A fuel cell system can convert chemical energy directly into electricity. Early efforts to realize such energy conversion were already made around 1900. Over 100 years fuel cell technology has been studied in depth and now is driven mostly because of the energy needs and environmental issues.

Polymer electrolyte membrane (PEM) fuel cells, also known proton exchange membrane fuel cells, have such a name because the electrolyte is made of a polymer membrane. They have attracted more attention for their application in transport, stationary power generation and portable power. The schematic structure of PEM is given in Figure 1. It can be seen that the PEM consists of many components, one of which is a bipolar plate (BP). From the point of weight, volume and cost, the BP is much of importance in a cell system. It has a number of functions, such as even distribution of gases, prevention of gas escape, electrical connection between anode and cathode, and current collection[1, 2]. According to the major component elements, BPs can be categorized into three major styles: carbon-based, composite and metal-based, which are elaborated subsequently.

Figure 1 The schematic structure of a PEM fuel cell

2. Carbon-based bipolar plates
The traditional plate material is graphite. Graphite has a high electric conductivity, is chemically stable in the PEM fuel cell, can endure high temperatures making it suitable for the high-temperature PEM. However, one of undesirable aspects with graphite plates is the porous structure that makes them brittle and gas-permeable. Also, Making and maching of a graphite BP takes much money and time. The cost of graphite plates is about $200/kW far higher than the target figure $10-30/kW. In order to make the material gas-impermeable POCO impregnates the bulk material with a resin. This, however, reduces the maximum service temperature to 150℃[3].
Carbon/carbon plates are made from carbon and carbon fibers. A C-C composite is porous, so sealing is required. The common method is to cover the material surface with graphite carbon. Chemical vapour infiltration (CVI) is capable of it[4]. C-C BPs possess such properties as gas tightness, high electrical conductivity, low density about that half of graphite[5] as well as balanced flexural strength. The major weakness similar to that of graphite plates is that manufacturing of C-C BPs is time-consuming and costly.
Expanded natural graphite has completely displaced the synthetic graphite technology as the material of choice for proton exchange membrane (PEM)[6]. It has many advantages similar to that of synthetic graphite in terms of electrical conductivity and electrochemical stability and could also be made thinner, lighter weight and less brittle[7]. The flow field channels in expanded natural graphite can also be embossed which results in a cost-effective manufacturing process[8]. Expanded natural graphite can be used in almost any PEM, portable, micro or direct methanol fuel cell application and 85 percent of the fuel cell vehicles are using flow field plates from expanded natural graphite materials. Expanded graphite is a porous foil made of high surface area graphite particles. Thickness and porosity depend on the calendering process. Many researchers have been keenly interested in expanded graphite on account of its peculiar structure and excellent electrical conductivity and have used it to manufacture BPs by several methods[9, 10].

