The Stability Challenges of Oxygen Evolving Catalysts: Towards a Common Fundamental Understanding and Mitigation of Catalyst Degradation

Customized reaction route for ruthenium oxide towards stabilized water oxidation in high-performance PEM electrolyzers

The poor stability of Ru-based acidic oxygen evolution (OER) electrocatalysts has greatly hampered their application in polymer electrolyte membrane electrolyzers (PEMWEs). Traditional understanding of performance degradation centered on influence of bias fails in describing the stability trend, calling for deep dive into the essential origin of inactivation. Here we uncover the decisive role of reaction route (including catalytic mechanism and intermediates binding strength) on operational stability of Ru-based catalysts. Using MRuO_x (M = Ce⁴⁺, Sn⁴⁺, Ru⁴⁺, Cr⁴⁺) solid solution as structure model, we find the reaction route, thereby stability, can be customized by controlling the Ru charge. The screened SnRuO_x thus exhibits orders of magnitude lifespan extension. A scalable PEMWE single cell using SnRuO_x anode conveys an ever-smallest degradation rate of 53 μV h^-1 during a 1300 h operation at 1 A cm^-2.

Altering Oxygen Binding by Redox-Inactive Metal Substitution to Control Catalytic Activity: Oxygen Reduction on Manganese Oxide Nanoparticles as a Model System

Establishing generic catalyst design principles by identifying structural features of materials that influence their performance will advance the rational engineering of new catalytic materials. In this study, by investigating metal-substituted manganese oxide (spinel) nanoparticles, Mn₃ O₄ :M (M=Sr, Ca, Mg, Zn, Cu), we rationalize the dependence of the activity of Mn₃ O₄ :M for the electrocatalytic oxygen reduction reaction (ORR) on the enthalpy of formation of the binary MO oxide, Δ_f H°(MO), and the Lewis acidity of the M²⁺ substituent. Incorporation of elements M with low Δ_f H°(MO) enhances the oxygen binding strength in Mn₃ O₄ :M, which affects its activity in ORR due to the established correlation between ORR activity and the binding energy of *O/*OH/*OOH species. Our work provides a perspective on the design of new compositions for oxygen electrocatalysis relying on the rational substitution/doping by redox-inactive elements.

Al-Pt compounds catalyzing the oxygen evolution reaction

Al-Pt compounds have been systematically studied as electrocatalysts for the oxygen evolution reaction (OER). Considering the harsh oxidative conditions of the OER, all Al-Pt compounds undergo modifications during electrochemical experiments. However, the degree of changes strongly depends on the composition and crystal structure of a compound. In contrast to Al-rich compounds (Al₄Pt and Al₂₁Pt₈), which reveal strong leaching of aluminum, changes in other compounds (Al₂Pt, Al₃Pt₂, rt-AlPt, Al₃Pt₅, and rt-AlPt₃) take place only on the surface or in the near-surface region. Furthermore, surface modification leads to a change in the electronic structure of Pt, giving rise to the in situ formation of catalytically more active surfaces, which are composed of intermetallic compounds, Pt-rich Al_xPt_1-x phases and Pt oxides. Forming a compromise between sufficient OER activity and stability, Al₂Pt and Al₃Pt₂ can be considered as precursors for OER electrocatalysts.

Surface Modified CoCrFeNiMo High Entropy Alloys for Oxygen Evolution Reaction in Alkaline Seawater

Electrolysis of seawater is a promising technique to desalinate seawater and produce high-purity hydrogen production for freshwater and renewable energy, respectively. For the application of seawater electrolysis technique on a large scale, simplicity of manufacture method, repeatability of catalyst products, and stable product quality is generally required in the industry. In this work, a facile, one-step, and metal salt-free fabrication method was developed for the seawater-oxygen-evolution-active catalysts composed of CoCrFeNiMo layered double hydroxide array self-supported on CoCrFeNiMo high entropy alloy substrate. The obtained catalysts show improved performance for oxygen evolution reaction in alkaline artificial seawater solution. The best-performing sample delivered the current densities of 10, 50, and 100 mA cm−2 at low overpotentials of 260.1, 294.3, and 308.4 mV, respectively. In addition, high stability is also achieved since no degradation was observed over the chronoamperometry test of 24 h at the overpotential corresponding to 100 mA cm−2. Furthermore, a failure mechanism OER activity of multi-element LDHs catalysts was put forward in order to enhance catalytic performance and design catalysts with long-term durability.

