U.S. patent application number 14/122780 was filed with the patent office on 2014-05-15 for carbon dioxide enrichment device.
The applicant listed for this patent is Kazuhito Hashimoto, Adam Heller, Ryo Kamai, Shuji Nakanishi, Michio Suzuka, Takeyuki Yamaki, Yong Zhao. Invention is credited to Kazuhito Hashimoto, Adam Heller, Ryo Kamai, Shuji Nakanishi, Michio Suzuka, Takeyuki Yamaki, Yong Zhao.
Application Number | 20140131197 14/122780 |
Document ID | / |
Family ID | 47258788 |
Filed Date | 2014-05-15 |
United States Patent
Application |
20140131197 |
Kind Code |
A1 |
Suzuka; Michio ; et
al. |
May 15, 2014 |
CARBON DIOXIDE ENRICHMENT DEVICE
Abstract
A carbon dioxide enrichment device includes first and second gas
diffusion electrodes; an anion exchange membrane; and an
electrolytic solution partitioned by the anion exchange membrane.
The electrolytic solution contains solvent and solute, and the
solute is dissolved to form a dissolved inorganic carbon containing
carbonic acid, hydrogen carbonate ions, or carbonic acid ions. The
oxygen is consumed by an oxygen reduction reaction on the first gas
diffusion electrode, whereby, a dissolved inorganic carbon is
formed by a dissolution and ionization reaction of carbon dioxide
in the solvent. The dissolved inorganic carbon from the solute or
the dissolved inorganic carbon is transported to the second gas
diffusion electrode through the anion exchange membrane, and oxygen
is formed from the solvent near the second gas diffusion electrode
by an oxidation reaction of the solvent on the second gas diffusion
electrode, and carbon dioxide is formed from the dissolved
inorganic carbon.
Inventors: |
Suzuka; Michio; (Osaka,
JP) ; Kamai; Ryo; (Osaka, JP) ; Nakanishi;
Shuji; (Osaka, JP) ; Yamaki; Takeyuki; (Nara,
JP) ; Hashimoto; Kazuhito; (Tokyo, JP) ;
Heller; Adam; (Austin, TX) ; Zhao; Yong;
(Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Suzuka; Michio
Kamai; Ryo
Nakanishi; Shuji
Yamaki; Takeyuki
Hashimoto; Kazuhito
Heller; Adam
Zhao; Yong |
Osaka
Osaka
Osaka
Nara
Tokyo
Austin
Tokyo |
TX |
JP
JP
JP
JP
JP
US
JP |
|
|
Family ID: |
47258788 |
Appl. No.: |
14/122780 |
Filed: |
May 29, 2012 |
PCT Filed: |
May 29, 2012 |
PCT NO: |
PCT/JP2012/003506 |
371 Date: |
November 27, 2013 |
Current U.S.
Class: |
204/283 |
Current CPC
Class: |
B01D 53/32 20130101;
Y02E 60/368 20130101; Y02P 20/10 20151101; Y02E 60/36 20130101;
Y02P 20/126 20151101; B01D 2257/504 20130101; C01B 32/50 20170801;
C25B 11/03 20130101; Y02P 20/152 20151101; Y02P 20/151
20151101 |
Class at
Publication: |
204/283 |
International
Class: |
C25B 11/03 20060101
C25B011/03 |
Foreign Application Data
Date |
Code |
Application Number |
May 31, 2011 |
JP |
2011-122069 |
Oct 3, 2011 |
JP |
2011-219485 |
Claims
1. A carbon dioxide enrichment device, comprising: a first gas
diffusion electrode; a second gas diffusion electrode separated
from the first gas diffusion electrode; an anion exchange membrane
located between the first gas diffusion electrode and the second
gas diffusion electrode; and an electrolytic solution existing
between the first gas diffusion electrode and the second gas
diffusion electrode to be in contact with the first gas diffusion
electrode and the second gas diffusion electrode and to be
partitioned by the anion exchange membrane, wherein the
electrolytic solution contains a solvent and a solute dissolved in
the solvent, and the solute is dissolved in the solvent to form a
dissolved inorganic carbon containing at least one of carbonic
acid, hydrogen carbonate ions, and carbonic acid ions; oxygen is
consumed by an oxygen reduction reaction on the first gas diffusion
electrode, whereby a dissolved inorganic carbon is formed by a
dissolution and ionization reaction of carbon dioxide in the
solvent; the dissolved inorganic carbon derived from the solute or
the dissolved inorganic carbon formed on the first gas diffusion
electrode is transported to the second gas diffusion electrode
through the anion exchange membrane; and oxygen is formed from the
solvent in the vicinity of the second gas diffusion electrode by an
oxidation reaction of the solvent on the second gas diffusion
electrode, and carbon dioxide is formed from the dissolved
inorganic carbon.
2. The carbon dioxide enrichment device according to claim 1,
wherein a molar ratio of carbon dioxide and oxygen to be emitted
from the second gas diffusion electrode is in the range of from
1:0.1 to 1:10.
3. The carbon dioxide enrichment device according to claim 1,
wherein the anion exchange membrane is a permselective membrane of
monovalent ions.
4. The carbon dioxide enrichment device according to claim 1,
wherein an electrolytic solution on a first diffusion electrode
side partitioned by the anion exchange membrane has a pH of 7 to
12, wherein an electrolytic solution on a second diffusion
electrode side partitioned by the anion exchange membrane has a pH
of 6 to 12, and wherein a difference between the pH of the
electrolytic solution on the first diffusion electrode side and
that of the electrolytic solution on the second diffusion electrode
side is in the range of from -4 to -0.01.
5. The carbon dioxide enrichment device according to claim 1,
wherein an electrolyte of the electrolytic solution contains any
one of Li.sup.+, Na.sup.+, and K.sup.+ as cations, and contains
HCO.sub.3.sup.- or CO.sub.3.sup.2- as anions.
6. The carbon dioxide enrichment device according to claim 1,
wherein the first gas diffusion electrode and the second gas
diffusion electrode comprise a polytetrafluoroethylene (PTFE)
layer, a porous conductor, and an electrode catalyst.
7. The carbon dioxide enrichment device according to claim 6,
wherein the electrode catalyst contains a metal complex or a
catalytic component of the metal complex, the metal complex
containing any one of a polymer of one or more monomers selected
from the group consisting of diaminopyridine, triaminopyridine,
tetraminopyridine, a diaminopyridine derivative, a triaminopyridine
derivative, and a tetraminopyridine derivative; or a modified
product of the polymer; or a catalytic metal; and the electrode
catalyst satisfying at least one of the following (i) and (ii): (i)
the content of metal coordinated to a nitrogen atom, analyzed by
X-ray photoelectron spectroscopy, is 0.4 mol % or more, and (ii)
the existence of metal coordinated to a nitrogen atom is recognized
by X-ray photoelectron spectroscopy, and the content of the
nitrogen atom is 6.0 mol % or more.
8. The carbon dioxide enrichment device according to claim 6,
wherein the electrode catalyst contains a polymer of one or more
monomers selected from the group consisting of diaminopyridine,
triaminopyridine, and tetraminopyridine; or a fired metal complex
obtained by firing a polymer metal complex composed of a catalytic
metal; or a catalyst component of the fired metal complex.
9. The carbon dioxide enrichment device according to claim 1,
wherein the electrolytic solution on at least one of the first
diffusion electrode side and the second diffusion electrode side is
a polymer gel electrolyte.
10. The carbon dioxide enrichment device according to claim 1,
wherein the electrolytic solution on the first diffusion electrode
side contains a carbonic anhydrase catalyst facilitating a reaction
of the below-mentioned [Chemical Formula 1]:
CO.sub.2+H.sub.2O.fwdarw.HCO.sub.3.sup.-+H.sup.+. [Chemical Formula
1]
Description
TECHNICAL FIELD
[0001] The present invention relates to a device capable of
enriching carbon dioxide by causing dissolution and release of
carbon dioxide in an electrolytic solution utilizing an
oxygen-generating/oxygen-reducing electrochemical reaction.
BACKGROUND ART
[0002] Carbon dioxide is a substance widely distributed on earth,
accounting for 0.04% of the atmosphere, which is a compound widely
used for industrial applications. Specific examples of industrial
use of carbon dioxide include foaming gas for carbonated drinks,
bath salts, and fire extinguishing agents; dry ice for cooling; and
air for emergency replenishment of automobiles. Carbon dioxide in a
supercritical state is also used as an extracting solvent for
caffeine, and is further used in a laser that is used in the
industrial field, and a carbonic acid gas laser that is used as a
medical laser knife. It is also used as a substitute for a
chlorofluorocarbon refrigerant in a CO.sub.2 refrigerant
compressor.
[0003] In the agricultural field, carbon dioxide is used as a
carbon dioxide fertilizer for facilitating the growth of plants
such as strawberry in forcing culture, and water plant in a water
tank for admiration, and is also used in controlled atmosphere (CA)
storage for fresh agricultural products.
[0004] As mentioned above, carbon dioxide has been popularly used,
and there has hitherto been a technique of a carbon dioxide
facilitated transport membrane utilizing a difference in a
permeability rate of a porous polymer membrane as mentioned in
Non-Patent Document 1, or a technique using a solid molten salt as
mentioned in Patent Document 1, as a technique of enriching carbon
dioxide. In the carbon dioxide facilitated transport membrane,
there exists a need to pressurize a gas to high pressure of about
200 kPa or higher against the carbon dioxide facilitated transport
membrane so as to enrich carbon dioxide, and to reduce the pressure
of the side where the enriched gas permeates. Even in the case of
the technique using a solid molten salt, there existed a need to
maintain the device at high temperature of about 600.degree. C. so
as to drive the device since the molten salt is used. As mentioned
above, there has never existed a device capable of enriching carbon
dioxide with low energy consumption without requiring a large-scale
apparatus.
