U.S. patent application number 13/472943 was filed with the patent office on 2012-11-08 for redox flow battery.
This patent application is currently assigned to Sumitomo Electric Industries, Ltd.. Invention is credited to Yongrong Dong, Takahiro Kumamoto, Toshio Shigematsu.
Application Number | 20120282509 13/472943 |
Document ID | / |
Family ID | 44861554 |
Filed Date | 2012-11-08 |
United States Patent
Application |
20120282509 |
Kind Code |
A1 |
Shigematsu; Toshio ; et
al. |
November 8, 2012 |
REDOX FLOW BATTERY
Abstract
A redox flow (RF) battery is provided that performs charge and
discharge by supplying a positive electrode electrolyte and a
negative electrode electrolyte to a positive electrode cell and a
negative cell, respectively. Each of the positive and negative
electrode electrolytes contains a vanadium (V) ion as active
material. At least one of the positive and negative electrode
electrolytes further contains another metal ion, for example, a
manganese ion that exhibits a higher redox potential than a V ion
or a chromium ion that exhibits a lower redox potential than a V
ion. Even in cases where the RF battery is nearly fully charged,
side reactions such as generation of oxygen has or hydrogen gas due
to water decomposition and oxidation degradation of an electrode
can be suppressed since the above-mentioned another metal ion
contained together with the V ion is oxidized or reduced in the
late stage of charge.
Inventors: |
Shigematsu; Toshio;
(Osaka-shi, JP) ; Dong; Yongrong; (Osaka-shi,
JP) ; Kumamoto; Takahiro; (Osaka-shi, JP) |
Assignee: |
Sumitomo Electric Industries,
Ltd.
Osaka-shi
JP
|
Family ID: |
44861554 |
Appl. No.: |
13/472943 |
Filed: |
May 16, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13505281 |
May 1, 2012 |
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PCT/JP2011/060228 |
Apr 27, 2011 |
|
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13472943 |
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Current U.S.
Class: |
429/109 ;
429/105 |
Current CPC
Class: |
H01M 2300/0088 20130101;
Y02E 60/10 20130101; H01M 8/188 20130101; H01M 2300/0091 20130101;
Y02E 60/50 20130101; H01M 2/30 20130101; H01M 2/40 20130101 |
Class at
Publication: |
429/109 ;
429/105 |
International
Class: |
H01M 4/36 20060101
H01M004/36 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 27, 2010 |
JP |
2010-102747 |
Apr 27, 2010 |
JP |
2010-102748 |
Apr 27, 2010 |
JP |
2010-102749 |
Claims
1. A redox flow battery performing charge and discharge by
supplying a positive electrode electrolyte and a negative electrode
electrolyte to a battery cell, each of said positive electrode
electrolyte and said negative electrode electrolyte containing a
vanadium ion, and at least one of said positive electrode
electrolyte and said negative electrode electrolyte further
containing at least one of a metal ion higher in redox potential
than the vanadium ion and a metal ion lower in redox potential than
the vanadium ion.
2. The redox flow battery according to claim 1, wherein at least
said positive electrode electrolyte further contains a metal ion
higher in redox potential than the vanadium ion.
3. The redox flow battery according to claim 1, wherein each of
said positive electrode electrolyte and said negative electrode
electrolyte further contains a metal ion higher in redox potential
than the vanadium ion.
4. The redox flow battery according to claim 1, wherein said metal
ion higher in redox potential is at least one type of metal ions
selected from a manganese ion, a lead ion, a cerium ion, and a
cobalt ion, and a total concentration of said metal ion higher in
redox potential in an electrolyte containing said metal ion higher
in redox potential is not less than 0.1M and not more than 5M.
5. The redox flow battery according to claim 1, wherein said metal
ion higher in redox potential is at least one type of manganese
ions of a divalent manganese ion and a trivalent manganese ion.
6. The redox flow battery according to claim 1, wherein the
electrolyte containing said metal ion higher in redox potential
contains at least one type of manganese ions of a divalent
manganese ion and a trivalent manganese ion, and tetravalent
manganese.
7. The redox flow battery according to claim 1, wherein at least
said positive electrode electrolyte further contains a metal ion
higher in redox potential than the vanadium ion, and at least said
negative electrode electrolyte further contains a metal ion lower
in redox potential than the vanadium ion.
8. The redox flow battery according to claim 1, wherein at least
one of said positive electrode electrolyte and said negative
electrode electrolyte further contains each of the metal ion higher
in redox potential than the vanadium ion and the metal ion lower in
redox potential than the vanadium ion.
9. The redox flow battery according to claim 1, wherein said metal
ion higher in redox potential is at least one type of metal ions
selected from a manganese ion, a lead ion, a cerium ion, and a
cobalt ion, said metal ion lower in redox potential is at least one
type of metal ions of a chromium ion and a zinc ion, and each of a
total concentration of the metal ions higher in redox potential in
an electrolyte containing said metal ions higher in redox potential
and a total concentration of the metal ions lower in redox
potential in an electrolyte containing said metal ions lower in
redox potential is not less than 0.1M and not more than 5M.
10. The redox flow battery according to claim 1, wherein said metal
ion higher in redox potential is at least one type of manganese
ions of a divalent manganese ion and a trivalent manganese ion, and
said metal ion lower in redox potential is at least one type of
chromium ions of a divalent chromium ion and a trivalent chromium
ion.
11. The redox flow battery according to claim 1, wherein the
electrolyte containing said metal ion higher in redox potential
contains at least one type of manganese ions of a divalent
manganese ion and a trivalent manganese ion, and tetravalent
manganese, and said metal ion lower in redox potential is at least
one type of chromium ions of a divalent chromium ion and a
trivalent chromium ion.
12. The redox flow battery according to claim 1, wherein at least
said negative electrode electrolyte further contains a metal ion
lower in redox potential than the vanadium ion.
13. The redox flow battery according to claim 1, wherein each of
said positive electrode electrolyte and said negative electrode
electrolyte further contains a metal ion lower in redox potential
than the vanadium ion.
14. The redox flow battery according to claim 1, wherein said metal
ion lower in redox potential is at least one type of metal ions of
a chromium ion and a zinc ion, and a total concentration of the
metal ions lower in redox potential in an electrolyte containing
said metal ions lower in redox potential is not less than 0.1M and
not more than 5M.
15. The redox flow battery according to claim 1, wherein said metal
ion lower in redox potential is at least one type of chromium ions
of a divalent chromium ion and a trivalent chromium ion.
16. The redox flow battery according to claim 1, wherein each of
said positive electrode electrolyte and said negative electrode
electrolyte contains a sulfate anion.
17. The redox flow battery according to claim 1, wherein a solvent
of each of said positive electrode electrolyte and said negative
electrode electrolyte is an H.sub.2SO.sub.4 aqueous solution, and a
sulfuric acid concentration of each of said positive electrode
electrolyte and said negative electrode electrolyte is not more
than 5M.
18. The redox flow battery according to claim 1, wherein an
operation is performed such that a state of charge of an
electrolyte of an electrode containing at least one of said metal
ion higher in redox potential and said metal ion lower in redox
potential exceeds 90%.
Description
TECHNICAL FIELD
[0001] The present invention relates to a redox flow battery
containing a vanadium ion as active material, and particularly to a
redox flow battery capable of improving an energy density as
compared to the conventional vanadium redox flow battery.
BACKGROUND ART
[0002] As a way to combat global warming, introduction of new
energy such as solar photovoltaic power generation and wind power
generation has been promoted in recent years throughout the world.
Since outputs of such power generation are affected by the weather,
it is predicted that introduction on a large scale will cause
problems with operation of power systems such as difficulty in
maintaining frequencies and voltages. As a way to solve such
problems, installation of large-capacity storage batteries for
smoothing output variations, storing surplus power, and load
leveling is expected.
[0003] A redox flow battery is one of large-capacity storage
batteries. In a redox flow battery, a positive electrode
electrolyte and a negative electrode electrolyte are supplied to a
battery cell having a membrane interposed between a positive
electrode and a negative electrode, to charge and discharge the
battery. An aqueous solution containing a water-soluble metal ion
having a valence which changes by oxidation-reduction is
representatively used as the electrolytes, and such a metal ion is
used as active material. In recent years, the most widely studied
type is a vanadium redox flow battery in which a vanadium (V) ion
is used as active material for each of the positive electrode and
the negative electrode (for example, Patent Literatures 1 and 2).
