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 (SnO 2 · n H 2 O) and hydrated zirconia (ZrO 2 · n H 2 O), and their relationships were investigated. Both hydrates had many micropores with a pore radius below 1 nm. The average pore size of SnO 2 · n H 2 O was smaller than that of ZrO 2 · n H 2 O. The amount of hydrated water n was 0.7−1.2 for SnO 2 · n H 2 O and 1.0−1.7 for ZrO 2 · n H 2 O 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 SnO 2 · n H 2 O maintained a higher conductivity than that of ZrO 2 · n H 2 O under low RH. The high conductivity of SnO 2 · n H 2 O 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 SnO 2 · n H 2 O will be maintained even under low humidity (due to the high adsorption energy in the small micropore), and this was considered to result in the small dependence of conductivity on RH for SnO 2 · n H 2 O.