U.S. patent application number 14/690184 was filed with the patent office on 2015-08-06 for carbon dioxide reduction device and method for reducing carbon dioxide.
The applicant listed for this patent is PANASONIC CORPORATION. Invention is credited to Masahiro DEGUCHI, Hiroshi HASHIBA, Kazuhiro OHKAWA, Yuka YAMADA, Satoshi YOTSUHASHI.
Application Number | 20150218719 14/690184 |
Document ID | / |
Family ID | 51898052 |
Filed Date | 2015-08-06 |
United States Patent
Application |
20150218719 |
Kind Code |
A1 |
DEGUCHI; Masahiro ; et
al. |
August 6, 2015 |
CARBON DIOXIDE REDUCTION DEVICE AND METHOD FOR REDUCING CARBON
DIOXIDE
Abstract
A device for reducing CO.sub.2 by light, including: a cathode
chamber holding a first electrolyte solution that contains
CO.sub.2; an anode chamber holding a second electrolyte solution; a
proton conducting membrane disposed in a connecting portion between
these chambers; a cathode electrode; and an anode electrode. The
cathode electrode has a CO.sub.2 reduction reaction region composed
of a metal or a metal compound, and the anode electrode has a
photochemical reaction region composed of nitride semiconductors.
The photochemical reaction region of the anode electrode has a
multilayer structure of a GaN layer and an Al.sub.xGa.sub.1-xN
layer containing Mg (0<x.ltoreq.0.25). The content of Mg in the
Al.sub.xGa.sub.1-xN layer is 1.times.10.sup.15 or more and
1.times.10.sup.19 or less in terms of the number of Mg atom per
cm.sup.3. The anode electrode is disposed in such a manner that the
Al.sub.xGa.sub.1-xN layer can be exposed to light.
Inventors: |
DEGUCHI; Masahiro; (Osaka,
JP) ; YOTSUHASHI; Satoshi; (Osaka, JP) ;
HASHIBA; Hiroshi; (Osaka, JP) ; YAMADA; Yuka;
(Nara, JP) ; OHKAWA; Kazuhiro; (Saitama,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
PANASONIC CORPORATION |
Osaka |
|
JP |
|
|
Family ID: |
51898052 |
Appl. No.: |
14/690184 |
Filed: |
April 17, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/JP2014/002526 |
May 13, 2014 |
|
|
|
14690184 |
|
|
|
|
Current U.S.
Class: |
205/340 ;
204/252 |
Current CPC
Class: |
B01J 23/72 20130101;
B01J 27/24 20130101; C25B 9/08 20130101; C25B 11/0478 20130101;
B01J 23/75 20130101; B01J 23/745 20130101; B01J 23/52 20130101;
B01J 35/004 20130101; C25B 3/04 20130101; C25B 15/00 20130101; C25B
1/003 20130101 |
International
Class: |
C25B 15/00 20060101
C25B015/00; C25B 1/00 20060101 C25B001/00; C25B 3/04 20060101
C25B003/04 |
Foreign Application Data
Date |
Code |
Application Number |
May 13, 2013 |
JP |
2013-100944 |
Claims
1. A carbon dioxide reduction device for reducing carbon dioxide by
light energy, comprising: a cathode chamber holding a first
electrolyte solution that contains carbon dioxide; an anode chamber
holding a second electrolyte solution, connected to the cathode
chamber; a proton conducting membrane that is disposed in a
connecting portion between the anode chamber and the cathode
chamber so as to serve as a separator between the first electrolyte
solution and the second electrolyte solution and to conduct
hydrogen ions between the first and second electrolyte solutions; a
cathode electrode disposed in the cathode chamber so as to be in
contact with the first electrolyte solution; and an anode electrode
disposed in the anode chamber so as to be in contact with the
second electrolyte solution, wherein the cathode electrode has a
carbon dioxide reduction reaction region that is in contact with
the first electrolyte solution and is composed of a metal or a
metal compound, the anode electrode has a photochemical reaction
region that is in contact with the second electrolyte solution and
is composed of nitride semiconductors, the photochemical reaction
region of the anode electrode has a multilayer structure of a GaN
layer and an Al.sub.xGa.sub.1-xN layer containing Mg
(0<x.ltoreq.0.25), a content of Mg in the Al.sub.xGa.sub.1-xN
layer is 1.times.10.sup.15 or more and 1.times.10.sup.19 or less in
terms of the number of Mg atoms that are contained in a unit volume
(1 cm.sup.3) of the Al.sub.xGa.sub.1-xN layer, the anode electrode
is disposed in the anode chamber in such a manner that the
Al.sub.xGa.sub.1-xN layer in the photochemical reaction region can
be exposed to light, and the cathode electrode and the anode
electrode are electrically connected to each other without an
external power source interposed therebetween.
2. The carbon dioxide reduction device according to claim 1,
wherein the content of Mg in the Al.sub.xGa.sub.1-xN layer is
1.times.10.sup.16 or more and 1.times.10.sup.18 or less in terms of
the number of Mg atoms that are contained in the unit volume (1
cm.sup.3) of the Al.sub.xGa.sub.1-xN layer.
3. The carbon dioxide reduction device according to claim 1,
wherein the x has a value of 0.10 or more and 0.15 or less.
4. The carbon dioxide reduction device according to claim 1,
wherein the GaN layer is composed of an n-type GaN.
5. The carbon dioxide reduction device according to claim 1,
wherein a metal oxide containing Ni is disposed on the
Al.sub.xGa.sub.1-xN layer in the photochemical reaction region.
6. The carbon dioxide reduction device according to claim 5,
wherein the metal oxide is in the form of fine particles.
7. The carbon dioxide reduction device according to claim 1,
wherein the metal constituting the reduction reaction region
includes at least one selected from copper, gold, silver, tantalum,
and indium.
8. The carbon dioxide reduction device according to claim 1,
wherein the first electrolyte solution is an aqueous solution
containing at least one electrolyte selected from potassium
bicarbonate, sodium bicarbonate, potassium chloride, and sodium
chloride.
9. A method for reducing carbon dioxide using a carbon dioxide
reduction device, wherein the device is the carbon dioxide
reduction device according to claim 1, and the method comprises the
step of irradiating the Al.sub.xGa.sub.1-xN layer in the
photochemical reaction region of the anode electrode with light
having a wavelength of 365 nm or less, with the first electrolyte
solution and the second electrolyte solution being held in the
cathode chamber and the anode chamber respectively, so as to allow
generation of electrons and hydrogen ions to proceed in the
photochemical reaction region and to allow a reaction of reducing
carbon dioxide contained in the first electrolyte solution to
proceed in the reduction reaction region of the cathode
electrode.
10. The method for reducing carbon dioxide according to claim 9,
further comprising the step of introducing a gas containing carbon
dioxide into the first electrolyte solution held in the cathode
chamber.
11. The method for reducing carbon dioxide according to claim 9,
wherein the step is performed with the device being placed at room
temperature and atmospheric pressure.
12. The method for reducing carbon dioxide according to claim 9,
wherein the reaction of reducing carbon dioxide produces at least
one selected from methanol, ethanol, acetaldehyde, formic acid,
methane, ethylene, and carbon monoxide.
Description
[0001] This is a continuation of International Application No.
PCT/JP2014/002526, with an international filing date of May 13,
2014, which claims the foreign priority of Japanese Patent
Application No. 2013-100944, filed on May 13, 2013, the entire
contents of both of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present disclosure relates to a carbon dioxide reduction
device for reducing carbon dioxide by light energy and to a method
for reducing carbon dioxide using this device.
[0004] 2. Description of Related Art
[0005] Carbon dioxide (CO.sub.2) is a substance that plays an
important role in reserving carbon atoms in the global carbon
cycle. Given that CO.sub.2 is a reservoir of carbon atoms, it is
also a substance that can serve as a carbon source for various
carbon compounds as typified by organic compounds. However, since
CO.sub.2 is an energetically very stable substance, the use of
CO.sub.2 as a carbon source requires a high level of reduction
energy.
[0006] Meanwhile, there are growing concerns about the increase in
the CO.sub.2 concentration in the atmosphere due to the consumption
of fossil fuels such as coal, petroleum, and natural gas, and about
the global climate change (so-called global warming) due to the
increase in the CO.sub.2 concentration. Reduction of CO.sub.2 by
light energy has attracted attention not only from the viewpoint of
using CO.sub.2 as a carbon source but also from the viewpoint of
reducing the consumption of fossil fuels by the use of carbon
compounds converted from CO.sub.2 so as to suppress the climate
change.
[0007] Various methods have been tried to reduce CO.sub.2 by light
energy. The following documents disclose methods for reducing
CO.sub.2 by light energy.
[0008] JP 55(1980)-105625 A and JP 2526396 B2 each disclose a
method of using an oxide semiconductor such as titania or zirconia
as a catalyst for CO.sub.2 reduction, more specifically, a method
of irradiating a suspension obtained by dispersing the oxide
semiconductor powder in water with light while introducing CO.sub.2
into the suspension.
[0009] JP 3876305 B2 and JP 4158850 B2 each disclose a method of
using a composite compound of a metal component and a semiconductor
component such as a titanium compound, as a catalyst for CO.sub.2
reduction, more specifically, a method of introducing CO.sub.2 into
a suspension obtained by dispersing powder of the composite
compound in water and then irradiating the suspension with
light.
[0010] JP 2010-064066 A discloses a method of using, as a catalyst
for CO.sub.2 reduction, a catalyst in which a semiconductor and a
base material such as an organic rhenium complex or an organic
ruthenium complex are joined so that they can donate and accept
electrons to and from each other, more specifically, a method of
introducing CO.sub.2 into a suspension obtained by dispersing
powder of the catalyst in an organic solvent and then irradiating
the suspension with light.
[0011] JP 2011-094194 A discloses a photochemical reaction device
including an oxidation reaction electrode for oxidizing water to
produce oxygen and a reduction reaction electrode electrically
connected to the oxidation reaction electrode and for reducing
carbon dioxide to synthesize a carbon compound. JP 2011-094194 A
also discloses titania, tungsten oxide, and tantalum oxynitride as
the materials for the oxidation reaction electrode, and the
catalyst disclosed in JP 2010-064066 A as the material for the
reduction reaction electrode. In the device disclosed in JP
2011-094194 A, both electrodes are irradiated with light.
