Extending the lifetime of organic flow batteries via redox state management

Article abstract of DOI:10.1021/jacs.8b13295

Redox flow batteries based on quinonebearing aqueous electrolytes have emerged as promising systems for energy storage from intermittent renewable sources. The lifetime of these batteries is limited by quinone stability. Here, we confirm that 2,6-dihydroxyanthrahydroquinone tends to form an anthrone intermediate that 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 for anthrone formation is greater for anthraquinones with lower reduction potentials. We show that the decomposition can be substantially mitigated. We demonstrate that conditions minimizing anthrone formation and avoiding anthrone dimerization slow the capacity loss rate by over an order of magnitude. We anticipate that this mitigation strategy readily extends to other anthraquinone-based flow batteries and is thus an important step toward realizing renewable electricity storage through long-lived organic flow batteries.

Full text of DOI:10.1021/jacs.8b13295

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Communication
Extending the Lifetime of Organic Flow Batteries via Redox State Management
Marc-Antoni Goulet, Liuchuan Tong, Daniel A. Pollack, Daniel P. Tabor,
Eugene E. Kwan, Alán Aspuru-Guzik, Roy G Gordon, and Michael J. Aziz
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b13295 • Publication Date (Web): 04 Apr 2019
Downloaded from http://pubs.acs.org on April 4, 2019
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Extending the Lifetime of Organic Flow Batteries via Redox
State Management
Marc-Antoni Goulet1‡†, Liuchuan Tong2‡, Daniel A. Pollack3, Daniel P. Tabor2, Eugene E. Kwan2,
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Alan Aspuru-Guzik2**, Roy G. Gordon1,2 and Michael J. Aziz1
1Harvard John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA
2Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138, United States
3Department of Physics, Harvard University, Cambridge, MA 02138, United States
‡ co-first authors
Supporting Information Placeholder
31, 34
electrophilic aromatic substitution,39 ring-opening,17 and
ABSTRACT: Redox flow batteries based on quinone-bearing
disproportionation,25,
none of these pathways have been
33, 37
aqueous electrolytes have emerged as promising systems for
energy storage from intermittent renewable sources. The lifetime
of these batteries is limited by quinone stability. Here, we confirm
that 2,6-dihydroxyanthrahydroquinone (DHAHQ) tends to form
an anthrone intermediate that 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 for
anthrone formation is greater for anthraquinones with lower
reduction potentials. We show that the decomposition can be
substantially mitigated. We demonstrate that conditions
minimizing anthrone formation and avoiding anthrone
dimerization slow the capacity loss rate by over an order of
magnitude. We anticipate that this mitigation strategy readily
extends to other anthraquinone-based flow batteries and is thus an
important step toward realizing renewable electricity storage
through long-lived organic flow batteries.
linked quantitatively to capacity fade.
Here we identify irreversible dimerization as the mechanism of
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 through simple 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, the reactions and
potentials vs. SHE are:16
Negolyte: DHAQ2- + 2e- ⇌ DHAHQ4-
Posolyte: [Fe(CN)6]3– + e- ⇌ [Fe(CN)6]4-
–680 mV; (pH 14)
+510 mV.
In practice, negolyte decomposition 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 to approach the decadal service life that
will be necessary for large-scale grid storage applications.42
The cost of electricity generated from renewable sources such
as the sun and wind has become competitive with fossil
electricity.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 for generation.2-4 Redox flow batteries
(RFBs), whose energy and power capabilities can be scaled
independently, may enable cost-effective long-duration
discharge.4-9
Results & Discussion
To identify the chemical origin of negolyte 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 forms of 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 all-vanadium redox flow battery chemistry is currently the
most technologically developed but may not access much of the
9-13
grid storage market due to electrolyte cost constraints.6,
Emerging organic electrolytes comprising cheaper earth-abundant
elements may address this limitation.14-39 However, organic
electrolytes are more prone to molecular decomposition, which
can lead to a progressive loss of charge storage capacity.
