54 TheElectrochemicalSocietyInterface•Fall2010

Fig. 1. Power and energy densities of various EES systems.

Flow Batteriesby Trung Nguyen and Robert F. Savinell

Renewableenergysourcesincludingwind and solar can supply asignificant amount of electrical

energyintheUnitedStatesandaroundthe world. However, because of theirintermittent nature, the potential ofthese two energy sources can be fullyexploited only if efficient, safe, andreliable electrical energy storage (EES)systems are provided. EES will also becritical to improving the robustnessand efficiency of the grid by reducingpower surges and balancing the loadovertime.Arecentreport fromSandiaidentifiesover17majoropportunitiesofEEStomakeanimpactontheU.S.gridandimplementationofrenewables.1Forvery large energy storage applications,only pumped hydro and compressedgasarecosteffectiveatthistime.Thesetechnologies, however, are limitedby geography, while electrochemicalenergy storage devices such asbatteries, fuel cells/flow batteries, andelectrochemical capacitors are amongthe leading EES technologies for thefuture because of their scale-abilityandversatility.Theirpowerandenergydensity characteristics are shown inFig.1.2Capacitors,withtheirveryhighpower densities, low energy densities,andsub-secondresponsetimes,aremoresuitableforpowerqualitymanagement.Batteries and flow batteries/fuel cellshave the energy densities needed forlarge-scaleelectricalenergystorage.

Batteriesandflowbatteries/fuelcellsdiffer in two main aspects. First, in abattery, the electro-active materials arestored internally, and the electrodes atwhich the energy conversion reactionsoccur are themselves part of theelectrochemicalfuel.Thecharacteristicsof the negative and positive electrodesdetermine both the power density(e.g., electrical, transport, and catalyticproperties of the active material andnon-reactive materials) and the energydensity (e.g., mass of active materials)of thebattery.Asabatteryconverts itschemical energy to electrical energy,electrodes are consumed and undergosignificant physical and chemicalchanges which affect its electricalperformance. Second, because ofthe dual functions of the electrodesdescribedabove,aconventionalbatteryhasminimalornoscale-upadvantages.Instead, it can only be scaled-out.That is, ifmoreenergy isneeded, thenmore battery modules with identicalcomponentsarerequired.Astheamountof electro-active materials increasesin a battery, more current collectingmaterials, electrolyte, separators, andenclosure materials are also needed.Consequently, a battery can neverapproachitstheoreticalenergydensity.Furthermore, increasing the capacityof a battery almost always increasesinternal resistances and consequentlydecreasespowerdensityandefficiency.

Flow Batteries Classification

A flow battery is an electrochemicaldevice that converts the chemicalenergy in the electro-active materialsdirectlytoelectricalenergy,similartoaconventionalbatteryandfuelcells.Theelectro-activematerialsinaflowbattery,however, are stored mostly externallyin an electrolyte and are introducedintothedeviceonlyduringoperation.3Trueflowbatterieshaveallthereactantsand products of the electro-activechemicals storedexternal to thepowerconversiondevice.Systemsinwhichalltheelectro-activematerialsaredissolvedin a liquid electrolyte are called redox(for reduction/oxidation) flow batteries(RFBs). A schematic of a redox flowbattery system is shown in Fig. 2.Other true flow batteries might have agas species (e.g., hydrogen, chlorine)and liquid species (e.g., bromine).Rechargeable fuel cells like H2-Br2 andH2-Cl2 could be thought of as trueflow batteries. Systems in which oneor more electro-active components arestored internallyare calledhybridflowbatteries. Examples include the zinc–bromine and zinc–chlorine batteries.Similarly to conventionalbatteries, theenergy densities of these hybrid flowbatteries are limited by the amountof electro-active materials that can bestored within the batteries and theyhavelimitedscale-upadvantages.TableI shows some of the more well-knownflow battery systems. Although muchflow battery research dates back to the1970s, some research has continuedover the past several decades and thestate-of-thearthasbeenreviewedintherecentliterature.4

