SYSTEM AND METHOD FOR SENSING AND MITIGATING
HYDROGEN EVOLUTION WITHIN A FLOW BATTERY SYSTEM
BACKGROUND
1. Technical Field
[0001] This disclosure relates generally to flow batteries and, in particular, to a system
and method for sensing and mitigating hydrogen evolution within a flow battery system.
2. Background Information
[0002] A typical flow battery system includes a flow battery stack, an anolyte reservoir
and a catholyte reservoir. An anolyte solution is circulated between the anolyte reservoir and the
flow battery stack. A catholyte solution is circulated between the catholyte reservoir and the
flow battery stack.
[0003] During operation, the flow battery stack may convert electrical energy into
chemical energy, and store the chemical energy in the anolyte and catholyte solutions. Hydrogen
evolution within the anolyte solution, however, may also occur as the electrical energy is being
converted to chemical energy. The term "hydrogen evolution" describes a secondary reaction
where positively charged hydrogen ions combine with negatively charged electrons to form
hydrogen gas. The formation of hydrogen within the anolyte solution may decrease system
efficiency and may also create an imbalance between the states of charge of the anolyte and
catholyte solutions. It also may result in unsustainable changes to the composition of the
solutions, which may require these solutions to be replenished. There is a need for a system and
method for sensing and mitigating hydrogen evolution within a flow battery system.
BRIEF DESCRIPTION OF THE DRAWINGS
[0004] FIG. 1 illustrates a flow battery system;
[0005] FIG. 2 illustrates a cross-section of a flow battery cell;
[0006] FIG. 3 is a flow chart illustration of steps for charging a flow battery system in a
manner that mitigates hydrogen evolution;
[0007] FIG. 4 is a graphical illustration of a first curve showing sensed electrical current
versus time, and a second curve showing electrical potential of a first solution versus time; and
[0008] FIG. 5 illustrates an alternative embodiment flow battery system.
DETAILED DESCRIPTION
[0009] FIG. 1 illustrates a flow battery system 10. The flow battery system 10 includes a
first reservoir 12, a second reservoir 14, a first solution flow circuit 16, a second solution flow
circuit 1 , a flow battery stack 20, an electrochemical cell 22, a valve 24, a purge gas reservoir
26, a purge gas flow regulator 28, a power converter 30, and a controller 32.
[0010] The first reservoir 12 has an exterior reservoir wall 34 and contains a first solution
(e.g., a vanadium anolyte) within an interior reservoir cavity 36, where the first solution has a
first reversible reduction-oxidation ("redox") couple reactant (e.g., V + and/or V3+ ions). The
second reservoir 14 has an exterior reservoir wall 38 and contains a second solution (e.g., a
vanadium catholyte) within an interior reservoir cavity 40, where the second solution has a
second reversible redox couple reactant (e.g., V + and/or V5+ ions).
[001 1] The first and second solution flow circuits 16 and 18 each include a source
conduit 42, 44, a return conduit 46, 48 and a solution flow regulator 50, 52, respectively. The
solution flow regulator 50, 52 may include a variable speed pump connected, for example, inline
within the source conduit 42, 44.
[0012] The flow battery stack 20 includes one or more flow battery cells 54.
[0013] FIG. 2 illustrates a cross-section of one of the flow battery cells 54 shown in FIG.
1. Each flow battery cell 54 includes a first current collector 56, a second current collector 58, a
liquid-porous first electrode layer 60, a liquid-porous second electrode layer 62, and a separator
64 between the first and second electrode layers 60 and 62. The first electrode layer 60 may be
an anode, and the second electrode layer 62 may be a cathode. The separator 64 may be an ionexchange
membrane (e.g., a Nafion® polymer membrane manufactured by DuPont of
Wilmington, Delaware, United States). The electrode layers 60 and 62 are positioned between
the first and second current collectors 56 and 58. Additional examples of flow battery cells are
disclosed in PCT/US09/68681, and U.S. Patent Application Serial Nos. 13/084,156 and
13/023,101, each of which is incorporated by reference in its entirety.