Non-iridium-based electrocatalyst for durable acidic oxygen evolution reaction in proton exchange membrane water electrolysis

Iridium-based electrocatalysts remain the only practical anode catalysts for proton exchange membrane (PEM) water electrolysis, due to their excellent stability under acidic oxygen evolution reaction (OER), but are greatly limited by their high cost and low reserves. Here, we report a nickel-stabilized, ruthenium dioxide (Ni-RuO₂) catalyst, a promising alternative to iridium, with high activity and durability in acidic OER for PEM water electrolysis. While pristine RuO₂ showed poor acidic OER stability and degraded within a short period of continuous operation, the incorporation of Ni greatly stabilized the RuO₂ lattice and extended its durability by more than one order of magnitude. When applied to the anode of a PEM water electrolyser, our Ni-RuO₂ catalyst demonstrated >1,000 h stability under a water-splitting current of 200 mA cm^-2, suggesting potential for practical applications. Density functional theory studies, coupled with operando differential electrochemical mass spectroscopy analysis, confirmed the adsorbate-evolving mechanism on Ni-RuO₂, as well as the critical role of Ni dopants in stabilization of surface Ru and subsurface oxygen for improved OER durability.

Iridium Stabilizes Ceramic Titanium Oxynitride Support for Oxygen Evolution Reaction

Decreasing iridium loading in the electrocatalyst presents a crucial challenge in the implementation of proton exchange membrane (PEM) electrolyzers. In this respect, fine dispersion of Ir on electrically conductive ceramic supports is a promising strategy. However, the supporting material needs to meet the demanding requirements such as structural stability and electrical conductivity under harsh oxygen evolution reaction (OER) conditions. Herein, nanotubular titanium oxynitride (TiON) is studied as a support for iridium nanoparticles. Atomically resolved structural and compositional transformations of TiON during OER were followed using a task-specific advanced characterization platform. This combined the electrochemical treatment under floating electrode configuration and identical location transmission electron microscopy (IL-TEM) analysis of an in-house-prepared Ir-TiON TEM grid. Exhaustive characterization, supported by density functional theory (DFT) calculations, demonstrates and confirms that both the Ir nanoparticles and single atoms induce a stabilizing effect on the ceramic support via marked suppression of the oxidation tendency of TiON under OER conditions.

Carbon nanotube-titanium dioxide nanocomposite support for improved activity and stability of an iridium catalyst toward the oxygen evolution reaction

In order to improve the electrocatalytic activity and stability of an iridium (Ir) nanoparticle catalyst toward the oxygen evolution reaction (OER) in acidic electrolyte, carbon nanotube and titanium dioxide nanocomposites (CNT@TiO2) are presented as a high-performance support. TiO2 was synthesized on CNTs by using a novel layer-by-layer solution coating method that mimics atomic layer deposition (ALD) but is cost-effective and scalable. In the nanocomposites, CNTs serve as the electron pathways and the surface TiO2 layers protect CNTs from corrosion under the harsh OER conditions. Thus, CNT@TiO2 demonstrates excellent corrosion resistance as well as a high electrical conductivity (1.6 ± 0.2 S cm-1) comparable to that of Vulcan carbon (1.4 S cm-1). The interaction between Ir and TiO2 promotes the formation of Ir(iii) species, thereby enhancing the OER activity and stability of the Ir nanoparticle catalyst. Compared to commercial carbon-supported Ir (Ir/C) and Ir black catalysts, CNT@TiO2-supported Ir exhibits superior OER activity and stability.

Probing the Effects of Electrode Composition and Morphology on the Effectiveness of Silicon Oxide Overlayers to Enhance Selective Oxygen Evolution in the Presence of Chloride Ions

Seawater electrolysis offers significant logistical advantages over freshwater electrolysis but suffers from a fundamental selectivity problem at the anode. To prevent the evolution of toxic chlorine alongside the evolution of oxygen, a promising approach is the use of electrochemically inert overlayers. Such thin films can exert a perm-selective effect, allowing the transport of water and oxygen between the bulk electrolyte and the electrocatalytic buried interface while suppressing the transport of chloride ions. In this work, we investigate thin (5-20 nm) overlayer films composed of amorphous silicon oxide (SiO x ) and their application to suppressing the chlorine evolution reaction (CER) in favor of the oxygen evolution reaction (OER) during acidic saltwater electrolysis on three different types of electrodes. While SiO x overlayers are seen to be an effective barrier against the CER on well-defined, smooth Pt thin films, decreasing the CER activity roughly 20-fold, this ability has not been previously explored on Ir-based catalysts with a higher surface area relevant to industrial applications. On amorphous iridium oxide electrodes, the selectivity toward the CER versus the OER was marginally reduced from ∼98 to ∼94%, which was attributed to the higher abundance of defects in overlayers deposited on the rougher electrode. On the other hand, Ir-based anodes consisting of thick mixed metal oxide films supported on Ti showed a significant decrease in CER selectivity, from ∼100 to ∼50%, although this came at the cost of reduced activity toward the OER. These results show that the morphology and composition of the underlying electrode play important roles in the effectiveness of the selective overlayers and provide guidance for further development of high-surface-area OER-selective anodes.

