Corrosion stability of graphite/polymer composites was verified in a study of the time dependence of the potential drop across a plate. Various materials were investigated in a special bipolar plate corrosion test cell at Los Alamos National Laboratory (LANL). The tests revealed that the ohmic resistance of the composite material does not rise with time. This indicates that no preferential etching of the carbon occurs. Satisfactory corrosion stability of graphite/polymer composites is also obvious from the polarization curves. The corrosion current of the carbon-polymer does not exceed the value of 16 uAcm-2 specified in Department of Energy targets.
Another promising graphite-based bipolar plate material is graphite foil. Graphite foil is made from expanded graphite by compressing and compacting the plane layers. Upon application of pressure, e.g., by rolls, the adjacent plane layers interlock; thus a stable, self-supporting sheet can be formed without application of a binder. The starting material for the production of graphite foil is natural graphite or artificial graphite with a structure close to the ideal graphite plane-layers structure. By treatment with an oxidizing agent like a mixture of sulfuric and nitric acid at increased temperature, the graphite is transformed into a graphite salt. The graphite salt is washed and dried, and then expanded by rapid heating to temperatures of more than 800C. The intercalate substance evaporates within a few seconds, thereby opening the layer structure. The result is a worm-like structure with the interlayer distances increased by a factor of 80-200. The worm-like expanded structures are subsequently compressed by application of rolls or other suitable means. The thickness and density of the sheets can be adjusted by employing an adequate load in the compacting step. Common applications of graphite foil include gaskets, rupture disks, resistive heating elements, and heat and radiation shielding.
Graphite foil is characterized by a high degree of orientation of the graphite plane-layers resulting in a strong anisotropy of its electrical and thermal conductivity, i.e., conductivity in the thickness direction is remarkably lower than in the planar direction. Compared to the composites discussed before, this orientation effect is a major disadvantage from the viewpoint of the application as a bipolar plate in an electrochemical device. But the higher through-plane resistivity of graphite foil is offset by the ability to make the plates thinner because the foil is self-supporting, and by the lower contact resistance due to the soft character of the material.
Although the invention of graphite foil dates back to the end of the 19th century, its fuel cell application started only in the 1980s. Multilayer separator plates consisting of gas-impermeable inner carbon layers interposed between graphite sheets were proposed with the sheets acting as buffers to absorb any expansion or shrinkage of the carbon layers. The layers were joined together by an adhesive and subsequently calcined to form an integrated carbon body. With increasing progress in the development of automotive and portable fuel cell application in the 1990s, graphite foil as a lightweight alternative to conventional bipolar plate materials became more and more attractive to fuel cell manufacturers, and R&D efforts were refocused on bipolar plates made solely of graphite foil. For fuel cell operation the mechanical strength and the gas tightness of the graphite foil need to be increased. Both can be achieved by impregnation with a resin. Suitable resins comprise phenolic, furan, acrylic and epoxy. Several techniques can be used to apply flow field structures to graphite sheets.
The flakes in expanded graphite are of a size in the range of 0.3-5mm, i.e., by a factor of 5-10 larger than the synthetic graphite particles in the composite described before. Since particle size sets the limit for the dimensions of flow field structures, more fine flow field structures can be realized with the composites. Slternatively, the flow channels can be incorporated in the electrode support instead of the plate surfaces, thus allowing for thin graphite foil bipolar plates and reduced stack weight.
In summary, each of the graphite-based bipolar plate materials discussed in this article has its specific advantages and disadvantages, and therefore, will find its special fields of application. Generally, graphite foil will be the preferred material when weight reduction is the main issue, as in portable applications. On the other hand, graphite/polymer composite bipolar plates with optimized, subtle flow field designs will be employed for high-power applications with less constraint in space and weight, like stationary and residential ones. Thus, both materials should not be considered as “rivals” but as “specialties” for different applications.