U.S. patent application number 13/565619 was filed with the patent office on 2015-09-10 for synthesis of electroactive ionic liquids for flow battery applications.
This patent application is currently assigned to Sandia Corporation. The applicant listed for this patent is Travis Mark Anderson, David Ingersoll, Harry Pratt, Chad Staiger. Invention is credited to Travis Mark Anderson, David Ingersoll, Harry Pratt, Chad Staiger.
Application Number | 20150255823 13/565619 |
Document ID | / |
Family ID | 53938976 |
Filed Date | 2015-09-10 |
United States Patent
Application |
20150255823 |
Kind Code |
A1 |
Anderson; Travis Mark ; et
al. |
September 10, 2015 |
Synthesis of Electroactive Ionic Liquids for Flow Battery
Applications
Abstract
The present disclosure is directed to synthesizing metal ionic
liquids with transition metal coordination cations, where such
metal ionic liquids can be used in a flow battery. A cation of a
metal ionic liquid includes a transition metal and a ligand
coordinated to the transition metal.
Inventors: |
Anderson; Travis Mark;
(Albuquerque, NM) ; Ingersoll; David;
(Albuquerque, NM) ; Staiger; Chad; (Albuquerque,
NM) ; Pratt; Harry; (Albuquerque, NM) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Anderson; Travis Mark
Ingersoll; David
Staiger; Chad
Pratt; Harry |
Albuquerque
Albuquerque
Albuquerque
Albuquerque |
NM
NM
NM
NM |
US
US
US
US |
|
|
Assignee: |
Sandia Corporation
Albuquerque
NM
|
Family ID: |
53938976 |
Appl. No.: |
13/565619 |
Filed: |
August 2, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61515204 |
Aug 4, 2011 |
|
|
|
Current U.S.
Class: |
429/418 ;
252/182.1; 429/105; 429/107; 429/70 |
Current CPC
Class: |
H01M 8/20 20130101; Y02E
60/50 20130101; H01M 8/188 20130101; H01M 4/368 20130101; Y02E
60/10 20130101; H01M 4/364 20130101 |
International
Class: |
H01M 8/18 20060101
H01M008/18; H01M 8/20 20060101 H01M008/20; H01M 2/40 20060101
H01M002/40 |
Goverment Interests
STATEMENT OF GOVERNMENTAL INTEREST
[0002] This invention was developed under contract
DE-AC04-94AL85000 between Sandia Corporation and the U.S.
Department of Energy. The U.S. Government has certain rights in
this invention.
Claims
1. A metal ionic liquid comprising: a cation comprising a
transition metal selected from the group consisting of Fe, Cu, Mn,
Zn and combinations thereof and a ligand coordinated to the
transition metal, wherein the ligand comprises an amine functional
group and a hydroxyl functional group; and an anion selected from
the group consisting of 2-ethylhexanoate, hexafluorophosphate,
triflate, triflimide, tetrafluoroborate and combinations thereof,
the metal ionic liquid being a non-aqueous liquid.
2-3. (canceled)
4. The metal ionic liquid of claim 1, wherein the ligand is one of
ethanolamine or diethanolamine.
5. (canceled)
6. The metal ionic liquid of claim 1, wherein the nitrogen
containing functional group and the oxygen containing functional
group have localized dipoles.
7. The metal ionic liquid of claim 6, wherein the localized dipoles
produce an electronically asymmetric secondary coordination sphere
of the cation that perturbs pairing with the anion.
8. The metal ionic liquid of claim 1, wherein the cation has an
electronically asymmetric secondary coordination sphere that
perturbs pairing with the anion.
9. The metal ionic liquid of claim 1, wherein the metal ionic
liquid is an electroactive material and a solvent of at least one
of a catholyte or an anolyte in a flow battery.
10. A method of synthesizing a metal ionic liquid, comprising:
reacting a transition metal salt with a ligand to produce the metal
ionic liquid, wherein the transition metal salt comprises a
transition metal and an anion, and wherein the ligand comprises a
nitrogen containing functional group and an oxygen containing
functional group.
11. The method of claim 10, wherein the metal ionic liquid is
produced by a direct combination reaction of the transition metal
salt with the ligand.
12. The method of claim 10, wherein the nitrogen containing
functional group is an amine functional group and the oxygen
containing functional group is a hydroxyl functional group.
13. The method of claim 10, wherein the ligand is one of
ethanolamine or diethanolamine.
14. The method of claim 10, wherein the transition metal is one of
copper, iron, manganese, or zinc.
15. The method of claim 10, wherein the anion is one of
2-ethylhexanoate, hexafluorophosphate, triflate, triflimide, or
tetrafluoroborate.
16. The method of claim 10, further comprising varying a
physicochemical property of the metal ionic liquid based on
selection of at least one of the ligand or the anion, wherein the
physicochemical property is at least one of viscosity or
conductivity.
17. The method of claim 10, further comprising varying whether the
ligand coordinates to the transition metal through the nitrogen
containing functional group or the oxygen containing functional
group based on selection of the anion.
18. A redox flow battery comprising: a first electrode; a second
electrode; and an anion exchange membrane that separates the first
electrode and the second electrode; wherein: a first non-aqueous
metal ionic liquid flows through the first electrode and a second
non-aqueous metal ionic liquid, different from the first
non-aqueous metal ionic liquid, flows through the second electrode;
the first non-aqueous metal ionic liquid further comprises an anion
selected from the group consisting of 2-ethylhexanoate,
hexafluorophosphate, triflate, triflimide, tetrafluoroborate and
combinations thereof and a first cation selected from the group
consisting of Fe, Cu, Mn, Zn and combinations thereof, wherein the
first cation comprises a first ligand coordinated to a first
transition metal, wherein the first ligand comprises an amine
functional group and a hydroxyl functional group; and the second
non-aqueous metal ionic liquid further comprises the anion selected
from the group consisting of 2-ethylhexanoate, hexafluorophosphate,
triflate, triflimide, tetrafluoroborate and combinations thereof
and a second cation selected from the group consisting of Fe, Cu,
Mn, Zn and combinations thereof, wherein the second cation
comprises a second ligand coordinated to a second transition metal,
wherein the second ligand comprises an amine functional group and a
hydroxyl functional group.
19. The redox flow battery of claim 18, wherein the nitrogen
containing functional group is an amine functional group and the
oxygen containing functional group is a hydroxyl functional
group.
20. The redox flow battery of claim 18, wherein the first metal
ionic liquid is an electroactive material and a solvent of a
catholyte and the second metal ionic liquid is the electroactive
material and the solvent of an anolyte in a flow battery.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of U.S. Provisional
Patent Application Ser. No. 61/515,204, filed Aug. 4, 2011, and
entitled "SYNTHESIS OF ELECTROACTIVE IONIC LIQUIDS FOR FLOW BATTERY
APPLICATIONS", the entirety of which is incorporated herein by
reference.
BACKGROUND
[0003] Global energy consumption is projected to significantly
increase by mid-century, and this increased need may be partially
met through use of renewable energy sources. Due to the
intermittent nature of some of these renewable energy sources, such
as wind and solar, it is desirable to incorporate compatible
large-scale energy storage devices into the energy grid. Use of
such grid storage is also being driven by the evolving nature of
the grid (e.g., green grid, smart grid, distributed nature of the
grid, etc.) as well as by other technological developments
including vehicle electrification. Redox (reduction-oxidation) flow
batteries, a rechargeable system that uses redox states of various
species for charge/discharge purposes, represent a potential
approach for grid storage.
[0004] In conventional flow batteries, electrolyte (e.g., catholyte
and anolyte) that includes one or more dissolved electroactive
species oftentimes flows through an electrochemical cell that
reversibly converts chemical energy to electricity. The
electroactive components are dissolved in a solvent rather than
being in a solid state in such flow batteries. Additional
electrolyte can be stored external to the cell (e.g., in tanks),
and can be pumped through the cell or fed into the cell via
gravity. Thus, spent electrolyte in the cell can be recovered for
re-energization and replaced with electrolyte from the external
tanks While flow batteries may be charged and discharged without
degradation of performance, conventional flow batteries commonly
have low energy densities and include costly materials.
SUMMARY
[0005] Described herein are various technologies that pertain to
synthesizing metal ionic liquids with transition metal coordination
cations, where such metal ionic liquids can be used in a flow
battery. A cation of a metal ionic liquid includes a transition
metal and a ligand coordinated to the transition metal. Moreover,
the ligand includes a nitrogen containing functional group and an
oxygen containing functional group.
[0006] According to various embodiments, a metal ionic liquid can
be synthesized by reacting a transition metal salt with a ligand.
The reaction can be a single-step reaction. Moreover, the metal
ionic liquid can be produced by a direct combination reaction of
the transition metal salt with the ligand rather than a metathesis
reaction.
