Redox flow batteries (RFBs) have been widely recognized as the primary choice for large-scale energy storage due to their high energy efficiency, low cost, and versatile design of decoupled energy storage and power output. However, the broad deployment of RFBs in the power grid has long been plagued by the high cost and low energy density of existing inorganic metal-based electrodes. Redox-active organic molecules (ROMs), on the other hand, have recently been extensively explored as the potentials electrodes in RFBs for their potential low cost, abundant resources, and highly tunable structure. The energy density of RFBs is proportional to the number of electrons transferred per unit reaction, the concentration of active materials, and the cell voltage. Therefore, strategies to improve the energy density of RFBs could be categorized into three classes: (1) expanding the cell voltage; (2) maximizing the practical concentration of active materials; (3) realizing multi-redox process. Benefited by the highly tunable structure and properties of ROMs, the cell voltage of RFBs could be realized by lowering the redox potentials of anolytes or/and increasing the redox potentials of catholytes. To fully exploit the low-potential anolytes and high-potential catholytes, non-aqueous electrolytes with wider electrochemical potential windows (EPWs) are preferred over the aqueous systems. However, the solubility of most ROMs in commonly used non-aqueous electrolytes is very limited. Several effective strategies to improve the practical concentrations of ROMs have been proposed: (1) the solubility of ROMs could be easily tailored by adjusting the intermolecular interactions between ROMs and the interactions between ROMs and electrolytes via molecular engineering; (2) the redox-active eutectic systems remain liquid at or near room temperature, allowing us to reduce or completely remove the inactive solvent used in the conventional electrolyte of RFBs, which leads to an enhanced practical concentration of the redox-active components; (3) the semi-solid suspension achieves a high practical concentration of ROMs by combining the advantages of solid ROMs with high energy density and liquid electrolytes with flowability; (4) the redox-targeting approach breaks the solubility limitation by realizing remote charge exchange between the solid active materials deposited in the tanks and the current collectors of the electrochemical stacks via ROMs dissolved in electrolytes. The first three strategies employ a homogeneous flowing redox-active fluid which suffers from deteriorated physical and electrochemical properties as the practical concentration of ROMs increase, e.g., high viscosity, phase separation, and salt precipitation. The redox-targeting approach uses a hybrid flowing liquid/static solid system, which avoids the aforementioned unfavorable changes in electrolyte properties, however, this design introduces additional chemical reactions between the ROMs and the solid active materials, which may reduce the power output. Another efficient method to improve the energy density of RFBs without affecting the properties of the electrolyte is achieved by realizing the multi-redox process of ROMs, however, the generated high valence state ROMs are highly reactive. Further optimization of the structure of these ROMs is required to improve their lifetime at high valence states. In this perspective, we summarize the general working principle of the RFBs, highlight the recent developments of the ROMs in non-aqueous redox flow batteries (NRFBs), with an emphasis on the strategies to improve the energy density of NRFBs, and discuss the remaining challenges and future directions of the non-aqueous organic redox flow batteries (NORFBs).
Currently, water shortage is a globally prevalent issue, with approximately 1.5 billion people in over 80 countries in the world are facing a shortage of fresh water. Among them, 300 million people in 26 countries face daily water shortages. It is estimated that by 2025, billions of people will suffer due water shortage. The desalination of seawater and other water treatment technologies have been widely investigated to solve this problem. Recently, a lot of study have been carried out on the production of clean water via solar evaporation with new materials and technologies. Under the condition of illumination, the light-absorbing material converts solar energy directly into heat energy to realize rapid and large amount of water evaporation, after condensation, clean water was obtained. It is important that this technology can effectively remove salt, bacteria, and other pollutants from raw water, and the quality of the obtained water fully meets the drinking water quality standard set by the World Health Organization. This is an efficient, green, and low-cost method for solving the shortage of water resources. Three-dimensional (3D) graphene materials have excellent physical and chemical properties, high photothermal conversion efficiency, high solar absorption rate, rich internal micro- and nano-channels, good water transmission channels, and large surface water evaporation area; in addition, they can achieve an ultra-high water evaporation rate under solar irradiation. These properties are highly significant in the research and practical applications of photothermal water treatment. In this study, the research progress of 3D-graphene is discussed with regard to the following three aspects. 1) The main preparation method of 3D-graphene was investigated. The advantages and disadvantages of different preparation methods, such as self-assembly, template, and chemical vapor deposition methods were summarized and compared. It can provide reference for readers to choose the preparation method of 3D-graphene; 2) The basic principle of photothermal water evaporation is introduced in detail. The research progress of photothermal water evaporation was summarized based on pure graphene, graphene/polymer composites, and graphene/metal oxide composites. The evaporation properties of different materials were compared. The development, fabrication, and performance of small photothermal conversion devices are briefly introduced; 3) The water treatment of graphene photothermal water evaporation was investigated, and its limitations were analyzed and summarized. Consequently, the challenges faced by photothermal evaporation in theoretical research and the problems to be solved in practical production applications are finally prospected. This review is a valuable reference for the development of 3D-graphene materials and solar-thermal steam generation and water treatment.
