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/NiFeO_xH_y nanosheets supported on nickel foam (denoted as Ni(OH)₂/NiFeO_xH_y/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 NiFeO_xH_y 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 NiFeO_xH_y 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.