Synthesis and Characterization of Nonsubstituted and Substituted Proton-Conducting La6-xWO12-y

Mixed proton-electron conductors (MPEC) can be used as gas separation membranes to extract hydrogen from a gas stream, for example, in a power plant. From the different MPEC, the ceramic material lanthanum tungstate presents an important mixed protonic-electronic conductivity. Lanthanum tungstate La6-xWO12-y (with y = 1.5x + delta and x = 0.5-0.8) compounds were prepared with La/W ratios between 4.8 and 6.0 and sintered at temperatures between 1300 and 1500 degrees C in order to study the dependence of the single-phase formation region on the La/W ratio and temperature. Furthermore, compounds substituted in the La or W position were prepared. Ce, Nd, Tb, and Y were used for partial substitution at the La site, while Ir, Re, and Mo were applied for W substitution. All substituents were applied in different concentrations. The electrical conductivity of nonsubstituted La6-xWO12-y and for all substituted La6-xWO12-y compounds was measured in the temperature range of 400-900 degrees C in wet (2.5% H2O) and dry mixtures of 4% H-2 in Ar. The greatest improvement in the electrical characteristics was found in the case of 20 mol % substitution with both Re and Mo. After treatment in 100% H-2 at 800 degrees C, the compounds remained unchanged as confirmed with XRD, Raman, and SEM.