As one of the most promising energy storage technologies, flow batteries have received a lot of development and attention in recent years. With the industrialization of flow battery projects across the country and policy incentives for the development of flow battery technology, flow batteries will undoubtedly become an indispensable presence in the energy storage field.

As is well known, the most well-developed in the field of flow batteries is all vanadium flow batteries. VO2+/VO2+serves as the positive electrode active material of all vanadium flow batteries, and V2+/V3+serves as the negative electrode active material of all vanadium flow batteries. Through the oxidation-reduction reaction of positive and negative electrode active materials, electricity is generated, achieving the conversion of chemical energy. In liquid flow batteries, electrode materials are a very important link. Although they do not directly participate in the redox process as reactants, they provide a place for redox reactions. Good electrode materials undoubtedly promote the charging and discharging reactions of flow batteries, ensure the stability and service life of the battery structure, and thereby improve the overall operating efficiency and output power of flow batteries. In previous articles, we have reviewed and analyzed relevant patents in the field of all vanadium flow battery electrodes. Currently, carbon felt and graphite felt electrodes are mainly used, which have good conductivity, high stability, high specific surface area, and considerable cost advantages [1].

Carbon felt, also known as carbon fiber felt, refers to the carbonization of carbon fibers at a temperature of around 1000 degrees Celsius. After being made into carbon fiber felt, the carbon content is about 90%, and the usage temperature of carbon felt is also around 1000 degrees Celsius. The corresponding graphite felt is formed by heating carbon fiber felt in an anaerobic environment to above 2000 degrees Celsius, and its usage temperature can also reach around 2000 degrees Celsius. However, the electrochemical performance of the original carbon or graphite felt electrodes is not ideal, so it is often necessary to modify their surface to improve their reversibility in battery reactions, thereby improving the voltage efficiency and power density of all vanadium flow batteries. This article will mainly summarize the surface activity improvement process and related research of carbon felt electrodes in all vanadium flow batteries, which are currently widely cited. This content will be sent separately in four parts, mainly focusing on the modification of functional groups on the surface of carbon felt.

Surface functional group modification is mainly achieved by introducing active oxygen-containing functional groups to enhance the electrochemical activity and hydrophilicity of carbon felt electrodes, thereby promoting the reaction in all vanadium flow batteries. The commonly used methods include direct oxidation and strong acid oxidation. In addition, introducing oxygen-containing functional groups through electrochemical methods to attach to carbon felt to improve the electrochemical performance of carbon felt electrodes is also an important means of surface modification of carbon felt. In the electrochemical oxidation method, carbon felt is used as the anode and graphite is used as the cathode. Under acidic conditions, the negatively charged oxygen-containing functional groups are moved towards the anode and attached to the surface of the carbon felt, achieving the modification effect.

Liu Suqin et al. [2] obtained surface modified carbon felt by directly oxidizing it in heated air at 435 ℃ for 10 hours. The specific surface area and oxygen-containing functional groups on the surface of the carbon felt obtained through direct oxidation are significantly increased, thereby improving the electrochemical activity of the carbon felt electrode. The voltage efficiency and Coulombic efficiency of the carbon felt electrode prepared under 50 mA cm-2 charging and discharging conditions are as high as 89% and 95%, respectively. Wang Xinwei et al. [3] obtained surface modified carbon felt electrodes made of polyacrylonitrile based carbon felt at different temperatures after being heat treated at 450 ℃ for 2 hours. The electrode surface contains more chemically active groups, such as C-OH, C=O, - COOH, etc., which makes the battery have better reactivity.

After mild oxidation at 500 ℃ for 5 hours, the energy efficiency of the carbon felt electrode increased from 68% to 75%, and even after 500 cycles, the electrode still maintained its electrochemical activity. The improvement in efficiency is attributed to the mild oxidation modification, which increases the surface area of the carbon felt electrode and forms active functional groups on its surface.

Sun et al. [4] directly modified carbon felt with concentrated sulfuric acid. After heating in concentrated sulfuric acid for 5 hours, the number of oxygen-containing functional groups on the surface of the carbon felt increased. The reported internal resistance of this process was only 2.5 Ω cm-2, and its battery energy efficiency was also improved to a certain extent. Meanwhile, Wang Xinwei et al. [5] obtained surface modified carbon felt electrodes made of polyacrylonitrile based carbon felt at different temperatures and subjected to acid treatment for 5 hours under nitric acid. The electrode surface also introduced oxygen-containing groups, reducing the reaction potential and significantly improving the electrochemical activity of the electrode.

