Structural and transport properties of lanthanum tungstate with high La/W ratio: Suitability for proton-conducting solid oxide fuel cells operating at intermediate temperature

Systematic characterization of lanthanum tungstate (LWO) with a high La/W ratio was conducted to evaluate its suitability as a proton-conducting solid electrolyte for fuel cells operating at intermediate temperature. In this study, the crystal structure, ionic and electronic transport properties of LWO with a high La/W ratio (La/W = 6.7: LWO67), which had high proton conductivity, were investigated. Rietveld analysis using synchrotron powder X-ray diffraction confirmed the single-phase formation of LWO67 having a cubic crystal structure with the Fm3¯m space group. The coefficient of thermal expansion of LWO67 was also evaluated from Rietveld analysis at high temperatures. Thermogravimetric analysis indicated a good chemical stability of LWO67 under wet CO2 conditions. The observed electrical properties indicated that the proton conductivity was dominant under wet conditions below 873 K. The dependence of the electrical conductivity on oxygen partial pressure suggested that the electron and hole conductivities were suppressed sufficiently below 973 K. Based on the observed transport properties, the superiority of LWO67 applied to a proton-conducting solid electrolyte fuel cells (p-ITFCs) was also discussed. The optimized electrolyte thickness of LWO67 was several tens of micrometers, which is more suitable than perovskite-type proton conductors such as BaZrO3 and BaCeO3 for electrode-supported fuel cells, when they are compared in terms of power generation efficiency.