FLOW BATTERY WITH HYDRATED ION-EXCHANGE MEMBRANE
HAVING MAXIMUM WATER DOMAIN CLUSTER SIZES
BACKGROUND
[0001] 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.
[0002] 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 fluid electrolyte (sometimes
referred to as the anolyte) is delivered to the negative electrode and a positive fluid
electrolyte (sometimes referred to as the catholyte) 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 freely and rapidly 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, fluid electrolyte
solutions that include reactants that participate in reversible electrochemical reactions.
SUMMARY
[0003] 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 arranged between the first electrode and the second electrode. A supply/storage system
is external of the at least one cell and includes first and second vessels that are fluidly
connected with the at least one cell. First and second fluid electrolytes are located in the
supply/storage system. The electrolyte separator layer includes a hydrated polymeric ionexchange
membrane that has a carbon backbone chain and side chains extending from the
carbon backbone chain. The side chains include hydrophilic chemical groups with water
molecules attached by secondary bonding to form clusters of water domains. The clusters
have an average maximum cluster size no greater than 4 nanometers, with an average number
of water molecules per hydrophilic chemical group, l (lambda), being greater than zero. The
average maximum cluster size of no greater than 4 nanometers and the l (lambda) limit
migration of vanadium or iron ions across the hydrated ion-exchange membrane.
[0004] In another aspect, the hydrated ion-exchange membrane is a
perfluorosulfonic acid membrane that has perfluorinated carbon backbone chain and
perfluorinated carbon side chains that terminate in hydrophilic chemical groups. The average
number of water molecules per hydrophilic chemical group, l (lambda), is greater than zero
and less than or equal to 22.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] 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.
[0006] Figure 1 illustrates an example flow battery.
[0007] Figure 2 illustrates a selected portion of polymer of a hydrated ionexchange
membrane.
[0008] Figure 3 illustrates another example of a polymer of a hydrated ionexchange
membrane.
[0009] Figure 4 illustrates another example of a polymer of a hydrated ionexchange
membrane.
[0010] Figure 5 illustrates another example of a polymer of a hydrated ionexchange
membrane.
[0011] Figure 6 illustrates another example of a polymer of a hydrated ionexchange
membrane.
DETAILED DESCRIPTION
[0012] 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 can
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 time 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.
[0013] The flow battery 20 includes a fluid electrolyte 22 that has an
electrochemically active species 24 that functions in a redox pair with regard to an additional
fluid electrolyte 26 that has an electrochemically active species 28. The electrochemically
active species 24/28 are common and are based on ions of vanadium or iron, for example.
That is, in one example, the electrochemically active species 24/28 are differing oxidation or
valence states of vanadium, and in another example the electrochemically active species
24/28 are differing oxidation or valence states of iron. The fluid electrolytes 22/26 are liquid
solutions that include the electrochemically active species 24/28. The first fluid electrolyte 22
(e.g., the negative electrolyte) and the second fluid electrolyte 26 (e.g., the positive
electrolyte) are contained in a supply/storage system 30 that includes first and second vessels
32/34 and pumps 35.
[0014] The fluid electrolytes 22/26 are delivered using the pumps 35 to at least
one cell 36 of the flow battery 20 through respective feed lines 38 and are returned from the
cell 36 to the vessels 32/34 via return lines 40. The feed lines 38 and the return lines 40
connect the vessels 32/34 with first and second electrodes 42/44. Multiple cells 36 can be
provided as a stack.
[0015] The cell 36 includes the first electrode 42, the second electrode 44 spaced
apart from the first electrode 42, and a hydrated ion-exchange membrane 46 arranged
between the first electrode 42 and the second electrode 44. For example, the electrodes 42/44
are porous carbon structures, such as carbon paper or felt. In general, the cell or cells 36 can
include bipolar plates, manifolds and the like for delivering the fluid electrolytes 22/26
through flow field channels to the electrodes 42/44. The bipolar plates can be carbon plates,
for example. It is to be understood however, that other configurations can be used. For
example, the cell or cells 36 can alternatively be configured for flow-through operation where
the fluid electrolytes 22/26 are pumped directly into the electrodes 42/44 without the use of
flow field channels.
