Enhancing Selective Electrooxidation of 5-Hydroxymethylfurfural via Coordinating the Contradictory Role of Fe in Ni(II)/Ni(III) Redox Kinetics

Nickel-based materials have shown great potential as electrocatalysts for the 5-hydroxymethylfurfural oxidation reaction (HMFOR), owing to dynamic Ni2+/Ni3+ redox cycling. However, this redox process, which is critical for HMFOR kinetics, is prone to disruption. For example, Fe plays a paradoxical role: it facilitates the reduction of Ni3+ but simultaneously inhibits the oxidation of Ni2+. Here, we develop a hybrid electrocatalyst consisting of Ni(OH)2/NiFeOxHy nanosheets supported on nickel foam (denoted as Ni(OH)2/NiFeOxHy/NF), which exhibits satisfactory HMFOR performance, achieving a current density of 204 mA cm–2 at 1.45 V vs RHE along with complete HMF conversion and 92% Faradaic efficiency over 20 cycles. We strategically leveraged Fe incorporation to enhance the proton-coupled electron transfer process during HMF dehydrogenation, a critical step facilitated by Ni3+ reduction. This enhancement is attributed to the synergistic effect between NiFeOxHy and Ni(OH)2, which enhances HMF adsorption and increases interfacial nucleophilicity, thereby facilitating the capture of protons released from HMF. Although Fe incorporation partially suppresses Ni2+ oxidation, the abundant crystalline/amorphous boundaries, oxygen-deficient amorphous domains, and the integration of NiFeOxHy with Ni(OH)2 collectively increase the number of active Ni sites and compensate for the inhibitory effect of Fe. Furthermore, we propose two HMFOR pathways involving hydroxyl groups and protons on the Ni(OH)2 surface via Ni2+/Ni3+ redox chemistry and identify proton deintercalation as the dominant pathway through density functional theory calculations. This work presents a distinctive strategy that not only enhances HMFOR kinetics by leveraging the dual role of Fe in Ni-based redox chemistry but is also potentially applicable to other biomass-derived substrates.