U.S. patent application number 16/434238 was filed with the patent office on 2019-09-19 for electrochemical-based purification of electrolyte solutions, and related systems and methods.
The applicant listed for this patent is Vionx Energy Corporation. Invention is credited to Joseph T. Sullivan.
Application Number | 20190288317 16/434238 |
Document ID | / |
Family ID | 61829690 |
Filed Date | 2019-09-19 |
United States Patent
Application |
20190288317 |
Kind Code |
A1 |
Sullivan; Joseph T. |
September 19, 2019 |
Electrochemical-Based Purification of Electrolyte Solutions, and
Related Systems and Methods
Abstract
Methods and systems for removing impurities from electrolyte
solutions having three or more valence states. In some embodiments,
a method includes electrochemically reducing an electrolyte
solution to lower its valence state to a level that causes
impurities to precipitate out of the electrolyte solution and then
filtering the precipitate(s) out of the electrolyte solution. In
embodiments in which the electrolyte solution is desired to be at a
valence state higher than the precipitation valence state, a method
of the disclosure includes oxidizing the purified electrolyte
solution to the target valence.
Inventors: |
Sullivan; Joseph T.;
(Hanover, NH) |
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Applicant: |
Name |
City |
State |
Country |
Type |
Vionx Energy Corporation |
Woburn |
MA |
US |
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|
Family ID: |
61829690 |
Appl. No.: |
16/434238 |
Filed: |
June 7, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15726858 |
Oct 6, 2017 |
10333164 |
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16434238 |
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62405576 |
Oct 7, 2016 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 8/188 20130101;
C01G 31/00 20130101; C01B 3/00 20130101; H01M 8/0693 20130101; C01G
31/02 20130101 |
International
Class: |
H01M 8/06 20060101
H01M008/06; C01G 31/00 20060101 C01G031/00; H01M 8/18 20060101
H01M008/18; C01G 31/02 20060101 C01G031/02 |
Claims
1. A method of making a purified electrolyte solution, the method
comprising: providing an electrolyte solution having a
precipitation valence and containing at least one impurity that
precipitates out of the electrolyte solution when the valence of
the electrolyte solution is at or below the precipitation valence;
reducing the electrolyte solution to a valence below the
precipitation valence so as to cause the at least one impurity to
precipitate out of the electrolyte solution as a precipitate; and
removing a desired portion of the precipitate from the electrolyte
solution so as to make the purified electrolyte solution.
2. The method according to claim 1, wherein the electrolyte
solution comprises a vanadium-based electrolyte solution, and the
reducing of the electrolyte solution includes reducing the
vanadium-based electrolyte solution to a valence of less than
3.0.
3. The method according to claim 2, wherein the reducing of the
vanadium-based electrolyte solution to a valence of less than 3.0
includes reducing the vanadium-based electrolyte solution to a
valence of less than 2.5.
4. The method according to claim 2, wherein the reducing of the
vanadium-based electrolyte solution to a valence of less than 3.0
includes reducing the vanadium-based electrolyte solution to a
valence of substantially 2.0.
5. The method according to claim 1, wherein the reducing of the
electrolyte solution is performed using a hybrid electrochemical
cell.
6. The method according to claim 5, wherein the reducing of the
electrolyte solution utilizes hydrogen gas as a reductant.
7. The method according to claim 1, further comprising oxidizing
the purified electrolyte solution to a desired valence.
8. The method according to claim 7, wherein the purified
electrolyte solution comprises a purified vanadium-based
electrolyte solution and the oxidizing of the purified electrolyte
solution includes oxidizing the purified vanadium-based electrolyte
to the desired valence.
9. The method according to claim 8, wherein the desired valence is
substantially 3.5.
10. The method according to claim 8, wherein the desired valence is
substantially 4.5.
11. The method according to claim 8, wherein the desired valence is
substantially 2.5.
12. The method according to claim 1, further comprising oxidizing
the purified electrolyte solution to each of two valences so as to
create a plurality of purified valence-adjusted electrolyte
solutions of differing valence.
13. The method according to claim 12, wherein the purified
electrolyte solution comprises a purified vanadium-based
electrolyte solution and the oxidizing of the purified electrolyte
solution to each of two valences includes oxidizing a first portion
of the purified vanadium-based electrolyte solution to a valence of
greater than 3.5 and oxidizing a second portion of the purified
vanadium-based electrolyte solution to a valence of less than
2.5.
14. The method according to claim 13, wherein the oxidizing of a
first portion of the purified vanadium-based electrolyte solution
to a valence of greater than 3.5 includes oxidizing the first
portion to a valence of about 4.5 and oxidizing a second portion of
the purified vanadium-based electrolyte solution to a valence of
less than 2.5 includes oxidizing the second portion to a valence of
about 2.5.
15. The method according to claim 12, wherein the oxidizing of the
purified electrolyte solution is performed using a hybrid
electrochemical reduction cell.
16. The method according to claim 15, wherein the oxidizing of the
purified electrolyte solution utilizes the formation of hydrogen
gas from protons.
17. The method according to claim 16, wherein the reducing of the
electrolyte solution is performed using a hybrid electrochemical
reduction cell.
18. The method according to claim 17, wherein the reducing of the
electrolyte solution utilizes hydrogen gas output from the hybrid
electrochemical oxidation cell.
19. The method according to claim 12, wherein the reducing of the
electrolyte solution and the oxidizing of the purified electrolyte
solution are performed using an electrolyte-only electrochemical
cell having a reduction side and an oxidation side with the
electrolyte solution on the reduction side and the purified
electrolyte solution on the oxidation side.
20. The method according to claim 19, wherein each of the
electrolyte solution and purified electrolyte solution comprises a
vanadium-based electrolyte solution.
21. The method according to claim 1, wherein the providing of an
electrolyte solution comprises providing a vanadium-based
electrolyte solution.
22. The method according to claim 21, wherein all vanadium in the
vanadium-based electrolyte solution comes substantially only from
mixing V.sub.2O.sub.5 with at least one strong acid.
23. The method according to claim 22, wherein all the vanadium in
the vanadium based electrolyte solution comes substantially from
V.sub.2O.sub.5 and V.sub.2O.sub.3, each mixed with at least one
strong acid.
24. The method according to claim 1, wherein the electrolyte
solution has a lowest possible valence and the reducing of the
electrolyte solution includes reducing the electrolyte solution to
about the lowest possible valence.
25. The method according to claim 1, wherein the precipitate is in
the form of particles having different sizes, and removing a
desired portion of the precipitate includes removing particles
having sizes larger than a predetermined size.
26. The method according to claim 1, further comprising, prior to
providing the electrolyte solution, identifying that the
electrolyte solution contains the at least one impurity that the
method removes to make the purified electrolyte solution.
27. The method according to claim 1, further comprising measuring
valence of the electrolyte solution to determine whether or not the
valence is at or below the precipitation valence so as to ensure
the precipitation of the at least one impurity.
28. The method according to claim 1, wherein: the purified
electrolyte solution will be used in a redox flow battery; and the
method is performed prior to the purified electrolyte solution
being installed into the redox flow battery.
29. The method according to claim 1, wherein: the purified
electrolyte solution is installed in a redox flow battery prior to
performing the method; and the method is performed using the redox
flow battery prior to placing the redox flow battery into
service.
30. The method according to claim 1, wherein removing a desired
portion of the precipitate includes mechanically or passively
separating the desired portion from the electrolyte solution.
