Title: Alkaline Quinone Flow Battery

27 Pages·2015·2.77 MB·English

RESEARCH | REPORTS
25.N.H.Schliebenetal.,J.Mol.Biol.349,801–813 32.K.Niefind,J.Müller,B.Riebel,W.Hummel,D.Schomburg, wrotethemanuscript;F.G.M.plannedtheexperiments,expressed
(2005). J.Mol.Biol.327,317–328(2003). andpurifiedtheAmDHs,performedthebiocatalyticreactions,and
26.K.Tauberetal.,Chemistry19,4030–4035(2013). analyzedthedata;T.K.performedthegenecloningofallAmDHs
27.SeesupplementarymaterialsonScienceOnline. ACKNOWLEDGMENTS andpurifiedtheADHs;N.S.S.andM.B.providedintellectualand
28.A.C.Price,Y.-M.Zhang,C.O.Rock,S.W.White,Structure12, Theresearchleadingtotheseresultsreceivedfundingfromthe technicalsupport;andM.B.andBASFprovidedtheADHs.We
417–428(2004). EuropeanUnion’sSeventhFrameworkProgrammeFP7/2007-2013 thankR.HeathforapreliminarykineticassayofthePh-AmDH.
29.ThePh-AmDHtoleratesdifferentbufferswell(i.e.,ammonium undergrantagreement266025(BIONEXGEN).Theprojectleading
chholmoroidteet,rfaomrmeraicteL,Bbvo-rAaDteH,scictroanttea,inascettwaoteM,ogx2a+lacteen)t.eHros;wtehveerre,ftohree, tRoestheaisrcahppCloicuanticoilnuhnadserretcheeivEeudrofupnedainngUnfrioomn’sthHeorEizuornop2e0a2n0 SUPPLEMENTARYMATERIALS
strongchelatorssuchasoxalate,citrate,andacetatemustbe researchandinnovationprogram(grantagreementno.638271, www.sciencemag.org/content/349/6255/1525/suppl/DC1
avoidedasbuffersalts.RemovaloftheseMg2+ionsinactivatesthe BioSusAmin).T.K.andN.S.S.receivedfundingfromUK MaterialsandMethods
enzyme(32). BiotechnologyandBiologicalSciencesResearchCouncil(BBSRC; Figs.S1toS12
30.BCo.mR.mBuonm.5m0a,ri1u4s9,5M3–.1S4c9h5ü5rm(2a0nn14,)A..S.Bommarius,Chem. BWBo/lfKso0n01R7e8s0e2a/r1c)h.NM.eJr.Tit.Aiswgarrdat.eSfuolmteoathsepeRcotsyaolfStohceierteysufoltrsa TRaebfeleresnSc1esto(3S32–036)
31. R.Cannio,M.Rossi,S.Bartolucci,Eur.J.Biochem.222, reportedherearethesubjectofaprovisionalpatentapplication. 30June2015;accepted14August2015
345–352(1994). Authorcontributions:F.G.M.andN.J.T.conceivedtheprojectand 10.1126/science.aac9283
BATTERIES energycomponentsofthesystembystoringthe
electro-activespeciesoutsidethebatterycontainer 5555
Alkaline quinone flow battery itself(3–5).Inaflowbattery,thepowerisgen- 01010101
eratedinadeviceresemblingafuelcell,which 2222
containselectrodesseparatedbyanion-permeable 8, 8, 8, 8,
2222
KaixiangLin,1QingChen,2MichaelR.Gerhardt,2LiuchuanTong,1SangBokKim,1 membrane.Liquidsolutionsofredox-activespe- er er er er
ciesarepumpedintothecell,wheretheycanbe bbbb
LouiseEisenach,3AlvaroW.Valle,3DavidHardee,1RoyG.Gordon,1,2* chargedanddischarged,beforebeingreturnedto mmmm
MichaelJ.Aziz,2*MichaelP.Marshak1,2* storage inanexternalstoragetank.Scalingthe ptepteptepte
amountofenergytobestoredthusinvolvessim- eeee
Sbetotwraegeenotfhephinotteorvmolittatiecnatnsdupwpilnydoefltehcetrsiecitreyninewbaabttleerrieessocuoruceldssaonldvevtahreiabmleisdmeamtcahndp.roFblolewm pbalyttemriaeksinagrelabragseerdtoannkmse(Ftailg.io1nAs).inExaicstidinicgsfololuw- on Son Son Son S
batteriespermitmoreeconomicallong-durationdischargethansolid-electrodebatteriesby tion,butchallengeswithcorrosivity,hydrogenevo- g g g g
usingliquidelectrolytesstoredoutsideofthebattery.Wereportanalkalineflowbatterybased lution,kinetics,materialscostandabundance,and orororor
onredox-activeorganicmoleculesthatarecomposedentirelyofEarth-abundantelements efficiencythusfarhavepreventedlarge-scalecom- ag.ag.ag.ag.
