FLOW BATTERY WITH VOLTAGE-LIMITING DEVICE
BACKGROUND
[0001] This disclosure relates to flow batteries for selectively storing and discharging
electric energy.
[0002] Flow batteries, also known as redox flow batteries or redox flow cells, are
designed to convert electrical energy into chemical energy that can be stored and later released
when there is demand. As an example, a flow battery may be used with a renewable energy
system, such as a wind-powered system, to store energy that exceeds consumer demand and later
release that energy when there is greater demand.
[0003] A typical flow battery includes a redox flow cell that has a negative electrode
and a positive electrode separated by an electrolyte layer, which may include a separator, such as
an ion-exchange membrane. A negative liquid electrolyte is delivered to the negative electrode
and a positive liquid electrolyte is delivered to the positive electrode to drive electrochemically
reversible redox reactions. Upon charging, the electrical energy supplied causes a chemical
reduction reaction in one electrolyte and an oxidation reaction in the other electrolyte. The
separator prevents the electrolytes from mixing but permits selected ions to pass through to
complete the redox reactions. Upon discharge, the chemical energy contained in the liquid
electrolytes is released in the reverse reactions and electrical energy can be drawn from the
electrodes. Flow batteries are distinguished from other electrochemical devices by, inter alia, the
use of externally-supplied, liquid electrolyte solutions that include reactants that participate in
reversible electrochemical reactions.
SUMMARY
[0004] Disclosed is a flow battery that includes at least one cell that has a first
electrode, a second electrode spaced apart from the first electrode and an electrolyte separator
layer that is arranged between the first electrode and the second electrode. A storage portion is
fluidly connected with the at least one cell. At least one liquid electrolyte includes an
electrochemically active specie and is selectively deliverable to the at least one cell. An electric
circuit is coupled with the first electrode and the second electrode. The circuit includes a voltagelimiting
device that is configured to limit a voltage potential across the first electrode and the
second electrode in response to a transition of the at least one cell from an inactive, shut-down
mode with respect to an active, charge/discharge mode.
[0005] Also disclosed is a method of controlling corrosion in a flow battery. The
method includes transitioning a flow battery to or from an inactive, shut-down mode with respect
to an active, charge/discharge mode. The flow battery includes at least one cell having a first
electrode, a second electrode spaced apart from the first electrode, and an electrolyte separator
layer arranged between the first electrode and the second electrode. A voltage potential across
the first electrode and the second electrode of the flow battery is limited during the transitioning
using a voltage-limiting device arranged in an electric circuit that is electrically coupled with the
first electrode and the second electrode
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] The various features and advantages of the present disclosure will become
apparent to those skilled in the art from the following detailed description. The drawings that
accompany the detailed description can be briefly described as follows.
[0007] Figure 1 shows an example flow battery having a voltage-limiting device.
[0008] Figure 2 shows another example flow battery having a voltage-limiting device
with a plurality of resistors.
[0009] Figure 3 shows another example of a flow battery having a voltage-limiting
device and gas sources to provide a cover gas.
[0010] Figure 4 shows an example method of controlling corrosion in a flow battery.
DETAILED DESCRIPTION
[0011] Figure 1 schematically shows portions of an example flow battery 20 for
selectively storing and discharging electrical energy. As an example, the flow battery 20 may be
used to convert electrical energy generated in a renewable energy system to chemical energy that
is stored until a later time when there is greater demand at which the flow battery 20 then
converts the chemical energy back into electrical energy. The flow battery 20 can supply the
electric energy to an electric grid, for example. As will be described, the disclosed flow battery
20 includes features for enhanced corrosion protection.
[0012] The flow battery 20 includes a liquid electrolyte 22 that has an
electrochemically active specie 24 that functions in a redox pair with regard to an additional
liquid electrolyte 26 and electrochemically active specie 28. For example, the electrochemically
active species 24 and 28 are based on vanadium, bromine, iron, chromium, zinc, cerium, lead or
combinations thereof. In embodiments, the liquid electrolytes 22 and 26 are aqueous solutions
that include one or more of the electrochemically active species 24 and 28. Alternatively, a
single one of the liquid electrolytes 22 or 26 can be used in combination with a gaseous reactant.
