Oxygen Evolution Reaction Kinetics Research Articles

Surface hydroxylation engineering to boost oxygen evolution reaction on IrO2/TiO2 for PEM water electrolyzer

Dynamic evolutionary hypervalent Irx+ species (HVI) plays a decisive role in promoting the catalytic activity towards acidic oxygen evolution reaction (OER) on Ir-based electrocatalysts, but regulating the efficient formation of HVI remains a big challenge. Herein we propose surface hydroxylation engineering to accelerate the formation of HVI along the OER process on the OH-rich IrO2/TiO2 electrocatalyst. In-situ/operando spectroscopies demonstrate that the high concentration OH ligand accelerates the formation of HVI. DFT calculation clarifies that the dynamically evolved HVI benefits to weakening the adsorption free energy and thus boosting the OER kinetics. Differential electrochemical mass spectrometry with 18O isotope labelling experiment further unveils that the OH ligand directly participates in the OER cycle, facilitating the rapid oxidation of Ir3+ to Ir5+ and the O-O bond formation. PEM water electrolyzer with the optimized IrO2/TiO2 electrocatalyst delivers a low cell voltage of 1.787 V at 2 Acm−2 with an inaccessible low Ir usage of ca. 0.08 g/kW, while maintaining a good stability over 350 h, with an estimated cost of US$0.88 kg−1 of H2, much lower than 2026 US-DOE target.

Cation-Triggered Growth of Nanowires for Enhanced Oxygen Evolution Reaction.

The oxygen evolution reaction (OER), which occurs in a variety of energy-related devices, necessitates optimization of the reaction pathways for efficient and scalable deployment. Nevertheless, fully harnessing the advanced structure of synthetic electrocatalysts remains a significant challenge due to the inevitable surface reconstruction process during OER. Here we present an efficient and flexible method to control the surface reconstruction process by engineering an electrolyte containing trace Co2+ cation. This controllable reconstruction process enhances fast charge transfer, facilitates electroactive species transport, and exposes the inner active site, significantly improving the OER kinetics. An impressive 60% increase in current density at an applied potential of 2.2 V (vs RHE) confirms its remarkable contribution to the performance. The identification of cation-triggered reconstruction for the formation of a well-defined surface provides a novel insight into understanding electrolyte engineering and offers a viable pathway to address activity and stable concerns in electrocatalysts.

Built-in electric field construction and lattice oxygen activation for boosting alkaline electrochemical water/seawater oxidation

Oxygen evolution reaction (OER) remains a bottleneck for hydrogen production by water-alkali electrolysis at high industrial current density, but the structure–activity relationship of the catalyst and the underlying catalytic mechanism are still debated, which always limits the design of efficient catalysts. Herein, we report a proof-of-concept hierarchical hydroxide/sulfide (NiFe LDH/Ni3S2) heterostructure as model catalyst to simultaneously adjust the electronic states and activate lattice oxygen. As-activated NiFe LDH/Ni3S2 electrode displays promising OER capability with an ultra-low overpotential of 295 mV at the industrial current density of 500 mA·cm−2. The interface-induced built-in electric field effect promotes the asymmetric charge distribution on both sides. In-situ/ex-situ characterization demonstrates the adjusted intermediates adsorption/desorption and accelerated OER kinetics due to rapid electron transfer. A series of electrochemical probe experiments reveal that the OER mechanism of NiFe LDH/Ni3S2 follows the lattice oxygen mechanism. Especially, the super-strong wettability electrode design and structural advantages effectively enhance mass transfer and promote O2 desorption in alkaline water/seawater splitting systems. This work provides an avenue to break through OER limitations for efficient electrocatalytic water oxidation.

Magnesium-promoted rapid self-reconstruction of NiFe-based electrocatalysts toward efficient oxygen evolution

The acceleration of active sites formation through surface reconstruction is widely acknowledged as the crucial factor in developing high-performance oxygen evolution reaction (OER) catalysts for water splitting. Herein, a simple one-step corrosion method and magnesium (Mg)-promoted strategy are reported to develop the NiFe-based catalyst with enhanced OER performance. The Mg is introduced in NiFe materials to preparate a “pre-catalyst” Mg-Ni/Fe2O3. In-situ Raman shows that Mg doping would accelerate the self-reconstruction of Ni/Fe2O3 to form active NiOOH species during OER. In-situ infrared indicates that Mg doping benefits the formation of *OOH intermediate. Theoretical analysis further confirms that Mg doping can optimize the adsorption of oxygen intermediates, accelerating the OER kinetics. Accordingly, the Mg-Ni/Fe2O3 catalyst exhibits excellent OER performance with overpotential of 168 mV at 10 mA cm−2. The anion exchange membrane water electrolyzer achieved 200 mA cm−2 at voltage of 1.53 V, showing excellent stability over 500 h as well. This work demonstrates the potential of Mg-promoted strategy in regulating the activity of transition metal-based OER electrocatalysts.