[0007] In various embodiments, an anion of a metal ionic liquid can
be 2-ethylhexanoate, hexafluorophosphate, triflate, triflimide, or
tetrafluoroborate. Moreover, in accordance with an example, a
ligand of a metal ionic liquid can include an amine functional
group and a hydroxyl functional group. Further, a physiochemical
property of an metal ionic liquid can be varied based on selection
of the anion and/or the ligand.
[0008] Further, a flow battery can include a metal ionic liquid.
Accordingly, the metal ionic liquid can be an electroactive
material and a solvent of an electrolyte (e.g., catholyte, anolyte)
in the flow battery. The metal ionic liquid can provide higher
energy densities for the flow battery as compared to a flow battery
where an electroactive material is dissolved in a solvent.
Moreover, the metal ionic liquid can have negligible vapor pressure
and can be non-corrosive.
[0009] The above summary presents a simplified summary in order to
provide a basic understanding of some aspects of the systems and/or
methods discussed herein. This summary is not an extensive overview
of the systems and/or methods discussed herein. It is not intended
to identify key/critical elements or to delineate the scope of such
systems and/or methods. Its sole purpose is to present some
concepts in a simplified form as a prelude to the more detailed
description that is presented later.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIGS. 1-3 illustrate exemplary structural formulas of
various exemplary cations in exemplary metal ionic liquids.
[0011] FIG. 4 illustrates an exemplary diagrammatic representation
of an electronically asymmetric secondary coordination sphere of a
cation of a metal ionic liquid.
[0012] FIGS. 5-6 illustrate Cu{NH(CH.sub.2CH.sub.2OH).sub.2}.sub.6
cations with differing ligand coordinations.
[0013] FIG. 7 illustrates an exemplary redox flow battery that
includes metal ionic liquids.
[0014] FIG. 8 illustrates a weight change of Example 1, a heat flow
of Example 1, and a weight change of
NH(CH.sub.2CH.sub.2OH).sub.2.
[0015] FIG. 9 illustrates results of a differential scanning
calorimetry measurement of Example 1 performed using a liquid
N.sub.2 quench cooling accessory.
[0016] FIG. 10 illustrates infrared spectra of Example 1 and
infrared spectra of NH(CH.sub.2CH.sub.2OH).sub.2.
[0017] FIG. 11 illustrates Raman spectra (633 nm laser) of
NH(CH.sub.2CH.sub.2OH).sub.2, Example 1, and
Fe(CF.sub.3SO.sub.3).sub.3.
[0018] FIG. 12 illustrates ultraviolet-visible spectroscopic data
of Example 1 in 1-butyl-3-methyl-imidazolium hexafluorophosphate
(BMI-PF.sub.6).
[0019] FIG. 13 illustrates cyclic voltammograms of Example 1, which
were performed with a glassy carbon working electrode at four
different scan rates.
[0020] FIG. 14 illustrates a Osteryoung square wave voltammogram of
Example 1 with 1 mV steps, sweep width amplitude of 25 mV, and
sweep width frequency of 15 Hz.
[0021] FIG. 15 illustrates infrared spectra of copper
2-ethylhexanoate, infrared spectra of Example 2, and infrared
spectra of ethanolamine.
[0022] FIG. 16 illustrates infrared spectra of Example 3A, infrared
spectra of Example 3B, and infrared spectra of Example 3C, each
compared to infrared spectra of neat diethanolamine.
[0023] FIG. 17 illustrates solution .sup.13C NMR spectra of Example
5 and ethanolamine dissolved in CD.sub.3CN.
[0024] FIGS. 18-22 illustrate various cyclic voltammograms of the
Examples.
DETAILED DESCRIPTION
[0025] Various technologies pertaining to synthesizing ionic
liquids with transition metal coordination cations, where such
metal ionic liquids can be used in a flow battery, are now
described with reference to the drawings, wherein like reference
numerals are used to refer to like elements throughout. In the
following description, for purposes of explanation, numerous
specific details are set forth in order to provide a thorough
understanding of one or more aspects. It may be evident, however,
that such aspect(s) may be practiced without these specific
details. In other instances, well-known structures and devices are
shown in block diagram form in order to facilitate describing one
or more aspects. Further, it is to be understood that functionality
that is described as being carried out by certain system components
may be performed by multiple components. Similarly, for instance, a
component may be configured to perform functionality that is
described as being carried out by multiple components.
[0026] Moreover, the term "or" is intended to mean an inclusive
"or" rather than an exclusive "or." That is, unless specified
otherwise, or clear from the context, the phrase "X employs A or B"
is intended to mean any of the natural inclusive permutations. That
is, the phrase "X employs A or B" is satisfied by any of the
following instances: X employs A; X employs B; or X employs both A
and B. In addition, the articles "a" and "an" as used in this
application and the appended claims should generally be construed
to mean "one or more" unless specified otherwise or clear from the
context to be directed to a singular form.
[0027] Set forth herein is a family of metal ionic liquids
(MetILs), which are synthesized in a single-step reaction (e.g.,
from low-cost precursors). The metal ionic liquids include
transition metal coordination cations and weakly coordinating
anions. Examples of the anions include 2-ethylhexanoate,
hexafluorophosphate, triflate, triflimide, and tetrafluoroborate.
The metal ionic liquids can simultaneously act as a solvent and
electroactive material of a catholyte or an anolyte in a flow
battery.
[0028] The transition metal-based ionic liquid can be prepared in a
single-step reaction by reacting a transition metal salt with a
ligand. The transition metal salt and the ligand can be combined
and heated to produce the metal ionic liquid, for instance. The
metal ionic liquid is produced by a direct combination reaction of
the transition metal salt with the ligand as opposed to a
metathesis reaction. A metathesis reaction yields a secondary
product while a direct combination reaction does not yield a
secondary product; accordingly, since a secondary product is not
generated by a direct combination reaction, such secondary product
need not be isolated and discarded when generating the metal ionic
liquid (e.g., isolation may be costly and flow battery performance
may be detrimentally impacted by impurities). The synthesis can be
scalable and can facilitate varying physicochemical properties of
the metal ionic liquid based on selection of the transition metal
salt and/or the ligand.
[0029] The transition metal salt includes a transition metal and an
anion. Examples of the anion include 2-ethylhexanoate,
hexafluorophosphate (PF.sub.6.sup.-), triflate
(CF.sub.3SO.sub.3.sup.-), triflimide
[(CF.sub.3SO.sub.2).sub.2N.sup.-], and tetrafluoroborate
(BF.sub.4.sup.-). Examples of the transition metal included in the
transition metal salt described below include copper, iron,
manganese, and zinc (e.g., copper, iron, manganese, and zinc
salts); however, it is to be appreciated that the transition metal
included in the transition metal salt can be cobalt, cerium,
nickel, or substantially any other transition metal.
[0030] Moreover, the ligand includes a nitrogen containing
functional group and an oxygen containing functional group. The
nitrogen containing functional group can be an amine functional
group and the oxygen containing functional group can be a hydroxyl
functional group; thus, the ligand can be an amino alcohol.
Examples of the ligand set forth below include ethanolamine (EA)
and diethanolamine (DEA); yet, it is contemplated that
substantially any other ligand that includes a nitrogen containing
functional group and an oxygen containing functional group is
intended to fall within the scope of the hereto appended claims.
For instance, the ligand can be a straight chain ligand, a branched
chain ligand, a cyclic ligand, or the like.
[0031] Examples of the metal ionic liquid include
Fe{NH(CH.sub.2CH.sub.2OH).sub.2}.sub.6[CF.sub.3SO.sub.3].sub.3,
Cu{NH.sub.2CH.sub.2CH.sub.2OH}.sub.6[CH.sub.3(CH.sub.2).sub.3CH(C.sub.2H.-
sub.5)CO.sub.2].sub.2,
Cu{NH(CH.sub.2CH.sub.2OH).sub.2}.sub.6[CH.sub.3(CH.sub.2).sub.3CH(C.sub.2-
H.sub.5)CO.sub.2].sub.2,
Cu{NH(CH.sub.2CH.sub.2OH).sub.2}.sub.6[CF.sub.3SO.sub.3].sub.2,
Cu{NH(CH.sub.2CH.sub.2OH).sub.2}.sub.6[(CF.sub.3SO.sub.2).sub.2N].sub.2,
Mn{NH(CH.sub.2CH.sub.2OH).sub.2}.sub.6[CF.sub.3SO.sub.3].sub.2, and
Zn{NH.sub.2CH.sub.2CH.sub.2OH}.sub.6[CF.sub.3SO.sub.3].sub.2. Other
examples of the metal ionic liquid include
Ce{NH.sub.2CH.sub.2CH.sub.2OH}.sub.8[CF.sub.3SO.sub.3].sub.3,
Cu{NH(CH.sub.2CH.sub.2OH).sub.2}.sub.6[BF.sub.4].sub.2,
Co{NH.sub.2CH.sub.2CH.sub.2OH}.sub.6[CF.sub.3SO.sub.3].sub.2, and
Ni{NH.sub.2CH.sub.2CH.sub.2OH.sub.2}.sub.8[CF.sub.3SO.sub.3].sub.2.