andarenontoxic,nonflammable,andsafeforuseinresidentialandcommercialenvironments. mercialization.Theuseofanthraquinonesinan mmmm
Thebatteryoperatesefficientlywithhighpowerdensitynearroomtemperature.Theseresults acidic aqueous flow battery can dramatically cececece
demonstratethestabilityandperformanceofredox-activeorganicmoleculesinalkaline nnnn
eeee
flowbatteries,potentiallyenablingcost-effectivestationarystorageofrenewableenergy. cicicici
T 1UDneivpearrstmitye,n1t2oOfxCfohredmSisttrreyeta,nCdaCmhbermidigcea,lMBiAolo0g2y1,3H8,arUvSaArd. w.sw.sw.sw.s
hecostofphotovoltaic(PV)andwindelec- tostoreelectricalenergy;however,traditional,en- 2HarvardJohnA.PaulsonSchoolofEngineeringandApplied wwww
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tlarircgietysthbaasrdrireorpspteodgseottminugcmhothstatofonoueroefltehce- cfolorsfeadrbtoaottsehrioerstmaadiunrtaatiinonditsochaadregqeuaattpeleyarkepgouwlaeter 3SHcaiernvcaerds,C2o9lleOgxefo,rCdamStbreriedtg,eC,aMmAbr0id2g13e,8M,UAS0A2.138,USA. m m m m
tinritceirtmyfirttoemnctyh(e1s–e3)r.eBnaetwtearibelsepsroouvridceesaimstehaenisr wbainttderoierssoclaanrpinodweepreonudtepnuttly(1s,c2a).leInthceonptorwasetr,falonwd *gedCourodr([email protected])edminigstaryu.thhaorrv.aEr-dm.eadiul:a([email protected]);asm.hicahraveal.rmda.ersdhuak(@Mc.Jo.loAr.a)d;o. ed froed froed froed fro
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Fig.1.Cyclicvoltammetryofelectrolyteandcellschematic.(A)Schematic text.Themolecularstructuresofoxidizedandreducedspeciesareshown
ofcell inchargemode.Cartoon on topof the cell represents sourcesof oncorrespondingreservoirs.(B)Cyclicvoltammogramof2mM2,6-DHAQ
electricalenergyfromwindandsolar.Curvedarrowsindicatedirectionof (darkcyancurve)andferrocyanide(goldcurve)scannedat100mV/son
electronflow,andwhitearrowsindicateelectrolytesolutionflow.Bluear- glassycarbon electrode; arrows indicate scan direction. Dotted line rep-
rowindicatesmigrationofcationsacrossthemembrane.Essentialcom- resentsCVof1MKOHbackgroundscannedat100mV/songraphitefoil
ponents of electrochemical cells are labeled with color-coded lines and electrode.