In this regard, the flow battery 20 has at least one liquid electrolyte (22 or 26) that has an
electrochemically active specie (24 or 28).
[0013] The liquid electrolytes 22 (e.g., an anolyte) and 26 (e.g., a catholyte) are
contained in a storage portion 30 that includes respective storage tanks 32 and 34. As shown, the
storage tanks 32 and 34 are substantially equivalent cylindrical storage tanks; however, the
storage tanks 32 and 34 can alternatively have other shapes and sizes.
[0014] The liquid electrolytes 22 and 26 are delivered (e.g., pumped) to one or more
cells 36 of the flow battery 20 through respective feed lines 38 and are returned from the cell or
cells 36 to the storage tanks 32 and 34 via return lines 40. The cell 36 includes a first electrode
42, a second electrode 44 spaced apart from the first electrode 42, and an electrolyte separator
layer 46 arranged between the first electrode 42 and the second electrode 44. In general, the cell
or cells 36 can include bipolar plates, manifolds and the like for delivering the liquid electrolytes
22 and 26 through flow field channels to the electrodes 42 and 44. However, it is to be
understood that other configurations can be used. For example, the cell or cells 36 can
alternatively be configured for flow-through operation where the liquid electrolytes 22 and 26
are pumped directly into the electrodes 42 and 44 without the use of flow field channels.
[0015] The electrolyte separator layer 46 can be an ionic-exchange membrane, or a
micro-porous polymer membrane, or an electrically insulating microporous matrix of a material
like SiC that prevents the liquid electrolytes 22 and 26 from rapidly mixing but permits selected
ions to pass through to complete the redox reactions while electrically isolating the two
electrodes 42 and 44. Multiple cells 36 can be provided in a stack.
[0016] The liquid electrolytes 22 and 26 are delivered to the cell 36 to either convert
electrical energy into chemical energy or, in the reverse reaction, convert chemical energy into
electrical energy that can be discharged. The electrical energy is transmitted to and from the cell
36 through an electric circuit 48 that is electrically coupled with the electrodes 42 and 44. The
electric circuit 48 includes a voltage-limiting device 50. As will be described, the voltage
limiting-device 50 is configured to limit a voltage potential V across the electrodes 42 and 44
upon transition of the cell or cells 36 to or from an inactive, shut-down mode with respect to an
active, charge/discharge mode.
[0017] The flow battery 20 has several modes of use, including the inactive, shut
down mode. The modes are represented by different physical states of the flow battery 20. For
example, the flow battery 20 has an active, charge/discharge mode in which the liquid
electrolytes 22 and 26 are continuously circulated from the storage tanks 32 and 34 through the
cell or cells 36 and back into the storage tanks 32 and 34. In the charge/discharge mode, the flow
battery is either being charged with electrical energy from the electric circuit 48 or is discharging
electrical energy to the electric circuit 48. Additionally, the cell or cells 36 are substantially or
completely filled with the liquid electrolytes 22 and 26. For example, during the
charge/discharge mode, the porous volume of the cell or cells are ideally 100% filled with the
liquid electrolytes 22 and 26 and, at a minimum, are 90% filled with the liquid electrolytes 22
and 26.
[0018] The flow battery 20 is transitioned from the charge/discharge mode into an
inactive, shut-down mode when not in use for charging or discharging, or vice versa for start-up.
There can be numerous different procedures for shut-down of the flow-battery 20, depending
upon which of numerous inactive, shut-down modes is selected. In one example, the liquid
electrolytes 22 and 26 are static in the inactive, shut-down mode, at least with regard to active
pumping, such that the liquid electrolytes 22 and 26 do not flow through the cell or cells 36 and
are primarily held in the respective storage tanks 32 and 34.