The vanadium redox flow battery is currently put in practical use
and expected to be continuously used in the future.
CITATION LIST
Patent Literature
PTL 1: Japanese Patent No. 3143568
PTL 2: Japanese Patent Laying-Open No. 2003-157884
SUMMARY OF INVENTION
Technical Problem
[0004] However, it is difficult for the conventional vanadium redox
flow battery to achieve a further improvement in the energy
density.
[0005] Generally, batteries are desired to have a higher energy
density. In order to increase the energy density, for example, it
is conceivable to raise the solubility of the active material in
the electrolyte and to raise the utilization rate of the
electrolyte, that is, the utilization rate of the metal ion
contained as active material in the electrolyte. The
above-described utilization rate means the actually available
battery capacity (discharge capacity) with respect to the
theoretical battery capacity (Ah) of the above-mentioned metal ion,
that is, the difference between the battery capacity in the lower
limit state of charge (SOC) and the battery capacity in the upper
limit state of charge.
[0006] However, when the above-described utilization rate is raised
as much as possible for charging, in other words, when the state of
charge is increased, in the late stage of charge, the positive
electrode undergoes a side reaction such as generation of oxygen
resulting from water decomposition and deterioration of electrodes
(particularly, made of carbon materials) while the negative
electrode undergoes a side reaction such as generation of hydrogen
resulting from water decomposition since an aqueous solution is
utilized for an electrolyte as described above in the typical
configuration of the redox flow battery.
[0007] The above-described side reactions bring about a lot of
harmful effects such as (1) a current loss (a loss caused by the
fact that a part of the quantity of electricity (Ah) used during
charge is not used for a battery reaction (valence change) but is
used for another reaction such as decomposition of water and the
like) is caused to decrease the battery efficiency; (2) a
difference between the states of charge of the positive and
negative electrodes is caused, leading to a reduction in the
available battery capacity; (3) deterioration of electrodes causes
a shortened battery lifetime; and the like. Accordingly, when the
battery is actually operated, the voltage at which charge is
stopped (upper limit charge voltage) is determined so as to use the
battery to such a degree that the above-described side reaction
does not occur. For example, in order to suppress the
above-described side reactions, Patent Literature 1 proposes that a
pentavalent V ion in the positive active material is 90% or less at
the end of charge while Patent Literature 2 proposes that charge is
to be continued such that a divalent V ion in the negative active
material is 94% or less.
[0008] However, the cell resistance is increased in the long-term
use. Accordingly, when the voltage at which charge is to be stopped
is set at a constant value without being changed from the beginning
of its use, the cell resistance is increased, so that the state of
charge at the start of its use cannot be maintained. Therefore, the
voltage at which charge is stopped is to be increased over time in
order to ensure a prescribed state of charge. Consequently, it
becomes difficult to ensure a high state of charge without
generating oxygen gas and hydrogen gas for a long period of
time.
[0009] From the viewpoint of suppression of a side reaction, it is
difficult in the current situation to keep the state of charge of a
vanadium ion in the electrolyte at 90% or higher for a long period
of time, and therefore, the vanadium ion cannot be sufficiently
utilized. For that reason, in the conventional vanadium redox flow
battery, it is difficult to achieve the utilization rate of the
vanadium ion at 90% or higher, and still higher. Thus, an
improvement in the energy density is limited.
[0010] An object of the present invention is to provide a redox
flow battery that can improve an energy density.
Solution to Problem
[0011] In the conventional vanadium redox flow battery, only a
vanadium ion is used as a metal ion serving as active material. On
the other hand, the present inventors have surprisingly found that
the utilization rate of a vanadium ion can be greatly improved as
compared to the conventional vanadium redox flow battery, for
example, by causing the electrolyte containing a vanadium ion as
active material to contain metal ions such as a manganese (Mn) ion
that is higher in oxidation-reduction potential (hereinafter simply
referred to as potential) than the vanadium ion on the positive
electrode side and a metal ion such as a chromium (Cr) ion that
exhibits a lower redox potential than the vanadium ion on the
negative electrode side, together with the vanadium ion. This is
considered to result from the reasons described below.
[0012] In the redox flow battery using the electrolyte containing a
vanadium ion as active material, the following reaction occurs in
each electrode upon charging. The standard potentials at the time
of occurrence of the reaction in each electrode are also shown.
[0013] Charge (positive electrode):
V.sup.4+.fwdarw.V.sup.5++e.sup.- Potential: about 1.0V
(V.sup.4+/V.sup.5+)
[0014] Charge (negative electrode):
V.sup.3++e.sup.-.fwdarw.V.sup.2+Potential: about -0.26V
(V.sup.3+/V.sup.2+)
[0015] Furthermore, the following side reaction may occur in the
late stage of charge. Also shown in this case is the standard
potential at the time of occurrence of each reaction when the
electrode made of carbon material is utilized.
[0016] Charge (positive electrode):
H.sub.2O.fwdarw.(1/2)O.sub.2+2H.sup.++2e.sup.- [0017] Potential:
about 1.2V (actual potential: about 2.0V)
[0017] C (carbon)+O.sub.2.fwdarw.CO.sub.2+4e.sup.- [0018]
Potential: about 1.2V (actual potential: about 2.0V)
[0019] Charge (negative electrode):
H.sup.++e.sup.-.fwdarw.(1/2)H.sub.2 [0020] Potential: about 0V
(actual potential: about -0.5V)
[0021] In the actual operation, an overvoltage depending on the
used electrode material is required, in which case the potential at
the time of occurrence of the actual side reaction on the positive
electrode side tends to be higher than the standard value. For
example, when the electrode material is carbon material, the
potential at the time of carbon reaction or water decomposition is
about 2V, which is higher than about 1V that is the potential at
the time of occurrence of battery reaction in the positive
electrode. Therefore, an oxidation reaction of a vanadium ion
(V.sup.4+.fwdarw.V.sup.5+) mainly occurs in the positive electrode
during charge as described above. However, when the charge voltages
rises in the late stage of charge to cause the potential of the
positive electrode to be relatively high, generation of oxygen gas
and oxidation degradation of electrodes (carbon) may occur together
with the above-described oxidation reaction of the vanadium ion.
Furthermore, this side reaction also leads to deterioration of the
battery characteristics.
[0022] Furthermore, in the actual operation, the potential at the
time of occurrence of the actual side reaction on the negative
electrode side tends to be lower than the standard value, depending
on the used electrode material. For example, in the case where the
electrode material is carbon material, a hydrogen overvoltage is
relatively large, with the result that the potential at the time of
generation of hydrogen is approximately -0.5V, which further
exhibits a lower redox potential than approximately -0.26V that is
the potential at the time of occurrence of the battery reaction in
the negative electrode. Therefore, during charge, a reduction
reaction of the vanadium ion (V.sup.3+.fwdarw.V.sup.2+) mainly
occurs as described above in the negative electrode. However, when
the charge voltage rises in the late stage of charge to cause the
potential of the negative electrode to be relatively low, hydrogen
gas may be generated simultaneously with the above-described
reduction reaction of the vanadium ion.
[0023] In contrast, the following is the case where the positive
electrode electrolyte contains, in addition to a vanadium ion, a
metal ion higher in redox potential than a vanadium ion. For
example, the potential of Mn.sup.2+/Mn.sup.3+ is approximately
1.5V, which is higher than the potential of
V.sup.4+/V.sup.5+(approximately 1.0V). In this case, however, this
potential exists on the lower side with respect to the actual
potential (approximately 2V) at the time of occurrence of a side
reaction on the positive electrode side such as generation of
oxygen gas resulting from water decomposition or electrode
oxidation as described above. Accordingly, for example, when a
divalent manganese ion (Mn.sup.2+) is contained, an oxidation
reaction of Mn.sup.2+ is to first occur before occurrence of the
side reaction on the positive electrode side such as generation of
oxygen gas described above. In other words, in the late stage of
charge, together with the oxidation reaction of V.sup.4+ that is a
main reaction of the battery, an oxidation reaction of Mn.sup.2+
also occurs as a part of the battery reaction. The oxidation
reaction of the metal ion different from the vanadium ion occurs,
so that the above-described side reaction on the positive electrode
side can be suppressed.
