosheets (IGNSs) from coal tar by MgO templating combined with in-situ KOH activation. The IGNSs were obtained after the impurity removal by repeated washing with dilute acid. The as-synthesized IGNSs feature high specific surface area of up to 2887 m2∙g−1 and abundant hierarchical short pores, which provide abundant active sites for ion adsorption, supply plentiful channels for fast ion transport and boost electrical conductivity. As electrodes for SCs, IGNSs manifest high specific capacitance value of up to 313 F∙g−1 at 0.05 A∙g−1, good rate capability of 261 F∙g−1 at 20 A∙g−1, and excellent cycle stability with 92.7% of initial capacitance retained after 10000 cycles in 6 mol∙L−1 KOH aqueous electrolyte. This study provides a facile method for large-scale production of IGNSs from aromatic hydrocarbon molecules for use in high-performance energy storage devices.
The sol-gel method, developed for over 150 years, is a conventional route for designing and preparing various kinds of metal oxide materials. In the sol-gel method, different chemical agents are homogenously mixed together in aqueous or organic solutions. During the evaporation of the solvents, the solution transforms to sol and gel through polycondensation or polyesterification reaction, and the dried gel is obtained after the complete evaporation of the solvent. Then, the dried precursor is often heat-treated in air at high temperature to induce the formation of oxide materials, especially the multi-component oxide materials that are difficult to prepare using other methods. Recently, new developments have been achieved in the sol-gel method. The application of the sol-gel method has been extended to the preparation of metallic nanomaterials, especially the alloy nanocrystals. For instance, the sol-gel method can be used to prepare CoPt and FePt hard magnetic alloy nanocrystals; CoCrCuNiAl high-entropy alloy nanocrystals; Ni3Fe and Cu3Pt alloy nanocrystals with equilibrium-ordered crystalline phases; and Ni, Cu, Bi, Sb, Te, Ag, Pt, and Pd monometallic nanocrystals. This article reviews the recent progresses in the sol-gel method for designing and preparing metallic and alloy nanocrystals, as well as the detailed experimental procedures and the different metallic nanocrystals that can be obtained by the sol-gel method. The crystalline phase formed in the final calcined products can be determined from the thermodynamic calculations of the sol-gel method. The thermodynamic model involves the calculation of the Gibbs free energy change of the reaction between the metallic oxide and reducing gases, such as hydrogen. A negative change and a positive change in the Gibbs free energy of the reaction correspond to the formation of metallic and alloy crystalline phases, or oxide crystalline phase, respectively. Based on the thermodynamic calculations and the relationship between the Gibbs free energy and standard electrodynamic potential of the chemical reaction, a new parameter, metal oxide standard electrode potential, was proposed. This electrode potential is different from the conventional standard metal electrode potential. A metallic crystalline phase is obtained if the electrode potential of the corresponding metal oxide is positive, while a metal oxide crystalline phase is obtained if the electrode potential of the metal oxide is negative. We also discuss the possible applications, including the magnetic and electrocatalytic applications, of the metallic and alloy nanocrystals that have been obtained by the sol-gel method. Finally, the future prospects of the application of the sol-gel method in designing metallic and alloy nanocrystals are discussed.
Organic field effect transistors (OFETs) have great potential in flexible sensor and display driver applications. However, there are immense challenges in the development of large-area and high-quality thin-film fabrications. In this article, we introduce a method to fabricate patterned organic semiconductor films by oxygen plasma treatment and the synergy of Marangoni and coffee-ring effects. The procedure is as follows: First, we spin-coated the cyclic transparent amorphous fluoropolymers (CYTOP) on the substrate in the form of a hydrophobic layer. Then, parts of the substrate surface were treated with plasma and modified to make them hydrophilic. By comparing the water contact angle on the plasma treated surface with that on the untreated surface, we optimized the treating time to get a relatively uniform water contact angle on a different region of the substrate surface. The plasma treated substrate was dipped into 2, 7-dioctylbenzothieno[3, 2-b]benzothiophene (C8-BTBT) solution with methylbenzene and carbon tetrachloride as a mixed solvent, and then lifted from it. So the mixed solution flowed down rapidly on the hydrophobic portion of the surface, leaving droplet on the hydrophilic portion. Subsequently, the droplet started evaporating under the synergy of Marangoni and coffee-ring effects. Based on the difference between the hydrophilic and hydrophobic portions on the substrate surface, we successfully obtained the patterned C8-BTBT thin films on the substrate. Furthermore, the solvent ratio was optimized while growing the C8-BTBT film to adjust the boiling point of the solution, which was due to a fully covered surface was obtained. From the grazing-incidence X-ray diffraction (GIXRD) measurement of the films with three different concentrations, we observed that increasing in the concentration of the solution yielded different molecular orientations. Based on the three films, OFETs with bottom gate and top contact structure were fabricated. Moreover, the mobility and the on/off current ratio became more uniform with the progressive increase in the concentration of the solution. This may be attributed to the increase in the number of different molecular orientations and charge transfer channels. Although the increase in the number of different molecular orientations might lead to the decrease in mobility, it could improve the alignment of the electric field and also increase π–π stacking direction of the molecules, which promote highly uniform device performance distribution. Since uniform distribution of device performance is significant for practical applications, we believe the transistors that are fabricated at the highest concentration are better than those generated at lower concentrations. Thus, on the 5 cm × 5 cm substrate, it is observed that the average mobility of the transistors is 7.9 cm2·V-1·s-1, and all the devices have threshold voltages less than -2 V with the on/off current ratio of 104. This work is significant for the fabrication of large-area and high-performance thin films and transistors.
Lithium metal is the most promising anode material for Li (ion) batteries from the viewpoint of energy density because of its high theoretical specific capacity (3860 mAh∙g-1, 2061 mAh∙cm−3) and low reduction potential (−3.04 V vs standard hydrogen electrode (SHE)). Lithium has been used as an anode material for lithium metal batteries since the 1970s. However because of the serious reaction between Li and non-aqueous electrolytes, the large volume expansion during Li plating, and the formation of Li dendrites during cycling, Li batteries with Li metal anodes show very low Coulombic efficiency (CE) and are easily short-circuited. This limits the widespread commercialization of Li metal anodes for Li batteries. Motivated by our previous study on the development of a Li carbon nanotube (Li-CNT) composite anode material, in this study, we prepared a Si-loaded Li carbon nanotube composite (Li-CNT-Si) via a facile molten impregnation method. The introduction of Si nanoparticles increased the Li content of the composite, thus increasing its specific capacity (the specific capacity of the Li-CNT composite increased from 2000 mAh∙g-1 to 2600 mAh∙g-1 with the addition of 10% Si (mass fraction)). Moreover, Si nanoparticles decreased the polarization for Li plating/stripping, resulting in an improved electrochemical performance. The Li-CNT-Si composite showed the merits of the Li-CNT composite with the advantages of limited electrode volume expansion and negligible Li dendrite formation during cycling. Furthermore, the Si nanoparticles filled the pores inside the Li-CNT microspheres, thus preventing the electrolyte from flowing into the microspheres to corrode the Li metal present inside them. Hence, the incorporation of Si nanoparticles improved the CE of the composite anode. When the 10% Si-loaded Li-CNT-Si composite was used as an anode and coupled with a commercial LiFePO4 cathode, the resulting battery showed more than 900 stable cycles in an ether-based electrolyte at a charge/discharge rate of 1C (0.7 mA∙cm-2) corresponding to a CE of 96.7%, which is considerably higher than those of the Li-CNT (90.1%) and Li metal foil (79.3%) anodes obtained under the same conditions. We believe that the Li-CNT-Si composite prepared in this study is a promising anode material for Li secondary batteries having high energy density, particularly for those employing Li-free cathodes, e.g., Li-sulfur and Li-oxygen batteries.
In the past few decades, new energy industries have developed rapidly due to the threat of the depletion of non-renewable resources. Among them, the lithium-ion battery has attracted significant attention of various researchers. However, lithium-ion batteries are limited by the uneven distribution of lithium resources and high cost. Sodium, which is in the same periodic group as Lithium, can help alleviate the problems related to the limited development of lithium ion batteries owing to the shortage of lithium resources. Sodium ion batteries are cheap, with varying choice of electrolytes, and have relatively stable electrochemical performances. However, the radius of a sodium ion is larger than that of a lithium ion, leading to slow ion transportation as well as changes in the volume of the host material during the charging and discharging processes. Therefore, compared with existing lithium ion batteries, sodium ion battery anode materials are very limited. Moreover, most sodium ion battery electrode materials have low specific capacities and poor cycle retention rates. Among these, ternary metal oxides, which have two different cations and can reversibly react with sodium ions are promising high-capacity anode materials for sodium ion batteries. In this study, Ti2Nb2O9 nanosheets are obtained by ion-exchange and chemical-delaminate methods. The carbon-coated Ti2Nb2O9 nanosheets are obtained after hydrothermal coating with sucrose and calcination. From the thermogravimetric analysis (TG) curve, the carbon content in the composites is calculated to be approximately 8.0%. Owing to the rich reactive sites and a short ion transport pathway, the Ti2Nb2O9/C electrode delivers a high reversible capacity of 265.2 mAh·g-1 at a current density of 50 mA·g-1. Even at a high current density of 500 mA·g-1, the electrode exhibits an excellent electrochemical performance with a reversible capacity of 160.9 mAh·g-1 after 200 cycles (capacity retention of 75.3%). Additionally, the Ti2Nb2O9/C nanosheets exhibit high reversible capacities of 251.3, 224.6, 197.4, 176.3, and 156.5 mAh·g-1 at the current densities of 100, 200, 500, 1000, and 2000 mA·g-1, respectively. It is demonstrated through the use X-ray photoelectron spectroscopy (XPS) that the following process involving the transfer of four electrons occurs: Ti4+/Ti3+, Nb5+/Nb4+, during the charging and discharging process of the Ti2Nb2O9 electrode in the voltage range of 0.01–3.0 V. The theoretical specific capacity of Ti2Nb2O9 in this process is calculated to be 252 mAh·g-1, corresponding to the electrochemical data. Overall, this study demonstrates that the Ti2Nb2O9/C anode nanosheets have an excellent charge-discharge performance, cycle stability, and rate performance in sodium ion batteries, thereby providing a feasible choice for sodium ion battery anode materials.
In this study, a novel silicon carbide/platinum/cadmium sulfide (SiC/Pt/CdS) Z-scheme heterojunction nanorod is constructed using a simple chemical reduction-assisted hydrothermal method, in which Pt nanoparticles are anchored at the interface of SiC nanorods and CdS nanoparticles to induce an electron-hole pair transfer along the Z-scheme transport path. Multiple characterization techniques are used to analyze the structure, morphology, and properties of these materials. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) results show that the SiC/Pt/CdS materials with good crystal structure are successfully synthesized. Transmission electron microscopy reveals that Pt nanoparticles grow between the interfaces of SiC nanorods and CdS nanoparticles. UV-Vis diffuse reflectance spectroscopy shows that the as-prepared Z-scheme heterojunction samples have a wider light absorption range in comparison with pristine CdS materials. Photoluminescence spectroscopy and the transient photocurrent response further demonstrate that the SiC/Pt/CdS nanorod sample with an optimal molar ratio possesses the highest electron-hole pair separation efficiency. The loading amount of CdS on the surface of SiC/Pt nanorods is effectively adjusted by controlling the molar ratio of SiC and CdS to achieve the optimal performance of the SiC/Pt/CdS nanorod photocatalysts. The optimal H2 evolution capacity is achieved at SiC : CdS = 5 : 1 (molar ratio) and the maximum H2 evolution rate reaches a high value of 122.3 µmol·h−1. In addition, scanning electron microscopy, XRD, and XPS analyses show that the morphology and crystal structure of the SiC/Pt/CdS photocatalyst remain unchanged after three cycles of activity testing, indicating that the SiC/Pt/CdS nanocomposite has a stable structure for H2 evolution under visible light. To prove the Z-scheme transfer mechanism of electron-hole pairs, selective photo-deposition technology is used to simultaneously carry out the photo-reduction deposition of Au nanoparticles and photo-oxidation deposition of Mn3O4 nanoparticles in the photoreaction. The experimental results indicate that during photocatalysis, the electrons in the conduction band of CdS participate mainly in the reduction reaction, and the holes in the valence band of SiC are more likely to undergo the oxidation reaction. The electrons in the conduction band of SiC combine with the holes in the valence band of CdS to form a Z-scheme transport path. Therefore, a possible Z-scheme charge migration path in SiC/Pt/CdS nanorods during photocatalytic H2 production is proposed to explain the enhancement in the activity. This study provides a new strategy for synthesizing a Z-scheme photocatalytic system based on SiC nanorods. Based on the characterization results, it is determined that SiC/Pt/CdS nanocomposites are highly efficient, inexpensive, easy to prepare, and are stable structures for H2 evolution under visible light with outstanding commercial application prospects.
Compressible supercapacitor is a promising flexible energy storage device in view of its excellent capacitive performance, which is recoverable at different compression states. The compressible electrode constitutes the core component that largely determines the performance of a compressible supercapacitor. Commercial polymer sponges are highly compressible materials because most of them are composed of elastic and interconnected polyurethane fibers. However, polymer sponges cannot be directly used as supercapacitor electrodes due to their non-conductive polymer framework. In contrast, carbon sponge (CS) derived from melamine sponge has superior compressible property and exhibits substantially improved conductivity compared to commercial polymer sponge. However, the low specific surface area of CS leads to low specific capacitance, which severely limits its application as compressible supercapacitor electrodes. Currently, pseudocapacitive materials are grown on the conductive CS framework to form hybrid electrodes with improved specific capacitance. Among various pseudocapacitive electrode, iron oxides have attracted considerable attentions due to their natural abundance, high theoretical specific capacitance, and negative working potential. Moreover, the much higher specific capacitance than that of carbon electrodes makes iron oxides one of promising negative candidates for configuring an asymmetric supercapacitor. Herein we report the successful growth of α-Fe2O3 nanosheets on CS by electrodeposition followed by low-temperature thermal annealing. The α-Fe2O3 on CS displays typical nanosheet morphology with mass loading ranging from 3.4 to 6.7 mg·cm-3 that can be facilely controlled by extending the deposition time from 4 to 16 h. The CS-Fe2O3 electrode retains 90% of its geometric height even after manual compression for 100 cycles. Moreover, the CS-Fe2O3 can withstand 60% strain even at Fe2O3 mass loading as high as 6.5 mg·cm-3. The performance of the CS-Fe2O3 electrode at different strains was systematically investigated in 3.0 mol·L-1 KOH aqueous electrolyte by cyclic voltammetry (CV), galvanostatic charge-discharge, and electrochemical impedance spectroscopy (EIS) in a three-electrode system. Our results show that the CS-Fe2O3 composite electrode produces lower specific capacitance at lower strain. The EIS characterization and IR drop results indicate that this is due to the larger internal resistance arising from looser contact of electrode with the current collector and longer ion diffusion length. Particularly, the CS-Fe2O3-12 electrode delivers a maximum specific capacitance of 294 F·g-1 at a current density of 1.0 A·g-1, 1.7-times higher than that of CS substrate. Assuming that the specific capacitance of CS-Fe2O3-12 is derived from the double-layer capacitance of CS and the pseudocapacitance of Fe2O3, the capacitance of Fe2O3 nanosheets in the hybrid is calculated to be as high as 421 F·g-1, much higher than most of recently reported results, showing that the sheet-like structure with more exposed active sites and short ion pathways could dramatically improve the utilization efficiency of the electrode for reversible faradaic reactions. More importantly, the CS-Fe2O3-12 electrode retains 87% of its initial capacitance after continuous charge-discharge at 5.0 A·g-1 for 10000 cycles, showing promising application as a stable and compressible supercapacitor electrode.
