Oxygen reduction reaction mechanism on PrSrCo_2-xFe_xO_5+d (x = 0, 1, 2) and Sm_0.2Ce_0.8O_1.9 composite cathodes for intermediate-temperature solid oxide fuel cells

Perovskite-type mixed ionic and electronic conductors are finding applications in intermediate-temperature solid oxide fuel cells. Here, we study the effects of substituting Co with Fe on the crystal structure and electrical and electrochemical properties of PrSrCo_2-xFe_xO_5+d (PSCF, x = 0, 1, 2). The electrochemical performance of symmetrical half-cells of PrSrCo_2-xFe_xO_5+d and Sm_0.2Ce_0.8O_1.9 composite cathodes with the Sm_0.2Ce_0.8O_1.9 electrolyte was measured using electrochemical impedance spectroscopy at 600-750 °C. The oxygen reduction reaction (ORR) mechanism was analyzed as a function of temperature and oxygen partial pressure (pO₂) using the distribution function of relaxation times (DFRT). The electrical conductivity measurements of the PSCF pellets were in the range of 100-1300 S cm^-1. The PSCF (x = 1) - Sm_0.2Ce_0.8O_1.9 composite cathode yielded the lowest area specific resistance (ASR) value of 0.07 Ω cm² at 750 °C for ORR. DFRT and pO₂ studies showed that impedance arcs corresponding to the high frequency could be attributed to the oxygen ion transfer resistance while the medium-low frequency impedance arcs could be correlated with the charge transfer resistance at the cathode/gas interface followed by ion incorporation. This study demonstrates that the partial substitution of Fe on the Co site improves the ORR activity of PSCF perovskites.