Exceptionally active iridium evolved from a pseudo-cubic perovskite for oxygen evolution in acid

Development of Perovskite Oxide-Based Electrocatalysts for Oxygen Evolution Reaction

Perovskite oxides are studied as electrocatalysts for oxygen evolution reactions (OER) because of their low cost, tunable structure, high stability, and good catalytic activity. However, there are two main challenges for most perovskite oxides to be efficient in OER, namely less active sites and low electrical conductivity, leading to limited catalytic performance. To overcome these intrinsic obstacles, various strategies are developed to enhance their catalytic activities in OER. In this review, the recent developments of these strategies is comprehensively summarized and systematically discussed, including composition engineering, crystal facet control, morphology modulation, defect engineering, and hybridization. Finally, perspectives on the design of perovskite oxide-based electrocatalysts for practical applications in OER are given.

Restructuring highly electron-deficient metal-metal oxides for boosting stability in acidic oxygen evolution reaction

The poor catalyst stability in acidic oxidation evolution reaction (OER) has been a long-time issue. Herein, we introduce electron-deficient metal on semiconducting metal oxides-consisting of Ir (Rh, Au, Ru)-MoO₃ embedded by graphitic carbon layers (IMO) using an electrospinning method. We systematically investigate IMO’s structure, electron transfer behaviors, and OER catalytic performance by combining experimental and theoretical studies. Remarkably, IMO with an electron-deficient metal surface (Ir^x+; x > 4) exhibit a low overpotential of only ~156 mV at 10 mA cm⁻² and excellent durability in acidic media due to the high oxidation state of metal on MoO₃. Furthermore, the proton dissociation pathway is suggested via surface oxygen serving as proton acceptors. This study suggests high stability with high catalytic performance in these materials by creating electron-deficient surfaces and provides a general, unique strategy for guiding the design of other metal-semiconductor nanocatalysts.

The poor catalyst stability for oxygen evolution in acidic media has been a long-time issue. Here, authors demonstrate iridium on MoO₃ exhibits a low overpotential for oxygen evolution and excellent durability in acidic media due to the high oxidation state of iridium metal on MoO₃.

Tuning Reconstruction Level of Precatalysts to Design Advanced Oxygen Evolution Electrocatalysts

Surface reconstruction engineering is an effective strategy to promote the catalytic activities of electrocatalysts, especially for water oxidation. Taking advantage of the physicochemical properties of precatalysts by manipulating their structural self-reconstruction levels provide a promising methodology for achieving suitable catalysts. In this review, we focus on recent advances in research related to the rational control of the process and level of surface transformation ultimately to design advanced oxygen evolution electrocatalysts. We start by discussing the original contributions to surface changes during electrochemical reactions and related factors that can influence the electrocatalytic properties of materials. We then present an overview of current developments and a summary of recently proposed strategies to boost electrochemical performance outcomes by the controlling structural self-reconstruction process. By conveying these insights, processes, general trends, and challenges, this review will further our understanding of surface reconstruction processes and facilitate the development of high-performance electrocatalysts beyond water oxidation.

Recent advances in activating surface reconstruction for the high-efficiency oxygen evolution reaction

A climax in the development of cost-effective and high-efficiency transition metal-based electrocatalysts has been witnessed recently for sustainable energy and related conversion technologies. In this regard, structure-activity relationships based on several descriptors have already been proposed to rationally design electrocatalysts. However, the dynamic reconstruction of the surface structures and compositions of catalysts during electrocatalytic water oxidation, especially during the anodic oxygen evolution reaction (OER), complicate the streamlined prediction of the catalytic activity. With the achievements in operando and in situ techniques, it has been found that electrocatalysts undergo surface reconstruction to form the actual active species in situ accompanied with an increase in their oxidation state during OER in alkaline solution. Accordingly, a thorough understanding of the surface reconstruction process plays a critical role in establishing unambiguous structure-composition-property relationships in pursuit of high-efficiency electrocatalysts. However, several issues still need to be explored before high electrocatalytic activities can be realized, as follows: (1) the identification of initiators and pathways for surface reconstruction, (2) establishing the relationships between structure, composition, and electrocatalytic activity, and (3) the rational manipulation of in situ catalyst surface reconstruction. In this review, the recent progress in the surface reconstruction of transition metal-based OER catalysts including oxides, non-oxides, hydroxides and alloys is summarized, emphasizing the fundamental understanding of reconstruction behavior from the original precatalysts to the actual catalysts based on operando analysis and theoretical calculations. The state-of-the-art strategies to tailor the surface reconstruction such as substituting/doping with metals, introducing anions, incorporating oxygen vacancies, tuning morphologies and exploiting plasmonic/thermal/photothermal effects are then introduced. Notably, comprehensive operando/in situ characterization together with computational calculations are responsible for unveiling the improvement mechanism for OER. By delivering the progress, strategies, insights, techniques, and perspectives, this review will provide a comprehensive understanding of the surface reconstruction in transition metal-based OER catalysts and future guidelines for their rational development.