Yet, it is to be appreciated that the claimed subject matter is not
limited to the foregoing examples.
[0032] Ionic liquids, including those that comprise transition
metal elements (metal ionic liquids), are a class of highly
modifiable molten salts; for instance, ionic liquids can be salts
with melting points below 100.degree. C. Ionic liquids can have
features such as high thermal stability, negligible vapor pressure,
wide electrochemical window, and the ability to dissolve a range of
organic and inorganic compounds; such features make ionic liquids
attractive for a wide range of applications (e.g., solvents,
electrically conducting fluids, etc.). Many of the properties of
ionic liquids can be systematically varied by subtle compositional
and structural changes. Further, it may be desirable to develop
ionic liquids that simultaneously exhibit low viscosity and high
conductivity for flow batteries.
[0033] Large, structurally asymmetric organic cations are often
found in conventional ionic liquids, including those utilized as
electrochemical solvents, because they lower the melting point by
reducing the lattice energy of the crystalline salt. Recently, some
conventional ionic liquids include transition metal-based anions.
Examples include compounds including imidazolium cations with
tetrahedral halogenoferrates and phosphonium cations with various
cobaltates as well as ionic liquids including alkyl ammonium,
phosphonium, or imidazolium salts of polyoxotungstate clusters.
According to other examples, some conventional ionic liquids
include transition metal-based cations. By way of illustration,
some ionic liquids have Ag(H.sub.2N--R).sub.2' or
Zn(H.sub.2N--R).sub.4.sup.2- (R=alkyl group) cations, and there are
also a number of compounds that include ferrocenyl-functionalized
cations.
[0034] In contrast to conventional ionic liquids which oftentimes
include cations with low structural symmetry, the metal ionic
liquids set forth herein include cations that have electronically
asymmetric secondary coordination spheres that perturb pairing with
anions. The nitrogen containing functional group and the oxygen
containing functional group of a cation of the metal ionic liquid
can have localized dipoles (e.g., the amine functional group and
the hydroxyl function group are polarizable); thus, the localized
dipoles can produce the electronically asymmetric secondary
coordination sphere of the cation. Partial positive and negative
charges can be sufficiently distributed in a secondary coordination
sphere of the cation to limit interaction with an anion while
simultaneously keeping electrons sufficiently mobile to either add
charge to or remove charge from the transition metal ion at the
center of the cation complex. Further, the electronically
asymmetric cations can lower the melting point of the metal ionic
liquids.
[0035] According to an example, FIG. 1 illustrates an exemplary
structural formula of a cation in an exemplary metal ionic liquid,
namely
Cu{NH.sub.2CH.sub.2CH.sub.2OH}.sub.6[CH.sub.3(CH.sub.2).sub.3CH(C.sub.2H.-
sub.5)CO.sub.2].sub.2. The cation has a pseudo-octahedral
structural symmetry (with an almost insect-like motif). As
depicted, NH.sub.2CH.sub.2CH.sub.2OH is coordinated to the
transition metal (e.g., Cu) through an amine functional group.
[0036] By way of another example, FIG. 2 illustrates an exemplary
structural formula of a cation in
Mn{NH(CH.sub.2CH.sub.2OH).sub.2}.sub.6[CF.sub.3SO.sub.3].sub.2
(e.g., another exemplary metal ionic liquid). Again, the cation
shown in FIG. 2 has a pseudo-octahedral structural symmetry.
Further, NH(CH.sub.2CH.sub.2OH).sub.2 is coordinated to the
transition metal (e.g., Mn) through an amine functional group.
[0037] In accordance with yet a further example, FIG. 3 illustrates
an exemplary structural formula of a cation in
Fe{NH(CH.sub.2CH.sub.2OH).sub.2}.sub.6[CF.sub.3SO.sub.3].sub.3
(e.g., another exemplary metal ionic liquid). Similar to the
examples shown in FIGS. 1 and 2, the cation depicted in FIG. 3 has
a pseudo-octahedral structural symmetry. In contrast to the
examples of FIGS. 1 and 2, the ligand
(NH(CH.sub.2CH.sub.2OH).sub.2) of the cation in
Fe{NH(CH.sub.2CH.sub.2OH).sub.2}.sub.6[CF.sub.3SO.sub.3].sub.3 is
coordinated to the transition metal (e.g., Fe) through a hydroxyl
functional group rather than an amine functional group.
[0038] FIG. 3 further depicts a primary coordination sphere 300 and
a secondary coordination sphere 302 of the cation in
Fe{NH(CH.sub.2CH.sub.2OH).sub.2}.sub.6[CF.sub.3SO.sub.3].sub.3 .
The amine and hydroxyl functional groups of the ligand are
polarizable. Accordingly, the polarizable amine and hydroxyl
functional groups distal to the iron cation can create an
electronically asymmetric secondary coordination sphere 302 that
can perturb ion pairing (e.g., pairing with
[CF.sub.3SO.sub.3].sub.3). By perturbing ion pairing, the
polarizable amine and hydroxyl groups can lower electronic symmetry
of the secondary coordination sphere 302.
[0039] Turning to FIG. 4, illustrated is an exemplary diagrammatic
representation 400 of an electronically asymmetric secondary
coordination sphere of a cation 402 of a metal ionic liquid. A
portion of the diagrammatic representation 400 of the
electronically asymmetric secondary coordination sphere is cut away
to show the structure of the cation 402. The cation 402 can have a
symmetric structure. Moreover, the amine and hydroxyl functional
groups of the cation 402 are polarizable, and thus, localized
dipoles can be created by such functional groups, resulting in the
electronically asymmetric secondary coordination sphere. The
dipoles can asymmetrically distribute charge in the secondary
coordination sphere. While the cation 402 overall has a net
positive charge, in the diagrammatic representation 400, pentagons
404 can each represent an arbitrary negative charge and hexagons
406 can each represent an opposite arbitrary positive charge,
thereby illustrating the asymmetric charge distribution due to the
polarizable amine and hydroxyl functional groups. Further, the
localized negative charges distributed through the electronically
asymmetric secondary coordination sphere can repulse the anion of
the metal ionic liquid. Thus, the asymmetric charge distribution
(as opposed to physical shape) of the cation 402 can mitigate
pairing between the cation 402 and the anion.
[0040] Moreover, a physiochemical property of the metal ionic
liquid can be varied based at least in part on selection of the
ligand and/or the anion. Such physiochemical property that can be
varied can be viscosity, conductivity, or a combination thereof,
for example. For instance, whether the ligand coordinates to the
transition metal through the nitrogen containing functional group
or the oxygen containing function group can be based on selection
of the anion. FIGS. 5-6 depict
Cu{NH(CH.sub.2CH.sub.2OH).sub.2}.sub.6 cations with differing
ligand coordinations. In FIG. 5, the ligands coordinate to the
transition metal (e.g., Cu) through amine functional groups. In
contrast, in FIG. 6, the ligands coordinate to the transition metal
(e.g., Cu) through the hydroxyl functional group. According to an
example pertaining to the Cu{NH(CH.sub.2CH.sub.2OH).sub.2}.sub.6
cation, when the anion is 2-ethylhexanoate or triflimide, the
ligands coordinate through the amine functional group as shown in
FIG. 5. Following this example, when the anion is triflate, the
ligands coordinate through the hydroxyl functional group as shown
in FIG. 6. Further following this example, viscosity of the metal
ionic liquid can correspond to the ligand coordination. For
instance, the cation of FIG. 6 has both amine and hydroxyl
functional groups in the secondary coordination sphere, and thus,
the cation of FIG. 6 has lower symmetry as compared to the cation
of FIG. 5. The lower symmetry leads to a decrease in ion pairing,
which leads to lower viscosity. Thus, by way of illustration, the
ligand and anion (and transition metal) can be selected to provide
a metal ionic liquid with low viscosity, high conductivity, and the
ability to reversibly store charge (e.g., through changes in
oxidation state of one or more metals incorporated into the
molecular formula of the metal ionic liquid) for use in a flow
battery; yet, the claimed subject matter is not so limited.
