Mixed pronton–electron conducting properties of Yb doped strontium cerate

Abstract

The electrical conductivity and hydrogen permeation properties of \({\hbox{Sr}\hbox{Ce}_{0.8}\hbox{Yb}_{0.2}\hbox{O}_{3-d}}\) membranes were studied as a function of temperature and \({P_{{\rm H}_{2}}}\) gradient. The bulk conductivity of \({\hbox{Sr}\hbox{Ce}_{0.8}\hbox{Yb}_{0.2}\hbox{O}_{3-d}}\) was an order of magnitude higher than the grain boundary conductivity over the temperature range 100–250 °C in feed gas of 4% H2/balance He (pH2O = 0.03 atm). The significantly lower grain boundary conductivity indicates that larger-grained materials might be more suitable for proton transport. The hydrogen flux through the membranes is proportional to thickness down to 0.7 mm. The hydrogen permeation flux increases with an increase in \({{P_{{\rm H}_{2}}}}\) gradient where the increase in hydrogen flux was explained by an increase in electron conduction as a function of temperature. The ambipolar conductivity calculated from hydrogen permeation fluxes shows the same \({{P_{{\rm H}_{2}}}}\) and \({{P_{{\rm O}_{2}}}}\) dependence as electron concentrations. The hydrogen and oxygen potential dependence of the ambipolar conductivity (\({\log \sigma_{\rm amb} =\log P_{\rm H_2}^{1/2} }\), \({\log \sigma_{\rm amb} =\log P_{{\rm O}_{2}}^{1/4} }\)) was understood from the defect structure. From this, it was confirmed that hydrogen permeation might be limited by electron transport at wet reducing atmosphere. From the temperature dependence of the electronic conductivity, the activation energy calculated at wet reducing conditions is 0.63 eV.