A Study On The Exfoliated Graphite/Resin Composite Bipolar Plate In PEM Fuel Cells

PEMFC is an electrical device which directly converts chemical energy of fuel and oxidant to electricity. Compared with heat engine and rechargeable battery, it has a series of merits, such as high efficiency of energy transform, high dependability, high energy density, low emission, easy to start, low noise, low radiation, strong disguise and modularized structure, etc.Bipolar plate is one of the important parts in fuel cells which distributes fuel and oxidant to the membrane electrode and provides electrically connection between neighboring single cells. High conductivity and gas tightness are the basic requirements for bipolar plate materials. At present there are mainly two kinds of bipolar plate in PEMFC: graphite and metal. Graphite bipolar plate is made via machine tooling, it has high conductivity and is strongly corrosion-resist, with shortcomings including high cost and poor mechanical strength. Metal bipolar plate usually has high mechanical strength and can be made much thinner, but special measures have to be taken to prevent metal from corrosion and yet maintaining its high conductivity. Composite bipolar plates combining the merits of graphite and metal should be a trend in the future, as indicated by recent effects in bipolar plate research and development.In this study, low-priced bipolar plates composed of natural graphite and polypropylene, were proposed and basic research on the related materials and manufacturing techniques were carried out.The influences of various factors, including polypropylene particle-size, its melt flowing index and particle-size of graphite, on the electrical conductivity and gas-tightness of the plates were studied. A mechanical treatment method has been found which improves composition of expanded graphite with polypropylene. Investigation has also been conducted on process conditions, including heating temperature, holding time, molding pressure, cooling pressure, and the uniformity of mixing. Through orthogonal experiment with factors of heating temperature, holding time, molding pressure, cooling pressure and melt flowing index, were also designed and conducted.Optical microscope, SEM and metallurgical microscope were used to observe the texture and status of PP and graphite inside the composite plates. Preliminary understanding has been achieved of the main factors influencing the performances of the composite plates. The composite plates through the novel combination of expanded graphite and PP show higher electrical conductivity and lower H2 permeability compared with plates of pure flexible graphite. They should also be much less expensive than the conventional plates of hard graphite. This can be expected to lead to new PEM fuel cell stack that are more competitive and of wider application.