[0012] JP 05(1993)-311476 A and JP 07(1995)-188961 A each disclose
an electrochemical reduction device including an anode electrode
made of an oxide semiconductor such as titania and a cathode
electrode having a specific structure made of a specific metal, and
a method for reducing CO.sub.2 on the cathode electrode by
irradiating the anode electrode with light in this device. The
device disclosed in JP 05(1993)-311476 A or JP 07(1995)-188961 A
requires an external power source such a solar cell or a
potentiostat disposed between the anode electrode and the cathode
electrode.
[0013] WO 2012/046374 A1 discloses a method for reducing CO.sub.2
on a cathode electrode by irradiating an anode electrode with light
in a device including the anode electrode whose surface has an area
of a nitride semiconductor such as gallium nitride or aluminum
gallium nitride and the cathode electrode made of a metal or a
metal compound. The method disclosed in WO 2012/046374 A1 does not
require an external power source between the anode electrode and
the cathode electrode.
[0014] WO 2006/082801 A1 discloses a nitride-based semiconductor
photocatalyst represented by the formula
Al.sub.yGa.sub.1-x-yIn.sub.xN (x-y.ltoreq.0.45,
0.ltoreq.x.ltoreq.1, and 0.ltoreq.y.ltoreq.1) for use as an
electrode for a device for producing acidic water and alkaline
water, although it does not disclose a method for reducing CO.sub.2
by light energy.
SUMMARY OF THE INVENTION
[0015] The widespread use of devices and methods for reducing
CO.sub.2 by light energy to convert CO.sub.2 into carbon compounds,
like plant photosynthesis, would be very useful for industrial
development and global environmental protection. However, in
conventional devices and methods, the efficiency of reducing
CO.sub.2 by light energy to convert it into carbon compounds
without the use of any external power source is not necessarily
high enough.
[0016] One non-limiting and exemplary embodiment of the present
disclosure provides a CO.sub.2 reduction device for reducing
CO.sub.2 by light energy and a method for reducing CO.sub.2 by
light energy, in which CO.sub.2 can be reduced without the use of
any external power source and CO.sub.2 can be converted into carbon
compounds with higher efficiency than conventional devices and
methods.
[0017] Additional benefits and advantages of the disclosed
embodiments will be apparent from the specification and Figures.
The benefits and/or advantages may be individually provided by the
various embodiments and features of the specification and drawings
disclosure, and need not all be provided in order to obtain one or
more of the same.
[0018] In one general aspect, the techniques disclosed here feature
a CO.sub.2 reduction device for reducing CO.sub.2 by light energy.
This device includes: a cathode chamber holding a first electrolyte
solution that contains CO.sub.2; an anode chamber holding a second
electrolyte solution, connected to the cathode chamber; a proton
conducting membrane that is disposed in a connecting portion
between the anode chamber and the cathode chamber so as to serve as
a separator between the first electrolyte solution and the second
electrolyte solution and to conduct hydrogen ions between the first
and second electrolyte solutions; a cathode electrode disposed in
the cathode chamber so as to be in contact with the first
electrolyte solution; and an anode electrode disposed in the anode
chamber so as to be in contact with the second electrolyte
solution. The cathode electrode has a CO.sub.2 reduction reaction
region that is in contact with the first electrolyte solution and
is composed of a metal or a metal compound. The anode electrode has
a photochemical reaction region that is in contact with the second
electrolyte solution and is composed of nitride semiconductors. The
photochemical reaction region of the anode electrode has a
multilayer structure of a GaN layer and an Al.sub.xGa.sub.1-xN
layer containing Mg (0<x.ltoreq.0.25). The content of Mg in the
Al.sub.xGa.sub.1-xN layer is 1.times.10.sup.15 or more and
1.times.10.sup.19 or less in terms of the number of Mg atoms that
are contained in a unit volume (1 cm.sup.3) of the
Al.sub.xGa.sub.1-xN layer. The anode electrode is disposed in the
anode chamber in such a manner that the Al.sub.xGa.sub.1-xN layer
in the photochemical reaction region can be exposed to light. The
cathode electrode and the anode electrode are electrically
connected to each other without an external power source interposed
therebetween.
[0019] These general and specific aspects may be implemented using
a system, a method, and a computer program, and any combination of
systems, methods, and computer programs.
[0020] The CO.sub.2 reduction device and the method for reducing
CO.sub.2 of the present disclosure are the device and method for
reducing CO.sub.2 by light energy, in which CO.sub.2 can be reduced
without the use of any external power source and CO.sub.2 can be
converted into carbon compounds with higher efficiency than
conventional devices and methods.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1A is a cross-sectional view schematically showing an
example of an anode electrode included in a CO.sub.2 reduction
device of the present disclosure.
[0022] FIG. 1B is a cross-sectional view schematically showing
another example of the anode electrode included in a CO.sub.2
reduction device of the present disclosure.
[0023] FIG. 1C is a cross-sectional view schematically showing
still another example of the anode electrode included in a CO.sub.2
reduction device of the present disclosure.
[0024] FIG. 1D is a cross-sectional view schematically showing even
still another example of the anode electrode included in a CO.sub.2
reduction device of the present disclosure.
[0025] FIG. 2A is a cross-sectional view schematically showing an
example different from the example given above, of the anode
electrode included in a CO.sub.2 reduction device of the present
disclosure.
[0026] FIG. 2B is a cross-sectional view schematically showing an
example different from the example given above, of the anode
electrode included in a CO.sub.2 reduction device of the present
disclosure.
[0027] FIG. 2C is a cross-sectional view schematically showing an
example different from the example given above, of the anode
electrode included in a CO.sub.2 reduction device of the present
disclosure.
[0028] FIG. 2D is a cross-sectional view schematically showing an
example different from the example given above, of the anode
electrode included in a CO.sub.2 reduction device of the present
disclosure.
[0029] FIG. 3 is a diagram schematically showing an example of the
CO.sub.2 reduction device of the present disclosure and an example
of the CO.sub.2 reduction method of the present disclosure using
this device.
[0030] FIG. 4 is a diagram showing the amounts of CO.sub.2 reduced
per unit time evaluated in Examples 1 to 3 and Comparative Example
1.
[0031] FIG. 5 is a diagram showing the relationship between the
content of Mg atoms in an AlGaN layer of the anode electrode and
the amount of CO.sub.2 reduced per unit time, evaluated in Example
7.
DETAILED DESCRIPTION
[0032] A first aspect of the present disclosure provides a CO.sub.2
reduction device for reducing CO.sub.2 by light energy, including:
a cathode chamber holding a first electrolyte solution that
contains CO.sub.2; an anode chamber holding a second electrolyte
solution, connected to the cathode chamber; a proton conducting
membrane that is disposed in a connecting portion between the anode
chamber and the cathode chamber so as to serve as a separator
between the first electrolyte solution and the second electrolyte
solution and to conduct hydrogen ions between the first and second
electrolyte solutions; a cathode electrode disposed in the cathode
chamber so as to be in contact with the first electrolyte solution;
and an anode electrode disposed in the anode chamber so as to be in
contact with the second electrolyte solution, wherein the cathode
electrode has a CO.sub.2 reduction reaction region that is in
contact with the first electrolyte solution and is composed of a
metal or a metal compound, the anode electrode has a photochemical
reaction region that is in contact with the second electrolyte
solution and is composed of nitride semiconductors, the
photochemical reaction region of the anode electrode has a
multilayer structure of a GaN layer and an Al.sub.xGa.sub.1-xN
layer containing Mg (0<x.ltoreq.0.25), a content of Mg in the
Al.sub.xGa.sub.1-xN layer is 1.times.10.sup.15 or more and
1.times.10.sup.19 or less in terms of the number of Mg atoms that
are contained in a unit volume (1 cm.sup.3) of the
Al.sub.xGa.sub.1-xN layer, the anode electrode is disposed in the
anode chamber in such a manner that the Al.sub.xGa.sub.1-xN layer
in the photochemical reaction region can be exposed to light, and
the cathode electrode and the anode electrode are electrically
connected to each other without an external power source interposed
therebetween.
[0033] A second aspect of the present disclosure provides the
CO.sub.2 reduction device according to the first aspect, wherein
the content of Mg in the Al.sub.xGa.sub.1-xN layer is
1.times.10.sup.16 or more and 1.times.10.sup.18 or less in terms of
the number of Mg atoms that are contained in the unit volume (1
cm.sup.3) of the Al.sub.xGa.sub.1-xN layer.
[0034] A third aspect of the present disclosure provides the
CO.sub.2 reduction device according to the first or second aspect,
wherein the x has a value of 0.10 or more and 0.15 or less.
[0035] A fourth aspect of the present disclosure provides the
CO.sub.2 reduction device according to any one of the first to
third aspects, wherein the GaN layer is composed of an n-type
GaN.
[0036] A fifth aspect of the present disclosure provides the
CO.sub.2 reduction device according to any one of the first to
fourth aspects, wherein a metal oxide containing Ni is disposed on
the Al.sub.xGa.sub.1-xN layer in the photochemical reaction
region.
[0037] A sixth aspect of the present disclosure provides the
CO.sub.2 reduction device according to the fifth aspect, wherein
the metal oxide is in the form of fine particles.
[0038] A seventh aspect of the present disclosure provides the
CO.sub.2 reduction device according to any one of the first to
sixth aspects, wherein the metal constituting the reduction
reaction region includes at least one selected from copper, gold,
silver, tantalum, and indium.
[0039] An eighth aspect of the present disclosure provides the
CO.sub.2 reduction device according to any one of the first to
seventh aspects, wherein the first electrolyte solution is an
aqueous solution containing at least one electrolyte selected from
potassium bicarbonate, sodium bicarbonate, potassium chloride, and
sodium chloride.
[0040] A ninth aspect of the present disclosure provides a method
for reducing CO.sub.2 using a CO.sub.2 reduction device, wherein
the device is the CO.sub.2 reduction device according to any one of
the first to eighth aspects, and the method includes the step of
irradiating the Al.sub.xGa.sub.1-xN layer in the photochemical
reaction region of the anode electrode with light having a
wavelength of 365 nm or less, with the first electrolyte solution
and the second electrolyte solution being held in the cathode
chamber and the anode chamber respectively, so as to allow
generation of electrons and hydrogen ions to proceed in the
photochemical reaction region and to allow a reaction of reducing
CO.sub.2 contained in the first electrolyte solution to proceed in
the reduction reaction region of the cathode electrode.
