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Cite This: J. Am. Chem. Soc. 2019, 141, 8014−8019

Extending the Lifetime of Organic Flow Batteries via Redox StateManagement
Marc-Antoni Goulet,†,∥,⊥ Liuchuan Tong,‡,∥ Daniel A. Pollack,§ Daniel P. Tabor,‡
Susan A. Odom,†,∇ Alań Aspuru-Guzik,‡,¶ Eugene E. Kwan,*,‡ Roy G. Gordon,*,†,‡and Michael J. Aziz*,†

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†

Harvard John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138,
United States‡
Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, United States§
Department of Physics, Harvard University, Cambridge, Massachusetts 02138, United States∇
Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506, United States
S Supporting Information*

tion,19,22,31,40,41 nucleophilic substitution,25,31,34 electrophilicaromatic substitution,39 ring-opening,17 and disproportionation,25,33,37 none of these pathways have been linked
quantitatively to capacity fade.
Here we identify irreversible dimerization as the mechanismof capacity loss in a RFB utilizing the inexpensive redox
couples of 2,6-dihydroxyanthraquinone (DHAQ) and potassium salts of iron hexacyanide (Fe(CN)6). We further
demonstrate that capacity loss can be suppressed throughsimple modiﬁcations of the battery operating conditions.
Under ideal conditions, discharging a ﬂow battery involves
the reversible oxidation and concurrent reduction of the low
potential (negolyte) and high potential (posolyte) active
species, respectively. In a DHAQ/Fe(CN)6 ﬂow battery, thereactions and potentials vs SHE are16

ABSTRACT: Redox ﬂow batteries based on quinonebearing aqueous electrolytes have emerged as promisingsystems for energy storage from intermittent renewablesources. The lifetime of these batteries is limited byquinone stability. Here, we conﬁrm that 2,6-dihydroxyanthrahydroquinone tends to form an anthrone intermediatethat is vulnerable to subsequent irreversible dimerization.
We demonstrate quantitatively that this decomposition
pathway is responsible for the loss of battery capacity.
Computational studies indicate that the driving force foranthrone formation is greater for anthraquinones withlower reduction potentials. We show that the decomposition can be substantially mitigated. We demonstratethat conditions minimizing anthrone formation andavoiding anthrone dimerization slow the capacity loss
rate by over an order of magnitude. We anticipate that thismitigation strategy readily extends to other anthraquinone-based ﬂow batteries and is thus an important step
toward realizing renewable electricity storage throughlong-lived organic ﬂow batteries.

Negolyte: DHAQ2 − + 2e− F DHAHQ4 −Posolyte: [Fe(CN)6 ]3 − + e− F [Fe(CN)6 ]4 −+510 mVIn practice, decomposition of the negolyte active species causes
the battery capacity to fade at ∼5−8%/day. As this rate limits
the lifetime to the order of 1 week, identifying and inhibiting
the mechanism of capacity loss is critical for the battery toapproach the decadal service life that will be necessary forlarge-scale grid storage applications.42

T

he cost of electricity generated from renewable sources
such as the sun and wind has become competitive withelectricity derived from fossil fuels.1 Nonetheless, the widespread adoption of intermittent renewable electricity requires
new methods for the reliable storage and delivery of electricity
over long periods when these sources are unavailable forgeneration.2−4 Redox ﬂow batteries (RFBs), whose energy andpower capabilities can be scaled independently, may enable
cost-eﬀective long-duration discharge.4−9
The all-vanadium redox ﬂow battery chemistry is currently
the most technologically developed but may not access muchof the grid storage market due to electrolyte cost
constraints.6,9−13 Emerging organic electrolytes comprisingcheaper earth-abundant elements may address this limitation.14−39 However, organic electrolytes are more prone tomolecular decomposition, which can lead to a progressive lossof charge storage capacity. Although several studies haveproposed decomposition pathways, such as Michael addi© 2019 American Chemical Society−680 mV; (pH 14)■

RESULTS AND DISCUSSION
To identify the chemical origin of negolyte active species degradation,43 we cycled a sample of DHAQ electrolyte repeatedly
over the course of several days in a symmetric cell and analyzed
its composition (Figure S1). In addition to deprotonated formsof DHAQ and its intended reduction product, dihydroxyanthrahydroquinone (DHAHQ), we identiﬁed the decomposition products 2,6-dihydroxyanthrone (DHA) and its dimers((DHA)2) (see Scheme 1).
The identiﬁcation of DHA was enabled by precisely
controlling cycling conditions (Figure S2). The structure of
DHA was suggested by high resolution mass spectrometry andReceived: December 22, 2018
Published: April 4, 20198014

DOI: 10.1021/jacs.8b13295
J. Am. Chem. Soc. 2019, 141, 8014−8019

Communication

Journal of the American Chemical Society
Scheme 1. Intended Operation and Decomposition Pathway
of a DHAQ/Fe(CN)6 Flow Batteryaa

DHA is depicted in its presumed protonation state at pH 14 as
DHA3−.

conﬁrmed by an exact match to the 1H NMR spectrum and
HPLC retention time of an authentic sample (Figures S3−S5).
When samples containing DHA were subjected to electrochemical oxidation (Figure S6), mass spectrometric peaks
corresponding to (DHA)2 were observed (Figure S7). The
assignment of (DHA)2 was conﬁrmed by the isolation of puresamples by preparative HPLC and subsequent NMR analysis(Figures S8 and S9).
Beck and Heydecke observed anthrone formation during
electrochemical reduction of unsubstituted anthraquinone
under highly acidic conditions and proposed homogeneous
disproportionation as the mechanism by which this transformation occurs.44 Two observations suggest that DHAHQ in
alkaline solution undergoes disproportionation as well. First,
increasing the reduction overpotential beyond the potential
required to reduce DHAQ to DHAHQ does not increase therate of capacity loss.43 Additionally, increasing proportions of
DHA are formed when DHAHQ is held away from theelectrodes in a sealed vial for increasing durations (FigureS10). This disproportionation would occur according to thefollowing proposed reaction:

Figure 1. (A) Cyclic voltammetry of 10 mM synthesized DHA at pH
14 under a nitrogen atmosphere. After aeration, redox activity isrecovered through conversion of DHA to DHAQ. A separate 10 mM
DHAQ sample is shown for comparison. (B) (DHA)2 has a higher
redox potential than DHAQ, which leads to poor battery performance. Cyclic voltammograms were obtained separately for DHAQ and(DHA)2, both at 5 mM in 1 M KOH. The (DHA)2 sample contained
a ∼5% DHAQ impurity.

DHAQ (Figure 1B) decreases the overall potential of thebattery prohibitively.11 Second, even with strong overpotentials, cycling accesses only two electrons per molecule(Figure S16B). Thus, at twice the molecular weight of DHAQ,(DHA)2 has half the capacity by mass. Finally, (DHA)2 isunstable at pH 14 (Figure S17). These ﬁndings indicate thatthe conversion of DHAQ to (DHA)2 engenders a progressive
loss of battery capacity.
To establish that irreversible dimerization is the primarymechanism of capacity loss, we quantitatively correlated thedecrease in concentrations of DHAQ and DHAHQ with theextent of capacity loss. We subjected a typical ﬂow battery
containing 0.1 M DHAQ/DHAHQ to symmetric cell-cyclingat pH 14 (8 cycles over ∼5 h). When cycling was stopped andthe capacity-limiting side held in a reduced state for 43 h, thebattery capacity decreased by 15% (Figure 2) when corrected
for aliquot volume removal. When cycling with ±350 mV
discharge overpotential was resumed, the ﬁrst oxidative
discharge half-cycle included the transient capacity associatedwith the irreversible conversion of DHA to (DHA)2. 1H NMR
analysis of an aliquot of negolyte removed after aging revealed
that the total concentration of DHAQ had declined by 16%.
This decrease was mass-balanced by an increase in DHA

2DHAHQ4 − + 2H 2O → DHAQ2 − + DHA3 − + 3OH−

Under the highly alkaline conditions within the negolytecompartment (Figure S11), facile one-electron oxidation of the
DHA carbanion, followed by radical dimerization, would leadto (DHA)2. Indeed, pure samples of DHA are completely andirreversibly converted to (DHA)2 when subjected to electrochemical oxidation under alkaline and anaerobic conditions(Figure S12). However, under aerobic conditions, oxidation of
DHA back to DHAQ can be favored over dimerization(Figures 1A and S13), particularly at low DHA concentration(Figures S14 and S15). Precedents for the oxidation ofanthrone species to anthraquinone and dianthrone species may
be found in refs 45−56.
The irreversible conversion of DHAHQ to (DHA)2 has adeleterious eﬀect on battery operation due to the replacementof the anthraquinone moiety with a benzophenone functionalgroup (Figures 1B and S16A). Although (DHA)2 is also redoxactive, its use in a RFB is impractical for several reasons. First,
the higher reduction potential of (DHA)2 with respect to8015

DOI: 10.1021/jacs.8b13295
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Journal of the American Chemical Society

than that of DHAQ and is approximately 100 times morestable in terms of RFB lifetime.34 In addition to the associateddecrease in battery voltage that this higher reduction potential
entails, 2,6-DBEAQ is synthesized from DHAQ, increasing its
cost.34 Strategies that can increase the lifetime of anthraquinones independently of their redox potential would therefore
be valuable.
Two simple changes to the operating conditions of thebattery may be used to greatly decrease the rate of capacity
loss. Because the degradation pathway is initiated bydisproportionation of DHAHQ, we hypothesized that avoiding
high states of charge would decrease the amount of DHA
formed (Figure S21), thereby extending battery life. When a
DHAQ/Fe(CN)6 ﬂow battery was cycled at 100 mA/cm2 witha 1.25 V cutoﬀ that accessed 88% of the theoretical capacity,
the observed capacity declined at only 0.14%/day (Figures 4A
and S22A). By contrast, under more typical conditions with a
1.6 V cutoﬀ that accessed 99.9% of the theoretical capacity, theobserved capacity declined at 5.6%/day. Similar results wereobtained by using identical potential conditions but with a
Coulombic cutoﬀ to restrict the SOC range (Figure S23).

