IrOx·nH2O with lattice water-assisted oxygen exchange for high-performance proton exchange membrane water electrolyzers

Vacancy Engineering Strategy Releases the Electrocatalytic Oxygen Evolution Reaction Activity of High-Entropy Oxides

The sluggish kinetics of oxygen evolution reaction (OER) poses a great challenge to the industrial promotion of electrocatalytic water splitting and zinc-air battery. Herein, we demonstrate that the kinetic limitation of the OER imposed by a conventional adsorbate evolution mechanism can be successfully overcome through activating lattice oxygen in the electrocatalyst. For example, incorporating aluminum (Al) into high-entropy oxides (HEO) remarkably enhances the oxygen vacancy concentration, facilitates the generation of reactive oxygen species, and promotes the deprotonation during the electrochemical OER process, thereby boosting the kinetic reaction. This defect engineering strategy effectively decreases the energy barrier associated with the lattice oxygen oxidation and optimizes the configurational entropy of HEO, resulting in remarkable structural stability. Consequently, the developed HEO decorated with Al (HEO-Al) achieves an overpotential of ∼206 mV at 10 mA cm-2 in water electrolysis and a power density (∼20 mW cm-2) in rechargeable zinc-air battery, with long-term stability of 100 h, realizing an optimal balance between electrocatalytic activity and stability. More importantly, the performances of HEO-Al are significantly superior to those of the HEO counterpart (∼260 mV, ∼1.5 mW cm-2) and commercial ruthenium oxide (∼359 mV, ∼5 mW cm-2), showing great competitiveness and application prospect. These results offer essential inspiration for other electrochemical applications dominated by the OER at the same time.

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.

Atomically Dispersed High-Valent d0-Metal Breaks the Activity-Stability Trade-Off in Proton Exchange Membrane Water Electrolysis

Green hydrogen production via proton exchange membrane water electrolysis (PEMWE) faces economic feasibility challenges, primarily due to its reliance on noble metal catalysts. While cost-effective Ru-based catalysts show promise as alternatives to expensive Ir-based catalysts for an anodic oxygen evolution reaction, their long-term performance is compromised by overoxidation at high current densities. In addressing this challenge, we present a cooperative dual-site strategy for atomic-scale incorporation of high-valent d0-metal cations into RuO2. This synthesis results in uniformly distributed Ru-O-d0metal bonds, effectively reconciling the activity and stability trade-off. Leveraging these effects, our optimized Ta1/RuO2 catalyst demonstrates exceptional performance, with a low overpotential of 164 ± 2 mV and stable operation for 1000 h at 100 mA cm-2. In practical PEMWE systems, Ta1/RuO2 achieves 1.58 V at 2 A cm-2, surpassing the 2026 Department of Energy target, and maintains remarkable stability over 650 h at 500 mA cm-2. This breakthrough offers a highly active and durable PEMWE system suitable for industrial-scale applications.

Engineering high-density microcrystalline boundary with V-doped RuO2 for high-performance oxygen evolution in acid

Designing efficient acidic oxygen evolution catalysts for proton exchange membrane water electrolyzers is challenging due to a trade-off between activity and stability. In this work, we construct high-density microcrystalline grain boundaries (GBs) with V-dopant in RuO2 matrix (GB-V-RuO2). Our theoretical and experimental results indicate this is a highly active and acid-resistant OER catalyst. Specifically, the GB-V-RuO2 requires low overpotentials of 159, 222, and 300 mV to reach 10, 100, and 1500 mA cm-2geo in 0.5 M H2SO4, respectively. Operando EIS, ATR-SEIRAS FTIR and DEMS measurements reveal the importance of GBs in stabilizing lattice oxygen and thus inhibiting the lattice oxygen mediated OER pathway. As a result, the adsorbate evolution mechanism pathway becomes dominant, even at high current densities. Density functional theory analyses confirm that GBs can stabilize V dopant and that the synergy between them modulates the electronic structure of RuO2, thus optimizing the adsorption of OER intermediate species and enhancing electrocatalyst stability. Our work demonstrates a rational strategy for overcoming the traditional activity/stability dilemma, offering good prospects of developing high-performance acidic OER catalysts.

Self-replenishing Ni-rich stainless-steel electrode toward oxygen evolution reaction at ampere-level

In the past few decades, tremendous attention has been devoted to enhancing the activity of oxygen evolution reaction (OER) catalysts for hydrogen production, while the cost and long-term stability of catalysts, which can play an even more important role in industrialization, have been much less emphasized. Herein, we engineered an OER electrode from abundant stainless steel (SS) via facile approaches, and the obtained electrode consists of a Ni-rich oxide surface layer with a Fe-rich metal substrate. An outstanding activity was observed with an overpotential of 316 mV at 100 mA cm-2 in 1 M KOH electrolyte. Additionally, an electrode self-replenishing concept is proposed in which a Ni-rich catalyst layer can be regenerated from a metallic substrate due to the difference in diffusion and dissolution rates of metal oxides/hydroxides, and this regeneration is validated by various characterizations. A recorded degradation rate of 0.012 was observed at 1000 mA cm-2 for 1000 h. The facile engineering of OER electrodes from SS combined with the self-replenishing catalyst can potentially address the cost, activity, and long-term stability barriers.

Rapid synthesis of metastable materials for electrocatalysis

Metastable materials are considered promising electrocatalysts for clean energy conversions by virtue of their structural flexibility and tunable electronic properties. However, the exploration and synthesis of metastable electrocatalysts via traditional equilibrium methods face challenges because of the requirements of high energy and precise structural control. In this regard, the rapid synthesis method (RSM), with high energy efficiency and ultra-fast heating/cooling rates, enables the production of metastable materials under non-equilibrium conditions. However, the relationship between RSM and the properties of metastable electrocatalysts remains largely unexplored. In this review, we systematically examine the unique benefits of various RSM techniques and the mechanisms governing the formation of metastable materials. Based on these insights, we establish a framework, linking RSM with the electrocatalytic performance of metastable materials. Finally, we outline the future directions of this emerging field and highlight the importance of high-throughput approaches for the autonomous screening and synthesis of optimal electrocatalysts. This review aims to provide an in-depth understanding of metastable electrocatalysts, opening up new avenues for both fundamental research and practical applications in electrocatalysis.