Quenching-induced anion defects and precise Ru doping on Co3O4/CoN heterostructures for efficient overall water splitting performance

The difficulty of nitride modification is to develop simple and efficient strategies to induce defects and efficiently capture Ru atoms. With these in mind, this work innovatively constructed a Ru-Co3O4/CoN-L catalyst with abundant anion defects (oxygen vacancies (VO) and nitrogen vacancies (VN)) using the nitridation-quenching-Ru doping strategy. Surprisingly, the porous structure provided more active sites, and the VN and VO were conducive to promoting the anchoring of Ru atoms. These significantly improved the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) performances of the Ru-Co3O4/CoN/NF-L catalyst. The density functional theory results showed that the anion defects optimized the hydrogen adsorption capacity of the Ru active sites for the HER. Furthermore, Ru dopants and anion defects reduced the OER energy barrier of the Co-active sites, accelerating the HER and OER kinetics. This study proposes a new concept for defect construction and nitride-structure optimization.

A Cu doped RuO2 catalyst for efficient and durable acidic water oxidation

AbstractProducing hydrogen by water electrolysis in acidic system is essential for advancing the proton‐exchange membrane water electrolyzer (PEMWE) and maintaining the global sustainability. However, excavating highly active and durable catalysts for anodic oxygen evolution reaction (OER) in acid medium remains a great challenge due to the sluggish kinetics of OER and the severe catalyst dissolution. In this work, we developed a Cu doped RuO2 catalyst (Cu‐RuO2@CP) through simple drop casting and low temperature pyrolysis procedures. The incorporation of Cu into RuO2 executes complicated responsibilities: improving OER activity by raising more oxygen vacancies and accelerating the charge transfer between solid‐liquid interface; strengthening the long‐term durability via suppressing the over‐oxidation of RuO2 into soluble RuO4; lowering the catalyst cost by replacing with less expensive Cu. Hence, the Cu‐RuO2@CP exhibits excellent activity and stability in 0.5 M H2SO4 electrolyte, requiring a low overpotential of 200 mV to achieve 10 mA cm−2 and preserving stability for almost 500 h. Moreover, the overpotential is as low as 397 mV even when operating under an industrial‐level current density of 1000 mA cm−2, paving a new way to develop high performance OER electrocatalyst in acidic environment to promote the large‐scale utilization of PEMWE.

Unraveling the Role of Particle Size and Nanostructuring on the Oxygen Evolution Activity of Fe-Doped NiO.

Nickel-based oxides and oxyhydroxide catalysts exhibit state-of-the-art activity for the sluggish oxygen evolution reaction (OER) under alkaline conditions. A widely employed strategy to increase the gravimetric activity of the catalyst is to increase the active surface area via nanostructuring or decrease the particle size. However, the fundamental understanding about how tuning these parameters influences the density of oxidized species and their reaction kinetics remains unclear. Here, we use solution combustion synthesis, a low-cost and scalable approach, to synthesize a series of Fe0.1Ni0.9O samples from different precursor salts. Based on the precursor salt, the nanoparticle size can be changed significantly from ∼2.5 to ∼37 nm. The OER activity at pH 13 trends inversely with the particle size. Using operando time-resolved optical spectroscopy, we quantify the density of oxidized species as a function of potential and demonstrate that the OER kinetics exhibits a second-order dependence on the density of these species, suggesting that the OER mechanism relies on O-O coupling between neighboring oxidized species. With the decreasing particle size, the density of species accumulated is found to increase, and their intrinsic reactivity for the OER is found to decrease, attributed to the stronger binding of *O species (i.e., a cathodic shift of species energetics). This signifies that the high apparent OER activity per geometric area of the smaller nanoparticles is driven by their ability to accumulate a larger density of oxidized species. This study not only experimentally disentangles the influence of the density of oxidized species and intrinsic kinetics on the overall rate of the OER but also highlights the importance of tuning these parameters independently to develop more active OER catalysts.