Figure 2. Pausing redox cycling in the reduced state adversely aﬀects
battery capacity. Symmetric-cell cycling (see SI) for 8 cycles, followed
by a 43 h pause, leads to a large drop in capacity. The ﬁrst oxidative
discharge after the pause shows a transient amount of capacity
corresponding to the conversion of accumulated DHA to (DHA)2.

concentration of 16% (as determined by NMR analysis). Bothquantities correspond well to the observed loss in capacity of
15%. In addition, all of the DHA formed was measurably
converted to (DHA)2 (Figures S18−S20). These observations
strongly implicate anthrone formation and subsequent
dimerization as the primary source of capacity loss. Anthrone
and dianthrone species have also been reported to be productsof the chemical reduction of anthraquinones and of theelectrochemical reduction of anthraquinones in acidic aqueous
solutions.44,57−65
The ﬁnding that irreversible dimerization is linked to theredox chemistry of anthraquinones may limit the ongoingsearch for quinones with lower potentials. This is because any
decrease in the reduction potential of the quinone/hydroquinone couple may simultaneously increase the propensity forthe hydroquinone to disproportionate into anthrone andquinone. This hypothesis is supported both by calculations(Figure 3; PM7/COSMO on a subset of molecules from ref41) and the experimental properties of 4,4′-((9,10-anthraquinone-2,6-diyl)dioxy)dibutyrate (2,6-DBEAQ).34 This negolyte
active species has a reduction potential that is 180 mV higher

Figure 4. (A) Limiting the state of charge reduces the rate of capacity
loss in a negolyte-limited DHAQ/Fe(CN)6 full cell. Over the ﬁrst 1.6
days, the operating state of charge was limited to 88% of thetheoretical capacity (i.e., 88% DHAHQ and 12% DHAQ), and thecapacity faded at only 0.14%/day. The right-hand segment reﬂects
typical operating conditions (cycling to 99.9% of theoretical capacity),
and the capacity faded at 5.6%/day. (B) Symmetric cell cycling inwhich the capacity-limiting side (5 mL of 0.1 M DHAQ in 1.2 M
KOH) demonstrates recovery of 70% of lost capacity after aeration ofdischarged electrolyte.

Figure 3. Trade-oﬀ between the reduction potential of the quinonehydroquinone couple and the thermodynamic stability of thehydroquinone form toward anthrone formation (Ntotal = 858
quinones). The mean absolute errors (previously benchmarked in
ref 41) of the predicted DHAQ potentials is ∼0.07 V.
8016

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Journal of the American Chemical Society*maziz@harvard.edu*ekwan@fas.harvard.edu

These observations demonstrate that operating conditions thataccess most, but not all, of the theoretical capacity greatly
extend battery lifetime.
Although the strategy of avoiding high states of charge
reduces the amount of DHA formation, it does not eliminate itcompletely unless impractically low (less than ∼60%) states ofcharge are utilized (Figure S21). To divert the remaining DHA
from irreversible dimerization and prevent the associated lossof capacity, we used lower oxidation overpotentials to slow therate of DHA dimerization. We then added molecular oxygen tofavor the oxidation of DHA back to DHAQ over DHA
dimerization (Scheme 1). We tested this strategy by symmetriccell cycling of a DHAQ electrolyte under anaerobic conditions
within its full SOC range (0−99.9%) but with an oxidative
overpotential of 200 mV. The capacity decreased by 13%(Figures 4B and S22B) over 2 days of cycling. At this point, theelectrolyte was removed from the glovebox in the discharged(oxidized) state, aerated, and returned to service. This
operation resulted in the recovery of approximately 70% of
the lost capacity when cycling was resumed. In a large-scale
ﬂow battery, the same process might be made more eﬀectiveby purging the DHAQ negolyte reservoir with air after eachfull discharge to exploit the high DHAQ recovery rate at low
DHA concentrations. Aeration could be optimized to matchthe DHA concentration to avoid unnecessary current eﬃciency
loss (Figure S24).
Our ﬁndings establish that the progressive loss of capacity ina DHAQ-based ﬂow battery is primarily due to anthrahydroquinone disproportionation followed by irreversible
anthrone dimerization. Computational analysis indicates thatthe propensity for anthrahydroquinones to disproportionate ishighly correlated to the redox potential of the anthraquinone.
Therefore, the use of anthraquinones with lower reduction
potentials, enabling apparently higher cell voltages, may belimited by a decrease in stability. This correlation suggests thatongoing eﬀorts to improve quinone-based negolytes might
proﬁtably be redirected toward the manipulation of properties
such as their solubility and synthetic costs, rather than redox
potential alone.
We have also shown that the loss of battery capacity canlargely be ameliorated by avoiding high states of charge and bynegolyte aeration. We estimate that the combination of these
two strategies would reduce the capacity fade rate ofanthraquinone-based ﬂow batteries by approximately 2 ordersof magnitude, from 5.6%/day to (0.3)(0.14%/day) = 0.042%/
day. These modiﬁed operating procedures should not
substantially increase the estimated large-scale capital cost(∼$20/kWh for the DHAQ negolyte).66 By comparison, theUnited States Department of Energy has set a $150/kWh goalfor grid-based energy storage systems. With further optimization of the battery operating conditions, this goal may beattained in the near future.

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ORCID

Liuchuan Tong: 0000-0001-6211-6322
Daniel P. Tabor: 0000-0002-8680-6667
Susan A. Odom: 0000-0001-6708-5852
Alán Aspuru-Guzik: 0000-0002-8277-4434
Roy G. Gordon: 0000-0001-5980-268X
Michael J. Aziz: 0000-0001-9657-9456
Present Addresses⊥

Form Energy Inc., Somerville, Massachusetts 02143, UnitedStates¶Department of Chemistry and Department of Computer
Science; Vector Institute for Artiﬁcial Intelligence, Universityof Toronto, Toronto, Ontario M5S 1A1, Canada
Author Contributions∥

The authors declare the following competing ﬁnancial
interest(s): Harvard University has ﬁled a patent application
based on the methods described here.

■

ACKNOWLEDGMENTS
This research was supported by U.S. DOE award DE-AC0576RL01830 through PNNL subcontract 428977, by U.S. DOE
ARPA-E award DE-AR-0000767, by Innovation Fund Denmark via the Grand Solutions project “ORBATS” ﬁle no. 704600018B, by the Massachusetts Clean Energy Technology
Center, and by the Harvard School of Engineering and Applied
Sciences. D.A.P. acknowledges funding support from the NSF
Graduate Research Fellowship Program, no. DGE1144152 andDGE1745303. A.A.-G. acknowledges support from the Canada
150 Research Chair Program. We thank Prof. Cyrille
Costentin, Prof. Luke M. Davis, Dr. Eugene S. Beh, Prof.
Sergio Granados-Focil, and Prof. Alison E. Wendlandt for
helpful discussions and Eric Fell for experimental support.

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ASSOCIATED CONTENT* Supporting InformationS

The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.8b13295.

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These authors contributed equally.

