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

Abstract

Rare-earth tungstates (La_28−yW_4+yO_54+δ□_2−δ) have attracted attention recently because of their relatively high proton-electron conductivity and high stability in a CO₂ environment. Since doping on the tungsten-site may increase the conductivity, a new series of compounds with composition La_5.5W_1−xM_xO_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 pO₂, pH₂O and pD₂O 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_5.5W_0.9Ti_0.1O_11.25−δ, in wet N₂ at 800 °C.