The first type of metal materials used in metal bipolar plates includes precious metal materials such as gold, titanium, and platinum, which have good corrosion resistance and conductivity, but their prices are extremely expensive. The second type is lead metal bipolar plates, which have the disadvantage of being quite toxic and easily oxidized on the surface. Finally, there is a type of stainless steel bipolar plate, which has low cost and strong machinability, but it is also very prone to corrosion in acidic environments and high potentials, so its surface needs to be modified. The modification methods include thermal spraying, screen printing, physical vapor deposition, chemical vapor deposition, electroplating, chemical plating, etc. However, the process is complex and the production cost is high, which is not suitable for large-scale production. Although the service life of the modified stainless steel plate has been improved, it cannot work stably in highly corrosive electrolytes such as all vanadium flow batteries. Overall, metal bipolar plates cannot adapt well to flow battery systems with corrosive electrolytes due to various drawbacks, such as all vanadium flow batteries; For less corrosive liquid flow systems, such as zinc bromide batteries, titanium plates with titanium carbide prepared on the surface can be used as bipolar plates [6].
Graphite bipolar plates are obtained by first preparing graphite plates and then cutting and polishing them. They have good conductivity and are stable under acidic conditions, making them suitable as bipolar plate materials for all vanadium flow batteries. The preparation process of non porous pure graphite plates usually involves mixing graphite powder, crushed coke, and graphitized resin or asphalt. The graphite is then heated to 2500-2700 ℃ in a graphitization furnace according to a certain program for graphitization treatment, resulting in non porous graphite blocks. After cutting and polishing, a graphite bipolar plate with a thickness of 2-5mm is finally obtained. The conductivity of pure graphite bipolar plates can usually reach 1200 S/cm, but they are prone to breakage during processing and installation, which also limits their application in large-scale industry.
Polymer carbon composite bipolar plates possess excellent processing properties and mechanical strength of polymer resins, as well as excellent conductivity of carbon materials, and can reduce the preparation cost of bipolar plates. Graphite based composite bipolar plates, which are prepared by injection molding or molding methods using graphite as conductive filler and polymer resin as auxiliary materials, are also widely used in all vanadium flow batteries. At present, graphite bipolar plates and graphite based composite bipolar plates are commonly used in flow battery systems with corrosive electrolytes, such as all vanadium flow batteries. Graphite bipolar plates have outstanding characteristics such as chemical stability, corrosion resistance, high conductivity, and low density. However, their processing is complex, production costs are high, and the space for cost reduction is very limited. Their production efficiency is also low. These disadvantages mean that graphite bipolar plates cannot meet the corresponding requirements in large-scale production and are difficult to apply in practical industries.
Compared to other materials, graphite based composite materials can selectively add auxiliary conductive carbon materials, reinforcing materials, and resins as binders to the graphite main material, while maintaining the excellent conductivity and chemical stability of graphite materials, and can effectively reduce costs. It is considered a type of material that is expected to be widely used in liquid flow batteries. Fan Yongsheng et al. prepared bipolar plates for all vanadium flow batteries using PVDF and graphite as substrates, and measured the conductivity and bending strength of the bipolar plates. The results show that when the amount of expanded graphite used is 50%, the bipolar plate produced has excellent conductivity (92 S/cm) and mechanical strength (51 MPa) at the same time. And it was found that under the same graphite mass, smaller graphite particles have a larger contact area and better dispersion, which can form more conductive pathways in the bipolar plate, thereby increasing the conductivity of the bipolar plate [3]. Meanwhile, another article also systematically investigated the influence of conductive filler types, contents, particle size, and dispersion methods on the performance of plastic composite bipolar plates. The research process obtained the following conclusions: by comparing the performance of bipolar plates prepared by different processing methods, it was found that the solution processing method and suspension processing method have similar conductive effects, but the suspension processing method has simple operation, short processing time, low cost, and is easy to carry out industrial batch production. On the other hand, adding 400 mesh 50% (wt) expandable graphite bipolar plates has good mechanical strength, can completely block vanadium ion penetration, and has excellent oxidation resistance. It is expected to be applied in the actual development process of all vanadium flow batteries [4].