[0041] The metal ionic liquids can be used for flow battery energy
storage. The metal ionic liquid is a non-aqueous liquid. Moreover,
use of the metal ionic liquid in a flow battery can potentially
provide wider voltage windows, higher charge cycle efficiency,
decreased temperature sensitivity, and increased cycle life as
compared to conventional flow batteries. Thus, the metal ionic
liquids with reducing-oxidizing (redox) transition metal species
can be incorporated into a flow battery cell configuration. A
difference in potentials of two different metal ionic liquids can
be relied upon to establish a cell voltage. The ionically
conductive metal ionic liquids can act as both an electroactive
material and a solvent of an electrolyte (e.g., catholyte and/or
anolyte) in the flow battery. Moreover, since the metal ionic
liquids can have negligible vapor pressure, safety issues related
to cell pressurization can be mitigated.
[0042] With reference to FIG. 7, illustrated is an exemplary redox
flow battery 700 that includes metal ionic liquids. The redox flow
battery includes two electrodes (e.g., a cathode and an anode),
namely, electrode 702 and electrode 704 (collectively referred to
as electrodes 702-704). The two electrodes 702-704 are separated by
an anion exchange membrane 706. Two electrolytes flow through the
electrodes 702-704 (e.g., catholyte flows through the cathode and
anolyte flows through the anode).
[0043] More particularly, the electrolytes included in the redox
flow battery 700 are metal ionic liquids. Thus, a first metal ionic
liquid flows through the electrode 702 and a second metal ionic
liquid flows through the electrode 704. The first metal ionic
liquid can include an anion and a first cation (e.g., a first
ligand coordinated with a first transition metal), and the second
metal ionic liquid can include the anion and a second cation (e.g.,
a second ligand coordinated with a second transition metal, where
the first and second transition metals can be the same or
different, and the first and second ligands can be the same or
different). Reduction and oxidation reactions can occur in the
first and second metal ionic liquids.
[0044] The flow battery 700 further includes a tank 708 and a tank
710 in which the metal ionic liquids are stored. Moreover, the flow
battery 700 can include a pump 712 and a pump 714. The pump 712 can
cause the first metal ionic liquid to flow from the tank 708 into
the electrode 702, where the first metal ionic liquid can flow
through the electrode 702 and return to the tank 708. Similarly,
the pump 714 can cause the second metal ionic liquid to flow from
the tank 710 into the electrode 704, where the second metal ionic
liquid can flow through the electrode 704 and return to the tank
710.
[0045] Further, a circuit can be completed between the electrode
702 and the electrode 704 through a component 716. If the component
716 is an electrical power source, then the redox flow battery 700
can be charged. If the component 716 is an electrical power load,
then the redox flow battery 700 can be discharged.
[0046] Conventional redox flow batteries commonly include
electrolytes with electroactive materials (e.g., metal salts)
dissolved in aqueous solvents. However, water in aqueous solvents
can hydrolyze above 1.5 Volts; thus, these conventional flow
batteries typically do not support potentials above 1.5 Volts.
Moreover, aqueous solvents can be corrosive. Moreover, when a
non-aqueous solvent is used instead for conventional electrolytes,
lower amounts of electroactive materials oftentimes are able to be
dissolved, thus resulting in lower energy densities.
[0047] In contrast, in the redox flow battery 700, the metal ionic
liquids act as the electroactive materials and the solvents of the
electrolytes (e.g., catholyte and anolyte). Thus, the metal ionic
liquids can support higher energy densities compared to
electrolytes of conventional redox flow batteries. Further, the
metal ionic liquids have negligible vapor pressure and are
non-corrosive. Moreover, the metal ionic liquids can allow for
potentials above 1.5 Volts.
EXAMPLES
[0048] Set forth below are seven exemplary metal ionic liquids
(referred to as Examples 1, 2, 3A, 3B, 3C, 4, and 5) that include
an anion and a cation that comprises a transition metal and a
ligand coordinated to the transition metal, where the ligand
includes a nitrogen containing functional group and an oxygen
containing functional group. The seven exemplary metal ionic
liquids are
Fe{NH(CH.sub.2CH.sub.2OH).sub.2}.sub.6[CF.sub.3SO.sub.3].sub.3
(Example 1),
Cu{NH.sub.2CH.sub.2CH.sub.2OH}.sub.6[CH.sub.3(CH.sub.2).sub.3CH(C.sub-
.2H.sub.5)CO.sub.2].sub.2 (Example 2),
Cu{NH(CH.sub.2CH.sub.2OH).sub.2}.sub.6[CH.sub.3(CH.sub.2).sub.3CH(C.sub.2-
H.sub.5)CO.sub.2].sub.2 (Example 3A),
Cu{NH(CH.sub.2CH.sub.2OH).sub.2}.sub.6[CF.sub.3SO.sub.3].sub.2
(Example 3B),
Cu{NH(CH.sub.2CH.sub.2OH).sub.2}.sub.6[(CF.sub.3SO.sub.2).sub.2N].su-
b.2 (Example 3C),
Mn{NH(CH.sub.2CH.sub.2OH).sub.2}.sub.6[CF.sub.3SO.sub.3].sub.2
(Example 4), and
Zn{NH.sub.2CH.sub.2CH.sub.2OH}.sub.6[CF.sub.3SO.sub.3].sub.2
(Example 5). It is to be appreciated, however, that other metal
ionic liquids that similarly include an anion and a cation having a
transition metal and a ligand coordinated to the transition metal,
where the ligand includes the nitrogen and oxygen containing
functional groups, are intended to fall within the scope of the
hereto appended claims.
[0049] According to an example, the metal ionic liquids can be
synthesized by mixing a metal salt with six equivalents of either
EA or DEA. Additional heating drives the reactions to completion. A
color change is observed upon coordination of the ligand for
Examples 1, 2, 3A, 3B, 3C, and 4. The reactions are readily
scalable and an analytically pure product is obtained in
quantitative yield with a single step. Further, the metal to ligand
ratio can be altered to four in the case of divalent copper and
zinc, since these metals can have six- or four-coordinate
geometries. While copper 2-ethylhexanoate and zinc triflate will
react with four equivalents of EA, the resulting compounds are
extremely viscous and decompose quickly (e.g., less than one day)
in air.
[0050] Example syntheses of the seven exemplary compounds are
described below. In these exemplary syntheses, all starting
materials were from Aldrich or Alfa Aesar.
Fe(CF.sub.3SO.sub.3).sub.3 was recrystallized from hot
CH.sub.3CN.
Synthesis of Example 1
Fe{NH(CH.sub.2CH.sub.2OH).sub.2}.sub.6[CF.sub.3SO.sub.3].sub.3
[0051] A 2.00 g (3.98 mmol) sample of recrystallized
Fe(CF.sub.3SO.sub.3).sub.3 was added to 2.51 g (23.9 mmol) of
NH(CH.sub.2CH.sub.2OH).sub.2 in a 20 mL glass vial, thoroughly
mixed for several minutes, and then heated to 115.degree. C. in an
oven for 15-20 minutes. Additional mixing was then performed by
ultrasound for 5 minutes and the solution was again heated to
115.degree. C. for 15-20 minutes (yield >99.9%). IR (ATR,
4000-500 cm.sup.-1): 3440 (m), 3309 (sh), 3094 (w), 2938 (w), 2862
(m), 1608 (m), 1453 (m), 1273 (sh), 1240 (s), 1224 (s), 1161 (s),
1061 (s), 1024 (s), 811 (m), 760 (m), 635 (s), 574 (m), and 515
(m). Raman (1800-200 cm.sup.-1): 1464 (m), 1227 (w), 1033 (s), 878
(w), 817 (w), 761 (s), 577 (m), 517 (w), 351 (m), and 317 (m).
Electronic spectral data (400-700 nm, in BMI-PF.sub.6) [.lamda., nm
(.epsilon., M.sup.-1 cm.sup.-1)]: 477 (63). Density: 1.44.+-.0.06 g
mL.sup.-1. Magnetic susceptibility: .chi..sub.meas=5.78 BM at 295
K. Anal. Calcd. (%) for
Fe(NH(CH.sub.2CH.sub.2OH).sub.2).sub.6(CF.sub.3SO.sub.3).sub.3 : C,
28.6; H, 5.87; N, 7.41; F, 15.1; Fe, 4.9; S, 8.5. Found: C, 28.0;
H, 5.76; N, 7.33; F, 14.7; Fe, 5.0; S, 8.8. [MW=1134 g
mol.sup.-1].
Synthesis of Example 2
Cu{NH.sub.2CH.sub.7CH.sub.2OH}.sub.6[CH.sub.3(CH.sub.2).sub.3CH(C.sub.2H.s-
ub.5)CO.sub.2].sub.2
[0052] A 2.00 g (5.72 mmol) amount of Cu[(2-Et)C.sub.5COO].sub.2
(where (2-Et)C.sub.5COO is 2-ethylhexanoate) was added to
ethanolamine (2.06 g, 34.3 mmol) in a 20 mL glass vial, quickly
heated to .about.200.degree. C., and continuously stirred for
approximately 10 minutes without additional heating. IR (ATR,
4000-500 cm.sup.-1): 3234 (m), 3151 (m), 2955 (w), 2927 (m), 2871
(w), 2857 (w), 1738 (m), 1548 (s), 1456 (m), 1399 (s), 1312 (m),
1231 (m), 1161 (w), 1067 (s), 1032(s), 869 (m), 802 (m), 761 (w),
729 (w), 673 (w), 521 (w), and 486 (w). Anal. Calcd. (%): C, 47.0;
Cu, 8.87; H, 10.1; N, 11.7. Found: C, 47.3; Cu, 8.8; H, 9.9; N,
11.5. [MW=716.5 g mol.sup.-1].
