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 CoOx to the active CoO2 phase. Here, we demonstrate that introducing oxygen vacancies into CoOx significantly enhances its oxidation kinetics. The oxygen vacancy-rich CoOx supported on copper foam (CoOx/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 CoO2 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 CoO2 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.