Tailoring Lattice Oxygen Binding in Ruthenium Pyrochlores to Enhance Oxygen Evolution Activity

Engineering 3d-2p-4f Gradient Orbital Coupling to Enhance Electrocatalytic Oxygen Reduction

The development of highly efficient and economical materials for the oxygen reduction reaction (ORR) plays a key role in practical energy conversion technologies. However, the intrinsic scaling relations exert thermodynamic inhibition on realizing highly active ORR electrocatalysts. Herein, a novel and feasible gradient orbital coupling strategy for tuning the ORR performance through the construction of Co 3d-O 2p-Eu 4f unit sites on the Eu₂ O₃ -Co model is proposed. Through the gradient orbital coupling, the pristine ionic property between Eu and O atoms is assigned with increased covalency, which optimizes the e_g occupancy of Co sites, and weakens the OO bond, thus ultimately breaking the scaling relation between *OOH and *OH at Co-O-Eu unit sites. The optimized model catalyst displays onset and half-wave potential of 1.007 and 0.887 V versus reversible hydrogen electrode, respectively, which are higher than those of commercial Pt/C and most Co-based catalysts ever reported. In addition, the catalyst is found to possess superior selectivity and durability. It also reveals better cell performance than commercial noble-metal catalysts in Zn-air batteries in terms of high power/energy densities and long cycle life. This study provides a new perspective for electronic modulation strategy by the construction of gradient 3d-2p-4f orbital coupling.

Probing Dynamic Self-Reconstruction on Perovskite Fluorides toward Ultrafast Oxygen Evolution

Exploring low cost, highly active, and durable electrocatalysts for oxygen evolution reaction (OER) is of prime importance to boost energy conversion efficiency. Perovskite fluorides are emerging as alternative electrocatalysts for OER, however, their intrinsically active sites during real operation are still elusive. Herein, the self-reconstruction on newly designed NiFe coupled perovskite fluorides during OER process is demonstrated. In situ Raman spectroscopy, ex situ X-ray absorption spectroscopy, and theoretical calculation reveal that Fe incorporation can significantly activate the self-reconstruction of perovskite fluorides and efficiently lower the energy barrier of OER. Benefiting from self-reconstruction and low energy barrier, the KNi0.8 Fe0.2 F3 @nickel foam (KNFF2@NF) electrocatalyst delivers an ultralow overpotential of 258 mV to afford 100 mA cm-2 and an excellent durability for 100 h, favorably rivaling most the state-of-the-art OER electrocatalysts. This protocol provides the fundamental understanding on OER mechanism associated with surface reconstruction for perovskite fluorides.

Pyrochlores for Advanced Oxygen Electrocatalysis

Fuel cells (FCs), water electrolyzers (WEs), unitized regenerative fuel cells (URFCs), and metal-air batteries (MABs) are among the emerging electrochemical technologies for energy storage, fuel (H₂), oxidant (O₂), and clean energy production. Their commercial applications are hindered by the low oxygen reduction reaction/oxygen evolution reaction (ORR/OER) bifunctional activity (for URFCs and MABs), OER selectivity (brine electrolysis in seawater and Martian environments), and high cost of the benchmark electrocatalysts (OER: RuO₂, IrO₂ and ORR: Pt/C) which affects the performance and affordability of the devices. Low-cost electrocatalysts with highly symmetric ORR/OER bifunctional activity and high OER selectivity are crucial for large-scale FC, WE, URFC, and MAB application. Recent studies have revealed that tuning the structure of pyrochlore oxides provides a pathway to enhancing OER and ORR activity over a wide range of pH. Pyrochlore oxides commonly contain a cubic A₂B₂O_7-x structure with two types of tetrahedrally coordinated O atoms containing (1) A-O-A and (2) A-O-B types with a cationic radii mismatch of r_A/r_B > 1.5 and propensity toward oxygen vacancy formation. The variety of pyrochlore oxides and their tunable properties make them attractive for a wide spectrum of applications. Among all the metal oxides, Ru-based pyrochlores (e.g., Pb₂Ru₂O_7-x) exhibit the best bifunctional oxygen electrocatalytic activity, i.e., low bifunctionality index (BI), in alkaline medium. Furthermore, pyrochlores exhibit high OER selectivity in brine electrolytes due to the presence of surface oxygen vacancies, making them suitable for space applications (brine electrolysis on Mars) and coastal hydrogen generation. Their bifunctional activity and selectivity can be further amplified by (1) substituting "A" and "B" sites of pyrochlores (AA'BB'O_7-x), (2) tuning metal oxidation states of A and B by varying synthesis conditions, and (3) modulating oxygen vacancy concentration, each of which yield favorable structural and electronic variations. In recent years, research on the synthesis and understanding of pyrochlores has significantly enhanced their viability, offering a new horizon in the quest for economical and active electrocatalysts. However, an account that focuses on critical developments in this field is still lacking.In this Account, we focus on the recent development of a variety of pyrochlore electrocatalysts to understand intrinsic structure-activity-selectivity-stability relationships in these materials. Recent developments and applications of pyrochlore-based electrocatalysts are discussed under the following headings: (1) modulation of crystal and electronic structure of pyrochlores, (2) structure-activity-stability relationships of different pyrochlores for OER and ORR, (3) development of OER-selective pyrochlores for brine electrolysis, and (4) the application of pyrochlores in electrochemical devices. Finally, we highlight some unaddressed issues such as the precise identification of active sites, which can be addressed in the future through advanced in situ and ex situ characterization techniques coupled with the density functional theory-based analyses. This Account provides foundational understanding to guide the comprehensive development of highly active, selective, stable and low-cost structurally engineered pyrochlores for high performance electrochemical devices.

