Tuning proton-coupled electron transfer by crystal orientation for efficient water oxidization on double perovskite oxides

Anthraquinones-based photocatalysis: A comprehensive review

In recent years, there has been significant interest in photocatalytic technologies utilizing semiconductors and photosensitizers responsive to solar light, owing to their potential for energy and environmental applications. Current efforts are focused on enhancing existing photocatalysts and developing new ones tailored for environmental uses. Anthraquinones (AQs) serve as redox-active electron transfer mediators and photochemically active organic photosensitizers, effectively addressing common issues such as low light utilization and carrier separation efficiency found in conventional semiconductors. AQs offer advantages such as abundant raw materials, controlled preparation, excellent electron transfer capabilities, and photosensitivity, with applications spanning the energy, medical, and environmental sectors. Despite their utility, comprehensive reviews on AQs-based photocatalytic systems in environmental contexts are lacking. In this review, we thoroughly describe the photochemical properties of AQs and their potential applications in photocatalysis, particularly in addressing key environmental challenges like clean energy production, antibacterial action, and pollutant degradation. However, AQs face limitations in practical photocatalytic applications due to their low electrical conductivity and solubility-related secondary contamination. To mitigate these issues, the design and synthesis of graphene-immobilized AQs are highlighted as a solution to enhance practical photocatalytic applications. Additionally, future research directions are proposed to deepen the understanding of AQs' theoretical mechanisms and to provide practical applications for wastewater treatment. This review aims to facilitate mechanistic studies and practical applications of AQs-based photocatalytic technologies and to improve understanding of these technologies.

Promoting Oxygen Evolution Electrocatalysis by Coordination Engineering in Cobalt Phosphate

Understanding the structure-activity correlation is an important prerequisite for the rational design of high-efficiency electrocatalysts at the atomic level. However, the effect of coordination environment on electrocatalytic oxygen evolution reaction (OER) remains enigmatic. In this work, the regulation of proton transfer involved in water oxidation by coordination engineering based on Co3(PO4)2 and CoHPO4 is reported. The HPO4 2- anion has intermediate pKa value between Co(II)-H2O and Co(III)-H2O to be served as an appealing proton-coupled electron transfer (PCET) induction group. From theoretical calculations, the pH-dependent OER properties, deuterium kinetic isotope effects, operando electrochemical impedance spectroscopy (EIS) and Raman studies, the CoHPO4 catalyst beneficially reduces the energy barrier of proton hopping and modulates the formation energy of high-valent Co species, thereby enhancing OER activity. This work demonstrates a promising strategy that involves tuning the local coordination environment to optimize PCET steps and electrocatalytic activities for electrochemical applications. In addition, the designed system offers a motif to understand the structure-efficiency relationship from those amino-acid residue with proton buffer ability in natural photosynthesis.

The mechanism of water oxidation using transition metal-based heterogeneous electrocatalysts

The water oxidation reaction, a crucial process for solar energy conversion, has garnered significant research attention. Achieving efficient energy conversion requires the development of cost-effective and durable water oxidation catalysts. To design effective catalysts, it is essential to have a fundamental understanding of the reaction mechanisms. This review presents a comprehensive overview of recent advancements in the understanding of the mechanisms of water oxidation using transition metal-based heterogeneous electrocatalysts, including Mn, Fe, Co, Ni, and Cu-based catalysts. It highlights the catalytic mechanisms of different transition metals and emphasizes the importance of monitoring of key intermediates to explore the reaction pathway. In addition, advanced techniques for physical characterization of water oxidation intermediates are also introduced, for the purpose of providing information for establishing reliable methodologies in water oxidation research. The study of transition metal-based water oxidation electrocatalysts is instrumental in providing novel insights into understanding both natural and artificial energy conversion processes.