Highly Efficient and Durable Anode Catalyst Layer Constructed with Deformable Hollow IrOx Nanospheres in Low-Iridium PEM Water Electrolyzer

Reducing iridium packing density (gIr cm-3 electrode) represents a critical pathway to lower geometric Ir loading in proton exchange membrane water electrolyzers (PEMWEs), yet conventional approaches often cause performance issues of anode catalyst layer due to decreased structural stability and limited electron/mass transport efficiency. Here, we present deformable hollow IrOx nanospheres (dh-IrOx) as a structural-engineered catalyst architecture that achieves an ultralow Ir packing density (20% of conventional IrO2 nanoparticle-based electrodes) while maintaining high catalytic activity and durability at reduced Ir loadings. Scalable synthesis of dh-IrOx via a hard-template method-featuring precise SiO2 nanosphere templating and conformal Ir(OH)3 coating-enables batch production of tens of grams. Through cavity dimension and shell thickness optimization, dh-IrOx demonstrates excellent mechanical resilience to necessary electrode fabrication stresses, including high-shear agitation, ultrasonic processing and hot-pressing. In the anode catalyst layer, the quasi-ordered close packing of dh-IrOx nanospheres simultaneously maximizes electrochemically active surface area, suppresses particle migration and agglomeration, and establishes percolated electron highways and rapid mass transport channels. The architected anode delivers high PEMWE performance (e.g., 1 A cm-2 @1.60 V and 2 A cm-2 @1.75 V@80 °C), while demonstrating excellent operational durability with <1.5% voltage loss over 3000 h.

Low-Ir-Content Ir0.10Mn0.90O2 Solid Solution for Highly Active Oxygen Evolution in Acid Media

Iridium (Ir)-based materials are the most widely used oxygen evolution reaction (OER) electrocatalysts in proton exchange membrane water electrolysis (PEMWE). However, their commercial application suffers from high cost and insufficient activity. To optimize the atom utilization efficiency of Ir, the aim is to engineer and develop a rutile-structured solid solution catalyst with minimal Ir content, which is identified through a phase boundary. Here, Ir0.10Mn0.90O2 represents the lowest Ir content in the desired IrO2-MnO2 solid solution. The Ir0.10Mn0.90O2 catalyst exhibits outstanding OER performance in acidic electrolytes, reaching a remarkable mass activity of 1135 A g-1 Ir at an overpotential of 300 mV, which is ≈50 times higher than that of a commercial IrO2 catalyst. Additionally, it demonstrates excellent stability at a current density of 200 mA cm-2 over 120 h during PEMWE operations. Density functional theory (DFT) calculations indicate that the hydroxylation process can be efficiently promoted by the electron-withdrawing on Ir sites in Ir0.10Mn0.90O2, contributing to the enhancement of OER activity.

Engineering High-Density Grain Boundaries in Ru0.8Ir0.2Ox Solid-Solution Nanosheets for Efficient and Durable OER Electrocatalysis

The oxygen evolution reaction (OER) in proton exchange membrane water electrolyzers (PEMWE) has long stood as a formidable challenge for green hydrogen sustainable production, hindered by sluggish kinetics, high overpotentials, and poor durability. Here, these barriers are transcended through a novel material design: strategic engineering of high-density grain boundaries within solid-solution Ru0.8Ir0.2Ox ultrathin nanosheets. These carefully tailored grain boundaries and synergistic Ir─Ru interactions, reduce the coordination of Ru atoms and optimize the distribution of charge, thereby enhancing both the catalytic activity and stability of the nanosheets, as verified by merely requiring an overpotential of 189 mV to achieve 10 mA cm-2 in acidic electrolyte. In situ electrochemical techniques, complemented by theoretical calculations, reveal that the OER follows an adsorption evolution mechanism, demonstrating the pivotal role of grain boundary engineering and electronic modulation in accelerating reaction kinetics. Most notably, the Ru0.8Ir0.2Ox exhibits outstanding industrial-scale performance in PEMWE, reaching 4.0 A cm-2 at 2 V and maintaining stability for >1000 h at 500 mA cm-2. This efficiency reduces hydrogen production costs to $0.88 kg-1. This work marks a transformative step forward in designing efficient, durable OER catalysts, offering a promising pathway toward hydrogen production technologies and advancing the global transition to sustainable energy.

Proton-Conducting, Vacancy-Rich HxIrOy Nanosheets for the Fabrication of Low-Ionomer-Dependent Anode Catalyst Layer in PEM Water Electrolyzer

The anode catalyst layer is composed of catalytically functional IrOx and protonic conducting ionomer and largely dictates catalytic performance of proton exchange membrane water electrolyzer (PEMWE). Here, we report a new type of anode nanocatalyst that possesses both IrOx's catalytic function and high proton conductivity that traditional anode catalysts lack and demonstrate its ability to construct high-performance, low-ionomer-dependent anode catalyst layer, the interior of which-about 85% of total catalyst layer-is free of ionomers. The proton-conducting anode nanocatalyst is prepared via protonation of layered iridate K0.5(Na0.2Ir0.8)O2 and then exfoliation to produce cation vacancy-rich, 1 nm-thick iridium oxide nanosheets (labeled as □-HxIrOy). Besides being a proton conductor, the □-HxIrOy is found to have abundant catalytic active sites for the oxygen evolution reaction due to the optimization of both edge and in-plane iridium sites by multiple cation vacancies. The dual functionality of □-HxIrOy allows the fabrication of low-iridium-loading, low-ionomer-dependent anode catalyst layer with enhanced exposure of catalytic sites and reduced electronic contact resistance, in contrast to common fully mixed catalyst/ionomer layers in PEMWE. This work represents an example of realizing the structural innovation in anode catalyst layer through the bifunctionality of anode catalyst.

Recent Development of Ir- and Ru-Based Electrocatalysts for Acidic Oxygen Evolution Reaction

Proton exchange membrane (PEM) water electrolyzers are one type of the most promising technologies for efficient, nonpolluting and sustainable production of high-purity hydrogen. The anode catalysts account for a very large fraction of cost in PEM water electrolyzer and also determine the lifetime of the electrolyzer. To date, Ir- and Ru-based materials are types of promising catalysts for the acidic oxygen evolution reaction (OER), but they still face challenges of high cost or low stability. Hence, exploring low Ir and stable Ru-based electrocatalysts for acidic OER attracts extensive research interest in recent years. Owing to these great research efforts, significant developments have been achieved in this field. In this review, the developments in the field of Ir- and Ru-based electrocatalysts for acidic OER are comprehensively described. The possible OER mechanisms are first presented, followed by the introduction of the criteria for evaluation of the OER electrocatalysts. The development of Ir- and Ru-based OER electrocatalysts are then elucidated according to the strategies utilized to tune the catalytic performances. Lastly, possible future research in this burgeoning field is discussed.

Vacancy and Dopant Co-Constructed Active Microregion in Ru-MoO3- x/Mo2AlB2 for Enhanced Acidic Hydrogen Evolution

Accurate identification of catalytic active regions is crucial for the rational design and construction of hydrogen evolution catalysts as well as the targeted regulation of their catalytic performance. Herein, the low crystalline-crystalline hybrid MoO3- x/Mo2AlB2 with unsaturated coordination and rich defects is taken as the precursor. Through the Joule heating reaction, the Ru-doped MoO3- x/Mo2AlB2 catalyst is successfully constructed. Building on the traditional view that individual atoms or vacancies act as active sites, this article innovatively proposes the theory that vacancies and doped atoms synergistically construct active microregions, and multiple electron-rich O atoms within the active microregions jointly serve as hydrogen evolution active sites. Based on X-ray absorption fine structure analysis and first-principles calculations, there is a strong electron transfer among Ru atoms, Mo atoms, and O atoms, leading to extensive O atoms with optimized electronic structure in the active microregions. These O atoms exhibit an H* adsorption free energy close to zero, thereby enhancing the catalytic activity for hydrogen evolution. This work provides a brand-new strategy for the design and preparation of electrocatalytic materials and the systematic regulation of the local electronic structure of catalysts.

