Water electrolysis on La(1-x)Sr(x)CoO(3-δ) perovskite electrocatalysts

Recent advances in perovskite oxides as electrode materials for supercapacitors

Owing to the high power density and ultralong cycle life, supercapacitors represent an alternative to electrochemical batteries in energy storage applications. However, the relatively low energy density is the main challenge for supercapacitors in the current drive to push the entire technology forward to meet the benchmark requirements for commercialization. To effectively solve this issue, it is crucial to develop electrode materials with excellent electrochemical performance since the electrode used is closely related to the specific capacitance and energy density of supercapacitors. With the unique structure, compositional flexibility, and inherent oxygen vacancy, perovskite oxides have attracted wide attention as promising electrode materials for supercapacitors. In this review, we summarize the recent advances in perovskite oxides as electrode materials for supercapacitors. Firstly, the structures and compositions of perovskite oxides are critically reviewed. Following this, the progress in various perovskite oxides, including single perovskite and derivative perovskite oxides, is depicted, focusing on their electrochemical performance. Furthermore, several optimization strategies (i.e., modulating the stoichiometry of the anion or cation, A-site doping, B-site doping, and constructing composites) to improve their electrochemical performance are also discussed. Finally, the significant challenges facing the advancement of perovskite oxide electrodes for supercapacitor applications and future outlook are proposed.

Hydrogen production from water electrolysis: role of catalysts

As a promising substitute for fossil fuels, hydrogen has emerged as a clean and renewable energy. A key challenge is the efficient production of hydrogen to meet the commercial-scale demand of hydrogen. Water splitting electrolysis is a promising pathway to achieve the efficient hydrogen production in terms of energy conversion and storage in which catalysis or electrocatalysis plays a critical role. The development of active, stable, and low-cost catalysts or electrocatalysts is an essential prerequisite for achieving the desired electrocatalytic hydrogen production from water splitting for practical use, which constitutes the central focus of this review. It will start with an introduction of the water splitting performance evaluation of various electrocatalysts in terms of activity, stability, and efficiency. This will be followed by outlining current knowledge on the two half-cell reactions, hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), in terms of reaction mechanisms in alkaline and acidic media. Recent advances in the design and preparation of nanostructured noble-metal and non-noble metal-based electrocatalysts will be discussed. New strategies and insights in exploring the synergistic structure, morphology, composition, and active sites of the nanostructured electrocatalysts for increasing the electrocatalytic activity and stability in HER and OER will be highlighted. Finally, future challenges and perspectives in the design of active and robust electrocatalysts for HER and OER towards efficient production of hydrogen from water splitting electrolysis will also be outlined.

Elucidating intrinsic contribution of d-orbital states to oxygen evolution electrocatalysis in oxides

Although numerous studies on oxide catalysts for an efficient oxygen evolution reaction have been carried out to compare their catalytic performance and suggest new compositions, two significant constraints have been overlooked. One is the difference in electronic conduction behavior between catalysts (metallic versus insulating) and the other is the strong crystallographic surface orientation dependence of the catalysis in a crystal. Consequently, unless a comprehensive comparison of the oxygen-evolution catalytic activity between samples is made on a crystallographically identical surface with sufficient electron conduction, misleading interpretations on the catalytic performance and mechanism may be unavoidable. To overcome these limitations, we utilize both metallic (001) LaNiO₃ epitaxial thin films together with metal dopants and semiconducting (001) LaCoO₃ epitaxial thin films supported with a conductive interlayer. We identify that Fe, Cr, and Al are beneficial to enhance the catalysis in LaNiO₃ although their perovskite counterparts, LaFeO₃, LaCrO₃, and LaAlO₃, with a large bandgap are inactive. Furthermore, semiconducting LaCoO₃ is found to have more than one order higher activity than metallic LaNiO₃, in contrast to previous reports. Showing the importance of facilitating electron conduction, our work highlights the impact of the near-Fermi-level d-orbital states on the oxygen-evolution catalysis performance in perovskite oxides.

Ultrathin vanadium hydroxide nanosheets assembled on the surface of Ni-Fe-layered hydroxides as hierarchical catalysts for the oxygen evolution reaction

Developing state-of-the-art non-noble metal catalysts for the oxygen evolution reaction holds a key to the production of electrolytic hydrogen. Herein, self-supported hierarchical NiFe LDH/VO(OH)2 nanoflowers/nanosheets grown on a Ni foam have been synthesized via a two-step hydrothermal method. Numerous fine VO(OH)2 nanosheets grown on NiFe LDH nanoflowers enlarge the contact area for the electrolyte penetration and facilitate ion diffusion, while the three-dimensional structure of the material also provides an extensive active surface area and plentiful accessible active sites. Moreover, the strong synergistic interaction between VO(OH)2 and NiFe LDHs subtly modulates the electronic environment, accelerating the electron/charge transfer. As a result, the catalyst exhibits excellent electrochemical performance for OER giving a voltage of 1.51 V to achieve the current density of 100 mA cm-2 and possessed a Tafel slope of 65 mV dec-1 in 1.0 M KOH. In addition, the material exhibited remarkable long-term durability and stability during the 40 h measurement. This investigation provides a promising strategy for rationally designing high-efficiency metal electrocatalysts with hierarchical multi-dimensional nanostructures for OER.