[0019] In a further example, the cell or cells 36 are at least partially drained of the
liquid electrolytes 22 and 26 in the shut-down mode such that the cell or cells 36 are partially or
fully empty. For example, the cell or cells 36 are more than 90% empty with regard to liquid
electrolytes 22 and 26 in the porous volume of the cell or cells 36. In this case, the porous
volumes of the cell or cells 36 are mostly filled with gases from the head-space of the storage
tanks displaced by the liquid volume drained from the cells. Air may also infiltrate the system
and enter the cells when the cells are drained of liquid.
[0020] In a further example, the cell or cells 36 are at least partially drained of the
liquid electrolytes 22 and 26 in the shut-down mode, as described above, and the empty volume
is provided with a chemically and electrochemically inert cover gas. For example, the cover gas
includes nitrogen, argon, helium or combinations thereof. In a further example, the cover gas has
a purity of 90% or greater of the given gas or gases. The cover gas serves to blanket the cell or
cells 36 in a relatively chemically inert environment to facilitate the prevention of air or oxygen
entering into the cell or cells 36 and protect against oxidation corrosion or self-discharge
reactions with the electrochemically active species..
[0021] In a further example, the cell or cells 36 are partially or fully filled with the
liquid electrolytes 22 and 26 in the shut-down mode and permitted to self-discharge to a
discharged state. The discharged liquid electrolytes 22 and 26 are held, without active pumping
circulation, in the cell or cells 36 during the period of the inactive, shut-down mode. The
discharged liquid electrolytes 22 and 26 serve to blanket the cell or cells 36 to limit exposure to
air or oxygen that may infiltrate the flow battery 20.
[0022] If in the inactive, shut-down mode, the flow battery 20 must be started to
bring it out of the inactive, shut-down mode and into the active, charge/discharge mode.
Conversely, if in the charge/discharge mode, the flow battery 20 must be stopped or shut-down
to bring it into the inactive, shut-down mode. For example, the start-up period begins with the
initiation of circulation of the liquid electrolytes 22 and 26 from the storage tanks 32 and 34 and
ends once the liquid electrolytes 22 and 26 substantially or completely fill the cell or cells 36 as
described above in the active charge/discharge mode. The shut-down period begins with the
stopping of circulation of the liquid electrolytes 22 and 26 from the storage tanks 32 and 34 and
ends once the start-up period is initiated.
[0023] The flow battery 20 is susceptible to corrosion during these transition periods
of starting-up or shutting-down, which can debit the performance and useful life of the flow
battery 20. The type and degree of corrosion can depend on which of the inactive, shut-down
modes are used. For example, in the inactive, shut-down mode with the cell or cells 36 at least
partially empty, the free volume of the cell or cells 36 can contain air and moisture that has
infiltrated the flow battery 20. A minor amount of hydrogen may also be present from
spontaneous self-discharging of the liquid electrolytes 22 and 26 during the inactive, shut-down
mode. Upon start-up, the liquid electrolytes 22 and 26 are fed into the cell or cells 36. As the
liquid electrolytes 22 and 26 fill the feed inlet of cell or cells 36, they generate a cell voltage, as
is the case during normal operation. However, this cell voltage is also present at the exits of the
cells, since the cell components are good electrical conductors, yet the liquid electrolytes 22 and
26 have not yet reached the cell exits during the start-up period, and air/oxygen and water are
present at the exits. If the cell voltage exceeds a threshold, the electric potential generated in the
electrodes 42 and 44 can drive undesired corrosion reactions between the oxygen, water, and/or
hydrogen and the materials of the cell or cells 36, such as to convert carbon of carbon-based
components to carbon dioxide. In one example, the threshold in the flow battery 20 is 1.5 volts
per cell for driving carbon corrosion reactions.
[0024] Somewhat similarly, in the inactive, shut-down modes that have the
discharged liquid electrolytes 22 and 26 held in the cell or cells 36 or that use the cover gas, the
electric potential at the exit of the cell or cells 36 can exceed a threshold that drives undesired
corrosion reactions. The threshold, and thus the severity of the corrosion reactions, can vary
depending upon the selected inactive, shut-down mode and the length of the shut-down period.
Similar phenomenon can occur upon shutting down from the charge/discharge mode into the
shut-down mode.