[0024] Alternatively, the following is the case where the negative
electrode electrolyte contains, in addition to a vanadium ion, a
metal ion lower in redox potential than the vanadium ion. For
example, the potential of Cr.sup.3+/Cr.sup.2+ is approximately
-0.42V that is lower than the potential of V.sup.3+/V.sup.2+
(approximately -0.26V). In this case, however, this potential
exists on the higher side with respect to the actual potential
(approximately -0.5V) at the time of occurrence of the side
reaction on the negative electrode side such as generation of
hydrogen gas described above. Accordingly, for example, in the case
where a trivalent chromium ion (Cr.sup.3+) is contained, a
reduction reaction of Cr.sup.3+ is to first occur before occurrence
of the above-described side reaction on the negative electrode
side. In other words, in the late stage of charge, together with
the reduction reaction of V.sup.3+ that is a main reaction of the
battery, reduction reaction of Cr.sup.3+ also occurs as part of the
battery reaction. The reduction reaction of the metal ion different
from the vanadium ion occurs, so that the above-described side
reaction on the negative electrode side can be suppressed.
[0025] As described above, in the case where the positive electrode
electrolyte contains not only a vanadium ion but also a metal ion
higher in redox potential than the vanadium ion, and in the case
where the negative electrode electrolyte contains not only a
vanadium ion but also a metal ion lower in redox potential than the
vanadium ion, the above-described side reaction hardly occurs or
substantially does not occur, for example, even when charge is
performed such that the state of charge of the electrolyte in each
of the positive electrode and the negative electrode exceeds 90%.
Therefore, in the embodiment where the above-described metal ion is
contained, it is considered that the vanadium ion in the
electrolyte can be fully utilized repeatedly with stability as
compared to the conventional vanadium redox flow battery. Thus, the
utilization rate of the vanadium ion is enhanced in this way,
thereby allowing improvement in the energy density. The present
invention is based on the above-described findings.
[0026] The present invention relates to a redox flow battery
performing charge and discharge by supplying a positive electrode
electrolyte and a negative electrode electrolyte to a battery cell.
Each of the positive electrode electrolyte and the negative
electrode electrolyte contains a vanadium ion. Furthermore, at
least one of the positive electrode electrolyte and the negative
electrode electrolyte further contains at least one of a metal ion
higher in redox potential than a vanadium ion and a metal ion lower
in redox potential than the vanadium ion.
[0027] The redox flow battery according to the present invention
having the above-described configuration allows suppression of the
side reaction in the late stage of charge even when charge is
performed until the state of charge of the electrolyte in at least
one of the positive electrode and the negative electrode reaches
nearly 100%. Specifically, for example, on the positive electrode
side, oxidation of another metal ion (specifically, a metal ion
higher in redox potential than a vanadium ion on the positive
electrode side) contained together with a vanadium ion allows
suppression of the side reaction such as generation of oxygen gas
resulting from water decomposition and oxidation degradation of the
electrode as described above. For example, on the negative
electrode side, reduction of another metal ion (specifically, a
metal ion lower in redox potential than a vanadium ion on the
negative electrode side) contained together with a vanadium ion
allows suppression of the side reaction such as generation of
hydrogen gas as described above. Accordingly, as compared to the
conventional redox flow battery that can only raise the state of
charge to at most approximately 90% due to the side reaction
occurring in the late stage of charge, the redox flow battery
according to the present invention can raise the state of charge of
the electrolyte in at least one of the electrodes to nearly 100%.
The state of charge can be raised in this way, thereby allowing an
increase in the utilization rate of the vanadium ion in the
electrolyte. Accordingly, the redox flow battery according to the
present invention can improve the energy density as compared to the
conventional case.
[0028] Furthermore, since the redox flow battery according to the
present invention can suppress the side reaction as described
above, it can also effectively suppress various defects (decreased
battery efficiency, decreased battery capacity, shortened lifetime)
caused by the side reaction. Thus, since the redox flow battery
according to the present invention is not only excellent in battery
characteristics but also capable of increasing the durability, high
reliability can be ensured for a long period of time.
[0029] Examples of a representative embodiment of the present
invention will be described as follows. In each of the following
embodiments, a metal ion higher in redox potential than a vanadium
ion exists at least in the positive electrode electrolyte, and a
metal ion lower in redox potential than a vanadium ion exists at
least in the negative electrode electrolyte, so that the side
reaction in the late stage of charge can be effectively suppressed
as described above, thereby allowing an increase in the utilization
rate of the vanadium ion.
[0030] (1) The embodiment in which at least the positive electrode
electrolyte contains a vanadium ion and a metal ion higher in redox
potential than the vanadium ion while the negative electrode
electrolyte contains the vanadium ion.
[0031] (2) The embodiment in which each of the positive electrode
electrolyte and the negative electrode electrolyte contains a
vanadium ion and a metal ion higher in redox potential than the
vanadium ion.
[0032] (3) The embodiment in which at least the positive electrode
electrolyte contains a vanadium ion, a metal ion higher in redox
potential than the vanadium ion and a metal ion lower in redox
potential than the vanadium ion while the negative electrode
electrolyte contains the vanadium ion.
[0033] (4) The embodiment in which at least the positive electrode
electrolyte contains a vanadium ion, a metal ion higher in redox
potential than the vanadium ion and a metal ion lower in redox
potential than the vanadium ion while at least the negative
electrode electrolyte contains a vanadium ion and a metal ion
higher in redox potential than the vanadium ion.
[0034] (5) The embodiment in which the positive electrode
electrolyte contains a vanadium ion while at least the negative
electrode electrolyte contains a vanadium ion and a metal ion lower
in redox potential than the vanadium ion.
[0035] (6) The embodiment in which each of the positive electrode
electrolyte and the negative electrode electrolyte contains a
vanadium ion and a metal ion lower in redox potential than the
vanadium ion.
[0036] (7) The embodiment in which the positive electrode
electrolyte contains a vanadium ion while at least the negative
electrode electrolyte contains a vanadium ion, a metal ion higher
in redox potential than the vanadium ion and a metal ion lower in
redox potential than the vanadium ion.
[0037] (8) The embodiment in which at least the positive electrode
electrolyte contains a vanadium ion and a metal ion lower in redox
potential than the vanadium ion while at least the negative
electrode electrolyte contains a vanadium ion, a metal ion higher
in redox potential than the vanadium ion and a metal ion lower in
redox potential than the vanadium ion.
[0038] Particularly, it is preferable to provide the embodiment in
which at least the positive electrode electrolyte further contains
a metal ion higher in redox potential than the vanadium ion while
at least the negative electrode electrolyte further contains a
metal ion lower in redox potential than the vanadium ion, since the
side reaction in the late stage of charge described above is
further effectively suppressed, thereby allowing a further increase
in the utilization rate of the vanadium ion. This embodiment can
also be configured such that the positive electrode electrolyte
further contains a metal ion lower in redox potential than the
vanadium ion or such that the negative electrode electrolyte
further contains a metal ion higher in redox potential than the
vanadium ion.
