U.S. patent application number 15/698329 was filed with the patent office on 2018-09-20 for carbon dioxide electrolytic device.
This patent application is currently assigned to KABUSHIKI KAISHA TOSHIBA. The applicant listed for this patent is KABUSHIKI KAISHA TOSHIBA. Invention is credited to Ryota Kitagawa, Yuki KUDO, Satoshi Mikoshiba, Akihiko Ono, Yoshitsune Sugano, Jun Tamura, Eishi Tsutsumi, Masakazu Yamagiwa.
Application Number | 20180265440 15/698329 |
Document ID | / |
Family ID | 59799302 |
Filed Date | 2018-09-20 |
United States Patent
Application |
20180265440 |
Kind Code |
A1 |
KUDO; Yuki ; et al. |
September 20, 2018 |
CARBON DIOXIDE ELECTROLYTIC DEVICE
Abstract
A carbon dioxide electrolytic device of an embodiment includes:
an anode part including an anode which oxidizes water or hydroxide
ions to produce oxygen; a cathode part including a cathode which
reduces carbon dioxide to produce a carbon compound, a cathode
solution flow path which supplies a cathode solution to the
cathode, and a gas flow path which supplies carbon dioxide to the
cathode; a separator which separates the anode part and the cathode
part; and a differential pressure control unit which controls a
differential pressure between a pressure of the cathode solution
and a pressure of the carbon dioxide so as to adjust a production
amount of the carbon dioxide produced by a reduction reaction in
the cathode part.
Inventors: |
KUDO; Yuki; (Yokohama,
JP) ; Ono; Akihiko; (Kita, JP) ; Yamagiwa;
Masakazu; (Yokohama, JP) ; Tsutsumi; Eishi;
(Kawasaki, JP) ; Sugano; Yoshitsune; (Kawasaki,
JP) ; Kitagawa; Ryota; (Setagaya, JP) ;
Tamura; Jun; (Chuo, JP) ; Mikoshiba; Satoshi;
(Yamato, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KABUSHIKI KAISHA TOSHIBA |
Minato-ku |
|
JP |
|
|
Assignee: |
KABUSHIKI KAISHA TOSHIBA
Minato-ku
JP
|
Family ID: |
59799302 |
Appl. No.: |
15/698329 |
Filed: |
September 7, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C25B 3/04 20130101; Y02E
60/366 20130101; C25B 15/02 20130101; C25B 1/00 20130101; Y02E
60/36 20130101; C25B 1/10 20130101; C25B 9/08 20130101; C07C 29/153
20130101 |
International
Class: |
C07C 29/153 20060101
C07C029/153; C25B 3/04 20060101 C25B003/04 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 14, 2017 |
JP |
2017-048928 |
Claims
1. A carbon dioxide electrolytic device comprising: a cathode part
including a cathode to reduce carbon dioxide and thus produce a
carbon compound, a cathode solution flow path to supply a cathode
solution to the cathode, and a gas flow path to supply carbon
dioxide to the cathode; an anode part including an anode to oxidize
water or hydroxide ions and thus produce oxygen and an anode
solution flow path to supply an anode solution to the anode; a
separator to separate the anode part and the cathode part; a power
supply to pass an electric current between the anode and the
cathode; a first pressure control unit to control a pressure of the
cathode solution flowing in the cathode solution flow path; a
second pressure control unit to control a pressure of the carbon
dioxide flowing in the gas flow path; a detection unit to detect a
production amount of the carbon compound produced by a reduction
reaction in the cathode part; and a differential pressure control
unit to control a differential pressure between a pressure of the
cathode solution and a pressure of the carbon dioxide so as to
adjust the production amount of the carbon compound detected in the
detection unit.
2. The device according to claim 1, wherein the differential
pressure control unit controls the first and second pressure
control units so that an absolute value of the differential
pressure between the pressure of the cathode solution and the
pressure of the carbon dioxide is 0.1 kPa or more to 100 kPa or
less.
3. The device according to claim 1, wherein the differential
pressure control unit controls the first and second pressure
control units so that the pressure of the carbon dioxide is larger
than the pressure of the cathode solution.
4. The device according to claim 1, wherein the anode has a first
surface in contact with the separator and a second surface facing
the anode solution flow path so that the anode solution is in
contact with the anode, and wherein the cathode has a first surface
facing the cathode solution flow path and a second surface facing
the gas flow path, and the cathode solution flow path is disposed
between the separator and the cathode so that the cathode solution
is in contact with the separator and the cathode.
5. The device according to claim 4, wherein the cathode has a gas
diffusion layer disposed on the second surface side and a catalyst
layer disposed on the first surface side and constituted of a
cathode catalyst provided on the gas diffusion layer.
6. The device according to claim 5, wherein the cathode catalyst
contains at least one metal selected from the group consisting of
Au, Ag, Cu, Pt, Pd, Ni, Co, Fe, Mn, Ti, Cd, Zn, In, Ga, Pb, and Sn,
and has at least one selected from the group consisting of
nanoparticles of the metal, a nanostructure of the metal, nanowires
of the metal, and a composite body in which the nanoparticles are
supported by carbon particles, carbon nanotubes, or graphene.
7. The device according to claim 4, wherein the anode includes a
base material having at least one selected from the group
consisting of a mesh material, a punching material, a porous body,
and a metal fiber sintered body, and wherein the anode has the base
material constituted of an anode catalyst or a catalyst layer
constituted of an anode catalyst provided on a surface of the base
material.
8. The device according to claim 7, wherein the base material is
constituted of a metal material containing at least one selected
from the group consisting of Ti, Ni, and Fe, and the anode catalyst
is constituted of a metal material containing at least one metal
selected from the group consisting of Ni, Fe, Co, Mn, La, Ru, Li,
Ir, In, Sn, and Ti, or an oxide material containing the metal.
9. The device according to claim 1, wherein the anode solution and
the cathode solution contain at least one ion selected from the
group consisting of a hydroxide ion, a hydrogen ion, a potassium
ion, a sodium ion, a lithium ion, a chloride ion, a bromide ion, an
iodide ion, a nitrate ion, a sulfate ion, a phosphate ion, a borate
ion, and a hydrogen carbon ion.