SCIENCE sciencemag.org 25SEPTEMBER2015(cid:127)VOL349ISSUE6255 1529
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reducebatterycosts (6,7);however,the useof whichwefindexhibitsaroom-temperaturesol- (CVs)of2,6-DHAQandferro/ferricyanidepredict
bromineintheotherhalfofthesystemprecludes ubilityof>0.6Min1MKOH.Thissystemcan anequilibriumcellpotentialof1.2Vuponcom-
deploymentinresidentialcommunitiesowingto achievepowerdensitiesof>0.45Wcm−2atroom binationofthesetwohalf-reactions(Fig.1B).A
toxicityconcerns. temperatureand0.7Wcm−2at45°C. quantitativeanalysisoftheCVof2,6-DHAQat
Wedemonstratethatquinone-basedflowbat- TheuseofalkalineelectrolyteexploitspHasa pH14(fig.S2)revealedredoxbehaviorconsistent
teriescanbeadaptedtoalkalinesolutions,where parametertoshiftthethermodynamicpotentials withtwoone-electronreductionsatpotentials
hydroxylatedanthraquinonesarehighlysoluble ofproton-dependentreactionstomorenegative separatedbyonly0.06 V,witharapidkineticrate
andbrominecanbereplacedwiththenontoxic values.Inacidsolutions,AQundergoesatwo- similartothatofquinonesinacid(6).Thisbe-
ferricyanideion(8,9)—afoodadditive(10).Func- electrontwo-protonreductionatasinglepoten- haviorraisesinterestingquestionsaboutthere-
tionalization of 9,10-anthraquinone (AQ) with tial,whichshiftstomorenegativevaluesasthe lationshipbetweenquinoneredoxandhydrogen
electron-donatinggroupssuchasOHhasbeen pHincreases(6).WhenthepHexceeds12,the bonding(13).
showntolowerthereductionpotentialandex- reduction potential of 2,6-DHAQ becomes pH- Cell testing was performed at 20°C with so-
pandthebatteryvoltage(6).Inalkalinesolution, independentbecausethereducedspeciesisgen- lutionsof0.5M2,6-DHAQdipotassiumsaltand
theseOHgroupsaredeprotonatedtoprovidesol- eratedinitsfullydeprotonatedform(fig.S1).In 0.4MK Fe(CN) ,bothdissolvedin1MKOH.
4 6
ubilityandgreaterelectrondonationcapability, contrastwiththepH-dependentelectrochemical Thesesolutionswerepumpedthroughaflowcell
whichresultsinanincreaseintheopen-circuit behavior ofquinones(negative terminal),the constructedfromgraphiteflowplatesandcarbon
voltage(OCV)of47%overthepreviouslyreported ferro/ferricyanideredoxcouple(positiveterminal) paperelectrodes,whichwereseparatedbyaNafion
system.Becausefunctionalizationawayfromthe hasapH-independentredoxpotential.Thiscon- membrane.Achargingcurrentof0.1Acm−2was
ketonegroupprovidesmoleculeswiththehighest trastingpHdependencecanbeexploitedthrough appliedtochargethecell,andpolarizationcurves
solubility(11,12),weinitiallytargetedcommercially thedevelopmentoflow–reductionpotentialqui- were measured at 10, 50, and 100% states of
available2,6-dihydroxyanthraquinone(2,6-DHAQ), nones at high pH. The cyclic voltammograms quinonecharge(SOC).TheOCVis1.2Vat50%
Fig.2.Cellperformance.(A)Cellopen-circuitvoltageversusstateofcharge.Allpotentialsweretakenwhencellvoltagestabilizedtowithin±1mV.One
hundredpercentSOCwasreachedbypotentiostaticholdingat1.5Vuntilthecurrentdecreasedtobelow20mA/cm2.(BandC)Cellvoltageandpowerdensity
versuscurrentdensityat20°and45°C,respectively,at10,50,and~100%SOC.Electrolytecomposition:At20°C,0.5M2,6-DHAQand0.4Mferrocyanide
wereusedinnegativeelectrolyteandpositiveelectrolyte,respectively.At45°C,bothconcentrationsweredoubled.Inbothcases,potassiumhydroxidecontent
startedat1Mforbothsidesinthefullydischargedstate.
Fig.3.Cellcyclingperformance.(A)Representativevoltageversustimecurvesduring100charge-dischargecyclesat0.1A/cm2,recordedbetweenthe
10thand19thcycles.(B)Capacityretention,currentefficiency,andenergyefficiencyvaluesof100cycles.Normalizedcapacityisevaluatedbasedonthe
capacityofthefirstchargeanddischargecycle.