Boosting Acidic Water Oxidation on Hematite Photoanodes with Synergistic Effects of Ce‐Doped Co3O4 Nanoparticles

AbstractPhotoelectrochemical (PEC) water splitting is pivotal for addressing the clean and renewable energy demands. However, developing low‐cost, efficient, and robust non‐precious metal catalysts for the acidic oxygen evolution reaction (OER) remains a significant challenge. In this study, we introduced Ce to modulate the electronic structure of Co3O4, enhancing the stability of Co3O4 without compromising its activity. Ce‐doped Co3O4 nanoparticles were electrodeposited onto Ti‐doped hematite surfaces (Ce : Co3O4/Fe2O3), significantly enhancing the PEC performance for water splitting under acidic conditions. These catalysts achieved high photocurrent densities and exhibited prolonged stability. Functioning as p‐type semiconductors, the Ce : Co3O4 nanoparticles not only boosted light absorption but also formed a p‐n heterojunction with the Ti‐doped hematite. This heterojunction generated a built‐in electric field that facilitated the separation and transfer of photogenerated carriers, thereby improving charge separation efficiency. Additionally, these nanoparticles expanded the active surface area of hematite and served as a co‐catalyst, markedly accelerating the OER kinetics. The Ce : Co3O4/Fe2O3 photoanode achieved an impressive photocurrent density of 1.08 mA cm−2 at 1.23 VRHE in acidic media, demonstrating its enhanced activity and stability.

Recycled Cathodes in Rechargeable Aqueous Batteries as Ready-Made Electrodes for Oxygen Evolution Catalysis.

The development of a low-cost and efficient oxygen evolution reaction (OER) electrode is of critical importance for water electrolysis technologies. The general approach to achieving a high-efficiency OER electrode is to regulate catalytic material structures by synthetic control. Here we reported an orthogonal approach to obtaining the OER electrode without intentional design and synthesis, namely, recycling MnO2 cathodes from failed rechargeable aqueous batteries and investigating them as ready-made catalytic electrodes. The recycled MnO2 cathode showed very little Zn2+ storage capacity but surprisingly high OER activity with a low overpotential of 307 mV at 10 mA cm-2 and a small Tafel slope of 77.9 mV dec-1, comparable to the state-of-the-art RuO2 catalyst (310 mV, 86.9 mV dec-1). In situ electrochemical and theoretical studies jointly revealed that the accelerated OER kinetics of the recycled MnO2 electrode was attributed to the enlarged active surface area of MnO2 and optimized electronic structure of Mn sites. This work suggests failed battery cathodes as successful catalysis electrodes for sustainable energy development.

Mesoporous graphene-supported nanostructured nickel telluride for efficient electrocatalytic oxygen evolution reaction

The electrocatalytic oxygen evolution reaction (OER) imposes significant constraints on the operational overpotential of electrochemical energy devices, such as water electrolyzers, metal-air batteries, and fuel cells. However, the sluggish kinetics of OER reactions hinder these applications, which has prompted efforts to design more effective electrocatalysts. In this study, we have synthesized NiTe via in-situ growth into mesoporous graphene (MG), with varying nominal weight percentages (5- wt%, 10- wt%, and 15- wt%) of NiTe. The resulting NiTe/MG nanohybrids displayed hexagonally structured NiTe nanoparticles with a predominant (101) surface that promotes the OER kinetics. The wrinkled 3D mesoporous graphene scaffold facilitated the attachment and dispersion of NiTe particles of nanoscale on its surface, preventing their agglomeration. The porous characteristics of the MG significantly increased the high surface area of the NiTe/MG nanohybrids (583–671 m2 g−1), promoting enhanced accessibility of active sites and mass transport beneficial for the OER. Moreover, the altered electronic structure of the NiTe in the MG nanohybrid indicates enhancing O2 desorption during the OER. The optimized composition of the 10 wt% NiTe-loaded MG nanohybrid electrocatalyst demonstrates superior OER activity compared to other nanohybrids. It exhibits a small overpotential of 250 mV at 10 mA cm−2, a low Tafel slope of 63 mV dec−1 and maintained stability in 1 M KOH (pH = 14) as the electrolyte for a prolonged period. This enhanced OER activity is ascribed to the synergistic effect of MG’s high conductivity and the improved intrinsic activity of NiTe within the nanohybrid structure.

Improving the Oxygen Evolution Reaction Kinetics in Zn-Air Battery by Iodide Oxidation Reaction.