[0016] The hydrated ion-exchange membrane 46 prevents the fluid electrolytes
22/26 from freely and rapidly mixing but permits selected ions to pass through to complete
the redox reactions while electrically isolating the electrodes 42/44. In this regard, the fluid
electrolytes 22/26 are generally isolated from each other during normal operation, such as in
charge, discharge and shutdown states. In one example, the hydrated ion-exchange membrane
46 has a specific conductivity of 0.01 to 0.2 S/cm at 25 °C under 100% relative humidity. In a
further example, the hydrated ion-exchange membrane 46 has an average thickness of 25-178
micrometers and is relatively uniform in thickness.
[0017] The fluid electrolytes 22/26 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 42/44.
[0018] Figure 2 schematically illustrates selected portions of a polymer of the ionexchange
membrane 46. The polymer includes a carbon backbone chain, schematically
represented at 50 and side chains schematically represented at 52 (one representative side
chain shown) that extend from the carbon backbone chain 50. Each of the side chains 52
includes a hydrophilic chemical group schematically represented at 54 with water molecules
56 attached by secondary bonding to the hydrophilic chemical group 54. The hydrophilic
chemical group 54 and the attached water molecules 56 form water domains 58 that have an
average number of water molecules per hydrophilic chemical group 54, l (lambda), that is
greater than zero. For example, l (lambda) is the number of water molecules per hydrophilic
chemical group at 30°C and is less than or equal to 22 or is less than or equal to 14.
[0019] Figure 3 schematically shows another example polymer of an ionexchange
membrane 146. 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, the polymer is or includes
perfluorosulfonic acid that has a perfluorinated carbon backbone chain 150 and
perfluorinated carbon side chains 152 that terminate in a sulfonic acid group, -SO 3H. Water
molecules 156 are attached by secondary bonding to the sulfonic acid group 154 to form
water domains 158 that have an average number of water molecules per hydrophilic chemical
group, l (lambda), that is greater than zero. In a further example, l (lambda) is also less than
or equal to 22 or is less than or equal to 14.
[0020] As shown in Figure 4, the water domains 158 (or alternatively 58) form
clusters 162 of neighboring water domains 158 that can conduct protons and, if large, allow
migration of vanadium or other ions, across the hydrated ion-exchange membrane 146. An
average maximum cluster size 164 of the clusters 162 is no greater than 4 nanometers.
Without being bound to any particular theory, at low water sorption levels, the domains 58
disperse and separate from each other in the polymer matrix such that the clusters 162 are
small or non-existent. When water sorption levels are higher, the size and concentration of
the water domains 58 increase to a certain level and form the clusters 162. Relatively small
hydrophilic channels can form between two adjacent clusters 162. The size of the channels
can be 1 nanometer or smaller.
[0021] For vanadium and other electrochemically active species, there can be a
trade-off in a flow battery between conductivity across the ion-exchange membrane and
selectivity with respect to ions (e.g., vanadium ions) migrating through the ion-exchange
membrane. That is, high conductivity is desired to conduct protons through the ion-exchange
membrane and high selectivity is desired to prevent migration of the vanadium or other
electrochemically active specie ions through the ion-exchange membrane.
[0022] The selected average maximum cluster size 164 with l (lambda) greater
than zero of the ion-exchange membrane 46/146 provides a relatively high conductivity and
relatively high selectivity that is designed for the flow battery 20. Furthermore, the design of
the ion exchange membrane 46/146 for the flow battery 20 can essentially be "locked in"
because the flow battery 20 operates at a design operational temperature that is below the
glass transition temperature of the wet polymer of the ion-exchange membrane 46/146. For
example, fuel cells, which utilize two gaseous reactants (e.g., air and hydrogen), typically
operate at temperatures that exceed the glass transition temperature of their ion-exchange
membranes. Thus, the operational temperature in a fuel cell dictates the polymeric structure
by essentially thermally erasing any prior polymeric structure that may have resulted from
processing of the ion-exchange membrane. However, in the flow battery 20 that has a design
operating temperature below the glass transition temperature of the wet ion-exchange
membrane 46/146, the structure of the polymeric ion-exchange membrane is retained and can
thus be designed, as disclosed herein, for use in the flow battery 20 with regard to having
good conductivity and selectivity.