Notes

Experimental details (PDF)

AUTHOR INFORMATION

Corresponding Authors*gordon@chemistry.harvard.edu
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(46) Iwata, M.; Kuzuhara, H. Oxidation of Anthracenols andAnthrone to Anthraquinones with Oxygen Mediated by Copper(II)Ion and Imidazole. Bull. Chem. Soc. Jpn. 1985, 58, 1609−1610.
(47) Cavey, D.; Caron, J.-C.; Shroot, B. Anthralin: Chemical
instability and glucose-6-phosphate dehydrogenase inhibition. J.
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Dithranol, Singlet Oxygen and Unsaturated Fatty Acids. Arch. Pharm.
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Communication

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(50) Martinmaa, J.; Vanhala, L.; Mustakallio, K. K. Free radical
intermediates produced by autoxidation of 1,8-dihydroxy-9-anthrone(dithranol) in pyridine. Experientia 1978, 34, 872−873.
(51) Davies, A. G.; Hawari, J. A.-A.; Whitefield, M. Generation and
E.S.R. spectrum of the 1,8-dihydroxy-9-Anthron-10-yl radical.
Tetrahedron Lett. 1983, 24 (41), 4465−4468.
(52) Geiger, W. 1,8-Dihydroxyanthrone and two isomeric 1,1’,8,8’Tetrahydroxy-10,10’-bianthrones. Chem. Ber. 1974, 107 (9), 2976−
2984.
(53) Dabestani, R.; Hall, R. D.; Sik, R. H.; Chignell, C. F.
Spectroscopic studies of cutaneous photosensitizing agents−XV.
anthralin and its oxidation product 1,8-dihydroxyanthraquinone.
Photochem. Photobiol. 1990, 52 (5), 961−971.
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Potential Oxidation of Anthracene in Acetonitrile. J. Am. Chem. Soc.
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emodinanthrone oxygenase in fungus Aspergillus terreus. Biochem. Int.
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(56) Chung, J.-y.; Fujii, I.; Harada, S.; Sankawa, U.; Ebizuka, Y.
Expression, Purification, and Characterization of AknX Anthrone
Oxygenase, Which Is Involved in Aklavinone Biosynthesis in
Streptomyces galilaeus. J. Bacteriol. 2002, 184 (22), 6115−6122.
(57) Wendlandt, A. E.; Stahl, S. S. Quinones in Hydrogen Peroxide
Synthesis and Catalytic Aerobic Oxidation Reactions. In Liquid Phase
Aerobic Oxidation Catalysis: Industrial Applications and Academic
Perspectives, 1st ed.; Stahl, S. S., Alsters, P. L., Eds.; Wiley-VCH Verlag
GmbH & Co. KGaA: Weinheim, Germany, 2016; pp 221−237.
(58) Perkin, A. G.; Whattam, T. W. XXXVI.Some products of the
reduction of 2-hydroxyanthraquinone. J. Chem. Soc., Trans. 1922, 121,289−300.
(59) Goodall, F. L.; Perkin, A. G. LVIII.Reduction products of thehydroxyanthraquinones. Part V. J. Chem. Soc., Trans. 1924, 125, 470−
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Electrochem. 1985, 15 (5), 771−773.
(61) Shi, Z.-W.; Li, Y.-Z.; Li, Y.; Lu, G.-Y.; Liu, S.-H. 1,1′,8,8′Tetramethoxy-10,10′-bianthrone. Acta Crystallogr., Sect. E: Struct. Rep.
Online 2004, E60 (12), o2275−o2277.
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(63) Prinz, H.; Wiegrebe, W.; Mü l ler, K. Syntheses ofanthracenones. 1. Sodium dithionite reduction of peri-substituted
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Revision. J. Org. Chem. 1996, 61 (8), 2857−2860.
(65) Müller, K.; Leukel, P.; Ziereis, K.; Gawlik, I. Antipsoriatic
Anthrones with Modulated Redox Properties. 2. Novel Derivatives ofChrysarobin and IsochrysarobinAntiproliferative Activity and 5Lipoxygenase Inhibition. J. Med. Chem. 1994, 37 (11), 1660−1669.
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Addition/Correction

Correction to “Extending the Lifetime of Organic Flow Batteries via
Redox State Management”
Marc-Antoni Goulet, Liuchuan Tong, Daniel A. Pollack, Daniel P. Tabor, Susan A. Odom,
Alán Aspuru-Guzik, Eugene E. Kwan,* Roy G. Gordon,* and Michael J. Aziz*
J. Am. Chem. Soc. 2019, 141 (20), 8014−8019. DOI: 10.1021/jacs.8b13295

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he additional data presented herein is intended to revisit
a previous claim based upon Figure 4 of the original
publication. More extensive trials now expose a clear trend
between the restriction of negolyte state of charge (SOC) andthe capacity fade rate of the DHAQ/Fe(CN)6 ﬂow battery.
The authors now consider the fade rate of 0.14%/day claimed
in the main text to be an outlier, as depicted by the red cross inFigure 1 below, and the data in Figure S23 of the original
Supporting Information to be more representative of theachievable fade rate with an 88% SOC restriction. Upon
further investigation, it was determined that cell cycling historyis the most likely source of the discrepancy. The cell andelectrolyte used in the experiment shown in Figure 4 of theoriginal publication had undergone prior cycling, thereby
accumulating the DHA decomposition product and slowing
the rate of further decomposition; in contrast, the data for
Figure S23 come from a pristine cell and electrolyte. Thisconsideration was overlooked in the original analysis and canbe an important direction for future study.
The overall conclusions of the original study remainunchanged. The present data aﬀect only the degree to which
the SOC restriction strategy minimizes the formation of theanthrone decomposition product of the fresh DHAQ negolyte.
More speciﬁcally, the data herein indicate that a 10-fold (from4.5 to 0.49%/day at 80% SOC restriction) or 15-fold (from 4.5
to 0.29%/day at 50% SOC restriction) decrease in capacity loss
rate can be reproducibly achieved through SOC restriction
alone, but not a 40-fold decrease (from 5.6 to 0.14%/day), as
originally published. In addition, the aggregate data in Figure 1,
collected under various conditions over an entire year, indicate
the robustness of the trend to seasonal variability (∼5 °C).
Although most of the data in Figure 1 were obtained with a
Coulombic capacity cutoﬀ as in Figure S23, a few of the cellruns at 50% and 80% SOC restriction were obtained with apotential-controlled SOC cutoﬀ similar to that used in Figure 4of the original publication. Within error, these diﬀerent cutoﬀ
conditions produce indistinguishable results, thereby showingthat the type of cutoﬀ condition is not the cause of the original
discrepancy. In addition, the aggregate data contain cell runs at
two diﬀerent electrolyte concentrations, which suggests thatthe lifetime extension strategy is applicable to practical battery
concentrations.
© XXXX American Chemical Society

Figure 1. Aggregate cycling data from pristine cells and electrolytes
elucidating a trend in capacity fade rate as a function of DHAQ
negolyte SOC restriction. All data were obtained by comparing
capacity immediately before and after SOC-restricted segments. All
cells were cycled galvanostatically with potential holds and composedof one of two possible electrolyte formulations consisting of either 0.1
M DHAQ in 1.2 M KOH as negolyte vs 0.07 M K4[Fe(CN)6] and0.01 M K3[Fe(CN)6] in 1 M KOH as posolyte, or 0.5 M DHAQ in 2
M KOH as negolyte vs 0.35 M K4[Fe(CN)6] and 0.05 M
K3[Fe(CN)6] in 1 M KOH as posolyte, where the posolytes generally
have 6× the volume and therefore >2× the capacity of the negolytes.
The green triangle data point at 88% SOC restriction is from Figure
S23 in the Supporting Information of the original publication, whereas
the red cross represents the result from Figure 4 of the original maintext. Data points at 50%, 80%, 98%, and 99% SOC restriction eachconsist of at least ﬁve cell runs.

A

https://doi.org/10.1021/jacs.1c05529
J. Am. Chem. Soc. XXXX, XXX, XXX−XXXJournal of the American Chemical Society■

pubs.acs.org/JACS

Addition/Correction

ACKNOWLEDGMENTS
We are deeply saddened at the loss of Prof. Susan A. Odom on
April 18, 2021.

B

https://doi.org/10.1021/jacs.1c05529
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Supporting Information
Extending the Lifetime of Organic Flow Batteries via Redox
State Management
Marc-Antoni Goulet,†,||,⊥ Liuchuan Tong,‡,|| Daniel A. Pollack,§ Daniel P. Tabor,‡ Susan A.
Odom,†, Alán Aspuru-Guzik,‡,¶ Eugene E. Kwan,*,‡ Roy G. Gordon,*,†,‡ and Michael J. Aziz*,††Harvard John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138,

United States‡Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, United States§Department of Physics, Harvard University, Cambridge, Massachusetts 02138, United StatesDepartment of Chemistry, University of Kentucky, Lexington, Kentucky 40506, United States||These authors contributed equally.

