Strain Regulation to Optimize the Acidic Water Oxidation Performance of Atomic-Layer IrOx

Advanced Ir-Based Alloy Electrocatalysts for Proton Exchange Membrane Water Electrolyzers

Proton exchange membrane water electrolyzer (PEMWE) coupled with renewable energy to produce hydrogen is an important part of clean energy acquisition in the future. However, the slow kinetics of the oxygen evolution reaction (OER) hinder the large-scale application of PEM water electrolysis technology. To deal with the problems existing in the PEM electrolyzer and improve the electrolysis efficiency, substantial efforts are invested in the development of cost-effective and stable electrocatalysts. Within this scenario, the different OER reaction mechanisms are first discussed here. Based on the in-depth understanding of the reaction mechanism, the research progress of low-iridium noble metal alloys is reviewed from the aspects of special effects, design strategies, reaction mechanisms, and synthesis methods. Finally, the challenges and prospects of the future development of high-efficiency and low-precious metal OER electrocatalysts are presented.

Decoding the entropy-stabilized matrix of high-entropy layered double hydroxides: Harnessing strain dynamics for peroxymonosulfate activation and tetracycline degradation

The current understanding of the mechanism of high-entropy layered double hydroxide (LDH) on enhancing the efficiency of activating peroxymonosulfate (PMS) remains limited. This work reveals that a strong strain effect, driven by high entropy, modulates the structure of FeCoNiCuZn-LDH (HE-LDH) as evidenced by geometric phase analysis (GPA) and density functional theory (DFT) calculations. Compared to FeCoNiZn-LDH and FeCoNi-LDH with weaker strain effects, the high entropy-driven strain effect in HE-LDH shortens metal-oxygen-hydrogen (MOH) bond lengths, allows system to be in a constant steady state during catalysis, reduces the leaching of active M-OH sites, and enhances the adsorption capacity of these sites and the excess strain strength of the interfacial stretches the IO-O of the PMS, facilitates reactive oxygen species (·OH, SO4·-, 1O2 and O2·-) generation, and thereby improving the efficiency of PMS in degrading tetracycline (TC). Consequently, HE-LDH demonstrated a 90% TC degradation within 3 min, maintained over 92% TC removal across a wide pH range (3-11), and achieved over 90% degradation performance after 6 cycles. This study reports the first use of high-entropy LDH material as a non-homogeneous catalyst and provides insights into the extremely different catalytic behaviors of high entropy mechanisms for the activation of PMS.

Local compressive strain-induced anti-corrosion over isolated Ru-decorated Co3O4 for efficient acidic oxygen evolution

Enhancing corrosion resistance is essential for developing efficient electrocatalysts for acidic oxygen evolution reaction (OER). Herein, we report the strategic manipulation of the local compressive strain to reinforce the anti-corrosion properties of the non-precious Co3O4 support. The incorporation of Ru single atoms, larger in atomic size than Co, into the Co3O4 lattice (Ru-Co3O4), triggers localized strain compression and lattice distortion on the Co-O lattice. A comprehensive exploration of the correlation between this specific local compressive strain and electrocatalytic performance is conducted through experimental and theoretical analyses. The presence of the localized strain in Ru-Co3O4 is confirmed by operando X-ray absorption studies and supported by quantum calculations. This local strain, presented in a shortened Co-O bond length, enhances the anti-corrosion properties of Co3O4 by suppressing metal dissolutions. Consequently, Ru-Co3O4 shows satisfactory stability, maintaining OER for over 400 hours at 30 mA cm-2 with minimal decay. This study demonstrates the potential of the local strain effect in fortifying catalyst stability for acidic OER and beyond.

Efficient detection of carbendazim using an electrochemical sensor for a novel NiFeLDH@HsGY-NH2/MWCNTs heterostructure with lattice-strain

Carbendazim (CBZ) is widely used for crop protection and its residues threaten human health and the environment. Therefore, developing an effective electrocatalyst is important for the extremely sensitive detection of CBZ. Lattice-strain engineering is an effective strategy to change its electronic structure and ultimately optimize the catalytic performance of materials, which can be used as a modification method to improve the detection performance of electrochemical sensors. Herein, a NiFeLDH@HsGY-NH2/MWCNTs heterojunction with strain effect is prepared by the electrostatic self-assembly method. The structure, morphology, composition, crystallinity and electrochemical performance of NiFeLDH@HsGY-NH2/MWCNTs are analyzed using various instrumental techniques, in which geometric phase analysis (GPA) and X-ray diffraction (XRD) images confirm the lattice-strain generated in NiFeLDH@HsGY-NH2/MWCNTs. The results indicate that the prepared electrochemical sensor exhibited an excellent response for carbendazim (CBZ) in the linear range of 0.05-50.00 μM with a detection limit of 10.00 nM (S/N = 3) under the optimal detection conditions. By analyzing the reasons for the improvement of the catalytic performance of the composite material, it is found that the composite of MWCNTs not only improves the conductivity of NiFeLDH but also regulates the electronic structure of metal atoms through double effects. This study provides new insights into the design of efficient and low-cost catalysts to facilitate electrochemical sensor applications.

Optimal Electrocatalyst Design Strategies for Acidic Oxygen Evolution

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

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.

Toward Realistic Models of the Electrocatalytic Oxygen Evolution Reaction

The electrocatalytic oxygen evolution reaction (OER) supplies the protons and electrons needed to transform renewable electricity into chemicals and fuels. However, the OER is kinetically sluggish; it operates at significant rates only when the applied potential far exceeds the reversible voltage. The origin of this overpotential is hidden in a complex mechanism involving multiple electron transfers and chemical bond making/breaking steps. Our desire to improve catalytic performance has then made mechanistic studies of the OER an area of major scientific inquiry, though the complexity of the reaction has made understanding difficult. While historically, mechanistic studies have relied solely on experiment and phenomenological models, over the past twenty years ab initio simulation has been playing an increasingly important role in developing our understanding of the electrocatalytic OER and its reaction mechanisms. In this Review we cover advances in our mechanistic understanding of the OER, organized by increasing complexity in the way through which the OER is modeled. We begin with phenomenological models built using experimental data before reviewing early efforts to incorporate ab initio methods into mechanistic studies. We go on to cover how the assumptions in these early ab initio simulations─no electric field, electrolyte, or explicit kinetics─have been relaxed. Through comparison with experimental literature, we explore the veracity of these different assumptions. We summarize by discussing the most critical open challenges in developing models to understand the mechanisms of the OER.

