Proton conduction in rare-earth ortho-niobates and ortho-tantalates

Abstract

Some oxides contain sufficient equilibrium concentrations of protons in wet atmospheres to show useful proton conduction at elevated temperatures¹. As an example, Y-doped BaCeO₃ has shown promising performance as a thin-film electrolyte in fuel cells at intermediate temperatures (400–600 ^∘C)². In contrast to proton-conducting polymers (for example, Nafion(R)) and acid salts (for example, CsHSO₄), such oxidic ceramics are stable at sufficiently elevated temperatures that electrode kinetics are fast and insensitive to poisoning, but they tend to be basic (Ba-based or Sr-based) compounds with poor chemical and mechanical stability³. In search of more stable proton-conducting materials, we have investigated several acceptor-doped rare-earth ortho-niobates and ortho-tantalates, RE_1−xA_xMO₄ (M=Nb,Ta). We show that this class of materials shows mixed protonic, native ionic and electronic conduction depending on conditions. Both the low-temperature monoclinic and high-temperature tetragonal polymorphs show proton conduction. The proton conductivity is dominant in wet atmospheres below roughly 800^∘C and the highest proton conductivity of approximately 10⁻³Scm⁻¹ was found for Ca-doped LaNbO₄. These transport characteristics can be used in sensors and fuel cells provided that the electrolyte film thickness is in the micrometre range.