[0039] In addition, it becomes possible to provide the embodiment
in which the electrolyte in each of the positive electrode and the
negative electrode contains a vanadium ion, a metal ion higher in
redox potential than the vanadium ion and a metal ion lower in
redox potential than the vanadium ion, and representatively, the
embodiment in which the electrolytes in both of the electrodes
contain the same metal ion species. In the embodiment in which
metal ion species in the both positive and negative electrode
electrolytes are the same or partially the same, specific effects
as described below may be achieved. Specifically, (1) the metal ion
higher in redox potential in the positive electrode electrolyte and
the metal ion lower in redox potential in the negative electrode
electrolyte each move to a counter electrode, to cause a relative
decrease in the metal ion essentially reacting on each electrode,
so that it becomes possible to effectively avoid or suppress a
decreased effect of suppressing the side reaction. (2) Even when
liquid transfer occurs over time in accordance with
charge/discharge (the phenomenon in which the electrolyte in one
electrode moves to the other electrode) to cause variations in the
amount of the electrolyte in each electrode, mixture of the
electrolytes in both of the electrodes allows or facilitates the
variations to be readily corrected. (3) Manufacturability of the
electrolyte is excellent. In addition, in the embodiment in which
the metal ion species are the same or partially the same, the metal
ion higher in redox potential than the vanadium ion existing in the
negative electrode electrolyte and the metal ion lower in redox
potential than the vanadium ion existing in the positive electrode
electrolyte exist mainly for the electrolytes in both of the
electrodes to contain partially the same metal ion species, but do
not actively act as active materials. Accordingly, the
concentration of the metal ion higher in redox potential in the
negative electrode electrolyte and the concentration of the metal
ion higher in redox potential in the positive electrode electrolyte
may be differently set, and the concentration of the metal ion
lower in redox potential in the positive electrode electrolyte and
the concentration of the metal ion lower in redox potential in the
negative electrode electrolyte may be differently set. However,
when these respective concentrations are equally set, the
above-described effects (1) to (3) can be readily achieved.
[0040] It is preferable that the above-described metal ion higher
in redox potential and the above-described metal ion lower in redox
potential are water-soluble similarly to a vanadium ion or soluble
in an acid aqueous solution. It is preferable that the metal ion
higher in redox potential exists on the lower side than the actual
potential (approximately 2V) at the time when a side reaction
occurs on the positive electrode side. It is preferable that the
metal ion lower in redox potential exists on the higher side than
the actual potential (approximately -0.5V) at the time when a side
reaction occurs on the negative electrode side.
[0041] Examples of the above-described metal ion higher in redox
potential may include at least one type of metal ions, for example,
selected from a manganese (Mn) ion, a lead (Pb) ion, a cerium (Ce)
ion, and a cobalt (Co) ion. The standard potential of the
above-described metal ions is Mn.sup.2+/Mn.sup.3+: approximately
1.5V, Pb.sup.2+/Pb.sup.4+: approximately 1.62V,
Pb.sup.2+/PbO.sub.2: approximately 1.69V, Ce.sup.3+/Ce.sup.4+:
approximately 1.7V, and Co.sup.2+/Co.sup.3+: approximately 1.82V.
Thus, this potential is higher than the potential of the vanadium
ion on the positive electrode side: V.sup.4+/V.sup.5+
(approximately 1.0V), and lower than the potential of the
above-described side reaction on the positive electrode side
(approximately 2V). In addition to a vanadium ion, the electrolyte
in each of the positive electrode and the negative electrode may
contain one type of the above-described higher potential metal ion
or contain a plurality of types of combined higher potential metal
ions having different potentials.
[0042] Examples of the above-described metal ion lower in redox
potential may include at least one type of metal ions, for example,
of a chromium ion and a zinc ion. The standard potential of
chromium is Cr.sup.3+/Cr.sup.2+: approximately -0.42V, which is
lower than the potential of the vanadium ion on the negative
electrode side: V.sup.3+/V.sup.2+ (approximately -0.26V) and higher
than the potential of the above-described side reaction on the
negative electrode side (approximately -0.5V). On the other hand,
the standard potential of zinc is Zn.sup.2+/Zn (metal):
approximately -0.76V, which is lower than the potential of
V.sup.3+/V.sup.2+ (approximately -0.26V) and lower than the
potential of the above-described side reaction on the negative
electrode side. However, zinc is sufficiently high in hydrogen
overvoltage, and therefore, can cause a battery reaction. In
addition to a vanadium ion, the electrolyte in each of the positive
electrode and the negative electrode may contain one type of the
above-described lower potential metal ion or contain a plurality of
types of combined lower potential metal ions having different
potentials.
[0043] As for the above-described metal ions, by utilizing such
metal ions as allowing a reversible oxidation-reduction reaction
and at least functioning as positive electrode active material or
negative electrode active material, it becomes possible to decrease
the amount of the vanadium ions practically required to store a
prescribed electric power amount (kWh). Therefore, it is expected
in this case that metal ions used as active material can be
stabilized and supplied less expensively. The present inventors
have found that Mn.sup.3+ produced by oxidation reaction of
Mn.sup.2+ undergoes a reversible oxidation-reduction reaction in
the sulfuric acid solution, that is, Mn.sup.3+ oxidized during
charge may be used during discharge for the discharge reaction of
the battery (Mn.sup.3++e.sup.-.fwdarw.Mn.sup.2+), and, in addition
to a vanadium ion, a manganese ion can be repeatedly used as active
material. Furthermore, among the above-described metal ions, a
manganese ion is excellent in solubility. The above-described
chromium ion and zinc ion undergo a reversible oxidation-reduction
reaction in the sulfuric acid solution. Specifically, Cr.sup.2+ and
Zn (metal) reduced during charge are utilized during discharge for
discharge reaction (Cr.sup.2+.fwdarw.Cr.sup.3++e.sup.-,
Zn.fwdarw.Zn.sup.2++2e.sup.-) of the battery and can be repeatedly
used as active material. Therefore, it is preferable that the
above-described higher potential metal ions contain a manganese ion
while the above-described lower potential metal ions contain a
chromium ion and a zinc ion.
[0044] When the manganese ion is contained as the above-described
metal ion higher in redox potential, there may be a specific
embodiment in which at least one type of a manganese ion of a
divalent manganese ion and a trivalent manganese ion is contained.
By containing one of the above-described manganese ion, the
divalent manganese ion (Mn.sup.2+) exists during discharge and the
trivalent manganese ion (Mn.sup.3+) exists during charge, leading
to existence of both manganese ions through repeated charge and
discharge.
[0045] In the case of the electrolyte containing the manganese ion
as described above, it is considered that tetravalent manganese may
exist depending on the state of charge in the actual operation.
Therefore, as one embodiment according to the present invention, an
electrolyte containing the above-described metal ions higher in
redox potential contains at least one type of manganese ions of a
divalent manganese ion and a trivalent manganese ion, and
tetravalent manganese. In this case, Mn.sup.3+ is unstable, which
may cause a disproportionation reaction that produces Mn.sup.2+
(divalent) and MnO.sub.2 (tetravalent) in a manganese ion aqueous
solution. As a result of the study by the present inventors,
tetravalent manganese produced by the disproportionation reaction
is considered to be MnO.sub.2, but this MnO.sub.2 is considered to
be not entirely a solid precipitation but to exist in a stable
state in which the MnO.sub.2 seems to be at least partially
dissolved in the electrolyte. This MnO.sub.2 floating in the
electrolyte can be used repeatedly by being reduced to Mn.sup.2+
(discharged) through two-electron reaction during discharge,
namely, by serving as active material, to contribute to increase in
battery capacity. Accordingly, the present invention allows
existence of tetravalent manganese. In addition, when it is desired
to suppress precipitation of MnO.sub.2 by the disproportionation
reaction, for example, it is proposed that the operation is
performed such that the state of charge of positive electrode
manganese is not more than 90%, and preferably, equal to 70%, and
the acid concentration (for example, the sulfuric acid
concentration) of the electrolyte is increased when the solvent of
the electrolyte is an acid aqueous solution.
[0046] In the case where a chromium ion is contained as the
above-described metal ion lower in redox potential, as a more
specific embodiment, at least one type of chromium ions of a
divalent chromium ion and a trivalent chromium ion may be
contained. By containing any one of the chromium ions described
above, a trivalent chromium ion (Cr.sup.3+) exists during discharge
while a divalent chromium ion (Cr.sup.2+) exists during charge,
leading to existence of both chromium ions through repeated charge
and discharge. Chromium is easily treated since it exists always as
an ion in an aqueous solution with stability.
