Recommended Practices and Benchmark Activity for Hydrogen and Oxygen Electrocatalysis in Water Splitting and Fuel Cells

Modulation of the structure and morphology of NiCo2S4 by varying the anion types of nickel and cobalt salts to achieve high-rate supercapacitive performance

In this work, the structure and morphology of NiCo2S4 (NCS) were modulated by varying the anion types (Cl-, CH3COO-, and NO3-) of nickel and cobalt salts. Extensive material characterization and analyses revealed that the grain size of the obtained NCSs was determined by different solvation free energies, capping effects, and steric hindrance during the crystal growth process. Among these three anions, Cl-, with the smallest ionic size, exhibited the lowest capping effect, steric hindrance, and solvation free energy, leading to the largest average grain size of 15.34 nm for Cl--based NiCo2S4 (NCS-C). Moreover, the sea urchin-like morphology of NCS-C provided a high reaction interface for electrochemical energy storage. As a result, the specific capacitance of NCS-C could reach 1112.4 F/g at a current density of 6 A/g, retaining 692 F/g even at a high current density of 16 A/g. The assembled asymmetric supercapacitor could also deliver a high energy density of 23.4 Wh kg-1. This work highlights the significant influence of anion type on the structural and morphological evolution of NCS materials, providing new insights for the development of high-rate NCS-based electrode materials.

High-efficiency oxygen evolution catalysts: composite hexagonal structure SrCo1-yNiyO3-δ/Sr9Ni7O21

Transition metal perovskite oxides are regarded as ideal catalysts for the oxygen evolution reaction (OER) owing to the high tunability of the B-site active elements. Among them, SrCoO3 has been widely studied due to its high theoretical catalytic performance, but incomplete oxidation of the B-site elements results in poor intrinsic activity. Herein, we prepared a series of SrCo1-xNixO3-δ (x = 0-0.5) catalysts for the alkaline OER using a simple sol-gel method and optimized their catalytic performance by modulating the Co/Ni ratio and annealing temperature (950 °C and 1050 °C). Among the series, SrCo0.6Ni0.4O3-δ (annealed at 950 °C) with a composite hexagonal structure (SrCo1-yNiyO3-δ/Sr9Ni7O21, y = 0.1-0.2) exhibited the best OER activity, achieving an overpotential of 321 mV at 10 mA cm-2 (1 M KOH). The introduction of Ni ions and the presence of Sr9Ni7O21 not only enhance the Co4+/Co3+ and Ni3+/Ni2+ ratios but also promote the generation of more highly oxidative oxygen species (O22-/O-). Additionally, the abundant Ni3+ surface facilitates the formation of a highly active NiOOH phase during the OER, leading to a further reduction in the overpotential (after 1000 CV cycles, the overpotential on the nickel foam electrode decreased by 45 mV).

Delineating the multifunctional performance of Janus WSSe with nonmetals in water splitting and hydrogen fuel cell applications via first-principles calculations

The development of cost-effective and highly efficient multifunctional catalysts for water splitting and hydrogen fuel cells is crucial for advancing renewable energy technologies. This study employs density functional theory to investigate the electrocatalytic performance of Janus-type WSSe (JW) transition metal dichalcogenides in the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR). Additionally, the impact of nonmetal doping (NM = C, O, N, P) at the S and Se sites in the JW structure is explored. A cohesive energy of -5.82 eV per atom and minimal fluctuations in AIMD simulations over 10 ps at 300 K and 500 K confirm its structural stability. Although the pristine structure exhibits a high overpotential, NM doping substantially improves catalytic performance, making it more suitable for efficient energy conversion applications. The N doped JW system demonstrates exceptional multifunctional performance, with NS@JW showing overpotentials of 0.34 V (HER), 0.18 V (OER), and 0.14 V (ORR), while NSe@JW exhibits overpotentials of 0.35 V (HER), 0.46 V (OER), and 0.24 V (ORR). This outstanding performance results from bonding-antibonding interactions in intermediate adsorption, as confirmed by crystal orbital Hamiltonian population analysis. This comprehensive study highlights the potential of Janus-type WSSe and emphasizes the crucial role of NM doping in boosting catalytic efficiency, offering key insights for designing cost-effective, high-performance multifunctional electrocatalysts for energy conversion.

Enhancing Rechargeable Zinc-Air Batteries with Atomically Dispersed Zinc Iron Cobalt Planar Sites on Porous Nitrogen-Doped Carbon

Rechargeable zinc-air batteries (ZABs) face significant challenges in achieving both high power density and long-term stability, primarily due to limitations in catalytic materials for oxygen electrodes. Here, we present a trimetal planar heterogeneous metal catalyst featuring atomically dispersed ZnN4, FeN4, and CoN4 sites supported on a porous nitrogen-doped carbon substrate (ZnFeCo-NC) through a templating approach. By fine-tuning the content of each metal, the optimized ZnFeCo-NC-based ZAB achieves a high peak power density of 244 mW cm-2 and maintains durable performance for 500 h at 10 mA cm-2. Ab initio molecular dynamics simulations reveal that the ZnFeCo-NC catalyst configuration remains stable at 300 K during the oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) process. Further theoretical calculations demonstrate that the introduction of adsorbed OH groups effectively tunes the electronic structure redistribution of metal active sites, particularly improving the catalytic performance at the Fe site for ORR and the Co site for the OER. These findings provide insights into the rational design of high-performance electrocatalysts in energy storage technologies.

The role of carbon catalyst coatings in the electrochemical water splitting reaction

Designing inexpensive, sustainable, and high-performance oxygen-evolution reaction (OER) electrocatalysts is one of the largest obstacles hindering the development of new electrolyzers. Carbon-coated metal/metal oxide (nano)particles have been used in such applications, but the role played by the carbon coatings is poorly understood. Here, we use a carbon-coated catalyst comprising metal-oxide nanoparticles encapsulated within single-walled carbon nanotubes (SWNTs), to study the effects of carbon coatings on catalytic performance. Electrolyte access to the encapsulated metal oxides is shut off by plugging the SWNT ends with size-matched fullerenes. Our results reveal that the catalytic activity of the composite rivals that of the metal oxide, despite the fact that the metal oxides cannot access the bulk electrolyte. Moreover, the rate-determining step (RDS) of the OER matches that measured at empty SWNTs, indicating that electrocatalysis occurs on the carbon surface. Synergism between the encapsulated metal oxide and carbon coating was explored using electrochemical Raman spectroscopy and computational analysis, revealing that charge transfer from the carbon host to the metal oxide is key to the high electrocatalytic activity of carbon in this system; decreasing electron density on the carbon surface facilitates binding of -OH, accelerating the rate of the OER on the carbon surface.

Rational Design of Rare Earth-Based Nanomaterials for Electrocatalytic Reactions

Rare earth-based nanomaterials hold great promise for applications in the electrocatalysis field owing to their unique 4f electronic structure, adjustable coordination modes, and high oxophilicity. As a cocatalyst, the location of rare earth elements can alter the intrinsic properties of support, including coordination environments, electronic structure, and structure evolution under applied potentials in a variable manner, to potentially impact catalytic performance with respect to their activity, stability, and selectivity. Therefore, a comprehensive understanding of the effects of rare earth elements' location on local structure and reaction mechanisms is a prerequisite for designing advanced rare earth-based nanomaterials. In this review, the rare earth-based nanomaterials have been categorized into three main groups based upon the location of rare earth elements in the support, namely lattice, surface, and interface structure. We initially discuss recent advances and representing breakthroughs to realize controllable synthesis of rare earth-based nanomaterials. Next, we discuss the state-of-the-art rare earth-based nanomaterials and the structure modulation strategy employed to enhance their catalytic performance. Combined with advanced characterizations, the role of rare earth elements in reaction mechanisms and structure evolution process is also discussed. Finally, we further highlight the future research directions and remaining challenges for the development of rare earth-based nanomaterials in practical applications.