Anode Alchemy on Multiscale: Engineering from Intrinsic Activity to Impedance Optimization for Efficient Water Electrolysis

The past decade has seen significant progress in proton exchange membrane water electrolyzers (PEMWE), but the growing demand for cost-effective electrolytic hydrogen pushes for higher efficiency at lower costs. As a complex system, the performance of PEMWE is governed by a combination of multiscale factors. This review summarizes the latest progress from quantum to macroscopic scales. At the quantum level, electron spin configurations can be optimized to enhance catalytic activity. At the nano and meso scales, advancements in atomic structure optimization, crystal phase engineering, and heterostructure design improve catalytic performance and mass transport. At the macro scale, innovative techniques in gas bubble management and internal resistance reduction drive further efficiency gains under ampere-level operating conditions. These modifications at the quantum level cascade through meso- and macro-scales, affecting charge transfer, reaction kinetics, and gas evolution management. Unlike conventional approaches that focus solely on one scale-either at the catalyst level (e.g., atomic, or crystal modifications) or at the device level (e.g., porous transport layers design)-combining multiscale optimizations unlocks greater performance improvements. Finally, a perspective on future opportunities for multiscale engineering in PEMWE anode design toward commercial viability is offered.

Feasibility of Active and Durable Lattice Oxygen-Mediated Oxygen Evolution Electrocatalysts in Proton Exchange Membrane Water Electrolyzers Through d0 Metal Ion Incorporation

The primary hurdle faced in the practical application of proton exchange membrane water electrolyzer (PEMWE) involves improving the intrinsic kinetic activity of oxygen evolution reaction (OER) electrocatalysts while concurrently enhancing their durability. Although electrocatalysts based on lattice oxygen-mediated mechanism (LOM) have the potential to significantly enhance the activity in OER without being restricted by scaling relationships, they are neglected in acidic electrolytes due to limited durability. In this study, an innovative approach is presented to simultaneously promote the activation of lattice oxygen and improve the durability of LOM-based OER electrocatalysts by incorporating d0 metal ions into the RuO2 electrocatalyst. Leveraging the unique electronic properties of the d0 metal ion, the O 2p band center and Ru-O covalency of the electrocatalyst are successfully engineered, resulting in the change in OER mechanism. Furthermore, in a single cell of PEMWE, the LOM-based electrocatalyst demonstrates outstanding performance, achieving 3.0 A cm-2 at 1.81 V and maintaining durability for 100 h at 200 mA cm-2, surpassing commercial RuO2. This innovative strategy challenges the traditional viewpoint that suppressing lattice oxygen activation in OER is essential for enhancing PEMWE durability, offering new perspectives for the development of OER electrocatalysts in acidic electrolytes.

Local Oxygen Vacancy-Mediated Oxygen Exchange for Active and Durable Acidic Water Oxidation

Developing an active and durable acidic oxygen evolution reaction (OER) catalyst is vital for implementing a proton exchange membrane water electrolyzer (PEMWE) in sustainable hydrogen production. However, it remains dauntingly challenging to balance high activity and long-term stability under harsh acidic and oxidizing conditions. Herein, through developing the universal rare-earth participated pyrolysis-leaching approach, we customized the active and long lifespan pseudo-amorphous IrOx with locally ordered rutile IrO2 and unique defect sites (IrOx-3Nd). IrOx-3Nd achieved a low overpotential of 206 mV and long-term durability of 2200 h with a slow degradation rate of 0.009 mV  h-1 at 10 mA cm-2, and, more importantly, high efficiency in PEMWE (1.68 V at 1 A cm-2 for 1000 h) for practical hydrogen production. Utilizing in situ characterizations and theoretical calculations, we found that lattice oxygen vacancies (Ov) and contracted Ir-O in locally ordered rutile IrO2 induced the Ov-modulated lattice oxygen exchange process, wherein thermodynamically spontaneous occupation of surface hydroxyl groups on Ov and effective promotion of O─O coupling and lattice oxygen recovery accounted for enhanced activity and durability. This work underscores the importance of tailor-made local configuration in boosting activity and durability of OER catalyst and different insights into the promotion mechanism.

Establishing the Link Between Oxygen Vacancy and Activity Enhancement in Acidic Water Oxidation of Trigonal Iridium Oxide

Developing durable IrO2-based electrocatalysts with high oxygen evolution reaction (OER) activity under acidic condition is crucial for proton exchange membrane electrolyzers. While oxygen defects are considered potentially important in OER, their direct relationship with catalytic activity has yet to be established. In this study, we introduced abundant oxygen vacancies through Re doping in 2D IrO2 (Re0.03Ir0.97O2), demonstrating their decisive role in enhancing OER performance. The Re0.03Ir0.97O2 catalyst exhibited excellent OER performance with an overpotential of 193 mV at 10 mA cm-2 and sustained activity for over 650 hours, significantly surpassing the undoped catalyst. Moreover, it maintained operation at a cell voltage of 1.70 V (~1200 mA cm-2) for over 140 hours without significant performance degradation. Theoretical calculations coupled with cyclic voltammetry, transient potential scanning and in situ characterizations confirmed the adsorbate evolving mechanism on Re0.03Ir0.97O2, as well as the critical role of Re-induced oxygen vacancies in enhancing OER performance. These findings highlight that oxygen defects directly influence OER activity, providing guidance for the application of oxygen vacancy engineering in electrocatalyst design.

Morphology-Guided Phase-Controlled Synthesis of Nickel Selenide Nanocrystals through Cation Exchange Reactions

The precise control of the crystal phase during the synthesis of nickel selenide (NixSey) nanocrystals is crucial, as crystal structure and composition significantly influence their reactivity, growth kinetics, and properties. The cation exchange (CE) method provides a versatile and robust approach for synthesizing nanomaterials, enabling precise control over phase, composition, and morphology. However, the application of this method for phase-controlled synthesis of NixSey nanocrystals has received limited research attention. Here, we present a morphology-guided CE method for the synthesis of spinel Ni3Se4 nanoparticles (NPs) and rhombic Ni3Se2 nanorods (NRs), wherein berzelianite Cu2-xSe NPs and NRs are employed as sacrificial templates for CE with Ni2+. This phase-controlled behavior, which is guided by morphology and dependent on the stacking length of the close-packed facets, relies on the rearrangement of the Se2- sublattice accompanied by CE, providing a unique and precise approach to controlling phase during nanocrystal synthesis. Additionally, the obtained Ni3Se4 NPs and Ni3Se2 NRs exhibit structure-dependent catalytic activities in the oxygen evolution reaction.