Perovskite Oxides Toward Oxygen Evolution Reaction: Intellectual Design Strategies, Properties and Perspectives

Developing electrochemical energy storage and conversion devices (e.g., water splitting, regenerative fuel cells and rechargeable metal-air batteries) driven by intermittent renewable energy sources holds a great potential to facilitate global energy transition and alleviate the associated environmental issues. However, the involved kinetically sluggish oxygen evolution reaction (OER) severely limits the entire reaction efficiency, thus designing high-performance materials toward efficient OER is of prime significance to remove this obstacle. Among various materials, cost-effective perovskite oxides have drawn particular attention due to their desirable catalytic activity, excellent stability and large reserves. To date, substantial efforts have been dedicated with varying degrees of success to promoting OER on perovskite oxides, which have generated multiple reviews from various perspectives, e.g., electronic structure modulation and heteroatom doping and various applications. Nonetheless, the reviews that comprehensively and systematically focus on the latest intellectual design strategies of perovskite oxides toward efficient OER are quite limited. To bridge the gap, this review thus emphatically concentrates on this very topic with broader coverages, more comparative discussions and deeper insights into the synthetic modulation, doping, surface engineering, structure mutation and hybrids. More specifically, this review elucidates, in details, the underlying causality between the being-tuned physiochemical properties [e.g., electronic structure, metal-oxygen (M-O) bonding configuration, adsorption capacity of oxygenated species and electrical conductivity] of the intellectually designed perovskite oxides and the resulting OER performances, coupled with perspectives and potential challenges on future research. It is our sincere hope for this review to provide the scientific community with more insights for developing advanced perovskite oxides with high OER catalytic efficiency and further stimulate more exciting applications.

Graphical Abstract

Bifunctional Square-Planar NiO4 Coordination of Topotactic LaNiO2.0 Films for Efficient Oxygen Evolution Reaction

The high-efficient and low-cost oxygen evolution reaction (OER) is decisive for applications of oxide catalysts in metal-air batteries, electrolytic cells, and energy-storage technologies. Delicate regulations of active surface and catalytic reaction pathway of oxide materials principally determine thermodynamic energy barrier and kinetic rate during catalytic reactions, and thus have crucial impacts on OER performance. Herein, a synergistic modulation of catalytically active surface and reaction pathway through facile topotactic transformations switching from perovskite (PV) LaNiO3.0 film to infinite-layer (IL) LaNiO2.0 film is demonstrated, which absolutely contributes to improving OER performance. The square-planar NiO4 coordination of IL-LaNiO2.0 brings about more electrochemically active metal (Ni+ ) sites on the film surface. Meanwhile, the oxygen-deficient driven PV- IL topotactic transformations lead to a reaction pathway converted from absorbate evolution mechanism to lattice-oxygen-mediated mechanism (LOM). The non-concerted proton-electron transfer of LOM pathway, evidenced by the pH-dependent OER kinetics, further boosts the OER activity of IL-LaNiO2.0 films. These findings will advance the in-depth understanding of catalytic mechanisms and open new possibilities for developing highly active perovskite-derived oxide catalysts.

Facilitating Proton Coupled Electron Transfer Reaction through the Interfacial Micro Electric Field with Fe─N4 ─C in FeMOFs Glass

The proton-coupled electron transfer(PCET) reaction plays a crucial role in the chemical transformation process andhas become one of the most concerned elementary reactions. However, the complex kinetics of PCET reaction, which requires the simultaneous transfer of protons and electrons, leads to the dilemma that thermodynamics and kinetics cannot bebalanced and restricts its further development. In this, an interface micro-electric field (IMEF) basedon Fe─N4 in FeMOFs (Fe-Based Metal-Organic Frameworks) glass is designed tosynchronize proton/electron interface behavior for the first time to realizeefficient PCET reaction and optimize reaction thermodynamics and kinetics. The IMEF facilitates the separation of photogenerated electrons and holes, and accelerates Fe(III)/Fe(II) cycle. Driven by near-surface electric field force, the protons near surfacemigrate to Fe sites and participate in Fe(IV)═O formation and reaction, lowering the reaction energy barrier. Based on the interface regulation ofIMEF, a high-efficiency PCET reaction is realized, and kinetic reactionrate constant of photocatalytic oxidation of emerging contaminants is increasedby 3.7 times. This study highlights a strategy for IMEFs to modulate PEC Treactions for a wide range of potential applications, including environmental and ecological applications.