[0025] To control or limit the electric potentials generated in the electrodes 42 and 44
during the transition period, the flow battery 20 includes the voltage-limiting device 50. The
voltage-limiting device 50 facilitates the reduction in the cell voltages during transition and
thereby reduces the undesired corrosion reactions that can occur. For instance, the voltagelimiting
device 50 can be a resistor, a rheostat, a controllable resistor such as a gate transistor, a
plurality of resistors that are electrically connected in parallel or series or even an external load,
such as a traditional rechargeable battery.
[0026] The voltage-limiting device 50 has an electric impedance that is selected to
correspond to the voltage generated by the particular flow battery 20 during the transition period.
For example, the selected electric impedance limits the cell or stack voltage but does not draw so
much current as to generate a starvation condition of the cell or cells 36 with regard to the
amount of liquid electrolytes 22 and 26 available for reaction. That is, a non-zero voltage
potential below the threshold for driving corrosion reactions is desired.
[0027] In a further example, an instantaneous desired amount of electric impedance
through the transition period as the liquid electrolytes 22 and 26 fill the cell or cells 36 changes.
Thus, the electric impedance of the voltage-limiting device 50 can be varied through the
transition period to maintain or control the electric potential with regard to an identified
threshold. For example, for a start-up period, the voltage-limiting device 50 can initially provide
a relatively low electric impedance that is then increased through the start-up period as the liquid
electrolytes 22 and 26 fill the cell or cells 36. The inverse may be used for a shut-down.
[0028] Figure 2 illustrates selected portions of another example flow battery 120. In
this disclosure, like reference numerals designate like elements where appropriate and reference
numerals with the addition of one-hundred or multiples thereof designate modified elements that
are understood to incorporate the same features and benefits of the corresponding elements. In
this example, an electric circuit 148 of the flow battery 120 includes a voltage-limiting device
150 that has a plurality of resistors 150a. The resistors 150a can be connected in series or in
parallel.
[0029] The voltage-limiting device 150 is in communication with a controller 152.
The controller 152 may also be connected with other components of the flow battery 120, such
as pumps, valves, gas sources, etc., to control the operation thereof. The controller 152 can
include software, hardware or both and is operable to control operation of the voltage-limiting
device 150 with respect to the resistors 150a. Given this description, one of ordinary skill in the
art will be able to provide appropriate hardware and/or programming to operate the controller
152 as described herein. In one example, the controller 152 can individually control each of the
resistors 150a between an ON (resistance) and OFF (no resistance) state to vary the overall
electric impedance of the voltage-limiting device 150 through the transition period.
[0030] Figure 3 illustrates selected portions of another example flow battery 220 that
is somewhat similar to the flow battery 20 depicted in Figure 1 but also includes gas sources 260.
The gas sources 260 can be fluidly connected with the respective storage tanks 32 and 34 and the
respective electrodes 42 and 44. The gas sources include one or more of the cover gases
described above and serve to provide the cover gas to a header volume of the storage tanks 32
and 34 and to the cell or cells 36. Alternatively, a single gas source can be used to provide the
cover gas to a header volume of the storage tanks 32 and 34 by fluidly connecting the gas
reservoirs at the top of the storage tanks 32 and 34. This single gas source can also be used to
provide the cover gas to both electrodes of the cell or cells 36.
[0031] Figure 4 shows an example method 470 of controlling a voltage potential in a
flow battery during a mode transition to control corrosion, such as in any of the flow batteries 20,
120 and 220 described herein. That is, the flow batteries 20, 120 and 220 embody the method
470 and vice versa. The method 470 includes step 472 of transitioning the flow battery to or from
the inactive, shut-down mode with respect to the active, charge/discharge mode and step 474 of
limiting a voltage potential across a first electrode and a second electrode of the flow battery
during and in response to the transitioning by using a voltage-limiting device arranged in an
electric circuit that is electrically coupled with the first electrode and the second electrode. The
method 470 can further include any or all of the operational actions described herein with regard
to the flow batteries 20, 120 and 220.