Synthesis of Example 3A
Cu{NH(CH.sub.2CH.sub.2OH).sub.2}.sub.6[CH.sub.3(CH.sub.2).sub.3CH(C.sub.2H-
.sub.5)CO.sub.2].sub.2
[0053] Copper(II) 2-ethylhexanoate (2.00 g, 5.72 mmol) was added to
diethanolamine (3.62 g, 34.3 mmol) in a 20 mL glass vial, quickly
heated to .about.200.degree. C., and continuously stirred for
approximately 10 minutes without additional heating. IR (ATR,
4000-500 cm.sup.-1): 3219 (s), 2926 (w), 2826 (w), 1739 (m), 1560
(s), 1455 (m), 1402 (m), 1377 (w), 1309 (w), 1230 (w), 1217 (w),
1204 (w), 1052 (s), 919 (w), 864 (w), 800 (m), 731 (w), 640 (w),
and 547 (w). Anal. Calcd. (%): C, 49.0; Cu, 6.48; H, 9.87; N, 8.57.
Found: C, 48.7; Cu, 6.8; H, 9.9; N, 8.5. [MW=980.8 g
mol.sup.-1].
Synthesis of Example 3B
Cu{NH(CH.sub.2CH.sub.2OH).sub.2}.sub.6[CF.sub.3SO.sub.3].sub.2
[0054] A 2.00 g (5.53 mmol) amount of Cu(OTf).sub.2 (where OTf is
CF.sub.3SO.sub.3) was added to diethanolamine (3.49 g, 33.2 mmol)
in a 20 mL glass vial, quickly heated to .about.200.degree. C., and
continuously stirred for approximately 10 minutes without
additional heating. IR (ATR, 4000-500 cm.sup.-1): 3406 (sh), 3285
(w), 2930 (w), 2875 (w), 1738 (m), 1612 (w), 1454 (m), 1365 (w),
1274 (sh), 1224 (w), 1161 (m), 1058 (m), 1025 (s), 943 (w), 880
(w), 814 (w), 758 (w), 636 (w), 573(m), and 515 (m). Anal. Calcd.
(%): C, 31.5; Cu, 6.40; F, 11.5; H, 6.70; N, 8.47; S, 6.46. Found:
C, 32.1; Cu, 6.5; F, 11.7; H, 6.8; N, 8.3; S, 6.8. [MW=992.5 g
mol.sup.-1].
Synthesis of Example 3C
Cu{NH(CH.sub.2CH.sub.2OH).sub.2}.sub.6[(CF.sub.3SO.sub.2).sub.2N].sub.2
[0055] A 2.00 g (3.21 mmol) amount of
Cu((CF.sub.3SO.sub.2).sub.2N).sub.2 was added to 2.02 g (19.3 mmol)
of diethanolamine in a 20 mL glass vial, quickly heated to
.about.200.degree. C., and continuously stirred for approximately
10 minutes without additional heating. IR (ATR, 4000-500
cm.sup.-1): 3522 (m), 3292 (m), 2940 (m), 2890 (m), 1456 (m), 1343
(s), 1325 (sh), 1228 (sh), 1191 (s), 1133 (m), 1050 (s), 983 (m),
913 (w), 873 (w), 813 (s), 791 (m), 740 (m), 653 (w), 610 (m), 569
(m), and 509 (m). Anal. Calcd. (%): C, 26.8; Cu, 5.06; F, 18.2; H,
5.30; N, 8.93; S, 10.2. Found: C, 26.1; Cu, 5.1; F, 17.7; H, 5.6;
N, 8.9; S, 9.9. [MW=1254.7 g mol.sup.-1].
Synthesis of Example 4
Mn{NH(CH.sub.2CH.sub.2OH).sub.2}.sub.6[CF.sub.3SO.sub.3].sub.2
[0056] Manganese(II) triflate (2.00 g, 5.66 mmol) was added to
diethanolamine (3.57 g, 40.0 mmol) in a 20 mL glass vial, quickly
heated to .about.200.degree. C., and continuously stirred for
approximately 10 minutes without additional heating. IR (ATR,
4000-500 cm.sup.-1): 3299 (m), 2940 (w), 2849 (w), 1738 (m), 1455
(m), 1365 (m), 1247 (m), 1225 (w), 1164 (s), 1121 (m), 1027 (s),
937 (m), 802 (w), 760 (w), 637 (s), 574 (w), 515 (m), and 440 (w).
Anal. Calcd. (%): C, 31.7; F, 11.6; H, 6.76; Mn, 5.58; N, 8.54; S,
6.52. Found: C, 30.6; F, 11.2; H, 6.8; Mn, 5.6; N, 8.8; S, 6.7.
[MW=983.9 g mol.sup.-1].
Synthesis of Example 5
Zn{NH.sub.2CH.sub.2CH.sub.2OH}.sub.6[CF.sub.3SO.sub.3].sub.2
[0057] Zinc(II) triflate (2.00 g, 5.50 mmol) was added to
ethanolamine (1.32 g, 22.0 mmol) in a 20 mL glass vial, quickly
heated to .about.175.degree. C., and continuously stirred for
approximately 10 minutes without additional heating. IR (ATR,
4000-500 cm.sup.-1): 3440 (sh), 3269 (m), 3172 (w), 2953 (m), 2892
(w), 1738 (m), 1545 (m), 1463 (w), 1366 (w), 1241 (w), 1224 (w),
1160 (s), 1066 (m), 1023 (s), 873 (s), 760 (m), 633 (s), 574 (m),
515 (s), and 443 (w). .sup.13C (100 MHz, CD.sub.3CN) .delta. 121.0
(q, J=317 Hz), 61.5, 43.8. Anal. Calcd. (%): C, 19.8; F, 18.8; H,
4.64; N, 9.22; S, 10.6; Zn, 10.8. Found: C, 20.3; F, 18.1; H, 4.5;
N, 8.9; S, 10.5; Zn, 11.0. [MW=607.9 g mol.sup.-1].
[0058] The seven exemplary compounds were analyzed as set forth
below. Magnetic susceptibility measurements were made on a Johnson
and Matthey MK-1 balance and Pascal's constants were used to obtain
a diamagnetic correction. The visible absorption spectra (300-700
nm) were collected on a Shimadzu UV-3600. Elemental analyses (C, H,
N, Cu, F, Fe, Mn, S, and Zn) were performed by Galbraith
Laboratories, Inc. Water content was measured using a Mettler
Toledo DL32 Karl Fischer coulometer. Viscosity measurements were
performed on a Brookfield DV-E viscometer.
[0059] For Example 1, thermal analysis was measured with a Mettler
Toledo TGA/DSC 1 (Ar flow) and a TA Instruments DSC 2010 (with a
quench cooling accessory, N.sub.2 flow) with 10.degree. C.
min.sup.-1 heating. Infrared spectra were recorded for Example 1 on
a Thermo Nicolet 380 FT-IR equipped with a Smart Orbit (Diamond)
ATR (attenuated total reflectance) accessory. Raman data were
acquired for Example 1 on a Thermo DXR with a 633 nm laser. X-ray
fluorescence was performed for Example 1 with a Thermo ARL QUANT'X
analyzer.
[0060] For Examples 2, 3A, 3B, 3C, 4, and 5, thermal analyses were
measured with a TA Instruments Q600 and Q2000 (with an RCS 90
refrigerated cooling system). Infrared spectra were recorded for
Examples 2, 3A, 3B, 3C, 4, and 5 on a Thermo Nicolet iS10 FT-IR
equipped with a Smart Orbit (Diamond) ATR accessory. Solution
.sup.13C NMR spectra were recorded for Examples 2, 3A, 3B, 3C, 4,
and 5 on a Bruker Avance DRX spectrometer operating at 100 MHz. The
chemical shifts are reported in the .delta. scale in ppm with the
solvent indicated as the internal reference. Coupling constants (J)
are reported in Hz and the splitting abbreviation used is q,
quartet.