Direct Regulation of Double Cation Defects at the A1A2 Site for a High-Performance Oxygen Evolution Reaction Perovskite Catalyst

Perovskites are one of the efficient catalysts for the oxygen evolution reaction (OER), and they belong to the primary ABO₃ in which the A site and B site are site-substituted, and oxygen vacancies are introduced. Further improvement of these complex perovskites is the next necessary topic for specific applications. Herein, two complex perovskites, La_0.6Sr_0.4Co_0.8Fe_0.2O_3-δ (LSCF) and Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF), are exploited as the examples to demonstrate the double cation defects-introduced method of A1 and A2 to supply superimposed enhancement of the activity and stability. This is based on the fact that the increased content of oxygen vacancies and coordination can balance the oxygen vacancy and B-site element oxidation state. The electrochemical measurements revealed that the optimized A-LSCF10 and A-BSCF10 both exhibit outstanding OER catalytic activity. A small Tafel slope (57 mV dec^-1) and a low overpotential (228 mV at 10 mA cm^-2) for A-LSCF10 (vs 93 mV dec^-1 and 345 mV at 10 mA cm^-2 for A-LSCF0), and a small Tafel slope (65 mV dec^-1) and an overpotential (242 mV at 10 mA cm^-2) for A-BSCF10 (vs 66 mV dec^-1 and 308 mV at 10 mA cm^-2 for A-BSCF0) are determined, as well as good stability for 24 h.

Fluorination-enabled Reconstruction of NiFe Electrocatalysts for Efficient Water Oxidation

Developing low-cost and efficient electrocatalysts to accelerate oxygen evolution reaction (OER) kinetics is vital for water and carbon-dioxide electrolyzers. The fastest-known water oxidation catalyst, Ni(Fe)O_xH_y, usually produced through an electrochemical reconstruction of precatalysts under alkaline condition, has received substantial attention. However, the reconstruction in the reported catalysts usually leads to a limited active layer and poorly controlled Fe-activated sites. Here, we demonstrate a new electrochemistry-driven F-enabled surface-reconstruction strategy for converting the ultrathin NiFeO_xF_y nanosheets into an Fe-enriched Ni(Fe)O_xH_y phase. The activated electrocatalyst shows a low OER overpotential of 218 ± 5 mV at 10 mA cm^-2 and a low Tafel slope of 31 ± 4 mV dec^-1, which is among the best for NiFe-based OER electrocatalysts. Such superior performance is caused by the effective formation of the Fe-enriched Ni(Fe)O_xH_y active-phase that is identified by operando Raman spectroscopy and the substantially improved surface wettability and gas-bubble-releasing behavior.

Amorphization mechanism of SrIrO3 electrocatalyst: How oxygen redox initiates ionic diffusion and structural reorganization

The use of renewable electricity to prepare materials and fuels from abundant molecules offers a tantalizing opportunity to address concerns over energy and materials sustainability. The oxygen evolution reaction (OER) is integral to nearly all material and fuel electrosyntheses. However, very little is known about the structural evolution of the OER electrocatalyst, especially the amorphous layer that forms from the crystalline structure. Here, we investigate the interfacial transformation of the SrIrO3 OER electrocatalyst. The SrIrO3 amorphization is initiated by the lattice oxygen redox, a step that allows Sr2+ to diffuse and O2- to reorganize the SrIrO3 structure. This activation turns SrIrO3 into a highly disordered Ir octahedral network with Ir square-planar motif. The final Sr y IrO x exhibits a greater degree of disorder than IrO x made from other processing methods. Our results demonstrate that the structural reorganization facilitated by coupled ionic diffusions is essential to the disordered structure of the SrIrO3 electrocatalyst.