[0032] In one further example related to start-up from the inactive, shut-down mode
to the active, charge/discharge mode, the voltage-limiting device 50/150 is optionally disengaged
in response to the start-up period ending. That is, the voltage-limiting device 50/150 ceases to
provide any impedance.
[0033] In one further example related to start-up from the inactive, shut-down mode
to the active, charge/discharge mode, the method 470 includes limiting or controlling the voltage
potential across the electrodes by controlling the active flow of the liquid electrolytes 22 and 26.
For example, the start-up includes feeding the liquid electrolytes 22 and 26 into the cell or cells
36 such that the liquid electrolytes 22 and 26 are fed one at a time. Feeding the liquid electrolytes
22 and 26 one at a time can lower the cell voltages generated by the inlet region of the cell or
cells 36 and thus reduces or avoids driving any corrosion reactions. In one example, the anolyte
22 is introduced first, followed by the catholyte 24, after the anolyte is exiting the cells.
[0034] In one further example related to stopping from the active, charge/discharge
mode inactive into the inactive, shut-down mode, the method 470 includes limiting or controlling
the voltage potential across the electrodes by stopping (i.e. no active pumping) the flow of the
liquid electrolytes 22 and 26 into the cell or cells 36 one at a time. For example, the catholyte 24
flow is stopped first, followed by the anolyte 22, after the cell voltages have decreased below the
cell voltage threshold.
[0035] In a further example related to stopping from the active, charge/discharge
mode into the inactive, shut-down mode, the voltage limiting device can be utilized to lower the
cell voltages more rapidly after flow of one or both of the electrolytes are stopped. As an
example, the after the catholyte 24 flow is stopped the voltage limiting device can be used to
bring the average cell voltage down to desired shut-down voltage (e.g., less than 0.2 V per cell)
and then the anolyte 22 is stopped. In this manner, the cells are shut down in a known state;
namely, to an electrochemical potential that is close to the anolyte potential, which is
significantly lower than the catholyte potential and will thereby minimize undesired oxidation
reactions that can occur during the shut-down period.
[0036] In a further example related to stopping from the active, charge/discharge
mode inactive into the inactive, shut-down mode, the method 470 can include filling the
electrodes 42 and 44 with the cover gas, as described herein.
[0037] Although a combination of features is shown in the illustrated examples, not
all of them need to be combined to realize the benefits of various embodiments of this disclosure.
In other words, a system designed according to an embodiment of this disclosure will not
necessarily include all of the features shown in any one of the Figures or all of the portions
schematically shown in the Figures. Moreover, selected features of one example embodiment
may be combined with selected features of other example embodiments.
[0038] The preceding description is exemplary rather than limiting in nature.
Variations and modifications to the disclosed examples may become apparent to those skilled in
the art that do not necessarily depart from the essence of this disclosure. The scope of legal
protection given to this disclosure can only be determined by studying the following claims.

What is claimed is:
1. A flow battery comprising:
at least one cell including a first electrode, a second electrode spaced apart from the first
electrode, and an electrolyte separator layer arranged between the first electrode and the second
electrode;
a storage portion fluidly connected with the at least one cell;
at least one liquid electrolyte including an electrochemically active specie, the at least one
liquid electrolyte being selectively deliverable to the at least one cell; and
an electric circuit electrically coupled with the first electrode and the second electrode,
the electric circuit including a voltage-limiting device configured to limit a voltage potential
across the first electrode and the second electrode in response to a transition of the at least one
cell to or from an inactive, shut-down mode with respect to an active, charge/discharge mode.
2. The flow battery as recited in claim 1, wherein the voltage-limiting device is selected
from the group consisting of a resistor or a plurality of resistors, a rheostat, a controllable resistor
and combinations thereof.
3. The flow battery as recited in claim 1, wherein the voltage-limiting device is a
rechargeable battery.
4. The flow battery as recited in claim 1, wherein the voltage-limiting device has a plurality
of resistors, and each of the plurality of resistors is individually controllable between ON/OFF
conditions.
