A Newly Designed Modular Znbr2 Single Cell Structure

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Article A Newly Designed Modular ZnBr2 Single Cell Structure

Zongqiang Pang 1,* , Yutao Gong 1, Ming Yuan 1 and Xin Li 2

1 College of Automation, Nanjing University of Posts and Telecommunications, Nanjing 210003, China; [email protected] (Y.G.); [email protected] (M.Y.) 2 NARI Technology Co., Ltd., Nanjing 211000, China; [email protected] * Correspondence: [email protected]; Tel.: +86-186-5291-9118

Received: 18 February 2020; Accepted: 28 April 2020; Published: 4 May 2020

Abstract: We describe a ZnBr2 single cell which has a highly modular symmetrical structure. With designed polyethylene shell frames, membrane frame and composite titanium-carbon felt electrodes, it has a higher energy density and is more flexible compared with traditional flow batteries. We repeatedly tested its performance, which showed good tightness, high reliability and a high energy efficiency of 75%. Due to the special symmetrical structure and modular design, it is easy to assemble and disassemble, which makes it suitable as a test platform for electrodes, membranes and electrolyte performance testing. The designed modular flow cell has low cost and high energy density, and can provide good guidance for flow battery research.

Keywords: composite electrodes; energy density; ﬂow battery; ﬂexibility; membrane frame; zinc bromide

1. Introduction Energy storage is the key technology to promote the replacement of traditional energy types and closely affects the development of new energy technology, making how to build efficient, reliable and cost-effective large-scale energy storage systems critical for societal development [1,2]. Due to flow batteries’ long cycle life, deep discharge depth, high safety, low cost and high energy efficiency, they are considered an important long-term energy storage mode [3]. Theoretically, the flow battery capacity is only determined by the volume of the electrolyte stored in the tanks. According to the different active substances, flow batteries can be divided into vanadium, zinc/bromine, sodium polysulfide/bromine, iron/chromium, etc. Among of them, the vanadium redox battery (VRB) and zinc-bromine flow battery (ZBFB) are two of the most widely used flow batteries. Different from VRB, ZBFB has higher energy density, lower building cost and better environmentally friendliness [4–6]. The ZBFB system is based on the deposition of zinc at the negative electrode and bromine evolution at the positive electrode and consists of the cell, electrolyte tanks and pumps [7]. As shown in Figure1, different from conventional batteries, the active materials of ZBFB at both electrodes are the same ZnBr2 solution that is stored in two external tanks and separately circulated inside the flow cell by two magnetic pumps [4]. In order to reduce the self-discharge inside the flow cell, the ZnBr2 solutions inside two half-cells are separated by an exchange membrane to prevent the two solutions from mixing. The basic electrochemical reactions of a ZBFB are as follows [8]:

Anode reaction : Zn2+ + 2e Zn (1)

Cathode reaction : 2Br− Br2 + 2e (2) 2+ Net cell reaction : Zn + 2Br− Zn + Br2 (3)

Batteries 2020, 6, 27; doi:10.3390/batteries6020027 www.mdpi.com/journal/batteries Batteries 2020, 6, x FOR PEER REVIEW 2 of 8 Batteries 2020, 6, 27 2 of 8 Batteries 2020, 6, x FOR PEER REVIEW 2 of 8

Figure 1. Schematic diagram of the ZnBr2 single cell.

