Conductive polymer-based composite electrode materials are composed of polymer and conductive materials. Common matrix materials include polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), ABS resin, etc. Common conductive materials include metal or carbon materials such as carbon black and graphite. In practical application, conductive materials are added to the resin matrix through special processing technology, so that the plastic becomes a composite electrode material with conductive property.

Influence of polymer matrix on composite electrode materials

Polymers mainly play the role of bonding, protecting and stabilizing conductive materials, and also affect the mechanical properties, temperature resistance and other parameters of conductive composites. The molecular weight, crystallinity, flexibility of the main chain and compatibility with conductive fillers of polymer will affect the conductivity and other comprehensive properties of the whole electrode materials.

The small relative molecular weight of polymer matrix leads to the low frame strength after drying and poor physical properties. On the contrary, if the relative molecular weight is too large, it will affect the toughness of the material and the material become brittle. Meanwhile, the increase of the thickness of the resin among conductive particles will also reduce the conductivity. Generally, conductive fillers are more likely to exist in places with poor crystallization. Therefore, the stronger the crystallinity of the polymer matrix, the greater the filler concentration in the non crystalline area, the smaller the spacing between conductive particles, and the better the conductivity. In addition, the interface effect between the conductive filler and the polymer will also affect the conductivity. Lower viscosity of the polymer can make conductive filler more evenly dispersed in the polymer, and the conductive network can be formed with a small amount of conductive materials. For example, polypropylene with low surface tension can conduct electricity when the content of conductive filler is 5%, while nylon needs 30% of conductive filler for electric conduction.

Effects of conductive materials on composite electrode materials

Different conductive materials lead to different conductive performance, even for the same same conductive media, the conductivity will be different due to different manufacturing processes. Carbon black is a kind of nano scale conductive material with low price and stable conductivity. In addition, the carbon black industry is relatively mature, and various brands of carbon black products provide a a wide range of choices. It has become the most widely used conductive filler. The modification of carbon black materials is also the focus of research. For example, after high-temperature treatment, the specific surface area can be increased and the chemical properties of its surface can be changed. The conductive and mechanical properties of composite materials can be improved by coupling agent treatment. Adding appropriate dispersant can make carbon black particles more evenly dispersed in the polymer.

When conductive fillers are added to the polymer, it’s difficult to form a uniform distribution. Only when the spacing between carbon particles is small enough and chain-type conductive channels are formed, the composite materials can have conductive properties. On the other hand, when the amount of conductive material increases to a certain critical value, the bulk resistivity of the composite electrode material will suddenly decrease. This critical value is known as percolation threshold. There are many factors affecting the percolation threshold, including the structure, characteristics, composite process and the interface effect between conductive fillers and polymers. The composite conductive plastic with carbon black as the conductive media can adjust conductive peformance in a wide range. The specific usage amount should be determined according to the specific model of carbon black and its percolation range in the polymer. How to improve the conductivity of composite electrode materials on the basis of reducing the amount of conductive fillers is the future research direction.

Manufacturing process of conductive polymer-based composite electrodes

At present, the mainstream composite electrode materials at home and abroad are manufactured by filling composite procedure, one-time molding is ideal method to reduce the technological process flow for mass production. Conductive fillers must be fully dried before preparing, in order to prevent water or other volatiles from appearing bubbles or other defects on the surface of finished products. In addition, the uniformity of dispersion in the polymer will also affect the final conductivity of composite electrodes, so the material should mixing well, and the distribution structure of the conductive filler may be affected during the process. In extruding process, try to ensure a small force and a low speed to ensure that the conductive structure won’t be damaged. Excessive shear stress will break the conductive chain and reduce the conductivity. In addition, the conductive structure can be effectively protected by increasing the molding temperature or melt index, reducing the viscosity is necessary. Decide the appropriate compression molding technology according to needs, extrusion process or injection molding process. Both processing technology and material combination should be based on different properties of conductive materials and polymers. Only thus can manufacture the best conductive polymer composite electrode materials for flow batteries.