Studies On The Stability Of Electrocatalysts In Direct Methanol Fuel Cells

Direct Methanol Fuel Cell (DMFC) becomes very attractive recently due to advantages such as high power densities, mild operating conditions and conveniences for carrying, which make it an ideal candidate for portable power sources. For commercial applications, high stabilities and long lifetimes are necessary. There are many factors that influence the lifetime of fuel cells. Among these, the stability of electrocatalysts is very important. In this dissertation, the degradation of electrocatalysts in DMFCs during operations is studied. Several important factors that influence the stability of electrocatalysts are examined.Results of the life-test show that, catalyst particles sintered during operations with the degree lower in anode than in cathode. A reasonable explanation is that the anode catalyst is the PtRu catalyst, in which Ru exists partly in the form of oxides or hydroxides, and these species inhibit the sintering process of Pt crystallites to some extent. The degree in ECA loss is higher than that in SSA loss during the life-test, both in anode and cathode. Part of ruthenium migrated through the membrane from the anode side to the cathode side, leading to the degradation of electrocatalysts.Several factors that influence the stability of electrocatalysts such as temperature, potential, intermediates and impurities are examined. The results showed that, increases in temperature and potential values accelerate the sintering process of catalyst particles. An elevated alloy degree of PtRu catalysts wasobserved after a discharging process at high temperature. The ruthenium loss from PtRu catalysts is severe at high potential values. In comparison with constant potentials, potential scans make electrocatalysts degraded more quickly. The existence of intermediates such as formaldehyde and formic acid hampers the methanol-oxidation reaction. The existence of Fe³⁺ and Cr³⁺ leads to a drastic cell performance loss, and the cell performance can not be fully recovered.Potential scan tests are used to rapidly evaluate the stability of electrocatalysts. The results show that, potential scans within low potential ranges are benign to the PtRu catalyst, but when the potential value exceeds 0.7V, a remarkable ruthenium loss will take place. The stability of the Pt/C electrocatalyst is improved by the oxidative treatment of the support.A polymer-graphite composite bipolar plate material is prepared. This material displays a high corrosion resistivity both in anode and cathode environments of DMFCs.