Oxygen Evolution Electrocatalysts for the Proton Exchange Membrane Electrolyzer: Challenges on Stability

Hydrogen generated by proton exchange membrane (PEM) electrolyzer holds a promising potential to complement the traditional energy structure and achieve the global target of carbon neutrality for its efficient, clean, and sustainable nature. The acidic oxygen evolution reaction (OER), owing to its sluggish kinetic process, remains a bottleneck that dominates the efficiency of overall water splitting. Over the past few decades, tremendous efforts have been devoted to exploring OER activity, whereas most show unsatisfying stability to meet the demand for industrial application of PEM electrolyzer. In this review, systematic considerations of the origin and strategies based on OER stability challenges are focused on. Intrinsic deactivation of the material and the extrinsic balance of plant-induced destabilization are summarized. Accordingly, rational strategies for catalyst design including doping and leaching, support effect, coordination effect, strain engineering, phase and facet engineering are discussed for their contribution to the promoted OER stability. Moreover, advanced in situ/operando characterization techniques are put forward to shed light on the OER pathways as well as the structural evolution of the OER catalyst, giving insight into the deactivation mechanisms. Finally, outlooks toward future efforts on the development of long-term and practical electrocatalysts for the PEM electrolyzer are provided.

Progress of Heterogeneous Iridium-Based Water Oxidation Catalysts

The water oxidation reaction (or oxygen evolution reaction, OER) plays a critical role in green hydrogen production via water splitting, electrochemical CO₂ reduction, and nitrogen fixation. The four-electron and four-proton transfer OER process involves multiple reaction intermediates and elementary steps that lead to sluggish kinetics; therefore, a high overpotential is necessary to drive the reaction. Among the different water-splitting electrolyzers, the proton exchange membrane type electrolyzer has greater advantages, but its anode catalysts are limited to iridium-based materials. The iridium catalyst has been extensively studied in recent years due to its balanced activity and stability for acidic OER, and many exciting signs of progress have been made. In this review, the surface and bulk Pourbaix diagrams of iridium species in an aqueous solution are introduced. The iridium-based catalysts, including metallic or oxides, amorphous or crystalline, single crystals, atomically dispersed or nanostructured, and iridium compounds for OER, are then elaborated. The latest progress of active sites, reaction intermediates, reaction kinetics, and elementary steps is summarized. Finally, future research directions regarding iridium catalysts for acidic OER are discussed.

Tailoring of Active Sites from Single to Dual Atom Sites for Highly Efficient Electrocatalysis

Single atom catalysts (SACs) have been attracting extensive attention in electrocatalysis because of their unusual structure and extreme atom utilization, but the low metal loading and unified single site induced scaling relations may limit their activity and practical application. Tailoring of active sites at the atomic level is a sensible approach to break the existing limits in SACs. In this review, SACs were first discussed regarding carbon or non-carbon supports. Then, five tailoring strategies were elaborated toward improving the electrocatalytic activity of SACs, namely strain engineering, spin-state tuning engineering, axial functionalization engineering, ligand engineering, and porosity engineering, so as to optimize the electronic state of active sites, tune d orbitals of transition metals, adjust adsorption strength of intermediates, enhance electron transfer, and elevate mass transport efficiency. Afterward, from the angle of inducing electron redistribution and optimizing the adsorption nature of active centers, the synergistic effect from adjacent atoms and recent advances in tailoring strategies on active sites with binuclear configuration which include simple, homonuclear, and heteronuclear dual atom catalysts (DACs) were summarized. Finally, a summary and some perspectives for achieving efficient and sustainable electrocatalysis were presented based on tailoring strategies, design of active sites, and in situ characterization.

Design of a Metal/Oxide/Carbon Interface for Highly Active and Selective Electrocatalysis

Sustainable energy-conversion and chemical-production require catalysts with high activity, durability, and product-selectivity. Metal/oxide hybrid structure has been intensively investigated to achieve promising catalytic performance, especially in neutral or alkaline electrocatalysis where water dissociation is promoted near the oxide surface for (de)protonation of intermediates. Although catalytic promise of the hybrid structure is demonstrated, it is still challenging to precisely modulate metal/oxide interfacial interactions on the nanoscale. Herein, we report an effective strategy to construct rich metal/oxide nano-interfaces on conductive carbon supports in a surfactant-free and self-terminated way. When compared to the physically mixed Pd/CeO₂ system, a much higher degree of interface formation was identified with largely improved hydrogen oxidation reaction (HOR) kinetics. The benefits of the rich metal-CeO₂ interface were further generalized to Pd alloys for optimized adsorption energy, where the Pd₃Ni/CeO₂/C catalyst shows superior performance with HOR selectivity against CO poisoning and shows long-term stability. We believe this work highlights the importance of controlling the interfacial junctions of the electrocatalyst in simultaneously achieving enhanced activity, selectivity, and stability.

