Double perovskites as a family of highly active catalysts for oxygen evolution in alkaline solution

Four Generations of Volcano Plots for the Oxygen Evolution Reaction: Beyond Proton-Coupled Electron Transfer Steps?

ConspectusDue to its importance for electrolyzers or metal-air batteries for energy conversion or storage, there is huge interest in the development of high-performance materials for the oxygen evolution reaction (OER). Theoretical investigations have aided the search for active material motifs through the construction of volcano plots for the kinetically sluggish OER, which involves the transfer of four proton-electron pairs to form a single oxygen molecule. The theory-driven volcano approach has gained unprecedented popularity in the catalysis and energy communities, largely due to its simplicity, as adsorption free energies can be used to approximate the electrocatalytic activity by heuristic descriptors.In the last two decades, the binding-energy-based volcano method has witnessed a renaissance with special concepts being developed to incorporate missing factors into the analysis. To this end, this Account summarizes and discusses the different generations of volcano plots for the example of the OER. While first-generation methods relied on the assessment of the thermodynamic information for the OER reaction intermediates by means of scaling relations, the second and third generations developed strategies to include overpotential and kinetic effects into the analysis of activity trends. Finally, the fourth generation of volcano approaches allowed the incorporation of various mechanistic pathways into the volcano methodology, thus paving the path toward data- and mechanistic-driven volcano plots in electrocatalysis.Although the concept of volcano plots has been significantly expanded in recent years, further research activities are discussed by challenging one of the main paradigms of the volcano concept. To date, the evaluation of activity trends relies on the assumption of proton-coupled electron transfer steps (CPET), even though there is experimental evidence of sequential proton-electron transfer (SPET) steps. While the computational assessment of SPET for solid-state electrodes is ambitious, it is strongly suggested to comprehend their importance in energy conversion and storage processes, including the OER. This can be achieved by knowledge transfer from homogeneous to heterogeneous electrocatalysis and by focusing on the material class of single-atom catalysts in which the active center is well defined. The derived concept of how to analyze the importance of SPET for mechanistic pathways in the OER over solid-state electrodes could further shape our understanding of the proton-electron transfer steps at electrified solid/liquid interfaces, which is crucial for further progress toward sustainable energy and climate neutrality.

Promotion of Acid-Water Oxidation by Lattice Distortion and Orbital Hybridization Induced by Ionic Dopant in Pyrochlore Y2Ru2O7

For acid-water oxidation, pyrochloric ruthenates are thought to be extremely effective electrocatalysts. In this work, through partial B-site replacement with larger M2+ cations, the electronic states of Y2Ru2O7 with strong electron correlations are reasonably managed, by which the inherent performance is tremendously promoted. Based on this, the improved Y2Ru1.9Sr0.1O7 electrocatalyst exhibits an outstanding durability and presents a highly inherent mass activity of 1915.1 A gRu-1 (at 1.53 V vs RHE). The enhanced oxygen-evolving reaction (OER) activity by ionic dopant in YRO pyrochlore can be attributed to two aspects, i.e., the lattice distortion induced inhibition of the grain coarsening, which results in a large surface area for YRO-M and increases the OER active sites, and the weakening of electron correlation via broadening of the Ru 4d bandwidths due to the increase of the average radius of B-site ions, which gives rise to an enhancement of conductivity and a strengthened hybridization between Ru 4d and O 2p orbitals and improves the reaction kinetics. The synergistic effects of lattice distortion and orbital hybridization promote the enhanced OER activity. The results would provide fresh concepts for the design of improved electrocatalysts and underscore the significance of managing the intrinsic performance through the dual modification of microstructure morphology and electronic structure.

Probing Surface Transformations of Lanthanum Nickelate Electrocatalysts during Oxygen Evolution Reaction

Monitoring the spontaneous reconstruction of the surface of metal oxides under electrocatalytic reaction conditions is critical to identifying the active sites and establishing structure-activity relationships. Here, we report on a self-terminated surface reconstruction of Ruddlesden-Popper lanthanum nickel oxide (La2NiO4+δ) that occurs spontaneously during reaction with alkaline electrolyte species. Using a combination of high-resolution scanning transmission electron microscopy (HR-STEM), surface-sensitive X-ray photoelectron spectroscopy (XPS), and soft X-ray absorption spectroscopy (sXAS), as well as electrochemical techniques, we identify the structure of the reconstructed surface layer as an amorphous (oxy)hydroxide phase that features abundant under-coordinated nickel sites. No further amorphization of the crystalline oxide lattice (beyond the ∼2 nm thick layer formed) was observed during oxygen evolution reaction (OER) cycling experiments. Notably, the formation of the reconstructed surface layer increases the material's oxygen evolution reaction (OER) activity by a factor of 45 when compared to that of the pristine crystalline surface. In contrast, a related perovskite phase, i.e., LaNiO3, did not show noticeable surface reconstruction, and also no increase in its OER activity was observed. This work provides detailed insight into a surface reconstruction behavior dictated by the crystal structure of the parent oxide and highlights the importance of surface dynamics under reaction conditions.

Unraveling the Evolution of Dynamic Active Sites of LaNixFe1-xO3 Catalysts During OER

Perovskites have attracted tremendous attention as potential catalysts for the oxygen evolution reaction (OER). It is well-known that the introduction of Fe into rare earth perovskites such as LaNiO3 enhances the intrinsic OER activity. Despite numerous studies on structure-property relationships, the origin of the activity and the nature of the active species are still elusive and unclear. In this work, we study a series of LaNixFe1-xO3 perovskites using in situ electrochemical surface-enhanced Raman spectroscopy and electron energy loss spectroscopy to decipher the surface evolution and formation of active species during OER. While the origin of the activity arises from NiOOH species formed from the active Ni centers in LaNiO3, our work shows that Fe serves as the active center in LaNi0.5Fe0.5O3 and forms Fe-O-Ni and FeOOH species during OER. The OER activity of LaFeO3 originates from FeOOH species, which interact with the soluble Ni species in the electrolyte forming an active electrode-electrolyte interface with high-valent stable surface iron species (Fe4+) and thereby improving the performance. Our work provides deeper insights into the synergistic effects of Ni and Fe on the catalytic activity, which in turn provides new design principles for perovskite catalysts for the OER.

