PGM-Free Cathode Catalysts for PEM Fuel Cells: A Mini-Review on Stability Challenges

Improving the Stability of Non-Noble-Metal M-N-C Catalysts for Proton-Exchange-Membrane Fuel Cells through M-N Bond Length and Coordination Regulation

An effective and universal strategy is developed to enhance the stability of the non-noble-metal M-N_x /C catalyst in proton exchange membrane fuel cells (PEMFCs) by improving the bonding strength between metal ions and chelating polymers, i.e., poly(acrylic acid) (PAA) homopolymer and poly(acrylic acid-maleic acid) (P(AA-MA)) copolymer with different AA/MA ratios. Mössbauer spectroscopy and X-ray absorption spectroscopy (XAS) reveal that the optimal P(AA-MA)-Fe-N catalyst with a higher Fe³⁺ -polymer binding constant possesses longer FeN bonds and exclusive Fe-N₄ /C moiety compared to PAA-Fe-N, which consists of ≈15% low-coordinated Fe-N₂ /N₃ structures. The optimized P(AA-MA)-Fe-N catalyst exhibits outstanding ORR activity and stability in both half-cell and PEMFC cathodes, with the retention rate of current density approaching 100% for the first 37 h at 0.55 V in an H₂ -air fuel cell. Density functional theory (DFT) calculations suggest that the Fe-N₄ /C site could optimize the difference between the adsorption energy of the Fe atoms on the support (E_ad ) and the bulk cohesive energy (E_coh ) relative to Fe-N₂ /N₃ moieties, thereby strongly stabilizing Fe centers against demetalation.

Chemical vapour deposition of Fe-N-C oxygen reduction catalysts with full utilization of dense Fe-N4 sites

Replacing scarce and expensive platinum (Pt) with metal-nitrogen-carbon (M-N-C) catalysts for the oxygen reduction reaction in proton exchange membrane fuel cells has largely been impeded by the low oxygen reduction reaction activity of M-N-C due to low active site density and site utilization. Herein, we overcome these limits by implementing chemical vapour deposition to synthesize Fe-N-C by flowing iron chloride vapour over a Zn-N-C substrate at 750 °C, leading to high-temperature trans-metalation of Zn-N₄ sites into Fe-N₄ sites. Characterization by multiple techniques shows that all Fe-N₄ sites formed via this approach are gas-phase and electrochemically accessible. As a result, the Fe-N-C catalyst has an active site density of 1.92 × 10²⁰ sites per gram with 100% site utilization. This catalyst delivers an unprecedented oxygen reduction reaction activity of 33 mA cm^-2 at 0.90 V (iR-corrected; i, current; R, resistance) in a H₂-O₂ proton exchange membrane fuel cell at 1.0 bar and 80 °C.

Theoretically probing the possible degradation mechanisms of an FeNC catalyst during the oxygen reduction reaction

For the FeNC catalyst widely used in the oxygen reduction reaction (ORR), its instability under fuel cell (FC) operating conditions has become the biggest obstacle during its practical application. The complexity of the degradation process of the FeNC catalyst in FCs poses a huge challenge when it comes to revealing the underlying degradation mechanism that directly leads to the decay in ORR activity. Herein, using density functional theory (DFT) and ab initio molecular dynamics (AIMD) approaches and the FeN₄ moiety as an active site, we find that during catalyzing the ORR, Fe site oxidation in the form of *Fe(OH)₂, in which 2OH* species are adsorbed on Fe on the same side of the FeN₄ plane, results in the successive protonation of N and then permanent damage to the FeN₄ moiety, which causes the leaching of the Fe site in the form of Fe(OH)₂ species and a sharp irreversible decline in the ORR activity. However, other types of OH* adsorption on Fe in the form of HO*FeOH and *FeOH intermediates cannot cause the protonation of N or any breaking of Fe-N bonds in the FeN₄ moiety, only inducing the blocking of the Fe site. Meanwhile, based on the competitive relationship between catalyzing the ORR and Fe site oxidation, we propose a trade-off potential (U ^RHE _TMOR) to describe the anti-oxidation abilities of the TM site in the TMN _X moiety during the ORR.

