U.S. patent application number 13/338791 was filed with the patent office on 2012-05-03 for redox flow batteries based on supporting solutions comprising a mixture of acids.
This patent application is currently assigned to BATTELLE MEMORIAL INSTITUTE. Invention is credited to Baowei Chen, Soowhan Kim, Liyu Li, Zimin Nie, Wei Wang, Guanguang Xia, Zhenguo Yang, Jianlu Zhang.
Application Number | 20120107660 13/338791 |
Document ID | / |
Family ID | 45870984 |
Filed Date | 2012-05-03 |
United States Patent
Application |
20120107660 |
Kind Code |
A1 |
Li; Liyu ; et al. |
May 3, 2012 |
Redox Flow Batteries Based on Supporting Solutions Comprising a
Mixture of Acids
Abstract
Redox flow battery systems having a supporting solution that
contains Cl" ions can exhibit improved performance and
characteristics. Furthermore, a supporting solution having mixed
SO.sub.4.sup.2- and Cl.sup.- ions can provide increased energy
density and improved stability and solubility of one or more of the
ionic species in the catholyte and/or anolyte. According to one
example, a vanadium-based redox flow battery system is
characterized by an anolyte having V.sup.2+ and V.sup.3+ in a
supporting solution and a catholyte having V.sup.4+ and V.sup.5+ in
a supporting solution. The supporting solution can contain Cl.sup.-
ions or a mixture of SO.sub.4.sup.2- and Cl.sup.- ions.
Inventors: |
Li; Liyu; (Richland, WA)
; Kim; Soowhan; (Richland, WA) ; Yang;
Zhenguo; (Richland, WA) ; Wang; Wei;
(Richland, WA) ; Zhang; Jianlu; (Richland, WA)
; Chen; Baowei; (Richland, WA) ; Nie; Zimin;
(Richland, WA) ; Xia; Guanguang; (Pasco,
WA) |
Assignee: |
BATTELLE MEMORIAL INSTITUTE
Richland
WA
|
Family ID: |
45870984 |
Appl. No.: |
13/338791 |
Filed: |
December 28, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12892693 |
Sep 28, 2010 |
|
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13338791 |
|
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Current U.S.
Class: |
429/105 |
Current CPC
Class: |
Y02E 60/10 20130101;
H01M 8/08 20130101; H01M 2300/0002 20130101; H01M 8/20 20130101;
H01M 2300/0005 20130101; Y02E 60/50 20130101; H01M 8/18 20130101;
H01M 2300/0011 20130101; H01M 8/188 20130101 |
Class at
Publication: |
429/105 |
International
Class: |
H01M 4/36 20060101
H01M004/36 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] This invention was made with Government support under
Contract DE-AC0576RL01830 awarded by the U.S. Department of Energy.
The Government has certain rights in the invention.
Claims
1. A redox flow battery system having a supporting solution, the
supporting solution comprising a mixture of SO.sub.4.sup.2- and
Cl.sup.- ions.
2. The system of claim 1, wherein the Cl to SO.sub.4.sup.2-
concentration ratio is between 1:100 and 100:1.
3. The system of claim 1, wherein the Cl.sup.- to SO.sub.4.sup.2-
concentration ratio is between 1:10 and 10:1.
4. The system of claim 1, wherein the Cl.sup.- to SO.sub.4.sup.2-
concentration ratio is between 1:3 and 3:1.
5. The system of claim 1, wherein the redox flow battery system is
a Fe/Cr system.
6. A redox flow battery system comprising a supporting solution,
the supporting solution characterized by containing a mixture of
SO.sub.4.sup.2- and F.sup.- ions.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 12/892,693, filed Sep. 28, 2010 and entitled "REDOX FLOW
BATTERIES BASED ON SUPPORTING SOLUTIONS CONTAINING CHLORIDE," which
is incorporated herein by reference.
BACKGROUND
[0003] A redox flow battery (RFB) stores electrical energy in
reduced and oxidized species dissolved in two separate electrolyte
solutions. The anolyte and the catholyte circulate through a cell
electrode separated by a porous membrane. Redox flow batteries are
advantageous for energy storage because they are capable of
tolerating fluctuating power supplies, repetitive charge/discharge
cycles at maximum rates, overcharging, overdischarging, and because
cycling can be initiated at any state of charge.
[0004] However, among the many redox couples upon which redox flow
batteries are based, a number of disadvantages exist. For example,
many systems utilize redox species that are unstable, are highly
oxidative, are difficult to reduce or oxidize, precipitate out of
solution, and/or generate volatile gases. In many ways, the
existing approaches to addressing these disadvantages have been ad
hoc and can include the imposition of restrictive operating
conditions, the use of expensive membranes, the inclusion of
catalysts on the electrodes, and/or the addition of external heat
management devices. These approaches can significantly increase the
complexity and the cost of the total system. Therefore, a need for
improved redox flow battery systems exists.
SUMMARY
[0005] The present invention includes redox flow battery systems
having a supporting solution that comprises Cl.sup.- anions. In one
embodiment, a vanadium-based redox flow battery system is
characterized by an anolyte comprising V.sup.2+ and V.sup.3+ in a
supporting solution and a catholyte comprising V.sup.4+ and
V.sup.5+ in a supporting solution. The supporting solution can
comprise Cl.sup.- ions or a mixture of SO.sub.4.sup.2- and Cl.sup.-
ions. The use of Cl.sup.- ions can improve the energy density and
the stability of an all-vanadium battery compared to the
traditional use of SO.sub.4.sup.2- ions.
[0006] Supporting solutions comprising both SO.sub.4.sup.2- and
Cl.sup.- ions can further improve the performance and
characteristics by all-vanadium batteries by increasing the
solubility of the vanadium cations as described in greater detail
below. In particular embodiments, the concentration ratio of
Cl.sup.- to SO.sub.4.sup.2- can be between 1:100 and 100:1. In
other embodiments, the ratio can be between 1:10 and 10:1. In still
other embodiments, the ratio can be between 1:3 and 3:1.
[0007] For all-vanadium batteries, the Cl.sup.- in the supporting
solution can improve stability of the vanadium cations. For
example, in traditional flow redox batteries, V.sup.5+ can tend to
form V.sub.2O.sub.3 at temperatures above 40.degree. C. However,
the presence of Cl.sup.- ions in the supporting solution can result
in the formation of VO.sub.2Cl(H.sub.2O).sub.2, a stable, neutral
species. Accordingly, embodiments of the present invention can
operate at cell temperatures greater than 40.degree. C. Preferably,
the cell temperature during operation is between -35.degree. C. and
60.degree. C. Furthermore, the embodiments of the present invention
can operate without thermal management devices actively regulating
the cell temperature. In conventional all-vanadium flow redox
batteries, thermal management devices are required to maintain the
battery below the cell temperature at which the V cations come out
of solution.