Figure 1. Schematic diagram of the ZnBr2 single cell. During the charging process,Figure 1. zinc Schematic ions are diagram redu cedof the to ZnBr zinc2 particlessingle cell and. deposit on the surface of theDuring anode theelectrode. charging At process,the same zinc time, ions the are bromine reduced ions to converted zinc particles into and bromine deposit by onthe the oxidation surface During the charging process, zinc ions are reduced to zinc particles and deposit on the surface reactionof the anode will be electrode. captured At by the the same ligand time, complex the bromine forming ions an converted oily complex into precipitate bromine by and the are oxidation stored of the anode electrode. At the same time, the bromine ions converted into bromine by the oxidation atreaction the bottom will be of captured the positive by the electrode ligand complex electrolyte. forming During an oily the complexdischarging precipitate process, and in arecontrast stored to at the reaction will be captured by the ligand complex forming an oily complex precipitate and are stored chargingbottom of process, the positive zinc electrode and bromide electrolyte. ions Duringare generated the discharging at the negative process, inand contrast positive to the electrodes, charging at the bottom of the positive electrode electrolyte. During the discharging process, in contrast to the respectively.process, zinc and bromide ions are generated at the negative and positive electrodes, respectively. charging process, zinc and bromide ions are generated at the negative and positive electrodes, Although ZBFB systems have have been studied for a long time, the problems of charge-discharge respectively. efficiencyefficiency and and battery battery charge charge are are still still not not solved solved ad adequatelyequately [9]. [9 ].In Inorder order to tocheck check the the performance performance of Although ZBFB systems have been studied for a long time, the problems of charge-discharge electrodes,of electrodes, membranes membranes and and electrolytes, electrolytes, we we have have toto design design and and build build many many single single cells cells which which is efficiency and battery charge are still not solved adequately [9]. In order to check the performance of complicated and a heavy waste. How How to to design design one one flexib flexiblele flow flow cell cell structure structure that that has has good good flexibility, flexibility, electrodes, membranes and electrolytes, we have to design and build many single cells which is high space utilization rate and great fluidfluid tightnesstightness is still a research focusfocus forfor ZBFBZBFB systems.systems. complicated and a heavy waste. How to design one flexible flow cell structure that has good flexibility, InIn this paper, we describedescribe a modularmodular ZnBrZnBr2 single cell structure, which adopts a symmetrical high space utilization rate and great fluid tightness is still a research focus for ZBFB systems. structure with a membrane sandwichedsandwiched between two shell frames [[10].10]. The symmetrical structure In this paper, we describe a modular ZnBr2 single cell structure, which adopts a symmetrical and modular design make it flexible flexible for assembly assembly and and disassembly, disassembly, so it is suitable to be use as a test structure with a membrane sandwiched between two shell frames [10]. The symmetrical structure platform for electrodes, membranes andand electrolyte performance testing. and modular design make it flexible for assembly and disassembly, so it is suitable to be use as a test platform for electrodes, membranes and electrolyte performance testing. 2. Experimental Experimental

2.1.2. Experimental Flow Frame ZnBr 2 Single Cell 2.1. Flow Frame ZnBr2 Single Cell The ZnBr2 single cell system includes a cell, three magnetic pumps, and two electrolyte tanks. 2.1. FlowThe ZnBrFrame2 ZnBrsingle2 Singlecell system Cell includes a cell, three magnetic pumps, and two electrolyte tanks. The most important part is the cell, which is the core component of the ZnBr2 cell system. A schematic The most important part is the cell, which is the core component of the ZnBr2 cell system. A schematic viewThe of the ZnBr cell2 issingle shown cell in system Figure 2includes. a cell, three magnetic pumps, and two electrolyte tanks. view of the cell is shown in Figure 2. The most important part is the cell, which is the core component of the ZnBr2 cell system. A schematic view of the cell is shown in Figure 2.

Figure 2. Schematic view of modular ZnBr2 single cell structure. 1. Shell frame, 2. Carbon felt, Figure 2. Schematic view of modular ZnBr2 single cell structure. 1. Shell frame, 2. Carbon felt, 3. 3. Membrane frame, 4. O-ring, 5. Titanium foam. Membrane frame, 4. O-ring, 5. Titanium foam. Figure 2. Schematic view of modular ZnBr2 single cell structure. 1. Shell frame, 2. Carbon felt, 3. TheMembrane cell is frame, composed 4. O-ring, of two 5. Titanium symmetrical foam. polyethylene shell frames, two composite electrodes and aThe membrane cell is composed frame, which of two are symmetrical squeezed together polyet inhylene series shell through frames, two two ﬂuorocarbon composite O-rings electrodes and and a membrane frame, which are squeezed together in series through two fluorocarbon O-rings and six stainlessThe cell steelis composed bolts. The of positivetwo symmetrical and negative polyet electrodeshylene shell are bothframes, composed two composite by titanium electrodes foam six stainless steel bolts. The positive and negative electrodes are both composed by titanium foam and a membrane frame, which are squeezed together in series through two fluorocarbon O-rings and six stainless steel bolts. The positive and negative electrodes are both composed by titanium foam Batteries 2020, 6, 27 3 of 8

