Correlation between Oxygen Vacancies and Oxygen Evolution Reaction Activity for a Model Electrode: PrBaCo2 O5+δ

Tailored (La0.2Pr0.2Nd0.2Tb0.2Dy0.2)2Ce2O7 as a Highly Active and Stable Nanocatalyst for the Oxygen Evolution Reaction

Designing highly active and robust catalysts for the oxygen evolution reaction is key to improving the overall efficiency of the water splitting reaction. It has been previously demonstrated that evaporation induced self-assembly (EISA) can be used to synthesize highly porous and high surface area cerate-based fluorite nanocatalysts, and that substitution of Ce with 50% rare earth (RE) cations significantly improves electrocatalyst activity. Herein, the defect structure of the best performing nanocatalyst in the series are further explored, Nd2Ce2O7, with a combination of neutron diffraction and neutron pair distribution function analysis. It is found that Nd3 + cation substitution for Ce in the CeO2 fluorite lattice introduces higher levels of oxygen Frenkel defects and induces a partially reduced RE1.5Ce1.5O5 + x phase with oxygen vacancy ordering. Significantly, it is demonstrated that the concentration of oxygen Frenkel defects and improved electrocatalytic activity can be further enhanced by increasing the compositional complexity (number of RE cations involved) in the substitution. The resulting novel compositionally-complex fluorite- (La0.2Pr0.2Nd0.2Tb0.2Dy0.2)2Ce2O7 is shown to display a low OER overpotential of 210 mV at a current density of 10 mAcm-2 in 1M KOH, and excellent cycling stability. It is suggested that increasing the compositional complexity of fluorite nanocatalysts expands the ability to tailor catalyst design.

Bifunctional Electrocatalysts for Overall and Hybrid Water Splitting

Electrocatalytic water splitting driven by renewable electricity has been recognized as a promising approach for green hydrogen production. Different from conventional strategies in developing electrocatalysts for the two half-reactions of water splitting (e.g., the hydrogen and oxygen evolution reactions, HER and OER) separately, there has been a growing interest in designing and developing bifunctional electrocatalysts, which are able to catalyze both the HER and OER. In addition, considering the high overpotentials required for OER while limited value of the produced oxygen, there is another rapidly growing interest in exploring alternative oxidation reactions to replace OER for hybrid water splitting toward energy-efficient hydrogen generation. This Review begins with an introduction on the fundamental aspects of water splitting, followed by a thorough discussion on various physicochemical characterization techniques that are frequently employed in probing the active sites, with an emphasis on the reconstruction of bifunctional electrocatalysts during redox electrolysis. The design, synthesis, and performance of diverse bifunctional electrocatalysts based on noble metals, nonprecious metals, and metal-free nanocarbons, for overall water splitting in acidic and alkaline electrolytes, are thoroughly summarized and compared. Next, their application toward hybrid water splitting is also presented, wherein the alternative anodic reactions include sacrificing agents oxidation, pollutants oxidative degradation, and organics oxidative upgrading. Finally, a concise statement on the current challenges and future opportunities of bifunctional electrocatalysts for both overall and hybrid water splitting is presented in the hope of guiding future endeavors in the quest for energy-efficient and sustainable green hydrogen production.

In Situ X-ray Absorption Spectroscopy of LaFeO3 and LaFeO3/LaNiO3 Thin Films in the Electrocatalytic Oxygen Evolution Reaction

We study the electrocatalytic oxygen evolution reaction using in situ X-ray absorption spectroscopy (XAS) to track the dynamics of the valence state and the covalence of the metal ions of LaFeO3 and LaFeO3/LaNiO3 thin films. The active materials are 8 unit cells grown epitaxially on 100 nm conductive La0.67Sr0.33MnO3 layers using pulsed laser deposition (PLD). The perovskite layers are supported on monolayer Ca2Nb3O10 nanosheet-buffered 100 nm SiNx membranes. The in situ Fe and Ni K-edges XAS spectra were measured from the backside of the SiNx membrane using fluorescence yield detection under electrocatalytic reaction conditions. The XAS spectra show significant spectral changes, which indicate that (1) the metal (co)valencies increase, and (2) the number of 3d electrons remains constant with applied potential. We find that the whole 8 unit cells react to the potential changes, including the buried LaNiO3 film.