Scalable Sacrificial Templating to Increase Porosity and Platinum Utilisation in Graphene-Based Polymer Electrolyte Fuel Cell Electrodes

Polymer electrolyte fuel cells hold great promise for a range of applications but require advances in durability for widespread commercial uptake. Corrosion of the carbon support is one of the main degradation pathways; hence, corrosion-resilient graphene has been widely suggested as an alternative to traditional carbon black. However, the performance of bulk graphene-based electrodes is typically lower than that of commercial carbon black due to their stacking effects. This article reports a simple, scalable and non-destructive method through which the pore structure and platinum utilisation of graphene-based membrane electrode assemblies can be significantly improved. Urea is incorporated into the catalyst ink before deposition, and is then simply removed from the catalyst layer after spraying by submerging the electrode in water. This additive hinders graphene restacking and increases porosity, resulting in a significant increase in Pt utilisation and current density. This technique does not require harsh template etching and it represents a pathway to significantly improve graphene-based electrodes by introducing hierarchical porosity using scalable liquid processes.

The influence of the anion exchange membrane on mass-transport limiting phenomena in bipolar interface fuel cells with Fe-N/C based cathode catalyst layers

Water management is a very important issue in low temperature fuel cells such as proton exchange membrane fuel cells (PEMFCs) or anion exchange membrane fuel cells. Within bipolar interface fuel cells, water management inhibits an even more critical role. The earlier work on bipolar interface fuel cells (BPIFCs), employing Fe–N/C on the cathode side for the oxygen reduction reaction (ORR) in an alkaline environment, demonstrated increased stability of the catalyst compared to the acidic environment of the conventional PEMFCs. However, for the BPIFCs, severe mass transport limitations (MTL) dramatically reduced the power output of the cell within a few hours. In the present work water transport processes are identified as the source of the observed MTL, after evaluating the performance data of BPIFCs, where the amount of directly deposited anion exchange membrane (AEM) material was varied. It can be seen that the BPIFCs with lower AEM content show an earlier onset of MTL than the cells prepared with higher AEM content. It is shown that the AEM can be used as a tool to regulate the influx rate of product water from the bipolar interface into the CCL and that flooding of the porous layers is identified as the main source of the observed MTL. This work paves the way for further development of BPIFCs using Fe–N/C at the cathode electrode, as novel cell design strategies can now focus exclusively on avoiding flooding phenomena.

The AEM layer content in a bipolar interface fuel cell enables the opportunity to regulate the influx rate of water into the porous layer.

Degradation and regeneration of Fe-N x active sites for the oxygen reduction reaction: the role of surface oxidation, Fe demetallation and local carbon microporosity

The severe degradation of Fe-N-C electrocatalysts during a long-term oxygen reduction reaction (ORR) has become a major obstacle for application in proton-exchange membrane fuel cells. Understanding the degradation mechanism and regeneration of aged Fe-N-C catalysts would be of particular interest for extending their service life. Herein, we show that the by-product hydrogen peroxide during the ORR not only results in the oxidation of the carbon surface but also causes the demetallation of Fe active sites. Quantitative analysis reveals that the Fe demetallation constitutes the main reason for catalyst degradation, while previously reported carbon surface oxidation plays a minor role. We further reveal that post thermal annealing of the aged catalysts can transform the oxygen functional groups on the carbon surface into micropores. These newly formed micropores not only help to increase the active-site density but also the intrinsic ORR activity of the neighbouring Fe-N4 sites, both contributing to complete activity recovery of aged Fe-N-C catalysts.

Ionic liquid-derived Fe, N, S, F multiple heteroatom-doped carbon materials for enhanced oxygen reduction reaction

