U.S. patent application number 17/078633 was filed with the patent office on 2021-04-22 for materials for high-performance aqueous organic redox flow batteries.
The applicant listed for this patent is UNIVERSITY OF SOUTHERN CALIFORNIA. Invention is credited to ROBERT ANISZFELD, LENA HOOBER-BURKHARDT, SANKARGANESH KRISHNAMOORTHY, ADVAITH MURALI, SRI NARAYAN, ARCHITH NIRMALCHANDAR, SURYA G. PRAKASH, BO YANG.
Application Number | 20210119240 17/078633 |
Document ID | / |
Family ID | 1000005307319 |
Filed Date | 2021-04-22 |
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United States Patent
Application |
20210119240 |
Kind Code |
A1 |
NARAYAN; SRI ; et
al. |
April 22, 2021 |
MATERIALS FOR HIGH-PERFORMANCE AQUEOUS ORGANIC REDOX FLOW
BATTERIES
Abstract
Quinones and related compounds for use in flow batteries are
provided. Many of these compounds are found to mitigate the effects
of crossover in a flow battery. Other structure for improving
battery performance is provided.
Inventors: |
NARAYAN; SRI; (ARCADIA,
CA) ; PRAKASH; SURYA G.; (HACIENDA HEIGHTS, CA)
; ANISZFELD; ROBERT; (LOS ANGELES, CA) ; YANG;
BO; (LOS ANGELES, CA) ; HOOBER-BURKHARDT; LENA;
(LOS ANGELES, CA) ; KRISHNAMOORTHY; SANKARGANESH;
(LOS ANGELES, CA) ; MURALI; ADVAITH; (LOS ANGELES,
CA) ; NIRMALCHANDAR; ARCHITH; (LOS ANGELES,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
UNIVERSITY OF SOUTHERN CALIFORNIA |
LOS ANGELES |
CA |
US |
|
|
Family ID: |
1000005307319 |
Appl. No.: |
17/078633 |
Filed: |
October 23, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15723071 |
Oct 2, 2017 |
10833345 |
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17078633 |
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62403004 |
Sep 30, 2016 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
Y02E 60/50 20130101;
H01M 4/96 20130101; H01M 8/20 20130101; H01M 2300/0014 20130101;
H01M 2300/0082 20130101; H01M 8/188 20130101; H01M 2250/10
20130101; H01M 2008/1095 20130101 |
International
Class: |
H01M 8/18 20060101
H01M008/18; H01M 8/20 20060101 H01M008/20 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] The invention was made with Government support under
Contract No. DE-AR0000337 awarded by the Department of Energy. The
Government has certain rights to the invention.
Claims
1-14. (canceled)
15. A flow battery comprising: a positive electrode; a positive
electrode electrolyte including water and a first redox couple, the
positive electrode electrolyte flowing over and contacting the
positive electrode, the first redox couple including an organic
compound having formula II and a reduction product of the organic
compound, the organic compound having formula II being reduced
during discharge; a negative electrode; and a negative electrode
electrolyte including water and a second redox couple, the negative
electrode electrolyte flowing over and contacting the positive
electrode, the second redox couple also including an organic
compound having formula II, a reduction product of the organic
compound having formula I being oxidized to the organic compound
during discharge: A-X--B II wherein: A is a substituted or
unsubstituted flavins with substituents, quinoxaline with and
without substituents, substituted anthraquinones, and substituted
naphthaquinones; and B is a substituted quinone.
16. The flow battery of claim 15 further comprising an ion exchange
membrane interposed between the positive electrode and the negative
electrode.
17. The flow battery of claim 16 wherein the ion exchange membrane
has an equivalent weight greater than 1000 with respect to
protogenic groups.
18. The flow battery of claim 15 wherein the positive electrode
and/or the negative electrode include multi-walled carbon
nanotubes.
19. The flow battery of claim 15 wherein the positive electrode and
the negative electrode each independently includes water-soluble
materials.
20. The flow battery of claim 19 wherein the water-soluble
materials include carbon and a binder material.
21. The flow battery of claim 15 wherein the positive electrode
electrolyte and the negative electrode electrolyte each
independently include a molecule with different redox properties in
various ranges of potentials.
22. A flow battery comprising: a positive electrode; a positive
electrode electrolyte including water and a first redox couple, the
positive electrode electrolyte flowing over and contacting the
positive electrode, the first redox couple including a first
organic compound and a reduction product of the first organic
compound, the first organic compound being reduced during
discharge: a negative electrode; and a negative electrode
electrolyte including water and a second redox couple, the negative
electrode electrolyte flowing over and contacting the positive
electrode, the second redox couple also including a second organic
compound, a reduction product of the second organic compound being
oxidized to the second organic compound during discharge, one or
both of the first organic compound and the second organic compound
including a compound selected from the group consisting of:
##STR00011## and combinations thereof wherein R.sub.4, R.sub.5,
R.sub.6, and R.sub.7 are each independently H, ##STR00012##
--SO.sub.3H, --SO.sub.3Na, --SO.sub.3K, --CO.sub.2H, --CO.sub.2Na,
--CO.sub.2K, --OH, --OMe, alkyl, (CH.sub.2CH.sub.2O).sub.nOMe,
NH.sub.2, NMe.sub.2, I, Cl, Br, F, --CF.sub.3, --CF.sub.2H,
--NO.sub.2, --N(R).sub.3.sup.+X.sub.1.sup.-, CCl.sub.3, --CN,
--PO.sub.3H.sub.2, --COOH, --O.sup.-M.sup.+.sub.2,
--SO.sub.3.sup.-1M.sup.+, --PO.sub.3.sup.2-M.sup.+.sub.2,
--COO.sup.-M.sup.+, --COOR, --CHO, or --COR where R is H or
C.sub.1-10 alkyl, M.sup.+ is a positively charged counter-ion, and
X.sub.1.sup.- is a negatively charge counter-ion.
23. A flow battery comprising: a positive electrode; a positive
electrode electrolyte including water and a first redox couple, the
positive electrode electrolyte flowing over and contacting the
positive electrode, the first redox couple including a first
organic compound and a reduction product of the first organic
compound, the first organic compound being reduced during
discharge: a negative electrode; and a negative electrode
electrolyte including water and a second redox couple, the negative
electrode electrolyte flowing over and contacting the positive
electrode, the second redox couple also including a second organic
compound, a reduction product of the second organic compound being
oxidized to the second organic compound during discharge, one or
both of the first organic compound and the second organic compound
including a compound selected from the group consisting of:
##STR00013## and combinations thereof.
24. The flow battery of claim 23 further comprising an ion exchange
membrane interposed between the positive electrode and the negative
electrode.
25. The flow battery of claim 24 wherein the ion exchange membrane
has an equivalent weight greater than 1000 with respect to
protogenic groups.
26. The flow battery of claim 23 wherein the positive electrode
and/or the negative electrode include multi-walled carbon
nanotubes.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. provisional
application Ser. No. 62/403,004 filed Sep. 30, 2016, the disclosure
of which is hereby incorporated in its entirety by reference
herein.
TECHNICAL FIELD
[0003] The present invention is related to metal-free organic redox
flow batteries.
BACKGROUND
[0004] The integration of electrical energy generated from solar
and wind power into the grid is faced with the challenge of
intermittent electricity output from these sources. This challenge
can be effectively met by storing the electricity during times of
excess production and releasing the electrical energy to the grid
during times of peak demand. Rechargeable batteries are very
attractive for energy storage because of their high energy
efficiency and scalability. Since grid-scale electrical energy
storage requires hundreds of gigawatt-hours to be stored, the
batteries for this application must be inexpensive, robust, safe
and sustainable. None of today's mature battery technologies meet
all of these requirements.
[0005] Accordingly, there is a need for improvements in redox flow
batteries that are inexpensive, robust, safe and sustainable.