[0047] The present invention may include an embodiment where at
least one of the total concentration of the metal ion higher in
redox potential in the electrolyte containing the above-described
metal ions higher in redox potential and the total concentration of
the metal ion lower in redox potential in the electrolyte
containing the above-described metal ions lower in redox potential
is not less than 0.1M and not more than 5M (M is a mol
concentration). More specifically, the present invention may
include an embodiment where the total concentration of the metal
ions higher in redox potential is not less than 0.1M and not more
than 5M when the positive electrode electrolyte contains the metal
ion higher in redox potential; an embodiment where the total
concentration of the metal ions lower in redox potential is not
less than 0.1M and not more than 5M when the negative electrode
electrolyte contains the metal ions lower in redox potential; and
an embodiment where the total concentration of the metal ions
higher in redox potential and the total concentration of the metal
ions lower in redox potential each are not less than 0.1M and not
more than 5M when the positive electrode electrolyte contains the
metal ions higher in redox potential and the negative electrode
electrolyte contains the metal ions lower in redox potential.
[0048] When the total concentration of each of the higher potential
metal ions and the lower potential metal ions existing in the
electrolyte of each of the positive electrode and the negative
electrode is less than 0.1M, oxidation reaction and reduction
reaction of the metal ions hardly occur, leading to difficulty in
achieving the effect of suppressing the above-described side
reaction by these oxidation reaction and reduction reaction.
Consequently, it becomes difficult to sufficiently improve the
energy density. The higher the total concentration of each of the
above-described metal ions is, the greater the above-described
effect of suppressing the side reaction is achieved and the more
the energy density is improved. In this case, however, the
solubility of the vanadium ion tends to decrease due to increased
metal ions. When each total concentration of the above-described
metal ions is not more than 1M, and further, not more than 0.5M,
the effects of suppressing the above-described side reaction and
the like can be achieved while the solubility of vanadium ion can
also be sufficiently ensured. Furthermore, when the solvent of the
electrolyte is an acid aqueous solution as described above and
contains a manganese ion, the acid concentration of the electrolyte
is increased to some extent, thereby allowing suppression of
precipitation of MnO.sub.2. In this case, however, the increased
acid concentration may cause a decrease in the solubility of metal
ions. Accordingly, the upper limit of the total concentration of
the metal ions in each of the electrodes is considered to be
5M.
[0049] The present invention includes an embodiment where both the
positive and negative electrode electrolytes contain a sulfate
anion (SO.sub.4.sup.2-).
[0050] As for the solvent of the electrolyte in each of the
positive electrode and the negative electrode, the aqueous solution
containing at least one type of a sulfate anion (SO.sub.4.sup.2-),
a phosphate anion (PO.sub.4.sup.3-) and a nitrate anion
(NO.sub.3.sup.-) can be suitably utilized. These acid aqueous
solutions can be expected to achieve several effects that (1) the
stability, the reactivity and the solubility of the vanadium ion
and the above-described metal ions in the electrolyte may be
improved; (2) the ion conductivity is increased and the internal
resistance of the battery is reduced, and (3) unlike when
hydrochloric acid (HCl) is used, chlorine gas is not generated.
Particularly, the embodiment where a sulfate anion
(SO.sub.4.sup.2-) is contained is preferable since the stability
and the reactivity of the vanadium ion and the above-described
metal ions can be improved as compared to the case where a
phosphate anion and a nitrate anion are contained. For the
electrolyte in each of the above-described electrodes to contain a
sulfate anion, for example, a sulfate salt containing a vanadium
ion and the above-described metal ions may be used.
[0051] The present invention includes an embodiment where the
solvent of each of the above-described positive and negative
electrode electrolytes is an aqueous solution of H.sub.2SO.sub.4.
In this case, it is preferable that the sulfuric acid concentration
of the electrolyte in each of the positive electrode and the
negative electrode is not more than 5M.
[0052] In addition to use of the sulfate salt as described above,
an H.sub.2SO.sub.4 aqueous solution (sulfuric acid aqueous
solution) is used as a solvent of the electrolyte, so that the
stability and the reactivity of the vanadium ion and the metal ion
can be improved while the internal resistance can also be reduced
as described above. However, when the sulfuric acid concentration
is too high, existence of the sulfate anion may lead to a decrease
in the solubility of the vanadium ion and the metal ions such as a
manganese ion and a chromium ion, and also lead to an increase in
the viscosity of the electrolyte. Accordingly, the sulfuric acid
concentration is preferably not more than 5M, in which case 1M to
4M can be readily available, and 1M to 3M is more preferable.
[0053] The present invention includes an embodiment where the
operation is carried out such that at least one of the state of
charge of the positive electrode electrolyte and the state of
charge of the negative electrode electrolyte exceeds 90%. More
specifically, it is preferable that the redox flow battery
according to the present invention is operated such that the state
of charge of the electrolyte of one of the positive electrode
electrolyte and the negative electrode electrolyte containing at
least one of the metal ions higher in redox potential and the metal
ions lower in redox potential exceeds 90%.
[0054] In the present invention, in the state where the positive
electrode electrolyte contains, in addition to a vanadium ion, a
metal ion higher in redox potential than the vanadium ion and the
state where the negative electrode electrolyte contains, in
addition to a vanadium ion, a metal ion lower in redox potential
than the vanadium ion, the side reaction can be suppressed as
described above even when charge is performed such that the state
of charge exceeds 90%. The state of charge is increased in this
way, the utilization rate of the vanadium ion can be effectively
raised. Particularly in the embodiment where the positive electrode
electrolyte contains the above-described metal ions higher in redox
potential and the negative electrode electrolyte contains the
above-described metal ions lower in redox potential, the state of
charge of each electrolyte in the positive electrode and the
negative electrode is increased to exceed 90%. Thus, it is expected
that the utilization rate of the vanadium ion can be more
effectively increased.
Advantageous Effects of Invention
[0055] The redox flow battery according to the present invention
can improve the energy density.
BRIEF DESCRIPTION OF DRAWINGS
[0056] FIG. 1 illustrates the operating principles of a battery
system including a redox flow battery according to the first
embodiment.
[0057] FIG. 2 illustrates the operating principles of a battery
system including a redox flow battery according to the second
embodiment.
[0058] FIG. 3 illustrates the operating principles of a battery
system including a redox flow battery according to the third
embodiment.
[0059] FIG. 4 shows a graph illustrating the relation between a
cycle time (sec) of charge and discharge and a battery voltage (V)
in an example system manufactured in Experimental Example 1.
[0060] FIG. 5 shows a graph illustrating the relation between a
cycle time (sec) of charge and discharge and a battery voltage (V)
in an example system manufactured in Experimental Example 4.
[0061] FIG. 6 shows a graph illustrating the relation between a
charge time (sec) and a battery voltage (V) in a comparison system
(I).
[0062] FIG. 7 shows a graph illustrating the relation between a
cycle time (sec) of charge and discharge and a battery voltage (V)
in a comparison system (II).
DESCRIPTION OF EMBODIMENTS
[0063] Referring to FIGS. 1 to 3, battery systems including redox
flow batteries according to the first to third embodiments will be
hereinafter schematically described. In FIGS. 1 to 3, the same
reference characters indicate components having the same names.
Metal ions other than a vanadium ion shown in FIGS. 1 to 3 are
merely illustrative examples. In FIGS. 1 to 3, a solid line arrow
indicates charge, and a broken line arrow indicates discharge.
[0064] Redox flow batteries 100 according to the first to third
embodiments have similar basic structures, which will be described
with reference to FIG. 1. Redox flow battery 100 is
representatively connected to a power generation unit (for example,
a solar photovoltaic power generator, a wind power generator, or a
common power plant) and to a load such as a power system or a
consumer through a power conditioning system (PCS), charged by the
power generation unit as a power supply source, and discharged to
provide power to the load. To be charged and discharged, the
following battery system including redox flow battery 100 and a
circulation mechanism (tanks, pipes, pumps) for circulating an
electrolyte through battery 100 is constructed.
