Study Of Methanol Electrooxidation And Simulation Of Its Nonelinear Dynamics

The direct methanol fuel cell (DMFC) which can directly use aqueous methanol as fuel is considered as a possible portable electrical source in the near future. To solve the problem of anode electro-catalyst poisoning in DMFC, underpotential deposition (upd) method was adopted to prepare upd-Ru/Pt and upd-Sn/Pt electrodes based on the dual function mechanism of modified atoms, and electrochemical measurements were performed in this paper. On the base of mechanism discussion, upd-RuSn/Pt was prepared and measured. We establish nonlinear dynamics model, make a linear stability analysis and numerical value simulation for the electrochemical oscillation phenomena observed in experiment.Underpotential deposition of ruthenium on a platinum electrode was adopted to obtain a submonolayer of ruthenium on platinum surface. Ru coverage area on Pt electrode was evaluated according to the hydrogen desorption peak of the cyclic voltammograms (CV) on Pt electrodes before and after upd-Ru modification from the potential of 70mV to 350mV (vs. Ag/AgCl) in 0.5 mol·L-1 H2SO4. The effects of the potential for formation of upd-Ru and coverage of upd-Ru adtoms on methanol oxidation were investigated and analyzed. It was shown that Ru adatoms could be underpotentially deposited on a platinum surface. The submonolayer of upd-Ru on the Pt electrode could enhance methanol electro-oxidation several times as large as that on the pure Pt electrode. It was also shown that as long as the amount of upd-Ru adatoms was controlled in a proper range, upd-Ru deposits would enhance the methanol oxidation independently of the deposition potential. The catalytic activity of binary Pt-Ru catalysts for methanol electro-oxidation was highly associated with the surface composition of Pt and Ru, but was free of whether Pt-Ru deposits appear in a form of alloy, bimetallic compounds, adatoms, or something else.Underpotential deposition of tin on platinum electrode was adopted to prepare upd-Sn/Pt electrode. Methanol oxidation on the upd-Sn/Pt electrode shifts toward a more negative potential than that on upd-Ru/Pt. In potential range from 0 to 0.22 V, the current of methanol oxidation on upd-Sn/Pt with underpotential deposition time of 100s is larger than that on upd-Ru/Pt. The action of Sn adatoms was investigated by surface-enhanced infrared absorption spectroscopy. The enhancement effects of upd-Sn adatoms ascribe to the important role of Sn in COB oxygenation. According to the facts that ruthenium and tin adatoms on the Pt surface play an enhancement role in methanol oxidation in a different potential range, we prepared upd-RuSn/Pt electrode by construction of upd-Ru/Pt at 0.58 V first and then by construction of upd-Sn at -0.17V sequentially and test its electro-catalytic activity. Larger current of methanol electro-oxidation was obtained on the upd-RuSn/Pt electrode due to some synergetic effects of the upd-Ru and upd-Sn adatoms.Based on dual path reaction mechanism, a nonlinear dynamics model reflecting the potential oscillation in electro-oxidation of methanol on Pt surface was established. The model involves three main variable parameters, electrode potential (e), surface coverage of carbon monoxide (x) and adsorbed water (y). The chemical reactions and electrode potential were coupled together through the rate constant ( ( ))k i = expaie?ei. The analysis to the established model discloses: there are different kinetics behaviors in different ranges of current densities. The chemical oscillation in methanol electro-oxidation is assigned to two aspects, one from poison mediate CO of methanol electro-oxidation, which is the induced factor of the chemical oscillation, the other from the oxygen-containing species such as H2Oa. The formation and disappearance of H2Oa deeply depends on the electrode potential, and directly cause the chemical oscillation. The established model makes clear that the potential oscillation in methanol electro-oxidation is the result of the feedback of electrode potential e on the reactions involving poison mediates CO and oxygen-containing species H2Oa. The numerical analysis of the established model successfully explains why the potential oscillation in methanol galvanostatic oxidation on a Pt electrode only happens at a certain range of current densities but not at any current density.