Batteries 2020, 6, x FOR PEER REVIEW 3 of 8 and carbon felt [11]. The whole structure is symmetrical and simple [12], it is easy to assemble or and carbon felt [11]. The whole structure is symmetrical and simple [12], it is easy to assemble or disassemble, which makes it suitable to be used as a standard test platform for performance testing of disassemble, which makes it suitable to be used as a standard test platform for performance testing electrodes, membranes and electrolytes [13,14]. of electrodes, membranes and electrolytes [13,14]. 2.2. Electrode and Membrane Frame 2.2. Electrode and Membrane Frame As shown in Figure3a, the titanium foam electrode which has a titanium terminal at the center is manufacturedAs shown by in 3DFigure printing. 3a, the The titanium calculated foam active electrode area of which the electrode has a titanium is 24.72 terminal cm2. Compared at the center with traditionalis manufactured graphite by electrodes, 3D printing. titanium The calculat foam electrodesed active havearea goodof the electrical electrode conductivity, is 24.72 cm good2. Compared rigidity andwith can traditional be machined graphite into electrodes, any desired titanium shape foam [15]. Withelectrodes the help have of good carbon electrical felt between conductivity, the titanium good foamrigidity electrode and can and be exchangemachined membrane, into any desired the internal shape resistance [15]. With inside the help the ﬂowof carbon cell decreases felt between because the carbontitanium felt foam hasa electrode better conductivity and exchange than the membrane, electrolyte. the Most internal importantly, resistance the carbon inside felt the can flow provide cell moredecreases active because reaction carbon sites. felt Furthermore, has a better theconducti titaniumvity foamthan the electrode electrolyte. integrate Most the importantly, electrode andthe currentcarbon felt collecting can provide column more together, active reaction so it’s not sites. necessary Furthermore, to use the a separate titanium current foam electrode collecting integrate column thatthe electrode can reduce and the current contact collecting resistance column between together, the electrode so it’s andnot thenecessary collecting to use column a separate [16]. The current role ofcollecting the membrane column is that to preventcan reduce the solutionsthe contact from resi thestance two between half-cells the from electrode mixing, and which the can collecting reduce thecolumn self-discharge [16]. The role of theof the ZBFB membrane system. is Weto preven designedt the asolutions membrane from frame the two and half-cells built it byfrom using mixing, one polyethylenewhich can reduce plate the as self-discharge shown in Figure of the3b. ZBFB The membranesystem. We we designed used in a thismembrane paper isframe a zbmm-600 and built microporousit by using one exchange polyethylene membrane plate as produced shown in by Figure Beijing 3b. Zbest The Powermembrane Technology we used Co., in this Ltd. paper (Beijing, is a China),zbmm-600 which microporous has a pore exchange size of less membrane than 10 nm. produced The designed by Beijing membrane Zbest Power frame hasTechnology the advantage Co, Ltd. of low(Beijing, cost andChina), strong which water has absorption. a pore size of less than 10 nm. The designed membrane frame has the advantage of low cost and strong water absorption.

(a) (b)

Figure 3. Photos of the cell structure ( a) and the rib-like microporous exchangeexchange membranemembrane ((b).).

A rib-likerib-like microporousmicroporous exchange exchange membrane membrane is fixedis fixed at theat the center center of the of membranethe membrane frame frame by laser by weldinglaser welding technology. technology. After After hundreds hundreds of assembly of assembly and performance and performance cycling cycling tests, tests, besides besides its good its good fluid sealingfluid sealing performance, performance, the designed the designed structure structure has another has another two distinct two advantages:distinct advantages: First, the First, channels the onchannels the microporous on the microporous exchange membraneexchange membrane can strengthen can strengthen the rigidity the of therigidity structure of the and structure prevent and the deformationprevent the deformation of the membrane of the caused membrane by the caused high fluid by flowthe high rate. fluid Second, flow the ra channelste. Second, can the increase channels the turbulencecan increase in the the turbulence cell and the in ion the reaction cell and areathe ion [17 ].reaction area [17].