Oxygen Migration Pathways in Layered LnBaCo2O6-δ (Ln = La - Y) Perovskites

Layered LnBaCo2O6-δ perovskites are important mixed ionic-electronic conductors, exhibiting outstanding catalytic properties for the oxygen evolution/reduction reaction. These phases exhibit considerable structural complexity, in particular, near room temperature, where a number of oxygen vacancy ordered superstructures are found. This study uses bond valence site energy calculations to demonstrate the key underlying structural features that favor facile ionic migration. BVSE calculations show that the 1D vacancy ordering for Ln = Sm-Tb could be beneficial at low temperatures as new pathways with reduced barriers emerge. By contrast, the 2D vacancy ordering for Ln = Dy and Y is not beneficial for ionic transport with the basic layered parent material having lower migration barriers. Overall, the key criterion for low migration barriers is an expanded ab plane, supported by Ba, coupled to a small Ln size. Hence, Ln = Y should be the best composition, but this is stymied by the low temperature 2D vacancy ordering and moderate temperature stability. The evolution of the oxygen cycling capability of these materials is also reported.

Easy conversion perovskite fluorides KCo1-xFexF3 for efficient oxygen evolution reaction

Herein, we report an easily oxidized Co-Fe perovskite fluoride as an efficient catalyst for the oxygen evolution reaction (OER). In situ Raman spectroscopy showed that the presence of F promotes reconstruction to form highly active (Co3+Fe3+)OOH, and the current density of 10 mA cm-2 can be achieved at the overpotential of only 118 mV in 1 M KOH aqueous solution. This work helps to understand the role of fluoride during the OER.

The formation of unsaturated IrOx in SrIrO3 by cobalt-doping for acidic oxygen evolution reaction

Electrocatalytic water splitting is a promising route for sustainable hydrogen production. However, the high overpotential of the anodic oxygen evolution reaction poses significant challenge. SrIrO3-based perovskite-type catalysts have shown great potential for acidic oxygen evolution reaction, but the origins of their high activity are still unclear. Herein, we develop a Co-doped SrIrO3 system to enhance oxygen evolution reaction activity and elucidate the origin of catalytic activity. In situ experiments reveal Co activates surface lattice oxygen, rapidly exposing IrOx active sites, while bulk Co doping optimizes the adsorbate binding energy of IrOx. The Co-doped SrIrO3 demonstrates high oxygen evolution reaction electrocatalytic activity, markedly surpassing the commercial IrO2 catalysts in both conventional electrolyzer and proton exchange membrane water electrolyzer.

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

Structured Catalysts and Catalytic Processes: Transport and Reaction Perspectives

The structure of catalysts determines the performance of catalytic processes. Intrinsically, the electronic and geometric structures influence the interaction between active species and the surface of the catalyst, which subsequently regulates the adsorption, reaction, and desorption behaviors. In recent decades, the development of catalysts with complex structures, including bulk, interfacial, encapsulated, and atomically dispersed structures, can potentially affect the electronic and geometric structures of catalysts and lead to further control of the transport and reaction of molecules. This review describes comprehensive understandings on the influence of electronic and geometric properties and complex catalyst structures on the performance of relevant heterogeneous catalytic processes, especially for the transport and reaction over structured catalysts for the conversions of light alkanes and small molecules. The recent research progress of the electronic and geometric properties over the active sites, specifically for theoretical descriptors developed in the recent decades, is discussed at the atomic level. The designs and properties of catalysts with specific structures are summarized. The transport phenomena and reactions over structured catalysts for the conversions of light alkanes and small molecules are analyzed. At the end of this review, we present our perspectives on the challenges for the further development of structured catalysts and heterogeneous catalytic processes.

Key role of subsurface doping in optimizing active sites of IrO2 for the oxygen evolution reaction

The oxygen evolution reaction (OER) over a family of metal-doped rutile IrO2 catalysts is theoretically investigated by controlling the species and position of doped elements. The subsurface substitution doping is demonstrated to efficiently regulate the eg-filling of surface iridium sites and lower the adsorption strength of oxygen intermediates, improving the catalytic activity for the OER. Finally, based on screening, subsurface Cu- and Li-doped IrO2 models stand near the top of the volcano plot and display high levels of structural stability toward acidic OER.

Computational chemistry for water-splitting electrocatalysis

Electrocatalytic water splitting driven by renewable electricity has attracted great interest in recent years for producing hydrogen with high-purity. However, the practical applications of this technology are limited by the development of electrocatalysts with high activity, low cost, and long durability. In the search for new electrocatalysts, computational chemistry has made outstanding contributions by providing fundamental laws that govern the electron behavior and enabling predictions of electrocatalyst performance. This review delves into theoretical studies on electrochemical water-splitting processes. Firstly, we introduce the fundamentals of electrochemical water electrolysis and subsequently discuss the current advancements in computational methods and models for electrocatalytic water splitting. Additionally, a comprehensive overview of benchmark descriptors is provided to aid in understanding intrinsic catalytic performance for water-splitting electrocatalysts. Finally, we critically evaluate the remaining challenges within this field.

From Charge to Spin: An In-Depth Exploration of Electron Transfer in Energy Electrocatalysis

Catalytic materials play crucial roles in various energy-related processes, ranging from large-scale chemical production to advancements in renewable energy technologies. Despite a century of dedicated research, major enduring challenges associated with enhancing catalyst efficiency and durability, particularly in green energy-related electrochemical reactions, remain. Focusing only on either the crystal structure or electronic structure of a catalyst is deemed insufficient to break the linear scaling relationship (LSR), which is the golden rule for the design of advanced catalysts. The discourse in this review intricately outlines the essence of heterogeneous catalysis reactions by highlighting the vital roles played by electron properties. The physical and electrochemical properties of electron charge and spin that govern catalysis efficiencies are analyzed. Emphasis is placed on the pronounced influence of external fields in perturbing the LSR, underscoring the vital role that electron spin plays in advancing high-performance catalyst design. The review culminates by proffering insights into the potential applications of spin catalysis, concluding with a discussion of extant challenges and inherent limitations.

Superaerophobic/Superhydrophilic Multidimensional Electrode System for High-Current-Density Water Electrolysis

Water electrolysis is emerging as a promising renewable-energy technology for the green production of hydrogen, which is a representative and reliable clean energy source. From economical and industrial perspectives, the development of earth-abundant non-noble metal-based and bifunctional catalysts, which can simultaneously exhibit high catalytic activities and stabilities for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), is critical; however, to date, these types of catalysts have not been constructed, particularly, for high-current-density water electrolysis at the industrial level. This study developed a heterostructured zero-dimensional (0D)-one-dimensional (1D) PrBa0.5Sr0.5Co1.5Fe0.5O5+δ (PBSCF)-Ni3S2 as a self-supported catalytic electrode via interface and morphology engineering. This unique heterodimensional nanostructure of the PBSCF-Ni3S2 system demonstrates superaerophobic/superhydrophilic features and maximizes the exposure of the highly active heterointerface, endowing the PBSCF-Ni3S2 electrode with outstanding electrocatalytic performances in both HER and OER and exceptional operational stability during the overall water electrolysis at high current densities (500 h at 500 mA cm-2). This study provides important insights into the development of catalytic electrodes for efficient and stable large-scale hydrogen production systems.

