Polymer plastic electrode plates are one of the important components of all Vanadium redox flow batteries. Not only insulate positive and negative electrolytes, but also establish a current path between the anode and cathode in series. Polymer conductive plates can be combined with graphite felt electrodes to form an integrated composite plate materials, taking into good electrochemical and mechanical properties. Currently, the composite conductive plastics for bipolar plates are prepared by mixing carbon based conductive materials and polymer materials, and then add blockers and release agents, common polymer resins include PP, PE, PVDF, CPE, PVC, etc, common conductive fillers include graphite, carbon black, metal powder, etc.

Advantages of plastic electrode plates

The traditional electrode plate of vanadium battery is mainly made of graphite, which is usually mixed with graphite powder or asphalt, after high temperature treatment of several thousand degrees, further cutting and polishing, it has many advantages of good electrical conductivity, low resistivity, good density. The disadvantages include high manufacturing costs and high brittleness of materials. The assembly of battery stack for all vanadium flow battery seems simple, but in reality there is a high requirement for pressure resistance performance of electrode plate material. The electrode and the bipolar plate belong to hard connection, require a certain pressure to reduce contact resistance. Poor fit can reduce the stack efficiency. Therefore, when assembling vanadium battery stacks, uneven pressure assembly may cause graphite bipolar plate deformation or fragmentation, which directly result in leakage of battery electrolyte. This phenomenon will lead to degradation of battery capacity and even safety accidents. In general, the greater the power and reaction area of the stack, the greater the difficulty of stack assembly process. Therefore, graphite electrode plates are more suitable for small electric stacks and not suitable for large-scale vanadium battery applications.

Use conductive plastic bipolar plates to replace graphite bipolar plates, is one of the current improvement methods. Add a portion of polymer mixture to carbon based materials, can greatly imporve tensile strength and bending strength of bipolar plates, and the cost of composite bipolar plates is reduced by nearly half. The corrosion resistance of polymer conductive plates is also much better than that of conventional graphite or metal electrode plates. Due to the strong oxidizing and reducing electrolytes on both sides of electrode plate, the electrode plate materials require good chemical inertness in long-term operation. Another advantage brought by the conductive polymer electrode plate is that it can be welded to the guide frame plate, no need any sealing ring assembly to prevent leakage, reduce a possible electrolyte leakage point. Lighter weight can reduce the overall weight of battery stack and convenience to transport. For a large energy storage project, using plastic electrode plates can reduce total costs and increase market competitiveness. In addition, carbon plastic composite electrode plates can be applicable to different types and specifications of flow batteries by changing formulation and manufacturing process of materials, and can adjust electrical conductivity in a wide range, with strong adaptability.

Performance and cost of plastic electrode plates

All Vanadium flow batteries require high demands on performance of bipolar plates including high conductivity, high mechanical strength, good corrosion resistance, etc. However, blindly pursuing high performance will also lead to high costs and cannot satisfy the needs of industrialization. Generally speaking, electrical conductivity and mechanical strength are in a restrictive relation. How to maximize electrical performance while maintaining the strength of the composite material, the proportion and distribution for conductive fillers and polymers play a crucial role. In conventional conductive composite plate, conductive materials will be wrapped by polymers to form an independent region. New material formulations and processes are used to add high content of conductive particles to connect isolated conductive regions to form a complete conductive network skeleton. The conductivity of composite materials will largely depend on the dispersion status of polymers and conductive materials, and final conductive structures, conductive particles with smaller sizes are more likely to be uniformly dispersed in the polymer body. By fully mixing the fillers and optimizing conductive structure, the lower content of conductive fillers not only meets the requirements for conductive properties of battery stack, but also has better mechanical properties and significantly reduces production costs.

Never talk about performance regardless of cost. No matter metal electrode plate or graphite electrode plate, the cost are much higher than plastic electrode plates in preparation process. Polymer-carbon composite electrode plates can be mass produced through common plastic processing methods such as injection, molding, extrusion, calendering and others. Less than half the cost of graphite bipolar plates. They have strong market competitiveness and significantly reduce the whole battery costs. The tensile strength and bending strength are greater than 14 mpa, it’s diffcult to crack during press mounting process, making up for the shortcomings of graphite electrode plate such as large brittleness and poor mechanical properties. Plastic electrode plates made of PE or PP, which can be selected based on different materials of the plate frame for laser welding, fully ensure that the plates is free of liquid leakage. Carbon based composite bipolar plates can bring better performance through flexible design, and can also reduce long-term costs through manufacturing improvements in the future. In fact, the application of polymer conductive plates in vanadium batteries is no longer in the experimental testing phase, but has been put into large-scale energy storage.

Research direction of plastic polymer conductive plates

Plastic polymer conductive plates have many advantages of high mechanical strength, corrosion resistance, cheaper cost, light weight, long service life and so on. However, plastic electrode plates also have some disadvantages in VRFBs, for example, their electrical conductivity is not as effective as metal or graphite electrode plates, which means that they cannot store as much energy as metal electrodes. In addition, in order to obtain better electrical conductivity, more conductive materials are added to result in a serious decline in the mechanical properties. Carbon cloth materials can be used as a reinforcement skeleton to balance the longitudinal and lateral stress of composite material. The impact strength, hardness, and tensile strength of sandwich-type bipolar plates have been greatly improved. Researchers have conducted extensive research on composite bipolar plates of VRFB to improve the efficiency of plastic electrodes and reduce their cost of production, maximize their electrical conductivity without adding conductive fillers, and make them a viable option for all Vanadium redox flow batteries.