Thermal analysis (TA) is a technology that can be applied to evaluate the relationship between the physical properties of substances and temperature changes under programmed temperature control. It has been widely used in many fields and is particularly useful for determining the thermal stability and service life of polymers and other materials, the stability of drugs, and the danger of flammable and explosive materials. Simultaneously, the mechanism of dehydration, decomposition, and degradation of inorganic materials or dissociation of complexes can be studied and the decomposition rates of environmental pollutants can be estimated. Recently, TA kinetics has become the most extensively studied topic in TA research. The main purpose of kinetic analysis is to obtain the three kinetic triplets of a reaction process, namely, activation energy Ea, pre-exponential factors A, and and mechanism function f(a). For a solid-state reaction, many mathematical models and corresponding data processing methods can be used for the study of TA kinetics. These methods can be classified as either isothermal or non-isothermal methods and further divided into integral and differential methods in the form of the kinetic equation. These equations can be divided into a single scanning rate and multiple scanning rate methods (isoconversion method) by the operation method. The isoconversion method can calculate activation energies without the mechanism function, and the complexity of the reaction can be determined by the change in activation energy as a function of conversion rate. Therefore, the International Confederation for Thermal Analysis and Calorimetry (ICTAC) recommends the isoconversion method for processing TA data. Because of the limitation of traditional isoconversion methods, novel isoconversion methods have been proposed over the past 10 years. The relationship among the existing dynamic analysis methods must be complementary, instead of competitive, because the reliability of the analysis results can be improved only through complementarity. Further efforts to popularize modern integral and differential methods with equal conversion rates are essential. Herein, the progress in isoconversion method development is briefly introduced. A novel kinetic equation and seven new isoconversion methods are reviewed, and the characteristics and limitations of these methods are discussed. In addition, the development trends and prospects of TA kinetics research methods are highlighted. We suggest that the Arrhenius formula should be modified on the basis of the relationship between the rate constant and temperature. The rate equation that is more suitable for non-isothermal and heterogeneous reactions should be used. The mechanism of multi-step solid-state reactions should be studied in depth, and unified standards must be adopted for the study of thermal decomposition kinetics. This represents imminent and important progress in the study of TA kinetics.
With the increasing energy demands for electronic equipment, numerous studies have been conducted to achieve higher energy conversion and develop storage devices such as metal-air batteries, water splitting devices, and fuel cells. All these devices are related to the oxygen evolution reaction (OER) and/or oxygen reduction reaction (ORR). Currently, platinum group metals (PGMs) or their oxides are the most active electrocatalysts for OER and ORR. However, the high cost and scarcity of these noble metals hinder their widespread application. Therefore, the development of a low-cost electrocatalyst that exhibits catalytic performance comparable to or better than that of PGMs is essential.
Metal-organic frameworks (MOFs) are a new class of porous materials constructed from metal ions and organic linkers. MOF materials have diverse metal centers. In addition, organic ligands containing various heteroatoms can change the microenvironment of these metal centers. Moreover, the size, morphology, and porosity of MOF materials can be precisely tuned. These advantages of MOF are beneficial for electrocatalytic reactions. However, MOF is generally considered to be a poor electrocatalyst and is rarely used in the field of electrocatalysis because of its low electrical conductivity. To increase the electrical conductivity of MOF, high-temperature calcination or hybridization with conductive supports is necessary. However, high-temperature calcination may sacrifice the intrinsic molecular metal active sites of MOFs, whereas hybridization with conductive supports may block their inherent micropores. The development of MOF materials with high electrical conductivity is vital for electrocatalysis.
Herein, we report a two-dimensional conductive MOF based on copper foam growth (Cu3HITP2/CF, where HITP = 2, 3, 6, 7, 10, 11-hexaaminotriphenylene hexahydrochloride, CF = copper foam), which has high electrical conductivity and excellent catalytic stability and can be used as a bi-functional electrocatalyst in OER and ORR. In addition, this catalyst does not require heat treatment or the addition of a conductive agent. We first electroplated needle-shaped Cu(OH)2 nanowires onto the surface of a blank copper foam, and then immersed it in a solution of HITP to convert it into Cu3HITP2 at 65 ℃. To confirm its physicochemical properties, the as-synthesized Cu3HITP2/CF was characterized and analyzed by X-ray diffraction, infrared spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. The morphology was characterized by scanning and transmission electron microscopy. The as-synthesized Cu3HITP2/CF maintained a two-dimensional needle-like morphology during the reaction and could be stably operated in an alkaline solution. The overpotential at 10 mA·cm-2 in the OER was only 1.53 V, and the current density did not decrease significantly after 24 h. The Faraday efficiency was as high as 96.84%, and only 1.57% of the by-product H2O2 was produced. In addition, during the ORR, the half-wave potential of Cu3HITP2/CF reached 0.75 V and its activity did not decrease significantly after 2000 cycles of voltammetric scanning. Moreover, its electron transfer number was 3.85, with 5.7% H2O2 generation. Comparative experiments with powder Cu3HITP2 showed that Cu3HITP2 grown on copper foam had a larger electrochemical specific surface area and exhibited superior OER and ORR properties, which was due to its two-dimensional needle-like morphology. In general, this study not only provides a method for in-situ growth of MOF materials on copper foam but also provides new ideas for developing two-dimensional conductive MOF materials in the field of electrocatalysis.
In order to meet high-performance propulsion system requirements for aerospace technology and severe future restrictions on hydrazine use, research on non-toxic, high-performance, and low-cost propulsion technology is urgently needed. The N2O-C2 hydrocarbon monopropellant NOFBX (Nitrous Oxide Fuel Blend) provides significant benefits for meeting these criteria and has become a focus of increased research in recent years. In this study, a chemical kinetic model for NOFBX combustion that integrates the reduced C2 sub-mechanism, the N2O sub-mechanism in the literature, and the N2O/CH species reaction mechanism has been developed. The present mechanism consists of 52 species and 325 elementary reactions. For better predictions of ignition and combustion characteristics, the kinetic parameters of the sensitive reactions with comparatively high rate constant uncertainties have been revised. The present model has been validated against published experimental data, including flow reactor results on N2O/H2O/N2 mixture decomposition, shock tube ignition delay times on N2O/C2 hydrocarbons diluted with N2 or Ar mixtures, heat flux of flat flame laminar flame speeds on N2O/C2H2 diluted with N2 mixtures, and Bunsen flame laminar flame speeds on N2O/C2H4 diluted with N2 mixtures. Additionally, this study compares the new model to other published small hydrocarbon fuel kinetic models with a NOx sub-mechanism. The experimental validations show that the present model accurately captures the nitrous oxide decomposition process and precisely predicts N2O, O2, NO, and NO2 vital species concentration distributions. For all N2O-C2 hydrocarbon fuel systems (ethane-, ethylene-, and acetylene-nitrous oxide), the ignition delay times predicted by the present model are in good agreement with the experimental data. Furthermore, at a wider range of initial temperatures (1100-1700 K), initial pressures (0.1-1.6 MPa), and equivalence ratios (0.5-2.0) for the ignition delay times of ethylene-nitrous oxide, the present model exhibits improved predictions of experimental data. For the laminar flame speeds of N2O-C2H2 and N2O-C2H4 mixtures, the present model generally exhibits satisfactory predictions of the experimental data over the whole range of equivalence ratios (0.6-2.0). However, at initial pressure 0.1 MPa and equivalence ratios of 1.0-1.6 for N2O-C2H4 laminar flame speeds, the present model slightly underestimates experimental data. Considering the much higher uncertainty of the measured laminar flame speeds by the Bunsen flame method, this discrepancy is acceptable. Due to the small scale, full experimental validations and good applicability, the present model can be used to further research on multi-dimensional combustion simulation in NOFBX engine combustors.
Hydrogen (H2), a clean and sustainable energy carrier, is regarded as one of the most promising alternatives to carbon-based fuels. Hydrogen can be generated in a more sustainable way from renewable energy sources via electrocatalytic water splitting. However, the high cost and low abundance of the benchmarking platinum-based hydrogen evolution reaction (HER) catalysts hinder their widespread applications. Thus, developing highly efficient, stable, and low-cost electrocatalysts to replace platinum for HER is imperative, but remains a challenging task. Recently, efforts have been devoted to developing non-noble HER electrocatalysts, including transition metal carbides, oxides, phosphides, and sulfides. However, traditional synthetic strategies cannot effectively control active sites and the catalysts tend to aggregate under high temperature. Recently, polyoxometalates (POMs) have been applied as precursors for the preparation of non-noble HER electrocatalysts as they contain discrete metal-oxygen clusters with well-defined structures. POMs are typically composed of oxygen ligands and high-valent metal ions such as Ⅴ(Ⅴ), Mo(Ⅵ), and W(Ⅵ), which can serve as Ⅴ, Mo, and W sources to produce the corresponding metal carbides, oxides, phosphides, and sulfides by pyrolysis at high temperature. Some POMs may also contain a series of redox-active heteroatoms, which are usually named hetero-polyoxometalates. These can serve as precursors to electrocatalysts with uniform heteroatom doping. Moreover, direct applications of POMs as molecular catalysts in HER have, in recent years, received rapidly growing attention. This is because POMs not only serve as mediators or molecular catalysts to facilitate the HER, but can also be deposited on the electrode surface to catalyze the HER. However, the interpretation that HER catalytic activity enhancement is due to the intrinsic catalytic properties of the electrodeposited polyoxometalate or the deposition of small amounts of platinum has been highly debated. Reviewing these studies may help us understand the intrinsic active sites as well the intrinsic HER mechanism of POMs and POMs-derived catalysts, and thus design more efficient HER catalysts. This review, therefore, focuses on recent progress in the applications of POMs and their derivatives in electrocatalytic HER. Firstly, basic HER mechanisms for common metal catalysts and POMs molecular catalysts are discussed along with challenges in the field of HER. Next, applications of POMs molecular catalysts and POMs-derived catalysts in HER are summarized. Finally, some perspectives of POMs-based catalysts/pre-catalysts for electrocatalytic HER are proposed.
The fabrication of compact, continuous, and large-scale metal organic framework (MOF) membranes with high permeability and H2/CO2 selectivity remains challenging because of the wake interaction between the MOF membrane and the substrate. In addition, substrates with smooth and plain surfaces and suitable pore size are required to prepare high-quality MOF membranes because it is difficult to obtain dense and continuous MOF membranes on a substrate with large pores and rough surfaces. To overcome these challenges, numerous MOF membrane growth methods have emerged, including in situ (direct) growth, secondary (seeded) growth, and layer-by-layer growth methods as well as electrostatic spinning and the chemical modification of the substrate. Among these methods, usage of substrates suitable for surface-functionalization is a promising technique. Herein, Al2O3 was selected as the substrate and was coated with PIM-1 (one polymer of intrinsic microporosity), followed by carboxylation of PIM-1 to furnish a large number of carboxyl groups on the surface. In situ growth of the MOF membrane using the interactions between the carboxyl group and the metal yielded two types of compact, continuous, and large-scale polymer-supported MOF membranes (PIM-1-COOH/ZIF-8 and PIM-1-COOH/HKUST-1). Furthermore, the fabricated polymer-supported MOF membrane structures were investigated by powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). Gas separation experiments were performed to explore the gas permeability and selectivity of the prepared MOF membranes. The XRD characterization confirmed the pure phase and high crystallinity of the MOF membranes. The SEM images showed that the MOF membranes were compact and continuous with a tight combination between the MOF crystal membrane and the substrate. Gas separation measurements showed that both MOF membranes exhibited high H2 permeability and selectivity for H2/CO2. For the PIM-1-COOH/ZIF-8 and PIM-1-COOH/HKUST-1 membranes, the 1 : 1 binary mixtures gas separation factors of H2/CO2 calculated as the gas molar ratios in the permeate and retentate side 7.32 and 9.69, respectively, at room temperature and atmospheric pressure. The H2/CO2 mixture separation factors of the two MOF membranes exceeded the corresponding Knudsen constants (4.7), with H2 permeances higher than 3.16 × 10-6 and 1.14 × 10-6 mol·m-2·s-1·Pa-1, respectively. The ideal separation factors of H2/CO2 of both MOF membranes calculated as the ratio of single gas permeances were 7.70 and 12.04, respectively, with the respective H2 permeances of up to 3.73 × 10-6 and 3.86 × 10-6 mol·m-2·s-1·Pa-1. Because of their outstanding characteristics, these novel MOF membranes can be widely used in the fields of H2 purification and separation.
Propylene is widely used as a raw material for producing polypropylene, acrylonitrile, propylene oxide, etc. Typical manufacturing processes for propylene (steam cracking and FCC process) are over-reliant on petroleum resources and cannot meet the rapidly growing global demands. New routes for producing propylene from non-oil resources, particularly methanol-to-propylene (MTP) technology, have attracted increasingly more attention, where a fixed-bed reactor is used and ZSM-5 zeolite is the best alternative catalyst. However, structural optimization of ZSM-5 to enhance the lifetime and propylene selectivity and a deep understanding of the mechanism of the MTP reaction are still considerable challenges. For the conventional ZSM-5 zeolite, carbon deposition preferentially occurs near the outer surface of the zeolite particles because of the high acid density on the external surface, which accelerates the deactivation by blocking the outer pore openings, especially in a long-term MTP reaction. Large amounts of external strong acids also promote secondary reactions, such as hydrogen transfer reactions, resulting in a decrease in propylene selectivity. To study the effects of strong and weak acid distributions of ZSM-5 zeolite on the MTP reaction, two series of boron-modified ZSM-5 zeolites were designed: B-Al-ZSM-5 zeolites by one-step synthesis and Al-ZSM-5@B-ZSM-5 core-shell zeolites by two-step synthesis. These were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM) mapping, N2 physical adsorption-desorption, temperature-programmed desorption of ammonia (NH3-TPD) and 1, 3, 5-triisopropylbenzene (TIPB) cracking, and B1-Al-ZSM-5 and Al@B1-ZSM-5, B2-Al-ZSM-5 and Al@B2-ZSM-5, and B3-Al-ZSM-5 and Al@B3-ZSM-5 samples in the two series were found to have similar texture properties, acid amounts and acid strengths, but different B and Al elemental distributions and acid distributions. We used these two sets of samples to compare the effect of different strong and weak acid distributions—a uniform distribution and a gradient distribution of strong and weak acids on the performance of the MTP reaction. The results showed that samples with a uniform distribution of strong and weak acids have higher propylene selectivity due to lower strong and weak acid densities, whereas samples with a gradient acid distribution have a longer catalytic lifetime in the MTP reaction due to the absence of strong acid density and higher weak acid density on the outer surface. The different acid distributions lead to two different carbon deposition modes. Carbon deposition of the sample with the uniform acid distribution preferentially formed on the outer surface, resulting in rapid deactivation by blocking external micropores and leaving the internal active centers not fully utilized. However, for the sample with the gradient acid distribution, the carbon-blocking rate of the external surface considerably decreased, which increased the time that the reactant molecules had to enter the internal micropores. Thus, the utilization rate of the active centers and the catalytic lifetime of the Al-ZSM-5@B-ZSM-5 core-shell sample considerably increased.