Realizing a high OER activity in new single-atom catalysts formed by introducing TMNx (x = 3 and 4) units into carbon nanotubes using high-throughput calculations

Exploring highly efficient electrocatalysts for the oxygen evolution reaction (OER) is of great significance for hydrogen production through water splitting. By means of high-throughput density functional theory (DFT) calculations, we investigated the OER catalytic activity of a series of one-dimensional carbon nanotube (CNT)-based systems containing TMN4 or TMN3 functional units. Through the screening of 3d/4d/5d transition metals (TMs) from Group IVB to Group VIII, eight newly obtained TMNx@CNT (x = 3 and 4) systems were found to exhibit excellent OER activity, with very low overpotentials in the range 0.29-0.51 V, where the Co, Rh, Ir, Ti, Fe, and Ru atoms could be used as active sites. It was found that under the framework of TMN3@CNTs, the pre-adsorption of some species from water dissociation on the relevant TM sites (TM = Ti, Fe, and Ru) could lead to a high OER catalytic activity, which was different from the general situation where OER reactions directly occur on the clean surfaces of the remaining systems with Co/Rh/Ir metal centers. Moreover, the catalytic mechanisms were analyzed in detail. This work can be conducive to obtaining low-cost and high-performance OER single-atom electrocatalysts based on excellent CNT nanomaterials.

Crystal-facet-dependent surface transformation dictates the oxygen evolution reaction activity in lanthanum nickelate

Electrocatalysts are the cornerstone in the transition to sustainable energy technologies and chemical processes. Surface transformations under operation conditions dictate the activity and stability. However, the dependence of the surface structure and transformation on the exposed crystallographic facet remains elusive, impeding rational catalyst design. We investigate the (001), (110) and (111) facets of a LaNiO3-δ electrocatalyst for water oxidation using electrochemical measurements, X-ray spectroscopy, and density functional theory calculations with a Hubbard U term. We reveal that the (111) overpotential is ≈ 30-60 mV lower than for the other facets. While a surface transformation into oxyhydroxide-like NiOO(H) may occur for all three orientations, it is more pronounced for (111). A structural mismatch of the transformed layer with the underlying perovskite for (001) and (110) influences the ratio of Ni2+ and Ni3+ to Ni4+ sites during the reaction and thereby the binding energy of reaction intermediates, resulting in the distinct catalytic activities of the transformed facets.

Electrochemical Etching Switches Electrocatalytic Oxygen Evolution Pathway of IrOx /Y2 O3 from Adsorbate Evolution Mechanism to Lattice-Oxygen-Mediated Mechanism

Oxygen evolution reaction (OER) plays key roles in electrochemical energy conversion devices. Recent advances have demonstrated that OER catalysts through lattice oxygen-mediated mechanism (LOM) can bypass the scaling relation-induced limitations on those catalysts through adsorbate evolution mechanism (AEM). Among various catalysts, IrOx , the most promising OER catalyst, suffers from low activities for its AEM pathway. Here, it is demonstrated that a pre-electrochemical acidic etching treatments on the hybrids of IrOx and Y2 O3 (IrOx /Y2 O3 ) switch the AEM-dominated OER pathway to LOM-dominated one in alkali electrolyte, delivering a high performance with a low overpotential of 223 mV at 10 mA cm-2 and a long-term stability. Mechanism investigations suggest that the pre-electrochemical etching treatments create more oxygen vacancies in catalysts due to the dissolution of yttrium and then provide highly active surface lattice oxygen for participating OER, thereby enabling the LOM-dominated pathway and resulting in a significantly increased OER activity in basic electrolyte.

Reconstructed Ir‒O‒Mo species with strong Brønsted acidity for acidic water oxidation

Surface reconstruction generates real active species in electrochemical conditions; rational regulating reconstruction in a targeted manner is the key for constructing highly active catalyst. Herein, we use the high-valence Mo modulated orthorhombic Pr₃Ir_1-xMo_xO₇ as model to activate lattice oxygen and cations, achieving directional and accelerated surface reconstruction to produce self-terminated Ir‒O_bri‒Mo (O_bri represents the bridge oxygen) active species that is highly active for acidic water oxidation. The doped Mo not only contributes to accelerated surface reconstruction due to optimized Ir‒O covalency and more prone dissolution of Pr, but also affords the improved durability resulted from Mo-buffered charge compensation, thereby preventing fierce Ir dissolution and excessive lattice oxygen loss. As such, Ir‒O_bri‒Mo species could be directionally generated, in which the strong Brønsted acidity of O_bri induced by remaining Mo assists with the facilitated deprotonation of oxo intermediates, following bridging-oxygen-assisted deprotonation pathway. Consequently, the optimal catalyst exhibits the best activity with an overpotential of 259 mV to reach 10 mA cm_geo^-2, 50 mV lower than undoped counterpart, and shows improved stability for over 200 h. This work provides a strategy of directional surface reconstruction to constructing strong Brønsted acid sites in IrO_x species, demonstrating the perspective of targeted electrocatalyst fabrication under in situ realistic reaction conditions.