[0014] Referring to FIGS. 1 and 2, the source conduit 42 fluidly connects the first
reservoir 12 to the flow battery stack 20 such that the first current collector 56 and/or the first
electrode layer 60 in each flow battery cell 54 receives the first solution. The return conduit 46
reciprocally connects the flow battery stack 20 to the first reservoir 2 such that the first
reservoir 12 receives the first solution from the first current collector 56 and/or the first electrode
layer 60 in each flow battery cell 54. The source conduit 44 fluidly connects the second
reservoir 14 to the flow battery stack 20 such that the second current collector 58 and/or the
second electrode layer 62 in each flow battery cell 54 receives the second solution. The return
conduit 48 reciprocally connects the flow battery stack 20 to the second reservoir 14 such that
the second reservoir 14 receives the second solution from the second current collector 5 and/or
the second electrode layer 62 in each flow battery cell 54.
[0015] Referring to FIG. 1, the electrochemical cell 22 may be configured as a hydrogen
sensor. The electrochemical cell 22 includes a gas-porous first electrode layer 65, a gas-porous
second electrode layer 66, a separator 68, and a current sensor 70. The first electrode layer 65
may be an anode, and the second electrode layer 66 may be a cathode. The separator 68 may be
a proton-exchange or anion exchange electrolyte layer. The separator 68 is configured between
the first and second electrode layers 65 and 66 such that the electrode layers 65 and 66 and the
separator 68 may form a fuel cell 72. Other examples of a fuel cell are disclosed in U.S. Patent
Nos. 5,156,929 and 6,617,068, each of which is hereby incorporated by reference in its entirety.
The current sensor 70 is electrically connected between the first and second electrode layers 65
and 66.
[0016] The valve 24 may be a one-way check valve. The valve 24 fluidly connects the
first reservoir 2 to the electrochemical cell 22 and, in particular, a top region of the interior
reservoir cavity 36 to the first electrode layer 65. The first electrode layer 65 therefore is located
outside of the exterior reservoir wall 34 in the embodiment shown in FIG. 1.
[0017] The purge gas reservoir 26 contains purge gas such as, for example, nitrogen (N2)
gas. Other examples of purge gas include inert gases such as carbon dioxide (C0 2) gas, Argon
gas, etc.
[001 ] The purge gas flow regulator 28 may include a variable speed pump or an
electronically actuated valve (e.g., a one-way valve). The purge gas flow regulator 28 fluidly
connects the purge gas reservoir 26 to the first reservoir 12.
[0019] The power converter 30 may include a two-way power converter or a pair of one
way power converters. The power converter 30 may be configured as, for example, a two-way
power inverter or a two-way DC/DC converter connected to a DC bus (not shown). The power
converter 30 is electrically connected to the flow battery stack 20. For example, the power
converter 30 may be electrically connected to the first and second current collectors 56 and 58.
[0020] The controller 32 may be implemented using hardware, software, or a
combination thereof. The hardware may include, for example, one or more processors, a
memory, analog and/or digital circuitry, etc. The controller 32 is in signal communication (e.g.,
hardwired or wirelessly connected) with the flow regulators 50 and 52, the current sensor 70, the
purge gas flow regulator 28 and the power converter 30.
[0021] The flow battery system 10 may be operated in an energy storage mode to store
energy in the first and second solutions, or in an energy discharge mode to discharge energy
from the first and second solutions. During both modes of operation, the controller 32 signals
the solution flow regulator 50 to circulate the first solution between the first reservoir 12 and the
flow battery stack 20 through the first solution flow circuit 16. The controller 32 signals the
solution flow regulator 52 to circulate the second solution between the second reservoir 14 and
the flow battery stack 20 through the second solution flow circuit 18. The controller 32 also
signals the power converter 30 to exchange electrical current with (e.g., provide electrical current
to, or receive electrical current from) the flow battery stack 20 and, thus, the flow battery cells 54
at a rate that corresponds to a selected current density within the cells 54. The term "current
density" describes a ratio of (i) total current delivered to or drawn from the flow battery stack 20
to (ii) an active area (not shown) of one of the flow battery cells 54, and in particular, of the
separator 64 (see FIG. 2). Alternatively, the electrical energy may be exchanged such that there
is a substantially constant exchange of power between the power converter 30 and the flow
battery stack 20, or the voltage of the power converter may be held constant. Or, any
combination of galvanostatic, potentiostatic, or constant power modes may be utilized.
