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.

The protonic ceramic electrolysis cell NBN–BCZDy|BCZDy|Ni–BCZDy (where NBN = Nd_1.95Ba_0.05NiO_4+δ, BCZDy = BaCe_0.3Zr_0.5Dy_0.2O_3−δ) has been successfully designed and tested for carrying out the CO₂ electrochemical reduction.