Effect of pore-wall chemistry on proton conductivity in mesoporous titanium dioxide

In this paper we describe how the conductivity of a mesoporous TiO2 membrane is strongly affected by the chemistry of the pore walls. We have studied the effect of site density, state of surface protonation, and surface modification in samples with a fixed pore structure. Pore structure was kept fixed by firing all samples at the same temperature. Changing the surface site density (number of water molecules per square nanometer) from 5.5 to 5.7 leads to an increase in conductivity from 8.00 x 10(-3) to 1.00 x 10(-2) Ohm(-1) cm(-1) at 25 degrees C and 81% relative humidity (RH). The effect of the state of protonation was studied by pretreating wafers at pH 1.5 and equilibrating them with solutions at pH 2.5 and 4.0. This variable (protonation state of the material) was found to have an even stronger effect on conductivity. Surface modification was achieved by adsorbing phosphate anions from solutions with different pH. It was observed that even a very small degree of phosphate loading (0.71 ions/nm(2)) leads to an increase in conductivity from 8.27 x 10(-3) to 9.66 x 10(-3) Ohm(-1) cm(-1) at pH 2.5. The conductivity of our materials, especially those treated at pH 1.5, is very close to that of Nafion, a polymeric material used as a proton conducting membrane in fuel cell systems. The lower cost and higher hydrophilicity of our materials make them potential substitutes for costlier hydrophobic polymeric membranes in fuel, cells.