[0022] During the energy storage mode of operation, the electrical energy provided to
the flow battery stack 20 from the power converter 30 is converted to chemical energy. The
conversion process occurs through electrochemical reactions in the first solution and the second
solution, and a transfer of non-redox couple reactants (e.g., H+ ions) from the first solution to the
second solution across each of the flow battery cells 54 and, in particular, each of the separators
64. The chemical energy is then stored in the first and second solutions, which are respectively
stored in the first and second reservoirs 12 and 14. During the energy discharge mode of
operation, the chemical energy stored in the first and second solutions is converted back to
electrical current through reverse electrochemical reactions in the first solution and the second
solution, and the transfer of the non-redox couple reactants from the second solution to the first
solution across each of the flow battery cells 54. The electrical current is then provided to the
power converter 30 from the flow battery stack 20.
[0023] Hydrogen evolution may occur within the first solution during the energy storage
mode of operation when, for example, the first solution has reached an especially high state of
charge (e.g., greater than approximately 90% of the V+3 ions have been converted to V+2 ions).
The term "hydrogen evolution" describes a secondary reaction to the desired energy storage
process where positively charged hydrogen ions combine with negatively charged electrons. For
example, instead of the desired energy storage reaction (2V + 2e ® 2V ) occurring, the
following secondary hydrogen evolution reaction occurs: 2H+ + 2e ® H2. The electrons are
produced by the reaction in the second solution (e.g., 2V+4 ® 2V+5 + 2e~). Disadvantageously,
the formation of hydrogen within the first solution may decrease system efficiency since the
electrical energy is not converted into the stored chemicals (i.e., the redox couples).
Additionally, the secondary reaction can result in an imbalance between the states of charge of
the first solution and the second solution.
[0024] FIG. 3 illustrates a method for charging the flow battery system 10 in a manner
that mitigates hydrogen evolution. For ease of explanation, the following description begins
with an assumption that (i) the state of charge of the first solution is less than approximately
80%, and/or (ii) little or no hydrogen evolution is occurring within the first solution. Referring
to FIGS. 1 and 3, in step 300, the controller 32 signals the power converter 30 to provide
electrical power (e.g., constant current) to the flow battery stack 20 such that the flow battery
cells 54 are operated at a first energy input rate.. The electrical power may be controlled at a
substantially constant current, power, and/or voltage.
[0025] In step 302, the controller 32 signals the purge gas flow regulator 28 to provide
purge gas to the first reservoir 12. The injected purge gas creates a positive pressure within the
first reservoir 1 such that the purge gas and other gases within the first reservoir 12 flow
through the valve 24 and into the first electrode 65. The positive pressure as well as the valve 24
reduce / prevent a backflow of gas (e.g., air) from entering the first reservoir 12 from the
electrochemical cell 22. In an alternative embodiment, step 302 may be omitted where
overpressure created by the evolution of hydrogen pushes gases within the first reservoir 1
through the valve 24 and into the first electrode 65.
[0026] In step 304, reactant (e.g., air) is provided to the second electrode 66. The
reactant may be provided via an electronically actuated reactant regulator (not shown), or by
diffusion where the second electrode is simply exposed to ambient air.
[0027] Hydrogen may form within the first solution through hydrogen evolution when,
for example, the first solution has reached a relative high (e.g., > 80-90%) state of charge. In
step 306, the electrochemical cell 22, which may be operated with a relatively low potential (e.g.,
0.2 volts) and/or a relatively low resistance across the electrodes 65 and 66, generates an
electrical current when hydrogen formed by hydrogen evolution is provided to the first electrode
65 along with the purge gas. The electrical current is generated through electrochemical
reactions on either side of the separator 68, as in a fuel cell 72.
[0028] In step 308, the current sensor 70 senses the electrical current generated by the
electrochemical reaction between the hydrogen and the reactant, and provides a current signal
indicative of the sensed electrical current to the controller 32.
[0029] In step 310, the controller 32 processes the current signal, and provides a control
signal to the power converter 30. The current signal may be processed, for example, by
comparing a value of the curre it signal to one or more threshold values. Each threshold value is
indicative of a predetermined current signal value.