10. The device according to claim 1, wherein the carbon compound to
be produced by a reduction reaction of the carbon dioxide contains
at least one selected from the group consisting of carbon monoxide,
methane, ethane, ethylene, methanol, ethanol, and ethylene glycol.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is based upon and claims the benefit of
priority from Japanese Patent Application No. 2017-048928, filed on
Mar. 14, 2017; the entire contents of which are incorporated herein
by reference.
FIELD
[0002] Embodiments described herein relate generally to a carbon
dioxide electrolytic device.
BACKGROUND
[0003] In recent years, there has been a concern for depletion of
fossil fuel such as petroleum or coal, and expectation for
sustainably-usable renewable energy has been rising. As the
renewable energy, a solar cell, wind power generation, and the like
can be cited. Because these depend on weather and a natural
situation in a power generation amount, there is a problem that
stable supply of electric power is difficult. Therefore, there has
been made an attempt to store the electric power generated by the
renewable energy in a storage battery and stabilize the electric
power. However, when the electric power is stored, there are
problems that a cost is required for the storage battery and a loss
occurs at a time of storage.
[0004] For such points, attention is being given to a technology of
performing water electrolysis using the electric power generated by
the renewable energy to produce hydrogen (H.sub.2) from water or
reducing carbon dioxide (CO.sub.2) electrochemically to convert it
into a chemical substance (chemical energy) such as a carbon
compound such as carbon monoxide (CO), a formic acid (HCOOH),
methanol (CH.sub.3OH), methane (CH.sub.4), an acetic acid
(CH.sub.3COOH), ethanol (C.sub.2H.sub.5OH), ethane
(C.sub.2H.sub.6), or ethylene (C.sub.2H.sub.4). When these chemical
substances are stored in a cylinder or a tank, as compared with
when the electric power (electric energy) is stored in the storage
battery, there are advantages that a storage cost of energy can be
reduced and a storage loss is also small.
[0005] As a configuration of a carbon dioxide electrolytic device,
for example, three configurations indicated below are being
studied. As a first configuration, there can be cited a
configuration which includes an electrolytic bath accommodating an
electrolytic solution in which carbon dioxide (CO.sub.2) has been
absorbed, an anode (oxidation electrode) and a cathode (reduction
electrode) immersed in the electrolytic solution, and a separator
such as an ion exchange membrane disposed so as to separate the
anode and the cathode. As a second configuration, there can be
cited a configuration which includes a cathode solution flow path
disposed along one surface of a cathode, a CO.sub.2 gas flow path
disposed along the other surface of the cathode, an anode solution
flow path disposed along one surface of an anode, and a separator
disposed between the cathode solution flow path and the anode
solution flow path. As a third configuration, similarly to a solid
polymer fuel cell, there can be cited a configuration in which an
ion exchange membrane is disposed between an anode and a cathode
and a CO.sub.2 gas flow path is disposed along the other surface of
the cathode.
[0006] Among the above-described configuration examples of the
carbon dioxide electrolytic device, in the first configuration
example, in an electrolysis operation at about 10 mA/cm.sup.2 or
more, an overvoltage loss is large, resulting in a large cell
voltage, and therefore there is a problem that electrolysis
efficiency at a high current density is low. In the second
configuration example, a cell voltage can be reduced more than that
in the first configuration example, and the electrolysis efficiency
can be improved. However, there is a problem that selectivity of a
product to be obtained by a reduction reaction on a cathode side is
low and variations exist in the electrolysis efficiency in the
second configuration example. Note that in the third configuration
example, development, selection, and the like of an ion exchange
membrane suitable for electrolysis of CO.sub.2 are required.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 is a view illustrating a carbon dioxide electrolytic
device of an embodiment.
[0008] FIG. 2 is a sectional view illustrating an electrolysis cell
of the carbon dioxide electrolytic device illustrated in FIG.
1.
[0009] FIG. 3 is a view illustrating one example of an anode
solution flow path in the electrolysis cell illustrated in FIG.
2.
[0010] FIG. 4 is a view illustrating one example of a cathode
solution flow path in the electrolysis cell illustrated in FIG.
2.
[0011] FIG. 5 is a view illustrating the other example of the
cathode solution flow path in the electrolysis cell illustrated in
FIG. 2.
[0012] FIG. 6 is a view illustrating one example of a CO.sub.2 gas
flow path in the electrolysis cell illustrated in FIG. 2.
[0013] FIG. 7 is a view illustrating one example of a cathode in
the electrolysis cell illustrated in FIG. 2.
[0014] FIG. 8 is a view illustrating the other example of the
cathode in the electrolysis cell illustrated in FIG. 2.
[0015] FIG. 9 is a view schematically illustrating a reaction in
the cathode in the electrolysis cell illustrated in FIG. 2.
[0016] FIG. 10 is a chart illustrating time changes in a cell
voltage, an anode potential, and a cathode potential by using a
carbon dioxide electrolytic device in an example.
DETAILED DESCRIPTION
[0017] According to the embodiments of the present invention, there
is provided a carbon dioxide electrolytic device that includes: a
cathode part including a cathode to reduce carbon dioxide and thus
produce a carbon compound, a cathode solution flow path to supply a
cathode solution to the cathode, and a gas flow path to supply
carbon dioxide to the cathode; an anode part including an anode to
oxidize water or hydroxide ions and thus produce oxygen and an
anode solution flow path to supply an anode solution to the anode;
a separator to separate the anode part and the cathode part; a
power supply to pass an electric current between the anode and the
cathode; a first pressure control unit to control a pressure of the
cathode solution flowing in the cathode solution flow path; a
second pressure control unit to control a pressure of the carbon
dioxide flowing in the gas flow path; a detection unit to detect a
production amount of the carbon compound produced by a reduction
reaction in the cathode part; and a differential pressure control
unit to control a differential pressure between a pressure of the
cathode solution and a pressure of the carbon dioxide so as to
adjust the production amount of the carbon compound detected in the
detection unit.