Harnessing Coordination Microenvironment of Metal-bis(dithiolene) Sites for Modulating Electrocatalytic CO2 Reduction by Metal-Organic Framework

Nature's metalloenzymes inspire biomimetic catalysts for the CO2 reduction reaction (CO2RR), particularly using metal-bis(dithiolene) ([MS4]) cores in frameworks. While prior research focused on tuning the chelating atoms of Ni-centered sites or [NiS4] in metal-organic frameworks (MOFs), how different metal centers affect the electronic structure and catalytic activity is often overlooked. Notably, reported [NiS4] molecular analogues exhibits a Faradaic efficiency (FE) of less than 70% for the major carbon product and shows operational stability for only about 4 hours (say falling FE and current density beyond). In this study, MOFs are used to host [MS4] units with varying central metals (M = Ni, Cu, Co, Fe) to assess how the metal center affects electrocatalytic CO2RR. Among the studied MS4-In MOFs, NiS4-In demonstrates the best performance, achieving a FECO of 88.54% and operational stability greater than 6 hours-significantly outlasting the ≈200 seconds of the [NiS4] molecule. This work underscores the importance of frameworks in stabilizing [MS4] units and highlights [MS4] as essential for CO2 binding and reduction, with [NiS4] exhibiting optimal catalytic performance due to its favorable electronic properties. This findings clarify how substituting the metal center within the framework enhances electronic structure and coordination, leading to improved electrocatalytic performance.

Perchlorate Fusion-Hydrothermal Synthesis of Nano-Crystalline IrO2: Leveraging Stability and Oxygen Evolution Activity

Iridium oxides are the state-of-the-art oxygen evolution reaction (OER) electrocatalysts in proton-exchange-membrane water electrolyzers (PEMWEs), but their high cost and scarcity necessitate improved utilization. Crystalline rutile-type iridium dioxide (IrO2) offers superior stability under acidic OER conditions compared to amorphous iridium oxide (IrOx). However, the higher synthesis temperatures required for crystalline phase formation result in lower OER activity due to the loss in active surface area. Herein, a novel perchlorate fusion-hydrothermal (PFHT) synthesis method to produce nano-crystalline rutile-type IrO2 with enhanced OER performance is presented. This low-temperature approach involves calcination at a mild temperature (300 °C) in the presence of a strong oxidizing agent, sodium perchlorate (NaClO4), followed by hydrothermal treatment at 180 °C, yielding small (≈2 nm) rutile-type IrO2 nanoparticles with high mass-specific OER activity, achieving 95 A gIr -1 at 1.525 VRHE in ex situ glass-cell testing. Most importantly, the catalyst displays superior stability under harsh accelerated stress test conditions compared to commercial iridium oxides. The exceptional activity of the catalyst is confirmed with in situ PEMWE single-cell evaluations. This demonstrates that the PFHT synthesis method leverages the superior intrinsic properties of nano-crystalline IrO2, effectively overcoming the typical trade-offs between OER activity and catalyst stability.

Atomic Cobalt-Doped Palladium Metallene toward Efficient Oxygen Reduction Electrocatalysis

Designing a high-efficiency catalyst for the cathode oxygen reduction reaction (ORR) in fuel cells still faces enormous challenges due to the stringent requirements for high power density and long-term durability. Palladium (Pd) metallene, on account of its unique properties and high Pd utilization efficiency, is recognized as a prospective candidate for enhancing the ORR catalytic performance. Herein, we present atomic cobalt (Co)-doped Pd metallene (Co-Pdene), featuring an ultrathin and highly curved morphology, developed via a straightforward wet-chemical approach for efficient ORR electrocatalysis in alkaline media. Resulting from the metallene structure and transition metal Co doping, the Co-Pdene catalyst demonstrates exceptional electrocatalytic performance, achieving an electrochemical mass activity (MA) of 3.14 A per milligram palladium at 0.85 V while maintaining structural integrity over 30000 potential cycles. Theory simulations (DFT) manifest that the single-atom Co sites optimize the electronic structure of palladium in the Co-Pdene, thereby lowering the theoretical overpotential to 0.29 V. This work proposes an innovative design strategy of single-atom transition metal-doped Pd metallene as a highly efficient ORR electrocatalyst.

K D, Sankar R, Cyue ZW, Pong WF, Chen CW. In Situ Identification of Spin Magnetic Effect on Oxygen Evolution Reaction Unveiled by X-ray Emission Spectroscopy

Manipulating the spin ordering of the oxygen evolution reaction (OER) catalysts through magnetization has recently emerged as a promising strategy to enhance performance. Despite numerous experiments elaborating on the spin magnetic effect for improved OER, the origin of this phenomenon remains largely unexplored, primarily due to the difficulty in directly distinguishing the spin states of electrocatalysts during chemical reactions at the atomic level. X-ray emission spectroscopy (XES), which provides information sensitive to the spin states of specific elements in a complex, may serve as a promising technique to differentiate the onset of OER catalytic activities from the influence of spin states. In this work, we employ the in situ XES technique, along with X-ray absorption spectroscopy (XAS), to investigate the interplay between atomic/electronic structures, spin states, and OER catalytic activities of the CoFe2O4 (CFO) catalyst under an external magnetic field. This enhancement is due to the spin magnetic effect that facilitates spin-selective electron transfer from adsorbed OH- reactants, which strongly depends on the spin configurations of the tetrahedral-(Td) and octahedral-(Oh) sites of both Fe and Co ions. Our result contributes to a comprehensive understanding of magnetic field-assisted electrocatalysis at the atomic level and paves the way for designing highly efficient OER catalysts.

A Programmable Nanoparticle Conversion Pathway to Monodisperse Polyelemental High Entropy Alloy, Intermetallic, and Multiphase Nanoparticles

Polyelemental nanoparticles (PE NPs), those consisting of four or more elements, exhibit unique properties from synergistic compositional effects. Examples include high entropy alloys, high entropy intermetallics, and multiphase types, including Janus and core-shell architectures. Although colloidal syntheses offer excellent structural control for mono- and bi-elemental compositions, achieving the same control for PE NPs remains challenging. Here, this challenge is addressed with a NP conversion strategy wherein different types of PE NPs - including high entropy alloy, high entropy intermetallic, and multiphase Janus nanoparticles - are achieved through thermal transformation of readily synthesized colloidal core-shell NPs. Through systematic variations in stoichiometry and metal identity to the core-shell precursor NPs, along with atomistic simulations that probe phase stabilities, we deduce that the final mixing states of the various NPs are governed by the balance between the enthalpy and entropy of mixing. Moreover, our annealing method allows us to trap NPs at intermediate states of mixing, creating distinct surface ensembles that were evaluated as catalysts for the hydrogen evolution reaction. This study is the first, to our knowledge, to report colloidally derived precursor NPs enabling the synthesis of all types of PE NPs in a single process. This NP conversion strategy offers a general route to diverse PE NPs.

Oxygen Vacancies Can Drive Surface Transformation of High-Entropy Perovskite Oxide for the Oxygen Evolution Reaction as Probed with Scanning Probe Microscopy

Epitaxial transition-metal oxide perovskite catalysts form a highly active catalyst class for the oxygen evolution reaction (OER). Understanding the origin of chemical dissolution and surface transformations during the OER is important to rationally design effective catalyst. These changes arise from complex interactions involving dynamic restructuring and electronic/structural adaptations. Although initial instability is common, surfaces can reach equilibrium through chemical transformations. High entropy perovskite oxides (HEPOs), which incorporate multiple 3d metal cations in near-equimolar ratios, have emerged as promising catalysts due to their enhanced OER activity compared to single-cation variants, attributed to their high configurational entropy and compositional flexibility. To advance HEPO catalyst applications, understanding the mechanisms governing their surface (in)stability is important. In this work, we examine surface degradation in epitaxial La(Cr,Mn,Fe,Co,Ni)O3-δ thin films before and after OER using complementary scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). STM reveals tip-induced degradation of as-grown films under positive bias, attributed to oxygen anion removal and charge trapping-induced lattice degradation, demonstrating its utility as a probe for surface stability dynamics. Post-OER XPS analysis shows irreversible surface transformations from the initial epitaxial phase, characterized by 3d-metal leaching and formation of La and d-metal (oxy)hydroxides. Our findings indicate that oxygen vacancies and lattice strain trigger structural breakdown in these multi-cation perovskite surfaces during the OER, leading to surface restructuring and diminished catalytic performance compared to the as-grown epitaxial HEPO phase. This work identifies oxygen leaching as the likely primary driver of surface transformation during the OER. We show that STM offers an important tool to probe the transformation even before operando conditions, which can find use in similar material studies.