Iridium Doped Pyrochlore Ruthenates for Efficient and Durable Electrocatalytic Oxygen Evolution in Acidic Media

Developing highly active, durable, and cost-effective electrocatalysts for the oxygen evolution reaction (OER) is of prime importance in proton exchange membrane (PEM) water electrolysis techniques. Ru-based catalysts have high activities but always suffer from severe fading and dissolution issues, which cannot satisfy the stability demand of PEM. Herein, a series of iridium-doped yttrium ruthenates pyrochlore catalysts is developed, which exhibit better activity and much higher durability than commercial RuO₂ , IrO₂ , and most of the reported Ru or Ir-based OER electrocatalysts. Typically, the representative Y₂ Ru_1.2 Ir_0.8 O₇ OER catalyst demands a low overpotential of 220 mV to achieve 10 mA cm^-2 , which is much lower than that of RuO₂ (300 mV) and IrO₂ (350 mV). In addition, the catalyst does not show obvious performance decay or structural degradation over a 2000 h stability test. EXAFS and XPS co-prove the reduced valence state of ruthenium and iridium in pyrochlore contributes to the improved activity and stability. Density functional theory reveals that the potential-determining steps barrier of OOH* formation is greatly depressed through the synergy effect of Ir and Ru sites by balancing the d band center and oxygen intermediates binding ability.

Promoting biomass electrooxidation via modulating proton and oxygen anion deintercalation in hydroxide

The redox center of transition metal oxides and hydroxides is generally considered to be the metal site. Interestingly, proton and oxygen in the lattice recently are found to be actively involved in the catalytic reactions, and critically determine the reactivity. Herein, taking glycerol electrooxidation reaction as the model reaction, we reveal systematically the impact of proton and oxygen anion (de)intercalation processes on the elementary steps. Combining density functional theory calculations and advanced spectroscopy techniques, we find that doping Co into Ni-hydroxide promotes the deintercalation of proton and oxygen anion from the catalyst surface. The oxygen vacancies formed in NiCo hydroxide during glycerol electrooxidation reaction increase d-band filling on Co sites, facilitating the charge transfer from catalyst surface to cleaved molecules during the 2^nd C-C bond cleavage. Consequently, NiCo hydroxide exhibits enhanced glycerol electrooxidation activity, with a current density of 100 mA/cm² at 1.35 V and a formate selectivity of 94.3%.

Developing catalysts for biomass electrooxidation are critical in electric refinery. The reaction mechanism, however, is still ambiguous. Here, the authors reveal how proton and oxygen anion deintercalation in hydroxide determine the elementary reaction steps in a model reaction of glycerol oxidation.

