On the complexity of proton conduction phenomena

The proton transport mechanisms in systems containing water and in cubic perovskite-type oxides are analyzed in detail in terms of chemical interactions. Some features of proton transport in hydroxides and systems containing oxo-acid anions or heterocycles as proton solvents are also described. Despite the differences between the rather complex proton conduction mechanisms in these systems, significant structural and dynamical variations of hydrogen bond interactions as a result of a good balance of chemical interactions in a wide range of configuration space have been identified as common features. These allow for high rates of proton transfer and structural reorganization, the two reactions involved in long-range proton transport. From the analyses and additional information on mesoscopic effects, suggestions are also made for the optimization of the mobility of protonic defects in hydrated, acidic polymers and perovskite-type oxides, which are interesting as separator materials for fuel cells. For the former, the importance of the microstructure is emphasized, while, for the latter, the prospects for III–III perovskites are stressed.