Most redox flow batteries consistof two separate electrolytes, onestoring the electro-active materials forthe negative electrode reactions andthe other for the positive electrodereactions. (To prevent confusion, thenegativeelectrodeistheanodeandthepositiveelectrodeisthecathodeduringdischarge. It is understood that theywill be reversed during charge.) Boththefreshandspentelectrolytesmaybecirculatedandstoredinasinglestoragetank as shown in Fig. 2 or separatelyto control the concentrations of theelectro-activematerial.Anionselectivemembrane is often used to preventmixing or cross-over of the electro-active specieswhich result in chemicalshort-circuit of electro-active materials.Only the common counter ion carrieris allowed to cross the membrane. Forexample, in the bromine-polysulfidesystem,asNa2S2isconvertedtoNa2S4at

TheElectrochemicalSocietyInterface•Fall2010 55

theanodeandBr2isconvertedto2Br-atthecathode,theexcessNa+ ionsat theanodeareallowedtocrosstothecathodetomaintainelectroneutralitycondition.Similarly, in the vanadium system, asV+2 isoxidizedtoV+3at theanodeandV+5 is reduced to V+4 at the cathode,hydronium ions are transported acrossa proton conducting membrane fromtheanode to thecathode. In this case,however, sometimes a microporousnon-selective membrane separator canbeusedsincemostofthecurrentmightbe carried by high mobility protons intheacidelectrolyteandsincethecross-over of the common vanadium cationlowers efficiency but does not causea permanent loss of capacity. (Note:the distinction of the flow electrolytedoes not take into account the fixedelectrolyte within the membraneseparatorofmanyflowbatteries.)

A fuel cell might be considered as atype of flow battery in that the powerconversion component is independentof the chemical energy capacity of thedevice.Mostfuelcells,however,cannotbe reversed electrically efficiently, andtherefore cannot be used effectively asan electrical energy storage device; sotheyareconsideredasachemicalenergyconversiondeviceonly.

Advantages and Disadvantages

With the electrolyte and electro-active materials stored externally, trueflow batteries have many advantages,one of which is the separation ofthe power and energy requirements.The electrodes, not being part of theelectrochemical fuel, can be designedto have optimal power acceptanceand delivery properties (e.g., catalytic,electrical, and transport) without theneed to also maximize energy storagedensity. Furthermore, the electrodesdo not undergo physical and chemicalchanges during operation (becausethey do not contain active materials),thusleadingtomorestableanddurableperformance. Therefore, engineeredmicrostructures developed to optimizeperformance can be maintained overthe lifetime of the device: with longerlifetimes,thecapitalcostsofthebatterysystem can be amortized over a longerperiod,andwithawiderstate-of-chargeoperatingwindow,thequantityofactivematerialrequiredtodeliverpowerovertheentirerequireddurationofdischargecanbeminimized.

The energy capacity requirement ofa flow battery is addressed by the sizeof the external storage components.Consequently, a redox flow batterysystem could approach its theoreticalenergy density as the system is scaledup to a point where the weight orvolume of the battery is small relativeto that of the stored fuel and oxidant.A conventional analogous system isthe internalcombustionenginesystemin which the power is determined by

System Reactions Ecell o Electrolyte

Redox Anode/Cathode

All Vanadium 3 Anode: V2+ V3+ + e-

Cathode: VO2+ + e- VO2+

1.4 V H2SO4/H2SO4

Vanadium-Polyhalide 5 Anode: V2+ V3+ + e-

Cathode: ½ Br2 + e- Br-

1.3 V VCl3-HCl/NaBr-HCl

Bromine-Polysulfide 6 Anode: 2 S2

2- S42- + 2e-

Cathode: Br2 + 2e- 2 Br-

1.5 V NaS2/NaBr

Iron-Chromium 7 Anode: Fe2+ Fe3+ + e-

Cathode: Cr3+ + e- Cr2+

1.2 V HCl/HCl

H2-Br2 8 Anode: H2 2H+ + 2e-

Cathode: Br2 + 2e- 2Br-

1.1 V HBrPEM*

PEM*

-

Hybrid

Zinc-Bromine Anode: Zn Zn2+ + 2e-

Cathode: Br2 + 2e- 2 Br-

1.8 V ZnBr2/ZnBr2

Zinc-Cerium 9 Anode: Zn Zn2+ + 2e-

Cathode: 2Ce4+ + 2e- 2Ce3+

2.4 V CH3SO3H

(both sides)

*Polymer Electrolyte Membrane

Table I. Characteristics of Some Flow Battery Systems.

the size of the engine and the energydensityisdeterminedbythesizeofthefueltank.