PRIOR ART DOCUMENTS
Patent Documents
[0005] Patent Document 1: JP 11-28331 A
Non-Patent Documents
[0005] [0006] Non-Patent Documents: R. Yegani et. al., J. Membr.
Sci., 291, 157 (2007).
DISCLOSURE OF THE INVENTION
Problems to be Solved by the invention
[0007] The solutions reported in the above prior art documents
require a great deal of energy, that is, there exists a need to
apply heat during desorption (release) of carbon dioxide or to
maintain high temperature during driving, and had a problem that it
is impossible to achieve both enrichment performance of carbon
dioxide and low energy consumption.
[0008] In light of the above circumstance, the present invention
has been made and an object thereof is to provide a carbon dioxide
enrichment device that has high enrichment performance, and also
enables a significant reduction in energy required during
driving.
Means for Solving the Problems
[0009] The carbon dioxide enrichment device according to the
present invention is characterized by comprising: a first gas
diffusion electrode functioning as a cathode; a second gas
diffusion electrode separated from the first gas diffusion
electrode functioning as an anode; an anion exchange membrane
located between the first gas diffusion electrode and the second
gas diffusion electrode; and an electrolytic solution existing
between the first gas diffusion electrode and the second gas
diffusion electrode to be in contact with the first gas diffusion
electrode and the second gas diffusion electrode and to be
partitioned by the anion exchange membrane, being characterized in
that the electrolytic solution contains a solvent and a solute
dissolved in the solvent, and the solute is dissolved in the
solvent to form a dissolved inorganic carbon containing at least
one of carbonic acid, hydrogen carbonate ions, and carbonic acid
ions; oxygen is consumed by an oxygen reduction reaction on the
first gas diffusion electrode, whereby, a dissolved inorganic
carbon is formed by a dissolution and ionization reaction of carbon
dioxide in the solvent; the dissolved inorganic carbon derived from
the solute or the dissolved inorganic carbon formed on the first
gas diffusion electrode is transported to the second gas diffusion
electrode through the anion exchange membrane; and oxygen is formed
from the solvent in the vicinity of the second gas diffusion
electrode by an oxidation reaction of the solvent on the second gas
diffusion electrode, and also carbon dioxide is formed from the
dissolved inorganic carbon. In other words, when a voltage is
applied between the first gas diffusion electrode and the second
gas diffusion electrode, and carbon dioxide and oxygen are
introduced into the first gas diffusion electrode, a reaction
occurs as shown in the below-mentioned [Chemical Formula 2] on this
first gas diffusion electrode. HCO.sub.3.sup.- formed by the scheme
as shown in the [Chemical Formula 2] permeates through the anion
exchange membrane and the electrolytic solution, whereby, a
reaction occurs as shown in the below-mentioned [Chemical Formula
3] on the second gas diffusion electrode, and then carbon dioxide
and oxygen are emitted from the second gas diffusion electrode.
CO.sub.2+H.sub.2O.fwdarw.H.sup.++HCO.sub.3.sup.-
O.sub.2+4H.sup.++4e.sup.-.fwdarw.2H.sub.2O [Chemical Formula 2]
2H.sub.2O.fwdarw.O.sub.2+4H.sup.++4e.sup.-
HCO.sub.3.sup.-+H.sup.+.fwdarw.H.sub.2O+CO.sub.2 [Chemical Formula
3]
[0010] As used herein, "dissolved inorganic carbon" means at least
one selected from the group consisting of carbonic acid, hydrogen
carbonate ions, and carbonic acid ions, formed by dissolving carbon
dioxide in a solvent.
[0011] As used herein, "enrichment" means that the concentration of
a specific gas is made higher than that in an initial state, and
"carbon dioxide enrichment device" means a device capable of making
the concentration of carbon dioxide higher than that in an initial
state with high selectivity.
[0012] In the carbon dioxide enrichment device according to the
present invention, a molar ratio of carbon dioxide and oxygen to be
emitted from the second gas diffusion electrode is in the range of
from 1:0.1 to 1:10. Whereby, it enables penetration of carbon
dioxide while maintaining an influence of driving of the device on
the oxygen concentration at low level.
[0013] In the carbon dioxide enrichment device according to the
present invention, the anion exchange membrane is preferably a
permselective membrane of monovalent ions. Whereby, it enables
selective permeation of HCO.sub.3.sup.-, leading to achievement of
high selective permeation of CO.sub.2. Suppression of permeation of
divalent anions facilitates the movement of monovalent anions
HCO.sub.3.sup.-.
[0014] In the carbon dioxide enrichment device according to the
present invention, an electrolytic solution on a first diffusion
electrode (cathode) side partitioned by the anion exchange membrane
preferably has a pH of 7 to 12, and also an electrolytic solution
on a second diffusion electrode (anode) side partitioned by the
anion exchange membrane preferably has a pH of 6 to 12, and a
difference between the pH of the electrolytic solution on the first
diffusion electrode side and that of the electrolytic solution on
the second diffusion electrode side is preferably in the range of
from -4 to -0.01. Whereby, a difference in concentration of
OH.sup.- or H.sup.+ arises between the electrolytic solution on the
first diffusion electrode (cathode) side and the electrolytic
solution on the second diffusion electrode (anode) side, and thus
facilitating diffusion of anions HCO.sub.3.sup.-. As a result, it
becomes possible to drive the device at lower applied voltage.
[0015] In the carbon dioxide enrichment device according to the
present invention, an electrolyte of the electrolytic solution
preferably contains any one of Li.sup.+, Na.sup.+, and K.sup.+ as
cations, and contains HCO.sub.3.sup.- or CO.sub.3.sup.2- as anions.
Whereby, an electrode reaction corresponding to bias is likely to
occur, and thus electrode overvoltage decreases and also selective
CO.sub.2 permeation occurs.
[0016] In the carbon dioxide enrichment device according to the
present invention, the first gas diffusion electrode and the second
gas diffusion electrode preferably comprise a
polytetrafluoroethylene (PTFE) layer, a porous conductor, and an
electrode catalyst, thus enabling to impart water repellency to an
electrode surface, and to prevent moisture from flowing out of the
device. Inclusion of an electrode catalyst enables suppression of
overvoltage of a reduction reaction of oxygen, leading to a
decrease in device driving voltage.
[0017] In the carbon dioxide enrichment device according to the
present invention, the electrode catalyst contains a metal complex
or a catalytic component of the metal complex, the metal complex
containing any one of a polymer of one or more monomers selected
from the group consisting of diaminopyridine, triaminopyridine,
tetraminopyridine, a diaminopyridine derivative, a triaminopyridine
derivative, and a tetraminopyridine derivative; or a modified
product of the polymer; or a catalytic metal; and the electrode
catalyst also satisfying at least one of the following (i) and
(ii):
[0018] (i) the content of metal coordinated to a nitrogen atom,
analyzed by X-ray photoelectron spectroscopy, is 0.4 mol % or more,
and
[0019] (ii) the existence of metal coordinated to a nitrogen atom
is recognized by X-ray photoelectron spectroscopy, and also the
content of the nitrogen atom is 6.0 mol % or more.
[0020] In the carbon dioxide enrichment device according to the
present invention, the electrode catalyst contains a polymer of one
or more monomers selected from the group consisting of
diaminopyridine, triaminopyridine, and tetraminopyridine; or a
fired metal complex obtained by firing a polymer metal complex
composed of a catalytic metal; or a catalyst component of the fired
metal complex.
[0021] In the carbon dioxide enrichment device according to the
present invention, a specific surface area of the porous conductor
is preferably 1 m.sup.2/g or more, in the BET adsorption
measurement. The specific surface area is more preferably 30
m.sup.2/g, still more preferably 100 m.sup.2/g or more, and yet
more preferably 500 m.sup.2/g or more. Whereby, a reaction area in
the electrode can be increased, thus enabling an increase in
current density of an oxidation-reduction reaction of oxygen
required to drive the device.
[0022] In the carbon dioxide enrichment device according to the
present invention, the electrode catalyst may be platinum,
nickel-doped carbon nanotube, tungsten oxide doped with copper or
nickel, or titanium oxide.
[0023] In the carbon dioxide enrichment device according to the
present invention, the electrolytic solution on at least one of the
first diffusion electrode side and the second diffusion electrode
side is a polymer gel electrolyte. Whereby, leakage of the
electrolytic solution from the device can be suppressed, thus
enabling a device having high durability.
[0024] In the carbon dioxide enrichment device according to the
present invention, the electrolytic solution on the first diffusion
electrode side contains a carbonic anhydrase catalyst facilitating
a reaction of the below-mentioned [Chemical Formula 1]. Whereby, an
ionization rate of Chemical Formula 1, thus enabling formation of
HCO.sub.3.sup.- at a higher rate.
