Engineering high-valent Ni3+–O active sites through Pd incorporation in NiCo-MOF/Ni-foam for enhanced 5-hydroxymethylfurfural electrooxidation

Abstract

Electro-oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) enables the co-production of green hydrogen under ambient conditions, yet anode catalysts must simultaneously deliver high activity and selectivity and low cost. Nickel-based materials are regarded as the most cost-effective candidates for the 5-hydroxymethylfurfural oxidation reaction (HMFOR) because of their variable valence and natural abundance; however, the Ni²⁺ → Ni³⁺ transformation suffers from high overpotential and sluggish kinetics, while the low density of active sites and weak HMF adsorption restrict current density, selectivity, and stability. Here, a Pd/NiCo-metal–organic-framework/Ni-foam (Pd/NiCo-MOF/NF) composite electrode was fabricated by hydrothermal growth of NiCo-MOF on NF, followed by electrodeposition of Pd clusters. In situ electrochemical reconstruction of the MOF generates high-density Ni/Co oxyhydroxides, and interfacial Pd–NiOOH coupling lowers the deprotonation barrier of Ni²⁺–OH, accelerates Ni²⁺ → Ni³⁺ oxidation, enhances HMF and OH* adsorption, and suppresses the oxygen evolution reaction (OER). At 1.5 V vs. RHE, the HMFOR current density doubles; over five consecutive cycles, HMF conversion remains about 98%, FDCA yield >95%, and faradaic efficiency (FE) >95%. Theoretical calculations confirm that Pd incorporation significantly reduces the Gibbs free energy of key reaction steps. This carbon-free, scalable MOF-derived electrode strategy offers a new paradigm for overcoming the intrinsic bottlenecks of Ni-based HMFOR catalysts.