Dual role of carbon in the catalytic layers of perovskite/carbon composites for the electrocatalytic oxygen reduction reaction

Abstract Perovskite oxides are promising materials for the ORR in alkaline media. However, catalytic layers prepared from perovskite powders suffer from high Ohmic losses and low catalyst utilization. An addition of carbon to the catalytic layers greatly improves the performance of the electrodes in the ORR. In this work composite thin film electrodes comprised of a perovskite oxide (either LaCoO3 or La0.8Sr0.2MnO3) and pyrolytic carbon of the Sibunit family were investigated in aqueous 1 M NaOH electrolyte using cyclic voltammetry and rotating disc electrode (RDE) method with the objective to unveil the influence of carbon on the catalyst utilization and on the ORR electrocatalysis. By systematically varying the oxide to carbon ratio we arrive to the conclusion on the dual role of carbon in composite electrodes. On the one hand, it is required to improve the electrical contact between perovskite particles and the current collector, and to ensure maximum utilization of the perovskite surface. On the other hand, carbon plays an active role in the ORR by catalyzing the O2 reduction to H2O2. Composite electrodes catalyze the 4e− ORR in contrast to carbon which is only capable of catalyzing the 2e− reduction. For LaCoO3 composite electrodes, carbon is responsible for the catalysis of the first steps of the ORR, the role of LaCoO3 being largely limited to the hydrogen peroxide decomposition and/or reduction. For La0.8Sr0.2MnO3 composite electrodes, along with the catalysis of the chemical decomposition and/or reduction of H2O2 produced on carbon, the perovskite also significantly contributes to the first steps of the ORR. The results of this work suggest that the ORR on the carbon and the oxide components of composite cathodes must be considered as coupled reactions whose contributions cannot be always separated, and that neglecting the contribution of carbon to the ORR electrocatalysis may lead to erroneous values of the catalytic activity of perovskite materials.