The pivot to achieve high current density for biomass electrooxidation: Accelerating the reduction of Ni3+ to Ni2+

Electrochemical oxidation is a promising method to convert 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) with high selectivity at room temperature. Developing catalysts with high current densities is crucial for the industrial application of HMF electrooxidation reaction (HMFOR). Herein, we demonstrate that the current densities exhibited by the LSV curves are falsely high and propose to evaluate catalyst activity using anodic peak current density rather than overpotential for the comparison of actual efficiency. Besides, the reduction of Ni3+ to Ni2+ accompanied by proton coupled electron transfer (PCET) process of HMF dehydrogenation is confirmed as the rate-determining step affecting the conversion efficiency of HMF rather than Ni2+ electrooxidated to Ni3+. Theoretical calculations reveal differences in the hydrogen transfer energy barriers of PCET process. Furthermore, Ni(OH)2/NF with appropriate Cr doping shows superb activity with actual current densities of over 230 mA cm-2 at HMF concentrations of 10 mM and over 360 mA cm-2 at 20 mM (the ever highest current densities for HMFOR), and obtain a yield of more than 98% of FDCA.