Proton-Conducting Properties and Microstructure of Hydrated Tin Dioxide and Hydrated Zirconia

The microstructure (specific surface area and pore size distribution), amount of hydrated water, and proton conductivity were examined for hydrated tin dioxide (SnO2·nH2O) and hydrated zirconia (ZrO2·nH2O), and their relationships were investigated. Both hydrates had many micropores with a pore radius below 1 nm. The average pore size of SnO2·nH2O was smaller than that of ZrO2·nH2O. The amount of hydrated water n was 0.7−1.2 for SnO2·nH2O and 1.0−1.7 for ZrO2·nH2O under a relative humidity (RH) of ∼0 to 95% at 150 °C. Proton conductivity was approximately 10-2 S cm-1 under 95% RH at 150 °C for both hydrates. The conductivity decreased with decreasing RH, but SnO2·nH2O maintained a higher conductivity than that of ZrO2·nH2O under low RH. The high conductivity of SnO2·nH2O at high RH despite low hydrated water content was assumed to be due to the high electronegativity of Sn and resulting high concentration of dissociated protons. The hydrated water in the small micropores of SnO2·nH2O will be maintained ev...