Heteroatoms doped carbon catalysts have been intensively studied to take the place of Platinum based catalysts for oxygen reduction reaction (ORR) because of their ideal catalytic activity. Herein, the microporous-mesoporous carbon material catalysts doped with Fe, N, S and F were synthesized through a plain one-pot pyrolysis method with ionic liquid 1-butyl-3-methyli-midazolium bis((trifluoromethyl)sulfonyl)imide ([Bmim][TF₂N]) and melamine as precursors. Electrochemical analysis shows that the ORR activity and stability of the obtained catalysts are obviously better than Pt/C under alkaline condition. Meanwhile, the catalysts show similar ORR activity and much better durability in 0.1 M HClO₄comparing to Pt/C. Moreover, the tolerance of methanol in both basic and acid solutions is greatly better than Pt/C. The high activity is ascribed to the large specific surface area, porous structure and the synergistic effect between S, F, pyridine N, graphite N and Fe-N_x. The high stability possibly comes from the appropriate graphitization and the carbon-coating effect. The strategy proposed here has the advantages of facile, low cost, high efficiency and easy large-scale production, which provides new ideas for the preparation of high-performance Fe-N-C electrocatalysts.

Pore Modification and Phosphorus Doping Effect on Phosphoric Acid-Activated Fe-N-C for Alkaline Oxygen Reduction Reaction

The price and scarcity of platinum has driven up the demand for non-precious metal catalysts such as Fe-N-C. In this study, the effects of phosphoric acid (PA) activation and phosphorus doping were investigated using Fe-N-C catalysts prepared using SBA-15 as a sacrificial template. The physical and structural changes caused by the addition of PA were analyzed by nitrogen adsorption/desorption and X-ray diffraction. Analysis of the electronic states of Fe, N, and P were conducted by X-ray photoelectron spectroscopy. The amount and size of micropores varied depending on the PA content, with changes in pore structure observed using 0.066 g of PA. The electronic states of Fe and N did not change significantly after treatment with PA, and P was mainly found in states bonded to oxygen or carbon. When 0.135 g of PA was introduced per 1 g of silica, a catalytic activity which was increased slightly by 10 mV at −3 mA/cm² was observed. A change in Fe-N-C stability was also observed through the introduction of PA.

Metal-Organic Frameworks and Metal-Organic Gels for Oxygen Electrocatalysis: Structural and Compositional Considerations

Increasing demand for sustainable and clean energy is calling for the next-generation energy conversion and storage technologies such as fuel cells, water electrolyzers, CO₂ /N₂ reduction electrolyzers, metal-air batteries, etc. All these electrochemical processes involve oxygen electrocatalysis. Boosting the intrinsic activity and the active-site density through rational design of metal-organic frameworks (MOFs) and metal-organic gels (MOGs) as precursors represents a new approach toward improving oxygen electrocatalysis efficiency. MOFs/MOGs afford a broad selection of combinations between metal nodes and organic linkers and are known to produce electrocatalysts with high surface areas, variable porosity, and excellent activity after pyrolysis. Some recent studies on MOFs/MOGs for oxygen electrocatalysis and their new perspectives in synthesis, characterization, and performance are discussed. New insights on the structural and compositional design in MOF/MOG-derived oxygen electrocatalysts are summarized. Critical challenges and future research directions are also outlined.

Proton Exchange Membrane (PEM) Fuel Cells with Platinum Group Metal (PGM)-Free Cathode

Proton exchange membrane (PEM) fuel cells have gained increasing interest from academia and industry, due to its remarkable advantages including high efficiency, high energy density, high power density, and fast refueling, also because of the urgent demand for clean and renewable energy. One of the biggest challenges for PEM fuel cell technology is the high cost, attributed to the use of precious platinum group metals (PGM), e.g., Pt, particularly at cathodes where sluggish oxygen reduction reaction takes place. Two primary ways have been paved to address this cost challenge: one named low-loading PGM-based catalysts and another one is non-precious metal-based or PGM-free catalysts. Particularly for the PGM-free catalysts, tremendous efforts have been made to improve the performance and durability-milestones have been achieved in the corresponding PEM fuel cells. Even though the current status is still far from meeting the expectations. More efforts are thus required to further research and develop the desired PGM-free catalysts for cathodes in PEM fuel cells. Herein, this paper discusses the most recent progress of PGM-free catalysts and their applications in the practical membrane electrolyte assembly and PEM fuel cells. The most promising directions for future research and development are pointed out in terms of enhancing the intrinsic activity, reducing the degradation, as well as the study at the level of fuel cell stacks.