[0023] The average maximum cluster size 164 no greater than 4 nanometers, with
an average number of water molecules per hydrophilic chemical group, l (lambda), provides
a relatively small size of the clusters 162 and is operable to prevent or reduce migration of the
electrochemically active species, such as vanadium, across the hydrated ion-exchange
membrane 46/146. Vanadium or other electrochemically active species can migrate through
the channels and clusters 162 if they are too large in a flow battery application. Thus, the
hydrated ion-exchange membrane 146 provides good conductivity and good selectivity with
regard to the electrochemically active species in the flow battery 20.
[0024] The hydrated ion-exchange membranes herein can be fabricated using
various techniques. As an example, the formation of an ion-exchange membrane having a
perfluorinated or non-perfluorinated carbon backbone and perfluorinated or nonperfluronated
carbon side chains to provide a base, starting membrane is known and therefore
not described in further detail herein. Typically, a perfluorinated membrane is cast or
extruded, and boiled in a 3% solution of H2O2 for about one hour to remove residue
monomers and solvents, boiled in 0.5M H2SO4 solution for about one hour to obtain a fully
acidized form of the polymer. The membrane can be further boiled in deionized water for
about one hour to remove any residue of the sulfuric acid. In this initial state, absent any
further processing, the membrane does not have a l (lambda) of less than or equal to 22 and
an average maximum cluster size of no greater than 4 nanometers. The membrane is thus
further processed to provide the disclosed l (lambda) of less than or equal to 22 and an
average maximum cluster size of no greater than 4 nanometers using any or all of the belowdescribed
techniques. The cluster sizes and l (lambda) can be determined experimentally
using small-angle X-ray scattering techniques, nuclear magnetic resonance techniques and
differential scanning calorimetry techniques.
[0025] In one example, the membrane is perfluorosulfonic acid polymer and can
be annealed below its glass transition temperature. For instance, the membrane is annealed at
a temperature of 80°-100°C for 24 hours. The annealing reduces extra free volume in the
membrane and thus shrinks the average maximum cluster sizes. The annealing also frees
water and thus can be used to reduce l (lambda). The annealing will also reduce proton
conductivity but will increase selectivity with respect to the electrochemically active species,
such as vanadium ions.
[0026] Alternatively or additionally, the membrane is perfluorosulfonic acid
polymer and can be processed at a temperature below its melting temperature and above its
glass transition temperature to decrease free volume and increase crystallinity. In one
example, the ion-exchange membrane is treated at a temperature of 120°-140°C. Such a
treatment reduces the average maximum cluster size and decreases l (lambda). The increase
in crystallinity also serves to "lock in" the polymeric structure, making the membrane more
resistant to changes in the average cluster size and l (lambda).
[0027] Additionally or alternatively, the membrane is perfluorosulfonic acid
polymer and can be physically altered, such as by stretching, to further increase
crystallization. For example, the membrane is extruded through an orifice that generates
tensile stresses on the membrane and thus aligns the polymeric chains to achieve a higher
level of crystallinity.
[0028] Additionally or alternatively, the polymer of the membrane is
perfluorosulfonic acid polymer and can be cured at a higher temperature during fabrication.
Curing at higher temperatures induces a higher level of crystallinity. In one example based
upon the polymeric structure shown in Figure 3, the polymer is cured at a temperature of
120°C, 135°C, 150°C or 165°C.
[0029] Additionally or alternatively, the polymeric structure is perfluorosulfonic
acid polymer and can be altered to provide a higher equivalent weight ("EW"), which is the
weight in grams of the polymer required to neutralize one mole of a base and can be
represented in units of grams or commonly as "g/eq." Typical perfluorosulfonic acid
polymers have an EW of about 1100. However, the EW can be increased to 1200, 1400 or
1500 to thus decrease the hydrophilic domain concentration in the polymer matrix and
thereby reduce the average maximum cluster size. In one example, the polymer has an
equivalent weight of 800 g/eq or greater to provide the polymer with l (lambda) of less than
or equal to 22 and an average maximum cluster size of no greater than 4 nanometers.
