An exploration of the electrocatalytic activity of nickel boride nanocrystals in the oxidation of 5-HMF

In this work, we investigated the inherent electrocatalytic activity of nickel borides in an important reaction in the context of electrochemical valorization of biomass as the oxidation of hydroxymethylfurfural (5-HMF) to furan dicarboxylic acid (FDCA). For this purpose, nickel borides (Ni_xB, x = 2 and 3) in the form of phase-pure nanocrystals (NCs) were synthesized through a solid-state synthesis method, supported on carbon paper and then tested as electrocatalysts for the oxidation of hydroxymethylfurfural (pH 12.9 or 13.9, 1.8 V vs. RHE, 3 h) by comparing their activity to that of Ni nanocrystals of similar average particle size (36-39 nm). Ni₃B NCs achieved the highest 5-HMF conversion and Faradaic efficiency towards 5-HMF oxidation (Conv._5-HMF = 70%, FE = 94%), which is a markedly better performance compared to Ni₂B NCs (Conv._5-HMF = 57%, FE = 72%) and to Ni nanoparticles (Conv._5-HMF = 58%, FE = 65%), thus unequivocally demonstrating for the first time the superior activity brought about by Ni₃B. Based on a combination of physicochemical and electrochemical characterization (XPS, SEM, TEM, C_dl analysis), the better performance of the Ni₃B-based electrocatalyst is attributed to differences in surface composition compared to the Ni₂B-based electrocatalyst and to differences in terms of electrochemical surface area and/or bulk chemical features compared to the Ni-based electrocatalyst. Notably, these results were achieved with a remarkably low electrocatalyst loading (0.05 mg cm⁻²), leading to significantly higher turnover frequency compared to state-of-the-art nickel boride electrocatalysts for this reaction. A kinetic study showed that Ni_xB NCs catalyze the electrosynthesis of FDCA from 5-HMF both through a direct and indirect mechanism, with the contribution of each changing as a function of the pH of the electrolyte.