A pattern bipolar plate (BP) is designed to decrease the electrical contact resistance between the BP and carbon felt electrode (CFE) by increasing the BP's specific surface area through hot pressing the spherical crown pattern on the surface of the BP, and the carbon felt is pressed into the pattern by the stack tightening force. The contact area growth rate is 18.3% of a single cell which has 48cm2 active area. The areal specific resistance (ASR) of conventional BP and pattern BP is 1309 mΩ·cm2 and 946 mΩ·cm2 at 20% electrode compression ratio, the pattern plate is 27% lower than the conventional plate. In the single battery charge/discharge cycle test, battery assembled with the pattern BP shows a 1% higher EE than the conventional BP

Jiang, Shan

Li, Quanlong

Wang, Shiyu

Wang, Zixue

Wu, Jingbo

English

Electronics Science Technology and Application (E-Journal)

Volume 9 Issue2 - 49 -ANew Structure Bipolar Plate for Vanadium Flow BatteryShan Jiang1, Zixue Wang1, Quanlong Li2, Jingbo Wu1, Shiyu Wang11. Dalian Rongke Power Integration Co., Ltd. Dalian 116025, China.2. Dalian Rongke Power Co., Ltd. Dalian 116025, China. Abstract: A pattern bipolar plate (BP) is designed to decrease the electrical contact resistance between the BP and carbon feltelectrode (CFE) by increasing the BP's specific surface area through hot pressing the spherical crown pattern on the surfaceof the BP, and the carbon felt is pressed into the pattern by the stack tightening force. The contact area growth rate is 18.3%of a single cell which has 48cm2 active area. The areal specific resistance (ASR) of conventional BP and pattern BP is 1309mΩ·cm2 and 946 mΩ·cm2 at 20% electrode compression ratio, the pattern plate is 27% lower than the conventional plate. Inthe single battery charge/discharge cycle test, battery assembled with the pattern BP shows a 1% higher EE than theconventional BP.Keywords: Vanadium Flow Battery; Bipolar Plate; Contact Resistance1. IntroductionThe vanadium redox flow battery (VRFB) is one of the most promising energy storage system (ESS) with variousadvantages such as non-explosiveness, unlimited power capacity and a long lifetime [1]. The VRFB system is shown in Figure1. The single cell is composed of two bipolar plates, two electrodes, two frames and a membrane. The charge and dischargeof VRFBs occur by the potential difference of vanadium ions through reduction and oxidation (redox) reaction. The cathodicreaction of VRFBs is as follows:VO㔰   㔰H    e  ↔ VO㔰    H㔰OThe anodic reaction of VRFBs is as follows:V㔰  ↔ V3    e The energy efficiency and voltage efficiency of batteries are mainly affected by the ohmic resistance, concentrationpolarization and electrochemical polarization. The ohmic resistance is the main influencing factor accounting for 50%~70%including the bulk resistance and contact resistance of electrolyte, membrane, electrode, bipolar plate and so on.Electronics Science Technology and Application- 50 -The bipolar plates separate each single cell and provide electrical current paths to the adjacent cells in a VRFB stack.Therefore, bipolar plates should have high electrical conductivity and low permeability to vanadium sulfuric acid electrolytes[2,3]. At present, there are graphite BPs, metal BPs and composite BPs used in flow battery. Graphite BPs have high electricalconductivity and stable chemical properties, but the BPs are fabricated mainly through machining of thin graphite plates,which are vulnerable to breakage during machining and handling and to leakage due to high porosity[4]. Metal BPs have highelectrical conductivity and stable chemical properties, but the BPs require coating process such as gold or titanium nitratecoating to prevent corrosion of metal in the acid environment of electrolyte, which incurs high cost and lowVolume 9 Issue2 - 51 -productivity[5].Composite BPs have exellent corrosion resistance, high liquid resistance, high strength and low cost, whichhave greater advantages than graphite and metal BPs. However, the problem of relatively low electrical conductivity shouldbe overcome compared to that of metal or graphite BPs due to its high electrical contact resistance.In order to reduce the electrical contact resistance of composite BPs. Jun Woo Lim et al. developed a carbonfiber/polyethylene bipolar plate-carbon felt electrode assembly for vanadium redox flow batteries[1]. In their study, a car-bon fiber/polyethylene composite BP-CFE (carbon felt electrode) assembly has been developed to decrease the contact resis-tance between the BP and CFE by embedding the CFE into the PE matrix of the carbon fiber composite BP throughlocal thermoplastic welding process to decrease the electrical contact resistance. The developed BP-CFE assemblyachieved 84% of energy efficiency, which is higher than that of the conventional BP of 82%.In Jaeheon Choe'sstudy, a corrugated carbon/epoxy composite bipolar plate for