Recently, the problems of environmental pollution and energy shortages arise along with the rapid development of the economy, and gradually becoming significant challenges faced by society. To realize truly sustainable development, novel environment-friendly clean energy technologies need to be developed. The fuel cell is a chemical device that can directly convert the chemical energy of a fuel and an oxidant into electrical energy via an electrochemical reaction. The electrochemical reaction is usually clean and complete, and rarely produces harmful substances. Therefore, fuel cells are considered to be one of the most promising clean energy technologies. Fuel cells can be classified based on their electrolytes: alkaline fuel cells (AFC), phosphoric acid fuel cells (PAFC), molten carbonate fuel cells (MCFC), solid oxide fuel cells (SOFC), and proton exchange membrane fuel cells (PEMFC). Although there have been many studies regarding the materials and reactions of fuel cells, direct spectroscopic evidence to understand the reaction mechanisms in the electrodes is lacking. Raman spectroscopy, as a non-destructive molecular spectroscopy technique with ultra-high sensitivity, is suitable for studying fuel cell materials. Over the past decade, the development of surface-enhanced Raman spectroscopy (SERS) and shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) has overcome the material and morphology limitations of traditional Raman spectroscopy. The extraordinary progress in SHINERS has enabled researchers to acquire high-quality Raman spectra for many types of materials instead of only on the surface of noble metals such as Au, Ag, and Cu. This strategy can also be applied to trace intermediate reactants on electrodes to fully understand the reaction mechanism of the fuel cell. Although many kinds of characterization methods including X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and X-ray diffraction (XRD) also exhibit excellent sensitivity for studying electrode reactions, they require material pretreatments and long-duration experiments. Compared with the abovementioned methods, SERS and SHINERS show better performance for in situ experiments, which will aid in the rational design of catalysts and electrode materials with higher efficiency. This article provides an overview of the basic concepts of fuel cells, as well as SERS and SHINERS. In addition, the application of Raman spectroscopy, SERS, and SHINERS in fuel cell development is discussed along with future prospects.

Hydrogen oxygen fuel cells and water electrolysis serves as two important systems for realizing the recycling of hydrogen energy, which involves two crucial electrochemical reactions, the hydrogen oxidation reaction (HOR) and the hydrogen evolution reaction (HER). The kinetics of HOR/HER in alkaline media is 2 or 3 orders of magnitude slower than that in acidic media, which is the main bottleneck that hinders the development of alkaline membrane fuel cells and alkaline water electrolysis. Thus, clarifying the underlying difference of HOR/HER activity in alkaline and acid electrolytes, and exploring the alkaline HOR/HER mechanism are the significant challenges for widely commercial application of low temperature alkaline energy conversion devices. Here, this paper briefly reviews the related explanations and controversies about the alkaline HOR/HER mechanism in recent years, including bifunctional mechanism, hydrogen binding energy (HBE) theory and electronic effect. The bifunctional mechanism emphasizes the influence of water dissociation and OH adsorption on HER and HOR, respectively, which possesses guiding significance for designing and fabricating composite catalysts. The HBE theory stresses that H_ad is the key reaction intermediate of HOR/HER, and other external factors, such as electrode potential, pH, ions and so on, affect the HOR/HER mechanism and kinetics by disturbing HBE. HBE is widely considered to be the only activity descriptor of HOR/HER. The electronic effect emphasizes the role of catalysts' composition and reaction intermediates in regulating electronic structure of active sites and changing HOR/HER mechanism. It provides an effective strategy to construct active sites and optimize catalytic activity. In addition, we summarize the theoretical simulation methods of electrochemical interface and their applications in exploring HOR/HER mechanism. In-depth theoretical simulation of HOR/HER mechanism requires the establishment of a more reasonable explicit solvation model on electrode/electrolyte interface and the combination of density functional theory (DFT), ab initio molecular dynamics (AIMD), and microkinetic model, to calculate the electronic structure and the dynamic processes of electrode/electrolyte interface, such as bond breaking and formation, solvent recombination, and proton migration in the electric double layer during reaction process, and then to analyze the HOR/HER mechanism and reaction kinetics under different electrode potentials and electrolytes. The present review is helpful for understanding the ongoing developments of HOR/HER mechanism. And the combination of experiment and theoretical calculation can be employed to explore the pH-dependence of HOR/HER deeply, and design novel HOR/HER catalysts with high activity and stability.

