Alkaline Quinone Flow Battery with Long Lifetime at pH 12

41 Pages·2017·2.6 MB·English

by  Kwabi, David

Article
Alkaline Quinone Flow Battery with Long
Lifetime at pH 12
DavidG.Kwabi,KaixiangLin,
YunlongJi,...,Ala´n
Aspuru-Guzik,RoyG.Gordon,
MichaelJ.Aziz
[email protected]
HIGHLIGHTS
Near-neutralchemistryfor
rechargeable,aqueous-soluble,
organicredox-flowbatteries
Capacityfaderateof(cid:1)0.01%/
day,thelongestlifetimefora
quinone-basedelectrolyte
Meetstechnicalcriteriafor
commercializationofredox-flow
batteries
Thisworkdemonstratesanew,organicredox-flowbattery(RFB)thatoutlivesits
predecessors,offeringthelongest-livedhigh-performanceorganicflowbatteryto
date.Itappearstobethefirstaqueous-solubleorganicRFBchemistrytomeetall
thetechnicalcriteriaforcommercialization.Thepotentiallowreactantand
membranecostsofthischemistryofferthepotentialforRFBsofthistypetobe
usedcosteffectivelyatthegigawattscaleinordertoenablemassivepenetration
ofintermittentrenewableelectricity.
Kwabietal.,Joule2,1894–1906
September19,2018ª2018TheAuthors.
PublishedbyElsevierInc.
https://doi.org/10.1016/j.joule.2018.07.005
Article
Alkaline Quinone Flow Battery
with Long Lifetime at pH 12
David G. Kwabi,1,4 Kaixiang Lin,2,4 Yunlong Ji,2 Emily F. Kerr,2 Marc-Antoni Goulet,1
Diana De Porcellinis,1 Daniel P. Tabor,2 Daniel A. Pollack,3 Ala´n Aspuru-Guzik,2 Roy G. Gordon,1,2
and Michael J. Aziz1,5,*
SUMMARY Context &Scale
Wedemonstratealong-lifetime,aqueousredox-flowbatterythatcanoperate Electricitygenerationfrom
atapHaslowas12whilemaintaininganopen-circuitvoltageofover1V.We renewablesourcessuchassolar
functionalized2,6-dihydroxyanthraquinone(2,6-DHAQ)withhighlyalkali-solu- andwindcaneliminatefossil-fuel-
ble carboxylate terminal groups. The resulting negative electrolyte material basedsystemsiftheirintermittent
4,40-((9,10-anthraquinone-2,6-diyl)dioxy)dibutyrate(2,6-DBEAQ)wassixtimes outputcanberegulatedusing
moresolublethan2,6-DHAQatpH12.Symmetriccellcyclingwith2,6-DBEAQ safe,cost-effectiveenergy
onbothsidesofthecelldemonstratesacapacityfaderateof<0.01%/dayand storage.Aqueous-solubleorganic
<0.001%/cycle. By pairing 2,6-DBEAQ with a potassium ferri-/ferrocyanide redox-flowbatteries(RFBs)area
positiveelectrolyteandutilizinganon-fluorinatedmembrane,thisnear-neutral potentiallysafer,lessexpensive
flowbatteryshowsacapacityfaderatethatisthelowestofanyquinoneand alternativetolithiumionbatteries
rivalsthelowesteverreportedforanyflowbatteryintheabsenceofrebalanc- andvanadiumflowbatteriesfor
ingprocesses.Thisresultaddstheimportantattributeoflongcalendarlifeto long-dischargedurationstorage.
quinone-based redox-flow batteries, which may enable massive penetration Wedemonstratealong-lifetime,
ofintermittentrenewableelectricity. organic-molecule-basedRFBthat
canoperateinweakalkaline
INTRODUCTION conditionswhilemaintainingan
open-circuitvoltageofover1V.
Thecostofwindandphotovoltaicelectricityhasdroppedsomuchthatoneofthe
Thenegativeelectrolyte
greatest technical barriers to their widespread substitution for fossil electricity is
comprisesananthraquinone
theirintermittency. Cost-effective, safe, and scalablestationaryelectricitystorage
functionalizedwithsolubilizing
couldsolvethisproblem.Traditionalenclosedbatteriessuchaslithiumionbatteries
carboxylategroupsandthe
are common, but they cannot cost effectively store enough energy for the long
positiveelectrolytecomprisesthe
discharge durations at rated power that appear to be necessary for regulating
foodadditivepotassium
the intermittency of renewables. Redox-flow batteries (RFBs) comprise one class
ferrocyanide.TheRFBcan
ofenergystoragesystemsparticularlysuitedforlong-durationdischargebecause
operatewithaninexpensive
energy-storingspeciesareheldinliquidforminexternaltanksthatareseparated
membraneandshowsacapacity
fromthepowergenerationstack.Consequently,energyandpowerratingscanbe
faderatethatrivalsthelowest
scaled independently, and discharge durations (energy:power ratios) of several
everreportedforanyRFBinthe
hourstodayscanbeachievedattheratedpower.