[0041] A tenth aspect of the present disclosure provides the method
for reducing CO.sub.2 according to the ninth aspect, further
including the step of introducing a gas containing carbon dioxide
into the first electrolyte solution held in the cathode
chamber.
[0042] An eleventh aspect of the present disclosure provides the
method for reducing CO.sub.2 according to the ninth or tenth
aspect, wherein the step is performed with the device being placed
at room temperature and atmospheric pressure.
[0043] A twelfth aspect of the present disclosure provides the
method for reducing CO.sub.2 according to any one of the ninth to
eleventh aspects, wherein the reaction of reducing carbon dioxide
produces at least one selected from methanol, ethanol,
acetaldehyde, formic acid, methane, ethylene, and carbon
monoxide.
[0044] Conventionally, methods for reducing CO.sub.2 by light
energy are known. In a method (see JP 55(1980)-105625 A, JP 2526396
B2, JP 3876305 B2, JP 4158850 B2, or JP 2010-064066 A) in which a
suspension is prepared by dispersing semiconductor powder in a
solution containing CO.sub.2 so as to allow the powder to act as a
catalyst for CO.sub.2 reduction, since carriers (electrons and
holes) generated in the catalyst by light irradiation are easily
recombined before they are used to reduce CO.sub.2, highly
efficient CO.sub.2 reduction cannot be achieved. On the other hand,
in a method (see JP 2011-094194 A, JP 05(1993)-311476 A, or JP
07(1995)-188961 A) in which an oxidation reaction electrode (anode
electrode) for oxidizing water to evolve oxygen and a reduction
reaction electrode (cathode electrode) electrically connected to
the anode electrode and for reducing carbon dioxide to synthesize a
carbon compound, since electrons and holes generated at the
electrode by light irradiation are immediately separated and their
recombination is suppressed, more efficient CO.sub.2 reduction is
expected.
[0045] Here, in the CO.sub.2 reduction reaction using light energy,
the amount of reduction products obtained by the CO.sub.2 reduction
depends on the level of photoelectromotive force generated in an
anode electrode as an electrode to be irradiated with light (i.e.,
a photochemical electrode) and on the amount of carriers generated
by photoexcitation of this electrode. The above-described
suppression of the recombination of carriers increases the amount
of carriers generated. However, when the anode electrode is made of
an oxide semiconductor such as titania, the energy level of
electrons photoexcited at the electrode does not sufficiently reach
an energy level necessary for reducing CO.sub.2. Therefore, it is
necessary to dispose an external power source such as a solar cell
or a potentiostat between the anode electrode and the cathode
electrode.
[0046] In contrast, in the device and the method of the present
disclosure, the use of a nitride semiconductor in the anode
electrode increases the energy level of excited electrons, and thus
allows the CO.sub.2 reduction reaction to proceed without the help
of an external power source to increase the potential. In addition,
in the device and the method of the present disclosure, a
multilayer structure of a GaN layer and an Al.sub.xGa.sub.1-xN
layer (0<x.ltoreq.0.25) containing a specific amount of Mg is
adopted. Thereby, the built-in potential formed at the interface
between these layers, that is, the magnitude of the internal
electric field, increases. This increase further suppresses the
recombination of carriers generated by photoexcitation, and thus
increases the level of the photoelectromotive force in the anode
electrode and the amount of carriers generated. This means that the
device and the method of the present disclosure each have a
mechanism for reducing the loss of carriers excited in the anode
electrode as a photochemical electrode and for increasing the level
of the photoelectromotive force in this electrode. Such a mechanism
has not been disclosed so far. The effect of this mechanism is
observed, for example, as an increase in the value of a current
flowing from the anode electrode to the cathode electrode upon
irradiation with light (i.e., an increase in the amount of carriers
supplied from the anode electrode to the cathode electrode). This
effect contributes to the achievement of more efficient CO.sub.2
reduction in the device and the method of the present
disclosure.
[0047] WO 2006/082801 A1 discloses an electrode made of
Mg-containing Al.sub.yGa.sub.1-x-yIn.sub.xN (x-y.ltoreq.0.45,
0.ltoreq.x.ltoreq.1, and 0.ltoreq.y.ltoreq.1). However, this
electrode is a cathode electrode, and in WO 2006/082801 A1, water
reduction reaction is allowed to proceed by irradiating the cathode
electrode with light. In contrast, in the device and the method of
the present disclosure, a Mg-containing Al.sub.yGa.sub.1-xN layer
is used in the anode electrode and water oxidation reaction is
allowed to proceed by irradiating the anode electrode with light.
Therefore, the device and the method according to the present
disclosure are based on a technical idea completely different from
that of the disclosure of WO 2006/082801 A1.
[0048] [CO.sub.2 Reduction Device]
[0049] (Anode Electrode) FIG. 1A to FIG. 1D each show an example of
an anode electrode used in the device and the method of the present
disclosure. In the anode electrode, carriers (electrons and holes)
are generated by light irradiation. The generated electrons move to
the cathode electrode electrically connected to the anode
electrode. The generated holes are used for water oxidation
reaction at the anode electrode, and hydrogen ions (protons)
generated in the reaction move diffusively toward the cathode
electrode through an anode-side electrolyte solution (second
electrolyte solution), a proton conducting membrane disposed in a
connecting portion between an anode chamber and a cathode chamber,
and a cathode-side electrolyte solution (first electrolyte
solution). At the cathode electrode, CO.sub.2 reacts with electrons
and protons so as to allow the CO.sub.2 reduction reaction at this
electrode to proceed. Focusing on this generation of carriers by
light irradiation, this anode electrode is a photochemical
electrode for CO.sub.2 reduction. Focusing on the formation of
oxygen by water oxidation reaction, this anode electrode is an
oxygen generating electrode.
[0050] An anode electrode 10a shown in FIG. 1A is a multilayer body
of a Mg-containing Al.sub.xGa.sub.1-xN layer 11, a GaN layer 12, an
electrically conductive substrate 13, and an electrode layer
14.
[0051] The Al.sub.xGa.sub.1-xN layer 11 is the layer where carriers
(electrons and holes) are generated by light irradiation. In other
words, upon absorption of light in the Al.sub.xGa.sub.1-xN layer
11, photoexcitation occurs therein and thereby carriers are
generated. The generated carriers contribute to oxidation-reduction
reaction as described above. The holes generated in the
Al.sub.xGa.sub.1-xN layer 11 move to the surface of the anode
electrode 10a, typically to the surface of the Al.sub.xGa.sub.1-xN
layer 11, and oxidizes water in contact with the anode electrode
10a to produce protons and oxygen. The protons thus produced move
diffusively in the second electrolyte solution with which the anode
electrode 10a is in contact, and oxygen as a gas leaves the anode
electrode 10a.
[0052] The value of the band gap, that is, the width of the band
gap of the Al.sub.xGa.sub.1-xN layer 11 is 3.4 eV or more.
Therefore, the Al.sub.xGa.sub.1-xN layer 11 of the anode electrode
10a needs to be irradiated with light having a wavelength of 365 nm
or less whose energy is equal to or higher than that corresponding
to this band gap.
[0053] The content (doping concentration) of Mg in the
Al.sub.xGa.sub.1-xN layer 11 is 1.times.10.sup.15 or more and
1.times.10.sup.19 or less in terms of the number of Mg atoms that
are contained in a unit volume (1 cm.sup.3) of the
Al.sub.xGa.sub.1-xN layer 11 (hereinafter also referred to as "the
number of atoms per cm.sup.3").
[0054] Desirably, the content of Mg in the Al.sub.xGa.sub.1-xN
layer 11 is 1.times.10.sup.16 or more and 1.times.10.sup.18 or less
in terms of the number of Mg atoms that are contained in the unit
volume (1 cm.sup.3) of the Al.sub.xGa.sub.1-xN layer 11. In this
case, the effect obtained by adding Mg is further enhanced.
Specifically, the photoelectromotive force and the carrier
utilization efficiency in the Al.sub.xGa.sub.1-xN layer 11 are
enhanced, and thereby the efficiency of the CO.sub.2 reduction in
the device and the method of the present disclosure is further
enhanced.
[0055] When the content of Mg in the Al.sub.xGa.sub.1-xN layer is
less than 1.times.10.sup.15 in terms of the number of atoms per
cm.sup.3, the effect of adding Mg cannot be obtained. On the other
hand, when the content of Mg in the Al.sub.xGa.sub.1-xN layer is
more than 1.times.10.sup.19 in terms of the number of atoms per
cm.sup.3, the characteristics of the Al.sub.xGa.sub.1-xN layer
change and the photoelectromotive force and the carrier utilization
efficiency rather decrease in this layer (that is, in the anode
electrode 10a).
[0056] The Al.sub.xGa.sub.1-xN constituting the Al.sub.xGa.sub.1-xN
layer 11 has a composition satisfying the formula
0<x.ltoreq.0.25. The value of x in this range is suitable when a
readily available light source (such as the sun and a xenon lamp)
is used for irradiation of light having a wavelength of 365 nm or
less. The value of x is desirably 0.10 or more and 0.15 or less.
This desirable range of x values is particularly suitable when a
common xenon lamp is used for the above-mentioned light
irradiation. It should be understood that a xenon lamp may also be
used as a light source when the x value is outside this desirable
range.
[0057] The depth in the Al.sub.xGa.sub.1-xN layer 11 that the light
having a wavelength of 365 nm or less can reach (i.e., the distance
from the irradiated surface of this layer 11) is approximately 100
nm, although it depends on the band gap value of
Al.sub.xGa.sub.1-xN. The depth (i.e., the thickness of the light
absorption region of the Al.sub.xGa.sub.1-xN layer 11) is parallel
to the irradiated surface of the layer 11. In view of this, the
thickness of the Al.sub.xGa.sub.1-xN layer 11 is desirably 70 nm or
more and 1000 nm or less, and more desirably 80 nm or more and 200
nm or less.
[0058] The GaN layer 12 is the layer for enhancing the carrier
utilization efficiency in the Al.sub.xGa.sub.1-xN layer 11 (that
is, in the anode electrode 10a) based on the multilayer structure
of the GaN layer 12 and the Al.sub.xGa.sub.1-xN layer 11.