A Review of Rechargeable Zinc-Air Batteries: Recent Progress and Future Perspectives

Zinc-air batteries (ZABs) are gaining attention as an ideal option for various applications requiring high-capacity batteries, such as portable electronics, electric vehicles, and renewable energy storage. ZABs offer advantages such as low environmental impact, enhanced safety compared to Li-ion batteries, and cost-effectiveness due to the abundance of zinc. However, early research faced challenges due to parasitic reactions at the zinc anode and slow oxygen redox kinetics. Recent advancements in restructuring the anode, utilizing alternative electrolytes, and developing bifunctional oxygen catalysts have significantly improved ZABs. Scientists have achieved battery reversibility over thousands of cycles, introduced new electrolytes, and achieved energy efficiency records surpassing 70%. Despite these achievements, there are challenges related to lower power density, shorter lifespan, and air electrode corrosion leading to performance degradation. This review paper discusses different battery configurations, and reaction mechanisms for electrically and mechanically rechargeable ZABs, and proposes remedies to enhance overall battery performance. The paper also explores recent advancements, applications, and the future prospects of electrically/mechanically rechargeable ZABs.

Structural and electronic characterization of fluorine-doped La0.5Sr0.5CoO3-δ using electron energy-loss spectroscopy

Perovskite oxides, ABO3, are potential catalysts for the oxygen evolution reaction, which is important in the production of hydrogen as a sustainable energy resource. Optimizing the chemical composition of such oxides by substitution or doping with additional elements is an effective approach to improving the activity of such catalysts. Here, we characterized the crystal and electronic structures of fluorine-doped La0.5Sr0.5CoO3-δ particles using scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS). High-resolution STEM imaging demonstrated the formation of a disordered surface phase caused by fluorine doping. In addition, spatially resolved EELS data showed that fluorine anions were introduced into the interiors of the particles and that Co ions near the surfaces were slightly reduced by fluorine doping in conjunction with the loss of oxygen ions. Peak fitting of energy-loss near-edge structure data demonstrated an unexpected nanostructure in the vicinity of the surface. An EELS characterization comprising elemental mapping together with an energy-loss near-edge structure analysis indicated that this nanostructure could not be assigned to Co-based materials but rather to the solid electrolyte BaF2. Complementary structural and electronic characterizations using STEM and EELS as demonstrated herein evidently have the potential to play an increasingly important role in elucidating the nanostructures of functional materials.

P-Incorporation Induced Enhancement of Lattice Oxygen Participation in Double Perovskite Oxides to Boost Water Oxidation

Activating the lattice oxygen in the catalysts to participate in the oxygen evolution reaction (OER), which can break the scaling relation-induced overpotential limitation (> 0.37 V) of the adsorbate evolution mechanism, has emerged as a new and highly effective guide to accelerate the OER. However, how to increase the lattice oxygen participation of catalysts during OER remains a major challenge. Herein, P-incorporation induced enhancement of lattice oxygen participation in double perovskite LaNi0.58 Fe0.38 P0.07 O3-σ (PLNFO) is studied. P-incorporation is found to be crucial for enhancing the OER activity. The current density reaches 1.35 mA cmECSA -2 at 1.63 V (vs RHE), achieving a sixfold increase in intrinsic activity. Experimental evidences confirm the dominant lattice oxygen participation mechanism (LOM) for OER pathway on PLNFO. Further electronic structures reveal that P-incorporation shifts the O p-band center by 0.7 eV toward the Fermi level, making the states near the Fermi level more O p character, thus facilitating LOM and fast OER kinetics. This work offers a possible method to develop high-performance double perovskite OER catalysts for electrochemical water splitting.

Cobalt-free layered perovskites RBaCuFeO5+δ (R = 4f lanthanide) as electrocatalysts for the oxygen evolution reaction

Co-based perovskite oxides are intensively studied as promising catalysts for electrochemical water splitting in an alkaline environment. However, the increasing Co demand by the battery industry is pushing the search for Co-free alternatives. Here we report a systematic study of the Co-free layered perovskite family RBaCuFeO5+δ (R = 4f lanthanide), where we uncover the existence of clear correlations between electrochemical properties and several physicochemical descriptors. Using a combination of advanced neutron and X-ray synchrotron techniques with ab initio DFT calculations we demonstrate and rationalize the positive impact of a large R ionic radius in their oxygen evolution reaction (OER) activity. We also reveal that, in these materials, Fe3+ is the transition metal cation the most prone to donate electrons. We also show that similar R3+/Ba2+ ionic radii favor the incorporation and mobility of oxygen in the layered perovskite structure and increase the number of available O diffusion paths, which have an additional, positive impact on both, the electric conductivity and the OER process. An unexpected result is the observation of a clear surface reconstruction exclusively in oxygen-rich samples (δ > 0), a fact that could be related to their superior OER activity. The encouraging intrinsic OER values obtained for the most active electrocatalyst (LaBaCuFeO5.49), together with the possibility of industrially producing this material in nanocrystalline form should inspire the design of other Co-free oxide catalysts with optimal properties for electrochemical water splitting.

dz2 Band Links Frontier Orbitals and Charge Carrier Dynamics of Single-Atom Cocatalyst-Aided Photocatalytic H2 Production

The Cu single-atom catalyst (SAC) supported on TiO2 exhibits outstanding efficacy in photocatalytic hydrogen evolution. The precise operational mechanism remains a subject of ongoing debate. The focus resides with the interplay linking heightened catalytic activity, dynamic valence state alterations of Cu atoms, and their hybridization with H2O orbitals, manifested in catalyst color changes. Taking anatase TiO2 (101) as a prototypical surface, we perform ab initio quantum dynamics simulation to reveal that the high activity of the Cu-SAC is due to the quasi-planar coordination structure of the Cu atom after H2O adsorption, allowing it to trap photoexcited hot electrons and inject them into the hybridized orbital between Cu and H2O. The observed alterations in the valence state and the coloration can be attributed to the H atom released during H2O dissociation and adsorbed onto the lattice O atom neighboring the Cu-SAC. Notably, this adsorption of H atoms puts the Cu-SAC into an inert state, as opposed to an activating effect reported previously. Our work clarifies the relationship between the high photocatalytic activity and the local dynamic atomic coordination structure, providing atomistic insights into the structural changes occurring during photocatalytic reactions on SACs.

