Electrochemical upgrading of 5-hydroxymethylfurfural via a defect-rich NiCo2O4 array

Regulation of the surface evolution and kinetic behavior of interfacial molecules by defect engineering is significant for the efficient production of the value-added chemical 2,5-furandicarboxylic (FDCA) through the electrochemical oxidation of 5-hydroxymethylfurfural (HMFOR). Herein, a precatalyst array (NiCo₂O₄-mV) assembled from NiCo₂O₄ nanoparticles (∼10 nm) with mixed-ionic defect species (m-Ds) was synthesized. In situ Raman spectroscopy shows that as the potential increases, the partial surface of NiCo₂O₄-mV formed an NiCo₂O₄/NiCoO(OH) heterojunction on the catalyst surface. Operando ATR-SEIRAS and DE-MS measurements further reveal that the built-in electric field derived from this heterojunction leads to a decrease in the coverage of adsorbed water molecules at the electrode–electrolyte interface, thereby promoting the adsorption and efficient mass transfer of HMF molecules, ultimately obtaining an industrial-level current density (1 A@1.636 V). This work further elucidates the structure–activity relationship for defect-rich precatalysts in the electrooxidation of organic compounds.