Presumably, an increase in the built-in potential formed at the
interface between the Al.sub.xGa.sub.1-xN layer 11 and the GaN
layer 12 contributes to this enhancement.
[0059] Addition of an impurity (dopant) can reduce the electrical
resistance of the GaN layer 12. This GaN layer 12 can serve as an
electron conducting layer for efficiently transporting electrons
that are one type of the carriers generated in the
Al.sub.xGa.sub.1-xN layer 11 by light irradiation. In this case, in
the above-described multilayer structure of the anode electrode
10a, the light absorbing layer (Al.sub.xGa.sub.1-xN layer 11) and
the electron conducting layer (GaN layer 12) are functionally
separated. This separation promotes the extraction and transport of
the carriers (electrons) generated in the light absorbing layer,
and thus an even higher level of photoelectromotive force and an
even higher carrier utilization efficiency are achieved.
[0060] In view of the function as an electron conducting layer, the
GaN layer 12 is desirably a layer having a lower electrical
resistance than the Al.sub.xGa.sub.1-xN layer 11. This GaN layer 12
is a layer converted to n-type by addition of an impurity, that is,
a layer composed of n-type GaN. The impurity (dopant) is silicon
(Si), for example. The n-type includes n+-type.
[0061] The anode electrode 10a has a photochemical reaction region
that is composed of nitride semiconductors and that comes into
contact with the second electrolyte solution when it is
incorporated into a CO.sub.2 reduction device. When the
Al.sub.xGa.sub.1-xN layer 11 in this region is irradiated with
light, generation of carriers proceeds. The structure of the anode
electrode 10a is not limited as long as the photochemical reaction
region has a multilayer structure of the Al.sub.xGa.sub.1-xN layer
11 and the GaN layer 12. In the anode electrode 10a shown in FIG.
1A, the above-described multilayer structure is formed over the
entire electrode 10a, and one of the principal surfaces of the
electrode 10a can serve entirely as a photochemical reaction
region. The above-described multilayer structure may be formed in a
part of the anode electrode 10a, which means that a part of the
principal surface of the anode electrode 10a may be used as a
photochemical reaction region.
[0062] In the anode electrode 10a shown in FIG. 1A, the
above-described multilayer structure is formed on one of the
principal surfaces of the substrate 13. In an anode electrode for
use in the CO.sub.2 reduction device of the present disclosure, the
above-described multilayer structure may be formed on both
principal surfaces of the substrate 13.
[0063] The electrically conductive substrate 13 serves as a layer
not only for increasing the strength, shape retention, and ease of
handling of the anode electrode 10a but also efficiently
transporting electrons of the carriers generated in the
Al.sub.xGa.sub.1-xN layer 11.
[0064] The electrically conductive substrate 13 is composed of, for
example, single-crystal gallium nitride (GaN), gallium oxide
(Ga.sub.2O.sub.3), single-crystal silicon (Si), silicon carbide
(SiC), zinc oxide (ZnO), or zirconium boride (ZrB.sub.2).
[0065] The anode electrode for use in the device and the method of
the present disclosure does not necessarily need a substrate like
the electrically conductive substrate 13. The device can use an
anode electrode including the electrically conductive substrate 13,
as needed.
[0066] The electrode layer 14 is a layer composed of an
electrically conductive material, and serves as a terminal for
extracting, from the anode electrode 10a, electrons of the carriers
generated in the Al.sub.xGa.sub.1-xN layer 11. The structure of the
electrode layer 14 is not limited as long as it serves as the
terminal. As for the position of the electrode layer 14, in the
example shown in FIG. 1A, the electrode layer 14 is formed on the
entire surface of the electrically conductive substrate 13 opposite
to the surface thereof facing the GaN layer 12. However, the
electrode layer 14 may be formed on a part of this opposite
surface. The electrode layer 14 may be formed on the surface of the
electrically conductive substrate 13 facing the GaN layer 12 in
such a manner that the electrode layer 14 and the GaN layer 12 do
not overlap each other, or the electrode layer 14 may be formed on
the GaN layer 12 in such a manner that the electrode layer 14 and
the Al.sub.xGa.sub.1-xN layer 11 do not overlap each other (for the
latter case, see FIG. 2A).
[0067] The electrode layer 14 is composed of, for example, a metal
or a metal compound. The metal is, for example, gold (Au), silver
(Ag), copper (Cu), aluminum (Al), titanium (Ti), or an alloy of
these metals.
[0068] The electrode layer 14 may be composed of a single
electrically conductive film or may be a multilayer body composed
of a plurality of electrically conductive films. The material
constituting the electrically conductive film is, for example, the
above-mentioned metal or metal compound.
[0069] When the electrically conductive substrate 13 is composed of
a semiconductor or when the electrode layer 14 is formed on the
semiconductor GaN layer 12, it is desirable that the electrode
layer 14 or the surface (or the film) of the electrode layer 14
that is in contact with the electrically conductive substrate 13 or
the GaN layer 12 be composed of a material with a low interfacial
resistance to these semiconductors. This material is Ti, for
example.
[0070] The anode electrode for use in the device and the method of
the present disclosure does not necessarily need the electrode
layer 14. The device can use an anode electrode including the
electrode layer 14, as needed.
[0071] In the device and the method of the present disclosure, the
Al.sub.xGa.sub.1-xN layer 11 in the photochemical reaction region
is irradiated with light. Therefore, when the Al.sub.xGa.sub.1-xN
layer 11 serves as the irradiated surface of the anode electrode
10a, the efficiency of the carrier generation is high. When the
Al.sub.xGa.sub.1-xN layer 11 is used as the irradiated surface of
the anode electrode 10a, the Al.sub.xGa.sub.1-xN layer 11, the GaN
layer 12, the electrically conductive substrate 13, and the
electrode layer 14 are stacked in this order from the irradiated
surface of the anode electrode 10a.
[0072] In the anode electrode for use in the device and the method
of the present disclosure, a metal oxide may be disposed on the
Al.sub.xGa.sub.1-xN layer 11 in the photochemical reaction region.
This metal oxide serves as a co-catalyst for increasing the
efficiency of oxygen generation in the photochemical reaction
region of the anode electrode. This metal oxide also serves as a
protective layer of the Al.sub.xGa.sub.1-xN layer 11. The metal
oxide contains, for example, nickel (Ni) and desirably contains Ni
as a main component. The main component refers to a component whose
content is the highest. The content is usually 50 wt. % or more,
desirably 60 wt. % or more, and more desirably 70 wt. % or more.
The metal oxide may also be nickel oxide (NiO.sub.x, typically NiO
or Ni.sub.2O.sub.3).
[0073] FIG. 1B to FIG. 1D each show an example of the anode
electrode in which a metal oxide is disposed on the
Al.sub.xGa.sub.1-xN layer 11 in the photochemical reaction
region.
[0074] An anode electrode 10b shown in FIG. 1B has the same
structure as the anode electrode 10a of FIG. 1A, except that a
surface coating layer 15 composed of the above-mentioned metal
oxide is disposed on the Al.sub.xGa.sub.1-xN layer 11 so as to
cover the layer 11. This surface coating layer 15 is a layer having
the property of transmitting at least a part of light within a
bandwidth of wavelengths of 365 nm or less. In order to ensure this
light transmitting property, the thickness of the surface coating
layer 15 is desirably 10 nm or less. The surface coating layer 15
may contain fine particles of a metal or a metal oxide. In this
case, the function as a co-catalyst and the function as a
protective layer of the surface coating layer 15 are enhanced. The
metal is, for example, Ni or a Ni alloy containing Ni as a main
component. The metal oxide is, for example, the above-mentioned
metal oxide, and may be the same as or different from the metal
oxide of the surface coating layer 15.
[0075] An anode electrode 10c shown in FIG. 1C has the same
structure as the anode electrode 10a of FIG. 1A, except that a
surface coating layer 15 composed of the above-mentioned metal
oxide is disposed on the Al.sub.xGa.sub.1-xN layer 11 so as to
partially cover the layer 11 (to partially expose the layer 11).
The surface coating layer 15 of the anode electrode 10c is the same
as the surface coating layer 15 of the anode electrode 10b of FIG.
1B, except that the surface of the Al.sub.xGa.sub.1-xN layer 11 is
covered in a different manner. In the anode electrode 10c, a
plurality of surface coating layers 15 of various shapes and sizes
may be arranged regularly or randomly, or a plurality of surface
coating layers 15 of the same shape and size may be arranged
regularly or randomly.
[0076] An anode electrode 10d shown in FIG. 1D has the same
structure as the anode electrode 10a of FIG. 1A, except that fine
particles 16 composed of the above-mentioned metal oxide are
arranged on the Al.sub.xGa.sub.1-xN layer 11. In the anode
electrode 10d, the function of the metal oxide as a co-catalyst is
further enhanced. For more detailed explanation of the metal oxide
that is disposed on the Al.sub.xGa.sub.1-xN layer 11 of the anode
electrode and is in the form of fine particles, see U.S. patent
application Ser. No. 13/453,669 filed by the present inventors. The
specification of this US patent application is incorporated herein
by reference.
[0077] In the anode electrode 10d, a plurality of fine particles 16
of various shapes and sizes may be arranged regularly or randomly,
or a plurality of fine particles 16 of the same shape and size may
be arranged regularly or randomly.
[0078] Both the surface coating layer(s) 15 and the fine particles
16 may be disposed on the Al.sub.xGa.sub.1-xN layer 11.
[0079] The method for forming the anode electrode is not limited.
The Al.sub.xGa.sub.1-xN layer 11, the GaN layer 12, and the
electrode layer 14 can each be formed on the substrate 13 by a
known thin film forming technique. The thin film forming technique
is not limited to a specific one. In order to form the GaN layer 12
on the substrate 13 and to form the Al.sub.xGa.sub.1-xN layer 11 on
the GaN layer 12 thus formed, for example, metal-organic
vapor-phase epitaxy, molecular-beam epitaxy, or sputtering can be
used. In order to form the electrode layer 14, for example, vacuum
deposition, electron beam evaporation, or sputtering can be
used.