Comprehensive Understandings into Complete Reconstruction of Precatalysts: Synthesis, Applications, and Characterizations

Reconstruction induced by external environment (such as applied voltage bias and test electrolytes) changes catalyst component and catalytic behaviors. Investigations of complete reconstruction in energy conversion recently receive intensive attention, which promote the targeted design of top-performance materials with maximum component utilization and good stability. However, the advantages of complete reconstruction, its design strategies, and extensive applications have not achieved the profound understandings and summaries it deserves. Here, this review systematically summarizes several important advances in complete reconstruction for the first time, which includes 1) fundamental understandings of complete reconstruction, the characteristics and advantages of completely reconstructed catalysts, and their design principles, 2) types of reconstruction-involved precatalysts for oxygen evolution reaction catalysis in wide pH solution, and origins of limited reconstruction degree as well as design strategies/principles toward complete reconstruction, 3) complete reconstruction for novel material synthesis and other electrocatalysis fields, and 4) advanced in situ/operando or multiangle/level characterization techniques to capture the dynamic reconstruction processes and real catalytic contributors. Finally, the existing major challenges and unexplored/unsolved issues on studying the reconstruction chemistry are summarized, and an outlook for the further development of complete reconstruction is briefly proposed. This review will arouse the attention on complete reconstruction materials and their applications in diverse fields.

Constant Change: Exploring Dynamic Oxygen Evolution Reaction Catalysis and Material Transformations in Strontium Zinc Iridate Perovskite in Acid

While iridium-based perovskites have been identified as promising candidates for the oxygen evolution reaction (OER) in proton exchange membrane (PEM) electrolyzer applications, an improved fundamental understanding of these highly dynamic materials under reaction conditions is needed to inform more robust future catalyst design. Herein, we study the highly active SrIr_0.8Zn_0.2O₃ perovskite for the OER in acid by employing electrochemical experiments with in situ and ex situ characterization techniques to understand the dynamic nature of this material at both short and long time scales. We observe initial intrinsic OER activity improvement with electrochemical cycling as well as an initial increase of Ir oxidation state under OER conditions via in situ X-ray absorption spectroscopy. We discover that the SrIr_0.8Zn_0.2O₃ perovskite experiences an OER-induced metal to insulator transition (MIT) with extensive electrochemical cycling, caused by surface reorganization and changes to the material crystallinity that occur with exposure to an acidic and oxidizing environment. Our novel identification of an OER-induced MIT for iridate perovskites reveals an additional stability concern for iridate catalysts which are known to experience material dissolution challenges; this work ultimately aims to inform future catalyst material design for PEM water electrolysis applications.

Spin pinning effect to reconstructed oxyhydroxide layer on ferromagnetic oxides for enhanced water oxidation

Producing hydrogen by water electrolysis suffers from the kinetic barriers in the oxygen evolution reaction (OER) that limits the overall efficiency. With spin-dependent kinetics in OER, to manipulate the spin ordering of ferromagnetic OER catalysts (e.g., by magnetization) can reduce the kinetic barrier. However, most active OER catalysts are not ferromagnetic, which makes the spin manipulation challenging. In this work, we report a strategy with spin pinning effect to make the spins in paramagnetic oxyhydroxides more aligned for higher intrinsic OER activity. The spin pinning effect is established in oxide_FM/oxyhydroxide interface which is realized by a controlled surface reconstruction of ferromagnetic oxides. Under spin pinning, simple magnetization further increases the spin alignment and thus the OER activity, which validates the spin effect in rate-limiting OER step. The spin polarization in OER highly relies on oxyl radicals (O∙) created by 1^st dehydrogenation to reduce the barrier for subsequent O-O coupling.