Mechanisms of the Accelerated Li+ Conduction in MOF-Based Solid-State Polymer Electrolytes for All-Solid-State Lithium Metal Batteries

Solid polymer electrolytes (SPEs) for lithium metal batteries have garnered considerable interests owing to their low cost, flexibility, lightweight, and favorable interfacial compatibility with battery electrodes. Their soft mechanical nature compared to solid inorganic electrolytes give them a large advantage to be used in low pressure solid-state lithium metal batteries, which can avoid the cost and weight of the pressure cages. However, the application of SPEs is hindered by their relatively low ionic conductivity. In addressing this limitation, enormous efforts are devoted to the experimental investigation and theoretical calculations/simulation of new polymer classes. Recently, metal-organic frameworks (MOFs) have been shown to be effective in enhancing ion transport in SPEs. However, the mechanisms in enhancing Li+ conductivity have rarely been systematically and comprehensively analyzed. Therefore, this review provides an in-depth summary of the mechanisms of MOF-enhanced Li+ transport in MOF-based solid polymer electrolytes (MSPEs) in terms of polymer, MOF, MOF/polymer interface, and solid electrolyte interface aspects, respectively. Moreover, the understanding of Li+ conduction mechanisms through employing advanced characterization tools, theoretical calculations, and simulations are also reviewed in this review. Finally, the main challenges in developing MSPEs are deeply analyzed and the corresponding future research directions are also proposed.

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.

Sample Fabrication of BiOCl Nanosheets with Low Specific Surface Area for Efficient Photocatalytic Degradation of Organic Wastewater

BiOCl photocatalyst with excellent performance has been prepared by a simple liquid-solid phase transition method. Three BiOCl-x (x = 0.5, 0.75, 1.0) photocatalysts were obtained by changing the amount of HCl in the preparation process. The main dominant crystal planes are (001), (002), and (003). Their forbidden bandwidths are reduced to 2.81, 2.89, and 2.84 eV. The samples were characterized by X-ray diffractometer, high-resolution field emission scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, Fourier infrared spectrometry, UV-vis diffuse reflection spectrometer, and UV-vis spectrophotometer. The degradation mechanism of BiOCl-x on Rhodamine B(RhB) has been investigated by quenching experiments on active materials. ·O2- was the primary degradation agent. When the dosage of HCl was 0.75 mL, the degradation effect of RhB was the best under the same experimental conditions. In visible light, RhB was almost completely degraded within 15 min, demonstrating an excellent photocatalytic degradation efficiency.

Rapid Defect Engineering in FeCoNi/FeAl2O4 Hybrid for Enhanced Oxygen Evolution Catalysis: A Pathway to High-Performance Electrocatalysts

Rational modulation of surface reconstruction in the oxygen evolution reaction (OER) utilizing defect engineering to form efficient catalytic activity centers is a topical interest in the field of catalysis. The introduction of point defects has been demonstrated to be an effective strategy to regulate the electronic configuration of electrocatalysts, but the influence of more complex planar defects (e.g., twins and stacking faults), on their intrinsic activity is still not fully understood. This study harnesses ultrasonic cavitation for rapid and controlled introduction of different types of defects in the FeCoNi/FeAl2O4 hybrid coating, optimizing OER catalytic activity. Theoretical calculations and experiments demonstrate that the different defects optimize the coordination environment and facilitate the activation of surface reconstruction into true catalytic activity centers at lower potentials. Moreover, it demonstrates exceptional durability, maintaining stable oxygen production at a high current density of 300 mA cm-2 for over 120 hours. This work not only presents a novel pathway for designing advanced electrocatalysts but also deepens our understanding of defect-engineered catalytic mechanisms, showcasing the potential for rapid and efficient enhancement of electrocatalytic performance.

Designing 3d Transition Metal Cation-Doped MRuOx As Durable Acidic Oxygen Evolution Electrocatalysts for PEM Water Electrolyzers

The continuous dissolution and oxidation of active sites in Ru-based electrocatalysts have greatly hindered their practical application in proton exchange membrane water electrolyzers (PEMWE). In this work, we first used density functional theory (DFT) to calculate the dissolution energy of Ru in the 3d transition metal-doped MRuOx (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) to evaluate their stability for acidic oxygen evolution reaction (OER) and screen out ZnRuOx as the best candidate. To confirm the theoretical predictions, we experimentally synthesized these MRuOx materials and found that ZnRuOx indeed displays robust acidic OER stability with a negligible decay of η10 after 15 000 CV cycles. Of importance, using ZnRuOx as the anode, the PEMWE can run stably for 120 h at 200 mA cm-2. We also further uncover the stability mechanism of ZnRuOx, i.e., Zn atoms doped in the outside of ZnRuOx nanocrystal would form a "Zn-rich" shell, which effectively shortened average Ru-O bond lengths in ZnRuOx to strengthen the Ru-O interaction and therefore boosted intrinsic stability of ZnRuOx in acidic OER. In short, this work not only provides a new study paradigm of using DFT calculations to guide the experimental synthesis but also offers a proof-of-concept with 3d metal dopants as RuO2 stabilizer as a universal principle to develop high-durability Ru-based catalysts for PEMWE.

Ru Single Atoms Integrated into Cobalt Oxide Spinel Structure with Interstitial Carbon for Enhanced Electrocatalytic Water Oxidation

Oxygen evolution reaction (OER) plays a critical role in energy conversion technologies. Significant progress has been made in alkaline conditions. In contrast, it remains a challenge to develop stable OER electrocatalysts in acidic conditions. Herein, a new strategy is reported to stabilize single atoms integrated into cobalt oxide spinel structure with interstitial carbon (Ru0.27Co2.73O4), where the optimized Ru0.27Co2.73O4 exhibits a low overpotential of 265, 326, and 367 mV to reach a current density of 10, 50, and 100 mA cm2, respectively. More importantly, Ru0.27Co2.73O4 has long-term stability of up to 100 h, representing one of the most stable OER electrocatalysts. X-ray adsorption spectroscopy (XAS) characterization and density functional theory (DFT) calculations jointly demonstrate that the significant catalytic performance of Ru0.27Co2.73O4 is due to the synergistic effect between the Ru and Co sites and the bridging O ligands, as well as the significant reduction of the OER energy barrier. This work provides a new perspective for designing and constructing efficient non-noble metal-based electrocatalysts for water splitting.

Tungsten-Iron-Ruthenium Ternary Alloy Immobilized into the Inner Nickel Foam for High-Current-Density Water Oxidation

The pursuit of highly-active and stable catalysts in anodic oxygen evolution reaction (OER) is desirable for high-current-density water electrolysis toward industrial hydrogen production. Herein, a straightforward yet feasible method to prepare WFeRu ternary alloying catalyst on nickel foam is demonstrated, whereby the foreign W, Fe, and Ru metal atoms diffuse into the Ni foam resulting in the formation of inner immobilized ternary alloy. Thanks to the synergistic impact of foreign metal atoms and structural robustness of inner immobilized alloying catalyst, the well-designed WFeRu@NF self-standing anode exhibits superior OER activities. It only requires overpotentials of 245 and 346 mV to attain current densities of 20 and 500 mA cm-2, respectively. Moreover, the as-prepared ternary alloying catalyst also exhibits a long-term stability at a high-current-density of 500 mA cm-2 for over 45 h, evidencing the inner-immobilization strategy is promising for the development of highly active and stable metal-based catalysts for high-density-current water oxidation process.

