U.S. patent application number 16/409072 was filed with the patent office on 2020-01-23 for nonaqueous redox flow battery electrolyte comprising an ionic liquid with a metal cation coordinated to redox-active ligands.
The applicant listed for this patent is National Technology & Engineering Solutions of Sandia, LLC. Invention is credited to Travis Mark Anderson, Harry Pratt, Leo J. Small.
Application Number | 20200028196 16/409072 |
Document ID | / |
Family ID | 64271972 |
Filed Date | 2020-01-23 |
United States Patent
Application |
20200028196 |
Kind Code |
A1 |
Small; Leo J. ; et
al. |
January 23, 2020 |
Nonaqueous Redox Flow Battery Electrolyte Comprising an Ionic
Liquid with a Metal Cation Coordinated to Redox-Active Ligands
Abstract
Nonaqueous redox flow batteries (RFB) hold the potential for
high energy density grid scale storage, though are often limited by
the solubility of the redox-active species in their electrolytes. A
systematic approach enables an increase the concentration of
redox-active species in electrolytes for nonaqueous RFB, starting
from a metal-coordination-cation-based ionic liquid. As an example,
starting with an ionic liquid consisting of a metal coordination
cation (MetIL), ferrocene-containing ligands and iodide anions can
be substituted into the original MetIL structure, enabling a nearly
4.times. increase in capacity compared to original MetIL structure.
Application of this strategy to other chemistries, optimizing
electrolyte melting point and conductivity could yield >10 M
redox-active electrons.
Inventors: |
Small; Leo J.; (Albuquerque,
NM) ; Anderson; Travis Mark; (Albuquerque, NM)
; Pratt; Harry; (Albuquerque, NM) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
National Technology & Engineering Solutions of Sandia,
LLC |
Albuquerque |
NM |
US |
|
|
Family ID: |
64271972 |
Appl. No.: |
16/409072 |
Filed: |
May 10, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
15597474 |
May 17, 2017 |
10305133 |
|
|
16409072 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 2300/0022 20130101;
H01M 8/188 20130101; H01M 8/1016 20130101 |
International
Class: |
H01M 8/18 20060101
H01M008/18; H01M 8/1016 20060101 H01M008/1016 |
Goverment Interests
STATEMENT OF GOVERNMENT INTEREST
[0002] This invention was made with Government support under
Contract No. DE-NA0003525 awarded by the United States Department
of Energy/National Nuclear Security Administration. The Government
has certain rights in the invention.
Claims
1. A nonaqueous redox flow battery electrolyte, comprising an ionic
liquid comprising a redox-active metal cation coordinated to a
plurality of aminoalcohol or dialcoholamine ligands and at least
one redox-active ligand, and at least one redox-active anion,
wherein the metal cation comprises a transition metal ion.
2. The electrolyte of claim 1, wherein the transition metal ion
comprises iron, copper, or manganese.
3. The electrolyte of claim 1, wherein the aminoalcohol ligand
comprises ethanolamine, butanolamine, or hexanolamine.
4. The electrolyte of claim 1, wherein the at least one
redox-active ligand comprises a metallocene.
5. The electrolyte of claim 4, wherein the metallocene comprises
ferrocene.
6. The electrolyte of claim 1, wherein the at least one
redox-active ligand comprises a transition metal coordinated to a
bipyridine group.
7. The electrolyte of claim 6, wherein the at least one
redox-active ligand comprises tris(2,2'-bipyridine)nickel(II) or
tris(2,2'-bipyridine)iron(II).
8. The electrolyte of claim 1, wherein the at least one
redox-active ligand comprises a quinone,
(2,2,6,6-tetramethyl-piperidin-1-yl)oxyl, aniline, or
methylviologen.
9. The electrolyte of claim 1, wherein the at least one
redox-active anion comprises iodide, ferricyanide,
polyoxometallate, or peroxosulfate.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of U.S. application Ser.
No. 15/597,474, filed May 17, 2017, is which incorporated herein by
reference.
FIELD OF THE INVENTION
[0003] The present invention relates to redox flow batteries and,
in particular, to a nonaqueous redox flow battery electrolyte
comprising an ionic liquid with a metal cation coordinated to
redox-active ligands.
