Air electrodes for reversible protonic ceramic electrochemical cells: fundamental principles, optimization strategies, advanced characterization, and future perspectives

Reversible protonic ceramic electrochemical cells (R-PCECs) have emerged as promising systems for clean power generation and energy storage, distinguished by their fuel flexibility, high efficiency, and environmental friendliness. However, their practical implementation is hindered by the sluggish kinetics of the air electrode during oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER), a consequence of complex multi-carrier transport and high activation energy barriers. Given the limited performance of most perovskites when utilized as air electrodes, an in-depth investigation into the multi-species transport mechanisms and the strategic modification of air electrode materials are of paramount importance for enhancing the performance of R-PCECs. In this review, we first introduce the commonly used perovskite materials for R-PCEC air electrodes and their internal transport mechanisms for H⁺, O^2-, and e^-. We then elucidate the fundamental processes of the electrode reactions and systematically summarize the key factors influencing electrode performance. Building upon this foundation, we further review the state-of-the-art strategies for modifying perovskite air electrodes developed in recent years. Subsequently, a range of advanced characterization techniques employed to investigate the electrode reaction processes and mechanisms are summarized, providing insights into the underlying principles of the material modifications. Finally, we offer a perspective on future research directions and challenges in this field. This review is intended to provide guidance for the rational design of highly efficient and stable air electrode materials, with the ultimate goal of advancing the development of R-PCEC technology.