Enhanced overall water electrolysis on a bifunctional perovskite oxide through interfacial engineering

In situ investigation of ruthenium doped lanthanum nickel titanium double perovskite and its exsolution behaviour

Exsolution, an innovative method for fabricating perovskite-based oxides decorated with metal nanoparticles, has garnered significant interest in the fields of catalyst fabrication and electrochemical devices. Although dopant exsolution from single perovskite structures has been extensively studied, the exsolution behaviour of double perovskite structures remains insufficiently understood. In this study, we synthesized B-site double perovskite Ru-doped lanthanum nickel titanates with a 7.5 at% A-site deficiency, and systematically investigated the exsolution process that formed nickel metal nanoparticles on the material surface, across a broad reduction temperature range of 350-1000 °C. Both Ex situ and in situ characterization revealed that small, uniform Ni nanoparticles exsolved at low temperatures, whereas the exsolution of ruthenium required higher reduction temperatures beyond 1000 °C. Within the reduction temperature range of 350-500 °C, a notable finding is the reconstruction of exsolved nanoparticles, implying that Ni particles exist in a thermodynamically metastable state. Electrochemical impedance spectroscopy (EIS) showed a decreased area specific resistance (ASR) during the progress of exsolution. The increase in current density of a full solid oxide cell (SOC) in electrolysis mode and the doubling of peak power density in fuel cell mode attributed to the exsolution of Ni nanoparticles highlight the potential application of metal exsolution in electrode materials for SOCs.

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

Se-Doped Ni5P4 Nanocatalysts for High-Efficiency Hydrogen Evolution Reaction

Increasing energy consumption and environmental pollution problems have forced people to turn their attention to the development and utilization of hydrogen energy, which requires that hydrogen energy can be efficiently prepared. However, the sluggish kinetics of hydrogen evolution reaction (HER) requires higher overpotential. It is urgent to design and fabricate catalysts to drive the procedure and decrease the overpotential of HER. It is well known that platinum catalysts are the best for HER, but their high cost limits their wide application. Transition metals such as Fe, Co, Mo and Ni are abundant, and transition metal phosphides are considered as promising HER catalysts. Nevertheless, catalysts in powder form are very easily soluble in the electrolyte, which leads to inferior cycling stability. In this work, Ni5P4 anchored on Ni foam was doped with Se powder. After SEM characterization, the Ni5P4-Se was anchored on Ni foam, which circumvents the use of the conductive additives and binder. The Ni5P4-Se formed a porous nanosheet structure with enhanced electron transfer capability. The prepared Ni5P4-Se exhibited high electrochemical performances. At 10 mA cm−2, the overpotential was only 128 mV and the Tafel slope is 163.14 mV dec−1. Additionally, the overpotential was stabilized at 128 mV for 30 h, suggesting its excellent cycling stability. The results show that Se doping can make the two phases achieve a good synergistic effect, which makes the Ni5P4-Se catalyst display excellent HER catalytic activity and stability.

Perovskite Oxides as Electrocatalysts for Hydrogen Evolution Reaction

Hydrogen generation through electrocatalytic splitting of water, i.e., hydrogen evolution reaction (HER), is an attractive method of converting the electricity generated from renewable sources into chemical energy stored in hydrogen molecules. A wide variety of materials have been studied in an effort to develop efficient and cost-effective electrocatalysts that can replace the traditional platinum/carbon catalyst. One family of functional materials that holds promise for this application is perovskite oxides. This mini-review discusses some of the progress made in the development of HER electrocatalysts based on perovskite oxides in the past decade. Given the diverse range of possible compositions of perovskite oxides, various studies have focused on compositional modifications to develop single-phase catalysts, whereas others have investigated heterostructures and composites that take advantage of synergistic interactions of different compounds with perovskite oxides. The recent advances indicate that this family of materials have great potential for utilization in HER electrocatalysis.

In Situ Grown CoMn2 O4 3D-Tetragons on Carbon Cloth: Flexible Electrodes for Efficient Rechargeable Zinc-Air Battery Powered Water Splitting Systems

The integration of energy conversion and storage systems such as electrochemical water splitting (EWS) and rechargeable zinc-air battery (ZAB) is on the vision to provide a sustainable future with green energy resources. Herein, a unique strategy for decorating 3D tetragonal CoMn₂ O₄ on carbon cloth (CMO-U@CC) via a facile one-pot in situ hydrothermal process, is reported. The highly exposed morphology of 3D tetragons enhances the electrocatalytic activity of CMO-U@CC. This is the first demonstration of such a bifunctional activity of CMO-U@CC in an EWS system; it achieves a nominal cell voltage of 1.610 V @ 10 mA cm^-2 . Similarly, the fabricated rechargeable ZAB delivers a specific capacity of 641.6 mAh g_zn ^-1 , a power density of 135 mW cm^-2 , and excellent cyclic stability (50 h @ 10 mA cm^-2 ). Additionally, a series of flexible solid-state ZABs are fabricated and employed to power the assembled CMO-U@CC-based water electrolyzer. To the best of the authors' knowledge, this is the first demonstration of an in situ-grown binder-free CMO-U@CC as a flexible multifunctional electrocatalyst for a built-in integrated rechargeable ZAB-powered EWS system.