Promoting Atomically Dispersed MnN4 Sites via Sulfur Doping for Oxygen Reduction: Unveiling Intrinsic Activity and Degradation in Fuel Cells

Carbon supported and nitrogen coordinated single Mn site (Mn-N-C) catalysts are the most desirable platinum group metal (PGM)-free cathode catalysts for proton-exchange membrane fuel cells (PEMFCs) due to their insignificant Fenton reactions (vs. Fe), earth abundances (vs. Co), and encouraging activity and stability. However, current Mn-N-C catalysts suffer from high overpotential due to low intrinsic activity and less dense MnN₄ sites. Herein, we present a sulfur-doped Mn-N-C catalyst (Mn-N-C-S) synthesized through an effective adsorption-pyrolysis process. Using electron microscopy and X-ray absorption spectroscopy (XAS) techniques, we verify the uniform dispersion of MnN₄ sites and confirm the effect of S doping on the Mn-N coordination. The Mn-N-C-S catalyst exhibits a favorable oxygen reduction reaction (ORR) activity in acidic media relative to the S-free Mn-N-C catalyst. The corresponding membrane electrode assembly (MEA) generates enhanced performance with a peak power density of 500 mW cm^-2 under a realistic H₂/air environment. The constant voltage tests of fuel cells confirm the much-enhanced stability of the Mn-N-C-S catalyst compared to the Fe-N-C and Fe-N-C-S catalysts. The electron microscopy and Fourier transform XAS analyses provide insights into catalyst degradation associated with Mn oxidation and agglomeration. The theoretical calculation elucidates that the promoted ORR activity is mainly attributed to the spatial effect stemmed from the repulsive interaction between the ORR intermediates and adjacent S dopants.

Dynamically Unveiling Metal-Nitrogen Coordination during Thermal Activation to Design High-Efficient Atomically Dispersed CoN4 Active Sites

We elucidate the structural evolution of CoN₄ sites during thermal activation by developing a zeolitic imidazolate framework (ZIF)-8-derived carbon host as an ideal model for Co²⁺ ion adsorption. Subsequent in situ X-ray absorption spectroscopy analysis can dynamically track the conversion from inactive Co-OH and Co-O species into active CoN₄ sites. The critical transition occurs at 700 °C and becomes optimal at 900 °C, generating the highest intrinsic activity and four-electron selectivity for the oxygen reduction reaction (ORR). DFT calculations elucidate that the ORR is kinetically favored by the thermal-induced compressive strain of Co-N bonds in CoN₄ active sites formed at 900 °C. Further, we developed a two-step (i.e., Co ion doping and adsorption) Co-N-C catalyst with increased CoN₄ site density and optimized porosity for mass transport, and demonstrated its outstanding fuel cell performance and durability.

Fe-N-C with Intensified Exposure of Active Sites for Highly Efficient and Stable Direct Methanol Fuel Cells

Fe-N-C catalysts are promising candidates to replace expensive and scarce Pt-based catalysts for oxygen reduction reaction (ORR) in fuel cell devices. Herein, simultaneous improvement of activity and stability of Fe-N-C is achieved through exposing active sites via a surface modification strategy. Concretely, EDTAFe groups are anchored on the external surface of zeolitic imidazolate framework-8 (ZIF-8) through size limitation, followed by pyrolysis to obtain ZIF@EDTAFe-1%-950, whose surface active site density increases more than 1.7 times as detected by X-ray photoelectron spectroscopy (XPS) and ⁵⁷Fe Mössbauer spectra. Consequently, 1.7 times improvement of active site utilization efficiency in electrochemical measurements and more than 2 times performance enhancement in direct methanol fuel cells (DMFCs) are achieved due to facilitated mass transport as revealed by oxygen gain voltage and electrochemical impedance spectroscopy (EIS). Furthermore, through engineering robust drainage channels around exposed active sites to alleviate flooding, the assembled DMFC exhibits better stability than that of Pt/C in the first 3 h and remains 83.9% voltage after 24 h at 100 mA cm^-2.