[0008] Further still, vanadium cation concentrations in batteries
of the present invention can exceed those of traditional
SO.sub.4.sup.2--based batteries. In some embodiments, the vanadium
cation concentration is greater than 0.5M. In others, the vanadium
cation concentration is greater than 1.7M. In still others, the
vanadium cation concentration is greater than 2.5M.
[0009] In a preferred embodiment, the state of charge condition is
greater than 0% and less than 100% during operation. In other
words, the batteries are preferably not operated to full charge or
discharge states.
[0010] In another embodiment of the present invention, a redox flow
battery having a supporting solution comprising ions comprises an
anolyte comprising V.sup.2+ and V.sup.3+ in the supporting
solution, a catholyte comprising Fe.sup.2+ and Fe.sup.3+ in the
supporting solution, and a membrane separating the anolyte and the
catholyte. The anolyte and catholyte can comprise V cations and Fe
cations, respectively, or the anolyte and catholyte can each
contain both V and Fe cations in a mixture. In some instances, the
concentrations of the Fe cations and/or the V cations can be
greater than 0.5M.
[0011] Relative to some highly oxidative redox couples, The Fe and
V couple is less aggressive. Accordingly, expensive
oxidation-resistant membranes such as sulfonated
tetrafluoroethylene based fluoropolymer-copolymers are not
necessary. On a cost basis, other less expensive options can be
preferable. Accordingly, some embodiments of the Fe/V battery
system comprise hydrocarbon-based membranes or micro-porous
separators. One example of a hydrocarbon membrane includes, but is
not limited to a sulfonated poly(phenylsulfone) membrane. Other ion
exchange membranes can be suitable.
[0012] In another embodiment, the Fe/V battery system comprises
electrodes, which do not contain a redox catalyst, in contact with
the anolyte and the catholyte. Redox catalysts are sometimes
necessary for species that are difficult to reduce and/or oxidize
and can include metals or metal oxides. Redox catalysts are
preferably absent from the electrodes used in embodiments of the
present invention.
[0013] Some embodiments of the Fe/V battery system operate at cell
temperatures below 60.degree. C. In other embodiments, the system
operates at cell temperatures between -20.degree. C. and 50.degree.
C. In preferred embodiments, the system does not include a heat
management device actively regulating the cell temperature. In
particular, no heat management device is utilized to heat the Fe/V
battery system.
[0014] In the Fe/V battery systems, supporting solutions comprising
both SO.sub.4.sup.2- and Cl.sup.- ions can further improve the
performance and characteristics by increasing the solubility of the
cations as described in greater detail below. In particular
embodiments, the concentration ratio of Cl.sup.- to SO.sub.4.sup.2-
can be between 1:100 and 100:1. In other embodiments, the ratio can
be between 1:10 and 10:1. In still other embodiments, the ratio can
be between 1:3 and 3:1. Instances in which the supporting solution
comprises both Cl.sup.- to SO.sub.4.sup.2- and the anolyte and
catholyte both comprise V and Fe cations, the concentration of
V.sup.2+, V.sup.3+, Fe.sup.2+, and Fe.sup.3+ are greater than 1.5M.
Instances in which the anolyte comprises V cations and the
catholyte comprises Fe cations, the concentrations of V.sup.2+ and
V.sup.3+ are greater than 2M in the anolyte and concentrations of
Fe.sup.2+ and Fe.sup.3+ are greater than 2M in the catholyte.
[0015] In yet another embodiment, a redox flow battery system
comprises a supporting solution that comprises a mixture of
SO.sub.4.sup.2- and Cl.sup.-. As described elsewhere herein, a
supporting solution having mixed SO.sub.4.sup.2- and Cl.sup.- can
provide increased energy density and improved stability and
solubility of one or more of the ionic species in the catholyte
and/or anolyte, such as Fe.sup.2+, Fe.sup.3+, Cr.sup.2+, Cr.sup.3+,
and others. In particular embodiments, the concentration ratio of
Cl.sup.- to SO.sub.4.sup.2- can be between 1:100 and 100:1. In
other embodiments, the ratio can be between 1:10 and 10:1. In still
other embodiments, the ratio can be between 1:3 and 3:1. In still
other embodiments, other halogen ions can be mixed with
SO.sub.4.sup.2-, including but not limited to, F.sup.-.
[0016] The purpose of the foregoing abstract is to enable the
United States Patent and Trademark Office and the public generally,
especially the scientists, engineers, and practitioners in the art
who are not familiar with patent or legal terms or phraseology, to
determine quickly from a cursory inspection the nature and essence
of the technical disclosure of the application. The abstract is
neither intended to define the invention of the application, which
is measured by the claims, nor is it intended to be limiting as to
the scope of the invention in any way.
[0017] Various advantages and novel features of the present
invention are described herein and will become further readily
apparent to those skilled in this art from the following detailed
description. In the preceding and following descriptions, the
various embodiments, including the preferred embodiments, have been
shown and described. Included herein is a description of the best
mode contemplated for carrying out the invention. As will be
realized, the invention is capable of modification in various
respects without departing from the invention. Accordingly, the
drawings and description of the preferred embodiments set forth
hereafter are to be regarded as illustrative in nature, and not as
restrictive.
DESCRIPTION OF DRAWINGS
[0018] Embodiments of the invention are described below with
reference to the following accompanying drawings.
[0019] FIG. 1 is a graph of current versus voltage comparing
all-vanadium RFBs using chloride-containing and sulfate-containing
supporting solutions.
[0020] FIG. 2 compares thermodynamic equilibrium concentrations (a)
and equilibrium potentials (b) of chlorine and oxygen gases in
vanadium chloride RFB systems.
[0021] FIG. 3 compares cyclic performances of vanadium chloride RFB
systems and vanadium sulfate RFB systems.
[0022] FIG. 4 compares cyclic voltammetry curves of a
vanadium-chloride-sulfate solution and a vanadium sulfate
solution.
[0023] FIG. 5 is a graph of equilibrium concentrations of chlorine
in the positive side of a vanadium-chloride-sulfate cell at various
states of charge.
[0024] FIG. 6 is a diagram depicting structures of VO.sub.2.sup.-
in sulfuric acid (a) and in hydrochloric acid (b).
[0025] FIG. 7 is a graph of cyclic coulombic efficiency, voltage
efficiency, and energy efficiency for a vanadium-chloride-sulfate
RFB system.
[0026] FIG. 8 are cyclic voltammetry curves in a Fe/V and
Cl-containing solution using two different electrodes.
[0027] FIG. 9 contains graphs demonstrating the electrochemical
performance of an Fe/V redox flow cell using a Cl-containing
supporting solution.