Supported Iridium-based Oxygen Evolution Reaction Electrocatalysts - Recent Developments

The commercialization of acidic proton exchange membrane water electrolyzers (PEMWE) is heavily hindered by the price and scarcity of oxygen evolution reaction (OER) catalyst, i. e. iridium and its oxides. One of the solutions to enhance the utilization of this precious metal is to use a support to distribute well dispersed Ir nanoparticles. In addition, adequately chosen support can also impact the activity and stability of the catalyst. However, not many materials can sustain the oxidative and acidic conditions of OER in PEMWE. Hereby, we critically and extensively review the different materials proposed as possible supports for OER in acidic media and the effect they have on iridium performances.

Nanostructured IrO x supported on N-doped TiO2 as an efficient electrocatalyst towards acidic oxygen evolution reaction

Reducing the Ir consumption without compromising the catalytic performance for the oxygen evolution reaction (OER) is highly paramount to promote the extensive development of the environmentally-friendly solid polymer electrolyte water electrolysis (SPEWE) system. Herein, TiO2 is doped with N through facile NH3 gas treatment and innovatively employed to support IrO x nanoparticles towards acidic OER. N-doping action not only dramatically boosts the electrical conductivity and dispersing/anchoring effects of TiO2, but also effectively improves the electron-transfer procedure. As a result, the IrO x /N-TiO2 electrocatalyst exhibits prominent catalyst utilization, catalytic activity and stability. Specifically, the overpotential required to deliver 10 mA cm-2 is only 270 mV, and the mass activity climbs to 278.7 A gIr -1 @ 1.55 VRHE. Moreover, the single cell voltage is only 1.761 V @ 2.0 A cm-2 when adopting IrO x /N-TiO2 as the anode catalyst, which is 44 mV lower than that of the commercial IrO2 counterpart.

Highly Conductive and Mechanically Robust NiFe Alloy Aerogels: An Exceptionally Active and Durable Water Oxidation Catalyst

Poor stability of nanostructured electrocatalysts at rigorous industrial conditions significantly inhibits their performances in practical electrolyzers. Although many substrate-supported nanostructured electrocatalysts present attractive performance at small currents, they cannot sustain industry-level high current densities for long-term operation. Herein, by chemically organizing nanoscale electrocatalysts into a macroscopic substrate-free metallic alloy aerogel, this NiFe-based nano-catalyst achieves 1000-h durability at industrial-level current densities, with exceptionally high activities of 500 mA at the overpotential of only 281 mV. This NiFe alloy aerogel is constructed by a magnetic-field assisted growth and assembly of ferromagnetic NiFe nanoparticles, in which nanowires are loosely crosslinked by metallic joints. This alloy aerogel shows a high electric conductivity of 500 S m^-1 , structural stability for more than 1.5 years in alkaline electrolyte, and almost complete recovery after compression exceeding 50% strain for 1000 cycles. The excellent mechanical stability of this metallic aerogel behaves as the key contributor to the superior electrocatalytic stability under industrially relevant conditions. This work offers a paradigm for electrode design for the practical application of nano-catalysts in industrial alkaline water electrolysis.

Introducing Brønsted acid sites to accelerate the bridging-oxygen-assisted deprotonation in acidic water oxidation

Oxygen evolution reaction (OER) consists of four sequential proton-coupled electron transfer steps, which suffer from sluggish kinetics even on state-of-the-art ruthenium dioxide (RuO₂) catalysts. Understanding and controlling the proton transfer process could be an effective strategy to improve OER performances. Herein, we present a strategy to accelerate the deprotonation of OER intermediates by introducing strong Brønsted acid sites (e.g. tungsten oxides, WO_x) into the RuO₂. The Ru-W binary oxide is reported as a stable and active iridium-free acidic OER catalyst that exhibits a low overpotential (235 mV at 10 mA cm⁻²) and low degradation rate (0.014 mV h⁻¹) over a 550-hour stability test. Electrochemical studies, in-situ near-ambient pressure X-ray photoelectron spectroscopy and density functional theory show that the W-O-Ru Brønsted acid sites are instrumental to facilitate proton transfer from the oxo-intermediate to the neighboring bridging oxygen sites, thus accelerating bridging-oxygen-assisted deprotonation OER steps in acidic electrolytes. The universality of the strategy is demonstrated for other Ru-M binary metal oxides (M = Cr, Mo, Nb, Ta, and Ti).