Operando unveiling the activity origin via preferential structural evolution in Ni-Fe (oxy)phosphides for efficient oxygen evolution

Non-noble metal-based heteroatom compounds demonstrate excellent electrocatalytic activity for the oxygen evolution reaction (OER). However, the origin of this activity, driven by structure evolution effects, remains unclear due to the lack of effective in situ/operando techniques. Herein, we employ the operando quick-scan x-ray absorption fine structure (Q-XAFS) technique coupled with in situ controlled electrochemical potential to establish a structure-activity correlation of the OER catalyst. Using Ni-Fe bimetallic phosphides as a model catalyst, operando Q-XAFS experiments reveal that the structural transformation initiates at the preferential oxidation of Fe sites over Ni sites. The in situ-generated O-Fe-P structure serves as the origin of the enhanced electrocatalytic OER activity of the catalyst, a finding supported by theoretical calculations. This work provides crucial insights into understanding the reaction mechanism of the state-of-the-art Ni-Fe-based OER electrocatalysts, thus advancing the rational design of more efficient OER electrocatalysts.

Dynamic Deprotonation Enhancement Triggered by Accelerated Electrochemical Delithiation Reconstruction during Acidic Water Oxidation

The structure-dependent transition in reaction pathways during acidic oxygen evolution (OER) is pivotal due to the active site oxidation accompanied by the coordination environment changes. In this work, charge-polarized Ir-O-Co units are constructed in alkali metal cobalt oxides (LiCoO2, and Na0.74CoO2) to modify the lower Hubbard band. Benefiting from the accelerated delithiation reconstruction induced by the altered band structure, typical Ir-LiCoO2 produces high-valent Ir sites with unsaturated coordination through the charge compensation during OER. Oxygen atoms shared by trimetallic sites exhibit strong Bro̷nsted acidity, promoting proton migration for unsaturated Ir sites and dynamically enhancing deprotonation. Furthermore, the stable coordination environment, along with electron donation from Co sites, significantly improves the stability of Ir sites. The unique electrochemical activation results in a low overpotential of 190 mV at 10 mA cm-2 during acidic OER and delivers exceptional stability at 1 A cm-2 for 150 h with a slight voltage degradation in a proton exchange membrane electrolyzer. This work provides in-depth insights into the relationship between catalyst reconstruction and reaction mechanisms.

Advances in Oxygen Evolution Reaction Electrocatalysts via Direct Oxygen-Oxygen Radical Coupling Pathway

Oxygen evolution reaction (OER) is a cornerstone of various electrochemical energy conversion and storage systems, including water splitting, CO2/N2 reduction, reversible fuel cells, and rechargeable metal-air batteries. OER typically proceeds through three primary mechanisms: adsorbate evolution mechanism (AEM), lattice oxygen oxidation mechanism (LOM), and oxide path mechanism (OPM). Unlike AEM and LOM, the OPM proceeds via direct oxygen-oxygen radical coupling that can bypass linear scaling relationships of reaction intermediates in AEM and avoid catalyst structural collapse in LOM, thereby enabling enhanced catalytic activity and stability. Despite its unique advantage, electrocatalysts that can drive OER via OPM remain nascent and are increasingly recognized as critical. This review discusses recent advances in OPM-based OER electrocatalysts. It starts by analyzing three reaction mechanisms that guide the design of electrocatalysts. Then, several types of novel materials, including atomic ensembles, metal oxides, perovskite oxides, and molecular complexes, are highlighted. Afterward, operando characterization techniques used to monitor the dynamic evolution of active sites and reaction intermediates are examined. The review concludes by discussing several research directions to advance OPM-based OER electrocatalysts toward practical applications.

Ru0.1Mn0.9Ox Electrocatalyst for Durable Oxygen Evolution in Acid Seawater

Currently, direct electrolysis of seawater for green hydrogen production is primarily focused on neutral and alkaline systems. However, the precipitation of calcium and magnesium ions restricts the advancement of this technology. An acidic system can effectively address this issue. Given that Ru/Ir-based catalysts with high oxygen evolution reaction (OER) activity also exhibit high chlorine evolution reaction (CER) activity, acid seawater splitting requires anodes with higher selectivity and stability compared to the other two systems. In this study, we propose a non-precious Ru0.1Mn0.9Ox as the active anode for direct acid seawater electrolysis, which exhibits a high OER selectivity and remarkable stability for more than 1200 hours. Different from the Cl--free system, *Cl occupied on Ru sites could shift the OER active center to Mn on Ru0.1Mn0.9Ox, which prevents the lattice oxygen consumed on Ru and hinders the metal site dissolution. As the CER-insensitive catalytic center, Mn activated by the introduction of Ru can adsorb a substantial amount of *OH, creating an OER-favored local environment that inhibits CER. We introduce Cl--assisted transfer of OER active sites to CER-insensitive Mn as a fundamental strategy for achieving highly selective and durable oxygen evolution in acidic seawater.

Strong Heteroatomic Bond-Induced Confined Restructuring on Ir-Mn Intermetallics Enable Robust PEM Water Electrolyzers

Low-iridium acid-stabilized electrocatalysts for efficient oxygen evolution reaction (OER) are crucial for the market deployment of proton exchange membrane (PEM) water electrolysis. Manipulating the in situ reconstruction of Ir-based catalysts with favorable kinetics is highly desirable but remains elusive. Herein, we propose an atomic ordering strategy to modulate the dynamic surface restructuring of catalysts to break the activity/stability trade-off. Under working conditions, the strong heteroatom-bonded structure triggers rational surface-confined reconstruction to form self-stabilizing amorphous (oxy)hydroxides on the model Ir-Mn intermetallic (IMC). Combined in situ/ex situ characterizations and theoretical analysis demonstrate that the induced strong covalent Ir-O-Mn units in the catalytic layer weaken the formation barrier of OOH* and promote the preferential dynamic replenishment/conversion pathway of H2O molecules to suppress the uncontrollable participation of lattice oxygen (about 2.6 times lower than that of pure Ir). Thus, a PEM cell with Ir-Mn IMC as anode "pre-electrocatalyst" (0.24 mgIr cm-2) delivers an impressive performance (3.0 A cm-2@1.851 V@80 °C) and runs stably at 2.0 A cm-2 for more than 2,000 h with the cost of USD 0.98 per kg H2, further validating its promising application. This work highlights surface-confined evolution triggered by strong heteroatom bonds, providing insights into the design of catalysts involving surface reconstruction.