Multiscale Anion-Hybrid in Atomic Ni Sites for High-Rate Water Electrolysis: Insights into the Charge Accumulation Mechanism

Single-atom catalysts, characterized by transition metal-(N/O)4 units on nanocarbon (M-(N/O)4-C), have emerged as efficient performers in water electrolysis. However, there are few guiding principles for accurately controlling the ligand fields of single atoms to further stimulate the catalyst activities. Herein, using the Ni-(N/O)4-C unit as a model, we develop a further modification of the P anion on the outer shells to modulate the morphology of the ligand. The catalyst thus prepared possesses high activity and excellent long-term durability, surpassing commercial Pt/C, RuO2, and currently reported single-atom catalysts. Notably, mechanistic studies demonstrated that the pseudocapacitive feature of multiscale anion-hybrid nanocarbon is considerable at accumulating enough positive charge [Q], contributing to the high oxygen evolution reaction (OER) order (β) through the rate formula. DFT calculations also indicate that the catalytic activity is decided by the suitable barrier energy of the intermediates due to charge accumulation. This work reveals the activity origin of single atoms on multihybrid nanocarbon, providing a clear experiential formula for designing the electronic configuration of single-atom catalysts to boost electrocatalytic performance.

Thermal-Driven Orderly Assembly of Ir-atomic Chains on α-MnO2 with Enhanced Performance for Acidic Oxygen Evolution

The development of acid-stable oxygen evolution reaction electrocatalysts is essential for high-performance acidic water electrolysis. Herein, we report the results of one-dimensional (1D) nanorods (NRs) IrCeMnO@Ir containing ~20 wt . % Iridium (Ir) as an efficient anode electrocatalyst, synthesized via a one-step cation exchange strategy. Owing to the presence of 1D channels of the nanorod architecture and the unique electronic structure, the IrCeMnO@Ir exhibited 69 folds more mass activity than that of commercial IrO2 as well as over 400 h stability with only a 20 mV increase in overpotential. DFT calculations and control experiments demonstrated that CeO2 serves as an electron buffer to accelerate the kinetics of the rate-determined step for the significantly enhanced activity and suppress the over-oxidation of Ir species as well as their dissolution for impressively promoted stability under practical conditions. Our work opens up a feasible strategy to boost OER activity and stability simultaneously.

Lattice engineering of noble metal-based nanomaterials via metal-nonmetal interactions for catalytic applications

Noble metal-based nanomaterials possess outstanding catalytic properties in various chemical reactions. However, the increasing cost of noble metals severely hinders their large-scale applications. A cost-effective strategy is incorporating noble metals with light nonmetal elements (e.g., H, B, C, N, P and S) to form noble metal-based nanocompounds, which can not only reduce the noble metal content, but also promote their catalytic performances by tuning their crystal lattices and introducing additional active sites. In this review, we present a concise overview of the recent advancements in the preparation and application of various kinds of noble metal-light nonmetal binary nanocompounds. Besides introducing synthetic strategies, we focus on the effects of introducing light nonmetal elements on the lattice structures of noble metals and highlight notable progress in the lattice strain engineering of representative core-shell nanostructures derived from these nanocompounds. In the meantime, the catalytic applications of the light element-incorporated noble metal-based nanomaterials are discussed. Finally, we discuss current challenges and future perspectives in the development of noble metal-nonmetal based nanomaterials.

Strain Engineering of Unconventional Crystal-Phase Noble Metal Nanocatalysts

The crystal phase, alongside the composition, morphology, architecture, facet, size, and dimensionality, has been recognized as a critical factor influencing the properties of noble metal nanomaterials in various applications. In particular, unconventional crystal phases can potentially enable fascinating properties in noble metal nanomaterials. Recent years have witnessed notable advances in the phase engineering of nanomaterials (PEN). Within the accessible strategies for phase engineering, the effect of strain cannot be ignored because strain can act not only as the driving force of phase transition but also as the origin of the diverse physicochemical properties of the unconventional crystal phase. In this review, we highlight the development of unconventional crystal-phase noble metal nanomaterials within strain engineering. We begin with a short introduction of the unconventional crystal phase and strain effect in noble metal nanomaterials. Next, the correlations of the structure and performance of strain-engineered unconventional crystal-phase noble metal nanomaterials in electrocatalysis are highlighted, as well as the phase transitions of noble metal nanomaterials induced by the strain effect. Lastly, the challenges and opportunities within this rapidly developing field (i.e., the strain engineering of unconventional crystal-phase noble metal nanocatalysts) are discussed.

Continuous strain tuning of oxygen evolution catalysts with anisotropic thermal expansion

Compressive strain, downshifting the d-band center of transition metal oxides, is an effective way to accelerate the sluggish kinetics of oxygen evolution reaction (OER) for water electrolysis. Here, we find that anisotropic thermal expansion can produce compressive strains of the IrO6 octahedron in Sr2IrO4 catalyst, thus downshifting its d-band center. Different from the previous strategies to create constant strains in the crystals, the thermal-triggered compressive strains can be real-timely tuned by varying temperature. As a result of the thermal strain accelerating OER kinetics, the Sr2IrO4 exhibits the nonlinear lnjo - T-1 (jo, exchange current density; T, absolute temperature) Arrhenius relationship, resulting from the thermally induced low-barrier electron transfer in the presence of thermal compressive strains. Our results verify that the thermal field can be utilized to manipulate the electronic states of Sr2IrO4 via thermal compressive strains downshifting the d-band center, significantly accelerating the OER kinetics, beyond the traditional thermal diffusion effects.

The Promising Seesaw Relationship Between Activity and Stability of Ru-Based Electrocatalysts for Acid Oxygen Evolution and Proton Exchange Membrane Water Electrolysis

The development of electrocatalysts that are not reliant on iridium for efficient acid-oxygen evolution is a critical step towards the proton exchange membrane water electrolysis (PEMWE) and green hydrogen industry. Ruthenium-based electrocatalysts have garnered widespread attention due to their remarkable catalytic activity and lower commercial price. However, the challenge lies in balancing the seesaw relationship between activity and stability of these electrocatalysts during the acid-oxygen evolution reaction (OER). This review delves into the progress made in Ru-based electrocatalysts with regards to acid OER and PEMWE applications. It highlights the significance of customizing the acidic OER mechanism of Ru-based electrocatalysts through the coordination of adsorption evolution mechanism (AEM) and lattice oxygen oxidation mechanism (LOM) to attain the ideal activity and stability relationship. The promising tradeoffs between the activity and stability of different Ru-based electrocatalysts, including Ru metals and alloys, Ru single-atomic materials, Ru oxides, and derived complexes, and Ru-based heterojunctions, as well as their applicability to PEMWE systems, are discussed in detail. Furthermore, this paper offers insights on in situ control of Ru active sites, dynamic catalytic mechanism, and commercial application of PEMWE. Based on three-way relationship between cost, activity, and stability, the perspectives and development are provided.

