Long-Term Stability Challenges and Opportunities in Acidic Oxygen Evolution Electrocatalysis

Simple and Scalable Introduction of Single-Atom Mn on RuO2 Electrocatalysts for Oxygen Evolution Reaction with Long-Term Activity and Stability

Electrochemical oxygen evolution reaction (OER) is the bottleneck for realizing renewable powered green hydrogen production through water splitting due to the challenges of electrode stability under harsh oxidative environments and electrolytes with extreme acidity and basicity. Here, we introduce a single-atom manganese-incorporated ruthenium oxide electrocatalyst via a facile impregnation approach for catalyzing the OER across a wide pH range, while solving the stability issues of RuO2. The modified catalyst maintains stability for over 1000 h, delivering a current density of 10 mA cm-2 at a 213 mV overpotential in acid (pH 0), 570 mV in potassium bicarbonate (pH 8.8), and 293 mV in alkaline media (pH 14), demonstrating exceptional durability under various conditions. When used as an anode for realistic water-splitting systems, Mn-modified RuO2 performs at 1000 mA cm-2 with a voltage of 1.69 V (Nafion 212 membrane) for proton-exchange membrane water electrolysis, and 1.84 V (UTP 220 diaphragm) for alkaline water electrolysis, exhibiting low degradation and verifying its substantial potential for practical applications.

Single-Atom Ru-Triggered Lattice Oxygen Redox Mechanism for Enhanced Acidic Water Oxidation

Activating the oxygen anionic redox presents a promising avenue for developing highly active oxygen evolution reaction (OER) electrocatalysts for proton-exchange membrane water electrolyzers (PEMWE). Here, we engineered a lattice-confined Ru single atom dispersed on a lamellar manganese oxide (MnO2) cation site. The strong Ru-O bond induced an upward shift in the O 2p band, enhancing metal-oxygen covalency and reshaping the OER mechanism toward lattice oxygen oxidation pathway with increased activity. In situ spectral characterization combined with density functional theory (DFT) calculations revealed that electron transfer from Mn to Ru alleviates the Jahn-Teller effect within the MnO6 octahedral structure, stabilizing the lattice. The layered Ru/MnO2 architecture also promotes the rapid replenishment of oxygen vacancies, preventing structural collapse. As a result, the optimized Ru/MnO2 electrocatalyst achieves an OER overpotential of only 179 mV at 10 mA cm-2 in 0.5 M H2SO4, along with exceptional durability over 1000 h at 100 mA cm-2. Moreover, the Ru/MnO2-based PEM device requires only 1.71 V to reach 1 A cm-2 and shows a durability of 500 h at 500 mA cm-2.

Synergistic niobium and manganese co-doping into RuO2 nanocrystal enables PEM water splitting under high current

Low-cost ruthenium-based catalysts with high activity have emerged as promising alternatives to iridium-based counterparts for acidic oxygen evolution reaction (OER) in proton exchange membrane water electrolyzers (PEMWE), but the poor stability under high current density remains as a key challenge. Here, we utilize the synergistic complementary strategy of introducing earth-abundant Mn and Nb dopants in ruthenium dioxide (RuO2) for Nb0.1Mn0.1Ru0.8O2 nanoparticle electrocatalyst that exhibits a low overpotential of 209 mV at 10 mA cm-2 and good stability of > 400 h at 0.2 A cm-2 in 0.5 M H2SO4. Significantly, a PEMWE device fabricated with Nb0.1Mn0.1Ru0.8O2 anode can operate continuously at least for 1000 h at 0.5 A cm-2 with 59 μV h-1 decay rate. Operando Raman spectroscopy analysis, differential electrochemical mass spectroscopy measurements, X-ray absorption spectroscopy analysis and theoretical calculations indicate that OER reaction on Nb0.1Mn0.1Ru0.8O2 primarily follows the adsorbate evolution mechanism with much favorable energy barrier accompanied by a locally passivated lattice oxygen mechanism (AEM-LPLOM) and the co-existed Nb and Mn in RuO2 crystal lattice could not only stabilize the lattice oxygen, but also relieve the valence state fluctuation of Ru site to stabilize the catalyst during the reaction.

Topology-Sensitive Spin-Selecting Super-Exchange Interactions at Half-Antiperovskites with Orderly-Oxidized Semimetals for Acidic Water Oxidation

Developing inexpensive catalysts for acidic water oxidation plays a critical role in energy conversions and storages, but which have to trade off their catalytic activity and electrochemical stability. Different from traditional nano-engineering, herein we focus onto manipulating the spin-dependent topological quantum interactions to facilitate acidic water splitting at half-antiperovskite (Ni2Co1In2S2) with orderly-oxidized Kagome lattices, in which spatial wave functions of the triangular metal units (M3) are spontaneously converted to the anti-symmetry feature due to the symmetry breaking from controllable bridged-oxygen decorations (M3O), leading to a spin-selecting-dependent electronic reconfiguration. This topology-sensitive symmetry breaking makes the super-exchange interactions among neighboring metal sites switch to the indirect out-plane configuration (M─O─M) from the direct in-plane form (M─M), meanwhile the spin-dependent transformation from antiferromagnetic states to ferromagnetic states causes these metal sites to demonstrate a semi-metallic property for optimizing their bonding interactions with reactants. As a result, the capacity of carrier migration and intermediate diffusion in the inner Helmholtz region is significantly improved, which makes mass activity increase to 20.5 A g-1 at an overpotential of ≈356 mV, demonstrating an obvious superiority over the other state-of-the-art catalysts without noble metals. This work provides a new insight for designing topology-dependent catalysts to better understand spin-related reaction kinetics.

Effect of ionic-bonding d0 cations on structural durability in barium iridates for oxygen evolution electrocatalysis

Iridium has the exclusive chemistry guaranteeing both high catalytic activity and sufficient corrosion resistance in a strong acidic environment under anodic potential. Complex iridates thus attract considerable attention as high-activity electrocatalysts with less iridium utilization for the oxygen evolution reaction (OER) in water electrolyzers using a proton-exchange membrane. Here we demonstrate the effect of chemical doping on the durability of hexagonal-perovskite Bax(M,Ir)yOz-type iridates in strong acid (pH ~ 0). Some aliovalent cations are directly visualized to periodically locate at the octahedral sites bridging the two face-sharing [Ir2O9] dimer or [Ir3O12] trimers in hexagonal-perovskite polytypes. In particular, highly ionic bonding of the d0 Nb5+ and Ta5+ cations with oxygen anions results in notable suppression of lattice oxygen participation during the OER and thus effectively preserves the connectivity between the [Ir3O12] trimers without lattice collapse. Providing an in-depth understanding of the correlation between the electronic structure and bonding nature in crystals, our work suggests that proper control of chemical doping in complex oxides promises a simple but efficient tool to realize OER electrocatalysts with markedly improved durability.

Tuning the Catalytic Activity of MoS2-x-NbSx Heterostructure Nanosheets for Bifunctional Acidic Water Splitting

Developing durable electrocatalysts with high activity for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in acidic media is critically important for clean power production. In this study, MoS2-x-NbSx heterostructure nanosheets are synthesized from a solid-state reaction method followed by liquid phase exfoliation, and their catalytic performance is optimized. The MoS2-x-NbSx heterostructure nanosheets with optimal precursors ratio exhibit promising attributes for applications in the HER and OER compared to pristine MoS2 and Nb under the same conditions. The MoS2-x-NbSx heterostructure nanosheets catalyst on glassy carbon electrodes shows the minimum overpotential of 159 mV for HER and 295 mV for OER at a current density of 10 mA cm-2 in 0.5 m H2SO4. This research offers valuable insights into the fabrication of heterostructure nanosheets and evaluates their potential as effective electrocatalysts for water splitting compared with pristine 2D materials in an acid environment.

