Bipolar plates binder

Thermoplastic carbon composite materials are a favonrable material combination for bipolar plates becanse they can be mannfactnred by the mass production process of injection monlding [102]. Electrical condnctivity of a carbon composite requires a high content of carbon, nsnally a mixtnre of graphite and active coal. The percolation limit of the graphite in the polymer binder has to be exceeded, leading to direct contact between graphite particles. Additionally, a basic condnctivity of the polymer matrix by the smaller active carbon particles is achieved. The injection... 

As documented in and expressed by these various contributions, the topic Polymers for Fuel Cells is a vast one and concerns numerous synthetic and physico-chemical aspects, derived from the particular application as a solid polymer electrolyte. In this collection of contributions, we have emphasized work which has already led to tests of these polymers in the real fuel cell environment. There exist other synthetic routes for proton-conducting membrane preparation, which are not discussed in this edition. Furthermore, certain polymers are utilized as fuel-cell structure materials, e.g., as gaskets or additives (binder, surface coating) to bipolar plate materials. These aspects are not covered here. 

In general, there are two different types of bipolar plate technologies one based on metal and the other based on graphite as a conductive component. The latter can be pure (expanded) graphite or a graphite composite material containing a polymer binder. Metal plates offer excellent electrical and thermal conductivities and they can be machined to very small thicknesses. Additionally, they do not tend to form cracks or break even at small thicknesses and their nonporous structure offers good gas... 

In general, two different concepts of polymer binders can be applied in bipolar plates. First, the binder material can be polymerized or cross-linked in situ in the composite during molding of the plate (resin method). Second, a thermoplastic polymer material can be used in the compounding process (thermoplastic method). 

Typical thermoplastic polymers as binders in HT-PEM bipolar plates are polyphenylensulfide (PPS), polyetheretherketone (PEEK), and derivatives of polyphenylsulfone (PSU). Looking at the plastics pyramid (Fig. 19.1), all of these polymers are in the upper right region of high temperature stability and semi-crystallinity. These polymers safely resist the acid and oxygen contact and provide heat deflection temperature far above the operating point of a high temperature PEM fuel cell. 

Within the last years, considerable effort has been devoted to the development of innovative bipolar plate solutions at Eisenhuth. A broad variety of potential binder polymers have been tested. A major finding of the research activities is that semi-crystalline binders appear to be... 

Not only the binder polymer, but also the carbon fillers have to be characterized with respect to their thermal stability. In the next TGA example, several PPS-bonded bipolar plate samples with different contents of highly conductive carbon nanotubes (0-4 % CNT) are investigated by TGA. As shown in Fig. 19.5, the peak of thermal decomposition at approximately... 

Especially for composite bipolar plates the long-term stability of the plastic binder materials is a very appropriate classifier for the reliability of a bipolar plate. The quantification of the period of induction of the used polymer—as explained later—is essential as after this period a rapid aging of the polymer takes place resulting in a sudden decomposition of the polymeric matrix in the composite bipolar plates. 

Composite bipolar plates generally contain polymeric matrices as binder and filler materials such as graphite, carbon black, and others. As the degradation mechanism of these differ significantly they will be discussed separately in the following, starting with the polymeric matrices. 

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