Manipulating electron redistribution induced by asymmetric coordination for electrocatalytic water oxidation at a high current density

Electronic structure manipulation with regard to active site coordination is an effective strategy to improve the electrocatalytic oxygen evolution reaction (OER) activity. Herein, we present the structure-activity relationship between oxygen-atom-mediated electron rearrangement and active site coordination asymmetry. Ni²⁺ ions are introduced to FeWO₄ on Ni foam (NF) via self-substitution to break the symmetry of the FeO₆ octahedron and regulate d-electron structure of Fe sites. Structural regulation optimizes the adsorption energy of hydroxyl on the Fe sites and promotes the partial formation of hydroxyl oxide with high OER activity on the tungstate surface. Fe_0.53Ni_0.47WO₄/NF with the asymmetric FeO₆ octahedron of Fe sites can achieve an ultralow overpotential of 170 mV at 10 mA cm^-2 and 240 mV at 1000 mA cm^-2 with robust stability for 500 h at high current density under alkaline conditions. This research develops novel electrocatalysts with impressive OER performance and provides new insights into the design of highly active catalytic systems.

One-Step High-Temperature Electrodeposition of Fe-Based Films as Efficient Water Oxidation Catalysts

Electrolysis of water to produce hydrogen requires an efficient catalyst preferably made of cheap and abundant metal ions for the improved water oxidation reaction. An Fe-based film has been deposited in a single step by electrochemical deposition at temperatures higher than the room temperature. Until now, the electrodeposition of iron oxide has been carried out at 298 K or at lower temperatures under a controlled atmosphere to prohibit atmospheric oxidation of Fe²⁺ of the iron precursor. A metal inorganic complex, ferrocene, and non-aqueous electrolyte medium propylene carbonate have been used to achieve electrodeposition of iron oxide without the need of any inert or controlled atmosphere. At 298 K, the amorphous film was formed, whereas at 313 K and at higher temperatures, the hematite film was grown, as confirmed by X-ray diffraction. The transformation of iron of the ferrocene into a higher oxidation state under the experimental conditions used was further confirmed by X-ray photoelectron spectroscopy, ultraviolet-visible, and electron paramagnetic resonance spectroscopic methods. The films deposited at 313 K showed the best performance for water oxidation with remarkable long-term electrocatalytic stability and an impressive turnover frequency of 0.028 s^-1 which was 4.5 times higher than that of films deposited at 298 K (0.006 s^-1). The observed overpotential to achieve a current density of 10 mA cm^-2 was found to be 100 mV less for the film deposited at 313 K compared to room-temperature-derived films under similar experimental conditions. Furthermore, electrochemical impedance data revealed that films obtained at 313 K have the least charge transfer resistance (114 Ω) among all, supporting the most efficient electron transport in the film. To the best of our knowledge, this is the first-ever report where the crystalline iron-based film has been shown to be electrodeposited without any post-deposition additional treatment for alkaline oxygen evolution reaction application.

Crystalline-Dependent Discharge Process of Locally Enhanced Electrooxidation Activity on Ni2P

The state-of-the-art transition-based electrocatalysts in alkaline media generally suffer from unavoidable surface reconstruction during oxygen evolution reaction measurements, leading to the collapse and loss of the crystalline matrix. Low potential discharge offers a gentle way for surface reconstruction and thus realizes the manipulation of the real active site. Nevertheless, the absence of a fundamental understanding focus on this discharge region renders the functional phase, either the crystalline or amorphous matrix, for the controllable reconstruction still undecidable. Herein, we report a scenario to employ different crystalline matrices as electrocatalysts for discharge region reconstruction. The representative low crystalline Ni₂P (LC-Ni₂P) possesses a relatively weak surface structure compared with highly crystalline or amorphous Ni₂P (HC-Ni₂P or A-Ni₂P), which contributes abundant oxygen vacancies after the discharge process. The fast discharge behavior of LC-Ni₂P leads to the uniform distribution of these vacancies and thus endows the inner interface with reactant activating functionality. A high increase in current density of 36.7% is achieved at 2.32 V (vs RHE) for the LC-Ni₂P electrode. The understanding of the discharge behavior in this study, on different crystalline matrices, presents insights into the establishment of controllable surface reconstruction for an effective oxygen evolution reaction.