Environmentally Friendly and Earth-Abundant Self-Healing Electrocatalyst Systems for Durable and Efficient Acidic Water Splitting

Electrochemical water splitting under acidic conditions is an efficient route for green hydrogen production from renewable electricity. Its implementation on a globally relevant scale is hindered by the lack of abundant and low-cost electrocatalysts for the oxygen evolution reaction that can operate stably and efficiently under highly acidic anodic conditions. Here, we report the design of stable and efficient acidic OER electrocatalysts consisting of a self-healing bismuth (Bi)-based matrix hosting transition metal active sites. Comprehensive structural performance investigation of Co- and Ni-BiOx electrodes provides insights into the role of the electrolyte composition and pH in the self-healing mechanism under anodic conditions. Our best-performing [Co-Bi]Ox and [Ni-Bi]Ox anodes achieve over 200 h of continuous electrolysis at a catalytic current of 10 mA cm-2 with an overpotential of 590 and 670 mV at a pH of 1 in a 0.1 M H2SO4 electrolyte. Notably, while the [Bi]Ox matrix did not contribute to the catalytic activity, it was essential to stabilize the active Co and Ni sites during the acidic OER. Our findings provide a promising strategy for the engineering of earth-abundant materials for efficient acidic water splitting, as an alternative to the use of poorly scalable and expensive noble metal catalysts.

Atomic Ga triggers spatiotemporal coordination of oxygen radicals for efficient water oxidation on crystalline RuO2

Advancements in proton-exchange membrane water electrolyzer depend on developing oxygen evolution reaction electrocatalysts that synergize high activity with stability. Here, we introduce an approach aimed at elevating oxygen evolution reaction performance by enhancing the spatiotemporal coordination of oxygen radicals to promote efficient O-O coupling. A dense, single-atom configuration of oxygen radical donors within interconnected RuO2 nanocrystal framework is demonstrated. The stable oxygen radicals on gallium sites with adaptable Ga-O bonds are thermodynamically favorable to attract those from Ru sites, addressing dynamic adaptation challenges and boosting O-O coupling efficiency. The optimized catalyst achieves a low overpotential of 188 mV at 10 mA cm-2, operates robustly for 800 h at 100 mA cm-2 in acidic conditions, and shows a large current density of 3 A cm-2 at 1.788 V, with stable performance at 0.5 A cm-2 for 200 h, confirming its long-term viability in proton-exchange membrane water electrolyzer applications.

Preparation of Highly Efficient All-pH Bifunctional Water Electrolysis Catalysts Through a Surface Modification Strategy

Electrolytic hydrogen production from water is a very promising technology, and catalysts capable of efficient operation over a wide pH range are essential for energy storage and conversion. Herein, a trace Ru catalytic core restructures nickel foam (NF) under polymeric protection, with temperature gradient control forming HER-active metal monomers at low temperatures and OER-suitable oxides at high temperatures. It is demonstrated that the surface modification strategy can help NF to maintain its own backbone structure during the carbonation process and that the resulting catalysts possess excellent properties. The synthesized catalysts-Ru@NF-KPDA-550 exhibit the lowest OER overpotentials of 183 mV in 0.5 M H2SO4 and 151 mV in 1.0 M KOH, and Ru@NF-KPDA-350 exhibits the lowest HER overpotentials of 11.8 mV in 0.5 M H2SO4 and 13.4 mV in 1.0 M KOH for Ru@NF-KPDA-350 at 10 mA cm-2. The DFT simulations show that the synergistic interaction between Ru and Ni components, which optimizes their d-band centers, enhances the HER and OER pathways, thereby lowering activation barriers and boosting catalytic performance. This work provides a viable strategy for the design of pH-universal electrocatalysts for the overall water splitting.

Mesoporous Single-Crystalline Particles as Robust and Efficient Acidic Oxygen Evolution Catalysts

The scarcity of iridium (Ir) limits its widespread use in acidic oxygen evolution reaction (OER). Herein, mesoporous single-crystalline spinel Co3O4 with atomically dispersed low-valence-state Ir has been developed to enable Ir's efficient and stable utilization. The surface Pourbaix diagram suggests that under acidic OER conditions, O* fully covers both Co3O4(111) and (110) surfaces, passivating Co sites but enhancing Co3O4's structural stability, a benefit further improved by Ir doping. Mesopores offer numerous loading sites for Ir single atoms (13.8 wt %), which activate the originally O*-passivated Co3O4(111) surface by creating high-intrinsic-activity Co-Ir bridge sites; meanwhile, Ir and Co leaching rates are reduced to about 1/4 and 1/5, respectively, compared to conventional Ir/Co3O4 catalysts. Our catalyst exhibits a low η10 of 248 mV for over 100 h, showcasing its potential in water electrolysis.

Microenvironment Tailoring for Electrocatalytic CO2 Reduction: Effects of Interfacial Structure on Controlling Activity and Selectivity

The performance of the electrocatalytic CO2 reduction reaction (CO2RR) is highly dependent on the microenvironment around the cathode. Despite efforts to optimize the microenvironment by modifying nanostructured catalysts or microporous gas diffusion electrodes, their inherent disorder presents a significant challenge to understanding how interfacial structure arrangement within the electrode governs the microenvironment for CO2RR. This knowledge gap limits fundamental understanding of CO2RR while also hindering efforts to enhance CO2RR selectivity and activity. In this work, we investigate this knowledge gap using a tunable system featuring superhydrophobic hierarchical Cu nanowire arrays with microgrooves (NAMs). Adjusting the NAM structure tunes multiple synergistic effects in the microenvironment, which include stabilization of the microwetting state, confinement of CO*, improvement to local CO2 concentration, and modulation of the local pH. Notably, using mass transport modeling, we quantify the role of the gas-liquid-solid interface in boosting local CO2 concentrations within several microns of the interface itself. Leveraging these effects, we elucidate how CO* and H* competitively occupy active sites, influencing reaction pathways toward multicarbon products based on tuning the microenvironment. Consequently, we provide new insights into why the optimized configuration significantly increased CO2RR activity by 690% (as normalized by electrochemical active surface area), C2+ product selectivity by 72%, and Faradaic efficiency by 36%, compared to CO2RR with hydrophobic Cu foil. Based on these insights, our findings unlock new opportunities to engineer the CO2RR microenvironment through the rational organization of hierarchical interface materials in gas diffusion electrodes toward improved CO2RR selectivity and activity.

Modulating Built-In Electric Field Via N-Doped Carbon Dots for Robust Oxygen Evolution at Large Current Density

Constructing a built-in electric field (BIEF) within heterostructures has emerged as a compelling strategy for advancing electrocatalytic oxygen evolution reaction (OER) performance. Herein, the p-n type nanosheet array heterojunction Ni2P-NCDs-Co(OH)2-NF are successfully prepared. The variation in interaction affinity between nitrogen within N-doped carbon dots (NCDs) and Ni/Co induces charge redistribution between Co and Ni in the Ni2P-NCDs-Co(OH)2-NF-3 heterostructure, thereby enhancing the intensity of the BIEF, facilitating electron transfer, and markedly improving OER activity. The optimized electrocatalyst, Ni2P-NCDs-Co(OH)2-NF-3, demonstrates a remarkably low overpotential of 389 mV at 500 mA cm-2, alongsides a small Tafel slope of 65 mV dec-1, expansive electrochemical active surface area (ECSA), low impedance, outstanding stability exceeding 425 h at 500 mA cm-2, and a Faradaic efficiency of up to 96%. In situ Raman spectroscopy and density functional theoretical (DFT) calculations elucidate the OER mechanism, revealing that the enhanced BIEF optimizes the adsorption energy of Co3+ to OH- and weakened the desorption energy of oxygen during the reaction. The work ponieeringly employed the NCDs as a regulator of the BIEF, effectively tuning field intensity and achieving superior electrocatalytic OER performance under large current density, thus charting new pathways for the development of high-efficiency oxygen evolution electrocatalysts.