[0065] Redox flow battery 100 includes a positive electrode cell
102 having a positive electrode 104 therein, a negative electrode
cell 103 having a negative electrode 105 therein, and a membrane
101 separating cells 102 and 103 from each other, through which
ions permeate as appropriate. Positive electrode cell 102 is
connected to a tank 106 for a positive electrode electrolyte
through pipes 108, 110. Negative electrode cell 103 is connected to
a tank 107 for a negative electrode electrolyte through pipes 109,
111. Pipes 108, 109 include pumps 112, 113 for circulating the
electrolytes of the electrodes, respectively. In redox flow battery
100, the positive electrode electrolyte in tank 106 and the
negative electrode electrolyte in tank 107 are supplied to positive
electrode cell 102 (positive electrode 104) and negative electrode
cell 103 (negative electrode 105) through circulation,
respectively, through pipes 108 to 111 and pumps 112, 113, to
charge and discharge the battery through valence change reaction of
the metal ion serving as active materials in the electrolytes of
both electrodes.
[0066] Redox flow battery 100 representatively has a form referred
to as a cell stack, which includes a plurality of cells 102, 103
stacked therein. Cells 102, 103 are representatively structured
with a cell frame including a bipolar plate (not shown) having
positive electrode 104 arranged on one surface and negative
electrode 105 on the other surface, and a frame (not shown) having
a liquid supply hole for supplying the electrolytes and a liquid
drainage hole for draining the electrolytes, and formed on the
periphery of the bipolar plate. By stacking a plurality of cell
frames, the liquid supply holes and the liquid drainage holes form
a fluid path for the electrolytes, which is connected to pipes 108
to 111 as appropriate. The cell stack is structured by successively
and repeatedly stacking a set of the cell frame, positive electrode
104, membrane 101, negative electrode 105, and the cell frame. A
known structure may be used as appropriate as a basic structure of
the redox flow battery system.
[0067] In the redox flow battery according to the first embodiment,
the above-described positive electrode electrolyte and the
above-described negative electrode electrolyte each contain a
vanadium ion, in which the positive electrode electrolyte contains,
in addition to a vanadium ion, a metal ion higher in redox
potential than the vanadium ion (FIG. 1 shows a manganese ion by
way of example).
[0068] In the redox flow battery according to the second
embodiment, the above-described positive electrode electrolyte and
the above-described negative electrode electrolyte each contain a
vanadium ion. The positive electrode electrolyte further contains,
in addition to a vanadium ion, a metal ion higher in redox
potential than the vanadium ion (FIG. 2 shows a manganese ion by
way of example). The negative electrode electrolyte further
contains, in addition to a vanadium ion, a metal ion lower in redox
potential than the vanadium ion (FIG. 2 shows a chromium ion by way
of example).
[0069] In the redox flow battery according to the third embodiment,
the above-described positive electrode electrolyte and the
above-described negative electrode electrolyte each contain a
vanadium ion. In addition to a vanadium ion, the negative electrode
electrolyte further contains a metal ion lower in redox potential
than the vanadium ion (FIG. 3 shows a chromium ion by way of
example).
[0070] A more specific explanation will be hereinafter made with
reference to Experimental Examples. In each of Experimental
Examples described below, the redox flow battery system shown in
each of FIGS. 1 to 3 is structured as a basic configuration, in
which various types of electrolytes containing a vanadium ion were
prepared in each of the positive electrode and the negative
electrode to perform charge and discharge on various
conditions.
Experimental Example 1
[0071] The following was prepared as an example system according to
the first embodiment.
[0072] (Electrolyte)
[0073] As a positive electrode electrolyte, 6 ml (6 cc) of an
electrolyte having a vanadium ion (tetravalent) concentration of
1.65 M and a manganese ion (divalent) concentration of 0.5M was
prepared by dissolving sulfate salts (vanadium sulfate
(tetravalent) and manganese sulfate (divalent)) in the sulfuric
acid aqueous solution having a sulfuric acid concentration
(H.sub.2SO.sub.4aq) of 2.6M.
[0074] As a negative electrode electrolyte, 9 ml (9 cc) of an
electrolyte having a vanadium ion (trivalent) concentration of 1.7M
was prepared by dissolving sulfate salt (vanadium sulfate
(trivalent)) in the sulfuric acid aqueous solution
(H.sub.2SO.sub.4aq) having a sulfuric acid concentration of 1.75M.
The amount of the negative electrode electrolyte is set to be
greater than the amount of the positive electrode electrolyte, so
that the battery reaction on the positive electrode side (including
not only oxidation reaction of the vanadium ion but also oxidation
reaction of the manganese ion) can be sufficiently caused during
charge (which is the same in Experimental Example 2 described
later).
[0075] (Other Components)
[0076] A carbon felt was used for each of the positive and negative
electrodes, and an ion exchange membrane was used for the membrane.
The constituent materials of the electrode and the membrane can be
selected as appropriate. The electrode made of carbon felt have
advantages of (1) hardly generating oxygen gas and hydrogen gas on
the positive electrode side and the negative electrode side,
respectively, (2) having a relatively large surface area, and (3)
showing excellent circulation of the electrolyte. The ion exchange
membranes have advantages of (1) attaining excellent isolation of
the metal ions serving as active materials of each electrode, and
(2) having excellent permeability of an H.sup.+ ion (charge carrier
inside a battery).
[0077] Then, in this Experimental Example 1, a small single cell
battery including an electrode having an area of 9 cm.sup.2 was
manufactured, and the prepared electrolyte for each of the
above-described electrodes was used to perform charge at a constant
current of 630 mA (current density: 70 mA/cm.sup.2). More
specifically, the battery was charged until the state of charge
(SOC) of a vanadium ion in the positive electrode electrolyte
reached 124%. The above-described state of charge shows the
numerical value that is assumed to be set at 100 in the case where
only a vanadium ion was used as active material. Thus, the state of
charge exceeding 100% means that the state of charge of the
vanadium ion is approximately 100% and Mn.sup.2+ is changed to
Mn.sup.3+ (or tetravalent manganese) for charge. This charge was
then switched to discharge, which was followed by repetition of
charge and discharge on the same charge conditions as those
described above. FIG. 4 shows the relation between the cycle time
of charge and discharge and the battery voltage.
[0078] The vanadium redox flow battery system was constructed as
comparison systems. The basic configuration of each of the
comparison systems is the same as that of the above-described
example system, and therefore, configured in the similar manner to
the above-described example system except that the electrolyte and
the operating conditions were different. In this Experimental
Example 1, as a positive electrode electrolyte and a negative
electrode electrolyte, the vanadium electrolyte having a vanadium
ion (tetravalent) concentration of 1.7M in the positive electrode
and a vanadium ion (trivalent) concentration of 1.7M in the
negative electrode was prepared by dissolving vanadium sulfate
(tetravalent) in the sulfuric acid aqueous solution
(H.sub.2SO.sub.4aq) having a sulfuric acid concentration of 2.6M in
the positive electrode and dissolving vanadium sulfate (trivalent)
in the sulfuric acid aqueous solution (H.sub.2SO.sub.4aq) having a
sulfuric acid concentration of 1.75M in the negative electrode.
[0079] Then, in the comparison system (I), a small single cell
battery including an electrode having an area of 9 cm.sup.2 was
manufactured. Then, the above-described vanadium electrolyte was
used by 10 ml (10 cc) for each of the positive electrode and the
negative electrode, to perform charge at a constant current of 540
mA (current density: 60 mA/cm.sup.2). Furthermore, in the
comparison system (I), even when the state of charge of the
vanadium ion in the positive electrode electrolyte exceeded the
level equivalent to 100%, charge was continued for a while. FIG. 6
shows the relation between the charge time and the battery voltage
in the comparison system (I).
[0080] On the other hand, the comparison system (II) is configured
in the similar manner to the above-described comparison system (I)
except that the amount of the electrolyte and the operating
conditions are different. Specifically, the above-described
vanadium electrolyte was used by 7 ml (7 cc) for each of the
positive electrode and the negative electrode, to perform charge at
a constant current of 630 mA (current density: 70 mA/cm.sup.2).
Then, in the comparison system (II), charge was stopped and
switched to discharge at the point of time when the voltage reached
1.6V (the state of charge of the vanadium ion: 78%). Then, charge
and discharge were repeatedly performed in the similar manner. FIG.
7 shows the relation between the cycle time of charge and discharge
and the battery voltage in the comparison system (II).
