High porosity silica xerogels prepared by a particulate sol-gel route: pore structure and proton conductivity

The lower cost and higher hydrophilicity of silica xerogels could make them potential substitutes for perfluorosulfonic polymeric membranes in proton exchange membrane fuel cells (PEMFCs). For that purpose, we need to obtain micro or micro and mesoporous silica xerogels with a high porosity. The preparation of micro (<2 nm) and micro and mesoporous silica xerogels (2 < d(pore) (size) less than or equal to 10 nm) from particulate as oppossed to polymeric suspensions of silica using tetraethyl orthosilicate (TEOS) as precursor is used. Two techniques of varying packing density have been performed in this work: (1) Control of the aggregation degree in the sot by adjusting its pH before gelation (pH 5, 6 and 8) and (2) Mixture of sols with a different average particle size (particles formed under acid and base catalyzed reactions). Proton conductivity of the obtained xerogels was studied as a function of temperature and relative humidity (RH). High pore volume, high porosity and small pore size SiO2 xerogels have been achieved in the calcination temperature range from 250 to 550 degreesC. The calcined xerogels showed microporosity or micro and mesoporosity in the whole range of calcination temperatures. By mixing sols (molar ratio: acid/base = 4.8) porosities up to 54.7 +/- 0.1% are achieved, at 250 degreesC of firing temperature. According to EMF measurements, electrical transport is due to protons in this kind of materials. The proton conductivity of the studied xerogels increased linear with measured temperature. A S-shaped dependence of the conductivity with the RH was observed with the greatest increase noted between 58%) and 81% RH. Xerogels with a low porosity (40.8 +/- 0.1%) and an average pore size less than 2.0 mn showed lower values of proton conductivity than that of xerogels with a higher porosity and a higher average pore size in the whole range of temperature and RH. When silica xerogels, with the highest conductivity, are treated at pH 1.5, that property increased from 2.84 x 10(-3) +/- 5.11 x 10(-5) S/cm to 4.0 x 10(-3) +/- 7.2 x 10(-5) S/cm, at 81% RH and 80 degreesC. It indicates that the surface site density of these materials has a strong effect on conductivity. Proton conductivity values achieved are less than one order of magnitude lower than that of Nafion, under the same conditions of temperature and RH. (C) 2001 Elsevier Science B.V. All rights reserved.