Intermediate Sr2Co1.5Fe0.5O6-δ Tetragonal Structure between Perovskite and Brownmillerite as a Model Catalyst with Layered Oxygen Deficiency for Enhanced Electrochemical Water Oxidation

The generation of hydrogen in an environmentally benign way is highly essential to meet future energy demands. However, in the process of splitting water electrochemically, sluggish kinetics of the oxygen evolution reaction (OER) curtails its applicability, as it drags energy input. Herein, we synthesized Sr-Co-Fe-O oxides to optimize their OER activity by varying the Co/Fe ratio. Among them, Sr2Co1.5Fe0.5O6-δ exhibited the best OER catalytic activity in the series, with an overpotential of 318 mV at 10 mA cm-2 and Tafel slope of 44.8 mV dec-1. High-resolution neutron powder diffraction analysis identified an intermediate structure between the perovskite and brownmillerite, with alternating layers of disorderly orientated oxygen-deficient tetrahedra and fully stoichiometric octahedra. The unique stacking of tetrahedral and octahedral units facilitates desired interactions between the electrode surface and electrolyte. Theoretical calculations revealed that increased covalency of Co 3d and O 2p in Sr2Co1.5Fe0.5O6-δ oxide is another primary contributor to its augmented water oxidation ability. As a model for developing catalysts with such an intermediate structure, the synergetic effect of oxygen vacancy and hybridization between Co 3d and O 2p assured the Sr2Co1.5Fe0.5O6-δ oxide as a better catalyst for its enhanced OER activity.

Recent advances of noble-metal-free bifunctional oxygen reduction and evolution electrocatalysts

Bifunctional oxygen reduction and evolution constitute the core processes for sustainable energy storage. The advances on noble-metal-free bifunctional oxygen electrocatalysts are reviewed.

A bimetal hierarchical layer structure MOF grown on Ni foam as a bifunctional catalyst for the OER and HER

The as-synthesized NiFe-MOF-5 exhibited an overpotential of 168 mV at 10 mA cm⁻² for OER and a voltage of 1.57 V at 10 mA cm⁻² for overall water splitting, outperforming most non-noble metal catalysts reported in 1 M KOH.

Efficient Water Splitting Actualized through an Electrochemistry-Induced Hetero-Structured Antiperovskite/(Oxy)Hydroxide Hybrid

Exploring active, stable, and low-cost bifunctional electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is crucial for water splitting technology associated with renewable energy storage in the form of hydrogen fuel. Here, a newly designed antiperovskite-based hybrid composed of a conductive InNNi₃ core and amorphous InNi(oxy)hydroxide shell is first reported as promising OER/HER bifunctional electrocatalyst. Prepared by a facile electrochemical oxidation strategy, such unique hybrid (denoted as EO-InNNi₃ ) exhibits excellent OER and HER activities in alkaline media, benefiting from the inherent high-efficiency HER catalytic nature of InNNi₃ antiperovskite and the promoting role of OER-active InNi(oxy)hydroxide thin film, which is confirmed by theoretical simulations and in situ Raman studies. Moreover, an alkaline electrolyzer integrated EO-InNNi₃ as both anode and cathode delivers a low voltage of 1.64 V at 10 mA cm^-2 , while maintaining excellent durability. This work demonstrates the application of antiperovskite-based materials in the field of overall water splitting and inspires insights into the development of advanced catalysts for various energy applications.

Synthesis of LiNiO2 at Moderate Oxygen Pressure and Long-Term Cyclability in Lithium-Ion Full Cells

The widespread adoption of electric vehicles necessitates higher-energy-density and longer-life cathode materials for Li-ion batteries. LiNiO₂ offers a higher energy density at a lower cost than other high-Ni-content cathodes containing additional transition-metal ions. However, detrimental phase transformations and impedance growth, resulting from structural defects formed during synthesis, lead to poor cyclability and limit the practical viability of LiNiO₂. Herein, we demonstrate a considerably improved cycle life for LiNiO₂ by synthesizing it under a pressurized oxygen environment. The capacity retention in pouch-type full cells with a graphite anode after 1000 cycles is increased from 59 to 76% by applying a mere 1.7 atm of oxygen pressure during the synthesis of LiNiO₂. With iodometric titration and inductively coupled plasma optical emission spectroscopy analysis, we provide clear evidence that oxygen pressure during synthesis reduces the occurrence of lattice oxygen vacancies and increases the content of Ni³⁺ in LiNiO₂, improving its structural integrity and cyclability. Post-mortem analysis of the cycled cathodes provides insights into the sources of degradation occurring during long-term cycling. This work demonstrates a practically viable, synthetic approach combined with doping and coating to achieve improved performance with high-Ni layered oxide materials. Furthermore, this work represents the first report of extended cycling of LiNiO₂ in pouch full cells with graphite anode and will, therefore, serves as an important benchmark for future research on LiNiO₂.