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

Recent progress of Ni-based nanomaterials for the electrocatalytic oxygen evolution reaction at large current density

The precise design and development of high-performing oxygen evolution reaction (OER) for the production of industrial hydrogen gas through water electrolysis has been a widely studied topic. A profound understanding of the nature of electrocatalytic processes reveals that Ni-based catalysts are highly active toward OER that can stably operate at a high current density for a long period of time. Given the current gap between research and applications in industrial water electrolysis, we have completed a systematic review by constructively discussing the recent progress of Ni-based catalysts for electrocatalytic OER at a large current density, with special focus on the morphology and composition regulation of Ni-based electrocatalysts for achieving extraordinary OER performance. This review will facilitate future research toward rationally designing next-generation OER electrocatalysts that can meet industrial demands, thereby promoting new sustainable solutions for energy shortage and environment issues.

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.

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.

Oxygen-evolution reaction in the presence of cerium(IV) ammonium nitrate and iron (hydr)oxide: old system, new findings

Solar fuel production by photosynthetic systems strongly relies on developing efficient and stable oxygen-evolution catalysts (OECs). Cerium(IV) ammonium nitrate (CAN) has been the most commonly used sacrificial oxidant to investigate OECs. Although many metal oxides have been extensively investigated as OECs in the presence of CAN, mechanistic studies were rarely reported. Herein, first, Fe(III) (hydr)oxide (FeOxHy) was prepared by the reaction of Fe(ClO4)3 and KOH solution and characterized by some methods. Then, changes in Fe oxide in the presence of CAN during the OER were tracked using in situ Raman spectroscopy, in situ X-ray absorption spectroscopy, in situ visible spectroscopy, and in situ electron paramagnetic resonance spectroscopy. FeOxHy in the presence of CAN and during the OER converted to γ-Fe2O3 and [Fe(H2O)6]3+, and a small amount of oxygen was formed. A maximum turnover frequency and turnover number of 10-6 s-1 and 1.3 × 10-3 mol(O2)/mol(Fe) (for half an hour) in the presence of CAN (0.20 M) and FeOxHy were observed.

Dopant-Induced Surface Self-Etching of Cobalt Carbonate Hydroxide Boosts Efficient Water Splitting

Herein, vanadium-doped cobalt carbonate hydroxide, V-CoCH, was synthesized as efficient catalyst for water splitting. Vanadium species were partially dissolved in the early stages of the oxygen-evolution reaction (OER), inducing self-etching of the catalyst surface, which is helpful for catalyst surface reconstruction and resulted in a higher number of active sites and oxygen vacancies. The synergy between V-doping and oxygen vacancies improved the catalytic activity: V-CoCH showed an exceptional OER catalytic performance with an overpotential of 183 mV at 10 mA cm^-2 . The water-splitting cell consisting of V-CoCH only required 1.52 V to reach 10 mA cm^-2 . Theoretical calculations revealed that vanadium in V-CoCH played an important role in electron regulation of active sites. The oxygen vacancies had an important effect on improvement of the OER performance through not only the exposure of more active sites but also through modulation of the electronic structure. This work provides an effective strategy for constructing high-performance electrocatalysts.