SUMMARY
[0006] The present invention solved one or more problems of the
prior art by providing a flow battery that uses two different
water-soluble organic redox couples to store electrical energy. The
flow battery includes a positive electrode, a positive electrode
electrolyte, a negative electrode, and a negative electrode
electrolyte. The positive electrode electrolyte includes water and
a first redox couple with the positive electrode electrolyte
flowing over and contacting the positive electrode. The first redox
couple includes an organic compound having formula I and a
reduction product of the organic compound where the organic
compound having formula I is reduced during discharge. Similarly,
the negative electrode electrolyte includes water and a second
redox couple, the negative electrode electrolyte flowing over and
contacting the positive electrode. The second redox couple also
includes an organic compound having formula I where a reduction
product of the organic compound having formula I is oxidized during
discharge:
##STR00001##
wherein: p is 0, 1, 2, 3, or 4; o is 0, 1, 2, or 3; r is 0, 1, 2,
or 3; R.sub.1, R.sub.2, R.sub.3 are each independently --SO.sub.3H,
--SO.sub.3Na, --SO.sub.3K, --CO.sub.2H, --CO.sub.2Na, --CO.sub.2K,
--OH, --OMe, alkyl, (CH.sub.2CH.sub.2O).sub.nOMe, NH.sub.2,
NMe.sub.2, I, Cl, Br, F, --CF.sub.3, --CF.sub.2H, --NO.sub.2,
--N(R).sub.3.sup.+X.sub.1.sup.-, CCl.sub.3, --CN,
--PO.sub.3H.sub.2, --COOH, --SO.sub.3.sup.-M.sup.+,
--PO.sub.3.sup.2M.sup.+.sub.2, --COO.sup.-M.sup.+, --COOR, --CHO,
or --COR where R is H or C.sub.1-10 alkyl, M.sup.+ is a positively
charged counter-ion, and X.sub.1.sup.- is a negatively charge
counter-ion; X is --(CH.sub.2).sub.n, --(CH.sub.2OCH.sub.2).sub.n,
C.sub.4-15 branched alkyl chains or other substituted alkyl chains;
and n is 1 to 10.
[0007] In another embodiment, a flow battery is provided. The flow
battery includes a positive electrode, a positive electrode
electrolyte, a negative electrode, and a negative electrode
electrolyte. The positive electrode electrolyte includes water and
a first redox couple. The positive electrode electrolyte flows over
and contacts the positive electrode. The first redox couple
includes an organic compound having formula II and a reduction
product of the organic compound. Characteristically, the organic
compound having formula II is reduced during discharge. The
negative electrode electrolyte includes water and a second redox
couple. The negative electrode electrolyte flows over and contacts
the positive electrode. The second redox couple also includes an
organic compound having formula II. Characteristically, a reduction
product of the organic compound having formula II is oxidized to
the organic compound during discharge:
A-X--B II
wherein: A is a substituted or unsubstituted flavins with
substituents, quinoxaline with and without substituents,
substituted anthraquinones, and substituted naphthoquinones; and B
is a substituted quinone. The organic compound having formula II in
the first redox couple and the second couple can be the same or
different.
[0008] In another embodiment, a flow battery is provided. The flow
battery includes a positive electrode, a positive electrode
electrolyte, a negative electrode, and a negative electrode
electrolyte. The positive electrode electrolyte including water and
a first redox couple. The positive electrode electrolyte flows over
and contacts the positive electrode. The first redox couple
includes a first organic compound and a reduction product of the
first organic compound. Characteristically, the first organic
compound is reduced during discharge. The negative electrode
electrolyte includes water and a second redox couple. The negative
electrode electrolyte flows over and contacts the positive
electrode. The second redox couple also includes a second organic
compound. Characteristically, a reduction product of the second
organic compound is oxidized to the second organic compound during
discharge, one or both of the first organic compound and the second
organic compound including a compound selected from the group
consisting of:
##STR00002##
and combinations thereof wherein R.sub.4, R.sub.5, R.sub.6, and
R.sub.7 are each independently H,
##STR00003##
--SO.sub.3H, --SO.sub.3Na, --SO.sub.3K, --CO.sub.2H, --CO.sub.2Na,
--CO.sub.2K, --OH, --OMe, alkyl, (CH.sub.2CH.sub.2O).sub.nOMe,
NH.sub.2, NMe.sub.2, I, Cl, Br, F, --CF.sub.3, --CF.sub.2H,
--NO.sub.2, --N(R).sub.3.sup.+X.sub.1.sup.-, CCl.sub.3, --CN,
--PO.sub.3H.sub.2, --COOH, --SO.sub.3.sup.-M.sup.+,
--PO.sub.3.sup.2-M.sup.+.sub.2, --COO.sup.-M.sup.+,
--COO.sup.-M.sup.+, --COOR, --CHO, or --COR where R is H or
C.sub.1-10 alkyl, M.sup.+ is a positively charged counter-ion, and
X.sub.1.sup.- is a negatively charge counter-ion.
[0009] In still another embodiment, a flow battery that is
resistant to the Michael reaction is provided. The flow battery
includes a positive electrode, a positive electrode electrolyte,
negative electrode, and a negative electrode electrolyte. The
positive electrode electrolyte includes water and a first redox
couple. The positive electrode electrolyte flows over and contacts
the positive electrode. Characteristically, the first redox couple
includes a first organic compound and a reduction product of the
first organic compound where the first organic compound is reduced
during discharge. The negative electrode electrolyte includes water
and a second redox couple. The negative electrode electrolyte flows
over and contacts the positive electrode. Characteristically, the
second redox couple also includes a second organic compound. A
reduction product of the second organic compound is oxidized to the
second organic compound during discharge. One or both of the first
organic compound and the second organic compound include a compound
selected from the group consisting of:
##STR00004##
and combinations thereof.
[0010] Advantageously, the flow battery of this embodiment uses
only organic materials without any metals is expected to be
inexpensive, robust and eco-friendly for large-scale storage of
electrical energy generated by solar photovoltaic and wind
turbines. Advantageously, embodiments of the invention address the
problem of crossover by using a single molecule that is
bifunctional and can operate at both electrodes. Therefore, even if
crossover occurred from the positive to the negative side, the
performance will not be affected. Some embodiments address the
issue of crossover of redox active species by tailoring the size of
the molecules used at the positive to the negative side by the use
of bulky substituent groups. Transformation and degradation of the
redox active materials is reduced by using new molecular
architectures that do not undergo the Michael Reaction.
Characteristically, embodiments of the invention use identical
solution mixture of molecules of redox couples at both the positive
and negative electrodes and ion exchange membranes that have the
appropriate water content that ensures that the size of the
hydrophilic clusters is small enough to prevent molecular
crossover. Moreover, the present invention certain embodiments can
operate in an alkaline environment that allows the cost of the
battery to be lowered. Finally, certain electrode structures of the
invention allow for the increased utilization of the active
materials and improved transport of materials to the surface of the
electrode.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 provides a schematic illustration of a flow battery
that includes electrolytes that include quinones and
hydroquinones.
[0012] FIG. 2. Structure of Bi-functional redox material,
2-anthraquinonemethyl (p-benzoquinol sulfonic acid) or AMBSA for
short. A is the negative electrode portion and B the positive
electrode portion.
[0013] FIG. 3A. Reactions of the redox couple during charge and
discharge at the positive and negative electrode.
[0014] FIG. 3B. Synthesis of bifunctional redox molecule.
[0015] FIG. 3C. Synthesis of bifunctional redox molecule.
[0016] FIG. 4. Cyclic voltammogram of AMBSA showing two sets of
redox peaks.
[0017] FIG. 5. Rotating disk electrode study of AMBSA in sulfuric
acid electrolyte.
[0018] FIG. 6. Electrochemical properties of substituted
anthraquinones over the potential range of -0.5 V to 1.0 V vs. the
mercury sulfate reference electrode at a concentration of 1 mM.
[0019] FIG. 7. Rate of crossover of various substances across
NAFION.RTM. 117 membrane.