[0030] Additionally or alternatively, the side chains of the polymer include five
carbon atoms or less to provide the polymer with l (lambda) of less than or equal to 22 and
an average maximum cluster size of no greater than 4 nanometers. As shown in Table 1
below, for similar equivalent weight, vanadium permeability and proton conductivity trend
lower for polymers with shorter side chains (i.e., few carbon atoms) because shorter side
chains form smaller hydrophilic clusters. For example, NAFION has five carbon atoms with
a side chain structure similar to that shown in Fig. 3, and 3M PFSA has four carbon atoms
similar to the side chain structure shown in Fig. 5.
[0031] Table 1: Conductivity and Permeability of PFSA
[0032] Additionally or alternatively, as schematically shown in Figure 5, the ionexchange
membrane 246 includes cations (one representative cation shown) ionically bonded
to negative (electric polar moment) hydrophilic chemical groups 254. For example, the cation
is selected from lithium, sodium, potassium, rubidium, cesium and combinations thereof. In
particular, potassium, rubidium, cesium and combinations thereof have low hydrophilicity
and thus cause a lower amount of water molecules to attach to the hydrophilic chemical
group 254, which results in lowering l (lambda) and lowering the average maximum cluster
size. The cations can be introduced into the polymer by boiling the membrane in a solution
that includes the cations. For example, the membrane can be boiled in a 1M sodiumhydroxide
or potassium-hydroxide solution for approximately one hour, followed by boiling
in deionized water for one hour to incorporate either sodium or potassium into the membrane.
Other cations can be incorporated using a similar technique. The membrane can then be
annealed above its glass transition temperature, such as at 160°C for approximately six hours.
Additionally or alternatively, the ion-exchange membrane can include anions ionically
bonded to positive (electric polar moment) chemical groups. For example, the anions can
include chlorine and the positive chemical groups can include amine groups, NR2, wherein at
least one R is a non-hydrogen, such as an alkyl or aryl group. In another example, the amine
groups can be ammonium groups.
[0033] Additionally or alternatively, as shown in Figure 6, the hydrophilic
chemical group 354 can be a carboxylic acid group, -COOH. The ion-dipolar interactions of
carboxylic acid groups are weaker than sulfonic acid groups and thus provide relatively
smaller average maximum domain sizes. Additionally, the carboxylic acid groups permit
higher levels of crystallinity than sulfonic acid groups. For example, the polymer structure as
shown in Figure 3 may have a level of crystallinity of 12%, by weight, and the polymeric
structure shown in Figure 6 may have a crystallinity of 18%, by weight.
[0034] Additionally or alternatively, the ion-exchange membrane is cross-linked and
thus has cross-link chains attached at opposed ends to carbon backbone chains to provide the
polymer with l (lambda) of less than or equal to 22 and an average maximum cluster size of
no greater than 4 nanometers. The cross-linking constrains the movement of the chains and
thus decreases swelling of the membrane in electrolyte solution. This results in less water
uptake and smaller average maximum cluster size. A cross-linked ion-exchange membrane
also has lower vanadium permeability and lower proton conductivity than a non-crosslinked
analog. In one example, the cross-linked ion-exchange membrane is polysulfone. One
example polysulfone ion-exchange membrane is New SELEMION (Type Il-b, thickness of
140, provided by Asahi Glass). The polysulfone is crosslinked by accelerated electron
radiation. For instance, the accelerated electron radiation is conducted with avoltage of 150
keV, a current of 30 mA and a conveyer rate of 30 rn/min (25 kGy or 2.5 Mrad/pass). In
further examples, the cross-linking was conducted under doses of 5, 15, 20 and 40 Mrad.
Current efficiency and energy efficiency increase when using cross-linking polymer.
[0035] 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.
[0036] 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.

