As one of the key components for both Zinc-Bromine flow battery and Vanadium flow battery, bipolar plates are conductive plates in a flow battery stack that act as a positive electrode for one cell and a negative electrode for the next cell, and play the role of isolating the negative and positive electrolyte in the flow batteries. High-quality bi-polar plates must meet a variety of property requirements such as high conductivity, low permeability, corrosion resistance, high tensile strength and bending strength, high thermal conductivity, stable chemical and electrochemical properties, low thermal expansion, easy processing and molding, etc. Therefore, the quality of the bipolar plate will directly determine the power output and service life of the battery stack.

Common types of bipolar plate materials

1) Carbon/graphite bipolar plate materials

Advantages include strong corrosion resistance, good thermal conductivity and electrical conductivity, disadvantages include insufficient mechanical strength, it is easy to break during use, resulting in battery failure. In addition, it is not easy to shape ultra-thin bipolar plates (such as < 0.5mm). According to different processing technologies, it can be divided into machining molded carbon/graphite bipolar plates and injection molded carbon/graphite bipolar plates.

2) Metal bipolar plate materials

Metal bipolar plates have properties of high electrical conductivity, good mechanical strength and various production methods. Both gas flow channel and cooling water flow channel can be processed by stamping or etching technology, and also can be processed into thin plates to reduce battery volume. However the corrosion resistance of metal bipolar plates is poor. Besides, the large-scale production is limited due to their high costs such as gold, titanium and platinum. In particular, the penetration of metal ions into the proton exchange membrane will lead to the decrease of ionic conductivity and the corrosion layer will increase the contact resistance and further to reduce the battery performance.

3) Composite material bipolar plates

As thermoplastic polymer materials have properties of plasticity, flowability and adhesiveness at transition temperature, and also can be extruded, injected and molded. Plastic conductive bipolar plates with different shapes can be prepared based on the different molds. Therefore, the modes can be made for bipolar plate and flow channel which are suitable for technical requirements, and the corrosion-resistant polymer materials are selected to be compounded with conductive filling materials, The bipolar plate with flow field can be directly obtained by one-time composite molding process. Composite conductive plastic bipolar plate overcomes the disadvantages of metal plate and graphite plate, have strong corrosion resistance and good mechanical strength, and can meet the requirements of Redox-Flow flow battery.

Composite conductive plastic bipolar plates

Due to the limitations of metal electrode and carbon electrode, composite conductive plastic with good electrochemical performance and corrosion resistance, has become the most widely used bipolar plate materials for liquid flow battery. The composite plastic electrode is based on polymer materials, mixed and pressed with conductive fillers in a certain proportion to increase its conductivity.

1) Polymer material

Common polymer materials include polyethylene, nylon, polypropylene, rubber modified polypropylene, polytetrafluoroethylene, polyvinylidene fluoride, etc. In addition, in order to ensure good dispersion of the conductive filler in the bipolar plate, it is necessary to consider the compatibility between the polymer material and the conductive filler, the good chemical corrosion resistance and electrochemical corrosion resistance of polymer material are also very important. For example, in the liquid flow battery, the corrosion of the bipolar plate is mainly caused by the erosion of carbon by active oxygen atoms generated in the chemical reaction process. If the graphite and polymer are combined closely, the loss of carbon will be reduced to a certain extent.

2) Conductive filler

Conductive fillers include graphite powder, carbon black, metal powder, etc, and polymer resins include PP, PE, CPE, PVC, PVDF, etc. In order to improve the mechanical properties of carbon plastic bipolar plates, carbon fibers can also be added for better mechanical strength. Pure graphite has high conductivity, but it is easy to break when processing and installing, which is a major problem in large-scale industrial production. Graphite powder has excellent electrical performance, lubricity, dispersion and excellent chemical stability. The higher the purity of graphite powder, the better the electrical conductivity. The particle size of graphite powder also affects its electrical conductivity. It’s still one of the widely used conductive fillers in bipolar plates. Graphite can be also modified to further improve the conductivity.

3) Preparation and assembly of conductive plastic bipolar plates

At present, bipolar plates are generally prepared by mixing polymer materials and conductive fillers, after molding, injection, extrusion and casting into plates. Another method is to mix polymer materials and conductive fillers are, after plasticating, molding and then calendering process. Choose which kind of process route should be determined according to the product specifications and production capacity requirements. When the bipolar plate is applied for redox flow battery, the bipolar plate and shell frame must be assembled via high quality laser welding or other methods, the BP materials should be flat and have certain mechanical strength to ensure no electrolyte leakage during use.

At present, there is no uniform national standard for conductive plastic bipolar plates. Generally speaking, charge/discharge cycle number reflects its chemical corrosion resistance at some degree. Theoretically, the cycle life of the redox flow battery is infinite, but the number of charge and discharge cycles is limited by the life of separator membrane, but it should not be less than 6000.

Research and development of bipolar plates

At present, comparing oversea research level, gap is still exist in the research on bipolar plates. The mainly reason is that the domestic research on redox flow batteries started relatively late, and China is still at a catching stage on technology, so it is difficult to make a breakthrough in selection of process and raw materials. The bipolar plate requires low material cost and processing cost, light weight, thin plate, good mechanical properties, high surface and volume conductivity, low air permeability and corrosion resistance. Choosing appropriate bipolar plate materials and preparation technology can greatly improve the performance of the flow battery. In addition, at present the cost of redox flow battery is five times higher than lead-acid battery, and the cost of bipolar plate can account for about 40% of redox flow battery. Reducing the production cost of flow batteries is the basic condition for realizing industrialization and marketization, and also will be the key research direction of bipolar plate materials in the future.