While water electrolysis devices represent a technology for renewable energy, there are few stable catalysts that survive the acidic conditions. Here, authors enhance acidic oxygen evolution by introducing strong Brønsted acid sites into RuO₂ to accelerate bridging-oxygen-assisted deprotonation.

Recent Advances in Heterogeneous Electroreduction of CO2 on Copper-Based Catalysts

Facing greenhouse effects and the rapid exhaustion of fossil fuel, CO2 electrochemical reduction presents a promising method of environmental protection and energy transformation. Low onset potential, large current density, high faradaic efficiency (FE), and long-time stability are required for industrial production, due to economic costs and energy consumption. This minireview showcases the recent progress in catalyst design and engineering technology in CO2 reduction reaction (CO2RR) on copper based-catalysts. We focus on strategies optimizing the performance of copper-based catalysts, such as single-atom catalysts, doping, surface modification, crystal facet engineering, etc., and reactor design including gas diffusion layer, membrane electrode assembly, etc., in enhancing target electroreduction products including methane, methanol, ethylene, and C2+ oxygenates. The determination of the correlation and the developed technology might be helpful for future applications in the industry.

Sustainable oxygen evolution electrocatalysis in aqueous 1 M H2SO4 with earth abundant nanostructured Co3O4

Earth-abundant electrocatalysts for the oxygen evolution reaction (OER) able to work in acidic working conditions are elusive. While many first-row transition metal oxides are competitive in alkaline media, most of them just dissolve or become inactive at high proton concentrations where hydrogen evolution is preferred. Only noble-metal catalysts, such as IrO₂, are fast and stable enough in acidic media. Herein, we report the excellent activity and long-term stability of Co₃O₄-based anodes in 1 M H₂SO₄ (pH 0.1) when processed in a partially hydrophobic carbon-based protecting matrix. These Co₃O₄@C composites reliably drive O₂ evolution a 10 mA cm^-2 current density for >40 h without appearance of performance fatigue, successfully passing benchmarking protocols without incorporating noble metals. Our strategy opens an alternative venue towards fast, energy efficient acid-media water oxidation electrodes.

General Synthesis of Tube-like Nanostructured Perovskite Oxides with Tunable Transition Metal-Oxygen Covalency for Efficient Water Electrooxidation in Neutral Media

Hydrogen production from water electrolysis in neutral-pH electrolytes can not only avoid the corrosion and safety issues and expand the catalyst option but also potentially integrate with artificial photosynthesis and bioelectrocatalysis. However, heterogeneous catalysts that can efficiently negotiate the sluggish oxygen evolution reaction (OER) in neutral solutions are considerably lacking. Herein, we report a template-assisted strategy for the synthesis of 13 kinds of tube-like nanostructured perovskite oxides (TNPOs) with markedly high Brunauer-Emmett-Teller surface areas. By systematic examination of these TNPOs, we found that the OER activity of TNPOs in neutral solution exhibits a volcano shape as a function of the covalency of transition metal-oxygen bonds. Consequently, our designed Sm-doped LaCoO₃ catalyst yields a geometric current density of 8.5 mA cm^-2 at 1.75 V versus the reversible hydrogen electrode in 1 M phosphate buffer solution (pH 7) due to the optimized covalency of Co 3d and O 2p states, representing the most active noble-metal-free OER catalyst in neutral electrolytes reported as yet.

RuO2 electronic structure and lattice strain dual engineering for enhanced acidic oxygen evolution reaction performance

Developing highly active and durable electrocatalysts for acidic oxygen evolution reaction remains a great challenge due to the sluggish kinetics of the four-electron transfer reaction and severe catalyst dissolution. Here we report an electrochemical lithium intercalation method to improve both the activity and stability of RuO₂ for acidic oxygen evolution reaction. The lithium intercalates into the lattice interstices of RuO₂, donates electrons and distorts the local structure. Therefore, the Ru valence state is lowered with formation of stable Li-O-Ru local structure, and the Ru–O covalency is weakened, which suppresses the dissolution of Ru, resulting in greatly enhanced durability. Meanwhile, the inherent lattice strain results in the surface structural distortion of Li_xRuO₂ and activates the dangling O atom near the Ru active site as a proton acceptor, which stabilizes the OOH* and dramatically enhances the activity. This work provides an effective strategy to develop highly efficient catalyst towards water splitting.

While water splitting in acid offers higher operational performances than in alkaline conditions, there are few high-activity, acid-stable oxygen evolution electrocatalysts. Here, authors examine electrochemical Li intercalation to improve the activity and stability of RuO₂ for acidic water oxidation.