Reducing the Ir-O Coordination Number in Anodic Catalysts based on IrOx Nanoparticles towards Enhanced Proton-exchange-membrane Water Electrolysis

Due to the robust oxidation conditions in strong acid oxygen evolution reaction (OER), developing an OER electrocatalyst with high efficiency remains challenging in polymer electrolyte membrane (PEM) water electrolyzer. Recent theoretical research suggested that reducing the coordination number of Ir-O is feasible to reduce the energy barrier of the rate-determination step, potentially accelerating the OER. Inspired by this, we experimentally verified the Ir-O coordination number's role at model catalysts, then synthesized low-coordinated IrOx nanoparticles toward a durable PEM water electrolyzer. We first conducted model studies on commercial rutile-IrO2 using plasma-based defect engineering. The combined in situ X-ray absorption spectroscopy (XAS) analysis and computational studies clarify why the decreased coordination numbers increase catalytic activity. Next, under the model studies' guidelines, we explored a low-coordinated Ir-based catalyst with a lower overpotential of 231 mV@10 mA cm-2 accompanied by long durability (100 h) in an acidic OER. Finally, the assembled PEM water electrolyzer delivers a low voltage (1.72 V@1 A cm-2 ) as well as excellent stability exceeding 1200 h (@1 A cm-2 ) without obvious decay. This work provides a unique insight into the role of coordination numbers, paving the way for designing Ir-based catalysts for PEM water electrolyzers.

Ir-Sn pair-site triggers key oxygen radical intermediate for efficient acidic water oxidation

The anode corrosion induced by the harsh acidic and oxidative environment greatly restricts the lifespan of catalysts. Here, we propose an antioxidation strategy to mitigate Ir dissolution by triggering strong electronic interaction via elaborately constructing a heterostructured Ir-Sn pair-site catalyst. The formation of Ir-Sn dual-site at the heterointerface and the resulting strong electronic interactions considerably reduce d-band holes of Ir species during both the synthesis and the oxygen evolution reaction processes and suppress their overoxidation, enabling the catalyst with substantially boosted corrosion resistance. Consequently, the optimized catalyst exhibits a high mass activity of 4.4 A mgIr-1 at an overpotential of 320 mV and outstanding long-term stability. A proton-exchange-membrane water electrolyzer using this catalyst delivers a current density of 2 A cm-2 at 1.711 V and low degradation in an accelerated aging test. Theoretical calculations unravel that the oxygen radicals induced by the π* interaction between Ir 5d-O 2p might be responsible for the boosted activity and durability.

BiOCl Nanosheets with (001), (002), and (003) Dominant Crystal Faces with Excellent Light-Degradation Ability

Gray bismuth chloride nanosheets with a highly enhanced electric field intensity were prepared by a simple and efficient method. Their energy gap is reduced to 2.35 eV. The prepared nanosheets show high photocatalytic activity for the degradation of rhodamine B under visible light. The resulting samples were characterized by X-ray diffractometry, high-resolution scanning electron microscopy, X-ray photoelectron spectroscopy, infrared spectroscopy, UV-vis diffuse reflectance spectroscopy, specific surface area analysis, electrochemical analysis, electron paramagnetic resonance, and UV-vis spectroscopy. The photocatalytic activity of prepared BiOCl was evaluated by the degradation of RhB. The prepared BiOCl sample (0.5 g/L) could completely degrade RhB (10 mg/L) within 10 min, and its visible photocatalytic activity was 80 times that of the original white BiOCl. Superoxide radicals were the main active substance involved in organic degradation.

Lattice-Strain Engineering for Heterogenous Electrocatalytic Oxygen Evolution Reaction

The energy efficiency of metal-air batteries and water-splitting techniques is severely constrained by multiple electronic transfers in the heterogenous oxygen evolution reaction (OER), and the high overpotential induced by the sluggish kinetics has become an uppermost scientific challenge. Numerous attempts are devoted to enabling high activity, selectivity, and stability via tailoring the surface physicochemical properties of nanocatalysts. Lattice-strain engineering as a cutting-edge method for tuning the electronic and geometric configuration of metal sites plays a pivotal role in regulating the interaction of catalytic surfaces with adsorbate molecules. By defining the d-band center as a descriptor of the structure-activity relationship, the individual contribution of strain effects within state-of-the-art electrocatalysts can be systematically elucidated in the OER optimization mechanism. In this review, the fundamentals of the OER and the advancements of strain-catalysts are showcased and the innovative trigger strategies are enumerated, with particular emphasis on the feedback mechanism between the precise regulation of lattice-strain and optimal activity. Subsequently, the modulation of electrocatalysts with various attributes is categorized and the impediments encountered in the practicalization of strained effect are discussed, ending with an outlook on future research directions for this burgeoning field.

Intrinsic Strain-Mediated Ultrathin Ceria Nanoantioxidant

Metal oxide nanozymes have emerged as the most efficient and promising candidates to mimic antioxidant enzymes for treatment of oxidative stress-mediated pathophysiological disorders, but the current effectiveness is unsatisfactory due to insufficient catalytic performance. Here, we report for the first time an intrinsic strain-mediated ultrathin ceria nanoantioxidant. Surface strain in ceria with variable thicknesses and coordinatively unsaturated Ce sites was investigated by theoretical calculation analysis and then was validated by preparing ∼1.2 nm ultrathin nanoplates with ∼3.0% tensile strain in plane/∼10.0% tensile strain out of plane. Compared with nanocubes, surface strain in ultrathin nanoplates could enhance the covalency of the Ce-O bond, leading to increasing superoxide dismutase (SOD)-mimetic activity by ∼2.6-fold (1533 U/mg, in close proximity to that of natural SOD) and total antioxidant activity by ∼2.5-fold. As a proof of concept, intrinsic strain-mediated ultrathin ceria nanoplates could boost antioxidation for improved ischemic stroke treatment in vivo, significantly better than edaravone, a commonly used clinical drug.

Recent advances in iridium-based catalysts with different dimensions for the acidic oxygen evolution reaction

Proton exchange membrane (PEM) water electrolysis is considered a promising technology for green hydrogen production, and iridium (Ir)-based catalysts are the best materials for anodic oxygen evolution reactions (OER) owing to their high stability and anti-corrosion ability in a strong acid electrolyte. The properties of Ir-based nanocatalysts can be tuned by rational dimension engineering, which has received intensive attention recently for catalysis ability boosting. To achieve a comprehensive understanding of the structural and catalysis performance, herein, an overview of the recent progress was provided for Ir-based catalysts with different dimensions for the acidic OER. The promotional effect was first presented in terms of the nano-size effect, synergistic effect, and electronic effect based on the dimensional effect, then the latest progress of Ir-based catalysts classified into zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D) and three-dimensional (3D) catalysts was introduced in detail; and the practical application of some typical examples in the real PEM water electrolyzers (PEMWE) was also presented. Finally, the problems and challenges faced by current dimensionally engineered Ir-based catalysts in acidic electrolytes were discussed. It is concluded that the increased surface area and catalytic active sites can be realized by dimensional engineering strategies, while the controllable synthesis of different dimensional structured catalysts is still a great challenge, and the correlation between structure and performance, especially for the structural evolution during the electrochemical operation process, should be probed in depth. Hopefully, this effort could help understand the progress of dimensional engineering of Ir-based catalysts in OER catalysis and contribute to the design and preparation of novel efficient Ir-based catalysts.

