Free-Standing Hollow Electrospun Carbon Nanofiber Web As High-Performance Electrode for Vanadium Redox Flow Batteries
Authors: Zeng Lin , Jiang Haoran , Ren Yuxun , Xu Jianbo , Zhao Tianshou 
Published: February 2020

Vanadium redox flow batteries (VRFBs) as large-scale energy storage devices have considered being competent for the utilization of renewables to address their intermittency and fluctuation characteristics [1]. Although there are significant merits and some demonstrations with the power scale up to mega-kilowatt level have been proved, the broad market penetration of VRFBs is still hampered by their relatively high cost [2, 3]. To address this issue, the approaches to improve the cell performance, including the power densities and energy efficiencies, are highly required. Since boosting the cell performance will result in a reduced stack size, which will reduce the cost of the stack repeatable components, such as bipolar plates, separators, and electrodes [4]. Generally, the cell performance is highly related to the porous electrodes that provide the electrochemical reaction sites and transport the species. The conventional carbon electrodes are suffered from the poor electroactivity, thereby leading to a low energy efficiency.
In this regard, we here developed a free-standing hollow electrospun carbon nanofiber web (hECNFW) as a high-performance electrode for VRFBs. The hECNFW was fabricated using co-axial electrospinning of poly(acrylonitrile) shell solution and styrene-co-acrylonitrile core solution and subsequently carbonized at high temperature, as illustrated in Figure 1a. Contrary to the conventional carbon electrode consisting of carbon fibers with a diameter of several micrometers, the hECNFW was constituted by nanofibers with a diameter of 200-500 nm. Hence, the specific surface area was largely increased, thereby boosting the electrochemical kinetics for vanadium reaction. Meanwhile, some nitrogen-containing functional groups were remained in the matrix of carbon nanofibers although they were carbonized at temperature as high as 1000oC. The existence of the nitrogen residue was confirmed by X-ray photoelectron spectroscopy. The nitrogen groups in the carbon surface modified the adsorption behavior of vanadium ions and accelerated the conversion of vanadium ions among different chemical valences. Finally, the surface of hECNFW became more hydrophilic, which was beneficial for the adsorption of the vanadium ions.
The battery performance of VRFBs assembled with the hECNFW was preliminarily evaluated. As shown in Figure 1b, the battery can be operated at a high current density range. More importantly, at a current density of 200 mA cm-2, the battery achieved an energy efficiency of 83.7%, which was much higher than that of results reported previously. The performance can be further improved by optimizing the assembly work between the thin electrode and membrane. This work is under progress. Meanwhile, further characterizations, including the graphitization, the surface state and microstructure of hECNFW, will be presented at the meeting.
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
[1] G.L. Soloveichik, Chem Rev, 115 (2015) 11533-11558.
[2] W. Wang, Q.T. Luo, B. Li, X.L. Wei, L.Y. Li, Z.G. Yang, Adv Funct Mater, 23 (2013) 970-986.
[3] K.J. Kim, M.S. Park, Y.J. Kim, J.H. Kim, S.X. Dou, M. Skyllas-Kazacos, J Mater Chem A, 3 (2015) 16913-16933.
[4] M. Park, J. Ryu, J. Cho, Chem-Asian J, 10 (2015) 2096-2110.

Figure 1