CO.sub.2+H.sub.2O.fwdarw.HCO.sub.3.sup.-+H.sup.+ [Chemical Formula
1]
Effects of the Invention
[0025] According to the carbon dioxide enrichment device of the
present invention, in the constitution including a first gas
diffusion electrode; a second gas diffusion electrode; an anion
exchange membrane located between the first gas diffusion electrode
and the second gas diffusion electrode; and an electrolytic
solution existing between the first gas diffusion electrode and the
second gas diffusion electrode, that is partitioned by the anion
exchange membrane; when a voltage is applied between the first gas
diffusion electrode and the second gas diffusion electrode, and
carbon dioxide and oxygen are introduced into the first gas
diffusion electrode, a reaction occurs in this first gas diffusion
electrode, as shown in the below-mentioned [Chemical Formula 2],
and HCO.sub.3.sup.- formed by the [Chemical Formula 2] or
CO.sub.3.sup.2- formed by ionization, or H.sub.2CO.sub.3 formed by
equilibrium permeates through the electrolytic solution. Whereby, a
reaction occurs in the second gas diffusion electrode, as shown in
the below-mentioned [Chemical Formula 3], and thus discharging
carbon dioxide and oxygen from the second gas diffusion electrode,
leading to the achievement of carbon dioxide enrichment. Therefore,
the carbon dioxide enrichment device exert an excellent effect
capable of significantly reducing energy required during driving
since it has high carbon dioxide enrichment performance and there
is no need to heat during releasing carbon dioxide.
[0026] Accordingly, according to the present invention, it is
possible to provide a carbon dioxide enrichment device that has
high enrichment performance, and also enables a significant
reduction in energy required during driving.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 is a schematic sectional view illustrating an
embodiment of a carbon dioxide enrichment device according to the
present invention.
MODE FOR CARRYING OUT THE INVENTION
[0028] An embodiment of a carbon dioxide enrichment device is
illustrated in FIG. 1. The carbon dioxide enrichment device
includes a first gas diffusion electrode that functions as a
cathode (gas diffusion electrode 1); a second gas diffusion
electrode (gas diffusion electrode 2) disposed so as to be
separated from the first gas diffusion electrode (gas diffusion
electrode 1), that functions as an anode; an anion exchange
membrane 5; and an electrolytic solution 3.
[0029] The anion exchange membrane 5 exists between the gas
diffusion electrode 1 and the gas diffusion electrode 2 so as to be
gas separated from the diffusion electrode 1 and the gas diffusion
electrode 2. The electrolytic solution 3 swells the anion exchange
membrane 5, and exists between the gas diffusion electrode 1 and
the gas diffusion electrode 2. Namely, the gas diffusion electrode
1 and the gas diffusion electrode 2 are in contact with the
electrolytic solution 3, and a gas and an electrolytic solution
exist so that a three-phase interface of an electrode (solid
phase), an electrolytic solution (liquid phase or solid phase), and
a gas (vapor phase) containing carbon dioxide and oxygen is formed
on a surface of the gas diffusion electrode 1 and the gas diffusion
electrode 2, and thus enabling an electrode reaction due to the gas
and the electrolytic solution.
[0030] The anion exchange membrane 5 is a membrane that enables
selectively permeation anions, and particularly enables prevention
of the movement of cations contained in a supporting electrolyte,
or H.sup.+ contained in the solvent. Whereby, unbalanced charge
between an electrode 1 and an electrode 2 caused by an electrode
reaction is compensated only by the movement of anions, thus
facilitating the movement of HCO.sub.3.sup.-. Therefore, the
movement of cations is suppressed by portioning the electrolytic
solution 3 by the anion exchange membrane 5, thus achieving
permeation of HCO.sub.3.sup.- corresponding to an electrode
reaction.
[0031] Any anion exchange membrane may be used as the anion
exchange membrane 5 as long as it can exert a function that enables
permeation of only anions, but does not enable permeation of
cations. Examples thereof include NEOSEPTA AMX, AHA, and ACM
manufactured by Tokuyama Corporation. Preferably, the anion
exchange membrane is NEOSEPTA AMX manufactured by Tokuyama
Corporation.
[0032] The gas diffusion electrode 1 and the gas diffusion
electrode 2 have a structure including a catalyst layer made of a
porous conductor, one surface of which is subjected to water
repellent finishing, and the other surface of which includes an
oxygen reduction catalyst support thereon.
[0033] The specific surface area of the porous conductor is
preferably 1 m.sup.2/g or more, more preferably 30 m.sup.2/g or
more, still more preferably 100 m.sup.2/g or more, and yet more
preferably 500 m.sup.2/g or more, in the BET adsorption
measurement. In case the specific surface area satisfies the above
condition, a reaction area increases, thus enabling achievement of
more CO.sub.2 permeation amount. In case the specific surface area
is less than 1 m.sup.2/g, sufficient carbon dioxide enrichment
performance is not attained because of a small area of the
three-phase interface. In order to reduce voltage loss due to
surface resistance of the porous conductor, the lower the surface
resistance of the porous conductor, the better it is. The surface
resistance is preferably 1 k.OMEGA./.quadrature. or less, and more
preferably 200.OMEGA./.quadrature. or less. Preferred examples of
the porous conductor include a carbon sheet, a carbon cloth, and
the like.
[0034] Water repellent finishing can be performed by coating a
surface of a porous conductor with polytetrafluoroethylene (PTFE).
This water repellent finishing enables the gas diffusion electrode
1 and gas diffusion electrode 2 to have a property capable of
permeating a gas, but incapable of permeating water, and also the
gas diffusion electrodes have a feature that the gas can diffuse to
the catalyst layer.
[0035] The catalyst to be supported on the gas diffusion electrode
1 and the gas diffusion electrode 2 is particularly preferably a
material that catalyzes an oxidation-reduction reaction of oxygen.
Examples thereof include alloys or complexes containing at least
one metal selected from transition metals capable of acting as an
adsorption site of oxygen, such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni,
Cu, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Ir, Pt, and Au; or compounds
containing these metals as a dopant; carbon nanotube; and graphite.
Of these catalyst carriers, Pt, Pt/Ru, and carbon nanotube are
preferable because of having comparatively high performances in
light of catalyst performance. Pt/Ru is more preferable. In this
case, it is expected that resistance to CO (carbon monoxide)
poisoning is improved to obtain a device having long-term
stability.
[0036] Both the gas diffusion electrode 1 and the gas diffusion
electrode 2 are disposed so that the side, on which a catalyst is
supported, is in contact with the electrolytic solution 3, and that
the side subjected to water repellent finishing is in contact with
an external gas.
[0037] The gas diffusion electrode 1 and the gas diffusion
electrode 2 are connected to a DC power 4 through an external
circuit. It is necessary that DC voltage to be applied between the
gas diffusion electrode 1 and the gas diffusion electrode 2 is a
voltage that causes a reduction reaction of oxygen in the gas
diffusion electrode 1 (cathode), and also causes an oxidation
reaction of water in the gas diffusion electrode 2 (anode). When
using water as the solvent of the electrolytic solution 3, the
voltage is preferably a voltage that does not cause electrolysis of
water so as to permanently operate the device, and is preferably a
voltage not exceeding 1.2 V that is a voltage to be determined from
free energy of a decomposition reaction of water, where no
electrolysis does not theoretically occur. In case there is a loss
such as IR drop of an electrode or an electrolyte, 1.2 V or higher
voltage may be applied. In this case, the voltage is preferably 10
V or lower. The voltage is more preferably 5 V or lower, and still
more preferably 2 V or lower.
[0038] The device is driven by supplying a gas containing carbon
dioxide and oxygen to the gas diffusion electrode 1 from the
external atmosphere such as atmosphere. Therefore, the gas
diffusion electrode 1 may be provided so as to increase a contact
area with the external atmosphere.
[0039] It is preferred that the gas diffusion electrode 1 and the
gas diffusion electrode 2 are disposed oppositely each other. A
distance between the gas diffusion electrode 1 and gas diffusion
electrode 2, that face each other, is a distance wide enough to
prevent mutual contact between electrodes so as to reduce voltage
drop (IR drop) due to solution resistance as small as possible, and
both electrodes are preferably disposed in proximity as close as
possible. In case the electrolytic solution 3 has sufficiently high
ion concentration and also has small solution resistance, it is
possible to reduce voltage loss due to IR drop. If there is a fear
that both electrodes may be in contact with each other by close
proximity of both electrodes in terms of a structure of the device,
a separator may be inserted between the gas diffusion electrode 1
and the gas diffusion electrode 2. This separator preferably has
properties that make it to possible to contain an electrolytic
solution 3, and also has insulating properties. In order to prevent
diffusion properties of ions existing in the electrolytic solution
3 from causing deterioration, the higher the void ratio of the
separator, the better it is.
[0040] The solvent is preferably a solvent that absorbs carbon
dioxide to thereby cause ionization of carbon dioxide. Examples of
such solvent are alcohols, an organic solvent, an ionic liquid, and
the like, including water. Particularly preferred solvent is water,
or a mixed solvent containing water.
[0041] The solute to be used in the electrolytic solution 3 is
preferably a hydrogen carbonate or a carbonate of an alkali metal,
or a hydrogen carbonate or a carbonate of an alkali earth metal.
More specifically, the solute is NaHCO.sub.3, KHCO.sub.3,
LiHCO.sub.3, Na.sub.2CO.sub.3, K.sub.2CO.sub.3, or
Li.sub.2CO.sub.3.
[0042] The pH of the electrolytic solution 3 is preferably from 5
to 14. In order to adjust the pH of the electrolytic solution 3, an
alkali electrolyte is added to the electrolytic solution 3.
Preferred examples of the electrolyte used to adjust the pH include
NaOH, KOH, LiOH, and the like. In case the pH of the electrolytic
solution 3 is lower than 5, the absorption rate of carbon dioxide
drastically decreases and thus the absorption of carbon dioxide is
a rate-limiting factor. As a carbon dioxide enrichment device is
driven, the total inorganic carbon concentration in the
electrolytic solution 3 decreases, resulting in deterioration of
performance of the carbon dioxide enrichment device.
