Unraveling Adsorbed Hydroxyl‐Mediated 5‐Hydroxymethylfurfural Electro‐Oxidation on a Cation‐Defective Copper Oxide Catalyst

Copper oxide (CuO) is identified as a promising electrocatalyst for the oxidative upgrading of biomass derivatives, although the catalysis mechanism on CuO‐based catalysts remains poorly understood. In this work, we report that cation defects in CuO enable a new electro‐oxidation mechanism, designated as the adsorbed hydroxyl‐mediated (AHM) pathway. A cation‐defective CuO (CD‐CuO) is prepared by the self‐reconstruction of a vanadium‐doped cuprous sulfide (V‐Cu 2 S) precatalyst, which exhibits high product yield and Faradaic efficiency exceeding 90% for the electro‐oxidation of 5‐hydroxymethylfurfural (HMF) to 2,5‐furandicarboxylic acid (FDCA). By combining operando Raman spectroscopy with density functional theory‐based Raman spectrum simulations, we identify that hydroxyl species adsorbed on high‐valence copper sites (Cu 2+δ O‐OH*) act as a redox mediator for HMF oxidation. The cation defects are found to lower the energy barrier for Cu 2+δ O‐OH* formation and enhance HMF adsorption, thereby promoting catalytic activity. Furthermore, the high performance of CD‐CuO is well‐maintained for more than 100 h in an anion exchange membrane electrolyzer. This work provides new insights into the catalytic mechanism and structure–performance relationships of copper‐based catalysts.