Fuel cells have attracted much attention because of their high specific energy and low environmental load, but their commercial application is limited by the poor performance and high cost of the relevant electrode catalysts. The oxygen reduction reaction (ORR) is the key cathodic reaction in a fuel cell, and it plays an important role in the chemical energy conversion. However, the slow reaction kinetics, large reaction energy barrier, and low selectivity deteriorate the energy efficiency of fuel cells. Thus, rational design of low-cost electrocatalysts that show good activity is highly desirable for improving the performance of fuel cells. Although noble-metal-based electrocatalysts (e.g., Pt/C) show excellent catalytic activity for the ORR, their limited resources, high price, and low stability caused by the migration and agglomeration of nanoparticles on the surface of carbon supports have hindered their extensive application. Because of their excellent electrical conductivity and stability, carbon-based materials are widely used as substrates for electrode materials in the ORR. Heteroatom (e.g., nitrogen, phosphorus, sulfur)-doped carbon materials can influence the adsorption state of oxygen molecules and intermediates by changing the charge distribution of adjacent carbon atoms because of the difference in electronegativity and atomic radius between the heteroatoms and carbon atoms, thus promoting the ORR activity. Optimization of the structure and surface properties of carbon-based electrocatalysts has helped accelerate the four-electron reaction and reduce the overpotential in the ORR. Therefore, non-noble metal and heteroatom-doped carbon-based catalysts exhibit improved ORR activity. The dispersion of non-noble metals on carbon materials via the interaction of metal atoms with the neighboring nitrogen atoms or other heteroatoms produces high-density active sites in the carbon support, thus leading to high atomic utilization and significantly improving the electrocatalytic activity owing to the synergistic effect. This review focuses on the applications of carbon-based electrocatalysts in fuel cells, summarizing the design strategies and electrocatalytic activities of heteroatom-doped carbon-based catalysts with non-noble metals toward improving their ORR activity. Furthermore, the latest research progress in the field of carbon-based catalysts used as cathode catalysts in proton exchange membrane fuel cells (PEMFCs) and anion-exchange membrane fuel cells (AEMFCs) is integrated, and the direction of future development is addressed.

Oxygen reduction reaction (ORR) largely governs the overall performance of fuel cells. Commercial Pt/C has long been employed as the state-of-the-art electrocatalyst for ORR. The scarcity and high price of Pt, however, have restrained the broad application of fuel cells. Thus, it is crucial to substitute commercial Pt/C with non-precious metal or metal-free electrocatalysts. Among them, heteroatom-doped metal-free electrocatalysts (DMFEs) are promising candidates. Heteroatom doping can modify the electron distribution of carbon materials, generating active sites suitable for the adsorption and reduction of oxygen. Despite significant progress in recent years, high-performance DMFEs remain rare. It is possible to obtain improved ORR activity by the introduction of more active sites to DMFEs, in combination with a large specific surface area. Since Jasinski reported cobalt phthalocyanine is active for ORR more than half a century ago (Nature 1964, 201, 1212), tremendous investigations on metallomacrocycles as ORR electrocatalysts have been carried out. Nevertheless, few studies have further enriched the active sites of DMFEs by adding metallomacrocycles. Herein, we attempt to introduce metallomacrocycles to DMFEs, and to use templates to fabricate porous nanostructures with high specific surface areas. By controlling pH, positively charged aniline monomers can be adsorbed on the negatively charged surface of SiO₂ nanospheres via electrostatic interactions. After in situ polymerization of aniline monomers, a polyaniline (PANI) coated SiO₂ (SiO₂@PANI) composite was formed. To introduce more active components, Fe^Ⅲ tetrakis(4-methoxyphenyl) porphyrin (FeP) was deposited on the surface of SiO₂@PANI by rotary evaporation method. After pyrolysis and removal of the template, FeP-modified porous PANI-based electrocatalysts were synthesized. Remarkably, the resultant 40%FeP/PANI-18-700 electrocatalysts demonstrate a high ORR activity, in terms of a half-wave potential (E_1/2) of 0.843 V (vs. reversible hydrogen electrode (RHE)) in 0.1 mol·L^-1 KOH aqueous solution, which is better than that of most DMFEs, and comparable to that of commercial Pt/C. The improvement of the ORR activity likely originates from the abundant pore structure (18 nm average pore diameter, pore volume of 1.1 cm³·g^-1), large surface area (687.5 m²·g^-1), and high N content (6.4%). Only 25 mV degradation of E_1/2 was observed for 40%FeP/PANI-18-700 during the accelerated durability test, in contrast to a 74 mV negative shift of E_1/2 for commercial Pt/C. Additionally, a hydroxide exchange membrane fuel cell (HEMFC) fabricated with 40%FeP/PANI-18-700 as the cathode approaches a peak power density of 42 mW·cm^-2. The results exhibit 40%FeP/PANI-18-700 may have potential applications in HEMFCs. Our strategy highlights a new avenue for the design and synthesis of non-precious metal electrocatalysts toward ORR in alkaline media.

