Oxides with Mixed Protonic and Electronic Conductivity

Oxides with mixed protonic and p-type electronic conductivity (and typically containing also mobile oxygen vacancies) are important functional materials, e.g., for oxygen electrodes in protonic ceramic electrochemical cells or for permeation membranes. Owing to the presence of three carriers, their defect chemical behavior is complex. Deviations from ideal behavior (defect interactions) have to be taken into account, which are related to the partially covalent character of the transition metal–oxygen bonds. Compared to acceptor-doped Ba(Zr,Ce)O3− z electrolytes, perovskites with redox-active transition-metal cations typically show smaller degrees of hydration. Trends in the proton uptake of (Ba,Sr,La)(Fe,Co,Y,Zn)O3−δ perovskites are analyzed and correlated to structural features (local lattice distortions) and electronic properties (the position of oxygen states on an absolute energy scale). The proton mobility in such mixed-conducting perovskites is estimated. Specific aspects of the application of protonic and electronic mixed-conducting oxides in protonic ceramic electrochemical cells are discussed, and an overview of recent materials and device developments is given.