[0043] Regarding the pH of the electrolytic solution 3, an
electrolytic solution on a first diffusion electrode 1 (cathode)
side partitioned by the anion exchange membrane 5 preferably has a
pH of 7 to 12, and also an electrolytic solution 3 on a second
diffusion electrode 2 (anode) side partitioned by the anion
exchange membrane 5 preferably has a pH of 6 to 12, and also a
difference between the pH of the electrolytic solution 3 on the
first diffusion electrode 1 side and that of the electrolytic
solution 3 on the second diffusion electrode 2 side is preferably
from -4 to -0.01.
[0044] In case the pH of the cathode side is lower than 7, the
CO.sub.2 absorption rate drastically decreases and CO.sub.2 is less
likely to be absorbed, leading to a decrease in the permeation
amount.
[0045] In case the pH of the anode side is 6 or lower, the amount
of CO.sub.2 generated from the electrolytic solution 3
significantly increases and exceeds the adsorption amount on the
anode side to thereby release CO.sub.2, resulting in degradation of
the electrolytic solution 3. Therefore, in order to stably drive,
it is necessary that the pH of the anode side is 6 or higher.
[0046] The pH of both anode and cathode sides of the electrolytic
solution 3 is preferably 12 or lower. In case the pH is 12 or
higher, a current per unit area decreases due to an increase in
viscosity of the solution, thus causing a phenomenon in which the
permeation amount decreases.
[0047] A difference in the pH of the electrolytic solution 3
separated by an ion exchange membrane 5 is preferably from 0.01 to
4. The existence of the pH difference facilitates the movement of
hydrogen carbonate anions existing in the electrolytic solution 3
of the cathode to the anode side.
[0048] It is considered that the principle of the movement of
anions due to the pH difference is the same as that of a
concentration cell, and that inside the device goes into a state
where about 60 mV is applied per pH difference of 1.
[0049] In case the solute concentration is low, a supporting
electrolyte may be dissolved in the solvent so as to improve ionic
conductivity of the electrolytic solution 3. Preferred examples of
the electrolyte include ammonium salts such as tetrabutylammonium
perchlorate, tetraethylazanium hexafluorophosphate, imidazolium
salt, and pyridinium salt; and alkali metal salts such as lithium
perchlorate and potassium borofluoride. The electrolyte also
includes salts containing an alkali metal or an alkali earth metal
such as lithium, sodium, potassium or calcium, or an organic
compound having an amino group as a cation, and a halogen ion such
as chlorine or bromine, or sulfonium as an anion. In case the
electrolytic solution 3 has sufficient ionic conductivity, there is
no need to add a supporting electrolyte.
[0050] The electrolytic solution 3 may be gelled and fixed to a
predetermined position, or may be formed of a gelled electrolytic
solution (gelled electrolytic solution), or a polyelectrolyte.
Examples of a gelling agent for gelling the electrolytic solution 3
include a gelling agent, a polymerizable polyfunctional monomer,
and an oil gelling agent that utilize a technique such as a polymer
or polymer crossliking reaction. Commonly used substances are
applied as a gelled electrolytic solution and a polyelectrolyte,
and preferred examples thereof include a vinylidene fluoride-based
polymer such as polyvinylidene fluoride, an acrylic acid-based
polymer such as polyacrylic acid, an acrylonitrile-based polymer
such as polyacrylonitrile, a polyether-based polymer such as
polyethylene oxide, a compound having an amide structure in the
structure, and the like. In case the electrolytic solution 3 is
gelled or fixed, the total inorganic carbon concentration of a
gelled or fixed electrolytic solution in contact with the gas
diffusion electrode 1 and a gelled or fixed electrolytic solution
in contact with the gas diffusion electrode 2, that is calculated
by [Equation 1], the pH of the electrolyte, presence or absence and
kinds of the supporting electrolyte and the concentration thereof
may be different. As the total inorganic carbon concentration
becomes lower, a reverse reaction rate against an ionization
reaction of an equilibrium reaction shown in [Chemical Formula 4]
decreases. As the pH of the electrolytic solution becomes higher,
an acid dissociation constant pKa value of an equilibrium reaction
shown in [Chemical Formula 4] increases. Therefore, in order to
facilitate absorption of CO.sub.2 on the first gas diffusion
electrode side and to facilitate generation of a CO.sub.2 gas on
the gas diffusion electrode 2 side, it is preferred to decrease the
total inorganic carbon concentration and to increase the pH value
in the gelled or fixed electrolyte in contact with the gas
diffusion electrode 1 as compared with gelled or fixed electrolyte
in contact with the gas diffusion electrode 2. In this case, a
supporting electrolyte is preferably added to the gelled or fixed
electrolyte having smaller ionic conductivity.
(Total inorganic carbon
concentration)=[H.sub.2CO.sub.3]+[HCO.sub.3.sup.-]+[CO.sub.3.sup.2-]
[Equation 1]
CO.sub.2+H.sub.2OH.sup.++HCO.sub.3.sup.- [Chemical Formula 4]
[0051] In a state where a voltage is applied between a gas
diffusion electrode 1 and a gas diffusion electrode 2 by a DC power
4, if a gas containing carbon dioxide and oxygen is supplied to the
gas diffusion electrode 1, first, a dissolution and ionization
reaction of carbon dioxide occurs on the gas diffusion electrode 1
side, as shown in the following scheme.
CO.sub.2+H.sub.2O.fwdarw.H.sup.++HCO.sub.3.sup.-
Using hydrogen ions H.sup.+ formed by this reaction, an
oxygen-reducing electrochemical reaction occurs, as shown in the
following scheme.
O.sub.2+4H.sup.++4e.sup.-+2H.sub.2O
The higher the concentration of carbon dioxide existing on the gas
diffusion electrode 1 side, the more the reaction amount increases,
and the current value of the carbon dioxide enrichment device
increases.
[0052] Subsequently, the hydrogen carbonate HCO.sub.3.sup.- thus
formed is partially ionized to form carbonic acid ions
CO.sub.3.sup.2-, and also hydrogen carbonate HCO.sub.3.sup.- is
partially converted into carbonic acid H.sub.2CO.sub.3 by an
equilibrium reaction. The thus formed hydrogen carbonate
HCO.sub.3.sup.-, carbonic acid ions CO.sub.3.sup.2-, and carbonic
acid H.sub.2CO.sub.3 diffuse to the gas diffusion electrode 2 side
in the electrolytic solution 3 by concentration diffusion. Since
hydrogen carbonate HCO.sub.3.sup.-, carbonic acid ions
CO.sub.3.sup.2-, and carbonic acid H.sub.2CO.sub.3 exist in the
electrolytic solution 3, they undergo concentration diffusion,
together with ions and carbonic acid in the electrolytic solution
3.
[0053] In the vicinity of the gas diffusion electrode 2, hydrogen
carbonate HCO.sub.3.sup.- reaches the gas diffusion electrode 2 by
phoresis due to concentration diffusion and an electrostatic force.
On the gas diffusion electrode 2 side, an oxidation reaction of
water as shown in the following scheme occurs to generate
oxygen.
2H.sub.2O.fwdarw.O.sub.2+4H.sup.++4e.sup.-
This reaction causes an increase in the concentration of hydrogen
ions H.sup.+ in the vicinity of the gas diffusion electrode 2,
leading to a decrease in pH. Since this pH change significantly
shifts equilibrium among hydrogen carbonate HCO.sub.3.sup.-,
carbonic acid ions CO.sub.3.sup.2-, and carbonic acid
H.sub.2CO.sub.3 to the carbonic acid side, hydrogen ions H.sup.+
react with hydrogen carbonate HCO.sub.3.sup.- in the electrolytic
solution 3 in such manner as shown in the following schemes to form
carbon dioxide.
H.sup.++HCO.sub.3.sup.-.fwdarw.H.sub.2CO.sub.3
H.sub.2CO.sub.3.fwdarw.H.sub.2O+CO.sub.2
As a result, a mixed gas of oxygen and carbon dioxide is discharged
from the gas diffusion electrode 2 side. In case the atmospheric
concentration of carbon dioxide (0.04%) is supplied to a gas
diffusion electrode 1, a ratio of oxygen:carbon dioxide is enriched
to about 1:1 to 2:1.
[0054] In case the concentration of carbon dioxide on the gas
diffusion electrode 2 side is high, a reverse reaction of
equilibrium as shown in the following scheme is likely to occur,
leading to an increase in overvoltage for causing a reaction on the
gas diffusion electrode 2 side as an anode.
H.sub.2O+CO.sub.2.fwdarw.H.sup.++HCO.sub.3.sup.-
Therefore, in order to increase the reaction amount at the same
voltage, the lower the concentration of carbon dioxide on the gas
diffusion electrode 2 side, the better it is. The concentration is
preferably 5% or less. More preferably, the device is provided with
equipment, that generates an atmospheric current to thereby always
lower the concentration of carbon dioxide, on the gas diffusion
electrode 2 side.
[0055] Describing in detail the above-mentioned dissolution of
carbon dioxide as a gas in the electrolytic solution 3, a reaction
occurs first by a reaction as shown in the following scheme in
which carbon dioxide molecules are surrounded by hydrated
water.
CO.sub.2 (g).fwdarw.CO.sub.2 (aq)
[0056] Carbon dioxide dissolved in the electrolytic solution 3 is
partially converted into carbonic acid by the addition of water
molecules, as shown in the following scheme.