Activity-Stability Relationships in Oxide Electrocatalysts for Water Electrolysis

The oxygen evolution reaction (OER) is one of the key kinetically limiting half reactions in electrochemical energy conversion. Model epitaxial catalysts have emerged as a platform to identify structure-function-relationships at the atomic level, a prerequisite to establish advanced catalyst design rules. Previous work identified an inverse relationship between activity and the stability of noble metal and oxide OER catalysts in both acidic and alkaline environments: The most active catalysts for the anodic OER are chemically unstable under reaction conditions leading to fast catalyst dissolution or amorphization, while the most stable catalysts lack sufficient activity. In this perspective, we discuss the role that epitaxial catalysts play in identifying this activity-stability-dilemma and introduce examples of how they can help overcome it. After a brief review of previously observed activity-stability-relationships, we will investigate the dependence of both activity and stability as a function of crystal facet. Our experiments reveal that the inverse relationship is not universal and does not hold for all perovskite oxides in the same manner. In fact, we find that facet-controlled epitaxial La_0.6Sr_0.4CoO_3-δ catalysts follow the inverse relationship, while for LaNiO_3-δ, the (111) facet is both the most active and the most stable. In addition, we show that both activity and stability can be enhanced simultaneously by moving from La-rich to Ni-rich termination layers. These examples show that the previously observed inverse activity-stability-relationship can be overcome for select materials and through careful control of the atomic arrangement at the solid-liquid interface. This realization re-opens the search for active and stable catalysts for water electrolysis that are made from earth-abundant elements. At the same time, these results showcase that additional stabilization via material design strategies will be required to induce a general departure from inverse stability-activity relationships among the transition metal oxide catalysts to ultimately grant access to the full range of available oxides for OER catalysis.

The Ir-OOOO-Ir transition state and the mechanism of the oxygen evolution reaction on IrO2(110)

Carefully assessing the energetics along the pathway of the oxygen evolution reaction (OER), our computational study reveals that the "classical" OER mechanism on the (110) surface of iridium dioxide (IrO₂) must be reconsidered. We find that the OER follows a bi-nuclear mechanism with adjacent top surface oxygen atoms as fixed adsorption sites, whereas the iridium atoms underneath play an indirect role and maintain their saturated 6-fold oxygen coordination at all stages of the reaction. The oxygen molecule is formed, via an Ir-OOOO-Ir transition state, by association of the outer oxygen atoms of two adjacent Ir-OO surface entities, leaving two intact Ir-O entities at the surface behind. This is drastically different from the commonly considered mono-nuclear mechanism where the O₂ molecule evolves by splitting of the Ir-O bond in an Ir-OO entity. We regard the rather weak reducibility of crystalline IrO₂ as the reason for favoring the novel pathway, which allows the Ir-O bonds to remain stable and explains the outstanding stability of IrO₂ under OER conditions. The establishment of surface oxygen atoms as fixed electrocatalytically active sites on a transition-metal oxide represents a paradigm shift for the understanding of water oxidation electrocatalysis, and it reconciles the theoretical understanding of the OER mechanism on iridium oxide with recently reported experimental results from operando X-ray spectroscopy. The novel mechanism provides an efficient OER pathway on a weakly reducible oxide, defining a new strategy towards the design of advanced OER catalysts with combined activity and stability.

Effects of the Catalyst Dynamic Changes and Influence of the Reaction Environment on the Performance of Electrochemical CO2 Reduction

Electrochemical reduction of carbon dioxide (CO₂ ) is substantially researched due to its potential for storing intermittent renewable electricity and simultaneously helping mitigating the pressing CO₂ emission concerns. The major challenge of electrochemical CO₂ reduction lies on having good controls of this reaction due to its complicated reaction networks and its unusual sensitivity to the dynamic changes of the catalyst structure (chemical states, compositions, facets and morphology, etc.), and to the non-catalyst components at the electrode/electrolyte interface, in another word the reaction environments. To date, a comprehensive analysis on the interplays between the above catalyst-dynamic-changes/reaction environments and the CO₂ reduction performance is rare, if not none. In this review, the catalyst dynamic changes observed during the catalysis are discussed based on the recent reports of electrochemical CO₂ reduction. Then, the above dynamic changes are correlated to their effects on the catalytic performance. The influences of the reaction environments on the performance of CO₂ reduction are also discussed. Finally, some perspectives on future investigations are offered with the aim of understanding the origins of the effects from the catalyst dynamic changes and the reaction environments, which will allow one to better control the CO₂ reduction toward the desired products.