Inaflowbatterythereisinherentsafetyofstoringtheactivematerialsseparatelyfrom the reactive point source. Otheradvantages are quick response times(common to all battery systems), highelectricity-to-electricity conversionefficiency, no cell-to-cell equalizationrequirement, simple state-of-chargeindication (based on electro-activeconcentrations), low maintenance,tolerance to overcharge and over-discharge,andperhapsmostimportantly,the ability for deep discharges withoutaffecting cycle life.Thehybrid systemslikethoseinvolvingzincplatingdonotofferalltheseadvantages,butstillhavemanyofthedesirablefeaturesofatrueflowbattery.Themaindisadvantageofflowbatteriesistheirmorecomplicatedsystemrequirementsofpumps,sensors,flow and power management, andsecondary containment vessels, thusmaking them more suitable for large-scalestorageapplications.

If one examines the vanadium flowbattery system, one of the few redoxflow batteries that has been tested atthe utility scale, one estimates thatthe vanadium itself is a significantcost driver: cost analysis suggests thatthe vanadium material contributesbetween $50/kWh to $110/kWh at

current vanadium prices, or from 50to 100 percent of the aforementionedcost target of $100-200/kWh. Fromthis standpoint, identifying low-costredox couples with high solubility iscritical tomeeting theultimatemarketrequirements.

The other key cost driver is theconstructionoftheelectrochemicalcellitself.Theconstructioncostsofthecellscalewith the totalpower requirementof the application, but these costs aredirectly rated to the specific powerof the device itself–how effectivelythe materials are utilized. While flowbatteries ought to be able to operateat relatively high current densities, asconvectioncanbeemployed todeliverreactants to the electrode surface, flowbatteries have typically been operatedat ~50 mA/cm2, a current densityconsistent with conventional batterieswithoutconvection.Itisanticipatedthatelectrolytemanagementandcelldesigncan deliver at least a 5x improvementinpowerdensity, therebyreducingcellconstructioncostsbyclosetothatsamefactorof5.

Current Technical Barriers

Onlyafewflowbatterysystemshaveseen deployment. Consequently, thetechnologies are relatively new andunfamiliar. Further development will

56 TheElectrochemicalSocietyInterface•Fall2010

Fig. 2. Schematic of a redox flow battery system with electrodes shown in a discharge mode.

Ion Selective Mem

brane

Anode: M

+x

M+(x+n) + ne

-

Cathode: N

+y + ne

N +(y-n)

Porous Electrode

Porous Electrode

Positive Electrolyte

Storage

Negative Electrolyte

Storage

AC/DC Converter

− +

Power Sources (Wind/Solar) Customers

Charge Discharge

requireresearchactivitiesinthefollowingareas: (1.) low-cost, efficient, anddurableelectrodes;(2.)chemicallystableredox couples, having large potentialdifferences, with high solubilities ofboth oxidized and reduced species,and fast redox kinetics; (3.) highlypermselective and durable membranes;(4.) electrode structure and cell designthat minimize transport losses; (5.)designs with minimal pumping andshuntcurrentlosses;and(6.)largescalepower and system management andgrid integration. Overall, the primarybarriers to commercialization for largescale energy storage are round tripenergystorageefficiency,costforenergystorageintermsof$/kwh,andcostforthepowercapacityintermsof$/kw.

About the Authors

Trung nguyen isafullprofessorintheDepartmentofChemicalandPetroleumEngineeringatTheUniversityofKansas.Hehasover22yearsofbothindustrialandacademicexperienceinfuelcellandbattery technology, and has researchactivities that span from fundamentalstodevicestosystemsaspects.Hiscurrentresearch interest is in interfacial and

transport phenomena in fuel cells andbatteries, and mathematical modelingofelectrochemicalsystems.Hemaybereachedatcptvn@ku.edu.

roberT F. Savinell is the George S.Dively Professor of Engineering atCase Western Reserve University inCleveland, Ohio. His research interestsinclude understanding electrocatalysis,mass transfer, and interfacial processesin electrochemical systems, andapplicationsofthisknowledgetodeviceimprovement and development. Hemaybereachedatrfs2@case.edu.

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Storage for the Electricity Grid:Benefits and Market PotentialAssessmentGuide,”SandiaReportSAND2010-0815(Feb.2010).

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Nguyen and Savinell(continued from previous page)