[0020] FIG. 8. Charge and discharge of cell with a mixture of
DMDHMS and AQDS on both sides.
[0021] FIG. 9. Cluster of hydrophilic regions in NAFION.RTM.
membrane through which water soluble organic molecules are expected
to crossover.
[0022] FIG. 10. Capacity fade of 0.1 DMDHMS/0.1M AQDS cell with
NAFION.RTM. 117 and NAFION.RTM. 324 membranes.
[0023] FIG. 11. Properties of PSSA-PVDF membranes with low water
content and reduced rate of crossover.
[0024] FIG. 12. Schematic of the Organic Redox Flow Battery with
alkaline electrolyte.
[0025] FIG. 13. Cyclic voltammogram of 0.001 M DHA in 1 M potassium
hydroxide at a glassy carbon electrode.
[0026] FIG. 14. Cyclic voltammogram of 0.001 M AQDS in 1M potassium
hydroxide.
[0027] FIG. 15. Cycling experiment of organic redox flow battery
using alkaline solutions of 0.2M DHA, 0.2M AQDSH in 1M NaOH charged
and discharged at 8 mA/cm2.
[0028] FIG. 16. Performance of HQS/DHA cell using alkaline
electrolyte charged at 200 mA and discharged at 50 mA.
[0029] FIG. 17. Electrochemical behavior (a cyclic voltammogram) of
quinoxaline in alkaline electrolyte (1 M potassium hydroxide).
[0030] FIG. 18. Cyclic voltammogram of 6-methylquinoxaline in 1 M
potassium hydroxide.
[0031] FIG. 19. Flavin group of compounds with various substituents
for R=ribose,
[0032] FIG. 20A. Cyclic voltammogram of riboflavin-5'-phosphate
sodium hydrate in 1M potassium hydroxide.
[0033] FIG. 20B. Cyclic voltammogram of riboflavin in 1M potassium
hydroxide.
[0034] FIG. 21. Michael Reaction on Benzoquinonedisulfonic
acid.
[0035] FIG. 22. Electrochemical Properties of Selected Molecules
that are resistant to the Michael Reaction.
[0036] FIGS. 23A and 23B. Electrochemical properties of dimethyl
substituted ortho and para-benzoquinone monosulfonic acid.
[0037] FIG. 24. Electrochemical Properties of
dimethyldihydroxybenzene disulfonic acid.
[0038] FIG. 25. Cycling of redox flow cells with 0.5 M
2,5-dimethyl-1,4-dihydroxybenzene-3-sulfonic acid and 0.5 M
anthraquinone-2,7-disulfonic acid.
[0039] FIG. 26. Mechanism of the Michael Addition reaction.
[0040] FIG. 27. Compounds that resist Michael Addition and suitable
for use in alkaline electrolyte.
[0041] FIG. 28. Benzene 1,2-dihydroxy-4,6-dimethyl-3,5-disulfonic
acid Example of substituted compound with multiple functional
groups that can resist Michael Addition reaction.
[0042] FIG. 29. Electrochemical behavior of HQTC in 1 M potassium
hydroxide.
[0043] FIG. 30. The electrochemical behavior of HQTC and DHA in 1 M
potassium hydroxide showing the cell voltage that can be realized
from this type of redox flow battery.
[0044] FIG. 31. Cycling of purpurin-AQDS cell in alkaline
electrolyte.
[0045] FIG. 32. Performance of aqueous organic redox flow battery
with carbon-nanotube modified electrodes.
DETAILED DESCRIPTION
[0046] Reference will now be made in detail to presently preferred
compositions, embodiments and methods of the present invention,
which constitute the best modes of practicing the invention
presently known to the inventors. The Figures are not necessarily
to scale. However, it is to be understood that the disclosed
embodiments are merely exemplary of the invention that may be
embodied in various and alternative forms. Therefore, specific
details disclosed herein are not to be interpreted as limiting, but
merely as a representative basis for any aspect of the invention
and/or as a representative basis for teaching one skilled in the
art to variously employ the present invention.
[0047] Except in the examples, or where otherwise expressly
indicated, all numerical quantities in this description indicating
amounts of material or conditions of reaction and/or use are to be
understood as modified by the word "about" in describing the
broadest scope of the invention. Practice within the numerical
limits stated is generally preferred. Also, unless expressly stated
to the contrary: all R groups (e.g. R.sub.i where i is an integer)
include alkyl, lower alkyl, C.sub.1-6 alkyl, C.sub.6-10 aryl, or
C.sub.6-10 heteroaryl; single letters (e.g., "n" or "o") are 1, 2,
3, 4, or 5; percent, "parts of," and ratio values are by weight;
the term "polymer" includes "oligomer," "copolymer," "terpolymer,"
and the like; molecular weights provided for any polymers refers to
weight average molecular weight unless otherwise indicated; the
description of a group or class of materials as suitable or
preferred for a given purpose in connection with the invention
implies that mixtures of any two or more of the members of the
group or class are equally suitable or preferred; description of
constituents in chemical terms refers to the constituents at the
time of addition to any combination specified in the description,
and does not necessarily preclude chemical interactions among the
constituents of a mixture once mixed; the first definition of an
acronym or other abbreviation applies to all subsequent uses herein
of the same abbreviation and applies mutatis mutandis to normal
grammatical variations of the initially defined abbreviation; and,
unless expressly stated to the contrary, measurement of a property
is determined by the same technique as previously or later
referenced for the same property.
[0048] It is also to be understood that this invention is not
limited to the specific embodiments and methods described below, as
specific components and/or conditions may, of course, vary.
Furthermore, the terminology used herein is used only for the
purpose of describing particular embodiments of the present
invention and is not intended to be limiting in any way.
[0049] It must also be noted that, as used in the specification and
the appended claims, the singular form "a," "an," and "the"
comprise plural referents unless the context clearly indicates
otherwise. For example, reference to a component in the singular is
intended to comprise a plurality of components.
[0050] The term "comprising" is synonymous with "including,"
"having," "containing," or "characterized by." These terms are
inclusive and open-ended and do not exclude additional, unrecited
elements or method steps.
[0051] The phrase "consisting of" excludes any element, step, or
ingredient not specified in the claim. When this phrase appears in
a clause of the body of a claim, rather than immediately following
the preamble, it limits only the element set forth in that clause;
other elements are not excluded from the claim as a whole.
[0052] The phrase "consisting essentially of" limits the scope of a
claim to the specified materials or steps, plus those that do not
materially affect the basic and novel characteristic(s) of the
claimed subject matter.
[0053] With respect to the terms "comprising," "consisting of," and
"consisting essentially of," where one of these three terms is used
herein, the presently disclosed and claimed subject matter can
include the use of either of the other two terms.
[0054] As used herein "alkyl" refers to C.sub.1-20 inclusive,
linear (i.e., "straight-chain"), branched, saturated or at least
partially and in some cases fully unsaturated (i.e., alkenyl and
alkynyl) hydrocarbon chains, including for example, methyl, ethyl,
propyl, isopropyl, butyl, isobutyl, tert-butyl, pentyl, hexyl,
octyl, ethenyl, propenyl, butenyl, pentenyl, hexenyl, octenyl,
butadienyl, propynyl, butynyl, pentynyl, hexynyl, heptynyl, and
allenyl groups. "Branched" refers to an alkyl group in which a
lower alkyl group, such as methyl, ethyl or propyl, is attached to
a linear alkyl chain. "Lower alkyl" refers to an alkyl group having
1 to about 8 carbon atoms (i.e., a C.sub.1-8 alkyl), e.g., 1, 2, 3,
4, 5, 6, 7, or 8 carbon atoms. "Higher alkyl" refers to an alkyl
group having about 10 to about 20 carbon atoms, e.g., 10, 11, 12,
13, 14, 15, 16, 17, 18, 19, or 20 carbon atoms. The alkyl group can
be optionally substituted (i.e., a "substituted alkyl") with
another atom or functional group such as alkyl, substituted alkyl,
halogen, aryl, substituted aryl, alkoxyl, hydroxyl, nitro, amino,
alkylamino, dialkylamino, sulfate, mercapto, and the like.