Liu Suqin et al. [6] modified the surface of carbon felt with Prussian blue (PB) through electrochemical deposition. After PB modification, the positive electrode performance of PAN based carbon felt was significantly improved compared to before modification, with a peak potential difference reduced to 96 mV and a peak current density increased to 1.333 mA cm-2. The electrode has good reversibility and stable cycling performance, and can be used as a positive electrode for all vanadium redox flow batteries. The voltage and current efficiency of static vanadium batteries with PB and oxalic acid modified carbon felt as positive and negative electrodes were 83.28% and 96%, respectively, at a current density of 35 mA cm-2. Compared with batteries with unmodified electrodes, the voltage and current efficiency were improved by 13.89% and 7.2%, respectively.

Yue et al. [7] modified carbon felt (CFs) electrodes in vanadium redox flow batteries (VRFBs) using electrochemical oxidation in different weak acid solutions (citric acid, oxalic acid, and ethylenediaminetetraacetic acid). Among them, the single cell assembled with carbon felt electrodes after 2 hours of oxidation in ethylenediaminetetraacetic acid showed the best battery performance and energy efficiency, increasing from 81.4% to 85.4%. This is mainly due to the increase in oxygen content on the surface of the carbon felt electrode during treatment, and the modified carbon felt can also meet practical needs.

Ki et al. [8] studied the effect of surface treatment combining corona discharge and hydrogen peroxide (H2O2) on the electrochemical performance of carbon felt electrodes in vanadium redox flow batteries (VRFB). They successfully introduced high concentrations of oxygen-containing functional groups into the surface of the carbon felt electrode through a specially designed surface treatment to improve the energy efficiency of all vanadium flow batteries. The all vanadium flow battery with surface modified carbon felt electrode prepared by this process exhibits better wettability of the carbon felt electrode at high current density (148 mA cm-2), mainly due to the fact that the surface active oxygen-containing functional groups can facilitate faster charge transfer and better wettability. In addition, it claims that this method is more competitive than other surface treatments in terms of processing time, production cost, and electrochemical performance.

Ki et al. [9] also proposed a stable carbon felt with oxygen rich phosphate groups as an electrode preparation process for all vanadium redox flow batteries. It directly modifies the surface of carbon felt with ammonium hexafluorophosphate, and the - OH part with good hydrophilicity can form phosphate functional groups, successfully binding phosphorus to the surface of carbon felt. Carbon felt rich in phosphate groups exhibits excellent catalytic activity, which can effectively improve the electrochemical reactivity of the redox reactions of VO2+/VO2+(in cathode solution) and V2+/V3+(in anode solution). In addition, the occurrence of hydrogen precipitation reaction can be suppressed by minimizing the overpotential of the V2+/V3+redox reaction in the anode electrolyte of all vanadium flow batteries. Moreover, battery cycling tests using the prepared catalytic electrode showed that at a constant current density of 32 mA cm-2, the energy efficiency increased to 88.2% and 87.2% in the first and 20th cycles compared to the original electrode, which were 83.0% and 81.1%, respectively. This is mainly attributed to the faster charge transfer caused by the oxygen rich phosphate groups on the carbon felt electrode.

In order to improve the hydrophilicity and surface area of polyacrylonitrile bare carbon felt, increase the contact potential between vanadium, and reduce the overpotential generated by electrochemical reaction gaps, Lin et al. [10] prepared a high-performance carbon felt electrode for all vanadium redox flow battery systems by using low-temperature atmospheric pressure plasma treatment in air. The Brunauer Emmett Teller (BET) surface area of the modified carbon felt prepared by it is five times higher than that of the original felt. The modified carbon felt showed higher energy efficiency (EE) and voltage efficiency (VE) in single cell testing of all vanadium flow batteries at a constant current density of 160 mA cm-2, and maintained good performance at low temperatures. In addition, the results indicate a decrease in resistance between the electrolyte and the newly prepared carbon felt electrode. Due to the increased reactivity of vanadium ions on the treated carbon felt, the efficiency of all vanadium flow batteries with plasma modified carbon felt is much higher, and they exhibit better capacity under 100 cycles of constant current charging and discharging tests.

Kwang et al. [11] proposed modifying carbon felt through heat treatment. However, the heat treatment method can cause local damage to the surface of the carbon felt. Therefore, glucose was chosen as the coating material to protect the carbon felt during the heat treatment process and provide abundant functional groups as active sites in the redox reaction. The results showed that the glucose based carbon coating on the carbon felt exhibited a higher crystalline graphite structure than heat-treated carbon felt, and promoted electrochemical properties such as electron transfer kinetics and reversibility of redox reactions. The energy efficiency of carbon felt with glucose based carbon coating at 100 mA cm-2 was 82.79%, which was 2.0% higher than the original carbon felt.

Surface functionalization modification of carbon felt is an important means of modifying carbon felt electrodes for flow batteries. Introducing oxygen-containing functional groups through various means plays an important role in improving the operational efficiency and overall performance of all vanadium flow batteries. At present, the process methods for introducing functional groups are still being improved and developed. We believe that in the process of scientific exploration, introducing surface active functional groups through more convenient and feasible methods will help all vanadium flow batteries shine in the field of energy storage.



Reference materials

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