[0018] Hereinafter, a carbon dioxide electrolytic device of an
embodiment will be described with reference to the drawings. In the
embodiment presented below, substantially the same components are
denoted by the same reference signs, and a description thereof is
sometimes partially omitted. The drawings are schematic, and a
relationship between a thickness and a planar size, thickness
proportions of the respective portions, and the like are sometimes
different from actual ones.
[0019] FIG. 1 is a view illustrating a configuration of a carbon
dioxide electrolytic device according to the embodiment. FIG. 2 is
a sectional view illustrating a configuration of an electrolysis
cell in the electrolytic device illustrated in FIG. 1. A carbon
dioxide electrolytic device 1 illustrated in FIG. 1 includes: an
electrolysis cell 2; an anode solution supply system 100 which
supplies an anode solution to the electrolysis cell 2; a cathode
solution supply system 200 which supplies a cathode solution to the
electrolysis cell 2; a gas supply system 300 which supplies carbon
dioxide (CO.sub.2) gas to the electrolysis cell 2; a product
collection system 400 which collects a product produced by a
reduction reaction in the electrolysis cell 2; and a product
control system 500 which detects a type and a production amount of
the collected product and controls the product.
[0020] The electrolysis cell 2 includes an anode part 10, a cathode
part 20, and a separator 30 as illustrated in FIG. 2. The anode
part 10 includes an anode 11, an anode solution flow path 12, and
an anode current collector 13. The cathode part 20 includes a
cathode solution flow path 21, a cathode 22, a CO.sub.2 gas flow
path 23, and a cathode current collector 24. The separator 30 is
disposed so as to separate the anode part 10 and the cathode part
20. The electrolysis cell 2 is sandwiched by a pair of support
plates not illustrated, and further tightened by bolts or the like.
In FIG. 1 and FIG. 2, a reference sign 40 is a power supply which
passes an electric current through the anode 11 and the cathode 22.
The power supply 40 is connected via a current introduction member
to the anode 11 and the cathode 22. The power supply 40 is not
limited to a normal commercial power supply, battery, or the like,
and may supply electric power generated by renewable energy such as
a solar cell or wind power generation.
[0021] The anode 11 is an electrode (oxidation electrode) which
causes an oxidation reaction of water (H.sub.2O) in an anode
solution to produce oxygen (O.sub.2) or hydrogen ions (H.sup.+), or
causes an oxidation reaction of hydroxide ions (OH.sup.-) produced
in the cathode part 20 to produce oxygen (O.sub.2) or water
(H.sub.2O). The anode 11 preferably has a first surface 11a in
contact with the separator 30 and a second surface 11b facing the
anode solution flow path 12. The first surface 11a of the anode 11
is in close contact with the separator 30. The anode solution flow
path 12 supplies the anode solution to the anode 11, and is
constituted by a pit (groove portion/concave portion) provided in a
first flow path plate 14. The anode solution flows through in the
anode solution flow path 12 so as to be in contact with the anode
11. The anode current collector 13 is electrically in contact with
a surface on a side opposite to the anode 11 of the first flow path
plate 14 constituting the anode solution flow path 12.
[0022] To the first flow path plate 14, a solution inlet port and a
solution outlet port whose illustrations are omitted are connected,
and via these solution inlet port and solution outlet port, the
anode solution is introduced and discharged by the anode solution
supply system 100. For the first flow path plate 14, a material
having low chemical reactivity and high conductivity is preferably
used. As such a material, a metal material such as Ti or SUS,
carbon, or the like can be cited. Along the anode solution flow
path 12, as illustrated in FIG. 3, a plurality of lands (convex
portions) 15 are preferably provided. The lands 15 are provided for
mechanical retention and electrical continuity. The lands 15 are
preferably provided alternately to uniformize flow of the anode
solution. The above lands 15 make the anode solution flow path 12
serpentine. Moreover, also for a good discharge of the anode
solution in which oxygen (O.sub.2) gas is mixed, the lands 15 are
preferably provided alternately along the anode solution flow path
12 to make the anode solution flow path 12 serpentine.
[0023] The anode 11 is preferably mainly constituted of a catalyst
material (anode catalyst material) capable of oxidizing water
(H.sub.2O) to produce oxygen or hydrogen ions or oxidizing
hydroxide ions (OH.sup.-) to produce water or oxygen, and capable
of reducing an overvoltage of the above reaction. As such a
catalyst material, there can be cited a metal such as platinum
(Pt), palladium (Pd), or nickel (Ni), an alloy or an intermetallic
compound containing the above metals, a binary metal oxide such as
a manganese oxide (Mn--O), an iridium oxide (Ir--O), a nickel oxide
(Ni--O), a cobalt oxide (Co--O), an iron oxide (Fe--O), a tin oxide
(Sn--O), an indium oxide (In--O), a ruthenium oxide (Ru--O), a
lithium oxide (Li--O), or a lanthanum oxide (La--O), a ternary
metal oxide such as Ni--Co--O, Ni--Fe--O, La--Co--O, Ni--La--O, or
Sr--Fe--O, a quaternary metal oxide such as Pb--Ru--Ir--O or
La--Sr--Co--O, or a metal complex such as a Ru complex or a Fe
complex.
[0024] The anode 11 includes a base material having a structure
capable of moving the anode solution or ions between the separator
30 and the anode solution flow path 12, for example, a porous
structure such as a mesh material, a punching material, a porous
body, or a metal fiber sintered body. The base material may be
constituted of a metal such as titanium (Ti), nickel (Ni), or iron
(Fe), or a metal material such as an alloy (for example, SUS)
containing at least one of these metals, or may be constituted of
the above-described anode catalyst material. When the oxide is used
as the anode catalyst material, the anode catalyst material
preferably adheres to or is stacked on a surface of the base
material constituted of the above-described metal material to form
a catalyst layer. The anode catalyst material preferably has
nanoparticles, a nanostructure, a nanowire, or the like for the
purpose of increasing the oxidation reaction. The nanostructure is
a structure in which nanoscale irregularities are formed on a
surface of the catalyst material.
