Surface Fluorination to Boost the Stability of the Fe/N/C Cathode in Proton Exchange Membrane Fuel Cells

Inhibiting demetalation of ZnNC via bimetallic CoZn alloy for an efficient and durable oxygen reduction reaction

Inhibition of demetalation due to electrochemical dissolution of metal active centers is a major challenge for the real-world commercialization of transition metals and nitrogen co-doped carbon (MNC) material catalysts. This research utilized a microchannel reactor to synthesize zeolitic imidazolate framework-8@zeolitic imidazolate framework-67, resulting in a CoZn/ZnNC material produced through a core-shell pyrolysis strategy. Direct synergistic interaction of CoZn alloy nanoparticles and ZnNC improves the activity and durability of the oxygen reduction reaction. In proton-exchange membrane fuel cell tests, the CoZn/ZnNC achieved a peak power density of 380.1 mW/cm2. Furthermore, it demonstrated excellent stability in Zn-air battery charge/discharge cycles, lasting up to 480 h. Experimental tests and density functional theory calculations confirmed the presence of strong interactions between the CoZn alloy and ZnNC, which could inhibit demetalation by strengthening the ZnN bond. Furthermore, a moderate rise in the d-band center optimized the adsorption and desorption capacities of oxygen-containing intermediates (*O, *OH, and *OOH). Overall, this research presents a new strategy based on interactions between alloy nanoparticles and single atoms.

Breaking the Activity and Stability Trade-Off of Platinum-Free Catalysts for the Oxygen Reduction Reaction in Hydrogen Fuel Cells

Hydrogen fuel cells, which use hydrogen as fuel to generate electricity, hold great promises as future energy conversion devices for heavy-duty transport, due to their zero CO2 emissions, high energy conversion efficiency, and high power density. However, the adoption of hydrogen fuel cells has been slow due to their reliance on large amounts of costly and scarce platinum (Pt) for the oxygen reduction reaction. The replacement of Pt with Earth-abundant transition metals such as Fe, Co, Mn, and Sn with oxygen reduction reaction affinity has thus been a holy grail of electrocatalysis research. Pt-free catalysts must combine both high power density and high stability in hydrogen fuel cells to be considered viable alternatives to Pt. Despite promising progress on both fronts, a trade-off has emerged: Pt-free catalysts either achieve high power densities (≥1.5 W cm-2) but suffer from low stabilities (≥70% loss after 25 h) or more recently demonstrate improved stability (≤25% loss after 150 h), while delivering considerably lower power densities (<1 W cm-2) in hydrogen fuel cells. Herein, we summarize the recent progress in the synthesis of high power density M-N-C catalysts for hydrogen fuel cells and highlight the critical importance of uncovering the underlying mechanisms using operando methods. We then discuss the primary causes of catalyst degradation in hydrogen fuel cells and the most promising strategies to enhance the stability of the M-N-C catalysts. Finally, a roadmap is proposed to overcome the activity stability trade-off for Pt-free catalysts in hydrogen fuel cells.

Deactivation Mechanism and Mitigation Strategies of Single-Atom Site Electrocatalysts

Single-atom site electrocatalysts (SACs), with maximum atom efficiency, fine-tuned coordination structure, and exceptional reactivity toward catalysis, energy, and environmental purification, have become the emerging frontier in recent decade. Along with significant breakthroughs in activity and selectivity, the limited stability and durability of SACs are often underemphasized, posing a grand challenge in meeting the practical requirements. One pivotal obstacle to the construction of highly stable SACs is the heavy reliance on empirical rather than rational design methods. A comprehensive review is urgently needed to offer a concise overview of the recent progress in SACs stability/durability, encompassing both deactivation mechanism and mitigation strategies. Herein, this review first critically summarizes the SACs degradation mechanism and induction factors at the atomic-, meso- and nanoscale, mainly based on but not limited to oxygen reduction reaction. Subsequently, potential stability/durability improvement strategies by tuning catalyst composition, structure, morphology and surface are delineated, including construction of robust substrate and metal-support interaction, optimization of active site stability, fabrication of porosity and surface modification. Finally, the challenges and prospects for robust SACs are discussed. This review facilitates the fundamental understanding of catalyst degradation mechanism and provides efficient design principles aimed at overcoming deactivation difficulties for SACs and beyond.