Modern Catalytic Materials for the Oxygen Evolution Reaction

The oxygen evolution reaction (OER) has, in recent years, attracted great interest from scientists because of its prime role in a number of renewable energy technologies. It is one of the reactions that occurs during hydrogen production through water splitting, is used in rechargeable metal-air batteries, and plays a fundamental role in regenerative fuel cells. Therefore, there is an emerging need to develop new, active, stable, and cost-effective materials for OER. This review presents the latest research on various groups of materials, showing their potential to be used as OER electrocatalysts, as well as their shortcomings. Particular attention has been paid to metal-organic frameworks (MOFs) and their derivatives, as those materials offer coordinatively unsaturated sites, high density of transition metals, adjustable pore size, developed surface area, and the possibility to be modified and combined with other materials.

RF Sputtering of Gold Nanoparticles in Liquid and Direct Transfer to Nafion Membrane for PEM Water Electrolysis

Sputtering onto liquids is rapidly gaining attention for the green and controlled dry synthesis of ultrapure catalysts nanomaterials. In this study, we present a clean and single-step method for the synthesis of gold nanoparticles directly in polyethylene glycol (PEG) liquid using radio frequency (RF) magnetron sputtering and by subsequently transferring them to Nafion ionomer, fabricating a catalyst-coated membrane (CCM), an essential component of the proton exchange membrane water electrolyzer (PEMWE). The samples were systematically characterized at different stages of process development. The innovative transfer process resulted in a monodispersed homogeneous distribution of catalyst particles inside CCM while retaining their nascent nanoscale topography. The chemical analysis confirmed the complete removal of the trapped PEG through the process optimization. The electrochemical catalytic activity of the optimized CCM was verified, and the hydrogen evolution reaction (HER) in acidic media appeared outstanding, a vital step in water electrolysis toward H2 production. Therefore, this first study highlights the advantages of RF sputtering in liquid for nanoparticle synthesis and its direct application in preparing CCM, paving the way for the development of innovative membrane preparation techniques for water electrolysis.

Modulating Spatial Distributions of Single Atoms on Supports for Enhanced Oxygen Evolution

Single-atom catalysts (SACs) hold great promise in oxygen evolution reactions due to their ultrahigh atomic utilization rates and uniform active sites. The performance of SACs is closely related to the spatial distributions of single atoms on the supports. However, modulating the spatial distributions of single atoms on the supports is extremely challenging. Herein, we precisely anchored Ir single atoms onto the face sites (Ir1/F-CoOOH) and the edge sites (Ir1/E-CoOOH) of CoOOH. Ir single atoms with distinct spatial distributions on CoOOH exhibited different electronic structures but nearly identical coordination environments. Nevertheless, Ir1/E-CoOOH required an overpotential of only 220 mV to reach a current density of 10 mA cm-2, which was 80 mV lower than that of Ir1/F-CoOOH. Mechanistic studies demonstrated that Ir single atoms with distinct spatial distributions activated the supports through different mechanisms.

Engineering MXene Surface via Oxygen Functionalization and Au Nanoparticle Deposition for Enhanced Electrocatalytic Hydrogen Evolution Reaction

MXene, a family of 2D transition metal carbides and nitrides, presents promising applications in electrocatalysis. Maximizing its large surface area is key to developing efficient non-noble-metal catalysts for the hydrogen evolution reaction (HER). In this study, oxygen-functionalized Ti3C2Tx MXene (Ti3C2Ox) is synthesized and deposited gold nanoparticles (Au NPs) onto it, forming a novel composite material, Au-Ti3C2Ox. By selectively removing other functional groups, mainly -O functional groups are retained on the surface, directing electron transfer from Au NPs to MXene due to electronic metal-support interaction (EMSI), thereby improving the catalytic activity of the MXene surface. Additionally, the interaction between Au NPs and -O functional groups further enhanced the overall catalytic activity, achieving an overpotential of 62 mV and a Tafel slope of 40.1 mV dec-1 at a current density of -10 mA cm-2 in 0.5 m H2SO4 solution. Density functional theory calculations and scanning electrochemical microscopy with ≤150 nm resolution confirmed the enhanced catalytic efficiency due to the specific interaction between Au NPs and Ti3C2Ox. This work provides a surface modification strategy to fully utilize the MXene surface and enhance the overall catalytic activity of MXene-based catalysts.

Updating the sub-nanometric cognition of reconstructed oxyhydroxide active phase for water oxidation

Unveiling structure-activity correlations at the sub-nanoscale remains an essential challenge in catalysis science. During electrocatalysis, dynamic structural evolution drives the ambiguous entanglement of crystals and electrons degrees of freedom that obscure the activity origin. Here, we track the structural evolution of Ni-based model pre-catalysts (Ni(OH)2, NiS2, NiSe2, NiTe), detailing their catalytically active state during water oxidation via operando techniques and theoretical calculations. We reveal the sub-nanometric structural difference of NiO6 unit with a regular distortion in the reconstructed active phase NiOOH, codetermined by the geometric (bond lengths) and electronic (covalency) structure of the pre-catalysts on both spatial and temporal scales. The symmetry-broken active units induce the delicate balance of the p and d orbitals in NiOOH, further steering the modulation of catalytic intermediate configurations and mechanisms, with improved performance. This work recognizes the fine structural differences of the active phases from the sub-nanometer scale, and quantitatively explains their influence on activity. Our findings provide a more intuitive design framework for high-efficiency materials through targeted symmetry engineering of active units.

Operando evidence on the chirality-enhanced oxygen evolution reaction in intrinsically chiral electrocatalysts

Electrolytic hydrogen is identified as a crucial component in the desired decarbonisation of the chemical industry, utilizing renewable energy to split water into hydrogen and oxygen. Water electrolysis still requires important scientific advances to improve its performance and lower its costs. One of the bottlenecks in this direction is related to the sluggish anodic oxygen evolution reaction (OER). Producing anodes with competitive performance remains challenging due to the high energy losses and the harsh working conditions typically required by this complex oxidation process. Recent advancements point to spin polarization as an opportunity to enhance the kinetics of this spin-restricted reaction, yielding the paramagnetic O2 molecule. One powerful strategy deals with the generation of chiral catalytic surfaces, typically by surface functionalisation with chiral organic molecules, to promote the chiral-induced spin selectivity (CISS) effect during electron transfer. However, the relationship between optical activity and enhanced electrocatalysis has been established only from indirect experimental evidence. In this work, we have exploited operando electrochemical and spectroscopic tools to confirm the direct relationship between the faster OER kinetics and the optical activity of enantiopure Fe-Ni metal oxides when compared with that of achiral catalysts in alkaline conditions. Our results show the participation of chiral species as reactive intermediates during the electrocatalytic reaction, supporting the appearance of a mechanistic CISS enhancement. Furthermore, these intrinsically chiral transition-metal oxides maintain their enhanced activity in full cell electrolyser architectures at industrially relevant current densities.

