Recent advances and perspectives of fluorite and perovskite-based dual-ion conducting solid oxide fuel cells

High-temperature solid-state electrolyte is a key component of several important electrochemical devices, such as oxygen sensors for automobile exhaust control, solid oxide fuel cells (SOFCs) for power generation, and solid oxide electrolysis cells for H₂ production from water electrolysis or CO₂ electrochemical reduction to value-added chemicals. In particular, internal diffusion of protons or oxygen ions is a fundamental and crucial issue in the research of SOFCs, hypothetically based on either oxygen-ion-conducting electrolytes or proton-conducting electrolytes. Up to now, some electrolyte materials based on fluorite or perovskite structure were found to show certain degree of dual-ion transportation capability, while in available electrolyte database, particularly in the field of SOFCs, such dual-ion conductivity was seriously overlooked. Actually, few concerns arising to the simultaneous proton and oxygen-ion conductivities in electrolyte of SOFCs inevitably induce various inadequate and confusing results in literature. Understanding dual-ion transportation behavior in electrolyte is indisputably of great importance to explain some unusual fuel cell performance as reported in literature and enrich the knowledge of solid state ionics. On the other hand, exploration of novel dual-ion conducting electrolytes will benefit the development of SOFCs. In this review, we provide a comprehensive summary of the understanding of dual-ion transportation in solid electrolyte and recent advances of dual-ion conducting SOFCs. The oxygen ion and proton conduction mechanisms at elevated temperature inside oxide-based electrolyte materials are first introduced, and then (mixed) oxygen ion and proton conduction behaviors of fluorite and perovskite-type oxides are discussed. Following on, recent advances in the development of dual-ion conducting SOFCs based on fluorite and perovskite-type single-phase or composite electrolytes, are reviewed. Finally, the challenges in the development of dual-ion conducting SOFCs are discussed and future prospects are proposed.