Structural and electrical properties of Bi₃Y_0.9W_0.1O_6.15-La_0.8Sr_0.2MnO₃ (BiYWO-LSM) composites

Electrical conductivity and structural behavior in Bi₃Y_0.9W_0.1O_6.15-La_0.8Sr_0.2MnO₃ (BiYWO-LSM) composite cathodes for intermediate temperature solid-oxide fuel cells (IT-SOFCs) have been investigated. The ionically conducting component (BiYWO) was selected as it not only exhibits high oxide ion conductivity at intermediate temperatures, but also shows good long-term stability. The LSM component is a well-known electronic conductor, commonly used for SOFC cathodes and also shows high interfacial polarization effects. Both composite components exhibit similar thermal expansion coefficients. Composites of different molar ratios of components were prepared by sintering the components together at 850 °C. Electrical behavior was studied by a.c. impedance spectroscopy and transference number measurements were used to determine the ionic and electronic contributions to total conductivity. Percolation type behavior was observed for total conductivity and the percolation threshold value was determined to be in the bismuth rich region of the compositional range, due to the significant difference in the grain sizes of the two components. Ionic and electronic components of the total conductivity at, above and below the percolation threshold are discussed. Stability measurements on a composition close to the percolation threshold show some degree of conductivity decay on prolonged annealing at 650 °C, which is partly recoverable on heating to higher temperatures.