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334 Min-Young Kim et al. / J. Electrochem. Sci. Technol., 2021, 12(3), 330-338 charge/discharge device (WPG100e, WonATech Co., Ltd, Korea). The cell coulombic efficiency (CE) was defined as the discharge capacity divided by the charge capac- ity; the energy efficiency (EE) was defined as the dis- charge energy divided by the charge energy. Then the voltage efficiency (VE) was calculated from VE = EE/CE. 3. Results and Discussion 3.1 Characteristics of conductive adhesive films Fig. 4 shows the chemical stability of the non- woven EVA hot melt in electrolyte; it was checked whether this material is chemically stable after immersing in the electrolyte composition for VRBs for 720 hrs. In other words, Fig. 4 (a) shows the yel- low color as a bare electrolyte and EVA sheet used for VRBs before test, and Fig. 4 (b) shows the char- acteristics that the color of electrolyte is almost unchanged even after the test, indicating that this material is very chemically stable in electrolyte. Fig. 5 shows the surface morphology of each adhe- sive film by optical microscope and SEM, respec- tively. Fig. 5 (a) is a bare EVA film sample, which is a fine structure of EVA resin itself and has a white color in the form of a non-woven fabric. Fig. 5 (b) shows the microstructure of the bare EVA film sam- ple, and the thickness of the adhesive wire formed as a non-woven structure was made to be about 50 μm, showing a clean surface condition. Fig. 5 (d) show the state of carbon black coating on EVA nonwoven fabric as a microstructure of CB-EVA film sample. It seems that a large amount of carbon black is coated on the hot melt wire irregularly, but CB-EVA has a uniform black color as shown Fig. 5 (c). Fig. 5 (f) shows the microstructure of the CNT-EVA film sam- ple, where CNTs are uniformly coated on the EVA wire, and the coating layer has a relatively small amount of coating and looks thinner. Herein, since CNT is relatively expensive, the coating amount was adjusted to be relatively low. Thus, CNT-EVA has a light black color than CB-EVA. Fig. 6 shows the trend of through resistance according to the compaction pressure for each sample of the bare EVA film, CB-EVA film, and CNT-EVA film. This data was measured at room temperature by stacking and pressing four sheets of each sample using the equipment of KATECH (Korea Automotive Technology Institute), which was conducted at a rela- tively high pressure to remove the contact resistance factor between adhesive film layers. As a result, bare EVA film sample, which has no conductive material, has the highest resistance, and, CB-EVA film sample has the lowest through resistance than CNT-EVA film. In addition, CB-EVA film sample has little resistance change with increasing compaction pres- sure, but CNT-EVA film sample tends to be constant at a pressure of 220 kgf cm-2 or more, and bare EVA film at 280 kgf cm-2 or more. It can be seen that apparently the through resistance is more dependent on the coating content of conductive material than on the material type being coated. 3.2 Characteristics of electrode-bipolar plate assem- bly with conductive adhesive film Fig. 7 shows the typicial microstructure of cross section of electrode-bipolar plate assembly with CNT-EVA film for VRBs. That is, it can be seen that the film connects the electrode and the bipolar plate well by adhesion. However, some of the film is pulled in the direction of the electrode when the com- pression of assembly is released before being suffi- ciently cooled after thermal compression. And unlike the bipolar plate, since the electrode has a porous structure to allow the electrolyte to flow, the adhesive surface on the electrode side is not uniform, and the conductive composition of adhesive film is expected to affect the contact resistance between the electrode and the bipolar plate. In particular, it can be found Fig. 6. Through resistance of the laminated adhesive films under compaction pressure at 25 C. oPDF Image | Energy Efficiency Improvement of Vanadium Redox Flow Battery
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