[0055] As used herein "aryl" means a monovalent aromatic
hydrocarbon having a single ring (i.e., phenyl) or fused rings
(i.e., naphthalene). In a refinement, such aryl groups include from
6 to 12 carbon ring atoms. In another refinement, such aryl groups
include 6 to 10 carbon ring atoms. Representative aryl groups
include, by way of example, phenyl biphenyl, naphthyl, anthranyl,
and naphthalene-1-yl, naphthalene-2-yl, and the like. The term
"arylene" means a divalent aryl group.
[0056] As used herein "heteroaryl" means a monovalent aromatic
group having a single ring or two fused rings and containing in the
ring at least one heteroatom (typically 1 to 3 heteroatoms)
selected from nitrogen, oxygen or sulfur. In a refinement,
heteroaryl groups typically contain from 5 to 10 total ring atoms.
In a refinement, heteroaryl groups have from 6 to 16 total ring
atoms. In a refinement, the heteroaryl is a C.sub.5-12 heteroaryl.
Examples of heteroaryl include, but are not limited to, monovalent
species of pyrrole, imidazole, thiazole, oxazole, furan, thiophene,
triazole, pyrazole, isoxazole, isothiazole, pyridine, pyrazine,
pyridazine, pyrimidine, triazine, indole, benzofuran,
benzothiophene, benzimidazole, benzthiazole, quinoline,
isoquinoline, quinazoline, quinoxaline and the like, where the
point of attachment is at any available carbon or nitrogen ring
atom. Additional examples heteroaryl groups include, but are not
limited to, furanyl, thienyl, and pridinyl group. The term
"heteroarylene" means a divalent heteroaryl group.
[0057] Abbreviations:
[0058] "AQDS" means anthraquinone-2,7-disulfonic acid.
[0059] "BQDS" means 2,6-dimethyl-1,4-dihydroxy benzene
3-mono-sulfonic acid, 1,2-dihydroxybenzene-3,5-disulfonic acid.
[0060] "DHA" means dihydroxyanthraquinone.
[0061] "DMDHMS" means 2,6-dimethyl-1,4-dihydroxybenzene-3-sulfonic
acid.
[0062] "HQTC" means hydroquinonetetracarboxylic acid.
[0063] "MMO" means mixed metal oxide.
[0064] "p-TSA" means para-toluene sulfonic acid.
[0065] With reference to FIG. 1, a schematic illustration of a flow
battery that includes a pair of organic redox couples is provided.
Flow battery 10 includes battery cell 12 which includes positive
electrode 14, negative electrode 16, and polymer electrolyte
membrane 18. In the context of a flow cell, reduction occurs during
discharge at the positive electrode and oxidation occurs during
discharge at the negative electrode. Conversely, oxidation occurs
during charging at the positive electrode and reduction occurs
during charging at the negative electrode. Polymer electrolyte
membrane 18 is interposed between positive electrode 14 and
negative electrode 16. Positive electrode electrolyte 20 includes
water and a first redox couple 22. In FIG. 2, a first quinone redox
couple is depicted as an example. Positive electrode electrolyte 20
flows over and contacts positive electrode 14. During discharge of
the flow battery, the first organic compound Q.sup.1 is reduced to
the first reduction product H.sub.2Q.sup.1 of the first organic
compound. During charging of the flow battery, the first reduction
product H.sub.2Q.sup.1 is oxidized to the first organic compound
Q.sup.1. Negative electrode electrolyte 30 includes water and a
second redox couple 32. Negative electrode electrolyte 30 flows
over and contacts the negative electrode 16. In FIG. 1, a second
quinone redox couple is depicted as an example. The second redox
couple 32 includes a second organic compound Q.sup.2 and a
reduction product H.sub.2Q.sup.2 of the second organic compound.
During discharge, the reduction product H.sub.2Q.sup.2 is oxidized
to the second organic compound Q.sup.2.
[0066] In an embodiment, a redox flow battery that uses the same
organic redox molecule on both electrodes, i.e., for the first
redox couple 22 and the second redox couple 32 of FIG. 1. This
molecule is designed to have two separate functional redox parts.
One redox active part of the molecule serves as the positive
electrode material and the other redox active part as the negative
electrode material. The standard reduction potential for each of
these parts is distinctly different and identifiable with the redox
reaction occurring in each of these parts. Such a molecule has been
termed for the first time a "bi-functional redox active material".
These two parts are isolated by at least one atom and is linked to
the other parts by sigma bonds. The presence of pi-bonds or
delocalization or mixing of the electronic energy states of the two
sections of the molecule is avoided so that the standard electrode
potential of the molecule has two distinct values. Such a molecule
is embodied in the molecular structure shown below.
##STR00005##
wherein: p is 0, 1, 2, 3, or 4; o is 0, 1, 2, or 3; r is 0, 1, 2,
or 3; R.sub.1, R.sub.2, R.sub.3 are each independently --SO.sub.3H,
--SO.sub.3Na, --SO.sub.3K, --CO.sub.2H, --CO.sub.2Na, --CO.sub.2K,
--OH, --OMe, alkyl, (CH.sub.2CH.sub.2O).sub.nOMe, NH.sub.2,
NMe.sub.2, I, Cl, Br, F, --CF.sub.3, --CF.sub.2H, --NO.sub.2,
--N(R).sub.3.sup.+X.sub.1.sup.-, CCl.sub.3, --CN,
--PO.sub.3H.sub.2, --COOH, --O.sup.-M.sup.+.sub.2,
--SO.sub.3.sup.-M.sup.+, --PO.sub.3.sup.2-M.sup.+.sub.2,
--COO.sup.-M.sup.+, --COOR, --CHO, or --COR where R is H or
C.sub.1-10 alkyl, M.sup.+ is a positively charged counter-ion, and
X.sub.1.sup.- is a negatively charge counter-ion; X is
--(CH.sub.2).sub.n, --(CH.sub.2OCH.sub.2).sub.n, C.sub.4-15
branched alkyl chains or other substituted alkyl chains; and n is 1
to 10 (i.e., 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10). In a refinement, n
is 1, 2, 3, 4 or 5. It should be appreciated that multiple R groups
on the same ring can be different substituents specified from the
list provided selections. In a variation, one of the for the first
redox couple 22 and the second redox couple 32 includes a compound
having formula I. The counter electrode in this case can include a
quinone as disclosed by U.S. patent application Ser. No. 14/307,030
filed Jun. 17, 2014; the entire disclosure of which is hereby
incorporated by reference.
[0067] In a refinement, the R.sub.3 are each independently H,
--NO.sub.2, --CF.sub.3, or --SO.sub.3H. In one refinement, at least
one of R.sub.1, R.sub.2, R.sub.3 is an electron withdrawing group
that can increase the reduction potential of the compound having
the group. In a refinement, 1, 2, or 3 of R.sub.3 are electron
withdrawing groups. Examples of such electron withdrawing groups
include but are not limited to, --NO.sub.2,
--N(R).sub.3.sup.+X.sup.-, --CF.sub.3, CCl.sub.3, --CN,
--SO.sub.3H, --PO.sub.3H.sub.2, --COOH, --OH, --O.sup.-M.sup.+,
--SO.sub.3.sup.-M.sup.+, --PO.sub.3.sup.2-M.sup.+,
--COO.sup.-M.sup.+, --COOR, F, Cl, Br, --CHO, or --COR where R is H
or C.sub.1-10 alkyl, M.sup.+ is a positively charged counter-ion
(e.g., Na.sup.+, K.sup.+, and the like). As described herein, when
electron withdrawing groups are used as substituents for organic
compounds in the negative electrode electrolyte such groups can
provide further separation from the reduction potential of the
positive electrode electrolyte. In another refinement, R.sub.1,
R.sub.2 include one or more (e.g., 1, 2, 3 or 4) electron donating
group. Examples of such electron donating groups include, but are
not limited to, C.sub.1-10 alkyl, NH.sub.2, --NHR, --N(R).sub.2,
--O.sup.-M.sup.+.sub.2, --NHCOR, --OR, --CH.sub.3,
--C.sub.2H.sub.5, or phenyl where R is H or C.sub.1-10 alkyl and
M.sup.+ is a positively charged counter ion. In still another
refinement, at least one of R.sub.1, R.sub.2, R.sub.3 is a
functional group that increases water solubility. Examples of such
functional groups include, but are not limited to, --SO.sub.3H,
--PO.sub.3H.sub.2, --COOH, --OH, --O.sup.-M.sup.+,
--SO.sub.3.sup.-M.sup.+, --PO.sub.3.sup.2-M.sup.+.sub.2,
--COO.sup.-M.sup.+, pyridinyl, imidazoyl, or pyrroyl, where M.sup.+
is a positively charged counter ion (e.g., Na.sup.+, K.sup.+, or
the like).
