U.S. patent application number 15/259466 was filed with the patent office on 2017-09-21 for electrochemical reaction 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, Arisa YAMADA.
Application Number | 20170268118 15/259466 |
Document ID | / |
Family ID | 59855363 |
Filed Date | 2017-09-21 |
United States Patent
Application |
20170268118 |
Kind Code |
A1 |
ONO; Akihiko ; et
al. |
September 21, 2017 |
ELECTROCHEMICAL REACTION DEVICE
Abstract
An electrochemical reaction device, includes: an electrolytic
solution tank including a first storage part to store a first
electrolytic solution containing carbon dioxide, and a second
storage part to store a second electrolytic solution containing
water; a reduction electrode disposed in the first storing part; an
oxidation electrode disposed in the second storing part; a porous
body disposed in the first storing part; and a flow path connecting
the porous body and an outside of the electrolytic solution tank to
supply gas containing carbon dioxide to the porous body.
Inventors: |
ONO; Akihiko; (Kita, JP)
; MIKOSHIBA; Satoshi; (Yamato, JP) ; KITAGAWA;
Ryota; (Setagaya, JP) ; TAMURA; Jun; (Chuo,
JP) ; SUGANO; Yoshitsune; (Kawasaki, JP) ;
YAMADA; Arisa; (Kawasaki, JP) ; KUDO; Yuki;
(Yokohama, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kabushiki Kaisha Toshiba |
Minato-ku |
|
JP |
|
|
Assignee: |
Kabushiki Kaisha Toshiba
Minato-ku
JP
|
Family ID: |
59855363 |
Appl. No.: |
15/259466 |
Filed: |
September 8, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
Y02E 10/548 20130101;
C25B 9/10 20130101; Y02E 60/36 20130101; C25B 1/003 20130101; C25B
1/10 20130101; C25B 15/08 20130101 |
International
Class: |
C25B 9/10 20060101
C25B009/10; C25B 1/00 20060101 C25B001/00; C25B 15/08 20060101
C25B015/08; C25B 1/10 20060101 C25B001/10 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 18, 2016 |
JP |
2016-054732 |
Claims
1. An electrochemical reaction device, comprising: an electrolytic
solution tank including a first storage part to store a first
electrolytic solution containing carbon dioxide, and a second
storage part to store a second electrolytic solution containing
water; a reduction electrode disposed in the first storing part; an
oxidation electrode disposed in the second storing part; a porous
body disposed in the first storing part; and a flow path connecting
the porous body and an outside of the electrolytic solution tank to
supply gas containing carbon dioxide to the porous body.
2. An electrochemical reaction device, comprising: a first
electrolytic solution tank comprising a first storage part to store
a first electrolytic solution containing carbon dioxide, and a
second storage part to store a second electrolytic solution
containing water; a reduction electrode disposed in the first
storing part; an oxidation electrode disposed in the first storing
part; a second electrolytic solution tank comprising a third
storage part to store a third electrolytic solution containing
carbon dioxide higher in concentration than the first electrolytic
solution; a first flow path connecting the first storage part and
the third storage part; a porous body disposed in the third
electrolytic solution; and a second flow path connecting the porous
body and an outside of the second electrolytic solution tank to
supply gas containing carbon dioxide to the porous body.
3. The device of claim 2, further comprising: a pressure regulator
increasing a pressure in the third storage part.
4. The device of claim 2, further comprising: a third electrolytic
solution tank comprising a fourth storage part to store a fourth
electrolytic solution containing water; a third flow path
connecting the third storage part and the fourth storage part; a
second porous body disposed in the fourth electrolytic solution;
and a fourth flow path connecting the second porous body and an
outside of the third electrolytic solution tank to recover gas
containing oxygen from the fourth electrolytic solution via the
second porous body.
5. The device of claim 4, further comprising: a pressure regulator
increasing a pressure in the third storage part or the third flow
path and reducing a pressure in the fourth flow path.
6. The device of claim 1, wherein the porous body has a hydrophobic
property or water repellency.
7. The device of claim 1, wherein a pore size of the porous body is
1 .mu.m or less.
8. The device of claim 1, further comprising: a photoelectric
conversion body having a first surface connected to the reduction
electrode and a second surface connected to the oxidation
electrode.