Description
RELATED APPLICATION DATA
[0001] This application is a continuation of U.S. patent
application Ser. No. 15/726,858, filed on Oct. 6, 2017, entitled
"Electrochemical-Based Purification of Electrolyte Solutions, and
Related Systems and Methods", which application claims the benefit
of priority of U.S. Provisional Patent Application Ser. No.
62/405,576, filed on Oct. 7, 2016, and titled "Reduction-Based
Purification Of Electrolyte Solutions And Related Systems And
Methods". Each of the foregoing applications is incorporated herein
by reference in its entirety.
FIELD OF THE INVENTION
[0002] The present invention is related to purification and
production of electrolyte solutions. More particularly, the present
invention is directed to electrochemical-based purification of
electrolyte solutions, and related systems and methods.
BACKGROUND
[0003] Electrolytes for batteries and other uses generally need to
be free of impurities that are detrimental to those uses. For
example, in the context of a redox flow battery, each electrolyte
used needs to be free of impurities that foul components of the
battery. In a particular example, a vanadium redox flow battery
(VRFB) is a system that converts electrical energy into chemical
energy and then releases that chemical energy as electricity when
there is demand. This type of battery is often paired with a solar
and/or wind farm to help smooth out the power production
intermittency associated with these renewable energy sources.
[0004] A VRFB comprises an electrochemical cell that performs the
conversion between chemical and electrical energy. The
electrochemical cell includes a negative electrode, an electrolyte
separator (often a proton exchange membrane), and a negative
electrode. Two separate vanadium solutions are stored in individual
tanks--one tank contains a negative electrolyte solution that is
fed to the negative electrode, and the other tank contains a
positive electrolyte solution that is fed to the positive
electrode. During normal operation, the negative electrolyte
solution contains vanadium (II) and (III) ions, and the positive
electrolyte solution contains vanadium (IV) and (V) ions. During
charge, vanadium (III) ions are reduced to vanadium (II) ions in
the negative electrolyte solution at the negative electrode, and
vanadium (IV) ions are oxidized to vanadium (V) ions in the
positive electrolyte solution at the positive electrode; the
opposite happens during discharge.
[0005] When commissioning a new VRFB, a balanced electrolyte
solution of average valence of roughly 3.5, i.e., an electrolyte
solution of equal concentration of vanadium (III) and vanadium (IV)
ions, is transferred into both the negative and positive
electrolyte tanks. The battery is slowly charged until the negative
and positive electrolyte solutions are at the desired ratio of
vanadium (II)/(III) and vanadium (IV)/(V) ions, respectively. After
this initial charge, some impurities present in the negative
electrolyte solution will typically precipitate out as a solid
metal phase precipitate. These impurities include, but are not
limited to, As and Ge. These precipitates are detrimental to the
electrochemical cell, because they clog the negative electrode and
negatively impact battery performance.
[0006] The majority of conventional methods for making a
vanadium-based electrolyte solution involve one of two methods:
[0007] Method 1: Mixing V.sub.2O.sub.3 and V.sub.2O.sub.5 in a 3:1
molar ratio in excess acid to produce a solution of a 3.5 average
valence. [0008] Method 2: Using a VRFB electrochemical cell in
which the negative electrode is used to reduce a vanadium-based
electrolyte solution to a 3.5 valence and the positive electrode is
oxidizing a vanadium-based electrolyte solution that is
periodically or continually reduced using a chemical reducing
agent. This type of approach is needed because most organic
reducing agents are only able to reduce vanadium (V) ions to
vanadium (IV) ions (i.e. most organic reductants can't chemically
reduce vanadium (IV) ions to a lower valence).
[0009] Neither of these conventional methods adequately remove
impurities that often foul the electrochemical cell of a VRFB.
Rather, both methods require that the vanadium feedstocks have low
impurity content of select impurities for proper function and
inhibiting fouling in a VRFB system. Consequently, many
purification methods focus on the purification of the vanadium
feedstock. While many of these methods are effective, they consume
many chemicals and the final product commands a high cost
premium.
SUMMARY OF THE DISCLOSURE
[0010] In one implementation, the present disclosure is directed to
a method of making a purified electrolyte solution. The method
includes providing an electrolyte solution having a precipitation
valence and containing at least one impurity that precipitates out
of the electrolyte solution when the valence of the electrolyte
solution is at or below the precipitation valence; reducing the
electrolyte solution to a valence below the precipitation valence
so as to cause the at least one impurity to precipitate out of the
electrolyte solution as a precipitate; and mechanically separating
the precipitate out of the electrolyte solution so as to make the
purified electrolyte solution.
[0011] In another implementation, the present disclosure is
directed to a method of commissioning a redox flow battery having a
positive side and a negative side. The method includes providing an
electrolyte solution having an initial valence and at least four
available oxidation states comprising a set of higher oxidation
states and a set of lower oxidation states; reducing the
electrolyte solution to a valence below the initial valence to make
a reduced electrolyte solution having a valence within the set of
lower oxidation states; oxidizing a first portion of the reduced
electrolyte solution to make a positive-side electrolyte solution
having a valence within the set of higher oxidation states;
providing, for the commissioning, the positive-side electrolyte
solution for the positive side of the redox flow battery; and
providing, for the negative side of the redox flow battery for the
commissioning, a second portion of the reduced electrolyte solution
as a negative-side electrolyte solution having a valence within the
set of lower oxidation states.
[0012] In yet another implementation, the present disclosure is
directed to a system for making a purified electrolyte solution
from an electrolyte solution containing at least one impurity that
precipitates out of the electrolyte solution at or below a
precipitation valence. The system includes a reduction system that
includes an electrochemical reduction cell designed and configured
to electrochemically reduce the electrolyte solution based on a
reductant; a recirculation loop designed and configured to
recirculate at least a portion of the electrolyte solution to the
electrochemical reduction cell; a reduction-process control system
in operative communication with the reduction system so as to
control the valence of the electrolyte solution in the reduction
system to a desired value at or below the precipitation value so as
to cause the at least one impurity to precipitate out of the
electrolyte solution as a precipitate; and a solid/liquid
mechanical separation system designed and configured to remove at
least a portion of the precipitate out of the electrolyte solution
to make the purified electrolyte solution.