Direct formic acid fuel cells involve two significant half-reactions, namely formic acid electro-oxidation and oxygen reduction reaction, and the more sluggish and complex process for anode formic acid oxide also determine the whole fuel cells energy conversion efficiency. The most efficient catalysts rely on the noble metal of Pt and Pd based catalysts and compared with Pt catalysts, Pd catalyst is more appealing because of the low CO poisoning effect during the electro-oxidation of formic acid that mainly follows the direct pathway. The Pd alloy effect and support effect should be considered for the catalyst fabrication since the resulted structure and electronic modification could largely increase the catalytic performance. Graphene emerges out as novel support while the easy agglomeration and destruction of pure graphene do not guarantee promising merits. In this work, we demonstrated the formic acid oxidation ability boosting by facile coupling PdNi alloy and 3D graphene aerogel (PdNi/GA) via a freezing-drying/thermal annealing reduction approach. The PdNi alloy crystal structure was well confirmed by the X-ray diffractions technique and the alloy nanoparticles were successfully anchored on the 3D graphene aerogel surface. The electrochemical performance was evaluated for formic acid oxidation in the acid electrolyte. The PdNi/GA catalyst displayed a larger peak current density of 136 mA·cm^-2, which was 2 and 3.45 times greater than Pd/GA (68 mA·cm^-2) and Pd/C (39.4 mA·cm^-2), respectively. The anti-CO poisoning ability was measured by CO stripping technique, and a low onset potential of 0.49 V was found on PdNi/GA catalyst, about 120 mV less than that of Pd/GA catalyst, indicating the oxophilic property of Ni to assist formic acid oxidation via the bi-functional mechanism. The oxidation peak potential of 0.67 V was observed on PdNi/GA, about 40 mV less than that of Pd/C catalyst, indicating the merits of 3D structure of graphene. Moreover, PdNi/GA catalyst had the highest mass activity of 1699 mA·mg^-1 compared to Pd/GA (851 mA·mg^-1) and Pd/C (537 mA·mg^-1) catalysts. The specific activity of PdNi/GA was 2.6 mA·cm^-2, which was 1.94 and 2.7 times of Pd/GA (1.34 mA·cm^-2) and Pd/C (0.96 mA·cm^-2) catalysts. The highly improved catalytic performance could be due to the combined alloy and support effect. The PdNi/GA was a promising catalyst for application in the direct formic acid fuel cells.

Although platinum (Pt)-based catalysts are suffering from high costs and limited reserves, they are still irreplaceable in a short period of time in terms of catalytic performance. Structural optimization, composition regulation and carrier modification are the common strategies to improve the activity and stability of Pt-based catalyst. Strikingly, the morphological evolution of Pt-based electrocatalyst into nanoframes (NFs) have attracted wide attention to reduce the Pt consumption and improve the electrocatalytic activity simultaneously. Contrary to Pt-based solid nanocrystalline materials, Pt-based NFs have many advantages in higher atomic utilization, open space structure and larger specific surface area, which facilitate electron transfer, mass transport and weaken surface adsorption by more unsaturated coordination sites. Here we introduce the detailed preparation strategies of Pt-based NFs with different etching methods (oxidative etching, chemical etching, galvanic replacement and carbon monoxide etching), crystal structure evolution and formation mechanism, efficient applications for oxygen reduction reaction (ORR), methanol oxidation reaction (MOR) and ethanol oxidation reaction (EOR) in direct alcohol fuel cells (DAFCs). Based on the high-efficiency atom utilization, open space structure and diverse alloy composition, Pt-based NFs exhibit superior activity, stability and anti-poisoning than commercial counterparts in the application of DAFCs. The current challenges and future development of Pt-based NFs are prospected on the type of NFs materials, synthesis and etching methods, crystal control and catalytic performance. We propose a series of improvement mechanisms of Pt-based NFs, such as small size effect, high-energy facets, Pt-skin construction and Pt-C integration, thereby weakening the molecule absorption, increasing the Pt utilization, strengthening the intrinsic stability, and alleviating the metal dissolution and support corrosion. Additionally, the scale-up synthesis of catalytic materials, membrane electrodes assembly, and development of the start-stop system and the circulation system design are essential for the commercial application of Pt-based NFs and industrial manufacturing of DAFCs. More importantly, the reaction mechanism, active site distribution and dynamic changes in the catalytic material during the catalytic reaction are crucial to further explain the maintenance and evolution of catalytic performance, which will open a window to elucidate the improvement mechanism of the catalyst in the fuel cell reactions. This review work would promote continuous upgradations and understandings on Pt-based NFs in the future development of DAFCs.