CO.sub.2 (aq)+H.sub.2O (l).fwdarw.H.sub.2CO.sub.3 (aq)
[0057] A rate constant at 25.degree. C. of a forward reaction of
this equilibrium reaction is very low, for example, 0.039 s.sup.-1,
and a rate constant of a reverse reaction is 23 s.sup.-1. Carbonic
acid H.sub.2CO.sub.3 formed by the above reaction is ionized by an
acid dissociation reaction to form hydrogen carbonate
HCO.sub.3.sup.- and hydrogen ions H.sup.+, as shown in the
following scheme.
H.sub.2CO.sub.3 (aq).fwdarw.HCO.sub.3.sup.- (aq)+H.sup.+ (aq)
[0058] Hydrogen carbonate HCO.sub.3.sup.- is further ionized by an
acid dissociation reaction to form carbonic acid ions
CO.sub.3.sup.2-. Carbonic acid H.sub.2CO.sub.3, hydrogen carbonate
HCO.sub.3.sup.-, and carbonic acid ions CO.sub.3.sup.2- are in an
equilibrium state, and an existing ratio of the respective ions in
the electrolytic solution 3 is determined by the pH.
[0059] In order to enhance carbon dioxide absorption capacity of
the carbon dioxide enrichment device, the electrolytic solution 3
preferably contains a catalyst for a reaction capable of ionizing
carbon dioxide and water into hydrogen carbonate HCO.sub.3.sup.-
and hydrogen ions H.sup.+. Alternatively, it is preferred to
support a catalyst for a reaction capable of ionizing carbon
dioxide and water into hydrogen carbonate HCO.sub.3.sup.- and
hydrogen ions H.sup.+, on a surface on which an oxygen reduction
catalyst is supported, of the gas diffusion electrode 1. Preferred
examples of the catalyst for a reaction capable of ionizing carbon
dioxide and water into hydrogen carbonate HCO.sub.3.sup.- and
hydrogen ions H.sup.+ include a carbonic anhydrase, a
tetra-coordinated complex containing a zinc ion Zn.sup.2+ in the
center, and the like.
[0060] In the method of enriching carbon dioxide using the carbon
dioxide enrichment device, since a mixed gas at a normal
temperature supplied to a gas diffusion electrode 1 as a cathode is
discharged from a gas diffusion electrode 2 as an anode at a normal
temperature, and absorption of carbon dioxide into the device is
performed in a chemical manner, and also movement in the device
occurs by phoresis due to concentration diffusion and an
electrostatic force, there is no need to introduce a great deal of
energy. Therefore, it is possible to enrich carbon dioxide at low
cost while suppressing energy consumption.
[0061] It is preferred to use, as the electrode catalyst in the
present invention, a carbon-based catalyst using no platinum. Cost
reduction of the device can be expected by using no platinum.
Herein, the catalyst using no platinum will be described in
detail.
1. Electrode Catalyst
1-1. Summary
[0062] An electrode catalyst is characterized in that it contains a
specific metal complex as a catalyst component, and also has oxygen
reduction reaction (ORR) catalytic activity, durability, and
corrosion resistance that are equal to or higher than those of a
conventional electrode catalyst such as a Pt-based catalyst.
1-2. Constitution
[0063] An electrode catalyst contains, as a catalyst component, 1)
a metal complex having specific physical properties, or 2) a metal
complex obtained by subjecting a specific polymer metal complex to
a firing treatment. The constitution of the electrode catalyst of
the present invention will be specifically described below.
[0064] In the present invention, "metal complex" refers to a
compound composed of a polymer and/or a modified product thereof,
and a catalytic metal, ligands in the polymer or the modified
product thereof being coordinately bonded with the catalytic
metal.
[0065] As used herein, "fired metal complex" refers to a compound
obtained by subjecting a polymer metal complex to a firing
treatment. As used herein, "firing (treatment)" refers to a heat
treatment at high temperature.
[0066] As used herein, "polymer metal complex" refers to the metal
complex in a state of not being subjected to a firing
treatment.
[0067] "Metal complex" as simply designated herein refers to the
fired metal complex and the polymer metal complex regardless of
whether it has already subjected to a firing treatment.
[0068] In the electrode catalyst of the present invention, an
indispensable constituent functioning as a catalyst component is,
as mentioned below, a metal complex composed of a specific polymer
and/or a modified product thereof and a catalytic metal, or a fired
metal complex obtained by firing a metal complex composed of a
specific polymer and a catalytic metal. These metal complexes
serving as indispensable constituents in the electrode catalyst of
the present invention are comprehensively referred to as "2-4
aminopyridine polymer metal complex".
[0069] In the present invention, "2-4 aminopyridine polymer" is a
generic name of a compound obtained by polymerzing monomers such as
diaminopyridine (C.sub.5H.sub.7N.sub.3), triaminopyridine
(C.sub.5H.sub.8N.sub.4) and/or tetraminopyridine
(C.sub.5H.sub.9N.sub.5). "Polymer" as simply designated herein
refers to "2-4 aminopyridine polymer" unless otherwise specified.
"Modified product thereof" is a modified product of the polymer,
which refers to a compound and an oligomer that are obtained by
thermal decomposition of a polymer when a polymer metal complex is
fired.
[0070] The diaminopyridine, triaminopyridine, and tetraminopyridine
are compounds in which hydrogen atoms (H) of pyridine
(C.sub.5H.sub.5N) are respectively substituted with two, three or
fours amino groups (--NH.sub.2). The 2-4 aminopyridine polymer may
be composed of a monomer alone or a combination of two or more
monomers.
[0071] Examples of known position isomer of diaminopyridine include
2,3-diaminopyridine, 2,4-diaminopyridine, 2,5-diaminopyridine,
2,6-diaminopyridine, and 3,4-diaminopyridine; examples of known
position isomer of triaminopyridine include 2,3,4-triaminopyridine,
2,3,5-triaminopyridine, 2,3,6-triaminopyridine,
2,4,5-triaminopyridine, and 3,4,5-triaminopyridine; and examples of
known position isomer of tetraminopyridine include
2,3,4,5-tetraminopyridine, 2,4,5,6-tetraminopyridine, and
2,3,5,6-tetraminopyridine. Each monomer composing the 2-4
aminopyridine polymer may be any position isomer. The 2-4
aminopyridine polymer may be composed only of the same position
isomer, or different two or more position isomers.
[0072] In case the 2-4 aminopyridine polymer is composed of two or
more monomers and/or two or more position isomers, the position of
each monomer and/or position isomer in the 2-4 aminopyridine
polymer is not particularly limited as long as it is polymerizable.
For example, it may be polymerized so that a combination of
specific monomers is regularly repeated, or may be polymerized at
random.
[0073] Regarding the polymer metal complex, a ligand included in
the 2-4 aminopyridine polymer coordinates a catalytic metal.
Examples of the atom (ligating atom) that can serve as a ligand in
the polymer include a nitrogen atom of the pyridine ring and/or a
nitrogen atom of an amino group. Diaminopyridine, triaminopyridine
and tetraminopyridine include three, four and five nitrogen atoms
capable of serving as a ligand in a molecule. Therefore, the 2-4
aminopyridine polymer composed of these monomers contains a lot of
nitrogen atoms. Accordingly, it can coordinate a lot of catalytic
metals as compared with an electrode catalyst containing, as a
base, a metal complex composed of a polymer that coordinates a
conventional catalytic metal. According to this feature, the
electrode catalyst of the present invention can have high oxygen
reduction reaction (ORR) catalytic activity.
[0074] A preferred example of the 2-4 aminopyridine polymer
includes "diaminopyridine polymer" in which only diaminopyridine is
polymerized. The position isomer composing the diaminopyridine
polymer is not particularly limited and is preferably
2,6-diaminopyridine and/or 2,3-diaminopyridine. The reason is that
these position isomers can coordinate the catalytic metal in the
polymer in a more stable manner since nitrogen atoms (N) are most
proximally disposed each other. The diaminopyridine polymer is more
preferably a 2,6-diaminopyridine polymer in which only a
2,6-diaminopyridine monomer is polymerized.
[0075] The chemical polymerization reaction, that causes bonding of
the respective monomers composing the 2-4 aminopyridine polymer, is
not particularly limited, and is preferably anionic polymerization.
In case the polymer is a 2,6-diaminopyridine polymer, it is
presumed that the polymer includes, for example, chemical
structure(s) represented by the below-mentioned [Chemical Formula
5] and/or [Chemical Formula 6] through anionic polymerization of
2,6-diaminopyridine.
##STR00001##
[0076] As used herein, "catalytic metal" is a metal atom or metal
ion coordinated in a metal complex. In the 2-4 aminopyridine
polymer metal complex as a catalyst component of the present
invention, the catalytic metal is a substance that plays a role in
direct catalytic activity. The catalytic metal is not particularly
limited and is preferably a transition metal. Specific examples
thereof include atoms of titanium (Ti), vanadium (V), chromium
(Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper
(Cu), zirconium (Zr), niobium (Nb), molybdenum (Mo), ruthenium
(Ru), rhodium (Rh), palladium (Pd), silver (Ag), hafnium (Hf),
tantalum (Ta), tungsten (W), rhenium (Re), osnium (Os), iridium
(Ir), platinum (Pt), and gold (Au), or ions thereof. Persons
skilled in the art may appropriately select a proper catalytic
metal from these catalytic metals according to the intended
purposes, taking cost, supply amount, catalytic activity
efficiency, and the like into consideration. In the electrode
catalyst of the present invention, the catalytic metal is
preferably Cr, Mn, Fe, Co, Ni, and Cu. Of these electrode catalytic
metals, Fe and Co are preferred. The metal complex may be obtained
by coordinating a catalytic metal alone, or coordinating different
two or more catalytic metals. It is considered that Pt and Au are
rare and expensive and are therefore contrary to the object of the
present invention. However, since use of them in a state of being
coordinated in a metal complex enables relative reduction in use
amount of Pt as compared with a known Pt-based catalyst, it is
possible to achieve the object of the present invention. Therefore,
they can be included in the catalytic metal in the present
invention.