Cu-Based Multicomponent Metallic Compound Materials as Electrocatalyst for Water Splitting

In this study, Cu-based multicomponent metallic compound materials M-Cu (M = Mn, Fe, Co, Ni, and Pt) were studied as electrocatalytic materials for water splitting. Different metal materials attached to the copper foam substrate can change the valence states of copper and oxygen, resulting in the change of electronic structure of the materials, thus changing its catalytic activity.

Regulating the electronic structures of mixed B-site pyrochlore to enhance the turnover frequency in water oxidation

This paper describes the development of mixed B-site pyrochlore Y2MnRuO7 electrocatalyst for oxygen evolution reaction (OER) in acidic media, a challenge for the development of low-temperature electrolyzer for green hydrogen production. Recently, several theories have been developed to understand the reaction mechanism for OER, though there is an  uncertainty in most of the cases, due to the complex surface structures. Several key factors such as lattice oxygen, defect, electronic structure, oxidation state, hydroxyl group and conductivity were identified and shown to be important to the OER activity. The contribution of each factor to the performance however is often not well understood, limiting their impact in guiding the design of OER electrocatalysts. In this work, we showed mixed B-site pyrochlore Y2MnRuO7 catalyst exhibits 14 times higher turnover frequency (TOF) than RuO2 while maintaining a low overpotential of ~ 300 mV for the entire testing period of 24 h in acidic electrolyte. X-ray photoelectron spectroscopy (XPS) analysis reveals that this B-site mixed pyrochlore Y2MnRuO7 has a higher oxidation state of Ru than those of Y2Ru2O7, which could be crucial for improving OER performance as the broadened and lowered Ru 4d band resulted from the B-site substitution by Mn is beneficial to the OER kinetics.

The Pivotal Role of s-, p-, and f-Block Metals in Water Electrolysis: Status Quo and Perspectives

Transition metals, in particular noble metals, are the most common species in metal-mediated water electrolysis because they serve as highly active catalytic sites. In many cases, the presence of nontransition metals, that is, s-, p-, and f-block metals with high natural abundance in the earth-crust in the catalytic material is indispensable to boost efficiency and durability in water electrolysis. This is why alkali metals, alkaline-earth metals, rare-earth metals, lean metals, and metalloids receive growing interest in this research area. In spite of the pivotal role of these nontransition metals in tuning efficiency of water electrolysis, there is far more room for developments toward a knowledge-based catalyst design. In this review, five classes of nontransition metals species which are successfully utilized in water electrolysis, with special emphasis on electronic structure-catalytic activity relationships and phase stability, are discussed. Moreover, specific fundamental aspects on electrocatalysts for water electrolysis as well as a perspective on this research field are also addressed in this account. It is anticipated that this review can trigger a broader interest in using s-, p-, and f-block metals species toward the discovery of advanced polymetal-containing electrocatalysts for practical water splitting.

A Spinel Tin Ferrite with High Lattice-Oxygen Anchored on Graphene-like Porous Carbon Networks for Lithium-Ion Batteries with Super Cycle Stability and Ultra-fast Rate Performances

A new type of nano-SnFe₂O₄ with stable lattice-oxygen and abundant surface defects anchored on ultra-thin graphene-like porous carbon networks (SFO@C) is prepared for the first time by an interesting freezing crystallization salt template method. The functional composite has excellent rate performance and long-term cycle stability for lithium-ion battery (LIB) anodes due to the stable structure, improved conductivity, and shortened migrating distance for lithium-ions, which are derived from the higher lattice-oxygen of SnFe₂O₄, abundant porous carbon networks and surface defects, and smaller nanoparticles. Under the ultra-high current density of 10, 15, and 20 A g^-1 cycling for 1000 times, the SFO@C can provide high reversible capacities of 522.2, 362.5, and 361.1 mAh g^-1, respectively. The lithium-ion storage mechanism of the composite was systematically studied for the first time by in situ X-ray diffraction (XRD), ex situ XRD and scanning electron microscopy (SEM), and density functional theory (DFT) calculations. The results indicate that the existence of Li₂O and metallic Fe during the lithiation/delithiation process is a key reason for reducing the initial lithium-ion storage reversibility but increasing the rate performance and capacity stability in the subsequent cycles. DFT calculations show that lithium-ions are more easily adsorbed on the (111) crystal plane with a much lower adsorption energy of -7.61 eV than other planes, and the Fe element is the main acceptor of electrons. Moreover, the kinetics investigation indicates that the lithium-ion intercalation and deintercalation in SFO@C are mainly controlled by the pseudocapacitance behavior, which is favorable to enhancing the rate performance. The research provides a new strategy for designing LIB electrode materials with a stable structure and outstanding lithium-ion storage performance.