[0080] In order to dispose the metal oxide (the surface coating
layer(s) 15 and/or the fine particles 16) on the
Al.sub.xGa.sub.1-xN layer 11, for example, a method (metal particle
coating) in which a solution containing metal oxide particles, such
as a slurry solution in which the particles are dispersed, is
applied, heated and dried, or a method (metal organic compound
decomposition) in which a solution of an organic metal compound is
applied by spin coating or the like, and the compound is heated and
decomposed, can be used.
[0081] In the case where the anode electrode for use in the device
and the method of the present disclosure includes a substrate, the
substrate may be the electrically conductive substrate shown in
each of FIG. 1A to FIG. 1D or an insulating substrate composed of
an insulating material. The insulating substrate serves as a layer
for increasing the strength, shape retention, and ease of handling
of the anode electrode. FIG. 2A to FIG. 2D each show an example of
the anode electrode including an insulating substrate.
[0082] Anode electrodes 20a, 20b, 20c, and 20d shown in FIG. 2A,
FIG. 2B, FIG. 2C, and FIG. 2D have the same structures as the anode
electrodes 10a, 10b, 10c, and 10d shown in FIG. 1A, FIG. 1B, FIG.
1C, and FIG. 1D, respectively, except that an insulating substrate
23 is used instead of the electrically conductive substrate 13 and
that the electrode layer 14 serving as a terminal for extracting
electrons generated in the Al.sub.xGa.sub.1-xN layer 11 from the
anode electrode is disposed on the GaN layer 12 instead of on the
substrate 23 due to its insulating property.
[0083] The insulating substrate 23 is composed of, for example,
sapphire (typically, single-crystal sapphire) or high-resistance
silicon.
[0084] The method for forming the anode electrodes 20a to 20d each
including the insulating substrate 23 is the same as the method for
forming the anode electrodes 10a to 10d each including the
electrically conductive substrate 13.
[0085] The shape of the anode electrode is not limited, but, it may
be plate-shaped.
[0086] The anode electrode may optionally include a member other
than the above-described members as long as it has the
above-described photochemical reaction region where carriers
(electrons and holes) are generated by light irradiation, the
generated electrons can be extracted from the anode electrode in
order to supply them to the cathode electrode, and the generated
holes and water react with each other in the photochemical reaction
region to produce oxygen and protons.
[0087] (Cathode Electrode)
[0088] The cathode electrode is composed of a metal or a metal
compound. The cathode electrode has a structure capable of
receiving electrons generated by photoexcitation in the anode
electrode. The cathode electrode has a CO.sub.2 reduction reaction
region where CO.sub.2 contained in the first electrolyte solution
is reduced by reaction between the electrons received and protons
contained in the first electrolyte solution. The cathode electrode
has a structure capable of supplying the received electrons to the
reduction reaction region. The structure of the cathode electrode
is not limited as long as these conditions are satisfied. For
example, the cathode electrode may include a portion composed of an
insulating material.
[0089] The metal that can constitute the cathode electrode, in
particular, the metal that can constitute the CO.sub.2 reduction
reaction region is, for example, at least one selected from Cu, Au,
Ag, tantalum (Ta), and indium (In). The metal may be an alloy. The
metal compound that can constitute the cathode electrode, in
particular, the metal compound that can constitute the CO.sub.2
reduction reaction region is, for example, at least one selected
from tantalum carbide and tantalum nitride. The cathode electrode
may include any of these metals or metal compounds, as the whole or
a part of the surface of the cathode electrode, only in the
CO.sub.2 reduction reaction region. In this case, the other part of
the cathode electrode is composed of an arbitrary electrically
conductive material and/or an arbitrary insulating material. The
other part is, for example, the substrate of the cathode electrode.
The substrate is composed of, for example, glass or glassy carbon.
Glassy carbon has electrical conductivity. The cathode electrode
may have a structure in which particles or fine particles of the
metal or the metal compound are dispersedly arranged on the surface
of the substrate. In this case, these particles or fine particles
serve as the CO.sub.2 reduction reaction region.
[0090] In addition, when the reduction reaction region of the
cathode electrode is composed of any of these metals or the metal
compounds, it is possible to obtain, as a CO.sub.2 reduction
product, at least one selected from hydrocarbons such as methane
and ethylene, alcohols such as methanol and ethanol, organic acids
such as formic acid, aldehydes such as acetaldehyde, and carbon
monoxide. It is also possible to selectively produce a CO.sub.2
reduction product by selecting the type of the metal or the metal
compound. For example, when the reduction reaction region is
composed of Cu, hydrocarbon and/or alcohol is obtained as a
CO.sub.2 reduction product. When the reduction reaction region is
composed of In, formic acid is obtained selectively as a CO.sub.2
reduction product. Presumably, this is because the state in which
CO.sub.2 molecules are adsorbed to the reduction reaction region
varies depending on the type of the metal constituting the
region.
[0091] The shape of the cathode electrode is not limited. For
example, the cathode electrode is plate-shaped. From the viewpoint
of increasing the effective reaction area, it is desirable that the
cathode electrode have the shape of a plate having fine
irregularities on its surface or a porous plate.
[0092] The method for forming the cathode electrode is not limited,
and any known method can be used. In the case where the
above-mentioned metal or metal compound is used only for the
CO.sub.2 reduction reaction region, for example, the region is
formed on the whole or a part of the surface of the substrate, and
in this case, any known thin film formation technique and fine
particle formation technique can be used to form the region.
[0093] (CO.sub.2 Reduction Device)
[0094] FIG. 3 shows an example of a CO.sub.2 reduction device of
the present disclosure. A device 300 of FIG. 3 includes a cathode
chamber 302, an anode chamber 305, and a proton conducting membrane
306. The cathode chamber 302 and the anode chamber 305 are
connected to each other in a connecting portion 313. The proton
conducting membrane 306 is disposed in the connecting portion 313
between the cathode chamber 302 and the anode chamber 305.
[0095] A first electrolyte solution 307 that contains CO.sub.2 is
held in the cathode chamber 302. A second electrolyte solution 308
is held in the anode chamber 305. A cathode electrode 301 is
disposed in the cathode chamber 302 so as to be in contact with the
first electrolyte solution 307. An anode electrode 304 is disposed
in the anode chamber 305 so as to be in contact with the second
electrolyte solution 308.
[0096] The cathode electrode 301 is the cathode electrode for use
in the device and method of the present disclosure, as described
above, having a CO.sub.2 reduction reaction region therein. The
anode electrode 304 is the anode electrode for use in the device
and method of the present disclosure, as described above, having a
photochemical reaction region therein. The cathode electrode 301 is
disposed in the cathode chamber 302 so that at least a part of the
CO.sub.2 reduction reaction region (desirably the entire CO.sub.2
reduction reaction region) in the cathode electrode 301 be in
contact with the first electrolyte solution 307. The anode
electrode 304 is disposed in the anode chamber 305 so that at least
a part of the photochemical reaction region (desirably the entire
photochemical reaction region) in the anode electrode 304 be in
contact with the second electrolyte solution 308. In the example
shown in FIG. 3, both the electrodes 301 and 304 are partially
immersed in the electrolyte solutions 307 and 308 respectively. The
electrodes 301 and/or the electrode 304 may be entirely immersed in
the electrolyte solution(s).
[0097] In addition, the anode electrode 304 is disposed in the
anode chamber 305 in such a manner that the Al.sub.xGa.sub.1-xN
layer in the photochemical reaction region can be exposed to light.
In the example shown in FIG. 3, the anode chamber 305 is provided
with a window (not shown) at a part of the chamber, and the
photochemical reaction region of the anode electrode 304 is
irradiated with light emitted from a light source 303 and passing
through the window.
[0098] The cathode electrode 301 and the anode electrode 304 are
electrically connected to each other by electrode terminals 310 and
311 and a wire 312 connecting the terminals 310 and 311. There is
no external power source such as a solar cell or a potentiostat
connected between the cathode electrode 301 and the anode electrode
304. This means that the cathode electrode 301 and the anode
electrode 304 are electrically connected to each other without any
external power source interposed therebetween. The wire 312 serves
as a path of electrons generated by photoexcitation in the
photochemical reaction region of the anode electrode 304.
[0099] The proton conducting membrane 306 serves as a separator
between the first electrolyte solution 307 and the second
electrolyte solution 308 and separates the first and second
electrolyte solutions 307 and 308 from each other. This means that
in the device 300, the first electrolyte solution 307 in the
cathode chamber 302 and the second electrolyte solution 308 in the
anode chamber 305 are not mixed together as long as the proton
conducting membrane 306 normally functions. The electrolyte
solutions 307 and 308 and the proton conducting membrane 306 serve
as a proton diffusion path.
[0100] In the device 300, upon irradiation of the photochemical
reaction region of the anode electrode 304 as a photochemical
electrode with light, carriers (electrons and holes) are generated
and oxygen is generated. As described above, based on a multilayer
structure of a GaN layer and an Al.sub.xGa.sub.1-xN layer
containing a specific amount of Mg in the photochemical reaction
region, it is possible to achieve a high level of
photoelectromotive force and a high carrier utilization efficiency
in the anode electrode 304. Thereby, in the device 300, CO.sub.2
can be reduced to carbon compounds with high efficiency without the
use of any external power source. It should be understood that
these carbon compounds do not contain CO.sub.2 itself. Two or more
carbon compounds can be produced by CO.sub.2 reduction.
[0101] Electrons generated in the anode electrode 304 move to the
reduction reaction region of the cathode electrode 301 and react
with CO.sub.2 to reduce CO.sub.2 in that region. As described
above, the structure of this region composed of a metal or a metal
compound also contributes to the high CO.sub.2 reduction efficiency
in the device 300.
[0102] The shape of the cathode electrode 301 and that of the anode
electrode 304 are not particularly limited. The device 300 may two
or more cathode electrodes 301 and/or two or more anode electrodes
304.
[0103] The material constituting the cathode chamber 302 and that
constituting the anode chamber 305 are not limited as long as they
are not significantly corroded by the electrolyte solutions held in
these chambers. The material is, for example, a metal like
stainless steel, a glass, a resin, or a composite material of
these. However, for the material of the anode chamber 305, light
irradiation of the photochemical reaction region of the anode
electrode 304 has to be considered. However, in the case where the
light source 303 is disposed inside the anode chamber 305, such a
consideration is not always necessary.