Bubble Evolution in Rechargeable Zinc-Air Batteries: Mechanisms, Mitigation Strategies, and Future Prospects

Rechargeable zinc-air batteries (ZABs) are considered among the most promising power sources for portable electronic devices and electric vehicles due to their high energy density, environmental friendliness, and low production costs. However, a major challenge hindering their commercialization is the issue of gas evolution during operation. The electrochemical reactions at both the zinc electrode (hydrogen evolution reaction, HER) and the air electrode (oxygen evolution reaction, OER) result in the formation of gas bubbles. These bubbles reduce the active surface area, increase internal resistance, and disrupt the uniformity of electrochemical reactions, ultimately causing a decrease in battery performance or even failure. This work summarizes the mechanisms behind gas evolution in ZABs and explores various strategies to mitigate bubble formation, including optimizing electrode materials, pulse strategies, and structure designs. Finally, we discuss approaches to suppress gas evolution reactions and outline future research directions to enhance the performance of ZABs.

Atomically Dispersed Metal-Nitrogen-Carbon Catalysts for Acidic Oxygen Reduction Reaction

Designing efficient and cost-effective electrocatalysts toward oxygen reduction reaction (ORR) under demanding acidic environments plays a critical role in advancing proton exchange membrane fuel cells (PEMFCs). Metal-nitrogen-carbon (M-N-C) catalysts with atomically dispersed metals have gained attention for their affordability, excellent catalytic performance, and distinctive features including consistent active sites and high atomic utilization. Over the past decade, significant achievements have been made in this field. This review offers a comprehensive summary of the latest developments in atomically dispersed M-N-C catalysts for ORR in acidic environments along with their applications in PEMFCs. The ORR mechanisms, PEMFC configuration, and operational principles are presented first, followed by an in-depth discussion of strategies to improve the activity and stability of the PEMFC using atomically dispersed M-N-C catalysts at the cathode. Lastly, this review highlights the unresolved challenges and proposes future research pathways for advancing high-performance atomically dispersed M-N-C catalysts and PEMFCs.

Advanced Architectures of Air Electrodes in Zinc-Air Batteries and Hydrogen Fuel Cells

The air electrode is an essential component of air-demanding energy storage/conversion devices, such as zinc-air batteries (ZABs) and hydrogen fuel cells (HFCs), which determines the output power and stability of the devices. Despite atom-level modulation in catalyst design being recently achieved, the air electrodes have received much less attention, causing a stagnation in the development of air-demanding equipment. Herein, the evolution of air electrodes for ZABs and HFCs from the early stages to current requirements is reviewed. In addition, the operation mechanism and the corresponding electrocatalytic mechanisms of ZABs are summarized. In particular, by clarifying the air electrode interfaces of ZABs at different scales, several approaches to improve the air electrode in rechargeable ZABs are reviewed, including innovative electrode structures and bifunctional oxygen catalysts. Afterward, the operating mechanisms of proton-exchange-membrane fuel cells (PEMFCs) and anion-exchange-membrane fuel cells (AEMFCs) are explained. Subsequently, the strategies employed to enhance the efficiency of the membrane electrode assembly (MEA) in PEMFCs and AEMFCs, respectively, are highlighted and discussed in detail. Last, the prospects for air electrodes in ZABs and HFCs are considered by discussing the main challenges. The aim of this review is to facilitate the industrialization of ZABs and HFCs.

Generalized Encapsulations of ZIF-Based Fe-N-C Catalysts with Controllable Nitrogen-Doped Carbon for Significantly-Improved Stability Toward Oxygen Reduction Reaction

The vigorous development of efficient platinum group metal-free catalysts is considerably important to facilitate the universal application of proton exchange membrane fuel cells. Although nitrogen-coordinated atomic iron intercalated in carbon matrix (Fe-N-C) catalysts exhibit promising catalytic activity, the performance in fuel cells, especially the short lifetime, remains an obstacle. Herein, a highly-active Fe-N-C catalyst with a power density of >1 w cm^-2 and prolonged discharge stability with a current density of 357 mA cm^-2 after 40 h of constant voltage discharge at 0.7 V in H₂ -O₂ fuel cells using a controllable and efficient N-C coating strategy is developed. It is clarified that a thicker N-C coating may be more favorable to enhance the stability of Fe-N-C catalysts at the expense of their catalytic activity. The stability enhancement mechanism of the N-C coating strategy is proven to be the synergistic effect of reduced carbon corrosion and iron loss. It is believed that these findings can contribute to the development of Fe-N-C catalysts with high activity and long lifetimes.

