Mixed pronton-electron conducting properties of Yb doped strontium cerate

The electrical conductivity and hydrogen permeation properties of SrCe0.8Yb0.2O3-d membranes were studied as a function of temperature and P-H2 gradient. The bulk conductivity of SrCe0.8Yb0.2O3-d was an order of magnitude higher than the grain boundary conductivity over the temperature range 100-250 degrees C in feed gas of 4% H-2/balance He (pH(2)O = 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 PH2 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-H2 and P-O2 dependence as electron concentrations. The hydrogen and oxygen potential dependence of the ambipolar conductivity (log sigma(amb) = log P-H2(1/2), log sigma(amb) = log P-O2(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.