Graphite-based bipolar plates (1)

The bipolar plate material is a solid polymer fuel cell stack has to fulfill multiple, in part even contradictory, requirements arising from both the operational and the manufacturing perspective. It must be available at reasonable costs. Operational requirements like low electrical resistivity and corrosion stability are satisfactorily met by plates made of impregnated graphite. However, graphite is expensive; machining the intricate structures of flow fields, coolant passages, etc. from a graphite blank is difficult and the plates must be made rather thick to ensure mechanical strength. The choice of metals is limited to expensive corrosion-resistant ones like titan or highly alloyed steels, or a costly noble-metal coating must be applied to a lower cost metal substrate. More-over, the high densities of metals prevent stack weight reduction.

When a material has to fulfill a complex function and is subject to a variety of requirements, composites often offer a promising option. By employing a composite material, the package of requirements can be united and split up into the different constituents. Graphite/polymer composites first appeared in the 1960s as bipolar plate material for phosphoric acid fuel cells and electrolyzers and later on for solid polymer fuel cells. Success was found in these applications because of the combination of the high corrosion resistance of graphite and easier processability and lower material costs of the over-all composites. Such a composite can be considered as a highly filled plastic or, from another point of view, as a percolation network of graphite particles in a polymer matrix. The percolation network comprises interconnected conductive particle chains facilitating the transfer of electrons. This conduction mechanism can be considered as ohmic.

Although the polyer is the minor component, the graphite/polymer composite at least partly retains the processability of the polymer and can, therefore, be manufactured into shaped articles by typical plastic processing techniques like extruding, compression molding or injection molding.

Another advantage of the composite approach is the wide variety of possible combinations between conductive and polymer components which allows the design of tailored materials for special applications. The conductive component can be selected from artificial graphite powder, graphite flakes and/or graphite nanofibers, natural graphite, expanded graphite and mixtures thereof. Expanded natural graphite is also the precursor for the production of graphite foil, which is described later in this chapter as another alternative bipolar plate material. The polymer component of the composite can be selected from either thermoset or thermoplastic polymers. The choice of the polymer depends on the available processing technology and the desired fuel cell application. For example low cost thermoplastics with limited thermal stability can be employed in near-ambient temperature portable applications. Furthermore, by proper choice of the polymer the hydrophilicity or hydrophobicity of the plate can be adjusted.

Following this rather general description of the composite approach to bipolar plate materials and its major advantages, the carbonaceous and polymer materials employed in these composites and the technologies used in their processing are now described in further details.

In choosing the polymer components, the following features are important:

  • thermal stability at fuel cell operation temperature
  • low thermal expansion
  • stability against humidity
  • chemical stability in the presence of fuel, oxidant and product water, which may be slightly acid
  • no release of constituents which are prone to electrooxidation.
  • Fast processing and low viscosity
  • Large binder efficiency
  • Low permeability to avoid leakage of fuel and oxidant
  • Appropriate hydrophilicity/hydrophobicity

The choice of the carbonaceous component is mainly determined by the following requirements:

  • high electronic and thermal conductivity
  • chemical stability in the presence of fuel, oxidant and product water
  • electrochemical stability
  • processability, which is affected by several characteristics of the graphite such as particle size and size distribution, particle shape, packing density, specific surface area and binder absorption capability; particle size between about 10 and 200 um at narrow particle size distribution was found to yield a microscopically uniform distribution in the polymer; small particle size and low specific surface area are helpful in reducing the viscosity of the compound; the lower the binder absorption capability the lower the viscosity which allows more graphite particles to be dispersed within the binder.
  • Isotropy: the lower the aspect ratio of the particles, the lower is their tendency to take on a plannar orientation during lateral material flow in the molding process, which would result in low through-plane conductivity.
  • Mechanical reinforcement can be brought by addition of fibers, e.g., graphite fibers, but such particles are of high aspect ratio; therefore the ratio of planar-orientated fibrous particles facilitating mechanical reinforcement and particles with random orientation facilitating through-plane conductivity must be adjusted carefully
  • Purity: most harmful are metallic impurities, which can poison the catalyst and the membrane, and sulfur

Often the conductive component of the composite consists of synthetic graphite. Synthetic graphite is produced by thermal treatment of carbon powder or carbon compounds. The precursor material, an amorphous or liquid carbonaceous material or a hydrocarbon gas, is at first decomposited by pyrolysis yielding an intermediate carbon product, which is graphitized by thermal treatment at 2500-3000C. The degree of graphitization strongly depends on the starting material, it is the higher the more mobile the molecules are within the pyrolysis intermediate, i.e., the easier they can rearrange into the graphite structure. After purification, the graphite is milled to the desired grade. Natural graphites from different deposits vary widely in crystallinity, carbon content, refractoriness, flakiness and other properties. Carbon enrichment is achieved by flotation. To obtain the desired grade, the graphite is crushed, grinded and, if necessary, milled. Graphite waste, from the production of electrodes fro steel mills, can be recycled for the production of bipolar plates. Even the graphite from the bipolar plates of scrapped fuel cells can be recycled this way.

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