2.3. Electrolyte Configuration Conﬁguration The electrolyte electrolyte configuration conﬁguration is is also also very very impo importantrtant for fora ZBFB a ZBFB system, system, which which is related is related with withmany many problems problems during during charging, charging, such as such electrode as electrode corrosion, corrosion, high self-discharge high self-discharge rate, zinc rate, dendrite zinc dendriteformation, formation, etc. [18] According etc. [18] to According [19], we set to our [19 ZnBr], we2 solution set our molar ZnBr2 concentrationsolution molar as 2 concentration mol/L, which ashas 2 good mol/ conductivityL, which has and good lower conductivity corrosion rate, and and lower the corrosiontotal volume rate, of andthe electrolyte the total volumeis 1 L. In of order the electrolyteto prevent the is 1 bromine L. In order from to preventtransferring the bromineinto the zinc from half-cell, transferring we chose into themethylethylmorpholinium zinc half-cell, we chose methylethylmorpholiniumbromide (MEM) as our complexing bromide (MEM) agent. asZinc our bromide complexing (ZnBr agent.2), potassium Zinc bromide chloride (ZnBr (KCl)2), potassium are both chloridefrom NARI (KCl) Technology are both from Co, NARILtd. (Nanjing, Technology China). Co., Ltd. 2 M (Nanjing, ZnBr2 in China). deionized 2 M water ZnBr2 wasin deionized used as waterelectrolyte, was used 0.5 M as MEM electrolyte, and KCl 0.5 were M MEM added and into KCl the were solution added to intoadjust the the solution desired to concentration adjust the desired [20]. concentration [20]. Batteries 2020,, 6,, 27x FOR PEER REVIEW 44 of of 8

3. Results and Discussion 3. Results and Discussion

3.1. Pre-Preparation of the System Cycle

Figure 4 shows a photo of our modular ZnBr2 single cell system. Before the performance test, we Figure4 shows a photo of our modular ZnBr 2 single cell system. Before the performance test, wefirst first fill fillboth both reservoirs reservoirs with with some some deionized deionized water water and and turn turn on on all all the the magnetic magnetic pumps pumps (MP-6R, Shanghai XinxishanXinxishan Co., Co, Ltd., Ltd., Shanghai, Shanghai, China) China) at constantat constant room room temperature temperature to clean to theclean whole the systemwhole andsystem check and for check leaks. for According leaks. According to Sun’s to research Sun’s resear resultsch for results the flow for the rate flow in ZBFB rate in system, ZBFB 100system, mL/min 100 ismL/min a better is fluida better flow fluid rate flow which rate can which hinder can excessive hinder excessive production production of zinc dendritesof zinc dendrites [21], so [21], we set so ourwe fluidset our flow fluid rate flow as 100 rate mL as/min 100 inmL/min this paper. in this After pape twor. After hours two of deionized hours of waterdeionized circulation, water circulation, we confirm thewe singleconfirm cell’s the stabilitysingle cell’s and fluidstability tightness, and fluid then tightness, we fill the then configured we fill the electrolyte configured into electrolyte both reservoirs into andboth add reservoirs more bromine and add into more the bromine positive into reservoir the positive to make reservoir the bromine to make concentration the bromine around concentration 0.5 mol/L. around 0.5 mol/L.

