Improved thermal expansion and electrochemical performance of La1−xSrxFe0.7Ni0.3O3−δ cathodes for intermediate-temperature SOFCs

Abstract

Perovskite oxides of composition La1−xSrxFe0.7Ni0.3O3−δ (LSFN-x, 0 ≤ x ≤ 0.4) were evaluated as candidate cathode materials for intermediate-temperature solid oxide fuel cells (IT-SOFCs). Their crystal structures, electrical conductivity, thermal expansion coefficient (TEC), and electrochemical performance were investigated as a function of their Sr content. An orthorhombic structure was obtained when 0.3 equivalents of Ni were substituted at the Fe sites, resulting in LaFe0.7Ni0.3O3−δ, and the crystal structure was transformed from orthorhombic to hexagonal upon the substitution of Sr at La sites. Sr doping was found to increase the TEC of the LSFN material, resulting in a good thermal match with the commonly used Gd0.1Ce0.9O2−δ (GDC) electrolyte. The LSFN-0.2 also exhibited good chemical compatibility with the GDC electrolyte at temperatures <1000 °C. A gradual increase in electrical conductivity with increasing Sr content was observed because of the increase in the concentration of electronic defects. The cathode electrochemical performance was investigated by AC impedance spectroscopy. Among the LSFN series, LSFN-0.2 exhibited the lowest polarization resistance value of 0.065 Ω cm2 at 800 °C. The substitution of Sr can promote charge transfer and the adsorption/dissociation of oxygen molecules, thereby decreasing the polarization resistance of the LSFN material. X-ray photoelectron spectroscopy measurements were conducted to elucidate the cause of the performance improvement. The results indicated that the LSFN-0.2 material has potential applications as cathode material for IT-SOFCs.