Modelling Methods and Validation Techniques for CFD Simulations of PEM Fuel Cells

The large-scale adoption of fuel cells system for sustainable power generation will require the combined use of both multidimensional models and of dedicated testing techniques, in order to evolve the current technology beyond its present status. This requires an unprecedented understanding of concurrent and interacting fluid dynamics, material and electrochemical processes. In this review article, Polymer Electrolyte Membrane Fuel Cells (PEMFC) are analysed. In the first part, the most common approaches for multi-phase/multi-physics modelling are presented in their governing equations, inherent limitations and accurate materials characterisation for diffusion layers, membrane and catalyst layers. This provides a thorough overview of key aspects to be included in multidimensional CFD models. In the second part, advanced diagnostic techniques are surveyed, indicating testing practices to accurately characterise the cell operation. These can be used to validate models, complementing the conventional observation of the current–voltage curve with key operating parameters, thus defining a joint modelling/testing environment. The two sections complement each other in portraying a unified framework of interrelated physical/chemical processes, laying the foundation of a robust and complete understanding of PEMFC. This is needed to advance the current technology and to consciously use the ever-growing availability of computational resources in the next future.

Single-Atom Catalysts: A Sustainable Pathway for the Advanced Catalytic Applications

A heterogeneous catalyst is a backbone of modern sustainable green industries; and understanding the relationship between its structure and properties is the key for its advancement. Recently, many upscaling synthesis strategies for the development of a variety of respectable control atomically precise heterogeneous catalysts are reported and explored for various important applications in catalysis for energy and environmental remediation. Precise atomic-scale control of catalysts has allowed to significantly increase activity, selectivity, and in some cases stability. This approach has proved to be relevant in various energy and environmental related technologies such as fuel cell, chemical reactors for organic synthesis, and environmental remediation. Therefore, this review aims to critically analyze the recent progress on single-atom catalysts (SACs) application in oxygen reduction reaction, oxygen evolution reaction, hydrogen evolution reaction, and chemical and/or electrochemical organic transformations. Finally, opportunities that may open up in the future are summarized, along with suggesting new applications for possible exploitation of SACs.

Highly Graphitized Fe-N-C Electrocatalysts Prepared from Chitosan Hydrogel Frameworks

The development of platinum group metal-free (PGM-free) electrocatalysts derived from cheap and environmentally friendly biomasses for oxygen reduction reaction (ORR) is a topic of relevant interest, particularly from the point of view of sustainability. Fe-nitrogen-doped carbon materials (Fe-N-C) have attracted particular interest as alternative to Pt-based materials, due to the high activity and selectivity of Fe-Nx active sites, the high availability and good tolerance to poisoning. Recently, many studies focused on developing synthetic strategies, which could transform N-containing biomasses into N-doped carbons. In this paper, chitosan was employed as a suitable N-containing biomass for preparing Fe-N-C catalyst in virtue of its high N content (7.1%) and unique chemical structure. Moreover, the major application of chitosan is based on its ability to strongly coordinate metal ions, a precondition for the formation of Fe-Nx active sites. The synthesis of Fe-N-C consists in a double step thermochemical conversion of a dried chitosan hydrogel. In acidic aqueous solution, the preparation of physical cross-linked hydrogel allows to obtain sophisticated organization, which assure an optimal mesoporosity before and after the pyrolysis. After the second thermal treatment at 900 °C, a highly graphitized material was obtained, which has been fully characterized in terms of textural, morphological and chemical properties. RRDE technique was used for understanding the activity and the selectivity of the material versus the ORR in 0.5 M H2SO4 electrolyte. Special attention was put in the determination of the active site density according to nitrite electrochemical reduction measurements. It was clearly established that the catalytic activity expressed as half wave potential linearly scales with the number of Fe-Nx sites. It was also established that the addition of the iron precursor after the first pyrolysis step leads to an increased activity due to both an increased number of active sites and of a hierarchical structure, which improves the access to active sites. At the same time, the increased graphitization degree, and a reduced density of pyrrolic nitrogen groups are helpful to increase the selectivity toward the 4e- ORR pathway.