[0028] FIG. 10 shows cyclic Coulombic efficiency, voltage
efficiency, and energy efficiency (a) as well as cell charge
capacity and charge energy density change (b) for a Fe/V cell
employing S-Radel as membrane
DETAILED DESCRIPTION
[0029] The following description includes the preferred best mode
as well as other embodiments of the present invention. It will be
clear from this description of the invention that the invention is
not limited to these illustrated embodiments but that the invention
also includes a variety of modifications and embodiments thereto.
Therefore the present description should be seen as illustrative
and not limiting. While the invention is susceptible of various
modifications and alternative constructions, it should be
understood, that there is no intention to limit the invention to
the specific form disclosed, but, on the contrary, the invention is
to cover all modifications, alternative constructions, and
equivalents falling within the spirit and scope of the invention as
defined in the claims.
[0030] FIGS. 1-10 show a variety of embodiments and aspects of the
present invention. Referring first to FIG. 1, current versus
voltage data is plotted for vanadium ion redox couples using either
chloride or sulfate supporting solutions. Three redox couples were
observed in the chloride system, indicating that two redox couples
(VO.sup.2+/ VO.sub.2.sup.+ for the positive and V.sup.2+/ V.sup.3+
for the negative) can be employed for a redox flow battery.
Electrochemical reversibility of the V.sup.4+/V.sup.5+ couple was
similar to that of a sulfate system, but that of the
V.sup.2+/V.sup.3+ was significantly improved in the chloride
system. For example, the peak potential difference is 0.351 V in
the sulfate system and 0.509 V in the chloride system.
[0031] According to quantum chemistry calculations, the V.sup.5+
species in the chloride solution forms VO.sub.2Cl (H.sub.2O).sub.2,
which is a more stable neutral species than
[VO.sub.2(H.sub.2O).sub.3].sup.+, the species commonly formed in
the sulfate solution. However, V.sup.2+, V.sup.3+ and V.sup.4+ in
the chloride solution have a similar structure to that in the
sulfate solution. Based on the above, the half cell reaction shown
in Eq. (2) for the positive pole describes well the
electrochemistry. The standard potential of this half cell reaction
is expected to be slightly higher than that of the conventional
sulfate system resulting from a different V.sup.5+ species. By
forming this new structure, the thermal stability of the V.sup.5+
in the chloride solution was significantly improved.
##STR00001##
[0032] In the chloride system, oxygen and chlorine gas evolution
during charging can reduce columbic efficiency. Referring to FIG.
2(a), equilibrium concentrations of chlorine or oxygen estimated
from thermodynamic equilibrium for Eq. (1) and (4), and Eq. (1) and
(5), respectively, are shown as a function at the state of charge
(SOC) at various temperatures. It should be noted that hypochlorite
can be negligible because the equilibrium constant of Eq. (6) is
6.35E-13 at 25.degree. C. The actual concentrations of the chlorine
should be lower than the values depicted in FIG. 2(a) due to
complex formation. Within a typical operation window of redox flow
batteries (i.e., SOC of 20.about.80%), the chlorine concentration
is negligible even at 40.degree. C. However, gas evolution may be
significant at SOC values approaching 100%.
[0033] Chlorine has much higher solubility in water than oxygen;
Henry's constant of chlorine and oxygen in water at 25.degree. C.
is 0.062 mol/L-atm and 0.0013 mol/L-atm, respectively. Assuming
partial pressure of oxygen and chlorine is 0.1 bar, the equilibrium
potential of Eq. (4) and (5) was calculated for 2.3 M. V in 10 M
total chloride system, and is shown in FIG. 2(b) as a function of
SOC. Based on the data, VO.sub.2.sup.+ is thermodynamically stable
from oxygen evolution below an 80% SOC, and from chlorine evolution
below a 98% SOC. To maintain saturation of chlorine in the
electrolyte solution, the flow battery is preferably operated in a
closed system. A closed system is also advantageous to prevent
rapid oxidation of V.sup.2+ and V.sup.3+ by air and to minimize
electrolyte loss.
2Cl.sup.-Cl.sub.2+2e E.degree.=1.36V vs. NHE (4)
2H.sub.2OO.sub.2+4H.sup.++4e E.degree.=1.23V vs. NHE (5)
Cl.sub.2+H.sub.2O2H.sup.++Cl.sup.-+ClO.sup.- (6)
[0034] In addition to thermodynamic equilibrium, electrode
overpotential can contribute to gas evolution. The equilibrium
potential of reaction (4) is higher than that of reaction (5), but
oxygen evolution can be negligible compared to chlorine evolution
because of a higher overpotential on the electrode. For example,
the chlorine evolution overpotential on a graphite porous electrode
was 0.12 V at 25.degree. C. at charge current of 22 mA/cm.sup.2 for
a Zn/Cl.sub.2 battery (see N. Watanabe, T. Touhara, New Mat. New
Processes, 1 (1981) 62). This overpotential was higher than that of
the oxidation reaction in Eq. (2) above. Therefore, the chlorine
evolution reaction can be negligible except for an SOC of
.about.100%. Because the electrode overpotential of chlorine
evolution decreases with increasing temperature, charging is
preferably controlled below SOC of 90.about.95% to prevent chlorine
evolution, especially at elevated temperature.
[0035] The thermal stabilities of different vanadium ion species in
either sulfate or chloride supporting solutions are summarized in
Table 1. In the sulfate system, with more than 1.7 M vanadium,
V.sup.2+ and V.sup.4+ experienced precipitation at low temperatures
(-5.degree. C. and 25.degree. C.), and V.sup.5+ suffered from
precipitation at 40.degree. C. In the chloride system, thermal
stability was significantly improved. V.sup.2+, V.sup.4+ and
V.sup.5+ were stable for more than 10 days in the temperature
ranges of -5 and 50.degree. C. for 2.3 M vanadium. According to
nuclear magnetic resonance data (not shown), V.sup.5+ in the
sulfate solution exists as a form of
[VO.sub.2(H.sub.2O).sub.3].sup.+. With increasing temperature, this
complex decomposed into VO(OH).sub.3 and H.sub.3O.sup.+, and then
VO(OH).sub.3 is converted into a precipitate of
V.sub.2O.sub.5.3H.sub.2O. As mentioned elsewhere herein, V.sup.5+
is believed to exist as a stable neutral form of
VO.sub.2Cl(H.sub.2O).sub.2 in the chloride solution. Regardless,
the supporting solutions comprising Cl.sup.- can enable better
stability at higher temperature.