[0025] The cathode 22 is an electrode (reduction electrode) which
causes a reduction reaction of carbon dioxide (CO.sub.2) or a
reduction reaction of a carbon compound produced thereby to produce
a carbon compound such as carbon monoxide (CO), methane (CH.sub.4),
ethane (C.sub.2H.sub.6), ethylene (C.sub.2H.sub.4), methanol
(CH.sub.3OH), ethanol (C.sub.2H.sub.5OH), or ethylene glycol
(C.sub.2H.sub.6O.sub.2). In the cathode 22, simultaneously with the
reduction reaction of carbon dioxide (CO.sub.2), a side reaction in
which hydrogen (H.sub.2) is produced by a reduction reaction of
water (H.sub.2O) is sometimes caused. The cathode 22 has a first
surface 22a facing the cathode solution flow path 21 and a second
surface 22b facing the CO.sub.2 gas flow path 23. The cathode
solution flow path 21 is disposed between the cathode 22 and the
separator 30 so that the cathode solution is in contact with the
cathode 22 and the separator 30.
[0026] The cathode solution flow path 21 is constituted by an
opening portion provided in a second flow path plate 25. To the
second flow path plate 25, a solution inlet port and a solution
outlet port whose illustrations are omitted are connected, and via
these solution inlet port and solution outlet port, the cathode
solution is introduced and discharged by the cathode solution
supply system 200. The cathode solution flows through in the
cathode solution flow path 21 so as to be in contact with the
cathode 22 and the separator 30. For the second flow path plate 25
constituting the cathode solution flow path 21, a material having
low chemical reactivity and having no conductivity is preferably
used. As such a material, there can be cited an insulating resin
material such as an acrylic resin, polyetheretherketone (PEEK), or
a fluorocarbon resin.
[0027] In the cathode 22, the reduction reaction of CO.sub.2 occurs
mainly in a portion in contact with the cathode solution.
Therefore, to the cathode solution flow path 21, as illustrated in
FIG. 4, the opening portion having a large opening area is
preferably applied. However, in order to enhance mechanical
retention and electrical connectivity, as illustrated in FIG. 5, a
land (convex portion) 26 may be provided in the cathode solution
flow path 21. The land 26 in the cathode solution flow path 21 is
provided in a center portion of the cathode solution flow path 21,
and is retained to the second flow path plate 25 by a bridge
portion 27 thinner than the land 26 by so as not to prevent the
cathode solution in the cathode solution flow path 21 from flowing
through. When the land 26 is provided in the cathode solution flow
path 21, the number of lands 26 is preferably small in order to
reduce cell resistance.
[0028] The CO.sub.2 gas flow path 23 is constituted by a pit
(groove portion/concave portion) provided in a third flow path
plate 28. For the third flow path plate 28 constituting the
CO.sub.2 gas flow path, a material having low chemical reactivity
and high conductivity is preferably used. As such a material, the
metal material such as Ti or SUS, carbon, or the like can be cited.
Note that in each of the first flow path plate 14, the second flow
path plate 25, and the third flow path plate 28, an inlet port and
an outlet port for a solution or gas, screw holes for tightening,
and the like, whose illustrations are omitted, are provided.
Further, in front of and behind each of the flow path plates 14,
25, and 28, packing whose illustration is omitted is sandwiched as
necessary.
[0029] To the third flow path plate 28, a gas inlet port and a gas
outlet port whose illustrations are omitted are connected, and via
these gas inlet port and gas outlet port, CO.sub.2 gas or gas
containing CO.sub.2 (sometimes collectively referred to simply as
CO.sub.2 gas) is introduced and discharged by the gas supply system
300. The CO.sub.2 gas flows through in the CO.sub.2 gas flow path
23 so as to be in contact with the cathode 22. Along the CO.sub.2
gas flow path 23, as illustrated in FIG. 6, a plurality of lands
(convex portions) 29 are preferably provided. The lands 29 are
provided for the mechanical retention and the electrical
continuity. The lands 29 are preferably provided alternately, and
this makes the CO.sub.2 gas flow path 23 serpentine similarly to
the anode solution flow path 12. The cathode current collector 24
is electrically in contact with a surface on a side opposite to the
cathode 22 of the third flow path plate 28.
[0030] The cathode 22 has a gas diffusion layer 22a and a cathode
catalyst layer 22b provided thereon as illustrated in FIG. 7.
Between the gas diffusion layer 22a and the cathode catalyst layer
22b, as illustrated in FIG. 8, a porous layer 22c denser than the
gas diffusion layer 22a may be disposed. As illustrated in FIG. 9,
the gas diffusion layer 22a is disposed on the CO.sub.2 gas flow
path 23 side, and the cathode catalyst layer 22b is disposed on the
cathode solution flow path 21 side. The cathode catalyst layer 22b
preferably has catalyst nanoparticles, a catalyst nanostructure, or
the like. The gas diffusion layer 22a is constituted by carbon
paper, carbon cloth, or the like, for example, and subjected to
water repellent treatment. The porous layer 22c is constituted by a
porous body whose pore size is smaller than that of the carbon
paper or the carbon cloth.
[0031] As illustrated in a schematic view in FIG. 9, in the cathode
catalyst layer 22b, the cathode solution or ions are supplied and
discharged from the cathode solution flow path 21, and in the gas
diffusion layer 22a, the CO.sub.2 gas is supplied from the CO.sub.2
gas flow path 23 and further a product by the reduction reaction of
the CO.sub.2 gas is discharged. By subjecting the gas diffusion
layer 22a to moderate water repellent treatment, the CO.sub.2 gas
reaches the cathode catalyst layer 22b mainly owing to gas
stirring. The reduction reaction of CO.sub.2 or the reduction
reaction of a carbon compound produced thereby occurs in the
vicinity of a boundary between the gas diffusion layer 22a and the
cathode catalyst layer 22b, a gaseous product is discharged mainly
from the CO.sub.2 gas flow path 23, and a liquid product is
discharged mainly from the cathode solution flow path 21.
