Preparation Of Expanded Graphite/Polyimide Composite Bipolar Plates

Proton exchange membrane fuel cell (PEMFC) is considered as the most competitive alternative to traditional energy sources for its advantages, such as high energy conversion efficiency, environmental benignity, quick startup and silent operation. Bipolar plate (BPP), as one of the main components of a fuel cell stack, accounts for 30% to 40% of the total cost and 70% to 80% of the total weight; therefore its low-cost and lightweight become a key factor for PEMFC’s market application. In terms of the material composition, there are three types of BPP:graphite, metal and composite. Composite BPP has the advantages of low density, ease in machining, low cost and good corrosion resistance; it may be the best choice for BPP. For the application of composite BPP, in this dissertation the composition and the molding process of expanded graphite (EG)/polyimide (PI) composite BPP have been optimized, the improvement of its manufacturing process has been explored and pilot-scale manufacture has been conducted.Composite BPP for PEMFC was prepared using EG as the primary conductivity filler and PI as the matrix by dry mixing and compression molding processes. Differential scanning calorimetry (DSC) was used to determine the PI softening and curing temperatures. Effects of mixing time, molding process, and the ratio between EG and PI on properties of the composite BPP were studied. The results showed that the appropriate dry mixing time is 10 min; the best conditions for the molding process are preheating 110 ℃ for 0.5 h, then compressing to 80 MPa for 10 min while holding the temperature at 110 ℃, following by curing at temperature of 226 ℃ for 2 h. The optimum ratio of the ingredients is 60wt% EG and 40wt% PI, and the in-plane conductivity and flexural strength were 160.77 S·cm-1 and 64.62 MPa.To improve the conductivity of EG/PI composite BPP, the feasibility of adding the second conductive fillers such as graphene and sphere graphite (SG) was investigated preliminarily. Graphene was obtained by high temperature (1050 ℃) reduction of the graphite oxide prepared by Hummers method. The homemade graphene was characterized by SEM, XRD and Raman; for comparison, commercial graphene has been characterized as well. The results showed that both of them had been reduced incompletely and multilayer structure has been found. When used such graphene as the second conductive filler, the in-plane conductivity and mechanical strength of the composite materials couldn’t be improved as expected. Then the SG was selected as the second conductive filler; results showed that the adding of 5% of SG enabled the in-plane conductivity and flexural strength of the composite sheet increased by 29.8% and 17.3%, respectively.In order to improve the mixing uniformity of the EG and PI, the dissertation introduced an electrostatic assistant mixing process. EG and PI in different containers were charged with opposite static electricity respectively with a electrostatic generator. And then the PI was added into the EG, mixed by a three-dimensional motion mixer. BPPs prepared by the mixed materials were test, and the results showed that the in-plane conductivity and flexural strength of the composite bipolar plate were significantly improved. However, the further experiments showed that the environment factors significantly affected on the EG and PI charging process.Based on lab results, pilot-scale manufacturing was conducted in the dissertation to determine the parameters of thermal molding process. Pre-curing BPP was pressed to 4 mm with 8 MPa and preheated at 110 ℃ for 0.5 h, and then increased the compression pressure to80 MPa for 15 min, following by reducing the pressure to 30-40 MPa and heating 1 h at 230 ℃ for curing. Then a certain number of the BPPs with active area of 50 cm2 were prepared. At last, a single cell with commercial MEA and the homemade EG/PI composite BPP was assembled for performance test. The results showed that the cell acquired the maximum power density of 347.2 m W·cm-2 at current density of 800 mA · cm-2, only 62.8% of that of non-porous graphite BPP single cell under the same current density, one reason is the composite material conductivity less than non-porous graphite plate, on the other hand is the different dimension between the two types of BPP. That can be improved by process and designing optimization in the future.