[0104] The internal shape of the cathode chamber 302 and that of
the anode chamber 305 are not particularly limited.
[0105] The cathode chamber 302 and/or the anode chamber 305 may be
structured to have a sealable interior space. The sealing of the
interior space of the chamber is achieved by a valve, for
example.
[0106] The first electrolyte solution 307 is not limited as long as
it can contain CO.sub.2, is proton-conductive, does not
significantly inhibit (desirably does not inhibit) CO.sub.2
reduction reaction at the cathode electrode 301, and does not
significantly corrode (desirably does not corrode) the cathode
electrode 301. The first electrolyte solution 307 is typically an
aqueous solution. The first electrolyte solution 307 is, for
example, an aqueous solution containing at least one electrolyte
selected from potassium bicarbonate, sodium bicarbonate, potassium
chloride, and sodium chloride.
[0107] The types of the carbon compounds generated by CO.sub.2
reduction and the ratio of the compounds generated may vary
depending on the type of the electrolyte.
[0108] The concentration of the electrolyte in the first
electrolyte solution 307 is desirably 1 mol/L or more, and more
desirably 3 mol/L or more. The upper limit of the concentration is
not particularly limited, and it is 5 mol/L, for example.
[0109] The first electrolyte solution 307 contains CO.sub.2. The
concentration of CO.sub.2 contained therein is not limited.
Desirably, the first electrolyte solution 307 containing CO.sub.2
dissolved therein is acidic.
[0110] The device 300 can be operated using the first electrolyte
solution 307 previously containing CO.sub.2. The device 300 can be
operated while a gas containing CO.sub.2 is supplied into the first
electrolyte solution 307. In the example shown in FIG. 3, the
device 300 is operated while a gas containing CO.sub.2 is supplied
into the first electrolyte solution 307 through a gas supply pipe
309. The gas containing CO.sub.2 may be pure CO.sub.2 (100%
CO.sub.2 gas).
[0111] The second electrolyte solution 308 is not limited as long
as it is proton-conductive, does not significantly inhibit
(desirably does not inhibit) the photochemical reaction at the
anode electrode 304, and does not significantly corrode (desirably
does not corrode) the anode electrode 304. The second electrolyte
solution 308 is typically an aqueous solution. The second
electrolyte solution 308 is, for example, an aqueous sodium
hydroxide solution.
[0112] The concentration of the electrolyte in the second
electrolyte solution 308 is desirably 1 mol/L or more, and more
desirably 5 mol/L or more. The upper limit of the concentration is
not particularly limited, and it is 8 mol/L, for example.
Desirably, the second electrolytic solution 308 is strongly
basic.
[0113] The material constituting the proton conducting membrane 306
is not limited as long as the membrane 306 is permeable to protons
and can serve as a separator between the first and second
electrolyte solutions. Desirably, the proton conducting membrane
306 is a membrane impermeable to the electrolytes contained in the
electrolyte solutions 307 and 308. The material is, for example, a
proton conducting polymer material, and a specific example thereof
is perfluorocarbon sulfonic acid like Nafion (registered
trademark).
[0114] It is only necessary that the proton conducting membrane 306
be thick enough to ensure the strength required to serve as a
separator between the first and second electrolyte solutions, and
the thickness of the membrane 306 is 50 to 200 .mu.m, for
example.
[0115] The light source 303 emits light having energy required to
allow generation of carriers by photoexcitation to proceed in the
photochemical reaction region of the anode electrode 304. More
specifically, the light source 303 emits light of a wavelength of
365 nm or less (light having a wavelength of 365 nm or less). The
light source 303 may emit continuous light containing light
component of wavelengths of 365 nm or less. Instead, the light
source 303 may be a light source, e.g., a laser, that emits
monochromatic light containing light components within a wavelength
range of 365 nm or less. Desirably, the light source 303 emits
light of a wavelength of 250 nm or more and 325 nm or less.
[0116] The light source 303 is, for example, a xenon lamp, a
deuterium lamp, a mercury lamp, or a metal halide lamp. Sunlight
also can be used as the light source 303.
[0117] The method for irradiating the photochemical reaction region
of the anode electrode 304 with light from the light source 303 is
not limited. In the case where the light source 303 is placed
outside the anode chamber 305, the anode chamber 305 needs to have
a window for introducing the light emitted from the light source
303 into the anode chamber 305. The light source 303 may be placed
inside the anode chamber 305.
[0118] The use of the device of the present disclosure is not
limited. The device of the present disclosure can be applied to any
use that requires or desires CO.sub.2 reduction. Specific examples
of the use include formation of carbon compounds from CO.sub.2 as a
carbon source, such as carbon monoxide and/or organic compounds
like alcohol, aldehyde, carboxylic acid, hydrocarbon, and formation
of oxygen.
[0119] From another aspect, specific examples of the use include
removal of CO.sub.2 from a closed space and supply of oxygen into
the space. The device of the present disclosure can also be applied
to reduction of CO.sub.2 in the atmosphere to suppress global
warming (not only direct reduction of CO.sub.2 but also reduction
of CO.sub.2 emissions by reducing fossil fuel consumption using
CO.sub.2 as a carbon source), production of oxygen (artificial
photosynthesis) as an alternative to plant photosynthesis, etc.
[0120] [CO.sub.2 Reduction Method]
[0121] The method of the present disclosure is a method for
reducing CO.sub.2 using the photochemical electrode described above
as an anode electrode. For example, in the method of the present
disclosure, CO.sub.2 is reduced by the device of the present
disclosure described above. Thereby, CO.sub.2 can be reduced more
efficiently than before not using any external power source but
using light energy.
[0122] The method of the present disclosure can be performed by the
CO.sub.2 reduction device 300 shown in FIG. 3. An example of the
method of the present disclosure is described with reference to
FIG. 3.
[0123] As shown in FIG. 3, the Al.sub.xGa.sub.1-xN layer 11 in the
photochemical reaction region of the anode electrode 304 is
irradiated with light having a wavelength of 365 nm or less, with
the first electrolyte solution 307 and the second electrolyte
solution 308 being held in the cathode chamber 302 and the anode
chamber 305 respectively, so as to allow generation of electrons
and protons to proceed in that region. Protons are generated by the
reaction between water and holes generated in the
Al.sub.xGa.sub.1-xN layer 11 in the photochemical reaction region.
With this reaction, a reaction of reducing CO.sub.2 contained in
the first electrolyte solution 307 by electrons generated in the
photochemical reaction region of the anode electrode 304 and
protons contained in the first electrolyte solution 307 is allowed
to proceed in the reduction reaction region of the cathode
electrode 301. The generation of electrons and protons at the anode
electrode 304 and the reduction of CO.sub.2 at the cathode
electrode 301 can proceed simultaneously.
[0124] Desirably, the irradiated light has a wavelength of 250 nm
or more and 325 nm or less.
[0125] The method of the present disclosure may further include the
step of introducing a gas containing CO.sub.2 into the first
electrolyte solution 307 held in the cathode chamber 302. The
method for supplying the gas containing CO.sub.2 into the first
electrolyte solution 307 is not limited. In the example shown in
FIG. 3, the gas containing CO.sub.2 is supplied into the first
electrolyte solution 307 through the gas supply pipe 309 having one
end immersed in the first electrolyte solution 307. This step may
be performed during the operation of the device 300. That is, it is
possible to reduce CO.sub.2 while supplying the gas containing
CO.sub.2 into the first electrolyte solution 307. This step may
also be performed before the device 300 is operated. Desirably, the
gas containing CO.sub.2 is supplied into the first electrolyte
solution 307 before the device 300 is operated so as to start the
operation of the device 300 after the first electrolyte solution
307 containing a sufficient amount of CO.sub.2 is prepared.
[0126] In the method of the present disclosure, at least one
selected from alcohols such as methanol and ethanol, aldehydes such
as acetaldehyde, organic acids such as formic acid, hydrocarbons
such as methane and ethylene, and carbon monoxide is obtained by
the above-described reaction of reducing CO.sub.2. The carbon
compound generated by the CO.sub.2 reduction can be selected by,
for example, the structure of the cathode electrode 301, the type
of the first electrolyte solution 307, etc.
[0127] In the method of the present disclosure, the CO.sub.2
reduction can be performed with the device 300 being placed at room
temperature and atmospheric pressure. This means that the method of
the present disclosure does not necessarily require a special
environment (such as a high-temperature and high-pressure
environment).
[0128] Any optional step may be performed in addition to the steps
described above as long as the effects of the present invention can
be obtained.
[0129] The application of the method of the present disclosure is
not limited. Specific examples of the application are as described
above as specific examples of the use of the device of the present
disclosure.
EXAMPLES
[0130] Hereinafter, the CO.sub.2 reduction device and the CO.sub.2
reduction method of the present disclosure are described in more
detail with reference to Examples. The device and the method of the
present disclosure are not limited by the following Examples.
Example 1
[0131] In Example 1, a multilayer body of an electrode layer, an
electrically conductive substrate, a GaN layer, and a Mg-containing
Al.sub.xGa.sub.1-xN layer was used as an anode electrode. On the
Al.sub.xGa.sub.1-xN layer of this multilayer body (on the surface
of the Al.sub.xGa.sub.1-xN layer opposite to the surface thereof
facing the GaN layer), fine particles of Ni-containing metal oxide
were dispersedly arranged, as shown in FIG. 1D. The electrically
conductive substrate was a single-crystal GaN substrate doped with
high-concentration Si (with a thickness of about 0.4 mm). The GaN
layer was a Si-doped n*-type GaN layer (with a thickness of 3.0
.mu.m and a Si doping concentration of 4.0.times.10.sup.18 in terms
of the number of atoms per cm.sup.3). The thickness of the
Al.sub.xGa.sub.1-xN layer was 100 nm, the value of x was 0.10, and
the Mg doping concentration was 1.0.times.10.sup.17 in terms of the
number of atoms per cm.sup.3. The metal oxide fine particles were
fine particles of nickel oxide (with a diameter of several tens of
nanometers to several micrometers), and they were dispersedly
arranged on the Al.sub.xGa.sub.1-xN layer in such a manner that the
Al.sub.xGa.sub.1-xN layer is partially exposed. A solution
containing a Ni compound dispersed therein was applied onto the
surface of the Al.sub.xGa.sub.1-xN layer and then fired, and
thereby the metal oxide fine particles were arranged on the surface
of this layer. The number of fine particles placed thereon was
about 1.times.10.sup.8 to 1.times.10.sup.10 per unit area of 1
cm.sup.2.