Formation of electron-deficient Ni in a Nb/NiFe-layered double hydroxide nanoarray via electrochemical activation for efficient water oxidation

The intrinsically sluggish kinetics of the oxygen evolution reaction (OER) at the anode poses a formidable challenge to the industrial application of water electrolysis, although NiFe-based oxides and hydroxides have emerged as promising anodic candidates. Within this framework, we report the synthesis of Nb-doped NiFe-layered double hydroxides (Nb/NiFe-LDH) via a straightforward one-step hydrothermal approach. Notably, Nb doping maintained the structural integrity of the NiFe-LDH framework and it enhanced the valence state of the active Ni species during the electrochemical activation process, which accelerated the concomitant reconstruction kinetics of the LDH phase. As a result, Nb/NiFe-LDH demonstrated a remarkable overpotential of 198 mV to attain a current density of 10 mA cm-2 in an alkaline electrolyte. This work proposes a novel doping strategy for enhancing the performance of OER electrocatalysts.

Recent Advances in Polymer Electrolyte Membrane Water Electrolyzer Stack Development Studies: A Review

Polymer electrolyte membrane water electrolyzers have significant advantages over other electrolyzers, such as compact design, high efficiency, low gas permeability, fast response, high-pressure operation (up to 200 bar), low operating temperature (20-80 °C), lower power consumption, and high current density. Moreover, polymer electrolyte membrane water electrolyzers are a promising technology for sustainable hydrogen production due to their easy adaptability to renewable energy sources. However, the cost of expensive electrocatalysts and other construction equipment must be reduced for the widespread usage of polymer electrolyte membrane water electrolyzer technology. In this review, recent improvements made in developing the polymer electrolyte membrane water electrolyzer stack are summarized. First, we present a brief overview of the working principle of polymer electrolyte membrane water electrolyzers. Then, we discuss the components of polymer electrolyte membrane water electrolyzers (base materials such as membranes, gas diffusion layers, electrocatalysts, and bipolar plates) and their particular functions. We also provide an overview of polymer electrolyte membrane water electrolyzer's material technology, production technology, and commercialization issues. We finally present recent advancements of polymer electrolyte membrane water electrolyzer stack developments and their recent developments under different operating conditions.

Regulation of Electron and Mass Transport Pathways in Efficient and Stable Low-Loading PEM Water Electrolyzers

Improving the utilization of iridium in proton exchange membrane (PEM) water electrolyzer is critical in reducing their cost for future development. Titanium dioxide (TiO2) has notable electrochemical stability at high operating potential and has been developed as a promising support of iridium-based OER nano-catalysts. However, limited by insufficient conductivity, the iridium content on TiO2 support catalysts is normally above 50 wt.%. Herein, support is provided for iridium on conductivity-enhanced TiO2 for low-iridium-loading PEMWE, successfully reducing the iridium content to 28 wt.% by the regulation of electron transport pathway. A new ionomer distribution strategy is then applied to the Ir@Pt@TiO2 catalyst layer to release the iridium sites and regulate the local mass transport pathways in the anode. This work reveals that the catalyst-ionomer interface played an important role in activity and stability in the anode of PEMWE. Building a thin and uniform ionomer distribution on supports with iridium exposure can result in continuous proton and electron transport pathways, promoting bubble escape, and exposing more effective active sites during reaction situations. This work provides a novel perspective for future research on the catalyst-ionomer interface and mass transport in PEMWEs.

Spillover of active oxygen intermediates of binary RuO2/Nb2O5 nanowires for highly active and robust acidic oxygen evolution

Over-oxidation of surface ruthenium active sites of RuOx-based electrocatalysts leads to the formation of soluble high-valent Ru species and subsequent structural collapse of electrocatalysts, which results in their low stability for the acidic oxygen evolution reaction (OER). Herein, a binary RuO2/Nb2O5 electrocatalyst with abundant and intimate interfaces has been rationally designed and synthesized to enhance its OER activity in acidic electrolyte, delivering a low overpotential of 179 mV at 10 mA cm-2, a small Tafel slope of 73 mV dec-1, and a stabilized catalytic durability over a period of 750 h. Extensive experiments have demonstrated that the spillover of active oxygen intermediates from RuO2 to Nb2O5 and the subsequent participation of lattice oxygen of Nb2O5 instead of RuO2 for the acidic OER suppressed the over-oxidation of surface ruthenium species and thereby improved the catalytic stability of the binary electrocatalysts.

Effectiveness of strain and dopants on breaking the activity-stability trade-off of RuO2 acidic oxygen evolution electrocatalysts

Ruthenium dioxide electrocatalysts for acidic oxygen evolution reaction suffer from mediocre activity and rather instability induced by high ruthenium-oxygen covalency. Here, the tensile strained strontium and tantalum codoped ruthenium dioxide nanocatalysts are synthesized via a molten salt-assisted quenching strategy. The tensile strained spacially elongates the ruthenium-oxygen bond and reduces covalency, thereby inhibiting the lattice oxygen participation and structural decomposition. The synergistic electronic modulations among strontium-tantalum-ruthenium groups both optimize deprotonation on oxygen sites and intermediates absorption on ruthenium sites, lowering the reaction energy barrier. Those result in a well-balanced activity-stability profile, confirmed by comprehensive experimental and theoretical analyses. Our strained electrode demonstrates an overpotential of 166 mV at 10 mA cm-2 in 0.5 M H2SO4 and an order of magnitude higher S-number, indicating comparable stability compared to bare catalyst. It exhibits negligible degradation rates within the long-term operation of single cell and PEM electrolyzer. This study elucidates the effectiveness of tensile strain and strategic doping in enhancing the activity and stability of ruthenium-based catalysts for acidic oxygen evolution reactions.

Switchable Acidic Oxygen Evolution Mechanisms on Atomic Skin of Ruthenium Metallene Oxides

RuO2 has been considered as a promising, low-cost, and highly efficient catalyst in the acidic oxygen evolution reaction (OER). However, it suffers from poor stability due to the inevitable involvement of the lattice oxygen mechanism (LOM). Here, we construct a unique metallene-based core-skin structure and unveil that the OER pathway of atomic RuO2 skin can be regulated from the LOM to an adsorbate evolution mechanism by altering the core species from metallene oxides to metallenes. This switch is achieved without sacrificing the number of active sites, enabling Pd@RuO2 metallenes to exhibit outstanding acidic OER activity with a low overpotential of 189 mV at 10 mA cm-2, which is 54 mV lower than that of the counterpart PdO@RuO2 metallenes. Additionally, they also exhibit robust stability with negligible activity decay over 100 h at 50 mA cm-2, outperforming most reported RuO2-based catalysts. Multiple spectroscopic analyses and theoretical calculations demonstrate that the Pd-metallene core, acting as an electron donor, increases the migration energy of subsurface oxygen atoms and optimizes the adsorption energy of intermediates on the active Ru sites, enabling a switch in the reaction mechanism. Such a unique metallene-based core-skin structure offers a novel way for tuning the catalytic behaviors of electrocatalysts.