Iron and Nickel Substituted Perovskite Cobaltites for Sustainable Oxygen Evolving Anodes in Alkaline Environment

Perovskite oxides have great flexibility in their elemental composition, which is accompanied by large adjustability in their electronic properties. Herein, we synthesized twelve perovskite oxide-based catalysts for the oxygen evolution reaction (OER) in alkaline media. The catalysts are based on the parent oxide perovskite Ba0.5Gd0.8La0.7Co2O6-δ (BGLC587) and are synthesized through the sol-gel citrate synthesis route. To reduce the demand on cobalt (Co), but also increase the intrinsic catalytic activity of BGLC587 for the OER, we substitute Co on the B-site with certain amounts of Fe and Ni, synthesizing catalysts of the general formula Ba0.5Gd0.8La0.7Co2-x-yFexNiyO6-δ. A plethora of physicochemical and electrochemical methods suggest that an Fe content between 30 % and 70 % increases the intrinsic catalytic activity of BGLC587, while Tafel slopes in combination with in-situ Raman spectroscopy suggest the rate determining step is likely a proton-exchange reaction, progressing possibly through the lattice oxygen mechanism (LOM). We apply one of the optimized, Co-substituted perovskites in a monolithic, photovoltaic (PV)-driven electrolysis cell and we achieve an initial solar-to-hydrogen (STH) conversion efficiency of 10.5 % under one sun solar simulated illumination.

Ferromagnetic transformation of α-Fe2O3via Co doping for efficient water oxidation under magnetic field

The introduction of a magnetic field has great potential for enhancing oxygen evolution reaction (OER) performance, but it requires the catalyst to be ferromagnetic and there is a lack of understanding of the fundamental mechanisms. Herein, we achieved the transformation of non-ferromagnetic α-Fe2O3 into ferromagnetic Co0.14FeOx by Co doping. Compared to α-Fe2O3, the OER activity of Co0.14FeOx considerably increased under a magnetic field of 3000 Oe and is proportional to the magnetic field strength within a certain range. This can be ascribed to the external magnetic field enhancing the degree of the magnetic moment ordering of Co0.14FeOx, which improved electron spin polarization.

A Review of Surface Reconstruction and Transformation of 3d Transition-Metal (oxy)Hydroxides and Spinel-Type Oxides during the Oxygen Evolution Reaction

Developing efficient and sustainable electrocatalysts for the oxygen evolution reaction (OER) is crucial for advancing energy conversion and storage technologies. 3d transition-metal (oxy)hydroxides and spinel-type oxides have emerged as promising candidates due to their structural flexibility, oxygen redox activity, and abundance in earth's crust. However, their OER performance can be changed dynamically during the reaction due to surface reconstruction and transformation. Essentially, multiple elementary processes occur simultaneously, whereby the electrocatalyst surfaces undergo substantial changes during OER. A better understanding of these elementary processes and how they affect the electrocatalytic performance is essential for the OER electrocatalyst design. This review aims to critically assess these processes, including oxidation, surface amorphization, transformation, cation dissolution, redeposition, and facet and electrolyte effects on the OER performance. The review begins with an overview of the electrocatalysts' structure, redox couples, and common issues associated with electrochemical measurements of 3d transition-metal (oxy)hydroxides and spinels, followed by recent advancements in understanding the elementary processes involved in OER. The challenges and new perspectives are presented at last, potentially shedding light on advancing the rational design of next-generation OER electrocatalysts for sustainable energy conversion and storage applications.

Achieving Higher Activity of Acidic Oxygen Evolution Reaction Using an Atomically Thin Layer of IrOx over Co3O4

The development of electrocatalysts with reduced iridium (Ir) loading for the oxygen evolution reaction (OER) is essential to produce low-cost green hydrogen from water electrolysis under acidic conditions. Herein, an atomically thin layer of iridium oxide (IrOx) has been uniformly dispersed onto cobalt oxide (Co3O4) nanocrystals to improve the efficient use of Ir for acidic OER. In situ characterization and theoretical calculations reveal that compared to the conventional IrOx cluster, the atomically thin layer of IrOx shows stronger interaction with the Co3O4 and consequently higher OER activity due to the Ir-O-Co bond formation at the interface. Equally important, the facile synthetic method and the promising activity in the proton exchange membrane water electrolyzer, reaching 1 A cm-2 at 1.7 V with remarkable durability, enable potential scale-up applications. These findings provide a mechanistic understanding for designing active, stable and lower-cost electrocatalysts with well-defined structures for acidic OER.

Facilitating alkaline hydrogen evolution kinetics via interfacial modulation of hydrogen-bond networks by porous amine cages

The electrode-electrolyte interface is pivotal in the electrochemical kinetics. However, modulating the electrochemical interface at the atomic or molecular level is challenging due to the lack of efficient interfacial regulators. Here, we employ a porous amine cage as an interfacial modifier to Pt cluster in a confining configuration, largely enhancing alkaline HER kinetics by facilitating charge transfer. In situ electrochemical surface-enhanced Raman spectra, in combination with the ab initio molecular dynamics simulation, elucidates that the interaction between water and the -NH- moiety of cage frame softens the H-bonds net of interfacial water, making it more flexible for charge transfer. Moreover, our investigation pinpointed that the -NH- moiety acted as a pump for charge transfer by Grotthuss mechanism, lowering the kinetic barrier for hydrogen adsorption. Our findings highlight the strategy of establishing a soft-confining interfacial modifier by porous cage, offering opportunities to optimize electrochemical interfaces and promote reaction kinetics in a targeted way.

Leveraging Iron in the Electrolyte to Improve Oxygen Evolution Reaction Performance: Fundamentals, Strategies, and Perspectives

Electrochemical water splitting is a pivotal technology for storing intermittent electricity from renewable sources into hydrogen fuel. However, its overall energy efficiency is impeded by the sluggish oxygen evolution reaction (OER) at the anode. In the quest to design high-performance anode catalysts for driving the OER under non-acidic conditions, iron (Fe) has emerged as a crucial element. Although the profound impact of adventitious electrolyte Fen+ species on OER catalysis had been reported forty years ago, recent interest in tailoring the electrode-electrolyte interface has spurred studies on the controlled introduction of Fe ions into the electrolyte to improve OER performance. During the catalytic process, scenarios where the rate of Fen+ deposition on a specific host material outruns that of dissolution pave the way for establishing highly efficient and dynamically stable electrochemical interfaces for long-term steady operation. This review systematically summarizes recent endeavors devoted to elucidating the behaviors of in situ Fe(aq.) incorporation, the role of incorporated Fe sites in the OER, and critical factors influencing the interplay between the electrode surface and Fe ions in the electrolyte environment. Finally, unexplored issues related to comprehensively understanding and leveraging the dynamic exchange of Fen+ at the interface for improved OER catalysis are summarized.

Synthesis, structure, oxygen evolution reaction (OER) and visible-light assisted organic reaction studies on A2M2TeB2O10 (A = Ba and Pb; M = Mg, Zn, Co, Ni, Cu, and Fe)

Compounds with the general formula A2M2TeB2O10 (A = Ba and Pb; M = Mg, Zn, Co, Ni, Cu, and Fe) have been synthesised via solid-state techniques and characterised. The structure exhibits M2B2O10 layers connected by TeO6 octahedra giving rise to a three-dimensional structure with voids, where Ba2+ ions reside. Substitution of Mg by transition elements (M = Co, Ni, and Cu) in Ba2Mg2TeB2O10 and (Ba0.5Pb1.5)Mg2TeB2O10 gives rise to interesting colored compounds. NIR reflectivity studies indicated that white-colored compounds exhibited good NIR reflectivity, which was is comparable to that of TiO2. Dielectric studies indicated reasonable values with low dielectric loss at low frequencies. The cobalt-substituted compounds Ba2(MgCo)TeB2O10 and (Ba0.5Pb1.5)(MgCo)TeB2O10 were explored towards the oxygen evolution reaction (OER) in an alkaline medium. The compound (Ba0.5Pb1.5)(MgCo)TeB2O10 was found to be a good electrocatalyst for the OER with a faradaic efficiency of ∼96%. The Cu-substituted compound Ba2(Mg1.5Cu0.5)TeB2O10 was found to be a good photocatalyst for the formation of α-chloroketones under visible light in the presence of molecular oxygen.