IrO2 Nanoparticle-Decorated Ir-Doped W18O49 Nanowires with High Mass Specific OER Activity for Proton Exchange Membrane Electrolysis

The oxygen evolution reaction (OER) severely limits the efficiency of proton exchange membrane (PEM) electrolyzers due to slow reaction kinetics. IrO₂ is currently a commonly used anode catalyst, but its large-scale application is limited due to its high price and scarce reserves. Herein, we reported a practical strategy to construct an acid OER catalyst where Iridium oxide loading and iridium element bulk doping are realized on the surface and inside of W₁₈O₄₉ nanowires by immersion adsorption, respectively. Specifically, W_0.7Ir_0.3O_y has an overpotential of 278 mV at 10 mA·cm^-2 in 0.1 M HClO₄. The mass activity of 714.10 A·g_Ir^-1 at 1.53 V vs. the reversible hydrogen electrode (RHE) is 80 times that of IrO₂, and it can run stably for 55 h. In the PEM water electrolyzer device, its mass activity reaches 3563.63 A·g_Ir^-1 at the cell voltage of 2.0 V. This improved catalytic performance is attributed to the following aspects: (1) The electron transport between iridium and tungsten effectively improves the electronic structure of the catalyst; (2) the introduction of iridium into W₁₈O₄₉ by means of elemental bulk doping and nanoparticles supporting for the enhanced conductivity and electrochemically active surface area of the catalyst, resulting in extensive exposure of active sites and increased intrinsic activity; and (3) during the OER process, partial iridium elements in the bulk phase are precipitated, and iridium oxide is formed on the surface to maintain stable activity. This work provides a new idea for designing oxygen evolution catalysts with low iridium content for practical application in PEM electrolyzers.

Stability and Evolution of the Crystal Structure of TbBaCo2O6-δ During Thermal Oxygen Release/Uptake

A-site ordered double perovskites with the general formula LnBaCo₂O_6-δ (where Ln is a lanthanide element) present electrical and electrocatalytic properties that make them attractive as possible ceramic electrode materials for solid oxide cells or alkaline electrolyzers. The properties are highly influenced by the anion vacancy concentration, which is strongly related to the Co-oxidation state, and their location in the structure. Awareness of the stable phases is essential to synthesize, evaluate, and optimize the properties of LnBaCo₂O_6-δ oxides at operating conditions in different applications. TbBaCo₂O_6-δ are representative oxides of these layered perovskite systems. The present article reports a study of TbBaCo₂O_6-δ by electron diffraction, high-resolution electron microscopy, and powder neutron diffraction experiments at different temperatures. The synthesis of TbBaCo₂O_6-δ in air and slow cooling to room temperature (RT) at 5 °C h^-1 leads to samples formed by distinct phases with different oxygen contents and crystal structures. The 122 and 112 phases (with a_p × 2a_p × 2a_p and a_p × a_p × 2a_p unit cells, respectively, with a_p being the lattice parameter of the simple cubic perovskite structure) are predominant in quasi-equilibrium prepared samples (cooled at RT at 1 °C h^-1) or prepared in Ar flow and quenched to RT. The evolution of the crystal structure of TbBaCo₂O_6-δ during thermal oxygen release/uptaking consists of modulation from the 122 phase to the 112 phase (or vice versa during uptaking) by creation/occupation of anion vacancies within the TbO_1-δ planes. Anion vacancies are not detected in the oxygen crystallographic position different from those located within the TbO_1-δ planes even at the highest temperatures, supporting the 2D character of the high anion conduction of the LnBaCo₂O_6-δ oxides.

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

Exploring low cost, highly active, and durable electrocatalysts for oxygen evolution reaction (OER) is of prime importance to boost energy conversion efficiency. Perovskite fluorides are emerging as alternative electrocatalysts for OER, however, their intrinsically active sites during real operation are still elusive. Herein, the self-reconstruction on newly designed NiFe coupled perovskite fluorides during OER process is demonstrated. In situ Raman spectroscopy, ex situ X-ray absorption spectroscopy, and theoretical calculation reveal that Fe incorporation can significantly activate the self-reconstruction of perovskite fluorides and efficiently lower the energy barrier of OER. Benefiting from self-reconstruction and low energy barrier, the KNi_0.8 Fe_0.2 F₃ @nickel foam (KNFF2@NF) electrocatalyst delivers an ultralow overpotential of 258 mV to afford 100 mA cm^-2 and an excellent durability for 100 h, favorably rivaling most the state-of-the-art OER electrocatalysts. This protocol provides the fundamental understanding on OER mechanism associated with surface reconstruction for perovskite fluorides.

