1. Manufacturing of Carbon Felt Electrodes

Polyacrylonitrile (PAN) and artificial silk are the two most commonly used precursors in the manufacturing of graphite felt/carbon felt. The manufacturing process of carbon felt materials includes needle punching and graphitization, among which needle punching is an important step in determining the structure and thickness uniformity of the produced carbon felt. The process of needling involves hooking fibers with a barb on the needle and inserting them vertically, achieving compaction and mutual fusion through the rearrangement of fibers. Subsequently, the graphitization step is carried out, and carbon felt or graphite felt is obtained by heat treatment at different temperatures. The processing temperature of carbon felt is about 1200-1600 ℃, and the processing temperature of graphite felt is about 2000-2600 ℃.


The various precursors and processing parameters used in the manufacturing process of carbon felt can affect the graphitization stage of carbon felt materials, leading to differences between carbon felt and graphite felt. The physical parameters of carbon felt made from different precursors vary significantly. Studies have found that the resistivity of GF based on artificial silk is 0.023 Ω cm, which is much higher than the resistivity of PAN varieties (0.0038 Ω cm). The electrochemical activity of carbon felt produced by different manufacturers may also differ significantly due to their different distribution of hydroxyl groups (- OH) on the carbon surface, as well as the content and defect concentration of quaternary nitrogen groups.

Polarization in general batteries can be divided into ohmic polarization, electrochemical polarization, and concentration polarization, which correspond to ohmic internal resistance, electrochemical polarization internal resistance, and concentration polarization internal resistance, respectively. The internal resistance generated by Ohmic polarization mainly depends on the properties and parameters of the electrode itself, including material resistivity, carbon felt porosity, thickness, contact area, etc. Generally speaking, the higher the resistivity, the smaller the porosity, the thicker the thickness, and the smaller the contact area, the greater the Ohmic impedance of the carbon felt electrode. Therefore, for carbon felt electrodes, under ideal conditions, low resistivity, high porosity, small thickness, and large contact area can reduce the Ohmic impedance during the reaction process of all vanadium flow batteries. Of course, if considering the Ohmic resistance throughout the entire battery, it is also necessary to consider the resistance through the electrolyte and membrane during ion transport, the resistance in the bipolar plate, and the contact resistance between battery components.


In fact, the resistance of the electrode and the contact resistance between the bipolar plate and the electrode largely depend on the degree of electrode compression. As the degree of electrode compression increases, its ohmic internal resistance will decrease. However, an increase in electrode compression can also lead to a decrease in electrode porosity, which is not conducive to electrolyte transport and increases Ohmic internal resistance. The trade-off between these two effects results in an optimal compression ratio of approximately 20% to achieve the highest energy efficiency. In actual batteries, due to the layout of the flow channels, the thickness of the electrode frame is usually greater than 3 millimeters. Therefore, the carbon felt or graphite felt electrodes of traditional vanadium flow batteries with flow structures must be relatively thick (about 3-6 millimeters), which leads to the high Ohmic resistance of VFB, resulting in the operating current density of VFB being less than 150 mA cm-2 while maintaining energy efficiency of not less than 80%. According to relevant research reports, the Ohmic polarization ratio of all vanadium flow batteries using carbon or graphite felt accounts for approximately 64% of all polarizations. In addition, due to the proportional relationship between Ohmic overpotential and operating current, Ohmic losses will become more severe when operating in high power/current regions. Therefore, in order to improve the power density of VFB, the electrode thickness should be minimized as much as possible to reduce Ohmic polarization. Current research indicates that reducing electrode thickness will shorten the distance of ion and electron transport, leading to a decrease in Ohmic polarization. On the other hand, it also reduces the surface area and permeability of the electrode, corresponding to an increase in electrochemical polarization and concentration polarization. Therefore, it is necessary to balance these three effects to determine the optimal electrode thickness of various electrode materials with different electrocatalytic activities and pore structures, in order to achieve optimal battery performance.


The electrochemical polarization resistance mainly depends on the electrochemical reaction process of the battery. The carbon felt electrode itself has certain catalytic activity, but the catalytic activity is limited, which will generate a large electrochemical polarization impedance. Therefore, for flow batteries, especially for all vanadium flow batteries operating at higher current densities, it is necessary to modify the electrode material to improve the electrocatalytic activity and electrochemical reversibility. So currently, most research has focused on various modification methods such as introducing surface functional groups, optimizing microstructure, increasing active surface area, and introducing electrocatalysts on the surface of carbon felt electrodes, in order to effectively improve the electrochemical activity of vanadium ion redox reactions. The adjustment of its parameters and process in the manufacturing process of carbon felt electrodes mentioned earlier is also aimed at reducing its electrochemical polarization impedance and improving the activity of carbon felt electrodes.

The concentration polarization caused by insufficient mass transfer of active substances during the reaction process is also a major limiting factor in the development of high-power density all vanadium flow batteries, as faster reaction rates require more active substances. As is well known, increasing the electrolyte flow rate can enhance mass transfer and reduce concentration polarization, but this method also increases pump energy consumption, thereby reducing the overall energy efficiency of all vanadium flow battery systems. Therefore, currently, most efforts are made to improve electrode permeability and reduce concentration polarization by optimizing the pore size, pore distribution, and pore shape of porous structures. For conventional all vanadium flow batteries, the low flow resistance caused by the high porosity of thick CF or GF electrode materials results in less concentration polarization compared to the other two polarizations. But using appropriate flow channels can improve the flow distribution through the electrodes and reduce flow resistance, thereby reducing pump energy consumption.

In addition, the use of thin electrodes has become a development trend for obtaining higher power density VFB electrodes, as it can greatly reduce the Ohmic polarization of VFB. However, it simultaneously reduces the actual surface area and increases flow resistance, leading to an increase in electrochemistry and concentration polarization. Therefore, it is necessary to develop electrode materials with higher electrocatalytic activity and appropriate pore structure. In addition, the use of a zero gap structure must be accompanied by bipolar plates and flow fields. Currently, most public reports use graphite plates with flow fields as bipolar plates. However, brittleness and high cost limit its industrial application. Therefore, developing carbon plastic composite bipolar plates with high conductivity is the key to the industrial application of this structure.


For concentration polarization, due to the small concentration polarization of VFB with CF or GF electrodes, there is less attention paid to improving the concentration polarization of VFB. However, as the electrode thickness decreases and the flow resistance increases, concentration polarization will increase. At this point, in addition to designing flow channels on the surface of the bipolar plate, optimizing porous structures such as pore size, pore distribution, and pore shape, increasing electrode permeability is also a good solution strategy. Especially the design of multi-level pore structure can effectively solve the problems of high flow resistance, low porosity, and active surface area caused by thin electrodes. In the following research, optimizing the pore size of graded pores, including macropores as electrolyte transport channels and small pores as active sites for redox reactions, is crucial for further reducing concentration polarization.

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