Transport number measurements and fuel cell testing of undoped and Mo-substituted lanthanum tungstate

The partial protonic, oxide ion and electronic conductivities of lanthanum tungstate (La28−xW4+xO54+δ with x = 1, LWO54) and 30% molybdenum substituted lanthanum tungstate (Mo-LWO54) have been extracted using the EMF method. LWO54 is a relatively pure ionic conductor up to ∼800 °C; above that temperature electrons start to contribute significantly (under reducing conditions). The maximum protonic conductivity is ∼2-3 × 10−3 S cm−1, and protonic conductivity dominates the ionic regime under wet conditions below ∼700 °C. Above this temperature, oxide ion conductivity dominates the ionic contribution. The protonic and oxide ion conductivities for Mo-substituted LWO54 are quite similar to the Mo-free material, while the electronic conductivity is almost one order of magnitude higher for the former in wet 5% H2/Ar. From this report is clear that nominally undoped lanthanum tungstate is most suited for applications as electrolyte in proton conducting SOFCs, while Mo-LWO54 presents improved mixed proton-electron conductivity for use as dense membrane for hydrogen separation.