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The success of Solid Oxide Fuel Cells (S.O.F.C) rests heavily on material selection. The performances of several compounds were investigated as possible anode materials, starting with reduced titanates such as the magnesium titanate and zirconium titanate. These compositions, although possessing many qualities beneficial for use as an anode material, were found to be too unstable for practical use. For this reason further work concentrated on stable, zirconia based, compounds with exhibited mixed conduction under reducing atmospheres. The mobility of electronic carriers is considered to be much higher than that of ionic defects, therefore, promising mixed conductors can be formed by doping a good ionic conductor with a small concentration of transition metal ions. Zirconia based mixed conductors were studied for two reasons. Firstly, zirconia stabilised in the cubic defect fluorite structure, exhibits a high level of ionic conductivity. Secondly, it is the most common electrolyte material for an 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. Large defect fluorite solid solutions in the systems Y2O3-ZrO2-Nb2O5, Yb2O3-ZrO2-Nb2O5 and CaO-ZrO2-Nb2O5 were established, which enabled the effects of composition, dopant size and charge on conduction to be investigated. These effects were shown to be linked to structure. From these results and comparisons with the Y2O3-ZrO2-TiO2 system, optimum, mixed conducting, compositions were established. The sample Y0.25Ti0.15Zr0.60O1.875 exhibited the best mixed conducting properties to date, obtained for compositions based on zirconia. |