Unraveling the Synergistic Effect of Heteroatomic Substitution and Vacancy Engineering in CoFe2O4 for Superior Electrocatalysis Performance

Metal ion substitution and anion exchange are two effective strategies for regulating the electronic and geometric structure of spinel. However, the optimal location of foreign metallic cations and the exact role of these metals and anions remain elusive. Herein, CoFe₂O₄-based hollow nanospheres with outstanding oxygen evolution reaction activity are prepared by Cr³⁺ substitution and S^2- exchange. X-ray absorption spectra and theoretical calculations reveal that Cr³⁺ can be precisely doped into octahedral (Oh) Fe sites and simultaneously induce Co vacancy, which can activate adjacent tetrahedral (Td) Fe³⁺. Furthermore, S^2- exchange results in structure distortion of Td-Fe due to compressive strain effect. The change in the local geometry of Td-Fe causes the *OOH intermediate to deviate from the y-axis plane, thus enhancing the adsorption of the *OOH. The Co vacancy and S^2- exchange can adjust the geometric and electronic structure of Td-Fe, thus activating the inert Td-Fe and improving the electrochemical performance.

Activating lattice oxygen in NiFe-based (oxy)hydroxide for water electrolysis

Transition metal oxides or (oxy)hydroxides have been intensively investigated as promising electrocatalysts for energy and environmental applications. Oxygen in the lattice was reported recently to actively participate in surface reactions. Herein, we report a sacrificial template-directed approach to synthesize Mo-doped NiFe (oxy)hydroxide with modulated oxygen activity as an enhanced electrocatalyst towards oxygen evolution reaction (OER). The obtained MoNiFe (oxy)hydroxide displays a high mass activity of 1910 A/gmetal at the overpotential of 300 mV. The combination of density functional theory calculations and advanced spectroscopy techniques suggests that the Mo dopant upshifts the O 2p band and weakens the metal-oxygen bond of NiFe (oxy)hydroxide, facilitating oxygen vacancy formation and shifting the reaction pathway for OER. Our results provide critical insights into the role of lattice oxygen in determining the activity of (oxy)hydroxides and demonstrate tuning oxygen activity as a promising approach for constructing highly active electrocatalysts.

A "Pre-Constrained Metal Twins" Strategy to Prepare Efficient Dual-Metal-Atom Catalysts for Cooperative Oxygen Electrocatalysis

Dual-metal-atom-center catalysts (DACs) are a novel frontier in oxygen electrocatalysis, boasting functional and electronic synergies between contiguous metal centers and higher catalytic activities than single-atom-center catalysts. However, the definition and catalytic mechanism of DACs configurations remain unclear. Here, a "pre-constrained metal twins" strategy is proposed to prepare contiguous FeN₄ and CoN₄ DACs with homogeneous conformations embedded in a N-doped graphitic carbon (FeCo-DACs/NC). A programmable phthalocyanines dimer is used as a structural moiety to anchor the bimetallic sites (containing Co and Fe) in a metal-organic framework (MOF) to achieve delocalized dispersion before pyrolysis. The resultant FeCo-DACs/NC exhibits excellent electrochemical performance in oxygen electrocatalysis and rechargeable Zn-air batteries. Theoretical calculations demonstrate that the synergetic interaction of adjacent metals optimizes the d-band center position of metal centers and balances the free energy of the *O intermediate, thereby improving the oxygen electrocatalytic activity. This work opens up an avenue for the rational design of DACs with tailored electronic structures and uniform geometric configurations.

Pyrochlore-structural Nd2Ta2O5N2 Photocatalyst with Absorption Edge of over 600 nm for Z-scheme Overall Water Splitting

A pyrochlore-structural oxynitride Nd2Ta2O5N2 with a visible light absorption edge of ca. 620 nm was explored for photocatalytic water splitting. Dual functions of Nd2Ta2O5N2 were confirmed by proton reduction and...

