Molten Salt Flux Synthesis, Crystal Facet Design, Characterization, Electronic Structure, and Catalytic Properties of Perovskite Cobaltite

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

Triggering photocatalytic performance of La2CoxMn2-xO6 via heat activation

The La-based perovskite (LaBO3) exhibits excellent optical properties. However, its valence band (VB) potential is not sufficiently positive to reach the oxidation potential required for the cleavage of chemical bonds (such as benzylic C-H), limiting its application in photocatalysis. Herein, we report the unconventional effects of heat activation on the reduction of the dissociation energy of benzylic C-H and aqueous H-O, thereby triggering the photocatalytic activity of La2CoxMn2-xO6 perovskites. Additionally, we demonstrate that photocatalysis is the main contributor to substrate conversion in the selective oxidation of toluene and reduction of CO2. Particularly, La2Co1.5Mn0.5O6 shows excellent performance with a product yield of 550.00 mmol gcat-1 and a toluene conversion of 22,866.67 μmol gcat-1 h-1. To the best of our knowledge, this is the highest reported product yield for the selective oxidation of benzylic C-H bond of toluene. Our findings provide insight into the specific role of heat activation in photocatalysis, which is crucial for breaking and overcoming the VB barrier to realize challenging reactions.

Colossal Dielectric Perovskites of Calcium Copper Titanate (CaCu3 Ti4 O12 ) with Low-Iridium Dopants Enables Ultrahigh Mass Activity for the Acidic Oxygen Evolution Reaction

Oxygen evolution reaction (OER) under acidic conditions becomes of significant importance for the practical use of a proton exchange membrane (PEM) water electrolyzer. In particular, maximizing the mass activity of iridium (Ir) is one of the maiden issues. Herein, the authors discover that the Ir-doped calcium copper titanate (CaCu₃Ti₄O₁₂, CCTO) perovskite exhibits ultrahigh mass activity up to 1000 A g_Ir ^-1 for the acidic OER, which is 66 times higher than that of the benchmark catalyst, IrO₂ . By substituting Ti with Ir in CCTO, metal-oxygen (M-O) covalency can be significantly increased leading to the reduced energy barrier for charge transfer. Further, highly polarizable CCTO perovskite referred to as "colossal dielectric", possesses low defect formation energy for oxygen vacancy inducing a high number of oxygen vacancies in Ir-doped CCTO (Ir-CCTO). Electron transfer occurs from the oxygen vacancies and Ti to the substituted Ir consequentially resulting in the electron-rich Ir and -deficient Ti sites. Thus, favorable adsorptions of oxygen intermediates can take place at Ti sites while the Ir ensures efficient charge supplies during OER, taking a top position of the volcano plot. Simultaneously, the introduced Ir dopants form nanoclusters at the surface of Ir-CCTO, which can boost catalytic activity for the acidic OER.

Solid-State Reaction Synthesis of Nanoscale Materials: Strategies and Applications

Nanomaterials (NMs) with unique structures and compositions can give rise to exotic physicochemical properties and applications. Despite the advancement in solution-based methods, scalable access to a wide range of crystal phases and intricate compositions is still challenging. Solid-state reaction (SSR) syntheses have high potential owing to their flexibility toward multielemental phases under feasibly high temperatures and solvent-free conditions as well as their scalability and simplicity. Controlling the nanoscale features through SSRs demands a strategic nanospace-confinement approach due to the risk of heat-induced reshaping and sintering. Here, we describe advanced SSR strategies for NM synthesis, focusing on mechanistic insights, novel nanoscale phenomena, and underlying principles using a series of examples under different categories. After introducing the history of classical SSRs, key theories, and definitions central to the topic, we categorize various modern SSR strategies based on the surrounding solid-state media used for nanostructure growth, conversion, and migration under nanospace or dimensional confinement. This comprehensive review will advance the quest for new materials design, synthesis, and applications.

Controlling the Cation Exsolution of Perovskite to Customize Heterostructure Active Site for Oxygen Evolution Reaction

Perovskite oxides are an important class of oxygen evolution reaction (OER) catalysts offering an ordered atomic arrangement and a highly flexible electronic structure. Currently, understanding and adjusting the dynamic reconstruction of perovskite during the OER process remains a formidable challenge. Here, we report the artificial construction of a heterostructure by the cation exsolution of perovskite to control the active site formation and reconstruction. The deliberately made La deficiency in LaNiO₃ perovskite facilitates the original segregation of NiO from the parent matrix and forms a well-defined interface between perovskite parent and NiO exsolution phase. The dynamic formation process of such heterojunction was studied by density functional theory computation and high quality imaging characterization. Due to the valence redistribution of Ni ions caused by the interfacial electron transfer, the in situ formed LaNiO₃/NiO heterostructure displays high electroactivity. Therefore, the LaNiO₃/NiO heterostructure exhibits a dynamic surface evolution feature with the generation of the highly active NiOOH layer under a low anodic potential (∼1.35 V vs RHE) during the OER process, which is very different from the conventional LaNiO₃ with a stoichiometry and NiO catalysts. With the newly formed heterostructure, the reconstructed catalysts impart a 4.5-fold increase in OER activity and a 3-fold improvement in stability against La and Ni dissolution during the OER process. This work provides a feasible interface engineering strategy for artificially controlling the reconstruction of the active phase in high-performance perovskite-based electrocatalytic materials.