Present Addresses⊥ Form Energy, Inc., Somerville, Massachusetts 02143, United States¶ Department of Chemistry and Department of Computer Science; Vector Institute for Artificial Intelligence, University ofToronto, Toronto, Ontario M5S 1A1, Canada

Table of Contents
Experimental methods................................................................................................................................... 1
Capacity fade of DHAQ symmetric cell .......................................................................................................... 3
Formation and detection of 2,6-dihydroxyanthrone (DHA) .......................................................................... 4
Identification of DHA ..................................................................................................................................... 5
Oxidation of DHA ........................................................................................................................................... 8
Formation and detection of (DHA)2 ............................................................................................................... 9
Identification of (DHA)2................................................................................................................................10
Time-dependence of DHA formation ..........................................................................................................12
pH dependence of DHA 1H NMR spectrum .................................................................................................13
Synthesis of (DHA)2 via electrochemical oxidation of DHA .........................................................................14
Recovery of DHAQ via aeration of DHA .......................................................................................................15
Concentration dependent production of (DHA) 2 ........................................................................................16
Concentration effects on capacity fade .......................................................................................................17
Characteristics of (DHA)2 cycling .................................................................................................................18
Stability of (DHA)2 ........................................................................................................................................19
Quantitative analysis of DHA and (DHA)2 formation and correlation with capacity loss............................20
DHA formation vs state of charge of DHAQ ................................................................................................23
Capacity fade mitigation strategies .............................................................................................................24
Potentiostatic cycling with Coulombic SOC cutoff ......................................................................................25
Cycling of DHAQ under aerobic conditions .................................................................................................26
Theoretical investigation of anthrone formation susceptibility ..................................................................27
Annex (Details of NMR spectroscopic analysis)...........................................................................................28
Characterization of DHA ..........................................................................................................................28
Characterization of (DHA)2 ......................................................................................................................28

Experimental methods
Cell cycling
All flow cell cycling tests were performed with a 5 cm2 cell (Fuel Cell Tech, Albuquerque, NM)
equipped with POCO sealed graphite flow plates with serpentine flow fields. Each side of the cell
contained a stack of four sheets of Sigracet SGL 39AA porous carbon paper that had been
pretreated by baking in air at 400 °C for 24 h. In all cases, the cell was assembled with a Nafion
N117 membrane that had been pretreated by immersion in a 1 M KOH aqueous solution for atleast 72 hours. In all cases, Viton (PVDF) gaskets were used to seal the electrodes between themembranes and graphite flow fields. Flow of electrolytes was forced with a Cole-Parmer
Masterflex L/S peristaltic pump, which required a small length of Viton peristaltic tubing. All othertubing and electrolyte reservoirs were made from chemically resistant fluorinated ethylene
propylene (FEP).
Potentiostatic cycling of all volumetrically-unbalanced compositionally-symmetric cells was
performed in a glovebox with < 2 ppm of oxygen with a Biologic VSP 300 potentiostat. Unlessotherwise specified, the potential limits were set to ±200 mV to access > 99% of the available
DHAQ capacity. As explained below, the oxidation potential was increased to +350 mV in somecases in order to oxidize any DHA present in the electrolyte. Due to the starting 50% SOC, thesepotential limits can be considered similar to overpotentials being divided for each side of the cell.
In all symmetric cells, the non-capacity-limiting side comprised twice the volume of 50% SOC
electrolyte as the capacity-limiting side. For example, if the capacity-limiting side contained 5 mLof 50% SOC 0.1 M DHAQ/DHAHQ and 1.2 M KOH, then the non-capacity-limiting side contained
10 mL of the same electrolyte. Due to these unbalanced volumes, the overpotentials are not split
exactly evenly between the capacity-limiting and non-capacity-limiting sides of the cell.
Electrolyte preparation
Unless otherwise specified, DHAQ electrolytes were prepared as aqueous solutions at 0.1 M
concentration with 1.2 M KOH in order to deprotonate the DHAQ and afford a pH 14 solution.
For symmetric cell testing, this electrolyte was charged in a full cell against potassium
ferrocyanide until the current decayed to a steady state value less than 1 mA cm-2 and was thenmixed in equal proportions with uncharged material in order to make a 50% SOC electrolyte.
1H NMR spectroscopy

Unless otherwise specified, NMR analysis of cycled electrolytes was performed by diluting 80 µLof the DHAQ electrolyte in 720 µL of D2O containing approximately 10 mM of sodium

1methanesulfonate (δ 2.6 ppm) as an internal standard. Spectra were recorded on a Varian INOVA
500 MHz spectrometer unless otherwise specified.

General information for synthesis
1H

NMR and 13C NMR spectra were recorded on Varian INOVA 500 spectrometers unlessotherwise specified. NMR spectra were recorded in solutions of deuterated dimethyl sulfoxide(DMSO-d6) with the residual dimethyl sulfoxide (δ 2.49 ppm for 1H NMR and δ 39.5 ppm for 13C
NMR) taken as the internal reference and deuterated water (D2O) with the residual water (δ 4.79
ppm for 1H NMR) taken as the internal reference. All solvents and reagents were purchased from
Sigma-Aldrich or Alfa Aesar and were used as received unless otherwise specified.

High resolution LC-MS
High-resolution LC-MS analysis was performed in the Small Molecule Mass Spectrometry Facility
at Harvard University on a Bruker Impact II q-TOF with internal calibration sodium formate
clusters. Liquid chromatography was performed on an Agilent 1290 Infinity HPLC using an Allure
PFPP column (5 µm particle size, 150 x 2.1 mm) at a flow rate of 0.4 mL/min, and the following
elution conditions were applied (solvent A = 0.1% v/v formic acid in water; solvent B = 0.1% v/v
formic acid in acetonitrile): 95% solvent A for 2 min, a gradient increasing from 5% to 60% solvent
B in solvent A over 13 min, a gradient increasing to 95% solvent B over 5 min, a gradientdecreasing to 5% solvent B in solvent A over 0.1 min, and 95% solvent A for 4.9 min. The ESI mass
spectra were recorded in negative ionization mode.

Preparative-scale HPLC
Liquid chromatography was performed using a Thermo Scientific Hypersil GOLD aQ column (5 µm
particle size, 250 x 20 mm) at a flow rate of 10 mL/min, and the following elution conditions wereapplied (solvent A = 0.1% v/v formic acid in water; solvent B = 0.1% v/v formic acid in acetonitrile):
80% solvent A for 2 min, a gradient increasing from 20% to 70% solvent B in solvent A over 5 min,a gradient increasing to 85% solvent B over 10 min, a gradient increasing to 95% solvent B over 4
min, a gradient decreasing to 20% solvent B in solvent A over 1 min, and 80% solvent A for 2 min.
The eluent fractions were collected and lyophilized for subsequent LC-MS confirmation.

2

Capacity fade of DHAQ symmetric cellFig. S1 depicts results of a volumetrically-unbalanced compositionally-symmetric cell cycling
experiment performed potentiostatically with overpotentials of ±200 mV, accessing > 99% of thetheoretical DHAQ capacity.1 Cycling was continued for several days (363 cycles) in order toachieve significant capacity loss and perform post-mortem electrolyte analysis. A fade rate of
approximately 8%/day was observed in this experiment.

Figure S1. Unbalanced compositionally-symmetric cell cycling of 0.1 M DHAQ in a 1.2 M KOH
solution, demonstrating loss of capacity over time.

3Formation and detection of 2,6-dihydroxyanthrone (DHA)
Fig. S2a depicts results of a symmetric cell cycling experiment that began with a strong
discharging (oxidation) overpotential of +350 mV, initially accessing > 99% of the theoretical
DHAQ capacity. Near day 0.5 of cycling, the oxidation overpotential was lowered to +200 mV,
leading to a sudden drop in capacity. Cycling was continued with this oxidative overpotential forseveral days, and aliquots were taken periodically in the discharged state for 1H NMR
spectroscopy. The presence and increasing intensity of new peaks in the NMR spectrum indicatesthe accumulation of another chemical species besides DHAQ.
a

b

100
98

Capacity (%)

96

0.35 V94

0.20 V

92
90
88
86
0.0

0.5

1.0

Time (days)

Figure S2. (a) Symmetric cell cycling of 0.1 M DHAQ in a 1.2 M KOH solution, showing change incapacity and fade rate when oxidation overpotential is changed from +350 to +200 mV. (b)Continued cycling at +200 mV oxidation overpotential leads to broadening of DHAQ peaks and to
the appearance of additional peaks of increasing intensity in the 1H NMR spectrum in D2O.
(Bottom spectrum is DHAQ, middle spectrum is electrolyte cycled for 1 day, top spectrum iselectrolyte cycled for 5 days).