Zinc-air batteries (ZABs) have garnered considerable attention as a highly promising contender in the field of energy storage and conversion. Nevertheless, their performance is considerably impeded by the proliferation of dendrites on the Zinc anode and the slow kinetics of the redox reaction on the air cathode. Herein, taking Ag30%@LaCoO3 (Ag30%@LCO) heterojunction catalyst as the cathode, it is demonstrated that adding KI additives to the alkaline electrolyte can not only enhance the oxygen electrocatalytic reaction but also inhibit the formation of zinc anode dendrites, thereby achieving a comprehensive improvement in the performance of ZABs. Under the action of the KI additive, the optimized Ag30%@LCO catalyst shows a decreased overpotential from 460 to 220mV at j = 10mA cm-2, while the assembled ZAB shows reduced charging potential (1.8V), and long cycle stability (180h). Furthermore, the morphology characterization results indicate a reduction in dendrites on the Zn anode. Both experimental and calculated results indicate that the presence of I- as a reaction modifier alters the trajectory of the conventional oxygen evolution reaction, resulting in a more thermodynamically favorable pathway. The introduction of KI additives as electrolytes provides a straightforward approach to developing comprehensively improved ZABs.

Bimetallic Fe/Ni-BTC MOF decorated MXene hybrid for improved oxidation of water

Electrochemical production of green hydrogen via water-splitting is an emerging technology to generate a substitute source of energy. However, due to the slow kinetics of oxygen evolution reaction (OER), high cost, lesser availability and easy oxidation of noble metal-based electrocatalysts led the explorers to find an efficient and low-cost electrocatalysts. In the current communication, we have developed a synthesis strategy for the preparation of hybrid electrocatalyst composed of bimetallic (iron, nickel) based metal organic framework (FeNiBTC MOF) and MXene (Ti3C2Tx) via solvothermal reaction. Thanks to the ultrathin heterostructure with high electrical conductivity of MXene, with abundant active sites of FeNiBTC MOF, the as-prepared hybrid electrocatalyst FeNiBTC@MXene, leads the high efficiency OER in an alkaline environment. FeNiBTC MOF was impeccably decorated on the surfaces of MXene nanosheets with different FeNiBTC to MXene ratios and was characterized via pXRD, FESEM/EDS, XPS and BET. The optimized hybrid structured catalyst (FeNiBTC@Mx-3) revealed the best performance for OER with a low overpotential of 210 mV vs. RHE at a current density of 10 mA/cm2 and a Tafel plot value of 38.4 mV/dec and stable up to 1000th CV cycles.

Ion- and surface-sensitive interactions during oxygen evolution reaction in alkaline media

Abstract Clean and sustainable energy has turned towards electrochemical water splitting as a viable solution in minimizing carbon emissions. Electrolysis of water converts electrical energy to chemical energy, through the production of hydrogen and oxygen gases, which can be harnessed for potential applications without contributing to greenhouse emissions. While this energy storage process shows great potential, its efficiency is hindered by the sluggish kinetics of the oxygen evolution reaction (OER). As a result, its widespread application in green electrolytic technologies is limited hence investigations on improving OER kinetics are of utmost importance. Recent research breakthroughs indicate that alkali metal cations are more than passive observers. They play complex roles in the electric double layer (EDL), which positively influences the OER kinetics. The presence of numerous ions and their combinations presents a challenge of complexity. This study aims to delve into the impact of alkali metal cations on OER activity due to the variance in their hydration energies. Specific investigations focusing on different alkali metal cations in solution, such as Li+, Na+, and K+, was conducted on RuO2 to gain a deeper understanding of how these ions interact with both reactants and intermediate species in the reaction kinetics. Traditional electrochemical tests, including cyclic voltammetry (CV), linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), and accelerated degradation test (ADT) measurements were employed to elucidate critical aspects such as surface activation, electric double layer interactions, catalytic activity and stability, ohmic resistance, and mass and charge transport.

Unveiling the kinetics of oxygen evolution reaction in defect-engineered B/P-incorporated cobalt-oxide electrocatalysts

Defect-rich transition-metal oxide electrocatalysts hold great promise for alkaline water electrolysis due to their enhanced activity and stability. This study presents a new strategy that significantly improve the OER activity of Co-oxide nanosheets through incorporation of B and P (B/P-CoOx NS), eventually leading to abundant surface defects and oxygen vacancies. The B/P-CoOx NS demonstrates low overpotential of 220 mV to achieve 10 mA/cm2. The electrochemical and kinetic studies coupled with conventional morphological and structural characterizations, reveal that various crystalline defects like vacancies, dislocations, twin planes, and grain boundaries play crucial roles in promoting the OH− ion adsorption, the formation of intermediates, and the desorption of oxygen molecules. The industrial viability of the developed electrocatalyst is substantiated through assessments under harsh industrial conditions of 6 M KOH at 60 °C in a zero-gap single-cell alkaline electrolyzer which achieves 1 A/cm2 at 1.95 V. Chronoamperometry tests (100 h) highlight remarkable robustness of the electrocatalyst. This work establishes a new strategy to fabricate defect-rich OER electrocatalysts, setting a precedent to achieve better OER rates with non-noble materials.