Effect of Proton Transfer on Electrocatalytic Water Oxidation by Manganese Phosphates

The effect of proton transfer on water oxidation has hardly been measurably established in heterogeneous electrocatalysts. Herein, two isomorphous manganese phosphates (NH₄ MnPO₄  ⋅ H₂ O and KMnPO₄  ⋅ H₂ O) were designed to form an ideal platform to study the effect of proton transfer on water oxidation. The hydrogen-bonding network in NH₄ MnPO₄  ⋅ H₂ O has been proven to be solely responsible for its better activity. The differences of the proton transfer kinetics in the two materials indicate a fast proton hopping transfer process with a low activation energy in NH₄ MnPO₄  ⋅ H₂ O. In addition, the hydrogen-bonding network can effectively promote the proton transfer between adjacent Mn sites and further stabilize the Mn^III -OH intermediates. The faster proton transfer results in a higher proportion of zeroth-order in [H⁺ ] for OER. Thus, proton transfer-affected electrocatalytic water oxidation has been measurably observed to bring detailed insights into the mechanism of water oxidation.

Toward Superb Perovskite Oxide Electrocatalysts: Engineering of Coupled Nanocomposites

A significant issue that bedeviled the commercialization of renewable energy technologies, ranging from low-temperature water electrolyzers to high-temperature solid oxide cells, is the lack of high-performance catalysts. Among various candidates that could tackle such a challenge, perovskite oxides are rising-star materials because of their unique structural and compositional flexibility. However, single-phase perovskite oxides are challenging to satisfy all the requirements of electrocatalysts concurrently for practical applications, such as high catalytic activity, excellent stability, good ionic and electronic conductivities, and superior chemical/thermo-mechanical robustness. Impressively, perovskite oxides with coupled nanocomposites are emerging as a novel form offering multifunctionality due to their intrinsic features, including infinite interfaces with solid interaction, tunable compositions, flexible configurations, and maximum synergistic effects between assorted components. Considering this new configuration has attracted great attention owing to its promising performances in various energy-related applications, this review timely summarizes the leading-edge development of perovskite oxide-based coupled nanocomposites. Their state-of-art synthetic strategies are surveyed and highlighted, their unique structural advantages are highlighted and illustrated through the typical oxygen reduction reaction and oxygen evolution reactions in both high and low-temperature applications. Opinions on the current critical scientific issues and opportunities in this burgeoning research field are all provided.

Understanding of Oxygen Redox in the Oxygen Evolution Reaction

The electron-transfer process during the oxygen evolution reaction (OER) often either proceeds solely via a metal redox chemistry (adsorbate evolution mechanism (AEM), with metal bands around the Fermi level) or an oxygen redox chemistry (lattice oxygen oxidation mechanism (LOM), with oxygen bands around the Fermi level). Unlike the AEM, the LOM involves oxygen redox chemistry instead of metal redox, which leads to the formation of a direct oxygen-oxygen (OO) bond. As a result, such a process is able to bypass the rate-determining step, that is, OO bonding, in AEM, which highlights the critical advantage of LOM as compared to the conventional AEM. Thus, it has been well reported that LOM-based catalysts are able to demonstrate higher OER activities as compared to AEM-based catalysts. Here, a comprehensive understanding of the oxygen redox in LOM and all documented and possible characterization techniques that can be used to identify the oxygen redox are reviewed. This review will interpret the origins of oxygen redox in the reported LOM-based electrocatalysts and the underlying science of LOM-induced surface reconstruction in transition metal oxides. Finally, perspectives on the future development of LOM electrocatalysts are also provided.

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.

