Investigation of oxygen reduction reaction kinetics on Sm _0.5Sr_0.5CoO_3-δ cathode supported on Ce_0.85Sm_0.075Nd_0.075O_2-δ electrolyte

Sm_0.5Sr_0.5CoO_3-δ (SSC) cathode prepared by a glycine-nitrate process (GNP) is investigated for solid oxide fuel cells (SOFCs) based on Ce_0.85Sm_0.075Nd _0.075O_2-δ (SNDC) electrolyte. SSC forms cubic perovskite structure after being annealed at 1100 °C for 5 h. SSC cathode and SNDC electrolyte can retain their own structure and there is no reaction between the two compositions. The microstructure of the cathode and the interfaces between cathodes and SNDC electrolytes are studied by scanning electron microscopy (SEM) after sintering at various temperatures. Impedance spectroscopy measurements reveal that area specific resistances (ASRs) of SSC-SNDC30 cathode are much lower than those of SSC cathode. Kinetics of oxygen reduction reaction (ORR) on porous SSC cathode is investigated by analysis of impedance spectra. Medium-frequency conductivities show no dependency on oxygen partial pressure (PO2), which can be attributed to the oxygen ions transfer across the electrode/electrolyte interface. The dependencies of low-frequency conductivities on oxygen partial pressure (PO2) vary in the range from ca. 0.31 to ca. 0.34 and increase with the increasing temperatures. The low-frequency electrode process is a mixing process involving oxygen reduction reaction related to atomic oxygen and oxygen ions conduction step together with total charge-transfer step. IR-compensated current density (i)-overpotential (η) relationship is established and the exchange current densities i_o originated from high-field approximations are much higher than those of low-field approximations and a.c. impedance data under OCV state. It demonstrates the polarization overpotential has great effect on the kinetics of ORR. The polarization current is observed to increase with time in the long-term stability measurement, which can be ascribed to the propagation process of oxygen vacancies.