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 supply/storage system external of the at least one cell, the supply/storage system
including first and second vessels fluidly connected with the at least one cell; and
first and second fluid electrolytes in the supply/storage system,
wherein the electrolyte separator layer includes a hydrated ion-exchange membrane of
a polymer comprising a carbon backbone chain and side chains extending from the carbon
backbone chain, the side chains including hydrophilic chemical groups with water molecules
attached by secondary bonding thereto to form clusters of water domains, the clusters having
average maximum cluster sizes no greater than 4 nanometers, with an average number of
water molecules per hydrophilic chemical group, l (lambda), being greater than zero, the
average maximum cluster size of no greater than 4 nanometers and the l (lambda) limiting
migration of vanadium or iron ions across the hydrated ion-exchange membrane.
2. The flow battery as recited in claim 1, wherein the carbon backbone chain is
perfluorinated.
3. The flow battery as recited in claim 1, wherein the hydrophilic chemical groups are
sulfonic acid groups, -SO 3H.
4. The flow battery as recited in claim 1, wherein the polymer includes perfluorosulfonic
acid.
5. The flow battery as recited in claim 1, wherein the hydrophilic chemical groups are
carboxylic acid groups, -COOH.
6. The flow battery as recited in claim 1, wherein the hydrophilic chemical groups are
terminal end groups of the side chains.
7. The flow battery as recited in claim 1, wherein l (lambda) is less than or equal to 22.
8. The flow battery as recited in claim 1, wherein the hydrated ion-exchange membrane
has a specific conductivity of 0.01 to 0.2 S/cm.
9. The flow battery as recited in claim 1, wherein the hydrated ion-exchange membrane
has an average thickness of 25 - 178 micrometers.
10. The flow battery as recited in claim 1, wherein the first and second fluid electrolytes
include an electrochemically active specie of vanadium.
11. The flow battery as recited in claim 1, wherein the hydrated ion-exchange membrane
includes at least one of cations and anions bonded to the hydrophilic groups.
12. The flow battery as recited in claim 11, wherein the hydrated ion-exchange membrane
includes cations, the cations are selected from the groups consisting of lithium, sodium,
potassium, rubidium, cesium and combinations thereof.
13. The flow battery as recited in claim 11, wherein the hydrated ion-exchange membrane
includes cations, the cations are selected from the groups consisting of potassium, rubidium,
cesium and combinations thereof.
14. The flow battery as recited in claim 11, wherein the hydrated ion-exchange membrane
includes anions, the anions including chlorine.
15. The flow battery as recited in claim 1, wherein the hydrated ion-exchange membrane
has a percent crystallinity of at least 6%.
16. The flow battery as recited in claim 1, wherein a maximum design operating
temperature of the hydrated ion-exchange membrane is below the glass transition temperature
of the polymer.
17. The flow battery as recited in claim 1, wherein the side chains each include 5 or fewer
carbon atoms.
18. The flow battery as recited in claim 1, wherein the polymer has an equivalent weight
of 800 g/eq or greater.
19. The flow battery as recited in claim 1, wherein the polymer is cross-linked.
20. The flow battery as recited in claim 1, wherein the hydrophilic chemical groups
include, respectively, an anion group bonded with a positive chemical group.
21. The flow battery as recited in claim 20, wherein the positive chemical group is an
amine group.
22. 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 supply/storage system external of the at least one cell, the supply/storage system
including first and second vessels fluidly connected with the respective first and second flow
fields; and
first and second fluid electrolytes in the supply/storage system,
wherein the electrolyte separator layer includes a hydrated perfluorosulfonic acid ionexchange
membrane comprising a perfluorinated carbon backbone chain and perfluorinated
carbon side chains extending from the perfluorinated carbon backbone chain, the
perfluorinated carbon side chains terminating in hydrophilic chemical groups with water
molecules attached by secondary bonding thereto to form clusters of water domains, the
clusters having average maximum cluster sizes no greater than 4 nanometers, with an average
number of water molecules per hydrophilic chemical group, l (lambda), being greater than
zero and less than or equal to 22.