Strain and Surface Engineering of Multicomponent Metallic Nanomaterials with Unconventional Phases

Multicomponent metallic nanomaterials with unconventional phases show great prospects in electrochemical energy storage and conversion, owing to unique crystal structures and abundant structural effects. In this review, we emphasize the progress in the strain and surface engineering of these novel nanomaterials. We start with a brief introduction of the structural configurations of these materials, based on the interaction types between the components. Next, the fundamentals of strain, strain effect in relevant metallic nanomaterials with unconventional phases, and their formation mechanisms are discussed. Then the progress in surface engineering of these multicomponent metallic nanomaterials is demonstrated from the aspects of morphology control, crystallinity control, surface modification, and surface reconstruction. Moreover, the applications of the strain- and surface-engineered unconventional nanomaterials mainly in electrocatalysis are also introduced, where in addition to the catalytic performance, the structure-performance correlations are highlighted. Finally, the challenges and opportunities in this promising field are prospected.

Surface and near-surface engineering design of transition metal catalysts for promoting water splitting

Transition metal catalysts are widely used in the field of hydrogen production via water electrolysis. The surface state and near-surface environment of the catalysts greatly affect the efficiency of hydrogen production. Therefore, the rational design of surface engineering and near-surface engineering of transition metal catalysts can significantly improve the performance of water electrolysis. This review systematically introduces surface engineering strategies, including heteroatom doping, vacancy engineering, strain regulation, heterojunction effect, and surface reconstruction. These strategies optimize the surface electronic structure of the catalysts, expose more active sites, and promote the formation of highly active species, ultimately enhancing water electrolysis performance. Furthermore, near-surface engineering strategies, such as surface wettability, three-dimensional structure, high-curvature structure, external field assistance, and extra ion addition, are thoroughly discussed. These strategies expedite the mass transfer of reactants and gas products, improve the local chemical environment near the catalyst surface, and contribute toward achieving an industrial-level current density for overall water splitting. Finally, the key challenges faced by surface engineering and near-surface engineering of transition metal catalysts are highlighted and potential solutions are proposed. This review offers essential guidelines for the design and development of efficient transition metal catalysts for water electrolysis.

Electrocatalysts for the Oxygen Evolution Reaction in Acidic Media

The well-established proton exchange membrane (PEM)-based water electrolysis, which operates under acidic conditions, possesses many advantages compared to alkaline water electrolysis, such as compact design, higher voltage efficiency, and higher gas purity. However, PEM-based water electrolysis is hampered by the low efficiency, instability, and high cost of anodic electrocatalysts for the oxygen evolution reaction (OER). In this review, the recently reported acidic OER electrocatalysts are comprehensively summarized, classified, and discussed. The related fundamental studies on OER mechanisms and the relationship between activity and stability are particularly highlighted in order to provide an atomistic-level understanding for OER catalysis. A stability test protocol is suggested to evaluate the intrinsic activity degradation. Some current challenges and unresolved questions, such as the usage of carbon-based materials and the differences between the electrocatalyst performances in acidic electrolytes and PEM-based electrolyzers are also discussed. Finally, suggestions for the most promising electrocatalysts and a perspective for future research are outlined. This review presents a fresh impetus and guideline to the rational design and synthesis of high-performance acidic OER electrocatalysts for PEM-based water electrolysis.

Recent Advances in Water-Splitting Electrocatalysts Based on Electrodeposition

Green hydrogen is being considered as a next-generation sustainable energy source. It is created electrochemically by water splitting with renewable electricity such as wind, geothermal, solar, and hydropower. The development of electrocatalysts is crucial for the practical production of green hydrogen in order to achieve highly efficient water-splitting systems. Due to its advantages of being environmentally friendly, economically advantageous, and scalable for practical application, electrodeposition is widely used to prepare electrocatalysts. There are still some restrictions on the ability to create highly effective electrocatalysts using electrodeposition owing to the extremely complicated variables required to deposit uniform and large numbers of catalytic active sites. In this review article, we focus on recent advancements in the field of electrodeposition for water splitting, as well as a number of strategies to address current issues. The highly catalytic electrodeposited catalyst systems, including nanostructured layered double hydroxides (LDHs), single-atom catalysts (SACs), high-entropy alloys (HEAs), and core-shell structures, are intensively discussed. Lastly, we offer solutions to current problems and the potential of electrodeposition in upcoming water-splitting electrocatalysts.

In Situ Activation Endows Orthorhombic Fluorite-Type Samarium Iridium Oxide with Enhanced Acidic Water Oxidation

Developing electrochemical catalysts for acidic water oxidation with improved activity and stability has been the key to the further popularization of proton exchange membrane electrolyzers. In this work, an orthorhombic fluorite-type samarium iridium oxide (Sm3IrO7) catalyst is synthesized by a simple solid-state reaction. After in situ activation, the as-prepared Sm3IrO7 exhibits higher mass activity and durability than that of commercial IrO2. The in-depth analyses indicate the formation of amorphous IrOx species on the surface to evolve to a new heterostructure IrOx/Sm3IrO7, along with Sm leaching during the in situ activation process. More importantly, strong electronic interactions exist between newborn IrOx species and remaining Sm3IrO7, leading to the compressed Ir-O bonds in IrOx compared to commercial IrO2, thus reducing the energy barrier for oxygen evolution reaction (OER) intermediates to improve the OER process. Based on the above-mentioned analyses, it is speculated that the actual active species for enhanced acidic water oxidation should be IrOx/Sm3IrO7, rather than Sm3IrO7 itself. Theoretical calculations confirm that the optimal energy level path of IrOx/Sm3IrO7 follows the lattice oxygen mechanism, and the energy level of surface Ir 5d orbitals is lower than O 2p orbitals in IrOx/Sm3IrO7, enabling it a superior OER activity.