[0013] In still yet another implementation, the present disclosure
is directed to a system for making a valence-adjusted electrolyte
solution from an electrolyte solution having at least four
oxidation states. The system includes a reduction system that
includes an electrochemical reduction cell designed and configured
to electrochemically reduce the electrolyte solution based on a
reductant; a recirculation loop designed and configured to
recirculate at least a portion of the electrolyte solution to the
electrochemical reduction cell; a reduction-process control system
in operative communication with the reduction system so as to
control the valence of the electrolyte solution in the reduction
system to a desired value; and an output designed, configured, and
located to output the electrolyte solution as previously reduced
electrolyte solution at about the desired value; and an oxidation
system that includes an electrochemical oxidation cell designed and
configured to electrochemically oxidize the previously reduced
electrolyte solution based on an oxidant; a recirculation loop
designed and configured to recirculate at least a portion of the
previously reduced electrolyte solution to the electrochemical
oxidation cell; an oxidation-process control system in operative
communication with the oxidation system so as to control the
valence of the previously reduced electrolyte solution in the
oxidation system to a desired final value; and an output designed,
configured, and located to output the previously reduced
electrolyte solution at about the desired final value.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] For the purpose of illustrating the invention, the drawings
show aspects of one or more embodiments of the invention. However,
it should be understood that the present invention is not limited
to the precise arrangements and instrumentalities shown in the
drawings, wherein:
[0015] FIG. 1 is a graph of valence versus time for an example
electrolyte formation/purification and valence adjustment process
in which the example electrolyte solution has four oxidation states
ranging from +2 to +5;
[0016] FIG. 2 is a schematic diagram of an example continuous
electrolyte formation/purification and valence-adjustment
process;
[0017] FIG. 3A is a diagram illustrating an example construction of
the reduction cell of FIG. 2;
[0018] FIG. 3B is a diagram illustrating an example operation of
the reduction cell having the construction of FIG. 3A, showing that
a vanadium electrolyte is reduced and hydrogen gas is oxidized;
[0019] FIG. 4A is a diagram illustrating an example construction of
the oxidation cell of FIG. 2;
[0020] FIG. 4B is a diagram illustrating an example operation of an
oxidation cell having the construction of FIG. 4A, showing that a
vanadium electrolyte is oxidized while protons are reduced to
hydrogen gas;
[0021] FIG. 5 is a schematic diagram of the reduction/purification
sub-process used in experimental lab-scale testing;
[0022] FIG. 6A is a graph of cell operating and open-circuit
voltages versus time (hours) in the reduction cell of FIG. 5 for
Batch 1 containing 3 liters of impure electrolyte containing high
levels of As (5 ppm) and Ge (0.2 ppm) and having a starting valence
of 3.5;
[0023] FIG. 6B is a graph of pressure versus time (hours) for Batch
1 for pressure just upstream of the filter prior to the reduction
cell of FIG. 5 and the pressure at the inlet to the reduction
cell;
[0024] FIG. 7A is a graph of cell operating and open-circuit
voltages versus time (hours) of the reduction cell of FIG. 5 for
Batch 2 containing 3 liters of impure electrolyte containing high
levels of As (5 ppm) and Ge (0.2 ppm) and having a starting valence
of 3.5;
[0025] FIG. 7B is a graph of pressure versus time (hours) for Batch
2 for pressure just upstream of the filter prior to the reduction
cell of FIG. 5 and the pressure at the inlet to the reduction
cell;
[0026] FIG. 8 is a schematic diagram of the oxidation sub-process
used in the experimental lab-scale testing;
[0027] FIG. 9A is a graph of voltage versus time (hours) for the
oxidation of the Batch 1 electrolyte in the oxidation sub-process
of FIG. 8;
[0028] FIG. 9B is a graph of current density versus time (hours) in
the oxidation cell of FIG. 8;
[0029] FIG. 10 is a graph of negative electrode pressure versus
time (hours) for both of the as-made electrolyte of Batch 1 and the
electrolyte of Batch 1 after reduction/purification using the
experimental process;
[0030] FIG. 11A is a graph of cell and reference cell voltages
versus time (hours) for Batch 1 after reduction/purification using
the experimental process;
[0031] FIG. 11B is a graph illustrating the Coulombic efficiency,
energy efficiency, and vanadium utilization over time (hours) for
the purified Batch 1;
[0032] FIG. 11C is a graph of pressure versus time (hours) showing
negative-side and positive-side electrolyte pressures;
[0033] FIG. 12A is a diagram illustrating the reduction sub-process
for a purification system that uses an electrochemical cell that
accepts VRB electrolyte on both positive and negative
electrodes;
[0034] FIG. 12B is a diagram illustrating the oxidation sub-process
for a purification system that uses an electrochemical cell that
accepts VRB electrolyte on both positive and negative
electrodes;
[0035] FIG. 13 is a diagram illustrating an electrochemical cell
that can be used in each of the reduction and oxidation
sub-processes illustrated, respectively, in FIGS. 12A and 12B;
and
[0036] FIG. 14 is a diagram illustrating the reactions present in
the electrochemical cell of FIG. 13.
DETAILED DESCRIPTION
[0037] In some aspects, the present disclosure is directed to
methods and systems for removing impurities from an electrolyte
solution using a chemical reduction and filtration process. A
particularly useful embodiment is described below in the context of
a vanadium redox flow battery (VRFB) in which one or more
impurities in the vanadium-based electrolyte solution that would in
the normal course clog the electrochemical cell of the VRFB during
normal use are removed. An oxidation process, such as an
electrochemical oxidation process, can be used after filtration to
adjust the valence of the reduced electrolyte solution to a desired
value, often an electrolyte 3.5 valence.
[0038] In other aspects, the present disclosure is directed to
methods and systems directed to and relating to commissioning a
redox flow battery (RFB). In a particular embodiment, an
electrolyte solution is first reduced to a relatively low oxidation
valence and then oxidized to make negative and positive electrolyte
solutions having differing valences. The negative and positive
electrolyte solutions are installed into the negative and positive
sides of an RFB, respectively, thereby eliminating the need for the
typical battery-charging process that accompanies conventional
battery commissioning. These and other aspects of the present
disclosure are described and exemplified below in detail.
[0039] Referring now to the drawings, FIG. 1 illustrates an overall
process of making an electrolyte solution having a desired valence
by reduction-based purification followed by filtration and
oxidation-based valence adjustment. To understand the purification
process, i.e., the electrochemical reduction and filtering shown in
FIG. 1, it must be understood that the purification at issue
involves the removal of solid precipitates that form from
impurities in the electrolyte solution at a relatively low vanadium
valence. In some electrolyte solutions, certain impurities are in
aqueous states when the valence of the electrolyte solution is
relatively high but are transformed to their solid forms when the
valence of the electrolyte solution is sufficiently low. Depending
on the use of the electrolyte solution, the solids formed when the
electrolyte solution's valence is relatively low can be detrimental
to that use. For example, in the context of an RFB, impurity solids
on the negative side of the RFB's electrochemical cell can clog the
negative electrode, causing the electrical performance of the RFB
to degrade as the negative electrode becomes increasingly clogged
by the solids over time.
[0040] Using a VRFB as an example, in order to manufacture
electrolyte solutions for a VRFB cost effectively, an inexpensive
source of vanadium oxides is required. Generally, low price
vanadium oxides have high levels of impurities that end up in the
electrolyte solutions and have a large negative impact on VRFB
performance. As noted above, the most detrimental impurities are
the ones that are aqueous in the as-received electrolyte solution,
but will precipitate out as solids that clog the negative electrode
after the system's initial charge. In other words, the most
detrimental impurities are soluble/aqueous in an electrolyte
solution of vanadium (III)/(IV) ions but are insoluble (i.e. form
solid precipitates) in a solution of vanadium (II)/(III) ions. The
purification process illustrated in FIG. 1 can readily be used to
filter the precipitatable impurities out of the electrolyte
solution prior to using the solution in a VRFB.
[0041] Referring particularly to FIG. 1, this figure illustrates
purification and valence-adjustment performed in accordance with
the present disclosure in the context of an example electrolyte
solution in which the electrolyte has four possible oxidation
states, here, +2, +3, +4, and +5. As seen in zone 100 of FIG. 1,
the purification process involves first reducing the electrolyte
solution to a valence that is lower than the valence at which the
target impurity(ies) precipitate(s) out of the electrolyte
solution. The starting valence in zone 100 can be anywhere from 3.5
to 5.0, depending on the vanadium oxide feedstock used; in FIG. 1
we assume V.sub.2O.sub.5 is the sole feedstock and the resulting
starting valence is 5.0. In the example illustrated, the
electrolyte solution is reduced approximately to its lowest
oxidation state (here, an oxidation state of (II) (2.0)). For a
vanadium-based electrolyte solution, for example, target impurities
precipitate out of the solution at a valence at or below a valance
of 3.0. Consequently, in some embodiments, the reduction need only
take the valence of the electrolyte solution below about 3.0, such
as 2.9 to 2.0. However, a lower valence can be desirable to
accelerate the reduction process. The reduction can be performed
using any suitable process, such as electrochemical reduction using
an electrochemical cell. In one example, a hybrid electrochemical
cell may be used, wherein one half-cell utilizes a VRB electrolyte,
and the other half-cell uses a non-VRB reductant. In another
example, the electrochemical cell may be configured to accept a VRB
electrolyte on both halves of the cell.
