Effect of Graphite Nanofibers on Poly(methyl methacrylate) Nanocomposites for Bipolar Plates

Abstract

In this work, high-aspect-ratio graphite nanofibers (GNFs) were used to improve the electrical, thermal, and mechanical properties of the poly(methyl methacrylate) (PMMA) polymer, as well as those of PMMA composites suitable for use in bipolar plates. In the result, an electrical percolation threshold for the composites was formed between 1 and 2 wt% GNF content. This threshold was found to be influenced strongly by the three separate stages of the meltblending process. The composites exhibited higher thermal and mechanical properties and lower thermal shrinkage compared with the neat PMMA. Thus, GNFs were demonstrated to have positive impacts on the thermo-mechanical properties of PMMA composites and showed, thereby, reasonable potential for use in composites employed in the fabrication of bipolar plates.Key Words: Graphite nanofibers, Polymer, Composite materials, Thermal properties, Mechanical properties IntroductionThe polymer electrolyte membrane fuel cell (PEMFC) energy system can convert hydrogen and oxygen to electricity, leaving water as the only byproduct, not surprisingly then, this system is of great interest from an environmental point of view. [1][2][3][4] The most significant components of the PEMFC stack are the bipolar plates, 5,6 which account for about 80% of the total weight and 45% of the stack cost. They are designed for many functions, such as uniform distribution of reactants in active areas, heat removal from those areas, current carriage from cell to cell, and prevention of reactant and coolant leakages. The bipolar plates must also possess good thermal stability and low contact resistance.The bipolar plates are a multi-functional component in a PEMFC stack. They provide the electrical connection from cell-to-cell and they separate the reactive gases. On the anode side of the plates, hydrogen gas is consumed to produce electrons and protons as follows:The electrons are collected at the anode and the protons enter the electrolyte (oxidation of hydrogen). On the other side of the plates, i.e. at the cathode, oxygen gas combines with electrons from the cathode and protons from the electrolyte to produce water (reduction of oxygen) as follows:In addition, the bipolar plates serve the following functions: (i) they facilitate water management with in the cell; (ii) they enable heat transfer; (iii) they support thin membranes and electrodes; (iv) they withstand the clamping forces of the stack assembly.8 Therefore, the material requirements, such as in-plane electrical conductivity (S․cm -1 ), dimensional stability (%), flexural strength (MPa), tensile strength (MPa), impact strength (kgf․cm/cm), etc. should be satisfied for the construction of a bipolar plate.Traditionally, the most commonly used bipolar plate material has been graphite. Among the advantages of graphite are excellent resistance to corrosion and low bulk resistivity. The disadvantages are its cost, difficulty in machining, and brittleness.9,10 Given graphite's brittleness, the bipolar plate must be fabricated to a thickness on...