Mesoporous α-Fe2O3 membranes as proton conductors: Synthesis by microwave-assisted sol–gel route and effect of their textural characteristics on water uptake and proton conductivity

Abstract

This paper reports novel inorganic proton exchange electrolytes based on mesoporous α-Fe2O3 ceramic membranes. The unsupported mesoporous hematite proton ceramic membranes with high specific surface areas, high pore volumes, and narrow pore size distributions have been synthesized from a hydrolytic ferric oxide polymer prepared by a microwave-assisted sol–gel route. The effect of their textural characteristics on water uptake and proton conductivity has been studied.Microwave heating allows us to obtain homogeneous hydrosols at short times (2s) meanwhile conventional heating gives rise to inhomogeneities in the hydrosol independently of heating times. Electron diffraction TEM observations show that the xerogels calcined at 200°C are polycrystalline in nature and correspond to α-Fe2O3 in accordance with the two characteristic vibrations in hematite observed by FTIR spectroscopy. According to EMF measurements, proton transport is observed in these ceramic membranes and shows an Arrhenius dependence on temperature for all relative humidities studied. A sigmoidal dependence of the water uptake and the proton conductivity with the RH at a constant temperature was observed with the greatest increase detected between 58% and 81% RH. The pore volume and the average pore size of the α-Fe2O3 mesoporous ceramic membranes seem to be the main factors which influence the water uptake and consequently the proton conductivity in the studied ranges. The membranes with the largest pore volume, and the largest pore size have the highest water uptake and in turn the highest values of proton conductivity in the whole range of relative humidities (RHs). 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.