Recent Progress in Balancing the Activity, Durability, and Low Ir Content for Ir-Based Oxygen Evolution Reaction Electrocatalysts in Acidic Media

Proton exchange membrane (PEM) electrolysis faces challenges associated with high overpotential and acidic environments, which pose significant hurdles in developing highly active and durable electrocatalysts for the oxygen evolution reaction (OER). Ir-based nanomaterials are considered promising OER catalysts for PEM due to their favorable intrinsic activity and stability under acidic conditions. However, their high cost and limited availability pose significant limitations. Consequently, numerous studies have emerged aimed at reducing iridium content while maintaining high activity and durability. Furthermore, the research on the OER mechanism of Ir-based catalysts has garnered widespread attention due to differing views among researchers. The recent progress in balancing activity, durability, and low iridium content in Ir-based catalysts is summarized in this review, with a particular focus on the effects of catalyst morphology, heteroatom doping, substrate introduction, and novel structure development on catalyst performance from four perspectives. Additionally, the recent mechanistic studies on Ir-based OER catalysts is discussed, and both theoretical and experimental approaches is summarized to elucidate the Ir-based OER mechanism. Finally, the perspectives on the challenges and future developments of Ir-based OER catalysts is presented.

Be Aware of Transient Dissolution Processes in Co3O4 Acidic Oxygen Evolution Reaction Electrocatalysts

Recently, cobalt-based oxides have received considerable attention as an alternative to expensive and scarce iridium for catalyzing the oxygen evolution reaction (OER) under acidic conditions. Although the reported materials demonstrate promising durability, they are not entirely intact, calling for fundamental research efforts to understand the processes governing the degradation of such catalysts. To this end, this work studies the dissolution mechanism of a model Co3O4 porous catalyst under different electrochemical conditions using online inductively coupled plasma mass spectrometry (online ICP-MS), identical location scanning transmission electron microscopy (IL-STEM), and differential electrochemical mass spectrometry (DEMS). Despite the high thermodynamics tendency reflected in the Pourbaix diagram, it is shown that the cobalt dissolution kinetics is sluggish and can be lowered further by modifying the electrochemical protocol. For the latter, identified in this study, several (electro)chemical reaction pathways that lead to the dissolution of Co3O4 must be considered. Hence, this work uncovers the transient character of cobalt dissolution and provides valuable insights that can help to understand the promising stability of cobalt-based materials in already published works and facilitate the knowledge-driven design of novel, stable, abundant catalysts toward the OER in an acidic environment.

Electron-Donating Zr Induces Suppressed Ru Over-Oxidation and Accelerated Deprotonation Process Toward Efficient and Durable Water Electrolysis

The scarcity of cost-effective and durable iridium-free anode electrocatalysts for the oxygen evolution reaction (OER) poses a significant challenge to the widespread application of the proton exchange membrane water electrolyzer (PEMWE). To address the electrochemical oxidation and dissolution issues of Ru-based electrocatalysts, an electron-donating modification strategy is developed to stabilize WRuOx under harsh oxidative conditions. The optimized catalyst with a low Zirconium doping (Zr, 1 wt.%) enhances durability noticeably, with a 77% reduction in degradation rate in the durability test of 10 mA cm-2 in 0.5 m H2SO4. When integrated into a homemade PEMWE device, the Zr-doped catalyst achieves excellent long-term stability, lasting up to 650 h at 100 mA cm⁻2. Additionally, the electronic modulation from the Zr modification leads to superior activity with a low overpotential of 208 mV at 10 mA cm-2. Theoretical calculation results further reveal that electron-donating Zr modification effectively suppresses Ru overoxidation and lattice oxygen participation, maintaining a robust structure during acidic OER. This modification also promotes deprotonation through stronger Brønsted acid sites, significantly improving both long-term stability and activity.

Importing Atomic Rare-Earth Sites to Activate Lattice Oxygen of Spinel Oxides for Electrocatalytic Oxygen Evolution

Spinel oxides have emerged as highly active catalysts for the oxygen evolution reaction (OER). Owing to covalency competition, the OER process on spinel oxides often follows an arduous adsorbate evolution mechanism (AEM) pathway. Herein, we propose a novel rare-earth sites substitution strategy to tune the lattice oxygen redox of spinel oxides and bypass the AEM scaling relationship limitation. Taking NiCo2O4 as a model, the incorporation of Ce into the octahedral site induces the formation of Ce-O-M (M=Ni, Co) bridge, which triggers charge redistribution within NiCo2O4. The developed Ce-NiCo2O4 exhibits remarkable OER activity with a low overpotential, satisfactory electrochemical stability, and good practicability in anion-exchange membrane water electrolyzer. Theoretical analyses reveal that OER on Ce-NiCo2O4 surface follows a more favorable lattice oxygen mechanism (LOM) pathway and non-concerted proton-electron transfers compared to pure NiCo2O4, as also verified by pH-dependent behavior and in situ Raman analysis. The 18O-labeled electrochemical mass spectrometry provides direct evidence that the oxygen released during the OER originates from the lattice oxygen of Ce-NiCo2O4. We discover that electron delocalization of Ce 4f states triggers charge redistribution in NiCo2O4 through the Ce-O-M bridge, favoring antibonding state occupation of Ni-O bonding in [Ce-O-Ni] unit site, thereby activating lattice oxygen redox of NiCo2O4 in OER. This work provides a new perspective for designing highly active spinel oxides for OER and offers significant insights into the rare-earth-enhanced LOM mechanism.

Research Progress of High-entropy Oxides for Electrocatalytic Oxygen Evolution Reaction

High-entropy oxides (HEOs), similar to high-entropy materials (HEMs), have "four-core effects", i. e., high-entropy effect, delayed diffusion effect, lattice distortion effect and cocktail effect, which have attracted more and more attention in the scientific field of renewable energy technology due to their unique structural characteristics, variable chemical composition and corresponding functional properties. HEOs have become potential candidates for electrocatalytic oxygen evolution reaction (OER), which is a key half reaction for electrolytic CO2, nitrogen reduction, and water electrolysis. However, the precise synthesis of HEOs with a wide range of components and structures is challenging, not to mention their active and stable operation for OER. In this paper, we review the recent advancements in the electrocatalytic oxygen evolution facilitated by HEOs in water electrolysis. We analyze these developments from the perspectives of activity and stability in acid and alkaline conditions, respectively. Furthermore, we summarize the design from the aspect of element composition, structure, morphology, and catalyst-support interactions, along with related reaction mechanism of HEOs. Additionally, we discuss the current challenges faced by HEOs in the field of OER and suggest potential directions for the future development of HEOs beyond water electrolysis application.

Pioneering Built-In Interfacial Electric Field for Enhanced Anion Exchange Membrane Water Electrolysis

As a half-reaction in anion exchange membrane water electrolysis (AEMWE) technology, the hydrogen evolution reaction (HER) at the cathode is severely hindered by the sluggish reaction kinetics involved in additional water dissociation step, which results in large overpotentials and low energy conversion efficiency. Here, we develop a nano-heterostructure composed of ultra-thin W5N4 shells over Ni3N nanoparticles (Ni3N@W5N4) as efficient catalysts, in which built-in interfacial electric field (BIEF) is created owing to the distinct lattice arrangements and work functions of biphasic metal nitrides. The BIEF facilitates the electron localization around the interface and enables high valence W and more exposed binding sites in the surface W5N4 shell for accelerating the water dissociation step, ultimately leading to a remarkable reduction in the energy barriers of RDS from 1.40 eV to 0.26 eV. Theoretical calculations and operando X-ray absorption spectroscopy analysis results demonstrated that surface W5N4 serves as the active species for HER. Moreover, the ultra-thin shell characteristics enable the optimized W5N4 with enhanced intrinsic catalytic activity to be fully exposed as active sites. Consequently, the Ni3N@W5N4 exhibits exceptional performance in alkaline HER (60 mV@10 mA cm-2) and remarkable long-term stability (500 mA cm-2 for 100 hours). When employed as the cathode in the AEMWE device, the synthesized Ni3N@W5N4 demonstrates stable performance for 90 hours at a current density of 1 A cm-2.

