Fabrication and characterization of thermoplatic composite bipolar plates

Besides the Membrane-Electrode Assemblies (MEAs) and the Gas Diffusion Layer (GDLs), the bipolar plates (BPP) constitute the most important element of the Polymer Electrolyte Membrane Fuel Cell (PEMFC). The bipolar plates are a multifunctional component of the PEMFC assemblies (stacks), since they collect and conduct the electron from anode to cathode, they distribute the reactant gas (hydrogen and oxygen) uniformly over the active areas and they remove both heat and water generated during the process. To fulfill these requirements, the BPP have to be both electrically and thermally conductive, good corrosion resistant in fuel cell environment and they must have high strength, low permeability to hydrogen and tailored surface properties.

The use of metal BPP is limited by low resistance corrosion, high weight and cost (due to surface treatments to prevent corrosion) whereas the graphite one are brittle and expensive (due to machining). The BPP composites appear as attractive candidates provided they possess the functional properties and in addition they have low cost, easy manufacturing (without machining) and lightweight.

We manufacture two kinds of composite BPP made up by a mixture of graphite fillers (spheroids and platelets) and a PVDF binder. The first one is a dry blend from graphite fillers and PVDF powder. The second one called microcomposite is obtained from co-atomization of graphite fillers and a latex of submicrometer sized PVDF particles and from a subsequent drying. The BPP are directly fabricated from the dry mixtures by compression molding without post machining. The packing fillers of the composite is up to 85%vol. and the porosity is very low (a few %). The manufacturing conditions (evolution of temperature and pressure during the process) have been optimized owing to the measured electrical properties and porosity and they take into account the economical mass production (short process, high yield). The physical properties of the BPP composite (electrical conductivity, density, mechanical properties, microstructure from microscopic observations, surface properties) have been performed. In particular we present the electrical properties which have been measured directly on the BPP. The traditional approach assumes that the studied sample is semi infinite but experimental  conditions may not respect such assumption. A 3D simulation by finite element method has been developed and validated and this model allows to take into account the BPP geometric characteristics.