VRFBs was designed to increase the efficiency of the elec-trolyte flow and decrease the area-specific resistance (ASR) of the BP[5]. In Dongyoung Lee's study, soft release films wereinserted between the mold and the composite to prevent the formation of a resin-rich area and to expose carbon fibers on thesurface of the bipolar plate. The developed method considerably decreased the ASR of the carbon composite bipolar platewithout an expanded graphite coating[6]. Ki Hyun Kim et al. developed a carbon fabric/graphite hybrid composite BP com-posed of carbon composite with the expanded graphite coating layer for the VRFB to decrease the electrical contact resis-tance[7]. The energy efficiency of VRFB with the carbon/graphite hybrid composite BP was 86%, which was 6% higherthan that of the conventional graphite BP.In this study, a pattern bipolar plate has been developed to decrease the electrical contact resistance between the BP andCFE by increasing the BP's specific surface area through pattern-roll-forming. To evaluate the performances of the pattern BP,the specific surface area and the areal specific resistance (ASR) were measured. Also the mechanical properties test wasperformed to verify the BP strength. Finally, the performances of the pattern BP were measured and compared with those ofthe conventional bipolar plate during the charge/discharge cycles of a single cell.2. Experimental2.1 Design concept of pattern bipolar plateThe main factors affecting the contact resistance between BP and CFE are surface conductivity, contact area and con-tact pressure. In this paper, the contact resistance is reduced by increasing the contact area between BP and CFE. Theconventional structure of BP and CFE is shown in Figure 2.a. The contact surface between the BP and CFE is smooth and thecontact area can be easily calculated. In this paper, In order to increase the contact area, the specific surface area of thebipolar plate is increased by hot pressing the spherical crown pattern on the surface of the BP, and the carbon felt is pressedinto the pattern by the stack tightening force which is shown in Figure 2.b.The protrusion of the pattern bipolar plate is designed as a spherical crown with a radius of 330μm and a height of180μm. The surface area of the spherical crown is 3.73×10-3cm2 and the corresponding region of the conventional BP is2.72×10-3cm2, the growth rate is 37%.Electronics Science Technology and Application- 52 -(a)(b)Fig.2. Schematic diagrams of contact areas between the CFE and the BP; (a) smooth contact surface between CFE and theconventional BP, (b) pattern contact surface between CFE and the pattern BP.2.2 Pattern bipolar plate preparationThe process of material mixing system is shown in Fig.3. First, conductive materials, such as carbon black, graphiteand polypropylene and composite additives, such as lubricants, dispersants, coupling agents are weighted in a certainproportion. After high-speed mixing, a functional mixture is obtained which is then placed in the extrusion system bytransmission band.The bipolar plate extrusion system is shown in Fig.4. After mixing, the bipolar plate material is transported to the dryermachine through the feeder machine, and then the raw material is extruded into seven heating tubes by screws to be heated.The melted material is extruded through extruded mould. A pattern roller is arranged in front of the extruder head to presspattern on the extruded bipolar plate.Fig.3. Bipolar plate material mixing systemVolume 9 Issue2 - 53 -Fig.4. Bipolar plate extrusion systemThe main technological parameters of extrusion and rolling are shown in Table 1~2. The heating temperature of theheater tube is 180-190℃ and the temperature of extruded mould is 180-230℃ which is shown in Table 1. The pattern roller isinstalled in the distance from the extruder head 25 cm -35 cm which has a 120℃ temperature on the surface which is shownin Table 2. The clear pattern is pressed on the bipolar plate by constantly adjusting the gap between the upper and lower rolls.Electronics Science Technology and Application- 54 -Table1 The temperature of heater tubes and extruded moulds.Machine Section Temperature (℃)Heater tube 1 180Heater tube 2 180Heater tube 3 187Heater tube 4 187Heater tube 5 187Heater tube 6 187Heater tube 7 187Extruded mould 1 225Extruded mould 2 225Extruded mould 3 180Extruded mould 4 180Extruded mould 5 180Extruded mould 6 225Extruded mould 7 225Table 2 The main technological parameters of extrusion and rolling.Distance between extruded mouldand pattern roller(cm)Extrusion rate(m/s)Roller temperature(℃)25-35 0.08 120Volume 9 Issue2 - 55 -2.3 Apparent testThe spherical bulges on the surface of bipolar plate are clearly visible which is shown in Fig.5 and the bulgesdistribution is uniform which has no fragmentary or shedding. There are no apparent problems such as cracks, chromaticaberration, or ripple of the