Increasing climate change and environmental pollution caused by the excessive use of fossil fuels have prompted intensive research into clean and efficient renewable sources as a substitute for traditional fossil fuels. A very promising approach is to mimic the water splitting process that occurs in plants during photosynthesis, in order to convert solar energy into chemical energy. A successful water splitting reaction, which comprises two half reactions (water oxidation and the reduction of protons), can generate H2 and O2 from water. Hydrogen is a promising renewable energy carrier because of its clean combustion and high calorific value. Light-driven water splitting is considered to be a feasible way to transform water and solar energy into hydrogen energy. However, water oxidation is considered to be the bottleneck process of water splitting because it advances in a thermodynamically uphill manner with the involvement of 4e− and 4H+. Inspired by the nature of Mn4CaO5 in photosystem Ⅱ (PS Ⅱ), the comprehensive understanding of its key features for use in active molecular water oxidation catalysts (WOCs) remains challenging. Extensive effort has been devoted to researching and manufacturing the structure and biomimicking the catalytic activity of Mn4CaO5 clusters that contain the Mn3CaO4 cubane structure, for the construction of low-cost and robust WOCs. WOCs can be divided into heterogeneous and homogeneous catalysts. Although heterogeneous WOCs are convenient for recycling and are easily prepared on a large scale, homogeneous WOCs, especially complexes based on organic ligands or polyoxometalates (POMs), have more advantages owing to their catalytic efficiency, structural modifications, and mechanistic understanding. Thus, recently, some molecules with an M4O4 (M = transition metals, mainly Mn, Co, Ni, and Cu) cubic structure have been reportedly used as photocatalytic WOCs. In this review, we present an overview of the most important and recent advances based on M4O4 cubic WOCs that contain first-row transition metal cubanes for visible light-driven water oxidation. Our main focus is on the structure of cubane catalysts, including metal complexes, POMs, and a system containing BiVO4 or polymeric carbon nitride (PCN) as a photosensitizer, and cubic complexes as WOCs. Results have shown that the activity and stability of the catalyst can be tuned by the ligand stability, metal center, coordination environment, and other factors. This review will be helpful for designing new cubane catalysts for photocatalytic water oxidation that are highly efficient and stable.
Functionalized graphene has attracted significant interest over the past decade due to its unique physical properties and potential applications. Graphene oxide (GO), a readily scaled-up product, is a basic material for further functionalization. Using reductive processes, highly conductive reduced graphene oxide (RGO) can be obtained, which exhibits electrical and optical properties analogous to those of graphene. Moreover, due to the presence of oxygen-containing functional groups, its chemical reactivity and electronic properties can be easily tailored by chemical doping with nitrogen. However, developing strategies for doping graphene is challenging and the fundamental roles of the doping atom configuration and its environment on the resulting properties of graphene remain poorly understood. These properties are important for electrical and catalytic applications of graphene. Thus, synthesizing specific configurations of nitrogen-doped graphene and consequently investigating the electrical and catalytic properties of the product is imperative. Herein, we demonstrate an approach that allows for successful production of nitrogen-functionalized RGO using Schiff base condensation between the amino groups in an o-aryl diamine compound and the carbonyl groups in GO. Three typical nitrogen-containing species including o-phenylenediamine (OPD), 2, 3-diaminopyridine (23DAP), and bis(trifluoromethyl)-1, 2-diaminobenzene (BTFMDAB) were used for functionalizing the GO samples, and the corresponding RGO derivatives (OPD-RGO, 23DAP-RGO, and BTF-RGO) were obtained by thermal annealing. Pyrazine nitrogen was successfully introduced into graphitic framework, as confirmed by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectra, thermal gravimetric analysis (TGA), Raman, and X-ray photoelectron spectroscopy (XPS). Field-effect transistors (FETs) based on the BTF-RGO exhibited hole-dominated ambipolar field-effect behavior with a Dirac point at a 9 V gate voltage and hole mobilities up to 2.5 times that of RGO. The weak p-type doping effect originated from the strongly electron-withdrawing trifluoromethyl groups. By studying the OPD-RGO and 23DAP-RGO-based FETs, containing pyrazine nitrogen and mixed pyrazine/pyridine nitrogen, respectively, we found that pyrazine nitrogen provided weak n-type doping effects, while pyridine nitrogen exhibited weak p-type doping effects due to its electron-withdrawing ability. Enhanced p-type doping effect was accompanied by the introduction of groups with stronger electron-withdrawing ability into the graphitic framework. Impressively, pyridine nitrogen in the pyrazine nitrogen-doped RGO yielded a weak p-type doped graphene due to the electron-withdrawing effect of the pyridine nitrogen. Nitrogen-doped graphene can be finely tuned from weak n-type to weak p-type doping by adjusting the electron-withdrawing ability of o-aryl diamine compounds. This study demonstrates the effect of nitrogen configuration and its surrounding environment on the electrical properties of RGOs, providing additional possible applications.
Sodium-ion batteries (SIBs) have recently garnered considerable attention because of the greater abundance, wider distribution, and lower cost of Na compared to Li. However, the investigation is insufficient, mainly because Na+ is larger and heavier than Li+, thereby limiting the Na+ insertion and extraction ability from the host materials. Anodes with alloying reactions such as Sn, Ge, and Sb have been considered for SIBs owing to their high gravimetric and volumetric specific capacities. In this study, we devised a one-pot reaction strategy for the in-situ fabrication of a spherical porous nano-SnSb/C composite by employing aerosol spray pyrolysis, and subsequently applied it as an anode in SIBs. The products of spray pyrolysis generally feature three-dimensional spherical hierarchical structures, which are considered to be relatively stable and also act as high-packing-density electrode materials. Additionally, they can be easily handled during the fabrication of the electrode. By adjusting the precursor concentration of SnCl2·2H2O and SbCl3, different sizes for SnSb nanoparticles (10 and 20 nm) were obtained. The crystal structures and morphologies of the as-prepared samples were characterized using X-ray diffraction, field-emission scanning electron microscopy, and high-resolution transmission electron microscopy. Thermal gravimetric analysis was carried out to analyze the carbon content of SnSb/C composites by using a TG-DSC analyzer with a heating rate of 5 ℃·min-1 in air from 25 ℃ to 600 ℃. The specific surface areas of the microspheres were determined by Brunauer-Emmett-Teller analysis. X-ray photoelectron spectroscopy and Raman spectroscopy were used to investigate the studied materials. The micro-nanostructured composite is composed of SnSb nanoparticles (10 and 20 nm); moreover, the carbon content and size of SnSb nanograins could be controlled by altering the reaction conditions. Owing to its unique structure, the obtained nano-composite displays stable cycle performance and high rate capability as the anode for SIBs. The specific capacity of 10-SnSb/C was 722.1 mAh·g-1 at the first cycle, and the coulombic efficiency of the first cycle was 86.3%. The 10-SnSb/C was stable at different current densities of 100, 1000, and 3000 mA·g-1, and exhibited specific capacities of 607.7, 645.4 and 452.2 mAh·g-1, respectively. The reversible capacity reached 623 mAh·g-1 after 200 cycles at a current density of 1000 mA·g-1, and the capacity retention rate was 95%. The outstanding performance of SnSb/C was due to its distinctive nanostructure, which could effectively improve the utilization rate of active materials, facilitate the transportation of Na+ ions, and prevent the nanoparticle pulverization/agglomeration upon prolonged cycling. The facile synthesis technique and good performance would shed light on the practical development of SnSb/C nanocomposites as high rate capability and long cycle life electrodes for SIBs.
The photocatalytic reduction of CO2 has attracted considerable attention owing to the dual suppression of environmental pollution and energy shortage. The technology uses solar energy to convert carbon dioxide into hydrocarbon fuel, which is of great significance for achieving the carbon cycle. The development of low-cost photocatalytic materials is critical to achieving efficient solar energy to fuels conversion. One of the most commonly employed photocatalysts is TiO2. However, it suffers from broad band gap as well as the recombination of photo-excited holes and electron. Hence, in this work, we report the photochemical reduction of CO2 using rod-like PCN-222(Cu)/TiO2 composites as photocatalyst through a simple hydrothermal method, in which TiO2 nanoparticles are anchored at the interface of the SiC rod PCN-222(Cu). Multiple characterization techniques were used to analyze the structure, morphology, and properties of the PCN-222(Cu)/TiO2 composite. A series of characterizations including X-ray diffraction (XRD), scanning electron microscopy (SEM), diffuse reflectance spectroscopy (DRS), Fourier-transform infrared spectroscopy, photo-electrochemical, and photoluminescence (PL) confirm the successful preparation of PCN-222(Cu)/TiO2 composites. SEM reveals that the TiO2 nanoparticles are uniformly distributed on the surface of the rod-shaped PCN-222(Cu)/TiO2. XRD results show that PCN-222(Cu) and PCN-222(Cu)/TiO2 composite photocatalysts with good crystal structure were successfully synthesized. According to the DRS results, the prepared PCN-222(Cu)/TiO2 composite samples exhibit characteristic absorption peaks of metalloporphyrins in the visible region. PL spectroscopy, transient photocurrent response, and electrochemical impedance spectroscopy further confirm that the rod-like PCN-222(Cu)/TiO2 samples have high electron-hole pair separation efficiency. By controlling the mass ratio of PCN-222(Cu) and TiO2, the photocatalytic CO2 reduction performance test shows that the 10% PCN-222(Cu)/TiO2 composite achieves optimal catalytic performance, yielding 13.24 μmol·g−1·h−1 CO and 1.73 μmol·g−1·h−1 CH4, respectively. All the rod-like PCN-222(Cu)/TiO2 composites exhibit better photocatalytic CO2 activity than that of TiO2 nanoparticles or PCN-222(Cu) under the illumination of xenon lamps, which is attributed to charge transport and electron-hole separation capabilities. After three test cycles, the catalytic activity of PCN-222(Cu)/TiO2 photocatalyst was virtually unchanged. The reduction yield of the catalyst increased for 8 h under continuous illumination, indicating that PCN-222(Cu)/TiO2 composites have acceptable stability. The estimation of the band gap curve and the Mote-Schottky curve test show that the lowest unoccupied molecular orbital position of PCN-222(Cu) is more negative than the TiO2 of the conduction band; hence, a possible photocatalytic reaction mechanism of the PCN-222(Cu)/TiO2 composite is proposed. This study provides a new strategy for the integration of metal-organic frameworks and oxide semiconductors to construct efficient photocatalytic systems.
The assembly of two-component nanocrystals (NCs) such as metals, magnets, and semiconductors into binary nanocrystal superlattices (BNSLs) provides a fabrication route to novel classes of materials. BNSLs with certain structures can exhibit the combined and collective properties of their building blocks and are widespread in the fields of electronics and magnetic devices. As most studies have focused on combined two-component NCs of different sizes for self-assembling BNSLs, there are a few studies on single-component NCs of different sizes for the construction of BNSLs; this is especially true for Au NCs. Noble metallic Au NCs are an excellent candidate material because of their exceptional chemical stability, catalytic activity, process ability, and metallic nature; these characteristics provide them unique size-dependent optical and electronic properties as well as a wide variety of applications in sensing, imaging, electronic devices, medical diagnostics, and cancer therapeutics owing to their strong interactions with external electromagnetic fields. Therefore, it is important to develop a simple and efficient procedure to build BNSLs with different sizes of Au NCs. In our study, we synthesized monodispersed (size distribution < 10%) 6.0, 7.3, and 9.6 nm Au NCs using dodecanethiol-stabilized 3.7 nm Au NCs as seeds through a seed-growth method in oleylamine. The obtained Au NCs exhibited morphology and nanocrystallinity (single-domain and polycrystalline) similar to those of Au seeds. As the size of Au NCs increased from 3.7 to 6.0, 7.3, and 9.6 nm, the surface plasmon resonance peaks narrowed and indicated a red shift. The oleylamine-functionalized 6.0, 7.3, and 9.6 nm Au NCs were mixed with 3.7 nm Au NCs at certain concentration ratios. Au BNSLs with AB2 (hexagonal AlB2 structure), AB13 (NaZn13 structure), and AB (cubic NaCl structure) type were obtained through the solvent evaporation method. The (001) plane of the AlB2-type structure, (001) plane of the NaZn13-type structure, and (100) plane of NaCl-type structure superlattices were observed through transmission electron microscopy (TEM). The effective particle size ratios (γ= Dsmall/Dlarge) serve as the critical determining factor in the formation of the BNSLs. The effective particle size of NCs is equal to the sum of the metal core diameter and twice the thicknesses of the surface ligand. In our study, the effective particle size Dsmall (Au seed) is 5.7 nm; the effective particle sizes Dlarge (6.0, 7.3, and 9.6 nm Au NCs) are 9.0, 10.3, and 12.6 nm, respectively. The effective particle size ratios γ were therefore calculated to be 0.63, 0.55, and 0.45, respectively. The relevant space filling principle predicted the stability of the AlB2, NaZn13, and NaCl-type structures in the range of 0.482 < γ < 0.624, 0.54 < γ < 0.625, and γ < 0.458, respectively; the experimental results adequately matched the relevant space filling principle. The investigation of such a single nanocomponent as a building block is noteworthy with regard to the structures and properties of BNSLs as well as the potential development of novel meta-materials.
