A variety of oxides doped with elements of lower valence acquire hydroxyl-type defects when exposed, at high temperature, to atmospheres containing water vapour. Since the H of the hydroxyl groups was mobile, the oxides display protonic conductivity and may be used as electrolytes in sensors, H pumps, fuel cells, etc. The extent to which protonic defects form depended mainly upon the partial pressure of water vapour, temperature and basicity of the constituent oxides, while their mobility depended, among other factors, upon the metal–O bond length and bond energy. The defect equilibria that determine the protonic concentrations were considered, with emphasis on the regime of low O partial pressure. The measurement of the thermoelectric power and of the H+/D+ isotope effect in conductivity was proposed as a means of characterizing the conduction process.

Oxide-Based Protonic Conductors - Point Defects and Transport Properties. N.Bonanos: Solid State Ionics, 2001, 145[1-4], 265-74