Anodic Oxidation of Glucose on Platinum in Alkaline Media

Glucose as one of the small organic molecules is considered to be used as a fuel in the fuel cells since it can be directly oxidized at the lowtemperature. Aimed at learning the anodic oxidation behavior of glucose on platinum in alkaline media, cyclic voltammetry in the region of -0.9~0.4 Vvs SCE (saturated calomel electrode) was used to characterize the anodic oxidation process. The potential oscillation phenomena of glucose oxidation under constant current polarizationwere firstly presented. Adetailed studywas further performed byusingthe linearthe potential sweep technique. The appearance of three oxidation peaks in the cyclic voltametric curve means that glucose could be oxidized on the surface of platinum. The reaction products or intermediates could accumulate on the surface of platinumelectrodes, resulting in the increase of the overpotential. However, these adsorbed species can be furtheroxidized and removed at high enough potentials. The oscillation potential ranging form -0.9~0.0 V shows the potentials at which the adsorbed species inhibit the electrode activity, formation and oxidation. Glucose can be oxidized in steadystate at lower current densities and the oxidation potential is at -0.4 V. Since the formation and diffusion away of the products are in a equilibriumstate in that case, at higher current densities, the rate of the formation of inhibitive species increases and causes higher overpotentials to reach the critical potential value of about 0.0 V. The oscillation frequency is also related to the current density for glucose oxidation. The higher the current density, the faster the oscillation frequency is. The results reveal that the platinum electrodes could be easily poisoned by the intermediates produced during the process of glucose oxidation in an alkaline solution. The formation of the poisoning species is proportional to the current density used. The electrochemical oxidation of glucose involves the dissociate adsorption of glucose followed by the oxidation of hydrogen atom bound to the carbon C1atom in the glucose molecule. The further process involves the formation of gluconolactone that can be readily hydrolyzed togluconic acid. The gluconic acid exists as a salt in an alkaline solutions and the formation potential shifts to a more negative value than that in acidic solution due to the pHeffect. It is believed that the poisoning species could be the small fragments. For example, COwas formed by the breakdown and dehydrogenation of glucose during the electrochemical oxidation. In view of practical applications, the anodic potential must be low enongh to provide a reasonably large output voltage for awhole fuel cell. On the other hand, if an electrode cannot prevent the poisoning of any species in the solution, its lifetime is definitely short. The potential oscillation behavior is a precursor for a catalyst that cannot afford the long-term polarization owing to the poisoning. Once the oscillation starts, the electrode will eventually lose its activites.