Enhanced Acidic Oxygen Evolution Reaction Performance by Anchoring Iridium Oxide Nanoparticles on Co3O4

The sluggish kinetics of the anodic process, known as the oxygen evolution reaction (OER), has posed a significant challenge for the practical application of proton exchange membrane water electrolyzers in industrial settings. This study introduces a high-performance OER catalyst by anchoring iridium oxide nanoparticles (IrO2) onto a cobalt oxide (Co3O4) substrate via a two-step combustion method. The resulting IrO2@Co3O4 catalyst demonstrates a significant enhancement in both catalytic activity and stability in acidic environments. Notably, the overpotential required to attain a current density of 10 mA cm-2, a commonly used benchmark for comparison, is merely 301 mV. Furthermore, stability is maintained over a duration of 80 h, as confirmed by the minimal rise in overpotential. Energy spectrum characterizations and experimental results reveal that the generation of OER-active Ir3+ species on the IrO2@Co3O4 surface is induced by the strong interaction between IrO2 and Co3O4. Theoretical calculations further indicate that IrO2 sites loaded onto Co3O4 have a lower energy barrier for *OOH deprotonation to form desorbed O2. Moreover, this interaction also stabilizes the iridium active sites by maintaining their chemical state, leading to superior long-term stability. These insights could significantly impact the strategies for designing and synthesizing more efficient OER electrocatalysts for broader industrial application.

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.

Ensuring Stability of Anode Catalysts in PEMWE: From Material Design to Practical Application

Proton Exchange Membrane Water Electrolysis (PEMWE) has emerged as a clean and effective approach for the conversion and storage of renewable electricity, particularly due to its compatibility with fluctuating photovoltaic and wind power. However, the high cost and limited performance of iridium oxide catalysts (i. e. IrO2) used as anode catalyst in industrial PEM electrolyzers remain significant obstacles to widespread application. Although numerous low-cost and efficient alternative catalysts have been developed in laboratory research, comprehensive stability studies critical for industrial use are often overlooked. This leads to the failure of performance transfer from catalysts tested in liquid half-cell systems to those employed in PEM electrolyzers. This concept presents a thorough overview for the stability issues of anode catalysts in PEMWE, and discuss their degradation mechanisms in both liquid half-cell systems and PEM electrolyzers. We summarize the comprehensive protocols for assessment and characterization, analyze the effective strategies for stability optimization, and explore the opportunities for designing viable anode catalysts for PEM electrolyzers.

Multitasking-Effect Ca Ions Triggered Symmetry-Breaking of RuO2 Coordination for Acidic Oxygen Evolution Reaction

The development of highly active and stable electrocatalysts for the acid oxygen evolution reaction (OER) is both appealing and challenging. The generation of defects is an emerging strategy for improving the water oxidation efficiency. Herein, we introduced multitasking Ca ions to trigger oxygen vacancies in RuO2, resulting in vacancy-rich RuO2 (RuO2-Ov) nanoparticles with enhanced and sustainable OER activity. The oxygen vacancy in RuO2-Ov breaks the symmetry of the RuO6 octahedron, enhancing the d-band center of Ru and reducing the level of 4d-2p hybridization in Ru-O bonds. This effectively optimizes intermediate adsorption and inhibits Ru dissolution. The RuO2-OV catalyst achieves a current density of 10 mA/cm2 with an overpotential of only 198 mV, stabilizing for over 100 h (degradation rate: 0.2 mV/h). Its mass activity is 17.9 times higher than that of commercial RuO2. Our work highlights that multitasking atomic construction defect engineering effectively balances the seesaw relationship between catalytic activity and stability.

A Complex Oxide Containing Inherent Peroxide Ions for Catalyzing Oxygen Evolution Reactions in Acid

Proton exchange membrane water electrolyzers powered by sustainable energy represent a cutting-edge technology for renewable hydrogen generation, while slow anodic oxygen evolution reaction (OER) kinetics still remains a formidable obstacle that necessitates basic comprehension for facilitating electrocatalysts' design. Here, we report a low-iridium complex oxide La1.2Sr2.7IrO7.33 with a unique hexagonal structure consisting of isolated Ir(V)O6 octahedra and true peroxide O22- groups as a highly active and stable OER electrocatalyst under acidic conditions. Remarkably, La1.2Sr2.7IrO7.33, containing 59 wt % less iridium relative to the benchmark IrO2, shows about an order of magnitude higher mass activity, 6-folds higher intrinsic activity than the latter, and also surpasses the state-of-the-art Ir-based oxides ever reported. Combined electrochemical, spectroscopic, and density functional theory investigations reveal that La1.2Sr2.7IrO7.33 follows the peroxide-ion participation mechanism under the OER condition, where the inherent peroxide ions with accessible nonbonded oxygen states are responsible for the high OER activity. This discovery offers an innovative strategy for designing advanced catalysts for various catalytic applications.

Research Progress in Structure Evolution and Durability Modulation of Ir- and Ru-Based OER Catalysts under Acidic Conditions

Green hydrogen energy, as one of the most promising energy carriers, plays a crucial role in addressing energy and environmental issues. Oxygen evolution reaction catalysts, as the key to water electrolysis hydrogen production technology, have been subject to durability constraints, preventing large-scale commercial development. Under the high current density and harsh acid-base electrolyte conditions of the water electrolysis reaction, the active metals in the catalysts are easily converted into high-valent soluble species to dissolve, leading to poor structural durability of the catalysts. There is an urgent need to overcome the durability challenges under acidic conditions and develop electrocatalysts with both high catalytic activity and high durability. In this review, the latest research results are analyzed in depth from both thermodynamic and kinetic perspectives. First, a comprehensive summary of the structural deactivation state process of noble metal oxide catalysts is presented. Second, the evolution of the structure of catalysts possessing high durability is discussed. Finally, four new strategies for the preparation of stable catalysts, "electron buffer (ECB) strategy", combination strength control, strain control, and surface coating, are summarized. The challenges and prospects are also elaborated for the future synthesis of more effective Ru/Ir-based catalysts and boost their future application.

Atomically engineered interfaces inducing bridging oxygen-mediated deprotonation for enhanced oxygen evolution in acidic conditions

The development of efficient and stable electrocatalysts for water oxidation in acidic media is vital for the commercialization of the proton exchange membrane electrolyzers. In this work, we successfully construct Ru-O-Ir atomic interfaces for acidic oxygen evolution reaction (OER). The catalysts achieve overpotentials as low as 167, 300, and 390 mV at 10, 500, and 1500 mA cm-2 in 0.5 M H2SO4, respectively, with the electrocatalyst showing robust stability for >1000 h of operation at 10 mA cm-2 and negligible degradation after 200,000 cyclic voltammetry cycles. Operando spectroelectrochemical measurements together with theoretical investigations reveal that the OER pathway over the Ru-O-Ir active site is near-optimal, where the bridging oxygen site of Ir-OBRI serves as the proton acceptor to accelerate proton transfer on an adjacent Ru centre, breaking the typical adsorption-dissociation linear scaling relationship on a single Ru site and thus enhancing OER activity. Here, we show that rational design of multiple active sites can break the activity/stability trade-off commonly encountered for OER catalysts, offering good approaches towards high-performance acidic OER catalysts.

