Catalysts for the Oxygen Reduction from Heat-Treated Iron(III) Tetramethoxyphenylporphyrin Chloride: Structure and Stability of Active Sites

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.

Inhibiting carbon corrosion of cobalt-nitrogen-carbon materials via Mn sites for highly durable oxygen reduction reaction in acidic media

Cobalt-nitrogen-carbon (CoNxC) materials are regarded as promising low-cost electrocatalysts for the oxygen reduction reaction (ORR). However, their susceptibility to deactivation and poor stability in acidic media limits their practical applications. In this study, we develop cobalt (Co) and manganese (Mn) embedded in nitrogen-doped carbon (CoMnNxC) dual-site catalysts by incorporating Mn into CoNxC and leverage a synergistic dual-catalysis strategy to optimize both activity and stability. The dynamic evolution of *OOH intermediate on the catalyst surface is monitored via in situ Raman spectroscopy, confirming that Mn introduction modulates the reaction pathway. Due to electron transfer from Mn to the Co-Nx center in CoMnNxC, *OOH activation on the surface is enhanced, and the two-electron ORR process is inhibited. Consequently, the CoMnNxC catalyst exhibits excellent ORR activity (E1/2 = 0.76 V vs. reversible hydrogen electrode) and a very low hydrogen peroxide (H2O2) yield (<2.9 %) in acidic electrolyte. Additionally, the dynamic evolution of *OH on the Mn-Nx site confirms that Mn-Nx can serve as a potential catalytic site for the hydrogen peroxide reduction reaction (HPRR), facilitating H2O2 decomposition. Differential electrochemical mass spectrometry (DEMS) demonstrates that this parallel catalytic pathway effectively weaks the oxidative corrosion of H2O2 on the carbon carrier. The results indicate that the negative half-wave potential shift of CoMnNxC catalysts in acidic electrolyte after 10,000 accelerated durability tests (ADT) is only 11 mV. The synergistic dual-catalytic strategy proposed in this work offers a novel approach for designing high-efficiency and stable transition metal-nitrogen-carbon catalysts.

Chemical Kinetic Method for Active-Site Quantification in Fe-N-C Catalysts and Correlation with Molecular Probe and Spectroscopic Site-Counting Methods

Mononuclear Fe ions ligated by nitrogen (FeNx) dispersed on nitrogen-doped carbon (Fe-N-C) serve as active centers for electrocatalytic O2 reduction and thermocatalytic aerobic oxidations. Despite their promise as replacements for precious metals in a variety of practical applications, such as fuel cells, the discovery of new Fe-N-C catalysts has relied primarily on empirical approaches. In this context, the development of quantitative structure-reactivity relationships and benchmarking of catalysts prepared by different synthetic routes and by different laboratories would be facilitated by the broader adoption of methods to quantify atomically dispersed FeNx active centers. In this study, we develop a kinetic probe reaction method that uses the aerobic oxidation of a model hydroquinone substrate to quantify the density of FeNx centers in Fe-N-C catalysts. The kinetic method is compared with low-temperature Mössbauer spectroscopy, CO pulse chemisorption, and electrochemical reductive stripping of NO derived from NO2- on a suite of Fe-N-C catalysts prepared by diverse routes and featuring either the exclusive presence of Fe as FeNx sites or the coexistence of aggregated Fe species in addition to FeNx. The FeNx site densities derived from the kinetic method correlate well with those obtained from CO pulse chemisorption and Mössbauer spectroscopy. The broad survey of Fe-N-C materials also reveals the presence of outliers and challenges associated with each site quantification approach. The kinetic method developed here does not require pretreatments that may alter active-site distributions or specialized equipment beyond reaction vessels and standard analytical instrumentation.