[0068] The principal advantages of the compounds of the present
embodiment is that only one type of material is used in the
electrochemical cell, and thus, when crossover of the molecule from
the positive to the negative electrode occurs, the issue of
contamination of the sides is no longer relevant. An example of
such a molecule is what we have synthesized and shown in FIG. 2.
The synthetic procedure for this molecule and similar molecules is
also described in Schemes 1 and 2. In FIG. 2, a molecule with the
section marked A serves as the active material for the negative
electrode, and section B serves as the material for the positive
electrode. The two sections are linked by a methylene group. By
using a saturated linker, the reduction potentials of the sections
become separately observable. Therefore, such molecules can be
designed by selecting the properties of the individual A and B
moieties. The most useful molecules will have a separation in
electrode potential of >0.5 V to be useful in storing large
quantities of energy. Examples of such redox couples and their
reactions are shown in FIG. 3A. Synthetic procedure for the
molecule in FIGS. 3B and 3C and another variant of this type of
molecule are described in Schemes 1 and 2.
[0069] The molecule AMBSA shown in FIG. 2 has been synthesized. The
cyclic voltammogram of this molecule (FIG. 4) shows two distinct
peaks. Shown in FIG. 5 are the results of the testing of the new
AMBSA bifunctional redox material at a rotating disk electrode. The
two plateaus in current refer to the reduction of the anthraquinone
moiety and the oxidation of the hydroquinone moiety. AMBSA is only
one example of such a bi-functional redox couple. Any two moieties
that have redox properties that can be coupled through a covalent
bond that does not cause de-localization of the molecular orbitals
of the two redox couples will be able to function as a
bi-functional redox couple. For the molecule AMBSA let us designate
the moiety A as AQS and moiety B as BQS. Let the reduced forms of
moieties A and B be AQSH and BQSH. In an operating redox cell, the
redox states of the molecules are listed in Table 1.
TABLE-US-00001 TABLE 1 Redox States of the molecule AMBSA during
charge and discharge: Positive Negative Cell State electrode
electrode Voltage Fully Charged (BQS-AQS) (BQSH-AQSH) 0.8-1.0 Fully
Discharged (BQSH-AQS) (BQSH-AQS) 0.0
[0070] For these compounds, even if the materials crossover during
charge or discharge, they will be similar in nature to the
materials already present in the compartments. Therefore, no
additional purification or separation will be needed.
[0071] In another embodiment, the other bifunctional compounds to
be used in the positive and negative electrodes or electrolytes of
the flow cell depicted in FIG. 1 (i.e., for the first redox couple
22 and the second redox couple 32) are represented by:
A-X--B II
wherein A is a substituted or unsubstituted flavins with
substituents, quinoxaline with and without substituents,
substituted anthraquinones, and substituted naphthaquinones; and B
is a substituted quinone. In this family, the rings in the A part
can be substituted with 1, 2, 3, 4, 5, or 6 (or each integer from 0
up to the number of replaceable hydrogen atoms) substituents set
forth above with respect to the compound having formula I including
the electron donating and electron withdrawing groups disclosed
there. The B part of this molecule may be selected from substituted
quinones, with methyl, hydroxyl, chloride, fluoride and carboxylic
acid substituents rendered soluble by sulfonation. U.S. patent
application Ser. No. 14/307,030 filed Jun. 17, 2014 discloses
substituted quinones that can be used for the B part; the entire
disclosure of that patent application is hereby incorporated by
reference. Further other redox couple moieties with reversible
electrochemical behavior and different standard reduction
potentials that are about 0.4 V to 1.2 V apart can be combined to
yield a bifunctional redox active materials or electrolytes.
[0072] Bifunctional redox molecules can also be a molecule with
different redox properties in various ranges of potentials. This
type of molecule is typified by the substituted hydroxy
anthraquinones. In this regard, redox properties of several
substituted anthraquinones have been studies. Electrochemistry
experiments on the rotating disk electrode confirm the existence of
two diffusion limited regions separated by a significant potential
difference of 0.5 V to 1.0 V for certain types of substitutions
(FIG. 6). For these molecules, two different redox processes occur
at different potentials. Those molecules which have hydroxyl
substituents in positions such as 1,2- and 1, 4-are particularly
suitable for this purpose.
[0073] In Table 2 below we summarize the properties of these types
of substituted anthraquinones that can be used on both as positive
and negative electrode materials and others that have similar
moieties but are not suitable for this type of deployment. The use
of these molecules is suitable for use both acidic and alkaline
media as shown by the rotating electrode studies.
TABLE-US-00002 TABLE 2 Electrochemical properties of the
hydroxyl-substituted anthraquinones. HydroxyAnthraquinones in NaOH
E.sub.1/2(Volts) vs Compound MMO Di-Couple Potential
1,2,4-trihydroxy anthraquinone -0.84 V Yes, 840 mV difference
2,6-dihydroxy anthraquinone -0.8 V No 1,5-dihydroxy anthraquinone
-0.65 V No 1,4-dihydroxy anthraquinone -0.63 V Yes, 880 mV
difference difference 1,2-dihydroxy -0.73 V Yes, 1 V difference
anthraquinone 1,2-dihydroxy, 3-sulfonic acid -0.75 V Yes, 930 mV
difference anthraquinone
[0074] The description of the bifunctional redox active materials
for the flow batteries described herein is not restricted to
soluble materials. Insoluble materials can be used to form
electrode structures by combining with a conductive material such
as carbon and a binder material such as polyvinylidene fluoride.
Such electrodes can be assembled into batteries with aqueous or
non-aqueous liquid electrolytes, gelled electrolytes or solid
electrolytes. Such configurations include coin cells, pouch cells,
prismatic, spirally-wound and tubular designs.
[0075] In another embodiment, compounds for the first redox couple
22 and/or the second redox couple 32 of FIG. 1 that reduce
crossover between the anode and cathode compartments are provided.
The crossover of the redox active species across the cell is
largely determined by the molecular size. Examples of the redox
compounds of this embodiment include, but are not limited to,
para-toluene sulfonic acid (p-TSA), 2,6-dimethyl-1,4-dihydroxy
benzene 3-mono-sulfonic acid, 1,2-dihydroxybenzene-3,5-disulfonic
acid (BQDS), and anthraquinone-2,7-disulfonic acid (AQDS). The rate
of crossover of the various types of positive electrode materials
through a NAFION.RTM. 117 membrane was measured. The concentration
of p-TSA appearing by crossover through the membrane is found to be
much higher than that of the DMDHMS and BQDS (FIG. 6). AQDS was not
detected at all suggesting no measurable crossover in 192 hours.
The crossover rate measured over a period of 192 hours provided a
rank order of p-TSA>DMDHMS.apprxeq.BQDS>>AQDS. This rank
order correlates with molecular size. The molecules of smaller size
will diffuse more readily through the proton conducting membrane.