Regulation of Oxide Pathway Mechanism for Sustainable Acidic Water Oxidation

The advancement of acid-stable oxygen evolution reaction (OER) electrocatalysts is crucial for efficient hydrogen production through proton exchange membrane (PEM) water electrolysis. Unfortunately, the activity of electrocatalysts is constrained by a linear scaling relationship in the adsorbed evolution mechanism, while the lattice-oxygen-mediated mechanism undermines stability. Here, we propose a heterogeneous dual-site oxide pathway mechanism (OPM) that avoids these limitations through direct dioxygen radical coupling. A combination of Lewis acid (Cr) and Ru to form solid solution oxides (CrxRu1-xO2) promotes OH adsorption and shortens the dual-site distance, which facilitates the formation of *O radical and promotes the coupling of dioxygen radical, thereby altering the OER mechanism to a Cr-Ru dual-site OPM. The Cr0.6Ru0.4O2 catalyst demonstrates a lower overpotential than that of RuO2 and maintains stable operation for over 350 h in a PEM water electrolyzer at 300 mA cm-2. This mechanism regulation strategy paves the way for an optimal catalytic pathway, essential for large-scale green hydrogen production.

Advances in Stability of NiFe-Based Anodes toward Oxygen Evolution Reaction for Alkaline Water Electrolysis

Alkaline electrolysis plays a crucial role in sustainable energy solutions by utilizing electrolytic cells to produce hydrogen gas, providing a clean and efficient method for energy storage and conversion. Efficient, stable, and low-cost electrocatalysts for the oxygen evolution reaction (OER) are essential to facilitate alkaline water electrolysis on a commercial scale. Nickel-iron-based (NiFe-based) transition metal electrocatalysts are considered the most promising non-precious metal catalysts for alkaline OER due to their low cost, abundance, and tunable catalytic properties. Nevertheless, the majority of existing NiFe-based catalysts suffer from limited activity and poor stability, posing a significant challenge in meeting industrial applications. This also highlights a common situation where the emphasis on material activity receives significant attention, while the equally critical stability aspect is often underemphasized. Initiating with a comprehensive exploration of the stability of NiFe-based OER materials, this article first summarizes the debate surrounding the determination of active sites in NiFe-based OER electrocatalysts. Subsequently, the degradation mechanisms of recently reported NiFe-based electrocatalysts are outlined, encompassing assessments of both chemical and mechanical endurance, along with essential approaches for enhancing their stability. Finally, suggestions are put forth regarding the essential considerations for the design of NiFe-based OER electrocatalysts, with a focus on heightened stability.

Structure-Activity Relationships in Oxygen Electrocatalysis

Oxygen electrocatalysis, as the pivotal circle of many green energy technologies, sets off a worldwide research boom in full swing, while its large kinetic obstacles require remarkable catalysts to break through. Here, based on summarizing reaction mechanisms and in situ characterizations, the structure-activity relationships of oxygen electrocatalysts are emphatically overviewed, including the influence of geometric morphology and chemical structures on the electrocatalytic performances. Subsequently, experimental/theoretical research is combined with device applications to comprehensively summarize the cutting-edge oxygen electrocatalysts according to various material categories. Finally, future challenges are forecasted from the perspective of catalyst development and device applications, favoring researchers to promote the industrialization of oxygen electrocatalysis at an early date.

Stability of electrocatalytic OER: from principle to application

Hydrogen energy, derived from the electrolysis of water using renewable energy sources such as solar, wind, and hydroelectric power, is considered a promising form of energy to address the energy crisis. However, the anodic oxygen evolution reaction (OER) poses limitations due to sluggish kinetics. Apart from high catalytic activity, the long-term stability of electrocatalytic OER has garnered significant attention. To date, several research studies have been conducted to explore stable electrocatalysts for the OER. A comprehensive review is urgently warranted to provide a concise overview of the recent advancements in the electrocatalytic OER stability, encompassing both electrocatalyst and device developments. This review aims to succinctly summarize the primary factors influencing OER stability, including morphological/phase change and electrocatalyst dissolution, as well as mechanical detachment, alongside chemical, mechanical, and operational degradation observed in devices. Furthermore, an overview of contemporary approaches to enhance stability is provided, encompassing electrocatalyst design (structural regulation, protective layer coating, and stable substrate anchoring) and device optimization (bipolar plates, gas diffusion layers, and membranes). Hopefully, more attention will be paid to ensuring the stable operation of electrocatalytic OER and the future large-scale water electrolysis applications. This review presents design principles aimed at addressing challenges related to the stability of electrocatalytic OER.

Isolated Octahedral Pt-Induced Electron Transfer to Ultralow-Content Ruthenium-Doped Spinel Co3O4 for Enhanced Acidic Overall Water Splitting

The development of a highly active and stable oxygen evolution reaction (OER) electrocatalyst is desirable for sustainable and efficient hydrogen production via proton exchange membrane water electrolysis (PEMWE) powered by renewable electricity yet challenging. Herein, we report a robust Pt/Ru-codoped spinel cobalt oxide (PtRu-Co3O4) electrocatalyst with an ultralow precious metal loading for acidic overall water splitting. PtRu-Co3O4 exhibits excellent catalytic activity (1.63 V at 100 mA cm-2) and outstanding stability without significant performance degradation for 100 h operation. Experimental analysis and theoretical calculations indicate that Pt doping can induce electron transfer to Ru-doped Co3O4, optimize the absorption energy of oxygen intermediates, and stabilize metal-oxygen bonds, thus enhancing the catalytic performance through an adsorbate-evolving mechanism. As a consequence, the PEM electrolyzer featuring PtRu-Co3O4 catalyst with low precious metal mass loading of 0.23 mg cm-2 can drive a current density of 1.0 A cm-2 at 1.83 V, revealing great promise for the application of noniridium-based catalysts with low contents of precious metal for hydrogen production.

Microenvironment Engineering of Heterogeneous Catalysts for Liquid-Phase Environmental Catalysis

Environmental catalysis has emerged as a scientific frontier in mitigating water pollution and advancing circular chemistry and reaction microenvironment significantly influences the catalytic performance and efficiency. This review delves into microenvironment engineering within liquid-phase environmental catalysis, categorizing microenvironments into four scales: atom/molecule-level modulation, nano/microscale-confined structures, interface and surface regulation, and external field effects. Each category is analyzed for its unique characteristics and merits, emphasizing its potential to significantly enhance catalytic efficiency and selectivity. Following this overview, we introduced recent advancements in advanced material and system design to promote liquid-phase environmental catalysis (e.g., water purification, transformation to value-added products, and green synthesis), leveraging state-of-the-art microenvironment engineering technologies. These discussions showcase microenvironment engineering was applied in different reactions to fine-tune catalytic regimes and improve the efficiency from both thermodynamics and kinetics perspectives. Lastly, we discussed the challenges and future directions in microenvironment engineering. This review underscores the potential of microenvironment engineering in intelligent materials and system design to drive the development of more effective and sustainable catalytic solutions to environmental decontamination.

Extremely Active and Robust Ir-Mn Dual-Atom Electrocatalyst for Oxygen Evolution Reaction by Oxygen-Oxygen Radical Coupling Mechanism

A novel Ir-Mn dual-atom electrocatalyst is synthesized by a facile ion-exchange method by incorporating Ir in SrMnO3, which yields an extremely high activity and stability for the oxygen evolution reaction (OER). The ion exchange process occurs in a self-limitation way, which favors the formation of Ir-Mn dual-atom in the IrMnO9 unit. The incorporation of Ir modulates the electronic structure of both Ir and Mn, thereby resulting in a shorter distance of the Ir-Mn dual-atom (2.41 Å) than the Mn-Mn dual-atom (2.49 Å). The modulated Ir-Mn dual-atom enables the same spin direction O (↑) of the adsorbed *O intermediates, thus facilitating the direct coupling of the two adsorbed *O intermediates to release O2 via the oxygen-oxygen radical coupling mechanism. Electrochemical tests reveal that the Ir-SrMnO3 exhibits a superior OER's activity with a low overpotential of 207 mV at 10 mA cm-2 and achieves a mass specific activity of 1100 A gIr -1 at 1.5 V. The proton-exchange-membrane water electrolyzer with the Ir-SrMnO3 catalyst exhibits a low electrolysis voltage of 1.63 V at 1.0 A cm-2 and a stable 2000-h operation with a decay of only 15 μV h-1 at 0.5 A cm-2.