As an important component in electrodes, the choice of an appropriate binder is significant when fabricating lithium-ion batteries (LIBs) with good cycle stability and rate capability, which are used in numerous applications, especially portable electronics and eco-friendly electric vehicles (EVs). Semi-crystalline poly(vinylidene fluoride) (PVDF), which is a traditional and widely used binder, cannot efficiently accommodate the volume changes observed in the anode during the charge-discharge process while binding all the components in the electrode together, which results in increased internal cell resistance, detachment of the electrode components, and capacity fading. Herein, we have investigated a highly polar and elastomeric polyacrylonitrile-butadiene (NBR) rubber for use as a binder in LIBs, which can accommodate graphite particles of different shapes compared to semi-crystalline PVDF. Prior to our electrochemical tests, NBR was analyzed using thermogravimetric analysis (TGA) and X-ray diffraction (XRD), showing good thermal stability and an amorphous morphology. NBR is more conformable to irregular surfaces, which results in the formation of a homogeneous passivation layer on both spherical and flaky graphite particles to effectively suppress any electrolyte side reactions, further allowing more uniform and fast Li ion diffusion at the electrolyte/electrolyte interface. As a result, the electrochemical performance of both spherical and flaky shape graphite electrodes was significantly improved in terms of their first cycle Coulombic efficiency (CE) and cycle stability. With comparative specific capacity, the first cycle CE of the NBR-based spherical and flaky graphite electrodes were 87.0% and 85.5%, compared to 85.3% and 82.6% observed for their corresponding PVDF-based electrodes, respectively. After 1000 discharge-charge cycles at 1C, the capacity retention of the NBR-based graphite electrodes was significantly higher than that of PVDF-based electrodes. This was attributed to the good stability of the solid electrolyte interphase (SEI) formed on the graphite electrodes and the high stretching ability of the elastomeric NBR binder, which help to accommodate the repeated volume fluctuation of graphite observed during long-term charge-discharge cycling. Electrochemical impedance spectroscopy (EIS) and microscopic analysis (SEM and TEM) were carried out to investigate the formation and evolution of the SEI layers formed on the spherical and flaky graphite electrodes. The results show that thin, homogeneous, and stable SEI layers are formed on the surface of both spherical and flaky graphite electrodes prepared using the NBR binder. When compared to the PVDF-based graphite electrodes, the graphite electrodes constructed using NBR showed decreased resistance in the SEI layer and faster charge transfer, thus enhancing the electrode kinetics for Li ion intercalation/deintercalation. Our study shows that the electrochemical performance of spherical and flaky graphite electrodes prepared using the NBR binder is significantly improved, demonstrating that NBR is a promising binder for these electrodes in LIBs.
SSZ-13 zeolite with a chabazite (CHA) topology structure has important applications for methanol to olefin (MTO) conversion and selective catalytic reduction of nitrogen oxides (NOx) by ammonia (NH3-SCR) to reduce diesel engine exhaust emissions. It has been reported that the Al-rich SSZ-13 zeolite can be used to tune the selectivity of olefins in the MTO reaction, and significantly enhance NO conversions at lower temperatures in NH3-SCR. Thus, the aluminum content and distribution as well as the corresponding acidity in SSZ-13 zeolite determine the catalytic performance of the zeolite for different catalytic reactions. Herein, quantum chemical computing using density functional theory (DFT) combined with multinuclear solid-state nuclear- magnetic-resonance (NMR) experiments were performed to investigate the correlation of Al location and Brønsted acidity of H-SSZ-13 zeolite with the Si/Al ratio varying from 5.8 to 25. The most favorable acid site in the 1Al model is O(1)―H in which a proton is bonded with the O(1) atom near the isolated Al atom of the zeolite framework. Nevertheless, energy differences were rather small when comparing the substitution energies of an Al atom replacing a Si atom in the zeolite framework with a proton located in different O sites. As the Si/Al ratio decreased, the Al-rich SSZ-13 zeolite contained more Al substitutions in its framework. This system exhibited the lowest substitution energy when two Al atoms were located at the diagonal of the same six-membered ring for the Al-Si-Si-Al (NNNN) sequence in the framework of the Al-rich SSZ-13 zeolite. However, for the Al-Si-Al (NNN) sequence, the most favorable distribution involved two Al atoms located in different six-membered rings of the double six-membered ring units (D6R). The proton affinities (PA), NH3 desorption energies, and 1H NMR chemical shifts after d3-acetonitrile adsorption were calculated in the most stable models to characterize the Brønsted acid strength of the SSZ-13 zeolite with different Si/Al ratios. All computing results suggested that the Al-rich SSZ-13 zeolite exhibited weaker Brønsted acid strength than that of the Si-rich counterpart due to the presence of Si(2Al) groupings with the NNN sequence in the framework. Quantitative 29Si magic-angle spinning (MAS) NMR measurements after deconvolution demonstrated that the content of Si(2Al) groupings in the Al-rich SSZ-13 was > 43%. The 1H MAS NMR experiments after d3-acetonitrile adsorption showed that the chemical shift of the bridging hydroxyls in the Al-rich SSZ-13 moved to the lower field, further confirming that it had a weaker Brønsted acid strength than the Si-rich counterpart.
Over the past decades, advances in science and technology have greatly benefitted the society. However, the exploitation of fossil fuels and excessive emissions of polluting gases have disturbed the balance of the normal carbon cycle, causing serious environmental issues and energy crises. Global warming caused by heavy CO2 emissions is driving new attempts to mitigate the increase in the concentration of atmospheric CO2. Significant efforts have been devoted for CO2 conversion. To date, the electroreduction of CO2, which is highly efficient and offers a promising strategy for both storing energy and managing the global carbon balance, has attracted great attention. In addition, the electrosynthesis of value-added C2+ products from CO2 addresses the need for the long-term storage of renewable energy. Therefore, developing catalysts that function under ambient conditions to produce C2 selectively over C1 products will increase the utility of renewable feedstocks in industrial chemistry applications. Recently, great progress has been made in the development of materials for electrocatalytic CO2 reduction (ECR) toward C2+ products; however, some issues (e.g., low selectivity, low current efficiency, and poor durability) remain to be addressed. In addition, the elementary reaction mechanism of each C2+ product remains unclear, contributing to the blindness of catalyst design. In this regard, the development of proposed mechanisms of ECR toward C2+ products is summarized herein. The key to generating C2+ products is improving the chances of C―C coupling. Test conditions significantly influence the reaction path of the catalyst. Thus, three different paths that that are most likely to occur during ECR to C2+ products are proposed, including the CO, CO-COH, and CO-CO paths. In addition, typical material regulatory strategies and technical designs for ECR toward C2+ products (e.g. crystal facet modulation, defect engineering, size effect, confinement effects, electrolyzer design, and electrolyte pH) are introduced, focusing on their effects on the selectivity, current efficiency, and durability. The four strategies for catalyst design (crystal facet modulation, defect engineering, size effect, and confinement effect) primarily affect the selectivity of the ECR via adjustment of the adsorption of reaction intermediates. The last two strategies for technique design (electrolyzer design and electrolyte pH) contributing greatly toward improving the current efficiency than selectivity. Finally, the challenges and perspectives for ECR toward C2+ products and their future prospects are discussed herein. Therefore, breakthroughs in the promising field of ECR toward the generation of C2+ products are possible when these catalyst design strategies and mechanisms are applied and novel designs are developed.
Photocatalytic technology can effectively solve the problem of increasingly serious water pollution, the core of which is the design and synthesis of highly efficient photocatalytic materials. Semiconductor photocatalysts are currently the most widely used photocatalysts. Among these is graphitic carbon nitride (g-C3N4), which has great potential in environment management and the development of new energy owing to its low cost, easy availability, unique band structure, and good thermal stability. However, the photocatalytic activity of g-C3N4 remains low because of problems such as wide bandgap, weakly absorb visible light, and the high recombination rate of photogenerated carriers. Among various modification strategies, doping modification is an effective and simple method used to improve the photocatalytic performance of materials. In this work, Cu/g-C3N4 photocatalysts were successfully prepared by incorporating Cu2+ into g-C3N4 to further optimize photocatalytic performance. At the same time, the structure, morphology, and optical and photoelectric properties of Cu/g-C3N4 photocatalysts were analyzed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy, UV-Vis diffuse reflectance spectroscopy (DRS), and photoelectric tests. XRD and XPS were used to ensure that the prepared photocatalysts were Cu/g-C3N4 and the valence state of Cu was in the form of Cu2+. Under visible light irradiation, the photocatalytic activity of Cu/g-C3N4 and pure g-C3N4 photocatalysts were investigated in terms of the degradation of RhB and CIP by comparing the amount of introduced copper ions. The experimental results showed that the degradation ability of Cu/g-C3N4 photocatalysts was stronger than that of pure g-C3N4. The N2 adsorption-desorption isotherms of g-C3N4 and Cu/g-C3N4 demonstrated that the introduction of copper had little effect on the microstructure of g-C3N4. The small difference in specific surface area indicates that the enhanced photocatalytic activity may be attributed to the effective separation of photogenerated carriers. Therefore, the enhanced photocatalytic degradation of RhB and CIP over Cu/g-C3N4 may be due to the reduction of carrier recombination rate by copper. The photoelectric test showed that the incorporation of Cu2+ into g-C3N4 could reduce the electron-hole recombination rate of g-C3N4 and accelerate the separation of electron-hole pairs, thus enhancing the photocatalytic activity of Cu/g-C3N4. Free radical trapping experiments and electron spin resonance indicated that the synergistic effect of superoxide radicals (O2•−), hydroxyl radicals (•OH) and holes could increase the photocatalytic activity of Cu/g-C3N4 materials.
Exploring economical and efficient photocatalysts for hydrogen production is of great significance for alleviating the energy and environmental crisis. In this study, 3D In2O3 nanostructures with appropriate self-assembly degrees were obtained using a facile hydrothermal strategy. To study the significance of 3D In2O3 nanostructures with appropriate self-assembly degrees in photocatalytic hydrogen production, the photocatalytic performances of samples were evaluated based on the amount of hydrogen gas release under visible-light irradiation (λ > 400 nm) and simulated solar light illumination. Interestingly, the 3D In2O3-150 nanostructured photocatalyst (hydrothermal temperature was 150 ℃, denoted as In2O3-150) exhibited extremely superior photocatalytic hydrogen evolution activity, which may have been caused by their unique structure to improve light reflection and gas evolution. The special structure can enhance light harvesting and induce more carriers to participate in photocatalytic hydrogen production. Despite possessing similar 3D nanostructures, the In2O3-180 photocatalyst exhibited poor photocatalytic activity. This may have been caused by the high self-assembly degree, which can hinder light irradiation and isolate a portion of the water. In addition, the 3D nanostructures could effectively make uniform the carrier migration direction, which is from the interior to the rod end. However, the direction of carrier migration of the In2O3-110 photocatalyst could transfer in various directions, whereas the In2O3-130 photocatalyst could transfer to both ends of the rod. This might cause partial migration to counteract each other. The compact cluster rod-like structure of In2O3-180 might prevent the light from exciting the carrier effectively. Through a photocatalytic recycling test, the 3D In2O3-150 nanostructured photocatalyst exhibited outstanding photochemical stability. This work highlights the importance of controlling the self-assembly degree of 3D In2O3 nanostructures and explores the performances of 3D In2O3 nanostructured photocatalysts in hydrogen production under visible light and simulated solar light.
The rapid development of batteries, especially lithium-ion batteries, has dramatically changed our daily lives. From portable electronics to electric vehicles and smart grids, batteries are extensively used in many fields and are difficult to be replaced in terms of their excellent energy and power densities. The advancement of battery technology requires the thorough understanding of electrochemical reaction mechanisms, which strongly depends on the collaboration of researchers from different fields. Magnetic resonance spectroscopy includes the important techniques of nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR), and the former is suitable for studying light elements commonly found in batteries including Li, Na, P and O, while the latter is suitable for studying heavier transition metals such as Co, Mn, Fe and V. In addition, NMR and EPR are capable of quantitatively analysis in a nondestructive manner regardless of sample crystallinity. Hence, NMR and EPR spectroscopies have allowed for significant research progress and have become increasingly important for battery research over the past three decades. Herein, we will provide our perspective of magnetic resonance methods and first summarize the main interactions and the Hamiltonian forms of solid-state NMR and EPR (dipole-dipole interaction, electric quadrupole interaction, chemical shift, and hyperfine interaction). Subsequently, we summarize the important and frequently-used methods of solid-state NMR and EPR spectroscopies and introduce their representative applications in metal ion battery research (mainly lithium- and sodium-ion batteries). Specifically, we introduce the basic principles and representative applications of (ⅰ) MQMAS (multiple-quantum magic angle spinning), (ⅱ) pjMATPASS (MAT = magic-angle turning, PASS = phase-adjusted sideband separation, and pj = projection), (ⅲ) WURST-CPMG (WURST = wide band uniform rate smooth truncation, CPMG = Carr-Purcell Meiboom-Gil), (ⅳ) 2D homonuclear correlation and exchange (2D EXSY), (ⅴ) 2D homonuclear correlation based on dipole coupling (i.e. RFDR), (ⅵ) perpendicular mode EPR, (ⅶ) parallel mode EPR, (ⅷ) in situ NMR, and (ⅸ) in situ EPR. In addition, we briefly introduce representative applications of 2D heteronuclear correlation (i.e. CP-HETCOR), pulsed field gradient NMR, spin-lattice relaxation (SLR), spin alignment echo (SAE), DFT calculations, and dynamic nuclear polarization (DNP). Previous reviews regarding the application of magnetic resonance technology in battery research are almost all reported in terms of the classification of battery materials. In other words, they are written from the perspective of applications in cathode, anode, and electrolyte research. Herein, we summarize from the perspective of solid-state NMR and EPR methods, which may be beneficial for the readers to fully understand the value of these important technologies. We believe this review can serve as a guide to solve challenges related to using solid-state NMR and EPR spectroscopies in battery research.
Biological systems can be regarded as complex and open thermodynamic systems. All processes involved in biological growth and metabolism are accompanied by material and energy exchange. During metabolism, energy in the organisms is released in the form of heat, i.e., metabolic heat, which is the basis for development in the field of biothermochemistry. The calorimetric method considers the thermal effects produced by the various forms of action as the research object, to reveal the law of energy change and quantitative energy conversion. Studying the thermodynamic processes of complex biological systems and related reactions through microcalorimetry and thermodynamic methods reflects the intrinsic laws of life-related processes macroscopically and intrinsically. With the tremendous development and progress in microcalorimetry in terms of the temperature measurement accuracy, stability of temperature control, automation, and multi-functionalization, calorimetry has been widely used in life sciences. It can be used to describe macroscopic processes such as ecosystems and biological evolution, observe organismal and cell growth, examine mitochondrial metabolism, and study problems at the molecular level, including enzymatic reactions and interactions between small molecules and biomacromolecules. Herein, the application of biomass calorimetry in the life sciences is reviewed. The status and progress of biomass calorimetry at different biological and structural levels, such as the ecosystem, biological, organ, cellular, subcellular, and molecular levels are introduced. For example, soil microbial metabolic activity is a universal index for evaluating soil quality. The growth and metabolism of organisms as well as the physical and chemical processes of substances in soil are often accompanied by heat release, which is usually a non-selective signal. The use of isothermal microcalorimetry to nonspecifically monitor and record soil microbial metabolic characteristics has promoted the study of microbial metabolism in complex soil systems. The application of calorimetry to the study of tissues and organs mainly involves the calorimetric study of isolated animal and plant tissues and organs. Calorimetry of animal and microbial cells is considered the most common application of calorimetry in life sciences research. It mainly involves the classification and identification of bacteria, their growth and metabolism, inhibition mechanisms of drugs on microbial growth, principles of kinetics, and the thermodynamic characteristics of microbial growth and metabolism. However, owing to the lack of specificity of biomass calorimetry and the lack of direct access to information at the molecular level, more applications of calorimetry combined with other analytical techniques (especially in biology, medicine, and pharmacy) are needed in the future.