Sr-Stabilized IrMnO2 Solid Solution Nano-Electrocatalysts with Superior Activity and Excellent Durability for Oxygen Evolution Reaction in Acid Media

The development of cost-effective catalysts for oxygen evolution reaction (OER) in acidic media is of paramount importance. This work reports that Sr-doped solid solution structural ultrafine IrMnO2 nanoparticles (NPs) (≈1.56 nm) on the carbon nanotubes (Sr-IrMnO2 /CNTs) are efficient catalysts for the acidic OER. Even with the Ir use dosage 3.5 times lower than that of the commercial IrO2 , the Sr-IrMnO2 /CNTs only need an overpotential of 236.0 mV to drive 10.0 mA cm-2 and show outstanding stability for >400.0 h. Its Ir mass activity is 39.6 times higher than that of the IrO2 at 1.53 V. The solid solution and Sr-doping structure of Sr-IrMnO2 are the main origin of the high catalytic activity and excellent stability of the Sr-IrMnO2 /CNTs. The density function theory calculations indicate that the solid solution structure can promote strong electronic coupling between Ir and Mn, lowering the energy barrier of the OER rate-determining step. The Sr-doping can enhance the stability of Ir against the chemical corrosion and demetallation. Water electrolyzers and proton exchange membrane water electrolyzers assembled with the Sr-IrMnO2 /CNTs show superb performance and excellent durability in the acid media.

Application of response surface methodology for optimization of the test condition of oxygen evolution reaction over La0.8Ba0.2CoO3 perovskite-active carbon composite

The Experimental Design was applied to optimize the electrocatalytic activity of La0.8Ba0.2CoO3 perovskite oxide/Active Carbon composite material in the alkaline solution for the Oxygen Evolution Reaction. After the preparation of La0.8Ba0.2CoO3, and structural characterizations, the experimental design was utilized to determine the optimal amount of the composite material and testing conditions. The overpotential was defined as the response variable, and the mass ratio of perovskite/active carbon, Potassium hydroxide (KOH) concentration, and Poly(vinylidene fluoride) (PVDF) amount were considered effective parameters. The significance of model terms is demonstrated by P-values less than 0.0500. The proposed prediction model determined the optimal amounts of 0.665 mg of PVDF, a KOH concentration of 0.609 M, and A perovskite/Active Carbon mass ratio of 2.81 with 308.22 mV overpotential (2.27% greater than the actual overpotential). The stability test of the optimized electrode material over 24 h suggests that it could be a good candidate electrocatalyst for OER with reusability potential.

Triggered lattice-oxygen oxidation with active-site generation and self-termination of surface reconstruction during water oxidation

To master the activation law and mechanism of surface lattice oxygen for the oxygen evolution reaction (OER) is critical for the development of efficient water electrolysis. Herein, we propose a strategy for triggering lattice-oxygen oxidation and enabling non-concerted proton-electron transfers during OER conditions by substituting Al in La0.3Sr0.7CoO3-δ. According to our experimental data and density functional theory calculations, the substitution of Al can have a dual effect of promoting surface reconstruction into active Co oxyhydroxides and activating deprotonation on the reconstructed oxyhydroxide, inducing negatively charged oxygen as an active site. This leads to a significant improvement in the OER activity. Additionally, Al dopants facilitate the preoxidation of active cobalt metal, which introduces great structural flexibility due to elevated O 2p levels. As OER progresses, the accumulation of oxygen vacancies and lattice-oxygen oxidation on the catalyst surface leads to the termination of Al3+ leaching, thereby preventing further reconstruction. We have demonstrated a promising approach to achieving tunable electrochemical reconstruction by optimizing the electronic structure and gained a fundamental understanding of the activation mechanism of surface oxygen sites.

Recent Progress and Future Prospects of Anions O-site Doped Perovskite Oxides in Electrocatalysis for Various Electrochemical Systems

With the rapid development of novel energy conversion and storage technologies, there is a growing demand for enhanced performance in a wide range of electrocatalysts. Perovskite oxides (ABO3 ) have caused widespread concerns due to their excellent electrocatalytic properties, low cost, stable and reliable performance. In recent years, the research on anion O-site doping of perovskite oxides has been a cynosure, which is considered as a promising route for enhancing performance. However, a systematic review summarizing the research progress of anion-doped perovskite oxides is still lacking. Therefore, this review mainly introduces the elements and strategies of various common anions doped at O-site of perovskite oxides, analyzes their influence on the physical and chemical properties of perovskites, and separately concludes their applications in electrocatalysis. This review will provide ideas and prospects for the development of subsequent anion doping strategies for high performance perovskite oxides.

Interface Engineering a High Content of Co3+ Sites on Co3O4 Nanoparticles to Boost Acidic Oxygen Evolution

Non-noble metal oxides have emerged as potential candidate electrocatalysts for acidic oxygen evolution reactions (OERs) due to their earth abundance; however, improving their catalytic activity and stability simultaneously in strong acidic electrolytes is still a major challenge. In this work, we report Co3O4@carbon core-shell nanoparticles on 2D graphite sheets (Co3O4@C-GS) as mixed-dimensional hybrid electrocatalysts for acidic OER. The obtained Co3O4@C-GS catalyst exhibits a low overpotential of 350 mV and maintains stability for 20 h at a current density of 10 mA cm-2 in H2SO4 (pH = 1) electrolyte. X-ray photoelectron and X-ray absorption spectroscopies illustrate that the higher content of Co3+ sites boosts acidic OER. Operando Raman spectroscopy reveals that the catalytic stability of Co3O4@C nanoparticles during the acidic OER is enhanced by the introduction of graphite sheets. This interface engineering of non-noble metal sites with high valence states provides an efficient approach to boost the catalytic activity and enhance the stability of noble-metal-free electrocatalysts for acidic OER.

