Proton Conduction and Fuel Cell Using the CuFe-Oxide Mineral Composite Based on CuFeO₂ Structure

High ionic conductivity has attracted considerable attention for use in energy applications, such as solid oxide fuel cells (SOFCs). Novel proton conductivity was discovered in a delafossite-based CuFe-oxide composite from a natural mineral. Low-temperature (400-550 °C) SOFCs (LTSOFCs) using the natural CuFe-oxide composite as an electrolyte were demonstrated. Power densities of 775 mW cm^-2 were achieved at 550 °C for the CuFeO₂ electrolyte, 423 mW cm^-2 for the CuFe-oxide from the natural mineral, and 672 mW cm^-2 by further introducing proton-dominating conduction into the CuFe-oxide natural material. We found that the CuFeO₂ delafossite structure played a key role in high ionic conductivity. Theoretical calculations indicated that enhanced proton transport through the Cu-O layer was associated with a low activation energy (0.23-0.26 eV) and rapid transport kinetics, which were consistent with the experimental results. Both theoretical and experimental results demonstrated that the CuFeO₂ delafossite material possessed advantages for high ionic conductors and LTSOFCs. Fundamental studies and a deeper scientific understanding from this work provide a new theoretical approach to design novel material families and functions based on natural mineral materials for ionic conductors and advanced applications, resulting in a new generation of LTSOFCs.