Figure 4. A photo of our novel ZnBr2 single cell system. Figure 4. A photo of our novel ZnBr2 single cell system. 3.2. Electrolyte Configuration 3.2. Electrolyte Configuration Before the first cycle charge-discharge test, it is better to circulate the electrolyte for two hours to completelyBefore soakthe first the cycle microporous charge-discharge exchange test, membrane. it is better After to circulate two hours the of electrolyte circulation, for we two start hours the etoffi completelyciency test withsoak ourthe microporou battery capacitys exchange tester (EBC-A10 membrane.+, ZKETECH,After two hour Zhuozhis of circulation, Electronic we Technology, start the Co.,efficiency Ltd., Shanghai,test with our China). battery For capacity the charging tester (EBC-A10+, process, using ZKETECH, constant Zhuozhi current chargingElectronic mode, Technology, we set theCo, chargingLtd., Shanghai, current China). density For value the charging at about proces 30 mAs,/cm using2. Byconstant changing current the chargingcharging time,mode, we we test set 2 the charging efficiency current under didensityfferent value cell capacities. at about 30 At mA/cm the end. By of changing every charge the charging cycle, we time, set onewe minutetest the relaxationefficiency timeunder for different the single cell cell capacities. to reach a stable At th statee end and of then every collect charge the opencycle, circuit we set voltage. one minute For the dischargingrelaxation time process, for the we single also use cell constant to reach discharging a stable state mode and andthen set collect the first the stage open discharge circuit voltage. current For as 0.75the discharging A and end voltage process, as we 0.3 V.also When use theconstant cell’s voltagedischarging is lower mode than and 0.3 set V, thethe first first stage stage discharging discharge processcurrent as stops 0.75 and A and the end terminal voltage voltage as 0.3 V. returns When to the a highercell’s voltage value. is Then lower we than start 0.3 the V, the second first stage discharging process process, stops in which and the we terminal set discharge voltage current returns as to 0.2 a Ahigher and endvalue. voltage Then aswe 0.3 start V forthe furthersecond deepstage dischargedischarging tests, process, and that in which cycle repeats. we set discharge current as 0.2 A and end voltage as 0.3 V for furtherAs deep shown discharge in Figure tests,5, we and tested that acycle series repeats. of capacities of the single cell from 0.5 Ah to 4 Ah at roomAs temperature. shown in Figure For every 5, we capacity,tested a series we perform of capacities three charge-dischargeof the single cell from tests 0.5 and Ah calculate to 4 Ah the at average.room temperature. As we know, For energy every ecapacity,fficiency characterizeswe perform three the degree charge-discharge of recovery oftests stored and energy,calculate while the voltageaverage. e ffiAsciency we know, (VE) energy reflects efficiency the internal characterize resistances of the the degree cell, moreover, of recovery a high of stored Coulomb energy, efficiency while (CE),voltage which efficiency is related (VE) reflects with electrodes’ the internal reversibility resistance of during the cell, the moreover, charge-discharge a high Coulomb reaction efficiency process, indicates(CE), which that is the related composite with titanium-carbon electrodes’ reversibility felt electrodes during have the high charge-discharge conductivity and reaction stability process, [22,23]. indicates that the composite titanium-carbon felt electrodes have high conductivity and stability [22,23]. Batteries 2020, 6, x FOR PEER REVIEW 5 of 8 Batteries 2020, 6, 27 5 of 8

Figure 5. The eﬃciency test results of the modular ZnBr2 single cell.