Iridium oxide nanoribbons with metastable monoclinic phase for highly efficient electrocatalytic oxygen evolution

Metastable metal oxides with ribbon morphologies have promising applications for energy conversion catalysis, however they are largely restricted by their limited synthesis methods. In this study, a monoclinic phase iridium oxide nanoribbon with a space group of C2/m is successfully obtained, which is distinct from rutile iridium oxide with a stable tetragonal phase (P42/mnm). A molten-alkali mechanochemical method provides a unique strategy for achieving this layered nanoribbon structure via a conversion from a monoclinic phase K_0.25IrO₂ (I2/m (12)) precursor. The formation mechanism of IrO₂ nanoribbon is clearly revealed, with its further conversion to IrO₂ nanosheet with a trigonal phase. When applied as an electrocatalyst for the oxygen evolution reaction in acidic condition, the intrinsic catalytic activity of IrO₂ nanoribbon is higher than that of tetragonal phase IrO₂ due to the low d band centre of Ir in this special monoclinic phase structure, as confirmed by density functional theory calculations.

A Highly Active, Long-Lived Oxygen Evolution Electrocatalyst Derived from Open-Framework Iridates

The acidic oxygen evolution reaction underpins several important electrical-to-chemical energy conversions, and this energy-intensive process relies industrially on iridium-based electrocatalysts. Here, phase-selective synthesis of metastable strontium iridates with open-framework structure and their unexpected transformation into a highly active, ultrastable oxygen evolution nano-electrocatalyst are presented. This transformation involves two major steps: Sr²⁺ /H⁺ ion exchange in acid and in situ structural rearrangement under electrocatalysis conditions. Unlike its dense perovskite-structured polymorphs, the open-framework iridates have the ability to undergo rapid proton exchange in acid without framework amorphization. The resulting protonated iridates further reconstruct into ultrasmall, surface-hydroxylated, (200) crystal plane-oriented rutile nanocatalyst, instead of the common amorphous IrO_x H_y phase, during acidic oxygen evolution. Such microstructural characteristics are found to benefit both the oxidation of hydroxyls and the formation of OO bonds in electrocatalytic cycle. As a result, the open-framework iridate derived nanocatalyst gives a comparable catalytic activity to the most active iridium-based oxygen evolution electrocatalysts in acid, and retains its catalytic activity for more than 1000 h.

Balancing Activity and Stability in Spinel Cobalt Oxides through Geometrical Sites Occupation towards Efficient Electrocatalytic Oxygen Evolution

Designing active and stable oxygen evolution reaction (OER) catalysts are vitally important to various energy conversion devices. Herein, we introduce elements Ni and Mn into (Co)_tet (Co₂ )_oct O₄ nanosheets (NSs) at fixed geometrical sites, including Mn_oct , Ni_oct , and Ni_tet , to optimize the initial geometrical structure and modulate the CoCo₂ O₄ surface from oxygen-excess to oxygen-deficiency. The pristine (Ni,Mn)-(Co)_tet (Co₂ )_oct O₄ NSs shows excellent OER activity with an overpotential of 281.6 mV at a current density of 10 mA cm^-2 . Moreover, without damaging their initial activity, the activated (Act)-(Ni,Mn)-(Co)_tet (Co₂ )_oct O₄ NSs after surface reconstruction exhibit long-term stability of 100 h under 10 mA cm^-2 , 50 mA cm^-2 , or even 100 mA cm^-2 . The optimal balance between electroactivity and stability leads to remarkable OER performances, providing a pivotal guideline for designing ideal electrocatalysts and inspiring more works to focus on the dynamic change of each occupation site component.

Iridium single atoms incorporated in Co3O4 efficiently catalyze the oxygen evolution in acidic conditions

Designing active and stable electrocatalysts with economic efficiency for acidic oxygen evolution reaction is essential for developing proton exchange membrane water electrolyzers. Herein, we report on a cobalt oxide incorporated with iridium single atoms (Ir-Co3O4), prepared by a mechanochemical approach. Operando X-ray absorption spectroscopy reveals that Ir atoms are partially oxidized to active Ir>4+ during the reaction, meanwhile Ir and Co atoms with their bridged electrophilic O ligands acting as active sites, are jointly responsible for the enhanced performance. Theoretical calculations further disclose the isolated Ir atoms can effectively boost the electronic conductivity and optimize the energy barrier. As a result, Ir-Co3O4 exhibits significantly higher mass activity and turnover frequency than those of benchmark IrO2 in acidic conditions. Moreover, the catalyst preparation can be easily scaled up to gram-level per batch. The present approach highlights the concept of constructing single noble metal atoms incorporated cost-effective metal oxides catalysts for practical applications.

Multistage Electron Distribution Engineering of Iridium Oxide by Codoping W and Sn for Enhanced Acidic Water Oxidation Electrocatalysis

Developing efficient and robust anodic electrocatalysts to implement the proton-exchange membrane (PEM) electrolyzer is critical for hydrogen generation. Nevertheless, the only known applicable anode catalyst IrO_x in PEM electrolyzers still requires high overpotential due to the weak binding energy between oxygen intermediates and active sites, limiting its wide applications. Herein, a ternary Ir_0.7 W_0.2 Sn_0.1 O_x nanocatalyst synthesized through a sol-gel strategy, exhibits a low overpotential of 236 mV (10 mA cm^-2 _geo ) for thoxygen evolution reaction (OER), accompanied with robust durability over 220 h at 1 A cm^-2 _geo in 0.5 m H₂ SO₄ . Moreover, the optimized Ir_0.7 W_0.2 Sn_0.1 O_x delivers a prominent mass activity of 722.7 A g^-1 _Ir at 1.53 V (vs RHE), which is around 34 times higher compared with that of IrO_x . The mircrostructural analyses reveal that codoping of W and Sn stabilizes Ir with a valence state lower than 4+ through multistage charge redistribution, avoiding the overoxidation of Ir above 1.6 V versus RHE and enhancing the acidic OER performance. Additionally, density functional theory calculations reveal that codoping of W and Sn moves the d band center of Ir to the Fermi level, thus enhancing the binding energies of oxygen intermediates with Ir sites and decreasing the energy barrier toward acidic OER.

Tensile-Strained RuO2 Loaded on Antimony-Tin Oxide by Fast Quenching for Proton-Exchange Membrane Water Electrolyzer

Future energy demands for green hydrogen have fueled intensive research on proton-exchange membrane water electrolyzers (PEMWE). However, the sluggish oxygen evolution reaction (OER) and highly corrosive environment on the anode side narrow the catalysts to be expensive Ir-based materials. It is very challenging to develop cheap and effective OER catalysts. Herein, Co-hexamethylenetetramine metal-organic framework (Co-HMT) as the precursor and a fast-quenching method is employed to synthesize RuO2 nanorods loaded on antimony-tin oxide (ATO). Physical characterizations and theoretical calculations indicate that the ATO can increase the electrochemical surface areas of the catalysts, while the tensile strains incorporated by quenching can alter the electronic state of RuO2 . The optimized catalyst exhibits a small overpotential of 198 mV at 10 mA cm-2 for OER, and keeps almost unchanged after 150 h chronopotentiometry. When applied in a real PEMWE assembly, only 1.51 V is needed for the catalyst to reach a current density of 1 A cm-2 .

