Theoretical Studies on Proton Transfer among a High Density of Acid Groups: Surface of Zirconium Phosphate with Adsorbed Water Molecules

To examine the origin of the high proton conductivity of hybrid electrolytes, composites of an inorganic grain and an organic electrolyte polymer, the reaction mechanism for proton transfer at the surface of water-adsorbed zirconium phosphate, α-Zr(HPO4)2·H2O (ZrP), has been theoretically investigated as a first step in our research. Reaction paths and activation energies, which determine the proton conductivity, are examined by quantum chemistry calculations. In particular, the effects of adsorbed water molecules and phosphate groups on the proton conductivity are analyzed by examining the interaction energies between the phosphate groups and the adsorbed water molecules, the charges of the protons and oxygen atoms and also the O−O distances along the proton-transfer paths. As a result, it has been clearly shown that the interactions between the phosphate groups at the ZrP surface and the adsorbed water molecules are relatively large and a strong hydrogen-bond network is generated locally. Because of the strong interactions, water molecules can be attached to the ZrP surface and the O−O distance becomes shorter than that in bulk water systems. Because of the short O−O distances and the delocalized charge of each atom, the activation energy of proton transfer at the ZrP surface decreases and causes high proton conductivity even under conditions of high temperature and low humidity. On the basis of the above studies, the origin of the high proton conductivity of hybrid electrolytes is also discussed.