Water oxidation to triplet oxygen requires a spin polarization process for faster kinetics. Here, the authors show an interface spin pinning effect between ferromagnetic oxides and reconstructed oxyhydroxide surface layer, where the spin ordering in paramagnetic oxyhydroxide catalyst layer can be tuned to improve the intrinsic activity.

Electrocatalysis for the Oxygen Evolution Reaction in Acidic Media: Progress and Challenges

The oxygen evolution reaction (OER) is the efficiency-determining half-reaction process of high-demand, electricity-driven water splitting due to its sluggish four-electron transfer reaction. Tremendous effects on developing OER catalysts with high activity and strong acid-tolerance at high oxidation potentials have been made for proton-conducting polymer electrolyte membrane water electrolysis (PEMWE), which is one of the most promising future hydrogen-fuel-generating technologies. This review presents recent progress in understanding OER mechanisms in PEMWE, including the adsorbate evolution mechanism (AEM) and the lattice-oxygen-mediated mechanism (LOM). We further summarize the latest strategies to improve catalytic performance, such as surface/interface modification, catalytic site coordination construction, and electronic structure regulation of catalytic centers. Finally, challenges and prospective solutions for improving OER performance are proposed.

Spin-polarized oxygen evolution reaction under magnetic field

The oxygen evolution reaction (OER) is the bottleneck that limits the energy efficiency of water-splitting. The process involves four electrons’ transfer and the generation of triplet state O₂ from singlet state species (OH^- or H₂O). Recently, explicit spin selection was described as a possible way to promote OER in alkaline conditions, but the specific spin-polarized kinetics remains unclear. Here, we report that by using ferromagnetic ordered catalysts as the spin polarizer for spin selection under a constant magnetic field, the OER can be enhanced. However, it does not applicable to non-ferromagnetic catalysts. We found that the spin polarization occurs at the first electron transfer step in OER, where coherent spin exchange happens between the ferromagnetic catalyst and the adsorbed oxygen species with fast kinetics, under the principle of spin angular momentum conservation. In the next three electron transfer steps, as the adsorbed O species adopt fixed spin direction, the OER electrons need to follow the Hund rule and Pauling exclusion principle, thus to carry out spin polarization spontaneously and finally lead to the generation of triplet state O₂. Here, we showcase spin-polarized kinetics of oxygen evolution reaction, which gives references in the understanding and design of spin-dependent catalysts.

Here, authors demonstrate the ferromagnetic catalyst to facilitate spin polarization in water oxidation reaction. They find the ferromagnetic-exchange-like behaviour between the ferromagnetic catalyst and the adsorbed oxygen species.

Recent Development of Oxygen Evolution Electrocatalysts in Acidic Environment

The proton exchange membrane (PEM) water electrolysis is one of the most promising hydrogen production techniques. The oxygen evolution reaction (OER) occurring at the anode dominates the overall efficiency. Developing active and robust electrocatalysts for OER in acid is a longstanding challenge for PEM water electrolyzers. Most catalysts show unsatisfied stability under strong acidic and oxidative conditions. Such a stability challenge also leads to difficulties for a better understanding of mechanisms. This review aims to provide the current progress on understanding of OER mechanisms in acid, analyze the promising strategies to enhance both activity and stability, and summarize the state-of-the-art catalysts for OER in acid. First, the prevailing OER mechanisms are reviewed to establish the physicochemical structure-activity relationships for guiding the design of highly efficient OER electrocatalysts in acid with stable performance. The reported approaches to improve the activity, from macroview to microview, are then discussed. To analyze the problem of instability, the key factors affecting catalyst stability are summarized and the surface reconstruction is discussed. Various noble-metal-based OER catalysts and the current progress of non-noble-metal-based catalysts are reviewed. Finally, the challenges and perspectives for the development of active and robust OER catalysts in acid are discussed.