Emerging Electrocatalysts for Water Oxidation under Near-Neutral CO2 Reduction Conditions

Electrocatalytic CO₂ reduction reaction (CO₂ RR), which produces valuable fuels and chemicals under near-neutral conditions, offers a renewable approach to alleviate the global energy crisis as well as the increasing concerns on climate change. However, to implement this strategy, one of the major challenges, the sluggish kinetics of the paired oxygen evolution reaction (OER) at anode, needs to be surmounted. It is therefore highly desirable to explore high-performance and cost-effective OER electrocatalysts suitable for CO₂ RR conditions, which is very different from those widely investigated under acidic or alkaline conditions. In this review, the ongoing development of OER electrocatalysts under near pH-neutral CO₂ -saturated (bi)carbonate solutions are highlighted and the future opportunities are discussed. It is started with a brief introduction on OER paired with CO₂ RR, the relevant theoretical tools such as density functional theory (DFT) and particularly machine learning (ML), and the operando characterization techniques. Then, there are some detailed discussions of recent progress on the rational design of OER electrocatalysts under CO₂ RR conditions ranging from noble-metal oxides to nonprecious metal phosphides, carbonates, (hydro)oxides, and so on. Finally, a perspective for developing OER electrocatalysts integrated with CO₂ electroreduction is proposed.

Self-reconstruction of cationic activated Ni-MOFs enhanced the intrinsic activity of electrocatalytic water oxidation

Here, the cation modified Ni-MOFs catalysts was cleverly designed. And the multilayer FeNi-LDH with enrich active sites derived by in-situ self-reconstruction in alkali solution was considered to be the decisive factor driving the OER reaction.

Quenching as a Route to Defect-Rich Ru-Pyrochlore Electrocatalysts toward the Oxygen Evolution Reaction

Defects have a significant impact on the electrocatalysts performance. Introducing defect structures in metal oxides such as pyrochlores and perovskites has proved to be an effective strategy to enhance electrocatalytic activity. However, it is hard to build numerous defect sites in such high-temperature oxides due to the strong metal-oxygen bonds and the so-called self-purification effect, which becomes increasingly important as the particle size reduced to the nanoscale. Here, a facile strategy is demonstrated to fabricate defect-rich yttrium ruthenate oxides Y₂ Ru₂ O_{7- δ} with the pyrochlore structure (denoted D_rich -YRO) by the liquid nitrogen (<-196 °C) quenching. Owing to the almost instantaneous cooling in oxygen-deficient condition, a large number of defects-including oxygen vacancies, grain boundaries, pores and surficial disorder-are preserved in the room temperature material and act as electrocatalytic active sites for oxygen evolution. As a result, D_rich -YRO shows excellent catalytic activity and high electrochemical stability, along with a high performance in the operation of proton exchange membrane electrolyzer. The quenching strategy employed in this work provides a facile approach for constructing defect-rich structures in high-temperature oxides and should lead to new applications in energy conversion and storage devices for such materials.

How Do Oxygen Vacancies Influence the Catalytic Performance of Two-Dimensional Nb2 O5 in Lithium- and Sodium-Oxygen Batteries?

Alkali metal-oxygen batteries possess a higher specific capacity than alkali-ion batteries and stand out as the most competitive next-generation energy source. The core reaction mechanism of the battery is mainly the formation of alkali metal oxide during the discharge process and the decomposition of these oxides during the charge process. A large number of researchers have devoted themselves to seeking promising catalysts for the reaction. Two-dimensional Nb₂ O₅ was discovered to be a highly potential catalyst that can promote the reaction of alkali-metal-oxygen batteries, but few studies focus on it. In this study, the catalytic performance of both pristine Nb₂ O₅ and oxygen-deficiency modified Nb₂ O₅ was investigated. Furthermore, the effect of oxygen defects on catalytic performance was analyzed from multiple angles, namely, the reaction mechanism, d-band center theory, and the diffusion behavior of alkali metals. The exploration revealed the microscopic mechanism of oxygen deficiency affecting the alkali-metal battery reaction and provided a theoretical basis for quantitatively changing the d-band center of the catalyst through oxygen deficiency to ultimately change the performance of the catalyst.