absenceofrebalancing
processes.Thisresultaddsthe
The RFBs farthest along the commercial pathway are based on charge storage
importantattributeoflong
of vanadium ions in acidic electrolytes, but they have limited potential for
calendarlifetoquinone-based
widespreadadoptionduetothehighcostandlowearthabundanceofvanadium.1
RFBs.
As a consequence, RFBs based on redox-active organic and organometallic
reactants2–6 are receiving much research interest, as they are potentially much
less expensive than their vanadium-based counterparts; in addition, organic
reactants can be chemically modified through selective functionalization in order
to improve aspects of performance such as voltage, rate capability, and energy
density.
1894 Joule2,1894–1906,September19,2018ª2018TheAuthors.PublishedbyElsevierInc.
ThisisanopenaccessarticleundertheCCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/).
OnedrawbackofmanyreportedorganicRFBs,however,istheirlowchemicalstabil-
ity because organic molecules are susceptible to degradation reactions such as
nucleophilic substitution, gem-diol formation, and self-polymerization. Several
organicandorganometallicRFBshavebeenreportedinthepast5years,operating
in highly acidic, highly alkaline, or neutral conditions, based on quinone,2,4,7
viologen,8–11 ferrocene,8 alloxazine,5 and nitroxide radical10,12–14 motifs. Most of
them, however, experience high temporal capacity fade rates of 0.1%–3.5%/day,
which limits their long-term use over many years. Such relatively short lifetimes
rendermostofthesechemistriesunsuitableforcommercialization,whilemotivating
further research into the development of organic molecules with high stability in
bothoxidizedandreducedredoxstates.
Here,wereportanewnegativeelectrolyte(negolyte)moleculesynthesizedfromour
previouslyreportedDHAQchemistrythatexhibitsrecordhighchemicalstability,lead-
ingtothelowesttemporalcapacityfadeamongquinone-basedflowbatteries,onthe
orderof(cid:2)5%/year.ThisisamongthelowestreportedforanyRFBintheabsenceofre-
balancing procedures. The approach involves the introduction of ether-linked alkyl
chains with solubilizing carboxylate functional groups onto an anthraquinone core
and affords highchemical stability and high solubility (0.6 M at pH 12;1.1 M at pH
14)inalkalineelectrolyte(TableS1).Wehypothesizedthatsolubilitywouldbeenhanced
by frustrating crystallization ifwe attachedsolubilizing groupsvia short hydrophobic
chains that were not so long that their hydrophobicity dominates. The solubility
enhancement reduces the concentration of supporting electrolyte by two orders
of magnitude without compromising the ionic conductivity of the system. Pairing
2,6-DBEAQwithaferro-/ferricyanide-basedpositiveelectrolyte(posolyte)resultsina
battery with an open-circuit voltage of 1.05 V and theoretical volumetric energy
densityof(cid:2)12Wh/LatpH14and (cid:2)17Wh/LatpH12(TableS2).Wefurthershow
thatthisO-alkylatedanthraquinonecanbeusedinaflowbatteryatpH12withaninex-
pensivehydrocarbon-basedmembranethatfeaturesexceptionallyhighpermselectivity
and low permeability of both the alkylated anthraquinone and ferricyanide species,
affording century-scale timescales for reactant crossover. These results highlight the
importanceofsyntheticapproachestoincreasethechemicalstabilityofactivespecies
andconstitutealeapforwardinrealizinghigh-performanceaqueousorganicRFBsthat
exhibitlongcycleandcalendarlifeatlowcapitalcost.
