Dynamic Interplay of Dissolved Ions and Solid Surfaces Enables Efficient Nonalkaline Biomass Electrooxidation

Earth-abundant transition-metal electrocatalysts often degrade in acidic or neutral media due to reconstruction and leaching, hindering their use for deep electrooxidation processes like the conversion of biomass-derived 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA), a key monomer for sustainable plastics. Here, we report that under nonalkaline conditions, cobalt ions (Co2+) and electrodeposited cobalt (oxy)hydroxide surfaces, maintained in a dynamic dissolution/deposition equilibrium, act synergistically to drive efficient HMF oxidation. Carboxylate anions, particularly propionate, stabilize dissolved Co2+ as active complexes that are enriched at the electrode interface, while the heterogeneous surfaces facilitate substrate adsorption. This cooperative mechanism enables deep oxidation to FDCA. By optimizing this equilibrium with a potassium propionate electrolyte, we achieved a 93.3% FDCA yield using only 1 mM Co2+. Coupling this anodic process with the oxygen reduction reaction (ORR) allows for low-voltage FDCA production from which FDCA precipitates directly at high concentrations without acidification. This synergy between homogeneous complexes and heterogeneous surfaces offers a new paradigm for stable and efficient electrocatalysis under mild conditions.