A Study On Ligh-induced Electrooxidation Behavior Of Small Organic Molecules

Small organic molecules, such as methanol, ethanol and formic acid, are widely used as the fuel for proton exchange membrane fuel cells (PEMFCs). Improvement of CO tolerance of a catalyst is a key issue in PEMFCs. In this work, the light-induced electrooxidation behavior of methanol, ethanol and formic acid has been investigated by energy-dispersive X-ray spectroscopy (EDS), cyclic voltammery (CV), chronopotentiometry, electrochemical impedance spectroscopy (EIS), potentiostatic method, etc. The main results are summarized as follows:The light irradiation shows more significant effect on methanol electrooxidation than ethanol. It has no effect on the hydrogen electrooxidation because there is no CO produced during hydrogen electrooxidation. Breaking of C-C bond in ethanol is the rate-determining step during ethanol electrooxidation on Pt so that light irradiation showed less effective on ethanol than on methanol.In order to clarify the photoelectrocatalysis mechanism, the oxidation reaction of methanol (MOR) on anode Pt catalyst has been investigated by CV, EIS, EDS, potentiostatic method, chronopotentiometry, etc. Mass transfer process is not critical to MOR. The anodic current significantly increased and the reaction impedance distinctly decreased when the electrode was irradiated. The reaction impedance decreased gradually with the increasing irradiation intensity. However, further increase in the irradiation intensity failed to produce a large increase in the anodic current. The photoelectrocatalytic active center was proved to be Pt because light irradiation did not improve the reactivity of methanol or water and the carbon support in the Pt/C catalyst did not affect the photochemical reaction. Monochromatic lights with wavelength from300nm to660nm demonstrate photoelectrocatalysis on MOR. The Photoelectrocatalysis are caused by,"CO oxidation mechanism" and "CO desorption mechanism". In "CO oxidation mechanism", light irradiation enhanced the formation of OHads on Pt at lower potentials, and consequently promoted the electrooxidation reaction of COads.In "CO desorption mechanism", the COads desorption from Pt surface was induced by light irradiation. Both of the two mechanisms lead to recover active Pt sites and the former was the major reason for the photoelectrocatalysis to MOR. The effectiveness of photoelectrocatalysis is related to anode potential. At low potential, only "CO desorption mechanism" worked. At high potential, the promotion of OHads formation by light irradiation was responsible for the increase of anodic current. The photoelectrocatalysis activity maximized at0.55V where the two mechanisms both worked. Additionally, when the concentration of methanol was0.7M and the loading dosage of Pt was0.86mg cm-2, the photoelectrocatalysis activity maximized.Formic acid, which is non-toxic and less permeation, was widely researched as a substitute for methanol in PEMFC. The photoelectrocatalysis oxidation of formic acid on Pt anode catalyst has been investigated. Monochromatic lights with wavelength from300nm to660nm all have photoelectrocatalysis activity during formic acid oxidation and decrease the reaction impedance as well. The formic acid oxidation on Pt undergoes two pathways named "direct pathway" and "CO pathway", which was called "dual-pathway" mechanism. Compared with methanol, the photoelectrocatalysis effectiveness of formic acid is lower because the CO poisoning is less serious. Compared with Pt/C, light irradiation had no effect on Pd/C because Pd did not suffer CO poisoning during formic acid oxidation.