Among the many types of redox flow batteries (RFBs), the vanadium redox flow battery (VRFB) has been regarded as one of the most viable chemistries for commercialization [1]. However, one of the most challenging issues that limits its commercialization process is the high capital cost, resulting from the low-performance power-pack. The porous carbon electrode design exerts a significant influence on the battery performance, primarily because it contributes to the system polarization through not only the ohmic polarization, but also through activation polarization and concentration polarization [2, 3]. The most commonly used materials for VRFB electrodes are carbon fiber-based materials (carbon felts/papers/clothes) for their high electronic conductivity, high hydraulic permeability, chemical and electrochemical inertness. However, issues associated with the use of these materials including low specific surface area and poor electrochemical activity limit the VRFB to be operated at a low current density (~50 mA cm^-2) [4].

In this work, we present a simple and cost-effective method to form different types of dual-scale porous electrodes via KOH activation of the fibers of different carbon materials, including carbon felts/papers/clothes. As shown in Fig. 1, the large pores (~10 μm), formed between carbon fibers, serve as the macroscopic pathways for high electrolyte flow rates, while the small pores (~5 nm), formed on carbon fiber surfaces, act as active sites for rapid electrochemical reactions. Hence, the dual-scale porous electrode is expected to deliver high surface area without any sacrificing in electrolyte permeability. Our preliminary results showed that Brunauer-Emmett-Teller specific surface area of these carbon materials is increased by more than 10 times and the battery assembled with dual-scale carbon-paper/cloth electrodes can even operate at 400 mA cm^-2 with energy efficiency over 80%. Moreover, the dual-scale carbon-felt electrode showed stable efficiencies during more than 1000 cycles. Further characterizations and optimizations of these dual-scale electrodes and comparisons between them will be disclosed at the meeting.

References:

[1] W. Wang, Q. Luo, B. Li, X. Wei, L. Li, Z. Yang, Advanced Functional Materials. 23 (2013) 970-986.

[2] A.Z. Weber, M.M. Mench, J.P. Meyers, P.N. Ross, J.T. Gostick, Q. Liu, J. Appl. Electrochem. 41 (2011) 1137-1164.

[3] M.L. Perry, A.Z. Weber, J. Electrochem. Soc. 163 (2016) A5064-A5067.

[4] B. Li, M. Gu, Z. Nie, Y. Shao, Q. Luo, X. Wei, X. Li, J. Xiao, C. Wang, V. Sprenkle, Nano letters. 13 (2013) 1330-1335.