Oxygen Reduction Electrocatalysts toward Practical Fuel Cells: Progress and Perspectives

Fuel cells are an incredibly powerful renewable energy technology, but their broad applications remains lagging because of the high cost and poor reliability of cathodic electrocatalysts for the oxygen reduction reaction (ORR). This review focuses on the recent progress of ORR electrocatalysts in fuel cells. More importantly, it highlights the fundamental problems associated with the insufficient activity translation from rotating disk electrode to membrane electrode assembly in the fuel cells. Finally, for the atomic-level in-depth information on ORR catalysts in fuel cells, potential perspectives are suggested, including large-scale preparation, unified assessment criteria, advanced interpretation techniques, advanced simulation and artificial intelligence. This review aims to provide valuable insights into the fundamental science and technical engineering for efficient ORR electrocatalysts in fuel cells.

Low-PGM and PGM-Free Catalysts for Proton Exchange Membrane Fuel Cells: Stability Challenges and Material Solutions

Fuel cells as an attractive clean energy technology have recently regained popularity in academia, government, and industry. In a mainstream proton exchange membrane (PEM) fuel cell, platinum-group-metal (PGM)-based catalysts account for ≈50% of the projected total cost for large-scale production. To lower the cost, two materials-based strategies have been pursued: 1) to decrease PGM catalyst usage (so-called low-PGM catalysts), and 2) to develop alternative PGM-free catalysts. Grand stability challenges exist when PGM catalyst loading is decreased in a membrane electrode assembly (MEA)-the power generation unit of a PEM fuel cell-or when PGM-free catalysts are integrated into an MEA. More importantly, there is a significant knowledge gap between materials innovation and device integration. For example, high-performance electrocatalysts usually demonstrate undesired quick degradation in MEAs. This issue significantly limits the development of PEM fuel cells. Herein, recent progress in understanding the degradation of low-PGM and PGM-free catalysts in fuel cell MEAs and materials-based solutions to address these issues are reviewed. The key factors that degrade the MEA performance are highlighted. Innovative, emerging material concepts and development of low-PGM and PGM-free catalysts are discussed.

Advanced Electrocatalysis for Energy and Environmental Sustainability via Water and Nitrogen Reactions

Clean and efficient energy storage and conversion via sustainable water and nitrogen reactions have attracted substantial attention to address the energy and environmental issues due to the overwhelming use of fossil fuels. These electrochemical reactions are crucial for desirable clean energy technologies, including advanced water electrolyzers, hydrogen fuel cells, and ammonia electrosynthesis and utilization. Their sluggish reaction kinetics lead to inefficient energy conversion. Innovative electrocatalysis, i.e., catalysis at the interface between the electrode and electrolyte to facilitate charge transfer and mass transport, plays a vital role in boosting energy conversion efficiency and providing sufficient performance and durability for these energy technologies. Herein, a comprehensive review on recent progress, achievements, and remaining challenges for these electrocatalysis processes related to water (i.e., oxygen evolution reaction, OER, and oxygen reduction reaction, ORR) and nitrogen (i.e., nitrogen reduction reaction, NRR, for ammonia synthesis and ammonia oxidation reaction, AOR, for energy utilization) is provided. Catalysts, electrolytes, and interfaces between the two within electrodes for these electrocatalysis processes are discussed. The primary emphasis is device performance of OER-related proton exchange membrane (PEM) electrolyzers, ORR-related PEM fuel cells, NRR-driven ammonia electrosynthesis from water and nitrogen, and AOR-related direct ammonia fuel cells.

57Fe-Enrichment effect on the composition and performance of Fe-based O2-reduction electrocatalysts

In this work, we study how the performance and composition of platinum-group metal free catalysts of the Fe–N–C type are affected upon employing ⁵⁷Fe-enriched precursors in their synthesis.

Short-range amorphous carbon nanosheets for oxygen reduction electrocatalysis

Selectively creating active sites that can work well in different media as much as possible remains an open challenge for the widespread application of sustainable metal air batteries and fuel cells. Herein, short-range amorphous nitrogen-doped carbon nanosheets (NCS) coupled with partially graphitized porous carbon architecture were reported, and were prepared via flexible salt-assisted calcination strategy and followed by a simple cleaning process. The short-range amorphous structure not only significantly promotes the exposure of electrochemically active sites of carbon defects with less protonation in acidic medium, but also maintains the structural stability and electron conduction of the NCS. This unique structure endows the NCS (0.832 V) with efficient ORR electrocatalytic performance with a high half-wave potential (E 1/2) comparable to that of commercial Pt/C (0.837 V) in alkaline electrolyte and an impressive E 1/2 of 0.64 V in harsh acidic medium, making it outstanding among the reported analogous metal-free carbon electrocatalysts. In addition, the NCS manifests robust stability for ORR electrocatalysis with little change in the catalytic activity after accelerated stability tests. This work will provide a feasible inspiration to the construction of carbon nanomaterials with high active site density for efficient energy conversion-related electrochemical reactions.

