Physical Organic Approach to Persistent, Cyclable, Low-Potential Electrolytes for Flow Battery Applications

Article abstract of DOI:10.1021/jacs.7b00147

The deployment of nonaqueous redox flow batteries for grid-scale energy storage has been impeded by a lack of electrolytes that undergo redox events at as low (anolyte) or high (catholyte) potentials as possible while exhibiting the stability and cycling lifetimes necessary for a battery device. Herein, we report a new approach to electrolyte design that uses physical organic tools for the predictive targeting of electrolytes that possess this combination of properties. We apply this approach to the identification of a new pyridinium-based anolyte that undergoes 1e^- electrochemical charge-discharge cycling at low potential (?1.21 V vs Fc/Fc⁺) to a 95% state-of-charge without detectable capacity loss after 200 cycles.

Full text of DOI:10.1021/jacs.7b00147

Communication
pubs.acs.org/JACS
Physical Organic Approach to Persistent, Cyclable, Low-Potential
Electrolytes for Flow Battery Applications
Christo S. Sevov,†,‡ David P. Hickey,†,§ Monique E. Cook,†,‡ Sophia G. Robinson,†,§ Shoshanna Barnett,†,‡
Shelley D. Minteer,*,†,§ Matthew S. Sigman,*,†,§ and Melanie S. Sanford*,†,‡
†Joint Center for Energy Storage Research, 9700 S. Cass Avenue, Argonne, Illinois 60439, United States
‡Department of Chemistry, University of Michigan, 930 N. University Avenue, Ann Arbor, Michigan 48109, United States
§Department of Chemistry, University of Utah, 315 South 1400 East, Salt Lake City, Utah 84112, United States
S
* Supporting Information
ABSTRACT: The deployment of nonaqueous redox flow
batteries for grid-scale energy storage has been impeded by
a lack of electrolytes that undergo redox events at as low
(anolyte) or high (catholyte) potentials as possible while
exhibiting the stability and cycling lifetimes necessary for a
battery device. Herein, we report a new approach to
electrolyte design that uses physical organic tools for the
predictive targeting of electrolytes that possess this
combination of properties. We apply this approach to
the identification of a new pyridinium-based anolyte that
undergoes 1e− electrochemical charge−discharge cycling at
low potential (−1.21 V vs Fc/Fc+) to a 95% state-of-
Figure 1. CV of 1+ (0.1 M TBAPF6 in CH3CN at 100 mV/s).
charge without detectable capacity loss after 200 cycles.
persistent organic electrolytes that undergo stable cycling in
acetonitrile at low potentials.
edox-flow batteries (RFBs) offer a potential solution to
Rthe unmet challenge of the large-scale integration of
intermittent renewable energy sources into the electrical grid.1
In an RFB, solvated anolyte and catholyte molecules are
pumped over inert electrodes to charge/discharge the battery
and are then stored in separate reservoirs. This design principle
means that RFBs can be inexpensively scaled by simply adding
more of the electroactive compounds to the external
reservoirs.2 Techno-economic modeling has implicated redox-
active organic molecules (ROMs) as particularly attractive
electrolytes for RFBs due to their relatively low cost and
molecular weights, as well as the feasibility of achieving high
solubility and significant electrochemical potential differences
between the anolyte and catholyte.3 However, even if all of
these techno-economic metrics are met, ROM-based electro-
lytes will not be viable unless they are long-lived in both the
charged and discharged states and undergo stable electro-
chemical cycling.