Proton exchange membrane fuel cells (PEMFCs) are considered one of the most promising technologies for efficient power generation in the 21st century. However, several challenges for the PEMFC power technology are associated with low operating temperature, such as complex water management and strict fuel purification. PEMFC operating at high temperature (HT, 100–200 ℃) has in recent years been recognized as a promising solution to meet these technical challenges. At present, HT-PEMFC based on phosphoric acid (PA)-doped polybenzimidazole (PBI) is considered to be the trend of PEMFC future development due to its good environmental tolerance and simplified water/ thermal management. In this HT-PEMFC system, the proton transfer in both catalyst layer (CL) and membrane relies on liquid PA. Thus, a proper amount of PA is required to impregnate the membrane and the CL in order to achieve good proton conductivities in an HT-PEMFC system. Therefore, reducing the loss of PA electrolyte in the membrane electrode assembly (MEA) is crucial to maintaining the good durability of HT-PEMFC. In this work, a Schiff base networks (SNW)-type covalent organic framework (COF) material is proposed as the CL additive to enhance the durability of HT-PEMFC. The well-defined porous structure and tailored functional groups endow the proposed COF network with not only excellent PA retention capacity but also good proton transfer ability, thus leading to the superior durability of the HT-PEMFC in an accelerated stress test (AST). After 100 h operation at heavy load (0.2 V) and high flow rate of air purge, the accumulative PA loss of the COF-based MEA was ~4.03 mg, which is almost an order of magnitude lower than that of the conventional MEA (~13.02 mg), consequently leading to a much lower degradation rate of current density (~0.304 mA·cm^-2·h^-1) than that of the conventional MEA (~1.01 mA·cm^-2·h^-1). Moreover, it was found that the electrode incorporating a proper amount (5%–10%, mass fraction) of the COF material possessed a higher electrochemical surface area (ECSA) and lower ohmic and charge transfer resistances, which further improved the performance of the HT-PEMFC. At the usual operating voltage of 0.6 V, the current density of the MEA containing 10% COF was up to 0.361 A·cm^-2, which is ~30% higher than that of the conventional MEA at 150 ℃, H₂/Air and ambient pressure. These results indicate that incorporating COF materials into CL is a promising strategy to enhance the performance and durability of HT-PEMFC.

High-temperature polymer exchange membrane fuel cells (HT-PEMFCs), promising and sustainable energy conversion devices, have received considerable attention ascribed to their high energy conversion efficiency and zero emission. Different from the traditional Nafion PEMFCs, the working temperature ranks from 120 to 250 ℃ for HT-PEMFCs; as a result, HT-PEFMCs show impressive merits, such as theoretically higher kinetics, simple water/heat management and better tolerance toward impurities in hydrogen fuel; especially the elimination of flooding issue in fuel cells. Moreover, the working temperature matches well with the temperature for hydrogen generation from methanol reforming revealing that the generated heat from HT-PEMFCs can be utilized for methanol reforming to generate hydrogen; in this case, hydrogen tank can be replaced by methanol reforming system for HT-PEMFCs leading to a higher safety. Similar to traditional Nafion PEMFCs, polymer electrolyte membrane (PEM) associated with two electrodes representing for anode and cathode compose the membrane electrode assembly (MEA). Electrocatalyst as heart of HT-PEMFCs significantly affects the output of fuel cells, especially the cathodic electrocatalyst since the oxygen reduction reaction (ORR) kinetics is substantially sluggish than hydrogen oxidation reaction (HOR). Phosphoric acid doped polybenzimidazole (PA-PBI) is the state-of-the-art PEM for HT-PEMFCs; while, due to the low interaction between PA and PBI, PA leaching to the catalyst layer is normally observed during the long-term operation resulting in blocking of active sites to reduce three-phase boundary (TPB); besides, oxygen dissolution/diffusion in PA is much lower compared to Nafion, thereby, lower fuel cell performance is customarily recorded than Nafion PEMFCs. Thus, construction of high-performance ORR electrocatalyst with exceptional tolerance toward phosphate and increasing of oxygen concentration at TPB are highly desirable to realize the commercialization of HT-PEMFCs. Additionally, the stability of electrocatalyst should be significantly considered because the coalescence of platinum (Pt) nanoparticles as well as carbon corrosion is accelerated at high working temperature. In this review, we have summarized the recently reported Pt, non-Pt and meta-free electrocatalysts in HT-PEMFCs application. Surficial modification, alloying effect as well as substrate effect have been invited to construct high-performance Pt electrocatalyst in phosphoric acid electrolyte since the adsorption of phosphate on Pt is alleviated by surface coating and modulation of electronic configuration of Pt. Due to the comparably lower interaction with phosphate than Pt and considerable catalytic activity toward ORR, non-Pt and metal-free electrocatalyst have also been systematically investigated as HT-PEMFCs cathodic electrocatalyst. Finally, the perspectives and challenges in HT-PEMFCs have been discussed.

