Hydrogen in oxides

The paper reviews the history and present understanding of protons in oxides; their defect chemistry, thermodynamics, and transport. Focus is put on correlations between hydration thermodynamics and other materials properties which may help to predict proton uptake and proton conduction in oxides. Also effects of defect association and the particular problem of high grain boundary resistance in high temperature proton conductors are addressed. In the second part, a number of experimental observations attributed to the presence of hydride ions under mildly reducing conditions are discussed in relation to the unlikelihood that general thermodynamic considerations predict of finding these species under such conditions.