Synergistic effect of perovskites and nitrogen-doped carbon hybrid materials for improving oxygen reduction reaction

A fundamental understanding of the electrochemical behavior of hybrid perovskite and nitrogen-doped (N-doped) carbon is essential for the development of perovskite-based electrocatalysts in various sustainable energy device applications. In particular, the selection and modification of suitable carbon support are important for enhancing the oxygen reduction reaction (ORR) of non-platinum group metal electrocatalysts in fuel cells. Herein, we address hybrid materials composed of three representative N-doped carbon supports (BP-2000, Vulcan XC-72 and P-CNF) with valid surface areas and different series of single, double and triple perovskites: Ba0.5Sr0.5Co0.8Fe0.2O3-δ, (Pr0.5Ba0.5)CoO3-δ, and Nd1.5Ba1.5CoFeMnO9-δ (NBCFM), respectively. The combination of NBCFM and N-doped BP-2000 produces a half-wave potential of 0.74 V and a current density of 5.42 mA cm-2 at 0.5 V versus reversible hydrogen electrode, comparable to those of the commercial Pt/C electrocatalyst (0.76 V, 5.21 mA cm-2). Based on physicochemical and electrochemical analyses, we have confirmed a significant improvement in the catalytic performance of low-conductivity perovskite catalyst in the ORR when nitrogen-doped carbon with enhanced electrical conductivity is introduced. Furthermore, it has been observed that nitrogen dopants play active sites, contributing to additional performance enhancement when hybridized with perovskite.

Electrocatalytic Water Oxidation Activity-Stability Maps for Perovskite Oxides Containing 3d, 4d and 5d Transition Metals

Improving catalytic activity without loss of catalytic stability is one of the core goals in search of low-iridium-content oxygen evolution electrocatalysts under acidic conditions. Here, we synthesize a family of 66 SrBO3 perovskite oxides (B=Ti, Ru, Ir) with different Ti : Ru : Ir atomic ratios and construct catalytic activity-stability maps over composition variation. The maps classify the multicomponent perovskites into chemical groups with distinct catalytic activity and stability for acidic oxygen evolution reaction, and highlights a chemical region where high catalytic activity and stability are achieved simultaneously at a relatively low iridium level. By quantifying the extent of hybridization of mixed transition metal 3d-4d-5d and oxygen 2p orbitals for multicomponent perovskites, we demonstrate this complex interplay between 3d-4d-5d metals and oxygen atoms in governing the trends in both activity and stability as well as in determining the catalytic mechanism involving lattice oxygen or not.

Highly Active Pyrochlore-Type Praseodymium Ruthenate Electrocatalyst for Efficient Acid-Water Oxidation

To produce directly combustible hydrogen from water, highly active, acid-resistant, and economical catalysts for oxygen evolution reaction (OER) are needed. An electrocatalyst based on praseodymium ruthenate (Pr2Ru2O7) is presented here that greatly outperforms RuO2 for acid-water oxidation. Specifically, at 10 mA cm-2, this electrocatalyst presents a low overpotential (η) of 213 mV and markedly superior stability. Moreover, Pr2Ru2O7 presents a significant rise in turnover frequency (TOF) and a highly intrinsic mass activity of 1618.8 A gRu-1 (η = 300 mV), exceeding the most commonly reported acid OER catalysts. Density functional theory calculations and electronic structure study demonstrate that the Ru 4d-band center related to the longer Ru-O bond with a large radius of Pr ion in this pyrochlore is lower than that in RuO2, which would optimize the binding between the adsorbed oxygen species and catalytic metal sites and enhance the catalytic intrinsic activity.

Identifying a Universal Activity Descriptor and a Unifying Mechanism Concept on Perovskite Oxides for Green Hydrogen Production

Producing indispensable hydrogen and oxygen for social development via water electrolysis shows more prospects than other technologies. Although electrocatalysts have been explored for centuries, a universal activity descriptor for both hydrogen-evolution reaction (HER) and oxygen-evolution reaction (OER) is not yet developed. Moreover, a unifying concept is not yet established to simultaneously understand HER/OER mechanisms. Here, the relationships between HER/OER activities in three common electrolytes and over ten representative material properties on 12 3d-metal-based model oxides are rationally bridged through statistical methodologies. The orbital charge-transfer energy (Δ) can serve as an ideal universal descriptor, where a neither too large nor too small Δ (≈1 eV) with optimal electron-cloud density around Fermi level affords the best activities, fulfilling Sabatier's principle. Systematic experiments and computations unravel that pristine oxide with Δ ≈ 1 eV possesses metal-like high-valence configurations and active lattice-oxygen sites to help adsorb key protons in HER and induce lattice-oxygen participation in the OER, respectively. After reactions, partially generated metals in the HER and high-valence hydroxides in the OER dominate proton adsorption and couple with pristine lattice-oxygen activation, respectively. These can be successfully rationalized by the unifying orbital charge-transfer theory. This work provides the foundation of rational material design and mechanism understanding for many potential applications.

Accelerated deprotonation with a hydroxy-silicon alkali solid for rechargeable zinc-air batteries

Transition metal oxides are promising electrocatalysts for zinc-air batteries, yet surface reconstruction caused by the adsorbate evolution mechanism, which induces zinc-ion battery behavior in the oxygen evolution reaction, leads to poor cycling performance. In this study, we propose a lattice oxygen mechanism involving proton acceptors to overcome the poor performance of the battery in the OER process. We introduce a stable solid base, hydroxy BaCaSiO4, onto the surfaces of PrBa0.5Ca0.5Co2O5+δ perovskite nanofibers with a one-step exsolution strategy. The HO-Si sites on the hydroxy BaCaSiO4 significantly accelerate proton transfer from the OH* adsorbed on PrBa0.5Ca0.5Co2O5+δ during the OER process. As a proof of concept, a rechargeable zinc-air battery assembled with this composite electrocatalyst is stable in an alkaline environment for over 150 hours at 5 mA cm-2 during galvanostatic charge/discharge tests. Our findings open new avenues for designing efficient OER electrocatalysts for rechargeable zinc-air batteries.

DFT-assisted low-dimensional carbon-based electrocatalysts design and mechanism study: a review

Low-dimensional carbon-based (LDC) materials have attracted extensive research attention in electrocatalysis because of their unique advantages such as structural diversity, low cost, and chemical tolerance. They have been widely used in a broad range of electrochemical reactions to relieve environmental pollution and energy crisis. Typical examples include hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), carbon dioxide reduction reaction (CO2RR), and nitrogen reduction reaction (NRR). Traditional "trial and error" strategies greatly slowed down the rational design of electrocatalysts for these important applications. Recent studies show that the combination of density functional theory (DFT) calculations and experimental research is capable of accurately predicting the structures of electrocatalysts, thus revealing the catalytic mechanisms. Herein, current well-recognized collaboration methods of theory and practice are reviewed. The commonly used calculation methods and the basic functionals are briefly summarized. Special attention is paid to descriptors that are widely accepted as a bridge linking the structure and activity and the breakthroughs for high-volume accurate prediction of electrocatalysts. Importantly, correlated multiple descriptors are used to systematically describe the complicated interfacial electrocatalytic processes of LDC catalysts. Furthermore, machine learning and high-throughput simulations are crucial in assisting the discovery of new multiple descriptors and reaction mechanisms. This review will guide the further development of LDC electrocatalysts for extended applications from the aspect of DFT computations.