[0042] Once the impurities precipitate out of the electrolyte
solution, in zone 104 of FIG. 1 solid precipitates are removed from
the solution. Precipitate removal can be performed using any
suitable removal means, such as via one or more porous filters
and/or one or more cyclones, among other things. Depending on the
use of the electrolyte solution, only the precipitates of a certain
size may need to be removed, such that the removal equipment can be
designed accordingly. For example, in a VRFB that includes a
negative electrode having a particular pore size, precipitates of a
certain size smaller than this pore size may not need to be
filtered, since they can flow through the negative electrode and
not clog it.
[0043] At an optional step, illustrated in zone 108 of FIG. 1, the
purified electrolyte solution, i.e., the solution having the
desired amount of precipitate removed, may be oxidized to a desired
valence. As an example and using a vanadium electrolyte, the
purified electrolyte solution may be oxidized so that it has a mix
of vanadium (III) and vanadium (IV) ions and a valence of about
3.5, which is the valence of conventional vanadium-based
electrolyte solution typically provided for commissioning VRFBs.
This mix is represented by line 112 in FIG. 1.
[0044] As another example using vanadium as the electrolyte, and as
shown in FIG. 1, purified electrolyte solution may be oxidized to
create two electrolyte solutions of differing valences, such as one
electrolyte solution of valence at or below 3.0, such as a valence
of 2.5, containing vanadium (II) and vanadium (III) ions and
another electrolyte solution of valence at or above 4.0, such as a
4.5 valence, containing vanadium (IV) and vanadium (V) ions. These
two electrolyte solutions are represented, respectively, in FIG. 1
by lines 116 and 120. In this example, these two electrolyte
solutions can be used, respectively, as the negative and positive
electrolyte solutions of a VRFB.
[0045] As mentioned above, when a VRFB is conventionally
commissioned, an electrolyte solution of 3.5 valence is provided to
both the negative and positive electrolyte tanks of the VRFB, and
then the battery is charged so that the negative-side electrolyte
solution settles at a valence of about 2.5 and the positive-side
electrolyte solution settles at a valence of about 4.5. However,
when the method of FIG. 1 is used to make the negative (such as a
2.5 valence) and positive (such as a 4.5 valence) electrolyte
solutions and these electrolyte solutions are added to the VRFB,
the commissioning charge step can be eliminated. It is noted that
if the feedstock electrolyte solution is sufficiently pure, i.e.,
is sufficiently devoid of precipitatable impurities, then the
impurity removal step can be eliminated in the process of making
the negative and positive electrolyte solutions. This embodiment
may be particularly useful when a high-oxidation-state material,
such as V.sub.2O.sub.5 is used as sole vanadium feedstock for
making the electrolyte.
[0046] In some embodiments, any of the methods represented by FIG.
1 may be performed at an electrolyte solution production facility
such that the resulting electrolyte solution(s) is/are transported
to the location of use, such as VRFB installation quite remote from
the production facility. However, the cost of transporting
electrolyte solutions can be quite high due to factors such as
distance, corrosivity of the electrolyte solutions, and the fact
that a large percentage of the weight of the solutions is due to
the solvents (e.g., acid and water) in the solutions. Consequently,
in other embodiments, any of the methods represented by FIG. 1 may
be performed at or in close proximity to the location of use. For
example, components, such as reduction equipment, precipitate
removal equipment, and oxidation equipment, of a system can be
containerized, palletized, or otherwise made readily transportable
so that the method(s) can be performed locally.
[0047] In some embodiments, electrochemical cells are used for
reduction and oxidation, and those cells need to be energized with
electricity to drive the reactions. In such embodiments that are
used proximate to the use locations of the fabricated electrolyte
solutions and in which the solutions are used for batteries for
renewable energy sources, such as wind turbines and solar farms,
the electricity needed for the reduction and/or oxidation processes
can be provided by the renewable energy sources.
[0048] Advantages of methods of the present disclosure over
existing methods include: [0049] No prior method utilizes
precipitation of impurities for removal. Precipitated impurities
are removed mechanically, for example, via filtration and/or a
hydrocyclone. [0050] No prior method utilizes both an
electrochemical oxidation and reduction of the electrolyte for the
formation and removal of key impurities. [0051] No prior method
uses electrochemical cells for reduction and oxidation of the VRB
electrolyte. An example of a cell for vanadium-based electrolyte
reduction is a hydrogen/VRB cell, where H.sub.2 gas is oxidized
(consumed) at the cathode and VRB electrolyte is reduced at the
anode. The oxidation cell is similar: H.sub.2 is produced on the
anode, and VRB electrolyte is oxidized on the cathode. [0052] No
other method allows the production of both separate anolyte and
catholyte solutions for a VRFB system using the same
electrochemical system.
[0053] FIG. 2 illustrates an example purification process 200 that
uses two electrochemical sub-processes, namely, a
reduction/purification sub-process 200(1) for reducing and
purifying the electrolyte solution and an oxidation sub-process
200(2) for adjusting the valence of the electrolyte solution to a
desired valence. Process 200 of this example enables the use of
vanadium oxides of a single oxidation state and of reduced purity.
The formation and purification of the electrolyte solution in
example process 200 is performed in a continuous manner, and the
process is designed to operate at room temperature.
[0054] Example process 200 removes impurities that are the most
detrimental to VRFB performance, namely, impurities that are
aqueous in a mixed vanadium (III)/(IV) solution but that will form
solid precipitates after charging to make vanadium (II)/(III)
solution. As noted above, it is these precipitates that can clog
the negative electrode. The purification performed by process 200
relies on the discovery that the most detrimental impurities will
precipitate out in a vanadium (II)/(III) solution and that these
impurities can be removed mechanically, such as by filtration
and/or cycloning, among other things. In this example, other
impurities, such as K, Na, and Al, are not removed in process 200,
because they remain as aqueous ions in a vanadium (II)/(III)
solution; however, the impact of these aqueous ions on battery
performance is negligible.
[0055] Electrochemical reduction and subsequent oxidation of the
vanadium-based electrolyte are critical to its formation and
purification. In this example, the reduction and oxidation of the
electrolyte solution are performed in two separate sub-processes,
each using one or more of its own electrochemical cells. It is
noted that for the sake of illustration, only a single
electrochemical cell is shown for each of the two sub-processes
200(1) and 200(2), though each sub-process could use two or more
electrochemical cells.
[0056] Reduction/purification sub-process 200(1), wherein
electrochemical reduction of the vanadium electrolyte occurs, is
performed by a "reduction cell" 204, such as a hybrid
electrochemical cell. Electricity (not illustrated) is provided to
operate reduction cell 204, and vanadium-based electrolyte solution
208 is reduced on the cathode 204C, and a reductant 212 is oxidized
on the anode 204A. In the illustrated example, reductant 212 is
H.sub.2 gas, which provides for a relatively simple and inexpensive
system. However, other reductants, such as water, formic acid,
ethylene glycol, among others, could be used for reductant 212.