Strontium Doped IrOx Triggers Direct O-O Coupling to Boost Acid Water Oxidation Electrocatalysis

The discovery of efficient and stable electrocatalysts for the oxygen evolution reaction (OER) in acidic conditions is crucial for the commercialization of proton-exchange membrane water electrolyzers. In this work, we propose a Sr(OH)2-assisted method to fabricate a (200) facet highly exposed strontium-doped IrOx catalyst to provide available adjacent iridium sites with lower Ir-O covalency. This design facilitates direct O-O coupling during the acidic water oxidation process, thereby circumventing the high energy barrier associated with the generation of *OOH intermediates. Benefiting from this advantage, the resulting Sr-IrOx catalyst exhibits an impressive overpotential of 207 mV at a current density of 10 mA cm-2 in 0.5 M H2SO4. Furthermore, a PEMWE device utilizing Sr-IrOx as the anodic catalyst demonstrates a cell voltage of 1.72 V at 1 A cm-2 and maintains excellent stability for over 500 hours. Our work not only provides guidance for the design of improved acidic OER catalysts but also encourages the development of iridium-based electrocatalysts with novel mechanisms for other electrocatalytic reactions.

Understanding the Dynamic Evolution of Active Sites among Single Atoms, Clusters, and Nanoparticles

Catalysis remains a cornerstone of chemical research, with the active sites of catalysts being crucial for their functionality. Identifying active sites, particularly during the reaction process, is crucial for elucidating the relationship between a catalyst's structure and its catalytic property. However, the dynamic evolution of active sites within heterogeneous metal catalysts presents a substantial challenge for accurately pinpointing the real active sites. The advent of in situ and operando characterization techniques has illuminated the path toward understanding the dynamic changes of active sites, offering robust scientific evidence to support the rational design of catalysts. There is a pressing need for a comprehensive review that systematically explores the dynamic evolution among single atoms, clusters, and nanoparticles as active sites during the reaction process, utilizing in situ and operando characterization techniques. This review aims to delineate the effects of various reaction factors on dynamic evolution of active sites among single atoms, clusters, and nanoparticles. Moreover, several in situ and operando techniques are elaborated with emphases on tracking the dynamic evolution of active sites, linking them to catalytic properties. Finally, it discusses challenges and future perspectives in identifying active sites during the reaction process and advancing in situ and operando characterization techniques.

Dynamic Redox Induced Localized Charge Accumulation Accelerating Proton Exchange Membrane Electrolysis

The sluggish anodic oxygen evolution reaction (OER) in proton exchange membrane (PEM) electrolysis necessitates applied bias to facilitate electron transfer as well as bond cleavage and formation. Traditional electrocatalysis focuses on analyzing the effects of electron transfer, while the role of charge accumulation induced by the applied overpotential has not been thoroughly investigated. To explore the influence mechanism of bias-driven charge accumulation, capacitive Mn is incorporated into IrO2 to regulate the local electronic structure and the adsorption behavior. The applied bias triggers dynamic redox reactions at the active sites, which introduce local charge accumulation on the surface of electrocatalyst. Under bias, Mn oxidation induced a noticeable pseudocapacitance in the pre-OER region, promoting the OER kinetics of iridium sites. Meanwhile, the increased oxygen vacancy formation energy further prevents the lattice oxygen activation. The PEM electrolyzer, equipped with optimal materials as an anode, operates at a low driving voltage of 1.637 V under 2.0 A cm-2, maintaining stable performance for over 800 h with a low degradation rate (19.4 µV h-1). This work provides insights into the performance of metal oxide catalysts in acidic environments and offers forward-looking strategies for enhancing the catalytic performance through dynamic redox induced capacitive behavior.

Bifunctional fluorine doped Ru/RuO2 clusters with dynamic electron modification and strong metal-support interaction boost proton exchange membrane water electrolyzer

The sluggish kinetics and inherent instability over the Ru/RuO2 clusters are still enormous challenges in proton exchange membrane (PEM) water electrolyzer. Herein, we innovatively report synergistic modulation of dynamic electron modification and strong metal-support interaction (SMSI) to activate and stabilize bifunctional fluorine doped Ru/RuO2 clusters anchored on carbon nanotube (CNT), thus achieving efficient and stable acidic overall water splitting. Theoretical and experimental studies found that surface metal-fluorine modification layer could dynamically regulate the interfacial electronic environment to stabilize and activate multiple active Ru species; and the SMSI between Ru/RuO2 cluster and CNT maintains stable electronic environment for dynamic electron modification and avoids migrating or shedding of active species in acidic environment. Therefore, the PEM electrolyzer assembled with optimal F5.5-Ru/RuO2@CNT can operate stably for 100 h at a high current density of 100 mA cm-2, which is the first time that bifunctional Ru-based nanocatalysts applied to PEM device at a high current density.

Controlled Structural Activation of Iridium Single Atom Catalyst for High-Performance Proton Exchange Membrane Water Electrolysis

Iridium single atom catalysts are promising oxygen evolution reaction (OER) electrocatalysts for proton exchange membrane water electrolysis (PEMWE), as they can reduce the reliance on costly Ir in the OER catalysts. However, their practical application is hindered by their limited stability during PEMWE operation. Herein, we report on the activation of Ir-doped CoMn2O4 in acidic electrolyte that leads to enhanced activity and stability in acidic OER for long-term PEMWE operation. In-depth material characterization combined with electrochemical analysis and theoretical calculations reveal that activating Ir-doped CoMn2O4 induces controlled restructuring of Ir single atoms to IrOx nanoclusters, resulting in an optimized Ir configuration with outstanding mass activity of 3562 A gIr-1 at 1.53 V (vs RHE) and enhanced OER stability. The PEMWE using activated Ir-doped CoMn2O4 exhibited a stable operation for >1000 h at 250 mA cm-2 with a low degradation rate of 0.013 mV h-1, demonstrating its practical applicability. Furthermore, it remained stable for more than 400 h at a high current density of 1000 mA cm-2, demonstrating long-term durability under practical operation conditions.

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.

Ir-O-Mn embedded in porous nanosheets enhances charge transfer in low-iridium PEM electrolyzers

Using metal oxides to disperse iridium (Ir) in the anode layer proves effective for lowering Ir loading in proton exchange membrane water electrolyzers (PEMWE). However, the reported low-Ir-based catalysts still suffer from unsatisfying electrolytic efficiency and durability under practical industrial working conditions, mainly due to insufficient catalytic activity and mass transport in the catalyst layer. Herein we report a class of porous heterogeneous nanosheet catalyst with abundant Ir-O-Mn bonds, achieving a notable mass activity of 4 A mgIr-1 for oxygen evolution reaction at an overpotential of 300 mV, which is 150.6 times higher than that of commercial IrO2. Ir-O-Mn bonds are unraveled to serve as efficient charge-transfer channels between in-situ electrochemically-formed IrOx clusters and MnOx matrix, fostering the generation and stabilization of highly active Ir3+ species. Notably, Ir/MnOx-based PEMWE demonstrates comparable performance under 10-fold lower Ir loading (0.2 mgIr cm-2), taking a low cell voltage of 1.63 V to deliver 1 A cm-2 for over 300 h, which positions it among the elite of low Ir-based PEMWEs.