RESULTSANDDISCUSSION
SynthesisandCyclicVoltammetryStudy
Figure1illustratesthesyntheticroute,chemicalstructures,andcyclicvoltammograms
(CVs)ofvariousisomersofDBEAQ.AllDBEAQisomerscanbesynthesizedfairlysimply
byanO-alkylationreactionfollowedbyhydrolysisoftheestertoaffordthehighlyalkali-
solublecarboxylicacidterminalgroups(Figure1A).Sincethesecondhydrolysisreaction
isalmost100%inyieldandtheby-productisavolatileorganic,i.e.,methanol,weexpect
thatfortheindustrialprocess,thenegolytecanbeprepareddirectlyafterthehydrolysis
stepandtheremovalofmethanolwithouttheadditionalcostincurredbytwounneces- 1HarvardSchoolofEngineeringandApplied
Sciences,Cambridge,MA02138,USA
sary acid-base treatments, namely, collection of the hydrolyzed DBEAQ product via
2DepartmentofChemistryandChemicalBiology,
acidification for characterization and deprotonation of DBEAQ with a designated
HarvardUniversity,Cambridge,MA02138,USA
amountofbaseforelectrolytepreparation.
3DepartmentofPhysics,HarvardUniversity,
Cambridge,MA02138,USA
Afterthefunctionalization,theresultingDBEAQisomers(Figures1BandS1–S3)showed 4Theseauthorscontributedequally
asignificantimprovementinsolubilityversustheirhydroxylcounterpartsatbothpH14 5LeadContact
andpH12.Forinstance,theroom-temperaturesolubilityof2,6-DBEAQexceeded1M *Correspondence:[email protected]
inpH14and0.5MinpH12KOHsolution(TableS1),asopposedto0.6M4and0.1M, https://doi.org/10.1016/j.joule.2018.07.005
Joule2,1894–1906,September19,2018 1895
Figure1. SynthesisandInitialCharacterizationofDBEAQ
(A)Syntheticapproachtofunctionalizehydroxyanthraquinonewithtetheredhighlyalkali-soluble
carboxylicgroups.
(B)Chemicalstructuresof1,2-,2,6-,and1,8-DBEAQ.
(C)Cyclicvoltammogramsof1,2-(blue),2,6-(gray),and1,8-(green)DBEAQ.Theredoxpotentialvs
SHEofeachisomerisindicated.
SeealsoFiguresS1–S3.
for2,6-DHAQundersimilarconditions,respectively.Severalavenuesexistforfurther
increasingreactant solubilityand the corresponding energy density,suchasthe use
of mixed cations in the electrolytes 15 or mixtures of different DBEAQ isomers or
differentmoleculeswithnearlythesamereductionpotential.
All the DBEAQ isomers showed very similar redox potentials between (cid:3)510 and
(cid:3)540mVversusstandardhydrogenelectrode(SHE)(Figure1C),whichwouldyield
batteryvoltagesabove1Vversusferri-/ferrocyanideposolyte.Despitetheincrease
inreductionpotentialcomparedwith2,6-DHAQ((cid:3)680mVversusSHE),theirCVs
exhibitedhighreversibilitywithredoxpeaks(cid:2)40mVapart,muchsmallerthanthe
(cid:2)90mVvaluefrom2,6-DHAQ.4ALevichanalysiswasusedtoobtainthediffusion
coefficientoftheoxidizedformof2,6-DBEAQ(1.58310(cid:3)6cm2/s;FigureS4),which
wasthenusedinaCVsimulationofitsredoxkinetics(FigureS5).Theresultsshow,in
agreementwithananalogousstudyofthe2,6-DHAQCV,thatthepeakshapeand
separationareconsistentwithtwoone-electronreductionstepsatdifferentpoten-
tials, E and E , whose values are modulated by the energetics of semiquinone
1 2
reduction (see Figure S6 and associated discussion in the Supplemental Informa-
tion). In comparison with 2,6-DHAQ, which exhibited an E -E separation of
1 2
60 mV,4 the 2,6-DBEAQ CV exhibits a much smaller E -E separation of 6 mV,
1 2
implying that 2,6-DHAQ is more thermodynamically susceptible to the formation
ofsemiquinoneradicalsthan2,6-DBEAQ.
ChemicalandElectrochemicalStabilityStudy
Wefirstdemonstratethat2,6-DBEAQhassuperiorchemicalstabilityoverotherqui-
nonesreportedtodate,notablyincluding2,6-DHAQ.Asadirectprobeoftheeffect
of O-alkylation of anthraquinone on its chemical stability, 2,6-DBEAQ was cycled
potentiostatically in a volumetrically unbalanced compositionally symmetric cell
1896 Joule2,1894–1906,September19,2018
Figure2. SymmetricCellCyclingof2,6-DBEAQatpH14
(A)Cellschematicforunbalancedcompositionallysymmetriccellcyclingandfullcellcycling.