Figure 5. The efficiency test results of the modular ZnBr2 single cell. The equations for calculating the battery’s efficiency are as follows: The equations for calculating the battery’s efficiency are as follows: Edischarge EE : η = 100% (4) E E η Echargedischarge × EE: E = 100% (4) E charge Adischarge CE : η = 100% (5) Q AA × η chargedischarge × CE: Q =100% (5) AEEcharge VE : η = 100% (6) V CE × EE We calculate the EE, CE and VE of ZnBrVE: singleη =100% cell× under different charging capacities. The high(6) 2 V CE Coulomb efficiency indicates that the flow cell has a lower self-discharge rate. Because of our use of a compositeWe calculate electrode, the the EE, internal CE and resistance VE of ZnBr becomes2 single smaller cell under which different is reflected charging in the capacities. VE results. The With high a fluidCoulomb flow rate efficiency of 100 mLindicates/min and that a currentthe flow density cell has of a 30 lower mA/ cmself-discharge2, the EE of ourrate. flow Because cell can of our reach use up of toa 75%composite at room electrode, temperature. the internal Compared resistance with Li’s becomes ZBFB stack smaller in awhich single is cell reflected module in [24 the], whichVE results. has traditionalWith a fluid structural flow rate design of 100 and mL/min a Nafion and/ PTFEa current composite density membrane,of 30 mA/cm our2, the cell EE structure of our flow has cell lower can cost,reach smaller up to size75% and at room higher temperature. current density. Compared with Li’s ZBFB stack in a single cell module [24], whichLaboratory-scale has traditional monolithic structural flowdesign batteries and a Nafion/PTFE typically use composite 50 cycles of membrane, constant current our cell cycling structure to testhas their lower stability cost, smaller [25,26]. size In orderand higher to get current a more intuitivedensity. comparison, we perform a 50-cycle battery charge-dischargeLaboratory-scale cycle testmonolithic with a constantflow batteries current typically density use of 3050 mAcycles/cm of2 asconstant described current in this cycling paper. to Astest shown their instability Figure [25,26].6, we perform In order a to cycle get testa more at 1.5 intuitive Ah of thecomparison, battery. The we totalperform amount a 50-cycle of released battery electricitycharge-discharge is the cell’s cycle capacity test with when a constant it reaches current the desired density voltage, of 30 mA/cm defined2 as:as described in this paper. As shown in Figure 6, we perform a cycle test at 1.5 Ah of the battery. The total amount of released electricity is the cell’s capacity when it reachesCd the= Id desiredTd voltage, defined as: (7)

Cd = IdTd (7) where Cd, Id and Td represent the discharging capacity, current, and duration of the discharge process, respectivelywhere Cd, I [d 27and]. InTd thisrepresent paper, theId of discharging the cell was ca setpacity, as 0.75 current, A, Td of an thed cellduration is 2 h. of Therefore, the discharge the formulas process, ofrespectively the battery energy[27]. In ( Ethisd.) andpaper, energy Id of densitythe cell ( wwas) can set be as expressed 0.75 A, T asd of follows: the cell is 2 h. Therefore, the formulas of the battery energy (Ed.) and energy density (w) can be expressed as follows: Ed = VdCd (8) Ed = VdCd (8) E w = d (9) mE w = d (9) where Vd represents the discharging voltage, which form the single cell is 1.37 V, and the actual reaction mass of the battery is about 50.8 g. According to calculations, the single cell has a EE of about 72% after where Vd represents the discharging voltage, which for the single cell is 1.37 V, and the actual reaction 50 test cycles at a current density of 30 mA/cm2, so the energy density of the single cell is 40.45 Wh/kg, mass of the battery is about 50.8 g. According to calculations, the single cell has a EE of about 72% which is higher than the average [28]. With a better microporous exchange membrane frame and better after 50 test cycles at a current density of 30 mA/cm2, so the energy density of the single cell is 40.45 Wh/kg, which is higher than the average [28]. With a better microporous exchange membrane frame Batteries 2020, 6, 27 6 of 8

Batteries 2020, 6, x FOR PEER REVIEW 6 of 8 Batteries 2020, 6, x FOR PEER REVIEW 6 of 8 electrode design, the EE should be even higher, and multiple cycle tests also confirm the stability and and better electrode design, the EE should be even higher, and multiple cycle tests also confirm the fluid tightness of our modular flow cell. stabilityand better and electrode fluid tightness design, of the our EE modular should flowbe even cell. higher, and multiple cycle tests also confirm the stability and fluid tightness of our modular flow cell.