BACKGROUND OF THE INVENTION
[0004] Nonaqueous redox flow batteries (RFBs) hold the potential
for high energy density grid scale storage. See R. M. Darling et
al., Energy Environ. Sci. 7, 3459 (2014); and B. R. Chalamala et
al., Proc. IEEE 102, 976 (2014). While aqueous chemistries are
limited to the 1.5 V potential window of water, many nonaqueous
electrolytes with stability ranges greater than 4 V exist, allowing
for increased cell voltages and corresponding energy densities. See
W. Wang et al., Adv. Funct. Mater. 23, 970 (2013); and A. Z. Weber
et al., J. Appl. Electrochem. 41, 1137 (2011). To best take
advantage of the wider potential window of nonaqueous electrolytes,
many groups have created highly reversible, electrochemically novel
molecules for non-aqueous catholyte and anolyte chemistries ranging
from complex, fully organic redox molecules with light weight high
current and high efficiencies, to redox-active organic ligands
complexed to metal ions, to redox activity centered in the cation
core, or to iodide anions. See J. Huang et al., Sci. Rep. 6, 32102
(2016); J. D. Milshtein et al., Energy Environ. Sci. 9, 3531
(2016); C. S. Sevov et al., J. Amer. Chem. Soc. 138, 15378 (2016);
C. S. Sevov et al., J. Amer. Chem. Soc. 127, 14465 (2015); C. S.
Sevov et al., Adv. Energ. Mater., 1602027 (2016); A. P. Kaur et
al., Energy Technol. 3, 476 (2015); L. Su et al., J. Electrochem.
Soc. 161, A1905 (2014); W. Wang et al., Chem. Commun. 48, 6669
(2012); R. A. Zarkesh et al., Dalton Trans. 45, 9962 (2016); J. Mun
et al., J. Electrochem. Soc. 15, A80 (2012); N. S. Hudak et al., J.
Electrochem. Soc. 162, A2188 (2015); S. Schaltin et al., Chem.
Commun. 52, 414 (2016); J. Suttil et al., J. Mater. Chem. A 3, 7929
(2015); M. Miller et al., J. Electrochem. Soc., 163, A578 (2016);
L. J. Small et al., J. Electrochem. Soc. 163, A5106 (2016); L.
Cosimbescu et al., Sci. Rep. 5, 14117 (2015); H.-S. Kim et al., J.
Power Sources 283, 300 (2015); C. Jia et al., Sci. Adv. 1, e1500886
(2015); and H. Chen and Y.-C. Lu, Adv. Energ. Mater., 1502183
(2016).
[0005] These chemistries often possess large cell voltages and
stable redox activity, though are limited by the solubility of the
redox-active species, with only a few exceeding 1 M. This limited
solubility severely hinders the widespread deployment of RFBs. At a
RFB energy density of 50 Wh/L, 230 times more volume is required to
house a fully charged RFB electrolyte than the same energy content
of natural gas. The fundamental difference between RFB electrolytes
and natural gas lies at the molecular level; every molecule of
natural gas participates in the energy-generating reaction,
compared to <5% for many RFB electrolytes. Therefore, a need
remains for a method to increase the energy density in RFB
electrolytes.
SUMMARY OF THE INVENTION
[0006] The present invention is directed to a method for increasing
the concentration of redox-active species in electrolytes for
nonaqueous redox flow batteries. In general, the improved
electrolyte can comprise a redox-active ionic liquid having a metal
coordination cation coordinated to a number of redox-active ligands
and an anion having an oxidation state, wherein the metal
coordination cation comprises a transition metal ion and at least
one the redox-active ligands comprises an aminoalcohol or a
dialcoholamine. For example, the transition metal ion can comprise
iron, copper, or manganese. The aminoalcohol can comprise
ethanolamine, butanolamine, hexanolamine, or other simple
aminoalcohol. For example, at least one of the redox-active ligands
can comprise a metallocene, such as ferrocene. For example, at
least one of the redox-active ligands can comprise a transition
metal coordinated to a bipyridine group, such as
tris(2,2'-bipyridine)nickel(II) or tris(2,2'-bipyridine)iron(II).
For example, at least one of the redox-active ligands can comprise
a quinone, (2,2,6,6-tetramethyl-piperidin-1-yl)oxyl, aniline, or
methylviologen. For example, the anion can comprise iodide,
ferricyanide, polyoxometallate, or peroxosulfate.