[0032] The cathode catalyst layer 22b is preferably constituted of
a catalyst material (cathode catalyst material) capable of reducing
carbon dioxide to produce a carbon compound and further reducing
the carbon compound produced thereby to produce a carbon compound
as necessary, and capable of reducing an overvoltage of the above
reaction. As such a material, there can be cited a metal such as
gold (Au), silver (Ag), copper (Cu), platinum (Pt), palladium (Pd),
nickel (Ni), cobalt (Co), iron (Fe), manganese (Mn), titanium (Ti),
cadmium (Cd), zing (Zn), indium (In), gallium (Ga), lead (Pb), or
tin (Sn), a metal material such as an alloy or an intermetallic
compound containing at least one of the above metals, a carbon
material such as carbon (C), graphene, CNT (carbon nanotube),
fullerene, or ketjen black, or a metal complex such as a Ru complex
or a Re complex. To the cathode catalyst layer 22b, various shapes
such as a plate shape, a mesh shape, a wire shape, a particle
shape, a porous shape, a thin film shape, and an island shape can
be applied.
[0033] The cathode catalyst material constituting the cathode
catalyst layer 22b preferably has nanoparticles of the
above-described metal material, a nanostructure of the metal
material, nanowires of the metal material, or a composite body in
which the nanoparticles of the above-described metal material are
supported by a carbon material such as carbon particles, carbon
nanotubes, or graphene. Applying catalyst nanoparticles, a catalyst
nanostructure, a catalyst nanowire, a catalyst nano-support
structure, or the like as the cathode catalyst material makes it
possible to enhance reaction efficiency of the reduction reaction
of carbon dioxide in the cathode 22.
[0034] The separator 30 is constituted of an ion exchange membrane
or the like capable of moving ions between the anode 11 and the
cathode 22 and separating the anode part 10 and the cathode part
20. As the ion exchange membrane, for example, a cation exchange
membrane such as Nafion or Flemion, or an anion exchange membrane
such as Neosepta, or Selemion can be used. As described later, when
an alkaline solution is used as the anode solution or the cathode
solution and it is assumed that hydroxide ions (OH.sup.-) move
mainly, the separator 30 is preferably constituted of the anion
exchange membrane. Also other than the ion exchange membrane, a
glass filter, a porous polymeric membrane, a porous insulating
material, or the like may be applied to the separator 30 as long as
they are a material capable of moving ions between the anode 11 and
the cathode 22.
[0035] The anode solution and the cathode solution are preferably a
solution containing at least water (H.sub.2O). Because carbon
dioxide (CO.sub.2) is supplied from the CO.sub.2 gas flow path 23,
the cathode solution may contain or need not contain carbon dioxide
(CO.sub.2). To the anode solution and the cathode solution, the
same solution may be applied or different solutions may be applied.
As a solution used as the anode solution and the cathode solution
and containing H.sub.2O, for example, an aqueous solution
containing an arbitrary electrolyte can be cited. As the aqueous
solution containing the electrolyte, for example, there can be
cited an aqueous solution containing at least one ion selected from
a hydroxide ion (OH.sup.-), a hydrogen ion (H.sup.+), a potassium
ion (K.sup.+), a sodium ion (Na.sup.+), a lithium ion (Li.sup.+), a
chloride ion (Cl.sup.-), a bromide ion (Br.sup.-), an iodide ion
(I.sup.-), a nitrate ion (NO.sub.3.sup.-), a sulfate ion
(SO.sub.4.sup.2), a phosphate ion (PO.sub.4.sup.2-), a borate ion
(BO.sub.3.sup.3-), and a hydrogen carbonate ion (HCO.sub.3.sup.-).
In order to reduce electrical resistance of the solution, as the
anode solution and the cathode solution, an alkaline solution in
which an electrolyte such as a potassium hydroxide or a sodium
hydroxide is dissolved in high concentration is preferably
used.
[0036] For the cathode solution, an ionic liquid which is made of
salts of cations such as imidazolium ions or pyridinium ions and
anions such as BF.sub.4.sup.- or PF.sub.6.sup.- and which is in a
liquid state in a wide temperature range, or its aqueous solution
may be used. As another cathode solution, there can be cited an
amine solution of ethanolamine, imidazole, pyridine, or the like,
or an aqueous solution thereof. As amine, any of primary amine,
secondary amine, and tertiary amine is applicable.
[0037] To the anode solution flow path 12 of the anode part 10, the
anode solution is supplied from the anode solution supply system
100. The anode solution supply system 100 circulates the anode
solution so that the anode solution flows through in the anode
solution flow path 12. The anode solution supply system 100 has a
pressure control unit 101, an anode solution tank 102, a flow rate
control unit (pump) 103, a reference electrode 104, and a pressure
gauge 105, and is constituted so that the anode solution circulates
in the anode solution flow path 12. The anode solution tank 102 is
connected to a gas component collection unit which collects a gas
component such as oxygen (O.sub.2) contained in the circulating
anode solution and is not illustrated. The anode solution, whose
flow rate and pressure are controlled in the pressure control unit
101 and the flow rate control unit 103, is introduced to the anode
solution flow path 12.
[0038] To the cathode solution flow path 21 of the cathode part 20,
the cathode solution is supplied from the cathode solution supply
system 200. The cathode solution supply system 200 circulates the
cathode solution so that the cathode solution flows through in the
cathode solution flow path 21. The cathode solution supply system
200 has a pressure control unit 201, a cathode solution tank 202, a
flow rate control unit (pump) 203, a reference electrode 204, and a
pressure gauge 205, and is constituted so that the cathode solution
circulates in the cathode solution flow path 21. The cathode
solution tank 202 is connected to a gas component collection unit
206 which collects a gas component such as carbon monoxide (CO)
contained in the circulating cathode solution. The cathode
solution, whose flow rate and pressure are controlled in the
pressure control unit 201 and the flow rate control unit 203, is
introduced to the cathode solution flow path 21.