Atomically dispersed Zn-Co-N-C catalyst boosting efficient and robust oxygen reduction catalysis in acid via stabilizing Co-N bonds

Transition metal supported N-doped carbon (M-N-C) catalysts for oxygen reduction reaction (ORR) are viewed as the promising candidate to replace Pt-group metal (PGM) for proton exchange membrane fuel cells (PEMFCs). However, the stability of M-N-C is extremely challenging due to the demetalation, H2O2 attack, etc. in the strongly oxidative conditions of PEMFCs. In this study, we demonstrate the universal effect of Zn on promoting the stability of atomically dispersed M-Nx/C (M = Co, Fe, Mn) catalysts and the enhancement mechanism is unveiled for the first time. The best-performing dual-metal-site Zn-Co-N-C catalyst exhibits a high half-wave potential (E 1/2) value of 0.81 V vs. reversible hydrogen electrode (RHE) in acid and outstanding durability with no activity decay after 15,000 accelerated degradation test (ADT) cycles at 60 °C, surpassing most reported Co-based PGM-free catalysts in acid media. For comparison, the Co-N-C in the absence of Zn suffers from a rapid degradation after ADT due to the demetalation and higher H2O2 yield. X-ray adsorption spectroscopy (XAS) and density functional theory (DFT) calculations suggest the more negative formation energy (by 1.2 eV) and increased charge transfer of Zn-Co dual-site structure compared to Co-N-C could strength the Co-N bonds against the demetalation and the optimized d-band center accounts for the improved ORR kinetics.

Review on the Degradation Mechanisms of Metal-N-C Catalysts for the Oxygen Reduction Reaction in Acid Electrolyte: Current Understanding and Mitigation Approaches

One bottleneck hampering the widespread use of fuel cell vehicles, in particular of proton exchange membrane fuel cells (PEMFCs), is the high cost of the cathode where the oxygen reduction reaction (ORR) occurs, due to the current need of precious metals to catalyze this reaction. Electrochemists tackle this issue in the short/medium term by developing catalysts with improved utilization or efficiency of platinum, and in the longer term, by developing catalysts based on Earth-abundant elements. Considerable progress has been achieved in the initial performance of Metal-nitrogen-carbon (Metal-N-C) catalysts for the ORR, especially with Fe-N-C materials. However, until now, this high performance cannot be maintained for a sufficiently long time in an operating PEMFC. The identification and mitigation of the degradation mechanisms of Metal-N-C electrocatalysts in the acidic environment of PEMFCs has therefore become an important research topic. Here, we review recent advances in the understanding of the degradation mechanisms of Metal-N-C electrocatalysts, including the recently identified importance of combined oxygen and electrochemical potential. Results obtained in a liquid electrolyte and a PEMFC device are discussed, as well as insights gained from in situ and operando techniques. We also review the mitigation approaches that the scientific community has hitherto investigated to overcome the durability issues of Metal-N-C electrocatalysts.

Heterogeneous M-N-C Catalysts for Aerobic Oxidation Reactions: Lessons from Oxygen Reduction Electrocatalysts

Nonprecious metal heterogeneous catalysts composed of first-row transition metals incorporated into nitrogen-doped carbon matrices (M-N-Cs) have been studied for decades as leading alternatives to Pt for the electrocatalytic O2 reduction reaction (ORR). More recently, similar M-N-C catalysts have been shown to catalyze the aerobic oxidation of organic molecules. This Focus Review highlights mechanistic similarities and distinctions between these two reaction classes and then surveys the aerobic oxidation reactions catalyzed by M-N-Cs. As the active-site structures and kinetic properties of M-N-C aerobic oxidation catalysts have not been extensively studied, the array of tools and methods used to characterize ORR catalysts are presented with the goal of supporting further advances in the field of aerobic oxidation.