(a) (b) (a) (b) Figure 6. ((a)) 0.75 0.75 A A constant constant current current discharge test (b) Single cell cycle test. Figure 6. (a) 0.75 A constant current discharge test (b) Single cell cycle test. 3.3. Internal Resistance of the Flow Cell 3.3. Internal Resistance of the Flow Cell With our homemade internal resistance test equipment,equipment, we measured the ﬂowflow cell’s internal resistance.With Through our homemade connecting internal a milliohm resistance resistor test oneq uipment,the flow ﬂow cell we and andmeasured applying applying the one one flow small small cell’s AC AC internal signal, signal, weresistance. tested the Through cell’s internal connecting resistance a milliohm during resistor repeatedrepeated on the charging flow cell and and discharging. applying one The small tested AC internal signal, resistancewe tested isthe composed cell’s internal of thethe resistance OhmicOhmic resistanceduringresistance repe andandated polarizationpolarization charging and resistance.resistance. discharging. The The Ohmic tested resistance internal includesresistance the is electrode,composed membrane,of the Ohmic electrolyte resistance an and andd other polarization contact resistance. resistance, The but Ohmic the polarization resistance resistanceincludes the is mainly electrode, caused membrane, by electrochemical electrolyte polarizationpo larizationand other and contact concentration resistance, polarization. but the polarization As shown inresistance Figure7 7,, is asas mainly thethe batterybattery caused capacitycapacity by electrochemical increases,increases, thethe po ionionlarization activityactivity and insideinside concentration thethe cellcell increasesincreases polarization. too,too, whichwhich As shown willwill resultin Figure in a 7, decrease as the battery of thethe internalinternal capacity resistanceresistance increases, thatthat the reaches reachesion activity 1.081.08 OhmsinsideOhms attheat fullfull cell capacitycapacity increases ofof too, 44 AhAh which [[29].29]. will result in a decrease of the internal resistance that reaches 1.08 Ohms at full capacity of 4 Ah [29].

Figure 7. Internal resistance test results of the ZnBr2 flow cell. FigureFigure 7.7.Internal Internal resistanceresistance testtest resultsresults of of the the ZnBr ZnBr22 flowflow cell.cell. 4. Conclusions 4. Conclusions A type of modular ZnBr2 single cell system was proposed, which adopts a newly membrane A type of modular ZnBr2 single cell system was proposed, which adopts a newly membrane A type of modular ZnBr2 single cell system was proposed, which adopts a newly membrane frame and channel structure design. We tested the performance of the designed ZnBr2 single cell, and frame and channel structure design. We tested the performance of the designed ZnBr2 single cell, frame and channel structure design. We tested the performance of the designed ZnBr2 single cell, and andthe test the results test results show show that after that hundreds after hundreds of test of cycl testes cyclesthe cell the has cell very has good very tightness, good tightness, a high energy a high the test results show that after hundreds of test cycles the cell has very good tightness, a high energy energyefficiency effi ofciency 75% ofand 75% high and reliability. high reliability. Compared Compared with the with traditional the traditional structural structural design, design, the flow the flow cell efficiency of 75% and high reliability. Compared with the traditional structural design, the flow cell cellcan canachieve achieve higher higher energy energy efficiency efficiency at higher at higher current current density density with withsmaller smaller size and size lower and lower cost. cost.The can achieve higher energy efficiency at higher current density with smaller size and lower cost. The Thewhole whole single single cell system cell system is very is veryeasy easyto assemble to assemble and disassemble, and disassemble, which which makes makes it suitable it suitable to be toused be whole single cell system is very easy to assemble and disassemble, which makes it suitable to be used usedas a test as aplatform test platform for designed for designed electrodes, electrodes, membranes membranes and electrolyte and electrolyte performance performance testing. testing. as a test platform for designed electrodes, membranes and electrolyte performance testing. Author Contributions: Z.P. and Y.G. conceived, performed the experiments, and wrote the paper. M.Y. and X.L. reviewedAuthor Contributions: and edited the Z.P. manuscript. and Y.G. Allconceived, authors performedread and approv the expeedriments, the manuscript. and wrote the paper. M.Y. and X.L. reviewed and edited the manuscript. All authors read and approved the manuscript. Batteries 2020, 6, 27 7 of 8

Author Contributions: Z.P. and Y.G. conceived, performed the experiments, and wrote the paper. M.Y. and X.L. reviewed and edited the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: The work was supported by the National Natural Science Foundation of China under Grant No.11604158 and 61701250, the State Key Laboratory of Mechanical System and Vibration under Grant No. MSV202018. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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