(Invited) Size, Composition Effects and Active State Formation of Nanostructured Cobalt and Iron-Cobalt Oxide-Based Catalysts during Oxygen Evolution Reaction

Abstract

The efficient water electrolysis is a key technology to establish a CO2-neutral, electrochemical (green) hydrogen production. Nonetheless, the electrocatalysts’ near-surface structure during the anodic oxygen evolution reaction (OER) is still largely unknown, which hampers knowledge-driven optimization. Here, we provide quantitative near-surface structural insights into oxygen-evolving CoOX(OH)Y nanoparticles by tracking their size-dependent catalytic activity down to 1 nm and their structural adaptation to OER conditions combining operando X-ray absorption spectroscopy and density functional theory calculations. We uncover a superior intrinsic OER activity of sub-5 nm nanoparticles and a size-dependent oxidation leading to a near-surface Co-O bond contraction and charge redistribution during OER.I will also report on the role of Fe in the performance of Co-oxide-based electrocalysts. By comparing epitaxial Co3O4(111), Co1+δFe2-δO4(111) (|δ|<=0.2), and Fe3O4(111) thin film electrocatalysts combining quasi in-situ preparation and characterization in ultra-high vacuum (UHV) with electrochemistry experiments. Interestingly, Co1+δFe2-δ O4(111) was found to be up to five times more active than Co3O4(111) and Fe3O4(111) with the activity depending acutely on the Co/Fe concentration ratio. Under OER conditions, all three oxides are covered by oxyhydroxide but the characteristics of this overlayer, including its thickness, stability, structural and chemical evolution was found to depend on the Fe content.