5. The flow battery as recited in claim 1, further comprising a controller in communication
with the voltage-limiting device, the controller being configured to control operation of the
voltage-limiting device with regard to an ON/OFF condition thereof.
6. The flow battery as recited in claim 5, wherein the controller is operable to control
operation of the voltage-limiting device with regard to an amount of electric impedance.
7. The flow battery as recited in claim 1, wherein, in the inactive, shut-down mode, there is
no active flow of the at least one liquid electrolyte from the storage portion to the at least one
cell.
8. The flow battery as recited in claim 7, wherein, in the inactive, shut-down mode, the at
least one cell is at least partially empty of the at least one liquid electrolyte.
9. The flow battery as recited in claim 7, wherein, in the inactive, shut-down mode, the at
least one cell includes a cover gas selected from the group consisting of air, nitrogen, argon,
helium, hydrogen and combinations thereof.
10. The flow battery as recited in claim 7, wherein, in the inactive, shut-down mode, the at
least one cell includes the at least one liquid electrolyte in a discharged state.
11. A method of controlling a voltage potential in a flow battery during a mode transition to
control corrosion, the method comprising:
(a) transitioning a flow battery to or from an inactive, shut-down mode with respect to an
active, charge/discharge mode, the flow battery including at least one cell having a first
electrode, a second electrode spaced apart from the first electrode, and an electrolyte separator
layer arranged between the first electrode and the second electrode; and
(b) limiting a voltage potential across the first electrode and the second electrode of the
flow battery during step (a) using a voltage-limiting device arranged in an electric circuit that is
electrically coupled with the first electrode and the second electrode.
12. The method as recited in claim 11, including limiting the voltage potential across the first
electrode and the second electrode to be below a predefined threshold voltage potential.
13. The method as recited in claim 11, wherein, in the inactive, shut-down mode, there is no
active flow of at least one liquid electrolyte of the flow battery from a storage portion of the flow
battery to the at least one cell.
14. The method as recited in claim 13, wherein the transitioning includes starting the flow
battery from the inactive, shut-down mode to the active, charge/discharge mode, and during the
starting of the flow battery, the at least one cell of the flow battery is partially empty of at least
one liquid electrolyte.
15. The method as recited in claim 13, wherein the transitioning includes starting the flow
battery from the inactive, shut-down mode to the active, charge/discharge mode, and during the
starting of the flow battery, the at least one cell includes a cover gas therein that is select from
the group consisting of air, nitrogen, argon, helium, hydrogen and combinations thereof.
16. The method as recited in claim 11, wherein the transitioning includes starting the flow
battery from the inactive, shut-down mode to the active, charge/discharge mode, and during the
starting of the flow battery, feeding a positive liquid electrolyte and a negative liquid electrolyte
to, respectively, the first electrode and the second electrode such that the positive liquid
electrolyte and the negative liquid electrolyte are fed one at a time.
17. The method as recited in claim 11, wherein the transitioning includes starting the flow
battery from the inactive, shut-down mode to the active, charge/discharge mode, and disengaging
the voltage-limiting device such that the voltage-limiting device ceases to limit the voltage
potential across the first electrode and the second electrode.
18. The method as recited in claim 11, wherein the transitioning includes stopping the flow
battery from the active, charge/discharge mode into the inactive, shut-down mode, and during the
stopping, stopping active flow of a positive liquid electrolyte and a negative liquid electrolyte
through, respectively, the first electrode and the second electrode such that active flow of the
positive liquid electrolyte and the negative liquid electrolyte are stopped one at a time.
19. The method as recited in claim 18, further including limiting the voltage potential across
the first electrode and the second electrode of the flow battery after stopping flow of one of the
positive liquid electrolyte and the negative liquid electrolyte and before stopping flow of the
other of the negative liquid electrolyte and the positive liquid electrolyte.
20. The method as recited in claim 11, wherein the transitioning includes stopping the flow
battery from the active, charge/discharge mode into the inactive, shut-down mode, and filling the
first electrode and the second electrode with a cover gas selected from the group consisting of
air, nitrogen, argon, helium, hydrogen and combinations thereof.