Tailoring Oxygen Reduction Reaction Pathway on Spinel Oxides via Surficial Geometrical-Site Occupation Modification Driven by the Oxygen Evolution Reaction

The oxygen reduction reaction (ORR) has been demonstrated as a critical technology for both energy conversion technologies and hydrogen peroxide intermediate production. Herein, an in situ oxygen evolution reaction (OER) surface evolution strategy is applied for changing the surface structure of MnCo₂ O₄ oxide with tetrahedral and octahedral cations vacancies to realize reaction pathway switching from 2e^- ORR and 4e^- ORR. Interestingly, the as-synthesized MnCo₂ O₄ -pristine (MnCo₂ O₄ -P) with the highest surficial Mn/Co octahedron occupation favors two electrons reaction routes exhibiting high H₂ O₂ selectivity (≈80% and reaches nearly 100% at 0.75 V vs RHE); after surface atoms reconstruction, MnCo₂ O₄ -activation (MnCo₂ O₄ -A) with the largest Mn/Co tetrahedron occupation present excellent ORR performance through the four-electron pathway with an ultrahigh onset potential and half-wave potential of 0.78 and 0.92 V, ideal mass activity (MA), and turnover frequencies (TOF) values. Density functional theory (DFT) calculations reveal the concurrent modulations of both Co and Mn by the surface reconstructions, which improve the electroactivity of MnCo₂ O₄ -A toward the 4e^- pathway. This work provides a new perspective to building correlation of OER activation-ORR property, bringing detailed understating for reaction route transformation, and thus guiding the development of certain electrocatalysts with specific purposes.

Equilibrated PtIr/IrOx Atomic Heterojunctions on Ultrafine 1D Nanowires Enable Superior Dual-Electrocatalysis for Overall Water Splitting

Dual-active-sites atomically coupled on ultrafine 1D nanowires (NWs) can offer synergic atomic heterojunctions (AHJs) and high atomic-utilization toward multipurpose and superior catalysis. Here, ≈2-nm-thick PtIr/IrO_x hybrid NWs are elaborately synthesized with equilibrated Pt/IrO_x AHJs as high-efficiency bifunctional electrocatalysts for overall water splitting. Mechanism studies reveal the atomically coupled Pt-IrO_x dual-sites are favorable for facilitating water dissociation, alleviating the binding of H* on Pt sites and inversely regulating the *OH adsorption and oxidation on bridge Ir-Ir sites. By simply equilibrating the Pt-IrO_x ratio, the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) can be substantially accelerated. In particular, Pt-rich PtIr/IrO_x -30 NWs attain 11-fold enhancements for HER compared to Pt/C in 1.0 m KOH, while IrO_x -rich PtIr/IrO_x -50 NWs express about five times mass activity referring to Ir/C for OER. Remarkably, the ratio-optimized PtIr/IrO_x NWs electrode couple achieves a durably continuous H₂ production under a substantially low cell voltage.

Single-site Pt-doped RuO2 hollow nanospheres with interstitial C for high-performance acidic overall water splitting

Realizing stable and efficient overall water splitting is highly desirable for sustainable and efficient hydrogen production yet challenging because of the rapid deactivation of electrocatalysts during the acidic oxygen evolution process. Here, we report that the single-site Pt-doped RuO₂ hollow nanospheres (SS Pt-RuO₂ HNSs) with interstitial C can serve as highly active and stable electrocatalysts for overall water splitting in 0.5 M H₂SO₄. The performance toward overall water splitting have surpassed most of the reported catalysts. Impressively, the SS Pt-RuO₂ HNSs exhibit promising stability in polymer electrolyte membrane electrolyzer at 100 mA cm^-2 during continuous operation for 100 hours. Detailed experiments reveal that the interstitial C can elongate Ru-O and Pt-O bonds, and the presence of SS Pt can readily vary the electronic properties of RuO₂ and improve the OER activity by reducing the energy barriers and enhancing the dissociation energy of ^*O species.

High-Entropy Alloys for Electrocatalysis: Design, Characterization, and Applications

High-entropy alloys (HEAs) are expected to function well as electrocatalytic materials, owing to their widely adjustable composition and unique physical and chemical properties. Recently, HEA catalysts are extensively studied in the field of electrocatalysis; this motivated the authors to investigate the relationship between the structure and composition of HEAs and their electrocatalytic performance. In this review, the latest advances in HEA electrocatalysts are systematically summarized, with special focus on nitrogen fixation, the carbon cycle, water splitting, and fuel cells; in addition, by combining this with the characterization and analysis of HEA microstructures, rational design strategies for optimizing HEA electrocatalysts, including controllable preparation, component regulation, strain engineering, defect engineering, and theoretical prediction are proposed. Moreover, the existing issues and future trends of HEAs are predicted, which will help further develop these high-entropy materials.

Self-Assembly of Ir-Based Nanosheets with Ordered Interlayer Space for Enhanced Electrocatalytic Water Oxidation

Iridium (Ir)-based electrocatalysts are widely explored as benchmarks for acidic oxygen evolution reactions (OERs). However, further enhancing their catalytic activity remains challenging due to the difficulty in identifying active species and unfavorable architectures. In this work, we synthesized ultrathin Ir-IrO_x/C nanosheets with ordered interlayer space for enhanced OER by a nanoconfined self-assembly strategy, employing block copolymer formed stable end-merged lamellar micelles. The interlayer distance of the prepared Ir-IrO_x/C nanosheets was well controlled at ∼20 nm and Ir-IrO_x nanoparticles (∼2 nm) were uniformly distributed within the nanosheets. Importantly, the fabricated Ir-IrO_x/C electrocatalysts display one of the lowest overpotential (η) of 198 mV at 10 mA cm^-2_geo during OER in an acid medium, benefiting from their features of mixed-valence states, rich electrophilic oxygen species (O^(II-δ)-), and favorable mesostructured architectures. Both experimental and computational results reveal that the mixed valence and O^(II-δ)- moieties of the 2D mesoporous Ir-IrO_x/C catalysts with a shortened Ir-O^(II-δ)- bond (1.91 Å) is the key active species for the enhancement of OER by balancing the adsorption free energy of oxygen-containing intermediates. This strategy thus opens an avenue for designing high performance 2D ordered mesoporous electrocatalysts through a nanoconfined self-assembly strategy for water oxidation and beyond.

Solvent-induced crystal-facet effect of nickel-cobalt layered double hydroxide for highly efficient overall water splitting

Two-dimensional layered materials have been universally acknowledged to be promising candidates for alternative precious metal in the field of catalysis. The crystal-facet effect is currently rare in the field of...

