Electric conductivities of PbWO
4- and PbMoO
4-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
PO2 down to 10
−12 atm, pure oxide ion conduction was maintained in the measured
PO2 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
PO2 with increasing molybdenum content. Comparing the two types of substitutions,
i.e. Pb
1−xLa
xMO
4+x/2 and Pb
1−xLa
2x/3MO
4 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 CH
4 gas as the fuel.
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