The optimisation of mixed conduction in potential S.O.F.C. anode materials

The success of the SOFC rests heavily on materials selection. In this work we address the optimisation of mixed conductivity in fluorite compounds in the search for new improved SOFC anodes based upon oxides. The mobility of electronic carriers is considered to be much higher than that of ionic defects, therefore, doping a good ionic conductor with a small concentration of reducible transition metal ions can form promising mixed conductors. Zirconia based mixed conductors were studied for two reasons. Firstly, zirconia, stabilised in the defect fluorite structure, exhibits a high level of ionic conductivity. Secondly it is the most common electrolyte material for a S.O.F.C. An anode based on zirconia would therefore be expected to offer a good physical compatibility with the electrolyte material and to exhibit a high ionic contribution to total conductivity. Work on the system ZrO₂-Nb₂O₅-Y₂O₃ showed that the influence of composition on conduction could be determined. This enabled the optimisation of both the electronic and ionic contributions to conduction by compositional selection. These factors were extended to explain conductivity behaviour observed in the comparable system ZrO₂-TiO₂-Y₂O₃. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.