Therefore, it is claimed that by modifying the size of the molecule
we can suppress or reduce crossover. The redox active molecules
that are prone to crossover may be modified in the size by either
applying substituent groups. Such substituent groups can be simple
methyl groups, sulfonic acid groups, alkyl groups, branched alkyls
such as tertiary butyl, aromatic groups such as phenyl or benzyl
groups, multi-ring aromatic groups such as naphthyl, anthracenyl
(with or without substituents) amino, quaternary amino, keto,
carboxyl and quinone groups. Examples of such redox active
molecules with substituents that are enhanced in molecular size
include the following compounds having formula II, III, and IV:
##STR00006##
wherein R.sub.4, R.sub.5, R.sub.6, and R.sub.7 are each
independently H,
##STR00007##
--SO.sub.3H, --SO.sub.3Na, --SO.sub.3K, --CO.sub.2H, --CO.sub.2Na,
--CO.sub.2K, --OH, --OMe, alkyl, (CH.sub.2CH.sub.2O).sub.nOMe,
NH.sub.2, NMe.sub.2, I, Cl, Br, F, --CF.sub.3, --CF.sub.2H,
--NO.sub.2, --N(R).sub.3.sup.+X.sub.1.sup.-, CCl.sub.3, --CN,
--PO.sub.3H.sub.2, --COOH, --PO.sub.3.sup.2-M.sup.+.sub.2, --COOR,
--CHO, or --COR where R is H or C.sub.1-10 alkyl, M.sup.+ is a
positively charged counter-ion, and X.sub.1.sup.- is a negatively
charge counter-ion. For compounds II and III, 1, 2, 3 or 4 of
R.sub.4, R.sub.5, R.sub.6, and R.sub.7 are the following cyclic
groups
##STR00008##
Similarly, for compounds IV, 1, 2 or 3 of R.sub.4, R.sub.5, and
R.sub.6 are these substituents. In a refinement, the cyclic groups
are substituted with 1, 2, 3, 4, 5, 6, 7, 8 or 9 (if there are
sufficient replaceable hydrogen atoms) of the following
substituents: independently --SO.sub.3H, --SO.sub.3Na, --SO.sub.3K,
--CO.sub.2H, --CO.sub.2Na, --CO.sub.2K, --OH, --OMe, alkyl,
(CH.sub.2CH.sub.2O).sub.nOMe, NH.sub.2, NMe.sub.2, I, Cl, Br, F,
--CF.sub.3, --CF.sub.2H, --NO.sub.2,
--N(R).sub.3.sup.+X.sub.1.sup.-, CCl.sub.3, --CN,
--PO.sub.3H.sub.2, --COOH, --SO.sub.3.sup.-M.sup.+,
--PO.sub.3.sup.2-M.sup.+.sub.2, --COOR, --CHO, or --COR where R is
H or C.sub.1-10 alkyl, M.sup.+ is a positively charged counter-ion,
and X.sub.1 is a negatively charge counter-ion. In a specific
refinement, the compound DMDHMS
(2,6-dimethyl-1,4-dihydroxybenzene-3-sulfonic acid) and
anthraquinone-2,7-disulfonic acid has been shown as an example of a
molecule with larger size with reduced crossover.
[0076] In another embodiment, the effects of crossover can be
mitigated by using a mixture of positive and negative electrolyte
materials in equimolar quantities at both the electrodes. When such
a mixture is charged only the positive electrolyte component will
undergo oxidation reaction at the positive electrode and the
negative electrolyte material will undergo reduction reactions at
the negative electrode. Since the concentration of the redox active
species is identical even if crossover occurs, the positive
electrolyte material occurred to the negative electrolyte side this
change in concentration will be balanced by back diffusion. Thus,
there might be self-discharge but no permanent capacity loss or
compositional change of the electrolyte. Any two redox couples that
qualify as positive and negative electrolyte materials can be mixed
in equimolar proportions and used as the electrolyte. We have shown
the feasibility of rechargeability of this type of mixed materials
using DMDHMS and AQDS (FIG. 8). Nearly 100% of the charge input was
returned during discharge.
[0077] In another variation, the effects of crossover are mitigated
by adjusting properties of the ion exchange membrane.
Advantageously, this variation can be deployed in each of the flow
batteries described herein. In this variation, the differences in
water content of the ion-exchange membranes is taken advantage of
to avoid crossover. Cation exchange membranes vary in their water
content based on the number of ionic groups present in the
membrane. The number of ionic groups is given by the equivalent
weight of the membrane, or the number of grams of the membrane that
contains a mole of sulfonic acid or other ionizable acid groups.
Table 3 shows the variation of water content of NAFION.RTM.
membranes with different equivalent weights.
TABLE-US-00003 TABLE 3 The number of molecules of water associated
with each ionic group in NAFION .RTM. and Dow membranes of
different equivalent weights (EW). Nafion Dow EW Mol EW Mol 850
35.1 800 36.8 1000 25.4 830 21.8 1200 19.9 1154 18.6 1400 16.9 1340
12.7 1600 14.0 1666 8.7
[0078] The cluster size of the ionic or hydrophilic domains also
varies considerably with the water content. The cluster size is of
the order of a few nanometers and is shown schematically in FIG. 9.
By reducing the water content, the cluster size can be reduced and
molecules larger than the cluster size can be prevented from
entering the membrane. Therefore, the equivalent weight of the ion
exchange membrane with respect to sulfonic acid groups is greater
than 1000 and typically between 1000 and 2000.
[0079] In a variation, the reduced rate of crossover of the redox
active molecules is achieved by using a membrane with a higher
equivalent weight of NAFION.RTM.. The membrane NAFION.RTM. 324, has
a 25-micron layer of equivalent weight 1500 and a 150-micron layer
of equivalent weight 1100. The crossover rate of DMDHMS through
such a membrane is just 0.65 of that through a homogenous
NAFION.RTM. 117 membrane that has an equivalent weight of 1100
throughout the membrane. FIG. 10 shows the advantage of using the
NAFION.RTM. 324 membrane over NAFION.RTM. 117 and benefit of
tailoring the water content of the membrane to reduce the crossover
rate and having a slower rate of decay of capacity.
[0080] Other membranes such as those fabricated from
polystyrenesulfonic acid and polyvinylidene fluoride (PSSA-PVDF)
that have low water content and have a low ionic cluster size and
will be useful in suppressing crossover of the redox molecules.
These membranes achieve low water content because of an
interpenetrating network of formed by the polymers. Such PSSA-PVDF
membranes have been fabricated and these membranes exhibit about
70% reduction in the crossover rate of methanol without compromise
of ionic conductivity (FIG. 11). Therefore, we claim that redox
flow batteries using PSSA-PVDF membranes and such other membranes
of the interpenetrating type or of low water content are preferred
improvements over the NAFION.RTM. 117 membrane. Similar membranes
can be prepared from sulfonated polyethersulfone (PES) and
sulfonate polyether-etherketone (PEEK).
[0081] In another embodiment, an organic redox flow battery using
alkaline electrolytes is provided. The Organic Redox Flow Battery
with Alkaline electrolyte (FIG. 17) consists of a cell in which the
positive and negative electrode is separated by a cation exchange
membrane (or ion conducting membrane). Such a membrane can conduct
cations such as protons, sodium and potassium ions. Other cations
such as cesium and lithium can also conduct across this membrane.
The membrane serves to separate the positive and negative
electrodes of the cell. An example of such a membrane is
NAFION.RTM., a DuPont product, consisting of a polymer made of
perfluorohydrocarbonether sulfonic acid.
[0082] The electrodes in the cell are constructed of an
electrically conductive, porous carbon structure made of either
carbon paper or graphite felt or similar materials that are made
from carbon and have the significant porosity to allow for the flow
of aqueous solutions and sufficient conductivity to offer a low
resistance for the flow of electrons. Electrode compartments
separated by the membrane are assembled together using tie rods.
