The Effects of Sr Content on the Performance of Nd_1-SrCoO_3-δ Air-Electrode Materials for Intermediate Temperature...

The potential of the perovskite system Nd_1-SrCoO_3-δ (x = 1/3 and 2/3) as cathode material for solid oxide fuel cells (SOFCs) has been investigated via detailed structural, electrical, and electrochemical characterization. The average structure of x = 1/3 is orthorhombic with a complex microstructure consisting of intergrown, adjacent, perpendicularly oriented domains. This orthorhombic symmetry remains throughout the temperature range 373-1073 K, as observed by neutron powder diffraction. A higher Sr content of x = 2/3 leads to stabilization of the cubic perovskite with a homogeneous microstructure and with a higher oxygen vacancy content and cobalt oxidation state than the orthorhombic phase at SOFC operation temperature. Both materials are p-type electronic conductors with high total conductivities of 690 and 1675 S·cm^-1 at 473 K in air for x = 1/3 and 2/3, respectively. Under working conditions, both compounds exhibit similar electronic conductivities, since x = 2/3 loses more oxygen on heating than x = 1/3, associated with a greater loss of p-type charger carriers. However, composite cathodes prepared with Nd_1/3Sr_2/3CoO_3-δ and Ce_0.8Gd_0.2O_2-δ present lower ASR values (0.10 Ω·cm² at 973 K in air) than composites prepared with Nd_2/3Sr_1/3CoO_3-δ and Ce_0.8Gd_0.2O_2-δ (0.34 Ω·cm²). The high activity for the oxygen electrochemical reaction at intermediate temperatures is likely attributable to a large disordered oxygen-vacancy concentration, resulting in a very promising SOFC cathode for real devices.