[0007] As an example of the invention, starting with an ionic
liquid consisting of a metal coordination cation (MetIL),
ferrocene-containing ligands and iodide anions can be substituted
into the original MetIL structure. While chemical structures can be
drawn for molecules with 10 M redox-active electrons (RAE),
practical limitations such as melting point and phase stability
constrain the structures to 4.2 M RAE, a 2.3.times. improvement
over the original MetIL. Referred to as "MetILs.sup.3" herein,
these ionic liquids possess redox activity in the cation core,
ligands, and anions. Throughout all compositions, infrared
spectroscopy shows the ethanolamine-based ligands primarily
coordinate to the Fe.sup.2+ core via hydroxyl groups. Calorimetry
indicates a profound change in thermophysical properties, not only
in melting temperature, but also in suppression of a cold
crystallization only observed in the original MetIL. Square wave
voltammetry reveals redox processes characteristic of each
molecular location. Testing a laboratory-scale RFB demonstrated
Coulombic efficiencies >96% and increased voltage efficiencies
due to more facile redox kinetics, effectively increasing capacity
4.times.. Application of this strategy to other chemistries,
optimizing melting point and conductivity, could yield >10 M
RAE, making nonaqueous RFB a viable technology for grid scale
storage.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] The detailed description will refer to the following
drawings, wherein like elements are referred to by like
numbers.
[0009] FIG. 1 is a schematic illustration of MetILs.sup.3 of the
family Fe(EA).sub.6-x(FcEA).sub.x(OTf).sub.2-yI.sub.y. Redox
activity of the original MetIL (1a) can be increased by
substituting in either iodide anions (1b), ferrocene-containing
ligands (2a), or both iodide anions and ferrocene-containing
ligands (2b).
[0010] FIG. 2 is a graph showing the infrared (IR) spectrum of
ethanolamine compared to those of MetILs.sup.3 1a, 2a, 1b, and
2b.
[0011] FIG. 3 is a graph showing differential scanning calorimetry
(DSC) thermograms for MetILs.sup.3 1a, 2a, 1b, and 2b.
[0012] FIG. 4 is a graph showing square wave voltammograms (SWV) of
a carbon fiber microelectrode in 0.5 M lithium
bis(trifluoromethane)sulfonamide (LiNTf.sub.2) in propylene
carbonate (PC) with 10 mM iron (II) trifluoromethansulfonate
(Fe(OTf).sub.2), {[(2-hydroxyethyl)amino]carbonyl}ferrocene (FcEA),
or MetILs.sup.3 1a, 2a, 1b, and 2b.
[0013] FIGS. 5A-D are graph showing square wave cyclic
voltammograms of a carbon fiber microelectrode in 0.5 M LiNTf.sub.2
in PC with 10 mM MetILs.sup.3 (FIG. 5A) 1a, (FIG. 5B) 2a, (FIG. 5C)
1b, and (FIG. 5D) 2b.
[0014] FIG. 6 is a schematic illustration of a laboratory-scale
redox flow battery.
[0015] FIG. 7 is a graph showing charge-discharge curves of the
third cycle at 5 mA cm.sup.-2 for flow batteries with 200 mM
cobaltocenium hexafluorophosphate (CcPF.sub.6) anolyte and 100 mM
MetILs.sup.3 1a (solid line) compared to 2a (dashed line).
Supporting electrolyte for anolyte and catholyte was 0.5 M
LiNTf.sub.2 in PC at 25.degree. C.
[0016] FIGS. 8A-C are graphs showing performance of RFBs with
catholyte containing 100 mM MetILs.sup.3 1a (circles) compared to
2a (triangles). FIG. 8A is a graph of coulombic efficiency. FIG. 8B
is a graph of voltage efficiency. FIG. 8C is a graph of
electrochemical yield.
DETAILED DESCRIPTION OF THE INVENTION
[0017] A redox flow battery (RFB) is a rechargeable battery in
which a liquid electrolyte containing one or more dissolved
redox-active groups flows through an electrochemical cell that
reversibly converts chemical energy directly to electricity. Redox
refers to the chemical reduction and oxidation reactions employed
in the RFB to store energy in the ionic liquid electrolyte. The
amount of energy stored in the RFB is determined by the total
amount of redox-active groups available in the volume of
electrolyte solution. Therefore, electrolytes for RFBs can be
thought of as electrochemical fuel, where as much of the
electrolyte as possible is integrated into the energy-storing redox
process. Instead of the typical formula of
electrolyte=solvent+supporting salt+redox active molecule, the
electrolyte should be holistically designed so as to maximize
energy storage.