Silver Microdisc Array Electrode Chip for Urea Detection in Saliva Samples from Patients with Chronic Nephritis

Urea is an important biomarker for diagnosing various kidney and liver disorders. However, many existing methods rely on invasive blood sampling, which can potentially harm patients. Saliva has been recently recognized as a noninvasive and easily collectible alternative to blood for urea quantification. Electrochemical urea detection in saliva remains limited, with catalytic materials typically applied to the electrode surface via drop casting. This results in a random distribution of materials and potential aggregation on the electrode, which inevitably hinders the efficient mass transport of analytes, reducing both detection sensitivity and the utilization of catalytic materials. In this work, a silver nanoparticle (AgNP)-integrated microdisc array electrode chip was fabricated through the in situ growth of AgNPs on polydopamine (PDA) arrays, which were patterned using the microcontact printing (μCP) technique on an indium tin oxide (ITO) glassy substrate. The resulting AgNP microdisc array chip sensor exhibited much higher sensitivity toward urea sensing and greater material utilization as compared to traditional drop-cast electrodes, due to the enhanced mass transfer. Furthermore, the chip sensors demonstrated superior selectivity when challenged with potential interferences. More importantly, reliable urea quantification was achieved in clinical saliva samples from nephritis patients. These results indicate that the current sensor presents great opportunities for developing a noninvasive and sensitive liquid biopsy platform for urea determination in clinical diagnosis applications.

Advanced electrocatalysts for fuel cells: Evolution of active sites and synergistic properties of catalysts and carrier materials

Proton exchange-membrane fuel cell (PEMFC) is a clean and efficient type of energy storage device. However, the sluggish reaction rate of the cathode oxygen reduction reaction (ORR) has been a significant problem in its development. This review reports the recent progress of advanced electrocatalysts focusing on the interface/surface electronic structure and exploring the synergistic relationship of precious-based and non-precious metal-based catalysts and support materials. The support materials contain non-metal (C/N/Si, etc.) and metal-based structures, which have demonstrated a crucial role in the synergistic enhancement of electrocatalytic properties, especially for high-temperature fuel cell systems. To improve the strong interaction, some exciting synergistic strategies by doping and coating heterogeneous elements or connecting polymeric ligands containing carbon and nitrogen were also shown herein. Besides the typical role of the crystal surface, phase structure, lattice strain, etc., the evolution of structure-performance relations was also highlighted in real-time tests. The advanced in situ characterization techniques were also reviewed to emphasize the accurate structure-performance relations. Finally, the challenge and prospect for developing the ORR electrocatalysts were concluded for commercial applications in low- and high-temperature fuel cell systems.

Partial oxidation of Rh/Ru nanoparticles within carbon nanofibers for high-efficiency hydrazine oxidation-assisted hydrogen generation

Hydrazine oxidation reaction (HzOR), an alternative to oxygen evolution reaction, effectively mitigates hydrazine pollution while achieving energy-efficient hydrogen production. Herein, partially oxidized Ru/Rh nanoparticles embedded in carbon nanofibers (CNFs) are fabricated as a bifunctional electrocatalyst for hydrogen evolution reaction (HER) and HzOR. The presence of multiple components including metallic Ru and Rh and their oxides provides numerous electrochemically active sites and superior charge transfer properties, thus improving the electrocatalytic performance. Additionally, the confinement of the active components within CNFs further enhances structural stability. Consequently, the optimized electrocatalyst exhibits ultralow overpotentials of 16 mV at 10 mA cm-2 and 176 mV to reach an industry-level current density of 1 A cm-2 for HER, considerably outperforming the benchmark Pt/C catalyst. Furthermore, it shows an outstanding anodic HzOR activity, achieving a small potential of -0.019 V to generate 10 mAcm-2. A two-electrode overall hydrazine splitting (OHzS) cell prepared using the electrocatalyst operates at a compelling voltage that is 1.953 V lower than that of the overall water splitting (OWS) cell at 200 mA cm-2. Furthermore, the OHzS cell achieves a hydrogen production rate of 1.17 mmol h-1, which is 15-fold that of OWS. Additionally, Rh1Ru1Ox-CNFs-350 is used to construct a Zn-hydrazine battery with excellent performance. This study presents an effective system for achieving high-yielding green H2 production with low energy consumption while simultaneously addressing hydrazine pollution.

Impact of ionomers on porous Fe-N-C catalysts for alkaline oxygen reduction in gas diffusion electrodes

Alkaline exchange membrane fuel cells (AEMFCs) offer a promising alternative to the traditional fossil fuel due to their ability to use inexpensive platinum group metal (PGM)-free catalysts, which could potentially replace Platinum-based catalysts. Iron coordinated in nitrogen-doped carbon (Fe-N-C) single atom electrocatalysts offer the best Pt-free ORR activities. However, most research focuses on material development in alkaline conditions, with limited attention on catalyst layer fabrication. Here, we demonstrate how the oxygen reduction reaction (ORR) performance of a porous Fe-N-C catalyst is affected by the choice of three different commercial ionomers and the ionomer-to-catalyst ratio (I/C). A Mg-templated Fe-N-C is employed as a catalyst owing to the electrochemical accessibility of the Fe sites, and the impact of ionomer properties and coverage were studied and correlated with the electrochemical performance in a gas-diffusion electrode (GDE). The catalyst layer with Nafion at I/C = 2.8 displayed the best activity at high current densities (0.737 ± 0.01 VRHE iR-free at 1 A cm⁻²) owing to a more homogeneous catalyst layer, while Sustainion displayed a higher performance in the kinetic region at the same I/C. These findings provide insights into the impact of catalyst layer optimization to achieve optimal performance in Fe-N-C based AEMFCs.

Balancing Activity and Selectivity in Two-Electron Oxygen Reduction through First Coordination Shell Engineering in Cobalt Single Atom Catalysts

The electrochemical two-electron oxygen reduction reaction (2e- ORR) offers a potentially cost-effective and eco-friendly route for the production of hydrogen peroxide (H2O2). However, the competing 4e- ORR that converts oxygen to water limits the selectivity towards hydrogen peroxide. Accordingly, achieving highly selective H2O2 production under low voltage conditions remains challenging. Herein, guided by first-principles density functional theory (DFT) calculations, we show that modulation the first coordination sphere in Co single atom catalysts (Co-N-C catalysts with Co-NxO4-x sites), specifically the replacement of Co-N bonds with Co-O bonds, can weaken the *OOH adsorption strength to boost the selectivity towards H2O2 (albeit with a slight decrease in ORR activity). Further, by synthesizing a series of N-doped carbon-supported catalysts with Co-NxO4-x active sites, we were able to validate the DFT findings and explore the trade-off between catalytic activity and selectivity for 2e- ORR. A catalyst with trans-Co-N2O2 sites exhibited excellent catalytic activity and H2O2 selectivity, affording a H2O2 production rate of 12.86 m o l g c a t . - 1 h - 1 ${mol\ {g}_{cat.}^{-1}{h}^{-1}{\rm \ }}$ and an half-cell energy-efficiency of 0.07 m o l H 2 O 2 g c a t . - 1 J - 1 ${{mol}_{{H}_{2}{O}_{2}}\ {g}_{cat.}^{-1}\ {J}^{-1}}$ during a 100-hours H2O2 production test in a flow-cell.

Operando monitoring of the functional role of tetrahedral cobalt centers for the oxygen evolution reaction

The complexity of the intrinsic oxygen evolution reaction (OER) mechanism, particularly the precise relationships between the local coordination geometry of active metal centers and the resulting OER kinetics, remains to be fully understood. Herein, we construct a series of 3 d transition metal-incorporated cobalt hydroxide-based nanobox architectures for the OER which contain tetrahedrally coordinated Co(II) centers. Combination of bulk- and surface-sensitive operando spectroelectrochemical approaches reveals that tetrahedral Co(II) centers undergo a dynamic transformation into highly active Co(IV) intermediates acting as the true OER active species which activate lattice oxygen during the OER. Such a dynamic change in the local coordination geometry of Co centers can be further facilitated by partial Fe incorporation. In comparison, the formation of such active Co(IV) species is found to be hindered in CoOOH and Co-FeOOH, which are predominantly containing [CoIIIO6] and [CoII/FeIIIO6] octahedra, respectively, but no mono-μ-oxo-bridged [CoIIO4] moieties. This study offers a comprehensive view of the dynamic role of local coordination geometry of active metal centers in the OER kinetics.