Many ROMs, particularly those that store considerable
amounts of energy by undergoing redox reactions at low
(anolytes) or high (catholytes) potentials, have poor shelf-lives
in the charged state and undergo fast decomposition during
charge−discharge cycling.4 To date, no general strategies have
been developed for improving/optimizing the lifetime (herein
referred to as persistence) and cyclability of ROMs without
concomitantly decreasing their redox potential window.4f,5
Herein, we introduce a physical organic approach to designing
We selected anolyte candidate 1+ as a test case for our design
strategy. A recent report demonstrated that this acylpyridinium
derivative meets a number of techno-economic metrics for
nonaqueous RFB applications, including high solubility (1.5 M
in acetonitrile), low molecular weight per electron transferred
(142 g/mol of e−), and multiple redox events at relatively low
potentials [−1.08 and −1.78 V vs Fc/Fc+ (Fc = ferrocene),
Figure 1].6 However, the application of this molecule in
nonaqueous RFBs remains fundamentally limited by the short
lifetime of the radical 1,7 which is formed upon the 1e−
reduction of 1+. For example, 0.29 M solutions prepared
from isolated samples of 1 decay by ∼7% over 24 h at room
temperature; furthermore, ∼20% capacity fade is observed after
just 75 charge−discharge cycles between 1+ and 1. We
hypothesized that modeling tools based on physical organic
parameters could be used to identify second generation
anolytes with enhanced persistence while maintaining the low
redox potential associated with the parent molecule. Notably,
although such tools have been successfully applied in the fields
of drug development,8 catalyst design,9 and mechanistic
study,10 they have not yet been utilized for the development
of energy storage materials.
We implemented a five-step workflow to develop a
quantitative correlation between the structures of 4-acylpyr-
Received: January 5, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.7b00147
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Journal of the American Chemical Society
Communication
idinium derivatives and their redox potentials and persisten-
ce.9a,11 First, a training set of 12 acylpyridine radicals was
synthesized, and the rate of decomposition was measured for
each. Second, a set of physical-organic parameters was
identified to relate the experimental decomposition rates to
the electronic and steric properties of each molecule in the
training set. Third, a mathematical relationship was established
to identify the key structural and electronic features responsible
for persistence. Fourth, virtual extrapolation of the mathemat-
ical model was used to predict new anolytes with improved
properties.10a Finally, the predicted molecules were synthesized
and validated, ultimately leading to new anolytes with
dramatically enhanced persistence as well as lower redox
potentials.
Conversely, the less electron-rich N-aryl derivatives 6 and 7
exhibit enhanced lifetimes but more positive reduction
potentials. These data are all consistent with a ground state
electronic stabilization of the radical (which is reported to be
localized at the 4-position of the pyridine)13 by electron
withdrawing substituents. Furthermore, they reinforce the
general paradigm that persistence and potential are inversely
correlated.14 Although distal to the primary site of radical
density, steric hindrance at nitrogen itself has been previously
demonstrated to improve the persistence of related pyridine
radicals.15 Indeed, upon systematically increasing the size of the
nitrogen substituent (1, 8, 9, 10), we observe significant
enhancements in radical persistence. For example, the tert-
butyl-substituted benzoylpyridine 10 exhibits a 100-fold slower
rate of decomposition relative to the N-methyl analogue 1. In
this case, the dramatic improvement in radical lifetime does not
occur at the expense of potential, providing promising evidence
that persistence and redox potential can be decoupled through
the judicious modification of appropriate substituents.
We next focused on generating a predictive model for the
virtual evaluation of new pyridinium anolytes. Using the data
above, we calculated thermodynamic properties and identified
computationally derived structural descriptors for the training
set anolytes (Figure 3a). As substituent size appeared to be
influential, initial correlations were attempted using a variety of
traditional steric parameters (i.e., Charton and Sterimol).9b
Poor trends were observed in all cases (R2 < 0.60), prompting
the examination of alternative descriptors. This ultimately led
to the identification of a parameter describing the substituent
height out of the pyridine ring plane (Hst). This effectively
accounts for the combined steric effects of the substituents at
nitrogen, at C2, and at C6. Using a linear regression algorithm
Radicals 1−12 were selected for the initial training set,
because they represent a range of steric and electronic variation
at different sites on the acylpyridinium core (Figure 2). The
decomposition of each was monitored at 70 °C in CD3CN via
NMR spectroscopy. As an example, a 0.30 M solution of 1
degraded through three half-lives in a 12 h period with a second
order rate constant of 6.6 × 10−4 M−1 s−1 (Figure 2, top).12
A
number of trends are apparent from the training set. For
example, radicals bearing electron-donating substituents (e.g.,
2) generally have more negative potentials but also faster rates
of decomposition than electron deficient analogues (e.g., 4).