As a new fluorescent nanomaterial, carbon dots (CDs) have many advantages, such as uniform particle size distribution, good light stability, adjustable excitation-emission wavelength, and surface modification. Moreover, one of the fascinating characters of CDs is that they are considered to be low-toxic and eco-friendly alternatives in chemical and biological analyses. They have exhibited broad application prospects in the fields of analysis, detection, and bioimaging. Silkworm excrement is dried and easily available. A large number of hydroxyl and carboxyl compounds in silkworm excrement can be used as ideal starting materials for the preparation of CDs. Also, compounds containing nitrogen and sulfur in silkworm excrement can be used as nitrogen and sulfur sources; thus, when used in the preparation of CDs, silkworm excrement can impart many more unique properties to CDs. Nitrogen-containing CDs prepared by microwave synthesis have an average hydration diameter of 4.86 nm. Elemental analysis data show that the prepared CDs contained 59.84% carbon, 5.46% nitrogen, and 2.32% sulfur. XPS spectra reveals sulfur (2p), carbon (1s), nitrogen (1s), and oxygen (1s) in CDs. FTIR data demonstrate that the prepared CDs may contain hydroxyl, amino, carbonyl, sulfonic, ester, and ether functional groups as well as carbon-nitrogen structures. The XRD pattern of the CDs has a broader peak of the amorphous carbon phase at approximately 2θ = 24.6°, and only D bands (at ~1400 cm-1) can be obviously detected in the Raman spectra of CDs. The intensity of fluorescence emission peak of CDs increases first and then decreases with the increase in excitation wavelength. The maximum intensity of fluorescence emission shifts gradually with the red shift of the excitation wavelength, and the relationship between excitation and emission wavelengths is exponential. In the pH ranging from 2.18 to 10.24, the fluorescence emission intensity of CDs decreases gradually with the increase in pH, and the maximum fluorescence emission intensity shifts gradually with the increase in pH. There is a linear relationship between pH and maximum emission wavelength. The fluorescence emission intensity of CDs decreases gradually with the increase in metal ion concentration. Under neutral conditions, CDs can selectively detect Cu2+. Under acidic conditions, CDs can detect Cu2+, Fe3+, Al3+, Ni2+, and Fe2+ separately without interference from other ions. There is a Stern-Volmer linear relationship between metal ion concentration and fluorescence intensity. The intensity of the fluorescence emission peak of CDs decreases with the increase in temperature, which may be due to the non-radiative transition process caused by molecular thermal motion. There is a linear relationship between temperature and fluorescence intensity. The maximum fluorescence emission intensity of CDs gradually shifts with the increase in polarity of the dispersed solvents. There is a linear relationship between fluorescence intensity and empirical constant ET of solvent polarity. Compared with the reported CDs prepared from natural products, silkworm-excrement-based CDs have abundant surface groups although they do not have an obvious crystal structure, which makes them have excellent response to various environmental factors (pH, temperature, ion concentration, temperature, solvent polarity, etc.) in a wide range. Above all, the fluorescence property changes with multiple environmental parameters will facilitate a broad application of silkworm-excrement-based CDs in biodetection and imaging.
The fast-growing demand for safe energy storages with high power and energy density drives the continuous improvement of rechargeable Li-ion batteries (LIBs). In situ characterization is a potential way to understand the mechanism (metaphases, diffusion, kinetics, inhomogeneity etc.) of battery under operation conditions. Solid-state nuclear magnetic resonance (SS-NMR) is very sensitive to the local environment of 1H, 6, 7Li, 11B, 13C, 17O, 19F, 23Na, and 31P isotopes, which are widely used in battery materials, regardless of their ordering degree. In addition to providing well-resolved spectra obtained under fast magic angle spinning (MAS), NMR can effectively serve as a non-invasive tool to capture the evolution of electrodes/electrolyte upon charge/discharge electrochemical cycling. Subsequently, in situ NMR and imaging (MRI) have been developed for extending toward temporal and spatial dimensions in working batteries. Complementarily, highly sensitive electron paramagnetic resonance (EPR) and imaging (EPRI) have been employed to track and map the redox of transition metals and oxygen species (O2n−) within electrodes. The insights gained from in situ NMR/EPR and their imaging can serve as a guide for the structural design of energy storage materials and the fabrication of batteries with optimized performance. As such, this review summarizes the applications of both NMR and EPR in the field of battery community. In particular, we first introduce the combination of fast magic angle spinning and phase-adjusted sideband separation (pjMATPASS) to obtain highly resolved spectra for extreme broad signal mediated by unpaired electrons, which is usually found in battery materials, as well as isotope-oriented NMR to determine the Li pathway in the composite electrolyte by the aid of 6Li replacing 7Li in their transport pathway. Secondly, we introduce the combination of NMR/MRI measurement while battery under electrochemical cycling by (1) briefly summarizing the advantages and disadvantages of home-made cells (coin cell, bag cell, and cylindrical cell) developed for in situ NMR study; (2) using different isotopes for conducting in situ NMR on batteries: 7Li, 23Na, and 31P spectra; and (3) performing in situ MRI on electrolytes and electrodes with and without chemical shift information (CSI, S-ISIS, and stray-field MRI). Furthermore, in situ EPR determines and quantifies the evolution of active Li microstructure, transition metals, and oxygen species together with in situ EPRI mapping of the concentration of the paramagnetic center within a functioning battery. Finally, we point out the limitations and perspective of in situ NMR and EPR for cycling batteries in real-time. This review will provide illuminating insights on the magnetic technologies in the battery community and pave a way for carrying out NMR/EPR on functional materials.
Lithium ion batteries (LIBs) are becoming the most popular energy storage systems in our society. However, frequently occurring accidents of electrical cars powered by LIBs have caused increased safety concern regarding LIBs. Solid-state lithium batteries (SSLBs) are believed to be the most promising next generation energy storage system due to their better in-built safety mechanisms than LIBs using flammable organic liquid electrolyte. However, constructing the ionic conducting path in SSLBs is challenging due to the slow ionic diffusion of Li ion in solid-state electrolyte, particularly in the case of solid-solid contact between the solid materials. In this paper, we demonstrate the construction of an integrated electrolyte and cathode for use in SSLBs. An integrated electrolyte and cathode membrane is obtained via simultaneous electrospinning and electrospraying of a polyacrylonitrile (PAN) electrolyte and a LiFePO4 (LFP) cathode material respectively, for the cathode layer, followed by the electrospinning of PAN to prepare the electrolyte layer. The resultant integrated PAN-LFP membrane is flexible. Scanning electron microscopy and energy dispersive X-ray spectroscopy measurement results show that the electrode and electrolyte are in close contact with each other. After the integrated PAN-LFP membrane is filled with a succinonitrile-bistrifluoromethanesulfonimide (SN-LiTFSI) salt mixture, it is paired with a lithium foil metal anode electrode, and the resultant solid-state Li|PAN-LFP cell exhibits limited polarization and outstanding interfacial stability during long term cycling. That is, the Li|PAN-LFP cell presents a specific capacity of 160.8 mAh∙g−1 at 0.1C, and 81% of the initial capacity is maintained after 500 cycles at 0.2C. The solid-state Li|PAN-LFP cell also exhibits excellent resilience in destructive tests such as cell bending and cutting.
Honeycomb-patterned porous films are polymer films with regular pore arrays on their surfaces. Since the pioneering work of François et al. in 1994, in which they used the breath figure (BF) technique to prepare honeycomb films, these highly ordered porous films have been attracting increasing interest in the past decades. Researchers are interested in the well-ordered pore arrays as they show great potential for use in many areas including superhydrophobic materials, photoelectric materials, tissue engineering, biomedicine, gas sensors, micro-reactors, to name just a few. Previous studies in this area have mainly focused on the preparation of porous films with regular microstructures and the effect of polymer architecture and casting conditions (such as temperature, relative humidity, and solvents) on the morphology. During the past two decades, considerable work has been devoted to identifying the mechanism of generation of well-ordered pore arrays during the BF process. Although the exact mechanism of film formation remains unclear, highly ordered honeycomb films can be produced from various polymers or polymer blends. In other words, currently, preparation of honeycomb films is not a major challenge. More recent studies in this area have concentrated on fabricating smart honeycomb films with reversible surface morphologies and/or properties. These smart films possess not only the properties of ordinary honeycomb films but also unique "on-off" switching functions. The research on stimuli-responsive smart honeycomb films is quite interesting in terms of both fundamental research and practical applications. Theoretically, the different scales and regular pore arrays provide an ideal model for studying the surface properties of porous materials under external stimulation, which is helpful in designing structured surfaces with desirable properties. From an application perspective, the "on-off" switching behavior imparts the films with additional functions that show high potential in a wide range of applications such as cell adhesion and controlled release, protein adsorption and separation, controlled drug release, etc. Thus far, honeycomb films with stimuli-responsive reversible surface morphologies, wettability, and fluorescence spectra have been developed, and the stimulus triggers have been mainly concentrated on temperature, pH, light, solvents, and gases. The main focus of this study is to describe the recent advances in smart honeycomb films, including fabrication strategies, triggers, "on-off" switching mechanisms, responsive behaviors, and related applications. Moreover, special attention is given to discussing the advantages and disadvantages of smart honeycomb films based on different triggers and design of smart honeycomb film systems with improved response properties. This study also discusses the challenges that concern the future development of smart honeycomb films and suggests several means of addressing those challenges.
Renewable energy resources (such as wind and solar) are being increasingly utilized to overcome issues of energy shortage and environmental deterioration. However, the intrinsically fluctuant and intermittent character of renewable energy sources hinders their practical application; therefore, batteries have been developed to act as a link between renewable energy sources and consumers. Lithium-ion batteries have become the most advanced battery technology in the last three decades, and have successfully captured the electric vehicles market; however, many concerns have recently arisen about the vastly expanded demand for lithium resources, which contrasts with their limited reserves. In this context, sodium-ion batteries have emerged as a promising alternative because of their intercalation chemistry similar to that of lithium-ion batteries, and the abundance of Na resources in the Earth's crust. Like lithium-ion batteries, the performance and cost of sodium-ion batteries are determined primarily by their cathodes. Among the various cathode materials that have been reported for sodium-ion batteries, Na0.44MnO2 is regarded as one of the most promising because of its opened three-dimensional tunnel structure and good chemical stability; it has also been demonstrated in previous studies to have superior cycling stability at room temperature. In practical terms, commercial batteries are often used at high temperatures (above 40 ℃) in summer. Several Mn-based cathode materials for lithium-ion batteries, such as LiMn2O4 and LiNi0.5Mn1.5O4, exhibit severe capacity decay at high temperatures. Therefore, the evaluation of the Na0.44MnO2 cathode in sodium-ion batteries at high temperatures is critical for its further commercialization. In this study, a Na0.44MnO2 cathode is prepared by a facile solid-state method and its electrochemical performance at a high temperature is measured. The electrochemical tests show that the Na0.44MnO2 cathode has a capacity retention of 66.5% over 100 cycles and a low reversible capacity of 12.3 mAh∙g-1 at 10C (1C = 120 mAh∙g-1). To improve its performance at a high temperature, Al2O3-coated Na0.44MnO2 is prepared via a liquid-phase method, and the coating effect is evaluated by electrochemical measurements as well as morphological, structural, and chemical composition analyses. The results show that the electrochemical performance of uncoated Na0.44MnO2 at 55 ℃ is significantly improved after coating with Al2O3; the capacity retention after 100 cycles increases to 79.2%, and the discharge capacity at 10C is increased to 63.6 mAh∙g-1. The improved performance is clearly attributed to the Al2O3 coating, which effectively prevents direct contact of Na0.44MnO2 with the electrolyte and alleviates the dissolution of manganese at a high temperature, thus maintaining a stable electrode/electrolyte interface and reducing charge transfer resistance.
Triboluminescence is a fascinating luminescence phenomenon induced by mechanical stimuli. Triboluminescent materials have potential applications in lighting, displays, and sensing, owing to their distinctive modes of light generation. However, organic triboluminescent materials are severely limited, and their luminescence mechanism remains unclear. Herein, we found that the luminescent manganese(Ⅱ) complex [BPP]2[MnBr4] displayed interesting triboluminescence performance. A series of green emissive tetrahalomanganese(Ⅱ) complexes was rationally designed and synthesized. The associated single crystal structures revealed that all complexes consisted of one [MnX4]2− (X = Br or Cl) ion and two organic cationic ligands per unit cell, with a tetrahedral geometrical symmetry around the Mn(Ⅱ) ion. In addition, the photophysical properties of tetrahalomanganese(Ⅱ) complexes were easily tuned by varying the organic ligands or halogen ions, which is beneficial for these organic-inorganic hybrid structures. Under UV light irradiation, all tetrahalomanganese(Ⅱ) complexes in the solid state exhibited bright green luminescence and a broad featureless emission band at 450–650 nm. The time-resolved photoluminescent decay curves demonstrated that the emission lifetimes of the prepared tetrahalomanganese(Ⅱ) complexes ranged from 260.5 μs to 1.95 ms, which was attributed to phosphorescence. The long-lived emission was mainly due to the spin-forbidden nature of the metal center d–d (4T1(G)→ 6A1) radiative transition. Thermogravimetric analysis was performed to examine the thermodynamic stabilities of the tetrahalomanganese(Ⅱ) complexes. The thermal stabilities of manganese(Ⅱ) complexes with P-based ligands were higher than those of the complexes containing N-based ligands. Upon applying a force to the crystals, the tetrahalomanganese(Ⅱ) complexes all exhibited prominent triboluminescence that could be observed by the naked eye in the dark. Systematic analysis of the crystals showed that the TL activities of the manganese(Ⅱ) complexes were related to the intra- and inter-molecular C-H···X (X = Br or Cl) interactions. The intra- and inter-molecular C-H···X interactions significantly reduced the possible energy loss caused by molecular vibrations and rotations in the [MnX4]2− unit under mechanical stress, improving TL emission. Moreover, a comparison of photoluminescence and triboluminescence indicated that different excitation sources yielded two distinct luminescence processes: transition of excitons excited by illumination and recombination of electrons and holes on the surface driven by polarization charges. Overall, the results presented herein new opportunities for fundamental research based on the developed class of triboluminescent materials.
