Proton conductivity: Materials and applications

In this review the phenomenon of proton conductivity in materials and the elements of proton conduction mechanisms-proton transfer, structural reorganization and diffusional motion of extended moieties-are discussed with special emphasis on proton chemistry. This is characterized by a strong proton localization within the valence electron density of electronegative species (e.g., oxygen, nitrogen) and self-localization effects due to solvent interactions which allows for significant proton diffusivities only when assisted by the dynamics of the proton environment in Grotthuss and vehicle type mechanisms. In systems with high proton density, proton/proton interactions lead to proton ordering below first-order phase transition rather than to coherent proton transfers along extended hydrogen-bond chains as is frequently suggested in textbooks of physical chemistry. There is no indication for significant proton tunneling in fast proton conduction phenomena for which almost barrierless proton transfer is suggested to occur. Models of proton conductivity are applied to specific compounds comprising oxides, phosphates, sulfates, and water-containing systems. The importance of proton conductivity is emphasized for biological systems and in devices such as fuel cells, electrochemical sensors, electrochemical reactors, and electrochromic devices.