Effect of tri- and tetravalent metal doping on the electrochemical properties of lanthanum tungstate proton conductors

Rare-earth tungstates (La 28− y W 4+ y O 54+ δ 2− δ ) have attracted attention recently because of their relatively high proton-electron conductivity and high stability in a CO 2 environment. Since doping on the tungsten-site may increase the conductivity, a new series of compounds with composition La 5.5 W 1− x M x O 11.25− δ (M = Al, Ti and Zr; x = 0, 0.05 and 0.10) have been investigated. The crystal structure of these materials has been studied using X-ray and time-of-flight neutron powder diffraction by Rietveld analysis. The concentration of oxygen vacancies for hydration in the structure has been indirectly determined by thermogravimetric analysis, and the total conductivity in several p O 2 , p H 2 O and p D 2 O atmospheres has been studied by impedance spectroscopy. An increase in the conductivity is observed, ranging from 4.1 mS cm −1 for the undoped sample to 9.2 mS cm −1 for La 5.5 W 0.9 Ti 0.1 O 11.25− δ , in wet N 2 at 800 °C. Metal doping on lanthanum tungstates at W2 site was confirmed by TOF-NPD. TGA and EIS demonstrated proton conductivity.