Critical Role of Tetrahedral Coordination in Determining the Polysulfide Conversion Efficiency on Spinel Oxides

Understanding the structure-property relationship and the way in which catalysts facilitate polysulfide conversion is crucial for the rational design of lithium-sulfur (Li-S) battery catalysts. Herein, a series of NiAl2O4, CoAl2O4, and CuAl2O4 spinel oxides with varying Ni2+, Co2+, or Cu2+ tetrahedral and octahedral site occupancy are studied as Li-S battery catalysts. Combined with experimental and theoretical analysis, the tetrahedral site is identified as the most active site for enhancing polysulfide adsorption and charge transfer. This work demonstrates the geometric configuration dependence of spinel oxides for polysulfide conversion and highlights the role of the molecular orbital in determining the activity of cations in different geometries, thereby providing new insights into the rational design of Li-S battery catalysts.

Cluster Engineering in Water Catalytic Reactions: Synthesis, Structure-Activity Relationship and Mechanism

Four fundamental reactions are essential to harnessing energy from water sustainably: oxidation reduction reaction (ORR), oxygen reduction reaction (OER), hydrogen oxidation reaction (HOR), and hydrogen evolution reaction (HER). This review summarizes the research advancements in the electrocatalytic reaction of metal nanoclusters for water splitting. It covers various types of nanoclusters, particularly those at the size level, that enhance these catalytic reactions. The synthesis of cluster-based catalysts and the elucidation of the structure-activity relationships and reaction mechanisms are discussed. Emphasis is placed on utilizing atomically precise cluster materials and the interplay between the carrier and cluster in water catalysis, especially for applying catalytic engineering principles (such as synergy, coordination, heterointerface, and lattice strain engineering) to understand structure-activity relationships and catalytic mechanisms for cluster-based catalysts. Finally, the field of cluster water catalysis is summarized and prospected. We believe that developing cluster-based catalysts with high activity, excellent stability, and high selectivity will significantly promote the development of renewable energy conversion reactions.

Activity-Stability Relationships in Oxygen Evolution Reaction

The oxygen evolution reaction (OER) is a critical process in various sustainable energy technologies. Despite substantial progress in catalyst development, the practical application of OER catalysts remains hindered by the ongoing challenge of balancing high catalytic activity with long-term stability. We explore the inverse trends often observed between activity and stability, drawing on key insights from both experimental and theoretical studies. Special focus is placed on the performance of different electrodes and their interaction with acidic and alkaline media across a range of electrochemical conditions. This Perspective integrates recent advancements to present a thorough framework for understanding the mechanisms underlying the activity-stability relationship, offering strategies for the rational design of next-generation OER catalysts that successfully meet the dual demands of activity and durability.

Comparison of In Situ and Postsynthetic Formation of MOF-Carbon Composites as Electrocatalysts for the Alkaline Oxygen Evolution Reaction (OER)

Mixed-metal nickel-iron, NixFe materials draw attention as affordable earth-abundant electrocatalysts for the oxygen evolution reaction (OER). Here, nickel and mixed-metal nickel-iron metal-organic framework (MOF) composites with the carbon materials ketjenblack (KB) or carbon nanotubes (CNT) were synthesized in situ in a one-pot solvothermal reaction. As a direct comparison to these in situ synthesized composites, the neat MOFs were postsynthetically mixed by grinding with KB or CNT, to generate physical mixture composites. The in situ and postsynthetic MOF/carbon samples were comparatively tested as (pre-)catalysts for the OER, and most of them outperformed the RuO2 benchmark. Depending on the carbon material and metal ratio, the in situ or postsynthetic composites performed better, showing that the method to generate the composite can influence the OER activity. The best material Ni5Fe-CNT was synthesized in situ and achieved an overpotential (η) of 301 mV (RuO2η = 354 mV), a Tafel slope (b) of 58 mV/dec (RuO2b = 91 mV/dec), a charge transfer resistance (Rct) of 7 Ω (RuO2 Rct = 39 Ω), and a faradaic efficiency (FE) of 95% (RuO2 FE = 91%). Structural changes in the materials could be seen through a stability test in the alkaline electrolyte, and chronopotentiometry over 12 h showed that the derived electrocatalysts and RuO2 have good stability.

Insight Into Intermediate Behaviors and Design Strategies of Platinum Group Metal-Based Alkaline Hydrogen Oxidation Catalysts

Hydrogen oxidation reaction (HOR) can effectively convert the hydrogen energy through the hydrogen fuel cells, which plays an increasingly important role in the renewable hydrogen cycle. Nevertheless, when the electrolyte pH changes from acid to base, even with platinum group metal (PGM) catalysts, the HOR kinetics declines with several orders of magnitude. More critically, the pivotal role of reaction intermediates and interfacial environment during intermediate behaviors on alkaline HOR remains controversial. Therefore, exploring the exceptional PGM-based alkaline HOR electrocatalysts and identifying the reaction mechanism are indispensable for promoting the commercial development of hydrogen fuel cells. Consequently, the fundamental understanding of the HOR mechanism is first introduced, with emphases on the adsorption/desorption process of distinct reactive intermediates and the interfacial structure during catalytic process. Subsequently, with the guidance of reaction mechanism, the latest advances in the rational design of advanced PGM-based (Pt, Pd, Ir, Ru, Rh-based) alkaline HOR catalysts are discussed, focusing on the correlation between the intermediate behaviors and the electrocatalytic performance. Finally, given that the challenges standing in the development of the alkaline HOR, the prospect for the rational catalysts design and thorough mechanism investigation towards alkaline HOR are emphatically proposed.

Biphasic 1T/2H-MoS2 Nanosheets In Situ Vertically Anchored on Reduced Graphene Oxide via Covalent Coupling of the Mo-O-C Bond for Enhanced Electrocatalytic Hydrogen Evolution

Transition-metal dichalcogenides (TMDs) have recently emerged as promising electrocatalysts for the hydrogen evolution reaction owing to their tunable electronic properties. However, TMDs still encounter inherent limitations, including insufficient active sites, poor conductivity, and instability; thus, their performance breakthrough mainly depends on structural optimization in hybridization with a conductive matrix and phase modulation. Herein, a 1T/2H-MoS2/rGO hybrid was rationally fabricated, which is characterized by biphasic 1T/2H-MoS2 nanosheets in situ vertically anchored on reduced graphene oxide (rGO) with strong C-O-Mo covalent coupling. The rGO substrate improves the conductivity and ensures high-dispersed 1T/2H-MoS2 nanosheets to expose plentiful highly active edges. More importantly, the strong heterointerface electrical interaction by the C-O-Mo covalent bond can enhance the charge-transfer efficiency and reinforce structural stability. Furthermore, the integration with the appropriate 2H phase is in favor of stabilization of the metastable 1T phase; thus, the ratio of 1T and 2H was precisely regulated to balance activity and stability. With these advantages, the 1T/2H-MoS2/rGO catalyst presents a satisfactory activity and stability, as confirmed by the relatively low overpotential (268 and 140 mV at 10 mA cm-2) and the small Tafel slope (102 and 86 mV dec-1) in alkaline and acidic media, respectively. The theory calculations disclose that the electronic structure redistribution has been optimized via the strong coupled C-O-Mo heterointerface and phase interface, significantly reducing the adsorption free energy of hydrogen and improving intrinsic activity.

