PROTON CONDUCTIVITY - MATERIALS AND APPLICATIONS

In this review the phenomenon of proton conductivity in materials and the elements of proton conduction mechanisms-proton transfer, structur al reorganization and diffusional motion of extended moieties-are disc ussed with special emphasis on proton chemistry. This is characterized by a strong proton localization within the valence electron density o f electronegative species (e.g., oxygen, nitrogen) and self-localizati on effects due to solvent interactions which allows for significant pr oton diffusivities only when assisted by the dynamics of the proton en vironment in Grotthuss and vehicle type mechanisms. In systems with hi gh proton density, proton/proton interactions lead to proton ordering below first-order phase transition rather than to coherent proton tran sfers along extended hydrogen-bond chains as is frequently suggested i n textbooks of physical chemistry. There is no indication for signific ant proton tunneling in fast proton conduction phenomena for which alm ost barrierless proton transfer is suggested to occur. Models of proto n conductivity are applied to specific compounds comprising oxides, ph osphates, sulfates, and water-containing systems. The importance of pr oton conductivity is emphasized for biological systems and in devices such as fuel cells, electrochemical sensors, electrochemical reactors, and electrochromic devices.