Conduction Property of PbWO₄- and PbMoO₄-based Oxide Ion Conductors in Lower Oxygen Partial Pressures

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Electric conductivities of PbWO₄- and PbMoO₄-based oxide ion conductors have been measured in the reducing atmospheres to investigate the transport properties and to evaluate the applicability to the solid electrolytes of fuel cells. Although almost all the solid solutions showed conductivity enhancements due to appearance of n-type electronic conduction with decreasing P_O2 down to 10⁻¹² atm, pure oxide ion conduction was maintained in the measured P_O2 range of ∼10⁻¹⁸ atm at 900°C only for Pb_1−xLa_xWO_4+x/2 system. For the tungstate and molybdate mixed crystal systems, furthermore, electronic contribution appears at higher P_O2 with increasing molybdenum content. Comparing the two types of substitutions, i.e. Pb_1−xLa_xMO_4+x/2 and Pb_1−xLa_2x/3MO₄ systems (M=W, Mo and mixed one), the former system tended to keep oxide ions in lattice rather than the latter, whereas the former contains excess oxide ions at the interstitial sites. At the end of this paper, test fuel cells have been constructed using Pb_1−xLa_xWO_4+x/2 solid electrolyte and operated at 500°C employing wet CH₄ gas as the fuel.