(BandC)(B)Unbalancedcompositionallysymmetriccellcyclingof0.10M2,6-DBEAQand(C)
0.65M2,6-DBEAQ,showingcapacityasafunctionoftimeandtemporalcapacityfaderatesfrom
linearfitsofthelast5daysofcycling.Thecapacity-limitingsidewas5mL2,6-DBEAQ,whilethe
non-capacity-limitingsidewas10mL2,6-DBEAQ,bothatpH14.Capacitieswereobtainedbyfull
potentiostaticreductionandoxidationatG0.2Vofthecapacity-limitingside;thepotentialwas
switchedwhenthemagnitudeofthecurrentdensitydecayedto2mA/cm2.Notethatbothyaxis
scalesrepresentabout2%ofthecapacityofthecapacity-limitingside.
SeealsoFigureS7.
configuration. The symmetric cell configuration, in which the two sides have the
sameelectrolytecomposition,isasimpleanddirectprobeofchemicalandelectro-
chemicalstabilityofRFBreactantsandisdescribedinfullinaseparatereport.16Any
observedcapacityfadeisdirectlyrelatedtodeactivationofthereactantontheca-
pacity-limitingsideineitheritsoxidizedorreducedstatebecause(1)thereisnegli-
gible reactant crossover with symmetric compositions, (2) changes to membrane
resistance do not result in temporal capacity variations in potentiostatic cycling,
and(3)theunbalancedvolumespermitthecapacity-limitingsidetobetakentoits
limitingstatesofcharge(SOCs)despitepotentialsidereactions.
Figure2shows aschematic ofthecellsetup(Figure2A) and theresultsofunbal-
anced compositionally symmetric cell cycling for 2,6-DBEAQ at 0.10 and 0.65 M
for durations of 13 and 26 days, respectively (Figures 2B and 2C). In both cases,
2,6-DBEAQexhibitedtemporalcapacityfaderatesof<0.01%/dayor<3.0%/year,
which suggests a loss mechanism that is first order in 2,6-DBEAQ concentration.
Incontrast,2,6-DHAQcycledinasimilar cellconfigurationdemonstratedamuch
higher temporal fade rate of 5%/day (Figure S7). Temporal capacity fade rates
forneutralorganic/organometallicRFBshavebeensummarizedelsewhere.8Most
systems show fade rates in the range of 0.1%–3.5%/day. The temporal fade rate
of 2,6-DBEAQ is the lowest ever reported for a quinone-based electrolyte, with
the lowest previously reported being for 9,10-anthraquinone-2,6-disulfonate
(AQDS) at 0.1%–0.2%/day.17,18 It is on par with that of a recently reported
Joule2,1894–1906,September19,2018 1897
viologen-basedflowbattery,8whichexhibitedthehighestcapacityretentionratefor
anyflowbatteryintheabsenceofrebalancingprocesses,withatemporalcapacity
faderateof<0.01%/dayinsymmetriccelltesting.16
Theoriginofthehigherchemicalstabilityof2,6-DBEAQcomparedwith2,6-DHAQ
isnotcompletelyunderstood;however,wehighlighthereafewkeydistinctionsthat
may be responsible. Notably, it is the reduced form of 2,6-DHAQ that has been
showninapreviousstudytobeinvolvedinthelossofredoxactivity,whereasthe
reducedformof2,6-DBEAQisshownheretobequitestableevenattemperatures
up to 95(cid:4)C (see Supplemental Information). Our first observation is that, because
the redox potential of 2,6-DBEAQ is (cid:2)200 mV higher than that of 2,6-DHAQ,
2,6-DBEAQ should be more stable thermodynamically in its reduced form. Sec-
ondly,athighpH,weexpectboththereducedhydroquinonecoreandsolubilizing
groups to be deprotonated and therefore negatively charged in both molecules.
The closer proximity of the negatively charged deprotonated hydroxyl groups in
2,6-DHAQshouldleadtolargerintramolecularCoulombicrepulsionforces,which
maycontributetothisdestabilizationofthereducedformofthemolecule.Finally,
the greater susceptibility to the formation of semiquinone radicals in 2,6-DHAQ,
asdiscussedabove,mayalsobeinvolvedinitsdecreasedstability.
The alkyl chain functionalization, while drastically improving lifetime, does not avoid
decomposition altogether. By performing elevated temperature chemical stability
studies, wehaveidentifiedthatthe cleavageofg-hydroxybutyrateisinvolvedinthe
decompositionoftheoxidizedformof2,6-DBEAQ(FiguresS8andS9),andwehave
characterizedthetimecourseofg-hydroxybutyratecleavageatpH12andpH14and
at0.1Mand0.5Mconcentration(FiguresS10andS11).Theresultssuggestthatthe
half-life of 2,6-DBEAQ atroomtemperature, pH 14,and 0.1 M concentration inthe
oxidized form is on the order of 5 years, with substantially slower decomposition in
thereducedformrelativetotheoxidizedformandatpH12relativetopH14;theseob-
servationsareconsistentwithlongRFBlifetimeattypicaloperatingcellconditions.