Assembly and Valence Modulation of Ordered Bimetallic MOFs for Highly Efficient Electrocatalytic Water Oxidation

Metal synergy can enhance the catalytic performance, and a prefabricated solid precursor can guide the ordered embedding, of secondary metal source ions for the rapid synthesis of bimetallic organic frameworks (MM'-MOFs) with a stoichiometric ratio of 1:1. In this paper, Co-MOF-1D containing well-defined binding sites was synthesized by mechanical ball milling, which was used as a template for the induced introduction of Fe ions to successfully assemble the ordered bimetallic Co1Fe1-MOF-74@2 (where @2 denotes template-directed synthesis of MOF-74). Its electrocatalytic performance is superior to that of the conventional one-step-synthesized Co1Fe1-MOF-74@1 (where @1 denotes one-step synthesis of MOF-74), and the ratio of the two metal sources, Co and Fe, is close to 1:1. Meanwhile, the iron valence states (FeII and FeIII) in Co1Fe1-MOF-74@2 were further regulated to obtain the electrocatalytic materials Co1Fe1(II)-MOF-74@2 and Co1Fe1(III)-MOF-74@2. The electrochemical performance test results confirm that Co1Fe1(II)-MOF-74@2 regulated by valence state has a better catalytic performance than Co1Fe1(III)-MOF-74@2 in the oxygen evolution reaction (OER) process. This phenomenon is related to the gradual increase in the valence state of Fe ions in Co1Fe1(II)-MOF-74@2, which promotes the continuous improvement in the performance of the MOF before reaching the optimal steady state and makes the OER performance reach the optimum when the FeII/FeIII mixed-valence state reaches a certain proportion. This provides a new idea for the directed synthesis and optimization of highly efficient catalysts.

Hydrogen Electrode Reactions in Energy-Related Electrocatalysis Systems

Hydrogen electrode reactions, including hydrogen evolution reactions and hydrogen oxidation reactions, are fundamental and crucial within aqueous electrochemistry. Particularly in energy-related electrocatalysis processes, there is a consistent involvement of hydrogen-related electrochemical processes, underscoring the need for in-depth study. This review encompasses significant reports, delving into elementary steps and reaction mechanisms of hydrogen electrode reactions, as well as catalyst design strategies. In addition, we focus on the application of hydrogen electrode reaction mechanism in different energy-related electrocatalytic reactions, and the significance of the promotion and suppression of reaction kinetics in different reaction systems. It thoroughly elucidated the significance of these reactions and the need for a deeper understanding, offering a novel perspective for the future development of this field.

Iron Porphyrin-Based Composites for Electrocatalytic Oxygen Reduction Reactions

The oxygen reduction reaction (ORR) is one of the most critical reactions in energy conversion systems, and it facilitates the efficient conversion of chemical energy into electrical energy, which is necessary for modern technology. Developing efficient and cost-effective catalysts for ORRs is crucial for advancing and effectively applying renewable energy technologies such as fuel cells, metal-air batteries, and electrochemical sensors. In recent years, iron porphyrin-based composites have emerged as ideal catalysts for facilitating effective ORRs due to their unique structural characteristics, abundance, advances in synthesis, and excellent catalytic properties, which mimic natural enzymatic systems. However, many articles have focused on reviewing porphyrin-based frameworks or metalloporphyrins in general, necessitating research specifically addressing iron porphyrin. This review discusses iron porphyrin as an effective catalyst in ORRs. It provides a comprehensive knowledge of the application of iron porphyrin-based composites for electrocatalytic ORRs, focusing on their properties, synthesis, structural integration with conductive supports, catalytic mechanism, and efficacy. This review also discusses the challenges of applying iron porphyrin-based composites and provides recommendations to address these challenges.

Recognizing the reactive sites of SnFe2O4 for the oxygen evolution reaction: the synergistic effect of SnII and FeIII in stabilizing reaction intermediates

Among the reported spinel ferrites, the p-block metal containing SnFe2O4 is scarcely explored, but it is a promising water-splitting electrocatalyst. This study focuses on the reaction kinetics and atomic scale insight of the reaction mechanism of the oxygen evolution reaction (OER) catalyzed by SnFe2O4 and analogous Fe3O4. The replacement of FeIIOh sites with SnIIOh in SnFe2O4 improves the catalytic efficiency and various intrinsic parameters affecting the reaction kinetics. The variable temperature OER depicts a low activation energy (Ea) of 28.71 kJ mol-1 on SnFe2O4. Experimentally determined second-order dependence on [OH-] and the prominent kinetic isotope effect observed during the deuterium labelling study implies the role of hydroxide ions in the rate-determining step (RDS). Using density functional theory, the reaction mechanism on the (001) surface of SnFe2O4 and Fe3O4 is modelled. The DFT simulated free energy diagram for the reaction intermediates shows an adsorbate evolution mechanism (AEM) on both the ferrites' surfaces where the formation of *OOH is the RDS on SnFe2O4 while *O formation is the RDS on Fe3O4. In contrast to other spinel ferrites, where individual metal sites act independently, in case of SnFe2O4, a synergy between FeIIIOh and the neighbouring SnIIOh atoms is responsible for stabilizing the OER intermediates, enhancing the catalytic OER activity of SnFe2O4 as compared to isostructural Fe3O4.

How to Assess and Predict Electrical Double Layer Properties. Implications for Electrocatalysis

The electrical double layer (EDL) plays a central role in electrochemical energy systems, impacting charge transfer mechanisms and reaction rates. The fundamental importance of the EDL in interfacial electrochemistry has motivated researchers to develop theoretical and experimental approaches to assess EDL properties. In this contribution, we review recent progress in evaluating EDL characteristics such as the double-layer capacitance, highlighting some discrepancies between theory and experiment and discussing strategies for their reconciliation. We further discuss the merits and challenges of various experimental techniques and theoretical approaches having important implications for aqueous electrocatalysis. A strong emphasis is placed on the substantial impact of the electrode composition and structure and the electrolyte chemistry on the double-layer properties. In addition, we review the effects of temperature and pressure and compare solid-liquid interfaces to solid-solid interfaces.

Keys to Unravel the Stability/Durability Issues of Platinum-Group-Metal Catalysts toward Oxygen Evolution Reaction for Acidic Water Splitting

Proton exchange membrane (PEM) water electrolyzers stand as one of the foremost promising avenues for acidic water splitting and green hydrogen production, yet this electrolyzer encounters significant challenges. The primary culprit lies in not only the requirements of substantial platinum-group-metal (PGM)-based electrocatalysts (e.g., IrO x ) at the anode where sluggish oxygen evolution reaction (OER) takes place, but also the harsh high overpotential and acidic environments leading to severe performance degradation. The key points for obtaining accurate stability/durability information on the OER catalysts have not been well agreed upon, in contrast to the oxygen reduction reaction fields. In this regard, we herein reviewed and discussed the pivotal experimental variables involved in stability/durability testing (including but not limited to electrolyte, impurity, catalyst loading, and two/three-electrode vs membrane-electrode-assembly), while the test protocols are revisited and summarized. This outlook is aimed at highlighting the reasonable and effective accelerated degradation test procedures to unravel the acidic OER catalyst instability issues and promote the research and development of a PEM water electrolyzer.

Room Temperature Electrochemical-Shock Synthesis of Solid-Solution Medium-Entropy Alloy Nanoparticles for Hydrogen Evolution

For millennia, humankind has discovered great benefits in alloying materials. Over the past 100 years, a renaissance in nanoscience has cemented the importance of nanoparticles in a variety of fields ranging from energy storage and conversion to cell biology. While many synthetic strategies exist for nanoparticle and alloy nanoparticle formation, new methods are necessary to create nanoparticles under unprecedented conditions. Here, we demonstrate a simple technique to electrodeposit solid-solution alloy nanoparticles at room temperature. When metal salts of platinum, gold, and palladium are confined to nanodroplets suspended in oil, and then a nanodroplet collides with a sufficiently negative-biased electrode to reduce the metal salts at the mass-transfer limitation, solid-solution alloy nanoparticles form. High-angle annular dark-field scanning transmission electron microscopy and single atom energy dispersive X-ray spectroscopy confirm the solid-solution microstructure of the nanoparticles. The results also confirm the nanodroplet's ability to tune alloy microstructures from amorphous to solid-solution. We further extend our technique by adding salts of silver, which lead to the synthesis of polycrystalline medium-entropy alloys. Finally, we go on to show the application of our midentropy alloys toward renewable and clean energy devices by highlighting their electrocatalytic activity toward hydrogen evolution reaction. Our method is unrivaled in its simplicity and will find applications across various fields of study.