Since organic-inorganic halide perovskites were first used in the field of solar cells in 2009, they have emerged as the most promising high-efficiency and low-cost next-generation solar cells. However, even though conventional lead perovskite halide perovskite solar cells have achieved a record efficiency of 25.2%, there is scope for improvement in terms of the detrimental properties of their constituent heavy metals. In addition, their theoretic efficiencies are limited by the large bandgap. Tin perovskite has received considerable attention in recent years, due to its heavy-metal-free character and superior semiconductor properties, such as a suitable bandgap and a high carrier mobility. In order to fabricate tin perovskite solar cells (TPSCs) of high-efficiency, the major obstacles have to be overcome, including fast crystallization of tin perovskites, high p-type carrier concentration, and high defect density. Even if Sn2+ has similar electronic configuration as Pb2+, Sn2+ has two more active electrons, which render tin perovskite less stable. To deal with these problems many strategies are developed. Lewis bases, such as dimethyl sulfoxide, are widely used to slow down the crystallization rate of tin perovskite, while oxide protective layer and plentiful additives (e.g., SnF2, liquid formic acid, and hydrazine vapor) have been found to reduce their oxidation. Furthermore, low-dimension structure and device engineering have been verified effectively promote TPSCs performance. Owing to the aforementioned strategies, the efficiency and stabilities of TPSCs were improving rapidly over the past few years, which indicates that TPSCs are the most promising candidate of lead-free perovskite solar cells. Recently, the certified efficiency of TPSCs reached over 12%, which is the maximum value for lead-free perovskite solar cells. Herein, we discuss the crystal and band structures, as well as the optoelectronic properties of tin perovskites. Furthermore, recent representative studies on tin perovskite are introduced, along with the strategies employed to improve the conversion efficiency, including the achievements based on component modification, dimension control, crystallization engineering and device structure design. Finally, we highlight the challenges presented by tin perovskites and the possible paths to improve device performance.
Perovskite solar cell is a star of the new generation of photovoltaic technology with the greatest application potential due to its simple preparation process and rapid efficiency improvement. At present, the mainstream perovskite solar cell adopts p-i-n structure, using carrier transport materials to extract electrons and holes respectively, so as to realize electric energy output. However, the dependence of traditional p-i-n perovskite solar cell on electron transport layer and hole transport layer makes it not a cost-effective cell, and greatly increases the risk of device stability. Therefore, the design and preparation of perovskite p-n homojunction to realize carrier separation and transmission is considered as an important direction of structural innovation. In recent years, it has been reported frequently that perovskite photoelectric materials exhibit flexible conductivity from p-type, intrinsic to n-type depending on self-doping or external impurities doping. Furthermore, the perovskite p-n homojunction has been developed by a combined deposition method, which provide the possibility for designing and preparing perovskite homojunction solar cells (PHSCs). PHSCs abandon the traditional electron transport layer and hole transport layer, simplifying the device structure. It can not only improve the working stability and reduce the production cost, but also further release the application potential of perovskite solar cells in the field of flexibility and translucency, which can promote the practical popularization of perovskite solar cells. Nevertheless, the PHSCs is still in its infancy, and there are many technical problems to be solved which restrict its efficiency and stability improvement as well as its scale and industrial production. Firstly, the doping degree of perovskite materials should be further increased for high efficiency perovskite homojunction. It means that more accurate self-doping method and exogenous doping processes for heavy doping perovskite need to be developed. Secondly, the stability of the perovskite homojunction should be enhanced to promote the practical application, which requires us to start with the three aspects of inhibiting perovskite decomposition, blocking ion migration, and developing the supporting encapsulation technology to carry out relevant research programs. Thirdly, it is an important task for the industrialization of PHSCs to realize the large-scale preparation through combined deposition method, preservation transfer of perovskite films or superficial doping technology. In this paper, the research progress of PHSCs is reviewed in terms of p-type/n-type doping process and perovskite homojunction. The basic structure, working principle and existing technical problems of PHSCs are discussed in detail. This work has wide ranging impacts beyond solar cells, including emerging applications in light emission, photoelectric detector and neuromorphic computing.