Elevated Water Oxidation by Cation Leaching Enabled Tunable Surface Reconstruction.

Water electrolysis is one promising and eco-friendly technique for energy storage, yet its overall efficiency is hindered by the sluggish kinetics of oxygen evolution reaction (OER). Therefore, developing strategies to boost OER catalyst performance is crucial. With the advances in characterization techniques, an extensive phenomenon of surface structure evolution into an active amorphous layer was uncovered. Surface reconstruction in a controlled fashion was then proposed as an emerging strategy to elevate water oxidation efficiency. In this work, Cr substitution induces the reconstruction of NiFexCr2-xO4 during cyclic voltammetry (CV) conditioning by Cr leaching, which leads to a superior OER performance. The best-performed NiFe0.25Cr1.75O4 shows a ~1500 % current density promotion at overpotential η=300 mV, which outperforms many advanced NiFe-based OER catalysts. It is also found that their OER activities are mainly determined by Ni : Fe ratio rather than considering the contribution of Cr. Meanwhile, the turnover frequency (TOF) values based on redox peak and total mass were obtained and analysed, and their possible limitations in the case of NiFexCr2-xO4 are discussed. Additionally, the high activity and durability were further verified in a membrane electrode assembly (MEA) cell, highlighting its potential for practical large-scale and sustainable hydrogen gas generation.

Elevated Water Oxidation by Cation Leaching Enabled Tunable Surface Reconstruction

AbstractWater electrolysis is one promising and eco‐friendly technique for energy storage, yet its overall efficiency is hindered by the sluggish kinetics of oxygen evolution reaction (OER). Therefore, developing strategies to boost OER catalyst performance is crucial. With the advances in characterization techniques, an extensive phenomenon of surface structure evolution into an active amorphous layer was uncovered. Surface reconstruction in a controlled fashion was then proposed as an emerging strategy to elevate water oxidation efficiency. In this work, Cr substitution induces the reconstruction of NiFexCr2‐xO4 during cyclic voltammetry (CV) conditioning by Cr leaching, which leads to a superior OER performance. The best‐performed NiFe0.25Cr1.75O4 shows a ~1500 % current density promotion at overpotential η=300 mV, which outperforms many advanced NiFe‐based OER catalysts. It is also found that their OER activities are mainly determined by Ni : Fe ratio rather than considering the contribution of Cr. Meanwhile, the turnover frequency (TOF) values based on redox peak and total mass were obtained and analysed, and their possible limitations in the case of NiFexCr2‐xO4 are discussed. Additionally, the high activity and durability were further verified in a membrane electrode assembly (MEA) cell, highlighting its potential for practical large‐scale and sustainable hydrogen gas generation.

Amorphous/crystalline NiFe LDH hierarchical nanostructure for large-current-density electrocatalytic water oxidation

Amorphous structure shows more catalytic active sites but poor conductivity towards oxygen evolution reaction (OER). Herein, we fabricated a novel three-dimensional (3D) self-supported NiFe layered double hydroxides (LDH) catalytic material with amorphous/ crystalline interface, aiming to obtain both excellent catalytic activity and conductivity. This homogeneous hierarchical structure exhibits a high catalytic activity and robustness, giving the industrially required current density of 1000 mA cm−2 at an ultralow overpotential of 359.8 mV with 240-hour stability in 1 M KOH. In-situ electrochemical tests reveal that the amorphous/crystalline interface boosts the intermediates evolution and OER kinetics, which might be ascribed to considerable active sites and fast charge transfer. Moreover, the 3D structure has unique superhydrophilicity and superaerophobicity, which can further accelerate the electrolyte penetration, facilitate the bubbles desorption from the electrode, enhance mass transport, and thus improve OER performance at large current densities. This work provides a feasible way to develop high-performance electrocatalytic material via constructing homogeneous amorphous/crystalline interface.