Enhancement on PrBa0.5Sr0.5Co1.5Fe0.5O5 Electrocatalyst Performance in the Application of Zn-Air Battery

Due to the insufficient stability and expensive price of commercial precious metal catalysts like Pt/C and IrO2, it is critical to study efficiently, stable oxygen reduction reaction as well as oxygen evolution reaction (ORR/OER) electrocatalysts of rechargeable Zn-air batteries. PrBa0.5Sr0.5Co1.5Fe0.5O5 (PBSCF) double perovskite was adopted due to its flexible electronic structure as well as higher electro catalytic activity. In this study, PBSCF was prepared by the citrate-EDTA method and the optimized amount of PBSCF-Pt/C composite was used as a potential ORR/OER bifunctional electrocatalyst in 0.1 M KOH. The optimized composite exhibited excellent OER intrinsic activity with an onset potential of 1.6 V and Tafel slope of 76 mV/dec under O2-saturated 0.1 M KOH. It also exhibited relatively competitive ORR activity with an onset potential of 0.9 V and half-wave potential of 0.78 V. Additionally, Zn–air battery with PBSCF composite catalyst showed relatively good stability. All these results illustrate that PBSCF-Pt/C composite is a promising bifunctional electrocatalyst for rechargeable Zn-air batteries.

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.

Ion Intercalation in Lanthanum Strontium Ferrite for Aqueous Electrochemical Energy Storage Devices

Ion intercalation of perovskite oxides in liquid electrolytes is a very promising method for controlling their functional properties while storing charge, which opens up its potential application in different energy and information technologies. Although the role of defect chemistry in oxygen intercalation in a gaseous environment is well established, the mechanism of ion intercalation in liquid electrolytes at room temperature is poorly understood. In this study, the defect chemistry during ion intercalation of La_0.5Sr_0.5FeO_3-δ thin films in alkaline electrolytes is studied. Oxygen and proton intercalation into the La_1-xSr_xFeO_3-δ perovskite structure is observed at moderate electrochemical potentials (0.5 to -0.4 V), giving rise to a change in the oxidation state of Fe (as a charge compensation mechanism). The variation of the concentration of holes as a function of the intercalation potential is characterized by in situ ellipsometry, and the concentration of electron holes is indirectly quantified for different electrochemical potentials. Finally, a dilute defect chemistry model that describes the variation of defect species during ionic intercalation is developed.

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.

Interfacial Engineering of Metal/Metal Oxide Heterojunctions toward Oxygen Reduction and Evolution Reactions

Oxygen reduction reactions (ORR) and oxygen evolution reactions (OER) are two very important electrochemical processes for renewable energy conversion and storage devices. Electrocatalysts are needed to accelerate their sluggish kinetics to improve energy conversion efficiencies. Hence, extensive efforts have been devoted to the development of OER and ORR electrocatalysts with high activity and stability as well as low cost. Among these developed electrocatalysts, metal/metal oxide heterostructures attract a great deal of research interest because their catalytic performances can be tuned by interface engineering. In this Review, the latest achievements in interface engineering of metal/metal oxides heterostructures toward ORR and OER are described. The effects of the metal/metal oxide interface on catalysis are first discussed. Then, the approaches for interface engineering are illustrated. The developments of interface engineering in OER and ORR catalysis as well as bifunctional electrocatalysis are further introduced. Lastly, a perspective for future development of interface engineering in metal/metal oxide for OER and ORR is discussed.

Activating Lattice Oxygen in Perovskite Oxide by B-Site Cation Doping for Modulated Stability and Activity at Elevated Temperatures

Doping perovskite oxide with different cations is used to improve its electro-catalytic performance for various energy and environment devices. In this work, an activated lattice oxygen activity in Pr0.4 Sr0.6 Cox Fe0.9- x Nb0.1 O3- δ (PSCxFN, x = 0, 0.2, 0.7) thin film model system by B-site cation doping is reported. As Co doping level increases, PSCxFN thin films exhibit higher concentration of oxygen vacancies ( V o • • ) as revealed by X-ray diffraction and synchrotron-based X-ray photoelectron spectroscopy. Density functional theory calculation results suggest that Co doping leads to more distortion in FeO octahedra and weaker metaloxygen bonds caused by the increase of antibonding state, thereby lowering V o • • formation energy. As a consequence, PSCxFN thin film with higher Co-doping level presents larger amount of exsolved particles on the surface. Both the facilitated V o • • formation and B-site cation exsolution lead to the enhanced hydrogen oxidation reaction (HOR) activity. Excessive Co doping until 70%, nevertheless, results in partial decomposition of thin film and degrades the stability. Pr0.4 Sr0.6 (Co0.2 Fe0.7 Nb0.1 )O3 with moderate Co doping level displays both good HOR activity and stability. This work clarifies the critical role of B-site cation doping in determining the V o • • formation process, the surface activity, and structure stability of perovskite oxides.