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

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

Strain-induced creation and switching of anion vacancy layers in perovskite oxynitrides

Perovskite oxides can host various anion-vacancy orders, which greatly change their properties, but the order pattern is still difficult to manipulate. Separately, lattice strain between thin film oxides and a substrate induces improved functions and novel states of matter, while little attention has been paid to changes in chemical composition. Here we combine these two aspects to achieve strain-induced creation and switching of anion-vacancy patterns in perovskite films. Epitaxial SrVO₃ films are topochemically converted to anion-deficient oxynitrides by ammonia treatment, where the direction or periodicity of defect planes is altered depending on the substrate employed, unlike the known change in crystal orientation. First-principles calculations verified its biaxial strain effect. Like oxide heterostructures, the oxynitride has a superlattice of insulating and metallic blocks. Given the abundance of perovskite families, this study provides new opportunities to design superlattices by chemically modifying simple perovskite oxides with tunable anion-vacancy patterns through epitaxial lattice strain.

High Oxygen Evolution Activity of Tungsten Bronze Oxides Boosted by Anchoring of Co2+ at Nb5+ Sites Accompanied by Substantial Oxygen Vacancy

The participation of lattice oxygen in the oxygen evolution reaction (OER) process has been proved to be faster in kinetics than the mechanisms where only metal is involved, although activating the lattice oxygen in the traditional rigid structures remains a big challenge. In this work, efforts are devoted to exploring a new flexible structure that is competent in providing large amounts of oxygen vacancies as well as offering the freedom to manipulate the electronic structure of metal cations. This is demonstrated by anchoring low valence state Co at high valence state Nb sites in the tetragonal tungsten bronze (TTB)-structured Sr0.5Ba0.5Nb2- x Co x O6-δ , with different ratios of Co to Nb to optimize the Co substitution proportion. It is found that the occupation of Co in the Nb5+ sites gives rise to the generation of massive surface oxygen vacancies (Ovac), while Co itself is stabilized in Co2+ by adjacent Ovac. The coexistence of Ovac and LS Co2+ enables an oxygen intercalation mechanism in the optimal SBNC45 with specific activity at 1.7 V versus reversible hydrogen electrode that is 20 times higher than for the commercial IrO2. This work illuminates an entirely new avenue to rationally design OER electrocatalysts with ultrafast kinetics.

Dopants fixation of Ruthenium for boosting acidic oxygen evolution stability and activity

Designing highly durable and active electrocatalysts applied in polymer electrolyte membrane (PEM) electrolyzer for the oxygen evolution reaction remains a grand challenge due to the high dissolution of catalysts in acidic electrolyte. Hindering formation of oxygen vacancies by tuning the electronic structure of catalysts to improve the durability and activity in acidic electrolyte was theoretically effective but rarely reported. Herein we demonstrated rationally tuning electronic structure of RuO₂ with introducing W and Er, which significantly increased oxygen vacancy formation energy. The representative W_0.2Er_0.1Ru_0.7O_2-δ required a super-low overpotential of 168 mV (10 mA cm⁻²) accompanied with a record stability of 500 h in acidic electrolyte. More remarkably, it could operate steadily for 120 h (100 mA cm⁻²) in PEM device. Density functional theory calculations revealed co-doping of W and Er tuned electronic structure of RuO₂ by charge redistribution, which significantly prohibited formation of soluble Ru^x>4 and lowered adsorption energies for oxygen intermediates.

There is an increasing interest in understanding how defect chemistry can alter material reactivity. Here, authors tune the electronic structure of RuO₂ by introducing W and Er dopants that boost acidic oxygen evolution performances by limiting oxygen vacancy formation during catalysis.

Oxygen-deficient perovskites for oxygen evolution reaction in alkaline media: a review

Oxygen vacancies in complex metal oxides and specifically in perovskites are demonstrated to significantly enhance their electrocatalytic activities due to facilitating a degree of control in the material’s intrinsic properties. The reported enhancement in intrinsic OER activity of oxygen-deficient perovskites surfaces has inspired their fabrication via a myriad of schemes. Oxygen vacancies in perovskites are amongst the most favorable anionic or Schottky defects to be induced due to their low formation energies. This review discusses recent efforts for inducing oxygen vacancies in a multitude of perovskites, including facile and environmentally benign synthesis strategies, characterization techniques, and detailed insight into the intrinsic mechanistic modulation of perovskite electrocatalysts. Experimental, analytical, and computational techniques dedicated to the understanding of the improvement of OER activities upon oxygen vacancy induction are summarized in this work. The identification and utilization of intrinsic activity descriptors for the modulation of configurational structure, improvement in bulk charge transport, and favorable inflection of the electronic structure are also discussed. It is our foresight that the approaches, challenges, and prospects discussed herein will aid researchers in rationally designing highly active and stable perovskites that can outperform noble metal-based OER electrocatalysts.