TABLE-US-00001 TABLE 1 Comparison of thermal stability of V.sup.n+
for chloride and sulfate systems. Time for V.sup.n+ species
V.sup.n+ [M] H.sup.+ [M] SO.sub.4.sup.2- [M] Cl.sup.- [M] T
(.degree. C.) precipitation V.sup.2+ 2 6 5 0 -5 419 hr 2 6 5 0 25
Stable (>20 d) 2 6 5 0 40 Stable (>20 d) V.sup.3+ 2 4 5 0 -5
Stable (>20 d) 2 4 5 0 25 Stable (>20 d) 2 4 5 0 40 Stable
(>20 d) V.sup.4+ (VO.sup.2+) 2 6 5 0 -5 18 hr 2 6 5 0 25 95 hr 2
6 5 0 40 Stable (>20 d) V.sup.5+ (VO.sub.2.sup.+) 2 8 5 0 -5
Stable (>20 d) 2 8 5 0 25 Stable (>20 d) 2 8 5 0 40 95 hr 1.8
8.4 5 0 40 358 hr V.sup.2+ 2.3 5.4 0 10 -5 Stable (>20 d) 2.3
5.4 0 10 25 Stable (>20 d) 2.3 5.4 0 10 40 Stable (>20 d)
V.sup.3+ 1.5 3.0 0 7.5 -5 Stable (>20 d) 1.8 3.0 0 8.4 -5 124 hr
2.3 3.1 0 10 -5 96 hr 2.3 3.1 0 10 25 Stable (>20 d) 2.3 3.1 0
10 40 Stable (>20 d) V.sup.4+ (VO.sup.2+) 2.3 5.4 0 10 -5 Stable
(>20 d) 2.3 5.4 0 10 25 Stable (>20 d) 2.3 5.4 0 10 40 Stable
(>20 d) V.sup.5+ (VO.sub.2.sup.+) 2.3 7.7 0 10 -5 Stable (>20
d) 2.3 7.7 0 10 25 Stable (>20 d) 2.3 7.7 0 10 40 Stable (>20
d) 2.3 7.7 0 10 50 Stable (>10 d)
[0036] When operation of an all Cl.sup.- system occurs at, or
below, freezing temperatures (i.e., 0.degree. C.), the tank
containing the electrolyte is preferably insulated to maintain
waste heat from the flow battery, which can be approximately 20% of
total energy. Operation above the freezing temperature, energy
density can be improved by approximately 35% owing to higher
vanadium concentration compared to the sulfate system.
Stabilization of the V.sup.3+ species at the lower temperature can
be achieved by using a supporting solution containing both
SO.sub.4.sup.2- and Cl.sup.-, as is described in greater detail
elsewhere herein.
[0037] Typical energy efficiency of vanadium redox flow batteries
is about 80%; indicating 20% of the energy is released as waste
heat during each cycle. Assuming an adiabatic system, the
electrolyte temperature can increase by about 4.degree. C. per
cycle. The thermal stability of electrolytes at higher temperatures
can be a major concern, especially on hot days. For conventional
all vanadium sulfate systems, active thermal management devices
such as heat exchangers are commonly employed to maintain the cell
temperature below 40.degree. C. and to prevent precipitation of
V.sup.5+. An active thermal management system is not preferable and
is a significant parasitic energy loss. Embodiments of the present
invention based on vanadium and Cl-containing supporting solution
can be operated at a wide range of temperatures between 0 to
50.degree. C. without an active thermal management system,
improving significant system efficiency and also reducing cost.
[0038] Flow cell performance for different chloride and sulfate
systems were evaluated under the identical test conditions. The
results at different discharging current densities were summarized
in Table 2. Energy density of the chloride system was .about.38
Wh/L, 30% higher than that of the sulfate system, resulting from
the higher solubility of vanadium in the chloride solution. This
higher energy density can reduce the system cost by reducing tank
size and footprint. Columbic efficiency of the chloride system was
94.about.97% under operation of SOC between 0 and 100% (not
inclusive), comparable to that of the sulfate system, which was
95.about.97%.
TABLE-US-00002 TABLE 2 Comparison of discharging rate capability
for VSRFB (1.7M V in 5M total sulphate) and VCRFB (2.3M V in 10M
total chloride). Energy Capacity density* Efficiency CD (Ah) (Wh/L)
Coulomb Energy Voltage (mA/cm.sup.2) Cl.sup.- SO.sub.4.sup.2-
Cl.sup.- SO.sub.4.sup.2- Cl.sup.- SO.sub.4.sup.2- Cl.sup.-
SO.sub.4.sup.2- Cl.sup.- SO.sub.4.sup.2- 100 2.75 2.14 35.5 27.9
0.94 0.95 0.80 0.83 0.85 0.87 75 2.75 2.14 36.6 28.4 0.96 0.96 0.84
0.85 0.87 0.89 50 2.75 2.14 37.8 29.1 0.97 0.96 0.87 0.88 0.90 0.91
25 2.74 2.13 38.7 29.7 0.97 0.97 0.90 0.91 0.92 0.94 *Note that
energy density was calculated only by electrolyte volume.
[0039] Cyclic performance of both systems at ambient temperature
was also evaluated by cycling between 1.6V and 1.2V, which are
shown in FIG. 3. The capacities of both systems slightly decreased
with cycles because of different transport rate of vanadium species
across the membrane. This capacity loss can be recovered by
remixing and rebalancing anolyte and catholyte because a single
element of V is used for both solutions. Energy and coulombic
efficiencies for the chloride system was stable with cycles and
comparable to those of sulfate system. It can be concluded that the
novel all vanadium chloride flow battery can be stably operated in
a comparable energy efficiency to the sulfate system, while
delivering energy density of .about.38 Wh/L, 30% higher than the
sulfate system. Chlorine evolution or V.sup.5+ electrolyte
stability in the chloride solution was not an issue under closed
operation conditions.
[0040] Electrolyte for the all vanadium chloride systems described
above was prepared by dissolving V.sub.2O.sub.3 in concentrated HCl
(38%). The electrolyte for the all vanadium sulphate system was
fabricated by dissolving VOSO.sub.4.3.8 H.sub.2O in sulfuric acid
(95.8%).
[0041] Cyclic voltammetry (CV) tests for the chloride system were
conducted with identical graphite felts (.phi.=5 mm mm) used in
flow cell testing to identify redox couples and electrochemical
reversibility using Solartron 1287 potentiostat. The scan rate was
0.5 mV/s.
[0042] Cell performance of two different systems was measured using
a flow cell system under identical test conditions. The apparent
area of the graphite felt was 10 cm.sup.2 (2 cm.times.5 cm), in
contact with NAFION 117 membrane, a sulfonated tetrafluoroethylene
based fluoropolymer-copolymer. Other proton-exchange membranes can
be suitable. 2.3 M vanadium in 10 M total chloride solution and 1.7
M V in 5 M total sulphate solution were used for performance
comparison. Each electrolyte volume and flow rate was 50 mL and 20
mL/min, respectively. The effect of different discharging current
densities was evaluated in the first 5 cycles with the same
charging current of 50 mA/cm.sup.2. The flow cell was charged to
1.7 V and then discharged to 0.8 V. After that, the flow cell was
cycled between 1.6 V and 1.2 V at 50 mA/cm.sup.2.
