Effect of Pore-Wall Chemistry on Proton Conductivity in Mesoporous Titanium Dioxide

Abstract

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 × 10-3 to 1.00 × 10-2 Ω-1 cm-1 at 25 °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/nm2) leads to an increase in conductivity from 8.27 × 10-3 to 9.66 × 10-3 Ω-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.

Proton conductivity in nanopore silica xerogels

“…There is, therefore, a need for new proton-conducting materials in order to improve the commercialization of PEMFCs [4][5][6]. Future materials should: provide high proton conductivity at low temperature; be hydrophilic and lonics 9 (2003) mechanically, thermally, and chemically, stable: be impermeable to H: and O~.…”

Nanoporous Anatase Thin Films as Fast Proton‐Conducting Materials

“…[6][7][8][9][10][11][12] However, other types of xerogels and porous ceramic membranes, such as c-Al 2 O 3 or anatase, could also, in principle, display high values of proton conductivity. Until now, these materials have been prepared and electrically characterized only as bulk materials, [12][13][14] with silica being the lone exception. [10] For use in PEMFCs, potential electrolyte materials should provide high proton conductivity at low temperature, be hydrophilic, and display mechanical, thermal, and chemical stability.…”

“…The similar values of proton conductivity, lower cost, and higher hydrophilicity [13] of nanoporous anatase films make them potential substitutes for Nafion membranes in PEMFCs.…”

Mesoporous Transition Metal Oxide Ceramics

“…Crepaldi and co-workers then grafted organic molecules onto mesoporous zirconia films using dibenzoylmethane, phenylphosphonic acid, and ferrocenecarboxylic acid [83]. The modification of mesoporous titania surfaces with orthophosphates was also reported in the same year [84]. Subsequent research oriented toward applications in drug delivery, acid-base catalysis, and electrochemical sensors using transition metal oxides Fe 2 O 3 and Al 2 O 3 then followed [85][86][87].…”