bipolar plate. It shows that the rolling process is stable, and the pattern can be successfully pressedon the bipolar plate.Fig.5. The pattern bipolar plateBefore doing the inspection of the tread plate, prepare the sample plate as shown in Fig.6. The sample is placed on thecarrier platform which is vertical to the stage, and one side can be supported by a conductive support block. The sample andsupport block are fixed on the carrier platform with conductive tape.Fig.6. Sample preparation(a: Test sample,b: Conductive support block,c: carrier platform,d: Conductive tape)Then put the prepared sample into the SEM machine for observation at the condition shown in Table 3.Table 3 SEM condition.Vacuum degree（pa） Acceleration voltage（kV）4.8×10-5 5Electronics Science Technology and Application- 56 -2.4 Measurement of electrical propertiesThe areal specific resistance(ASR) of the pattern plate under different electrodecompression ratio was measured, using the experimental setup shown in Fig.7. The size of thepattern bipolar plate specimens was Φ50mm×10mm. The pattern BP specimen was placedbetween the CFEs and they were inserted between the two gold-coated copper electrodes thatwere connected to a power supply and a multi meter. The ASR was calculated with themeasured voltages under the electrode compression ratio of 20%,35%,50%. Theenvironmental temperature was controlled to be 25℃ under atmospheric pressure to preventthe thermal effect on the electrical resistance of the carbon fiber. The total resistance R(total)of the BP and the system resistance R(system) are defined by Eqs. (1) and (2), respectively.Then the areal specific resistance ASR(bipolar plate) of the BP defined by Eq. (3), iscalculated by subtracting the system resistance from the total resistance as follows.R total   㔰RAu Cu  CFE   㔰RCFE   㔰RCFE BP   RBP (1)R system   㔰RAu Cu  CFE   㔰RCFE (2)ASR bipolar plate㌳   㔰RCFE BP   RBP (3)whereR(total): Total resistance of the pattern bipolar plateR(system): System resistance of the measurement setupASR(bipolar plate): Areal specific resistance of the pattern bipolar plateRAu(Cu)-CFE: Interfacial contact resistance between the electrode and the carbon feltRCFE: Bulk resistance of the carbon feltRCFE-BP: Interfacial contact resistance between the carbon felt and the bipolar plateRBP: Bulk resistance of the pattern bipolar plateFig.7. Experimental steup for areal specific resistance measurementVolume 9 Issue2 - 57 -2.5 Mechanical strength measurementsThe pattern plate was cut to 100mm×10mm rectangular spline to test the bending strength according to Fig. 8, andcompared with the conventional bipolar plate. The strength of the bipolar plate is calculated by Eqs. (4).Fig.8. Material testing machine(1. pressure head, 2. sample, 3. support)σ  3P × L㔰b × δ㔰 4㌳whereσ: Bending strength/MPaP: Fracture load/NL: Distance between two feet of the support/mmb: The width of the sample/mmδ: The thickness of the sample/mm2.6 Charge/discharge testTo investigate the performance of the pattern bipolar plate, charge/discharge tests were performed using a single cell testequipment. Ion exchange membrane is Nafion212. The surface area of the electrode is 48cm2 and the compression ratioshould be controlled to 20% after assembly. The single cell test conditions are as follows. The temperature of constanttemperature water bath pot is set at 37℃ that would keep the battery temperature between 35℃ to 39℃. The electrolyte is80ml sulfuric acid solution of vanadium which has a vanadium concentration at about 1.65-1.66mol/L and flow rate is 70 ml /min. Charge and discharge at constant current density 80mA/cm2.3. Results and discussion3.1 Apparent testAs shown in Fig.9. the apparent appearance of the pattern can be clearly observed and the height of patterns can bemeasured. Seven points are randomly selected to be measured which is shown in Fig.10. The height of the seven points is 150to 200 μm. Considering the measurement error, the height is relatively uniform and the average is 180 μm which is shown inTable 4. The radius of the projections can be calculated to be 330.9 μm. According to Eq. (1), the surface area of theprojections (sphere cap) can be calculated to 0.003743cm2.S   㔰πrh (5)Electronics Science Technology and Application- 58 -The surface area of the pattern plate is 8.77 cm2 larger than the conventional 48 cm2 bipolar plate. In an ideal state, thearea of the electrode area is 8.77 cm2 larger than the conventional plate which is shown in Table 5.Fig.9. Electron microscope images of patternsFig.10. Measure the height of patternsTable 4 The height of seven patterns.Raised number height(μm)1 184.92 151.83 1764 179.15 199.96 183.17 184.1Average 179.8Volume 9 Issue2 - 59 -Table 5 Pattern plate surface area of 48 cm2 single cell.Bipolar plateSingleprojectionsurfacearea(cm2)Projectionnumber inelectrode areaProjectionsurfacearea(cm2)Total surfacearea（cm2）Increase