Regulating the electronic structures of mixed B-site pyrochlore to enhance the turnover frequency in water oxidation

This paper describes the development of mixed B-site pyrochlore Y2MnRuO7 electrocatalyst for oxygen evolution reaction (OER) in acidic media, a challenge for the development of low-temperature electrolyzer for green hydrogen production. Recently, several theories have been developed to understand the reaction mechanism for OER, though there is an  uncertainty in most of the cases, due to the complex surface structures. Several key factors such as lattice oxygen, defect, electronic structure, oxidation state, hydroxyl group and conductivity were identified and shown to be important to the OER activity. The contribution of each factor to the performance however is often not well understood, limiting their impact in guiding the design of OER electrocatalysts. In this work, we showed mixed B-site pyrochlore Y2MnRuO7 catalyst exhibits 14 times higher turnover frequency (TOF) than RuO2 while maintaining a low overpotential of ~ 300 mV for the entire testing period of 24 h in acidic electrolyte. X-ray photoelectron spectroscopy (XPS) analysis reveals that this B-site mixed pyrochlore Y2MnRuO7 has a higher oxidation state of Ru than those of Y2Ru2O7, which could be crucial for improving OER performance as the broadened and lowered Ru 4d band resulted from the B-site substitution by Mn is beneficial to the OER kinetics.

CO2/carbonate-mediated electrochemical water oxidation to hydrogen peroxide

Electrochemical water oxidation reaction (WOR) to hydrogen peroxide (H₂O₂) via a 2e⁻ pathway provides a sustainable H₂O₂ synthetic route, but is challenged by the traditional 4e⁻ counterpart of oxygen evolution. Here we report a CO₂/carbonate mediation approach to steering the WOR pathway from 4e⁻ to 2e⁻. Using fluorine-doped tin oxide electrode in carbonate solutions, we achieved high H₂O₂ selectivity of up to 87%, and delivered unprecedented H₂O₂ partial currents of up to 1.3 A cm⁻², which represents orders of magnitude improvement compared to literature. Molecular dynamics simulations, coupled with electron paramagnetic resonance and isotope labeling experiments, suggested that carbonate mediates the WOR pathway to H₂O₂ through the formation of carbonate radical and percarbonate intermediates. The high selectivity, industrial-relevant activity, and good durability open up practical opportunities for delocalized H₂O₂ production.

Electrochemical H₂O oxidation to H₂O₂ is challenged by the competitive O₂ evolution reaction. Here, the authors report a CO₂/carbonate mediation approach to steering the H₂O oxidation pathway from O₂ evolution to H₂O₂ generation.

Efficient and stable noble-metal-free catalyst for acidic water oxidation

Developing non-noble catalysts with superior activity and durability for oxygen evolution reaction (OER) in acidic media is paramount for hydrogen production from water. Still, challenges remain due to the inadequate activity and stability of the OER catalyst. Here, we report a cost-effective and stable manganese oxybromide (Mn_7.5O₁₀Br₃) catalyst exhibiting an excellent OER activity in acidic electrolytes, with an overpotential of as low as 295 ± 5 mV at a current density of 10 mA cm⁻². Mn_7.5O₁₀Br₃ maintains good stability under operating conditions for at least 500 h. In situ Raman spectroscopy, X ray absorption near edge spectroscopy, and density functional theory calculations confirm that a self-oxidized surface with enhanced electronic transmission capacity forms on Mn_7.5O₁₀Br₃ and is responsible for both the high catalytic activity and long-term stability during catalysis. The development of Mn_7.5O₁₀Br₃ as an OER catalyst provides crucial insights into the design of non-noble metal electrocatalysts for water oxidation.

While acidic water splitting offers a renewable means to obtain renewable hydrogen fuel, the catalysts needed to oxidize water often require expensive noble metals. Here, authors show manganese oxyhalides as acidic oxygen evolution electrocatalysts.

Research progress in improving the oxygen evolution reaction by adjusting the 3d electronic structure of transition metal catalysts

As a clean and renewable energy carrier, hydrogen (H₂) has become an attractive alternative to dwindling fossil fuels. The key to realizing hydrogen-based energy systems is to develop efficient and economical hydrogen production methods. The water electrolysis technique has the advantages of cleanliness, sustainability, and high efficiency, which can be applied to large-scale hydrogen production. However, the electrocatalytic oxygen evolution reaction (OER) at the anode plays a decisive role in the efficiency of hydrogen evolution during water splitting. Generally, noble metal catalysts (such as ruthenium and iridium) are considered to exhibit the best OER performance; however, they exhibit disadvantages such as high costs, limited reserves, and poor stability. Therefore, the research on highly efficient non-noble metal catalysts that can replace their noble metal counterparts has always been important. This review presents the recent advances in the preparation of high-performance OER electrocatalysts by regulating the electronic structure of 3d transition metals. First, we introduce the reaction mechanism of water splitting and the OER, which reveals the high requirement of the complex four-electron process of the OER. Second, the electron transfer mode and development progress of highly active transition metal electrocatalysts are used to summarize the research situation of transition metal OER catalysts in water splitting. Finally, the future development direction and challenges of transition metal catalysts are prospected based on the current research progress.