Fabrication and Applications of 3D Nanoarchitectures for Advanced Electrocatalysts and Sensors

For the last few decades, nanoscale materials and structures have been extensively studied and developed, making a huge impact on human sustainability. For example, the introduction of nanostructures has brought substantial development in electrocatalysts and optical sensing applications. However, there are still remaining challenges that need to be resolved to further improve their performance, reliability, and cost-effectiveness. Herein, long-range ordered 3D nanostructures and their design principles are introduced with an emphasis on electrocatalysts for energy conversion and plasmonic nanostructures for optical sensing. Among the various fabrication techniques, sequential solvent-injection-assisted nanotransfer printing is suggested as a practical fabrication platform for tunable long-range ordered 3D nanostructures composed of ultrahigh-resolution building blocks. Furthermore, the importance of understanding and controlling the 3D design parameters is discussed to realize more efficient energy conversion as well as effective surface-enhanced Raman spectroscopy analyses, suggesting new solutions for clean energy and healthcare issues.

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

Developing highly efficient and cost-effective oxygen evolution reaction (OER) electrocatalysts is critical for many energy devices. While regulating the proton-coupled electron transfer (PCET) process via introducing additive into the system has been reported effective in promoting OER activity, controlling the PCET process by tuning the intrinsic material properties remains a challenging task. In this work, we take double perovskite oxide PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ (PBSCF) as a model system to demonstrate enhancing OER activity through the promotion of PCET by tuning the crystal orientation and correlated proton diffusion. OER kinetics on PBSCF thin films with (100), (110), and (111) orientation, deposited on single crystal LaAlO₃ substrates, were investigated using electrochemical measurements, density functional theory (DFT) calculations, and synchrotron-based near ambient X-ray photoelectron spectroscopy. The results clearly show that the OER activity and the ease of deprotonation depend on orientation and follow the order of (100) > (110) > (111). Correlated with OER activity, proton diffusion is found to be the fastest in the (100) film, followed by (110) and (111) films. Our results point out a way of boosting PCET and OER activity, which can also be successfully applied to a wide range of crucial applications in green energy and environment.

Boosting Lattice Oxygen Oxidation of Perovskite to Efficiently Catalyze Oxygen Evolution Reaction by FeOOH Decoration

In the process of oxygen evolution reaction (OER) on perovskite, it is of great significance to accelerate the hindered lattice oxygen oxidation process to promote the slow kinetics of water oxidation. In this paper, a facile surface modification strategy of nanometer-scale iron oxyhydroxide (FeOOH) clusters depositing on the surface of LaNiO₃ (LNO) perovskite is reported, and it can obviously promote hydroxyl adsorption and weaken Ni-O bond of LNO. The above relevant evidences are well demonstrated by the experimental results and DFT calculations. The excellent hydroxyl adsorption ability of FeOOH-LaNiO₃ (Fe-LNO) can obviously optimize OH⁻ filling barriers to promote lattice oxygen-participated OER (LOER), and the weakened Ni-O bond of LNO perovskite can obviously reduce the reaction barrier of the lattice oxygen participation mechanism (LOM). Based on the above synergistic catalysis effect, the Fe-LNO catalyst exhibits a maximum factor of 5 catalytic activity increases for OER relative to the pristine perovskite and demonstrates the fast reaction kinetics (low Tafel slope of 42 mV dec⁻¹) and superior intrinsic activity (TOFs of ~40 O₂ S⁻¹ at 1.60 V vs. RHE).

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

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

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

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

Bulk and Surface Properties Regulation of Single/Double Perovskites to Realize Enhanced Oxygen Evolution Reactivity

Perovskite-based oxides have emerged as promising oxygen evolution reaction (OER) electrocatalysts. The performance is closely related to the lattice, electronic, and defect structure of the oxides, which determine surface and bulk properties and consequent catalytic activity and durability. Further, interfacial interactions between phases in a nanocomposite may affect bulk transportation and surface adsorption properties in a similar manner to phase doping except without solubility limits. Herein, we report the development of a single/double perovskite nanohybrid with limited surface self-reconstruction capability as an OER electrocatalyst. Such superior performance arises from a structure that maintains high crystallinity post OER catalysis, in addition to forming an amorphous layer following the self-reconstruction of a single perovskite structure during the OER process. In situ X-ray absorption near edge structure spectroscopy and high-resolution synchrotron-based X-ray diffraction reveal an amorphization process in the hybrid single/double perovskite oxide system that is limited in comparison to single perovskite amorphization, ensuring high catalytic activity.

