A fast ceramic mixed OH⁻/H⁺ ionic conductor for low temperature fuel cells

Low temperature ionic conducting materials such as OH⁻ and H⁺ ionic conductors are important electrolytes for electrochemical devices. Here we show the discovery of mixed OH⁻/H⁺ conduction in ceramic materials. SrZr_0.8Y_0.2O_3-δ exhibits a high ionic conductivity of approximately 0.01 S cm⁻¹ at 90 °C in both water and wet air, which has been demonstrated by direct ammonia fuel cells. Neutron diffraction confirms the presence of OD bonds in the lattice of deuterated SrZr_0.8Y_0.2O_3-δ. The OH⁻ ionic conduction of CaZr_0.8Y_0.2O_3-δ in water was demonstrated by electrolysis of both H₂¹⁸O and D₂O. The ionic conductivity of CaZr_0.8Y_0.2O_3-δ in 6 M KOH solution is around 0.1 S cm⁻¹ at 90 °C, 100 times higher than that in pure water, indicating increased OH⁻ ionic conductivity with a higher concentration of feed OH⁻ ions. Density functional theory calculations suggest the diffusion of OH⁻ ions relies on oxygen vacancies and temporarily formed hydrogen bonds. This opens a window to discovering new ceramic ionic conducting materials for near ambient temperature fuel cells, electrolysers and other electrochemical devices.