Figure 2. Decomposition studies of 1−16 in CD3CN at 70 °C. Plot of
[1] (blue, left axis) and 1/[1] (red, right axis) vs time (top). Redox
potentials and relative rates of decomposition of 1−16 (bottom).
Figure 3. (a) Illustration of the electronic and steric descriptors. (b)
Plot of predicted vs experimentally determined free energies for
●
○
decomposition of training ( ) set and validation ( ) set.
B
DOI: 10.1021/jacs.7b00147
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Journal of the American Chemical Society
Communication
to explore a range of parameters, a statistical correlation (R2 =
0.92) was found when Hst was used in combination with
computed electrochemical reduction potentials (Figure 3b).
This simple relationship illustrates that reduction potential and
radical persistence can indeed be decoupled via the
incorporation of appropriate substituents at nitrogen, at C2,
and/or at C6.
are predicted to have enhanced redox potentials and
persistence, (ii) have substituents that add minimal molecular
weight, and (iii) are synthetically accessible.
Among the modeled anolyte candidates, 17 was selected for
experimental testing, as it was predicted to exhibit high
persistence as well as a potential lower than that of any
derivative examined to date (predicted to be −1.23 V versus
Fc/Fc+, Figure 3a).16 Compound 17 also incorporates the first
examples of substituents at the C2 and C6 positions of the
pyridine ring, thereby providing a key test of the extrapolative
capabilities of the model. As predicted, 17 exhibits the lowest
experimental potential (−1.21 V vs Fc/Fc+) and one of the
longest radical lifetimes (krel = 0.56). The model’s ability to
With a tool for quantitative prediction in hand, we next
sought to test the extrapolative limits of the model by
identifying a radical with exceptional persistence (defined for
our purposes as t1/2 ≥ 1 month at 70 °C). We conducted a
directed virtual screen of 18 compounds, focusing on
maximizing ΔG‡ according to the physical descriptors in
dec
predict accurately the decomposition barrier of 17 (ΔG‡
=
the equation. This screen suggested that N-xylyl-substituted 16
should exhibit 3 orders of magnitude greater persistence than
the parent radical 1 and should thus persist for months at 70
°C. Experimentally, we found that 16 exhibits such high
persistence that it exceeds the capabilities of our standard
protocol to experimentally measure a rate of decomposition,
necessitating that measurements be performed over 1 week at
70 °C. This example highlights an extrapolation in rate constant
of over an order of magnitude from the limit of the original
training set, corresponding to 2.3 kcal/mol greater ΔG‡dec. To
understand better the origin of the observed persistence, we
obtained a single crystal X-ray structure of 16. As illustrated in
Figure 4a, the solid state structure of 16 shows that the
perpendicular xylyl substituent protects the C2/C6 positions of
the pyridine ring, which presumably slows the homocoupling of
two pyridine radicals.15a
pred
28.4 kcal/mol, ΔG‡
= 28.2 kcal/mol) represents an
exp
electronic extrapolation of nearly 100 mV beyond the training
set, while simultaneously demonstrating its ability to account
for a new substituent pattern.
Most important for the identification of desirable RFB
electrolytes is that the model accounts for nonintuitive effects
of chemical structure on persistence. For example, the N-
methyl analog 18 was predicted and experimentally validated to
exhibit an approximately 20-fold lower persistence than the N-
ethyl anolyte 17. This is in contrast to the N-methyl and N-
ethyl derivatives 1 and 8, respectively, which display nearly
identical lifetimes. Because the reduction potentials for 17 and
18 are essentially the same, the ∼2 kcal/mol enhanced stability
of 17 arises from a difference in kinetic stability, reflected by the
parameter Hst (Et = 9.06 Å vs Me = 5.16 Å). This appears to
result from gearing between the Et group and the 2,6
substituents, which forces this Et group above the plane of
the pyridine ring.