Development of Perovskite Oxide-Based Electrocatalysts for Oxygen Evolution Reaction

Perovskite oxides are studied as electrocatalysts for oxygen evolution reactions (OER) because of their low cost, tunable structure, high stability, and good catalytic activity. However, there are two main challenges for most perovskite oxides to be efficient in OER, namely less active sites and low electrical conductivity, leading to limited catalytic performance. To overcome these intrinsic obstacles, various strategies are developed to enhance their catalytic activities in OER. In this review, the recent developments of these strategies is comprehensively summarized and systematically discussed, including composition engineering, crystal facet control, morphology modulation, defect engineering, and hybridization. Finally, perspectives on the design of perovskite oxide-based electrocatalysts for practical applications in OER are given.

Non-Stoichiometry Induced Exsolution of Metal Oxide Nanoparticles via Formation of Wavy Surfaces and their Enhanced Electrocatalytic Activity: Case of Misfit Calcium Cobalt Oxide

Most heterogeneous catalytic reactions demand high density and yet spatially separated nanoparticles that are strongly anchored on the oxide surfaces. Such nanoparticles can be deposited or synthesized in situ via nonstoichiometric methods. To date, nanoparticles have been exsolved from perovskite oxide surfaces using nonstoichiometric processes. However, the density of the space-separated nanoparticles on the oxide surfaces is still low. And less attention is paid toward the changes that happen to the host during the nanoparticle exsolution process. In this work, we demonstrated in situ exsolution of ultrafine nanoparticles (∼5 nm) of either Co₃O₄ or Ca(OH)₂ via judicious control of nonstoichiometry in a misfit Ca₃Co₄O₉ (CCO). The nanoparticle density over the CCO surface reached as high as 8500/μm², which is significantly higher than previously reported values. High-resolution electron microscopy studies reveal the formation mechanism of Co₃O₄ nanoparticles over CCO, and the formation takes palace via the formation of wavy surfaces on the CCO. Defects caused by the nonstoichiometric synthesis created microstrain within the host CCO, resulting in making the new density of states near the Fermi energy. Further, the exsolution process turned the inert host (CCO) into electrocatalytically active toward water splitting. The nonstoichiometric samples obtained by shorter annealing times showed high electrocatalytic behavior for the hydrogen evolution (HER) and oxygen evolution (OER) reactions. The catalytic activity is further enhanced (reaching overpotential of 320 mV and 410 mV for HER and OER respectively, for a current density of 10 mA/cm²) by removing the surface nanoparticles. The observation indicates that the active sites that are produced during the nonstoichiometric synthesis also present in the bulk of the CCO (host). We believe that similar nonstoichiometric synthesis can be applied to a wide variety of tricomponent systems, and they could endow the hosts with novel properties for applications such as catalysis and thermoelectrics.

Progress of Exsolved Metal Nanoparticles on Oxides as High Performance (Electro)Catalysts for the Conversion of Small Molecules

Utilizing electricity and heat from renewable energy to convert small molecules into value-added chemicals through electro/thermal catalytic processes has enormous socioeconomic and environmental benefits. However, the lack of catalysts with high activity, good long-term stability, and low cost strongly inhibits the practical implementation of these processes. Oxides with exsolved metal nanoparticles have recently been emerging as promising catalysts with outstanding activity and stability for the conversion of small molecules, which provides new possibilities for application of the processes. In this review, it starts with an introduction on the mechanism of exsolution, discussing representative exsolution materials, the impacts of intrinsic material properties and external environmental conditions on the exsolution behavior, and the driving forces for exsolution. The performances of exsolution materials in various reactions, such as alkane reforming reaction, carbon monoxide oxidation, carbon dioxide utilization, high temperature steam electrolysis, and low temperature electrocatalysis, are then summarized. Finally, the challenges and future perspectives for the development of exsolution materials as high-performance catalysts are discussed.

