Activated Nickel Foam Anodes for Sustainable Biomass Valorization: Competitive Oxidation of Organic Molecules vs the Oxygen Evolution

A systematic study on the competitive oxidation of glucose (Glc), xylose (Xyl), and 5-hydroxy-methyl-fur-fural (HMF) vs the oxygen evolution reaction (OER) was performed by coupling H-cell electrochemical experiments with <i>in situ</i> O₂ monitoring in the anodic chamber using an activated Ni foam as the anode. At a substrate concentration of 10 mM, multipotential steps showed similar OER onset potential values for Glc and Xyl (1.49 V_RHE), while the value for HMF was slightly lower (1.47 V_RHE). Chronoamperometry tests at 1.6 V_RHE (30 min) with varying concentrations showed that both Glc and Xyl oxidation reactions fully suppressed the OER at 30 mM, while 100 mM was required for HMF. A Langmuir-Hinshelwood analysis of the current-substrate concentration dependence revealed the slower kinetics and inhibitory effects impacting HMF oxidation, which account for the significant difference in performance with respect to both aldoses. Given its relevance as both a model and a promising substrate for membraneless electrolysis operation, Glc was further investigated in a long-term chronoamperometry experiment with <i>in situ</i> O₂ monitoring (15 h at 1.6 V_RHE, 30 mM Glc). The results suggested the feasibility of sustaining OER-free operational conditions for approximately 4 h from an initial Glc concentration of 100 mM. HPLC analysis indicated the presence of formate as the main coproduct of hydrogen via glucose electrolysis.