[0018] As shown in FIG. 1, increasing amounts of redox-active
groups can be incorporated into the ligands and anions of the ionic
liquid with a redox-active metal coordination cation (MetIL),
yielding MetILs.sup.3, a family of ionic liquids with redox
activity in the ion core, ligands, and anions. See T. Anderson et
al., Dalton Trans. 39, 8609 (2010). In this example,
{[(2-hydroxyethyl)amino]carbonyl}ferrocene (FcEA) can be
substituted for ethanolamine (EA) ligands of the metal coordination
cation, and iodide can be substituted for triflate anions.
Estimates suggest a density of 10 M redox-active electrons for
MetILs.sup.3 if all ligands were replaced by FcEA and all anions
were replaced by iodide. While practical limitations such as
melting point and viscosity limit the range of useful compositions,
this general strategy can be applied to a wide range of
redox-active ligands and ions. Therefore, a variety of anions,
cations, and ligands can be combined to readily increase the energy
density of single component chemistries which abound in the
literature.
[0019] In general, the present invention is directed to a RFB
electrolyte comprising a redox-active ionic liquid that comprises a
metal coordination cation coordinated to a number of redox-active
ligands and an anion. The metal coordination cation can comprise
any transition metal ion, such as iron, copper, or manganese. The
redox-active ligands can comprise an aminoalcohol and/or a
dialcoholamine. The alkyl group of the alcohol can vary. For
example, the aminoalcohol can comprise ethanolamine, butanolamine,
hexanolamine, etc. A variety of redox-active species can be
attached to or contained in the ligands. Many metallocenes, such as
ferrocene, can be used. Also, transition metals coordinated to
bypyridine groups can be used, such as
tris(2,2'-bipyridine)nickel(II) or tris(2,2'-bipyridine)iron(II).
Other families of redox-active species, such as quinones,
(2,2,6,6-tetramethyl-piperidin-1-yl)oxyl (TEMPO), aniline, or
methylviologen, can also be attached to the ligand. For example,
the anion can comprise iodide, ferricyanide, polyoxometallate, or
peroxosulfate.
[0020] As an example of the invention, MetILs.sup.3 of the family
Fe(EA).sub.6-x(FcEA).sub.x(OTf).sub.2-yI.sub.y were synthesized
with increasing amounts of FcEA and iodide, in order to obtain a
high energy density electrolyte. Specifically, MetILs.sup.3 were
synthesized in an argon-filled glovebox by thoroughly mixing
stoichiometric amounts of iron (II) trifluoromethansulfonate
(Fe(OTf).sub.2), iron (II) iodide, FcEA, and EA. FcEA was
synthesized according to the literature. See J. Banfic et al., Eur.
J. Inorg. Chem. 2014, 484 (2014). As FcEA concentrations were
increased, the melting temperature also increased. For compositions
with at least two FcEA ligands (x.gtoreq.2), melting temperatures
exceeded 100.degree. C., limiting their applicability for RFB
systems. For complete substitution of triflate by iodide, phase
separation was observed over the course of one month, though for a
single iodide substitution (MetIL.sup.3 1b) no phase separation was
seen over the following 6 months. Therefore, these extreme
compositions were avoided and the four ionic liquids shown in FIG.
1 were further characterized.
[0021] The physical properties of the resulting ionic liquids are
summarized in Table 1. As expected, the concentration of redox
active electrons (RAE) quickly increases as the FcEA ligands and
iodide anions are added. Generally, substitution of FcEA increased
the ionic liquid density, while substitution of iodide had no
statistical effect. MetILs.sup.3 2 b displayed the highest energy
density with 4.16 mol/L RAE, a 2.3.times. improvement over the
starting MetIL 1a, and a promising pathway forward to designing
high energy density electrolytes for RFB.
TABLE-US-00001 TABLE 1 Physical properties of the MetILs.sup.3.
"RAE" signifies redox-active electrons. Density, concentration, and
electrical conductivity measurements were recorded at 25.degree. C.
Compo- Density/g RAE/mol Conductivity/ T.sub.g/ T.sub.melt/
H.sub.melt/J sition cm.sup.-3 L.sup.-1 mS cm.sup.-1 .degree. C.