Stabilizing Highly Active Ru Sites by Suppressing Lattice Oxygen Participation in Acidic Water Oxidation

In hydrogen production, the anodic oxygen evolution reaction (OER) limits the energy conversion efficiency and also impacts stability in proton-exchange membrane water electrolyzers. Widely used Ir-based catalysts suffer from insufficient activity, while more active Ru-based catalysts tend to dissolve under OER conditions. This has been associated with the participation of lattice oxygen (lattice oxygen oxidation mechanism (LOM)), which may lead to the collapse of the crystal structure and accelerate the leaching of active Ru species, leading to low operating stability. Here we develop Sr-Ru-Ir ternary oxide electrocatalysts that achieve high OER activity and stability in acidic electrolyte. The catalysts achieve an overpotential of 190 mV at 10 mA cm^-2 and the overpotential remains below 225 mV following 1,500 h of operation. X-ray absorption spectroscopy and ¹⁸O isotope-labeled online mass spectroscopy studies reveal that the participation of lattice oxygen during OER was suppressed by interactions in the Ru-O-Ir local structure, offering a picture of how stability was improved. The electronic structure of active Ru sites was modulated by Sr and Ir, optimizing the binding energetics of OER oxo-intermediates.

IrCuNi Deeply Concave Nanocubes as Highly Active Oxygen Evolution Reaction Electrocatalyst in Acid Electrolyte

Proton exchange membrane water electrolyzer can sustainably and environmentally friendly produce hydrogen. However, it is hindered by the lack of high-performance anode catalysts for oxygen evolution reaction (OER) in acid electrolyte. Herein, IrCuNi deeply concave nanocubes (IrCuNi DCNCs) are successfully synthesized from the selective etching of the facet of cubic nanoparticles, and they significantly boost the OER. The obtained IrCuNi DCNCs show high activity toward OER in the acidic electrolyte, which only requires an overpotential of 273 mV to achieve the OER current density of 10 mA cm^-2 at a low Ir loading of 6.0 μg_Ir cm^-2. The precious metal based mass activity is 6.6 A mg_Ir^-1 at 1.53 V, which is 19 times as high as that of pristine Ir. It demonstrates that the outstanding catalytic performance is beneficial from the well-defined multimetal concave nanostructures, which may shed light on the fabrication of efficient water electrolyzers.

Grain-Boundary-Engineered La2CuO4 Perovskite Nanobamboos for Efficient CO2 Reduction Reaction

Electroreduction of carbon dioxide (CO₂RR) has been regarded as a promising approach to realize the production of useful fuels and to decrease greenhouse gas levels simultaneously, where high-efficiency catalysts are required. Herein, we report La₂CuO₄ nanobamboo (La₂CuO₄ NBs) perovskite with rich twin boundaries showing a high Faraday efficiency (FE) of 60% toward ethylene (C₂H₄), whereas bulk La₂CuO₄ exhibits a FE_CO of 91%. X-ray absorption spectroscopy (XAS) reveals that the Cu in La₂CuO₄ NBs is in the Cu²⁺ state, and no obvious change can be observed during the catalytic process, as monitored by in situ XAS. Density functional theory calculations reveal that the superior FE_C2H4 of La₂CuO₄ NBs originates from the active (113) surfaces with intrinsic strain. The formation of gap states annihilates the electron transfer barrier of C-C coupling, resulting in the high FE_C2H4. This work provides a new perspective for developing efficient perovskite catalysts via grain boundary engineering.

Active Phase on SrCo1-x Fe x O3-δ (0 ≤ x ≤ 0.5) Perovskite for Water Oxidation: Reconstructed Surface versus Remaining Bulk

Perovskite oxides based on earth-abundant transition metals have been extensively explored as promising oxygen evolution reaction (OER) catalysts in alkaline media. The (electro)chemically induced transformation of their initially crystalline surface into an amorphous state has been reported for a few highly active perovskite catalysts. However, little knowledge is available to distinguish the contribution of the amorphized surface from that of the remaining bulk toward the OER. In this work, we utilize the promoting effects of two types of Fe modification, i.e., bulk Fe dopant and Fe ions absorbed from the electrolyte, on the OER activity of SrCoO_3-δ model perovskite to identify the active phase. Transmission electron microscopy and X-ray photoelectron spectroscopy confirmed the surface amorphization of SrCoO_3-δ as well as SrCo_0.8Fe_0.2O_3-δ after potential cycling in Fe-free KOH solution. By further cycling in Fe-spiked electrolyte, Fe was incorporated into the amorphized surface of SrCoO_3-δ (SrCoO_3-δ + Fe³⁺), yielding approximately sixfold increase in activity. Despite the difference in remaining perovskites, SrCoO_3-δ + Fe³⁺ and SrCo_0.8Fe_0.2O_3-δ exhibited remarkably similar activity. These results reflect that the in situ developed surface species are directly responsible for the measured OER activity, whereas the remaining bulk phases have little impact.