[0057] FIG. 3A illustrate an example construction of a reduction
cell 300 that can be used for reduction cell 204 of FIG. 2.
Referring to FIG. 3A, example reduction cell 300 includes an
electrolyte flow field 304, a gas flow field 308 (here, for H.sub.2
gas), a proton-exchange membrane 312, a carbon-paper electrode 316,
and a gas diffusion layer 320 coated with an H.sub.2 oxidation
reaction catalyst (not shown). Electrolyte solution 208 (FIG. 2) is
flowed into electrolyte flow field 304, while reductant 212, here,
H.sub.2 gas, is flowed into gas flow field 308. FIG. 3B illustrates
the operation example reduction cell 300, with block 324
representing the anode side of the reduction cell receiving H.sub.2
reductant 212, block 328 representing the cathode side of the
reduction cell receiving electrolyte solution 208, and block 332
representing proton-exchange membrane 312 that allows hydrogen
protons to pass from the cathode side to the anode side, with the
flow of electrons following suit. FIG. 3B also illustrates the
reduction of vanadium-based electrolyte solution 208 and the
oxidation of H.sub.2 reductant 212. Those skilled in the art will
readily understand, the construction of reduction cell 300
illustrated in FIG. 3A is merely an example and that other
constructions may be used as desired.
[0058] Referring again to FIG. 2, reduction/purification
sub-process 200(1) performs two functions: 1) it creates vanadium
(II) ions that aid in the chemical dissolution and reduction of an
electrolyte 216, here V.sub.2O.sub.5 powder, and 2) it precipitates
out the most deleterious impurities 220 and enables their removal
via mechanical means (e.g., filtration and/or cycloning). In the
example shown in FIG. 2, system 200 includes one or more suitable
filters, here a coarse filter 224 and a pair of fine filters 228(1)
and 228(2). The result is a purified electrolyte solution 208P that
is purified to the extent that some or all of the precipitated
impurities 220 have been removed, again, here, by filters 224,
228(1), and 228(2).
[0059] Reduction/purification process 200(1) may be controlled by
an appropriate controller 264 that controls the process. Controller
264 may include any suitable hardware, such as a programmable logic
controller, general purpose computer, application-specific
integrated circuit, or any other hardware device(s) capable of
executing a suitable control algorithm. Many types of hardware
suitable for controller 264 are well known in the art. In one
example, controller 264 is configured, via software or otherwise,
to control the valence of the electrolyte solution flowing out of
reduction cell 204, here electrolyte solution 208(2.0), by
controlling the flow of impure electrolyte solution 208 from tank
252 into the reduction cell. In this example, inputs to the control
algorithm include user settings, such as the electrical current
within reduction cell 204, and a valence measurement of the
electrolyte in tank 252 by a suitable valence sensor (not shown).
For example, with a fixed cell current, controller 264 determines
whether or not the measured valence is below a target value (e.g.,
the precipitation valence) and outputs one or more control signals
that control one or more flow-control devices (not shown) that in
turn control the flow of impure electrolyte solution 208 into
reduction tank 252. Such flow-control devices may be, for example,
one or more pumps, one or more valves, or one or more devices that
changes the flow of impure feedstock into tank 252, or any
combination of these, among others. In other embodiments,
controller 264 can be configured to control both cell current and
flow of impure electrolyte solution 208 so as to control the
valence of the electrolyte solution within tank 252. Those skilled
in the art will readily understand how to create a suitable
algorithm for the control scheme selected for controller 264 based
on this disclosure and for the type of hardware used.
[0060] Oxidation sub-process 200(2) in this example uses
electrochemical oxidation to oxidize the purified electrolyte
solution to bring it up to its desired valence state (such as near
a 3.5 valence, near a 2.5 valence, and/or near a 4.5 valence,
depending on the desired use). In this example, a hybrid
electrochemical cell, referred to herein as an "oxidation cell"
232, is used to drive the oxidation in response to electricity (not
illustrated) provided to the cell. Purified vanadium-based
electrolyte solution 208P is oxidized on the anode 232A, and an
oxidant 236 is reduced on the cathode 232C. In one example, protons
are reduced to form H.sub.2 gas on cathode 232C. This is
particularly convenient when H.sub.2 gas is used as reductant 212
in the reduction cell 204 as mentioned above. After oxidation, the
valence-adjusted electrolyte solution(s) may be optionally
transferred to a VRFB, as illustrated at box 240.
[0061] FIG. 4A illustrate an example construction of a reduction
cell 400 that can be used for oxidation cell 232 of FIG. 2.
Referring to FIG. 4A, example oxidation cell 400 includes an
electrolyte flow field 404, an oxidant flow field 408 (here, for
H.sub.2 gas), a proton-exchange membrane 412, an oxidation-side
carbon-paper electrode 416, a reduction-side carbon-paper electrode
420, and a gas diffusion layer 424 coated with an H.sub.2 oxidation
reaction catalyst (not shown). Electrolyte solution 208 (FIG. 2) is
flowed into electrolyte flow field 404, while oxidant 236, here,
protons, are formed in the oxidant flow field 408. FIG. 4B
illustrates the operation of example oxidation cell 400, with block
428 representing the cathode side of the oxidation cell that
receives H.sub.2O 236, block 432 representing the anode side of the
oxidation cell that receives electrolyte solution 208P, and block
436 representing proton-exchange membrane 412 that allows hydrogen
protons to pass from the cathode side to the anode side, with the
flow of electrons following suit. FIG. 4B also illustrates the
oxidation of vanadium-based electrolyte solution 208P and the
reduction of the protons into H.sub.2 gas, which is removed by
H.sub.2O 236. Those skilled in the art will readily understand the
construction of reduction cell 400 illustrated in FIG. 4A is merely
an example and that other constructions may be used as desired.
[0062] Inputs for example process 200 of FIG. 2 are: [0063]
Materials: [0064] Generally, a solvent 244 for dissolving metal
oxides 216, here V.sub.2O.sub.5 powder. For the vanadium compound
of the illustrated example, this solvent is composed of one or more
strong acids, such as sulfuric acid and/or hydrochloric acid. In
other embodiments, other solvents, such as hydrobromic acid, and
chloric acid, among others, can be used. [0065] If the forgoing
solvent is not a polar solvent, a polar solvent 248. For the
illustrated vanadium compound, water is used. [0066] An
electrolyte, such as electrolyte 216. In the present example, the
electrolyte is vanadium pentoxide (V.sub.2O.sub.5) powder. In other
embodiments one or more vanadium oxides may be used, such as
vanadium (III) oxide (V.sub.2O.sub.3) alone or in combination with
vanadium (V) oxide (V.sub.2O.sub.5). In other embodiments, other
electrolytes, such as iron-chrome flow battery, and all uranium
flow battery, among others, can be used. [0067] A proton-donating
reductant, such as reductant 212. In the present example, H.sub.2
gas is used. In other embodiments, another reductant, such as
hydrogen, water, formic acid, ethylene glycol, H.sub.2O.sub.2,
among others, can be used. [0068] A proton-consuming oxidant, such
as oxidant 232. In this example, protons were reduced to H.sub.2
gas. An alternative could be to use oxygen or Air, where O.sub.2 is
reduced to H.sub.2O. [0069] Electricity: [0070] Electricity (not
illustrated) provides the energy input into both the reduction cell
and oxidation cell.