[0061] For the seven exemplary compounds, conductivity measurements
were made using a Solartron 1255B frequency analyzer with a SI 1287
electrochemical potentiostat using a custom cell with .about.5 mm
in diameter Platinum (Pt) working and Pt counter electrodes in a
cofacial arrangement. This cell had a cell constant of .about.10,
as determined by measuring a solution of known specific
conductance. The sample was thermally equilibrated prior to
measurement by placing the cell in an instrumented Tenney
environmental chamber to control temperature. Cyclic voltammograms
were collected using a BAS100B potentiostat in a three-electrode
cell. The working electrode was freshly polished 3 mm diameter
glassy carbon, Pt wire was the counter electrode, and the reference
electrode was silver/silver chloride (Ag/AgCl) in
1-ethyl-3-methyl-imidazolium chloride (EMIC) in
1,2-dimethyl-3-propyl-imidazolium bis(trifluoromethylsulfonyl)
imide (DMPI-Im). Measurements were performed in a glove box under
argon (Ar).
Evaluation of Example 1
[0062] Example 1,
Fe{NH(CH.sub.2CH.sub.2OH).sub.2}.sub.6[CF.sub.3SO.sub.3].sub.3, was
synthesized in a single-step complexation reaction. Infrared and
Raman data suggested NH(CH.sub.2CH.sub.2OH).sub.2 primarily
coordinated to Fe(III) through alcohol groups. Example 1 had
temperature for glass transition (T.sub.g) and temperature for
decomposition (T.sub.d) values of -64.degree. C. and 260.degree.
C., respectively. Cyclic voltammetry revealed quasi-reversible
Fe(III)/Fe(II) reduction waves.
[0063] Example 1 was obtained by mixing the solid metal salt
Fe(CF.sub.3SO.sub.3).sub.3 with liquid NH(CH.sub.2CH.sub.2OH).sub.2
in a 1:6 stoichiometry. The reaction was exothermic but additional
heating drove the reaction to completion. A color change from
almost white to red-orange was observed upon coordination of
NH(CH.sub.2CH.sub.2OH).sub.2. The reaction was a direct combination
reaction rather than a metathesis reaction. If the color is green
the iron is in the +2 oxidation state instead of +3. Moreover, the
presence of two simultaneously present functional groups (alcohol
and amine of NH(CH.sub.2CH.sub.2OH).sub.2) was required to produce
the metal ionic liquid (e.g., assuming no change in hydrocarbon
chain length). Reaction of Fe(CF.sub.3SO.sub.3).sub.3 with either
CH.sub.3CH.sub.2NH.sub.2 or CH.sub.3CH.sub.2OH alone failed to
produce an ionic liquid.
[0064] Example 1 was evaluated by elemental analysis, viscometry,
thermogravimetric analysis--differential scanning calorimetry
(TGA-DSC), infrared, Raman, and ultraviolet-visible (UV-Vis)
spectroscopy. The electrochemical properties of Example 1 were
evaluated using cyclic voltammetry and impedance spectroscopy.
[0065] The stability of Example 1 was evaluated by
thermogravimetric analysis (TGA) coupled with differential scanning
calorimetry (DSC) and the results are shown in FIG. 8. In FIG. 8, a
weight change of Example 1 is illustrated as line 800, a heat flow
of Example 1 is illustrated as line 802, and a weight change of
NH(CH.sub.2CH.sub.2OH).sub.2 is illustrated as line 804. No change
in mass of Example 1 was observed up to 260.degree. C. Under
identical conditions, NH(CH.sub.2CH.sub.2OH).sub.2 alone started to
evaporate around 200.degree. C. This observation provided evidence
that (1) NH(CH.sub.2CH.sub.2OH).sub.2 was coordinated to Fe(III)
and that (2) Fe(CF.sub.3SO.sub.3).sub.3 was not simply dissolved in
NH(CH.sub.2CH.sub.2OH).sub.2 solvent. A DSC measurement of Example
1 was also performed using a liquid N.sub.2 quench cooling
accessory and the results are shown in FIG. 9 (illustrated as line
900). As depicted in FIG. 9, there was a T.sub.g at -64.degree. C.,
indicative of an amorphous glass reforming a liquid upon
heating.
[0066] FIG. 10 illustrates infrared spectra of Example 1 (line
1000) and infrared spectra of NH(CH.sub.2CH.sub.2OH).sub.2. The
infrared bands from 3500-3200 cm.sup.-1 and 3100-2700 cm.sup.-1
were associated with the alcohol and amine functional groups of
NH(CH.sub.2CH.sub.2OH).sub.2, respectively. The alcohol and amine
bands of Example 1 were blue-shifted by approximately 200 cm.sup.-1
and 30 cm.sup.-1, respectively, relative to
NH(CH.sub.2CH.sub.2OH).sub.2. The observed blue-shifts support the
TGA data that indicate NH(CH.sub.2CH.sub.2OH).sub.2 was coordinated
to Fe(III). In addition, the larger blue-shift of the alcohol group
relative to the amine indicates that NH(CH.sub.2CH.sub.2OH).sub.2
was preferentially coordinated through the alcohol groups.
[0067] FIG. 11 illustrates Raman spectra (633 nm laser) of
NH(CH.sub.2CH.sub.2OH).sub.2 (line 1100), Example 1 (line 1102),
and Fe(CF.sub.3SO.sub.3).sub.3 (line 1104). The
Fe(CF.sub.3SO.sub.3).sub.3 symmetrical deformation and stretching
frequencies associated with CF.sub.3 (780 cm.sup.-1) and SO.sub.3
(1075 cm.sup.-1) respectively, were red-shifted in Example 1. The
foregoing was consistent with displacement of
CF.sub.3SO.sub.3.sup.- by NH(CH.sub.2CH.sub.2OH).sub.2 around the
Fe(III) centers. In addition, there was a peak in Example 1 (line
1100) around 300 cm.sup.-1 not associated with either
CF.sub.3SO.sub.3.sup.- or NH(CH.sub.2CH.sub.2OH).sub.2. This peak
around 300 cm.sup.-1 was associated with a Fe--O bond due to
preferential coordination through the alcohol group of
NH(CH.sub.2CH.sub.2OH).sub.2.
[0068] FIG. 12 depicts UV-Vis spectroscopic data of Example 1 in
1-butyl-3-methyl-imidazolium hexafluorophosphate (BMI-PF.sub.6) as
represented by line 1200. The high optical absorption of Example 1
prevented direct acquisition of UV-Vis spectroscopic data. As a
result, Example 1 was dissolved in BMI-PF.sub.6. A single
.lamda..sub.max (wavelength of maximum absorption) was observed at
477 nm. The position of the .lamda..sub.max at 477 nm, along with
value of an extinction coefficient (.epsilon.=63), suggested this
peak may be attributed to an iron d-d transition and was consistent
with preferential coordination through the alcohol groups. There
were no changes to the infrared and Raman difference spectra
(subtraction of solvent peaks) of Example 1 in BMI-PF.sub.6, and
there was no shift in .lamda..sub.max as a function of the
concentration of Example 1, suggesting that PF.sub.6.sup.- was not
coordinating to Fe(III).
[0069] The specific conductivity (.sigma.) of Example 1 (with <2
ppm H.sub.2O) at 25.degree. C. was 207 .mu.S cm.sup.-1. The
activation energy of conduction (E.sub.a), measured from an
Arrhenius plot, was 13.1 kcal mol.sup.-1. In comparison, typical
activation energy values for conduction in aqueous and molten metal
salts are .about.3-5 kcal mol.sup.-1. The data suggests significant
ion pairing was present in Example 1.
[0070] Cyclic voltammograms (CVs) of Example 1, which were
performed with a glassy carbon working electrode at four different
scan rates, are illustrated in FIG. 13. A scan rate of 50 mV/s is
represented by line 1300, a scan rate of 100 mV/s is represented by
line 1302, a scan rate of 200 mV/s is represented by line 1304, and
a scan rate of 400 mV/s is represented by line 1308. The high
viscosity and low conductivity of Example 1 prevented the direct
acquisition of electrochemical data. As a result, CV measurements
were performed on a 0.011 M solution of Example 1 in BMI-PF.sub.6,
and stability was confirmed by UV-Vis measurements. Example 1
displayed several quasi-reversible waves associated with
Fe(III)/Fe(II) reduction and oxidation, which were more resolved at
slower scan rates. To further resolve these peaks, an Osteryoung
square wave voltammogram was acquired on Example 1 as depicted by
line 1400 in FIG. 14. FIG. 14 illustrates the Osteryoung square
wave voltammogram of Example 1 with 1 mV steps, sweep width
amplitude of 25 mV, and sweep width frequency of 15 Hz. The results
show there were three waves. An explanation for this behavior is
that the pendant --CH.sub.2CH.sub.2OH groups of the
NH(CH.sub.2CH.sub.2OH).sub.2 ligands were coordinating to adjacent
Fe(III) centers. This is consistent with the magnetic
susceptibility measurement of Example 1 at 25.degree. C. showing
there was a small amount of antiferromagnetic coupling, where the
measured magnetic susceptibility (.chi..sub.meas) was 5.78 BM at
295 K and the calculated magnetic susceptibility (.chi..sub.calc)
was 5.90 BM.
