Mechanistic Study of Glycerol Electro-oxidation on Ni(OH)₂/NiOOH electrodes

Glycerol oxidation reaction (GOR) is a promising valorisation route to upgrade the biodiesel by-product while co-producing green hydrogen at the cathode in electrolysers. However, the working mechanism of transition-metal-based catalysts such as Ni(OH)₂ remains poorly understood. Here, we employed a multi-operando spectroelectrochemical approach combining UV–Vis optical spectroscopy, soft X-ray absorption spectroscopy, and time-resolved stepped-potential spectroscopy to investigate the active oxidising species and charge-transfer dynamics under OER and GOR conditions. We identified the NiOOH (Ni3+) as the active species for GOR, whereas the formation of higher valent NiOO (Ni4+) species is completely suppressed in the presence of glycerol. The accumulation of surface adsorbed glycerol molecules is rate-determining, (τ ~ 27.9 s at 1.47 VRHE), occuring slower than the intrinsic catalytic step of glycerol reaction ( τ ~ 3.2 s at 1.47 VRHE), which involves oxidation and bond cleavage. In contrast, OER has significantly slower kinetics (τ ~167 s at 1.47 VRHE), resulting in the dominance of GOR and suppresion of oxygen evolution in the presence of glycerol. The potential-independent production of formic acid during GOR follows an apparent first-order dependence on NiOOH concentration, suggesting continuous C-C bond cleavage activated by reactive *O species. These findings link oxidising species with charge-transfer dynamics, providing insight for the rational design of Ni-based catalysts for glycerol and other biomass-derived molecule oxidations.