Recent Advances and Future Perspectives of Proton-Conducting Electrolytes for Reversible Solid Oxide Cells

Proton-conducting reversible solid oxide cells (P-RSOCs) are emerging as a transformative platform for efficient and flexible conversion between electricity and chemical fuels, including hydrogen and syngas. Their intermediate-temperature operation (400-600 °C) offers a compelling combination of high energy efficiency, rapid reaction kinetics, and strong compatibility with renewable electricity and industrial waste heat, positioning P-RSOCs as a promising technology for a carbon-neutral energy future. At the heart of these devices lies the proton-conducting electrolyte, which governs proton transport, chemical stability, interfacial compatibility, and long-term durability. Despite remarkable advances in electrolyte development, fundamental challenges in understanding and controlling proton transport, chemical stability, and electrode-electrolyte interactions continue to constrain practical deployment. This review presents a comprehensive and critical assessment of the current state of proton-conducting electrolytes for P-RSOCs, with emphasis on proton transport mechanisms, composition-structure-property relationships, stability limitations, and interfacial compatibility. We examine advances in perovskite-based and emerging alternative electrolyte families, while highlighting key strategies─including aliovalent doping, interfacial engineering, microstructure design, and advanced fabrication─for overcoming persistent limitations. Emerging operando characterization and computational approaches are further discussed as powerful tools for uncovering dynamic transport and degradation mechanisms and accelerating materials discovery. By integrating fundamental insights with materials-design strategies, this review identifies critical knowledge gaps and opportunities to guide the development of robust, high-performance electrolytes. Ultimately, we envision that such advances will help unlock the full potential of P-RSOCs as a versatile platform for sustainable energy conversion, storage, and renewable-fuel production.