Electrooxidation of monosaccharides on nickel: A microkinetic study of the electrode site heterogeneity

The electrochemical oxidation of monosaccharides on nickel-based catalysts is a promising route for sustainable energy conversion and biomass valorization into added-value products. Although participation of the Ni(OH) 2 /NiOOH redox couple in the organics oxidation is well documented, the nature of the active sites and the underlying mechanisms of monosaccharide electrooxidation remain insufficiently understood. Here, we investigate the electrooxidation of monosaccharides containing 3, 4, 5, and 6 carbon atoms by cyclic voltammetry on Ni electrodeposited on glassy carbon in Fe-free 0.1 M NaOH, over a monosaccharide concentration range of 1–5 mM. All molecules exhibit an oxidation ‘onset’ approximately 250 mV below the Ni(OH) 2 /NiOOH transition peak, with the shape of the “low-potential” oxidation wave strongly dependent on the molecular structure. To rationalize these observations, we develop a two-site microkinetic model that explicitly accounts for surface heterogeneity, comprising highly reactive “minority-site” (likely defect sites) and more abundant “majority-sites”, both supporting parallel Eley–Rideal and Langmuir–Hinshelwood pathways. Sensitivity analysis reveals that the Eley–Rideal pathway dominates in the “low-potential” interval, with the ‘onset’ potential primarily governed by the standard potential of the Ni(OH) 2 /NiOOH transition on the “minority-site”. The simulations semi-quantitatively reproduce the voltammetric response of glucose, fructose, and erythrose, highlighting the critical role of intrinsic surface site heterogeneity in monosaccharide electrooxidation and providing a transferable modeling framework for the rational design of advanced Ni-based electrocatalysts. • Electrooxidation of C3–C6 monosaccharides starts below the Ni(OH) 2 /NiOOH peak. • A two-site microkinetic model accurately captures characteristic features of experimental CVs. • The low-potential ‘onset’ is attributed to the presence of highly active “minority-sites” • Oxidation at “minority-sites” primarily proceeds through an Eley–Rideal pathway. • Oxidation at high potentials occurs via dual Eley–Rideal and Langmuir-Hinshelwood mechanism.