As an environmentally friendly energy storage device, all vanadium flow batteries have become widely used in fields such as wind power, peak regulation for power grid, UPS power supply, etc. The carbon felt electrode for a vanadium battery is a place where redox reactions occur, and doesn’t participate in reaction itself. They are fixed on both sides of proton exchange membrane for electron and proton exchange. After further acid immersion treatment and thermal heating treatment, graphite felt electrodes with higher carbonization degree can be obtained, greatly improve energy efficiency and electrochemical performance, make it an ideal choice for VRFB electrode materials.

Requirements for VRFB electrode materials

1) High conductivity. The conductivity of liquid flow battery decides its output efficiency and overall operating power. Taking carbon felt electrodes as an example, low resistivity, low thickness, high porosity and large contact area can effectively reduce the internal resistance in running, and reduce the electrochemical polarization during charging and discharging. A high surface area can increase the contact area between electrode and electrolyte, improve the reaction volume and efficiency for electrolyte. In addition, the ohmic resistance in overall stack is also affected by the resistance from electrolytes and membranes, the impedance of bipolar plates and the contact resistance between various components.

2) Excellent mechanical performance. High mechanical strength is a guarantee for stability of battery system structure. The transmission and uniform distribution of active substances can be fully ensured by optimizing porosity and network structure, without internal collapse of system. On the other hand, during the pressing process, it is also necessary to reduce contact resistance by increasing pressure. The material must have good toughness and compressibility to avoid deformation and fracture.

3) Cost effectiveness and environmental protection. Low-cost raw material and preparation process are important foundations for large-scale commercial use. Whether the extraction and processing of raw materials have an impact on the environment, is also the key to sustainable development and application of the electrode material.

Performance of Carbon Felt Electrode Materials for Vanadium Batteries

At present, common VFB electrode materials include two categories: metal and carbon. For detailed information, please refer to the article: Electrode Materials of VRFB, Carbon electrodes also include materials such as carbon felt, carbon paper, etc. The ohm impedance of carbon felt electrodes mainly depends on the impedance, porosity, thickness and contact area of the material itself. In addition, the resistance of the bipolar plate, polarization internal resistance, concentration polarization resistance and contact resistance between components also greatly effect the conductivity efficiency of whole battery stack.

In terms of contact resistance between electrode and bipolar plate, the ohm resistance will continuously decrease during the compression process. On the other hand, the higher the compression degree, the lower the porosity, and also affect the transmission of electrolyte and increase the internal resistance. In addition, it will increase pump consumption to reduce system efficiency. In order to strike balance, a proper compression ratio is choosed to achieve ideal overall efficiency during actual assembly. Generally, the carbon or graphite felt electrodes of Vanadium batteries with a flow channel structure are relatively thick and have a high internal resistance. In order to improve the power density of VRFB, thinner electrodes are used to shorten the transmission distance of ions and electrons, and reduce ohm polarization effect. However, the surface area and permeability of electrodes will also decrease, lead to an increase in electrochemical polarization resistance and concentration polarization resistance. Therefore, to balance these three effects, continuous test for battery performance is essential to determine the right thickness of electrode. In practical applications, modification treatment for carbon felt materials is helpful to improve electrochemical reversibility and catalytic activity, which can reduce the impact of thin electrodes on electrochemical polarization. The permeability of the electrode can be improved by optimizing porous structure such as pore shape, pore size and pore distribution, to reduce the impact of concentration polarization.

Manufacturing process of carbon felt/graphite felt electrodes

Based on different raw materials, carbon felt can be mainly divided into three parts: polyacrylonitrile(PAN)-based carbon felt, viscose-based carbon felt, and asphalt-based carbon felt. Among them, the cost-effective and high-quality polyacrylonitrile(PAN)-based carbon fiber is popular in the market. The primary preparation of carbon felt is fiber cutting and lapping, and then forming it through needle punching process. Repeatedly and vertically pierce the needle with a barb into the fiber mesh which is compressed. When the needle is withdrawn, the inserted fibers will remove from barb and remain in the fiber web, The fibers are mixed with each other to form a carbon fiber felt with a certain strength and uniform thickness. The next step is preoxidation, and obtain carbon or graphite felt through carbonization at different temperatures. Generally, the heat treatment temperature of carbon felt is above 1000 degrees Celsius, and the heat treatment temperature of graphite felt is above 2000 degrees Celsius. During the manufacturing process, precursor materials and processing technology play a important role in parameters at graphitization stage. Studies have shown that the resistivity of viscose-based carbon felt is higher than that based on PAN. The carbon felt produced by different manufacturers varies in many aspects including distribution of hydroxyl groups (-OH) on carbon surface, defect concentration and the content of quaternary nitrogen groups, which can also result in obvious differences in electrochemical activity of carbon felt for all vanadium redox flow battery.