Advanced Cobalt-Free Perovskite Oxide-Based Air Electrodes for Protonic Ceramic Electrolysis Cells

Protonic ceramic electrolysis cells (PCECs) are regarded as a promising technology for producing green hydrogen due to the lower operational temperatures, higher conversion efficiencies, enhanced durability/safety, no requirement of hydrogen drying system and simplified water management over the traditional oxygen-ion-conducting solid oxide electrolysis cells. Nevertheless, the overall performance of PCECs is hindered by the slow kinetics of oxygen evolution reaction (OER) and insufficient triple conductivities (H⁺/O^2-/e^-) of air electrodes (e.g., perovskite oxides) at lower temperatures. Although remarkable advancements have been achieved about the development of cobalt-based perovskite air electrodes (highly active but instable), designing optimal and highly efficient/durable air electrodes for PCECs remains challenging. Herein, an in-time and critical review about the advances in designing cobalt-free air electrodes for PCECs is presented by highlighting the fundamental mechanisms, performance-influencing factors, and the superiority of cobalt-free perovskite oxides over cobalt-based counterparts. Several distinct design strategies to boost the OER activity/durability and thermo-mechanical compatibility of cobalt-free air electrodes for PCECs are also proposed. Finally, current limitations/contradictions, existed challenges/controversies and future directions are presented. This review aims to provide critical insights for the rational design of high-efficiency and long-lasting air electrodes for PCECs to realize widespread applications.