Vanadium redox flow batteries (VRFBs) have attracted great attention for large-scale energy storage to stabilize the fluctuated and intermittent renewable energies, attributed to their advantages including decoupled energy and power, fast response, excellent reversibility, no cross-contamination and long cycle life [1,2]. However, the widespread commercial application of VRFBs is greatly hindered by the high capital cost. One commonly used way to decrease the high capital cost is to operate the battery at a high current density with high energy efficiency, because the enhanced current density can lead to the reduced stack size, which in turns decreases the usage of stack components. In this regard, tremendous efforts have been taken to modify the graphite felt to enhance the battery performances. Especially, etching the graphite felt surface shows great promise to fabricate the high-performance electrode, because it can effectively increase the active surface area and oxygen-functional groups. However, most of the reported works use high-temperature method to activate the graphite felt surface, which needs a great deal of additional energy to achieve the high temperature and has some special requirements towards devices, such as good temperature resistance, corrosion resistance and leakproofness, greatly increasing the cost to manufacture the electrodes. In addition, the high-temperature activation process is complicated and sometimes the N₂ or argon protective atmosphere is required, making it difficult to mass production. Therefore, other works try to activate the graphite felt at room temperature, such as by Fenton’s reagent [3], modified Hummers method [4] and electrochemical activation [5]. Although promising, the additional energy, such as electrical energy, is still essential in many room-temperature activation methods. In addition, the adoption of concentrated acid inevitably leads to the corrosion of devices and safety concerns. More importantly, the state-of-art room-temperature activation methods are hard to compete with high-temperature activation methods, primarily due to their poor battery performance, which greatly limits the commercial application.

In this work, we propose a novel room-temperature activation method, which is simple, no need for additional energy, no special requirements for devices and suitable for mass production, to fabricate the cost-effective, highly active and stable electrodes for vanadium redox flow batteries. The XPS and BET tests show that the activated graphite felt has increased oxygen-functional groups and effective surface areas. Then, the activation time and concentrations are systematically optimized. In the battery tests, the VRFBs with the prepared graphite felt electrodes deliver an energy efficiency of 80.9% at the high current density of 250 mA cm^-2, and can be stably cycled for more than 500 cycles with a ~70% capacity retention. Further experimental results will be disclosed at the meeting.

Acknowledgement

The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. T23-601/17-R)

References

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[2] H. Jiang, W. Shyy, M. Wu, L. Wei, T. Zhao, J. Power Sources. 365 (2017) 34-42.

[3] C. Gao, N. Wang, S. Peng, S. Liu, Y. Lei, X. Liang, S. Zeng, H. Zi, Electrochim. Acta. 88 (2013) 193-202.

[4] X. Wu, H. Xu, Y. Shen, P. Xu, L. Lu, J. Fu, H. Zhao, Electrochim. Acta. 138 (2014) 264-269.

[5] W. Zhang, J. Xi, Z. Li, H. Zhou, L. Liu, Z. Wu, X. Qiu, Electrochim. Acta. 89 (2013) 429-435.