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SOC;itsdependenceonSOCisshowninFig.2A. however,alltheelectro-activemoleculesremain impedancespectroscopy(fig.S6).Inbothcases,
Thepolarizationcurves(Fig.2B)shownosignof negativelychargedinallchargestates,leadingto the r contributes more than 70% of the ASR
hf
redoxkineticlimitationsandexhibitapeakgal- a dramaticdecrease inthe degree of crossover andisindeedthelimitingfactortothecellcur-
vanicpowerdensityexceeding0.4Wcm−2. duringcellcycling.Cyclicvoltammetryoftheferro/ rentandpoweroutputs.Ther isdominatedby
hf
Thecellwascycledataconstantcurrentden- ferricyanide electrolyte collected at the end of theresistanceofthemembrane,whichisanor-
sityof±0.1Acm−2for100cycles(Fig.3A).The cycling showed no evidence of the presence of derofmagnitudehigherthantheresistanceof
current efficiency exceeded 99%, with a stable 2,6-DHAQ. This observation places an upper thesamemembraneinapH0acidsolution(14).
round-tripenergyefficiencyof84%.A0.1%loss limitoncrossoverof0.8%oftheDHAQ,imply- Thesluggishkineticsofthehydrogenevolution
incapacitypercyclewasobservedduringcycling, ingacrossovercurrentdensityof<2.5mAcm−2 reactioninalkalinesolutiononcarbonelectrodes
which appears to be a continuous loss of elec- (fig.S4).Finally,hydraulicleakagewasinvesti- resultsinalargerpracticalstabilitywindowin
trolyte over the 100 cycles. Three possible loss gated because an apparent but unquantifiable baseratherthaninacid(fig.S7).Consequently,
mechanismswereexplored:chemicaldecompo- smalldecreaseinfluidlevelswasobservedinthe quinoneswithsubstantiallymorenegativereduc-
sition,electrolyte crossoverthrough themem- reservoirs.Aftercellcycling,thecellwaswashed tionpotentialsarefeasibleasnegativeelectrolyte
brane,andleakagefromthepumpingsystem. withdeionizedwateruntilnocolorationofeluent materials.Preliminaryinvestigationsintothesyn-
Chemicalandelectrochemicalstability studies couldbeobserved.Thecellwasthendisassembled; thesis of different hydroxy-substituted anthra-
showed that the negative electrolyte is stable. colorationwasfoundonthegaskets,indicating quinonessuggestthatfurtherincreasesincell
Tenmillimolar2,6-DHAQwasheatedin5MKOH thelikelysiteofelectrolyteleakage(fig.S5).This potentialarepossible.Self-condensationreactions
solution at 100°C for 30 days and was charac- source of capacity loss—equivalent to roughly ofsubstitutedbenzeneyield2,3,6,7-tetrahydroxy-
terized by proton nuclear magnetic resonance eightdropsinoursystem—isexpectedtobecome AQ(THAQ)(15)and1,5-dimethyl-2,6-DHAQ(15-
(NMR).Cyclednegativeelectrolytewasalsocol- negligibleassystemsizeisscaledup. DMAQ)(figs.S8andS9).TheCVsofthesespecies
lected and characterized by the same method; By increasing the temperature to 45°C, the in1MKOHsuggestcellpotentialsversusferri/
bothstudiesshowednodegradationproductat peakgalvanicpowerdensityincreasesfrom0.45 ferrocyanideapproaching1.35V(Fig.4,AandB),
the sensitivity level of 1% (fig. S3). Membrane to~0.7Wcm−2(Fig.2C),asthecellarea-specific whichexceedsthatofmanyaqueousrechargeable
crossovercontaminationhasbeenacommonchal- resistance(ASR)decreasesfromabout0.878 to batteries(Fig.4C).
lengeinacid-basedredoxflowbatteries,where 0.560ohmcm2,estimatedfromthelinearparts The cyanide ions in both ferro- and ferri-
mostelectro-activemoleculesareeitherneutral ofthepolarizationcurvesinFig.2.Mostofthis cyanideareboundtootightlytobereleasedun-
orpositiveandtendtomigratethroughproton- ASRdecreasecomesfromachangeinthehigh- dermostconditions.Consequently,itisnontoxic
conductivemembranes(5).Inthisalkalinesystem, frequencyASR(r )measuredbyelectrochemical inbothoxidizedandreducedformsandiseven
hf
Fig.4.Molecularstructureandcyclicvoltammetryof2,6-DHAQderiva-
tives.(A and B) Molecularstructures and CVof 2,3,6,7-THAQ (pink) and
1,5-DMAQ(olive),respectively,plottedalongwithferrocyanide(orangecurve)
scannedat100mV/songlassycarbonelectrode.Both2,6-DHAQderivatives/
ferrocyanidecouplesshowedhigherequilibriumpotentialthan2,6-DHAQ/
ferrocyanide.(C)Selectedaqueoussecondarybatteriesshowingvoltageand
flowstatus.Literaturedatafrom(3)and(16).
