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Min-Young Kim et al. / J. Electrochem. Sci. Technol., 2021, 12(3), 330-338 331 Fig. 1. Schematic diagram of VRB single cell before and after assembly of bipolar plate-electrode: bipolar plate (a), electrode (b), separator (c). dium redox flow battery because of preventing corrosion of bipolar plate by electrolyte and securing a certain level of electron conductivity for reducing the internal resistance of the battery [6-7]. The carbon bipolar plate has been fabricated by cutting of carbon from carbon bar or compression molding using mix- ture of a polymer binder and graphite. The contact of the existing electrode/bipolar plate is determined by the fastening pressure during assembly in the form of single cell/stack. At this time, depending on the fas- tening pressure of the cell/stack, a trade-off problem has occurred between the contact resistance of the two parts, bipolar plate and electrode, and the fluidity of the electrolyte. Therefore, various studies have been conducted to reduce the contact resistance between the electrode and the bipolar plate [8-10]. As the most representative example, M. Kazaco et al. suggested a process of thermally bonded graphite felt on a carbon-plastic composite bipolar plate, and further improvements have been continued [11-14]. In addition, P. Qian et al. have conducted a study of applying an adhesive conducting layer between the bipolar plate and the electrode to bind the two parts with heat and pressure [15]. Herein, the bipolar plate is fabricated using flexible graphite, which is differ- ent from the existing studies using impermeable graphite. Moreover, adhesive conducting layer, which plays a vital role in electrode/bipolar assembly, is composed of thermo-plastic phenol formaldehyde resin (PF), carbon black, and graphite powder. According to the results of previous studies [15], when a thin flexible bipolar plate of carbon plastic composite type was applied on the graphite felt as electrode, several disadvantages have been come up such as high bulk resistance in bipolar plate, contact resistance between bipolar plate and graphite felt, and high manufacturing cost; moreover, contact resis- tance can be reduced by introducing an adhesive con- ducting layer composed of a PF-based conductive composite material between a thin flexible bipolar plate and graphite felt. In this study, we focused on effect of ethylene- vinyl-acetate (EVA) based conductive adhesive film (CAF) by coating conductive material such as carbon black (CB) and carbon nanotube (CNT). That is, CAF, which is non-woven fabric of hot melt adhesive (web type) coated CB (CB-EVA) and CNT (CNT- EVA), was fabricated by wet process. Bare EVA, CB- EVA, and CNT-EVA films were prepared and the through resistance of each film was measured. Moreover, they were applied between electrode (graphite felt) and the bipolar plates. Finally, the contact resistance, between the electrode and bipolar plate, and energy efficiency of VRB single cells were investigated by charging and discharging cycle in more detail. 2. Experimental 2.1 Fabrication and characteristics of conductive adhesive films Three kind of conductive adhesive films, a non- woven EVA (ethylene-vinyl- acetate) material alone (bare EVA), coating of carbon black (CB-EVA) and coating of CNT (Carbon Nano Tube) (CNT-EVA) on bare EVA film, were prepared by wet dispersion process. To fabricate the bare EVA, CB-EVA and CNT-EVA film samples based on a non-woven hot melt EVA sheet, as starting materials, a non-woven hot melt adhesive (EVA W80, JCC KOREA. Co., Ltd, Korea), carbon black (Ketjen black 600JD, Mit- subishi Chemical, Japan), and CNT (WMCS-I-O, WONIL Corporation, Korea) were prepared, respec- tively. Firstly, 0.2 wt% CB was put into an ethanol solvent for CB suspension. The carbon black suspension by wet dispersion process was prepared by mixing carbon black (CB) with an ethanol solvent to have a concentration of 0.2 wt% for CB-EVA film sample and the conductivePDF Image | Energy Efficiency Improvement of Vanadium Redox Flow Battery
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