4

Identification of DHA
The capacity-limiting electrolyte utilized in the experiment of which results are depicted in Fig.
S2 was collected after several days of further cycling with a +200 mV oxidation overpotential.
Initial HPLC-MS analysis suggested primarily one major compound in addition to DHAQ, namely
2,6-dihydroxyanthrone (DHA). To verify the identification of DHA as the major side-product of
DHAQ cell cycling at this potential, DHA was synthesized (Fig. S3) and confirmed to have the same
retention time in HPLC-MS (Fig. S4). In addition, NMR spectroscopy was also performed on a
mixture of synthesized DHA and DHAQ to simulate the cycled electrolyte. The NMR spectrum of
DHA-spiked DHAQ (Fig. S5) qualitatively matches the spectrum of discharged cycled DHAQ (Fig.
S2b).
a

b

Figure S3. (a) Chemical synthesis of DHA. (b) 1H NMR spectrum of synthetic DHA in DMSO-d6.
2,6-Dihydroxyanthrone (DHA). The synthesis of DHA (Fig. S3a) was carried out according to areported procedure2 with modifications. 2,6-dihydroxyanthraquinone (DHAQ, 2.40 g, 10.0 mmol)and stannous chloride (15.2 g, 80.0 mmol) were refluxed for 7 h in 80 mL of concentratedhydrochloric acid. The resulting solution was cooled to room temperature, and the precipitate
was collected by vacuum filtration. The collected solid was washed with DI water. The productwas dried in vacuo to afford 2.20 g pale yellow powder (97% yield).
1H NMR (600 MHz, DMSO-d ) δ 10.41 (s, 1H), 9.69 (s, 1H), 8.03 (d, J = 8 Hz, 1H), 7.51 (d, J = 3 Hz,
61H), 7.35 (d, J = 8 Hz, 1H), 7.06 (dd, J = 8 Hz, 3 Hz, 1H), 6.87 (m, 2H), 4.22 (s, 2H) HRMS (ESI) m/z:
calcd for C14H9O3 [M-H]-, 225.0552; found, 225.0556.
5

a

b

c

Figure S4. LC-MS traces of (a) pristine DHAQ, (b) cycled DHAQ in discharged state, and (c)synthetic DHA standard. The peak at 10.2 min was identified as DHA. The LC trace of the puresynthetic DHA standard also included a peak at 11.4 min corresponding to DHAQ due to partial
oxidation during material handling in alkaline solution.

6

Figure S5. 1H NMR spectra (500 MHz, D2O) of DHAQ (bottom spectrum), pristine DHA (2ndspectrum from bottom), a mixture of pristine DHA and DHAQ (3rd spectrum from bottom), DHAQ
cycled in a symmetric cell configuration and discharged at +200 mV oxidation potential (topspectrum). The mixed sample was prepared by diluting 40 µL of each 0.1 M electrolyte in 720 µL
of D2O, whereas all other samples were prepared as described in the Experimental Methods
section.

7

Oxidation of DHA
A sample of cycled DHAQ electrolyte, also utilized in the experiment of which results are depicted
in Fig. S2b, was discharged at +200 mV overpotential; the 1H NMR spectrum revealed peaks
matching those of DHA and broad peaks matching those of DHAQ (Fig. S6a, bottom spectrum).
When the same electrolyte was discharged at +350 mV, the 1H NMR spectrum revealed sharp(rather than broad) peaks matching those of DHAQ and peaks later confirmed to match those of(DHA)2 but no peaks matching those of DHA (Fig. S6a, top spectrum). When a sample of synthetic
DHA was chemically oxidized by aeration, the 1H NMR spectrum indicates that most of the DHA
was converted to DHAQ and a smaller amount was converted to (DHA) 2 (Fig. S6b).

ab

Figure S6. 1H NMR spectra (500 MHz, D2O) of (a) a sample of cycled DHAQ electrolyte discharged
at +200 mV oxidation overpotential (bottom spectrum) and at +350 mV oxidation overpotential(top spectrum) and (b) a sample of synthetic DHA before aeration (bottom spectrum) and after
aeration (top spectrum).

8

Formation and detection of (DHA)2
HPLC-MS of a cycled DHAQ electrolyte after a strongly oxidative discharge (+350 mV) was applied,
showing disappearance of DHA and the appearance of new compounds with [M-H]- = 449.1028
and [M-H]- = 223.0404.
abc

MS/MS of m/z = 449

Figure S7. (a) LC-MS trace of DHAQ after prolonged cycling shows disappearance of the DHA peak
and appearance of an additional peak at 10.2 min with [M-H]- of 449.1028. (b) Mass spectrum of
the material eluted at 10.2 min in the LC trace. A [M-H]- of 449.1028 corresponds to a neutral
molecular formula C28H18O6. The majority of the ions existed in a fragment with [M-H]- of
223.04104. (c) The material with [M-H]-of 449.1028 was isolated and fragmented by MS/MS,
indicating that the signal with [M-H]- of 223.04104 observed by LC-MS corresponds to fragmentsof the material with [M-H]-of 449.1028.

9

Identification of (DHA)2
A sample with [M-H]- of 449.1028 was isolated from the DHAQ electrolyte by pHPLC(preparative HPLC), and its mass and retention time were confirmed by LC-MS. The structures
were subsequently assigned as (DHA)2 stereoisomers by NMR.

ab

Figure S8. LC-MS traces of (a) DHAQ after pHPLC separation of cycled DHAQ electrolyte and (b)DHA dimers (DHA)2 after pHPLC separation. (DHA)2 is pure by LC trace: The peaks at 7.38 min and10.83 min are due to impurities from solvents and were also present in blank samples.

10

Following separation from the DHAQ electrolyte by pHPLC, the sample with [M-H]- of 449.1028
was dissolved in DMSO-d6 and analyzed by NMR to confirm the chemical structures as (DHA)2
stereoisomers (Fig. S9). Full details of the NMR analysis can be found in the Annex at the end of
the SI.

Figure S9. Chemical structures of DHAQ decomposition products, (DHA)2, revealed by NMR
analysis.

11

Time-dependence of DHA formation
Fig. S10 depicts the fraction of DHA formed from a DHAQ electrolyte in three separate
experiments, identical in protocol to the one of which results are depicted in Fig. 2 and in Fig.
S18-S20, but with different pause durations in each case. Taken together, these experimentssuggest that the amount of DHA formed and the associated capacity loss both increase with theamount of time the DHAQ electrolyte is held in the reduced state of charge.

Conversion to DHA (%)

20

15

10

5

0
0

10

20

30

40

50

60

70

80

Time in reduced state (hours)

Figure S10. Amount of DHA formed during cycling pause while DHAQ electrolyte was held inreservoir in reduced state of charge.

12

pH dependence of DHA 1H NMR spectrum
In a nitrogen-filled glovebox, an aqueous solution of 0.1 M synthetic DHA and 1.2 M KOH was
prepared. An 80 μL aliquot was then dissolved in 720 μL of D2O within a sealed 5 mm Precision
Low Pressure/Vacuum Valve (LPV) NMR tube (Wilmad-LabGlass Product No. 535-LPV-7), and the
1H NMR spectrum was obtained within 1 hour (top spectrum). A separate 80 μL aliquot was thendissolved in 720 μL of 1 M KOD in D2O, and the 1H NMR spectrum was obtained under identicalconditions (bottom spectrum).

Figure S11. 1H NMR spectra of DHA at pH 13 (top spectrum) and pH 14 (bottom spectrum).

13

Synthesis of (DHA)2 via electrochemical oxidation of DHA
In a nitrogen-filled glovebox, an aqueous solution of 0.1 M synthetic DHA and 1.2 M KOH was
prepared. An 80 μL aliquot was then dissolved in 720 μL of D2O within a sealed LPV NMR tube,
and a 1H NMR spectrum was obtained within 1 hour (top spectrum). The DHA electrolyte wasthen electrochemically oxidized in a 5 cm2 flow cell vs DHAQ. When the oxidation current fell
below 5 mA, the cell was stopped, and another 80 μL aliquot was taken for 1H NMR analysis.
The result below indicates that DHA was completely converted to (DHA)2 by the electrochemical
oxidation, as verified by comparison of the integrals of the 1H NMR signals with chemical shiftsof 6-8 ppm, relative to that of a sodium methanesulfonate internal standard (δ 2.6 ppm, notshown in Fig. S12).

Figure S12. 1H NMR spectra (500 MHz, D2O) of 10 mM synthetic DHA (top spectrum) with a 5%
DHAQ impurity and of the DHA electrolyte after electrochemical oxidation (bottom spectrum),demonstrating conversion of DHA to (DHA)2. The total integral of the 1H NMR signals withchemical shifts of 6-8 ppm, relative to that of a sodium methanesulfonate internal standard (δ2.6 ppm, not shown), indicates complete conversion of DHA to (DHA)2.