Proton exchange membrane fuel cells (PEMFCs) generate electricity from hydrogen, powering a range of applications while emitting nothing but water. Therefore, PEMFCs are regarded as an environmentally friendly alternative to internal combustion engines for the future. Nevertheless, the high cost and scarcity of platinum (Pt) sources prevent the widespread adoption of fuel cells. With the development of fuel cell manufacturing technology, current Pt utilization has increased to a relatively high level of 0.2 g·kW^-1 in PEMFCs. However, according to the PGM market report from Johnson Matthey (2020), current Pt utilization in fuel cells is still too low to meet the need for its large-scale application in the automotive industry, unless the Pt utilization can be further reduced to an ultra-low level (0.01 g·kW^-1). Therefore, higher Pt mass activity and higher Pt utilization must be realized in membrane electrode assemblies (MEA) to achieve ultra-low Pt loadings and a reduced Pt usage. Many key variables affect the performance of MEA, such as the activity of electrocatalysts, conductivity and distribution of ionomers, gas diffusion in carbon papers, and the thickness of the proton exchange membrane. For example, a wide variety of highly promising catalysts have been developed, such as shape-controlled Pt nanocrystals, Pt alloy/de-alloys, core-shells, the synergetic effect of active supports, single atom/single-atom layer catalysts for improving the utilization of Pt, and anti-poisoning catalysts. However, the super-high activity of a Pt catalyst is elusive in a real fuel cell because of the lack of a fundamental understanding of the reaction interface structure and mass transfer properties in real cells. For instance, the recently developed Pt-Ni nanoframes that exhibited an extremely high mass activity of 5.7 A·mg^-1 for the oxygen reduction reaction (ORR) in a liquid half-cell only showed about one-tenth the activity in a real fuel cell (0.76 A·mg^-1 Pt at 0.90 V). To achieve widespread adoption of Pt in fuel cells, we urgently need to explore new combinations of electrocatalysts, ionomers, gas diffusion layers, and proton exchange membranes. Taking into account all these factors, recent advances have enhanced the performance of MEA, such as a neural-network-like catalyst structure for higher Pt utilization, a highly order-structured with vertically aligned carbon nanotubes as a highly ordered catalyst layer that exhibits higher mass transfer efficiency, a novel anti-flooding electrode, a higher oxygen permeability and ionic conductivity ionomer, and an ultrathin MEA with low Pt loading that exhibits higher fuel cell output efficiency. This review mainly focuses on the recent progress in fuel cell cathode performance at ultra-low Pt loadings. To achieve the ultimate goal of Pt utilization (0.01 g·kW^-1), further efforts to accelerate this progress are urgently needed, including improving catalytic performance by using highly active and stable supports, decreasing the gas diffusion resistance, enhancing the water management in the catalytic layer, improving the anti-poisoning property, and establishing an integrated ultra-thin and low platinum film electrode.