.degree. C. mol.sup.-1 1a 1.29 1.79 0.315 -68.7 16.4 13.7 2a 1.37
2.94 -- -57.4 70.5 14.2 1b 1.28 3.07 0.210 -68.7 0.9 0.554 2b 1.42
4.16 -- -56.6 85.4 15.2
IR Spectroscopy
[0022] Successful coordination of the ligands to the metal ion
center is seen by shifts in the O--H stretching peaks of the
MetILs.sup.3 compared to neat ethanolamine. Full IR spectra from
each ionic liquid are compared to EA in FIG. 2, with specific
shifts from the O--H (2860 cm.sup.-1) and N--H (3354 cm.sup.-1)
peaks tabulated in Table 2. The larger shift in the position of the
O--H stretch compared to that of the N--H stretch lends itself to
the conclusion that the hydroxyl end of the EA is coordinated to
the Fe.sup.2+ center. This result is consistent with previous
reports coupling a ferrocene group onto the amine end of the EA,
allowing the FcEA to mimic the way the EA coordinates to the
Fe.sup.2+ center. See H. Pratt et al., Dalton T. 40, 11396
(2011).
TABLE-US-00002 TABLE 2 Shifts in IR peak locations for O--H and
N--H stretches of MetILs.sup.3 1a, 2a, 1b, and 2b as compared to
neat ethanolamine. O--H/ N--H/ Composition cm.sup.-1 cm.sup.-1 1a
+20 -7 2a +16 -7 1b +18 -7 2b +15 -8
Thermophysical Properties
[0023] The thermophysical properties of the MetILs.sup.3 were
interrogated with differential scanning calorimetry. The resulting
thermograms are plotted in FIG. 3, and relevant data compiled in
Table 1. All MetILs.sup.3 displayed a glass transition starting
between -70 and -50.degree. C. The substitution of EA by FcEA
increased the midpoint of the glass transition from -68.7 to
-57.4.degree. C., while the substitution of triflate by iodide did
not significantly influence this transition temperature. That the
anions did not influence the glass transition temperature suggests
that this phenomenon is predominantly controlled by the ligands of
the MetILs.sup.3.
[0024] Upon further heating the MetILs.sup.3 a cold crystallization
was seen at -17.8.degree. C. for 1a. That is, the MetILs.sup.3 was
supercooled and crystallized upon heating. Similar behavior has
been observed for several imidazolium-based ionic liquids. See C.
P. Fredlake et al., J. Chem. Eng. Data 49, 954 (2004). Addition of
either FcEA or iodide was sufficient to suppress this cold
crystallization, allowing the MetILs.sup.3 to remain a supercooled
glass until the melting point was achieved.
[0025] The substitution of EA by FcEA increased both the shape and
width of the melting temperature. While MetILs.sup.3 1a displayed a
sharp melting temperature with an onset at 16.4.degree. C.,
addition of FcEA increased this onset temperature to 70.5.degree.
C., and only slightly increased the enthalpy of melting from 13.7
to 14.2 J mol.sup.-1. These increases are likely due to increased
molecular entanglement and the higher molar masses of the FcEA
ligands compared to those of EA. Substitution of triflate anions by
iodide significantly lowered the onset of melting from 16.4 in 1a
to 0.9.degree. C. in 1b. Additionally, the heat of melting
decreased from 13.7 to 0.554 J mol.sup.-1. This decrease in both
melting temperature and enthalpy of melting suggests that the
iodide anions significantly weaken the hydrogen bonding network of
the MetILs.sup.3, enabling extreme supercooling and requiring less
thermal energy to achieve the liquid state. Addition of both iodide
and FcEA, however, resulted in the highest melting temperature.
While the peak in melting temperature remained similar for 2a and
2b the width of the melting region was much narrower for the
iodide-containing MetIL.sup.3 2 b, resulting in a higher onset
temperature for melting.
Electrochemical Evaluation
[0026] The electrochemical behavior of these redox-active ionic
liquids was characterized using square wave voltammetry (SWV), a
technique well-suited for differentiating overlapping redox
processes while simultaneously rejecting background (non-Faradaic)
currents. See J. Osteryoung and R. Osteryoung, Anal. Chem. 57, 101A
(1985). In these voltammograms, three distinct electrochemical
processes were observed, consistent with electrochemical activity
in the MetIL.sup.3: (1) ion core, (2) ligand, and (3) anion. At the
ion core, Fe.sup.2+ is oxidized to Fe.sup.3+, releasing a single
electron. Likewise, the ferrocene-containing ligand is oxidized to
ferrocenium, providing a single electron. The iodide anion can also
be oxidized, forming triiodide and donating two electrons.
