Recent advances and emerging design paradigms in proton-conducting mixed ionic-electronic conductors

Proton-conducting mixed ionic-electronic conductors (MIECs) are central to electrochemical devices such as protonic ceramic fuel cells and steam electrolysis cells. Most existing MIECs were developed within a defect-chemistry-based framework, where electronic conductivity is introduced into hydration-type proton-conducting oxides via transition-metal doping. Although this strategy enables excellent performance at high temperatures, operation at intermediate temperatures is intrinsically limited by the solubility and diffusivity of protons. A recent alternative design strategy is based on hydrogen dissolution in oxide semiconductors. In these hydrogenation-type MIECs, proton generation and transport are governed primarily by electronic structure, allowing the coupled control of carrier density and mobility through band structure and electron delocalization. This review contrasts the two design principles that are based on defect chemistry and electronic structure, examines their respective advantages and limitations, and proposes a reverse-design strategy to create new classes of solid electrolytes. As a convergence of solid state ionics and semiconductor science, the proposed strategy offers a guiding framework for next-generation proton-conducting materials.