Ultrathin and Conformal Depletion Layer of Core/Shell Heterojunction Enables Efficient and Stable Acidic Water Oxidation

Ru-based electrocatalysts hold great promise for developing affordable proton exchange membrane (PEM) electrolyzers. However, the harsh acidic oxidative environment of the acidic oxygen evolution reaction (OER) often causes undesirable overoxidation of Ru active sites and subsequent serious activity loss. Here, we present an ultrathin and conformal depletion layer attached to the Schottky heterojunction of core/shell RuCo/RuCoOx that not only maximizes the availability of active sites but also improves its durability and intrinsic activity for acidic OER. Operando synchrotron characterizations combined with theoretical calculations elucidate that the lattice strain and charge transfer induced by Schottky heterojunction substantially regulate the electronic structures of active sites, which modulates the OER pathway and suppresses the overoxidation of Ru species. Significantly, the closed core/shell architecture of the RuCo/RuCoOx ensures the structure integrity of the Schottky heterojunction under acidic OER conditions. As a result, the core/shell RuCo/RuCoOx Schottky heterojunction exhibits an unprecedented durability up to 250 0 h at 10 mA cm-2 with an ultralow overpotential of ∼170 mV at 10 mA cm-2 in 0.5 M H2SO4. The RuCo/RuCoOx catalyst also demonstrates superior durability in a proton exchange membrane (PEM) electrolyzer, showcasing the potential for practical applications.

Electrocatalytic Aromatic Alcohols Splitting to Aldehydes and H2 Gas

Selective electrocatalytic transformation of alcohols to aldehydes offers an efficient and environmentally friendly platform for the simultaneous production of fine chemicals and pure hydrogen gas. However, traditional alcohol oxidation reactions (AORs) in aqueous electrolyte unavoidably face competitive reactions (e.g., water oxidation and overoxidations reactions) for the presence of active oxygen species from water oxidation, causing an unwanted decrease in final efficiency and selectivity. Here, we developed an integrated all-solid proton generator-transfer electrolyzer to trigger the pure alcohol splitting reaction (ASR). In this splitting process, only O-H and C-H bonds can be cleaved at the proton generator (Pt nanoparticles), thereby completely avoiding all competitive reactions involving oxygen active species to give a > 99% selectivity to aldehydes. The as-generated protons are transported to the cathode by a three-dimensional (3D) conducting network (assemblies of ionomers and carbon spheres) for efficient hydrogen production. Unlike the poor selectivity (<22%) and durability (<3 h) of a conventional AOR electrolyzer, this ASR electrolyzer could be continuously operated at a low cell voltage of 1.2 V for at least 10 days to give a high Faradaic efficiency of 80-93% for aldehyde production.

Nest-Scheme RuIrLa Nanocrystals by NP-to-NP Oriented Assembly: Coherent Strain Fields-Driven Band Structure Splitting for Efficient Acidic Water Oxidation

Atomic substructure engineering provides new opportunities for the designing newly and efficient catalysts with diverse atom ensembles, trimmed electron bands, and way-out coordination environments, creating unique contributing to concertedly catalyze water oxidation, which is of great significance for proton exchange membrane water electrolysis (PEMWE). Herein, nest-scheme RuIrLa nanocrystals with dense coherent interfaces as built-in substructures are firstly fabricated by using commercial ZnO particles as acid-removable templates, through a La-stabilized coherent epitaxial growth of nanoparticles (NPs). The obtained nests exhibit a low overpotential of 198 mV at 10 mA cm-2, and the RuIrLa||Pt/C module equipped in PEMWE operates stably at a cell voltage potential of 1.69 V at 100 mA cm-2 in 0.5 M H2SO4 for 55 h, which is far beyond the current IrO2||Pt/C. Within the nests, the position at the interface shows high tensile/compressive strain, significantly reducing the OER activation energy. More importantly, the La termination-stabilized coherent interfaces within the nests creates a unique self-healing process for the outstanding long-term stability. This work provides a promising substructure engineering to develop efficient catalysts with abundant substructures, such as coherent interfaces, dislocations, or grain boundaries, thereby realizing concerted improvement of activity and durability toward water oxidation.

Optimal Electrocatalyst Design Strategies for Acidic Oxygen Evolution

Hydrogen, a clean resource with high energy density, is one of the most promising alternatives to fossil. Proton exchange membrane water electrolyzers are beneficial for hydrogen production because of their high current density, facile operation, and high gas purity. However, the large-scale application of electrochemical water splitting to acidic electrolytes is severely limited by the sluggish kinetics of the anodic reaction and the inadequate development of corrosion- and highly oxidation-resistant anode catalysts. Therefore, anode catalysts with excellent performance and long-term durability must be developed for anodic oxygen evolution reactions (OER) in acidic media. This review comprehensively outlines three commonly employed strategies, namely, defect, phase, and structure engineering, to address the challenges within the acidic OER, while also identifying their existing limitations. Accordingly, the correlation between material design strategies and catalytic performance is discussed in terms of their contribution to high activity and long-term stability. In addition, various nanostructures that can effectively enhance the catalyst performance at the mesoscale are summarized from the perspective of engineering technology, thus providing suitable strategies for catalyst design that satisfy industrial requirements. Finally, the challenges and future outlook in the area of acidic OER are presented.

Integrative catalytic pairs for efficient multi-intermediate catalysis

Single-atom catalysts (SACs) have attracted considerable research interest owing to their combined merits of homogeneous and heterogeneous catalysts. However, the uniform and isolated active sites of SACs fall short in catalysing complex chemical processes that simultaneously involve multiple intermediates. In this Review, we highlight an emerging class of catalysts with adjacent binary active centres, which is called integrative catalytic pairs (ICPs), showing not only atomic-scale site-to-site electronic interactions but also synergistic catalytic effects. Compared with SACs or their derivative dual-atom catalysts (DACs), multi-interactive intermediates on ICPs can overcome kinetic barriers, adjust reaction pathways and break the universal linear scaling relations as the smallest active units. Starting from this active-site design principle, each single active atom can be considered as a brick to further build integrative catalytic clusters (ICCs) with desirable configurations, towards trimer or even larger multi-atom units depending on the requirement of a given reaction.

Breaking the Bottleneck of Activity and Stability of RuO2-Based Electrocatalysts for Acidic Oxygen Evolution

Electrochemical acidic oxygen evolution reaction (OER) is an important part for water electrolysis utilizing a proton exchange membrane (PEM) apparatus for industrial H2 production. RuO2 has garnered considerable attention as a potential acidic OER electrocatalyst. However, the overoxidation of Ru active sites under high potential conditions is usually harmful for activity and stability, thereby posing a challenge for large-scale commercialization, which needs effective strategies to circumvent the leaching of Ru and further activate Ru sites. Herein, a Mini-Review is presented to summarize the recent developments regarding the activation and stabilization of the Ru active sites and lattice oxygen through the modulation of the d-band center, coordination environment, bridged heteroatoms, and vacancy engineering, as well as structural protection strategies and reaction pathway optimization to promote the acidic OER activity and stability of RuO2-based electrocatalysts. This Mini-Review offers a profound understanding of the design of RuO2-based electrocatalysts with greatly enhanced acidic OER performances.