Inter-relationships between Oxygen Evolution and Iridium Dissolution Mechanisms

The widespread utilization of proton exchange membrane (PEM) electrolyzers currently remains uncertain, as they rely on the use of highly scarce iridium as the only viable catalyst for the oxygen evolution reaction (OER), which is known to present the major energy losses of the process. Understanding the mechanistic origin of the different activities and stabilities of Ir-based catalysts is, therefore, crucial for a scale-up of green hydrogen production. It is known that structure influences the dissolution, which is the main degradation mechanism and shares common intermediates with the OER. In this Minireview, the state-of-the-art understanding of dissolution and its relationship with the structure of different iridium catalysts is gathered and correlated to different mechanisms of the OER. A perspective on future directions of investigation is also given.

Self-healing oxygen evolution catalysts

Electrochemical and photoelectrochemical water splitting offers a scalable approach to producing hydrogen from renewable sources for sustainable energy storage. Depending on the applications, oxygen evolution catalysts (OECs) may perform water splitting under a variety of conditions. However, low stability and/or activity present challenges to the design of OECs, prompting the design of self-healing OECs composed of earth-abundant first-row transition metal oxides. The concept of self-healing catalysis offers a new tool to be employed in the design of stable and functionally active OECs under operating conditions ranging from acidic to basic solutions and from a variety of water sources.

Large scale sustainable energy storage by water splitting benefits from performing the oxygen evolution reaction under a variety of conditions. Here, the authors discuss self-healing catalysis as a new tool in the design of stable and functionally active catalysts in acidic to basic solutions, and a variety of water sources

Strategies for Electrochemically Sustainable H2 Production in Acid

Acidified water electrolysis with fast kinetics is widely regarded as a promising option for producing H2 . The main challenge of this technique is the difficulty in realizing sustainable H2 production (SHP) because of the poor stability of most electrode catalysts, especially on the anode side, under strongly acidic and highly polarized electrochemical environments, which leads to surface corrosion and performance degradation. Research efforts focused on tuning the atomic/nano structures of catalysts have been made to address this stability issue, with only limited effectiveness because of inevitable catalyst degradation. A systems approach considering reaction types and system configurations/operations may provide innovative viewpoints and strategies for SHP, although these aspects have been overlooked thus far. This review provides an overview of acidified water electrolysis for systematic investigations of these aspects to achieve SHP. First, the fundamental principles of SHP are discussed. Then, recent advances on design of stable electrode materials are examined, and several new strategies for SHP are proposed, including fabrication of symmetrical heterogeneous electrolysis system and fluid homogeneous electrolysis system, as well as decoupling/hybrid-governed sustainability. Finally, remaining challenges and corresponding opportunities are outlined to stimulate endeavors toward the development of advanced acidified water electrolysis techniques for SHP.

Electrochemical Nitrate Production via Nitrogen Oxidation with Atomically Dispersed Fe on N-Doped Carbon Nanosheets

Electrocatalytic N₂ oxidation (NOR) into nitrate is a potential alternative to the emerging electrochemical N₂ reduction (NRR) into ammonia to achieve a higher efficiency and selectivity of artificial N₂ fixation, as O₂ from the competing oxygen evolution reaction (OER) potentially favors the oxygenation of NOR, which is different from the parasitic hydrogen evolution reaction (HER) for NRR. Here, we develop an atomically dispersed Fe-based catalyst on N-doped carbon nanosheets (AD-Fe NS) which exhibits an exceptional catalytic NOR capability with a record-high nitrate yield of 6.12 μ mol mg^-1 h^-1 (2.45 μ mol cm^-2 h^-1) and Faraday efficiency of 35.63%, outperforming all reported NOR catalysts and most well-developed NRR catalysts. The isotopic labeling NOR test validates the N source of the resultant nitrate from the N₂ electro-oxidation catalyzed by AD-Fe NS. Experimental and theoretical investigations identify Fe atoms in AD-Fe NS as active centers for NOR, which can effectively capture N₂ molecules and elongate the N≡N bond by the hybridization between Fe 3d orbitals and N 2p orbitals. This hybridization activates N₂ molecules and triggers the subsequent NOR. In addition, a NOR-related pathway has been proposed that reveals the positive effect of O₂ derived from the parasitic OER on the NO₃^- formation.