Structural Variations of Metal Oxide-Based Electrocatalysts for Oxygen Evolution Reaction

Electrocatalytic oxygen evolution reaction (OER), an important electrode reaction in electrocatalytic and photoelectrochemical cells for a carbon-free energy cycle, has attracted considerable attention in the last few years. Metal oxides have been considered as good candidates for electrocatalytic OER because they can be easily synthesized and are relatively stable during the OER process. However, inevitable structural variations still occur to them due to the complex reaction steps and harsh working conditions of OER, thus impending the further insight into the catalytic mechanism and rational design of highly efficient electrocatalysts. The aim of this review is to disclose the current research progress toward the structural variations of metal oxide-based OER electrocatalysts. The origin of structural variations of metal oxides is discussed. Based on some typical oxides performing OER activity, the external and internal factors that influence the structural stability are summarized and then some general approaches to regulate the structural variation process are provided. Some operando methods are also concluded to monitor the structural variation processes and to identify the final active structure. Additionally, the unresolved problems and challenges are presented in an attempt to get further insight into the mechanism of structural variations and establish a rational structure-catalysis relationship.

Electrocatalytic Oxygen Evolution Reaction in Acidic Conditions: Recent Progress and Perspectives

The electrochemical oxygen evolution reaction (OER) is an important half-cell reaction in many renewable energy conversion and storage technologies, including electrolyzers, nitrogen fixation, CO₂ reduction, metal-air batteries, and regenerative fuel cells. Among them, proton exchange membrane (PEM)-based devices exhibit a series of advantages, such as excellent proton conductivity, high durability, and good mechanical strength, and have attracted global interest as a green energy device for transport and stationary sectors. Nevertheless, with a view to rapid commercialization, it is urgent to develop highly active and acid-stable OER catalysts for PEM-based devices. In this Review, based on the recent advances in theoretical calculation and in situ/operando characterization, the OER mechanism in acidic conditions is first discussed in detail. Subsequently, recent advances in the development of several types of acid-stable OER catalysts, including noble metals, non-noble metals, and even metal-free OER materials, are systematically summarized. Finally, the current key issues and future challenges for materials used as acidic OER catalysis are identified and potential future directions are proposed.

Iridium in Tungsten Trioxide Matrix as an Efficient Bi-Functional Electrocatalyst for Overall Water Splitting in Acidic Media

Electrocatalytic water splitting in acidic media is a promising strategy for grid scale production of hydrogen using renewable energy, but challenges still exist in the development of advanced catalysts with both high activity and stability. Herein, it is reported that iridium doped tungsten trioxide (Ir-doped WO₃ ) with arrayed structure and confined Ir sites is an efficient and durable bi-functional catalyst for overall acidic water splitting. A low overpotential (258 mV) is required to achieve an oxygen evolution reaction current density of 10 mA cm^-2 in 0.5 m H₂ SO₄ solution. Meanwhile, Ir-doped WO₃ processes a similar intrinsic activity to Pt/C toward hydrogen evolution reaction. Overall water splitting using the bi-functional Ir-doped WO₃ catalyst shows low cell voltages of 1.56 and 1.68 V to drive the current densities of 10 and 100 mA cm^-2 , respectively, with only 16 mV decay observed after 60 h continuous electrolysis under the current density of 100 mA cm^-2 . Structural analysis and density functional theory calculation indicate that the adjusted coordination environment of Ir within the crystalline matrix of WO₃ contributes to the high activity and durability.

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.

Chloride Reduction of Mn3+ in Mild Hydrothermal Synthesis of a Charge Ordered Defect Pyrochlore, CsMn2+Mn3+F6, a Canted Antiferromagnet with a Hard Ferromagnetic Component

Geometrically frustrated systems play an important role in studying new physical phenomena and unconventional thermodynamics. Charge ordered defect pyrochlores AM²⁺M³⁺F₆ offer a convenient platform for probing the interplay between electron distribution over M²⁺ and M³⁺ sites and structural distortions; however, they are limited to compounds with M^2+/3+ = V, Fe, Ni, and Cu due to difficulties in the simultaneous stabilization of other 3d elements in the +2 and +3 oxidation states. Herein, we employ Cl^- anions under hydrothermal conditions for the mild reduction of Mn₂O₃ in concentrated HF to obtain the CsMn²⁺Mn³⁺F₆ composition as a phase pure sample and study its properties. The magnetism of CsMn₂F₆ was characterized by measuring the magnetic susceptibility and isothermal magnetization data, and a magnetic transition to a canted antiferromagnet state was found at 24.1 K. We determined the magnetic structure of CsMn₂F₆ using powder neutron diffraction, which revealed successive long-range ordering of the Mn²⁺ and Mn³⁺ sites that is accompanied by a second transition. The role and strength of magnetic exchange interactions were characterized using DFT calculations.