Finally, we sought to establish whether the high persistence
of 17 translates to stable electrochemical cycling, a necessity for
applications in RFBs. Thus, we subjected 17+ to 1e− charge−
discharge cycling in a symmetric electrochemical cell and
compared the results to those of 1+, the progenitor of this
study. Importantly, this experimental design was selected in
order to isolate the charge−discharge reactions to those of the
anolyte. This allows an evaluation of the anolyte’s electro-
chemical cycling stability without complications that can arise
in a complete redox flow battery (e.g., catholyte and membrane
compatibility as well as crossover).17 Galvanostatic cycling was
performed on a 10 mM solution of 17+ with reticulated vitreous
carbon electrodes at 2.5C (Figure 5a). Upon the 1e− reduction
of 17+ to 17, an average state-of-charge of 95% was reached,
and the anolyte discharged with 98% Coulombic efficiency
(Figure 5b). No loss in storage capacity was observed for 17+
after 200 cycles, and the cycling potentials are consistent with
the redox potential measured by CV. These results are in
marked contrast to cycling data for 1+, which shows 35%
capacity loss over the same time. The excellent cyclability of
17+ at even lower potentials than the parent molecule 1+
highlights the utility of this approach for ROM development/
optimization.
Consistent with the existing paradigm, the high persistence of
anolyte 16 mainly results from a ground-state stabilization,
which leads to an undesirably high reduction potential of −0.94
V vs Fc/Fc+. However, the utility of our parameter-based
modeling tool is that we can now rapidly identify compounds
with high charged-state persistence without sacrificing low
potential. As depicted in Figure 4, plotting the lifetimes of
charged anolytes 1−16 as a function of their potential reveals a
quadrant diagram with corners that represent combinations of
high/low potential and high/low persistence. The most
desirable anolytes reside in the upper left quadrant. Targeting
this region, we virtually evaluated a variety of molecules that (i)
In summary, we demonstrate that physical organic modeling
of an organic electrolyte can deliver new ROMs that possess a
previously elusive combination of properties for nonaqueous
RFB applications. Importantly, although this approach was
successfully demonstrated for the development of 17+, it is not
specific to this class of electrolyte. As such, we anticipate that it
should prove more broadly applicable to the design and
Figure 4. (a) X-ray structures of 16+ and 16. (b) Quadrant diagram of
radical half-life as a function of redox potential.
C
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Journal of the American Chemical Society
Communication
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.7b00147.
́
■
S
(7) 1− and 1+ persist as 0.70 M solutions for over 4 days.
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Data for C20H18NO, BF4 (CIF)
Data for C20H18NO (CIF)
Experimental procedures, characterization, and spectral
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AUTHOR INFORMATION
■
Corresponding Authors
*mssanfor@umich.edu
*minteer@chem.utah.edu
*sigman@chem.utah.edu
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ORCID
Shelley D. Minteer: 0000-0002-5788-2249
Matthew S. Sigman: 0000-0002-5746-8830
Melanie S. Sanford: 0000-0001-9342-9436
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
(16) 17+ has a solubilty of greater than 0.70 M in acetonitrile.
(17) (a) Montoto, E. C.; Nagarjuna, G.; Hui, J.; Burgess, M.; Sekerak,
■
This work was supported by the Joint Center for Energy
Storage Research (JCESR) a Department of Energy, Energy
Innovation Hub. We thank Dr. J. M. Kampf for X-ray
crystallography, and the NSF (CHE-0840456) for X-ray
instrumentation.
N. M.; Hernan
Cheng, K. J.; Lewis, J. A.; Moore, J. S.; Rodríguez-Lop
Chem. Soc. 2016, 138, 13230. (b) Burgess, M.; Moore, J. S.; Rodriguez-
Lopez, J. Acc. Chem. Res. 2016, 49, 2649. (c) Nagarjuna, G.; Hui, J.;
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