Layered transition metal oxides (LTMO) for oxygen evolution reactions and aqueous Li-ion batteries

This perspective paper comprehensively explores recent electrochemical studies on layered transition metal oxides (LTMO) in aqueous media and specifically encompasses two topics: catalysis of the oxygen evolution reaction (OER) and cathodes of aqueous lithium-ion batteries (LiBs). They involve conflicting requirements; OER catalysts aim to facilitate water dissociation, while for cathodes in aqueous LiBs it is essential to suppress water dissociation. The interfacial reactions taking place at the LTMO in these two distinct systems are of particular significance. We show various strategies for designing LTMO materials for each desired aim based on an in-depth understanding of electrochemical interfacial reactions. This paper sheds light on how regulating the LTMO interface can contribute to efficient water splitting and economical energy storage, all with a single material.

Ca substitution improves the catalytic activity of perovskite LaCoO3 toward toluene: comprehension of electronic structure alteration

For perovskite La1-xCaxCoO3 (Ca-x, x = 0-0.3), Ca-0.2 with the closest O p band center to the Fermi level, displays the best catalytic activity for toluene oxidation. The O p band center determines the reducibility and active oxygen content. This finding is beneficial for the design of highly active perovskite catalysts.

Synergistic Activation of Inert Iron Oxide Basal Planes through Heterostructure Formation and Doping for Efficient Hydrogen Evolution

Iron oxides have emerged as a very promising and cost-effective alternative to precious metal catalysts for hydrogen production. However, the inert basal plane of iron oxides needs to be activated to enhance their catalytic efficiency. In this study, we employed heterostructure engineering and doped nickel to cooperatively activate the basal planes of iron oxide (Ni-Fe2 O3 /CeO2 HSs) to achieve high hydrogen evolution reaction (HER) activity. The Ni-Fe2 O3 /CeO2 HSs electrocatalyst demonstrates excellent basic HER activity and stability, such as an extremely low overpotential of 43 mV at 10 mA cm-2 current density and corresponding Tafel slope of 58.6 mV dec-1 . The increase in electrocatalyst activity and acceleration of hydrogen precipitation kinetics arises from the dual modulation of Ni doping and heterostructure, which not only modulates the electrocatalyst's electronic structure, but also increases the number and exposure of active sites. Remarkably, the generation of heterogeneous structure makes the catalyst se. The Ni-doped catalyst has not only increased HER activity but also low-temperature resistance. These results suggest that the synergistic activation of inert iron oxide basal planes through heterostructure formation and doping is a feasible strategy. Furthermore, for efficient electrocatalytic water splitting, this technique can be extended to other non-noble metal oxides.

Atomic Positional Embedding-Based Transformer Model for Predicting the Density of States of Crystalline Materials

The rapid advancement of machine learning has revolutionized quite a few science fields, leading to a surge in the development of highly efficient and accurate materials discovery methods. Recently, predictions of multiple related properties have received attention, with a particular emphasis on spectral properties, where the electronic density of states (DOS) stands out as the fundamental data with enormous potential to advance our understanding of crystalline materials. Leveraging the power of the Transformer framework, we introduce an Atomic Positional Embedding-Based Transformer (APET), which surpasses existing state-of-the-art models for predicting ab initio DOS. APET utilizes atomic periodical positions as its positional embedding, which incorporates all of the structural information in a crystal, providing a more complete and accurate representation. Furthermore, the interpretability of APET enables us to discover the underlying physical properties of materials with greater precision and accuracy.

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.

Metal-Doped C3B Monolayer as the Promising Electrocatalyst for Hydrogen/Oxygen Evolution Reaction: A Combined Density Functional Theory and Machine Learning Study

The development of high-efficiency electrocatalysts for hydrogen evolution reduction (HER)/oxygen evolution reduction (OER) is highly desirable. In particular, metal borides have attracted much attention because of their excellent performances. In this study, we designed a series of metal borides by doping of a transition metal (TM) in a C3B monolayer and further explored their potential applications for HER/OER via density functional theory (DFT) calculations and machine learning (ML) analysis. Our results revealed that the |ΔG*H| values of Fe-, Ag-, Re-, and Ir-doped C3B are approximately 0.00 eV, indicating their excellent HER performances. On the other hand, among all the considered TM atoms, the Ni- and Pt-doped C3B exhibit excellent OER activities with the overpotentials smaller than 0.44 V. Together with their low overpotentials for HER (<0.16 V), we proposed that Ni/C3B and Pt/C3B could be the potential bifunctional electrocatalysts for water splitting. In addition, the ML method was employed to identify the important factors to affect the performance of the TM/C3B electrocatalyst. Interestingly, the results showed that the OER performance is closely related to the inherent properties of TM atoms, i.e., the number of d electrons, electronegativity, atomic radius, and first ionization energy; all these values could be directly obtained without DFT calculations. Our results not only proposed several promising electrocatalysts for HER/OER but also suggested a guidance to design the potential TM-boron (TM-B)-based electrocatalysts.

Enhanced Activity and Stability of Heteroatom-Doped Carbon/Bimetal Oxide for Efficient Water-Splitting Reaction

The research community is actively exploring ways to create cost-efficient and high-performing electrocatalysts for the oxygen evolution reaction. In this investigation, an innovative technique was employed to produce heteroatom-doped carbon containing NiCo oxides, i.e., HC/NiCo oxide@800, in the form of a three-dimensional hierarchical flower. This method involved the reduction of a bimetallic (Ni, Co) metal-organic framework, followed by carefully controlled oxidative calcination. The resulting porous flower-like structure possess numerous advantages, such as expansive specific surface areas, excellent conductivity, and multiple electrocatalytic active sites for both hydrogen and oxygen evolution reactions. Moreover, the presence of oxygen vacancies within HC/NiCo oxide@800 significantly enhances the conductivity of the NiCo substance, thus expediting the kinetics of both the processes. These benefits work together synergistically to enhance the electrocatalytic performance of HC/NiCo oxide@800. Empirical findings reveal that HC/NiCo oxide@800 electrocatalysts demonstrate exceptional catalytic activity, minimal overpotential, and remarkable stability when deployed for both hydrogen evolution and oxygen evolution reactions in alkaline environments. This investigation introduces a fresh avenue for creating porous composite electrocatalysts by transforming metal-organic frameworks with controllable structures. This approach holds promise for advancing electrochemical energy conversion devices by facilitating the development of efficient and customizable electrocatalytic materials.

