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

In this paper we describe how the conductivity of a mesoporous TiO2 membran e is strongly affected by the chemistry of the pore walls. We have studied the effect of site density, state of surface protonation, and surface modif ication in samples with a fixed pore structure. Pore structure was kept fix ed 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 l eads 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 t he state of protonation was studied by pretreating wafers at pH 1.5 and equ ilibrating them with solutions at pH 2.5 and 4.0. This variable (protonatio n state of the material) was found to have an even stronger effect on condu ctivity. Surface modification was achieved by adsorbing phosphate anions fr om solutions with different pH. It was observed that even a very small degr ee of phosphate loading (0.71 ions/nm(2)) leads to an increase in conductiv ity from 8.27 x 10(-3) to 9.66 x 10(-3) Ohm(-1) cm(-1) at pH 2.5. The condu ctivity 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 membra ne in fuel cell systems. The lower cost and higher hydrophilicity of our ma terials make them potential substitutes for costlier hydrophobic polymeric membranes in fuel, cells.