Rational Design of Atomically Dispersed Metal Site Electrocatalysts for Oxygen Reduction Reaction

Future renewable energy supply and a cleaner Earth greatly depend on various crucial catalytic reactions for the society. Atomically dispersed metal site electrocatalysts (ADMSEs) have attracted tremendous research interest and are considered as the next-generation promising oxygen reduction reaction (ORR) electrocatalysts due to the maximum atom utilization efficiency, tailorable catalytic sites, and tunable electronic structures. Despite great efforts have been devoted to the development of ADMSEs, the systematic summary for design principles of high-efficiency ADMSEs is not sufficiently highlighted for ORR. In this review, the authors first summarize the fundamental ORR mechanisms for ADMSEs, and further discuss the intrinsic catalytic mechanism from the perspective of theoretical calculation. Then, the advanced characterization techniques to identify the active sites and effective synthesis methods to prepare catalysts for ADMSEs are also showcased. Subsequently, a special emphasis is placed on effective strategies for the rational design of the advanced ADMSEs. Finally, the present challenges to be addressed in practical application and future research directions are also proposed to overcome the relevant obstacles for developing high-efficiency ORR electrocatalysts. This review aims to provide a deeper understanding for catalytic mechanisms and valuable design principles to obtain the advanced ADMSEs for sustainable energy conversion and storage techniques.

Identification of the Catalytically Dominant Iron Environment in Iron- and Nitrogen-Doped Carbon Catalysts for the Oxygen Reduction Reaction

For large-scale utilization of fuel cells in a future hydrogen-based energy economy, affordable and environmentally benign catalysts are needed. Pyrolytically obtained metal- and nitrogen-doped carbon (MNC) catalysts are key contenders for this task. Their systematic improvement requires detailed knowledge of the active site composition and degradation mechanisms. In FeNC catalysts, the active site is an iron ion coordinated by nitrogen atoms embedded in an extended graphene sheet. Herein, we build an active site model from in situ and operando ⁵⁷Fe Mössbauer spectroscopy and quantum chemistry. A Mössbauer signal newly emerging under operando conditions, D4, is correlated with the loss of other Mössbauer signatures (D2, D3a, D3b), implying a direct structural correspondence. Pyrrolic N-coordination, i.e., FeN₄C₁₂, is found as a spectroscopically and thermodynamically consistent model for the entire catalytic cycle, in contrast to pyridinic nitrogen coordination. These findings thus overcome the previously conflicting structural assignments for the active site and, moreover, identify and structurally assign a previously unknown intermediate in the oxygen reduction reaction at FeNC catalysts.

Fe-Fe3C Functionalized Few-Layer Graphene Sheet Nanocomposites for an Efficient Electrocatalyst of the Oxygen Reduction Reaction

Preparation of a high-efficiency, low-cost, and environmentally friendly non-precious metal catalyst for the oxygen reduction reaction (ORR) is highly desirable in fuel cells. Herein, a Fe-Fe₃C-functionalized few-layer graphene sheet (Fe/Fe₃C/FLG) nanocomposite was fabricated through the vacuum heat treatment technique using ferric nitrate and glucose as the precursors and exhibited a high-performance ORR electrocatalyst. Multiple characterizations confirm that the nanosized Fe particles with the Fe₃C interface are uniformly distributed in the FLGs. Electrocatalytic kinetics investigation of the nanocomposite indicates that the electron transfer process is a four-electron pathway. The formation of the Fe₃C interface between the Fe nanoparticles and FLGs may promote the electron transfer from the Fe to FLGs. Furthermore, the Fe/Fe₃C/FLG nanocomposite not only exhibits high ORR catalytic activity but also displays desirable stability. Consequently, the obtained Fe/Fe₃C/FLG nanocomposite might be a promising non-precious, cheap, and high-efficiency catalyst for fuel cells.