Gaskets are used to prevent leakage of solutions to the outside of
the cell. Current can be drawn from the cell through the electrical
contacts on the outer surface of the electrode compartments.
[0083] Each of the electrode compartments has flow passages (also
called flow fields) that allow the electrolyte to permeate the
electrode structures. The electrolyte is alkaline and is typically
sodium hydroxide or potassium hydroxide dissolved in water at
concentrations ranging from 0.01 to 7 moles/liter.
[0084] The organic redox couples are dissolved in this alkaline
electrolyte. The alkaline solution of two different kinds of redox
couples are circulated past the electrodes. The redox couple with a
more negative standard reduction potential functions as the
negative electrode, while that with a more standard reduction
potential serves as the positive electrode. Pumps are used to
circulate the alkaline electrolyte through the cell.
[0085] The redox couples must be soluble in the alkaline
electrolyte and must can undergo facile electron transfer. For
example, electrochemical studies on the molecule 1,
8-dihydroxyanthraquinone (DHA) dissolved in a 1 M solution of
sodium hydroxide suggest a reversible electrochemical reaction of
this redox couple (FIG. 18). The two symmetrical current peaks in
the cyclic voltammogram indicate reversible electrochemical
reactions of oxidation and reduction. The cyclic voltammogram
indicates that the reduction potential for this redox couple is
about -0.7 V vs. the mercury/mercuric oxide reference electrode.
Based on these results, we may expect DHA with substituent groups
such as additional hydroxyl groups or methyl, chloro, cyano or
sulfonic acid groups to have the desired electrochemical
characteristics for charge and discharge.
[0086] Similarly, anthraquinone-2,6-disulfonic acid (AQDS) also
exhibits reversible electrochemical behavior in an alkaline
electrolyte of 1 M sodium hydroxide (FIG. 19). The reduction
potential for this couple in alkaline electrolyte is about -0.35 V
vs. MMO.
[0087] Based on the electrode potentials, the DHA can serve as the
negative electrolyte and the AQDS can serve as the positive
electrolyte. The cell constructed with these two redox
couple-containing solutions circulated past the electrode was found
to be capable of undergoing charge and discharge many times (FIG.
20). The cell was charged and discharged at 8 mA/cm.sup.2. The
value of cell voltage was consistent with that expected from the
reduction potential of the redox couples. The expected value of
cell voltage was the difference between the reduction potentials of
-0.7 V and -0.35 V, which is 0.35 V. The discharge voltages were
about 0.2 V and the charge voltages were about 0.4 V.
[0088] Similarly, hydroquinone sulfonic acid (HQS) could be used as
a positive electrode redox couple and combined with DHA to form a
redox flow battery. The performance of such a cell during charge
and discharge is shown in FIG. 21.
[0089] While these compounds are not suitable for operation in
acidic media because of the attendant polymerization reactions,
these compounds perform well in alkaline media. The electrochemical
behavior of quinoxaline is shown in FIG. 22. The symmetrical
current peaks suggest reversible electrochemical behavior
suitability for redox flow batteries. The reduction potentials are
quite negative relative to the last example of DHA. Consequently,
this may be combined with AQDS to form a redox flow battery.
[0090] The electrochemical properties of a substituted quinoxaline
in alkaline media that is suitable for redox flow battery operation
is shown in FIG. 23.
[0091] In another embodiment, compounds for the first redox couple
22 and/or the second redox couple 32 of FIG. 1 flavins. The flavins
refer to the family of compounds with the basic structure of the
isoalloxazine ring system as in FIG. 24.
##STR00009##
wherein: R.sub.8, R.sub.9, R.sub.10, R.sub.11, R.sub.12 are each
independently H, NH.sub.2, --NHR, --N(R).sub.2, --NHCOR, --OR,
--CH.sub.3, --C.sub.2H.sub.5, --SO.sub.3H, --PO.sub.3H.sub.2,
--COOH, --OH, --N(R).sub.3.sup.+X.sup.-, --CF.sub.3, CCl.sub.3,
--CN, --COOR, F, Cl, Br, --CHO, --COR--O.sup.-M.sup.+,
--SO.sub.3.sup.-M.sup.+, --PO.sub.3.sup.2-M.sup.+.sub.2, pyridinyl,
imidazoyl, pyrroyl, or phenyl where R is H or C.sub.1-10 alkyl and
M.sup.+ is a positively charged counter.
[0092] In one refinement, at least one of R.sub.8, R.sub.9,
R.sub.10, R.sub.11, R.sub.12 is an electron withdrawing group that
can increase the reduction potential of the compound having the
group. In a further refinement, 1, 2, 3, 4, or 5 of R.sub.8,
R.sub.9, R.sub.10, R.sub.11, R.sub.12 are electron withdrawing
groups. Examples of such electron withdrawing groups include but
are not limited to, --NO.sub.2, --N(R).sub.3.sup.+X.sup.-,
--CF.sub.3, CCl.sub.3, --CN, --SO.sub.3H, --PO.sub.3H.sub.2,
--COOH, --OH, --O.sup.-M.sup.+, --SO.sub.3.sup.-M.sup.+,
--PO.sub.3.sup.2-M.sup.+.sub.2, --COOR, F, Cl, Br, --CHO, --COR
where R is H or C.sub.1-10 alkyl, M.sup.+ is a positively charged
counter-ion (e.g., Na.sup.+, and the like). As described herein,
when electron withdrawing groups are used as substituents for
organic compounds in the negative electrode electrolyte such groups
can provide further separation from the reduction potential of the
positive electrode electrolyte. In one refinement, at least one of
R.sub.8, R.sub.9, R.sub.10, R.sub.11, R.sub.12 is an electron
donating group that can decrease the reduction potential. In a
further refinement, 1, 2, 3, 4, or 5 of R.sub.8, R.sub.9, R.sub.10,
R.sub.11, R.sub.12 are electron donating groups. Examples of such
electron donating groups include but are not limited to, C.sub.1-10
alkyl, NH.sub.2, --NHR, --N(R).sub.2, --NHCOR, --OR, --CH.sub.3,
--C.sub.2H.sub.5, or phenyl where R is H or C.sub.1-10 alkyl and
M.sup.+ is a positively charged counter-ion (e.g., Na.sup.+,
K.sup.+, or the like). As described herein, when electron donating
groups are used as substituents for organic compounds in the
positive electrode electrolyte such groups can provide further
separation from the reduction potential of the negative electrode
electrolyte. In still another refinement, at least one of R.sub.8,
R.sub.9, R.sub.10, R.sub.11, R.sub.12 is a functional group that
increases water solubility. In a further refinement, 1, 2, 3, 4, or
5 of R.sub.8, R.sub.9, R.sub.10, R.sub.11, R.sub.12 are such
functional groups. Examples of such functional groups include, but
are not limited to, --SO.sub.3H, --PO.sub.3H.sub.2, --COOH, --OH,
--O.sup.-M.sup.+, --SO.sub.3.sup.-M.sup.+,
--PO.sub.3.sup.2M.sup.+.sub.2, --COO.sup.-M.sup.+.sub.2, pyridinyl,
imidazoyl, and pyrroyl, where M.sup.+ is a positively charged
counter-ion (e.g., Na.sup.+, K.sup.+, and the like). This family of
compounds is also suitable for use in alkaline media as a redox
flow battery electrolyte. Substitutions at positions 6, 9 and 10
are useful in changing the reduction potential of the redox
couples. The behavior of two types of flavins is shown in FIGS. 25A
and 25B.
[0093] In another embodiment, deterioration of redox flow batteries
by the Michael's Reaction is mitigated. Redox flow batteries that
use organic molecules with a propensity to undergo the Michael
reaction and transform during cycling. These transformations
involve electrochemical oxidation of the hydroxyl group to the
quinone, followed by the addition of water to form reduced
products. The addition of nucleophiles such as water to the
.alpha., .beta.-unsaturated carbonyl compounds in a 1,4-fashion is
called the Michael reaction which can be acid or base catalyzed.
