Electronic Modulation of Ni-Mo-O Porous Nanorods by Co Doping for Selective Oxidation of 5-Hydroxymethylfurfural Coupled with Hydrogen Evolution

Abstract: Fossil fuel depletion and environmental deterioration have created an urgent need to develop renewable and clean energy. Biomass, a sustainable organic carbon source, can meet the huge demand for energy and chemicals. Among them, 5-hydroxymethylfurfural (HMF) is an important biomass-derived platform molecule, which can be converted into various high-value chemicals. One of its oxidation products, 2,5-furandicarboxylic acid (FDCA), is expected to replace terephthalic acid as a raw material for the synthesis of bio-based degradable plastics. The electrooxidation of HMF emerges as a promising green route for preparing FDCA due to its advantages of mild conditions, fast reaction rate, and high selectivity. The theoretical potential of the HMF electrooxidation reaction (HMFOR, 0.3 V vs. reversible hydrogen electrode, RHE) is also lower than that of the oxygen evolution reaction (OER, 1.23 V vs. RHE). Coupling anodic HMFOR with cathodic hydrogen evolution reaction (HER) is expected to simultaneously produce valuable FDCA and reduce the cell voltage of hydrogen (H2) evolution. However, the construction of efficient and stable bifunctional catalysts for HMFOR-assisted H2 production is still challenging. In this study, Co-doped Ni-Mo-O porous nanorodsgrown on a nickel foam (Co-NiMoO/NF) is prepared by simple hydrothermal and calcination methods for both HMFOR and HER. Results of electrocatalytic studies indicate that Co-NiMoO/NF exhibits enhanced performance for HMFOR (E10/100 = 1.31/1.37 V vs. RHE) and HER (E-10/-100 = -35/-123 mV vs. RHE) and shows durable HMFOR/HER stability. In particular, Co-NiMoO/NF maintains high FDCA selectivity (~99.2%) and Faradaic efficiency (~95.7%) for 40 successive cycles at 1.36 V vs. RHE for HMFOR. Conversely, Co-NiMoO/NF maintains stable operation at -200 mA∙cm-2 for 50 h with no significant activity attenuation for HER. When coupled as a bifunctional electrode for overall HMF splitting, Co-NiMoO/NF reaches an electric flux of 50 mA∙cm-2 at 1.48 V, which is 290 mV lower than that of the overall water splitting. This confirms that the HMFOR-assisted H2 production over Co-NiMoO/NF significantly reduces the energy consumption. Moreover, the two-electrode system maintains good FDCA selectivity (97.6%) for 10 cycles at 1.45 V, implying good stability of HMFOR-assisted H2 evolution. The remarkable catalytic performance of Co-NiMoO/NF could be due to the introduction of Co, which optimizes the electronic structure of Ni-Mo-O and adsorption behaviors of the reactants, thereby enhancing the intrinsic activity and stability of the catalyst. Meanwhile, the porous nanorod structure enhanced the mass transport of substrates and desorption of bubbles, thereby elevating the HMFOR/HER kinetics. This study provides useful insights for designing efficient and durable bifunctional catalysts for HMFOR and HER.