Performance of SrSc_0.2Co_0.8O_3-δ + Sm_0.5Sr_0.5CoO_3-δ mixed-conducting composite electrodes for oxygen reduction at intermediate temperatures

In order to improve the electrical conductivity of the SrSc_0.2Co_0.8O_3-δ (SrScCo) electrode, a composite of 70 wt% SrSc_0.2Co_0.8O_3-δ and 30 wt% Sm_0.5Sr_0.5CoO_3-δ (SrScCo + SmSrCo) was prepared and investigated for electrochemical oxygen reduction at intermediate temperatures. The phase reaction between SrScCo and SmSrCo and its effect on the electrical conductivity, oxygen vacancy concentration and oxygen mobility were examined by XRD, 4-probe DC conductivity measurement, iodometry titration and O₂-TPD experiment, respectively. The results showed that the composite reached a maximum conductivity around 123 S cm^-1 at 600 °C, nearly five times that of SrScCo. AC impedance results showed that the electron charge-transfer process was greatly improved by forming the composite electrode, while the oxygen-ion charge-transfer process was somewhat deteriorated. By firing at 1000 °C for 2 h, a SOFC with the SrScCo + SmSrCo cathode and thin-film SDC electrolyte delivered peak power densities of 1100 and 366 mW cm^-2 at 600 and 500 °C, respectively, which were only modestly lower than those of a similar cell with a pure SrScCo cathode.