Hydrogen motion in oxides: from insulators to bronzes

The understanding of the diffusion of hydrogen is critical to a number of technologies, such as electrochromics, fuel cells and sensors, as well as in the study of corrosion/embrittlement phenomena. The motion of hydrogen species is reviewed here by comparing three different systems. First, the mobility of the counter-ions in the insulating naturally occurring silicates, exemplified by vermiculite, where single crystals are available, is described. Such materials have been suggested as the electrolyte for batteries and fuel cells. Second, a series of semiconducting transition metal oxides, predominantly of tungsten and molybdenum are considered. Finally, diffusion of protons in the metallic conducting tungsten bronzes is considered. In no case is ionic mobility very rapid unless a solvent such as water is present.