Engineering of the d-Band Center of Perovskite Cobaltite for Enhanced Electrocatalytic Oxygen Evolution

Great efforts have been made to understand and upgrade the kinetically sluggish oxygen evolution reaction (OER). In this study, a series of V-doped LaCoO₃ (V-LCO) OER electrocatalysts with optimized d-band centers are fabricated. When utilized as an electrode for the OER, as-formed LaCo_0.8 V_0.2 O₃ (V-LCO-II) requires an overpotential of only 306 mV to drive a geometrical catalytic current density of 10 mA cm^-2 . Furthermore, at a given overpotential of 350 mV, the OER current density of V-LCO-II is about 22 times that of pure LaCoO₃ (LCO) nanoparticles. Tailoring of the d-band center by V doping facilitates the adsorption of OER intermediates and promotes the formation of amorphous active species on the surface of LCO through the exchange interaction between high-spin V⁴⁺ and low-spin Co²⁺ . This work may create new opportunities for developing other highly active OER catalysts through d-band center engineering.

Probing Active Sites and Reaction Intermediates of Electrocatalysis Through Confocal Near-Infrared Photoluminescence Spectroscopy: A Perspective

Electrocatalytic reactions such as oxygen evolution (OER) and oxygen reduction reactions (ORR) are one of the most complex heterogeneous charge transfer processes because of the involvement of multiple proton-coupled-electron transfer steps over a narrow potential range and the formation/breaking of oxygen-oxygen bonds. Obtaining a clear mechanistic picture of these reactions on some highly active strongly-correlated oxides such as MnO_x, NiO_x, and IrO_x has been challenging due to the inherent limitations of the common spectroscopic tools used for probing the reactive intermediates and active sites. This perspective article briefly summarizes some of the key challenges encountered in such probes and describes some of unique advantages of confocal near-infrared photoluminescence (NIR-PL) technique for probing surface and bulk metal cation states under in-situ and ex-situ electrochemical polarization studies. Use of this technique opens up a new avenue for studying changes in the electronic structure of metal oxides occurring as a result of perturbation of defect equilibria, which is crucial in a broad range of heterogeneous systems such as catalysis, photocatalysis, mineral redox chemistry, and batteries.

Self-Assembled Ruddlesden-Popper/Perovskite Hybrid with Lattice-Oxygen Activation as a Superior Oxygen Evolution Electrocatalyst

The oxygen evolution reaction (OER) is pivotal in multiple gas-involved energy conversion technologies, such as water splitting, rechargeable metal-air batteries, and CO₂ /N₂ electrolysis. Emerging anion-redox chemistry provides exciting opportunities for boosting catalytic activity, and thus mastering lattice-oxygen activation of metal oxides and identifying the origins are crucial for the development of advanced catalysts. Here, a strategy to activate surface lattice-oxygen sites for OER catalysis via constructing a Ruddlesden-Popper/perovskite hybrid, which is prepared by a facile one-pot self-assembly method, is developed. As a proof-of-concept, the unique hybrid catalyst (RP/P-LSCF) consists of a dominated Ruddlesden-Popper phase LaSr₃ Co_1.5 Fe_1.5 O_10-δ (RP-LSCF) and second perovskite phase La_0.25 Sr_0.75 Co_0.5 Fe_0.5 O_3-δ (P-LSCF), displaying exceptional OER activity. The RP/P-LSCF achieves 10 mA cm^-2 at a low overpotential of only 324 mV in 0.1 m KOH, surpassing the benchmark RuO₂ and various state-of-the-art metal oxides ever reported for OER, while showing significantly higher activity and stability than single RP-LSCF oxide. The high catalytic performance for RP/P-LSCF is attributed to the strong metal-oxygen covalency and high oxygen-ion diffusion rate resulting from the phase mixture, which likely triggers the surface lattice-oxygen activation to participate in OER. The success of Ruddlesden-Popper/perovskite hybrid construction creates a new direction to design advanced catalysts for various energy applications.

Coupling amorphous cobalt hydroxide nanoflakes on Sr2Fe1.5Mo0.5O5+δ perovskite nanofibers to induce bifunctionality for water splitting

Developing cost-effective, stable and environmentally friendly catalysts is of prime importance for the commercial application of overall water splitting. Perovskite oxides have emerged as one of the promising bifunctional catalysts for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). However, their bifunctional activity, especially towards HER, is still not meeting the anticipated energy efficiency. Herein, we highlight a facile and efficient surface modification approach for boosting the bifunctionality of perovskites for overall water splitting. The construction of amorphous cobalt hydroxide (Co(OH)2) on the Sr2Fe1.5Mo0.5O6-δ (SFM) surface is conducted via an atomic layer deposition (ALD) technology. The optimized crystalline core-amorphous shell structure only needs 384 mV to reach a current density of 10 mA cm-2 for the OER and 322 mV at -10 mA cm-2 for the HER in alkaline media. The optimized catalytic activity is probably due to the unique structure and the synergistic effect between Co(OH)2 and SFM, resulting in the large electrochemical surface area, abundant oxygen vacancies and fast electron transfer. The cell assembled with Co(OH)2/SFM-NF as both cathode and anode electrodes delivers a low voltage of 1.60 V to achieve 10 mA cm-2 and remarkable stability over 68 h in practical operation, offering a viable alternative for overall water splitting.