Unveiling the Hidden Energy Profiles of the Oxygen Evolution Reaction via Machine Learning Analyses

The oxygen evolution reaction (OER) is a crucial electrochemical process for hydrogen production in water electrolysis. However, due to the involvement of multiple proton-coupled electron transfer steps, it is challenging to identify the specific elementary reaction that limits the rate of the OER. Here we employed a machine-learning-based approach to extract the reaction pathway exhaustively from experimental data. Genetic algorithms were applied to search for thermodynamic and kinetic parameters using the current-electrochemical potential relationship of the OER. Interestingly, analysis of the datasets revealed the energy state distributions of reaction intermediates, which likely originated in the interactions among intermediates or the distribution of multiple sites. Through our exhaustive analyses, we successfully uncovered the hidden energy profiles of the OER. This approach can reveal the reaction pathway to activate for efficient hydrogen production, which facilitates the design of catalysts.

In Situ Exsolution of Ba3(VO4)2 Nanoparticles on a V-Doped BaCoO3-δ Perovskite Oxide with Enhanced Activity for Electrocatalytic Hydrogen Evolution

The successful preparation of a perovskite-based heterostructure is important for broadening the applications of perovskites in the field of electrocatalysis, especially in a hydrogen evolution reaction (HER). Nevertheless, the limited active sites of perovskites severely hindered the HER properties. Herein, an in situ exsolution method was used to construct a nanocomposite based on V-doped BaCoO_3-δ decorated with Ba₃(VO₄)₂ (BVCO19) for alkaline HER. The exsolved Ba₃(VO₄)₂ can induce more Co⁴⁺ ions on BaCoO_3-δ, which serves as active sites for the release of H₂. Meanwhile, by regulating the valency of V and Co species, the catalyst can reach a charge balance by generating more oxygen vacancies, which greatly facilitates the adsorption and dissociation of H₂O molecules. The synergistic effect between the oxygen vacancies and high-valence Co⁴⁺ leads to an enhanced HER performance of BVCO19. The as-obtained catalyst delivers a low overpotential of 194 mV at 10 mA cm^-2 as well as impressive stability for 100 h in alkaline media, which outperforms pristine BaCoO_3-δ and most of the nonprecious-based perovskite oxides. This work provides new insights into the preparation of perovskite-based heterostructure for boosting HER.

Distilling universal activity descriptors for perovskite catalysts from multiple data sources via multi-task symbolic regression

Developing activity descriptors via data-driven machine learning (ML) methods can speed up the design of highly active and low-cost electrocatalysts. Despite the fact that a large amount of activity data for electrocatalysts is stored in the literature, data from different publications are not comparable due to different experimental or computational conditions. In this work, an interpretable ML method, multi-task symbolic regression, was adopted to learn from data in multiple experiments. A universal activity descriptor to evaluate the oxygen evolution reaction (OER) performance of oxide perovskites free of calculations or experiments was constructed and reached high accuracy and generalization ability. Utilizing this descriptor with Bayesian-optimized parameters, a series of compelling double perovskites with excellent OER activity were predicted and further evaluated using first-principles calculations. Finally, the two ML-predicted nickel-based perovskites with the best OER activity were successfully synthesized and characterized experimentally. This work opens a new way to extend machine-learning material design by utilizing multiple data sources.

Enhanced OER Performance and Dynamic Transition of Surface Reconstruction in LaNiO3 Thin Films with Nanoparticles Decoration

In an electrocatalytic process, the cognition of the active phase in a catalyst has been regarded as one of the most vital issues, which not only boosts the fundamental understanding of the reaction procedure but also guides the engineering and design for further promising catalysts. Here, based on the oxygen evolution reaction (OER), the stepwise evolution of the dominant active phase is demonstrated in the LaNiO₃ (LNO) catalyst once the single-crystal thin film is decorated by LNO nanoparticles. It is found that the OER performance can be dramatically improved by this decoration, and the catalytic current density at 1.65 V can be enhanced by ≈1000% via ≈10⁹ cm^-2 nanoparticle adhesion after extracting the contribution of surface enlargement. Most importantly, a transition of the active phase from LNO to NiOOH via surface reconstruction with the density of LNO nanoparticles is demonstrated. Several mechanisms in terms of this active phase transition are discussed involving lattice orientation-induced change of the surface energy profile, the lattice oxygen participation, and the A/B-site ions leaching during OER cycles. This study suggests that the active phases in transition metal-based OER catalysts can transform with morphology, which should be corresponding to distinct engineering strategies.

Accelerating the Discovery of Metastable IrO2 for the Oxygen Evolution Reaction by the Self-Learning-Input Graph Neural Network

The discovery of active and stable catalysts for the oxygen evolution reaction (OER) is vital to improve water electrolysis. To date, rutile iridium dioxide IrO₂ is the only known OER catalyst in the acidic solution, while its poor activity restricts its practical viability. Herein, we propose a universal graph neural network, namely, CrystalGNN, and introduce a dynamic embedding layer to self-update atomic inputs during the training process. Based on this framework, we train a model to accurately predict the formation energies of 10,500 IrO₂ configurations and discover 8 unreported metastable phases, among which C2/m-IrO₂ and P62-IrO₂ are identified as excellent electrocatalysts to reach the theoretical OER overpotential limit at their most stable surfaces. Our self-learning-input CrystalGNN framework exhibits reliable accuracy, generalization, and transferring ability and successfully accelerates the bottom-up catalyst design of novel metastable IrO₂ to boost the OER activity.