SCIENCE sciencemag.org 25SEPTEMBER2015(cid:127)VOL349ISSUE6255 1531
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permittedforuseasafoodadditive(10).Theuseof data.K.L.,Q.C.,M.R.G.,M.P.M.,R.G.G.,andM.J.A.wrotethe SupplementaryText
ferrocyanideoffersnotableadvantagesoverbro- paper,andallauthorscontributedtorevisingthepaper. Fig.S1toS9
minebecauseitisnonvolatileandnoncorrosive, References(17–22)
SUPPLEMENTARYMATERIALS
allowingsimplerandlessexpensivematerialsof
www.sciencemag.org/content/349/6255/1529/suppl/DC1 7April2015;accepted24August2015
construction.Inaddition,thesetri-andtetra-
MaterialsandMethods 10.1126/science.aab3033
anionicorganometallicmoleculesexhibitlowcross-
overratesthroughcation-exchangemembranes.
Theresultsreportedhereinhighlighttheability
ofhydroxy-substitutedanthraquinoneandferro- H-BONDING CATALYSIS
cyanidetofunctionasstableflowbatteryelectro-
–
lytesinalkalinesolution.Theuseoforganicand O H hydrogen bonding promotes
organometallic coordination complexes in base,
ratherthanaqueousmetalionsinacid,resolves a –
H-atom transfer from C H bonds
seriouscost,corrosion,andsafetyconcernsof
previousflowbatterychemistries.Alkalineflow
batteriescancompensateforhighermembrane for C-alkylation of alcohols
resistancewithhighervoltage,leadingtoperfor-
mance similar to that of their acidic counter-
parts.Inaddition,quinone-ferrocyanidealkaline JennaL.Jeffrey,*JackA.Terrett,*DavidW.C.MacMillan†
chemistryavoidsthemembranecrossover,cor-
rosivity,toxicity,andregulationsassociatedwith Theefficiencyandselectivityofhydrogenatomtransferfromorganicmoleculesareoften
bromine.Thisreducedcorrosivitycanleadtoa difficulttocontrolinthepresenceofmultiplepotentialhydrogenatomdonorsandacceptors.
substantiallylowermaterialscostbecausemany Here,wedescribethemechanisticevaluationofamodeofcatalyticactivationthat
components can be made of inexpensive poly- accomplishesthehighlyselectivephotoredoxa-alkylation/lactonizationofalcoholswith
olefinorpoly(vinylchloride)plastics. methylacrylateviaahydrogenatomtransfermechanism.Ourstudiesindicateaparticular
roleoftetra-n-butylammoniumphosphateinenhancingtheselectivityforaC–Hbondsin
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no.DE-AR0000348andtheHarvardJohnA.Paulson transitionstate.In1987,Robertsnotedthatcer- bond(19),thestrengthofwhichisreflectedinthe
SchoolofEngineeringandAppliedSciences.Methods,along tainelectrophilicradicals(suchast-butoxyl)pre- 13Cnuclearmagneticresonance(NMR)chemical
withanyadditionalextendeddatadisplayitemsand ferentially abstract hydrogen from electron-rich shiftandtheone-bond13C–1Hcouplingconstant
sourcedata,areavailableinthesupplementarymaterials; C–Hbonds,whereasnucleophilicradicals(such (1J )(20,21).Inparticular,itwasfoundthata
referencesuniquetothesesectionsappearonlyintheonline CH
versionofthepaper.M.P.M.,K.L.,R.G.G.,andM.J.A. 1kJ/molincreaseintheenthalpyoftheH-bond
formulatedtheproject.K.L.,S.B.K.,andD.H.synthesized, resultedina0.2-Hzdecreasein1J forhexafluo-
CH
analyzed,andpurifiedthecompounds.K.L.,L.T.,andS.B.K. MerckCenterforCatalysis,PrincetonUniversity,Princeton, roisopropanolcomplexedtovariousamines(20).