Effective Approaches for Designing Stable M-Nx /C Oxygen-Reduction Catalysts for Proton-Exchange-Membrane Fuel Cells

The large-scale commercialization of proton-exchange-membrane fuel cells (PEMFCs) is extremely limited by their costly platinum-group metals (PGMs) catalysts, which are used for catalyzing the sluggish oxygen reduction reaction (ORR) kinetics at the cathode. Among the reported PGM-free catalysts so far, metal-nitrogen-carbon (M-N_x /C) catalysts hold a great potential to replace PGMs catalysts for the ORR due to their excellent initial activity and low cost. However, despite tremendous progress in this field in the past decade, their further applications are restricted by fast degradation under practical conditions. Herein, the theoretical fundamentals of the stability of the M-N_x /C catalysts are first introduced in terms of thermodynamics and kinetics. The primary degradation mechanisms of M-N_x /C catalysts and the corresponding mitigating strategies are discussed in detail. Finally, the current challenges and the prospects for designing highly stable M-N_x /C catalysts are outlined.

General Carbon-Supporting Strategy to Boost the Oxygen Reduction Activity of Zeolitic-Imidazolate-Framework-Derived Fe/N/Carbon Catalysts in Proton Exchange Membrane Fuel Cells

The oxygen reduction reaction (ORR) activity of the Fe/N/Carbon catalysts derived from the pyrolysis of zeolitic-imidazolate-framework-8 (ZIF-8) has been still lower than that of commercial Pt-based catalysts utilized in the proton exchange membrane fuel cells (PEMFCs) due to low density of accessible active sites. In this study, an efficient carbon-supporting strategy is developed to enhance the ORR efficiency of the ZIF-derived Fe/N/Carbon catalysts by increasing the accessible active site density. The enhancement lies in (i) improving the accessibility of active sites via converting dodecahedral particles to graphene-like layered materials and (ii) enhancing the density of FeN_x active sites via suppressing the formation of nanoparticles as well as providing extra spaces to host active sites. The optimized and efficient Fe/N/Carbon catalyst shows a half-wave potential (E_1/2) of 0.834 V versus reversible hydrogen electrode in acidic media and produces a peak power density of 0.66 W cm^-2 in an air-fed PEMFC at 2 bar backpressure, outperforming most previously reported Pt-free ORR catalysts. Finally, the general applicability of the carbon-supporting strategy is confirmed using five different commercial carbon blacks. This work provides an effective route to derive Fe/N/Carbon catalysts exhibiting a higher power density in PEMFCs.

Facilitating the acidic oxygen reduction of Fe-N-C catalysts by fluorine-doping

As the alternatives to expensive Pt-based materials for the oxygen reduction reaction (ORR), iron/nitrogen co-doped carbon catalysts (FeNC) with dense FeN_x active sites are promising candidates to promote the commercialization of proton exchange membrane fuel cells. Herein, we report a synthetic approach using perfluorotetradecanoic acid (PFTA)-modified metal-organic frameworks as precursors for the synthesis of fluorine-doped FeNC (F-FeNC) with improved ORR performance. The utilization of PFTA surfactants causes profound changes of the catalyst structure including F-doping into graphitic carbon, increased micropore surface area and Brunauer-Emmett-Teller (BET) surface area (up to 1085 m² g^-1), as well as dense FeN_x sites. The F-FeNC catalyst exhibits an improved ORR activity with a high E_1/2 of 0.83 V (VS. RHE) compared to the pristine FeNC material (E_1/2 = 0.80 V). A fast decay occurs in the first 10 000 potential cycles for the F-FeNC catalyst, but high durability is still maintained up to another 50 000 cycles. Density functional theory calculations reveal that the strongly withdrawing fluorine atoms doped on the graphitic carbon can optimize the electronic structure of the FeN_x active center and decrease the adsorption energy of ORR intermediates.

Non-PGM Electrocatalysts for PEM Fuel Cells: A DFT Study on the Effects of Fluorination of FeNx-Doped and N-Doped Carbon Catalysts

Fluorination is considered as a means of reducing the degradation of Fe/N/C, a highly active FeN_x-doped disorganized carbon catalyst for the oxygen reduction reaction (ORR) in PEM fuel cells. Our recent experiments have, however, revealed that fluorination poisons the FeN_x moiety of the Fe/N/C catalytic site, considerably reducing the activity of the resulting catalyst to that of carbon only doped with nitrogen. Using the density functional theory (DFT), we clarify in this work the mechanisms by which fluorine interacts with the catalyst. We studied 10 possible FeN_x site configurations as well as 2 metal-free sites in the absence or presence of fluorine molecules and atoms. When the FeN_x moiety is located on a single graphene layer accessible on both sides, we found that fluorine binds strongly to Fe but that two F atoms, one on each side of the FeN_x plane, are necessary to completely inhibit the catalytic activity of the FeN_x sites. When considering the more realistic model of a stack of graphene layers, only one F atom is needed to poison the FeN_x moiety on the top layer since ORR hardly takes place between carbon layers. We also found that metal-free catalytic N-sites are immune to poisoning by fluorination, in accordance with our experiments. Finally, we explain how most of the catalytic activity can be recovered by heating to 900 °C after fluorination. This research helps to clarify the role of metallic sites compared to non-metallic ones upon the fluorination of FeN_x-doped disorganized carbon catalysts.

