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.