[0030] FIG. 4 is a graphical illustration of (i) a first curve 400 showing the sensed
electrical current versus time, and (ii) a second curve 402 showing electrical potential (vs. a
hydrogen reference electrode) of the first solution versus time. An example of a threshold value
is shown at time t33 where the sensed electrical current becomes greater than zero, which
corresponds to when hydrogen begins to form within the first solution. Another threshold value
is shown at time t3 (e.g., where the sensed electrical current is equal to approximately 1.8 amps),
which corresponds to when the first solution becomes overcharged (i.e., reaches approximately
100% state of charge).This is also where the electrical potential of first solution levels off
because the side reaction (hydrogen evolution) begins consuming substantially all of the current,
which is not desirable and is why a method to detect this condition before it occurs is
advantageous. FIG. 4 therefore illustrates how the electrochemical cell 22 may detect hydrogen
evolution before, for example, it becomes excessive.
[003 1] The detection of hydrogen evolution within the first solution does not depend on
which flow battery cell, or cells, in the flow battery stack are generating hydrogen since the first
solution (with hydrogen, if generated) returns to the first reservoir 12. Whereas if one tries to use
the flow-battery cell potential, one would need to measure the cell voltages of each of the
individual cells, as well as measure the half-cell potentials using a reference electrode. Such a
method, however, requires a lot of instrumentation and data collection. The control signal is
provided to the power converter 30 to control the exchange of electrical power between the
power converter 30 and the flow battery cells 54 as a function of the electrical current generated
by the electrochemical cell 22. For example, where the current signal value is greater than or
equal to one or more of the threshold values, the control signal may be used to (i) iteratively or
continuously reduce power (e.g., the current density) within the flow battery cells 54, or (ii) stop
the exchange of electrical power between the power converter 30 and the flow battery cells 54.
The current density may be reduced, for example, to a predetermined level that corresponds to a
respective one of the thresholds met by the current signal value. Alternatively, the current density
may be reduced as a function of the electrical current being generated by the electrochemical cell
22 such that as the current signal value increases, the current density decreases.
[0032] The formation of hydrogen within the first solution due to hydrogen evolution
may be mitigated using the aforesaid method. For example, when the current sensor 70 initially
senses an electrical current caused by an electrochemical reaction between the hydrogen and the
reactant, the controller 32 may signal the power converter 30 to stop providing electrical current
to the flow battery stack 20 to prevent additional formation of hydrogen. In another example,
when the current sensor 70 initially senses the electrical current, the controller 32 may signal the
power converter 30 to reduce the electrical current being provided to the flow battery stack 20 to
reduce the hydrogen evolution rate. However, once the current signal value is greater than or
equal to a specified threshold value (e.g., where the rate of hydrogen production is considered to
be excessive), the controller 32 may signal the power converter 30 to stop providing electrical
current to the flow battery stack 20 to prevent the first solution from becoming overcharged and
thus from generating excessive hydrogen.. The aforesaid method may also improve the safety of
the flow battery system 10 by consuming the hydrogen gas produced by hydrogen evolution
through the electrochemical reaction within the electrochemical cell 22.
[0033] In some embodiments, the controller 32 may additionally or alternatively process
the sensed current signal to determine how much hydrogen is being formed within the first
solution using Faraday's Law. The hydrogen that is consumed in the first electrode 65, for
example, is equal to I/2F, where I is the current sensed by the sensor 70 and F is Faraday
constant (96,485 coulombs/mol).
[0034] FIG. 5 illustrates an alternative embodiment of a flow battery system 510. In
contrast to the flow battery system 10 shown in FIG. 1, the first electrode 65 is located within the
exterior reservoir wall 34, while the second electrode 66 remains located outside of the exterior
reservoir wall 34. This configuration may reduce the complexity of the system since, for
example, the valve 24 may be omitted. The purge gas reservoir 26 may also be omitted since the
likelihood of air entering the first electrode 65 and flowing into the first reservoir 12 is greatly
reduced as the first electrode 65 is sealed within the first reservoir 12. The electrodes 65 and 66
and the separator 68 may also be designed and configured in such a way to ensure that most of
the water produced drains back to the first reservoir 36, if desired.
