Assessment of layered La2-x(Sr,Ba)xCuO4-δ oxides as potential cathode materials for SOFCs.

In this paper, selected layered cuprates with La 2-x (Sr,Ba) x CuO 4-δ formula are evaluated as candidate cathode materials for Solid Oxide Fuel Cells. Two synthesis routes, a typical solid state reaction and a sol-gel method yield well-crystallized La 1.5 Sr 0.5 CuO 4-δ , La 1.6 Ba 0.4 CuO 4-δ and La 1.5 Sr 0.3 Ba 0.2 CuO 4-δ materials having tetragonal I 4/ mmm space group, but differing in morphology of the powder. Fine powders obtained using sol-gel route seem to be more suitable for preparation of the porous cathode layers having good adhesion on the solid electrolyte, but powders obtained after the solid state route can be also successfully utilized. Investigations of structural and transport properties, the oxygen nonstoichiometry and its change with temperature, thermal expansion, as well as chemical and thermal stability are systematically performed, to evaluate and compare basic physicochemical properties of the oxides. At room temperature the average valence state of copper is found to be in 2.2–2.35 range, indicating oxygen deficiency in all of the compounds, which further increases with temperature. The conducted high-temperature X-ray diffraction tests reveal moderate, but anisotropic thermal expansion of La 2-x (Sr,Ba) x CuO 4-δ , with higher expansion at temperatures above 400 °C occurring along a- axis, due to the oxygen release. However, the corresponding chemical expansion effect is small and the materials possess moderate thermal expansion in the whole studied temperature range. All compounds show relatively high electrical conductivity at the elevated temperatures, related to the Cu 2+ /Cu 3+ charge transfer, with the highest values recorded for La 1.5 Sr 0.5 CuO 4-δ . Comprehensive studies of chemical stability of the selected La 1.5 Sr 0.5 CuO 4-δ material with La 0.8 Sr 0.2 Ga 0.8 Mg 0.2 O 3-δ solid electrolyte revealed complex behavior, with stability being dependent apart from temperature, also on morphology of the powders. A model describing such behavior is presented. While it is possible to minimize reactivity and characterize electrochemical properties of the La 1.5 Sr 0.5 CuO 4-δ -based cathode layer, usage of the buffer layer is indispensable to maintain full stability. It is shown that mutual chemical compatibility of La 1.5 Sr 0.5 CuO 4-δ and commonly used La 0.4 Ce 0.6 O 2-δ buffer layer material is excellent, with no reactivity even at 1000 °C for prolonged time. Laboratory-scale fuel cell with the La 1.5 Sr 0.5 CuO 4-δ cathode sintered at the optimized temperature is able to deliver 0.16 W cm −2 at 800 °C while fueled with wet hydrogen. [ABSTRACT FROM AUTHOR]