3. Composite bipolar plates
Composite plates can be classified as metal- or carbon-based. Los Alamos National Laboratory has developed a metal-based composite BP which combines porous graphite, polycarbonate plastic and stainless steel[11]. Producing porous graphite is not as time consuming or expensive as producing non-porous graphite plates. The stainless steel and polycarbonate parts provide impermeability. Stainless steel also provides rigidity. The graphite resists corrosion and the polycarbonate provides chemical resistance and can be molded to any shape.
Graphite composite material is a mixture of different types of graphite and carbon powders, fibres and a polymeric resin. Graphite composite BPs have been made using thermoplastic (polypropylene[12], polyethylene[13], polyvinylidene fluoride[14]) or themosetting resins (phenolics[15], epoxies[16], and vinyl esters[17]).
The advantages of a thermoplastic material are the low cost, high availability, high chemical resistance, good mechanical properties and gas-impermeability[18, 19], short cycle times in an injection moulding processing and easily recyclable[20]. SGL Technologies uses PVDF for its BMA5 material[21]. A major disadvantage of thermoplastic-based graphite composites is their low application temperatur. Another weak point is the limited creep resistance. Liquid crystalline polymer(LCP)/graphite materials are potentially alternative for BP due to their low viscosity and thus easy injection moldability. The low viscosity also avails carrying more graphite. Dupont[22] uses a LCP as the polymeric binder. Ticona[23] reports that it has introduced the world’s first PEM fuel cell prototype made solely of engineering thermoplastics. According to Ticona, the cost per kW for a new cell stack drops by nearly 74% to $1050 from $4000 required in aluminum, gold-coated stainless steel, graphite, or thermoset graphite composites. The LCP BPs can be injection molded without the need of costly machining and other finishing stages and then a relatively short cycle time is allowed, says Ticona. Further, the LCP is able to contain a carbon loading exceeding 85% and still process well. In addition, the LCP has an excellent high-temperature resistance and long endurance in the aggressive environment.
Possible thermosetting resins include epoxies and vinyl esters. Advantages of thermoset polymers are the good chemical resistance, the creep resistance and low cost for phenolic and epoxy resins[24]. Highly conductive epoxy composites developed by synergistic combinations of carbon black and expandable graphite were reported in the reference[16]. Also, this material is of light weight, low cost, and mechanical integrity at higher temperature than 150ºC. Compared with those made of epoxies, BPs produced from vinyl ester have high corrosion resistance, balanceable combination of conductive and mechanical properties and can be produced readily[2, 25]. In spite of these attractive properties, thermosetting polymers are relatively low with respect to conductivity and mechanical strength.

4. Metal-based bipolar plates
Metal-based flow plates can be made from crystalline or amorphous alloys. Major issues with metals are the extent of corrosion and contact resistance due to the formation of surface passivation film. Noble metals like gold and platinum possess very similar properties to Poco graphite and in some cases showed more superior performance. However, the high cost of these metals has limited their commercial application[26, 27]. Promising candidates are aluminum, titanium, stainless steel and nickel. In the article[28] it is reported that flow plates are in constant contact with highly acidic solutions(pH less than 0 when in direct contact with the membrane). In such an environment metals are prone to corrosion and release multi-valent cations, which can increase membrane resistance and poison the electrocatalyst. Both aluminum and titanium have an excellent corrosion resistance resulting from a stable oxide layer, but the latter is more expensive. Davies et al.[29] have shown that the contact resistance of stainless steels tends to decrease with increasing content of nickel and chromium because the oxygen-rich film decreases. However, for a long-term use coating is needed. Possible coatings are carbon-based, metal-based or obtained from surface treatment. Wang et al.[30] reported that substrate steel has a significant influence on the behavior of SnO2:F coating. Similar to graphite, a significant drawback in the mass production of BPs from metals is the machining or etching of channels.
Amorphous alloys have invited researchers’ attention on account of more excellent properties than their crystalline counterparts, such as high strength-to-density ratio, high corrosion resistance, high hardness and superplasticity in the supercooled liquid region[31, 32]. These unique properties result from the dense and random atomic configurations and would make amorphous alloys potential candidate materials of BPs in PEM fuel cells. Recently reported metallic glasses are Fe- and Ni-based. The problems considered most with them are their thermal stability, contact resistance and corrosion behavior. Inoue et al.[33] developed a Ni-based glassy alloy, of which the corrosion rate in sulfuric acid was about 3 digits lower than SUS316L. Even after 350h durability test the deterioration of the morphology of the glassy separator could hardly be recognized. For Fe- and Ni-based materials exploited in the references[34, 35], they are thermally stable because of their high glass transition temperature Tg and high reduced glass transition temperature Tg/T1 (T1 is liquidus temperature)[36]. Contact resistance tests showed that the interfacial contact resistance will not be an issue of concern upon high compaction load. Although these preliminary results are encouraging, further work will go on. Improvement of proerties and long-term test are needed. To prepare bulk amorphous metal materials, new ways or systems need to be investigated.

5. Conclusions
For a long endurance, such as in stationary applications, the graphite-based composites are best suited. For applications where size and weight are more important than endurance, metal plates and expanded foil plates are an attractive option. If strength is an additional criterion then only metal plates are a feasible possibility. In case that more balanced combination of properties is a concern, polymer composites will be the most possible candidate.

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