High-temperature proton-conducting materials constitute a unique class of oxide materials, which are able to exhibit protonic conductivity under hydrogen-containing atmospheres. Besides being of great fundamental interest, such oxide systems possess practical significance because they can achieve high protonic conductivity levels. This opens the possibility of using proton-conducting materials as electrolytes for a wide range of intermediate- and high-temperature solid oxide electrochemical devices. Recent advances in the field of solid oxide proton-conducting materials that belong to the class of perovskite-based materials (such as doped BaCeO₃, BaZrO₃, BaCeO₃–BaZrO₃, SrCeO₃, and LaScO₃) and to other classes of materials (such as doped Ba₂In₂O₅, CeO₂, and LaNbO₄) are presented and analyzed in this review. In order to highlight the most appropriate materials for applications in electrochemical devices, the analysis is devoted to the establishment of correlations between compositional and structural characteristics and their transport, thermal and stability properties.

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.

Protonic ceramic electrochemical cells (including fuel cells (PCFCs) and electrolysis cells (PCECs)) are positioned as an eco-friendly means for realizing energy/chemical conversion at low (below 500 °C) and intermediate (500–800 °C) temperatures; as a result, R&D of PCFCs and PCECs are compatible with hydrogen energy and CO₂ utilization programs that play an increasing role in global environmental practice. However, along with ionic transport, the majority of proton-conducting ceramic materials also exhibit electronic transport under oxidizing conditions and elevated temperatures. This feature negatively affects the performance of cells due to the short-circuit effect leading to a reduction in faradaic and energy efficiencies. In response, in order to achieve a compromise between high performance and high efficiency, the present review article aims at revealing the main factors contributing to undesirable electronic transport of materials used in PCFCs and PCECs, as well as possible solutions leading to its suppression for improving their efficiency.