The oxygen reduction reaction (ORR) is one of the most important electrochemical reactions in energy conversion and storage technologies, such as fuel cells and metal–air batteries. However, the sluggish kinetics of the ORR is a key factor limiting the performance of these energy storage and conversion devices. Perovskite oxides are a promising family of electrocatalysts for the ORR because of their unique physical and chemical properties, such as variable crystal structure and non-stoichiometric chemistry. Studies have shown that the catalytic properties of perovskite oxides in the ORR are largely related to oxygen vacancies, which alter their electronic and crystal structures and surface chemistry. This review summarizes recent research advances on understanding the role of oxygen vacancies of the ABO₃ perovskite oxides in catalyzing the ORR. With a brief introduction of perovskite oxides, approaches to creating oxygen vacancies in the ABO₃ perovskite oxides and the role of oxygen vacancies in improving their catalytic performance for the ORR are discussed. Research perspectives in this important area are highlighted.

Energy crisis and environmental problems caused by the conventional combustion of fossil fuels boost the development of renewable and sustainable energies. H₂ is regarded as a clean fuel for many applications and it also serves as an energy carrier for many renewable energy sources, such as solar and wind power. Among all the technologies for H₂ production, steam electrolysis by solid oxide electrolysis cells (SOECs) has attracted much attention due to its high efficiency and low environmental impact, provided that the needed electrical power is generated from renewable sources. However, the deployment of SOECs based on conventional oxygen-ion conductors is limited by several issues, such as high operating temperature, hydrogen purification from water, and electrode stability. To avoid these problems, proton-conducting oxides are proposed as electrolyte materials for SOECs. This review paper provides a broad overview of the research progresses made for proton-conducting SOECs, summarizing the past work and finding the problems for the development of proton-conducting SOECs, as well as pointing out potential development directions.

A high-entropy ceramic oxide is used as the cathode for the first time for proton-conducting solid oxide fuel cells (H-SOFCs). The Fe_0.6Mn_0.6Co_0.6Ni_0.6Cr_0.6O₄ (FMCNC) high-entropy spinel oxide has been successfully prepared, and the in situ chemical stability test demonstrates that the FMCNC material has good stability against CO₂. The first-principles calculation indicates that the high-entropy structure enhances the properties of the FMCNC material that surpasses their individual components, leading to lower O₂ adsorption energy for FMCNC than that for the individual components. The H-SOFC using the FMCNC cathode reaches an encouraging peak power density (PPD) of 1052 mW·cm⁻² at 700 °C, which is higher than those of the H-SOFCs reported recently. Additional comparison was made between the high-entropy FMCNC cathode and the traditional Mn_1.6Cu_1.4O₄ (MCO) spinel cathode without the high-entropy structure, revealing that the formation of the high-entropy material allows the enhanced protonation ability as well as the movement of the O p-band center closer to the Fermi level, thus improving the cathode catalytic activity. As a result, the high-entropy FMCNC has a much-decreased polarization resistance of 0.057 Ω·cm² at 700 °C, which is half of that for the traditional MCO spinel cathode without the high-entropy design. The excellent performance of the FMCNC cell indicates that the high-entropy design makes a new life for the spinel oxide as the cathode for H-SOFCs, offering a novel and promising route for the development of high-performance materials for H-SOFCs.

In the past decade, intensive research on proton-conducting oxide materials has provided a basis for the development of intermediate-temperature protonic ceramic electrochemical cells, which constitute a real alternative to conventional cells based on oxygen-conducting electrolytes. To achieve both high efficiency and excellent performance, not only electrolytes but also electrode materials should be carefully selected considering their functional properties. Compared to the traditional ABO₃ perovskite electrode materials, Ln₂NiO_4+δ with a layered structure has unique advantages (high chemical stability, mechanical compatibility, improved oxygen transport, and hydration ability), and thus is now becoming a hot topic in this field, offering both scientific and practical interests. However, a comprehensive and in-depth review is still lacking in the literature to date. Accordingly, this work presents a comprehensive overview of the prospects of layered nickelates (Ln₂NiO_4+δ, where Ln = La, Nd, and Pr) as one of the most attractive oxygen (steam) electrode materials for protonic ceramic electrochemical cells. In particular, the crystalline features, defect structure, stability, chemical properties, and mechanical compatibility of this class of materials, contributing to their transport functionality, are discussed with the primary emphasis on revealing the relationship between the composition of the materials and their properties. The presented systematic results reveal the main strategies regarding the utilisation of Ln₂NiO_4+δ-based electrodes and existing gaps related to fundamental and applied research aspects.