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 content, we have already introduced the goal of improving electrode performance by introducing surface functional groups, while this content will mainly focus on the modification of carbon nanotubes on the surface of carbon felt.

Surface modification of carbon felt with high conductivity, thermal stability, and specific surface area of carbon nanotubes can effectively improve the overall conductivity, thermal stability, and specific surface area of carbon felt, while improving its hydrophilicity and surface resistance. Meanwhile, the hollow structure of carbon nanotubes also provides more active sites for vanadium ion reactions [1].

Park M et al. [2] reported a process of in-situ growth of carbon nanofibers or carbon nanotubes by vapor deposition (CVD) to modify the surface of carbon felt. This method uses acetylene as the carbon source for vapor deposition and metal nickel as the catalyst for the entire process. Moreover, the in-situ grown carbon nanofibers and carbon nanotubes exhibit a synergistic effect. The high conductivity of carbon nanotubes and the surface defects associated with carbon nanofibers provide more reaction sites for the flow cell reaction, which is beneficial for the redox reaction of vanadium ions in all vanadium flow cells. However, due to its complexity, the cost of this process is relatively high.

Wei et al. [3] prepared a carbon felt loaded carbon nanotube catalyst composite electrode for vanadium redox flow batteries. It uses perfluorosulfonic acid polymer (Nafion) as an agglomerant to prepare carbon felt carbon nanotube composite electrodes containing short carboxyl groups, and the use of Nafion can effectively ensure the stability of carbon nanotubes on carbon fibers. Uniformly dispersed and adhered carbon nanotubes on carbon felt can improve electrode stability and achieve excellent catalytic performance. The results indicate that the electrochemical activity of carbon felt electrodes modified by this process is significantly improved, and the reversibility of VO2+/VO2+and V2+/V3+redox pairs is greatly increased. Compared with the original carbon felt electrode, the modified carbon felt electrode exhibited higher Coulombic efficiency (93.9%) and energy efficiency (82.0%) in all vanadium flow single cells.

Zhang et al. [4] synthesized 3D graphene nanowall modified carbon felt using in-situ microwave plasma enhanced chemical vapor deposition method and used it as the positive electrode for vanadium redox flow batteries. The carbon fibers in the carbon felt are wrapped by vertically grown graphene nanowalls, which not only increases the electrode specific area but also exposes high-density graphene edge states, exhibiting good catalytic activity for vanadium ions. The results showed that the reaction rate of VO2+/VO2+redox pairs in all vanadium flow batteries using this new electrode was increased by three times and the energy efficiency was increased by 11% compared to unmodified carbon felt electrodes. And it exhibits excellent stability during battery operation. After 100 cycles of charging and discharging, the electrode not only does not show significant morphological changes, but can also be reused in other batteries with the same performance (energy efficiency value can still be maintained at ≈ 90% after 100 cycles).

Saleem et al. etched carbon felt through cobalt oxide catalysis, resulting in the formation of carbon nanorods on the surface containing carbon felt fibers. Unlike the traditional multi-step process of growing nanostructures on carbon felt, this method optimizes the surface morphology by adjusting the etching temperature, treatment time, and catalyst type. The felt loaded with the catalyst is directly heat treated in the air and can produce well arranged nanorods on its fibers. The carbon felt obtained through catalytic etching process exhibits better surface wettability compared to the original carbon felt, with a specific surface area increased by about twice, thereby improving the kinetics of vanadium redox reaction. When used as an electrode for all vanadium redox flow batteries, the carbon felt with a nanorod structure can maintain 80% capacity after 100 charge/discharge operations at 150 mA cm-2, while the unetched carbon felt can maintain 48% capacity under the same conditions.

Mohammad et al. [6] used different concentrations of reduced graphene oxide to modify carbon felt electrodes and observed that the carbon felt modified with 2 mg/ml graphene modification solution had the best electrochemical performance. The peak currents of the anodic and cathodic redox reactions reached 45.3 mA and 21.1 mA, respectively, while the values of the original graphene felt were 14.2mA and 4.7 mA, respectively. The addition of graphene enhanced the linear diffusion coefficient of the graphene modified carbon felt by more than twice, significantly reducing the charge transfer resistance (from 459.3 Ω· cm2 to 94.2 Ω· cm2), promoting the diffusion process.