MembraneandFullCellStudies
Ion-conductivemembranesplayacriticalroleinRFBsystemsingoverningthetrans-
portofcounterionsacrossthemembranes(ionicconductivity)andthesimultaneous
transportofredox-activespecies(membranecrossover).Inacompositionallyasym-
metriccell,e.g.,2,6-DBEAQ-ferrocyanidesystem,capacityfaderatecanbegreatly
exacerbatedbytheirrecoverablecrossoverofthereactanttotheothersideofthe
electrolyte.19Tolimitcapacityfadebythismechanism,whileretaininghighpermse-
lectivity,wesurveyedarangeofcation-exchangemembranes,includingtheindustry
standardNafion-basedperfluorosulfonicacid(PFSA)membranes,bycharacterizing
boththeirK+conductivityandpermeabilityof2,6-DBEAQandferricyanide(more
permeable than ferrocyanide) (see Experimental Procedures for the customized
labsetup).E-600seriesmembranes,whichcompriseanon-fluorinated,sulfonated
polyaryletherketone-copolymer backbone, delivered the best performance. The
membrane displayed a low area specific resistance (ASR) of (cid:2)1 U∙cm2 in 1 M K+
solution, which is comparable with Nafion 212. It also showed an extremely
low 2,6-DBEAQ and ferricyanide permeability of 5.26 3 10(cid:3)13 cm2/s and 4.4 3
10(cid:3)12 cm2/s, respectively (Figure S12), which are at least an order of magnitude
lowerthanNafion212systems.Tofurtherprovethelowpermeabilityofredox-active
species,weconstructedalowconcentration2,6-DBEAQ-ferri-/ferrocyanidefullcell
at pH 14, using a Fumasep E-620 (K) membrane. Over a period of galvanostatic
cyclingtestingfor4days((cid:2)880cycles),thecellshowedimmeasurablylowcapacity
fadeandacurrentefficiencyaround(cid:2)99.94%(FigureS13).Thistestisprimafacie
1898 Joule2,1894–1906,September19,2018
evidence that the low 2,6-DBEAQ permeabilities measured ex situ using the
nutatingtablesetuptranslatetonegligiblecrossoverof2,6-DBEAQinanoperating
cell and that additional contributions to crossover that are not present in the
nutating table tests, such as electromigration and pressure from active pumping
ofelectrolytes,20areunimportant.Theselowpermeabilities,togetherwiththefact
that Fumasep E-600 series membranes do not contain fluorine, imply that their
useinaDBEAQ-ferri-/ferrocyanidefullcellaffordsapotentiallyrobustconfiguration
foralong-lastingRFBwithlowpowercost(in$/kW)forfullyinstalledsystems(seeA
NoteonDBEAQ/DHAQPermeabilityinSupplementalInformation).
To reduce the corrosivity of the system and the ferricyanide decomposition rate,
whichisexacerbatedathighpH,21,22weperformedthefullcelltestsof2,6-DBEAQ
atamoremoderateelectrolyteconditionofpH12.Symmetriccellcyclingof1,2-
(Figure S14A) and 1,8-DBEAQ (Figure S14B) at pH 12 showed higher temporal
faderates—greaterthan0.1%and10%/day,respectively—than2,6-DBEAQatthe
samepH(<0.01%/day,FigureS15)andwerethereforenottestedinfullcellexper-
iments.Theoreticalcalculationssuggestthattheseisomersaremorethermodynam-
icallysusceptibletohydroxideorwater-inducedg-hydroxybutyratecleavagethan
2,6-DBEAQ(seeTableS4).Giventhe0.6Msolubilitymeasuredfor2,6-DBEAQat
pH12(TableS2),subsequentcelltestswereperformedat0.5Minordertoexamine
theperformanceofafullcellwithreasonableenergydensity.FromthePourbaixdi-
agramof2,6-DBEAQ(FigureS16A),itsreductionpotentialbecomespHindepen-
dentabovepH11.5andisnotexpectedtochangeduringcellcycling.Polarization
andcapacityutilizationmeasurementswithanegolytecontaining0.5M2,6-DBEAQ
areshowninFigure3.Polarizationstudies(Figure3A)atroomtemperatureshowed
anear-linearrelationshipbetweencurrentdensityandvoltageatcurrentscloseto
theopen-circuitvoltage(OCV),whichincreasedfrom0.97Vat10%SOCto1.12V
at 100% SOC (Figure 3B). Between 80% and 90% of the polarization ASR is ac-
counted for by the high-frequency resistance measured using electrochemical
impedance spectroscopy (EIS), which largely reflects membrane resistance.