Although several studies have proposed decomposition pathways,
such as Michael addition,19, 22, 31, 40-41 nucleophilic substitution,25,
The identification of DHA was enabled by precisely controlling
cycling conditions (Figure S2). The structure of DHA was
suggested by high resolution mass spectrometry and 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),
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mass spectrometric peaks corresponding to (DHA)2 were
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observed (Figure S7). The assignment of (DHA)2 was confirmed
by the isolation of pure samples by preparative HPLC and
subsequent NMR analysis (Figure S8 & S9).
DHAQ
DHA
Aerated DHA
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-1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2
Potential vs SHE (V)
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DHAQ
(DHA)2
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Scheme 1. Intended operation and decomposition pathways of a
DHAQ-Fe(CN)6 flow battery.
Beck and Heydecke have observed anthrone formation during
electrochemical reduction of unsubstituted anthraquinone under
highly acidic conditions and have proposed homogeneous
disproportionation as the mechanism by which this 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 the rate of capacity loss.43
Additionally, increasing proportions of DHA are formed when
DHAHQ is held away from the electrodes in a sealed vial for
increasing durations (Figure S10). This disproportionation would
occur according to the following proposed reaction:
-0.1
-1.2
-1.0
-0.8
-0.6
-0.4
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Potential vs SHE (V)
Figure 1. (A) Cyclic voltammetry of 10 mM synthesized DHA at
pH 14 under a nitrogen atmosphere. After aeration, redox activity
is recovered 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.
2 DHAHQ4- + 2 H2O → DHAQ2- + DHA3- + 3 OH–
Under the highly alkaline conditions within the negolyte
compartment, DHA exists as a mixture of anthrone and anthrol
forms (Figure S11). Facile one-electron oxidation of the DHA
carbanion, followed by radical coupling, would lead to (DHA)2.
Indeed, pure samples of DHA are completely and irreversibly
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 (Figure S14 and S15).†
The irreversible conversion of DHAHQ to (DHA)2 has a
deleterious effect on battery operation due to the replacement of
the anthraquinone moiety with a benzophenone functional group
(Figure 1B and S16A). Although (DHA)2 is also redox-active, its
use in a RFB is impractical for several reasons. First, the higher
reduction potential of (DHA)2 with respect to DHAQ (Figure 1B)
decreases the overall potential of the battery 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 is unstable at pH 14 (Figure S17). These
findings indicate that the conversion of DHAQ to (DHA)2
engenders a progressive loss of battery capacity.
† Precedents for the oxidation of anthrone species to
anthraquinone and dianthrone species may be found in refs.45-56
.
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Restricting SOC improves
capacity retention
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0.0
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Time (days)
Figure 2. Pausing redox cycling in the reduced state adversely
affects battery capacity. Symmetric-cell cycling (see SI) for eight
cycles, followed by a 43-hour 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.
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To establish that irreversible dimerization is the primary
mechanism of capacity loss, we quantitatively correlated the
decrease in concentrations of DHAQ and DHAHQ with the extent
of capacity loss. We subjected a typical flow battery containing
0.1 M DHAQ/DHAHQ to symmetric cell-cycling at pH 14 (8
cycles over ~5 hours). When cycling was stopped and the
capacity-limiting side held in a reduced state for 43 hours, the
battery 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 associated with the irreversible
conversion of DHA to (DHA)2. NMR analysis of an aliquot of
negolyte removed during aging revealed that the total
concentration of DHAQ had declined by 16%. This decrease was
mass-balanced by an increase in DHA concentration of 16% (as
determined by NMR analysis). Both quantities 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
followed by subsequent dimerization as the primary source of
capacity loss. Anthrone and dianthrone species have also been
reported to be products of the chemical reduction of
anthraquinones and of the electrochemical reduction of
anthraquinones in acidic aqueous solutions.44, 57-65
Aeration recovers capacity
0.0
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1.0
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Time (days)
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 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 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 of discharged electrolyte.