[0027] In FIG. 4, anodic SWV for each of the MetILs.sup.3 diluted
to 10 mM in PC are compared to Fe(OTf).sub.2 and free FcEA. It is
readily seen that when the Fe(OTf).sub.2 is coordinated with EA to
form 1a, the redox potential dramatically shifts from 1768 to 418
mV vs. Ag/AgCl. This observation is consistent with previous
reports, where diethanolamine ligands were shown to control the
redox potential of the MetIL core more strongly than the identity
(e.g. Fe, Cu, Mn) of the ion itself. See H. Pratt et al., Dalton
Trans. 40, 11396 (2011). Throughout all of the MetILs.sup.3 tested
in FIG. 4 a peak is observed at E.sup.1/2=420.+-.17 mV vs. Ag/AgCl.
For 2b a distinct peak is observed near 1.9 V, suggesting that not
all of the Fe(OTf).sub.2 has coordinated to the EA to form the
MetIL.sup.3. Repeated stirring and heating of the MetILs.sup.3
during synthesis were unable to remove this peak for 2b.
[0028] As shown in FIG. 4, SWV of the FcEA ligand displays a sharp
peak at 1392 mV vs. Ag/AgCl. Upon coordination of FcEA to the
Fe.sup.2+ center of 2a, the FcEA peak shifts to E.sup.1/2=1118 mV,
and decreases in intensity from 4.55 to 2.67 mA cm.sup.-2. The
decrease in peak current is attributed to a decrease in the
diffusion constant of the bulky, relatively heavy MetILs.sup.3
complex compared to free FcEA. For 2b, a shoulder is seen at
potentials characteristic of uncoordinated FcEA, again suggesting
not all FcEA has coordinated to form 2b.
[0029] Substitution of triflate anions for iodide results in a peak
on the SWV at E.sup.1/2=924 mV vs. Ag/AgCl for 2a, and a shoulder
at a similar location of the MetIL 2b. Across all MetILs.sup.3
voltammograms presented in FIG. 4, the relative peak heights
suggest that the relative redox kinetics from (slowest) Fe.sup.2+
core <iodide<FcEA ligands (fastest).
[0030] The quasi-reversibility of the individual redox processes
were examined under sequential anodic and cathodic SWV scans,
presented in FIG. 5. As expected, a single current peak is observed
for MetIL 1a, with peak separation of 52 mV. Peak separations for
core Fe.sup.2+ oxidation in all other MetILs.sup.3 were within
.+-.4 mV. For 2a, redox processes for both Fe.sup.2+ ion core and
FcEA are observed at E.sup.1/2=444 and 1118 mV vs. Ag/AgCl,
respectively. Here the FcEA peak separation is 28 mV. For 1 b,
redox processes for both Fe.sup.2+ ion core and iodide anions are
recorded at E.sup.1/2=410 and 924 mV vs. Ag/AgCl, respectively. For
2b, the core Fe.sup.2+ oxidation is seen at E.sup.1/2=406 mV vs.
Ag/AgCl, while a complex peak centered around 1.2 V likely contains
redox processes for iodide oxidation, and oxidation of FcEA both
free and coordinated to the metal center. Resynthesizing 2b with
different mixing orders of reagents or heating or stirring for
longer times was not found to eliminate the high potential shoulder
associated with free FcEA. While lack of coordination for some FcEA
is not ideal, these species remain still redox active and
participate at a higher redox potential, effectively increasing the
energy density if applied to a battery.
Flow Battery Testing
[0031] MetILs.sup.3 1a and 2a were employed as catholytes in the
laboratory-scale RFB shown in FIG. 6. To increase the MetILs.sup.3
limited ionic conductivity as reported in the Table 1, the
MetILs.sup.3 were diluted to 0.1 M in propylene carbonate (PC) with
0.5 M lithium bis(trifluoromethane)sulfonamide (LiNTf.sub.2). For
proof of concept, 0.2 M cobaltocenium hexafluorophosphate
(CcPF.sub.6) was chosen as the anolyte (also in 0.5 M LiNTf.sub.2
in PC), and both electrolytes maintained at 25.degree. C. and
cycled at 5 mA cm.sup.-2 between 0.25 and 2.5 V. During discharge,
cobaltocenium is oxidized on the negative or anode side of the
battery and MetIL.sup.3 is reduced on the positive or cathode side
of the battery. The direction of the current and the chemical
reactions are reversed during charging.