Identification of Catalytic Active Sites for Durable Proton Exchange Membrane Fuel Cell: Catalytic Degradation and Poisoning Perspectives

Recent progress in synthetic strategies, analysis techniques, and computational modeling assist researchers to develop more active catalysts including metallic clusters to single-atom active sites (SACs). Metal coordinated N-doped carbons (M-N-C) are the most auspicious, with a large number of atomic sites, markedly performing for a series of electrochemical reactions. This perspective sums up the latest innovative and computational comprehension, while giving credit to earlier/pioneering work in carbonaceous assembly materials towards robust electrocatalytic activity for proton exchange membrane fuel cells via inclusive performance assessment of the oxygen reduction reaction (ORR). M-N_x -C_y are exclusively defined active sites for ORR, so there is a unique possibility to intellectually design the relatively new catalysts with much improved activity, selectivity, and durability. Moreover, some SACs structures provide better performance in fuel cells testing with long-term durability. The efforts to understand the connection in SACs based M-N_x -C_y moieties and how these relate to catalytic ORR performance are also conveyed. Owing to comprehensive practical application in the field, this study has covered very encouraging aspects to the current durability status of M-N-C based catalysts for fuel cells followed by degradation mechanisms such as macro-, microdegradation, catalytic poisoning, and future challenges.

Recent Advances in ZIF-Derived Atomic Metal-N-C Electrocatalysts for Oxygen Reduction Reaction: Synthetic Strategies, Active Centers, and Stabilities

Exploring highly active, stable electrocatalysts with earth-abundant metal centers for the oxygen reduction reaction (ORR) is essential for sustainable energy conversion. Due to the high cost and scarcity of platinum, it is a general trend to develop metal-N-C (M-N-C) electrocatalysts, especially those prepared from the zeolite imidazolate framework (ZIF) to replace/minimize usage of noble metals in ORR electrocatalysis for their amazingly high catalytic efficiency, great stability, and readily-tuned electronic structure. In this review, the most pivotal advances in mechanisms leading to declined catalytic performance, synthetic strategies, and design principles in engineering ZIF-derived M-N-C for efficient ORR catalysis, are presented. Notably, this review focuses on how to improve intrinsic ORR activity, such as M-N_x -C_y coordination structures, doping metal-free heteroatoms in M-N-C, dual/multi-metal sites, hydrogen passivation, and edge-hosted M-N_x . Meanwhile, how to increase active sites density, including formation of M-N complex, spatial confinement effects, and porous structure design, are discussed. Thereafter, challenges and future perspectives of M-N-C are also proposed. The authors believe this instructive review will provide experimental and theoretical guidance for designing future, highly active ORR electrocatalysts, and facilitate their applications in diverse ORR-related energy technologies.

Atomically dispersed Fe-Nx species within a porous carbon framework: an efficient catalyst for Li-CO2 batteries

Li-CO₂ batteries are a promising energy storage system, while their practical application is still restricted by a lack of high-performance electrocatalysts for CO₂ reduction and evolution reaction. Herein, we propose a metal-organic-framework-derived Fe-N-C electrocatalyst for Li-CO₂ batteries. Within the Fe-N-C electrocatalyst, abundant Fe-N_x active sites at the molecular level were formed in the porous carbon framework, profiting from a host-guest chemistry strategy between Fe-mIm nanoclusters and metal organic framework precursors in the pyrolysis process. The confinement effect of the metal organic framework host was beneficial to limit the Fe-mIm nanoclusters at the molecular level, thus resulting in the formation of Fe-N_x sites with the high catalytic activity. Moreover, the as-prepared Fe-N-C catalyst is composed of dodecahedral nanoparticles stacking to form a unique three-dimensional structure with a large specific surface area and sufficient space, which not only favored the electron transport and CO₂/Li⁺ diffusion but also promoted the deposition of discharge product Li₂CO₃ to ensure a high capacity. Therefore, the Fe-N-C based Li-CO₂ battery exhibits high specific capacity (13 238 mA h g^-1), good rate capability and excellent cyclability (140 cycles). Therefore, these encouraging results suggest an effective approach to obtain high-performance Fe-N-C electrocatalysts for Li-CO₂ batteries.