Evaluation of Examples 2, 3A, 3B, 3C, 4, and 5
[0071] In Examples 2, 3A, 3B, 3C, 4, and 5, copper-, manganese-,
and zinc-based ionic liquids
(Cu{NH.sub.2CH.sub.2CH.sub.2OH}.sub.6[CH.sub.3(CH.sub.2).sub.3CH(C.sub.2H-
.sub.5)CO.sub.2].sub.2 (Example 2),
Cu{NH(CH.sub.2CH.sub.2OH).sub.2}.sub.6[CH.sub.3(CH.sub.2).sub.3CH(C.sub.2-
H.sub.5)CO.sub.2].sub.2 (Example 3A),
Cu{NH(CH.sub.2CH.sub.2OH).sub.2}.sub.6[CF.sub.3SO.sub.3].sub.2
(Example 3B),
Cu{NH(CH.sub.2CH.sub.2OH).sub.2}.sub.6[(CF.sub.350.sub.2).sub.2N].su-
b.2 (Example 3C),
Mn{NH(CH.sub.2CH.sub.2OH).sub.2}.sub.6[CF.sub.3SO.sub.3].sub.2
(Example 4), and
Zn{NH.sub.2CH.sub.2CH.sub.2OH}.sub.6[CF.sub.3SO.sub.3].sub.2
(Example 5)) were synthesized in single-step reactions Infrared
data suggested that ethanolamine preferentially coordinated to the
metal center through the amine group in Example 2 and the hydroxyl
group in Example 5. In addition, infrared data suggested that
diethanolamine coordinated through the amine group in Example 3A,
Example 3C, and Example 4 and the hydroxyl group in Example 3B.
Examples 2, 3A, 3B, 3C, 4, and 5 were viscous (>1000 cP) at room
temperature, but two of the Examples, namely Example 3C and Example
4, displayed specific conductivities that are reasonably high for
ionic liquids (>20 mS cm.sup.-1). Examples 2, 3A, 3B, 3C, 4, and
5 each displayed a glass transition (T.sub.g) below -50.degree. C.
The cyclic voltammograms (CVs) of Examples 2, 3A, 3B, and 3C
displayed a single quasi-reversible wave associated with
Cu(II)/Cu(I) reduction and re-oxidation, while the CV of Example 5
showed a wave attributed to Zn(II)/Zn(0) reduction and stripping.
Example 4 displayed reversible Mn(II)/Mn(III) oxidation and
re-reduction at 50 mV/s using a glassy carbon working
electrode.
[0072] Examples 2, 3A, 3B, 3C, 4, and 5 were synthesized by mixing
the metal salt (Cu[2-Et)C.sub.5COO].sub.2, Cu[OTf].sub.2,
Mn[OTf].sub.2, or Zn[OTf].sub.2) with six equivalents of either EA
or DEA. Additional heating drove the reactions to completion. A
color change was observed upon coordination of the ligand for
Examples 2, 3A, 3B, 3C, and 4, while a color change was not
observed for Example 5. The reactions were direct combination
reactions; thus, an analytically pure product was obtained in
quantitative yield with a single step. The metal to ligand ratio
was altered to four in the case of divalent copper and zinc, since
these metals can have six- or four-coordinate geometries. While
copper 2-ethylhexanoate and zinc triflate reacts with four
equivalents of EA, the resulting compounds were extremely viscous
and decomposed quickly (e.g., less than one day) in air.
[0073] Examples 2, 3A, 3B, 3C, 4, and 5 were evaluated by elemental
analysis, viscometry, TGA-DSC, infrared and UV-Visible
spectroscopy, and their electrochemical properties were evaluated
using impedance spectroscopy and cyclic voltammetry. In addition,
.sup.13C NMR was used to analyze Example 5.
[0074] Infrared spectroscopy was used to identify the preferred
mode of coordination (hydroxyl or amine) in each of the Examples.
Specifically, shifts in the stretching frequencies of the infrared
bands from 3500-3200 cm.sup.-1 (O--H) and 3100-2700 cm.sup.-1
(N--H) for the secondary amine, DEA, and from 3000-2800 cm.sup.-1
(O--H) and 3400-3100 cm.sup.-1 (N--H) for the primary amine, EA,
were measured. FIG. 15 illustrates infrared spectra of
Cu[(2-Et)C.sub.5COO].sub.2 (represented by line 1500), infrared
spectra of Example 2 (represented by line 1502), and infrared
spectra of EA (represented by line 1504). The hydroxyl and amine
bands of Example 2 were blue-shifted by 15 cm.sup.-1 and 60
cm.sup.-1, respectively, relative to EA. The larger shift of the
amine group relative to the hydroxyl suggested EA preferentially
coordinated through the amine. Similar observations were made for
Examples 3A, 3B, 3C, 4, and 5, and the results are summarized in
Table 1. Examples 1, 3B, and 5 preferentially coordinated through
the hydroxyl group(s), while Examples 2, 3A, 3C, and 4 coordinated
through the amine.
[0075] It is noted that although the cations of Examples 3A, 3B,
and 3C all included one Cu(II) and six EA ligands, the preferred
mode of coordination varied as a function of the anion (e.g.,
2-ethylhexanoate, triflate, or triflimide). FIG. 16 depicts
infrared spectra of Example 3A (illustrated by line 1602), infrared
spectra of Example 3B (illustrated by line 1604), and infrared
spectra of Example 3C (illustrated by line 1606) each compared to
infrared spectra of neat DEA (illustrated by line 1600).
Accordingly, FIG. 16 shows the variation in preferred mode of
coordination as a function of the anion (e.g., larger shifts of the
amine groups relative to the hydroxyl groups for Examples 3A and 3C
and larger shift of the hydroxyl group relative to the amine group
for Example 3B).
TABLE-US-00001 TABLE 1 Example Hydroxyl shift (cm.sup.-1) Amine
shift (cm.sup.-1) 1 200 30 2 15 60 .sup. 3A 10 80 .sup. 3B 50 10
.sup. 3C 5 20 4 5 30 5 30 5
[0076] FIG. 17 illustrates solution .sup.13C NMR spectra of Example
5 (represented by line 1702) and EA (represented by line 1700)
dissolved in CD.sub.3CN (with the solvent as the internal
reference). There were no changes in the UV-Vis spectrum of Example
5 (neat) and of Example 5 dissolved in CD.sub.3CN, suggesting that
CD.sub.3CN was not coordinated to zinc. There were upfield shifts
of 2.0 ppm and 1.0 ppm for the oxymethylene and aminomethylene
groups, respectively, of Example 5 relative to EA alone. The larger
shifts associated with the oxymethylene group in the .sup.13C NMR
spectra suggested coordination of the zinc was predominately
through the hydroxyl moiety of EA, and this was consistent with the
infrared data. The paramagnetic metal centers of Examples 2, 3A,
3B, 3C, and 4 prevented the acquisition of NMR on these
compounds.
[0077] Physicochemical properties of the Examples 2, 3A, 3B, 3C, 4,
and 5 as well as Example 1 are summarized in Table 2. Similar to
Example 1, high optical absorbance of Examples 2, 3A, 3B, 3C, and 4
prevented direct acquisition of UV-Vis spectroscopic data.
Therefore, Examples 2, 3A, 3B, 3C, and 4 were dissolved in
1-butyl-3-methyl-imidazolium hexafluorophosphate (BMI-PF6), and the
resulting solutions were monitored by infrared difference spectra
(subtraction of solvent peaks) to provide that PF.sub.6.sup.- was
not coordinating to the metal centers. Each compound displayed a
single .lamda..sub.max (wavelength of maximum absorption) in the
visible range attributable to d-d transitions with extinction
coefficients (.epsilon.) that ranged from 31-244 M.sup.-1
cm.sup.-1. The UV-Vis spectrum of 5 (neat) revealed a single
.lamda..sub.max at 312 nm.
TABLE-US-00002 TABLE 2 .epsilon. E.sub.a [M.sup.-1 .lamda..sub.max
.rho. [g .sigma. [mS [kcal T.sub.g T.sub.d .chi..sub.calc
.chi..sub.meas Example cm.sup.-1] [nm] .mu. [cP] mL.sup.-1]
cm.sup.-1] mol.sup.-1] [.degree. C.] [.degree. C.] [BM] [BM] 1 63
477 4482 1.44 0.207 13.1 -64 260 5.90 5.78 2 244 635 3383 1.14
0.045 12.2 -63 190 1.73 1.50 3A 31 648 12313 1.21 0.014 11.6 -54
230 1.73 1.72 3B 52 658 1295 1.30 0.067 11.2 -65 240 1.73 1.72 3C
96 643 13900 1.52 30.2 4.8 -54 255 1.73 1.70 4 105 384 11671 1.19
624 1.1 -52 215 5.92 5.63 5 neat 312 2533 1.38 0.341 14.6 -84 155 0
0
[0078] Magnetic susceptibility measurements (.chi..sub.meas) were
used to confirm the oxidation state of the metals, and results of
such measurements are summarized in Table 2. No oxidation state
changes were observed upon coordinating the ligands to the metal
center. Antiferromagnetic coupling was observed in Example 1 (iron
compound) and Example 4 (manganese compound) at room
temperature.