[0077] It is presumed to include, as the structure of the 2-4
aminopyridine polymer metal complex, a structure represented by the
below-mentioned [Chemical Formula 7] in the case of a metal complex
in which a 2,6-diaminopyridine polymer coordinates cobalt as a
catalytic metal (Co-2,6-diaminopyridine polymer, which is often
abbreviated herein to "CoDAPP").
##STR00002##
[0078] The 2-4 aminopyridine polymer metal complex of the present
invention is preferably obtained by a firing treatment of a polymer
metal complex. The reason is that a catalytic metal in the polymer
metal complex is stably coordinates to a nitrogen atom by the
firing treatment, and thus stable catalytic activity as well as
high durability and corrosion resistance can be obtained by
chemical hardening.
[0079] A mixing ratio of the 2-4 aminopyridine polymer to the
catalytic metal salt in the 2-4 aminopyridine polymer metal complex
may be selected so that a molar ratio of a raw monomer to a
catalytic metal atom becomes 3:1 to 5:1, and preferably 3.5:1 to
4.5:1.
[0080] "Firing temperature" for firing treatment is from 650 to
800.degree. C., preferably from 680 to 780.degree. C., more
preferably from 690 to 760.degree. C., and still more preferably
from 700 to 750.degree. C. The firing treatment can be performed by
a known method for a heat treatment of an electrode catalyst. For
example, a powder of a dry polymer metal complex may be fired under
a reducing gas atmosphere at the firing temperature for 30 minutes
to 5 hours, and preferably 1 to 2 hours. It is possible to use, as
a reducing gas, for example, ammonia.
[0081] As used herein, "specific physical properties" mean physical
properties exhibited by the 2-4 aminopyridine polymer metal complex
in the present invention, for example, at least one of the
below-mentioned properties (i) and (ii) obtained as a result in
which coordaination of a catalytic metal to a nitrogen atom becomes
stable in a polymer metal complex by firing the polymer metal
complex:
(i) the content of metal coordinated to a nitrogen atom analyzed by
X-ray photoelectron spectroscopy is 0.4 mol % or more, and (ii) the
existence of metal coordinated to a nitrogen atom is recognized by
X-ray photoelectron spectroscopy, and also the content of the
nitrogen atom is 6.0 mol % or more.
[0082] The content of metal coordinated to a nitrogen atom and the
content of the nitrogen atom are measured by X-ray photoelectron
spectroscopy. The content is the proportion thereof based on the
metal complex (based on 100 mol % of the metal complex).
[0083] The 2-4 aminopyridine polymer may be partially modified by
firing thereby to lose the form of a polymer. Such modification is
permitted as long as a fired metal complex can be used as an
electrode catalyst, and thus the 2-4 aminopyridine polymer metal
complex composing the electrode catalyst of the present invention
can contain a substance in which the 2-4 aminopyridine polymer was
modified by firing.
[0084] There is no particular limitation on the shape of the fired
metal complex. However, the larger the specific surface area per
unit area of the electrode catalyst to be supported on a surface of
an electrode, the better it is. The reason is that it is possible
to more enhance catalytic activity (mass activity) per unit area of
the electrode. Therefore, the shape is preferably a particle, and
particularly preferably a powder. The specific surface area of the
fired metal complex is preferably 100 m.sup.2/g or more, more
preferably 400 m.sup.2/g or more, and still more preferably 500
m.sup.2/g or more. Such specific surface area can be measured by a
nitrogen BET adsorption method.
[0085] The electrode catalyst containing a metal complex can
contain a catalyst component other than the above-mentioned fired
metal complexes. For example, known catalysts such as a CoTMPP
catalyst may be contained.
1-3. Effects
[0086] According to the electrode catalyst, it is possible to
coordinate a catalytic metal in a larger amount than that in the
case of an electrode catalyst derived from a metal complex, which
contains a conventional polymer and a catalytic metal. Whereby, it
is possible to have ORR catalytic activity and durability, that is
equal to or higher than, that of a known electrode catalyst such as
a Pt-based catalyst or a CoTMPP catalyst, and to reduce the use
amount thereof.
[0087] Since inexpensive metal such as Fe can be used as the
catalytic metal in place of Pt, it is possible to provide an
electrode catalyst at low production cost per unit mass, and also
to cope with an increase in supply amount of resources caused by
mass production of a fuel cell in future.
2. Method for Producing Electrode Catalyst
2-1. Summary
[0088] According to the present production method, an electrode
catalyst can be produced at low cost.
2-2. Method
[0089] The method for producing an electrode catalyst includes (a)
a polymerization step, (b) a polymer metal complex forming step,
and (c) a firing step. The respective steps will be specifically
described below.
(a) Polymerization Step
[0090] The "polymerization step" is the step of synthesizing a 2-4
aminopyridine polymer by anionic polymerization of diaminopyridine,
triaminopyridine and/or tetraminopyridine. Monomers to be
polymerized may be used alone, or two or three kinds of monomers
may be used in combination. There is no particular limitation on a
mixing molar ratio of the respective monomers when a combination of
two or more kinds of monomers is polymerized. Persons skilled in
the art may appropriately select while taking catalytic activity
into consideration. One example of preferred monomer use in the
polymerization step includes diaminopyridine alone.
[0091] A position isomer of the respective monomers used in the
polymerization is not particularly limited, and is preferably a
position isomer in which nitrogen atoms serving as a ligand in the
monomer molecule are proximally located. For example, when the
above-mentioned diaminopyridine is polymerized as the monomer,
2,3-diaminopyridine or 2,6-diaminopyridine in which nitrogen atoms
(N) are most proximally located are preferable among position
isomers of diaminopyridine.
[0092] In the present step, the monomer is polymerized by an
anionic polymerization reaction. The anionic polymerization
reaction may be performed using a known method that is
conventionally used in the relevant field. For example, the monomer
is deprotonated by reacting with a strong base solution, and then
polymerized by using the thus generated carbanions as a
nucleophilic agent. Examples of the base used in the strong base
solution include sodium hydroxide, potassium hydroxide, lithium
hydroxide, calcium hydroxide, and the like. The polymerization
temperature and polymerization time are not particularly limited as
long as the reaction proceeds. Usually, the present step can be
achieved by reacting at a temperature of 5 to 40.degree. C. for
about 5 to 48 hours.
[0093] After the polymerization reaction, a solvent is removed by
centrifugation or filtration to recover a polymer. The recovered
2-4 aminopyridine polymer is preferably washed with water
(including deionized water and distilled water), dried and then
used in the subsequent step.
[0094] In case the 2-4 aminopyridine polymer, that has already been
synthesized in advance, is used in the above method for producing
an electrode catalyst, production is started from the subsequent
polymer metal complex forming step without performing the present
step.
(b) Polymer Metal Complex Forming Step
[0095] The "polymer metal complex forming step" is the step of
mixing the 2-4 aminopyridine polymer with a catalytic metal salt to
thereby coordinate a catalytic metal to the polymer to form a
polymer metal complex. A nitrogen atom included in the 2-4
aminopyridine polymer serves as a ligand (ligating atom) and is
coordinately bonded with the catalytic metal to form a polymer
metal complex.
[0096] The "catalytic metal salt" is a salt of the catalytic metal
to be coordinated in the metal complex, and specific examples
thereof include a hydrochloride, a sulfate, a nitrate, a phosphate,
an acetate, and the like of a catalytic metal. The catalytic metal
in the present step may be metal having catalytic activity in the
electrode catalyst and is not particularly limited, and is
preferably a transition metal. Specific examples thereof include
transition metals mentioned in the first embodiment. Among these
salts, salts of Cr, Mn, Fe, Co, Ni, and Cu are suitable as the
catalytic metal salt of the present step, and a catalytic metal
salt of Fe or Co is particularly preferable. Specific examples
thereof include iron chloride, iron nitrate, iron sulfide, cobalt
nitrate, cobalt chloride, cobalt sulfate, and the like.
[0097] A mixing ratio of the 2-4 aminopyridine polymer to the
catalytic metal salt may be selected so that a molar ratio of a raw
monomer to a catalytic metal atom becomes 3:1 to 5:1, and
preferably 3.5:1 to 4.5:1. Namely, the polymer may be mixed with
the catalytic metal salt by selecting the mass of the polymer and
the catalytic metal salt so that a ratio of (number of moles of a
repeating unit composing the polymer):(number of moles of metal
contained in the catalytic metal salt) falls within the above
preferable range. These materials are mixed and dispersed in an
appropriate solvent, followed by well stirring, thus making it
possible to coordinate the catalytic metal or ions thereof in the
2-4 aminopyridine polymer. It is possible to use, as the medium,
water, ethanol, propanol, or a mixed solution obtained by using
them in combination, for example, a mixed solution of water and
ethanol, or water and (iso)propanol. The mixing temperature and the
mixing time are not particularly limited as long as the reaction
proceeds. Usually, the present step can be achieved by reacting at
a temperature of 50 to 70.degree. C. for about 30 minutes to 5
hours. In order to well mix the above two substances, ultrasonic
mixing may also be performed.