Direct evidence of boosted oxygen evolution over perovskite by enhanced lattice oxygen participation

The development of oxygen evolution reaction (OER) electrocatalysts remains a major challenge that requires significant advances in both mechanistic understanding and material design. Recent studies show that oxygen from the perovskite oxide lattice could participate in the OER via a lattice oxygen-mediated mechanism, providing possibilities for the development of alternative electrocatalysts that could overcome the scaling relations-induced limitations found in conventional catalysts utilizing the adsorbate evolution mechanism. Here we distinguish the extent to which the participation of lattice oxygen can contribute to the OER through the rational design of a model system of silicon-incorporated strontium cobaltite perovskite electrocatalysts with similar surface transition metal properties yet different oxygen diffusion rates. The as-derived silicon-incorporated perovskite exhibits a 12.8-fold increase in oxygen diffusivity, which matches well with the 10-fold improvement of intrinsic OER activity, suggesting that the observed activity increase is dominantly a result of the enhanced lattice oxygen participation.

Voltage- and time-dependent valence state transition in cobalt oxide catalysts during the oxygen evolution reaction

The ability to determine the electronic structure of catalysts during electrochemical reactions is highly important for identification of the active sites and the reaction mechanism. Here we successfully applied soft X-ray spectroscopy to follow in operando the valence and spin state of the Co ions in Li₂Co₂O₄ under oxygen evolution reaction (OER) conditions. We have observed that a substantial fraction of the Co ions undergo a voltage-dependent and time-dependent valence state transition from Co³⁺ to Co⁴⁺ accompanied by spontaneous delithiation, whereas the edge-shared Co-O network and spin state of the Co ions remain unchanged. Density functional theory calculations indicate that the highly oxidized Co⁴⁺ site, rather than the Co³⁺ site or the oxygen vacancy site, is mainly responsible for the high OER activity.

Dimensionality Control of Electrocatalytic Activity in Perovskite Nickelates

The dimensionality of the crystal structure plays an important role in the electronic structures of materials. Ruddlesden-Popper perovskite oxides offer an attractive platform for studying this role due to dimensional flexibility. The effects of dimensionality on physical properties in those oxides have been widely reported. However, the study of dimensional dependence on the chemical properties is still lacking. Here, we synthesized a series of Ruddlesden-Popper perovskite nickelates La_nSrNi_nO_3n+1 (n = 1, 2, 3, and ∞) to explore the role of dimensionality on oxygen-evolution reaction (OER) performance. As the dimensionality increased with n, the nickelates exhibited an enhanced OER activity. We found that the weakening of electron correlations among Ni 3d electrons by increasing the dimensionality induced an insulator-to-metal transition and a strengthened Ni-O hybridization, both of which accelerated the OER kinetics. This work sets up a bridge between the dimensionality and electrocatalysis, which provides guidance for designing highly efficient oxygen-evolving catalysts.

Cation insertion to break the activity/stability relationship for highly active oxygen evolution reaction catalyst

The production of hydrogen at a large scale by the environmentally-friendly electrolysis process is currently hampered by the slow kinetics of the oxygen evolution reaction (OER). We report a solid electrocatalyst α-Li₂IrO₃ which upon oxidation/delithiation chemically reacts with water to form a hydrated birnessite phase, the OER activity of which is five times greater than its non-reacted counterpart. This reaction enlists a bulk redox process during which hydrated potassium ions from the alkaline electrolyte are inserted into the structure while water is oxidized and oxygen evolved. This singular charge balance process for which the electrocatalyst is solid but the reaction is homogeneous in nature allows stabilizing the surface of the catalyst while ensuring stable OER performances, thus breaking the activity/stability tradeoff normally encountered for OER catalysts.

Renewable hydrogen production from water will require understanding and improving the oxygen evolution reaction (OER) on catalyst surfaces. Here, authors report α-Li₂IrO₃ to transform into a hydrated birnessite phase under OER conditions that exhibits enhanced OER performances and durabilities.