Atomic-Level Manipulations in Oxides and Alloys for Electrocatalysis of Oxygen Evolution and Reduction

As chemical reactions and charge-transfer simultaneously occur on the catalyst surface during electrocatalysis, numerous studies have been carried out to attain an in-depth understanding on the correlation among the surface structure and composition, the electrical transport, and the overall catalytic activity. Compared with other catalysis reactions, a relatively larger activation barrier for oxygen evolution/reduction reactions (OER/ORR), where multiple electron transfers are involved, is noted. Many works over the past decade thus have been focused on the atomic-scale control of the surface structure and the precise identification of surface composition change in catalyst materials to achieve better conversion efficiency. In particular, recent advances in various analytical tools have enabled noteworthy findings of unexpected catalytic features at atomic resolution, providing significant insights toward reducing the activation barriers and subsequently improving the catalytic performance. In addition to summarizing important surface issues, including lattice defects, related to the OER and ORR in this Review, we present the current status and discuss future perspectives of oxide- and alloy-based catalysts in terms of atomic-scale observation and manipulation.

Low-iridium electrocatalysts for acidic oxygen evolution

The widespread use of proton-exchange membrane water electrolysis is limited by the dynamically sluggish oxygen evolution reaction (OER), which is mediated by noble iridium-based materials as active and stable electrocatalysts. Significant efforts have been made to decrease the amount of iridium in OER catalysts without sacrificing their catalytic performances. In this frontier paper, we present the main common issues relevant to the iridium-catalyzed OER, including catalytically active species, catalytic mechanisms and activity-stability relation. We also take iridium-based perovskites as an example, and summarize the recent theoretical and experimental advances in available strategies that can lead to highly efficient, low-iridium oxygen evolution electrocatalysts under acidic conditions. Finally, we propose the remaining challenges and future directions for exploring acidic OER catalysts.

Heterostructure and Oxygen Vacancies Promote NiFe2 O4 /Ni3 S4 toward Oxygen Evolution Reaction and Zn-Air Batteries

Developing high-performance catalysts for oxygen evolution reaction (OER) is critical for the widespread applications of clean and sustainable energy through electrochemical devices such as zinc-air batteries and (photo)electrochemical water splitting. Constructing heterostructure and oxygen vacancies have demonstrated great promises to boost the OER performance. Herein, we report a facile strategy to fabricate hetero-structured NiFe₂ O₄ /Ni₃ S₄ nanorods, where NiFe₂ O₄ can be derived from Fe-based metal-organic frameworks (MOFs). The NiFe₂ O₄ /Ni₃ S₄ catalyst exhibited excellent OER performance, evidenced by an overpotential value of 357 mV at the current density of 20 mA cm^-2 , and a small Tafel slope of 87.46 mV dec^-1 in 1 M KOH, superior to the benchmark IrO₂ catalyst. Moreover, NiFe₂ O₄ /Ni₃ S₄ outperformed with regard to long-term durability for OER than IrO₂ . Such outstanding OER performance is mainly accounted by the interface between NiFe₂ O₄ and Ni₃ S₄ , and the presence of rich oxygen vacancies. When employed as air-cathode in zinc-air batteries, the NiFe₂ O₄ /Ni₃ S₄ decorated battery had a high round-trip efficiency of 62.1% at 10 h, and possessed long-term stability of >50 h. This study may pave the way for fabricating non-noble-metal-based cost-effective, efficient and durable electrocatalysts for OER, zinc-air batteries, and beyond.

Active Site Engineering in Porous Electrocatalysts

Electrocatalysis is at the center of many sustainable energy conversion technologies that are being developed to reduce the dependence on fossil fuels. The past decade has witnessed significant progresses in the exploitation of advanced electrocatalysts for diverse electrochemical reactions involved in electrolyzers and fuel cells, such as the hydrogen evolution reaction (HER), the oxygen reduction reaction (ORR), the CO₂ reduction reaction (CO₂ RR), the nitrogen reduction reaction (NRR), and the oxygen evolution reaction (OER). Herein, the recent research advances made in porous electrocatalysts for these five important reactions are reviewed. In the discussions, an attempt is made to highlight the advantages of porous electrocatalysts in multiobjective optimization of surface active sites including not only their density and accessibility but also their intrinsic activity. First, the current knowledge about electrocatalytic active sites is briefly summarized. Then, the electrocatalytic mechanisms of the five above-mentioned reactions (HER, ORR, CO₂ RR, NRR, and OER), the current challenges faced by these reactions, and the recent efforts to meet these challenges using porous electrocatalysts are examined. Finally, the future research directions on porous electrocatalysts including synthetic strategies leading to these materials, insights into their active sites, and the standardized tests and the performance requirements involved are discussed.