[0043] The electrolyte stability tests were carried out in
polypropylene tubes at -5, 25, 40, and 50.degree. C., using about 5
ml solution for each sample. During the stability tests, the
samples were kept static without any agitation, and were monitored
daily by naked eye for the formation of precipitation.
[0044] Referring to Table 3, which summarizes the stability of
V.sup.2+, V.sup.3+, V.sup.4+, and V.sup.5+ in sulfuric acid
solutions, conventional sulfuric acid-only vanadium redox flow
batteries (VRFB) can typically only be operated at cell
temperatures between 10.degree. C. and 40.degree. C. with vanadium
concentration in the electrolytes less than 1.7 M (with an energy
density <25 Wh/L). The electrochemical reactions of an all
vanadium sulfate redox flow battery are represented by the
following equations.
##STR00002##
TABLE-US-00003 TABLE 3 Stability of V.sup.n+ cations in
H.sub.2SO.sub.4 solution Time for V.sup.n+ specie V.sup.n+, M
H.sup.+, M SO.sub.4.sup.2-, M T, .degree. C. precipitation V.sup.2+
2 6 5 -5 Stable (>10 d) 2 6 5 25 Stable (>10 d) 2 6 5 40
Stable (>10 d) V.sup.3+ 2 4 5 -5 Stable (>10 d) 2 4 5 25
Stable (>10 d) 2 4 5 40 Stable (>10 d) V.sup.4+ (VO.sup.2+) 2
6 5 -5 18 hr 2 6 5 25 95 hr 2 6 5 40 Stable (>10 d) V.sup.5+
(VO.sup.2+) 2 8 5 -5 Stable (>10 d) 2 8 5 25 Stable (>10 d) 2
8 5 40 95 hr
[0045] As mentioned earlier, since the standard potential of
reaction 2Cl.sup.--2e=Cl.sub.2 (g) (E.degree.-1.36 V) is much
higher than that of Reaction (7), the supporting solution in a VRFB
system can comprise Cl.sup.- either as a SO.sub.4.sup.2- and
Cl.sup.- mixture or comprising Cl.sup.- as the only anion.
Moreover, as is described elsewhere herein, the use of mixed
SO.sub.4.sup.2 and Cl.sup.- in the supporting solution is not
limited to vanadium-based redox flow batteries. Chloride and
sulfate ions in the supporting solution can help stabilize
relatively higher concentrations of other cations as well.
[0046] FIG. 4 shows the cyclic voltammetry curve of a solution
containing 2.5 M VOSO.sub.4 and 6 M HCl. This curve is similar to
that of a solution containing 1.5 M VOSO.sub.4 and 3.5 M Referring
to FIG. 5, the equilibrium concentrations of Cl.sub.2 gas in a
vanadium sulfate-chloride catholyte solution (containing 2.5 M
vanadium, 2.5 M sulfate, and 6 M chloride) under different
state-of-charge (SOC) conditions were calculated according to
Reaction 12. Under normal flow battery operation conditions (i.e.
T<40.degree. C. and SOC<80%), the equilibrium concentration
of Cl.sub.2 gas is less than 10 ppm. Due to its high solubility in
water (0.57 g Cl.sub.2 per 100 g water at 30.degree. C.), most of
the Cl.sub.2 gas generated should be dissolved in the catholyte
solutions. At high temperatures, SOC values higher than 80% are
preferably avoided to minimize the Cl.sub.2 gas evolution.
Nevertheless, a closed system can be used to minimize continuous
Cl.sub.2 gas generation and to prevent Cl.sub.2 gas emission to the
environment. Such closed systems are commonly required for the
conventional vanadium sulfate flow battery system to prevent
oxidation of V.sup.2+ and V.sup.3+ by O.sub.2 in air, and to
prevent water loss from electrolyte solutions.
2VO.sub.2.sup.+(a)+4H.sup.+(a)+2Cl.sup.-(a)=2VO.sup.2+(a)+Cl.sub.2(g)+2H-
.sub.2O (12)
[0047] The stability of different V.sup.n+ cations in Cl-containing
solutions was evaluated at a temperature range of -5.degree. C. to
40.degree. C. The results are given in Table 4. More than 2.3 M
VOCl.sub.2 and VO.sub.2Cl were stabilized in .about.6 M HCl
solution over a temperature range of -5.degree. C. to 40.degree.
C., which is much higher than those in the sulfuric acid solution
(.about.1.5 M vanadium) over the same temperature range. The
Cl.sup.- anions appears to stabilize VO.sub.2.sup.+ and VO.sup.2+
cations in the solution. Similar to that in the H.sub.2SO.sub.4
solution, more than 2.3 M V.sup.2+ was also stabilized in .about.6
M HCl solution at -5.degree. C. to 40.degree. C. However, compared
to that in the H.sub.2SO.sub.4 solution, the stability of V.sup.3+
in HCl solution was decreased. At -5.degree. C., only about 1.5 M
V.sup.3+ could be stabilized in 3 M HCl, whereas more than 2 M
V.sup.3+ was stabilized in 2 M H.sub.2SO.sub.4 (see Table 4).
TABLE-US-00004 TABLE 4 Stability of V.sup.n+ cations in HCl
solution Time for V.sup.n+ specie V.sup.n+, M H.sup.+, M Cl.sup.-,
M T, .degree. C. precipitation V.sup.2+ 2.3 5.4 10 -5 Stable
(>10 d) 2.3 5.4 10 25 Stable (>10 d) 2.3 5.4 10 40 Stable
(>10 d) V.sup.3+ 1.5 3.0 7.5 -5 Stable (>10 d) 1.8 3.0 8.4 -5
124 hr 2.3 3.1 10 -5 96 hr 2.3 3.1 10 25 Stable (>10 d) 2.3 3.1
10 40 Stable (>10 d) V.sup.4+ (VO.sup.2+) 2.3 5.4 10 -5 Stable
(>10 d) 2.3 5.4 10 25 Stable (>10 d) 2.3 5.4 10 40 Stable
(>10 d) V.sup.5+ (VO.sub.2.sup.+) 2.3 7.7 10 -5 Stable (>10
d) 2.3 7.7 10 25 Stable (>10 d) 2.3 7.7 10 40 Stable (>10
d)
[0048] Based on the stability test results above, anions can help
stabilizing VO.sup.2+ and VO.sub.2.sup.+ cations, and
SO.sub.4.sup.2- anions can help stabilize V.sup.3+ cations. Both
Cl.sup.- and SO.sub.4.sup.2- anions can stabilize V.sup.2+ cations.
