Thermionic emission of protons across a grain boundary in 5 mol% Y-doped SrZrO3, a hydrogen pump

Given the fact that polycrystalline SrZrO3 doped with 5 mol% Y3+ (p-SZY5), a proton-conducting solid electrolyte with superior chemical stability, has attracted considerable attention over the past years due to its potential for various applications including hydrogen pumps, it is rather unusual that such little information is available in literature regarding the electrical nature of the grain boundary in this material, although the grain boundary may determine its proton conductivity. Here we report the results of our investigation on the conduction mechanism at the grain boundary in p-SZY5. We clearly demonstrate that the current–voltage (I–V) characteristic of the grain boundary is consistent with thermionic emission current over a back-to-back Schottky barrier, suggesting that such a potential barrier is present at the grain boundary in p-SZY5 to limit the internal proton current. More importantly, based on the capacitance–voltage relation, we were able to deduce the values of both the potential-barrier height and the dopant concentration in p-SZY5.

► We have investigated the I–V relations of grain boundaries in 5 mol% Y-doped SrZrO3. ► The current increases with the applied dc-bias in a non-linear manner. ► The ohmic-superohmic transition occurred where the bias is higher than Vth. ► The Schottky-type potential barrier at the grain boundary limits the proton current. ► The barrier height was determined from the capacitant–voltage relation.