The nucleophilic addition of water is accompanied by
re-aromatization and exchange of the proton as depicted by the acid
catalyzed reaction of FIG. 21. The Michael reaction necessitates
additional charge input for the re-oxidation of the product during
charging. The flow battery of this embodiment includes a positive
electrode, a positive electrode electrolyte, negative electrode,
and a negative electrode electrolyte. The positive electrode
electrolyte includes water and a first redox couple. The positive
electrode electrolyte flows over and contacts the positive
electrode. Characteristically, the first redox couple includes a
first organic compound and a reduction product of the first organic
compound where the first organic compound is reduced during
discharge. The negative electrode electrolyte includes water and a
second redox couple. The negative electrode electrolyte flows over
and contacts the positive electrode. Characteristically, the second
redox couple also includes a second organic compound. A reduction
product of the second organic compound is oxidized to the second
organic compound during discharge. One or both of the first organic
compound and the second organic compound include a compound
selected from the group consisting of:
##STR00010##
and combinations thereof. The Michael reaction does not occur with
these molecules because the aromatic ring is fully substituted or
the nucleophilic attack of the open positions on the ring is
sterically hindered. These molecules are claimed to provide an
improved performance through resistance to degradation during
cycling. We have measured the electrochemical properties of several
of these substituted molecules are shown in FIG. 22. Cyclic
voltammetry and studies using the rotating disk electrode confirm
the redox activity and the reversibility of these molecules that
are resistant to the Michael reaction. The results of the
electrochemical studies are shown in FIGS. 23, 24 and 25.
[0094] The resistance of the materials to undergo the Michael
reaction has been demonstrated by the cycling properties of organic
redox flow cells that use these substituted molecules. In FIG. 25
we show the charge and discharge curves for a cell using
2,5-dimethyl-1,4-dihydroxybenzene-3-sulfonic acid and
anthraquinone-2,7-disulfonic acid. The charge and discharge curves
for this cell showed a single plateau confirming just one reaction
at the positive and negative electrodes (FIG. 25). If the Michael
reaction occurred, then there would be multiple plateaus
corresponding to multiple redox reactions that accompany the
sequential hydroxylation and aromatization processes of the Michael
reaction. The charging process required only a stoichiometric
amount of anthraquinone-2,7-disulfonic acid further confirming that
the Michael reaction did not occur. The resistance to Michael
reaction continues through at least 100 cycles of charge and
discharge (FIG. 25). Therefore, substituted molecules such as the
ones shown in FIGS. 14, 15 and 25 are claimed to improve the
performance of the aqueous organic redox flow battery.
[0095] Redox molecules that are suitable for operation in alkaline
media have a structure that is not prone to the Michael addition
reaction. The Michael addition reaction is also called nucleophilic
1,4 addition. This reaction leads to formation of hydroxylated
products (FIG. 26). Compounds that have the
.alpha.,.beta.-unsaturated carbonyl structure with an open position
for attack by the hydroxyl as shown in FIG. 26, will not be stable
in acidic, neutral or alkaline media. While this reaction is not
rapid in acidic media, it occurs far more readily in alkaline
media. The reactions can occur sequentially on the hydroxylated
products until no more positions are available for Michael
addition. This type of reaction is undesirable as it leads to an
irreversible transformation of the redox couples. Therefore,
designing molecules that are robust to the Michael addition
reaction is necessary. The compounds set forth in FIG. 27 with
substituents in the 4-position relative to the carbonyl group
resist Michael addition. All positions that are prone to the
Michael addition reaction are substituted in the compounds shown in
FIG. 27. These compounds are representative of the quinone family.
The principle can be extended to other functional groups such as
sulfonic acids, phosphonic acids, chloro, fluoro, bromo, hydroxyl,
alkoxy, amino, amido, and alkyl substituents. An example of a
compound of this type with different functional groups is benzene
1,2-dihydroxy-4,6-dimethyl-3,5-disulfonic acid as shown in
structure in FIG. 28.
[0096] The electrochemical reversibility of such a Michael
addition-resistant compound is shown in FIG. 29 for
benzenedi-hydroxy tetracarboxylic acid also called
hydroquinonetetracarboxylic acid (HQTC). This compound shows
reversible electrochemical behavior with two different sets of
peaks corresponding to two steps of one-electron and one proton
transfer processes. The reduction potential of HQTC is such that it
can be combined with DHA to create a redox flow battery with a cell
voltage of 0.5 V (FIG. 30). Further HQTC can also be combined with
the molecules from the quinoxaline family, and the flavin family to
yield redox flow batteries with the cell voltage in the range of
0.5 to 1 V. 1,2,4-trihydroxyanthraquinone (Purpurin) is claimed to
be a negative electrode material suitable for operation in alkaline
media. A cell constructed with purpurin as the negative electrolyte
and AQDS as the positive electrolyte was found to cycle in a stable
manner for over 70 hours without any noticeable change in capacity
(FIG. 31).
[0097] A stack of cells that is used for such a redox flow battery
that uses alkaline electrolytes set forth above can be fabricated
from inexpensive materials such as stainless steel and nickel
plated mild steel. Unlike the acid systems, the dissolution of
metals is not a principal concern in alkaline media. Thus, metals
such as nickel-plated steel or stainless steel can be are
inexpensively stamped into bipolar plates to form the repeating
element of the stack. The electrode structures do now have to be
made of graphite, and the expensive felt electrodes may be replaced
by porous metallic structures considerably reducing the cost of the
stack.
[0098] In another variation, electrodes using nano-structured
materials are provided. Advantageously, this variation can be
deployed in each of the flow batteries described herein. Electrodes
modified with carbon nano-structured materials on graphite felt are
claimed to have improvement over conventional graphite felt
structures. With these nano-carbon-coated electrode structures we
can obtain high limiting currents and lower mass transport
polarization. Consequently, the efficiency of the redox flow
battery can be improved. Such electrode structures are fabricated
by immersing the graphite felt electrode such as the ones made by
SGL in a suspension of multi-walled carbon nanotubes (MWCNT). The
suspension of MWCNT is made in a dilute solution of NAFION.RTM.,
typically containing 5% by weight of the solution in
isopropanol/water mixture. After immersing the graphite felt in the
NAFION.RTM. solution, the felt is dried in an air oven at
125.degree. C. for an hour. This treatment allows the NAFION.RTM.
ionomer to bond the MWCNT with the graphite felt and also cause a
phase inversion of the perfluorinated ionomer chains. The process
of immersion and drying is repeated at least three times so that
the required loading in the range of 10 to 30% is achieved. Higher
loadings of MWCNT will block the pores of the graphite felt and
thus impede fluid transport.
[0099] Examples of such MWCNT treated electrode structures were
prepared and their surface area was measured using impedance
spectroscopy. The electrochemically active surface area of the
electrode with a geometric surface area was maximum with the 15%
MWCNT containing felt (Table 4). The area enhancement was about 100
times.
TABLE-US-00004 TABLE 4 Electrochemically Active surface area of
graphite felt (geometric surface area 25 cm2) resulting from
various modifications. Effect of Electrode Modifications on Active
Area Measured from Double Layer Capacitance Surface Area
Modification Method (sq. cm) Plain Felt 122.0 NAFION .RTM. 141.8
Carbon 30% 1694.9 15% CNT 2222.2 Heat Treated Felt 1337.8
[0100] As a consequence of the enhancement in electrochemical
active surface area with the carbon nanotube modified electrodes,
the current density for the redox flow cell could be increased all
the way to 600 mA/cm.sup.2 (FIG. 32).
[0101] While exemplary embodiments are described above, it is not
intended that these embodiments describe all possible forms of the
invention. Rather, the words used in the specification are words of
description rather than limitation, and it is understood that
various changes may be made without departing from the spirit and
scope of the invention. Additionally, the features of various
implementing embodiments may be combined to form further
embodiments of the invention.
* * * * *