Development of perovskite and intergrowth oxide cathodes for intermediate temperature solid oxide fuel cells
Solid oxide fuel cells (SOFC) offer the advantage of using less expensive oxide catalysts and hydrocarbon fuels directly, but chemical reactivity and thermal expansion mismatch at the conventional operating temperature of ~ 1000 o C pose serious problems. These difficulties have generated considerable interest in intermediate temperature (500-800 o C) SOFC, but the lower temperature leads to poor oxygen reduction reaction kinetics with the conventional cathode material, La1- xSrxMnO3. To address this issue, this dissertation focuses on the synthesis and characterization of alternative cathode materials based on Ln1-xSrxCoO3-δ perovskites and La3-xSrxFe2-yCoyO7-δ and LaSr3Fe3-yCoyO10-δ intergrowth oxides. Both the electrical conductivity and the oxide ion vacancy concentration decrease from Ln = La to Gd in Ln1-xSrxCoO3-δ, which leads to a decrease in the electrocatalytic activity for the oxygen reduction reaction. However, the thermal expansion coefficient (TEC) decreases from Ln = La to Gd due to a decreasing ionicity of the Ln-O bond and a suppression of the tendency to lose oxygen from the lattice. Therefore, Nd1-xSrxCoO3-δ with an intermediate size lanthanide ion offers a tradeoff between electrocatalytic activity and TEC, with the x = 0.4 sample exhibiting the highest catalytic activity without any interfacial reaction. The substitution of Fe or Mn for Co in Nd0.6Sr0.4CoO3-δ leads to a decrease in the oxygen non-stoichiometry, TEC, electrical conductivity, and electrocatalytic activity, but the decrease in catalytic activity is rapid with the Mn-doped system due to a faster decrease in the oxide ion and electronic conductivities. Interestingly, the incorporation of metallic Ag into porous Nd0.6Sr0.4Co0.5Fe0.5O3-δ improves the electrochemical performance due to an increased electronic conductivity and enhanced electrocatalytic activity. The electrical conductivity, oxygen vacancy concentration, TEC, and electrocatalytic activity increase with increasing Co content in the perovskite-related intergrowth oxide systems, LaSr3Fe3-yCoyO10-δ and Sr3-xLaxFe2-yCoyO7-δ. The increase in catalytic activity is due to an increase in the electronic and oxide ion conductivities. The intergrowth LaSr3Fe3-yCoyO10-δ cathodes offer electrochemical performances comparable to that of the well-known La0.6Sr0.4CoO3-δ cathode, but with an important advantage of significantly lower TEC.