The process that converts CO2 to value-added chemical fuels or industrial feedstocks is called the electrochemical carbon dioxide reduction reaction (CO2RR). When used in combination with renewable energy resources such as solar or wind, it represents one of the most promising strategies for transforming the intermittent renewable energy to chemical energy. However, because CO2 molecules are thermodynamically stable, their electrochemical reduction is kinetically challenging. CO2RR also has several different reaction pathways with a large spectrum of reduction products, making its selectivity problematic. It often requires the assistance of highly effective electrocatalysts with excellent activity, selectivity, and durability. Recently, palladium (Pd)-based nanomaterials have attracted considerable attention for CO2RR. They can enable the selective production of formic acid or formate (HCOOH or HCOO-) at near the theoretical equilibrium, as well as CO at a more negative potential. Unfortunately, the strong surface affinity of Pd toward CO often results in the deactivation of catalytic activity in the electrocatalytic process, in particular for formate production. Over recent years, extensive research effort has been invested into enhancing the electrochemical performances of Pd-based electrocatalysts. By controlling the size, morphology, and crystal surfaces of Pd nanocrystals, the distribution and structure of the atoms on the catalyst surface can be carefully engineered. For example, reducing the size of Pd nanoparticles has been found to significantly enhance the reaction activity and selectivity for the production of both CO and formate. The high-index crystal surfaces of Pd nanocrystals with low coordination numbers also generally show higher electrocatalytic activities. The design of Pd-based alloy nanostructures with tunable electronic structures represents another effective way to improve the electrochemical performance. Incorporation of non-precious metals can not only reduce the cost, but also effectively weaken the surface binding of CO. In addition, dispersing Pd nanoparticles on high-surface-area supports can increase the surface exposure of active sites and facilitate the formation of the electrochemical active phase. In this perspective, we provide an overview of the recent progress on nanostructured Pd-based catalysts for electrochemical CO2 reduction. First, we briefly introduce the CO2RR fundamentals as well as the reaction mechanism on Pd-based nanostructures. We then review a number of strategies to promote CO2RR performance, including utilizing the size effect, morphology effect, alloy effect, core-shell effect, and support effect. Finally, we conclude with a perspective on the future prospects of Pd-based CO2RR electrocatalysts, providing readers a snapshot of this rapidly evolving field.
In this perspective, we review the chemical information encoded in electron density and other ingredients used in semilocal functionals. This information is usually looked at from the functional point of view: the exchange density or the enhancement factor are discussed in terms of the reduced density gradient. However, what parts of a molecule do these 3D functions represent? We look at these quantities in real space, aiming to understand the electronic structure information they encode and provide an insight from the quantum chemical topology (QCT). Generalized gradient approximations (GGAs) provide information about the presence of chemical interactions, whereas meta-GGAs can differentiate between the different bonding types. By merging these two techniques, we show new insight into the failures of semilocal functionals owing to three main errors: fractional charges, fractional spins, and non-covalent interactions. We build on simple models. We also analyze the delocalization error in hydrogen chains, showing the ability of QCT to reveal the delocalization error introduced by semilocal functionals. Then, we show how the analysis of localization can help understand the fractional spin error in alkali atoms, and how it can be used to correct it. Finally, we show that the poor description of GGAs of isodesmic reactions in alkanes is due to 1, 3-interactions.
Semiconducting, two-dimensional (2D) transition metal dichalcogenides (TMDCs) such as molybdenum disulfide (MoS2) have attracted significant attention because of their unique properties and promising applications in electronic and optoelectronic devices. However, the controllable tuning of the properties of 2D MoS2 remains a key challenge with regard to its practical application. Among various approaches to addressing this issue, chemical doping is one of the most efficient. This review focuses on three major doping strategies, which are surface charge transfer, in-plane substitution and interlayer intercalation. We discuss the principles, latest progress and limitations of these doping approaches. Finally, we summarize the current challenges and opportunities associated with the chemical doping of 2D MoS2.
In response to energy shortages and environmental concerns, global energy consumption is transitioning from a reliance on fossil fuels to multiple, clean and efficient power sources. Energy storage is central to the development of electric vehicles and smart grids, and hence to the emerging nationally strategic industries. Today, lithium-ion batteries (LIBs) are among the most widely used energy storage devices in daily life, but they face a severe challenge to meet the rigorous requirements of energy/power density, cycle life and cost for electric vehicles and smart grids. The search for next-generation energy storage technologies with large energy density, long cycle life, high safety and low cost is vital in the post-LIB era. Consequently, lithium-sulfur and lithium-air batteries with high energy density, and safe, low-cost room-temperature sodium-ion batteries, have attracted increasing interest. In this article, we briefly summarize recent progress in next-generation rechargeable batteries and their key electrode materials, with a particular focus on Li-S, Li-air, and Na-ion batteries. The prospects for the future development of these new energy storage technologies are also discussed.
One of the most appealing ways to resolve the worldwide energy crisis and environmental pollution is by converting solar energy into storable chemical energy as hydrogen through solar water splitting. The redox reactions of photogenerated charge carriers occurring on the surface of photocatalysts during the process of solar water splitting are particularly complex. Owing to the high reaction overpotentials and sluggish desorption kinetics of gas products, surface reaction is the rate-determining step in the solar water splitting process. Therefore, a great deal of attention has been focused on this specific research area. The recent advances and prospects for future directions regarding the importance of surface reactions for solar water splitting are presented. The main strategies to enhance the surface water splitting reaction kinetics are summarized. The roles and classifications of surface cocatalysts, as well as the effects of passivating the surface states and coating surface protective layers, are discussed by integrating the principles of photocatalysis. Prospects for the future development of surface reaction research are also proposed.