Investigation of New B-Site-Disordered Perovskite Oxide CaLaScRuO6+δ: An Efficient Oxygen Bifunctional Electrocatalyst in a Highly Alkaline Medium

Energy storage and conversion driven by electro- or photocatalyst is a highly exciting field of research, and generations of effective and durable oxide catalysts have received much attention in this field. Here, we report A-site lanthanum-doped oxygen-rich quinary oxide CaLaScRuO_6+δ synthesized by adopting the solid-state reaction method and characterized by various techniques such as powder X-ray diffraction, neutron diffraction, energy-dispersive X-ray spectroscopy, inductively coupled plasma-atomic emission spectrometry, Raman spectroscopy, and temperature-programmed reduction in the presence of a hydrogen atmosphere (H₂-TPR). X-ray absorption study confirms the existence of mixed valent Ru ions in the structure, which enhances the oxygen stoichiometry for the partial balance of an extra cationic charge. Neutron powder diffraction and reduction of the material in a hydrogen atmosphere (H₂-TPR) can confirm the oxygen overstoichiometry of the catalyst. The present material works as an efficient and robust oxygen bifunctional electrocatalyst for ORR/OER (oxygen evolution reaction/oxygen reduction reaction) followed by four-electron transfer pathway in a strong (1 M KOH) alkaline medium. The catalytic nature of the designed structural and chemical flexible perovskite is a novel example of an electrocatalyst for the oxygen bifunctional activity.

Vertical Strain-Driven Antiferromagnetic to Ferromagnetic Phase Transition in EuTiO3 Nanocomposite Thin Films

Three-dimensional (3D) strain induced in self-assembled vertically aligned nanocomposite (VAN) epitaxial films provides an unrivaled method to induce very large strains in thin films. Here, by growing VAN films of EuTiO₃ (ETO)-Eu₂O₃ (EO) with different EO fractions, the vertical strain was systematically increased in ETO, up to 3.15%, and the Eu-Ti-Eu bond angle along ⟨111⟩ decreased by up to 1°, leading to a weakening of the antiferromagnetic interactions and switching from antiferromagnetic to ferromagnetic behavior. Our work has shown for the first time that Eu-Ti-Eu superexchange interactions play a key role in determining the magnetic ground state of ETO. More broadly, our work serves as an exemplar to show that multifunctionalities in strong spin-lattice coupling perovskite oxides can be uniquely tuned at the atomic scale using simple VAN structures.

Photoinduced Defect Engineering: Enhanced Photothermal Catalytic Performance of 2D Black In2 O3- x Nanosheets with Bifunctional Oxygen Vacancies

Photothermal CO₂ reduction technology has attracted tremendous interest as a solution for the greenhouse effect and energy crisis, and thereby it plays a critical role in solving environmental problems and generating economic benefits. In₂ O_{3- x} has emerged as a potential photothermal catalyst for CO₂ conversion into CO via the light-driven reverse water gas shift reaction. However, it is still a challenge to modulate the structural and electronic characteristics of In₂ O₃ to enhance photothermocatalytic activity synergistically. In this work, a novel route to activate inert In(OH)₃ into 2D black In₂ O_{3- x} nanosheets via photoinduced defect engineering is proposed. Theoretical calculations and experimental results verify the existence of bifunctional oxygen vacancies in the 2D black In₂ O_{3- x} nanosheets host, which enhances light harvesting and chemical adsorption of CO₂ molecules dramatically, achieving 103.21 mmol g_cat ^-1 h^-1 with near-unity selectivity for CO generation and meanwhile excellent stability. This study reveals an exciting phenomenon that light is an ideal external stimulus on the layered In₂ O₃ system, and its electronic structure can be adjusted efficiently through photoinduced defect engineering; it can be anticipated that this synthesis strategy can be extended to wider application fields.

Influence of Surface Oxygen Vacancies and Ruthenium Valence State on the Catalysis of Pyrochlore Oxides

Proton exchange membrane (PEM) water electrolysis is a promising energy storage solution by electrochemically splitting water into hydrogen fuel and oxygen. However, the sluggish kinetics, high operating potential, and corrosive acidic environment during the oxygen evolution reaction (OER) require the use of scarce and costly Ir-based oxides, tremendously hampering its large-scale commercialization. Hence, developing active and stable anode catalysts with reduced precious-metal usage is desperately essential. For the first time, we report a group of Y_2-xBa_xRu₂O₇ pyrochlore oxides and employ them in acid OER and PEM electrolyzers. We reveal the mechanism for the promoted OER performance of Ba-doped Y₂Ru₂O₇ in which partially replacing Y³⁺ by Ba²⁺ in Y₂Ru₂O₇ greatly facilitates the hole-doping effect, which generates massive oxygen vacancy and multivalence of Ru⁵⁺/Ru⁴⁺, thus boosting the OER performance of Y_2-xBa_xRu₂O₇. This work provides an effective method and paradigm for improving the electrocatalytic property of pyrochlore oxides.