[0039] To the CO.sub.2 gas flow path 23, the CO.sub.2 gas is
supplied from the gas supply system 300. The gas supply system 300
has a CO.sub.2 gas cylinder 301, a flow rate control unit 302, a
pressure gauge 303, and a pressure control unit 304. The CO.sub.2
gas, whose flow rate and pressure are controlled in the flow rate
control unit 302 and the pressure control unit 304, is introduced
to the CO.sub.2 gas flow path 23. The gas supply system 300 is
connected to the product collection system 400 which collects a
product in gas which has flowed through the CO.sub.2 gas flow path
23. The product collection system 400 has a gas/liquid separation
unit 401 and a product collection unit 402. A reduction product
such as CO or H.sub.2 contained in the gas which has flowed through
the CO.sub.2 gas flow path 23 is accumulated via the gas/liquid
separation unit 401 in the product collection unit 402.
[0040] Part of the reduction product accumulated in the product
collection unit 402 is sent to a reduction performance detection
unit 501 of the product control system 500. In the reduction
performance detection unit 501, a production amount and a
proportion of each product such as CO or H.sub.2 in the reduction
product are detected. The detected production amount and proportion
of each product are inputted to a data collection control unit 502
of the product control system 500. The data collection control unit
502 is electrically connected via bi-directional signal lines whose
illustration is partially omitted to the pressure control unit 101
and the flow rate control unit 103 of the anode solution supply
system 100, the pressure control unit 201 and the flow rate control
unit 203 of the cathode solution supply system 200, and the flow
rate control unit 302 and the pressure control unit 304 of the gas
supply system 300 in addition to the reduction performance
detection unit 501.
[0041] Each operation of the electrolysis cell 2, the power supply
40, the anode solution supply system 100, the cathode solution
supply system 200, and the gas supply system 300 is controlled by
the data collection control unit 502. That is, the data collection
control unit 502 controls the pressure control unit 201 of the
cathode solution supply system 200 and the pressure control unit
304 of the gas supply system 300 so as to adjust the production
amount and the proportion of each product detected in the reduction
performance detection unit 501, specifically so that the production
amount and the proportion of each product each become a desired
value. Thereby, a differential pressure between a pressure of the
cathode solution flowing through the cathode solution flow path 21
and a pressure of the CO.sub.2 gas flowing through the CO.sub.2 gas
flow path 23 is controlled. Because the differential pressure
between the pressure of the cathode solution and the pressure of
the CO.sub.2 gas affects the production amount and the proportion
of each product, controlling the differential pressure based on a
detection result of the reduction product makes it possible to
adjust the production amount and the proportion of each product in
a desired state.
[0042] When the differential pressure between the pressure of the
cathode solution and the pressure of the CO.sub.2 gas is too large,
there is a possibility that the CO.sub.2 gas pet mates the cathode
solution flow path 21 or the cathode solution permeates the
CO.sub.2 gas flow path 23. Both of these become a factor of
impairing the reduction reaction of CO.sub.2 in the cathode 22.
Therefore, an absolute value of the differential pressure between
the pressure of the cathode solution and the pressure of the
CO.sub.2 gas is preferably set to 100 kPa or less. Further, when
the absolute value of the differential pressure between the
pressure of the cathode solution and the pressure of the CO.sub.2
gas is too small, a function of adjusting the production amount and
the proportion of each product decreases, and therefore the
absolute value of the differential pressure is preferably 0.1 kPa
or more. The absolute value of the differential pressure is more
preferably 0.1 kPa or more to 10 kPa or less. Specific control
contents of the differential pressure between the pressure of the
cathode solution and the pressure of the CO.sub.2 gas will be
described later.
[0043] Next, an operation of the carbon dioxide electrolytic device
1 of the embodiment will be described. Here, a case of producing
carbon monoxide (CO) as the carbon compound is mainly described,
but the carbon compound as the reduction product of carbon dioxide
is not limited to carbon monoxide. The carbon compound may be
methane (CH.sub.4), ethane (C.sub.2H.sub.6), ethylene
(C.sub.2H.sub.4), methanol (CH.sub.3OH), ethanol
(C.sub.2H.sub.5OH), ethylene glycol (C.sub.2H.sub.6O.sub.2), or the
like as described above, and further carbon monoxide which is the
reduction product may be further reduced to produce the organic
compounds as described above. Further, as a reaction process by the
electrolysis cell 2, a case of mainly producing hydrogen ions
(H.sup.+) and a case of mainly producing hydroxide ions (OH.sup.-)
are considered, but it is not limited to either of these reaction
processes.
[0044] First, the reaction process in a case of mainly oxidizing
water (H.sub.2O) to produce hydrogen ions (H.sup.+) is described.
When an electric current is supplied from the power supply 40
between the anode 11 and the cathode 22, the oxidation reaction of
water (H.sub.2O) occurs in the anode 11 in contact with the anode
solution. Specifically, as indicated by the following (1) formula,
H.sub.2O contained in the anode solution is oxidized and oxygen
(O.sub.2) and hydrogen ions (H.sup.+) are produced.
2H.sub.2O.fwdarw.4H.sup.++O.sub.2+4e.sup.- (1)
[0045] H.sup.+ produced in the anode 11 moves in the anode solution
existing in the anode 11, the separator 30, and the cathode
solution in the cathode solution flow path 21 and reaches the
vicinity of the cathode 22. The reduction reaction of carbon
dioxide (CO.sub.2) occurs by electrons (e.sup.-) based on the
electric current which is supplied from the power supply 40 to the
cathode 22 and H.sup.+ which moves to the vicinity of the cathode
22. Specifically, as indicated by the following (2) formula,
CO.sub.2 supplied from the CO.sub.2 gas flow path 23 to the cathode
22 is reduced and CO is produced.
2CO.sub.2+4H.sup.++4e.sup.-.fwdarw.2CO+2H.sub.2O (2)
[0046] Next, the reaction process in a case of mainly reducing
carbon dioxide (CO.sub.2) to produce hydroxide ions (OH.sup.-) is
described. When an electric current is supplied from the power
supply 40 between the anode 11 and the cathode 22, in the vicinity
of the cathode 22, as indicated by the following (3) formula, water
(H.sub.2O) and carbon dioxide (CO.sub.2) are reduced and carbon
monoxide (CO) and hydroxide ions (OH.sup.-) are produced. The
hydroxide ions (OH.sup.-) diffuse in the vicinity of the anode 11,
and as indicated by the following (4) formula, the hydroxide ions
(OH.sup.-) are oxidized and oxygen (O.sub.2) is produced.