collectedandanalyzedtheNMRandmassspectroscopy NJ08544,USA.
data.K.L.,A.W.V.,andL.E.measuredsolubility.K.L.,Q.C., *Theseauthorscontributedequallytothiswork.†Corresponding Onthebasisofthesestudies,wereasonedthatthe
L.E.,andM.R.G.collectedandanalyzedtheelectrochemical author.E-mail:[email protected] efficiencyandselectivityofalcoholC–Hactivation
1532 25SEPTEMBER2015(cid:127)VOL349ISSUE6255 sciencemag.org SCIENCE
Alkaline quinone flow battery
Kaixiang Lin et al.
Science 349, 1529 (2015);
DOI: 10.1126/science.aab3033
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Supplementary Materials for

Alkaline Quinone Flow Battery

Kaixiang Lin, Qing Chen, Michael R. Gerhardt, Liuchuan Tong, Sang Bok Kim, Louise
Eisenach, Alvaro Valle, David Hardee, Roy G. Gordon*, Michael J. Aziz*, & Michael P.
Marshak*

*Correspondence to: [email protected] (M.J.A.); [email protected]
(R.G.G.); [email protected] (MPM)

This PDF file includes:

Materials and Methods
Supplementary Text
Figs. S1 to S9
1
Materials and Methods
Materials
All chemicals were purchased from Sigma Aldrich and used as received unless
stated otherwise. 2,6-dihydroxyanthraquinone (2,6-DHAQ) (90% purity) was purchased
from AK Scientific Inc. and was recrystallized from DMF-water mixture. 2-methyl-3-
hydroxybenzoic acid (98% purity) was purchased from AK Scientific Inc. and was used
as received. Nafion 212 and Sigracet SGL 10AA porous carbon paper were purchased
from Ion Power and both pretreated (see method below). The cell hardware and Poco
pyrosealed graphite flow plates were purchased from Fuel Cell Technologies.

Chemical characterization and preparation
Proton nuclear magnetic resonance (1H NMR) spectra were recorded using Varian
INOVA 500 (500 MHz) NMR spectrometers at 23 °C. Proton chemical shifts are
expressed in parts per million (ppm, δ scale) and are referenced to residual protium in the
NMR solvent (CDCl , δ 7.26 ppm; (CD ) CO, δ 2.05 ppm; (CD ) SO, δ 2.50 ppm;
3 3 2 3 2
CD OD, δ 3.31 ppm). Carbon nuclear magnetic resonance (13C NMR) spectra were
3
recorded using Varian INOVA 500 (125 MHz) NMR spectrometers at 23 °C. Carbon
chemical shifts are expressed in parts per million (ppm, δ scale) and are referenced to the
carbon resonances of the NMR solvent (CDCl , δ 77.16 ppm; (CD ) CO, 29.84 ppm;
3 3 2
(CD ) SO, 39.50 ppm; CD3OD, 49.00 ppm).
3 2
Mass spectrum was collected using a maXis impact UHR time-of-flight mass
spectrometer system (Bruker Daltonics Inc, Billerica, MA) equipped with an electrospray
ionization (ESI) source. The sample was dissolved in acetonitrile (5 uM) and analyzed in
negative mode.
2,3,6,7-tetrahydroxyanthraquinone (2,3,6,7-THAQ) was prepared using a previously
reported synthetic route (15). The overall yield was 29% (1H and 13C NMR spectra shown
in Fig. S8). 1,5-dimethyl-2,6-dihydroxyanthraquinone (1,5-DMAQ) was prepared by the
following method: AlCl (10 g) and NaCl (1 g) were mixed and heated to 200 oC to afford
3
a molten salt. 2-methyl-3-hydroxybenzoic acid (2 g) was slowly added in four portions
into the AlCl -NaCl melt. The mixture was continuously heated at 200 oC for 1 hour.