Carbon-supported Pd-Ir nanoalloys as cathodic catalyst for oxygen reduction reaction

Pd as cathodic catalyst exhibits relatively low current density in mixing kinetic/diffusion controlled region. To improve catalytic activity of Pd, alloying Pd with another transition metal is an effective approach. Here, we prepared carbon-supported Pd-Ir nanoalloys (Pd-Ir/C) through impregnation method. Four types of Pd-Ir/C were designed and referred as Pd19Ir/C, Pd9Ir/C, Pd3Ir/C, and PdIr/C using nominal Pd/Ir atomic ratios of 19:1, 9:1, 3:1, and 1:1, respectively. The results showed that Pd and Ir formed the nanoalloy structures and Pd-Ir/C exhibited significantly improved catalytic activity for oxygen reduction reaction than Pd/C, indicating that Ir had important effect on Pd-Ir nanoalloys.

Hydrogen Fuel Cell Road Vehicles and Their Infrastructure: An Option towards an Environmentally Friendly Energy Transition

The latest pre-production vehicles on the market show that the major technical challenges posed by integrating a fuel cell system (FCS) within a vehicle—compactness, safety, autonomy, reliability, cold starting—have been met. Regarding the ongoing maturity of fuel cell systems dedicated to road transport, the present article examines the advances still needed to move from a functional but niche product to a mainstream consumer product. It seeks to address difficulties not covered by more traditional innovation approaches. At least in long-distance heavy-duty vehicles, fuel cell vehicles (FCVs) are going to play a key role in the path to zero-emissions in one or two decades. Hence the present study also addresses the structuring elements of the complete chain: the latter includes the production, storage and distribution of hydrogen. Green hydrogen appears to be one of the potential uses of renewable energies. The greener the electricity is, the greater the advantage for hydrogen since it permits to economically store large energy quantities on seasonal rhythms. Moreover, natural hydrogen might also become an economic reality pushing the fuel cell vehicle to be a competitive and environmentally friendly alternative to the battery electric vehicle. Based on its own functional benefits for on board systems, hydrogen in combination with the fuel cell will achieve a large-scale use of hydrogen in road transport, as soon as renewable energies become more widespread. Its market will expand from large driving range and heavy load vehicles.

Single Cobalt Sites Dispersed in Hierarchically Porous Nanofiber Networks for Durable and High-Power PGM-Free Cathodes in Fuel Cells

Increasing catalytic activity and durability of atomically dispersed metal-nitrogen-carbon (M-N-C) catalysts for the oxygen reduction reaction (ORR) cathode in proton-exchange-membrane fuel cells remains a grand challenge. Here, a high-power and durable Co-N-C nanofiber catalyst synthesized through electrospinning cobalt-doped zeolitic imidazolate frameworks into selected polyacrylonitrile and poly(vinylpyrrolidone) polymers is reported. The distinct porous fibrous morphology and hierarchical structures play a vital role in boosting electrode performance by exposing more accessible active sites, providing facile electron conductivity, and facilitating the mass transport of reactant. The enhanced intrinsic activity is attributed to the extra graphitic N dopants surrounding the CoN₄ moieties. The highly graphitized carbon matrix in the catalyst is beneficial for enhancing the carbon corrosion resistance, thereby promoting catalyst stability. The unique nanoscale X-ray computed tomography verifies the well-distributed ionomer coverage throughout the fibrous carbon network in the catalyst. The membrane electrode assembly achieves a power density of 0.40 W cm^-2 in a practical H₂ /air cell (1.0 bar) and demonstrates significantly enhanced durability under accelerated stability tests. The combination of the intrinsic activity and stability of single Co sites, along with unique catalyst architecture, provide new insight into designing efficient PGM-free electrodes with improved performance and durability.