[0132] The GaN layer was formed on the single-crystal GaN substrate
by growing GaN thereon by metal-organic vapor-phase epitaxy. The
Mg-containing Al.sub.xGa.sub.1-xN layer was formed on the GaN layer
thus formed by growing Mg-containing Al.sub.xGa.sub.1-xN thereon by
metal-organic vapor-phase epitaxy.
[0133] The electrode layer was a multilayer body of Ti/Al/Au (with
a thickness of 500 nm). A multilayer body of the electrically
conductive substrate, the GaN layer, and the Mg-containing
Al.sub.xGa.sub.1-xN layer was formed, the nickel oxide fine
particles were arranged on the Al.sub.xGa.sub.1-xN layer of the
multilayer body, and then the electrode layer was formed on the
surface of the electrically conductive substrate opposite to the
surface thereof facing the GaN layer by electron beam evaporation.
The electrode layer was formed in such a manner that the Ti film
was in contact with the electrically conductive substrate to
increase the adhesion between the electrode layer and the
single-crystal GaN substrate and to reduce the interfacial
resistance therebetween.
[0134] On the other hand, a copper plate (with a thickness of 0.5
mm) was used as a cathode electrode.
[0135] A CO.sub.2 reduction device shown in FIG. 3 was fabricated
using the anode electrode and the cathode electrode prepared as
described above. More specific configuration and operation
conditions of the CO.sub.2 reduction device thus fabricated were as
follows.
[0136] Cathode Chamber
[0137] Cathode electrode: Copper plate (with a thickness of 0.5
mm)
[0138] First electrolyte solution: 180 cm.sup.3 of aqueous
potassium bicarbonate solution having a concentration of 3.0
mol/L
[0139] Area of the cathode electrode immersed in the first
electrolyte solution: about 4 cm.sup.2
[0140] Supply of CO.sub.2: CO.sub.2 was supplied into the first
electrolyte solution at a flow rate of 200 mL/minute for 30 minutes
through the gas supply pipe 309 shown in FIG. 3 before the anode
electrode was irradiated with light. After CO.sub.2 was supplied,
the cathode chamber was sealed to prevent leakage of CO.sub.2
outside the cathode chamber.
[0141] Anode Chamber
[0142] Anode electrode: Multilayer body prepared as above
[0143] Second electrolyte solution: 180 cm.sup.3 of aqueous sodium
hydroxide solution having a concentration of 5.0 mol/L
[0144] Light irradiation: A quartz glass window (not shown in FIG.
3) was provided in the anode chamber so as to irradiate the
Al.sub.xGa.sub.1-xN layer of the anode electrode with light from
outside the chamber.
[0145] Connection Between Anode Chamber and Cathode Chamber
[0146] The anode chamber and the cathode chamber were connected at
a distance of about 8 cm from each other. The area of the
connecting portion was about 12.5 cm.sup.3, and as a proton
conducting membrane serving as a separator between the first
electrolyte solution and the second electrolyte solution, a Nafion
membrane ("Nafion 117" with a thickness of about 180 .mu.m,
manufactured by DuPont) was disposed in the connecting portion.
[0147] Connection Between Anode Electrode and Cathode Electrode
[0148] The electrode layer of the anode electrode and the end of
the copper plate as the cathode electrode were electrically
connected by the wire 312 without any external power source like a
battery or a potentiostat disposed between these electrodes. An
ammeter was disposed between the anode electrode and the cathode
electrode to detect a current flowing between these electrodes upon
irradiation with light.
[0149] Light Source
[0150] As a light source, a xenon lamp (with a power of 300 W, a
light irradiation area of about 4 cm.sup.2, and an irradiation
light power of about 20 mW/cm.sup.2) was used. Light emitted from
this light source has a broad spectrum within a wavelength range of
365 nm or less.
[0151] In this reduction device, CO.sub.2 gas was supplied into the
cathode chamber and then the Al.sub.xGa.sub.1-xN layer of the anode
electrode was irradiated with light. As a result, a current flowing
from the cathode electrode to the anode electrode was detected,
that is, the flow of electrons from the anode electrode to the
cathode electrode was observed, and the evolution of a gas from the
surface of the Al.sub.xGa.sub.1-xN layer of the anode electrode was
observed. When the light irradiation was temporarily stopped, the
current was not detected and the evolution of the gas also stopped.
When the irradiation was resumed, the current was detected again
and the gas was evolved again. It was thus confirmed that upon
irradiation of the Al.sub.xGa.sub.1-xN layer of the anode electrode
with light, some kind of chemical reaction proceeds at the anode
electrode and the cathode electrode.
[0152] Next, the gas evolved at the anode electrode was found to be
oxygen when examined separately. After the light irradiation, in
order to identify carbon compounds contained in the first
electrolyte solution, the gas components of the compounds were
analyzed using a gas chromatograph (GC) (GC-4000 manufactured by GL
Sciences Inc.) and the liquid components of the compounds were
analyzed using a liquid chromatograph (LC) (LC-2010 manufactured by
Shimadzu Corporation) and GC with head-space sampler (HS-GC)
(GC-17A manufactured by Shimadzu Corporation and HS40 manufactured
by Perkin-Elmer Corporation). As a result, carbon monoxide and
formic acid were detected. It was thus confirmed that upon
irradiation of the Al.sub.xGa.sub.1-xN layer of the anode electrode
with light, CO.sub.2 contained in the first electrolyte solution in
the cathode chamber was reduced and carbon monoxide and formic acid
were produced. The amounts of carbon monoxide and formic acid
produced were determined by GC and LC. These amounts increased in
proportion to the time of light irradiation of the anode
electrode.
Comparative Example 1
[0153] A CO.sub.2 reduction device was fabricated in the same
manner as in Example 1, except that an Al.sub.xGa.sub.1-xN layer
not containing Mg was used for the anode electrode, and the device
thus fabricated was irradiated with light in the same manner as in
Example 1.
[0154] In Comparative Example 1, as in the case of Example 1, it
was confirmed that upon irradiation of the Al.sub.xGa.sub.1-xN
layer of the anode electrode with light, a gas was evolved from the
surface of the Al.sub.xGa.sub.1-xN layer of the anode electrode and
that carbon monoxide and formic acid were produced by the reduction
of CO.sub.2 contained in the first electrolyte solution in the
cathode chamber.
[0155] Next, the value of current flowing between the electrodes
during light irradiation in Example 1 was compared with that in
Comparative Example 1. As a result, the value of current in Example
1 was about twice the value of current in Comparative Example 1.
The amount of CO.sub.2 reduction products (carbon monoxide and
formic acid) produced in a given period of irradiation time in
Example 1 was compared with the amount of CO.sub.2 reduction
products produced in the same period of irradiation time in
Comparative Example 1. As a result, the production amount in
Example 1 was about twice the production amount in Comparative
Example 1. That is, the value of the reaction current and the
amount of the reaction products obtained when an anode electrode
having an Al.sub.xGa.sub.1-xN layer containing Mg was used were
about twice those obtained when an anode electrode having an
Al.sub.xGa.sub.1-xN layer not containing Mg was used, and thus it
was confirmed that CO.sub.2 can be reduced more efficiently by
using the former anode electrode.
Example 2
[0156] A CO.sub.2 reduction device was fabricated in the same
manner as in Example 1, except that fine particles of nickel oxide
were not arranged on the Al.sub.xGa.sub.1-xN layer of the anode
electrode, and the device thus fabricated was irradiated with light
in the same manner as in Example 1.
[0157] In Example 2, as in the case of Example 1, it was confirmed
that upon irradiation of the Al.sub.xGa.sub.1-xN layer of the anode
electrode with light, a gas was evolved from the surface of the
Al.sub.xGa.sub.1-xN layer of the anode electrode and that carbon
monoxide and formic acid were produced by the reduction of CO.sub.2
contained in the first electrolyte solution in the cathode
chamber.
[0158] Next, the value of current flowing between the electrodes
during light irradiation in Example 2 was compared with that in
Example 1. The value of current in Example 2 was almost equal to
that in Example 1. The amount of CO.sub.2 reduction products
(carbon monoxide and formic acid) produced in a given period of
irradiation time in Example 2 was compared with the amount of
CO.sub.2 reduction products produced in the same period of
irradiation time in Example 1. As shown in FIG. 4, the amount of
reduction products obtained in Example 2 was slightly smaller than
that in Example 1 but was sufficiently greater than that in
Comparative Example 1. FIG. 4 shows the amounts of CO.sub.2 reduced
(=the amount of CO.sub.2 reduction products produced) per unit time
in Examples 1 to 3, relative to the amount of CO.sub.2 reduced per
unit time in Comparative Example 1.
Example 3
[0159] In Example 3, a CO.sub.2 reduction device was fabricated in
the same manner as in Example 1, except that an insulating
substrate (single-crystal sapphire substrate with a thickness of
about 0.4 mm) was used instead of an electrically conductive
substrate as the substrate of the anode electrode and that the
electrode layer was placed on the GaN layer (see FIG. 2D) instead
of on the surface of the substrate opposite to the surface thereof
facing the GaN layer. The electrode layer was placed in such a
manner that the Ti film was in contact with the GaN layer to
increase the adhesion between the electrode layer and the GaN layer
and to reduce the interfacial resistance therebetween.
[0160] The Al.sub.xGa.sub.1-xN layer of the anode electrode in the
CO.sub.2 reduction device thus fabricated was irradiated with light
in the same manner as in Example 1. As a result, it was confirmed
that a gas was evolved from the surface of the Al.sub.xGa.sub.1-xN
layer of the anode electrode and that carbon monoxide and formic
acid were produced by the reduction of CO.sub.2 contained in the
first electrolyte solution in the cathode chamber.
[0161] Next, the value of current flowing between the electrodes
during light irradiation in Example 3 was compared with that in
Example 1. The value of current in Example 3 was almost equal to
that in Example 1. The amount of CO.sub.2 reduction products
(carbon monoxide and formic acid) produced in a given period of
irradiation time in Example 3 was compared with the amount of
CO.sub.2 reduction products produced in the same period of
irradiation time in Example 1. As shown in FIG. 4, the amount of
reduction products obtained in Example 3 was slightly smaller than
that in Example 1 but was sufficiently greater than that in
Comparative Example 1 and slightly greater than that in Example
2.
