Flow battery stacks are assembled in a filter press manner. Bipolar plates connect individual cells within the battery pack, isolating the electrolyte solution between adjacent cells and collecting the current generated by the electrode reactions on both sides of the bipolar plates. In addition, the electrodes in the stack require a certain degree of deformation, and the bipolar plates provide rigid support for them.

　　To achieve these functions, the bipolar plate material must have the following characteristics.

　　(1) Excellent conductivity, low ohmic resistance connecting individual cells, and easy current collection.

　　(2) Good mechanical strength and toughness, capable of supporting electrode materials well without fracturing or breaking under the clamping force of the sealed battery.

　　(3) Good density, preventing leakage and ensuring no intermixing of electrolyte solutions between adjacent cells.

　　(4) Good acid resistance and corrosion resistance. This is particularly important in all-vanadium flow battery systems, which use strong acid as the supporting electrolyte, and the pentavalent vanadium (VO) at the positive electrode has strong oxidizing properties. Furthermore, under high electrode potentials, a harsh environment of strong acid, strong chemical oxidation, and electrochemical oxidation is formed.

　　1. Types of Bipolar Plates

　　All-vanadium flow batteries typically use carbon felt or graphite felt with relatively large porosity as electrodes. The electrolyte solution flows inside the electrodes, and there is no need to etch flow fields on the surface of the bipolar plates like proton exchange membrane fuel cells. However, the surface of the bipolar plates must be smooth, and the contact resistance with the electrodes must be low. For all-vanadium flow battery bipolar plate materials, graphite plates and carbon-plastic composite plates are generally used.

　　1.1 Metal Bipolar Plates

　　Non-noble metal materials are easily corroded or form passivation films with poor conductivity in the strong acid and strong oxidizing environment of the all-vanadium system. Although metals such as platinum, gold, and titanium have good corrosion resistance, they are expensive and unsuitable for large-scale applications. People have used electroplating and chemical deposition methods to treat stainless steel materials to enhance their corrosion resistance and improve their service life as bipolar plates, but the effect is minimal, and they still cannot work stably for a long time in the operating environment of all-vanadium flow energy storage batteries. Therefore, metal materials are not suitable for bipolar plate materials in flow batteries with acidic electrolyte solutions, and there is currently little research on this type of material.

　　1.2 Graphite Bipolar Plates

　　Graphite materials exhibit excellent conductivity, acid corrosion resistance, and chemical and electrochemical stability under the operating conditions of all-vanadium flow batteries. Porous hard graphite plates are dense and effectively prevent electrolyte solution penetration. These characteristics make hard graphite plates suitable for all-vanadium flow battery bipolar plates.

　　However, the preparation process of hard graphite plates is quite complex and energy-intensive. It generally involves mixing stone powder with graphitizable resin or asphalt, undergoing a complex manufacturing process, and heating in a graphitization furnace according to a strict temperature program to 2500℃~2700℃ to prepare a porous or low-porosity graphite block containing only nano-scale pores. Then, it is cut and polished to prepare graphite plates of the required thickness. This process is time-consuming and costly. Moreover, the high brittleness and poor toughness of the non-porous graphite bipolar plate material make it prone to breakage during the clamping and sealing process when used in high-power stacks, limiting its application in flow batteries.

　　1.3 Carbon-Plastic Composite Bipolar Plates

　　Currently, carbon-plastic composite bipolar plates are used in high-power flow battery stacks. Carbon-plastic composite bipolar plate materials generally use conductive carbon powder (such as graphite powder, carbon black, carbon fiber, etc.) and thermoplastic resin (such as polyethylene, polyvinyl chloride, polypropylene, etc.) and retardants, release agents, etc., mixed evenly, and prepared by injection molding or compression molding. Carbon materials are used as conductive fillers to provide conductivity, and resins are used as binders to provide mechanical strength and fill the pores between the carbon materials.

　　Because the resin used as a binder in composite bipolar plates is not carbonized, the intrinsic resistance of the plates is much higher than that of graphite bipolar plates, and the contact resistance between carbon-plastic composite bipolar plates and electrodes is also higher than that between graphite bipolar plates and electrodes.

　　2. Bipolar Plate Manufacturing Process

　　The bipolar plate manufacturing process has the following important steps: molding-impregnation-curing-drying and leveling-dispensing-bonding-curing-inspection (processes vary among companies).

　　Composite Bipolar Plate Processing Technology:

　　Currently, injection molding and compression molding are two commonly used methods for processing composite bipolar plates. Transfer molding and reaction injection molding, developed from injection molding, are also used to process composite bipolar plates. Injection molding is a more common method than compression molding, but due to the influence of material fluidity, it cannot be used to prepare composite bipolar plates with low polymer content, resulting in poor conductivity of the bipolar plates.

　　Extrusion molding is a process in which the composite material is mixed and softened in a rotating barrel with a screw, then extruded into a mold, cooled and solidified for a period of time, the mold is opened, and the composite bipolar plate is ejected. This is usually a semi-automatic or even automated process.

　　Compression molding involves placing the mixed composite material in a mold, heating and applying a certain pressure in a hot press, maintaining the temperature and pressure for a certain time, and finally cooling, solidifying, and releasing the pressure to open the mold.

　　3. Suppliers

　　Currently, in the field of flow battery bipolar plates, graphite bipolar plates and carbon-plastic bipolar plates are the main types. To meet customized needs and control costs, many flow battery companies choose to manufacture their own bipolar plate materials, such as Heng'an Energy Storage, Xinxin Vanadium Titanium, Suqian Times, Kaifeng Times, and Meimiao Energy Storage, etc.

　　In addition, many bipolar plate process suppliers in the hydrogen energy field are also beginning to deploy flow battery bipolar plates.