Bipolar plates (BPs) are one of the key components of proton exchange membrane fuel cell (PEMFC) stacks. To ensure that such a stack operates stably, a BP needs to meet exhibit electrical conductivity, heat conduction, H₂ airtight, flexural strength, and durability. Based on these requirements, the BP should also be as thin as possible to reduce the overall cost of PEMFCs, while improving their volumetric energy density. A composite bipolar plate (CBP) exhibits the advantages of a low production cost, low processing difficulty, and corrosion resistance; it is produced using polymers and graphite as the main materials. Moreover, channel structures can be formed directly after a compression molding process. However, the trade-off that exists between electrical conductivity and flexural strength is a major challenge. The electrical conductivity of a CBP is realized through the network formed by graphite materials. Therefore, it not only depends on the filler concentration, but also on the network structure. At the same time, microstructures such as accumulation polymers and graphite/resin interface are directly related to the gas tightness and flexural strength of CBP. This review summarizes the conductive fillers and polymers that are commonly used for fabricating CBPs. The universal modification methods for both (fillers and polymers) are discussed, and a brief description of the conductive theoretical model has also been included. In addition, the advanced production technology of CBP is summarized, which includes the organization of the conductive network, elimination of the polymer on the plate surface, and preparation technology of the layered plates. The relationship between the production process and the performance of the plate was also analyzed. Some studies indicate that the conductive network can be optimized by combining kinds of carbon-based filler or electric field inducing, which could significantly promote the electrical conductivity of CBP. Flexural strength and H₂ permeation rates were increased by introducing carbon-based materials such as carbon fabric and graphite foil. The modification of the filler and polymer could facilitate their bonding with each other, which reduces agglomeration and increases the performance. It is worth noting that the structure had a notable influence on the performance of CBP, which was reflected in the filler/polymer interface or the hybrid layer structure. Based on this results, some ideas have been provided as the next steps that can be taken for the optimization and production of a CBP. We believe that the optimization of the CBP structure will be the key point for its future research.

Fuel cells are clean, efficient energy conversion devices that produce electricity from chemical energy stored within fuels. The development of fuel cells has significantly progressed over the past decades. Specifically, polymer electrolyte fuel cells, which are representative of proton exchange membrane fuel cells (PEMFCs), exhibit high efficiency, high power density, and quick start-up times. However, the high cost of PEMFCs, partially from the Pt-based catalysts they employ, hinders their diverse applicability. Hydroxide exchange membrane fuel cells (HEMFCs), which are also known as alkaline polymer electrolyte fuel cells (APEFCs), alkaline anion-exchange membrane fuel cells (AAEMFCs), anion exchange membrane fuel cells (AEMFCs), or alkaline membrane fuel cells (AMFCs), have attracted much attention because of their capability to use non-Pt electrocatalysts and inexpensive bipolar plates. The HEMFCs are structurally similar to PEMFCs but they use a polymer electrolyte that conducts hydroxide ions, thus providing an alkaline environment. However, the relatively sluggish kinetics of the hydrogen oxidation reaction (HOR) inhibit the practical application of HEMFCs. The anode catalyst loading needed for HEMFCs to achieve high cell performance is larger than that required for other fuel cells, which substantially increases the cost of HEMFCs. Therefore, low-cost, highly active, and stable HOR catalysts in the alkaline condition are greatly desired. Here, we review the recent achievements in developing such HOR catalysts. First, plausible HOR mechanisms are explored and HOR activity descriptors are summarized. The HOR processes are mainly controlled by the binding energy between hydrogen and the catalysts, but they may also be influenced by OH adsorption, interfacial water adsorption, and the potential of zero (free) charge. Next, experimental methods used to elevate HOR activities are introduced, followed by HOR catalysts reported in the literature, including Pt-, Ir-, Pd-, Ru-, and Ni-based catalysts, among others. HEMFC performances when employing various anode catalysts are then summarized, where HOR catalysts with platinum-group metals exhibited the highest HEMFC performance. Although the Ni-based HOR catalyst activity was higher than those of other non-precious metal-based catalysts, they showed unsatisfactory performance in HEMFCs. We further analyzed HEMFC performances while considering anode catalyst cost, where we found that this cost can be reduced by using recently developed, non-Pt HOR catalysts, especially Ru-based catalysts. In fact, an HEMFC using a Ru-based HOR catalyst showed an anode catalyst cost-based performance similar to that of PEMFCs, making the HEMFC promising for use in practical applications. Finally, we proposed routes for developing future HOR catalysts for HEMFCs.