[0081] Consequently, in the comparison system (I), the voltage
rapidly rose from around 1.6V to 2.6V or higher, as shown in FIG.
6. When charge was further continued, oxygen gas was generated from
the positive electrode while hydrogen gas was generated from the
negative electrode. When discharge was performed starting in such a
state to further repeat charge and discharge several times on the
similar conditions (charge was continued until the state of charge
exceeded 100%), there was a tendency that the internal resistance
of the battery was gradually increased and the battery capacity was
also decreased. When the cell was disassembled after completion of
the experiment, oxidation degradation of the carbon material
constituting the positive electrode was recognized.
[0082] On the other hand, in the comparison system (II), when the
upper limit voltage for charge was set at 1.6V, no generation of
oxygen gas or hydrogen gas occurred. Furthermore, although charge
and discharge were repeated several times, neither the internal
resistance of the battery was increased nor the battery capacity
was reduced. Thus, the operation could be repeatedly performed with
stability. However, in the comparison system (II), the battery
capacity that could be actually utilized is 20.4 minutes with
respect to the theoretical capacity of 30.4 minutes (the value
converted into discharge time based on the vanadium ion
concentration of 1.7M, 7 ml, 630 mA) while the utilization rate of
the vanadium ion is 67% (<90%).
[0083] On the other hand, in the example system, although the
voltage rises from around 1.6V as shown in FIG. 4, this voltage
rise is not so sharp but relatively moderate as compared to the
comparison system (I). It was also observed from the voltage
characteristics after the voltage reached 1.6 V or higher that,
during charge, further oxidation reaction of the vanadium ion
occurred in the positive electrode while oxidation reaction of the
manganese ion (divalent) occurred. Furthermore, unlike the
comparison system (I), in the example system, even when charge was
performed in the state where the state of charge of the positive
electrode exceeded the level equivalent to 100%, a rise of the
battery voltage was suppressed, and thus, at about 2V at most. In
addition, in the example system, it was confirmed that oxygen gas
was not generated and the electrode did not deteriorate when the
cell was disassembled after repetition of charge and discharge.
Furthermore, the discharge time (discharge capacity) of the example
system was 23.7 minutes, which was 93.7% with respect to the
theoretical capacity (25.3 minutes that is the value converted into
discharge time based on the vanadium ion concentration of 1.65M, 6
ml, 630 mA), corresponding to the utilization rate exceeding 90%.
Furthermore, it was also confirmed that even repetition of charge
and discharge did not cause a reduction in the battery capacity and
allowed a stable operation.
[0084] It can be said from the above-described Experimental Example
1 that when at least the positive electrode electrolyte contains,
in addition to a vanadium ion, a metal ion higher in redox
potential than the vanadium ion on the positive electrode side, the
utilization rate of the vanadium ion can be effectively increased
to improve the energy density.
Experimental Example 2
[0085] In Experimental Example 2, as a positive electrode
electrolyte, 6 ml (6 cc) of an electrolyte having a vanadium ion
(tetravalent) concentration of 1.65M and a manganese ion (divalent)
concentration of 0.5M was prepared by dissolving sulfate salts
(vanadium sulfate (tetravalent) and manganese sulfate (divalent))
in the sulfuric acid aqueous solution (H.sub.2SO.sub.4aq) having a
sulfuric acid concentration of 2.6M. As a negative electrode
electrolyte, 9 ml (9 cc) of an electrolyte having a vanadium ion
(trivalent) concentration of 1.7M and a manganese ion (divalent)
concentration of 0.5M was prepared by dissolving sulfate salts
(vanadium sulfate (trivalent) and manganese sulfate (divalent) in
the sulfuric acid aqueous solution (H.sub.2SO.sub.4aq) having a
sulfuric acid concentration of 1.65M. Other configurations were
similar to those of the example system in Experimental Example
1.
[0086] Then, a small single cell battery (electrode area: 9
cm.sup.2) similar to that of Experimental Example 1 was
manufactured and the prepared electrolyte of each of the positive
electrode and negative electrode was used to repeatedly perform
charge and discharge on the conditions similar to those of the
example system in Experimental Example 1. In this case, it was
confirmed that the behavior of the voltage characteristics of the
system in Experimental Example 2 was almost the same as that of the
example system in Experimental Example 1 while the utilization rate
could also be set to exceed 90%. Furthermore, it was confirmed also
in the system in Experimental Example 2 that oxygen gas was not
generated and the electrode did not deteriorate when the cell was
disassembled after repetition of charge and discharge.
[0087] Therefore, it can be said from Experimental Example 2 that
the utilization rate of the vanadium ion can be effectively raised
to improve the energy density by the electrolyte in each of the
positive and negative electrodes containing, in addition to a
vanadium ion, a metal ion higher in redox potential than the
vanadium ion on the positive electrode side.
Experimental Example 3
[0088] The following was prepared as an example system according to
the second embodiment.
[0089] As a positive electrode electrolyte, 6 ml (6 cc) of an
electrolyte having a vanadium ion (tetravalent) concentration of
1.65M, a manganese ion (divalent) concentration of 0.5M and a
chromium ion (trivalent) concentration of 0.1M was prepared by
dissolving sulfate salts (vanadium sulfate (tetravalent), manganese
sulfate (divalent) and chromium sulfate (trivalent)) in the
sulfuric acid aqueous solution (H.sub.2SO.sub.4aq) having a
sulfuric acid concentration of 2.6M.
[0090] As a negative electrode electrolyte, 6 ml (6 cc) of an
electrolyte having a vanadium ion (trivalent) concentration of
1.65M, a manganese ion (divalent) concentration of 0.5M and a
chromium ion (trivalent) concentration of 0.1M was prepared by
dissolving sulfate salts (vanadium sulfate (trivalent), manganese
sulfate (divalent) and chromium sulfate (trivalent)) in the
sulfuric acid aqueous solution (H.sub.2SO.sub.4aq) having a
sulfuric acid concentration of 1.75M.
[0091] A carbon felt was used for each of the positive and negative
electrodes, and an ion exchange membrane was used for the
membrane.
[0092] Then, in this Experimental Example 3, a small single cell
battery including an electrode having an area of 9 cm.sup.2 was
manufactured, and the above-described prepared electrolyte of each
of the electrodes was used to perform charge at a constant current
of 630 mA (current density: 70 mA/cm.sup.2). More specifically,
charge was performed until the state of charge (SOC) of the
vanadium ion of the electrolyte in each electrode reached the level
equivalent to 105%. The above-described state of charge shows a
numerical value that is assumed to be set at 100 in the case where
only a vanadium ion is used as active material. The state of charge
exceeding 100% means that, in addition to the fact that the state
of charge of the vanadium ion is approximately 100%, Mn.sup.2+ is
changed to Mn.sup.3+ (or tetravalent manganese) for charge in the
positive electrode while Cr.sup.3+ is changed to Cr.sup.2+ for
charge in the negative electrode. This charge was then switched to
discharge, which was followed by repetition of charge and discharge
on the same charge conditions as those described above. The
comparison system was configured as a comparison system (I) and a
comparison system (II) in Experimental Example 1.
[0093] Consequently, in the example system according to the second
embodiment, although the voltage rose from about 1.6V, this rise
was not so sharp but relatively moderate as compared to the
comparison system (I). It was also observed from the voltage
characteristics after the voltage reached 1.6V or higher that,
during charge, the positive electrode underwent further oxidation
reaction of the vanadium ion and oxidation reaction of the
manganese ion (divalent) while the negative electrode underwent
further reduction reaction of the vanadium ion and reduction
reaction of the chromium ion (trivalent). Furthermore, unlike the
comparison system (I), in the example system of the second
embodiment, even when charge was performed in the state where the
state of charge of each electrode exceeded the level equivalent to
100%, a battery voltage rise was suppressed, and thus, at about 2V
at most. In addition, in the example system according to the second
embodiment, it was confirmed that oxygen gas or hydrogen gas was
not generated while the electrode did not deteriorate when the cell
was disassembled after repetition of charge and discharge. Then, it
was also confirmed that the discharge time (discharge capacity) of
the example system according to the second embodiment shows a
utilization rate exceeding 90% with respect to the theoretical
capacity (25.3 minutes that is a value converted into the discharge
time based on the vanadium ion concentration of 1.65M, 6 ml, 630
mA). Furthermore, it was also confirmed that even repetition of
charge and discharge did not cause a reduction in the battery
capacity and allowed a stable operation.
