Fabrication And Properties Of Milled Carbon Fiber Reinforced Epoxide Resin/Graphite Material For Bipolar Plate

Proton exchange 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.The bipolar plate is an important part in PEMFC,which accounts for about 45%of PEMFC stack.While the high cost of PEMFC is a key obstacle to its wide application.Therefore,it is important to search new materials and manufacture methods for high performance and low cost bipolar plate.In this research,epoxy resin/graphite(EP/G for short) composite was prepared by low temperature and hot pressure molding formation.According to property demand on resin/graphite bipolar plate by Department of Energy(DOE,USA),the bending strength and conductivity were considered as major factors during the research.The effect of preparation technology on properties of composite was studied.Firstly,the effect of resin content,molding temperature and holding time on the properties of the bipolar plate composite is explored through a series of experiments arranged by uniform design,and the regression functions between properties and experimental factors are generated with the results,from which optimized conditions are calculated theoretically.Then single factor experiments are carried out to make refinements and supplements to the conclusions drawn from uniform design experiments.According to the work above,the optimized composition content and preparation condition is proposed as epoxy resin content of 10%,molding temperature of 275℃,holding time of 100min and the ratio of phenolic resin to epoxide resin is 3:8,under which the composite is obtained with bending strength of 53.11MPa and conductivity of 209.25S/cm,well measuring up to DOE targets of R/C(resin/carbon) composite for PEMFC bipolar plate manufacturing.In order to improve the properties of the EP/G bipolar plate composite,the milled carbon fiber was proposed to reinforce the composite.The research was made out on the influences of carbon fiber length,content and surface treatment on the properties of reinforced EP/G composite.The experimental results show that the bending strength and electrical conductivity of the composite reinforced by milled carbon fibers are optimum when the content of carbon fibers is 7%.The best temperature and soaking time is 250℃and 2 hours in vapor oxidation.The bending strength of the composite rises from 53.11MPa to 59.99MPa obviously,while the conductivity only descends from 209.25S/cm to 203.72S/cm slightly.Under liquid oxidation procession, the most important affecting factor on bending strength is UV radiation time and on electrical conductivity is pH value.Due to the great decline of the composite properties after the liquid phase oxidation treatment,the treatment is not suitable to treat carbon fibers.Through analyzing the curing mechanism of the EP/G composite,it is believed that some active groups of the surface of graphite would react with resin through cross linking reaction,and resulted in the combination on interface of resin by covalent bond, when the concentration of the resin was low.While when its concentration is proper, the curing reaction between EP resin and solidification agent(that is phenolic resin) would lead to the appearance of three-dimensional cross-linked structure,and realized their effective composition.By analyzing the surface of EP/G composite strengthened by milled carbon fibers,it can be concluded that both strong combination and weak combination existed in the interfaces between vapor oxidation carbon fiber and base material,and only weak combination existed in the interface between liquid phase oxidation carbon fiber and base material.According to the theory of fiber reinforced composite,an estimation formula of bending strength to the milled carbon fiber reinforced EP/G composite was given and proved by experimental results.The comparison result shows that the error between estimating values and experimental results was minimum was about 4.9%when the content of milled carbon fibers was 5%.Furthermore,evaluated by our theoretical estimation formula,much higher bending strength of EP/G composite reinforced by milled carbon fibers could be reached,so that there must exist a large developing space in EP/G composite reinforced by milled carbon fibers.