[0071] The final purified and adjusted electrolyte solution 208P+A
in the present example (i.e., a vanadium-based electrolyte solution
valence-adjusted to 3.5 valence) may contain: [0072] electrolyte
with a supporting acid solution (generally sulfuric acid and/or
hydrochloric acid); [0073] a balance of vanadium (III) and (IV)
ions in solution in generally a 1:1 ratio; and [0074] additional
additives for thermal stabilization.
[0075] In this example, reduction/purification sub-process 200(1)
involves the electrochemical reduction of vanadium electrolyte
solution 208. The chemical/electrochemical reactions for this
exemplary reduction/purification sub-process 200(1) are shown below
in Table I.
TABLE-US-00001 TABLE I Eqn. 1: V.sub.2O.sub.5(s) + 2H.sup.+
.fwdarw. V.sub.2O.sub.5 dissolution 2VO.sub.2.sup.+ + H.sub.2O Eqn.
2: 2VO.sub.2.sup.+ + 4V.sup.2+ + Chemical Reduction of V(V)
reduction 8H.sup.+ .fwdarw. 6V.sup.3+ + 4 H.sub.2O by V(II) Eqn. 3:
6V.sup.3+ + 6e- .fwdarw. 6V.sup.2+ Electrochemical reduction of
V(III) ion in reduction cell - cathode reaction Eqn. 4: 3H.sub.2
.fwdarw. 6H.sup.+ + 6e- Electrochemical oxidation of reductant in
reduction cell (H.sub.2 as example reductant) - anode reaction Eqn.
5: V.sub.2O.sub.5(s) + 4H.sup.+ + Net reaction for
reduction/purification 3H.sub.2 .fwdarw. 2V.sup.2+ + 5H.sub.2O
sub-process
[0076] One of the main challenges of using V.sub.2O.sub.5 powder
(electrolyte 216) is its limited solubility in strong acid
solutions. Instead of making a vanadium (V) electrolyte solution,
this exemplary process utilizes a reduced electrolyte solution of
predominantly vanadium (II) ions to both dissolve and reduce the
V.sub.2O.sub.5 powder. V.sub.2O.sub.5 powder, water, and acid are
slowly added to a well-mixed tank (tank 252 in FIG. 2) containing
electrolyte solution 208 of predominately vanadium (II) ions. The
valence of predominantly vanadium (II) solution 208 in tank 252 is
maintained via reduction cell 204. When the V.sub.2O.sub.5 powder
(metal oxide 216) is dissolved (Table I, Eqn. 1) and reduced (Table
I, Eqn. 2) in predominantly vanadium (II) solution 208, other
vanadium ions are oxidized (Table I, Eqn. 2). This oxidation of
vanadium ions is balanced using reduction cell 204, which
continuously reduces electrolyte solution 208 to maintain a
constant valence. Measurements from a set of sensors (not shown)
that measure the vanadium (II) and (III) concentration and the
total volume may be used to control the feed rates of
V.sub.2O.sub.5 (metal oxide 216), acid (solvent 244), and water
(polar solvent 248) into Tank 252, the electrical current into
reduction cell 204, and the extraction rate of approximately 2.0
valence electrolyte 208(2.0) from the outlet 2040 of the reduction
cell to a tank 256 of oxidation sub-process 200(2). Examples of
sensors for measuring the vanadium (II) and (III) concentration
include commercial off-the-shelf optical sensors and
electrochemical cells having reference electrodes, among
others.
[0077] The vanadium (II) ions in tank 252 serve two purposes.
First, they aid in the rapid dissolution and reduction of
V.sub.2O.sub.5 powder (metal oxide 216), as described previously.
Second, the vanadium (II) ions provide a reducing atmosphere to
reduce many of the most deleterious impurities (e.g., impurities
220) to their solid, neutral states. When in their solid states,
the impurities are filtered out using mechanical means, again, here
a series of course filter 224 and fine filter 228(1) placed
upstream of reduction cell 204. In the example shown, coarse filter
224, for example, an activated carbon filter, filters relatively
larger precipitated solids, and fine filters 228(1) and 228(2), for
example, PTFE hydrophilic filters, filter relatively smaller
precipitated particles. As noted above, in a VRFB context, the
level of filtration can be dependent on the pore size of the
negative electrode(s) used in the target VRFB.
[0078] In this example impurities 220 that precipitate out of
reduced electrolyte solution 208 can include, but are not limited
to, As and Ge metal precipitates. Filters 224, 228(1), and 228(2)
before reduction cell 204 serve two functions: they 1) remove
impurities 220 from electrolyte solution 208 and 2) protect the
reduction cell from the precipitated impurities. The second
function is successfully achieved if any of filters 224, 228(1),
and 228(2) has a smaller effective pore size than the carbon-paper
electrode of the reduction cell 204, such as carbon paper electrode
316 of FIG. 3A. The system of filters 224, 228(1), and 228(2) and
reduction cell 204 shown in FIG. 2 allows for the continuous
operation of the reduction cell. Methods that use
electroplating/electrowinning would require frequent chemical
cleaning of the cell or electrode replacement, temporarily shutting
down operation.
[0079] Reduction cell 204 operates by reducing vanadium (III) ions
at cathode 204(C) (Table I, Eqn. 3) and oxidizing reductant 212 at
anode 204(A). As noted above, in this example H.sub.2 gas is used
as reductant 212 (Table I, Eqn. 4), but as also noted above, other
chemical reductants could be used, such as water, formic acid, or
ethylene glycol, among others. In this example, reductant 212, here
H.sub.2 gas, is provided both from an H.sub.2 source 260 and an
oxidation cell 232 of oxidation sub-process 200(2), which produces
H.sub.2 gas. An example of construction of reduction cell 204 and
the appropriate half-cell reactions are shown in FIGS. 3A and 3B,
respectively. Downstream of outlet 2040 (FIG. 2) of reduction cell
204, a portion of purified electrolyte solution 208P is returned to
tank 252 to aid in the dissolution and reduction of V.sub.2O.sub.5
powder (metal oxide 216), and a portion is moved to oxidation
sub-process 200(2).
[0080] Oxidation sub-process 200(2) involves oxidizing purified
electrolyte solution 208P. The chemical/electrochemical reactions
for this exemplary process are shown below in Table II.
TABLE-US-00002 TABLE II Eqn. 6: H.sup.+ + e- .fwdarw. 1/2 H.sub.2
Hydrogen gas generation (cathode reaction) Eqn. 7: V.sup.3+ +
H.sub.2O .fwdarw. Vanadium electrolyte oxidation (anode VO.sup.2+ +
2H.sup.+ + e.sup.- reaction) Eqn. 8: V.sup.3+ + H.sub.2O .fwdarw.
Net reaction for oxidation cell process 1/2H.sub.2 + VO.sup.2+ +
H.sup.+
[0081] Purified electrolyte solution 208P of average valence that
is below the critical precipitation valence (i.e. below 2.9)
transferred from sub-process 200(1) into tank 256. Purified
electrolyte solution 208P in tank 256 is kept just below the final
desired valence (generally 3.5 in this example). Purified
electrolyte solution 208P from tank 252 is pumped into oxidation
cell 232, which oxidizes the purified electrolyte solution to the
desired final valence to make valence-adjusted electrolyte solution
208P+A. A portion of the output of oxidation cell 232 is returned
to tank 256 to maintain a constant valence, and, in the present
example, the remainder is transferred into a VRFB, as indicated by
box 240.