Apeakgalvanicpowerdensityof0.24W/cm2wasrealizedat100%SOC(FigureS17).
Thispowerdensityisabouthalfofthatpreviouslyreportedina2,6-DHAQ-ferrocy-
anidecell,4owingtothehigherOCVofthelatter(1.20Vasopposedto1.05Vat50%
SOC)andsmallerASR(0.858Ucm2asopposedto1.2Ucm2at50%SOC).When
voltageandcurrenthavealinear,ohmicrelationship,thepeakgalvanicpowerden-
sityisgivenbyp =V2 =r,whereV istheopen-circuitpotentialandristheASR.
max OC OC
The low permeability of the Fumasep membrane to the fastest-crossing species
(i.e.,200yearsrequiredfor50%lossthroughcrossoverofferricyanide)permits,in
principle,a4-foldreductioninthemembranethickness,whichwouldraisethepower
densitysignificantly.
Inordertoavoidtemporalvariationsinaccessiblecapacityduringfullcellcycling
caused by changes in membrane resistance,16 each galvanostatic half cycle was
finished witha potential holdatthepotentiallimit(1.4Vaftercharge,0.6Vafter
discharge)untilthemagnitudeofthecurrentdensityfellbelow2mA/cm2.Overa
5-daytestperiod,acapacityfaderateof0.05%/dayor0.001%/cyclewasobserved
(Figure4A).Aparallelcyclingtestwasperformedinwhichapotentiostaticcharge-
discharge cycle was executed after every 20 galvanostatic cycles; a capacity fade
rateof0.04%/daywasobservedinthatcase(Figure4B).Ithasbeenshownthatca-
pacityretentionratesusingbothcyclingprotocolsyieldvirtuallyidenticalresults.16
Thesefullcellmeasurements,however,showedroughly4-foldincreaseincapacity
fade compared with the <0.01%/day observed during symmetric cycling tests
Joule2,1894–1906,September19,2018 1899
Figure3. PolarizationMeasurementsof2,6-DBEAQFullCellatpH12
(A)Cellvoltageversusdischargecurrentdensityatroomtemperatureat10%,30%,50%,70%,90%,
and100%SOC.Electrolytescomprised5mLof0.5M2,6-DBEAQ(negolyte)atpH12(10mMKOH)
and38mLof0.3Mpotassiumferrocyanideand0.1Mpotassiumferricyanide(posolyte)atpH12.
(B)OCV,high-frequency,andpolarizationASRversusSOC.
(C)Galvanostaticchargeanddischargecurvesfrom25to250mA/cm2.Theverticaldashedlines
indicatethemaximumcapacityrealizedwithpotentiostaticchargeanddischargeatthevoltage
cutoffs(1.4and0.6V,respectively),aswellasthetheoreticalcapacity.
(D)Coulombicefficiency,round-tripenergyefficiency,andcapacityutilizationasapercentageof
potentiostaticcapacityversuscurrentdensity.
(Figures2Band2C),suggestingadditionalcapacityfademechanismsnotobserved
duringsymmetriccellstudies.In-depthchemicalandelectrochemicalanalysis(Fig-
ure S18) was performed to probe the chemical decomposition and crossover of
DBEAQfromthecapacity-limitingside.Electrolytesinacellcompositionallyiden-
ticaltothatinFigure4Abutcycledfor11daysweresubjectedtonuclearmagnetic
resonance(NMR)(FigureS18A)andCV(FigureS18B)analysis.Fromtheseresults,no
evidence of DBEAQ decomposition and crossover was found after examining the
negolyte and posolyte before and after cycling. Based on the detection limit of
NMR(0.1mMDBEAQ)andCVtechniquesundertheexperimentalconditionscho-
sen,theupperlimitofthecapacityfaderatecausedby2,6-DBEAQdecomposition
and/ormembranecrossoverwas(cid:2)0.01%/day,similartooursymmetriccellstudy.
