Fuel cell converts chemical energy into electrical energy for power generation, it’s a clean solution with high efficiency and low pollution. Comparing to storage battery, fuel cell has better energy density and power density. As one of the key components, the fuel cell bipolar plate plays the role of supporting the membrane electrode assembly(MEA), conducting heat and electric, and distributing reactive gases such as hydrogen and oxygen. It’s equivalent to the bone and blood vessels of fuel cell, and the cost accounts for 40% of the total cost of battery stack. For the industrialization of fuel cells, it’s necessary to develop bipolar plate materials with excellent performance and low price.

Overview of fuel cell

The fuel (H2) and oxidant (O2) in fuel cell are respectively delivered to each cell of stack to undergo oxidation-reduction reaction and finally achieve the purpose of producing electric energy. Compared with conventional dry cells and storage batteries, the fuel and oxidant of the fuel cell are stored in the storage tank outside. During running, the fuel and oxidant required to be continuously added to the tank. The power specification is determined by the battery stack, and capacity specification is determined by the amount of electrolyte. They are independent with each other. According to the type of electrolyte, fuel cells can be divided into alkaline fuel cell (AFC), molten carbonate fuel cell (MCFC), phosphoric acid fuel cell (PAFC), fixed oxide fuel cell (SOFC) and proton exchange membrane fuel cell (PEMFC).

PEMFC is the fifth generation fuel cell(FC). It is a low-temperature fuel cell which uses conductive polymer membrane as electrolyte, uses H2, methanol, natural gas as fuel, uses O2 and air as oxidant. It has many advantages of high energy conversion rate, environmental friendly, low-temperature startup, weak electrolyte corrosion, etc. PEMFC is mainly composed of proton exchange membrane(electrolyte), anode/cathode catalyst layer, anode/cathode gas diffusion layer and bipolar plates. The diffusion layer, catalyst layer and polymer electrolyte membrane are hot pressed to form membrane electrode assembly (MEA). Perfluorosulfonic acid membrane is mainly used to conduct hydrogen ions, block current transmission and block the reaction between anode and cathode. The catalyst layer on both sides is the place where oxidant and fuel occur redox reaction. The gas diffusion layer can offer gas transport channel, support the catalyst layer, stabilize and improve the electrode structure and water management. The bipolar plates are mainly used to isolate the reaction gas, transmit the reaction gas to the fuel cell through flow channel, support the membrane electrode collect and conduct current, also be responsible for the heat dissipation and drainage functions of fuel cell.

Types of fuel cell bipolar plates

Bipolar plates are mainly used to isolate oxidant and reductant in fuel cell, connect each single cell in series and play the role of current collector, and provide flow channels for H2 and O2 to make them evenly distributed. This requires that the bipolar plate material must have excellent electrical conductivity, thermal conductivity, corrosion resistance, gas resistance and mechanical strength. The common bipolar plate materials mainly include graphite materials, metal materials and composite materials.

1）Graphite-based bipolar plates

Non-porous graphite plates possess are broadly applied in fuel cell industry because of their good chemical stability and electrical and thermal conductivity. Generally prepared by graphite powder or carbon powder, and heated to prevent deformation. During high-temperature sintering, low viscosity and graphitizable resin materials is injected to prevent gas penetration. Because non-porous graphite plate is very brittle, it brings great difficulties to machining the flow channel, and the yield is very low. On the other hand, due to weak mechanical strength, it’s hard to be made very thin. The non porous graphite bipolar plate materials with greater thickness will also increase the volume and weight of the whole fuel cell stack. In addition, the complex manufacturing process also makes the cost very high.

To overcome the shortcoming of the non-porous graphite plate, the graphite powder or carbon powder is mixed with phenolic resin or epoxy resin in a certain proportion, metal debris or metal fine mesh can be added to increase the conductivity according to the needs, and the adhesive and fiber can also be added to increase the strength. The bipolar plates with flow channel can be directly obtained by injection molding pocess, and then graphitization can be carried out after molding. Injection molding graphite plate reduces the rate of rejects on machining and shortens the manufacturing cycle, but the plates are easy to deform during the process of graphitization which makes it difficult to ensure the accuracy. In addition, the high cost of graphitization limits its popularization.

2）Metallic bipolar plates

Stamping process of metallic bipolar plate for flow channel is easy, and have better electrical and thermal conductivity than graphite materials. Besides, the impermeability of metal will be a good barrier for oxidant and reductant. Generally, stainless steel, nickel, titanium and other metal materials are relatively common bipolar plate materials of PEMFC. However, for metal bipolar plates, corrosion or oxidation are easy to take place in a certain environment, and the corrosion perforation may cause the oxidant and fuel to mix and produce explosive substances. Oxidation will form a passivation layer on the surface of the plates, and increase contact resistance. At present, many researches focus on the treatment and modification of metal surface to make it have better corrosion resistance and lower contact resistance in fuel cell environment.

3）Composite bipolar plates

Metal based composite bipolar plates are also known as structure composite bipolar plate. Generally, thin metal plates are used as separators. Conductive adhesive is used to bond MEA components to metal plates, and graphite plate/felt or perforated carbon sheets are used as the flow channel plates. The bipolar plate combining graphite and metal possesses many advantages such as light weight, high strength and good corrosion resistance. However, due to the complex processing technology and high requirements for equipment, it’s difficult to control the cost.

Carbon based composite bipolar plates are also called material composite bipolar plates, which take carbon material as matrix and take polymer composite material as adhesive. The polymer(resin) and conductive fillers such as graphite, carbon fiber, graphene are mixed, melted, molded or injected to make conductive composite bipolar plates. Through adjusting the ratio of polymer and conductive material, carbon based composite plates can be produced to meet requirements on electrical and mechanical properties. In terms of processing technology, the injection molding method requires high material fluidity, and too low resin content is not allowed, that means the poor conductivity. The bipolar plate with high conductivity can be made by molding method, but not proper for mass production due to its low efficiency. At present, the extrusion method of bipolar plates is popular for continuous production.

Applications of fuel cell bipolar plates

As an efficient and clean energy storage solution, fuel cell has been widely used in mobile power supply, distributed power station, transportation and other civilian or military areas. As a core component, bipolar plates have also attracted more and more attention, especially in the field of fuel cell vehicles(FCV), graphite plates were first widely used. Later, Metallic plates gradually become the mainstream because metal plates can be processed thinner and lighter, and greatly improve the power density. Now, composite bipolar plates also occupy a large market share. In China, the battery stack based on compsite BPP has been applied to light buses. In fact, there is no BPP material or production process that can satisfy all applications. It’s necessary to comprehensively consider the specific application conditions, request on cost and comprehensive performance of battery stack to choose the right bipolar plates for fuel cell.