[0082] In this example, electrochemical oxidation cell 232 oxidizes
purified electrolyte solution 208P and reduces protons (i.e.,
produces H.sub.2 gas). Specifically, oxidation cell 232 oxidizes
vanadium (III) ions at the anode 232A via the half-reaction shown
in Table II, Equation 7, and reduces protons to form H.sub.2 gas at
the cathode 232C, as described by the half-reaction shown in Table
II, Equation 6. The net reaction for example oxidation cell 232 is
given in Table II, Equation 8. In oxidation cell 232, water is
circulated on the H.sub.2-producing side (i.e., cathode 232C), as
it helps wash away any vanadium ions that migrate over the membrane
232M to the cathode.
[0083] As described above, an alternative process could produce
purified valence-adjusted electrolyte solution 208P+A at any
desired valence. For example, oxidation cell 232 could oxidize a
first batch of purified valence-adjusted electrolyte solution
208P+A to 2.5 valence for the negative electrolyte solution of a
VRFB and a second batch of the purified valence-adjusted
electrolyte solution to 4.5 valence for the positive electrolyte
solution of the VRFB. Transferring these two separate solutions
into, respectively, the catholyte tank and the anolyte tank of a
VRFB would eliminate the need for the formation charging process
required in commissioning a new battery system.
[0084] In another embodiment, shown in FIGS. 12A and 12B, each of
one or more electrochemical cells, here electrochemical cell 1200
having a negative side 1200N (FIG. 12A) and a positive side 1200P
(FIG. 12B), accepts an electrolyte solution, such as a
vanadium-based electrolyte solution, on both the positive and
negative sides of the cell. This eliminates the need for a separate
reductant for the reduction/purification sub-process and a separate
oxidant for the oxidation sub-process. FIG. 13 illustrates and
example electrochemical cell 1300 that can be used as
electrochemical cell 1200 of FIGS. 12A and 12B. Referring to FIG.
13, electrochemical cell 1300 may have a symmetric design comprised
of negative and positive flow fields 1304N, and 1304P,
respectively, negative and positive carbon paper electrodes 1308N
and 1308P, respectively, and a proton-exchange membrane 1312
between the negative and positive sides of the cell.
[0085] In one example for purifying a vanadium-based electrolyte
solution, the starting impure electrolyte solution 1204 (FIG. 12A)
may be made of roughly equal amounts vanadium (III) and vanadium
(IV) and contains at least one impurity to be removed by the
process. Impure electrolyte solution 1204 could be made, for
example, using either of the methods described in the Background
section above (i.e., either Method 1 or Method 2). In the process
illustrated in FIGS. 12A and 12B, the negative electrode 1200N of
each electrochemical cell 1200 reduces electrolyte solution 1204 to
below a critical impurity precipitation valence (see, e.g., zone
104 of FIG. 1), while the positive electrode 1200P of each
electrochemical cell oxidizes vanadium (III) to vanadium (IV).
Upstream of the negative electrode(s) 1200N (FIG. 12A), one or more
filters, represented schematically at 1212 of FIG. 12A and which
can be the same as or similar to filters 224, 228(1), and 228(2) of
FIG. 2, capture precipitated impurities in the negative-side
electrolyte solution 1204N prior to them entering negative
electrode 1200N.
[0086] After passing through filter(s) 1212, a portion of this
now-purified electrolyte 1204P solution can, for example, be
transferred to a positive electrolyte tank 1216 (FIG. 12B). This
transfer process counteracts the electrochemical oxidation of the
fluid in the positive-side electrolyte loop 1220 (FIG. 12B) and
maintains an approximately 3.5 valence. Similarly, starting
electrolyte solution 1200 (FIG. 12A) of lower-purity is slowly
transferred to a negative-side electrolyte tank 1224. This transfer
chemically counteracts the electrochemical reduction and maintains
a valence at or below the critical precipitation valence. The
solution in positive-side electrolyte loop 1228 (FIG. 12B) (stored
in positive-side tank 1216) is free of the key contaminants that
would precipitate in a VRFB system. The positive-side purified and
oxidized electrolyte solution 1204P+O is free deleterious
impurities and is transferred to a holding tank 1232, where it may
be stored prior to use, for example, in a VRFB system. While this
embodiment adds additional material costs for the electrolyte due
to the chemical reducing agent or the reduced vanadium oxide, its
primary function is to remove key impurities from the electrolyte.
When individual electrochemical cells use the same basic
electrolyte solution on both their positive and negative sides,
these cells may be conveniently called "electrolyte-only
electrochemical cells."
[0087] It is noted that on the reduction/purification side (FIG.
12A), the reduction/purification process 1236 may be controlled by
an appropriate controller 1240 that controls the
reduction/purification sub-process. Like controller 264 of FIG. 2,
controller 1240 of FIG. 12A may include any suitable hardware, such
as programmable logic controller, general purpose computer,
application-specific integrated circuit, or any other hardware
device(s) capable of executing a suitable control algorithm. Many
types of hardware suitable for controller 1240 are well known in
the art. In one example, controller 1240 is configured, via
software or otherwise, to control the valence of the electrolyte
solution flowing out of negative side 1200N of reduction cell 1200,
here electrolyte solution 1204P by controlling the flow of impure
electrolyte solution 1204 from negative-side electrolyte tank 1224
into the reduction cell. In this example, inputs to the control
algorithm include user settings, such as the electrical current
within reduction cell 1200, and a valence measurement taken of the
electrolyte 1208 in the tank 1224 by a suitable valence sensor. For
example, with a fixed cell current, controller 1240 determines
whether or not the measured valence is below a target value (e.g.,
the precipitation valence) and outputs one or more control signals
that control one or more flow-control devices (not shown) that
control the flow of impure electrolyte solution 1204 into reduction
cell 1200. Such flow-control devices may be, for example, one or
more pumps, one or more valves, or one or more devices that changes
the flow impure feedstock into the negative-side electrolyte tank
1224, or any combination of these, among others. In other
embodiments, controller 1240 can be configured to control both cell
current and impure electrolyte solution flow so as to control the
valence of the electrolyte solution (here, solution 1208). Those
skilled in the art will readily understand how to create a suitable
algorithm for the control scheme selected for controller 1240 based
on this disclosure and considering the type of hardware used.
[0088] FIG. 14 illustrates the operation of example electrochemical
cell 1200 (FIGS. 12A and 12B), with block 1400 representing the
cathode side of the electrochemical cell that receives electrolyte
solution 1204 from negative side electrolyte tank 1224 (FIG. 12A),
block 1404 representing the anode side of the electrochemical cell
that receives electrolyte solution 1204 (mix) from positive-side
tank 1216 (FIG. 12B), and block 1408 representing proton-exchange
membrane 1312 (FIG. 13) that allows hydrogen protons to pass from
the cathode side to the anode side. Those skilled in the art will
readily understand the construction of electrochemical cell 1200
illustrated in FIGS. 12A and 12B is merely an example and that
other constructions may be used as desired.
[0089] Lab-Scale Experimental Results
[0090] Overview and Summary of Lab Scale Results
[0091] A purification process in accordance with aspects of the
present disclosure was demonstrated on the lab scale. Following is
an overview of that process. [0092] A desirable industrial-scale
system is a system that performs continuous electrolyte
formation/purification. However, for proof of concept at the lab
scale, a batch process was used. [0093] Reduction of the 3.5
valence initial electrolyte solution was done using a H.sub.2/VRB
hybrid electrochemical cell 500 (FIG. 5) identical to the cell
illustrated in FIG. 3A. [0094] Oxidation of the purified
electrolyte solution was performed in a hybrid electrochemical cell
800 (FIG. 8) identical to the cell illustrated in FIG. 4A.
