Studies On Direct Methanol Fuel Cell Anodic Catalysts

Direct methanol fuel cell (DMFC) is one of promising candidates as power sources in portable electronic products, electric vehicle etc,due to their inherent simplicity of design and operation, high energy efficiency, small weight and volume, and low pollution. The poor kinetics of anode reaction and methanol crossover in electrolyte are the two key issues hindering DMFC commercial application.In present dissertation, A new technology scheme, reversed micelles method, was firstly devised to prepare the anode catalysts of DMFC. By controlling the molar ratio of water to surfactant, it can be easily to obtain Pt/C catalysts with desirable Pt particle size, which is important for improving the catalytic activity of methanol oxidation. The Pt-Ru/C catalyst was also prepared with the reverse micelles method and exhibited a higher catalytic activity as compared with the catalyst Pt-Ru/Vulcan XC-72 (E-TEK). The overpotential of Pt-Ru/C electrode is 0.55 V (vs. SCE) at 150 mA/mg (Pt), about 70 mV lower than that of Pt-Ru/Vulcan XC-72 electrode.The effects of support on the activity of the supported catalyst were also investigated. The supported catalysts with mesocarbon microbeads (MCMB) as support were prepared for the first time. The effect of the support pretreatment on the catalyst performance of methanol electrooxidation was examined in detail. The Pt/MCMB catalyst with MCMB boiled in KOH for 1 h exhibits a high catalytic activity during the electrooxidation of methanol. The Pt-Ru/MCMB catalysts were also prepared. With half-cell measurement, the overpotential of Pt-Ru/MCMB electrode is 0.41 V (vs. SCE) at 300 mA cm-2/90oC, which is 170 mV lower than that of Pt-Ru/Vulcan XC-72 electrode. The result indicates that Pt-Ru/MCMB catalyst has higher catalytic activity than the catalyst with carbon black as support. All these results show that MCMB is a excellent catalyst support to be used in DMFC. The impedance spectra of Pt-Ru/MCMB catalysts were also studied. A loop reflecting an inductive behavior was observed at the low-frequency end only when the anode is at higher overpotential. Effects of Nafion content in the catalyst layer, the MEA hot press condition and the cell operation temperature on the impedance spectra of the Pt-Ru/MCMB electrode were also investigated. The results are helpful for the research of DMFC. On the polycrystalline Pt electrode, the effect of Cl- and F- adsorption on methanol electrooxidation was studied in H2SO4 solution. The methanol electrooxidation reaction was inhibited when the Cl- concentration was over 0.01 M, while F- had little effect on the reaction. Electrochemical Impedance Spectroscopy (EIS) measurements showed that Cl- had high adsorption capacitance on Pt electrode, which impeded the methanol adsorption on Pt electrode. The adsorption state of hydrogen formed from methanol electrooxidation on the polycrystalline Pt electrode surface in sulfuric acid and methanol solution had also been studied by the potential step experiments. The results indicated that the adsorption hydrogen was weakly bound and could be desorbed from the Pt electrode surface in the underpotential deposition (UPD) region. All these results provide rich information for understanding the methanol electrooxidation mechanism, and could be helpful to the development of DMFC.