[0098] When the reaction proceeds, the formed polymer metal complex
is precipitated in the solvent in the form of a solid. After
formation of the polymer metal complex, the solvent is removed by
centrifugation, filtration, or vaporization to recover the polymer
metal complex. In order to remove the uncoordinated catalytic
metal, the polymer metal complex may be washed with water
(including deionized water, and distilled water). The polymer metal
complex thus recovered may be optionally powderized using, for
example, a quartz mortar.
(c) Firing Step
[0099] The "firing step" is the step of firing the polymer metal
complex obtained in the polymer metal complex forming step under a
reducing gas atmosphere at high temperature to obtain a fired metal
complex. The catalytic metal moves in the polymer metal complex
during the present step to thereby prepare a high-durability
electrode-active component in which a catalytic metal is
coordinated in a stable manner.
[0100] The firing temperature is from 650 to 800.degree. C.,
preferably from 680 to 780.degree. C., more preferably from 690 to
760.degree. C., and still more preferably from 700 to 750.degree.
C. Firing at this temperature enables preparation of a fired metal
complex as a catalyst component having high oxidation-reduction
reaction (ORR) catalytic activity and durability.
[0101] In the same manner as in the first embodiment, an ammonia
gas can be used as a reducing gas.
[0102] The firing treatment can be performed by a known method for
a heat treatment of an electrode catalyst. For example, a powder of
a polymer metal complex may be fired under a reducing gas
atmosphere at the firing temperature for 30 minutes to 3 hours, and
preferably 1 to 2 hours.
[0103] After the firing treatment, the fired metal complex is
preferably subjected to a pickling treatment (pre-leaching) using a
hydrochloric acid, nitric acid or sulfuric acid solution so as to
remove an insoluble substance and an inert catalyst. After the
pickling treatment, the fired metal complex is well washed with
water (including deionized water and distilled water), recovered by
centrifugation or filtration, and then dried, thus making it
possible to obtain the objective fired metal complex.
[0104] The obtained fired metal complex is preferably powderized to
form fine particles using a quartz mortar so as to increase a
specific surface area.
[0105] The fired metal complex obtained in the present step is a
catalyst component and, therefore, it can also be used as the
electrode catalyst as it is.
2-3. Effects
[0106] According to the above electrode catalyst, it is possible to
provide an electrode catalyst having oxidation-reduction reaction
(ORR) catalytic activity, durability, and corrosion resistance,
that are equal to or higher than those of known electrode catalysts
such as a Pt-based catalyst and a CoTMPP catalyst, at low cost; and
a comparatively simple production method thereof.
3. Conductive Carrier, Support, and Supporting Method
3-1. Conductive Carrier
[0107] The "conductive carrier" refers to a substance that has
conductivity, and is also capable of supporting an electrode
catalyst. The material is not particularly limited as long as it is
a substance having the above-mentioned properties. Examples thereof
include a carbon-based substance, a conductive polymer, a
semiconductor, a metal, and the like.
[0108] As used herein, "carbon-based substance" refers to a
substance containing carbon (C) as a constituent. Examples thereof
include graphite, activated carbon, carbon powder (including, for
example, carbon black, VulcanXC-72R, acetylene black, furnace
black, and denka black), carbon fiber (including graphite felt,
carbon wool, and carbon woven fabric), carbon plate, carbon paper,
carbon disk, and fine structure substances such as carbon nanotube,
carbon nanohorn, and carbon naocluster.
[0109] As used herein, "conductive polymer" is a generic term of a
polymer compound having conductivity. Examples thereof include
aniline, aminophenol, diaminophenol, pyrrole, thiophene,
paraphenylene, fluorene, furan, acetylene, or a single monomer
including a derivative thereof as a structural unit, or a polymer
of two or more kinds of monomers. Specific examples thereof include
polyaniline, polyaminophenol, polydiaminophenol, polypyrrole,
polythiophene, polyparaphenylene, polyfluorene, polyfuran, and
polyacetylene.
[0110] Taking ease of availability, cost, corrosion resistance,
durability, and the like into consideration, suitable conductive
carrier is a carbon-based substance, and is not limited thereto in
the present invention.
[0111] The carrier may be composed of a single kind of a carrier,
or a combination of two or more kinds of carries. For example, it
is possible to use a carrier using a carbon-based substance in
combination of a conductive polymer, or a carrier using a carbon
powder as the same carbon-based substance in combination of a
carbon paper.
[0112] The shape of the carrier is not particularly limited as long
as the shape is capable of supporting the electrode catalyst of the
first embodiment on a surface. For the purpose of enhancing
catalytic activity (mass activity) per unit mass in an electrode
for fuel cell, the shape is preferably a powder or fiber shape
having a large specific surface area per unit mass. The reason is
that the carrier having a larger specific surface area can usually
ensure a wider supporting area, and thus making it possible to
enhance dispersibility of a catalyst component on a carrier surface
and to support a larger amount of the catalyst component on a
surface thereof. Accordingly, the shape of a fine particle like a
carbon powder, and the shape of a fine fiber like a carbon fiber
are suitable as the shape of the carrier. A fine powder having an
average particle size of 1 nm to 1 .mu.m is particularly
preferable. For example, carbon black having an average particle
size of about 10 nm to 300 .mu.m is suitable as a carrier of the
present step.
[0113] The carrier also includes a connection terminal with a
conducting wire for connecting a fuel cell electrode with an
external circuit.
3-2. Support
[0114] The "support" refers to a substance that itself has rigidity
and is callable of imparting a fixed shape to the electrode for
fuel cell of the present invention. In case the conductive carrier
has a powder shape, it is impossible to retain a fixed shape of the
electrode for fuel cell by using a conductive carrier including an
electrode catalyst supported thereon alone. In case the conductive
carrier is in a state of a thin layer, the carrier itself has no
rigidity. In such case, a fixed shape and rigidity of the electrode
are imparted by disposing a conductive carrier including an
electrode catalyst support thereon on a support surface.
[0115] However, the support is not an indispensable constituent of
the electrode for fuel cell of the present invention. For example,
in case the conductive carrier itself has a fixed shape and
rigidity, like a carbon disk, it is possible to retain a fixed
shape of the electrode for fuel cell by using a conductive carrier
including an electrode catalyst support thereon. An electrolyte
material itself may impart a fixed shape and rigidity to the
electrode for fuel cell in some cases. For example, a thin layer
electrode is bonded to both surfaces of a solid polymer electrolyte
membrane in PEFC. In such case, a support is not necessarily
needed. Accordingly, the support may be optionally added to the
electrode for fuel cell of the present invention.
[0116] The material of the support is not particularly limited as
long as the electrode has rigidity enough to retain a fixed shape.
The material may be either an insulator or a conductor. Examples of
the material as the insulation include glass, plastics, synthetic
rubbers, ceramics, or papers or vegetative pieces (including, for
example, wood piece), animal fragments (including, for example,
ossicle, shell, and sponge) subjected to a waterproofing treatment
or water repellent finishing. The support having a porous structure
is more preferable since a specific surface area for bonding a
conductive carrier including an electrode catalyst support thereon
increases, thus enabling an increase in mass activity of the
electrode. Examples of the support having a porous structure
include porous ceramics, porous plastics, animal fragments, and the
like. Examples of the material as the conductor include
carbon-based substances (including, carbon paper, carbon fiber, and
carbon bar), metal, conductive polymers, and the like. In case the
support is a conductor, it can function as a support and a current
collector by disposing a conductive carrier including an electrode
catalyst support thereon on a surface thereof.
[0117] In case the electrode for fuel cell of the present invention
includes a support, the shape of the support usually reflects the
shape of the electrode for fuel cell. The shape of the support is
not particularly limited as long as it can achieve the function of
the electrode. The shape may be appropriately determined according
to the shape of a fuel cell. Examples of the shape include
approximately plate (including thin layer), approximately column,
approximately sphere, or a combination thereof.
3-3. Methods
(1) Electrode Catalyst Supporting Method
[0118] It is possible to use, as the method of supporting an
electrode catalyst on a conductive carrier, a method known in the
relevant field. Examples thereof include a method of fixing a fired
metal complex on a conductive carrier surface using an appropriate
fixing agent. The fixing agent preferably has conductivity and is
not particularly limited. It is possible to use, as the fixing
agent, a conductive polymer solution prepared by dissolving the
conductive polymer in an appropriate solvent, a dispersion of
polytetrafluoroethylene (PTFE), and the like. Supporting of the
electrode catalyst on a conductive carrier can be achieved by
applying or spraying such fixing agent on a conductive carrier
surface and/or an electrode catalyst surface to thereby mix them,
or drying after impregnating in a solution of a fixing agent. It is
also possible to use a method in which a conductive carrier and a
fired metal complex are mixed in a solvent such as water, and then
a base such as sodium hydroxide is added to thereby precipitate a
fired metal complex on a conductive carrier surface, thus achieving
supporting.
(2) Method for Formation of Electrode for Fuel Cell
[0119] It is possible to use, as the method for formation of an
electrode for fuel cell, a method known in the relevant field. For
example, a conductive carrier including an electrode catalyst
supported thereon is mixed with a dispersion of PTFE (for example,
Nafion (registered trademark; DuPont) solution) and the mixture was
formed into an appropriate shape, followed by a heat treatment,
thus enabling formation of an electrode for fuel cell. In case an
electrode is formed on a surface of a solid polymer electrolyte
membrane or an electrolyte matrix layer, like PEFC or PAFC, the
mixed solution is formed into a sheet and a solution of a
fluororesin-based ion exchange membrane having proton conductivity
on a surface of the thus formed electrode sheet, to which a
membrane is bonded, is applied or impregnated, and then the sheet
is laid on both surfaces of the membrane, followed by hot pressing,
thus bonding to the membrane. It is possible to use, as the
fluororesin-based ion exchange membrane having proton conductivity,
for example, Nafion, Filemion (registered trademark; Asahi Glass
Co., Ltd.), and the like.
