This series of content 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. In the previous two articles, we have reviewed two methods of modification: surface functionalization of carbon felt and introduction of carbon nanotubes. Both of these methods are important means of modifying carbon felt electrodes for flow batteries. By introducing oxygen-containing functional groups or carbon nanotube materials through various means, the surface of carbon felt electrodes can be modified, thereby improving the operational efficiency and overall performance of all vanadium flow batteries. This content is the third in a four part series, mainly focusing on the modification of carbon felt electrodes by depositing metals or metal oxides on the surface of the carbon felt.

In the positive and negative reactions of vanadium flow batteries, there are often side reactions generated by hydrogen and oxygen, leading to electrolyte imbalance and a decrease in the Coulombic efficiency of the battery. Research has shown that depositing metal or transition metal oxides on the surface of carbon felt through certain means and methods can increase the overpotential of hydrogen and oxygen precipitation, thereby improving battery efficiency. The usual deposition methods include ion exchange, electrochemical deposition, and impregnation.

Wang et al. [1] reported a carbon felt surface modified Ir electrode obtained through pyrolysis reduction of H2IrCl6 process. The voltage efficiency of a single battery composed of Ir modified carbon felt reached 87.5% at 20 mA cm-2, and the energy efficiency reached 69.7%. Compared with the battery composed of unmodified carbon felt, the average voltage efficiency increased by 8.6%, and the average internal resistance of the battery decreased by 25%. However, due to the simultaneous reduction of overpotential for hydrogen and oxygen evolution by Ir, this process is not suitable for negative electrode applications and can only be used for positive electrode material modification. Kim et al. [5] reported a process of loading inexpensive metal oxide Mn3O4 on the surface of carbon felt electrodes by hydrothermal method. The modified carbon felt electrodes obtained have good catalytic effects on the electrochemical reaction rates of both positive and negative electrodes. This is mainly because the loaded Mn3O4 particles can not only inhibit the side reaction of oxygen precipitation, but also have strong catalytic effects on the electrode reaction of VO2+/VO2+and V2+/V3+pairs, thereby improving the voltage efficiency and Coulombic efficiency of vanadium batteries.

Wang Xinwei et al. [2] reported that the nickel metal surface modified carbon felt prepared by treating polyacrylonitrile carbon felt at 2000 ℃ with nickel nitrate solution for 24 hours showed significantly improved electrochemical performance and reversibility, making it particularly suitable as a negative electrode material for redox flow batteries. And the current density of the treated polyacrylonitrile carbon felt in the electrochemical reaction is improved to a certain extent, and its corrosion resistance is also better than that of the untreated carbon felt, thereby extending the service life of the electrode material.

Yang et al. [3] improved the electrochemical activity of carbon felt for V2+/V3+redox reactions by combining KOH etching pretreatment with uniform deposition of Bi nanoparticles. The functionalization of carbon felt through KOH activation pre-treatment resulted in an increase of up to 16.49% in the microporous structure and oxygen-containing functional groups on the surface. The microporous structure and high content of oxygen-containing functional groups of KOH etched carbon felt promote the uniform distribution of Bi nanoparticles on the surface of CFE, with an average particle size of 45 nm. The reported process significantly enhanced and reduced the charge transfer resistance of the carbon felt, resulting in a CFE Bi charge transfer resistance of 0.160 Ω cm 2, which is much lower than the 3.238 Ω cm 2 of the carbon felt under the same conditions. The electrochemical activity of its V2+/V3+redox pairs is significantly improved, thereby enhancing the efficiency of the battery. Using the prepared electrode as the negative electrode, the energy efficiency of the all vanadium flow battery reaches 79.3% at 160 mA cm-2, which is 36.2% higher than the efficiency of the battery using the original carbon felt.

Tung et al. [4] prepared a new carbon felt negative electrode by coating carbon black with titanium dioxide (TiO2). The addition of TiO2 material with good hydrophilicity can improve the wettability of the carbon felt electrode and reduce the surface resistance of the electrode. The results showed that the electrode loaded with 20 wt% TiO2 at a scanning rate of 0.006 V s-1 exhibited a specific capacitance of 186.2 F g-1, which was 55.5% and 12.2% higher than pure carbon electrode (119.7 F g-1) and commercial TiO2 (166.0 F g-1), respectively. Under the condition of a current density of 200 mA cm-2, the energy storage efficiency (η E=65.4%) of a single battery containing 20 wt% self-made TiO2/C carbon felt negative electrode is 16.0% and 6.1% higher than that of the negative electrode of the raw carbon felt (η E=56.4%) and the negative electrode containing commercial TiO2/C (η E=61.6%), respectively.