14

Recovery of DHAQ via aeration of DHA
In a nitrogen-filled glovebox, an aqueous solution of 100 mM synthetic DHA, 1 M KOH, and a 67
mM sodium methanesulfonate internal standard was prepared. Six dilutions were prepared at
10 mM DHA by diluting 10-fold in 1 M KOD in D2O, and six dilutions were prepared at 1 mM DHA
by diluting 100-fold in 1 M KOD in D2O. Two dilutions were prepared at 10 mM DHA and pH 13
by diluting 10-fold in D2O. The two samples at 10 mM DHA and pH 13 were prepared in sealed
LPV NMR tubes, and 1H NMR spectra were obtained. The remaining dilutions were removed fromthe glovebox, and samples at both DHA concentrations (10 mM and 1 mM) were oxidized bythree different conditions (with two replicates performed for each condition): (1) by exposing thesamples to air for several hours (“Open air”), (2) by repeatedly shaking and inverting the samples
in air for less than one minute (“Air shaken”), and (3) by exposing the samples to pure oxygenfrom a gas tank (“O2 bubbled”). The 1H NMR spectra of each of these samples were subsequently
obtained, and in each, the signal matching a single DHAQ proton was integrated with respect tothe internal standard. For each condition, the ratio of DHAQ formed relative to initial DHA wasdetermined by dividing this integral by twice the integral of the signal corresponding to a single
proton in the 1H NMR spectrum of unaerated DHA at pH 13 with respect to the internal standard.
(The factor of two in the denominator reflects that the DHAQ proton signal corresponds to two
equivalent protons, whereas the DHA proton signal corresponds to one. For each spectrum, theaverage integral of the two replicates was used, and the error bars in Fig. S13 correspond to thestandard deviation for each condition.)
Ratio of final DHAQ concentration
to initial DHA concentration

Figure S13. Dependence of fraction of DHAQ formed relative to initial DHA on initial DHA
concentration and method of oxygen exposure. The data demonstrate that the fraction of DHAQ
recovered varies based on the initial DHA concentration and method of oxygenation and thatwith sufficient oxygen exposure, greater than 90% recovery may be achieved.
15

Concentration dependent production of (DHA)2ab

Figure S14. 1H NMR spectra of (a) synthetic DHA at various concentrations dissolved in 1 M KOH
solution and (b) after 1 day of exposure to air. From top to bottom: 100 mM, 20 mM, 4 mM, 0.8
mM. Dashed lines show concentration-dependent shift of DHAQ signals. The greater relative
fraction of (DHA)2 signals in spectra of samples formed from higher concentration DHA isconsistent with a bimolecular oxidation reaction.

16

Concentration effects on capacity fadeFig. S15 depicts results of an experiment in which a symmetric cell was initially cycled with 0.1 M
DHAQ in a 1.2 M KOH solution, resulting in a fade rate of ~8%/day. The electrolyte wassubsequently diluted 10x with 1 M KOH, some volume was removed, and cycling continued. At
0.01 M DHAQ, the cell fade was ~4.2%/day. The electrolyte was diluted 10x again with 1 M KOH,
volume was again removed, and cycling continued. At 0.001 M DHAQ, the fade rate was~1.9%/day. These differences could be due to either a decrease in the rate ofdisproportionation/DHA formation or due to a decrease in the dimerization rate.
bacFigure S15. Symmetric cell cycling of (a) 6 mL of 0.1 M DHAQ in a 1.2 M KOH solution, showing acapacity fade rate of ~8.0%/day, (b) 6.5 mL of 0.01 M DHAQ in a 1.02 M KOH solution, showing acapacity fade rate of ~4.2%/day, and (c) 8 mL of 0.001 M DHAQ in a 1 M KOH solution, showing
a capacity fade rate of ~1.9%/day.

17

Characteristics of (DHA)2 cycling
After the 0.1 M DHA electrolyte utilized in the experiment of which results are depicted in Fig.
S12 was completely converted to 0.05 M (DHA)2, this electrolyte of (DHA)2 was cycledgalvanostatically as a posolyte against a DHAQ negolyte, with potential holds at +700 mV and-300 mV. Within these potential limits, the accessed capacity of approximately 48 C corresponds
to only 2 electrons transferred per molecule of (DHA)2. Although more capacity may be accessedby an even greater oxidation overpotential (according to Fig. 1B), this would result in even greater
voltage efficiency loss.

Figure S16. (a) A typical cycle from the cycling of 5 mL of 0.05 M (DHA)2 in a 1.2 M KOH solution
vs 0.1 M DHAQ in a 1.2 M KOH solution from -300 mV to +700 mV with 3 mA cm-2 current cutoff
limits. (b) Change in capacity vs. time.

18

Stability of (DHA)2
To monitor the stability of the (DHA)2 dimers, an aqueous solution of 0.1 M DHA and 1.2 M KOH
was prepared and aerated so that the DHA was converted to DHAQ and (DHA)2. The 1H NMR
spectrum of an 80 µL aliquot of the solution was obtained immediately after aeration. The
electrolyte was then stored for 5 days, after which an 1H NMR spectrum of another aliquot was
obtained, demonstrating a decrease in the integrals of the (DHA)2 signals.

DHA 30 min after aeration

DHA 5 days after aeration

Figure S17. 1H NMR spectra of DHA, 30 min (top) and 5 days (bottom) after aeration, which show
a decrease in integrals of (DHA)2 signals after 5 days in 1.2 M KOH solution. The integral of thesignal with chemical shift of 7.45 ppm decreases from 0.38 to 0.14 vs DHAQ, reflecting a loss ofover 50%.

19

Quantitative analysis of DHA and (DHA)2 formation and correlation withcapacity loss
Fig. 2 and Fig. S18 depict results of an experiment to quantitatively correlate the formation of
DHA and subsequent dimerization with capacity loss, which demonstrates the formation of DHA
when a DHAQ electrolyte was held in the reduced state of charge. An aliquot of electrolyte(Aliquot #1) was removed at the beginning of a pause in cycling. Though not shown due to scale,the first step after the pause is another reduction step involving 18 C (or 16% of the total capacity
before the pause), consistent with the disproportionation reaction involving the oxidation of
DHAHQ. In this case, the 43 hour pause in the reduced state of charge converted 16% of the
DHAHQ into DHA before cycling was resumed. DHA was not oxidized by the first discharge cycle
after the pause when +200 mV was applied (Aliquot #2) but was completely converted to (DHA)2
when +350 mV was applied (Aliquot #3). Overall, the cycling capacity decreased by 27 C during
the pause, a loss of 23%, corresponding to approximately 15% after correcting for the aliquots
removed for 1H NMR analysis. This capacity loss correlates well with the 16% loss of DHAQ
concentration and transient formation of DHA during the pause and with the conversion of DHA
to (DHA)2 thereafter (see Fig. S19). This experiment was repeated twice with different electrolyte
samples utilizing 12 hour and 71 hour pauses. The DHAQ concentration loss measured by 1H NMR
was 13% during the 12 hour pause and 17% during the 71 hour pause (see Fig. S10).
ab

Figure S18. Capacity fade observed when redox cycling was paused in the reduced state ofcharge. Both figures depict the same data as depicted in Fig. 2 but plotted as (a) complete charge
and discharge curves and (b) absolute capacity. After 8 cycles, cycling was stopped once thereduction current reached its limit, pumping was stopped with the electrolyte stored in its
reservoir, and Aliquot #1 was removed (and immediately aerated). Cycling was resumed after 43
hours, and the first oxidative discharge after the pause was performed in two steps. First, thesample was discharged at +200 mV overpotential, at which point Aliquot #2 was removed, andthen the discharge was continued at +350 mV until the current limit was reached, at which point
Aliquot #3 was removed. The capacity accessed after this discharge was proportional to theremaining concentration of DHAQ (when adjusted for removal of the aliquots).
20

DHAQ = 0.81

DHA = 0.07

DHAQ = 0.68
(DHA)2 = 0.08

Figure S19. Quantitative electrolyte species tracking by 1H NMR (aliquots defined in caption ofFig. S18), where numbers in black correspond to signal integrations relative to that of an internal
standard of sodium methanesulfonate (δ 2.6 ppm, not shown). The integral of the DHA signal(one proton) with a chemical shift of 7.82 ppm is 0.07, whereas that of the DHAQ signal with achemical shift of 7.62 ppm (two protons) has decreased by 0.13 after the DHA has been fully
converted to (DHA)2. Hence, the loss of DHAQ concentration is due to DHA formation. The (DHA)2
signals with chemical shift near 7.42 ppm (two protons total for each dimer) have an integral of
approximately 0.08 as well. Hence, all DHA appears to be electrochemically oxidized to (DHA)2(as demonstrated in Fig. S12). From the initial spectrum to the final (DHA)2 spectrum, nearly all
protons in the region shown are conserved, demonstrating that the loss of DHAQ has been massbalanced by conversion to DHA and subsequent conversion to (DHA)2.

21

9000000
Discharge at +200 mV

8000000

Discharge at +350 mV

7000000

Peak Intensity

6000000
5000000
4000000
3000000
2000000
1000000
0
DHAQ

DHAHQ

DHA

(DHA)2(DHA)2
fragment

SpeciesFigure S20. Corroboration by HPLC-MS of quantitative 1H NMR analysis of electrolyte species
depicted in Fig. S19. After discharging at +200 mV (blue), substantial amounts of DHA remain,
whereas the DHAHQ has been fully converted to DHAQ. After discharging at +350 mV (orange),both (DHA)2 and the fragment with [M-H]- of 223.04104 described in Fig. S7 appear in substantial
quantities.