B-Site Doping Boosts the OER and ORR Performance of Double Perovskite Oxide as Air Cathode for Zinc-Air Batteries

Double perovskite oxides are key players as oxygen evolution and oxygen reduction catalysts in alkaline media due to their tailorable electronic structures by doping. In this study, we synthesized B-site doped NdBaCoaFe2-aO5+δ (a=1.0, 1.4, 1.6, 1.8) electrocatalysts, systematically probed their bifunctionality and assessed their performance in zinc-air batteries as air cathodes. X-ray photoelectron spectroscopy analysis reveals a correlation between iron reduction and increased oxygen vacancy content, influencing electrocatalyst bifunctionality by lowering the work function. The electrocatalyst with highest cobalt content, NdBaCo1.8Fe0.2O5+δ exhibited a bifunctionality value of 0.95 V, outperforming other synthesized electrocatalysts. Remarkably, NdBaCo1.8Fe0.2O5+δ, demonstrated facilitated charge transfer rate in oxygen evolution reaction with four-electron oxygen reduction reaction process. As an air cathode in a zinc-air battery, NdBaCo1.8Fe0.2O5+δ demonstrated superior performance characteristics, including maximum capacity of 428.27 mA h at 10 mA cm-2 discharge current density, highest peak power density of 64 mW cm-2, with an enhanced durability and stability. It exhibits lowest voltage gap change between charge and discharge even after 350 hours of cyclic operation with a rate capability of 87.14 %.

Quantification of electrochemically accessible iridium oxide surface area with mercury underpotential deposition

Research drives development of sustainable electrocatalytic technologies, but efforts are hindered by inconsistent reporting of advances in catalytic performance. Iridium-based oxide catalysts are widely studied for electrocatalytic technologies, particularly for the oxygen evolution reaction (OER) for proton exchange membrane water electrolysis, but insufficient techniques for quantifying electrochemically accessible iridium active sites impede accurate assessment of intrinsic activity improvements. We develop mercury underpotential deposition and stripping as a reversible electrochemical adsorption process to robustly quantify iridium sites and consistently normalize OER performance of benchmark IrOx electrodes to a single intrinsic activity curve, where other commonly used normalization methods cannot. Through rigorous deconvolution of mercury redox and reproportionation reactions, we extract net monolayer deposition and stripping of mercury on iridium sites throughout testing using a rotating ring disk electrode. This technique is a transformative method to standardize OER performance across a wide range of iridium-based materials and quantify electrochemical iridium active sites.

Active site engineering of intermetallic nanoparticles by the vapour-solid synthesis: carbon black supported nickel tellurides for hydrogen evolution

The development and design of catalysts have become a major pillar of latest research efforts to make sustainable forms of energy generation accessible. The production of green hydrogen by electrocatalytic water splitting is dealt as one of the most promising ways to enable decarbonization. To make the hydrogen evolution reaction through electrocatalytic water splitting usable on a large scale, the development of highly-active catalysts with long-term stability and simple producibility is required. Recently, nickel tellurides were found to be an interesting alternative to noble-metal materials. Previous publications dealt with individual nickel telluride species of certain compositions due to the lack of broadly applicable synthesis strategies. For the first time, in this work the preparation of carbon black supported nickel telluride nanoparticles and their catalytic performance for the electrocatalytic hydrogen evolution reaction in alkaline media is presented. The facile vapour-solid synthesis strategy enabled remarkable control over the crystal structure and composition, demonstrating interesting opportunities of active site engineering. Both single- and multi-phase samples containing the Ni-Te compounds Ni3Te2, NiTe, NiTe2-x & NiTe2 were prepared. Onset potentials and overpotentials of -0.145 V vs. RHE and 315 mV at 10 mA cm-2 respectively were achieved. Furthermore, it was found that the mass activity was dependent on the structure and composition of the nickel tellurides following the particular order: Ni3Te2 > NiTe > NiTe2-x > NiTe2.

Trifunctional Graphene-Sandwiched Heterojunction-Embedded Layered Lattice Electrocatalyst for High Performance in Zn-Air Battery-Driven Water Splitting

Zn-air battery (ZAB)-driven water splitting holds great promise as a next-generation energy conversion technology, but its large overpotential, low activity, and poor stability for oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) remain obstacles. Here, a trifunctional graphene-sandwiched, heterojunction-embedded layered lattice (G-SHELL) electrocatalyst offering a solution to these challenges are reported. Its hollow core-layered shell morphology promotes ion transport to Co3S4 for OER and graphene-sandwiched MoS2 for ORR/HER, while its heterojunction-induced internal electric fields facilitate electron migration. The structural characteristics of G-SHELL are thoroughly investigated using X-ray absorption spectroscopy. Additionally, atomic-resolution transmission electron microscopy (TEM) images align well with the DFT-relaxed structures and simulated TEM images, further confirming its structure. It exhibits an approximately threefold smaller ORR charge transfer resistance than Pt/C, a lower OER overpotential and Tafel slope than RuO₂, and excellent HER overpotential and Tafel slope, while outlasting noble metals in terms of durability. Ex situ X-ray photoelectron spectroscopy analysis under varying potentials by examining the peak shifts and ratios (Co2+/Co3+ and Mo4+/Mo6+) elucidates electrocatalytic reaction mechanisms. Furthermore, the ZAB with G-SHELL outperforms Pt/C+RuO2 in terms of energy density (797 Wh kg-1) and peak power density (275.8 mW cm-2), realizing the ZAB-driven water splitting.

Recent Advances in Revealing the Electrocatalytic Mechanism for Hydrogen Energy Conversion System

In light of the intensifying global energy crisis and the mounting demand for environmental protection, it is of vital importance to develop advanced hydrogen energy conversion systems. Electrolysis cells for hydrogen production and fuel cell devices for hydrogen utilization are indispensable in hydrogen energy conversion. As one of the electrolysis cells, water splitting involves two electrochemical reactions, hydrogen evolution reaction and oxygen evolution reaction. And oxygen reduction reaction coupled with hydrogen oxidation reaction, represent the core electrocatalytic reactions in fuel cell devices. However, the inherent complexity and the lack of a clear understanding of the structure-performance relationship of these electrocatalytic reactions, have posed significant challenges to the advancement of research in this field. In this work, the recent development in revealing the mechanism of electrocatalytic reactions in hydrogen energy conversion systems is reviewed, including in situ characterization and theoretical calculation. First, the working principles and applications of operando measurements in unveiling the reaction mechanism are systematically introduced. Then the application of theoretical calculations in the design of catalysts and the investigation of the reaction mechanism are discussed. Furthermore, the challenges and opportunities are also summarized and discussed for paving the development of hydrogen energy conversion systems.

Strain-Controlled Galvanic Synthesis of Platinum Icosahedral Nanoframes and Their Enhanced Catalytic Activity toward Oxygen Reduction

The unique strain distribution on the surface of a Pd icosahedral nanocrystal is leveraged to control the sites for oxidation and reduction involved in the galvanic replacement reaction. Specifically, Pd is oxidized and dissolved from the center of each {111} facet due to its tensile strain, while the Pt(II) precursor adsorbs onto the vertices and edges featuring a compressive strain, followed by surface reduction and conformal deposition of the Pt atoms. Once the galvanic reaction is initiated, the {111} facets become more vulnerable to oxidation and dissolution, as the vertices and edges are protected by the deposited Pt atoms. The site-selected galvanic reaction naturally results in the formation of Pt icosahedral nanoframes covered by compressively strained {111} facets, which show enhanced catalytic activity and durability toward oxygen reduction relative to commercial Pt/C.