Wethereforehypothesizethatothercapacityfademechanisms,suchasprecipita-
tion of 2,6-DBEAQ in the posolyte after crossing over, might be operative but
untraceablebyNMRandCVtechniquesduetotheslowcapacityfaderate,which
corresponds to a total loss of <0.5% of 2,6-DBEAQ over a 6-day testing period.
Oneothersuchmechanismmightbeleakageofthenegolyteduetopooreradhe-
sionbetweenthethinnerFumasepmembraneandtheVitongasketinthefullcell
thanbetweenNafionN117andthegasketinthesymmetriccell.Indeed,thetotal
capacityfadeinFigure4Bcorrespondstoatotallossovertheentire6-daycycling
period of (cid:2)10 mL of negolyte volume, which is roughly one-fifth of a droplet.
When translated to an equivalent current density (0.5 mA/cm2), this negolyte loss
rateiswellwithinexpectationforseepageoftheelectrolyteintospacesbetween
1900 Joule2,1894–1906,September19,2018
Figure4. ExtendedFullCellCyclingof2,6-DBEAQat0.5MandpH12
(A)Currentefficiency(squares)andcharge(upward-pointingtriangles)anddischarge(downward-
pointingtriangles)capacityversustimeandcyclenumberforanegolyte-limited2,6-DBEAQ-
Fe(CN)6cell.Thecellwascycledgalvanostaticallyat100mA/cm2between1.4and0.6V,and
eachhalfcycleendedwithapotentiostaticholduntilthemagnitudeofthecurrentdensityfell
below2mA/cm2.Thenegolytecomprised5mLof0.5M2,6-DBEAQatpH12whiletheposolyte
comprised30mLof0.3Mpotassiumferrocyanideand0.1MpotassiumferricyanideatpH12.
(B)Evolutionofcharge(upward-pointingtriangles)anddischarge(downward-pointingtriangles)
capacityforextendedcellcyclingat100mA/cm2.Afterevery20thgalvanostaticcycle,a
potentiostaticcharge-dischargecyclewasperformed,thedischargecapacityofwhichisshown
(opencircles).Becausetheconcentrationofoxidized2,6-DBEAQisatitsmaximumaftereach
potentiostaticdischarge,thecapacityofthegalvanostaticchargestepimmediatelyfollowing
potentiostaticcharge-dischargeishigherthaninsubsequentcycles.Thisparticularcellshowed
only(cid:2)65%ofitstheoreticalcapacitybecausealargefractionofsaltwasleftoverfromthesynthesis
of2,6-DBEAQ.
thegasketsand/orinterfacebetweenthemembraneandgaskets,comparedwithan
analogousleakrateestimatedfromapreviousstudyofcapacityfadeinananthraqui-
none-basedflowbatterywiththiscellarchitecture(0.09–0.12mA/cm2).19
Quinones for aqueous flow batteries have been the subject of intensive research
since their recent debut in this application. This work demonstrates that the
simplefunctionalizationwithbulkychargedgroupscangreatlyextendthecalendar
life and cycle life; raise the solubility, leading to a theoretical energy density
of 17 Wh/L with possible routes to further increases; operate with fluoropolymer-
free membranes at pH as low as 12 while maintaining the voltage against
Fe(CN) 3(cid:3)/4(cid:3) above 1 V; and perform with peak power density of 0.24 W/cm2 at
6
roomtemperaturewithpossibleroutestofurtherincreases.Thesepropertiesappear
tomeetallofthetechnicalrequirementscommonlyunderstoodtobenecessaryfor
aqueousRFBcommercialization.
At the large production volumes necessary for gigawatt-scale grid storage, it is
possible that DBEAQ may approach the cost of other functionalized anthraqui-
nones,suchasDHAQandAQDS,thelatterofwhichwasrecentlyestimatedtobe
between$0.92/kgand$3.92/kg.23Using$2.40/kgasamid-rangecost,thisresults
inacapitalcostof$17.60/kWhforthe2-electron-acceptingDBEAQinthenegolyte
ina1.05Vbattery.Similarly,weestimatetheindustrial-scalecostofpotassiumferro-
cyanidetobearound$2.15/kg,24whichamountstoacapitalcostofabout$32/kWh
fortheposolyte(seeEstimated CostofElectrolytesintheSupplementalInforma-
tion). At $50/kWh, the total capital cost of these electrolytes stands at less than
one-thirdofthecostofanall-vanadiumelectrolyte,forwhichthevanadiumalone
iscurrentlypricedat$160/kWh.Usingafluorine-freemembranesuchasFumasep
E(cid:3)620(K)shouldresultinaproductofmembranecost(<$25/m2atlargeproduction
Joule2,1894–1906,September19,2018 1901
volumes)andASR(<1.5Ucm2)thatisbelow$5/mU,whichaddsnegligiblecostto
thesystem.25Thus,iftheDBEAQproductioncostatscaleturnsouttobenearlyas
lowasforAQDSandDHAQ,23,26thenthischemistrymayhaveseriouscommercial
potential, and DBEAQ-based flow batteries may be instrumental in accelerating
thepenetrationofwindandphotovoltaicelectricity.
