Effect Of Vanadium Lons On The Performance Of Direct Methanol Fuel Cells

Direct methanol fuel cell (DMFC) is a kind of new-fashioned and clean power source. Recently, DMFC has attracted widespread attentions, because of the advantages of the abundant source, low price, easy storage and transportation of the liquid methanol as well as high energy density and the simple structure; Furthermore, DMFC has wide application prospect in portable electronic devices such as mobile phone, digital camera and notebook PC computer. However, there are some problems for the commercialization of DMFC, such as the high cost, the low electrocatalytic activities of the anodic catalysts and the low poison tolerance. Many researchers reported that the performance of the anode catalyst of Pt could be improved by introducing other elements (e.g., Ru, Sn) or transition metal oxides (e.g., WOx, VOx). But the introduction of these species still exist high cost or bad performance stability problems.In order to better solve the problems, this thesis researched and developed a new method for improving the performance of anode catalysts of DMFC, the tolerance of CO poisoning and reduce the Cost of DMFC, through introducing an appropriate amount of vanadium ions to methanol electrolyte. This thesis mainly studied the promoting effect of Vanadium ions on the catalytic ability of the anodic Pt/C catalyst for methanol electrooxidation; Furthermore, the effect of Vanadium ions on the performance of a passive DMFC was investigated. The main contents and results of this thesis are as follows:1. The promoting effect of Vanadium ions on the catalytic ability of Pt/C catalyst for methanol electrooxidationMany studies show that introducing the metal oxide of VOx into the anode catalyst of DMFC can improve the electrocatalytic activity of the anode catalyst for methanol oxidation, inhibit CO poisoning, and reduce the dosage of precious metals. However, there are some problems for the metal oxide of VOx, such as bad conductive and easy dissolve in the DMFC, resulting in unconspicuous promotion effect and the poor stability for the anode catalysts and DMFC.According to this problem, we studied that the influence of a certain amount of vanadium ions introduced into the methanol electrolyte on the performance of methanol oxidation for the Pt/C catalyst prepared by conventional impregnation method. The results showed that the electrocatalytic performance for methanol oxidation can be promoted significantly by addition of an appropriate amount of vanadium ions. Moreover, the relationship between the catalytic performances and the concentration of vanadium ions is a "volcano-type" trend. By comparing the curves of hydrogen and CO adsorption/desorption experiments for the Pt/C and Pt/C-V-3 catalyst, it was discovered that there is an adsorption effect of vanadium ions on the Pt catalyst surface, which causes the decrease of electrochemical active site of the Pt catalyst. The adsorption of vanadium ions on Pt catalyst with low amount results in the unconspicuous promotion effect for the electeooxidation of methanol, while the adsorption of vanadium ions with high amount results in the excessive occupation of Pt active site. Therefore, when an appropriate amount of vanadium ions were introduced into the electrolytic solution, the maximize performance for the electrooxidation of methanol on Pt/C catalyst would be acquired. The reason for the improvement in the performance for the electrooxidation of methanol on Pt/C catalyst is believed that the existence of VO2+/V3+ redox pair can favor the electrooxidation of methanol and the introduction of vanadium ions can promote the removal of COad at the surface of Pt/C catalyst. Furthermore, we had a new discover that the enhancement mechanism of vanadium ions on the electrooxidation of methanol for the Pt/C catalyst is attributed to an electronic effect.2. The effect of Vanadium ions on the performance of DMFCBecause the electrocatalytic performance for methanol oxidation can be promoted significantly for the Pt/C anode catalyst of DMFC by addition of an appropriate amount of vanadium ions, so we have the hope to reduce the time of DMFC commercialization through the addition of an appropriate amount of vanadium ions. But the performance of the whole DMFC is affected by many complicated factors, not only decided by the Pt/C anode catalyst of DMFC. Therefore, the effects of methanol concentration, the catalyst loading and MEA fabrication method on the performance of passive DMFC are investigated. When DMFC adopted 3 mol·L-1 methanol solution, the catalyst loading of 8 mg·cm-2, and the spraying fabrication method of MEA in room temperature, the cell performance was optimal. Under the optimized condition, the comparison of the performance with and without vanadium ions for DMFC was evaluated. It was found that an appropriate amount of vanadium ions have enhancement effect on the performance of DMFC. When the concentration of vanadium ions is 5×10-5 mol·L-1 in the fuel of 3 mol·L-1 methanol solution, the maximum output power density of DMFC under the optimized condition was 22.57 mW·cm-2, which had increased by 6.04 mW·cm-2 compared with the DMFC without vanadium ions; Furthermore, when the DMFC discharged at the constant current of 60mA, the operation time was increased by 2.39 h compared with the DMFC without vanadium ions. Consequently, we have enough reasons to believe that an appropriate amount of vanadium ions can enhance the performance of DMFC, which provides a chance for reducing the catalyst loading of the MEA and the cost of DMFC. The improvement of the performance for DMFC should be attributed to the enhancement effect of an appropriate amount of vanadium ions, which promoted the electrooxidation of the anode catalyst for methanol and intermediate poisoned species COad of methanol.