Xia et al. [7] successfully manufactured graphene modified carbon fiber felt through a simple solution coating process and used it as the positive electrode for vanadium flow battery single cells. It was found that the immersion time of carbon felt in graphene/Nafion solution has a significant impact on its electrochemical activity towards VO2+/VO2+redox pairs. With an increase in immersion times (>5 times), a large amount of Nafion without electrochemical activity will be loaded on the surface of the graphene/carbon felt electrode, thereby inhibiting electrochemical activity. Compared with the vanadium flow battery single cell based on the original carbon felt, the battery assembled with its prepared electrodes exhibits lower polarization during the charging and discharging process, thus exhibiting higher voltage efficiency and energy efficiency. The electrode prepared by it remained above 80% after 500 charging/discharging test cycles in a cyclic test of 80 mA/cm2, and also improved by 4.8% and 6.7% in a cyclic test of 360 mA/cm2. Meanwhile, the power density of graphene modified carbon fiber felt based batteries is 39 mW/cm 2 higher than that of vanadium flow batteries based on the original carbon felt.

David et al. [8] synthesized a three-dimensional mesoporous graphene modified carbon felt electrode as an electrode for all vanadium flow batteries through a simple self-assembly interaction process. It reported that among all three-dimensional mesoporous graphene modified carbon felt electrodes, a 4 wt% three-dimensional mesoporous graphene modified carbon felt electrode exhibited the best electrochemical performance for VO2+/VO2+and V2+/V3+redox pairs. This is because the optimal graphene loading leads to an increase in the specific surface area and conductivity of the electrode, resulting in a decrease in activation polarization, a decrease in electrode overpotential, accessibility of more redox active sites, and rapid electron transfer redox reactions of vanadium. In addition, the charging/discharging test using a 4 wt% three-dimensional mesoporous graphene modified carbon felt electrode showed an excellent energy efficiency of 76.5% compared to ordinary carbon felt at a high current density of 100 mA cm-2, and its specific discharge capacity was increased by 110%. At the same time, it showed excellent cycling stability under 100 cycles without observable attenuation. The electrodes prepared have smaller peak potential intervals, higher peak current density, lower transfer resistance and discharge/charge overpotential, improved charging/discharge specific capacity, and outstanding voltage and energy efficiency, making them suitable as positive and negative electrode materials for all vanadium flow batteries.

Carbon nanotubes are widely used in the surface modification of carbon felt electrodes due to their excellent conductivity, thermal stability, and specific surface area. Through the modification of carbon nanotubes, in addition to improving the conductivity, thermal stability, and specific surface area of the electrodes to varying degrees, their hydrophilicity and surface resistance can also be improved, thereby improving the overall performance of carbon felt electrodes and achieving higher voltage efficiency and energy efficiency in all vanadium flow batteries. In addition, carbon materials such as graphene and oxidized graphene have also been attempted to be used for surface modification of carbon felt to varying degrees. At present, there is still extensive exploration and research on how to easily and effectively introduce carbon nanotubes into the surface of carbon felt electrodes. However, from the results obtained, a considerable number of studies have achieved surprising results. In the future, as the surface modification process of carbon nanotubes becomes more mature, it will greatly improve the overall operating efficiency of liquid flow batteries, thereby helping to reduce costs and promote their large-scale applications.



Reference materials

[1] Lin Hang Research on the Zinc Dendritic Resistance of Zinc Bromide Flow Battery by Carbon Felt Electrode Modification [D]. Northeast Electric Power University, 2019

[2] Park M, Jung YJ, Kim J, Lee H, Cho J.Synergistic effect of carbon nanofiber/nanotube composite catalyst on carbon felt electrode for high-performance all-vanadium redox flow battery.Nano Lett, 2013, 13:4833–4839.

[3] Wei G, Jia C, Liu J, et al. Carbon felt supported carbon nanotubes catalysts composite electrode for advanced redox flow battery application [J] Journal of Power Sources, 2012, 220: 185-192.

[4] Li W, Zhang Z, Tang Y, et al. Graphene nanowall decorated carbon felt with excellent electrical activity towards VO2+/VO2+couple for all advanced redox flow batteries [J] Advanced Science, 2016, 3 (4): 1500276

[5] Abbas S, Lee H, Hwang J, et al. A novel approach for forming carbon nanorods on the surface of carbon felt electrodes by catalytic etching for high performance advanced redox flow battery [J] Carbon, 2018, 128: 31-37.

[6] Moghim M H, Eqra R, Babaiee M, et al. Role of reduced graphene oxide as nano electron catalyst in carbon felt electrode of advanced redox flow battery [J] Journal of Electroanalytical Chemistry, 2017, 789: 67-75.

[7] Xia L, Zhang Q, Wu C, et al. Graphene coated carbon felt as a high performance electrode for all advanced redox flow batteries [J] Surface and Coatings Technology, 2019, 358: 153-158.

[8] Opar D O, Nankya R, Lee J, et al. Three dimensional mesoporous graphene modified carbon felt for high performance advanced redox flow batteries [J] Electrochimica Acta, 2020, 330: 135276.



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