Metal Substitution Steering Electron Correlations in Pyrochlore Ruthenates for Efficient Acidic Water Oxidation

Exploring the advanced oxygen evolution reaction (OER) electrocatalysts is highly desirable toward sustainable energy conversion and storage, yet improved efficiency in acidic media is largely hindered by its sluggish reaction kinetics. Herein, we rationally manipulate the electronic states of the strongly electron correlated pyrochlore ruthenate Y₂Ru₂O₇ alternative through partial A-site substitution of Sr²⁺ for Y³⁺, efficiently improving its intrinsic OER activity. The optimized Y_1.7Sr_0.3Ru₂O₇ candidate observes a highly intrinsic mass activity of 1018 A g_Ru^-1 at an overpotential of 300 mV with excellent durability in 0.5 M H₂SO₄ electrolyte. Combining synchrotron-radiation X-ray spectroscopic investigations with theoretical simulations, we reveal that the electron correlations in the Ru 4d band are weakened through coordinatively geometric regulation and charge redistribution by the exotic Sr²⁺ cation, enabling the delocalization of Ru 4d electrons via an insulator-to-metal transition. The induced Ru-O covalency promotion and band alignment rearrangement decreases the charge transfer energy to accelerate interfacial charge transfer kinetics. Meanwhile, the chemical affinity of oxygen intermediates is also rationalized to weaken the metal-oxygen binding strength, thus lowering the energy barrier of the overall reaction. This work offers fresh insights into designing advanced solid-state electrocatalysts and underlines the versatility of electronic structure manipulation in tuning catalytic activity.

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.

Hierarchical CoP Nanostructures on Nickel Foam as Efficient Bifunctional Catalysts for Water Splitting

A highly active and cheap catalyst is also key to hydrogen production by water splitting. However, most of the high-efficiency catalysts reported to date only are catalytically active for either the hydrogen evolution reaction (HER) or the oxygen evolution reaction (OER), which makes the development of multifunctional catalysts more meaningful. Here, for the first time, Co(CO₃ )_0.5 OH^. 0.11 H₂ O (CHCH) as precursor with different microstructures on the surface of nickel foam (NF) was obtained using a facile hydrothermal method. The CoP/NF catalyst was obtained after thermal phosphating that retained the microhierarchical structure of the precursor and greatly improved the catalytic performance, with a highly efficiency performance as HER and OER dual-functional catalyst. Density functional theory (DFT) calculations showed that the possible reason for the excellent performance of the CoP/NF layered structure is an increase in the number of of surface defects and an increased active surface area. The results reported in this paper show that CoP/NF, a layered bifunctional electrocatalyst, is a cost-effective and efficient water-splitting electrode. This finding can offer the opportunity for the commercial use of excess electric energy for large-scale water splitting hydrogen production.

Surface Lattice Oxygen Activation on Sr2Sb2O7 Enhances the Photocatalytic Mineralization of Toluene: from Reactant Activation, Intermediate Conversion to Product Desorption

Transition-metal oxide photocatalysis has attracted increasing attention in environmental remediation and solar energy conversion. Surface lattice oxygen is the key active site on the metal oxide, but its role and activation mechanism in the photocatalytic VOC mineralization are still unclear. In this work, we have demonstrated that Sr₂Sb₂O₇ exhibits an excellent photocatalytic activity and stability compared to TiO₂ (P25) in gaseous toluene mineralization because the lattice oxygen on Sr₂Sb₂O₇ can be activated efficiently. The lattice oxygen of Sr₂Sb₂O₇ promotes the adsorption and activation of O₂ and H₂O molecules and enhances the production of ^•O₂^- and ^•OH radicals, as confirmed by the electron spin resonance and DFT calculations. The in situ diffuse reflectance infrared Fourier transform spectroscopy spectra are applied to dynamically monitor the intermediate activation and selective conversion. Combined with DFT calculation, the role and the mechanism of lattice oxygen in photocatalysis have been revealed. Owing to the promoted surface lattice oxygen, the selectivity for benzoic acid formation is enhanced and final product desorption is promoted, which could largely advance the ring opening and mineralization of toluene. This work reveals the origin of lattice oxygen activation and the role for efficient VOC degradation at the atomic scale.