Geoinspired syntheses of materials and nanomaterials

The search for new materials is intimately linked to the development of synthesis methods. In the current urge for the sustainable synthesis of materials, taking inspiration from Nature's ways to process matter appears as a virtuous approach. In this review, we address the concept of geoinspiration for the design of new materials and the exploration of new synthesis pathways. In geoinspiration, materials scientists take inspiration from the key features of various geological systems and processes occurring in nature, to trigger the formation of artificial materials and nanomaterials. We discuss several case studies of materials and nanomaterials to highlight the basic geoinspiration concepts underlying some synthesis methods: syntheses in water and supercritical water, thermal shock syntheses, molten salt synthesis and high pressure synthesis. We show that the materials emerging from geoinspiration exhibit properties differing from materials obtained by other pathways, thus demonstrating that the field opens up avenues to new families of materials and nanomaterials. This review focuses on synthesis methodologies, by drawing connections between geosciences and materials chemistry, nanosciences, green chemistry, and environmental sciences.

Elucidating the influence of molten salt chemistries on the synthesis and stability of perovskites oxides

In this work, we investigate the synthesis of (La_0.8Sr_0.2)MnO₃ (LSM) in various molten salts to gain insight on the influence of molten salt ions for synthesizing materials critical for energy applications. LSM nanoparticles with a size range of ∼10-200 nm and with target stoichiometries were formed from oxide precursors via feeding into KNO₃. Furthermore, feeding precursors into the melt compared to mixing and heating from room temperature results in complete formation of LSM that was otherwise unattainable using conventional molten salt synthesis methods. In LiCl-KCl eutectic, the high Lux acidity of Li⁺ and Cl^- establishes a thermodynamic barrier that impedes Sr from reacting with other precursors in solution and increases Sr stability in the melt compared to the perovskite phase. As a result, LSM will not form in a LiCl-KCl eutectic under ambient conditions. Thus, this study further explicates the molten salt synthesis for perovskites and can serve as a guide for future syntheses.

Growth of LaCoO3 crystals in molten salt: effects of synthesis conditions

Herein, several typical reaction media for the synthesis of LaCoO₃ (LCO) crystals were examined by X-ray absorption spectroscopy (XAS).

Perovskite-Type Solid Solution Nano-Electrocatalysts Enable Simultaneously Enhanced Activity and Stability for Oxygen Evolution

A trade-off between catalytic activity and structural stability generally exists in oxygen evolution electrocatalysis, especially in acidic environment. This dilemma limits the development of higher-performance electrocatalysts that are required by next-generation electrochemical technologies. Here it is demonstrated that the inverse catalytic activity-structural stability relation can be broken by alloying catalytically inert strontium zirconate with the other catalytically active perovskite, strontium iridate. This strategy results in an alloyed perovskite electrocatalyst with simultaneously improved iridium mass activity and structural stability, by about five times, for the oxygen evolution reaction under acidic conditions. The experimental and theoretical results suggest that the alloying strategy generates multiple positive effects, mainly including the reduction of catalyst size, the decrease of catalyst covalency, and the weakening of surface oxygen-binding ability. The synergistic optimization of bulk and surface properties, as a result, enhances the intrinsic activity and availability of surface iridium sites, whilst significantly inhibiting the surface cation corrosion during electrocatalysis.

Optimizing the surface state of cobalt-iron bimetallic phosphide via regulating phosphorus vacancies

We regulated phosphorus vacancies in Co_0.68Fe_0.32P by Ar-plasma treatment, leading to a unique surface state.

Enhanced Iridium Mass Activity of 6H-Phase, Ir-Based Perovskite with Nonprecious Incorporation for Acidic Oxygen Evolution Electrocatalysis

One of the key objectives in PEM electrolysis technology is to reduce iridium loading and to improve iridium mass activity at the side of oxygen evolution electrocatalysis. 6H-phase, Ir-based perovskite (6H-SrIrO₃) is known to be a promising alternative to the IrO₂ catalyst, and developing effective strategies to further enhance its catalytic performance is needed. Here we present that a significant enhancement in electrocatalytic activity for the oxygen evolution reaction of 6H-SrIrO₃ can be achieved by cobalt incorporation. A suitable amount of cobalt dopants results in a decreased formation temperature of 6H-SrIrO₃ from 700 to 500 °C and thereby a decreased thickness of platelike particles for the material. Besides the morphological effect, the cobalt incorporation also increases the coverage of surface hydroxyl groups, regulates the Ir-O bond covalency, and modulates the oxygen p-band center of the material. This synergistic optimization of the morphological, surface, and electronic structures makes the cobalt-doped 6H-SrIrO₃ catalyst give a 3-fold increase in iridium mass activity for oxygen evolution reaction in comparison with the undoped 6H-SrIrO₃ under acidic conditions.

Black Phosphorus-Modified Co3O4 through Tuning the Electronic Structure for Enhanced Oxygen Evolution Reaction

Spinel Co₃O₄, consisting of two mixed valence states, Co²⁺ and Co³⁺, has attracted enormous interest as a promising electrocatalyst for oxygen evolution reaction (OER). Proper control on the relative proportion of Co²⁺/Co³⁺ in cobalt oxide can greatly tune the electronic structure and further optimize its catalytic performance. Herein, a hybrid coupling Co₃O₄ with black phosphorus (Co₃O₄@BP) is designed as an efficient catalyst for OER. Electron migration from BP to Co₃O₄ is achieved in Co₃O₄@BP, owing to the higher Fermi level of BP than that of Co₃O₄. Efficient electron transfer can not only create massive active sites with abundant Co²⁺ but also remarkably suppress the deterioration of BP. Particularly, the Co₃O₄@BP catalyst outperforms the pristine Co₃O₄ by over four times and is even 20 times higher than that of bare BP at a potential of 1.65 V versus reversible hydrogen electrode. Our finding provides insightful understanding for electronic engineering in Co₃O₄@BP by balancing advantages and utilizing drawbacks of Co₃O₄ and BP.