Accordingly, a sulfuric acid and hydrochloric acid mixture can
stabilize high concentrations of all four vanadium cations. Table 5
gives the stability of different V.sup.n+ cations in two mixed
SO.sub.4.sup.2- and Cl.sup.- solutions at -5.degree. C. to
40.degree. C. Without optimization, about 2.5 M of all four
V.sup.n+ cations were effectively stabilized in the 2.5 M
SO.sub.4.sup.2--6 M Cl.sup.- mixed acid solution. At a higher
vanadium concentration (3M), V.sup.2+, VO.sup.2+, and
VO.sub.2.sup.+ were also stabilized in the 3 M SO.sub.4.sup.2--6 M
Cl.sup.- mixed acid solution at -5.degree. C. to 40.degree. C.
However, V.sup.3+ was only stable for about 8 days at -5.degree. C.
Precipitation of VOCl was observed. Due to the large amount of heat
generation during the operation of a VRFB system, it is not
difficult to keep the cell temperature of the electrolytes higher
than -5.degree. C. even when the ambient temperature is -5.degree.
C. or lower. Also, since a VRFB system is always operated under 80
to 90% state-of-charge and state-of-discharge conditions, the
highest concentration of V.sup.3+ in a 3 M all vanadium flow
battery system is 2.7 M (mixing with 0.3 M V.sup.2+, at the end of
90% discharge) or 2.4 M (mixing with 0.6 M V.sup.2+, at the end of
80% discharge). Therefore, in one embodiment, by using a sulfuric
acid and hydrochloric acid mixture as the supporting solution, the
VRFB system uses a supporting solution with a total vanadium
concentration higher than 3 M.
TABLE-US-00005 TABLE 5 Stability of V.sup.n+ in the SO.sub.4.sup.2-
+ Cl.sup.- solutions V.sup.n+ specie V.sup.n+ [M] H.sup.+ [M]
SO.sub.4.sup.2- [M] Cl.sup.- [M] T (.degree. C.) Time for
precipitation V.sup.2+ 3 6 3 6 -5 Stable (>10 d) 2.5 6 2.5 6 -5
Stable (>10 d) 2.5 6 2.5 6 25 Stable (>10 d) 2.5 6 2.5 6 40
Stable (>10 d) 3 6 3 6 40 Stable (>10 d) V.sup.3+ 3 3 3 6 -5
192 hr (8 d) 2.5 3.5 2.5 6 -5 Stable (>10 d) 2.5 3.5 2.5 6 25
Stable (>10 d) 2.5 3.5 2.5 6 40 Stable (>10 d) 3 3 3 6 40
Stable (>10 d) V.sup.4+ (VO.sup.2+) 3 6 3 6 -5 Stable (>10 d)
2.5 6 2.5 6 -5 Stable (>10 d) 2.5 6 2.5 6 25 Stable (>10 d)
2.5 6 2.5 6 40 Stable (>10 d) 3 6 3 6 40 Stable (>10 d)
V.sup.5+ (VO.sub.2.sup.+) 3 9 3 6 -5 Stable (>10 d) 2.5 8.5 2.5
6 -5 Stable (>10 d) 2.5 8.5 2.5 6 25 Stable (>10 d) 2.5 8.5
2.5 6 40 Stable (>10 d) 3 9 3 6 40 Stable (>10 d) 2.7
V.sup.5+ + 0.3 V.sup.4+ 7.7 3 6 50 Stable (>10 d) 2.7 V.sup.5+ +
0.3 V.sup.4+ 7.7 3 6 60 Stable (>10 d)
[0049] At temperatures higher than 40.degree. C., in traditional
all-vanadium sulfate RFBs the stability of V.sup.5+ might decrease
due to the formation of V.sub.2O.sub.5. However, as shown in Table
5, embodiments of the present invention using mixed
SO.sub.4.sup.2-Cl.sup.- solutions exhibit excellent stability with
a mixture of 2.7 M V.sup.5+ and 0.3 M V.sup.4+ (corresponding to
90% of state-of-charge of a 3 M VRFB system) at temperatures as
high as 60.degree. C., indicating that Cl.sup.- anions can
effectively stabilize the VO.sub.2.sup.+ cations. As described
elsewhere herein, quantum chemistry calculations show that, in
Cl-containing solutions, a stable neutral species can form having
the formula VO.sub.2Cl(H.sub.2O).sub.2. Referring to FIG. 6, a
diagram depicts the molecular structure of
[VO.sub.2(H.sub.2O).sub.3].sup.+ and of VO.sub.2Cl(H.sub.2O).sub.2.
In this structure, one Cl.sup.- anion, two O.sup.2+ anions, and two
H.sub.2O molecules complex with one V.sup.5+ in the first
coordination shell. Without Cl.sup.- anions in the solution, two
O.sup.2- anions, and three H.sub.2O molecules complex with V.sup.5+
in the first coordination shell and a positively-charged specie
with [VO.sub.2(H.sub.2O).sub.3].sup.+ formula forms. Quantum
chemistry calculations also indicate that, at elevated
temperatures, this positively charged species is prone to convert
to V.sub.2O.sub.5-3H.sub.2O by de-protonation (Reaction 13) and
condensation (Reaction 14). The structural differences appear to
account for the much improved stability of VO.sub.2.sup.+ cations
in Cl.sup.--containing solutions. Due to the formation of stable
VO.sub.2Cl(H.sub.2O).sub.2 structure, the equilibrium concentration
of Cl.sub.2 gas in the catholyte solution should be lower than that
shown in FIG. 5.
[VO.sub.2(H.sub.2O).sub.3].sup.+.fwdarw.VO(OH).sub.3+[H.sub.3O].sup.+
(13)
2VO(OH).sub.3.fwdarw.V.sub.2O.sub.5-3H.sub.2O (14)
[0050] In embodiments comprising mixed SO.sub.4.sup.2-Cl.sup.-
solutions, the stability of V.sup.4+ is controlled by the
solubility of VOSO.sub.4, and the stability of V.sup.3+ is
controlled by the solubility of VOCl. The improvement of V.sup.4+
stability is due to the decrease of SO.sub.4.sup.2- concentration
in the solution, and the improvement of V.sup.3+ stability is due
to the decrease of Cl.sup.- concentration. V.sup.2+ cation is
stable in both Cl.sup.- and SO.sub.4.sup.2--containing
solutions.
[0051] In traditional all-vanadium sulfate RFBs, energy efficiency
is about 80%, which means about 20% of the total energy is lost as
waste heat during each cycle. For an adiabatic system, this heat
can raise the temperature of the whole system by about 5.degree. C.