The excited states of transition metal complexes with a wide range of photochemical and photophysical properties have attracted considerable attention recently. However, the luminescence property is affected by concentration quenching in practical applications. Aggregation-induced emission (AIE) is an effective strategy to solve this problem. In this work, a new imidazole-based N^C^N Pt(Ⅱ) metal complex, PtP2IM, with the AIE property was synthesized and characterized according to its single crystal structure. Under visible light, we found that the metal complex undergoes a photo-oxidation reaction with the generation of a new red-emitting, imidazole/benzoylimino-based N^C^N' Pt(Ⅱ) metal complex, PtPIMO, which was also confirmed by the crystal structure. Additional studies on the reaction process and conditions of this photo-oxidation reaction were conducted using different methods, such as NMR, UV-Vis spectroscopy, and so on. The experimental results showed that the change from PtP2IM to PtPIMO gradually occurred, and the new photochemical reaction was finally concluded as the C＝C double bond in either one of the two imidazole rings of the PtP2IM complex was attacked by oxygen to generate a new complex, PtPIMO, under photoirradiation in air. Electron paramagnetic resonance (EPR) measurements demonstrated the production of singlet oxygen, which is an excited state of oxygen with a high energy. Through the density functional theory (DFT) calculations, the electronic transition was determined to be a metal to ligand charge transfer (MLCT) in which more energy could transfer from the triplet excited state of PtP2IM to the ground-state oxygen to generate singlet oxygen (1O2) with a high intersystem crossing (ISC) efficiency due to the spin-orbit coupling of Pt heavy atoms. When large amounts of the singlet capture agent, triethylenediamine (TEDA) were added, the previously observed UV-Vis spectra change that corresponded to the photo oxidation reaction was not detected, which means that the photo-oxidation reaction observed in the case of PtP2IM was because of the oxidation by singlet oxygen. When oxygen was removed, excellent photostability and an obvious aggregation-induced emission (AIE) were observed for PtP2IM with the luminescent quantum efficiency of PtP2IM in solution and as a film at ~3% and ~20%, respectively. Based on the packing structure in the crystal, we observed that there were no strong intermolecular interactions, such as π-π or Pt-Pt interactions. Additionally, many intermolecular CH―π bonds between the two adjacent PtP2IM molecules were observed, which could effectively limit the rotation of the peripheral phenyl group linked to the imidazole ring. Thus, the AIE property of PtP2IM was attributed to the restricted intramolecular rotation (RIR) effect of the peripheral flexible phenyl group that was linked to the imidazole ring in the solid state in which the vibration of multiple peripheral benzene rings was effectively suppressed. This decreased the non-radiative transition rate and induced the high luminescent quantum efficiency. In the aggregation state, PtP2IM still demonstrated the photo-oxidation reaction by singlet oxygen. Thus, we report a new Pt(Ⅱ) metal complex, PtP2IM, with the AIE property that can undergo an uncommon photo-oxidation reaction in the photo-excitation state. This work aimed to elucidate the basic photochemical and photophysics of transition metal complexes with the AIE property.
high tensile strength, mobility, and thermal conductivity as well as clean surface. Hence, CNTs have been widely investigated for many potential applications, for example, as additives in composites and main components of integrated circuits. However, the former application widely used does not exploit their intrinsic properties, while the latter has only been demonstrated at the level of laboratory prototype devices. As the main factor determining future applications of CNTs is the ability to achieve their structure-controlled synthesis, this review first introduces a classification of CNT structures highlighting the potential difficulties associated with fine CNT structure control due to the similarities between different CNTs. Then, advances in the basic research and industrialization of CNTs in the past decades are summarized, including fine structure control, aggregation synthesis, and scale-up production. Catalysts are crucial for controlling the structure of CNTs, as their lifetime determines the CNT length and size (wall number and diameter), while their state and formation affects CNT chirality. Moreover, as the microscopic properties of individual CNTs often differ from their macroscale performance at industrial-scale production, their aggregation state should be carefully taken into consideration. Therefore, several methods were developed to realize different types of aggregates, such as lattice orientation for obtaining horizontally aligned CNT arrays, the use of catalysts with high density for the synthesis of vertical CNT arrays, direct deposition of CNT films, and even fabrication of very complex three-dimensional (3D) macrostructures. Furthermore, many efforts have been invested to promote CNT industrialization and develop various techniques to increase CNT production, including the fluidized bed method and floating method. Finally, the ideal synthesis of CNTs should combine structure control with scale-up preparation. To this aim, further theoretical understanding of the detailed CNT growth mechanism is still needed to clarify, for example, how CNT caps form at the atomic scale, which is the close matching relationship between CNTs and catalysts, and how the growth model affects the chirality preference of single-walled carbon nanotubes (SWNTs). Experimentally, different methods to grow SWNTs with a uniform structure should be further developed, focusing on catalyst design to increase temperature tolerance and achieve epitaxial growth of SWNT segments. On the other hand, the large-scale synthesis of SWNTs should also be reconsidered, for instance, by improving the growth equipment. In order to identify suitable applications for different CNT products, standards should be established and adopted. In addition to improving CNT synthesis, the driving force of the CNT industry in the future will be finding disruptive applications of CNTs, whose functions and contributions are irreplaceable. In conclusion, still much progress is needed to achieve the complete commercialization of CNTs in the future. Nevertheless, the rapid development and continuous attention given to this field may lead to growth opportunities in the CNT industry.
The development of the global economy has been accompanied by frequent oil spills caused by accidental leaks and industrial manufacturing, which have seriously threatened the aquatic environment and human health. Traditional methods for the treatment of oily wastewater include centrifugation, skimming, flotation, oil-absorbing technology, etc., which are limited by low separation efficiency as well as secondary pollution during the post-processing of oil absorption materials. Recently, separation technologies utilizing the special wettabilities of filtration membranes have been developed to enrich and recycle oils from wastewater. Among these, the fabrication of superhydrophilic/underwater superhydrophobic membranes have attracted intensive research interest, which can selectively allow the passage of water through the membrane while blocking the oils. However, microorganisms are more likely to breed on these hydrophilic surfaces, eventually leading to the blockage of the membranes. In this study, ZSM-5 zeolite crystals (MFI topological structure) were coated onto the stainless-steel meshes by means of seeding and secondary hydrothermal growth. Then, 70% of the total Na+ ions in the zeolite channels were substituted by Ag+ ions via an ion exchange process. The resultant membranes (Ag@ZCMFs) were superamphiphilic in air, with both water contact angle and oil contact angle of approximately 0°. However, they became superoleophobic when immersed in water, and the underwater oil contact angle reached 151.27° ± 4.34°. In terms of special wettability, Ag@ZCMF achieved efficient separation for various oil-water mixtures with separation efficiencies above 99%. The water flux and intrusion pressure of Ag@ZCMF depended on the diameter of pinholes in the membrane, which could be modulated by altering the time of secondary hydrothermal growth. For instance, the average diameter of pinholes in Ag@ZCMF with optimum secondary growth time of 14 h (Ag@ZCMF-14) reached approximately 21 μm, giving rise to the water flux and intrusion pressure of 54720 L·m-2·h-1 and 4357 Pa, respectively. The anti-corrosion test and rubbing test confirmed the high chemical and mechanical stability of Ag@ZCMF-14, respectively. The separation efficiency of Ag@ZCMF-14 remained stable during ten purification-regeneration cycles, and no obvious attenuation was observed, proving the high separation stability of Ag@ZCMF-14. Furthermore, the loaded Ag+ ions afforded the membrane excellent anti-biofouling activity, which could effectively inhibit the growth of both alga and bacteria in the operating environment, thus preventing membrane blockage during the oil-water separation process. In particular, the bacteriostatic rate of Ag@ZCMF-14 to Escherichia coli reached to 99.6%. These results demonstrate that Ag@ZCMFs with anti-biofouling activity has promising potential future applications in the removal of oil slicks from oily wastewater.
The considerable demand of robust solid-state nuclear magnetic resonance (NMR) sequences has been met by the development in solid-state NMR hardware and probe design, particularly for fast magic angle spinning (MAS). Fast MAS enhances spectral resolution, however, it makes many conventional methods unusable because of the need of significantly high radiofrequency (RF) field strength and the intrinsic inefficiencies under such condition. Dipolar-based homonuclear recoupling sequences are widely used for structural analysis, and radio-frequency driven recoupling (RFDR) is one of the most popular zero-quantum (ZQ) homonuclear recoupling sequence. Previous studies demonstrated that RFDR efficiency strongly depends on factors such as MAS frequency, resonance offset, RF field inhomogeneity, and chemical shift anisotropy (CSA). To alleviate these dependencies, different RFDR phase cycles have been proposed. To completely understand the principle of ZQ recoupling sequences and achieve uniform broadband homonuclear recoupling under fast MAS conditions, we herein utilize the theory of symmetry sequences and propose a series of RNN1 (N ≥ 4, N is even) sequences with various phase cycles under both moderate and fast MAS conditions. We simulated the influence of MAS rate, resonance offset, RF field strength, RF mismatch, and heteronuclear decoupling on ZQ homonuclear polarization transfer efficiency. We verified the ZQ dipolar recoupling efficiencies of various RN symmetry sequences using U-13C, 15N-labeled L-histidine and microcrystalline U-13C, 15N-labeled dynein light chain (LC8) protein. The basic R4 sequence showed the worst broadband ZQ polarization transfer performance theoretically and experimentally, while the basic R6 sequence could efficiently achieve ZQ dipolar recoupling within moderate bandwidth. Under low to moderate MAS conditions, high-power 1H decoupling could considerably enhance the polarization transfer efficiency, while homonuclear recoupling sans heteronuclear decoupling is recommended under fast MAS conditions. Super phase cycling enhanced ZQ polarization transfer efficiency and bandwidth and resulted in significantly reduced sensitivity to RF mismatch. RNixy3 and RNixy4 sequences with 6*N and 8*N phase cycling steps, respectively, were preferred. The R4ixy3 sequence with fewer phase cycling steps showed comparable, or even slightly better, performance to the R4ixy4 sequence. As shown in the simulations, by choosing proper RF field strengths, 1.5*ωr < ω1 < 3*ωr, uniform broadband ZQ recoupling with R4ixy3 or R4ixy4 sequences could be achieved under fast MAS conditions, which would be significant for the accurate determination of spatial proximities and internuclear distances. By prolonging the mixing time, the RN ZQ scheme could provide more cross peaks, where medium- to long-range spatial correlations could be included; these correlations are essential for structural determination in complex systems.
Artificial photosynthesis is an ideal method for solar-to-chemical energy conversion, wherein solar energy is stored in the form of chemical bonds of solar fuels. In particular, the photocatalytic reduction of CO2 has attracted considerable attention due to its dual benefits of fossil fuel production and CO2 pollution reduction. However, CO2 is a comparatively stable molecule and its photoreduction is thermodynamically and kinetically challenging. Thus, the photocatalytic efficiency of CO2 reduction is far below the level of industrial applications. Therefore, development of low-cost cocatalysts is crucial for significantly decreasing the activation energy of CO2 to achieving efficient photocatalytic CO2 reduction. Herein, we have reported the use of a Ni2P material that can serve as a robust cocatalyst by cooperating with a photosensitizer for the photoconversion of CO2. An effective strategy for engineering Ni2P in an ultrathin layered structure has been proposed to improve the CO2 adsorption capability and decrease the CO2 activation energy, resulting in efficient CO2 reduction. A series of physicochemical characterizations including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and atomic force microscopy (AFM) were used to demonstrate the successful preparation of ultrathin Ni2P nanosheets. The XRD and XPS results confirm the successful synthesis of Ni2P from Ni(OH)2 by a low temperature phosphidation process. According to the TEM images, the prepared Ni2P nanosheets exhibit a 2D and near-transparent sheet-like structure, suggesting their ultrathin thickness. The AFM images further demonstrated this result and also showed that the height of the Ni2P nanosheets is ca 1.5 nm. The photoluminescence (PL) spectroscopy results revealed that the Ni2P material could efficiently promote the separation of the photogenerated electrons and holes in [Ru(bpy)3]Cl2·6H2O. More importantly, the Ni2P nanosheets could more efficiently promote the charge transfer and charge separation rate of [Ru(bpy)3]Cl2·6H2O compared with the Ni2P particles. In addition, the electrochemical experiments revealed that the Ni2P nanosheets, with their high active surface area and charge conductivity, can provide more active centers for CO2 conversion and accelerate the interfacial reaction dynamics. These results strongly suggest that the Ni2P nanosheets are a promising material for photocatalytic CO2 reduction, and can achieve a CO generation rate of 64.8 μmol·h-1, which is 4.4 times higher than that of the Ni2P particles. In addition, the XRD and XPS measurements of the used Ni2P nanosheets after the six cycles of the photocatalytic CO2 reduction reaction demonstrated their high stability. Overall, this study offers a new function for the 2D transition-metal phosphide catalysts in photocatalytic CO2 reduction.
Partially reduced TiO2 nanomaterials have attracted significant interest because of their visible-light activity for catalysis and photodegradation. Herein, we prepared a partially reduced anatase TiO2 (Re-A-TiO2) nanoparticle material using a fast combustion method, demonstrating good activity toward decomposing methyl orange under visible light irradiation. The surface structure of the prepared material, after being surface-selectively 17O-labeled with H217O (17O-enriched water), was studied via 17O and 1H solid-state magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy and electron paramagnetic resonance (EPR) spectroscopy, and the obtained results were compared to those of non-reduced anatase TiO2 (A-TiO2). The EPR results showed that the concentrations of paramagnetic species (i.e., oxygen vacancies (OV) and Ti3+) in Re-A-TiO2 were much higher than that in A-TiO2, while the former was associated with a higher OV/Ti3+ ratio. The intensities of the EPR signals were significantly affected by the adsorbed water, and this phenomenon was explored in combination with 1H NMR spectroscopy. The 1H species on Re-A-TiO2 appeared at larger chemical shifts, denoting the increased acidity of the sample, and these 1H species on Re-A-TiO2 were more difficult to remove than those on A-TiO2. On the other hand, different features were observed for the signals arising from the two-coordinated oxygen atoms (μ2-O) in 17O NMR, suggesting a typical anatase TiO2(101) surface on A-TiO2, but a more complex surface environment for Re-A-TiO2. Furthermore, a larger amount of hydroxyl groups (OH) were observed on Re-A-TiO2 compared to that on A-TiO2, indicating a larger proportion of exposed (001) facets on Re-A-TiO2. However, the μ2-O signals broadened and became similar when the drying temperature was increased to 100 ℃, indicating a non-faceted anatase TiO2 surface in such conditions. Based on the EPR and NMR results, a significant fraction of the OH species is believed to be formed from the reaction of the paramagnetic centers and adsorbed water molecules. The 1H→17O cross polarization (CP) MAS and two-dimensional heteronuclear correlation (2D HETCOR) NMR spectra were used to verify the spatial proximity of the hydrogen and oxygen species, confirming the spectral assignments of a strongly adsorbed water and one type of surface OH species. In particular, the 1H NMR signals at approximately 11 ppm were ascribed to the hydrogen species in the intramolecular hydrogen bond. In summary, this study investigated the paramagnetic species and surface structure of anatase TiO2 materials by combining EPR along with 1H and 17O solid-state NMR spectroscopy. The differences in the surface structures of Re-A-TiO2 and A-TiO2 should be closely related to their different properties toward the photodegradation of methyl orange.