Ex Situ Reconstruction-Shaped Ir/CoO/Perovskite Heterojunction for Boosted Water Oxidation Reaction

The oxygen evolution reaction (OER) is the performance-limiting step in the process of water splitting. In situ electrochemical conditioning could induce surface reconstruction of various OER electrocatalysts, forming reactive sites dynamically but at the expense of fast cation leaching. Therefore, achieving simultaneous improvement in catalytic activity and stability remains a significant challenge. Herein, we used a scalable cation deficiency-driven exsolution approach to ex situ reconstruct a homogeneous-doped cobaltate precursor into an Ir/CoO/perovskite heterojunction (SCI-350), which served as an active and stable OER electrode. The SCI-350 catalyst exhibited a low overpotential of 240 mV at 10 mA cm^-2 in 1 M KOH and superior durability in practical electrolysis for over 150 h. The outstanding activity is preliminarily attributed to the exponentially enlarged electrochemical surface area for charge accumulation, increasing from 3.3 to 175.5 mF cm^-2. Moreover, density functional theory calculations combined with advanced spectroscopy and ¹⁸O isotope-labeling experiments evidenced the tripled oxygen exchange kinetics, strengthened metal-oxygen hybridization, and engaged lattice oxygen oxidation for O-O coupling on SCI-350. This work presents a promising and feasible strategy for constructing highly active oxide OER electrocatalysts without sacrificing durability.

A High-Entropy Oxide as High-Activity Electrocatalyst for Water Oxidation

High-entropy materials are an emerging pathway in the development of high-activity (electro)catalysts because of the inherent tunability and coexistence of multiple potential active sites, which may lead to earth-abundant catalyst materials for energy-efficient electrochemical energy storage. In this report, we identify how the multication composition in high-entropy perovskite oxides (HEO) contributes to high catalytic activity for the oxygen evolution reaction (OER), i.e., the key kinetically limiting half-reaction in several electrochemical energy conversion technologies, including green hydrogen generation. We compare the activity of the (001) facet of LaCr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2O_3-δ with the parent compounds (single B-site in the ABO₃ perovskite). While the single B-site perovskites roughly follow the expected volcano-type activity trends, the HEO clearly outperforms all of its parent compounds with 17 to 680 times higher currents at a fixed overpotential. As all samples were grown as an epitaxial layer, our results indicate an intrinsic composition-function relationship, avoiding the effects of complex geometries or unknown surface composition. In-depth X-ray photoemission studies reveal a synergistic effect of simultaneous oxidation and reduction of different transition metal cations during the adsorption of reaction intermediates. The surprisingly high OER activity demonstrates that HEOs are a highly attractive, earth-abundant material class for high-activity OER electrocatalysts, possibly allowing the activity to be fine-tuned beyond the scaling limits of mono- or bimetallic oxides.

Magnetic Field-Enhanced Electrocatalytic Oxygen Evolution on a Mixed-Valent Cobalt-Modulated LaCoO3 Catalyst

Extensive efforts to enhance the oxygen evolution reaction (OER) catalytic performance of transition metal oxides mainly concentrate on the extrinsic morphology tailoring, lattice doping, and electrode interface optimizing. Nevertheless, little room is left for performance improvement using these methods and an obvious gap still exists compared to the precious metal catalysts. In this work, a novel "mixed-valent cobalt modulation" strategy is presented to enhance the electrocatalytic OER of perovskite LaCoO₃ (LCO) oxide. The valence transition of cobalt is realized by ethylenediamine post reduction procedure at room temperature, which further induces the variation of magnetic properties for LCO catalyst. The optimized LCO catalyst with Co²⁺ /Co³⁺ of 1.98 % exhibits the best OER activity, and the overpotential at 10 mA cm^-2 current density is decreased by 170 mV compared pristine LCO. Impressively, the ferromagnetic LCO catalyst can perform magnetic OER enhancement. By application of an external magnetic field, the overpotential of LCO at 10 mA cm^-2 can be further decreased by 20 mV compared to that of under zero magnetic field, which arises from the enhanced energy states of electrons and accelerated electron transfer process driven by magnetic field. Our findings may provide a promising strategy to break the bottleneck for further enhancement of OER performance.

Achieving structural stability and enhanced electrochemical performance through Nb-doping into Li- and Mn-rich layered cathode for lithium-ion batteries

Although Li- and Mn-rich layered oxides are attractive cathode materials possessing high energy densities, they have not been commercialized owing to voltage decay, low rate capability, poor capacity retention, and high irreversible capacity in the first cycle. To circumvent these issues, we propose a Li_1.2Ni_0.13Co_0.13Mn_0.53Nb_0.01O₂ (Nb-LNCM) cathode material, wherein Nb doping strengthens the transition metal oxide (TM-O) bond and alleviates the anisotropic lattice distortion while stabilizing the layered structure. During long-term cycling, maintaining a wider LiO₆ interslab thickness in Nb-LNCM creates a favorable Li⁺ diffusion path, which improves the rate capability. Moreover, Nb doping can decrease oxygen loss, suppress the phase transition from layered to spinel and rock-salt structures, and relieve structural degradation. Nb doping results in less capacity contributions of Mn and Co and more reversible Ni and O redox reactions compared to pristine Li_1.2Ni_0.133Co_0.133Mn_0.533O₂ (LNCM), which significantly mitigates the voltage decay (Δ0.289 and Δ0.516 V for Nb-LNCM and LNCM, respectively) and ensures stable capacity retention (82.7 and 70.3% for Nb-LNCM and LNCM, respectively) during the initial 100 cycles. Our study demonstrates that Nb doping is an effective and practical strategy to enhance the structural and electrochemical integrity of Li- and Mn-rich layered oxides. This promotes the development of stable cathode materials for high-energy-density lithium-ion batteries.

Tailoring Spin State of Perovskite Oxides by Fluorine Atom Doping for Efficient Oxygen Electrocatalysis

Promoting the initially deficient but economical catalysts to high-performing competitors is important for developing superior catalysts. Unlike traditional nano-morphology construction methods, this work focuses on intrinsic catalytic activity enhancement via heteroatom doping strategies to induce lattice distortion and optimize spin-dependent orbital interaction to alter charge transfer between catalysts and reactants. Experimentally, a series of different concentrations of fluorine-doped lanthanum cobaltate (F_x -LaCoO₃ ) exhibiting excellent electrocatalytic activity is synthesized, including a low overpotential of 390 mV at j = 10 mA cm^-2 for OER and a large half-wave potential of 0.68 V for ORR. Meanwhile, the assembled rechargeable Zn-air batteries deliver an excellent performance with a large specific capacity of 811 mAh/g_Zn under 10 mA cm^-2 and stability of charge/recharge (120 h). Theoretically, taking advantage of density functional theory calculations, it is found that the prominent OER/ORR performance arises from the spin state transition of Co³⁺ (Low spin state (LS, t_2g ⁶ e_g ⁰ ) → Intermediate spin state (IS, t_2g ⁵ e_g ¹ ) and the mediated d-band center upshift by F atom incorporation. This work establishes a novel avenue for designing superior electrocatalysts in perovskite-based oxides by regulating spin states.