Due to the large amount of waste heat generation, stability of
electrolytes at high temperature range is a major concern,
especially during hot days. The embodiments of the present
invention encompassing all-vanadium RFBs utilizing mixed
SO.sub.4.sup.2-Cl.sup.- supporting solutions system can not only
improve the energy density, but can also expand the operation
temperature window from 10-40.degree. C. to -5-60.degree. C. During
the cold winter days, limited insulation can easily keep the
temperature of the system above -5.degree. C. Accordingly, in
preferred embodiments, no active heat management is needed
[0052] Several small VRFB cells were used to evaluate the
performances of two vanadium sulfate-chloride mixed systems (with
2.5 M and 3.0 M vanadium). For comparison, the performance of a
vanadium sulfate system (with 1.6 M vanadium) was also measured.
The results are summarized in Table 6. The sulfate-chloride mixed
systems show much higher energy density than the sulfate system.
Even with higher vanadium concentration, the all vanadium
sulfate-chloride mixed systems still showed similar energy
efficiency to that of the vanadium sulfate system. FIG. 7 provides
the cyclic coulombic efficiency, voltage efficiency, and energy
efficiency of the 2.5 M all vanadium sulfate-chloride mixed acid
system at different ambient temperatures. Stable performance was
observed with this new system. During a course of 20 days of
operation, the gas-phase pressures of the anolyte and catholyte
containers remained constant, indicating no significant gas
evolution occurred in the whole system. The viscosity and density
of a solution containing 2.5 M VOSO.sub.4 and 6 M HCl at 30.degree.
C. is 6.1 cP and 1.40 g/ml respectively, slightly lower than the
6.4 cP and 1.45 g/ml for a solution containing 2.0 M VOSO.sub.4 and
3.0 M H.sub.2SO.sub.4.
TABLE-US-00006 TABLE 6 Performance of all vanadium redox flow cells
using the mixed SO.sub.4.sup.2-Cl.sup.- supporting solutions Energy
Coulombic Energy Voltage Density Efficiency Efficiency Efficiency
Current of Wh/L .eta..sub.C .eta..sub.E .eta..sub.V Discharge,
2.5VS 3VS 1.6V 2.5VS 3VS 1.6V 2.5VS 3VS 1.6V 2.5VS 3VS 1.6V
mA/cm.sup.2 6Cl 6Cl 4.5S 6Cl 6Cl 4.5S 6Cl 6Cl 4.5S 6Cl 6Cl 4.5S 100
36.2 39.5 22.3 0.95 0.95 0.94 0.81 0.76 0.83 0.85 0.80 0.88 75 37.5
40.8 22.4 0.96 0.96 0.94 0.84 0.81 0.85 0.88 0.84 0.90 50 38.5 41.8
22.6 0.96 0.97 0.94 0.87 0.85 0.87 0.91 0.88 0.92 25 39.2 43.1 22.6
0.96 0.97 0.94 0.90 0.89 0.88 0.93 0.91 0.94 1. Cell Operation
conditions: 10 cm.sup.2 flow cell, Charged to 1.7 V by 50
mA/cm.sup.2 current. 2. 2.5VS 6HCl:2.5M V 2.5M SO.sub.4.sup.2- 6M
Cl.sup.-; 3VS6HCl:3M V 3M SO.sub.4.sup.2- 6M Cl.sup.-; 1.6V
4.5S:1.6M V 4.5M SO.sub.4.sup.2-.
[0053] The experiment details related to the all-vanadium RFBs
using mixed SO.sub.4.sup.2- Cl.sup.- supporting solutions are as
follows. The flow cells consisted of two graphite electrodes, two
gold-coated copper current collectors, two PTFE gaskets, and a
Nafion.RTM. 117 membrane. The active area of the electrode and the
membrane was about 10 cm.sup.2. An Arbin battery tester was used to
evaluate the performance of flow cells and to control the charging
and discharging of the electrolytes. A Solartron 1287 potentiostat
was employed for cyclic voltammetry (CV) experiments. The flow rate
was fixed at 20 mL/min, which was controlled by a peristaltic pump.
A balanced flow cell contained about 50 mL anolyte and 50 mL
catholyte.
[0054] For cell performance evaluation and electrolyte solution
preparation, the cell was normally charged at a current density of
50 mA/cm.sup.2 to 1.7 V and discharged to 0.8 V with a current
density of 25 to 100 mA/cm.sup.2. Cell cycling tests were performed
at 90% state-of-charge and state-of-discharge at a fixed charging
and discharging current density of 50 mA/cm.sup.2.
[0055] The electrolyte solutions of V.sup.2+, V.sup.3+, V.sup.4+
and V.sup.5+ used in this work were prepared electrochemically in
flow cells using VOSO.sub.4 (from Alfa Aesar) and VCl.sub.3 as
starting chemicals. VCl.sub.3 solutions were prepared by dissolving
V.sub.2O.sub.3 (from Alfa Aesar) in HCl solutions. The electrolyte
stability tests were carried out in polypropylene tubes at
-5.degree. C., ambient temperature, 40.degree. C., 50.degree. C.,
and 60.degree. C., using about 5 ml solution for each sample.
During the stability tests, the samples were kept static without
any agitation, and were monitored daily by naked eye for the
formation of precipitation. Solution viscosity was measured using a
Ubbelohde calibrated viscometer tube.
[0056] Thermodynamic calculations of reaction 2VO.sub.2.sup.+
(a)+4H.sup.+ (a)+2Cl.sup.- (a)=2VO.sup.2+ were carried out using
HSC Chemistry.RTM. 6.1 program from Outotec Research Oy. Quantum
chemistry calculations were carried out using the Amsterdam Density
Functional (ADF) program.
[0057] Yet another embodiment of the present invention encompasses
a redox flow battery system based on the redox couple of Fe and V.
In this system, the anolyte comprises V.sup.2+ and V.sup.3+ in the
supporting solution while the catholyte comprises Fe.sup.2+ and i
Fe.sup.3+ in the supporting solution. The redox reactions and their
standard potentials can be described as follows:
Fe.sup.2+-eFe.sup.3+ E.degree.0.77 V vs. NHE (15)
V.sup.3++eV.sup.2+ E.degree.=-0.25 V vs. NHE (16)
V.sup.3++Fe.sup.2+V.sup.2++Fe.sup.3+ E.degree.=1.02V vs. NHE
(17)
[0058] The Fe/V system of the present invention can provide
significant benefits while circumventing some of the intrinsic
issues of conventional systems. For example, certain embodiments of
the Fe/V system do not use catalysts and/or high-temperature
management systems, which add to the complexity and cost of the
system. Moreover the evolution of H.sub.2 gas during charging is
minimized since the working potential of V.sup.2+/ V.sup.3+ (-0.25
V) is considerably higher than that of others, such as Cr.sup.2+/
Cr.sup.3+(-0.41 V). In the catholyte, the Fe.sup.2+/ Fe.sup.3+
redox couple is electrochemically reversible and can be less
oxidative than other common ionic species, such as V.sup.4+/
V.sup.5+, which can result in higher stability at high temperatures
while avoiding expensive, oxidation-resistant membrane materials,
such as sulfonated tetrafluoroethylene based
fluoropolymer-copolymer.
