Based on different materials, bipolar plates can be divided into graphitic bipolar plates, metallic bipolar plates and composite bipolar plates, and Bpps with different materials have completely different manufacturing processes. Generally, metal bipolar plates are easy to process and realize mass production, but their disadvantage is that they are prone to corrosion and require high surface coating technology. The graphite bipolar plate has stable chemical properties, but it’s brittle and has poor mechanical properties, hard to process and costly. Composite bipolar plates combine the advantages of the first two, are of good corrosion resistance and easy to shape, but have some shortcomings in mechanical strength and conductivity. Besides, the reliability of long-term operation still needs to be verified.

Manufacturing process for metal bipolar plates

The manufacturing process of metal bipolar plates mainly includes several parts: forming, welding, coating and packaging. Common forming processes include stamping and hydraforming, stamping use special molds to apply pressure to metal plates on production line at mutiple workstations to obtain sheets with desired shape and size. The characteristic of stamping technology is high efficiency, suitable for continuous batch manufacturing. Hydraulic forming only requires a set of mold and plastic deformation occurs via liquid pressure, the final product has better surface quality and fewer rebound issues compared to stamping technology. Rubber-pad molding process is a flexible forming method proper for small size flow channels, consists of a rigid mold and rubber plates, which can solve problems such as wrinkles and cracks in both stamping and hydraulic processes. The disadvantage is that the rubber plates need to be replaced frequently. In addition, the other methods for forming metallic bipolar plates such as etching, machining or laser can be also applied for some factories which have low demand for production volume.

In general, the welding process adopts laser welding, which uses laser to weld along the sealing groove on bipolar plate, fuse two negative/positive single polar plates into one Bpp. In order to ensure good performance and durability of materials, it’s necessary to choose appropriate surface coating materials and processes. Before coating, use ultrasonics to clean the residual on the surface of plate. The coating material can be graphite, precious metals, or metal oxides. According to the different coating processes, it can be divided into several methods: electroplating, chemical plating, chemical vapor deposition(CVD) and physical vapor deposition (PVD). Currently, PVD is widely used through physical vapor methods such as sputtering or evaporation, result in a dense and strong corrosion resistant protective layer formed on surface of bipolar plate. Packaging is the final production process, and manufacturers will decide proper methods based on their own needs. Generally, sealing materials are filled into the sealing groove of bipolar plate, some factories also attach customized silicone rings to the bipolar plate, and others use dispensing technology to achieve the goal of sealing.

Preparation methods for graphite bipolar plates

As a porous material, graphite has defects in gas sealing and cannot be directly processed into bipolar plates. It requires high-temperature carbonization treatment and repeated impregnation with resin to make a nonporous graphite plate. Common preparation methods include machining processing, injection molding and compression molding.

The machining process first involves graphitizing the raw materials at high temperatures and make it into graphite embryos. Then, cut them into slices according to required specifications of bipolar plate, the graphite sheet after rough cutting is impregnated with resin to fill the pores inside graphite. The resin material can be phenolic resin or furan resin. After 24 hours immersion, the electrode plate can be polished for better surface finish and more precise dimensions. Finally, carve out flow channel on bipolar plates, and a high-precision engraving machine is the key point to ensure optimal dimensional tolerance and flow field quality. Due to the high brittleness of graphite electrode plates, this method is not suitable for processing very thin graphite sheet. In addition, bipolar plates produced by graphite processing machinery are difficult to meet the requirements of high-quality bipolar plates in terms of surface quality and size accuracy, and gradually being eliminated in the market.

The injection molding process first sends a mixture of graphite and polymer materials into the barrel of injection molding machine, fully mixed and granulated by a twin screw extruder. Then, the granular raw materials are heated, melted and presented in a flowing state, which are compressed and sprayed into the mold through a nozzle under the action of a plunger or screw. The resin is cooled and solidified in mold cavity to form the desired shape of BPP flow field and channel size. Finally, the graphite bipolar plate was obtained by demolding.

The compression molding process first mixes graphite powder and resin evenly and place them into a pre-heated mold. At a certain temperature, the material begins to melt and flow in the mold cavity. The upper mold slowly moves downwards to apply forming pressure to filler, and the entire mold is filled and air is expelled during the process of mold closure. After a certain period of cooling and solidification, open the mold and remove the part from mode. In this manufacturing process, temperature control, molding pressure and curing time are crucial for the quality of final bipolar plates.

Production process for composite bipolar plates

Composite bipolar plates generally use conductive fillers and resins as raw materials. Considering the electrical and mechanical properties of Bpps, they are mixed in a certain proportion. After plasticization, they are extruded into pellets, and then molded into plates through injection molding. Besides, they can also calendered or molded. The production process should be decided based on specifications and production volume of products. Common polymer materials include PP, PE, CPV, PVC, etc., and conductive fillers include graphite, carbon black, carbon fiber, metal powder, etc. The manufacturing process of composite bipolar plates is relatively mature, and can be mass-produced through universal plastic processing methods, greatly reducing costs. They are widely applied in fields of corrosive electrolyte flow batteries, such as VRFB, which have been popularly used.