Oxide-based protonic conductors: point defects and transport properties

A variety of oxides doped with elements of lower valence acquire hydroxyl-t ype defects when exposed, at high temperature, to atmospheres containing wa ter vapour. Since the hydrogen of the hydroxyl groups is mobile, the oxides display protonic conductivity and may be used as electrolytes in sensors, hydrogen pumps, fuel cells, etc. The extent to which protonic defects form depends mainly on the partial pressure of water vapour, temperature and bas icity of the constituent oxides, while their mobility depends, among other factors, on the metal-oxygen bond length and bond energy. The defect equili bria that determine the protonic concentrations are considered, with emphas is on the regime of low oxygen partial pressure. The measurement of the the rmoelectric power (TEP) and of the H+/D+ isotope effect in conductivity are discussed as a means of characterising the conduction process. (C) 2001 El sevier Science B.V. All rights reserved.