surfacearea(cm2)IncreaseratePattern plate 3.74×10-3 8625 32.3 56.8 8.8 18.3%Conventionalplate2.72×10-3 0 23.5 48 - -3.2 Measurement of electrical propertiesAs shown in Fig.11, the ASRs of pattern bipolar plate in different electrode compression ratio were lower than those ofconventional bipolar plate. ASR of two different bipolar plate both reduced in different degrees with the increase of electrodecompression ratio. The ASR of conventional bipolar plate and pattern bipolar plate is 1309 mΩ·cm2 and 946 mΩ·cm2 ,andthe pattern plate is 27% lower than the conventional plate. When the compression ratio increased to 50%, the ASR of twobipolar plates decreased to 444 mΩ·cm2 and 371 mΩ·cm2, and the pattern plate was 16% lower than the conventional plate. Itcan be seen that with the increase of the compression ratio, the ASR of the two kinds of plates is constantly approaching,because the reduction effect of the contact resistance between the bipolar plate and electrode caused by the increase of thespecific surface area of the bipolar plate is not obvious when the compression ratio is continuously increased.Fig.11. ASRs of conventional/pattern bipolar plate under different electrode compression ratioElectronics Science Technology and Application- 60 -3.3 Mechanical strength measurementsThe bending strength of the pattern bipolar plate is 41.5MPa, which is 8% lower than that of the conventional bipolarplate 45.3MPa, as shown in Table 6. This is because the bend-resist ability of the gaps among patterns on the surface of thepattern bipolar plate is weak where fracture occurs during the strength test.Table 6 Bending strength measurements.Pattern BP (MPa) Conventional BP (MPa)41.5 45.33.4 Charge/discharge testFrom the single cell charge/discharge test, the pattern bipolar plate achieved 79.4% of energy efficiency, which is higherthan that of the conventional bipolar plate of 78.5% as shown in Fig. 12. This is because the ohmic loss was reduced by thelower cell resistance due to the reduced total resistance compared to that of the conventional bipolar plate. Also, it was foundthat there was no degradation on the pattern bipolar plate after 500 cycles of the test. Therefore, the pattern bipolar plate issuitable for the VRFBs application, and it can replace the conventional bipolar plate.Fig.12. Charge/discharge efficiencies of the pattern bipolar plate and the conventional bipolar plate4. ConclusionIn this work, pattern bipolar plate was rolled to decrease the areal specific resistance (ASR) of vanadium redox flowbattery (VRFB). The apparent, mechanical strength and the electrical resistance were measured. Also, the single cellcharge/discharge tests were conducted for performance assessment.Volume 9 Issue2 - 61 -For single cell size, The surface area of the pattern plate is 8.77 cm2 larger than the conventional 48 cm2 bipolar plate.The areal specific resistance (ASR) of conventional BP and pattern BP is 1309 mΩ·cm2 and 946 mΩ·cm2 at 20% electrodecompression ratio, the pattern plate is 27% lower than the conventional plate. Experimental results showed that patternbipolar plate with 55% compression ratio showed the lowest areal specific resistance (ASR). The bending strength of thepattern bipolar plate is 41.5MPa, which is 8% lower than that of the conventional bipolar plate 45.3MPa.From the single cell charge/discharge test, the developed pattern bipolar plate achieved 79.4% of energy efficiency,which is higher than that of the conventional BP of 78.5%. Furthermore, there was no degradation in the developed BP-CFEassembly after 500 cycles of the test.References[1] Jun Woo Lim a, Dai Gil Lee b,Carbon fiber/polyethylene bipolar plate-carbon felt electrode assembly for vanadium redoxflow batteries (VRFB).[2] Blanc C. Modeling of a vanadium redox flow battery electricity storage system. École polytechnique fédérale deLausanne; 2009.[3] Larminie J, Dicks A. Fuel cell systems explained. Chichester: John Wiley & Sons Ltd; 2003.[4] Lee, NJ, Lee, SW, Kim, KJ, Kim, JH, Park, MS, Jeong, G, et al. Development of carbon composite bipolar plates forvanadium redox flow batteries. Bull Korean Chem Soc 2012;33(11):3589–92.[5] Jaeheon Choe, Ki Hyun Kim, Dai Gil Lee. Corrugated carbon/epoxy composite bipolar plate for vanadium redox flowbatteries. Composite Structures 119 (2015) 534–542.[6] Dongyoung Lee, Jun Woo Lim, Soohyun Nam. Method for exposing carbon fibers on composite bipolar plates.Composite Structures 134 (2015) 1–9.[7] Ki Hyun Kim, Bu Gi Kim, Dai Gil Lee. Development of carbon composite bipolar plate (BP) for vanadium redox flowbattery (VRFB). Composite Structures 109 (2014) 253–259.

A New Structure Bipolar Plate for Vanadium Flow Battery

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A New Structure Bipolar Plate for Vanadium Flow Battery

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Electronics Science Technology and Application (E-Journal)