Water electrolysis: from textbook knowledge to the latest scientific strategies and industrial developments

Replacing fossil fuels with energy sources and carriers that are sustainable, environmentally benign, and affordable is amongst the most pressing challenges for future socio-economic development. To that goal, hydrogen is presumed to be the most promising energy carrier. Electrocatalytic water splitting, if driven by green electricity, would provide hydrogen with minimal CO2 footprint. The viability of water electrolysis still hinges on the availability of durable earth-abundant electrocatalyst materials and the overall process efficiency. This review spans from the fundamentals of electrocatalytically initiated water splitting to the very latest scientific findings from university and institutional research, also covering specifications and special features of the current industrial processes and those processes currently being tested in large-scale applications. Recently developed strategies are described for the optimisation and discovery of active and durable materials for electrodes that ever-increasingly harness first-principles calculations and machine learning. In addition, a technoeconomic analysis of water electrolysis is included that allows an assessment of the extent to which a large-scale implementation of water splitting can help to combat climate change. This review article is intended to cross-pollinate and strengthen efforts from fundamental understanding to technical implementation and to improve the 'junctions' between the field's physical chemists, materials scientists and engineers, as well as stimulate much-needed exchange among these groups on challenges encountered in the different domains.

Correlating Orbital Composition and Activity of LaMnxNi1-xO3 Nanostructures toward Oxygen Electrocatalysis

The atomistic rationalization of the activity of transition metal oxides toward oxygen electrocatalysis is one of the most complex challenges in the field of electrochemical energy conversion. Transition metal oxides exhibit a wide range of structural and electronic properties, which are acutely dependent on composition and crystal structure. So far, identifying one or several properties of transition metal oxides as descriptors for oxygen electrocatalysis remains elusive. In this work, we performed a detailed experimental and computational study of LaMn_xNi_1–xO₃ perovskite nanostructures, establishing an unprecedented correlation between electrocatalytic activity and orbital composition. The composition and structure of the single-phase rhombohedral oxide nanostructures are characterized by a variety of techniques, including X-ray diffraction, X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, and electron microscopy. Systematic electrochemical analysis of pseudocapacitive responses in the potential region relevant to oxygen electrocatalysis shows the evolution of Mn and Ni d-orbitals as a function of the perovskite composition. We rationalize these observations on the basis of electronic structure calculations employing DFT with HSE06 hybrid functional. Our analysis clearly shows a linear correlation between the OER kinetics and the integrated density of states (DOS) associated with Ni and Mn 3d states in the energy range relevant to operational conditions. In contrast, the ORR kinetics exhibits a second-order reaction with respect to the electron density in Mn and Ni 3d states. For the first time, our study identifies the relevant DOS dominating both reactions and the importance of understanding orbital occupancy under operational conditions.

Rapid Surface Reconstruction of Amorphous Co(OH)2 /WOx with Rich Oxygen Vacancies to Promote Oxygen Evolution

Herein, a transition metal dissolution-oxygen vacancy strategy, based on dissolution of highly oxidized transition metal species in alkaline electrolyte, was suggested to construct a high-performance amorphous Co(OH)₂ /WO_x (a-CoW) catalyst for the oxygen evolution reaction (OER). The surface reconstruction of a-CoW and its evolution were described by regulating oxygen vacancies. With continuous dissolution of W species, oxygen vacancies on the surface were generated rapidly, the surface reconstruction was promoted, and the OER performance was improved significantly. During the surface reconstruction, W species also played a role in electronic modulation for Co. Due to its rapid surface reconstruction, a-CoW exhibited excellent OER performance in alkaline electrolyte with an overpotential of 208 mV at 10 mA cm^-2 and had long-term stability for at least 120 h. This work shows that the transition metal dissolution-oxygen vacancy strategy is effective for preparation of high-performance catalysts.