Substituted ceria materials for applications in solid oxide fuel cells

Abstract

Cerias, appropriately doped with trivalent rare earth ions in particular, can have high oxide ion conductivity and are attractive as both SOFC (solid oxide fuel cell) electrolytes and anodes. Here, four groups of candidate electrolyte materials were synthesised using a low temperature method in order to determine the effect of multiple doping on their microstructure and ionic conductivity. In an initial study, seven compositions of Ce₀.₈SmₓGd[sub]yNd[sub]zO₁.₉ (where x, y and z = 0.2, 0.1, 0.0667 or 0 and x + y + z = 0.2) were synthesised and the properties of multiply-doped materials were compared with the corresponding singly-doped parent materials. The effect of co-doping with Gd and Sm was investigated in more detail by preparing and studying five compositions of Ce₁₋₂ₓSmₓGdₓO₂₋ₓ (where x = 0.125, 0.1, 0.0875, 0.075 or 0.05) and seven compositions of Ce₀.₈₂₅SmₓGd₀.₁₇₅₋ₓO₁.₉₁₂₅ (where x = 0.175, 0.14, 0.105, 0.0875, 0.07, 0.035 or 0). The effect of additional doping with a divalent ion- Ca²⁺- was studied in six compositions of Ce[sub](0.825+y)Sm[sub](0.0875-y)Gd[sub](0.0875-y)Ca[sub]yO₁.₉₁₂₅ (where y = 0, 0.00875, 0.0175, 0.02625, 0.035 or 0.04375). The materials were characterised using scanning and transmission electron microscopy, inductively coupled plasma mass spectrometry and X-ray diffraction. Crystallite sizes were determined in the powders and relative densities and grain size distributions were obtained in sintered pellets. Total, bulk and grain boundary conductivities were obtained using impedance spectroscopy and corresponding activation energies and enthalpies of ion migration and defect association were calculated. The most promising material for SOFCs operating at intermediate temperatures was found to be Ce₀.₈₂₅Sm₀.₀₈₇₅Gd₀.₀₈₇₅O₁.₉₁₂₅ which had a total conductivity at 600 °C of 2.23 S m⁻¹. Lastly, doped ceria materials, primarily Ce₀.₈Sm₀.₂O₁.₉, were employed as catalytic supports for Pd and PdO nanoparticles and these were investigated as SOFC anode materials.