Oxygen vacancy enabling CoO2 formation: A key boost for efficient electrocatalytic 5-hydroxymethylfurfural oxidation over CoOx

The electrocatalytic oxidation of 5-hydroxymethylfurfural (HMF) offers a promising approach for producing high-value chemicals and hydrogen. While cobalt-based oxides are promising catalysts for the HMF oxidation reaction (HMFOR), their performance is limited by inefficient oxidation of CoO x to the active CoO 2 phase. Here, we demonstrate that introducing oxygen vacancies into CoO x significantly enhances its oxidation kinetics. The oxygen vacancy-rich CoO x supported on copper foam (CoO x /CF) achieves an impressive 98% HMF conversion with a Faradaic efficiency of 98.6% at 1.5 V vs. RHE. Operando Raman spectroscopy reveals that oxygen vacancies facilitate the preferential formation of γ-CoOOH over β-CoOOH during electrocatalysis, thereby promoting the generation of the active CoO 2 phase. Combining in situ infrared spectroscopy with density functional theory (DFT) calculations, we unambiguously establish the reaction pathway, which proceeds via the sequence of HMF→5-hydroxymethyl-2-furancarboxylic acid (HMFCA)→2-formyl-5-furancarboxylic acid (FFCA)→2,5-furandicarboxylic acid (FDCA), and reveal the pivotal role of the active CoO 2 species in accelerating hydroxyl radical oxidation. This work not only provides fundamental mechanistic insights into oxygen vacancy-mediated catalyst design but also offers a novel strategy for developing high-performance transition metal oxide electrocatalysts for biomass valorization.