Unveiling the Structure and Dissociation of Interfacial Water on RuO2 for Efficient Acidic Oxygen Evolution Reaction

Understanding the structure and dynamic process of interfacial water molecules at the catalyst-electrolyte interface on acidic oxygen evolution reaction (OER) kinetics is highly desirable for the development of proton exchange membrane water electrolyzers. Herein, we construct a series of p-block metal elements (Ga, In, Sn) doped RuO2 catalysts with manipulated electronic structure and Ru-O covalency to investigate the effect of electrochemical interfacial engineering on the improvement of acidic OER activity. Associated with operando attenuated total reflectance surface-enhanced infrared absorption spectroscopy measurements and theoretical analysis, we uncover the free-H2O enriched local environment and dynamic evolution from 4-coordinated hydrogen-bonded water and 2-coordinated hydrogen-bonded water to free-H2O on the surface of Ga-RuO2, are responsible for the optimized connectivity of hydrogen bonding network in the electrical double layer by promoting solvent reorganization. In addition, the structurally ordered interfacial water molecules facilitate high-efficiency proton-coupled electron transfer across the interface, leading to reduced energy barrier of the follow-up dissociation process and enhanced acidic OER performance. This work highlights the key role of structure and dynamic process of interfacial water for acidic OER, and demonstrates the electrochemical interfacial engineering as an efficient strategy to design high-performance electrocatalysts.

Constructing Highly Porous Low Iridium Anode Catalysts Via Dealloying for Proton Exchange Membrane Water Electrolyzers

Iridium (Ir) is the most active and durable anode catalyst for the oxygen evolution reaction (OER) for proton exchange membrane water electrolyzers (PEMWEs). However, their large-scale applications are hindered by high costs and scarcity of Ir. Lowering Ir loadings below 1.0 mgcm-2 causes significantly reduced PEMWE performance and durability. Therefore, developing efficient low Ir-based catalysts is critical to widely commercializing PEMWEs. Herein, an approach is presented for designing porous Ir metal aerogel (MA) catalysts via chemically dealloying IrCu alloys. The unique hierarchical pore structures and multiple channels of the Ir MA catalyst significantly increase electrochemical surface area (ECSA) and enhance OER activity compared to conventional Ir black catalysts, providing an effective solution to design low-Ir catalysts with improved Ir utilization and enhanced stability. An optimized membrane electrode assembly (MEA) with an Ir loading of 0.5 mgIr cm-2 generated 2.0 A cm-2 at 1.79 V, higher than the Ir black at a loading of 2.0 mgIr cm-2 (1.63 A cm-2). The low-Ir MEA demonstrated an acceptable decay rate of ≈40 µV h-1 during durability tests at 0.5 (>1200 h) and 2.0 A cm-2 (400 h), outperforming the commercial Ir-based MEA (175 µV h-1 at 2.0 mgIr cm-2).

Lewis Acid-Mediated Interfacial Water Supply for Sustainable Proton Exchange Membrane Water Electrolysis

The catalyst-electrolyte interface plays a crucial role in proton exchange membrane water electrolysis (PEMWE). However, optimizing the interfacial hydrogen bonding to enhance both catalytic activity and stability remains a significant challenge. Here, a novel catalyst design strategy is proposed based on the hard-soft acid-base principle, employing hard Lewis acids (LAs = ZrO2, TiO2, HfO2) to mediate the reconfiguration of interfacial hydrogen bonding, thereby enhancing the acidic oxygen evolution reaction (OER) performance of RuO2. Mechanistic analysis indicates that LAs prompt a directional evolution from a rigid hydrogen bonding network to free water, enhancing the trapping of interfacial water on the RuO2 surface, which continuously supplies reactants to the catalytic sites. Moreover, the interconnected hydrogen bonding network facilitates rapid proton transfer, reducing local acidity on the catalyst surface and preventing structural corrosion, thus significantly improving long-term stability. The tandem pathway of water supply and deprotonation transforms the dissolution mechanism of traditional Ru-based catalysts, emphasizing the widespread applicability. Consequently, ZrO2-RuO2 displays a significantly reduced overpotential of 170 mV and exhibits high durability, sustaining 1800 h at 10 mA cm-2 under acidic OER, and maintains robust activity for 100 h at 2 A cm-2 in PEMWE, outperforming most Ru/Ir-based catalysts.

Enriched Electrophilic Oxygen Species on Ru Optimize Acidic Water Oxidation

Ruthenium oxide (RuO2) is considered one of the most promising catalysts for replacing iridium oxide (IrO2) in the acidic oxygen evolution reaction (OER). Nevertheless, the performance of RuO2 remains unacceptable due to the dissolution of Ru and the lack of *OH in acidic environments. This paper reports a grain boundary (GB)-rich porous RuO2 electrocatalyst for the efficient and stable acidic OER. The involvement of GB regulates the valence state of Ru and weakens the interaction between Ru and O, effectively facilitating *OH adsorption and *OOH formation. Notably, achieved a record-high catalytic activity (145 mV at 10 mA cm-2) with a low Tafel slope (40.9 mV dec-1) and a remarkable mass activity of 332 mA mg-1 Ru at 1.5 V versus reversible hydrogen electrode is achieved. Additionally, the porous RuO2 exhibits superb stability with an ultra-low degradation rate of 26 µV h-1 over a 50-day durability test. This study opens a viable pathway for the development of efficient and robust Ru-based acidic OER electrocatalysts.

Corrosion-resistant NiFe anode towards kilowatt-scale alkaline seawater electrolysis

Development of large-scale alkaline seawater electrolysis requires robust and corrosion-resistant anodes. Here we propose engineering NiFe layered double hydroxide (LDH)-based anodes by incorporating a series of anions into the LDH interlayers. The most optimal NiFe LDH anode with intercalated phosphates demonstrates stable operation at a high current density of 1.0 A cm-2 for over 1000 hours in a 2 W-scale alkaline seawater electrolyzer (ASWE). Fundamental studies indicate that the basicity, indicated by pKa values, of the intercalated anions in NiFe LDH governs its oxygen evolution reaction activity and corrosion resistance. Highly basic anions (i.e., phosphates) securely anchor Fe sites and facilitate proton transfer to boost both durability and activity. Notably, we demonstrate the proof-of-concept for the NiFe anode in an industrial 1 kW-scale ASWE stack (1,081.2 cm2 anode area in total). This unit achieves a stable operating current density of 0.5 A cm-2 at about 2.0 V, twice that of the commercial alkaline pure water electrolyzer, contributing to an economically competitive hydrogen production cost of US$ 1.96 kgH2-1.

