Size-Dependent Activity Trends Combined with in Situ X-ray Absorption Spectroscopy Reveal Insights into Cobalt Oxide/Carbon Nanotube-Catalyzed Bifunctional Oxygen Electrocatalysis

Bifunctional oxygen electrocatalysts play a vital role in important energy conversion and storage devices. Cost-effective, abundant, and active Co-based materials have emerged as promising bifunctional electrocatalysts for which identifying catalytically active structures under reaction conditions and unraveling the structure–activity relationships are of critical importance. Here, we report the size-dependent (3–10 nm) structure and catalytic activity of bifunctional cobalt oxide nanoparticle (CoOx NP) catalysts for the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR). In situ X-ray absorption spectroscopy (XAS) revealed that the majority of NPs during OER and ORR were composed of the Co3O4 and CoOOH phases regardless of their particle sizes. The OER activity increased with decreasing NP size, which correlated to the increased oxidation state and larger surface area in smaller NPs, whereas the ORR activity was nearly independent of NP size. These particle size-dependent catalytic activities in conjunction with the in situ XAS results can provide insights into the CoOx-catalyzed bifunctional oxygen electrode reactions.