Multifunctional polymeric nanocomposites fabricated by incorporation of exfoliated graphene nanoplatelets and their application in bipolar plates for polymer electrolyte membrane fuel cells

The focus of this research is to investigate the potential of using exfoliated graphene nanoplatelets, GNP, as the multifunctional nano-reinforcement in fabricating polymer/GNP nanocomposites and then explore their prospective applications in bipolar plates for polymer electrolyte membrane (PEM) fuel cells. Firstly, HDPE (high density polyethylene)/GNP nanocomposites were fabricated using the conventional compounding method of melt-extrusion followed by injection molding. The mechanical properties, crystallization behaviors, thermal stability, thermal conductivity, and electrical conductivity of the resulting HDPE/GNP nanocomposites were evaluated as a function of GNP concentration. Results showed that HDPE/GNP nanocomposites exhibit equivalent flexural modulus and strength to HDPE composites filled with other commercial reinforcements but they have superior impact strength. By investigating the crystallization behavior of HDPE/GNP nanocomposites, it was found that GNP is a good nucleating agent at low loading levels and as a result can significantly increase crystallization temperature and crystallinity of HDPE. At high GNP loadings, however, the close proximity of GNP particles retards the crystallization process. The thermal stability and thermal conductivity of HDPE/GNP nanocomposites were significantly enhanced due to the excellent thermal properties of GNP. Meanwhile, results indicated that the percolation threshold of these nanocomposites prepared by the conventional melt-extrusion and injection molding is relatively high at around 10--15 vol% GNP loading. To enhance the electrical conductivity of HDPE/GNP nanocomposites, two special processing methods named solid state ball milling (SSBM) and solid state shear pulverization (SSSP) were studied. The mechanism by which SSBM and SSSP are capable of producing lower percolation or higher electrical conductivity is to coat the polymer surface by GNP platelets which facilitate the formation of conductive networks during injection molding. However, it was noted that the mechanical and thermal properties of the resulting nanocomposites were compromised at high GNP loadings. A wax coating method was thus applied which is capable of improving both the electrical and mechanical properties in the resulting HDPE/GNP nanocomposites due to a greatly enhanced GNP dispersion. The last but not least, the feasibility of using highly conductive GNP nanocomposites to substitute conventional metallic and graphite bipolar plates was discussed. Polymer/GNP nanocomposites for bipolar plates were made by SSBM and compression molding on account of its good processability and the resulted high electrical conductivity. HDPE/GNP bipolar plates were selected for low-temperature applications, while PPS (polyphenylene sulfide)/GNP bipolar plates were fabricated for a high-temperature usage. Because of the excellent mechanical, structural, thermal and electrical properties of GNP, it is believed that the bipolar plates made from GNP nanocomposites will allow lighter weight of PEM fuel cells with enhanced performance which are particularly suited for automotive applications.