Proton-exchange membrane fuel cells (PEMFCs) directly transform chemical energy into electrical energy with high energy density and zero carbon emissions, thereby offering a clean energy alternative for fossil fuels and vehicle electrification. However, the existing PEMFCs rely on Pt-based catalysts, especially at the cathode side wherein the sluggish oxygen reduction reaction (ORR) takes place, resulting in high cost and limiting their commercial applications. Therefore, there is a strong interest in developing platinum group metal-free (PGM-free) PEMFCs. Although impressive advancements have been made since metal-nitrogen-carbon (M-N-C) catalysts have been developed as promising candidates for low-cost cathode catalysts, PGM-free PEMFCs still suffer from insufficient activity and durability. Owing to the intricate structure of the tri-phase interface and mass transport limitation, the M-N-C catalysts with high ORR activity in rotating disk electrode (RDE) tests still suffer from unexpected problems such as showing low activity and undesired rapid degradation process in real fuel cell conditions. Therefore, a comprehensive understanding of the active sites and influences of the M-N-C catalyst structure and cathode structure on the PEMFC performance will promote the development of PGM-free PEMFCs. Herein, with an aim to increase the activity and durability of PEMFCs based on M-N-C catalysts, we summarize the recent progress in understanding the active sites of M-N-C catalysts and the relationships between the structures of catalysts/catalyst layers and device performances. At the catalyst level, multiple delicately designed synthetic strategies suggest that attractive device performances can be obtained by tailoring the intrinsic activity and density of the catalyst active sites while engineering the porosity of catalysts to improve the utilization of active sites. Additionally, integrating the catalyst ink into the cathode catalyst layers in PGM-free PEMFC is pivotal for transforming the impressive ORR performance of catalysts in the RDE test to fuel cell performance. Accordingly, the recent advances in the enhancement of mass transfer and charge transport to achieve remarkable fuel cell performance were also included by rationally designing ionomer contents, catalyst morphology, and fabrication process of cathodic catalyst layers. Moreover, durability is the Achilles heel of PEMFCs with M-N-C catalysts, which is currently far behind the commercial requirements. The possible degradation mechanisms and the recent progress in seeking the corresponding solutions are also discussed in this review, including the decomposition of metal species, protonation of nitrogen sites, corrosion of carbon support, and micropore flooding. Based on these insights, the perspective is proposed by articulating open challenges and opportunities in materials innovations and device engineering with an aim to achieve practical M-N-C based PEMFCs.

Proton exchange membrane fuel cells (PEMFCs) have attracted significant attention owing to their high conversion efficiency, high power density, and low pollution. Their performance is mainly governed by the oxygen reduction reaction (ORR) occurring at the cathode. Owing to the sluggish kinetics of ORR, a large amount of electrocatalysts, i.e., platinum (Pt), is required to accelerate the reaction rate and improve the performance of PEMFCs for practical applications. The use of Pt electrocatalysts inevitably increases the cost, thereby hindering the commercialization of PEMFCs. In addition, the activity and stability of the commercial Pt/C catalyst are still insufficient. Therefore, advanced electrocatalysts with high activity, good stability, and low cost are urgently needed. To date, some theoretical models, especially d-band center theory, have been proposed and guided the search for next-generation electrocatalysts with higher ORR activity. Based on these theories, several strategies and catalysts, especially Pt-based alloy catalysts, have been developed to accelerate ORR and improve the fuel cell performance. For instance, Pt–Ni octahedral nanoparticles (NPs) electrocatalysts have achieved remarkable ORR activity, with one order of magnitude higher activity than that of commercial Pt/C. However, PEMFCs are usually operated at a high voltage (0.6–0.8 V) and an acidic electrolyte, where the transition metals (M) are easily oxidized and etched away. The electronic effect induced by the introduction of M would be eliminated due to the dissolution of transition metals and the agglomeration of NPs, leading to the decay of ORR activity. Therefore, the long-term stability of oxygen reduction catalysts and fuel cells remains highly challenging. It is crucial to design an efficient and highly stable ORR catalyst to promote the application of PEMFCs. Aiming to the stability issues of fuel cell cathode catalysts, the current review summarizes the principles, strategies, and approaches for improving the stability of Pt-based catalysts. First, we introduce thermodynamic and kinetic principles that affect the stability of catalysts. Thermodynamic (such as cohesive energy, alloy formation energy, and segregation energy) and kinetic parameters (such as vacancy formation and diffusion barrier) regarding the structural stability of catalysts significantly affect the metal dissolution and atomic diffusion processes. In addition, these parameters seem to be associated with chemical bond energy to some extent, which could be employed as a descriptor for the stability of catalysts. Later, we outline some representative strategies and methods for improving catalyst stability, namely elemental doping, atomic arrangement engineering, chemical or physical confinement, and supporting material design. Finally, a brief summary and future research perspectives are provided.