Inorganic, organic, and biological materials have specific natural properties which mostly depend on their atomic structures. The properties of novel materials can be predicted based solely on knowing the structure fully. Thus, structure determination plays a very important role in chemistry, physics, and materials science. X-ray crystallography, including single-crystal X-ray diffraction (SCXRD) and powder X-ray diffraction (PXRD), remains an important technique for studying structures. However, SCXRD can only be applied to high-quality large single crystals without disorders/defects, whereas PXRD provides only one-dimensional information and reflections with the similar d-values will overlap, which makes it difficult to determine the unit-cell parameters, space groups, and accurate intensities. Another important technique for structural determination is electron crystallography (EC). As the electron is the probe, EC alone can be used for those crystals which are too small to be studied by SCXRD or too complex to be studied by PXRD. Electrons interact much more strongly with matter than X-rays; therefore, both electron diffractions (ED) patterns and high-resolution transmission electron microscopy (HRTEM) images can be obtained from nano-sized crystals. Although electron crystallography started later than X-ray crystallography, it has become a very important technique for structural analysis after several decades of development. Especially, three dimensional (3D) ED techniques have been developed, automated electron diffraction tomography (ADT) and rotation electron diffraction (RED), which allow for automated data collection without requiring considerable expertise on the operation of electron microscopes. In addition, the intensities of 3D ED data can be extracted and used for structure determination using specialized software developed for SCXRD. However, the strong interactions between electrons and materials also result in dynamic effects and beam damage. Although the dynamic effects in 3D electron diffraction techniques (ADT and RED) can be significantly reduced, some structures still pose problems for obtaining an initial model due to beam damage. Therefore, EC and X-ray crystallography have significant limitations. For many complicated crystals, a single technique is insufficient to solve the crystal structure and different techniques that supply complementary structural information must be used to obtain a complete structural determination. Herein, the application of X-ray crystallography combined with EC for the analysis of complex inorganic crystal structures will be introduced, covering issues associated with peak overlap, impurities, pseudo-symmetry and twinning, disordered frameworks, location guests, and aperiodic structures.
Energy components used in solid rocket propellants are beneficial for improving the energy performance, and their thermal decomposition characteristics significantly affect the combustion properties of the propellants. As a kind of energetic material with both high energy and low sensitivity (impact and friction), 5, 5'-bistetrazole-1, 1'-diolate (TKX-50) can effectively improve the energy and safety characteristics of solid propellants. Burning catalyst is another important component of solid propellants, which can significantly improve the burning rate of the propellant and reduce the pressure exponent. Among various burning catalysts, nanoscale transition metal oxides can promote the thermal decomposition of the energetic component, thus enhancing the combustion properties of the solid propellant. However, the catalytic effects of nanoscale transition metal oxides with different morphologies on the thermal decomposition of TKX-50 have rarely been studied. Based on the excellent catalytic activity of Fe2O3 for TKX-50 thermal decomposition, nano-Fe2O3 particles with spherical and tubular microstructures were used for TKX-50 thermal decomposition. The Fe2O3 nanoparticles were successfully fabricated via the solvothermal method and characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS) analyses. The XRD, FT-IR, and XPS results confirmed the successful fabrication of spherical and tubular Fe2O3 samples. The SEM and TEM images showed that the spherical Fe2O3 samples are composed of agglomerated Fe2O3 nanoparticles with an average particle size of 110 nm. In addition, the average diameter and length of hollow tubular Fe2O3 nanoparticles are 120 nm and 200 nm, respectively. The catalytic activities of spherical and tubular Fe2O3 for TKX-50 decomposition were studied by thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) methods. The DSC and TG-DTG curves showed that both tubular and spherical Fe2O3 could effectively promote TKX-50 thermal decomposition. The first thermal decomposition peak temperature (TFDP) of TKX-50 was reduced by 36.5 K and 26.3 K in the presence of tubular and spherical Fe2O3, respectively, at 10 K·min−1. The activation energy (Ea) of TKX-50, determined by the iso-conversional method, was significantly reduced in the presence of both tubular and spherical Fe2O3. The results indicated that the microstructure of the catalyst has a significant effect on its catalytic performance for TKX-50 thermal decomposition, and that tubular Fe2O3 with hollow microstructure possesses better catalytic activity than spherical Fe2O3. The excellent catalytic activity of tubular Fe2O3 can be attributed to the hollow microstructure, which has more active sites for TKX-50 thermal decomposition.
Alcohols fuel electro-oxidation is significant to the development of direct alcohols fuel cells, that are considered as a promising power source for portable electronic devices. Currently, the catalyst was restricted by the serious poisoning effect and high cost of noble metals. Developing low-cost Pt alloy with high performance and anti-CO poisoning ability was highly desired. In this work, PtCo-NC catalyst was synthesized by combining Pt nanoparticles with ZIF-67 after annealing in the tube furnace and the in situ generated N-doped carbon from ZIF-67 was functionalized to support the PtCo alloy nanoparticle. The structure and morphology were probed by X-ray diffraction, scanning electron microscope and transmission electron microscope, and the electrochemical performance was evaluated for alcohols of methanol and ethanol oxidation in the acid electrolyte. Compared with the reference sample of Pt/C, several times performance enhancement for alcohols fuel oxidation was found on PtCo-NC catalyst as well as the good catalytic stability. Specifically, the peak current density of PtCo-NC was 79.61 mA∙cm−2 for methanol oxidation, about 2.2 times higher than that of the Pt/C electrode (36.97 mA∙cm−2) and 2.5 times higher than that of the commercial Pt/C electrode (31.23 mA∙cm−2); it was 62.69 mA∙cm–2 for ethanol oxidation, about 1.65 times higher than that of Pt/C catalyst (37.99 mA∙cm−2) and commercial Pt/C electrode (37.77 mA∙cm−2). These catalytic performances were also much higher than some analogous catalysts developed for alcohols fuel oxidation. A much higher anti-CO poisoning ability was demonstrated by the CO stripping voltammetry experiment, in which the COad oxidation peak potential for PtCo-NC was 0.46 V, ca. 110 mV negative shift compared with Pt/C catalyst at 0.57 V. A strong electronic effect was indicated by the peak position shifting to the lower binding energy direction by 0.3 eV on PtCo-NC compared with Pt/C reference catalyst. According to the d-band center theory, the electron-enriched state of Pt will decrease the interaction strength of poisoning intermediates adsorbed on its surface; Moreover, according to the bifunctional catalytic mechanism, the presence of Co can form the adsorbed oxygen-containing species (―OH) more easily than Pt at low potentials, and this oxygen-species were helpful in the oxidation of COad at neighboring Pt sites. The high catalytic performance for alcohols fuel oxidation could be due to the largely improved anti-CO poisoning ability and the synergistic effect between the in situ formed PtCo nanoparticles and the N-doped carbon support.
In recent years, aqueous sodium-ion batteries (ASIBs) have experienced rapid development, and a series of cathode materials for ASIBs has been widely reported. Among these, Na0.44MnO2 possesses the most promising prospects due to its low cost, non-toxic nature, simple synthesis, and structural stability. However, the reported capacity of Na0.44MnO2 in aqueous electrolyte was ~40 mAh·g−1 (less than its theoretical capacity of 121 mAh·g−1), which limits its practical applications. Recently, we developed a novel alkaline Zn-Na0.44MnO2 dual-ion battery using Na0.44MnO2 as the cathode, a Zn metal sheet as the anode, and a 6 mol L−1 NaOH aqueous solution as the electrolyte. In this system, the Na0.44MnO2 electrode presented excellent electrochemical performance with high reversible capacity (80.2 mAh·g−1 at 0.5C) and outstanding cycling stability (73% capacity retention over 1000 cycles at 10C) in alkaline aqueous electrolyte. When the negative potential window was extended to 0.3 V, the Na0.44MnO2 electrode delivered an incredibly high capacity of 345.5 mAh·g−1, which far exceeded the theoretical capacity, but the cycling performance was extremely poor. In that study, X-ray diffraction (XRD) and inductively coupled plasma-atomic emission spectrometry (ICP-AES) analyses revealed that de-intercalation of Na+ and formation of Mn(OH)2 occurred during the discharge process, but the detailed electrochemical mechanism and structural evolution of this process remained unclear. In this study, we used ICP-AES to analyze the elemental composition of discharge products at different discharge depths and found that a small amount of Na+ ions extracted from Na0.44MnO2 electrode since Discharge-120 (corresponding to the discharge capacity of 120 mAh·g−1), and the extraction rate increased gradually with increasing discharge depth. Scanning electron microscope (SEM) and XRD analyses were also carried out to characterize the morphology and phase changes of Na0.44MnO2 electrode during discharge. The results show that the discharge of Na0.44MnO2 electrode in the voltage range 1.95–0.3 V could be divided into the three following steps: (1) the potential range above 1.0 V: Na+ ions de-intercalate reversibly into the tunnel structure of Na0.44MnO2; this discharge mechanism is consistent with that in non-aqueous and neutral aqueous sodium ion batteries. (2) The initial platform region at 1.0 V: in this step, protons (H+) began to insert into the Na+-vacancies in NaxMnO2, and the tunnel structure of NaxMnO2 was still maintained. (3) Subsequent slope region: when the Na+-vacancies in the tunnel structure were fully occupied by protons, further intercalation led to intensification of charge repulsion in the crystal structure. Thus, the tunnel structure collapsed to form a new Mn(OH)2 phase, accompanied by the release of Na+ from the structure. H+ has a smaller radius than Na+; therefore, it could insert into the smaller vacancies in Na0.44MnO2, resulting in higher specific capacity. However, the insertion of H+ will also cause structural damage, which seriously worsens the cycling stability of the Na0.44MnO2 electrode.
Biological systems can be regarded as complex and open thermodynamic systems. All processes involved in biological growth and metabolism are accompanied by material and energy exchange. During metabolism, energy in the organisms is released in the form of heat, i.e., metabolic heat, which is the basis for development in the field of biothermochemistry. The calorimetric method considers the thermal effects produced by the various forms of action as the research object, to reveal the law of energy change and quantitative energy conversion. Studying the thermodynamic processes of complex biological systems and related reactions through microcalorimetry and thermodynamic methods reflects the intrinsic laws of life-related processes macroscopically and intrinsically. With the tremendous development and progress in microcalorimetry in terms of the temperature measurement accuracy, stability of temperature control, automation, and multi-functionalization, calorimetry has been widely used in life sciences. It can be used to describe macroscopic processes such as ecosystems and biological evolution, observe organismal and cell growth, examine mitochondrial metabolism, and study problems at the molecular level, including enzymatic reactions and interactions between small molecules and biomacromolecules. Herein, the application of biomass calorimetry in the life sciences is reviewed. The status and progress of biomass calorimetry at different biological and structural levels, such as the ecosystem, biological, organ, cellular, subcellular, and molecular levels are introduced. For example, soil microbial metabolic activity is a universal index for evaluating soil quality. The growth and metabolism of organisms as well as the physical and chemical processes of substances in soil are often accompanied by heat release, which is usually a nonselective signal. The use of isothermal microcalorimetry to nonspecifically monitor and record soil microbial metabolic characteristics has promoted the study of microbial metabolism in complex soil systems. The application of calorimetry to the study of tissues and organs mainly involves the calorimetric study of isolated animal and plant tissues and organs. Calorimetry of animal and microbial cells is considered the most common application of calorimetry in life sciences research. It mainly involves the classification and identification of bacteria, their growth and metabolism, inhibition mechanisms of drugs on microbial growth, principles of kinetics, and the thermodynamic characteristics of microbial growth and metabolism. However, owing to the lack of specificity of biomass calorimetry and the lack of direct access to information at the molecular level, more applications of calorimetry combined with other analytical techniques (especially in biology, medicine, and pharmacy) are needed in the future.
The development of high-performance supercapacitor electrode materials is imperative to alleviate the ongoing energy crisis. Numerous transition metals (oxides) have been studied as electrode materials for supercapacitors owing to their low cost, environmental-friendliness, and excellent electrochemical performance. Among the developed binary transition metal oxides, manganese cobalt oxides typically show high theoretical capacitance and stable electrochemical performance, and are widely used in the electrode materials of supercapacitors. However, the poor conductivity and active material utilization of manganese cobalt oxide-based electrode materials limit their potential capacitance application. Cotton is mainly composed of organic carbon-containing materials, which can be transformed to carbon fibers after calcination. The resultant carbonaceous material exhibits a large specific surface area and good conductivity. Such advantages could potentially suppress the negative effects caused by the poor conductivity and small specific surface area of manganese cobalt oxides, thereby improving the electrochemical performance. Herein, we firstly deposited manganese cobalt oxides on cotton by a simple hydrothermal method, yielding a composite of manganese cobalt oxides and carbon fibers via subsequent calcination, to improve the electrochemical performance of the electrode material. X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller (BET), thermogravimetric analysis (TGA), and electrochemical characterizations were used to investigate the physical, chemical, and electrochemical properties of the prepared samples. The fabricated manganese cobalt oxides in the composite were uniformly dispersed on the carbon fiber surface, which increased the contact between the interface of the electrode material and electrolyte, and enhanced electrode material utilization. The electrode material was confirmed to have well contacted with the electrolyte during a contact angle test. Hence, a pseudo-capacitance reaction completely occurred on the manganese cobalt oxide material. Moreover, the addition of carbon fibers reduced the resistance of the material, resulting in excellent capacitive performance. The capacitance of the prepared composite was 854 F∙g-1 at a current density of 2 A∙g-1. The capacitance was maintained at 72.3% after 2000 cycles at a current density of 2 A∙g-1. These results indicate that the manganese cobalt oxide and carbon fiber composite is a promising electrode material for high-performance supercapacitors. The findings presented herein provide a strategy for coupling with carbon materials to enhance the performance of supercapacitor electrode materials based on manganese cobalt oxides. Thus, novel insights into the design of high-performance supercapacitors for energy management are provided.
With the development of clean and sustainable energy sources, the demand for large-scale electrochemical energy storage systems has rapidly increased over the last few years. Rechargeable Na-ion batteries (NIBs), one of the most promising energy storage technologies, have received a great deal of attention. Titanium-based P2-type layered oxides are attractive candidates for NIB anode materials, owing to their suitable redox potential, low cost, air stability and high safety. The exposed large interlayers of P2 configuration provide facile channels for Na+ insertion/extraction when employed as electrode materials for room temperature, non-aqueous NIBs. In this paper, a novel P2-type Na0.65Li0.13Mg0.13Ti0.74O2 is synthesized by a solid-state reaction method. An orthorhombic phase of Na0.9Mg0.45Ti1.55O2 is observed with the increase in calcination time. During the long calcination process, it is speculated that some lattice Na+ and Li+ of the previously formed P2 phase compound would be volatilized or extracted by O2, forming a low Na-content orthorhombic phase based on the layered host structure. In particular, when the precursor was calcined at 1273 K for 24 h, a perfect biphasic hybrid composite was synthesized. The Na storage performance of the pure P2 compound and hybrid composite were evaluated respectively in sodium half cells with voltage range of 0.2–2.5 V. The P2-type electrode can deliver a reversible capacity of 85.1 mAh·g-1 (theoretical capacity of approximately 108.5 mAh·g-1), whereas, the sample with the orthorhombic phase shows an enhanced initial reversible capacity of 96.3 mAh·g-1. Both of the curves are smooth with no observed plateau, indicating the good structural stability of the electrode during cycling. Thus, the hybrid composite exhibits better cycling performance (capacity retention of 89.7% vs. 84.4% for pure P2, after 400 cycles at current density of 1C) and better rate capability (56.6 mAh·g-1 at 5C vs 47.1 mAh·g-1 at 2C). These results can be attributed to the introduced second phase, which improves the electron and bulk ion conductivity and helps stabilize the structure. Therefore, this novel two-phase intergrowth composite could serve as a promising anode candidate for the large-scale energy storage application of NIBs. Moreover, this structural design strategy could be used for other layered oxides to improve their energy density and cycling stability.
