Overview Of Carbon Felt Electrode Modification For Flow Batteries (I) Surface Functionalization Modification

 

 

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

 

As we all know, the most developed in the field of flow batteries is the all-vanadium flow battery. VO2+/VO2+ is the positive active material of the all-vanadium flow battery, and V2+/V3+ is the negative active material of the all-vanadium flow battery. The redox reaction of the positive and negative active materials generates electrical energy and realizes the conversion of chemical energy.

 

In flow batteries, electrode materials are very important links. Although they do not directly participate in the redox process as reactants, they provide a place for redox reactions. Good electrode materials will undoubtedly promote the charge and discharge reaction of flow batteries, ensure the stability of the battery structure and service life, and thus 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 liquid 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 carbon fiber obtained by carbonization at a temperature of about 1000 degrees Celsius. The carbon content of carbon fiber felt is about 90%, and the operating temperature of carbon felt is about 1000 degrees Celsius.

 

Correspondingly, graphite felt is formed by heating carbon fiber felt to above 2000 degrees Celsius in an oxygen-free environment. Its operating temperature is also as high as about 2000 degrees Celsius.

 

However, the electrochemical performance of the original carbon felt or graphite felt electrode is not ideal. Therefore, it is often necessary to modify its surface to improve its reversibility in the battery reaction, thereby improving the voltage efficiency and power density of the all-vanadium liquid flow battery.

 

This article will mainly review the surface activity improvement process and related research of the all-vanadium liquid flow battery carbon felt electrode that are currently widely cited. This content will be divided into four parts and sent separately. This content mainly focuses on the modification of carbon felt surface functional groups.

 

Surface functional group modification is mainly achieved by introducing active oxygen-containing functional groups to improve the electrochemical activity and hydrophilicity of carbon felt electrodes, thereby promoting the reaction in all-vanadium liquid flow batteries.

 

Common methods include direct oxidation and strong acid oxidation. In addition, introducing oxygen-containing functional groups to carbon felt by electrochemical means to improve the electrochemical performance of carbon felt electrodes is also an important means of carbon felt surface modification.

 

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 move to the anode and attach to the surface of the carbon felt to achieve the modification effect.

 

Liu Suqin et al. [2] obtained surface-modified carbon felt by directly oxidizing carbon felt in heated air at 435°C for 10 hours. The specific surface area and surface oxygen-containing functional groups of the carbon felt obtained by direct oxidation are greatly increased, thereby improving the electrochemical activity of the carbon felt electrode. The voltage efficiency and coulombic efficiency of the carbon felt electrode prepared by the method are as high as 89% and 95% respectively under the condition of 50 mA cm-2.

 

Wang Xinwei et al. [3] obtained surface-modified carbon felt by heat-treating polyacrylonitrile-based carbon felt electrodes prepared at different temperatures at 450°C for 2h. The electrode surface contains more chemically active groups, such as C-OH, C=O, -COOH, etc., which makes the battery have better reaction activity.

 

Ki et al. [3] After mild oxidation of carbon felt electrodes at 500°C for 5 hours, the battery energy efficiency increased from 68% to 75%, and the electrode still maintained its electrochemical activity even after 500 cycles. The improvement in efficiency is attributed to the fact that mild oxidation modification increases the surface area of ​​the carbon felt electrode and forms active functional groups on its surface.

 

Sun et al. [4] directly used concentrated sulfuric acid to modify carbon felt. After heating in concentrated sulfuric acid for 5 h, the number of oxygen-containing functional groups on the surface of carbon felt increased. The internal resistance reported by this process was only 2.5 Ω cm-2, and its battery energy efficiency was also improved to a certain extent.

 

At the same time, Wang Xinwei et al. [5] obtained surface-modified carbon felt by treating polyacrylonitrile-based carbon felt electrodes prepared at different temperatures with nitric acid for 5 h. Oxygen-containing groups were also introduced on the electrode surface, which reduced the potential of the reaction and significantly improved the electrochemical activity of the electrode.

 

Liu Suqin et al. [6] used Prussian blue (PB) to modify the surface of carbon felt by electrochemical deposition. The positive electrode performance of PAN-based carbon felt after PB modification was significantly improved compared with that before modification. The peak potential difference was reduced to 96 mV, and the peak current density was increased to 1.333 mA cm-2. The electrode had good reversibility and stable cycle performance, and could be used as the positive electrode of all-vanadium redox flow battery.