Photo/Bio-Electrochemical Systems for Environmental Remediation and Energy Harvesting

Water and energy systems are interdependent: water is utilized in each stage of energy production, and energy is required to extract, treat, and deliver water for many uses. However, energy and water systems are usually developed and managed independently. In the quest to develop environmentally friendly and energy-efficient solutions for water and energy issues, photoelectrochemical (PEC) energy conversion and microbial electrochemical (MEC) systems show profound potential for addressing environmental remediation problems and harvesting energy simultaneously. Herein, PEC, MEC, and their variant hybrid systems toward energy conversion and environmental remediation are summarized and discussed.

Fundamental understanding of the acidic oxygen evolution reaction: mechanism study and state-of-the-art catalysts

The oxygen evolution reaction (OER), as the anodic reaction of water electrolysis (WE), suffers greatly from low reaction kinetics and thereby hampers the large-scale application of WE. Seeking active, stable, and cost-effective OER catalysts in acidic media is therefore of great significance. In this perspective, studying the reaction mechanism and exploiting advanced anode catalysts are of equal importance, where the former provides guidance for material structural engineering towards a better catalytic activity. In this review, we first summarize the currently proposed OER catalytic mechanisms, i.e., the adsorbate evolution mechanism (AEM) and lattice oxygen evolution reaction (LOER). Subsequently, we critically review several acidic OER electrocatalysts reported recently, with focus on structure-performance correlation. Finally, a few suggestions on exploring future OER catalysts are proposed.

Constructing an Adaptive Heterojunction as a Highly Active Catalyst for the Oxygen Evolution Reaction

Electrochemical water splitting is of prime importance to green energy technology. Particularly, the reaction at the anode side, namely the oxygen evolution reaction (OER), requires a high overpotential associated with OO bond formation, which dominates the energy-efficiency of the whole process. Activating the anionic redox chemistry of oxygen in metal oxides, which involves the formation of superoxo/peroxo-like (O₂ )^{n -} , commonly occurs in most highly active catalysts during the OER process. In this study, a highly active catalyst is designed: electrochemically delithiated LiNiO₂ , which facilitates the formation of superoxo/peroxo-like (O₂ )^{n -} species, i.e., NiOO*, for enhancing OER activity. The OER-induced surface reconstruction builds an adaptive heterojunction, where NiOOH grows on delithiated LiNiO₂ (delithiated-LiNiO₂ /NiOOH). At this junction, the lithium vacancies within the delithiated LiNiO₂ optimize the electronic structure of the surface NiOOH to form stable NiOO* species, which enables better OER activity. This finding provides new insight for designing highly active catalysts with stable superoxo-like/peroxo-like (O₂ )^{n -} for water oxidation.

Assembly of a Highly Active Iridium-Based Oxide Oxygen Evolution Reaction Catalyst by Using Metal-Organic Framework Self-Dissolution

The proton exchange membrane (PEM) electrolyzer for hydrogen production has multiple advantages but is greatly restricted by expensive iridium and sluggish oxygen evolution reaction (OER) kinetics. The most promising way to reduce the precious metal loading is to design and develop highly active Ir-based catalysts. In this study, a versatile approach is reported to prepare a hybrid in the form of a catalyst-support structure (Fe-IrO_x@α-Fe₂O₃, abbreviated Ir@Fe-MF) by utilizing the self-dissolving properties of Fe-MIL-101 under aqueous conditions. The formation of this hybrid is mainly due to the Ir⁴⁺ and released Fe³⁺ ions coprecipitated to assemble into Fe-IrO_x nanoparticles, and the Fe³⁺ released from the inward collapse of the metal-organic framework (MOF) spontaneously forms α-Fe₂O₃. The prepared Ir@Fe-MF-2 hybrid exhibits enhanced catalytic activity toward OER with a lower onset potential and Tafel slop, and only 260 mV overpotential is required to drive the current density to reach 10 mA cm^-2. The performed characterizations clearly indicate that the IrO₆ coordination structure is changed significantly by Fe incorporated into the IrO₂ lattice. The performed X-ray adsorption spectra (XAS) provides evidence that Ir 5d orbital degeneracy is eliminated because of multiple orbitals being semi-occupied in the presence of Fe, which is mainly responsible for the enhancement of OER activity. These findings open an opportunity for the design and preparation of more efficient OER catalysts of transition metal oxides by utilization of the MOF materials. It should be highlighted that a long-term stability of this catalyst run at a high current density in acidic conditions still faces great challenges.