High-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) have the unique advantages of fast electrode reaction kinetics, high CO tolerance, and simple water and thermal management at their operating temperature (140–200 ℃), which are considered as one of the important research directions of PEMFCs. Membrane electrode assemblies (MEAs), as the core component of HT-PEMFCs, are usually fabricated by sandwiching phosphoric acid (PA)-doped polymer membrane (HT-PEM) between two electrodes. Technically, high PA content is required in HT-PEMs to ensure fast proton conduction, since PA acts as a proton transport carrier, while a high content of PA decreases the interaction among polymer molecules, thus enhancing the movement of the polymer molecules and leading to a decrease in the mechanical strength of the polymer membranes. In addition, PA is driven into catalyst layers owing to capillary force caused by micropore structures, crack connectivity, and accessibility. The PA content in the electrodes is also affected by the hydrophilic/hydrophobic characteristics of the catalyst layers and the surface tension of the acid when it is in close contact with the catalyst layers. Furthermore, PA plays an important role in the construction of electrochemical triple-phase boundaries to promote electrochemical reactions in the catalyst layers. Simultaneously, as a liquid or "free molecule", the migration of PA may be accelerated by the current and the water produced, owing to the formation of charged phosphates or hydronium ions. This process encourages the redistribution of PA within the catalyst layers, and results in acid flooding of the catalytic layers and adsorption on the surface of the platinum catalyst, leading to increased mass transfer resistance for the gas reaction and reduced catalyst activity. Moreover, the increase in supplied absolute flow rate and the temperature elevation in the HT-PEMFC process could accelerate the evaporation of PA from the electrolyte membrane, resulting in a decrease in the stability of HT-PEMFC and corrosion of the metal end plate. Therefore, it is crucial to regulate the distribution and migration of PA in MEAs for the construction of HT-PEMFCs with high performance and stability. Hence, this paper reviews the research status of PA distribution in HT-PEM electrodes in recent years, and summarizes the corresponding regulations and optimization strategies as well as its future development trend.

Proton exchange membrane fuel cells (PEMFCs) are considered as one of the most promising energy conversion devices owing to their high power density, high energy conversion efficiency, environment-friendly merit, and low operating temperature. In the cathodic oxygen reduction reaction and anodic small-molecule oxidation reactions, Pt shows excellent catalytic activity. However, several factors limit the practical application of Pt nanoparticles in fuel cells, such as the high price of Pt, easy agglomeration during long-term cycling, and limited electrocatalytic performance. Alloying Pt with 3d-transition metal produces ligand and strain effects, which reduces the center of Pt-d band and weakens the binding strength of oxygen species, thereby improving the catalytic activity and reducing the cost. However, the performance of fuel cells degrades seriously because the transition metals tend to dissolve in acidic electrolytes. The disordered alloy transformed into ordered intermetallic nanoparticles can prevent the dissolution of transition metals. Ordered intermetallics have highly ordered atomic arrangements and strong Pt(5d)-M(3d) orbital interactions, which result in excellent stability in both acidic and alkaline electrolytes. Ordered intermetallic nanoparticles have attracted significant attention owing to their excellent electrocatalytic activity and stability, which can be attributed to controllable composition and structure. Pd has a similar electronic structure and lattice parameters to Pt, and has thus attracted significant attention. Several Pd-based ordered intermetallics have been synthesized, and they exhibit sufficient catalytic performance. This review discusses the recent progress in noble metal-based ordered intermetallic electrocatalysts based on the research status of our group over the years. First, the structural characteristics and characterization methods of ordered intermetallic nanoparticles are introduced, exhibiting approaches to distinguish ordered and disordered phases. Then, the controllable preparation of ordered nanoparticles is highlighted, including thermal annealing and direct liquid phase synthesis. The migration and interdiffusion of atoms in the ordering process is very difficult. High-temperature thermal annealing is the most commonly used method for preparing intermetallics, which can precisely control the composition and atomic ordered arrangement. However, thermal annealing can only produce thermodynamically stable spherical nanoparticles. Supports and coating layers are usually employed to prevent agglomeration of nanoparticles at high temperatures. Finally, the applications of ordered intermetallic nanoparticles in fuel cell electrocatalysts are reviewed, including the oxygen reduction reaction (ORR), hydrogen oxidation reaction (HOR), formic acid oxidation reaction (FAOR), methanol oxidation reaction (MOR), and ethanol oxidation reaction (EOR). In addition, the current challenges and future development directions of the catalysts are discussed and discussed to provide new ideas for the development of fuel cell electrocatalysts.