Composite Electrode Boosts the Activity of Ba0.5Sr0.5Co0.8Fe0.2O3-δ Perovskite and Carbon toward Oxygen Reduction in Alkaline Media

Herein we report the electrochemical activity and selectivity toward the oxygen reduction reaction (ORR) for composite electrodes made of Ba0.5Sr0.5Co0.8Fe0.2O3-delta (BSCF) perovskite oxide and Acetylene black (AB) carbon in alkaline media. The onset potential and the selectivity, i.e., hydroperoxide formation, toward the ORR exhibit a volcano type behavior as a function of the electrode composition. The HO2- formation measured by rotating ring disk electrode technique decreases from about 60% and 70% for the BSCF and the AB, respectively, to 28% for the electrode having a BSCF/AB weight ratio of 1.25 (values taken at 0.4 V vs RHE). Accordingly, the value for the overall transferred electrons is significantly larger (around 3.5) for the composite electrode compared to individual materials. Therefore, the overall results point toward a beneficial interaction between BSCF and AB. Different scenarios are considered; first an improved electrical connection of BSCF particle agglomerates upon carbon addition. However, the latter cannot fully explain the significant improvement in ORR activity and selectivity for the composite electrodes. By measuring the BSCF electrochemical activity toward HO2- reduction, we can exclude the occurrence of a synergistic ORR process where oxygen is first reduced on AB to OH2-, and then further electroreduced to OH- by the BSCF. Besides the activity toward hydroperoxide electroreduction, the potential of BSCF as catalyst for the hydroperoxide disproportionation reaction is also investigated, and even though a certain catalytic activity exists, the perovskite oxide shows rather low decomposition rate. These results might suggest that ligand (electronic) effects between the two materials also play a role in the enhanced ORR activity of the composite electrodes.