Toward Efficient Electrocatalytic Oxygen Evolution: Emerging Opportunities with Metallic Pyrochlore Oxides for Electrocatalysts and Conductive Supports

The design of active and stable electrocatalysts for oxygen evolution reaction is a key enabling step toward efficient utilization of renewable energy. Along with efforts to develop high-performance electrocatalysts for oxygen evolution reaction, pyrochlore oxides have emerged as highly active and stable materials that function as catalysts as well as conductive supports for hybrid catalysts. The compositional flexibility of pyrochlore oxide provides many opportunities to improve electrocatalytic performance by manipulating material structures and properties. In this Outlook, we first discuss the recent advances in developing metallic pyrochlore oxides as oxygen evolution catalysts, along with elucidation of their reaction mechanisms, and then introduce an emerging area of using pyrochlore oxides as conductive supports to design hybrid catalysts to further improve the OER activity. Finally, the remaining challenges and emerging opportunities for pyrochlore oxides as electrocatalysts and conductive supports are discussed.

Vacancy-engineered catalysts for water electrolysis

Vacancy-engineered electrocatalysts showing various effects on improving performances toward water electrolysis.

A review on fundamentals for designing oxygen evolution electrocatalysts

Electricity-driven water splitting can facilitate the storage of electrical energy in the form of hydrogen gas. As a half-reaction of electricity-driven water splitting, the oxygen evolution reaction (OER) is the major bottleneck due to the sluggish kinetics of this four-electron transfer reaction. Developing low-cost and robust OER catalysts is critical to solving this efficiency problem in water splitting. The catalyst design has to be built based on the fundamental understanding of the OER mechanism and the origin of the reaction overpotential. In this article, we summarize the recent progress in understanding OER mechanisms, which include the conventional adsorbate evolution mechanism (AEM) and lattice-oxygen-mediated mechanism (LOM) from both theoretical and experimental aspects. We start with the discussion on the AEM and its linked scaling relations among various reaction intermediates. The strategies to reduce overpotential based on the AEM and its derived descriptors are then introduced. To further reduce the OER overpotential, it is necessary to break the scaling relation of HOO* and HO* intermediates in conventional AEM to go beyond the activity limitation of the volcano relationship. Strategies such as stabilization of HOO*, proton acceptor functionality, and switching the OER pathway to LOM are discussed. The remaining questions on the OER and related perspectives are also presented at the end.

Charge transfer and spillover phenomena in ceria-supported iridium catalysts: A model study

Iridium-based materials are among the most active bifunctional catalysts in heterogeneous catalysis and electrocatalysis. We have investigated the properties of atomically defined Ir/CeO₂(111) model systems supported on Cu(111) and Ru(0001) by means of synchrotron radiation photoelectron spectroscopy, resonant photoemission spectroscopy, near ambient pressure X-ray photoelectron spectroscopy (NAP XPS), scanning tunneling microscopy, and temperature programmed desorption. Electronic metal-support interactions in the Ir/CeO₂(111) system are accompanied by charge transfer and partial reduction of CeO₂(111). The magnitude of the charge transfer depends strongly on the Ir coverage. The Ir/CeO₂(111) system is stable against sintering upon annealing to 600 K in ultrahigh vacuum (UHV). Annealing of Ir/CeO₂(111) in UHV triggers the reverse oxygen spillover above 450 K. The interaction of hydrogen with Ir/CeO₂(111) involves hydrogen spillover and reversible spillover between 100 and 400 K accompanied by the formation of water above 190 K. Formation of water coupled with the strong reduction of CeO₂(111) represents the dominant reaction channel upon annealing in H₂ above 450 K. The interaction of Ir/CeO₂(111) with oxygen has been investigated at moderate and NAP conditions. Additionally, the formation and stability of iridium oxide prepared by deposition of Ir in oxygen atmosphere was investigated upon annealing in UHV and under exposure to H₂. The oxidation of Ir nanoparticles under NAP conditions yields stable IrO_x nanoparticles. The stability of Ir and IrO_x nanoparticles under oxidizing conditions is hampered, however, by encapsulation by cerium oxide above 450 K and additionally by copper and ruthenium oxides under NAP conditions.