[0032] After the first three cycles, the lower voltage limit on the
RFB with 1a was decreased to 0 V in an attempt to increase cell
capacity. Charge-discharge curves from the 3rd cycle from each RFB
is plotted in FIG. 7. Upon charging both RFBs display a single
voltage plateau, though 2a required significantly less voltage and
charged to nearly 4.times. capacity given the same 2.5 V limit.
While only a 2.times. increase in cell capacity is expected from
the molecular formula, the more facile redox kinetics afforded by
the FcEA groups in 2a required less overpotential to achieve the
same 5 mA cm.sup.-2 charging current, resulting in a nearly
4.times. increase in capacity. Upon discharge, two distinct
plateaus are observed for 2a near 1.0 V and 0.25 V, attributed to
discharging the FcEA and Fe.sup.3+ core, respectively. As expected,
1a displayed only a single discharge plateau due to reduction of
the Fe.sup.3+ core.
[0033] The performance of RFBs with MetILs.sup.3 for 1a or 2a are
compared in FIGS. 8A-C. Both MetILs.sup.3 show an initial increase
in Coulombic efficiency in FIG. 8A from <50% to >95%
efficiency, though this increase is faster for 2a than 1a. On the
other hand, as shown in FIG. 8B, voltage efficiencies for these
RFBs were quite different, with the 2a composition showing 46.1%
efficiency after 20 cycles, compared to 6.4% efficiency for 1a. As
shown in FIG. 8C, the electrochemical yield (% of theoretical
capacity) performed similarly, with an initial rapid drop for the
first two cycles followed by a slow decline to 25.6% yield for 2a,
compared to 9.9% for 1a. Thus, it is concluded that the FcEA group
on the 2a MetIL.sup.3 enabled faster redox kinetics, resulting in
higher voltage efficiencies and electrochemical yields compared to
the original 1a. Both compositions, however, eventually reached
stable cycling conditions and >95% Coulombic efficiencies.
[0034] The electrochemical yield of the RFB is limited in a large
part due to crossover of cobaltocene species from the anolyte.
Cyclic voltammometry analysis of the MetILs.sup.3 catholyte after
testing showed significant concentrations of cobaltocene. The
inability of the anion conductive membrane (Fumasep.RTM. FAP-PK.
Fumasep.RTM. is a registered trademark of FuMA-Tech) to prevent
crossover of redox-active cations ultimately resulted in a
reduction of electrochemical yield, which prevented testing of
MetILs.sup.3 with redox-active iodide anions. If a
highly-conductive room temperature membrane highly selective
towards one non-redox active species (e.g. Li.sup.+) can be
identified, iodide-containing 2b might be successfully tested and
yield even higher capacities.
[0035] In summary, the energy density of an electrolyte for a RFB
can be increased by systematically adding redox-active ligands and
anions to MetILs.sup.3 of the family
Fe(EA).sub.6-x(FcEA).sub.x(OTf).sub.2-yI.sub.y. While addition of
these species did not change the general orientation of the ligands
about the ion core, the thermophysical properties of the
MetILs.sup.3 varied drastically as more redox activity was
incorporated into the structure. Electrochemical characterization
of these MetILs.sup.3 showed redox processes unique to the ion
core, ligands, and anions, confirming successful synthesis of the
MetILs.sup.3. Testing in a laboratory-scale RFB demonstrated that
the more facile redox kinetics afforded by the FcEA group of 2a
enabled nearly 4.times. increase in capacity compared to the RFB
with 1a cycled at the same composition. Application of this
strategy to other chemistries, optimizing electrolyte melting point
and conductivity could yield >10 M redox-active electrons,
though identification of a highly selective membrane will be
necessary to maintain all available capacity.
[0036] The present invention has been described as a method for
maximizing energy density in redox flow battery electrolytes. It
will be understood that the above description is merely
illustrative of the applications of the principles of the present
invention, the scope of which is to be determined by the claims
viewed in light of the specification. Other variants and
modifications of the invention will be apparent to those of skill
in the art.
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