 

The static vanadium battery with carbon felt modified with PB and oxalic acid as positive and negative electrodes respectively had a voltage and current efficiency of 83.28% and 96% at a current density of 35 mA cm-2, which were 13.89% and 7.2% higher than those of the battery using unmodified electrodes.

 

Yue et al. [7] used electrochemical oxidation to modify the carbon felt (CFs) electrodes in the all-vanadium redox flow battery (VRFB) in different weak acid solutions (citric acid, oxalic acid and ethylenediaminetetraacetic acid). Among them, the single cell assembled after the carbon felt electrode was oxidized in ethylenediaminetetraacetic acid for 2h had the best battery performance and energy efficiency, which increased from 81.4% to 85.4%. This was mainly due to the increase in oxygen content on the surface of the carbon felt electrode during the treatment. The modified carbon felt can also meet actual needs.

 

Ki et al. [8] studied the effect of surface treatment combined with corona discharge and hydrogen peroxide (H2O2) on the electrochemical performance of carbon felt electrodes for vanadium redox flow batteries (VRFBs). They successfully introduced high concentrations of oxygen-containing functional groups into the surface of carbon felt electrodes through specially designed surface treatment to improve the energy efficiency of all-vanadium flow batteries.

 

The all-vanadium flow battery with surface-modified carbon felt electrodes prepared by this process has better carbon felt electrode wettability at high current density (148 mA cm-2), which is mainly due to the fact that the surface active oxygen-containing functional groups can make the charge transfer faster and have better wettability.

 

In addition, it is claimed 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 performance carbon felt with oxygen-rich phosphate groups as an electrode preparation process for all-vanadium redox flow batteries. By directly modifying the carbon felt surface with ammonium hexafluorophosphate, the -OH part with good hydrophilicity can form phosphate functional groups and successfully bind phosphorus to the surface of the carbon felt.

 

The carbon felt rich in phosphate groups exhibits excellent catalytic effects and can effectively improve the electrochemical reactivity of the redox reactions of VO2+/VO2+ (in the cathode liquid) and V2+/V3+ (in the anode liquid).

 

In addition, the occurrence of hydrogen evolution reaction can be suppressed by minimizing the overpotential of the V2+/V3+ redox reaction in the anode electrolyte in the all-vanadium liquid flow battery. Moreover, the battery cycle test using the prepared catalytic electrode showed that at a constant current density of 32 mA cm-2, the energy efficiency in the 1st and 20th cycles was improved to 88.2 and 87.2% compared with 83.0 and 81.1% of the original electrode, respectively, which 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 ​​bare polyacrylonitrile carbon felt and increase the contact potential between vanadium to reduce the overpotential generated by the electrochemical reaction gap, Lin et al. [10] prepared a high-performance carbon felt electrode for all-vanadium redox flow battery system by treating it with low-temperature atmospheric pressure plasma in air.

 

The Brunauer-Emmett-Teller (BET) surface area of ​​the modified carbon felt prepared by them was five times higher than that of the original felt. The modified carbon felt showed higher energy efficiency (EE) and voltage efficiency (VE) in the all-vanadium flow battery single cell test at a constant current density of 160 mA cm -2, and maintained good performance at low temperatures.

 

In addition, the results showed that the resistance between the electrolyte and the newly prepared carbon felt electrode was also reduced. Due to the increased reactivity of vanadium ions on the treated carbon felt, the all-vanadium flow battery with plasma-modified carbon felt has much higher efficiency and shows better capacity under 100 cycles of constant current charge and discharge test.

 

Kwang et al. [11] proposed to modify carbon felt by heat treatment. However, heat treatment will cause local damage to the surface of carbon felt. Therefore, glucose was selected as the coating material to protect the carbon felt during heat treatment and provide abundant functional groups as active sites in redox reactions.

 

The results show that the glucose-based carbon coating on the carbon felt exhibits a higher crystalline graphite structure than the heat-treated carbon felt and promotes electrochemical properties such as electron transfer kinetics and reversibility of redox reactions. The energy efficiency of the carbon felt with glucose-based carbon coating is 82.79% at 100 mA cm-2, which is 2.0% higher than that of the original carbon felt.

 

The functional group modification of carbon felt surface is an important means to achieve the modification of carbon felt electrodes for flow batteries. The introduction of oxygen-containing functional groups by various means plays an important role in improving the operating efficiency and overall performance of all-vanadium flow batteries.

 

At present, the process for introducing functional groups is still being improved and developed. We believe that in the process of continuous advancement of scientific research, the introduction of surface active functional groups by simpler and easier methods will help all-vanadium flow batteries shine in the field of energy storage.