Dopant-Level Cobalt as an Electronic Promoter Stabilizes NiOOH for the Efficient Electrochemical Oxidation of 5-Hydroxymethylfurfural (HMF) to 2,5-Furandicarboxylic Acid (FDCA)

Abstract The electrochemical oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) is a promising route to biobased polymers using Ni-based hydroxide anodes. Although Co incorporation enhances performance, its mechanistic role in HMF oxidation remains unresolved. Here, NiCo hydroxides with precisely controlled Co contents were synthesized and evaluated. Trace Co incorporation (Ni99Co1) markedly enhanced FDCA production, achieving a production rate of up to 1.27 mM cm−2 h−1, a 48% increase over pristine Ni (0.86 mM cm−2 h−1), while maintaining the highest Faradaic efficiency (91%). Higher Co loadings instead favored the competing oxygen evolution reaction (OER), reflecting a mechanistic trade-off. Operando spectroscopies and theoretical calculations reveal that Co acts not as a catalytic site but as an electronic promoter: it lowers the Ni2+/Ni3+ transition barrier, stabilizes active NiOOH, and facilitates FDCA desorption. Excessive Co destabilizes NiOOH and accelerates OER. These findings establish dopant-level Co as a redox-modulating promoter for selective biomass electro-oxidation.