Stabilizing Fe-N-C Catalysts as Model for Oxygen Reduction Reaction

The highly efficient energy conversion of the polymer-electrolyte-membrane fuel cell (PEMFC) is extremely limited by the sluggish oxygen reduction reaction (ORR) kinetics and poor electrochemical stability of catalysts. Hitherto, to replace costly Pt-based catalysts, non-noble-metal ORR catalysts are developed, among which transition metal-heteroatoms-carbon (TM-H-C) materials present great potential for industrial applications due to their outstanding catalytic activity and low expense. However, their poor stability during testing in a two-electrode system and their high complexity have become a big barrier for commercial applications. Thus, herein, to simplify the research, the typical Fe-N-C material with the relatively simple constitution and structure, is selected as a model catalyst for TM-H-C to explore and improve the stability of such a kind of catalysts. Then, different types of active sites (centers) and coordination in Fe-N-C are systematically summarized and discussed, and the possible attenuation mechanism and strategies are analyzed. Finally, some challenges faced by such catalysts and their prospects are proposed to shed some light on the future development trend of TM-H-C materials for advanced ORR catalysis.

Recent Advances in Electrocatalysts for Proton Exchange Membrane Fuel Cells and Alkaline Membrane Fuel Cells

The rapid progress of proton exchange membrane fuel cells (PEMFCs) and alkaline exchange membrane fuel cells (AMFCs) has boosted the hydrogen economy concept via diverse energy applications in the past decades. For a holistic understanding of the development status of PEMFCs and AMFCs, recent advancements in electrocatalyst design and catalyst layer optimization, along with cell performance in terms of activity and durability in PEMFCs and AMFCs, are summarized here. The activity, stability, and fuel cell performance of different types of electrocatalysts for both oxygen reduction reaction and hydrogen oxidation reaction are discussed and compared. Research directions on the further development of active, stable, and low-cost electrocatalysts to meet the ultimate commercialization of PEMFCs and AMFCs are also discussed.

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.

Enhancement of Mass Transport for Oxygen Reduction Reaction Using Petal-Like Porous Fe-NC Nanosheet

Nitrogen-coordinated single-atom catalysts (SACs) have emerged as a new frontier for accelerating oxygen reduction reaction (ORR) owing to the optimal atom efficiency and fascinating properties. However, augmenting the full exposure of active sites is a crucial challenge in terms of simultaneously pursuing high metal loading of SACs. Here, petal-like porous carbon nanosheets with densely accessible Fe-N₄ moieties (FeNC-D) are constructed by combining the space-confinement of silica and the coordination of diethylenetriaminepentaacetic acid. The resulted FeNC-D catalyst possesses an enhanced mesoporosity and a balanced hydrophobicity/hydrophilicity, which can facilitate mass transport and advance the exposure of inaccessible Fe-N₄ sites, resulting in efficient utilization of active sites. By virtue of the petal-like porous architecture with maximized active site density, FeNC-D demonstrates superior ORR performance in a broad pH range. Remarkably, when utilized as the air cathode in Zn-air battery (ZAB) and microbial fuel cell (MFC), the FeNC-D-based device displays a large power density (356 mW cm^-2 for ZAB and 1041.3 mW m^-2 for MFC) and possesses remarkable stability, substantially outperforming the commercial Pt/C catalyst.

High temperature proton exchange membrane fuel cells: progress in advanced materials and key technologies

High temperature proton exchange membrane fuel cells (HT-PEMFCs) are one type of promising energy device with the advantages of fast reaction kinetics (high energy efficiency), high tolerance to fuel/air impurities, simple plate design, and better heat and water management. They have been expected to be the next generation of PEMFCs specifically for application in hydrogen-fueled automobile vehicles and combined heat and power (CHP) systems. However, their high-cost and low durability interposed by the insufficient performance of key materials such as electrocatalysts and membranes at high temperature operation are still the challenges hindering the technology's practical applications. To develop high performance HT-PEMFCs, worldwide researchers have been focusing on exploring new materials and the related technologies by developing novel synthesis methods and innovative assembly techniques, understanding degradation mechanisms, and creating mitigation strategies with special emphasis on catalysts for oxygen reduction reaction, proton exchange membranes and bipolar plates. In this paper, the state-of-the-art development of HT-PEMFC key materials, components and device assembly along with degradation mechanisms, mitigation strategies, and HT-PEMFC based CHP systems is comprehensively reviewed. In order to facilitate further research and development of HT-PEMFCs toward practical applications, the existing challenges are also discussed and several future research directions are proposed in this paper.

