In Situ X-ray Absorption Spectroscopy in Combination with Size-Dependent Activity Trends Reveals Insights into Cobalt Oxide Catalyzed Bifunctional Oxygen Electrocatalysis

Abstract

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. We investigated 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 was composed of the Co3O4 phase regardless of their particle sizes. The OER activity increased with decreasing NP size, which correlated to the enhanced oxidation state and larger surface area in smaller NPs, whereas the ORR activity was independent of NP size. These particle size-dependent catalytic activities in conjunction with the in situ XAS results can provide critical insights into the CoOx-catalyzed bifunctional oxygen electrode reactions.