Developing Catalysts for Membrane Electrode Assemblies in High Performance Polymer Electrolyte Membrane Water Electrolyzers

Extensive research is underway to achieve carbon neutrality through the production of green hydrogen via water electrolysis, powered by renewable energy. Polymer membrane water electrolyzers, such as proton exchange membrane water electrolyzer (PEMWE) and anion exchange membrane water electrolyzer (AEMWE), are at the forefront of this research. Developing highly active and durable electrode catalysts is crucial for commercializing these electrolyzers. However, most research is conducted in half-cell setups, which may not fully represent the catalysts' effectiveness in membrane-electrode-assembly (MEA) devices. This review explores the catalysts developed for high-performance PEMWE and AEMWE MEA systems. Only the catalysts reporting on the MEA performance were discussed in this review. In PEMWE, strategies aim to minimize Ir use for the oxygen evolution reaction (OER) by maximizing activity, employing metal oxide-based supports, integrating secondary elements into IrOx lattices, or exploring non-Ir materials. For AEMWE, the emphasis is on enhancing the performance of NiFe-based and Co-based catalysts by improving electrical conductivity and mass transport. Pt-based and Ni-based catalysts for the hydrogen evolution reaction (HER) in AEMWE are also examined. Additionally, this review discusses the unique considerations for catalysts operating in pure water within AEMWE systems.

Pinning effect of lattice Pb suppressing lattice oxygen reactivity of Pb-RuO2 enables stable industrial-level electrolysis

Ruthenium (Ru) is widely recognized as a low-cost alternative to iridium as anode electrocatalyst in proton-exchange membrane water electrolyzers (PEMWE). However, the reported Ru-based catalysts usually only operate within tens of hours in PEMWE because of their intrinsically high reactivity of lattice oxygen that leads to irrepressible Ru leaching and structural collapse. Herein, we report a design concept by employing large-sized and acid-resistant lattice lead (Pb) as a second element to induce a pinning effect for effectively narrowing the moving channels of oxygen atoms, thereby lowering the reactivity of lattice oxygen in Ru oxides. The Pb-RuO2 catalyst presents a low overpotential of 188 ± 2 mV at 10 mA cm-2 and can sustain for over 1100 h in an acid medium with a negligible degradation rate of 19 μV h-1. Particularly, the Pb-RuO2-based PEMWE can operate for more than 250 h at 500 mA cm-2 with a low degradation rate of only 17 μV h-1. Experimental and theoretical calculation results reveal that Ru-O covalency is reduced due to the unique 6s-2p-4d orbital hybridization, which increases the loss energy of lattice oxygen and suppresses the over-oxidation of Ru for improved long-term stability in PEMWE.

Molten Salt-Assisted Synthesis of Catalysts for Energy Conversion

A breakthrough in manufacturing procedures often enables people to obtain the desired functional materials. For the field of energy conversion, designing and constructing catalysts with high cost-effectiveness is urgently needed for commercial requirements. Herein, the molten salt-assisted synthesis (MSAS) strategy is emphasized, which combines the advantages of traditional solid and liquid phase synthesis of catalysts. It not only provides sufficient kinetic accessibility, but effectively controls the size, morphology, and crystal plane features of the product, thus possessing promising application prospects. Specifically, the selection and role of the molten salt system, as well as the mechanism of molten salt assistance are analyzed in depth. Then, the creation of the catalyst by the MSAS and the electrochemical energy conversion related application are introduced in detail. Finally, the key problems and countermeasures faced in breakthroughs are discussed and look forward to the future. Undoubtedly, this systematical review and insights here will promote the comprehensive understanding of the MSAS and further stimulate the generation of new and high efficiency catalysts.

The breakthrough of oxide pathway mechanism in stability and scaling relationship for water oxidation

An in-depth understanding of electrocatalytic mechanisms is essential for advancing electrocatalysts for the oxygen evolution reaction (OER). The emerging oxide pathway mechanism (OPM) streamlines direct O-O radical coupling, circumventing the formation of oxygen vacancy defects featured in the lattice oxygen mechanism (LOM) and bypassing additional reaction intermediates (*OOH) inherent to the adsorbate evolution mechanism (AEM). With only *O and *OH as intermediates, OPM-driven electrocatalysts stand out for their ability to disrupt traditional scaling relationships while ensuring stability. This review compiles the latest significant advances in OPM-based electrocatalysis, detailing design principles, synthetic methods, and sophisticated techniques to identify active sites and pathways. We conclude with prospective challenges and opportunities for OPM-driven electrocatalysts, aiming to advance the field into a new era by overcoming traditional constraints.

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.

Prediction of perovskite oxygen vacancies for oxygen electrocatalysis at different temperatures

Efficient catalysts are imperative to accelerate the slow oxygen reaction kinetics for the development of emerging electrochemical energy systems ranging from room-temperature alkaline water electrolysis to high-temperature ceramic fuel cells. In this work, we reveal the role of cationic inductive interactions in predetermining the oxygen vacancy concentrations of 235 cobalt-based and 200 iron-based perovskite catalysts at different temperatures, and this trend can be well predicted from machine learning techniques based on the cationic lattice environment, requiring no heavy computational and experimental inputs. Our results further show that the catalytic activity of the perovskites is strongly correlated with their oxygen vacancy concentration and operating temperatures. We then provide a machine learning-guided route for developing oxygen electrocatalysts suitable for operation at different temperatures with time efficiency and good prediction accuracy.

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.

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.

Recent Research on Iridium-Based Electrocatalysts for Acidic Oxygen Evolution Reaction from the Origin of Reaction Mechanism

As the anode reaction of proton exchange membrane water electrolysis (PEMWE), the acidic oxygen evolution reaction (OER) is one of the main obstacles to the practical application of PEMWE due to its sluggish four-electron transfer process. The development of high-performance acidic OER electrocatalysts has become the key to improving the reaction kinetics. To date, although various excellent acidic OER electrocatalysts have been widely researched, Ir-based nanomaterials are still state-of-the-art electrocatalysts. Hence, a comprehensive and in-depth understanding of the reaction mechanism of Ir-based electrocatalysts is crucial for the precise optimization of catalytic performance. In this review, the origin and nature of the conventional adsorbate evolution mechanism (AEM) and the derived volcanic relationship on Ir-based electrocatalysts for acidic OER processes are summarized and some optimization strategies for Ir-based electrocatalysts based on the AEM are introduced. To further investigate the development strategy of high-performance Ir-based electrocatalysts, several unconventional OER mechanisms including dual-site mechanism and lattice oxygen mediated mechanism, and their applications are introduced in detail. Thereafter, the active species on Ir-based electrocatalysts at acidic OER are summarized and classified into surface Ir species and O species. Finally, the future development direction and prospect of Ir-based electrocatalysts for acidic OER are put forward.