In Situ Observation of Nanoparticle Exsolution from Perovskite Oxides: From Atomic Scale Mechanistic Insight to Nanostructure Tailoring

Understanding and controlling the formation of nanoparticles at the surface of functional oxide supports is critical for tuning activity and stability for catalytic and energy conversion applications. Here, we use a latest generation environmental transmission electron microscope to follow the exsolution of individual nanoparticles at the surface of perovskite oxides, with ultrahigh spatial and temporal resolution. Qualitative and quantitative analysis of the data reveals the atomic scale processes that underpin the formation of the socketed, strain-inducing interface that confers exsolved particles their exceptional stability and reactivity. This insight also enabled us to discover that the shape of exsolved particles can be controlled by changing the atmosphere in which exsolution is carried out, and additionally, this could also produce intriguing heterostructures consisting of metal-metal oxide coupled nanoparticles. Our results not only provide insight into the in situ formation of nanoparticles but also demonstrate the tailoring of nanostructures and nanointerfaces.

Enhanced Iridium Mass Activity of 6H-Phase, Ir-Based Perovskite with Nonprecious Incorporation for Acidic Oxygen Evolution Electrocatalysis

One of the key objectives in PEM electrolysis technology is to reduce iridium loading and to improve iridium mass activity at the side of oxygen evolution electrocatalysis. 6H-phase, Ir-based perovskite (6H-SrIrO₃) is known to be a promising alternative to the IrO₂ catalyst, and developing effective strategies to further enhance its catalytic performance is needed. Here we present that a significant enhancement in electrocatalytic activity for the oxygen evolution reaction of 6H-SrIrO₃ can be achieved by cobalt incorporation. A suitable amount of cobalt dopants results in a decreased formation temperature of 6H-SrIrO₃ from 700 to 500 °C and thereby a decreased thickness of platelike particles for the material. Besides the morphological effect, the cobalt incorporation also increases the coverage of surface hydroxyl groups, regulates the Ir-O bond covalency, and modulates the oxygen p-band center of the material. This synergistic optimization of the morphological, surface, and electronic structures makes the cobalt-doped 6H-SrIrO₃ catalyst give a 3-fold increase in iridium mass activity for oxygen evolution reaction in comparison with the undoped 6H-SrIrO₃ under acidic conditions.

Intercalated Iridium Diselenide Electrocatalysts for Efficient pH-Universal Water Splitting

Developing bifunctional catalysts for both hydrogen and oxygen evolution reactions is a promising approach to the practical implementation of electrocatalytic water splitting. However, most of the reported bifunctional catalysts are only applicable to alkaline electrolyzer, although a few are effective in acidic or neutral media that appeals more to industrial applications. Here, a lithium-intercalated iridium diselenide (Li-IrSe₂ ) is developed that outperformed other reported catalysts toward overall water splitting in both acidic and neutral environments. Li intercalation activated the inert pristine IrSe₂ via bringing high porosities and abundant Se vacancies for efficient hydrogen and oxygen evolution reactions. When Li-IrSe₂ was assembled into two-electrode electrolyzers for overall water splitting, the cell voltages at 10 mA cm^-2 were 1.44 and 1.50 V under pH 0 and 7, respectively, being record-low values in both conditions.

Perovskite Oxide Based Electrodes for High-Performance Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) water splitting is an attractive strategy for the large-scale production of renewable hydrogen from water. Developing cost-effective, active and stable semiconducting photoelectrodes is extremely important for achieving PEC water splitting with high solar-to-hydrogen efficiency. Perovskite oxides as a large family of semiconducting metal oxides are extensively investigated as electrodes in PEC water splitting owing to their abundance, high (photo)electrochemical stability, compositional and structural flexibility allowing the achievement of high electrocatalytic activity, superior sunlight absorption capability and precise control and tuning of band gaps and band edges. In this review, the research progress in the design, development, and application of perovskite oxides in PEC water splitting is summarized, with a special emphasis placed on understanding the relationship between the composition/structure and (photo)electrochemical activity.