[0094] It can be said from the above-described Experimental Example
3 that when at least the positive electrode electrolyte contains,
in addition to a vanadium ion, a metal ion higher in redox
potential than the vanadium ion on the positive electrode side and
when at least the negative electrode electrolyte contains, in
addition to a vanadium ion, a metal ion lower in redox potential
than the vanadium ion on the negative electrode side, the
utilization rate of the vanadium ion can be effectively increased
to improve the energy density. Furthermore, it can be said that, in
the above-described Experimental Example 3, the metal ion species
in the electrolyte of each of the positive and negative electrodes
are partially the same, with the result that (1) a relative
decrease of the metal ions serving as active material hardly
occurs, thereby allowing further suppression of occurrence of the
side reaction; (2) variations in the liquid quantity resulting from
liquid transfer can be readily corrected; and (3) the
manufacturability of the electrolyte is excellent.
Experimental Example 4
[0095] The following was prepared as an example system according to
the third embodiment.
[0096] As a positive electrode electrolyte, 9 ml (9 cc) of an
electrolyte having a vanadium ion (tetravalent) concentration of
1.7M was prepared by dissolving sulfate salt (vanadium sulfate
(tetravalent)) in the sulfuric acid aqueous solution
(H.sub.2SO.sub.4aq) having a sulfuric acid concentration of
2.6M.
[0097] As a negative electrode electrolyte, 6 ml (6 cc) of an
electrolyte having a vanadium ion (trivalent) concentration of 1.7M
and a chromium ion (trivalent) concentration of 0.1M was prepared
by dissolving sulfate salts (vanadium sulfate (trivalent) and
chromium sulfate (trivalent)) in the sulfuric acid aqueous solution
(H.sub.2SO.sub.4aq) having a sulfuric acid concentration of 1.75M.
The amount of the positive electrode electrolyte is set to be
greater than the amount of the negative electrode electrolyte, so
that the battery reaction on the negative electrode side (including
not only reduction reaction of the vanadium ion but also reduction
reaction of the chromium ion) can be sufficiently caused during
charge (which is the same in Experimental Example 5 described
later).
[0098] A carbon felt was used for each of the positive and negative
electrodes, and an ion exchange membrane was used for the
membrane.
[0099] Then, in this Experimental Example 4, a small single cell
battery including an electrode having an area of 9 cm.sup.2 was
manufactured and the above-described prepared electrolyte in each
of the electrodes was used to perform charge at a constant current
of 630 mA (current density: 70 mA/cm.sup.2). More specifically,
charge was performed until the state of charge (SOC) of the
vanadium ion in the negative electrode electrolyte reached the
level equivalent to 109%. The above-described state of charge shows
a numerical value that is assumed to be set at 100 in the case
where only a vanadium ion was used as active material. Thus, the
state of charge exceeding 100% means that the state of charge of
the vanadium ion is approximately 100% and Cr.sup.3+ is changed to
Cr.sup.2+ for charge. This charge was then switched to discharge,
which was followed by repetition of charge and discharge on the
same charge conditions as those described above. FIG. 5 shows the
relation between the cycle time of charge and discharge and the
battery voltage. The comparison system was configured as a
comparison system (I) and a comparison system (II) of Experimental
Example 1.
[0100] Consequently, in the example system according to the third
embodiment, although the voltage rose from about 1.6V as shown in
FIG. 5, this rise was not so sharp but relatively moderate as
compared to the comparison system (I). It was also observed from
the voltage characteristics after the voltage reached 1.6V or
higher that, during charge, the negative electrode underwent
further reduction reaction of the vanadium ion and reduction
reaction of the chromium ion (trivalent). Furthermore, unlike the
comparison system (I), in the example system according to the third
embodiment, even when the charge was performed in the state where
the state of charge of the negative electrode exceeded the level
equivalent to 100%, a battery voltage rise was suppressed, and
thus, at about 2V at most. In addition, no generation of hydrogen
gas was observed in the example system according to the third
embodiment. Then, the discharge time (discharge capacity) of the
example system according to the third embodiment was 25.9 minutes
corresponding to 99.6% with respect to the theoretical capacity (26
minutes which is a value converted into the discharge time based on
the vanadium ion concentration of 1.75M, 6 ml, 630 mA). Thus, the
capacity of nearly 100% was achieved and the utilization rate
exceeding 90% was also achieved. Furthermore, it was also confirmed
that even repetition of charge and discharge did not cause a
reduction in the battery capacity and allowed a stable
operation.
[0101] It can be said from the above-described Experimental Example
4 that the utilization rate of the vanadium ion can be effectively
increased to improve the energy density by at least the negative
electrode electrolyte containing, in addition to a vanadium ion, a
metal ion lower in redox potential than the vanadium ion on the
negative electrode side.
Experimental Example 5
[0102] In Experimental Example 5, the electrolyte containing a
vanadium ion and a chromium ion was used as an electrolyte for each
of the positive electrode and the negative electrode. Specifically,
as a positive electrode electrolyte, sulfate salt (chromium sulfate
(trivalent)) was further used in addition to the same materials as
those in the example system of Experimental Example 4 to prepare 9
ml (9 cc) of an electrolyte having a vanadium ion (tetravalent)
concentration of 1.7M and a chromium ion (trivalent) concentration
of 0.1M. A negative electrode electrolyte similar to that in the
example system of Experimental Example 4 was prepared (a vanadium
ion (trivalent) concentration of 1.7M and a chromium ion
(trivalent) concentration of 0.1M, 6 ml (6 cc)). Other
configurations were the same as those in the example system of
Experimental Example 4.
[0103] Then, a small single cell battery similar to that in
Experimental Example 4 (an electrode area: 9 cm.sup.2) was
manufactured and the electrolyte in each of the prepared positive
and negative electrodes was used, to perform charge until the state
of charge of the vanadium ion reached the level equivalent to 110%
at a constant current of 630 mA (current density: 70 mA/cm.sup.2)
in the similar manner to Experimental Example 4. Then, the behavior
of the voltage characteristics of the system in Experimental
Example 5 showed almost the same behavior as that of the example
system in Experimental Example 4. Furthermore, the discharge time
of the system in Experimental Example 5 was 25 minutes, which was
98% with respect to the theoretical capacity (26 minutes). Thus, it
was confirmed that the battery capacity of nearly 100% was achieved
and the utilization rate exceeding 90% could also be achieved.
Furthermore, also in the system of Experimental Example 5,
repetition of charge and discharge still allowed a stable operation
and did not cause generation of hydrogen gas.
[0104] It can be said from Experimental Example 5 that the
utilization rate of the vanadium ion can be effectively increased
to improve the energy density also when the electrolyte in each of
the positive and negative electrodes contains, in addition to a
vanadium ion, a metal ion lower in redox potential than the
vanadium ion on the negative electrode side.
[0105] The present invention is not limited to the above-described
embodiments but can be modified as appropriate without deviation
from the contents of the present invention. For example, the type
and the concentration of the metal ion, the concentration of the
solvent of the electrolyte, and the like can be changed as
appropriate.
INDUSTRIAL APPLICABILITY
[0106] The redox flow battery according to the present invention
can be suitably used as a large-capacity storage battery for
stabilizing variations in power generation output, storing surplus
generated power, and load leveling for power generation of new
energy such as solar photovoltaic power generation and wind power
generation. The redox flow battery according to the present
invention can also be suitably used as a large-capacity storage
battery attached to a common power plant for voltage sag and power
failure prevention and for load leveling.
REFERENCE SIGNS LIST
[0107] 100 redox flow battery, 101 membrane, 102 positive electrode
cell, 103 negative electrode cell, 104 positive electrode, 105
negative electrode, 106 tank for positive electrode electrolyte,
107 tank for negative electrode electrolyte, 108, 109, 110, 111
pipe, 112, 113 pump.
* * * * *