3
After cool down, distilled water (100 mL) was slowly added to this solution, which was
then stirred at room temperature overnight. The dark yellow precipitate was collected on
frit glass filter and extracted with ethyl acetate (100 mL). Distilled water (50 mL) was
then added to this solution. Ethyl acetate layer was separated from aqueous layer by
separatory funnel and dried on magnesium sulfate. The organic phase was concentrated
(to approximately 25 mL) and cooled in an ice bath. The resulting precipitates were
filtered on celite and washed with cold ethyl acetate (50 mL). This filter cake was then
dissolved in acetone (50 mL) where ethyl acetate (50 mL) was added. This mixture was
gently heated to remove acetone. Precipitates from this resulting solution were removed
on celite. After concentrating the ethyl acetate filtrate, hexane was added to precipitate
out desired product which was then collected on frit glass filter and dried under vacuum
to afford pale yellow solid. The yield of this product was 12% (1H, 13C NMR spectra and
mass spectroscopy results are shown in Fig. S9).

Solubility measurement
2
Room temperature solubility in 1 M KOH solution was determined by measuring the
absorbance at 413 nm and comparing to an absorbance-vs.-concentration calibration
curve determined by preparing known concentrations of 2,6-DHAQ. UV-Vis
spectrophotometry measurements were performed using an Agilent Cary 60
spectrophotometer equipped with a Quantum Northwest T2 temperature regulator.
Appropriate aliquots of 2,6-DHAQ stock solution were added to 1 M KOH blank solution
and their UV-Vis absorbance spectra measured. A saturated solution of 2,6-DHAQ in 1
M KOH was prepared by adding 2,6-DHAQ potassium salt into 1 M KOH solution (10
mL) until a thin layer of precipitate formed, the remaining solution was diluted by known
proportions, and the absorbance of the resulting solution was compared to the calibration
curve.

Electrochemical characterization
A glassy carbon electrode was used for three-electrode cyclic voltammetry tests,
except in the electrochemical window tests where graphite foil was used to mimic the
condition of porous carbon paper. Rotating-disk electrode (RDE) experiments of 2,6-
DHAQ (1 mM) in 1 M KOH solution were performed using a BASi RDE-2 rotating-disk
electrode system (Fig. S2A). All tests were carried out using a Gamry Reference 3000
potentiostat, with a Pt counter electrode and a Ag/AgCl reference electrode (equilibrated
with 3M NaCl, 213 mV vs. standard hydrogen electrode). A Levich plot was constructed
from the RDE data by plotting the mass-transport limited current vs. the square root of
the rotation rate (Fig. S2B). The diffusion coefficient of 2,6-DHAQ was calculated for
each of three runs from the slope of the line fit to the Levich equation (17). The kinematic
viscosity was taken to be 1.08 × 10−6 m2 s−1 (18). The resulting value of the diffusion
coefficient is 4.8(2) × 10−6 cm2 s−1.

Membrane and electrode pretreatments
Pretreatment of Nafion ® 212 membrane was performed by first heating in 80 oC
de-ionized water for 20 minutes and then soaking in 5% hydrogen peroxide solution for
35 minutes. These pre-treated membranes were stored in 0.1 M KOH solution at room
temperature.
Sigracet ® SGL 10AA (~ 400 µm thickness) porous carbon paper was pretreated by
baking in air at 400 °C for 24 hours.

Full cell measurement
20 oC cycling study
The positive electrolyte was prepared by dissolving potassium ferrocyanide
trihydrate (2.56 g) in 1 M KOH solution (15 mL) to afford a 0.4 M ferrocyanide and 2.6
M potassium electrolyte solution. The negative electrolyte was prepared by dissolving
2,6-DHAQ (0.48 g) in 2 M KOH solution (4 mL) resulting a 0.5 M 2,6-DHAQ and 1 M
potassium electrolyte solution.
20 oC cell performance study
The positive electrolyte was prepared by dissolving potassium ferrocyanide
trihydrate (2.56 g) in 1 M KOH solution (15 mL) to afford a 0.4 M ferrocyanide and 2.6
M potassium electrolyte solution. The negative electrolyte was prepared by dissolving
3
2,6-DHAQ (0.6 g) in 2 M KOH solution (5 mL) resulting a 0.5 M 2,6-DHAQ and 1 M
potassium electrolyte solution.