Synthesis of graphitized hierarchical porous carbon supported transition-metal for electrochemical conversion

Carbon materials supported metal with high graphitization and hierarchical pore structure are emerging as promising catalysts in electrochemical conversion areas. However, a facile method to prepare this class of catalysts...

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.

Fe, P, N- and FeP, N-doped carbon hollow nanospheres: A comparison study toward oxygen reduction reaction electrocatalysts

In recent years, carbon materials co-doped with transition metals and heteroatoms have been widely used in the oxygen reduction reaction (ORR) as an alternative to platinum/carbon catalysts because of their high efficiency, low price, and appropriate sustainability. Herein, we report the synthesis of FeP, N-doped carbon (FeP, N-Carbon) hollow nanospheres (HNSs) and Fe, P, N-doped carbon (Fe, P, N-Carbon) HNSs. The FeP, N-Carbon was obtained via the pyrolysis of poly(o-phenylenediamine) (PoPD) HNSs in the presence of Fe(NO₃)₃ and phytic acid (PA), whereas Fe, P, N-Carbon was obtained by first pyrolyzing PoPD HNSs with Fe(NO₃)₃, followed by another cycle of pyrolysis with PA. Fe, P, N-Carbon exhibited a better ORR performance than FeP, N-Carbon, with an onset potential of 1.03 V, a half-wave potential of 0.89 V and a limiting current density of 5.75 mA cm^-2. The findings will provide insights into the controlled synthesis of transition-metal-heteroatom-codoped carbon nanomaterials for the development of advanced ORR electrocatalysts.

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.

Metal-Nitrogen-Carbon Catalysts of Specifically Coordinated Configurations toward Typical Electrochemical Redox Reactions

Metal-nitrogen-carbon (M-N-C) material with specifically coordinated configurations is a promising alternative to costly Pt-based catalysts. In the past few years, great progress is made in the studies of M-N-C materials, including the structure modulation and local coordination environment identification via advanced synthetic strategies and characterization techniques, which boost the electrocatalytic performances and deepen the understanding of the underlying fundamentals. In this review, the most recent advances of M-N-C catalysts with specifically coordinated configurations of M-N_x (x = 1-6) are summarized as comprehensively as possible, with an emphasis on the synthetic strategy, characterization techniques, and applications in typical electrocatalytic reactions of the oxygen reduction reaction, oxygen evolution reaction, hydrogen evolution reaction, CO₂ reduction reaction, etc., along with mechanistic exploration by experiments and theoretical calculations. Furthermore, the challenges and potential perspectives for the future development of M-N-C catalysts are discussed.

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.

A pyridinic Fe-N4 macrocycle models the active sites in Fe/N-doped carbon electrocatalysts

Iron- and nitrogen-doped carbon (Fe-N-C) materials are leading candidates to replace platinum catalysts for the oxygen reduction reaction (ORR) in fuel cells; however, their active site structures remain poorly understood. A leading postulate is that the iron-containing active sites exist primarily in a pyridinic Fe-N₄ ligation environment, yet, molecular model catalysts generally feature pyrrolic coordination. Herein, we report a molecular pyridinic hexaazacyclophane macrocycle, (phen₂N₂)Fe, and compare its spectroscopic, electrochemical, and catalytic properties for ORR to a typical Fe-N-C material and prototypical pyrrolic iron macrocycles. N 1s XPS and XAS signatures for (phen₂N₂)Fe are remarkably similar to those of Fe-N-C. Electrochemical studies reveal that (phen₂N₂)Fe has a relatively high Fe(III/II) potential with a correlated ORR onset potential within 150 mV of Fe-N-C. Unlike the pyrrolic macrocycles, (phen₂N₂)Fe displays excellent selectivity for four-electron ORR, comparable to Fe-N-C materials. The aggregate spectroscopic and electrochemical data demonstrate that (phen₂N₂)Fe is a more effective model of Fe-N-C active sites relative to the pyrrolic iron macrocycles, thereby establishing a new molecular platform that can aid understanding of this important class of catalytic materials.