[0162] As shown in FIG. 4, it was confirmed that more efficient
CO.sub.2 reduction can be achieved by the use of the Mg-containing
Al.sub.xGa.sub.1-xN layer in the anode electrode.
Example 4
[0163] A CO.sub.2 reduction device was fabricated in the same
manner as in Example 1, except that as a cathode electrode, an
electrode obtained by dispersing copper fine particles (with a
diameter of 20 nm to 100 nm) on the surface of a glassy carbon
substrate (Glassy Carbon (registered trademark) with a thickness of
0.5 mm, manufactured by Tokai Carbon, Co., Ltd.) in such a manner
that part of the surface of the substrate was exposed was used
instead of a copper plate, and the device thus fabricated was
irradiated with light in the same manner as in Example 1. A
dispersion of a copper compound was applied onto the surface of the
substrate by spin coating, dried to remove organic components, and
then fired in a reducing atmosphere. Thereby, the copper fine
particles were placed on the surface of the substrate. The number
of the fine particles placed thereon was about 1.times.10.sup.8 to
4.times.10.sup.9 per unit area of 1 cm.sup.2.
[0164] In Example 4, as in the case of Example 1, it was confirmed
that upon irradiation of the Al.sub.xGa.sub.1-xN layer of the anode
electrode with light, a gas was evolved from the surface of the
Al.sub.xGa.sub.1-xN layer of the anode electrode and that carbon
monoxide and formic acid were produced by the reduction of CO.sub.2
contained in the first electrolyte solution in the cathode chamber.
The value of current flowing between the electrodes during light
irradiation in Example 4 was compared with that in Example 1. The
value of current in Example 4 was almost equal to that in Example
1.
[0165] When a cathode electrode in which instead of copper fine
particles, fine particles of a copper-nickel alloy containing
traces of nickel component were placed was used, almost the same
results as the case where copper fine particles were placed was
obtained.
Example 5
[0166] A CO.sub.2 reduction device was fabricated in the same
manner as in Example 1, except that an indium plate (with a
thickness of 0.5 mm) was used instead of a copper plate as a
cathode electrode, and the device thus fabricated was irradiated
with light in the same manner as in Example 1.
[0167] In Example 5, as in the case of Example 1, it was confirmed
that a gas was evolved from the surface of the Al.sub.xGa.sub.1-xN
layer of the anode electrode upon irradiation of the
Al.sub.xGa.sub.1-xN layer of the anode electrode. The value of
current flowing between the electrodes during light irradiation in
Example 5 was compared with that in Example 1. As a result, the
value of current in Example 5 was almost equal to that in Example
1. On the other hand, after the light irradiation, the carbon
compounds contained in the first electrolyte solution were analyzed
by GC and LC, and as a result, it was confirmed that most of the
compounds was formic acid. This means that formic acid was
selectively produced as a CO.sub.2 reduction product by using
indium as a cathode electrode.
Example 6
[0168] A CO.sub.2 reduction device was fabricated in the same
manner as in Example 1, except that an aqueous potassium chloride
solution (having a concentration of 3.0 mol/L) was used instead of
an aqueous potassium bicarbonate solution as a first electrolyte
solution, and the device thus fabricated was irradiated with light
in the same manner as in Example 1.
[0169] In Example 6, as in the case of Example 1, it was confirmed
that upon irradiation of the Al.sub.xGa.sub.1-xN layer of the anode
electrode with light, a gas was evolved from the surface of the
Al.sub.xGa.sub.1-xN layer of the anode electrode. The value of
current flowing between the electrodes during light irradiation in
Example 6 was compared with that in Example 1. The value of current
in Example 6 was almost equal to that in Example 1. On the other
hand, after the light irradiation, the carbon compounds contained
in the first electrolyte solution were analyzed by GC and LC, and
as a result, it was confirmed that ethylene, alcohols such as
ethanol, and acetaldehyde were produced in addition to carbon
monoxide and formic acid observed in Example 1.
[0170] Almost the same results were obtained when an aqueous sodium
chloride solution was used as the first electrolyte solution.
Example 7
[0171] In Example 7, a plurality of anode electrodes including
Al.sub.xGa.sub.1-xN layers having different contents (different
doping concentrations) of Mg atoms were prepared. CO.sub.2
reduction devices were fabricated in the same manner as in Example
1, except that the anode electrodes having different Mg contents
were used, and the devices thus fabricated were irradiated with
light in the same manner as in Example 1.
[0172] In Example 7, as in the case of Example 1, it was confirmed
that upon irradiation of the Al.sub.xGa.sub.1-xN layer of the anode
electrode with light, a gas was evolved from the surface of the
Al.sub.xGa.sub.1-xN layer of the anode electrode and that carbon
monoxide and formic acid were produced by the reduction of CO.sub.2
contained in the first electrolyte solution in the cathode
chamber.
[0173] On the other hand, when the devices were irradiated with
light for a given period of time, the amounts of CO.sub.2 reduction
products (carbon monoxide and formic acid), that is, the amounts of
CO.sub.2 reduced in that period of time varied depending on the
types of the anode electrodes (the Mg contents in the
Al.sub.xGa.sub.1-xN layers) prepared in Example 7. FIG. 5 shows the
relationship between the Mg content in the Al.sub.xGa.sub.1-xN
layer and the amount of CO.sub.2 reduced per unit time. In FIG. 5,
the vertical axis indicates the values relative to the value (=1)
of the amount of CO.sub.2 that was reduced when an
Al.sub.xGa.sub.1-xN layer having a Mg content of 0, that is, an
Al.sub.xGa.sub.1-xN layer not containing Mg was used in an anode
electrode (i.e., the amount of CO.sub.2 reduced in Comparative
Example 1).
[0174] As shown in FIG. 5, relative to the amount of CO.sub.2 that
was reduced when an Al.sub.xGa.sub.1-xN layer not containing Mg was
used, the amount of CO.sub.2 reduced in Example 7 began to increase
rapidly when the Mg content in the Al.sub.xGa.sub.1-xN layer
reached and exceeded 1.times.10.sup.15 in terms of the number of
atoms per cm.sup.3. The value of the current flowing between the
electrodes also began to increase rapidly with the rapid change in
the amount of reduced CO.sub.2. The amount of reduced CO.sub.2 and
the current value peaked when the Mg content in the
Al.sub.xGa.sub.1-xN layer was 1.times.10.sup.17 in terms of the
number of atoms per cm.sup.3, and the amount of reduced CO.sub.2
and the current value decreased as the Mg content further
increased. When the Mg content in the Al.sub.xGa.sub.1-xN layer
exceeded 1.times.10.sup.19 in terms of the number of atoms per
cm.sup.3, the amount of reduced CO.sub.2 and the current value
decreased rapidly. This is presumably because an excessive amount
of Mg contained in the Al.sub.xGa.sub.1-xN layer had changed the
characteristics of the Al.sub.xGa.sub.1-xN layer, which affected
the utilization efficiency of carriers generated by
photoexcitation. As shown in FIG. 5, from the viewpoint of the
amount of reduced CO.sub.2, the Mg content in the
Al.sub.xGa.sub.1-xN layer was desirably 1.times.10.sup.16 or more
and 1.times.10.sup.18 or less in terms of the number of atoms per
cm.sup.3. However, the optimum value of the Mg content
(1.times.10.sup.17 in terms of the number of atoms per cm.sup.3 in
FIG. 5) may vary depending of the value of x in the composition of
the Al.sub.xGa.sub.1-xN layer as a base material because the
optimum value is influenced by the composition and the
characteristics of that Al.sub.xGa.sub.1-xN layer.
Example 8
[0175] In Example 8, 4 types of anode electrodes including
Al.sub.xGa.sub.1-xN layers having different compositions (having
different x values) were prepared. CO.sub.2 reduction devices were
fabricated in the same manner as in Example 1, except that the
anode electrodes having different compositions were used, and the
devices thus fabricated were irradiated with light in the same
manner as in Example 1. The x values were 0.05, 0.10, 0.15, and
0.20, respectively.
[0176] In Example 8, as in the case of Example 1, it was confirmed
that upon irradiation of the Al.sub.xGa.sub.1-xN layer of the anode
electrode with light, a gas was evolved from the surface of the
Al.sub.xGa.sub.1-xN layer of the anode electrode and that carbon
monoxide and formic acid were produced by the reduction of CO.sub.2
contained in the first electrolyte solution in the cathode chamber.
When the devices of Example 8 were irradiated with light for a
given period of time, the amounts of CO.sub.2 reduction products
produced in these devices including the above-mentioned anode
electrodes were almost the same as the amount of CO.sub.2 reduction
products produced in Example 1.
[0177] As shown in Examples 1 to 8 and Comparative Example 1, it
was confirmed that the use of an anode electrode including a
photochemical reaction region composed of nitride semiconductors,
more specifically, having a multilayer structure of a GaN layer and
an Al.sub.xGa.sub.1-xN layer containing (doped with) a specific
amount of Mg atoms, makes it possible to increase the value of
reaction current obtained by irradiation of the anode electrode
with light and to reduce CO.sub.2 with high efficiency at a cathode
electrode.
[0178] The present disclosure may be embodied in other forms
without departing from the spirit or essential characteristics
thereof. The embodiments disclosed in this specification are to be
considered in all respects as illustrative and not limiting. The
scope of the present disclosure is indicated by the appended claims
rather than by the foregoing description, and all changes which
come within the meaning and range of equivalency of the claims are
intended to be embraced therein.
INDUSTRIAL APPLICABILITY
[0179] The device of the present disclosure can be applied to all
industries that require CO.sub.2 reduction or desire CO.sub.2
reduction. These industries include the space industry, more
specifically removal of CO.sub.2 from a spacecraft or a space
station in the extraterrestrial space. This device can also be
applied to the production of a wide variety of substances that can
be obtained by CO.sub.2 reduction, such as alcohol, aldehyde,
carboxylic acid, hydrocarbon, carbon monoxide, and oxygen. This
device can further be applied to reduction of CO.sub.2 in the
atmosphere to suppress global warming, production of oxygen as an
alternative to plant photosynthesis, etc.
* * * * *