The finding that irreversible dimerization is linked to the redox
chemistry of anthraquinones may limit the ongoing search for
quinones with lower potentials. This is because any decrease in
the reduction potential of the quinone/hydroquinone couple may
simultaneously increase the propensity for the hydroquinone to
disproportionate into anthrone and quinone. This hypothesis is
supported both by calculations (Figure 3; PM7/COSMO on a
subset of molecules from ref. 41) and the experimental properties
of
4,4´-((9,10-anthraquinone-2,6-diyl)dioxy)dibutyrate
(2,6-
DBEAQ).34 This negolyte has a reduction potential that is 180 mV
higher than that of DHAQ and is approximately 100 times more
stable in terms of RFB lifetime.34 In addition to the associated
decrease 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.
Figure 3. Tradeoff between the reduction potential of the
quinone-hydroquinone couple and the thermodynamic stability of
the hydroquinone 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.
Two simple changes to the operating conditions of the battery
may be used to greatly decrease the rate of capacity loss. Because
the degradation pathway is initiated by disproportionation of
DHAHQ, we hypothesized that avoiding high states of charge
would decrease the amount of DHA formed (Figure S21), thereby
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extending battery life. When a DHAQ-Fe(CN)6 flow battery was
Roy G. Gordon (gordon@chemistry.harvard.edu); Michael J. Aziz
(maziz@harvard.edu); Eugene E. Kwan (ekwan@fas.harvard.edu)
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cycled at 100 mA/cm2 with a 1.25 V cutoff that accessed 88% of
the theoretical capacity, the observed capacity declined at only
0.14%/day (Figure 4A & S22A). In contrast, under more typical
conditions with a 1.6 V cutoff that accessed 99.9% of the
theoretical capacity, the observed capacity declined at 5.6%/day.
Similar results were obtained by using identical potential
conditions but with a coulombic cutoff to restrict the SOC range
(Figure S23). These observations demonstrate that operating
conditions that access most, but not all, of the theoretical capacity
greatly extend battery lifetime.
Present Addresses
†Form Energy Inc., Somerville, Massachusetts 02143, United
States
**Department of Chemistry and Department of Computer
Science; Vector Institute for Artificial Intelligence, University of
Toronto, Toronto, Ontario, M5S 1A1, Canada
9
Author Contributions
Although the strategy of avoiding high states of charge reduces
the amount of DHA formation, it does not eliminate it completely
unless impractically low (less than ~60%) states of charge are
utilized (Figure S21). To divert the remaining DHA from
irreversible dimerization and prevent the associated loss of
capacity, we used lower oxidation overpotentials to slow the rate
of DHA dimerization. We then added molecular oxygen to favor
the oxidation of DHA back to DHAQ over DHA dimerization
(Scheme 1). We tested this strategy by symmetric cell 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 was observed to decrease by 13% (Figure 4B &
S22B) over two days of cycling. At this point, the electrolyte 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 effective by purging the DHAQ negolyte
reservoir with air after each full discharge to exploit the high
DHAQ recovery rate at low DHA concentrations. Aeration could
be optimized to match the DHA concentration to avoid
unnecessary current efficiency loss (Figure S24).
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‡These authors contributed equally.
Notes
Harvard University has filed a patent application based on the
methods described here.
ACKNOWLEDGMENT
This research was supported by U.S. DOE award DE-AC05-
76RL01830 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. 7046-00018B,
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 and DGE1745303. 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. Susan A. Odom, Prof. Sergio Granados-
Focil, and Prof. Alison E. Wendlandt for helpful discussions and
Eric Fell for experimental support.
Our findings establish that the progressive loss of capacity in a
DHAQ-based flow battery is primarily due to anthrahydroquinone
disproportionation followed by irreversible anthrone dimerization.
Computational analysis indicates that the propensity for
anthrahydroquinones to disproportionate is highly correlated to
the redox potential of the anthraquinone. Therefore, the use of
anthraquinones with lower reduction potentials, enabling
apparently higher cell voltages, may be limited by a decrease in
stability. This correlation suggests that ongoing 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.
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