22

DHA formation vs state of charge of DHAQ
The state of charge (SOC) limit of the DHAQ electrolyte was demonstrated to strongly influence
the cycling capacity fade rate. To directly quantify the relationship between capacity fade rateand SOC limit, an aqueous electrolyte comprising 0.1 M DHAQ and 1.2 M KOH was charged in anoxygen-free glovebox to approximately 90, 80, 70, and 60% SOC (DHAHQ relative to total DHAQ
+ DHAHQ). Slow charging was performed potentiostatically at a minimal value of 1.25 V in order
to avoid overcharging the electrolyte in proximity to the electrodes. In order to reach 90% SOC,
the potential was increased to 1.3 V near the end of charge to reach the desired SOC. Within 1
hour of charging, aliquots were diluted with D 2O in sealed LPV NMR tubes, and the 1H NMR
spectra were obtained without exposure to oxygen. The resulting spectra (Fig. S21) depict
broadening of the peaks corresponding to DHAQ, whereas those corresponding to DHA are sharp
and diminishing in area as the SOC is decreased (spectrum at 60% SOC not shown due to lack ofquantifiable peaks). When quantified relative to a sodium methanesulfonate internal standard(δ 2.6 ppm, not shown), the amount of DHA observed at 70% SOC is approximately 33% and 16%of the amount observed at 80% and 90% SOC, respectively. Although an exact correspondencewith a cycling fade rate cannot be established due to variable charging conditions and the timedependence of DHA formation, this result demonstrates the sensitivity of DHA formation to SOC
and is consistent with the proposed disproportionation reaction described in the main text.

Figure S21. Relative concentration of DHA at different SOC of DHAQ. Integration of signals withchemical shift of 7.8 ppm reflect the relative quantities of DHA vs a sodium methanesulfonate
internal standard (δ 2.6 ppm, not shown).

23

Capacity fade mitigation strategies
Fig. S22 below contains the same data as depicted in Fig. 4 in the main text but plotted as absolute
capacity rather than relative capacity.
a

110

100

106

ChargeDischarge

104

Coulombic Efficiency

102change voltage
cutoff from1.25 to 1.6V to access>99% of the theoretical capacity

100
98
96

99

Efficiency (%)

Capacity (C)

108

94
92
90
88

98
0

1

2

Time (days)

b

140
135

Reduction
Oxidation

Capacity (C)

130
125
120
115
110
105

Aeration recovers capacity
100
0.0

0.5

1.0

1.5

2.0

2.5

3.0

Time (days)

Figure S22. (a) Limiting the state of charge reduces the rate of capacity loss in a negolyte-limited
DHAQ-Fe(CN)6 full cell. Over the first 1.6 days, the operating state of charge was limited to 88%
of the theoretical capacity (i.e., 88% DHAHQ and 12% DHAQ), and the capacity faded at only
0.14%/day. The right-hand segment reflects typical operating conditions (cycling to 99.9% of
theoretical capacity), and the capacity faded at 5.6%/day. (b) Symmetric cell cycling in which thecapacity-limiting side (5 mL of 0.1 M DHAQ in a 1.2 M KOH solution) demonstrates recovery of
70% of lost capacity after aeration of discharged electrolyte.

24Potentiostatic cycling with Coulombic SOC cutoff
Fig. S23 depicts results of an experiment in which 1.6 V potentiostatic cycling was initiallyperformed until the current was < 2 mA/cm2, corresponding to the first segment of thegraph. The cell lost 10 C in 2 days of full cycling, corresponding to approximately 4.8% loss perday. Then, in the second segment of the graph, the same voltage (1.6 V) was applied for charging
with a Coulomb cutoff set at 95 C (88% SOC based on the 107.8 C capacity from the end of thefirst segment). The cell was fully discharged every cycle at 0.6 V until the current was < 2mA/cm2.
The cell lost 2 C over 2 days. This number is calculated by the capacity at the end of the first
segment minus the capacity at the beginning of third segment. Using 107.8 C as the initial value,
this represents approximately 0.8%/day, much less than 4.8% loss per day, which demonstrates
the effect of SOC control. Although the SOC range is similar to that shown in Fig. 4A of the main
text, the fade rate is higher in this case. We hypothesize that the fast charging rate due to the 1.6
V potentiostatic condition creates a local SOC gradient near the electrodes and in the tubing,which exceeds the target SOC and therefore accelerates the decomposition.

Capacity (C)

120

110

100

90
88% SOC
cycling

100% SOC
cycling

80
0

1

2

3

100% SOC
cycling

45Time (Days)
Figure S23. Potentiostatic full cell cycling of 0.1 M DHAQ in a 1.2 M KOH solution vs 0.05 M
K4[Fe(CN)6] in a nitrogen atmosphere.

25

Cycling of DHAQ under aerobic conditions
As described in the main text, DHAQ capacity recovery can be achieved by aeration of theelectrolyte. Alternatively, this strategy could be achieved by operation of the flow battery in an
open-air environment, which also achieves a lower capacity fade rate and is demonstrated in Fig.
S24. In this case, the ‘protective’ effect of oxygen may be due to the aeration of DHA and recovery
of DHAQ, as previously demonstrated, but may also be due to the oxidation of DHAHQ, which
would decrease the accessed SOC and, correspondingly, the driving force for DHA formation. In
either case, this method comes at a performance cost because it engenders a loss of current
efficiency due to the continuous oxidation of the negolyte. For this reason, the method of
aeration after discharge demonstrated in Fig. 4 is preferable and could be achieved mosteffectively in an industrial application by adding an oxygen treatment step after each dischargeof the electrolyte.

Figure S24. Potentiostatic full cell cycling of 0.1 M DHAQ in a 1.2 M KOH solution vs 0.05 M
K4[Fe(CN)6] in a nitrogen atmosphere (fade rate = 2.8%/day) for 1 day, followed by an intermittent
nitrogen atmosphere for 2 days due to dewar replacement, and then an ambient air environment(fade rate = 0.75%/day) for the final 3 days.

26

Theoretical investigation of anthrone formation susceptibility
The values plotted in Fig. 3 in the main text were generated as follows. For a set of para-quinone
molecules taken from Ref. 41 (of the main text), all possible anthrone forms were generated. For
each molecule, up to 20 conformers were generated using the RDKit;3 these were optimized at
the PM7 COSMO level of theory.4 We then calculated the energy difference of the lowest energy
anthrone form and the hydroquinone form and report this energy relative to that of DHAHQ →DHA. The raw data for these calculations is provided on figshare with the following DOI:
10.6084/m9.figshare.7455338. An excel spreadsheet listing the molecules along with theircalculated properties is attached and a zipped version of the conformer and PM7 COSMO
calculations is provided.
In previous works,5 we have compared the accuracy of different quantum chemical methods(upon calibration) in predicting the thermodynamic quantities of reactions associated with redoxactive organic molecules, in particular quinones. Previously, we found that both DFT (B3LYP/6311+G(d,p) with PCM calculated at the B3LYP/6-31+G(d) optimized geometry) and PM7 COSMO
give comparable results for predicting the two-proton two-electron reduction potential ofquinones (mean absolute deviations of 0.04 V and 0.07 V, for B3LYP/6-311+G(d,p) with PCM andPM7 COSMO, respectively). In addition, we found for a set of ketone-gem diol equilibrium
constants, the mean absolute deviations in the calculated base-10 logarithm of this equilibrium
constant was 0.99 for B3LYP/6-311+G(d,p) with PCM and 1.23 for PM7 COSMO. This corresponds
to an error in the reaction energy of about 6.7 kJ mol-1 for PM7 COSMO. Given the scale of thequantities being examined in this study, PM7 COSMO strikes the best balance between speed
and accuracy.

27

Annex (Details of NMR spectroscopic analysis)
Characterization of DHA

1H (ppm)
13C (ppm)
#
Type
Hs
1
10.40
-br s
1
2
9.67
-br s
1
3
8.02
129.33d1
4
7.51
111.27m
1
5
7.35
129.99d1
6
7.06
120.95d1
7
6.88
115.39m
1
8
6.86
113.91
m
1
9
4.21
31.04
s
2
500 MHz, DMSO-d6, 25 °C
Quaternary Carbons: 182.10, 161.66, 156.13, 144.01, 132.49,
131.33, 123.64 ppm

Assignments were made by COSY, HSQC, and HMBC experiments. (Note: on a timescale of hours
in DMSO, this compound tautomerizes to its phenolic form.)

