Bifunctional oxygen electrocatalysts have great potentials for energy storage and conversion applications. Here we report a facile strategy toward bifunctional electrocatalysts by supporting mixed platinum and cobalt oxide nanoparticles on carbon black. The composite electrocatalyst (denoted as Pt+CoOx/C) is found to exhibit higher oxygen evolution reaction (OER) activity than CoOx/C, but slightly lower oxygen reduction reaction (ORR) activity than Pt/C in alkaline electrolytes. The bifunctional catalytic performance is ascribed to the in situ formation of heterogenous platinum-cobalt (hydro) oxide (Pt-CoOxHy) interfaces under the electrochemical reaction conditions. Calculated electrochemical phase (Pourbaix) diagrams suggest that the migration of Co species was driven by the relative stability of Pt-CoOxHy interfaces versus bulk (hydroxy) oxides or aqueous ions of cobalt. (C) The Author(s) 2019. Published by ECS.