Nanoparticle Ex-solution for Supported Catalysts: Materials Design, Mechanism and Future Perspectives

Supported metal catalysts represent one of the major milestones in heterogeneous catalysis. Such catalytic systems are feasible for use in a broad range of applications, including renewable energy devices, sensors, automotive emission control systems, and chemical reformers. The lifetimes of these catalytic platforms depend strongly on the stability of the supported nanoparticles. With this regard, nanoparticles synthesized via ex-solution process emphasize exceptional robustness as they are socketed in the host oxide. Ex-solution refers to a phenomenon which yields selective growth of fine and uniformly distributed metal nanocatalysts on oxide supports upon partial reduction. This type of advanced structural engineering is a game-changer in the field of heterogeneous catalysis with numerous studies showing the benefits of ex-solution process. In this review, we highlight the latest research efforts regarding the origin of the ex-solution phenomenon and the mechanism underpinning particle formation. We also propose research directions to expand the utility and functionality of the current ex-solution techniques.

Recent Advances of First d-Block Metal-Based Perovskite Oxide Electrocatalysts for Alkaline Water Splitting

First d-block metal-based perovskite oxides (FDMPOs) have garnered significant attention in research for their utilization in the water oxidation reaction due to their low cost, earth abundance, and promising activities. Recently, FDMPOs are being applied in electrocatalysis for the hydrogen evolution reaction (HER) and overall water splitting reaction. Numerous promising FDMPO-based water splitting electrocatalysts have been reported, along with new catalytic mechanisms. Therefore, an in-time summary of the current progress of FDMPO-based water splitting electrocatalysts is now considered imperative. However, few reviews have focused on this particular subject thus far. In this contribution, we review the most recent advances (mainly within the years 2014–2020) of FDMPO electrocatalysts for alkaline water splitting, which is widely considered to be the most promising next-generation technology for future large-scale hydrogen production. This review begins with an introduction describing the fundamentals of alkaline water electrolysis and perovskite oxides. We then carefully elaborate on the various design strategies used for the preparation of FDMPO electrocatalysts applied in the alkaline water splitting reaction, including defecting engineering, strain tuning, nanostructuring, and hybridization. Finally, we discuss the current advances of various FDMPO-based water splitting electrocatalysts, including those based on Co, Ni, Fe, Mn, and other first d-block metal-based catalysts. By conveying various methods, developments, perspectives, and challenges, this review will contribute toward the understanding and development of FDMPO electrocatalysts for alkaline water splitting.

Sucrose-Assisted Solution Combustion Synthesis of Doped Strontium Ferrate Perovskite-Type Electrocatalysts: Primary Role of the Secondary Fuel

The methodologies and experimental conditions used for the synthesis of cathode materials for electrochemical devices strongly influence their electrocatalytic performance. In particular, solution combustion synthesis is a convenient and versatile methodology allowing a fine-tuning of the properties of the material. In this work, we used for the first time a sucrose assisted-solution combustion synthesis for the preparation of Cerium and Cobalt-doped SrFeO3–δ electrocatalysts and we investigated the effect of polyethylene glycol (PEG) addition as a secondary fuel on their structural, microstructural, redox and electrochemical properties. The perovskite-type powders were characterized by X-ray diffraction coupled with Rietveld refinement, scanning, and high-resolution transmission electron microscopies, thermogravimetric analysis, nitrogen adsorption measurements, and temperature-programmed reduction. Electrical conductivity and overpotential measurements were performed after the deposition of the powders onto a Gd-doped ceria electrolyte pellet. Stable high-valence B-site cations were detected in the powders prepared from sucrose-PEG fuel mixtures, although a substantial improvement of the conductivity and a decrease of the overpotential values were obtained only with high molecular weight PEG. The superior electrochemical performance obtained using PEG with high molecular weight has been ascribed to a faster interaction of the powder with the oxygen gas phase favored by the nanometer-sized crystalline domains.

Smart Control of Composition for Double Perovskite Electrocatalysts toward Enhanced Oxygen Evolution Reaction

Double perovskites have emerged as efficient candidates for catalyzing the electrochemical oxygen evolution reaction (OER). Smart control of the composition of a B-site ordered double perovskite can lead to improved catalytic performance. By adopting a facile co-doping strategy, the OER-active elements are simultaneously introduced into the B-site and B'-site of a B-site-ordered double perovskite (A₂ BB'O₆ ), leading to an enhancement of the exposed reactive sites and an optimum surface chemical state. As a result, a model system built from the substitution of Co for Mo and Fe in the Sr₂ FeMoO_6-δ double perovskite (with a composition of Sr₂ Fe_0.8 Co_0.2 Mo_0.6 Co_0.4 O_6-δ ) shows significantly enhanced OER activity in alkaline media compared with the host material, requiring an overpotential of 345 mV to reach a 10 mA cm^-2 current density (catalyst loading≈0.232 mg_cat  cm^-2 _GEO ) and a cell voltage of 1.57 V to afford the same current density for the overall water splitting when coupled with a Pt/C cathode (catalyst loading≈2 mg cm^-2 ). It also demonstrates excellent electrochemical stability. The generalizability of the compositional control methodology has also been demonstrated in double perovskites incorporating transition metals other than Co (e.g., Ni).