A Study On The Carbon-based Composite Materials

Polymer electrolyte membrane fuel cell (PEMFC) is recognized as a new alternative to the present power sources because of its high efficiency, low emission, high energy and power density. Bipolar plate is an important part in FC, which distributes fuel and oxidant to the electrodes of FC and provides electronic connection between single cells. Its cost can account for as high as 40% of a FC stack, while the high cost of FC is a key obstacle to its wide application. Therefore, searching materials and manufacturing methods for high performance and low cost bipolar plate has been very important. With the above background, new materials and manufacturing methods for bipolar plates are explored in this study.The nature graphite/polyphenylene sulfide (NG/PPS) composite plates were manufactured using a sintering process. Compared with injection process commonly used, our method is more competitive for much lower equipment cost and high graphite content allowed in the plate, which then leads to lower cost of the plate and higher conductivity. Compared with moulding process, our method has higher efficiency.The heat-resistant PPS were selected to mix with the low-cost natural graphite. The variation of electrical properties, gas tightness, mechanical properties and corrosion resistance with sintering technological conditions and polymer content was studied. The property of sintered composite plates was compared with the property of molded composite plates, and the conductivity in high temperature was also studied. The sintered composite plate with PPS content 20wt% can meet the demand for the bipolar plate materials. The molded composite plate with PPS content 10～30wt% can meet the demand of the American DOE. Compared with the molded composite plates, the sintered composite plates have lower H2 permeability, higher flexural strength and lower conductivity, when the PPS content is the same in the tow type of composite plates. The conductivity of the tow kinds of composite plates at 200℃is higher than the conductivity at the room temperature.The contact resistance between the composite plate and the gas diffusion layer is high because of the nonconducting polymer in the composite plate, and the contact resistance increases rapidly with the polymer content increasing. In order to decrease the contact resistance, expanded graphite (EG) was used to modify the natural graphite/phenol formaldehyde (NG/PF) resin composite plate. Results show that the modification considerably reduced contact resistance versus bare NG/PF plates. The extant of the decrease in contact resistance is influenced by the expanded volume of EG used. The total electrical resistance, as expressed by volume resistance, can be reduced by applying EG layers to NG/PF composite plates. The reduction of total resistance is more remarkable for the composite plates with high PF content because the bulk resistance of the EG layer can be well compensated by the decrease of contact resistance at a proper range of EG layer thickness.Graphene oxide with high surface energy, high hydrophilic properties, good mechanical strength, can be used for preparation of nano composite membrane. So far, the study of graphen oxide is still in primary stage, and we have not seen the article about the difference between grapheme oxide and graphite oxide. For the fist time, we prepared grapheme oxide and graphite oxide and obtained the same and the difference by characterized them. We also adopted the graphene oxide in preparation of graphene oxide/polyvinyl alcohol (PVA) composite membrane, and investigated the properties.The surface functional groups of graphene oxide are the same as that of the graphite oxide. Ultrasonication can destroy the surface functional groups of the graphite oxide, as revealed by elemental analysis. The surface functional groups of the graphene oxide decomposed at 170℃, and the thermal stability of graphene oxide is lower than that of graphite oxide. Compared with graphite oxide, graphene oxide is much easier to distribute in water.Graphene oxide increases the glass temperature and the tensile strength of the PVA membrane. Graphene oxide/PVA composite membrane has anisotropic swelling property, but heat treatment would destroy this property. The oxygen functional groups of graphene oxide react with the hydroxyl groups of PVA after the composite membrane heated. Compared with the no heat treatment composite membrane (graphene oxide 0.1wt%), the elastic modulus of the heat treatment composite membrane at 150℃increases 168.59%.