Recent Progress on Stability of Layered Double Hydroxide-Based Catalysts for Oxygen Evolution Reaction

Water electrolysis is regarded as one of the most viable technologies for the generation of green hydrogen. Nevertheless, the anodic oxygen evolution reaction (OER) constitutes a substantial obstacle to the large-scale deployment of this technology, due to the considerable overpotential resulting from the retardation kinetics associated with the OER. The development of low-cost, high-activity, and long-lasting OER catalysts has emerged as a pivotal research area. Layered double hydroxides (LDHs) have garnered significant attention due to their suitability for use with base metals, which are cost-effective and exhibit enhanced activity. However, the current performance of LDHs OER catalysts is still far from meeting the demands of industrial applications, particularly in terms of their long-term stability. In this review, we provide an overview of the causes for the deactivation of LDHs OER catalysts and present an analysis of the various mechanisms employed to improve the stability of these catalysts, including the synthesis of LDH ultrathin nanosheets, adjustment of components and doping, dissolution and redeposition, defect creation and corrosion, and utilization of advanced carbon materials.

A robust inorganic binder against corrosion and peel-off stress in electrocatalysis

Electrochemical binders used to immobilize electrocatalysts on electrodes are essential in all fields of electrochemistry. However, conventional organic-based binders like Nafion generally suffer from oxidative decomposition at high potentials on anodic electrodes and have high charge transport resistivity. This work proposes the use of acidic redox-assisted deposition to form cobalt manganese oxyhydroxides (CMOH) as a solid-state inorganic binder. CMOH remains stable under high oxidative currents and ensures catalyst adhesion even under significant peel-off stress as shown by experiments involving the alkaline oxygen evolution reaction (OER) using RuO2 as a catalyst immobilized on a rotating disc electrode. While the molecular structure of Nafion decays significantly after 45 minutes under OER conditions at 3.86 V, the CMOH binder is able to support the powder catalysts (RuO2 and NiO x ) showing stability around 1000 mA cm-2 without significant current decay over 24 hours. The robust catalyst adhesion is a result of the formation of chemical bonds between the electrode and the binder and it can be further improved by increasing the applied loading of CMOH. Unlike Nafion, both the OER activity and the diffusion kinetics are not significantly affected by the CMOH binder. It has also been shown that using CMOH as a binder leads to lower charge transfer resistances R ct and higher electrochemical surface areas compared to systems using Nafion. This is partially due to the presence of metal sites in different oxidation states which has been shown to increase intrinsic conductivity, facilitating the charge hopping at the binder/electrocatalyst interface. With this, the present work provides a proof-of-concept for inorganic metal oxides as promising solid-state binders for a wide range of applications in electrochemistry, demonstrating CMOH's outstanding characteristic of strong adhesion to support other highly active but adhesion-weak electrocatalysts.

Regulating Strain and Electronic Structure of Indium Tin Oxide Supported IrOx Electrocatalysts for Highly Efficient Oxygen Evolution Reaction in Acid

The development of proton exchange membrane water electrolysis is a promising technology for hydrogen production, which has always been restricted by the slow kinetics of the oxygen evolution reaction (OER). Although IrOx is one of the benchmark acidic OER electrocatalysts, there are still challenges in designing highly active and stable Ir-based electrocatalysts for commercial application. Herein, a Ru-doped IrOx electrocatalyst with abundant twin boundaries (TB-Ru0.3Ir0.7Ox@ITO) is reported, employing indium tin oxide with high conductivity as the support material. Combing the TB-Ru0.3Ir0.7Ox nanoparticles with ITO support could expose more active sites and accelerate the electron transfer. The TB-Ru0.3Ir0.7Ox@ITO exhibits a low overpotential of 203 mV to achieve 10 mA cm-2 and a high mass activity of 854.45 A g-1noble metal at 1.53 V vs RHE toward acidic OER, which exceeds most reported Ir-based OER catalysts. Moreover, improved long-term stability could be obtained, maintaining the reaction for over 110 h at 10 mA cm-2 with negligible deactivation. DFT calculations further reveal the activity enhancement mechanism, demonstrating the synergistic effects of Ru doping and strains on the optimization of the d-band center (εd) position and the adsorption free energy of oxygen intermediates. This work provides ideas to realize the trade-off between high catalytic activity and good stability for acidic OER electrocatalysts.

Optimal Coatings of Co3O4 Anodes for Acidic Water Electrooxidation

Implementation of proton-exchange membrane water electrolyzers for large-scale sustainable hydrogen production requires the replacement of scarce noble-metal anode electrocatalysts with low-cost alternatives. However, such earth-abundant materials often exhibit inadequate stability and/or catalytic activity at low pH, especially at high rates of the anodic oxygen evolution reaction (OER). Here, the authors explore the influence of a dielectric nanoscale-thin oxide layer, namely Al2O3, SiO2, TiO2, SnO2, and HfO2, prepared by atomic layer deposition, on the stability and catalytic activity of low-cost and active but insufficiently stable Co3O4 anodes. It is demonstrated that the ALD layers improve both the stability and activity of Co3O4 following the order of HfO2 > SnO2 > TiO2 > Al2O3, SiO2. An optimal HfO2 layer thickness of 12 nm enhances the Co3O4 anode durability by more than threefold, achieving over 42 h of continuous electrolysis at 10 mA cm-2 in 1 m H2SO4 electrolyte. Density functional theory is used to investigate the superior performance of HfO2, revealing a major role of the HfO2|Co3O4 interlayer forces in the stabilization mechanism. These insights offer a potential strategy to engineer earth-abundant materials for low-pH OER catalysts with improved performance from earth-abundant materials for efficient hydrogen production.

Half-metallization Atom-Fingerprints Achieved at Ultrafast Oxygen-Evaporated Pyrochlores for Acidic Water Oxidation

The stability and catalytic activity of acidic oxygen evolution reaction (OER) are strongly determined by the coordination states and spatial symmetry among metal sites at catalysts. Herein, an ultrafast oxygen evaporation technology to rapidly soften the intrinsic covalent bonds using ultrahigh electrical pulses is suggested, in which prospective charged excited states at this extreme avalanche condition can generate a strong electron-phonon coupling to rapidly evaporate some coordinated oxygen (O) atoms, finally leading to a controllable half-metallization feature. Simultaneously, the relative metal (M) site arrays can be orderly locked to delineate some intriguing atom-fingerprints at pyrochlore catalysts, where the coexistence of metallic bonds (M─M) and covalent bonds (M─O) at this symmetry-breaking configuration can partially restrain crystal field effect to generate a particular high-spin occupied state. This half-metallization catalyst can effectively optimize the spin-related reaction kinetics in acidic OER, giving rise to 10.3 times (at 188 mV overpotential) reactive activity than pristine pyrochlores. This work provides a new understanding of half-metallization atom-fingerprints at catalyst surfaces to accelerate acidic water oxidation.

Single Entity Electrocatalysis

Making a measurement over millions of nanoparticles or exposed crystal facets seldom reports on reactivity of a single nanoparticle or facet, which may depart drastically from ensemble measurements. Within the past 30 years, science has moved toward studying the reactivity of single atoms, molecules, and nanoparticles, one at a time. This shift has been fueled by the realization that everything changes at the nanoscale, especially important industrially relevant properties like those important to electrocatalysis. Studying single nanoscale entities, however, is not trivial and has required the development of new measurement tools. This review explores a tale of the clever use of old and new measurement tools to study electrocatalysis at the single entity level. We explore in detail the complex interrelationship between measurement method, electrocatalytic material, and reaction of interest (e.g., carbon dioxide reduction, oxygen reduction, hydrazine oxidation, etc.). We end with our perspective on the future of single entity electrocatalysis with a key focus on what types of measurements present the greatest opportunity for fundamental discovery.