Synergistic Effect of Urea on Vitamin C Electro-Oxidation at NiOx/CoOx Binary Catalysts Supported on Graphene Nanosheets

Abstract

The electro-oxidation of ascorbate (AAO) in the presence of urea at NiOx/CoOx/GNs/GC electrode is investigated. Cyclic voltammetry measurements show that the peak current (I p) of AAO at NiOx/CoOx/GNs/GC electrode is ten times that of GNs/GC electrode. This highlights the role of catalyst composition and order of deposition on the activity of AAO. Interestingly, blending ascorbate (AH− ) with urea caused a significant promotion of AAO. Both the I p of AAO and the fuel utilization in the AH− /urea fuel blend solution are doubled when compared to those obtained at the same electrode in pure AH− solution (in addition to a favorable negative shift in the onset potential, E onset = 150 mV). Urea molecules are believed to act as anchoring molecules for AH− molecules at the NiOx/CoOx/GNs/GC surface with a favorable geometry facilitating its oxidation (by two-electron transfer process) to dehydroascorbate (DHA). Four interaction geometries are proposed, and comparative DFT calculations are conducted, displaying the relative stability and dipole moments of AH− in such a way that facilitates its adsorption and oxidation. These findings demonstrate the crucial role of the catalysts tailoring together with the selection of a smart blending fuel with AH−.