Turning Detrimental Effect into Benefits: Enhanced Oxygen Reduction Reaction Activity of Cobalt-Free Perovskites at Intermediate Temperature via CO₂-Induced Surface Activation

A minor amount of CO₂ in air usually causes a detrimental effect on oxygen activation over a solid oxide fuel cell (SOFC) cathode because insulating surface carbonate is easily formed, which inhibits charge transfer during the oxygen reduction reaction (ORR). In this study, we report that the detrimental effect due to the CO₂ interaction with perovskite oxide can be turned into a beneficial effect for facilitating ORR through tailoring the material composition of the perovskite. More specifically, for cobalt-free SrSc_0.025Nb_0.075Fe_0.9O_3-δ (SSNF), the exposure to the CO₂ atmosphere results in the formation of a minor amount of surface strontium carbonate mainly in the form of a nanofilm over the perovskite surface, which protects the electrode from further corrosion by CO₂, thus achieving a relatively stable performance even under a 10% CO₂-containing air atmosphere. When CO₂-free air is restored, the SrCO₃ is successfully decomposed at intermediate temperatures. As a result, the surface reaction kinetics is recovered to the initial degree while the charge transfer process is obviously improved. An area-specific resistance of only 0.07 ω cm² is achieved at 650 °C after the CO₂-induced surface activation, much smaller than the original value of 0.13 ω cm². In addition, the CO₂-treated electrode shows a fairly stable performance for ORR under a subsequent CO₂-free air atmosphere. To create such a beneficial effect, it is critical to tailor the degree of interaction of the perovskite surface with CO₂, while the benchmark Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) shows a too strong interaction with CO₂ with the formation of bulk-phase-like carbonate, which failed to decomposed even when restored with a CO₂-free atmosphere at intermediate temperatures, and as a result, worsened the ORR activity after the CO₂ treatment.