Fe-S dually modulated adsorbate evolution and lattice oxygen compatible mechanism for water oxidation

Simultaneously activating metal and lattice oxygen sites to construct a compatible multi-mechanism catalysis is expected for the oxygen evolution reaction (OER) by providing highly available active sites and mediate catalytic activity/stability, but significant challenges remain. Herein, Fe and S dually modulated NiFe oxyhydroxide (R-NiFeOOH@SO4) is conceived by complete reconstruction of NiMoO4·xH2O@Fe,S during OER, and achieves compatible adsorbate evolution mechanism and lattice oxygen oxidation mechanism with simultaneously optimized metal/oxygen sites, as substantiated by in situ spectroscopy/mass spectrometry and chemical probe. Further theoretical analyses reveal that Fe promotes the OER kinetics under adsorbate evolution mechanism, while S excites the lattice oxygen activity under lattice oxygen oxidation mechanism, featuring upshifted O 2p band centers, enlarged d-d Coulomb interaction, weakened metal-oxygen bond and optimized intermediate adsorption free energy. Benefiting from the compatible multi-mechanism, R-NiFeOOH@SO4 only requires overpotentials of 251 ± 5/291 ± 1 mV to drive current densities of 100/500 mA cm-2 in alkaline media, with robust stability for over 300 h. This work provides insights in understanding the OER mechanism to better design high-performance OER catalysts.

Supported IrO2 Nanocatalyst with Multilayered Structure for Proton Exchange Membrane Water Electrolysis

The design of a low-iridium-loading anode catalyst layer with high activity and durability is a key challenge for a proton exchange membrane water electrolyzer (PEMWE). Here, the synthesis of a novel supported IrO2 nanocatalyst with a tri-layered structure, dubbed IrO2@TaOx@TaB that is composed of ultrasmall IrO2 nanoparticles anchored on amorphous TaOx overlayer of TaB nanorods is reported. The composite electrocatalyst shows great activity and stability toward the oxygen evolution reaction (OER) in acid, thanks to its dual-interface structural feature. The electronic interaction in IrO2/TaOx interface can regulate the coverage of surface hydroxyl groups, the Ir3+/ Ir4+ ratio, and the redox peak potential of IrO2 for enhancing OER activity, while the dense TaOx overlayer can prevent further oxidation of TaB substrate and stabilize the IrO2 catalytic layers for improving structural stability during OER. The IrO2@TaOx@TaB can be used to fabricate an anode catalyst layer of PEMWE with an iridium-loading as low as 0.26 mg cm-2. The low-iridium-loading PEMWE delivers high current densities at low cell voltages (e.g., 3.9 A cm-2@2.0 V), and gives excellent activity retention for more than 1500 h at 2.0 A cm-2 current density.

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.

Boron-Induced Interstitial Effects Drive Water Oxidation on Ordered Ir-B Compounds

Interstitial filling of light atoms strongly affects the electronic structure and adsorption properties of the parent catalyst due to ligand and ensemble effects. Different from the conventional doping and surface modification, constructing ordered intermetallic structures is more promising to overcome the dissolution and reconstruction of active sites through strong interactions generated by atomic periodic arrangement, achieving joint improvement in catalytic activity and stability. However, for tightly arranged metal lattices, such as iridium (Ir), obtaining ordered filling atoms and further unveiling their interstitial effects are still limited by highly activated processes. Herein, we report a high-temperature molten salt assisted strategy to form the intermetallic Ir-B compounds (IrB1.1) with ordered filling by light boron (B) atoms. The B residing in the interstitial lattice of Ir constitutes favorable adsorption surfaces through a donor-acceptor architecture, which has an optimal free energy uphill in rate-determining step (RDS) of oxygen evolution reaction (OER), resulting in enhanced activity. Meanwhile, the strong coupling of Ir-B structural units suppresses the demetallation and reconstruction behavior of Ir, ensuring catalytic stability. Such B-induced interstitial effects endow IrB1.1 with higher OER performance than commercial IrO2, which is further validated in proton exchange membrane water electrolyzers (PEMWEs).

Accelerated Proton Transfer in Asymmetric Active Units for Sustainable Acidic Oxygen Evolution Reaction

The poor durability of Ru-based catalysts limits the practical application in proton exchange membrane water electrolysis (PEMWE). Here, we report that the asymmetric active units in Ru1-xMxO2 (M = Sb, In, and Sn) binary solid solution oxides are constructed by introducing acid-resistant p-block metal sites, breaking the activity and stability limitations of RuO2 in acidic oxygen evolution reaction (OER). Constructing highly asymmetric Ru-O-Sb units with a strong electron delocalization effect significantly shortens the spatial distance between Ru and Sb sites, improving the bonding strength of the overall structure. The unique two-electron redox couples at Sb sites in asymmetric active units trigger additional chemical steps at different OER stages, facilitating continuous proton transfer. The optimized Ru0.8Sb0.2O2 solid solution requires a superlow overpotential of 160 mV at 10 mA cm-2 and a record-breaking stability of 1100 h in an acidic electrolyte. Notably, the scale-prepared Ru0.8Sb0.2O2 achieves efficient PEMWE performance under industrial conditions. General mechanism analysis shows that the enhanced proton transport in the asymmetric Ru-O-M unit provides a new working pathway for acidic OER, breaking the scaling relationship without sacrificing stability.

Understanding the catalytic performances of metal-doped Ta2O5 catalysts for acidic oxygen evolution reaction with computations

The design of stable and active alternative catalysts to iridium oxide for the anodic oxygen evolution reaction (OER) has been a long pursuit in acidic water splitting. Tantalum pentoxide (Ta2O5) has the merit of great acidic stability but poor OER performance, yet strategies to improve its intrinsic OER activity are highly desirable. Herein, by using density functional theory (DFT) calculations combined with aqueous stability assessment from surface Pourbaix diagrams, we systematically evaluated the OER activity and acidic stability of 14 different metal-doped Ta2O5 catalysts. Apart from the experimentally reported Ir-doped Ta2O5, we computationally identified Ru- and Nb-doped Ta2O5 catalysts as another two candidates with reasonably high stability and activity in acidic OER. Our study also underscores the essence of considering stable surface states of catalysts under working conditions before a reasonable activity trend can be computationally achieved.

Aqueous alternating electrolysis prolongs electrode lifespans under harsh operation conditions

It is vital to explore effective ways for prolonging electrode lifespans under harsh electrolysis conditions, such as high current densities, acid environment, and impure water source. Here we report alternating electrolysis approaches that realize promptly and regularly repair/maintenance and concurrent bubble evolution. Electrode lifespans are improved by co-action of Fe group elemental ions and alkali metal cations, especially a unique Co2+-Na+ combo. A commercial Ni foam sustains ampere-level current densities alternatingly during continuous electrolysis for 93.8 h in an acidic solution, whereas such a Ni foam is completely dissolved in ~2 h for conventional electrolysis conditions. The work not only explores an alternating electrolysis-based system, alkali metal cation-based catalytic systems, and alkali metal cation-based electrodeposition techniques, and beyond, but demonstrates the possibility of prolonged electrolysis by repeated deposition-dissolution processes. With enough adjustable experimental variables, the upper improvement limit in the electrode lifespan would be high.