Novel Heteroatom-Doped Fe/N/C Electrocatalysts With Superior Activities for Oxygen Reduction Reaction in Both Acid and Alkaline Solutions

The exploration of noble metal-free catalysts with efficient electrochemical performance toward oxygen reduction reaction in the acid electrolyte is very important for the development of fuel cells technology. Novel pyrolyzed heteroatom-doped Fe/N/C catalysts have been regarded as the most efficient electrocatalytic materials for ORR due to their tunable electronic structure, and distinctive chemical and physical properties. Herein, nitrogen- and sulfur-doped (Fe/N/C and Fe/N/C-S) electrocatalysts were synthesized using ferric chloride hexahydrate as the Fe precursor, N-rich polymer as N precursor, and Ketjen Black EC-600 (KJ600) as the carbon supports. Among these electrocatalysts, the as prepared S and N-doped Fe/N/C-S reveals the paramount ORR activity with a positive half-wave potential value (E _1/2) 0.82 at 0.80 V vs. RHE in 0.1 mol/L H₂SO₄ solution, which is comparable to the commercial Pt/C (Pt 20 wt%) electrocatalyst. The mass activity of the Fe/N/C-S catalyst can reach 45% (12.7 A g^-1 at 0.8 V) and 70% (5.3 A g^-1 at 0.95 V) of the Pt/C electrocatalyst in acidic and alkaline solutions. As result, ORR activity of PGM-free electrocatalysts measured by the rotating-ring disk electrode method increases in the following order: Fe/N/C<Fe/N/C-S, in both basic and acidic medium. This scientific work offers a facile approach to design and synthesizes efficient heteroatom-doped catalytic materials for electrochemical reactions in energy devices.

Atomically dispersed metal–nitrogen–carbon catalysts for fuel cells: advances in catalyst design, electrode performance, and durability improvement

The review provides a comprehensive understanding of the atomically dispersed metal–nitrogen–carbon cathode catalysts for proton-exchange membrane fuel cell applications.

Progress in the Development of Fe-Based PGM-Free Electrocatalysts for the Oxygen Reduction Reaction

Development of alternative energy sources is crucial to tackle challenges encountered by the growing global energy demand. Hydrogen fuel, a promising way to store energy produced from renewable power sources, can be converted into electrical energy at high efficiency via direct electrochemical conversion in fuel cells, releasing water as the sole byproduct. One important drawback to current fuel-cell technology is the high content of platinum-group-metal (PGM) electrocatalysts required to perform the sluggish oxygen reduction reaction (ORR). Addressing this challenge, remarkable progress has been made in the development of low-cost PGM-free electrocatalysts synthesized from inexpensive, earth-abundant, and easily sourced materials such as iron, nitrogen, and carbon (Fe-N-C). PGM-free Fe-N-C electrocatalysts now exhibit ORR activities approaching that of PGM electrocatalysts but at a fraction of the cost, promising to significantly reduce overall fuel-cell technology costs. Herein, recent developments in PGM-free electrocatalysis, demonstrating increased fuel-cell performance, as well as efforts aimed at understanding the key limiting factor, i.e., the nature of the PGM-free active site, are summarized. Further improvements will be accomplished through the controlled and/or rationally designed synthesis of materials with higher active-site densities, while at the same time establishing methods to mitigate catalyst degradation.

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

In recent years, significant progress has been achieved in the development of platinum group metal-free (PGM-free) oxygen reduction reaction (ORR) catalysts for proton exchange membrane (PEM) fuel cells. At the same time the limited durability of these catalysts remains a great challenge that needs to be addressed. This mini-review summarizes the recent progress in understanding the main causes of instability of PGM-free ORR catalysts in acidic environments, focusing on transition metal/nitrogen codoped systems (M-N-C catalysts, M: Fe, Co, Mn), particularly MN_x moiety active sites. Of several possible degradation mechanisms, demetalation and carbon oxidation are found to be the most likely reasons for M-N-C catalysts/cathodes degradation.