Heterostructure of core-shell IrCo@IrCoOxas efficient and stable catalysts for oxygen evolution reaction

Although researches on non-noble metal electrocatalysts have been made some progress recently, their performance in proton exchange membrane water electrolyzer is still incomparable to that of noble-metal-based catalysts. Therefore, it is a more practical way to improve the utilization of precious metals in electrocatalysts for oxygen evolution reaction (OER) in the acidic medium. Herein, nanostructured IrCo@IrCoO_xcore-shell electrocatalysts composed of IrCo alloy core and IrCoO_xshell were synthesized through a simple colloidally synthesis and calcination method. As expected, the hybrid IrCo-200 NPs with petal-like morphology show the best OER activities in acidic electrolytes. They deliver lower overpotential and better electrocatalytic kinetics than pristine IrCo alloy and commercial Ir/C, reaching a low overpotential (j = 10 mA cm^-2) of 259 mV (versus RHE) and a Tafel slope of 59 mV dec^-1. The IrCo-200 NPs displayed robust durability with life time of about 55 h in acidic solution under a large current density of 50 mA cm^-2. The enhanced electrocatalytic activity may be associated with the unique metal/amorphous metal oxide core-shell heterostructure, allowing the improved charge transferability. Moreover, the *OH-rich amorphous shell functions as the active site for OER and prevents the further dissolution of the metallic core and thus ensures high stability.

Torsion strained iridium oxide for efficient acidic water oxidation in proton exchange membrane electrolyzers

Acidic oxygen evolution reaction is crucial for practical proton exchange membrane water splitting electrolysers, which have been hindered by the high catalytic overpotential and high loading of noble metal catalysts. Here we present a torsion-strained Ta_0.1Tm_0.1Ir_0.8O_2-δ nanocatalyst with numerous grain boundaries that exhibit a low overpotential of 198 mV at 10 mA cm^-2 towards oxygen evolution reaction in 0.5 M H₂SO₄. Microstructural analyses, X-ray absorption spectroscopy and theoretical calculations reveal that the synergistic effects between grain boundaries that result in torsion-strained Ir-O bonds and the doping induced ligand effect collectively tune the adsorption energy of oxygen intermediates, thus enhancing the catalytic activity. A proton exchange membrane electrolyser using a Ta_0.1Tm_0.1Ir_0.8O_2-δ nanocatalyst with a low mass loading of 0.2 mg cm^-2 can operate stably at 1.5 A cm^-2 for 500 hours with an estimated cost of US$1 per kilogram of H₂, which is much lower than the target (US$2 per kg of H₂) set by the US Department of Energy.

Elucidating the Strain-Vacancy-Activity Relationship on Structurally Deformed Co@CoO Nanosheets for Aqueous Phase Reforming of Formaldehyde

Lattice strain modulation and vacancy engineering are both effective approaches to control the catalytic properties of heterogeneous catalysts. Here, Co@CoO heterointerface catalysts are prepared via the controlled reduction of CoO nanosheets. The experimental quantifications of lattice strain and oxygen vacancy concentration on CoO, as well as the charge transfer across the Co-CoO interface are all linearly correlated to the catalytic activity toward the aqueous phase reforming of formaldehyde to produce hydrogen. Mechanistic investigations by spectroscopic measurements and density functional theory calculations elucidate the bifunctional nature of the oxygen-vacancy-rich Co-CoO interfaces, where the Co and the CoO sites are responsible for CH bond cleavage and OH activation, respectively. Optimal catalytic activity is achieved by the sample reduced at 350 °C, Co@CoO-350 which exhibits the maximum concentration of Co-CoO interfaces, the maximum concentration of oxygen vacancies, a lattice strain of 5.2% in CoO, and the highest aqueous phase formaldehyde reforming turnover frequency of 50.4 h^-1 at room temperature. This work provides not only new insights into the strain-vacancy-activity relationship at bifunctional catalytic interfaces, but also a facile synthetic approach to prepare heterostructures with highly tunable catalytic activities.

Advanced Transition Metal-Based OER Electrocatalysts: Current Status, Opportunities, and Challenges

Oxygen evolution reaction (OER) is an important half-reaction involved in many electrochemical applications, such as water splitting and rechargeable metal-air batteries. However, the sluggish kinetics of its four-electron transfer process becomes a bottleneck to the performance enhancement. Thus, rational design of electrocatalysts for OER based on thorough understanding of mechanisms and structure-activity relationship is of vital significance. This review begins with the introduction of OER mechanisms which include conventional adsorbate evolution mechanism and lattice-oxygen-mediated mechanism. The reaction pathways and related intermediates are discussed in detail, and several descriptors which greatly assist in catalyst screen and optimization are summarized. Some important parameters suggested as measurement criteria for OER are also mentioned and discussed. Then, recent developments and breakthroughs in experimental achievements on transition metal-based OER electrocatalysts are reviewed to reveal the novel design principles. Finally, some perspectives and future directions are proposed for further catalytic performance enhancement and deeper understanding of catalyst design. It is believed that iterative improvements based on the understanding of mechanisms and fundamental design principles are essential to realize the applications of efficient transition metal-based OER electrocatalysts for electrochemical energy storage and conversion technologies.

Doping regulation in transition metal compounds for electrocatalysis

In electrocatalysis, doping regulation has been considered as an effective method to modulate the active sites of catalysts, providing a powerful means for creating a large variety of highly efficient catalysts for various reactions. Of particular interest, there has been growing research concerning the doping of two-dimensional transition-metal compounds (TMCs) to optimize their electrocatalytic performance. Despite the previous achievements, mechanistic insights of doping regulation in TMCs for electrocatalysis are still lacking. Herein, we provide a systematic overview of doping regulation in TMCs in terms of background, preparation, impacts on physicochemical properties, and typical applications including the hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, CO₂ reduction reaction, and N₂ reduction reaction. Notably, we bridge the understanding between the doping regulation of catalysts and their catalytic activities via focusing on the physicochemical properties of catalysts from the aspects of vacancy concentrations, phase transformation, surface wettability, electrical conductivity, electronic band structure, local charge distribution, tunable adsorption strength, and multiple adsorption configurations. We also discuss the existing challenges and future perspectives in this promising field.

Interfacial engineering of CeO2 on NiCoP nanoarrays for efficient electrocatalytic oxygen evolution

Transition metal phosphides (TMP)-based oxygen evolution reaction (OER) catalysts constructed by interface engineering strategy have a broad prospect due to their low cost and good performance. Herein, a novel CeO₂/NiCoP nanoarray with intimate phosphide (NiCoP)-oxide (CeO₂) interface was developed via in situ generation on nickel foam (NF). This structure is conducive to increasing active sites and accelerating charge transfer, and may be conducive to regulating electronic structure and adsorption energy. As expected, optimal 1.4-CeO₂/NiCoP/NF delivers a low overpotential of 249 mV at the current density of 10 mA cm^-2 with a Tafel slope of 77.2 mV dec^-1. CeO₂/NiCoP/NF boasts one of the best OER catalytic materials among recently reported phosphides (TMP)-based OER catalysts and composite catalysts involving CeO₂. This work provides an effective strategy for the construction of hetero-structure with CeO₂ with oxygen vacancies to improve the OER performance of phosphides.