EXPERIMENTALPROCEDURES
SynthesisandChemicalCharacterization
Synthesisof4,40-((9,10-Anthraquinone-2,6-diyl)dioxy)dibutyricAcid
2,6-DHAQ was purchased from AK Scientific. Methyl 4-bromobutyrate was pur-
chased from VWR. All other chemicals were purchased from Sigma-Aldrich. All
chemicalswereusedasreceivedunlessspecifiedotherwise.
Dimethyl 4,40-((9,10-anthraquinone-2,6-diyl)dioxy)dibutyrate (1). 2,6-DHAQ was
first converted to its dipotassium salt (2,6-DHAQK ) by adding 2,6-DHAQ (5 g,
2
20.8 mmol) to a 250 mL oven-dried flask of dimethylformamide (250 mL). Under
vigorous stirring, potassium ethoxide (6.1 g, 72.9 mmol) was added. The mixture
solutionwasstirredatroomtemperaturefor15min.FortheO-alkylationreaction,
2,6-DHAQK (6.5 g, 20.8 mmol) was mixed with anhydrous K CO (14.3 g,
2 2 3
104mmol)andmethyl4-bromobutyrate(12.4mL,104mmol).Thereactionmixture
was then heated to 95(cid:4)C overnight. After cooling to 0(cid:4)C, deionized (DI) water
(150 mL) was added to the mixture to dissolve inorganic salt and to precipitate
the ester precursor of DBEAQ. The precipitate was vacuum filtered and washed
thoroughlywithDIwater(50mL).Theproductwasanalyzedby1HNMRandused
forthenextstepreactionwithoutfurtherpurification.Finalyield,87%.
4,40-((9,10-Anthraquinone-2,6-diyl)dioxy)dibutyricAcid(2). Theesterprecursorof
DBEAQ(1g,2.23mmol)wasaddedalongwithKOH(0.52g,9mmol)toaflaskfilled
withawater-isopropanolmixture(2:1v/v,60mL).Thesolutionwasstirredvigorously
andheatedto60(cid:4)Cfor12hr.Duringthereaction,allsolidsdissolved,andthesolu-
tionbecameadarkredcolor.Afterthereaction,thesolutionwastransferredtoa
larger500mLflaskanddilutedwithDIwater(200mL).Glacialaceticacidwasadded
until the pH of the solution dropped to 4. The mixture was stirred vigorously for
1 hr, followedby vacuumfiltrationand thoroughwashing withDI water (100mL).
Theproductwasvacuumdried,analyzedby1HNMR,andusedforelectrochemical
measurementwithoutfurtherpurification.Finalyield,99%.
The 1H NMR spectrum of 4,40-((9,10-anthraquinone-2,6-diyl)dioxy)dibutyric
acid is shown in Figure S1. 1,8- and 1,2-isomers were synthesized similarly, using
1,2-DHAQ and 1,8-DHAQ as precursors. Potassium tert-butoxide (8.16 g,
72.9 mmol) replaced potassium ethoxide to deprotonate the 1,2-DHAQ and 1,8-
DHAQisomersbecauseofthehigherpK ofthehydroxylgroupsoftheseisomers.
a
1HNMRspectraofthefinalproductsareshowninFiguresS2andS3,respectively.
ElectrochemicalCharacterization
CyclicVoltammetryandRotatingDiskElectrodeMeasurements
Glassycarbonwasusedastheworkingelectrodeforallthree-electrodeCVtests.
Rotating disk electrode experiments were conducted using a Pine Instruments
ModulatedSpeedRotatorAFMSRCEequippedwitha5mmdiameterglassycarbon
workingelectrode,aAg/AgClreferenceelectrode(BASi,pre-soakedin3MNaClso-
lution), and a graphite counter electrode. The electrode was rotated at a specific
speedwhilethevoltagewassweptlinearlyfrom(cid:3)0.4to(cid:3)0.11VversusAg/AgCl
(Figure S4A). The diffusion coefficient of the oxidized form of 2,6-DBEAQ was
1902 Joule2,1894–1906,September19,2018