2CO.sub.2+2H.sub.2O+4e.sup.-.fwdarw.2CO+40H.sup.- (3)
40H.sup.-.fwdarw.2H.sub.2O+O.sub.2+4e.sup.- (4)
[0047] In the above-described reaction processes in the cathode 22,
the reduction reaction of CO.sub.2 is considered to occur in the
vicinity of the boundary between the gas diffusion layer 22a and
the cathode catalyst layer 22b as described above. At this time,
when the pressure of the cathode solution flowing through the
cathode solution flow path 21 is larger than the pressure of the
CO.sub.2 gas flowing through the CO.sub.2 gas flow path 23,
production of H.sub.2 by the reduction reaction of H.sub.2O in the
cathode solution sometimes has superiority to production of CO by
the reduction reaction of CO.sub.2. In the above case, the
production amount and the proportion of H.sub.2 in the reduction
product increase and the production amount and the proportion of
intended CO decrease. In such a case, the differential pressure is
adjusted by the data collection control unit 502 functioning as a
differential pressure control unit so that the pressure of the
CO.sub.2 gas is larger than the pressure of the cathode solution,
thereby making the reduction reaction of CO.sub.2 preferentially
occur in the vicinity of the boundary between the gas diffusion
layer 22a and the cathode catalyst layer 22b. This makes it
possible to increase the production amount and the production
proportion of CO by the reduction reaction of CO.sub.2. The
specific differential pressure is preferably 0.1 kPa or more to 100
kPa or less, and more preferably 0.1 kPa or more to 10 kPa or less
as described above.
[0048] As described above, the differential pressure between the
pressure of the cathode solution flowing through the cathode
solution flow path 21 and the pressure of the CO.sub.2 gas flowing
through the CO.sub.2 gas flow path 23 is preferably adjusted so
that the pressure of the CO.sub.2 gas is larger than the pressure
of the cathode solution. However, this is not necessarily
restrictive. For example, when the gas diffusion layer 22a has high
water repellency and the cathode solution does not easily enter the
gas diffusion layer 22a, or when the CO.sub.2 gas easily leaks to
the cathode catalyst layer 22b side, an adjustment may be made so
that the pressure of the cathode solution is larger than the
pressure of the CO.sub.2 gas. In both cases, since the differential
pressure between the pressure of the cathode solution and the
pressure of the CO.sub.2 gas affects the production amount and the
proportion of each product, the differential pressure is adjusted
so that they each become a desired value, based on the production
amount and the proportion of each product detected in the reduction
performance detection unit 501. This makes it possible to obtain
the reduction product having desired production amount and
proportion.
[0049] Further, in both of the above-described reaction process in
which hydrogen ions (H.sup.+) are mainly produced and reaction
process in which hydroxide ions (OH.sup.-) are mainly produced,
oxygen (O.sub.2) is produced in the anode 11. At this time, for
example, in a cell structure in which a separator is sandwiched by
a cathode solution flow path and an anode solution flow path, air
bubbles of oxygen (O.sub.2) gas which occur in the anode 11 stay in
the anode solution flow path, and cell resistance between the anode
and the separator (ion exchange membrane or the like) increases,
and thereby a voltage variation of the anode is considered to
become large. In contrast to this, in the electrolysis cell 2 of
the embodiment, the anode solution flow path 12 is not disposed
between the anode 11 and the separator 30, and the anode 11 and the
separator 30 are brought in close contact with each other, and
therefore oxygen gas which occurs in the anode 11 is discharged to
the anode solution flow path 12 together with the anode solution.
Therefore, it is possible to prevent the oxygen gas from staying
between the anode 11 and the separator 30. Accordingly, it becomes
possible to suppress a variation in a cell voltage due to the
voltage variation of the anode.
[0050] Moreover, in the electrolysis cell 2 of the embodiment,
providing the lands 15 and the lands 29 along the anode solution
flow path 12 and the CO.sub.2 gas flow path 23 makes it possible to
increase a contact area between the anode 11 and the first flow
path plate 14 constituting the anode solution flow path 12 and a
contact area between the cathode 22 and the third flow path plate
28 constituting the CO.sub.2 gas flow path 23. Further, providing
the land 26 in the cathode solution flow path 21 makes it possible
to increase a contact area between the cathode 22 and the second
flow path plate 25 constituting the cathode solution flow path 21.
These make electrical continuity between the anode current
collector 13 and the cathode current collector 24 good while
enhancing mechanical retentivity of the electrolysis cell 2, and
make it possible to improve reduction reaction efficiency of
CO.sub.2, or the like.
EXAMPLE
[0051] Next, an example and its evaluation result will be
described.
Example 1
[0052] An electrolytic device illustrated in FIG. 1 and FIG. 2 was
fabricated as follows, and electrolysis performance of carbon
dioxide was examined. First, on carbon paper on which a porous
layer was provided, a cathode to which gold nanoparticle-supported
carbon particles were applied was produced by the following
process. A coating solution in which the gold
nanoparticle-supported carbon particles and pure water, a Nafion
solution, and ethylene glycol were mixed was produced. An average
particle diameter of the gold nanoparticle was 8.7 nm, and a
supported amount thereof was 18.9 mass %. An air brush was filled
with this coating solution, spray coating was performed using Ar
gas on the carbon paper on which the porous layer was provided.
Flowing water washing was performed by pure water for 30 minutes
after the coating, and thereafter organic matter such as ethylene
glycol was removed by oxidation through immersing in a hydrogen
peroxide solution. This was cut into a size of 2.times.2 cm to be
set as the cathode. Note that a coating amount of Au was estimated
at about 0.2 mg/cm.sup.2 from a mixing amount of the gold
nanoparticles and the carbon particles in the coating solution.
[0053] For an anode, an electrode in which IrO.sub.2 nanoparticles
which became a catalyst were applied to Ti mesh was used. As the
anode, the one in which IrO.sub.2/Ti mesh was cut into 2.times.2 cm
was used.
