A Highly Efficient Nickel Phosphate Electrocatalyst for the Oxidation of 5-Hydroxymethylfurfural to 2,5-Furandicarboxylic Acid

The electrocatalytic oxidation of 5-hydroxymethylfurfural (HMF), one of biomass-derived platform compounds, is an attractive route to produce 2,5-furandicarboxylic acid (FDCA), an important monomer of bio-based polyesters. Here, we successfully synthesized a highly efficient and stable nickel phosphate (Ni3 (PO4)2) electrocatalyst by a simple hydrothermal method for the electrocatalytic oxidation of HMF to FDCA, in which the yield of FDCA reached 94.2% and the Faraday efficiency was 93.5% in 0.1 M KOH. Compared with nickel hydroxide (Ni (OH)2), the introduction of phosphorus enhanced the charge transfer ability of Ni-based catalysts by EIS and Tafel slope analysis. At the same time, the Ni2+ species in Ni3(PO4)2 was more likely to be oxidized to active nickel species than that in Ni (OH)2 from XPS and in situ Raman results. Moreover, in situ attenuated total reflection-surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) over Ni3(PO4)2 were carried out to study the changes of nickel phosphate during the electrocatalytic oxidation of HMF. According to ATR-SEIRAS, XPS, and Raman results, it can be concluded that Ni2+ species under applied potential in KOH solution was transformed to the high-valence nickel species/Ni3+ species with P–OH and the ring-shaped P–O group. In addition, the phosphorus group was also involved in the electrocatalytic oxidation of HMF. Based on these evidences, a complete electrocatalytic oxidation process of HMF to FDCA on Ni3(PO4)2 was proposed.