Characterization of (DHA)2
Dimerization of the DHA monomer with no absolute or relative stereocontrol would result in thefollowing mixture:28

In the meso compound, there are two homotopic protons/carbons per site. For the pair ofenantiomers, each such pair is formally diastereotopic:

DFT calculations predict that each stereoisomer exists as a mixture of rotamers about the interring bond (electronic energies, B3LYP-D3(BJ)/cc-pVTZ/PCM(DMSO) // B3LYP/631G*/PCM(DMSO)). In the meso form, rotation is a chemical exchange process between
isochronous sites. In the DL form, to a very good approximation (see predictions below), rotation
converts the diastereotopic partners into one another.

29

Figure S25. 1H NMR spectra at 25 °C, showing that each peak is doubled (top, monomer; bottom,
dimer).

30

Figure S26. 13C NMR spectra at 25 °C, also showing that each peak is doubled (top, monomer;
bottom, dimer).

31

Figure S27. 1H NMR spectra of (DHA)2, showing further evidence of chemical exchange (top,monomer at 25 °C; middle, dimer at 25 °C; bottom, dimer at 85 °C).
Comment about rotational barrier:Consistent with these spectra, DFT calculations also indicate that rotation has a much larger
effect on chemical shift than stereoisomerism does (i.e., meso vs DL). First, the GIAO method wasused to obtain isotropic shifts for DHA at PBE0-D3(BJ)/cc-pVTZ/PCM(DMSO) using B3LYPD3(BJ)/6-31G*/PCM(DMSO) geometries. These isotropic shifts were then linearly scaled toexperimental shifts:
Proton shifts
NMR numbering

Expt
(ppm)

Isotropic
(ppm)

g16
numbering

Scaled(ppm)

Squared
error

3

8.02

22.6447

16

8.18

2.60E-02

4

7.51

23.2813

19

7.61

9.97E-03

5

7.35

23.6088

11

7.32

1.16E-03

6

7.06

24.061218

6.91

2.26E-02

7

6.88

24.1366

17

6.84

1.43E-03

32

8

6.86

24.2871

15

6.71

2.34E-02

9

4.21

26.9419

26,27

4.32

1.30E-02

slope-0.8976352819

intercept

28.50798478

RMSE

0.118

Carbon shifts
NMR numbering

Expt
(ppm)

Isotropic
(ppm)

g16
numbering

Scaled(ppm)

Squarederror

161.6619.96241

161.4874943

2.98E-02

113.9170.50642

112.4147251

2.24E+00

144.0135.9393

145.9759399

3.86E+00

123.6459.28964

123.3050274

1.12E-01

129.3351.23345

131.1267281

3.23E+00

115.3969.30486

113.581349

3.27E+00

182.1-0.81247

181.6575831

1.96E-01

132.4950.30838132.0249004

2.16E-01

131.3349.25379133.0488032

2.95E+00

129.9951.959910130.421375

1.86E-01

120.9563.489812

119.2270864

2.97E+00

156.1325.719713

155.8977774

5.39E-02

111.2771.759614

111.1980031

5.18E-03

31.04

153.462625

31.87320755

6.94E-01

slope-0.9708920795

intercept

180.8688304

RMSE

1.195717902

These scaling factors were then used to predict the shifts for both rotamers of the DL- and mesoforms. Canonical atom numbering was used:33

predicted
shifts (ppm)
meso

DL

(unsigned)

gaucheminus
atom
gauchenumber gauche minus
plus

gaucheplus

average gauche vs + difference

gauche vs +averages

ring 1ring 2ring 1ring 2

meso

DL

meso

DL

meso
vs DL

difference difference

1

6.994

7.414

7.297

6.997

6.998

7.295

0.420

0.299

7.204

7.147

0.359

0.057

3

6.256

7.079

7.044

6.258

6.262

7.044

0.823

0.784

6.667

6.652

0.804

0.015

4

5.672

7.718

7.704

5.541

5.545

7.705

2.046

2.161

6.695

6.624

2.103

0.071

5

7.240

4.713

4.858

7.266

7.257

5.089

2.527

2.288

5.977

6.118

2.408

0.141

7

6.755

6.569

6.599

6.787

6.771

6.466

0.186

0.247

6.662

6.656

0.216

0.006

8

7.741

7.557

7.551

7.852

7.846

7.520

0.184

0.314

7.649

7.692

0.249

0.044

10

4.603

4.549

4.577

4.598

4.582

4.607

0.054

0.007

4.576

4.591

0.030

0.015

1

110.155

110.499110.801 110.569 110.554 110.780

0.344

0.229 110.327 110.676

0.286

0.349

2

156.899

157.110156.898 156.793 156.800 156.879

0.211

0.092 157.005 156.843

0.152

0.162

3

116.077

119.118118.825 116.360 116.331 118.838

3.040

2.486 117.598 117.588

2.763

0.009

4

131.509130.440130.406 131.485 131.546 130.396

1.069

1.115 130.974 130.958

1.092

0.016

5

113.238

114.993114.899 113.301 113.203 115.476

1.755

1.935 114.116 114.219

1.845

0.104

6

161.739

159.381159.221 161.904 161.869 159.201

2.358

2.675 160.560 160.548

2.517

0.011

7

113.370

113.264113.348 113.512 113.450 112.781

0.107

0.416 113.317 113.273

0.261

0.044

8

129.845130.166129.502 129.436 129.599 129.323

0.322

0.105 130.005 129.465

0.213

0.541

1A

135.213135.917135.832 134.748 134.857 135.865

0.704

1.045 135.565 135.325

0.875

0.240

4A

128.965

134.287

134.877 128.901 128.998 135.013

5.322

5.996 131.626 131.947

5.659

0.321

5A

147.946141.587

141.535 147.512 147.461 141.655

6.360

5.891 144.766 144.541

6.125

0.226

8A

126.805

124.653

125.381 127.025 126.890 125.361

2.152

1.586 125.729 126.164

1.869

0.436

9

181.394180.858

181.019 181.333 181.450 181.049

0.536

0.357 181.126 181.213

0.447

0.087

10

56.39055.97256.1210.418

0.245 56.181

0.331

0.070

56.36056.38856.13756.251

34

From the last two columns, it is clear that rotation has a much larger effect on chemical shift thanstereochemistry. Thus, the NMR spectra at room temperature are dominated by rotational
effects, with each peak being slightly “doubled” by stereochemical effects.
Computational Structures:
Calculations were performed using Gaussian 16, revision A.03. Energies are in hartree (at B3LYPD3(BJ)/6-31G*/PCM(DMSO)). All structures were verified to be local minima or first-order saddle
points by frequency analysis as appropriate.
DHA-765.269653331
C -3.78253200
C -2.69090800
C -1.38558500
C -1.17834100
C -2.29046600
C -3.58086700
C 0.17633900
C 1.33174900
C 1.13998000
C 2.27190300
H 2.14572900
C 3.55552000
C 3.73483200
C 2.62320000
H -2.85782400
H -2.10596100
H -4.44234000
H 4.42079500
H 2.74410900
O 0.35213300
O 4.97155100
H 5.65081800
O -5.06842100
H -5.08058500
C -0.23371700
H -0.33177900
H -0.33172100-0.30739400-1.17708000-0.67938800
0.71159400
1.57627300
1.08466400
1.29490100
0.35840200-1.03288800-1.85695000-2.93648600-1.32363000
0.06861400
0.90288900-2.25196300
2.64499400
1.74420100-1.98199800
1.98000200
2.51792700
0.64952200-0.04431300-0.74630400-1.71807000-1.64972200-2.31376100-2.31347300-0.00002200-0.00001000
0.00002000
0.00002200
0.00002100
0.00000900
0.00000600
0.00002200-0.00000100-0.00004100-0.00006400-0.00002800-0.00003100-0.00001400-0.00002000
0.00003000
0.00002000-0.00000600-0.00003200-0.00002300
0.00008400-0.00065100-0.00006700-0.00005800
0.00011000-0.87017700
0.87062500

DL-gauche–-1529.35526268
C 2.05006000

0.81050100

2.66699100
35

C
C
C
C
C
C
C
C
C
H
C
C
C
H
H
H
H
H
O
O
H
O
H
C
H
C
C
C
C
C
C
C
C
C
C
H
C
C
C
H
H
H
H
H

1.60005800
0.28477100-0.60342100-0.13953700
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36

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O
H
O
H
C
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2.43293300 2.56498400 -0.62021900-2.09294800 3.99173100 -2.22982300-3.00900000 3.87394000 -2.53002300
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37

C
C
H
C
C
C
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H
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1.62448700
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38

H
C
H
C
C
C
C
C
C
C
C
C
C
H
C
C
C
H
H
H
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-5.23791900-0.23696200
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meso-gauche+-1529.35525686
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39

C
C
H
H
H
H
H
O
O
H
O
H
C
H
C
C
C
C
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C
C
C
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H
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4.54870000
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40

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