[0120] An electrode for fuel cell can be formed by applying a mixed
slurry of the mixed solution on a surface of a conductive support
such as a carbon paper, followed by a heat treatment.
[0121] The electrode may also be formed by applying a mixed ink or
mixed slurry of a solution (for example, Nafion solution) of a
proton conductive ion exchange membrane and a conductive carrier
including an electrode catalyst supported thereon on a surface of a
support, a solid polymer electrolyte membrane, or an electrolyte
matrix layer.
3-4. Effects
[0122] According to the present invention, it is possible to
provide an electrode catalyst having catalytic activity, durability
and corrosion resistance, which are equal to or higher than those
of a Pt-based catalyst, at low cost in a stable manner as compared
with a conventional Pt-based catalyst.
[0123] The present invention is not limited to the above-mentioned
embodiments and it will, of course, be understood that various
modifications can be made without departing from the scope of the
present invention.
EXAMPLES
[0124] The present invention will be specifically described by way
of Examples.
Example 1
Production of Gas Diffusion Electrode
[0125] A commercially available carbon paper (porosity of 70%,
thickness of 0.4 mm) was used as a conductive porous material. In
order to improve gas diffusivity, a solution containing 30% by
weight of polytetrafluoroethylene (PTFE) dispersed therein was
applied on one surface of the carbon paper by a bar coater method,
and then the resin was fixed to the carbon paper by firing in a
nitrogen atmosphere electric furnace at a temperature of
340.degree. C. for 20 minutes, and thus allowing to undergo water
repellent finishing.
[0126] A catalyst paste to be applied on the carbon paper was
prepared in the following manner. In a zirconia pot for ball mill,
a commercially available platinum-supported carbon black
(supporting 10 wt % Pt/Vulcan XC-72) was dispersed in 50 mL of a
mixed solvent (2-propanol/water=1/1) so that the content of the
carbon black becomes 100 mg. While stirring the dispersion, a
commercially available PTFE was added dropwise and mixed in the
form of a Polyflon dispersion (average particle size of 0.3 .mu.m).
PTFE was added so that a ratio of PTFE to the entire carbon black
becomes 1:5. The above dispersion containing PTFE added therein was
suction-filtered on the carbon paper, followed by heat sintering
through firing in a nitrogen atmosphere electric furnace at a
temperature of 340.degree. C. for 20 minutes to produce a porous
gas diffusion electrode 1 and a gas diffusion electrode 2.
(Preparation of Electrolytic Solution)
[0127] Sodium hydrogen carbonate NaHCO.sub.3 and sodium hydroxide
NaOH were dissolved in ion-exchange water, and the pH value was
variously changed under the condition where NaHCO.sub.3 is
saturated.
(Assembling of Device)
[0128] A gas diffusion electrode 1 and a gas diffusion electrode 2
were disposed oppositely each other, and an anion exchange membrane
5 (NEOSEPTA (registered trademark) AMX) was interposed into the
space therebetween, and then the space was filled with an
electrolytic solution 3. The electrolytic solution 3 was sealed so
as not to contact with the open air through the gas diffusion
electrode 1 and the gas diffusion electrode 2, and then these
electrodes were connected to a DC power 4 so that the gas diffusion
electrode 1 serves as a cathode and the gas diffusion electrode 2
serves as an anode. Thereby, the carbon dioxide enrichment device
was obtained. In order to enable observation of the amount of
carbon dioxide discharged from the gas diffusion electrode 2, a
glass container with a tube (having a volume that can achieve 8
mL/cm.sup.2) was attached to the gas diffusion electrode 2 side,
and then sealed with an O-ring so as not to leak the discharged
gas. A carbon dioxide detector (solid electrolyte sensor type,
resolution of 0.01%) was attached to a pipe portion of the glass
container so as not to leak the discharged gas. Room temperature
and the temperature of the system were adjusted to 25.degree.
C.
[0129] The gas diffusion electrode 1 was connected to an anode of a
DC power 4 and the gas diffusion electrode 2 was connected to a
cathode, and the space between both electrodes was filled with the
electrolytic solution 3 having a pH of 9.0 adjusted with NaOH, and
then DC voltage of 1.2 V was applied between both electrodes.
Discharge of carbon dioxide from the gas diffusion electrode 2 was
confirmed by application of a voltage. The amount of carbon dioxide
emitted from the gas diffusion electrode 2 was confirmed by
measuring the concentration of carbon dioxide in the glass
container attached to the gas diffusion electrode 2 using a carbon
dioxide detector. The results are shown in Table 1. The emission
amount was calculated by the following [Equation 2].
( Amount of emissions per unit area ) = ( Concentration of carbon
dioxide in glass container ) .times. ( Volume of glass container )
/ ( Area of gas diffusion electrode 2 surrounded by glass container
) [ Equation 2 ] ##EQU00001##
Examples 2 to 4
[0130] In the same manner as in Example 1, the pH of the
electrolytic solution 3 was variously changed by the addition of
NaOH and DC voltage of 1.2 V was applied between the gas diffusion
electrode 1 and gas diffusion electrode 2. The amount of carbon
dioxide emissions was confirmed by measuring the concentration of
carbon dioxide in the glass container attached to the gas diffusion
electrode 2 using a carbon dioxide detector. The results are shown
in Table 1.
Example 5
[0131] In the same manner as in Example 1, except that the
electrode catalyst in Example 1 was replaced by the below-mentioned
electrode catalyst using no platinum in Example 5, examination was
made.
Example 1
Preparation of Electrode Catalyst
Test Example 1
Preparation of Co-2,6-diaminopyridine Polymer (CoDAPP) Catalyst
[0132] A CoDAPP catalyst was prepared by the method according to
the second embodiment of the present invention. A
2,6-diaminopyridine monomer (Aldrich Corporation) was mixed with an
oxidizing agent ammonium peroxydisulfate (APS) (Wako Corporation)
in a molar ratio of 1:1.5, followed by mixing. Specifically, 5.45 g
of 2,6-diaminopyridine and 1 g of sodium hydroxide were dissolved
in 400 mL of distilled water, and then 27.6 g of APS and 100 mL of
water were added. The obtained mixture was stirred for 5 minutes
and 2,6-diaminopyridine was polymerized at room temperature for 12
hours. After polymerization reaction, the obtained black
precipitate was recovered by centrifugation at 3,000 rpm, and then
washed three times with distilled water. The precipitate was dried
under vacuum at 60.degree. C. for several hours to obtain a
2,6-diaminopyridine polymer.
[0133] Subsequently, 5.45 g of a 2,6-diaminopyridine polymer and
3.62 g of cobalt nitrate (Wako Pure Chemical Industries, Ltd.) were
suspended in a solution of 150 mL of water and ethanol (in a mixing
ratio of 1:1) so that a molar ratio of 2,6-diaminopyridine (raw
monomer) to cobalt (catalytic metal atom) becomes 4:1. In the same
manner, each amount of the 2,6-diaminopyridine polymer and cobalt
nitrate was calculated from the molar ratio so that each molar
ratio of 2,6-diaminopyridine to cobalt becomes 6:1, 8:1, and 10:1,
followed by mixing. The suspension was subjected to ultrasonic
mixing for 1 hour using sonicator ultrasonic probe systems (AS ONE
Corporation) and stirred at 60.degree. C. for 2 hours, and then the
solution was vaporized. The remaining powder of a polymer metal
complex composed of 2,6-diaminopyridine polymer and cobalt was
ground in a quartz mortar.
[0134] The polymer metal complex was fired under an ammonia gas
atmosphere at 700.degree. C. for 1.5 hours. The obtained fired
metal complex was subjected to an ultrasonic pickling treatment
(pre-leaching) using a 12N hydrochloric acid solution for 8 hours,
followed by removal of an insoluble substance and an inert
substance and further well washing with deionized water. Finally,
the fired metal complex as an electrode catalyst of the present
invention was recovered by filtration and dried at 60.degree.
C.
Comparative Example
[0135] Table 1 shows the results obtained by comparing performance
of the carbon dioxide enrichment device shown in FIG. 2 mentioned
in Example 1 with carbon dioxide enrichment performance of the
carbon dioxide facilitated transport membrane utilizing a
difference in a permeation rate of a porous polymer membrane
mentioned in Non-Patent Document 1, and carbon dioxide enrichment
performance of a device using a solid molten salt mentioned in
Patent Document 1.
TABLE-US-00001 TABLE 1 Emission amount Cathode Anode (.mu.L/minute
cm.sup.2) side pH side pH [25.degree. C., 1 atom] Example 1 8.3 8.3
12 Example 2 6.2 10 19 Example 3 11.3 11.3 13 Example 4 6.2 7.6 8
Example 5 8.3 8.3 14 Comparative -- -- 6 Example 1 Comparative --
-- -- (about 0) Example 2
[0136] As is shown in the results, carbon dioxide enrich membrane
performance was evaluated at normal temperature under normal
pressure which is the most important matter of driving at low
energy, and found that the device in the present invention has high
enrichment performance. Namely, it has been found that both high
carbon dioxide enrichment performance and low energy consumption
can be achieved in Examples 1 to 3.
DESCRIPTION OF REFERENCE NUMERALS
[0137] 1 First gas diffusion electrode (cathode) [0138] 2 Second
gas diffusion electrode (anode) [0139] 3 Electrolytic solution
[0140] 4 Power [0141] 5 Anion exchange membrane
* * * * *