[0054] The electrolysis cell 2 was produced by being stacked in
order of the cathode current collector 24, the CO.sub.2 gas flow
path 23 (the third flow path plate 28), the cathode 22, the cathode
solution flow path 21 (the second flow path plate 25), the
separator 30, the anode 11, the anode solution flow path 12 (the
first flow path plate 14), and the anode current collector 13 from
the top, being sandwiched by the support plates not illustrated,
and further being tightened by the bolts, as illustrated in FIG. 2.
For the separator 30, an anion exchange membrane (brand name:
Selemion, manufactured by ASAHI GLASS CO., LTD.) was used. The
IrO.sub.2/Ti mesh of the anode 11 was brought in close contact with
the anion exchange membrane. A thickness of the cathode solution
flow path 21 was set to 1 mm. Note that an evaluation temperature
was set to room temperature.
[0055] The electrolytic device illustrated in FIG. 1 was operated
under the following condition. CO.sub.2 gas was supplied to the
CO.sub.2 gas flow path of the electrolysis cell at 20 sccm, an
aqueous potassium hydroxide solution (concentration 1 M KOH) was
introduced to the cathode solution flow path at a flow rate of 5
mL/min, and the aqueous potassium hydroxide solution (concentration
1 M KOH) was introduced to the anode solution flow path at a flow
rate of 20 mL/min. A differential pressure between the CO.sub.2 gas
and the cathode solution was controlled so that a pressure of the
CO.sub.2 gas was 2.5 kPa larger than a pressure of the cathode
solution, so as to adjust a proportion of a reduction product.
Next, a 600 mA constant current (constant current density 150
mA/cm.sup.2) was passed between the anode and the cathode using the
power supply, an electrolytic reaction of CO.sub.2 was performed,
and a cell voltage, an anode potential, and a cathode potential at
that time were measured. Note that a Hg/HgO reference electrode
(+0.098 V vs. SHE) was used for potential measurement, and pH was
set to 13.65 to calculate an overvoltage. Part of gas outputted
from the CO.sub.2 gas flow path was collected, and production
amounts of CO gas to be produced by a reduction reaction of
CO.sub.2 and H.sub.2 gas to be produced by a reduction reaction of
water were analyzed by a gas chromatograph. From these gas
production amounts, a partial current density and Faraday's
efficiency which is a ratio between the entire current density and
the partial current density of CO or H.sub.2 were calculated.
[0056] FIG. 10 illustrates time changes in the cell voltage, the
anode potential, and the cathode potential. Table 1 presents an
average value of the cell voltage, an anode overvoltage, and a
cathode overvoltage between 300 seconds and 570 seconds when gas
collection is performed, and the Faraday's efficiency, the partial
current density, and electrolysis efficiency of CO and H.sub.2. As
presented in Table 1, good electrolysis performance having high
selectivity of CO, such as 2.76 V in the cell voltage, 83% in the
Faraday's efficiency of CO, and 40% in the electrolysis efficiency
of CO, was obtained.
TABLE-US-00001 TABLE 1 EXAMPLE 1 CELL VOLTAGE [V]* 2.76 ANODE
OVERVOLTAGE [V]* 0.53 CATHODE OVERVOLTAGE [V]* 0.39 CO FARADAY'S
EFFICIENCY [%] 83 H.sub.2 FARADAY'S EFFICIENCY [%] 13 CO PARTIAL
CURRENT DENSITY [mA/cm.sup.2] 125 H.sub.2 PARTIAL CURRENT DENSITY
[mA/cm.sup.2] 15.8 ELECTROLYSIS EFFICIENCY OF CO [%] 40
ELECTROLYSIS EFFICIENCY OF H.sub.2 [%] 5.6 ELECTROLYSIS EFFICIENCY
OF CO AND H.sub.2 [%] 46 *AVERAGE VALUE OF 300 s TO 570 s.
Reference Example 1
[0057] An electrolytic reaction of CO.sub.2 was performed similarly
to Example 1 except that the differential pressure between the
CO.sub.2 gas and the cathode solution was changed, and performance
was evaluated. The differential pressure between the CO.sub.2 gas
and the cathode solution was controlled at -0.6 kPa under a
condition in which the pressure of the cathode solution was larger.
Table 2 presents each of performance values found similarly to
Example 1. As presented in Table 2, low selectivity and low
electrolysis efficiency of CO as compared with Example 1, such as
3.12 V in the cell voltage, 21% in the Faraday's efficiency of CO,
and 9% in the electrolysis efficiency of CO were confirmed. From
these results, it was confirmed that improvement in the selectivity
and the electrolysis efficiency of CO was achieved by controlling
the differential pressure between the CO.sub.2 gas and the cathode
solution.
TABLE-US-00002 TABLE 2 REFERENCE EXAMPLE 1 CELL VOLTAGE [V]* 3.12
ANODE OVERVOLTAGE [V]* 0.51 CATHODE OVERVOLTAGE [V]* 0.43 CO
FARADAY'S EFFICIENCY [%] 21 H.sub.2 FARADAY'S EFFICIENCY [%] 25 CO
PARTIAL CURRENT DENSITY [mA/cm.sup.2] 31 H.sub.2 PARTIAL CURRENT
DENSITY [mA/cm.sup.2] 31.2 ELECTROLYSIS EFFICIENCY OF CO [%] 9
ELECTROLYSIS EFFICIENCY OF H.sub.2 [%] 9.9 ELECTROLYSIS EFFICIENCY
OF CO AND H.sub.2 [%] 19 *AVERAGE VALUE OF 300 s TO 570 s.
[0058] Note that configurations of the above-described embodiments
may be each applied in combination, and further may be partially
substituted. Herein, while certain embodiments of the invention
have been described, these embodiments have been presented by way
of example only, and are not intended to limit the scope of the
inventions. Indeed, the novel embodiments described herein may be
embodied in a variety of other forms; furthermore, various
omissions, substitutions, and changes in the form of the embodimens
described herein may be made without departing from the spirit of
the inventions. The accompanying claims and their equivalents are
intended to cover such forms or modifications as would fall within
the scope and spirit of the invention.
* * * * *