Photocatalysis based on visible light is an efficient and promising strategy to convert solar energy into chemical energy and solve the global issues of environmental pollution and energy shortages. CdS, as a visible light responsive semiconductor material, is widely used in photocatalysis and photoluminescence because of its simple synthesis, abundant raw materials, and appropriate bandgap structure. The inverse opal (IO) structure belonging to photonic crystal structure with unique three-dimensionally ordered macro-mesopore, which can tune the propagation direction of incident light and improve photocatalytic performance. Therefore, IO has attracted extensive attention for photocatalysis applications. Herein, CdS IO photonic crystal films were prepared by co-assembly using CdS nanocrystals and poly(styrene-methyl methacrylate-3-sulfopropyl methacrylate, potassium salt) (P(St-MMA-SPMAP)) emulsion. This method is widely used because it is simple and can rapidly prepare large photonic crystal films. The pore size of the IO structure was regulated by changing the diameter of the polymer. The IO structure was characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), ultraviolet-visible absorption spectroscopy (UV-Vis), and reflectance spectroscopy. The photocatalysis performance of three samples was evaluated via photocatalytic water splitting under visible light irradiation (λ ≥ 420 nm). The photocatalytic hydrogen production rate of the CdS IO film fabricated using a 310 nm P(St-MMA-SPMAP) template (CdS-310) was twice that of CdS nanoparticles (CdS-NPs) under visible light irradiation. This photocatalytic performance enhancement was ascribed to the hierarchically porous structure of the IO photonic crystal. On the one hand, the IO structure increased the propagation of photons in the photocatalytic material and improved sunlight utilization. On the other hand, the structure is conductive to transport and adsorption of molecules. In addition, the IO structure was composed of nanoparticles, providing more active sites for the photocatalytic reaction.
The biosafety of nanoparticles is gaining extensive attention due to their dichotomous effects in fields of biomedicine and atmospheric chemistry. A number of studies have been carried out focusing on the cytotoxicity of nanoparticles and their interactions with cells. However, the mechanism of nanoparticle–cell interactions remains unclear. Here, we review the latest progress in the study of nanoparticle-cell interactions from a cellular chemo-mechanical perspective. Cell mechanics play an important role in cell differentiation, proliferation, apoptosis, polarization, adhesion, and migration. An understanding of the effects of nanoparticles on cell mechanics is therefore needed in order to enhance comprehension of nanoparticle–cell interactions. Firstly, the main molecules and signal pathways related to mechanical chemistry are introduced from three perspectives: cell surface adhesion receptors, the cytoskeleton, and the nucleus. Specifically, integrins and cadherins play a critical role in sensing both the external mechanical force and the force of cell transmission. Actin and microtubules, which are two components of the cytoskeletal network, act as a bridge in mechanical conduction. The nucleus can also be mechanically stressed by the surrounding cytoskeleton through the contraction of the matrix. The nuclear envelope also plays important roles in sensing mechanical signals and in adjusting the morphology and function of the nucleus. We summarize the major nanoparticle-based tools used in the laboratory for the study of cell mechanics, which includes traction force microscopy, atomic force microscopy, optical tweezers, magnetic manipulation, micropillars, and force-induced remnant magnetization spectroscopy. In addition, we discuss the effects that nanoparticles have on cell mechanics. Nanoparticles interact with the adhesion of molecules on the cell membrane surface and on cell cytoskeletal proteins, which further affects the mechanical properties involved in cell stiffness, cell adhesion, and cell migration. Overall, the general conclusions regarding the effects of nanoparticles on cell mechanics are as follows: (1) Nanoparticles can affect cell adhesion by disrupting tight and adherent junctions, and by regulating cell-extracellular matrix adhesion; (2) Nanoparticles can interact with cytoskeletal proteins (actins and tubulins) leading to structural reorganization or disruption of microtubules and F-actin; (3) Cell stiffness changes with the structural reorganization of the cytoskeleton; (4) Cell migration ability can be affected through changes in the cytoskeleton, cell adhesion, and the expression of cell migration-related proteins/molecules. To develop the nano-biosafety evaluation system, future studies should attempt to gain a better understanding of the molecular mechanisms involved with regards to nanoparticles and cell mechanics. Ultimately, further development of new methods and technologies based on nano-mechanical chemistry for diagnosis and treatment purposes are expected, given the wide application of nanomaterials in the biomedical field.
Liquid marbles (LMs) are liquid droplets coated with a layer of lyophobic particles at the air-liquid interface. Since the pioneering work by Aussillous et al. in 2001, LMs have attracted significant attention owing to their facile fabrication, flexibility in the choice of the constituent particles and liquids, intriguing properties such as non-wetting and non-adhesive nature, satisfactory elasticity and stability, as well as promising applications in microfluidics, sensors, controlled release, and microreactors. The classical strategy for the preparation of LMs involves rolling a small volume of a droplet on a lyophobic powder bed for complete encapsulation of the liquid by the particles. In addition, various innovative methods, including electrostatic and coalescent approaches, have been developed for preparing special LMs with a complicated structure or morphology. Diverse materials such as water, surfactant solutions, liquid metals, reagents, blood, and even viscous adhesives have been employed as the internal liquid for the fabrication of LMs. Theoretically, any particulates such as lycopodium, polytetrafluoroethylene, Fe3O4, SiO2, and graphite grains can be employed as the outer coating, but they are usually required to be lyophobic with sizes of less than hundreds of microns. The unique structure of the particle-covered droplet and the dual solid-liquid characteristics endow LMs with some unique and interesting properties, especially the non-wetting and non-adhesive nature. As the lyophobic coating particles restrain the internal liquid from contacting the substrate, LMs can move easily across either solid or liquid surfaces, neither wetting the substrate nor contaminating the internal liquid. An equally fascinating property of LMs is their satisfactory stability, which is necessary for most of their applications. The high stability of LMs stems from the protection of the coating powders and is embodied in both good mechanical stability (remaining intact after being released from a certain height or under a certain compression) and long lifetime (greatly suppressing the evaporation of the internal liquid). These extraordinary properties make LMs promising candidates for use in multitudinous fields, especially droplet microfluidics and microreactors. The potential application of LMs in microfluidics is ascribed to their non-wetting, non-adhesive nature and other features such as an ability to float on a liquid surface, coalescence, split, a small force of rolling friction, and response to external forces. Notably, LMs hold great promise for applications in microreactions, because they can create a confined reaction microenvironment, minimize reagent usage, facilitate unhindered gas exchange between the internal liquid medium and the surrounding environment, and allow the entry/exit of the reactants/products. We herein review the recent advances in LMs, such as manufacturing techniques, formation mechanisms, physical properties, and emerging applications. In particular, much attention is paid to the factors affecting the stability of LMs and the potential strategies to increase their stability. Moreover, this review discusses the challenges in the future development of LMs, suggests several possible ways of addressing these challenges, and forecasts the future development directions. We believe that this review can help researchers gain a better understanding of LMs and promote their further advances.
Solvent molecules can significantly reduce the heat of detonation and stability of energetic metal-organic framework (EMOF) materials, and the development of solvent-free EMOFs has become an effective strategy to prepare high-energy density materials. In this study, a solvent-free EMOF, [Ag2(DTPZ)]n (1) (N% = 32.58%), was synthesized by reacting a high-energy ligand, 2, 3-di(1H-tetrazol-5-yl)pyrazine (H2DTPZ), with silver ions under hydrothermal conditions, and it was structurally characterized by elemental analysis, infrared spectroscopy, X-ray diffraction, and thermal analysis. In 1, the DTPZ2− ligands that adopted a highly torsional configuration bridged the Ag+ ions in an octadentate coordination mode to form a three-dimensional framework (ρ = 2.812 g∙cm−3). The large steric effect and strong coordination ability of DTPZ2− effectively prevented the solvent molecules from binding with the metal centers or occupying the voids of 1. Moreover, the strong π-π stacking interactions [centroid-centroid distance = 0.34461(1) nm] between the tetrazole rings in different DTPZ2− ligands provided a high thermal stability to the framework (Te = 619.1 K, Tp = 658.7 K). Thermal analysis showed that a one-step rapid weight loss with intense heat release primarily occurred during the decomposition of 1, suggesting potential energetic characteristics. Non-isothermal thermokinetic analyses (based on the Kissinger and Ozawa-Doyle methods) were performed using differential scanning calorimetry to obtain the thermoanalysis kinetic parameters of the thermodecomposition of 1 (Ea = 272.1 kJ·mol−1, Eo = 268.9 kJ·mol−1; lgA =19.67 s−1). The related thermodynamic parameters [enthalpy of activation (ΔH≠ = 266.9 kJ·mol−1), entropy of activation (ΔS≠ = 125.4 J·mol−1·K−1), free energy of activation (ΔG≠ = 188.3 kJ·mol−1)], critical temperature of thermal explosion (Tb = 607.1 K), and self-accelerating decomposition temperature (TSADT = 595.8 K) of the decomposition reaction were also calculated based on the decomposition peak temperature and extrapolated onset temperature when the heating rate approached zero. The results revealed that 1 featured good thermal safety, and its decomposition was a non-spontaneous entropy-driven process. The standard molar enthalpy for the formation of 1 was calculated to be (2165.99 ± 0.81) kJ·mol−1 based on its constant volume combustion energy determined using a precise rotating oxygen bomb calorimeter. Detonation and safety performance tests revealed that 1 was insensitive to impact and friction, and its heat of detonation (10.15 kJ·g−1) was higher than that of common ammonium nitrate explosives, such as octogen (HMX), hexogene (RDX), and 2, 4, 6-trinitrotoluene (TNT), indicating that 1 is a promising high-energy and insensitive material.
Silicon is a promising anode material for lithium-ion batteries (LIBs) because of its natural abundance, high theoretical capacity, and relatively low working potential for lithium storage. However, two main obstacles exist that hinder its commercial application. One is the large volume variation during prolonged cycling, which causes irreversible cracking and disconnection of the active mass from the current collector and subsequently rapid decay of capacity of the electrode. The other is its poor intrinsic electronic conductivity, which seriously restricts its rate performance. To date, strategies to improve its cycling stability and rate capability include rational designs of different Si nanostructures and the incorporation of conductive agents. In this study, we present a novel and effective method to fabricate a Si/C composite. Through hydrogen bonding and the electrostatic interaction between graphene oxides (GO) and acidized chitosans (Cs), a hybrid hydrogel was fabricated in which silicon nanoparticles and carbon nanotubes were encapsulated in situ. Following freeze-drying and subsequent calcination, a three-dimensional porous silicon/carbon nanotube/graphene (Si-CNT@G) nanocomposite was obtained. The phase, structure, and morphology of the sample were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and thermogravimetric analysis (TGA). The results show that the silicon nanoparticles were uniformly distributed in the graphene network, which was interwoven with carbon nanotubes. The resultant Si-CNT@G nanocomposite featured a porous three-dimensional conductive carbonaceous support, providing short pathways for electrons, conductive transport highways for lithium ions, a sufficient interface for contact of the electrolyte and electrode, and an effective buffer matrix to alleviate structural change during discharge/charge cycling. Benefiting from these particular features, the as-prepared Si-CNT@G nanocomposite exhibited superior lithium storage performance with high specific capacity and excellent long-term cycling stability when evaluated as an anode material for LIBs. For example, a high discharge capacity of 673.7 mAh·g−1 can be retained after 200 discharge/charge cycles at a current density of 500 mA·g−1 in the potential range of 0.01–1.20 V, with a decent capacity retention of 97%. Even when at a current density of 2000 mA·g−1, a high discharge capacity of 566.9 mAh·g−1 can still be retained. In contrast, the discharge capacity of pure silicon nanoparticles, when tested under the same conditions, was practically nil. These results suggest that the Si-CNT@G nanocomposite is a promising anode material for high-performance LIBs.
Platinum (Pt) is recognized as an excellent cocatalyst which not only suppresses the charge carrier recombination of the photocatalyst but also reduces the overpotential for photocatalytic H2 generation. Albeit of its good performance, the high cost and low abundance restricted the utilization of Pt in large-scale photocatalytic H2 generation. Pt based transition metal alloys are demonstrated to reveal enhanced activities towards various catalytic reactions, suggesting the possibility to substitute Pt as the cocatalyst. In the present work, Pt was partially substituted with Co, Ni, and Fe and Pt-M (M = Co, Ni, and Fe)/g-C3N4 composites were constructed through co-reduction of H2PtCl6 and transition metal salts by the reductant of ethylene glycol. The crystal structure and valence states were measured by X-ray diffractometer (XRD) and X-ray photoelectron spectrometer (XPS), respectively. The higher degree of XRD peaks and larger binding energies for Pt 4f5/2 and Pt 4f7/2 after incorporating Co2+ ions indicated that Co was successfully introduced into the lattice of Pt and Pt-Co bimetallic alloys was attained through the solvothermal treatment. The morphology was subsequently observed by transmission electron microscope (TEM), which showed a good dispersion of Pt-Co nanoparticles on the surface of g-C3N4. Meanwhile, the shrinkage of lattice fringe after introducing cobalt salt further confirmed the presence of Pt-Co bimetallic alloys. The UV-Vis absorption spectra of g-C3N4 and Pt, Pt-Co deposited g-C3N4 were subsequently performed. It was found that the absorption edges were all consistent for all three samples as anticipated, implying that the band gap energy was maintained after hybridizing with Pt or Pt-Co alloys. Furthermore, the photocatalytic H2 generation was carried out over the as-prepared composites with triethanolamine (TEOA) as sacrificial reagent. Under visible-light illumination, the1% (w) Pt2.5M/g-C3N4 (M = Co, Fe, Ni) composites all exhibited higher or comparable activity towards photocatalytic H2 generation when compared to 1% (w) Pt loaded counterpart. In addition, the atomic ratios of Pt/Co and the loading amount of Pt-Co cocatalyst were modified to optimize the photocatalytic performance, among which, 1% (w) Pt2.5Co/g-C3N4 composite revealed the highest activity with a 1.6-time enhancement. Electrochemical impedance spectra (EIS) and photoluminescence (PL) spectra indicated that the enhancement might be attributed to improved charge transfer from g-C3N4 to Pt2.5Co cocatalyst and inhibited charge carrier recombination in the presence of Pt2.5Co cocatalyst. Therefore, the present study demonstrates the great potential to partially replace Pt with low-cost and abundant transition metals and to fabricate Pt based bimetallic alloys as promising cocatalysts for highly efficient photocatalytic H2 generation.