[0059] In one example using mixed Fe and V reactant solutions, an
electrolyte for Fe/V redox flow battery tests was prepared by
dissolving VCl.sub.3 (99%) and FeCl.sub.2 (98%) in concentrated HCl
(38%). Cyclic voltammetry (CV) was carried out in Fe/V HCl
solutions with various concentrations to identify redox couples and
electrochemical reversibility using a SOLARTRON 1287 potentiostat
(SOLARTRON ANALYTICAL, USA). A Pt wire and Ag/AgCl electrode were
used as the counter and reference electrodes, respectively. Glassy
carbon electrodes and graphite felt (.phi.=5.5 mm) sealed onto a
metal current collector were used as the working electrodes. The
scan rate was 0.5 mV/s. identical graphite felts without redox
catalysts were used in both CV and flow cell testing.
[0060] Cell performance was measured under constant current methods
using a flow cell system. The apparent area of graphite felt was 10
cm.sup.2 (2 cm.times.5 cm), in contact with membrane. 1.25 M Fe/V
in 2.3 M HCl solution and 1.17 M Fe/V in 2.15 M HCl solution were
used with either a sulfonated tetrafluoroethylene based
fluoropolymer-copolymer (i.e., NAFION) Or a low-cost hydrocarbon
membrane such as sulfonated poly(phenylsulfone) membrane (i.e.,
S-RADEL), respectively. Each electrolyte volume and flow rate was
50 mL and 20 mL/min. The flow cell was charged to 1.3 V and then
discharged to 0.5 Vat a current density of 50mA/cm.sup.2.
[0061] The chemical stability of commercially available membranes
was determined by soaking them in 0.15 M Fe.sup.3+ and 7 M total
chloride solution at 40.degree. C., and in 0.1_M V.sup.5+ and 5 M
total sulfate solution for comparison. During the stability tests,
the samples were kept static without any agitation, and were
monitored daily by naked eye for changes of color indicating
oxidation of the membrane.
[0062] FIGS. 8(a) and (b) show CV results of 1.5 M Fe and V in a 1
M hydrochloric acid supporting solution using glassy carbon and
graphite felt electrode, respectively. The current density is
normalized to the geometrical area of the working electrode.
Similar CV spectra were observed on both the glassy carbon and
graphite felt working electrode with the graphite felt electrode
demonstrating higher over potential due to the low conductivity.
Two redox peaks were observed indicating two redox reactions,
Fe.sup.3+/Fe.sup.3+ for the positive and V.sup.2+/V.sup.3+ for the
negative. More importantly, no significant hydrogen evolution
current was observed at potentials below the V.sup.3+ reduction
peak, indicating that no significant gas evolution occurred at the
negative electrode during the charging process when employing a
V.sup.2+/ V.sup.3+ redox couple. Oxidation and reduction peaks
appear in the forward and reverse scans on the positive side, which
revealed a reversible redox couple of Fe.sup.3+/ Fe.sup.2+ with a
potential at approximately 0.5 V. Similarly, there is no anodic
current observed associated with evolution of the Cl.sub.2 and/or
O.sub.2 gas. Thus, the Fe.sup.3+/Fe.sup.2+ and V.sup.3+/ V.sup.2+
redox couples in chloride supporting solution can be used in the
negative and positive half cells according to embodiments of the
present invention.
[0063] FIG. 3 shows the results of Fe/V flow cell testing with a
NAFION 117 membrane. A plot of cell voltage versus time is given in
FIG. 3(a). FIG. 3(b) demonstrates the cell voltage profile with
respect to the cell capacity and the cell SOC. The SOC is
calculated against the maximum charge capacity. Referring to FIG.
3(b), the Fe/V redox flow cell can be charged and discharged to a
SOC in the range of 0.about.100%. A utilization ratio of close to
100% can be achieved. Up to 50 cycles, the Fe/V cell demonstrated
stable columbic efficiency of .about.97%, energy efficiency of
.about.78%, and voltage efficiency of .about.80% as shown in FIG.
3(c). The Fe/V cell also demonstrated excellent capacity and energy
density retention capability as shown in FIG. 3(d) with 0.1% loss
per cycle in charge capacity over 50 cycles.
[0064] Commercially available, low-cost membranes, including a
micro-porous separator, can be used in place of relatively
expensive NAFION (i.e., sulfonated tetrafluoroethylene based
fluoropolymer-copolymer) membranes. Suitable alternative membranes
can include, but are not limited to, hydrocarbon-based commercially
available ion-exchange membranes; for example, SELEMION.RTM. anion
exchange membrane (APS, from Asahi Glass, Japan), SELEMION.RTM.
cation exchange membrane (CMV, from Asahi Glass, Japan), and
sulfonated poly(phenylsufone) membrane (S-RADEL.RTM. (RADEL.RTM.
from Solvay Advanced Polymers, USA), and micro-porous separators
typically used in lithium battery, for example; CELGARD.RTM.
micro-porous separator (Celgard, USA).
[0065] The electrochemical performance of a Fe/V cell employing a
S-RADEL membrane was then evaluated using identical test protocols
to that of Nafion membrane. The cell performance data is shown in
FIG. 5 (a) and (b). Similar Coulombic efficiency, voltage
efficiency, and energy efficiency with that of Nafion membrane were
obtained.
[0066] In a preferred embodiment, the energy density of Fe/V RFB
systems can be improved by using a supporting solution comprising
SO.sub.4.sup.2-Cl.sup.- mixed ions to increase the reactant
concentration in the anolyte and catholyte. Referring to Table 7,
the solubility of Fe.sup.2+ and Fe.sup.3+ ions is higher in
H.sub.2SO.sub.4--HCl mixed acids than in hydrochloric acid.
TABLE-US-00007 TABLE 1 Stability of Fe.sup.n+ cations in the
H.sub.2SO.sub.4--HCl mixed solutions Fe.sup.n+ Time for specie
Fe.sup.n+, M H.sup.+, M SO.sub.4.sup.2-, M Cl.sup.-, M T, .degree.
C. precipitation Fe.sup.2+ 2 4 2 4 25 Stable (>6 d) Fe.sup.3+ 2
6 2 6 25 Stable (>6 d)
[0067] While a number of embodiments of the present invention have
been shown and described, it will be apparent to those skilled in
the art that many changes and modifications may be made without
departing from the invention in its broader aspects. The appended
claims, therefore, are intended to cover all such changes and
modifications as they fall within the true spirit and scope of the
invention.
* * * * *