Wang Wenbin et al. successfully prepared an all vanadium flow battery bipolar plate using solution intercalation composite method, with polypropylene (PP) and maleic anhydride grafted polypropylene (g-PP) as the matrix material and flake graphite (GP) as the conductive filler. The experimental results show that the addition of g-PP can uniformly disperse GP into the polymer matrix, significantly improving the compatibility between conductive fillers and the polymer matrix. The conductivity of the composite bipolar plate is 43.7 S/cm, and the corrosion current at 1.2 V is 1.9 × 10-3A · cm-2, which meets the requirements for the use of all vanadium flow battery bipolar plates. The experiment found that when the GP content in the bipolar plate is constant, the conductivity of the bipolar plate is proportional to the content of g-PP within a certain range. Moreover, the addition of g-PP not only uniformly disperses GP into the polymer matrix, forming a complete conductive network structure, but also forms a tight structure between GP and the polymer, improving the corrosion resistance of the bipolar plate. The electrochemical analysis of dynamic potential, constant potential, cyclic voltammetry, and EIS also showed that the corrosion resistance of self-made bipolar plates was significantly higher than that of commercial carbon plates; And when the content ratio of PP, g-PP, and GP is 1:2:7, not only is the conductivity of the bipolar plate comparable to that of the electrode (polyacrylonitrile based graphite felt), but it can also effectively collect current, and the comprehensive performance of the bipolar plate is the best, meeting the requirements for the use of VRB bipolar plates [5]. There is a contradiction between the amount of graphite filler in graphite based composite bipolar plates and the resin: a high graphite content means that the conductivity of the bipolar plate is improved, but the airtightness and mechanical strength of the bipolar plate will correspondingly decrease; A high resin content will increase the airtightness and mechanical strength of the composite material, but the conductivity of the bipolar plate will decrease. Therefore, researchers further improved the performance of bipolar plates by adding auxiliary fillers such as carbon fibers and carbon nanotubes. Yang Wenjun et al. prepared carbon plastic composite bipolar plates for vanadium batteries using high-density polyethylene resin (HDPE), ethylene propylene diene monomer (EPDM) as substrates, and carbon fiber (CF) and carbon nanotubes as fillers. The conductivity reached 58.3S/cm, but the tensile strength decreased with increasing carbon content. The carbon plastic composite bipolar plate prepared by this method has excellent corrosion resistance [7].
In addition, the integrated electrode bipolar plate has also received widespread attention. Integrated bipolar plate is a battery module that integrates electrodes and bipolar plates by directly pressing electrodes onto bipolar plate materials with thermal viscosity. This integrated structure can not only reduce the contact resistance between the electrode and the bipolar plate, but also reduce the assembly of the battery, making the electrolyte evenly distributed in the porous electrode. The key to the preparation of integrated electrode bipolar plates lies in the selection and manufacturing of bipolar plate materials. Usually, bipolar plate materials using thermoplastic polymer as the matrix have thermal viscosity and can be used as part of the bipolar plate in integrated electrode bipolar plates.
In summary, metal bipolar plates have good conductivity but are not corrosion-resistant, while pure graphite bipolar plates have good conductivity but high cost, and their mechanical properties need to be improved. Therefore, currently, the selection of bipolar plates tends to favor graphite based composite bipolar plates with good conductivity and corrosion resistance. However, there may be slight differences for different liquid flow battery systems, and the focus of consideration will also be correspondingly different. At present, the academic community is constantly conducting in-depth research on non graphite based composite bipolar plates, graphite based composite bipolar plate modification, and integrated electrode bipolar plates. It is believed that greater development and progress will be made under the accelerated commercialization of flow batteries.

Reference materials:
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[6] Shen Kaiyun, Wang Xuehua, Lu Miaomiao, Wang Hao, Wang Qiang. Preparation of bipolar plates for static zinc bromide batteries without separators [J]. Power Technology, 2020,44 (07): 980-982
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