[0035] While various embodiments of the flow battery system have been disclosed, it
will be apparent to those of ordinary skill in the art that many more embodiments and
implementations are possible within the scope of the flow battery system. Accordingly, the
present flow battery system is not to be restricted except in light of the attached claii i s and their
equivalents.
What is claimed is:
1. A flow battery system, comprising:
a first reservoir containing a first solution comprising a first reversible redox couple
reactant;
a plurality of flow battery cells that receive the first solution; and
a hydrogen sensor in fluid communication with the first reservoir.
2. The system of claim 1, wherein the hydrogen sensor comprises an electrochemical cell
that comprises
a first electrode that receives hydrogen from the first reservoir;
a second electrode that receives reactant; and
a current sensor that senses electrical current generated within the electrochemical
cell by an electrochemical reaction between the hydrogen and the reactant, and provides a
current signal indicative of the sensed electrical current.
3. The system of claim 2, wherein the electrochemical cell further comprises one of a proton
exchange electrolyte layer and an anion exchange electrolyte layer that separates the first
electrode from the second electrode.
4. The system of claim 2, wherein:
the first reservoir comprises an anolyte;
the first electrode comprises an anode; and
the second electrode comprises a cathode.
5. The system of claim 2, wherein the reactant comprises air.
6. The system of claim 2, wherein the first electrode is located outside of the first reservoir.
7. The system of claim 6, wherein the first electrode is fluidly connected to the first
reservoir through a one-way valve.
8. The system of claim 6, wherein the first reservoir receives purge gas such that the
hydrogen within the first reservoir flows to the first electrode.
9. The system of claim 2, wherein
the first electrode is located within an exterior reservoir wall of the first reservoir; and
the second electrode is located outside the exterior reservoir wall of the first reservoir.
10. The system of claim 1, further comprising a power converter that exchanges electrical
power with the flow battery cells as a function of a current signal provided by the hydrogen
sensor.
11. The system of claim 10, further comprising a controller that receives the current signal
and provides a control signal to the power converter to control a rate at which the power
converter exchanges electrical power with the flow battery cells.
1 . A method for mitigating hydrogen evolution within a flow battery system that comprises
a plurality of flow battery cells, a power converter and an electrochemical cell, the method
comprising:
providing hydrogen generated by the hydrogen evolution within the flow battery system
to the electrochemical cell;
providing reactant to the electrochemical cell;
generating a first electrical current through an electrochemical reaction between the
hydrogen and the reactant using the electrochemical cell; and
controlling an exchange of electrical power between the flow battery cells and the
power converter in response to the first electrical current.
13. The method of claim 12, wherein the reactant comprises air.
14. The method of claim 12, wherein the controlling of the exchange of the electrical power
comprises stopping the exchange of the electrical power between the flow battery cells and the
power converter when the first electrical current is greater than a threshold value.
15. The method of claim 14, wherein the threshold value corresponds to one of
where a first solution within the flow battery system has greater than approximately
ninety percent state of charge; and
where hydrogen begins to form within the first solution.
16. The method of claim 12, wherein the controlling of the exchange of electrical power
comprises one of
reducing current density within the flow battery cells when the first electrical current is
greater than a threshold value; and
reducing voltage across the flow battery cells when the first electrical current is greater
than the threshold value.
17. The method of claim 16, wherein the threshold value corresponds to one of
where a first solution within the flow battery system has greater than approximately
ninety percent state of charge; and
where hydrogen begins to form within the first solution.
18. The method of claim 1 , wherein the controlling of the exchange of electrical power
comprises:
sensing the first electrical current using a current sensor, and providing a current signal
indicative of the sensed first electrical current to a controller; and
processing the current signal to provide a control signal to the power converter.
19. The method of claim 12, wherein the hydrogen is provided to the electrochemical cell
from a reservoir in the flow battery system that contains a first solution having a first reversible
redox couple reactant.
20. The method of claim 19, further comprising directing the hydrogen from the reservoir to
the electrochemical cell through a valve.
21. The method of claim 19, further comprising purging the hydrogen from the reservoir such
that the hydrogen flows into the electrochemical cell.