Chemical diffusion of water in the high-temperature proton conductor Ba3Ca1.18Nb1.82O9-δ

Ba₃Ca_1.18Nb_1.82O_9-δ (also referred to as BCN18) is a promising high-temperature proton conductor. In the dry state it contains an abundant amount of vacancies in the oxygen sublattice. Water may enter the compound occupying the vacancies and supplying protons to the lattice. The conductivity of these protons may be used in future applications of high-temperature proton conductors as electrolytes for SOFCs and in sensors. Chemical diffusion of water in this compound is described by the coupled outward flux of Ovacancies and the inward flux of protons (OH^•). The chemical diffusivity is in general a complicated function of the concentrations and the individual diffusivities. In this work, the chemical diffusivity of water is determined by studying the kinetics of a) the weight increase in TG experiments, and b) of dilatometric length changes upon subjecting the samples to humid atmospheres. The chemical diffusivities determined using techniques a) and b) are in good agreement. Likewise, the present data are compatible with those determined in a previous thermal desorption study. The present data, however, only apply to situations where the initial proton concentration is rather low and where the proton content is not changed dramatically in the experiment. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997