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 SrSc0.025Nb0.075Fe0.9O3-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 Ba0.5Sr0.5Co0.8Fe0.2O3-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.