Cyanogroup functionalized sub-2 nm ultrafine Pt nanonetworks reinforce electrocatalytic hydrogen evolution in a broad pH range

Cyanogroup functionalized Pt ultrafine nanonetworks are synthesized via a facile one-pot oil bath heating method, and exhibit excellent HER performance in a broad pH range.

Carbon-Coated Tungsten Oxide Nanospheres Triggering Flexible Electron Transfer for Efficient Electrocatalytic Oxidation of Water and Glucose

Electrocatalytic oxidation of water (i.e., oxygen evolution reaction, OER) plays crucial roles in energy, environment, and biomedicine. It is a key factor affecting the efficiencies of electrocatalytic reactions conducted in aqueous solution, e.g., electrocatalytic water splitting and glucose oxidation reaction (GOR). However, electrocatalytic OER still suffers from problems like high overpotential, sluggish kinetics, and over-reliance on expensive noble-metal-based catalysts. Herein, 15 nm thick carbon-based shell coated tungsten oxide (CTO) nanospheres are loaded on nickel foam to form CTO/NF. An enhanced electrocatalytic OER is triggered on CTO/NF, with the overpotential at 50 mA cm^-2 (317 mV) and the Tafel slope (70 mV dec^-1) on CTO/NF lower than those on pure tungsten oxide (360 mV, 117 mV dec^-1) and noble-metal-based IrO₂ catalysts (328 mV, 96 mV dec^-1). A promoted electrocatalytic GOR is also achieved on CTO/NF, with efficiency as high as 189 μA mM^-1 cm^-2. The carbon-based shell on CTO is flexible for electron transfer between catalyst and reactants and provides catalytically active sites. This improves reactant adsorption and O-H bond dissociation on the catalyst, which are key steps in OER and GOR. The carbon-based shell on CTO retains the catalyst as nanospheres with a higher surface area, which facilitates OER and GOR. It is the multiple roles of the carbon-based shell that increases the electrocatalytic efficiency. These results are helpful for fabricating more efficient noble-metal-free electrocatalysts.

Pt-Co3O4 Superstructures by One-Pot Reduction/Precipitation in Bicontinuous Microemulsion for Electrocatalytic Oxygen Evolution Reaction

Bicontinuous microemulsions (BCME) were used to synthesize hierarchical superstructures (HSs) of Pt-Co3O4 by reduction/precipitation. BCMEs possess water and oil nanochannels, and therefore, both hydrophilic and lipophilic precursors can be used. Thus, PtAq-CoAq, PtAq-CoOi, PtOi-CoAq and PtOi-CoOi were prepared (where Aq and Oi stand for the precursor present in aqueous or oily phase, respectively). The characterization of the Pt-Co3O4-HS confirmed the formation of metallic Pt and Co3O4 whose composition and morphology are controlled by the initial pH and precursor combination, determining the presence of the reducing/precipitant species in the reaction media. The electrocatalytic activity of the Pt-Co3O4-HSs for oxygen evolution reaction (OER) was investigated using linear sweep voltammetry in 0.1 M KOH and compared with Pt-HS. The lowest onset overpotentials for Pt-Co3O4-Hs were achieved with PtOi-CoOi (1.46 V vs. RHE), while the lowest overpotential at a current density of 10 mA cm−2 (η10) was obtained for the PtAq-CoAq (381 mV). Tafel slopes were 102, 89, 157 and 92 mV dec−1, for PtAq-CoAq, PtAq-CoOi, PtOi-CoAq and PtOi-CoOi, respectively. The Pt-Co3O4-HSs showed a better performance than Pt-HS. Our work shows that the properties and performance of metal–metal oxide HSs obtained in BCMEs depend on the phases in which the precursors are present.

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.

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.

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.