Sheeraz et al. [5] successfully deposited clusters of SnO2 nanoparticles on the surface of carbon felt fibers through hydrothermal method, resulting in SnO2 nanoparticles modified carbon felt electrodes. The SnO2 deposited carbon felt electrode produced by it achieved an energy efficiency of 77.3% at a high current density of 150 mA cm-2 in an all vanadium flow battery, which increased the discharge capacity by 23.7% compared to the original electrode. And compared with the original carbon felt electrode of 50 mA cm-2, the cycling stability of the system has also been improved by nearly 2.7 times. The results indicate that SnO2 is used as an electrocatalyst for all vanadium redox flow battery systems, and the electrocatalytic activity of its nanoparticles helps to reduce overpotential, enabling faster charging/discharging reactions, especially for cathodic redox pairs (VO2+/VO2+). In addition, in the presence of SnO2 nanoparticles, the electrochemical surface area of the carbon felt also increased, and the all vanadium flow battery assembled with it also showed stable performance, with improved capacity retention compared to the original carbon felt electrode. This performance enhancement can be attributed to a decrease in activation potential, faster reaction kinetics, and a decrease in charge transfer resistance of vanadium redox reactions.

Abdulone et al. [6] modified commercial carbon felt using a simple precipitation method and neodymium oxide to enhance its electrochemical activity and stability towards VO2+/VO2+and V2+/V3+redox pairs, while reducing the degradation of the felt over time. It successfully optimized the quantity and distribution of neodymium oxide nanoparticles on the fiber surface. The results also indicate that the electrode modified with Nd2O3 exhibits significant performance improvements in energy efficiency and charging/discharging capacity, and has lower charge transfer resistance after 50 charging/discharging cycles. In addition, the carbon felt produced by it can restore its original performance after replacing the electrolyte after 50 cycles. The improvement in performance is related to the strong binding between the oxygen-containing functional groups on the surface of neodymium and fibers, which may serve as active sites for the redox reaction in all vanadium flow batteries.

Zhou et al. [7] achieved structural control of carbon felt by forming a nano catalytic layer on the surface of carbon fibers through a simple and effective copper oxide etching method. By carving nanopores on the surface of fibers, carbon felt can provide an expanded reaction surface area under rapidly flowing electrolytes without sacrificing mass transfer performance, and obtain rich defect sites and excellent structural stability properties. The battery assembled using nanopores and defective carbon felt electrodes exhibits excellent performance, with an energy efficiency of 85.1% at 320 mA cm-2, which is 21.8% higher than the original carbon felt. In addition, the liquid flow battery using this new electrode exhibits excellent long-term stability over up to 2000 cycles.

Deposition of metals or metal oxides is another important means of surface modification of carbon felt, which improves the performance of carbon felt electrodes through the special interaction between surface metals or metal oxides and the surface of carbon felt, thereby enhancing the electrochemical activity and stability of VO2+/VO2+and V2+/V3+redox pairs, and promoting the voltage efficiency and energy efficiency of all vanadium flow batteries.



Reference materials

[1] Wang WH, Wang XD.Investigation of Ir-modified carbon felt as the positive electrode of an all-vanadium redox flow battery.Electrochim Acta, 2007, 52:6755–6762.

[2] Wang Xinwei, Wang Shuangyin, Chen Jun. Nickel ion modification of carbon felt electrodes in vanadium flow batteries [J]. New Chemical Materials, 2014,42 (01): 107-109

[3] Lv Y, Zhang J, Lv Z, et al. Enhanced electrochemical activity of carbon felt for V2+/V3+redox reaction via combining KOH etched pretreatment with uniform position of Bi nanoparticles [J] Electrochimica Acta, 2017, 253: 78-84.

[4] Tseng T M, Huang R H, Huang C Y, et al. Carbon felt coated with titanium dioxide/carbon black composite as negative electrode for advanced redox flow battery [J] Journal of the Electrical Society, 2014, 161 (6): A1132

[5] Mehboob S, Ali G, Shin H J, et al. Enhancing the performance of all advanced redox flow batteries by decorating carbon felt electrodes with SnO2 nanoparticles [J] Applied energy, 2018, 229: 910-921.

[6] Fetyan A, El Nagar G A, Derr I, et al. A neodymium oxide nanoparticle doped carbon felt as promising electrode for advanced redox flow batteries [J] Electrochimica Acta, 2018, 268: 59-65.

[7] Zhou X, Zhang X, Lv Y, et al. Nano catalytic layer enriched carbon felt via copper oxide etching for advanced redox flow batteries [J] Carbon, 2019, 153: 674-681.

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