45 oC cell performance study
The positive electrolyte was prepared by heat dissolving potassium ferrocyanide
trihydrate (5.1 g) in 1 M KOH solution (15 mL) at 45 oC to afford a 0.8 M ferrocyanide
and 4.2 M potassium electrolyte solution. The negative electrolyte was prepared by heat
dissolving 2,6-DHAQ (1.2 g) in 3 M KOH solution (5 mL) at 45 oC resulting a 1 M 2,6-
DHAQ and 3 M potassium electrolyte solution.
For all full cell studies, the electrolytes were assembled in the fully discharged state.
Cell hardware from Fuel Cell Tech. (Albuquerque, NM) was used to assemble a
zero-gap flow cell configuration, similar to previous reports (19). Serpertine flow pattern
pyrosealed POCO graphite flow plates were used for both sides. A 5 cm2 geometric
surface area electrode comprised a stack of three sheets of Sigracet SGL 10AA porous
carbon paper. A sheet of pretreated Nafion 212 membrane served as the ion-selective
membrane. The rest of the space between the plates was gasketed by Teflon sheets. The
electrolytes were fed into the cell through PFA tubing, at a rate of 60 mL/min controlled
by Cole-Parmer Micropump gear pumps.
Before and during the tests, the electrolytes were purged with ultra-high purity argon
to ensure deaeration. Galvanostatic cycling was performed at ±0.1 A/cm2, with voltage
limits of 0.6 and 1.7 V, controlled by a Gamry 30K Booster potentiostat. To obtain the
polarization curves, the cell was first charged to the desired SOC, and then polarized via
linear sweep voltammetry at a rate of 100 mV/s. This method was found to yield
polarization curves very close to point-by-point galvanostatic holds, yet to impose
minimal perturbation to the SOC of the small-electrolyte-volume cell. EIS was performed
at 50% SOC, open-circuit potential, and 10 mA/cm2 AC current density, with frequency
ranging from 1 to 100,000 Hz.
Supplementary Text
Cyclic voltammogram modeling
Computation was performed using Mathematica 10.0.1.0 according to algorithms by
Oldham and Myland (20). The diffusivity of 2,6-DHAQ and both the one-electron and
two-electron reduction products were assumed to be 4.8 × 10−6 cm2 s−1, based on our
RDE studies. The temperature was 293 K. Time was discretized into 40 ms increments,
and the current at each time was computed based on the sum of previous currents as
described by Oldham (20).
To model the two-electron reduction process (Fig. S2C), the Butler-Volmer equation
with the number of electrons n = 2 was used in place of Oldham equation (4:5), and the
right-hand-side of equation (8:3) was multiplied by 2 to account for a two-electron
reduction (17, 20). The reduction potential was assumed to be equal to the voltage
equidistant from the two peaks on the measured cyclic voltammogram (−0.684 V vs
SHE) and the charge transfer coefficient α was assumed to be 0.5. The electrochemical
rate constant k was then varied from 10−3 cm s−1 to 10−5 cm s−1.
0
To model the two successive one-electron reductions (Fig. S2D), Oldham equations
(12:13) and (12:14) were used (20). The charge transfer coefficients α and α were
1 2
assumed to be equal to 0.5, and the electrochemical rate constants for the two reductions
were assumed equal. The values of the single rate constant and the first and second
4
reduction potentials were permitted to vary freely. A reasonable fit to the experimental
data was observed when the reduction potential of the first reduction E = −0.657 V vs
1
SHE, the reduction potential of the second reduction E = −0.717 V vs SHE, and the two
2
electrochemical rate constants k and k both equaled 7 × 10−3 cm s−1, a value very
0,1 0,2
close to that observed in other anthraquinone systems (6). The simulation was repeated
for k and k equal to 7 × 10−4 cm s−1 to establish a lower bound on possible k values.
0,1 0,2 0

Energy Density
The negative electrolyte of 0.5 M DHAQ, with two electrons per molecule, has a
charge capacity of 27 Ah/L. The positive electrolyte of 0.4 M ferrocyanide has a charge
capacity of 11 Ah/L. In the experimental cell used here, in which there is 50% more
positive electrolyte than needed to balance the negative electrolyte, the energy density is
6.8 Wh/L. However, eliminating the positive electrolyte volume excess and using
established methods of raising the ferrocyanide solubility to 1.4 M (21), the energy
density would become 18.8 Wh/L.

5