Surface site density and utilization of platinum group metal (PGM)-free Fe-NC and FeNi-NC electrocatalysts for the oxygen reduction reaction

Pyrolyzed iron-based platinum group metal (PGM)-free nitrogen-doped single site carbon catalysts (Fe-NC) are possible alternatives to platinum-based carbon catalysts for the oxygen reduction reaction (ORR). Bimetallic PGM-free M1M2-NC catalysts and their active sites, however, have been poorly studied to date. The present study explores the active accessible sites of mono- and bimetallic Fe-NC and FeNi-NC catalysts. Combining CO cryo chemisorption, X-ray absorption and 57Fe Mössbauer spectroscopy, we evaluate the number and chemical state of metal sites at the surface of the catalysts along with an estimate of their dispersion and utilization. Fe L3,2-edge X-ray adsorption spectra, Mössbauer spectra and CO desorption all suggested an essentially identical nature of Fe sites in both monometallic Fe-NC and bimetallic FeNi-NC; however, Ni blocks the formation of active sites during the pyrolysis and thus causes a sharp reduction in the accessible metal site density, while with only a minor direct participation as a catalytic site in the final catalyst. We also use the site density utilization factor, ϕ SDsurface/bulk , as a measure of the metal site dispersion in PGM-free ORR catalysts. ϕ SDsurface/bulk enables a quantitative evaluation and comparison of distinct catalyst synthesis routes in terms of their ratio of accessible metal sites. It gives guidance for further optimization of the accessible site density of M-NC catalysts.

Crystallinity dependence for high-selectivity electrochemical oxygen reduction to hydrogen peroxide

We report a semiconducting metal-organic graphene analog, Ni₃(HITP)₂, with an adjustable crystalline structure for mediating efficient selectivity toward hydrogen peroxide. The as-prepared 5-Ni₃(HITP)₂ catalyst with low crystallinity can electrocatalyze O₂ to H₂O₂ with a high selectivity of 80% over a wide potential range of 0.2-0.6 V vs. RHE and with a large mass activity of 292 A g_Ni^-1 at 0.25 V in 0.1 M KOH.

2 D Hybrid of Ni-LDH Chips on Carbon Nanosheets as Cathode of Zinc-Air Battery for Electrocatalytic Conversion of O2 into H2 O2

It remains great challenge to develop precious-metal-free electrocatalysts to implement high-activity electrochemical conversion of O₂ into value-added hydroperoxide species (HO₂ ^- ), which are vulnerable when exposed to various transition-metal-based catalysts. A strategy based on steric hindrance and layered nickel-based layered double hydroxide (Ni-LDH) induction has been developed for one-pot inlaying high-density ultrathin 2 D Ni-LDH chips on in situ-grown carbon nanosheets (Ni-LDH C/CNSs). The resulting material exhibits high electrocatalytic selectivity with a faradaic efficiency up to 95 % for oxygen reduction into peroxide and attains a fairly high mass activity of approximately 22.2 A g^-1 , outperforming most metal-based catalysts reported previously. Systematic studies demonstrate that the greatly increased defect concentration at Ni edge sites of Ni-LDH chips results in more active sites, which contributes a favorable thermodynamically neutral adsorption of OOH* and adsorbed H₂ O₂ molecules relatively weakly. Additionally, the modified CNSs effectively suppress H₂ O₂ decomposition and avoid O-O bond cleavage to produce H₂ O by steric effects. The synergistic effect of CNSs and Ni-LDH chips therefore leads to high activity and high selectivity in a two-electron pathway. A proof-of-concept zinc-air fuel cell is proposed and set up to demonstrate the feasibility of green synthesis of peroxide, generating an impressive H₂ O₂ production rate of 5239.67 mmol h^-1  g_cat. ^-1 .