Construction and enhancement of built-in electric field for efficient oxygen evolution reaction

The construction and regulation of built-in electric field (BIEF) are considered effective strategies for enhancing the oxygen evolution reaction (OER) performance of transition metal-based electrocatalysts. Herein, we present a strategy to regulate the electronic structure of nickel–iron layered double hydroxide (NiFe-LDH) by constructing and enhancing the BIEF induced by in-situ heterojunction transformation. This concept is demonstrated through the design and synthesis of Ag2S@S/NiFe-LDH (p-n heterojunction) and Ag@S/NiFe-LDH (Mott-Schottky heterojunction). Benefiting from the larger BIEF of Mott-Schottky heterojunction, efficient electron transfer occurs at the interface between silver (Ag) and NiFe-LDH. As a result, Ag@S/NiFe-LDH exhibits excellent OER performance, requiring only a 232 mV overpotential at 1 M KOH to achieve a current density of 100 mA cm−2, with a small Tafel slope of 73 mV dec-1, as well as excellent electrocatalytic durability. Density functional theory (DFT) calculations further verified that stronger BIEF in Mott-Schottky heterojunction enhances the electron interaction at the interfaces, reduces the energy barrier for the rate-determining step (RDS), and accelerates the OER kinetics. This work provides an effective strategy for designing catalyst with larger BIEF to enhance electrocatalytic activity.

Heterogenous Fe2P-NiFe layered double hydroxide nanostructures for boosting oxygen evolution reaction

One effective approach to improve the slow kinetics of oxygen evolution reaction (OER) for water splitting is to develop high-performance electrocatalysts. The Fe2P-NiFe LDH/NF catalyst with hetero-interfaces in this work was created using an easy two-step process that involved electrodeposition and hydrothermal processes. In the alkaline electrolyte, the as-prepared Fe2P-NiFe LDH/NF demonstrates improved OER performance. The Fe2P-NiFe LDH/NF catalyst only requires a low overpotential of 200 mV to achieve 10 mA cm−2, indicating the high efficiency for electrochemical reactions. Additionally, the Tafel slope of the catalyst is measured to be 59 mV dec−1. Even at a high current density of 100 mA cm−2, the overpotential of Fe2P-NiFe LDH/NF is only 250 mV, which is noticeably smaller than that of NiFe LDH/NF and commercial IrO2/NF. In addition, the current density of Fe2P-NiFe LDH/NF remains stable without any degradation during a 12 h durability test. By the SEM and TEM results, it showed that Fe2P with a crystalline-amorphous hybrid structure is grown at the edge of NiFe LDH nanosheets. The average length of Fe2P-NiFe LDH nanosheet is about 600 nm, which is longer than that of NiFe LDH nanosheets (about 400 nm). This increased size provides a larger specific area. As a result, more exposed active sites and a synergistic effect at the hetero-interface between Fe2P and NiFe LDH were achieved, which produces a distinct decrease in charge transfer resistance and promotes catalytic performance. Moreover, density functional theory (DFT) calculations emphasize the significance of rationally designed hetero-interfaces in increasing OER activity. It is found that the Fe2P-NiFe LDH/NF with a hetero-interface is an efficient OER catalyst for water splitting.

Advances in Noble Metal Electrocatalysts for Acidic Oxygen Evolution Reaction: Construction of Under-Coordinated Active Sites.

Renewable energy-driven proton exchange membrane water electrolyzer (PEMWE) attracts widespread attention as a zero-emission and sustainable technology. Oxygen evolution reaction (OER) catalysts with sluggish OER kinetics and rapid deactivation are major obstacles to the widespread commercialization of PEMWE. To date, although various advanced electrocatalysts have been reported to enhance acidic OER performance, Ru/Ir-based nanomaterials remain the most promising catalysts for PEMWE applications. Therefore, there is an urgent need to develop efficient, stable, and cost-effective Ru/Ir catalysts. Since the structure-performance relationship is one of the most important tools for studying the reaction mechanism and constructing the optimal catalytic system. In this review, the recent research progress from the construction of unsaturated sites to gain a deeper understanding of the reaction and deactivation mechanism of catalysts is summarized. First, a general understanding of OER reaction mechanism, catalyst dissolution mechanism, and active site structure is provided. Then, advances in the design and synthesis of advanced acidic OER catalysts are reviewed in terms of the classification of unsaturated active site design, i.e., alloy, core-shell, single-atom, and framework structures. Finally, challenges and perspectives are presented for the future development of OER catalysts and renewable energy technologies for hydrogen production.