Proton transport, water uptake and hydrogen permeability of nanoporous hematite ceramic membranes

For the first time, mesoporous acid-free hematite ceramic membranes have been studied as proton conductors. The xerogels after calcination at 300 degrees C for 1 h were mesoporous, as is mentioned above, with a BET surface area of 130 +/- 2 m(2) g(-1) an average pore diameter of 3.8 nm and a pore volume of 0.149 +/- 0.001 cc g(-1). A sigmoidal dependence of the conductivity and the water uptake with the RH at a constant temperature was observed. The conductivity of the ceramic membranes increased linearly with temperature for all relative humidities studied. The highest value of proton conductivity was found to be 2.76 x 10(-38) cm(-1) at 90 degrees C and 81% RH. According to the activation energy values, proton migration in this kind of materials could be dominated by the Grotthuss mechanism in the whole range of RH. The low cost and high hydrophilicity of these ceramic membranes make them potential substitutes for perfluorosulfonic polymeric membranes in proton exchange membrane (PEMFCs). In addition, since hydrogen permeability values are in the range of 10(-9) to 10(-10) mol cm(-1) s Pa, in order to fabricate oxide-based PEMs that are capable of keeping streams of H(2) and O(2) from mixing, a separation layer with pore sizes <2 nm whose pores are filled with water will be needed. (C) 2011 Elsevier B.V. All rights reserved.