Pyrolysis-driven synthesis of nanoscale carambola-like carbon decorated with atomically dispersed Fe sites toward efficient oxygen reduction reaction

We report the pyrolysis-driven structural evolution of nanocapsule-shaped Fe^III porphyrin MOFs into nanocarambolas decorated with atomically dispersed Fe suitable for the ORR.

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.

Insight into the Rapid Degradation Behavior of Nonprecious Metal Fe-N-C Electrocatalyst-Based Proton Exchange Membrane Fuel Cells

In the past few years, great progress has been made in nonprecious metal catalysts, which hold the potential as alternative materials to replace platinum in proton exchange membrane fuel cells. One type of nonprecious metal catalyst, Fe-N-C, has displayed similar catalytic activity as platinum in rotating disk electrode tests; however, rapid degradation of Fe-N-C catalyst-based fuel cells is always observed, which limits its practical application. Although considerable research has been devoted to study the degradation of the catalyst itself, rather less attention has been paid to the membrane electrode assembly that makes the mechanism of fuel cell degradation remain unclear. In this work, a high-performance Fe-N-C catalyst-based membrane electrolyte assembly is prepared and used to study its degradation mechanism. The fuel cell performs with an initial peak power density as high as 1.1 W cm^-2 but suffers a current loss of 52% at 0.4 V over 20 h only. The experimental and DFT calculation results indicate that Fe at active sites of catalysts is attacked by hydroxyl free radicals decomposed from H₂O₂, which is further leached out, causing an increase in activity loss. The ionomer of the catalyst layer and the membrane is further contaminated by the leached Fe ions, which results in an enlarged membrane resistance and cathode catalyst layer proton conduction resistance, greatly influencing the cell performance. In addition, it has been assumed in previous studies that the quick performance loss of Fe-N-C-based fuel cells is caused by water flooding within the catalyst layer, which is proved to be incorrect in our study through a dry-out experiment.

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.

57 Fe Mössbauer Spectroscopy Characterization of Electrocatalysts

This work addresses the importance of Mössbauer spectroscopy for the characterization of iron-containing electrocatalysts. The most important aspects of electrocatalysis and Mössbauer spectroscopy are summarized. Next, Fe-N-C catalysts and important conclusions made by this technique on preparation, active site identification and degradation are summarized. Furthermore, recent highlights derived for other iron-containing electrocatalysts are summarized.

Thermal Stability and Potential Cycling Durability of Nitrogen-Doped Graphene Modified by Metal-Organic Framework for Oxygen Reduction Reactions

Here we report a nitrogen-doped graphene modified metal-organic framework (N-G/MOF) catalyst, a promising metal-free electrocatalyst exhibiting the potential to replace the noble metal catalyst from the electrochemical systems; such as fuel cells and metal-air batteries. The catalyst was synthesized with a planetary ball milling method, in which the precursors nitrogen-functionalized graphene (N-G) and ZIF-8 are ground at an optimized grinding speed and time. The N-G/MOF catalyst not only inherited large surface area from the ZIF-8 structure, but also had chemical interactions, resulting in an improved Oxygen Reduction Reaction (ORR) electrocatalyst. Thermogravimetric Analysis (TGA) curves revealed that the N-G/MOF catalyst still had some unreacted ZIF-8 particles, and the high catalytic activity of N-G particles decreased the decomposition temperature of ZIF-8 in the N-G/MOF catalyst. Also, we present the durability study of the N-G/MOF catalyst under a saturated nitrogen and oxygen environment in alkaline medium. Remarkably, the catalyst showed no change in the performance after 2000 cycles in the N2 environment, exhibiting strong resistance to the corrosion. In the O2 saturated electrolyte, the performance loss at lower overpotentials was as low compared to higher overpotentials. It is expected that the catalyst degradation mechanism during the potential cycling is due to the oxidative attack of the ORR intermediates.