[0079] All of the compounds were viscous (at 25.degree. C. and less
than 2 ppm H.sub.2O) with viscosity (.mu.) numbers falling over a
wide range (1295-13900 cP, see Table 2). Although the cations of
Examples 3A, 3B, and 3C each included a divalent copper center
surrounded by six DEA ligands, the viscosity values span the
highest and lowest numbers observed. Since Examples 3A, 3B, and 3C
included different anions, it is shown that viscosity was strongly
influenced by the nature of the anion. A comparison of viscosity of
Examples 3A and 3C versus Example 3B suggested the cation also had
significant influence. Examples 3A and 3C displayed preferential
amine coordination leaving only hydroxyl groups in the secondary
coordination sphere. In contrast, Example 3B displayed primarily
hydroxyl coordination to the copper center, leaving both hydroxyl
and amine groups in the secondary coordination sphere (see FIG. 3).
As a result, the cation of Example 3B had lower symmetry than the
cations of Examples 3A and 3C. Lower symmetry cations can lead to a
decrease in ion pairing (and an increase in entropy) that in turn
leads to lower viscosity. It is noted that this trend holds for the
Examples set forth herein regardless of the metal. The Examples
containing DEA ligands that displayed preferential hydroxyl
coordination had an order of magnitude lower viscosity than those
with amine coordination. Although both Example 1 and Example 3B had
hydroxyl-coordinated DEA ligands, Example 1 had higher viscosity
than Example 3B because it had trivalent iron whereas Example 3B
had divalent copper and therefore lower charge density.
[0080] The specific conductivities of Examples 2, 3A, and 3B (at
25.degree. C. and less than 2 ppm H.sub.2O) were low (Table 2) and
were an order of magnitude lower than Example 1, while Example 5
was about the same as Example 1. This was consistent with the
viscosity of the compounds. Although Examples 3C and 4 were also
viscous, they displayed two and three orders of magnitude improved
conductivities, respectively. While this generally opposed the
trend that lower viscosity leads to higher conductivity,
aggregation and correlated ionic motions can lead to anomalies that
are not easily predicted by modeling and simulation studies. The
activation energies of conduction (E.sub.a), measured from
Arrhenius plots, ranged from 11.2-14.5 kcal mol.sup.-1 for the
Examples with low specific conductivity (Examples 1, 2, 3A, 3B, and
5) and 1.1-4.8 kcal mol.sup.-1 for the Examples with high
conductivity (Examples 3C and 4). These values were consistent with
the fact that typical activation energy values for highly
conductive aqueous and molten metal salts are .about.3-5 kcal
mol.sup.-1.
[0081] The stability of the Examples was evaluated by
thermogravimetric analysis (TGA) and the onset temperature for
decomposition (T.sub.d) was measured. The results are reported in
Table 2. Under identical conditions, the onset of mass loss for EA
and DEA is 170.degree. C. and 200.degree. C., respectively. The
T.sub.d values for the Examples reported in Table 2 were higher
than the pure ligand, suggesting that the ligands were coordinated
to the metal centers and the metal salts were not simply dissolved
in either EA or DEA. Differential scanning calorimetry (DSC) was
also used in order to determine melting (T.sub.m) and/or glass
transition (T.sub.g) temperatures. Each of the Examples displayed a
single T.sub.g below -50.degree. C. that was indicative of an
amorphous glass forming a liquid upon heating.
[0082] FIGS. 18-22 depict various cyclic voltammograms (CVs) of the
Examples and results are summarized in Table 3. FIG. 18 illustrates
CVs of Example 2 (represented by line 1800) and Example 3A
(represented by line 1802), performed on 0.1 M solutions in
BMI-PF.sub.6 with a glassy carbon (working) electrode (50 mV/s).
Both Example 2 and Example 3A were formed from
Cu[(2-Et)C.sub.5COO].sub.2 and displayed a single quasi-reversible
wave associated with Cu(II)/Cu(I) reduction and re-oxidation, but
the EA ligands of Example 2 provided improved reversibility over
Example 3A which instead contained DEA ligands. This result is
likely indicative of differences in the kinetics associated with
the coordination and disassociation of the two different ligands
particularly since Cu(II) tends to adopt a six-coordinate
tetragonal geometry whereas Cu(I) tends to be four-coordinate
(tetrahedral).
[0083] FIG. 19 shows CVs of Example 3A (represented by line 1902)
and Example 3C (represented by line 1900) in BMI-PF.sub.6 at a 50
mV/s scan rate and with a glassy carbon working electrode. Example
3A and Example 3C differed only in anion, and are shown under
identical conditions in FIG. 19. The single quasi-reversible
Cu(II)/Cu(I) wave observed in both complexes was less reversible in
Example 3A as a result of a negative shift in the reduction peak.
This was likely caused by increased ion pairing in
[(2-Et)C.sub.5COO].sup.< (Example 3A) relative to
[NTf.sub.2].sup.- (Example 3C).
[0084] FIG. 20 illustrates CVs of Example 4 in BMI-PF.sub.6 at
three different scan rates with a glassy carbon working electrode.
In FIG. 20, line 2000 represents a scan rate of 50 mV/s, line 2002
represents a scan rate of 100 mV/s, and line 2004 represents a scan
rate of 200 mV/s. In contrast to the copper Examples (Examples 2,
3A, 3B, and 3C), the manganese-based Example 4 displayed a single
reversible (at 50 mV/s scan rate) wave associated with
Mn(II)/Mn(III) oxidation and re-reduction (line 2000). The currents
increased approximately with the square root of the scan rate (up
to 200 mV/s), suggesting the process was diffusive.
[0085] FIG. 21 depicts a CV of Example 5 (seven cycles, 0.01 M in
BMI-PF.sub.6, Au working electrode, 100 mV/s) at 2100. A single
quasi-reversible wave associated with Zn(II)/Zn(0) reduction and
stripping was observed. The two-electron process was confirmed by
bulk electrolysis. It is noted that stripping was not observed with
a Pt working electrode as illustrated in FIG. 22, suggesting that
the surface chemistry of the working electrode influenced this
process. FIG. 22 shows CVs of Example 5 in BMI-PF.sub.6 at 100 mV/s
with a Pt working electrode (represented by line 2200) and an Au
working electrode (represented by line 2202).
TABLE-US-00003 TABLE 3 Example E.sub.pc (mV) E.sub.pa (mV) .DELTA.E
(mV) 2 -278 -34 244 .sup. 3A -410 112 522 .sup. 3B -328 238 566
.sup. 3C -82 125 207 4 -23 35 58 5 -563 -228 335
[0086] Generally, Examples 1, 2, 3A, 3B, 3C, 4, and 5 were
metal-containing ionic liquids (MetILs) prepared by the reaction of
metal salts (Fe[OTf].sub.3, Cu[(2-Et)C.sub.5COO]2 , Cu[OTf].sub.2,
Mn[OTf].sub.2, or Zn[OTf].sub.2) with six equivalents of either EA
or DEA. Single-step syntheses produced Examples 1, 2, 3A, 3B, 3C,
4, and 5. The results show that Examples 1, 2, 3A, 3B, 3C, 4, and 5
are members of a family of MetILs. Moreover, it has been
demonstrated that select physicochemical properties can be
systematically altered by the appropriate choice of ligand(s) and
anion(s). The Examples set forth herein are viscous, but Examples
3C and 4 displayed orders of magnitude improvement in conductivity
over Example 1, and Example 4 displayed reversible
electrochemistry.
[0087] Further, as used herein, the term "exemplary" is intended to
mean "serving as an illustration or example of something."
[0088] What has been described above includes examples of one or
more embodiments. It is, of course, not possible to describe every
conceivable modification and alteration of the above devices or
methodologies for purposes of describing the aforementioned
aspects, but one of ordinary skill in the art can recognize that
many further modifications and permutations of various aspects are
possible. Accordingly, the described aspects are intended to
embrace all such alterations, modifications, and variations that
fall within the spirit and scope of the appended claims.
Furthermore, to the extent that the term "includes" is used in
either the details description or the claims, such term is intended
to be inclusive in a manner similar to the term "comprising" as
"comprising" is interpreted when employed as a transitional word in
a claim.
* * * * *