Enhanced low-temperature proton conductivity in hydrogen-intercalated brownmillerite oxide

Solid oxide ionic conductors are employed in a wide range of energy-conversion applications, such as electrolytes in fuel cells. Typically, conventional ionic conductors based on metal oxides require elevated temperatures above approximately 500 °C to activate ionic transport, but the ability to operate at lower temperature could avoid mechanical instability and operating complexities. Here we report a solid oxide proton conductor, HSrCoO_2.5, which shows unusually high proton conductivity between 40 °C and 140 °C. The proton conductivity was between 0.028 S cm⁻¹ to 0.33 S cm⁻¹ in this temperature range, with an ionic activation energy of approximately 0.27 eV. Combining experimental results and first-principles calculations, we attribute these intriguing properties to the high proton concentration and the well-ordered oxygen vacancy channels granted by the hydrogen-intercalated brownmillerite crystalline structure. Our results open the possibility of using solid oxide materials as the proton-conducting electrolytes in low-temperature devices.