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

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, RE1−xAxMO4 (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.