Enhancing the Intrinsic Activity and Stability of Perovskite Cobaltite at Elevated Temperature Through Surface Stress

Perovskite-based oxides attract great attention as catalysts for energy and environmental devices. Nanostructure engineering is demonstrated as an effective approach for improving the catalytic activity of the materials. The mechanism for the enhancement, nevertheless, is still not fully understood. In this study, it is demonstrated that compressive strain can be introduced into freestanding perovskite cobaltite La_0.8 Sr_0.2 CoO_{3- δ} (LSC) nanofibers with sufficient small size. Crystal structure analysis suggests that the LSC fiber is characterized by compressive strain along the ab plane and less distorted CoO₆ octahedron compared to the bulk powder sample. Accompanied by such structural changes, the nanofiber shows significantly higher oxygen reduction reaction (ORR) activity and better stability at elevated temperature, which is attributed to the higher oxygen vacancy concentration and suppressed Sr segregation in the LSC nanofibers. First-principle calculations further suggest that the compressive strain in LSC nanofibers effectively shortens the distance between the Co 3d and O 2p band center and lowers the oxygen vacancy formation energy. The results clarify the critical role of surface stress in determining the intrinsic activity of perovskite oxide nanomaterials. The results of this work can help guide the design of highly active and durable perovskite catalysts via nanostructure engineering.

Surface Defect Engineering on Perovskite Oxides as Efficient Bifunctional Electrocatalysts for Water Splitting

The design of high-performance and cost-effective electrocatalysts for water splitting is of prime importance for efficient and sustainable hydrogen production. In this work, a surface defect engineering method is developed for optimizing the electrocatalytic activity of perovskite oxides for water electrolysis. A typical ferrite-based perovskite oxide material La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) is used and regulated by selective acid etching. The optimal parameters for the surface treatment are identified. An efficient bifunctional perovskite oxide, denoted LSCF-30, is prepared by selectively corroding the A-site Sr element in the surface region, which is found to not only increase the exposure and decrease the coordination of B-site metals but also effectively modulate the electronic structure of these metals. The crystal lattice of the perovskite bulk is kept constant during surface engineering, which ensures the structural stability of the perovskite catalyst. The findings demonstrate an effective strategy of surface defect engineering in enhancing the performance of perovskite oxide electrocatalysts for water splitting.

Selenic Acid Etching Assisted Vacancy Engineering for Designing Highly Active Electrocatalysts toward the Oxygen Evolution Reaction

Oxygen evolution electrocatalysts are central to overall water splitting, and they should meet the requirements of low cost, high activity, high conductivity, and stable performance. Herein, a general, selenic-acid-assisted etching strategy is designed from a metal-organic framework as a precursor to realize carbon-coated 3d metal selenides M_m Se_n (Co_0.85 Se_{1- x} , NiSe_{2- x} , FeSe_{2- x} ) with rich Se vacancies as high-performance precious metal-free oxygen evolution reaction (OER) electrocatalysts. Specifically, the as-prepared Co_0.85 Se_{1- x} @C nanocages deliver an overpotential of only 231 mV at a current density of 10 mA cm^-2 for the OER and the corresponding full water-splitting electrolyzer requires only a cell voltage of 1.49 V at 10 mA cm^-2 in alkaline media. Density functional theory calculation reveals the important role of abundant Se vacancies for improving the catalytic activity through improving the conductivity and reducing reaction barriers for the formation of intermediates. Although phase change after long-term operation is observed with the formation of metal hydroxides, catalytic activity is not obviously affected, which strengthens the important role of the carbon network in the operating stability. This study provides a new opportunity to realize high-performance OER electrocatalysts by a general strategy on selenic acid etching assisted vacancy engineering.