[0095] Reduction/Purification Sub-process Demonstration at
Lab-Scale
[0096] Two samples of an initial electrolyte solution having a
starting valence of 3.5, i.e., Batch 1 and Batch 2, were subjected
to a version of the reduction/purification process described above.
Each sample was 3 liters in volume and was reduced to a solution of
2.0 valence. The vanadium content of each sample was between 1.4
mol/liter and 1.65 mol/liter. The same reduction cell 500 (FIG. 5)
(no electrode replacement or membrane replacement) was used for
both samples. The construction of reduction cell 500 is identical
in construction and operation to reduction cell 300 as illustrated
in FIGS. 3A and 3B, respectively. The active area of reduction cell
500 (FIG. 5) was 23 cm.sup.2.
[0097] FIG. 5 illustrates the lab-scale reduction/purification
sub-process used in the testing of Batches 1 and 2. A hydrogen
cylinder 504 provided hydrogen to an H.sub.2/VRB cell 508 on the
positive electrode 508P. The dry hydrogen was humidified by passing
it through deionized water (DI) in a bubbler 512. A flow regulator
516 controlled the flow of hydrogen into H.sub.2/VRB cell 500. A
VRB electrolyte 520 was stored in a plastic tank 524. VRB
electrolyte 520 in tank 524 was recirculated through H.sub.2/VRB
cell 508 on the negative electrode 508N and passed through two
filters 528(1) and 528(2) prior to entering the cell. Recirculation
was performed by a peristaltic pump (not shown). A DC current (not
illustrated) was applied to H.sub.2/VRB cell 508, which oxidized
the hydrogen and reduced the vanadium in VRB electrolyte 520.
Filters 528(1) and 528(2) upstream of H.sub.2/VRB cell 508
consisted, respectively, of a coarse filter (activated carbon) and
a fine filter (hydrophilic PTFE filter with 0.5-micron pore size).
Cell voltages in H.sub.2/VRB cell 500 are shown in FIG. 6A for
Batch 1 and in FIG. 7A for Batch 2, and cell pressures are shown in
FIG. 6B for Batch 1 and in FIG. 7B for Batch 2. Open-circuit
voltages reported in FIGS. 6A and 7A were measured by periodically
removing DC current from the cell. The reduction/purification
sub-process appears to have been successful in both instances,
demonstrating two key concepts, namely: [0098] The pressure (FIGS.
6B and 7B) measured upstream of filters 528(1) and 528(2) (FIG. 5)
increased as the process continued. This pressure rise indicates
that filters 528(1) and 528(2) are successfully capturing
impurities that are precipitating. [0099] The pressure (FIGS. 6B
and 7B) just upstream of reduction cell 508 (FIG. 5) does not
increase during the sub-process, indicating that filters 528(1) and
528(2) protect the reduction cell from clogging because of
precipitated impurities.
[0100] Oxidation Sub-process Demonstration at Lab-Scale
[0101] Batch 1 of the purified electrolyte solution from the
reduction/purification sub-process was oxidized to a 3.5 valence
using an H.sub.2/VRB cell 800 (FIG. 8), which was identical in
construction and operation to the construction and operation of
oxidation cell 400 shown in FIGS. 4A and 4B, respectively. The
batch-type oxidation sub-process performed at the lab scale is
illustrated in FIG. 8. DI water was stored in a plastic tank 804,
where it is recirculated through H.sub.2/VRB cell 800 on the
negative electrode. The flow of DI water helped remove any vanadium
that migrated over to the negative electrode 800N. VRB electrolyte
808 was stored in plastic tank 812 and was recirculated through
H.sub.2/VRB cell 800 on the positive electrode 800P of the cell. A
DC current (not illustrated) was applied to H.sub.2/VRB cell 800,
which reduced protons to hydrogen gas and oxidized the vanadium in
VRB electrolyte 808. The voltage and current densities within
H.sub.2/VRB cell 800 are shown, respectively, in FIGS. 9A and 9B.
The active area of H.sub.2/VRB cell 800 (FIG. 8) was 23
cm.sup.2.
[0102] Verification of Electrolyte Purification at Lab-Scale
[0103] Batch 1 was tested in a sub-scale VRB system to verify that
the key impurities had been removed to an acceptable level in the
electrolyte. The performance of the electrolyte before and after
the purification process is shown in FIG. 10, which compares the
negative electrode pressures during operation before and after the
lab-scale purification process. Full cell operation metrics for the
purified Batch 1 electrolyte (voltages, cell resistances, cycle
performance, and pressures) are shown in FIGS. 11A to 11C. In FIG.
11B, "CE" stands for Coulombic efficiency, "EE" stands for energy
efficiency, and "Util" stands for utilization of vanadium. Data
represented in FIG. 11C are for negative-side pressure and
positive-side pressure with a lab-scale full VRB cell with
respective vanadium-based electrolyte solutions flowing on
corresponding respective sides of the cell.
[0104] An overview of the system parameters and specifications is
given below and illustrated in FIGS. 11A to 11C: [0105] Cycling
parameters: [0106] Open-circuit voltage at end of discharge=1.28 V
[0107] Open-circuit voltage at end of charge=1.52 V [0108] Max
charge cell voltage=1.6 V [0109] System Specifications: [0110] 1.46
mol/liter V [0111] 3.50 valence [0112] 2.8 liter total system
volume (positive and negative electrolytes) [0113] 2.times. Carbon
paper electrodes [0114] Proton exchange membrane [0115] 11 cm.sup.2
active area
[0116] The foregoing has been a detailed description of
illustrative embodiments of the invention. It is noted that in the
present specification and claims appended hereto, conjunctive
language such as is used in the phrases "at least one of X, Y and
Z" and "one or more of X, Y, and Z," unless specifically stated or
indicated otherwise, shall be taken to mean that each item in the
conjunctive list can be present in any number exclusive of every
other item in the list or in any number in combination with any or
all other item(s) in the conjunctive list, each of which may also
be present in any number. Applying this general rule, the
conjunctive phrases in the foregoing examples in which the
conjunctive list consists of X, Y, and Z shall each encompass: one
or more of X; one or more of Y; one or more of Z; one or more of X
and one or more of Y; one or more of Y and one or more of Z; one or
more of X and one or more of Z; and one or more of X, one or more
of Y and one or more of Z.
[0117] Various modifications and additions can be made without
departing from the spirit and scope of this invention. Features of
each of the various embodiments described above may be combined
with features of other described embodiments as appropriate in
order to provide a multiplicity of feature combinations in
associated new embodiments. Furthermore, while the foregoing
describes a number of separate embodiments, what has been described
herein is merely illustrative of the application of the principles
of the present invention. Additionally, although particular methods
herein may be illustrated and/or described as being performed in a
specific order, the ordering is highly variable within ordinary
skill to achieve aspects of the present disclosure. Accordingly,
this description is meant to be taken only by way of example, and
not to otherwise limit the scope of this invention.
[0118] Exemplary embodiments have been disclosed above and
illustrated in the accompanying drawings. It will be understood by
those skilled in the art that various changes, omissions and
additions may be made to that which is specifically disclosed
herein without departing from the spirit and scope of the present
invention.
* * * * *