9. The device of claim 1, further comprising: an ion exchange
membrane provided between the first storage part and the second
storage part.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is based upon and claims the benefit of
priority from Japanese Patent Applications No. 2016-054732, filed
on Mar. 18, 2016; the entire contents of all of which are
incorporated herein by reference.
FIELD
[0002] Embodiments described herein relate generally to an
electrochemical reaction device.
BACKGROUND
[0003] In recent years, there has been developed an artificial
photosynthesis technology which electrochemically converts sunlight
into chemical substances by modeling photosynthesis of plants from
viewpoints of energy problems and environmental problems. This is
because even if the sunlight is converted into chemical substances
at a land that is low in utility value such as a desert and is not
used for production of plants and then transported to a place away
therefrom, enough energy can be obtained. When the sunlight is
converted into the chemical substances and stored in a cylinder or
a tank, there are advantages that the energy storage cost can be
reduced and the amount of storage loss is small as compared to a
case where the sunlight is converted into electricity and stored in
a storage battery.
[0004] As a photoelectrochemical reaction device that
electrochemically converts sunlight to a chemical substance, there
has been known, for example, a two-electrode type device that
includes an electrode having a reduction catalyst for reducing
carbon dioxide (CO.sub.2) and an electrode having an oxidation
catalyst for oxidizing water (H.sub.2O), and in which these
electrodes are immersed in water with carbon dioxide dissolved
therein. In this case, the electrodes are electrically connected
via an electric wire or the like. In the electrode having the
oxidation catalyst, H.sub.2O is oxidized by light energy, whereby
oxygen (1/2O.sub.2) is obtained and a potential is obtained. In the
electrode having the reduction catalyst, by obtaining the potential
from the electrode in which the oxidation reaction is caused,
carbon dioxide is reduced and formic acid (HCOOH) or the like is
produced. As described above, in the two-electrode type device, the
reduction potential of carbon dioxide is obtained by two-stage
excitation, and therefore the conversion efficiency from the
sunlight to chemical energy is low.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] FIG. 1 is a schematic view illustrating a configuration
example of an electrochemical reaction device.
[0006] FIG. 2 is a schematic view illustrating a configuration
example of a photoelectric conversion cell.
[0007] FIG. 3 is a schematic view illustrating another
configuration example of the electrochemical reaction device.
[0008] FIG. 4 is a schematic view illustrating another
configuration example of the electrochemical reaction device.
[0009] FIG. 5 is a schematic view illustrating another
configuration example of the electrochemical reaction device.
[0010] FIG. 6 is a schematic view illustrating another
configuration example of the electrochemical reaction device.
[0011] FIG. 7 is a schematic view illustrating another
configuration example of the electrochemical reaction device.
[0012] FIG. 8 is a schematic view illustrating another
configuration example of the electrochemical reaction device.
[0013] FIG. 9 is a schematic view illustrating another
configuration example of the electrochemical reaction device.
DETAILED DESCRIPTION
[0014] An electrochemical reaction device in an embodiment,
includes: an electrolytic solution tank including a first storage
part to store a first electrolytic solution containing carbon
dioxide, and a second storage part to store a second electrolytic
solution containing water; a reduction electrode disposed in the
first storing part; an oxidation electrode disposed in the second
storing part; a porous body disposed in the first storing part; and
a flow path connecting the porous body and an outside of the
electrolytic solution tank to supply gas containing carbon dioxide
to the porous body.
[0015] Hereinafter, embodiments will be described with reference to
the drawings. Note that the drawings are schematic and, for
example, dimensions such as thickness and width of components may
differ from actual dimensions of the components. Besides, in the
embodiments, substantially the same components are denoted by the
same reference signs and the description thereof will be omitted in
some cases. A term of "connect" in the specification is not limited
to a case of connecting directly but may include a meaning of
connecting indirectly.
[0016] FIG. 1 is a schematic view illustrating a configuration
example of an electrochemical reaction device. The electrochemical
reaction device includes, as illustrated in FIG. 1, an electrolytic
solution tank 11, a reduction electrode 31, an oxidation electrode
32, a photoelectric conversion body 33, an ion exchange membrane 4,
a porous body 6, and a flow path 50.
[0017] The electrolytic solution tank 11 has a storage part 111 and
a storage part 112. The shape of the electrolytic solution tank 11
is not particularly limited as long as it is a solid shape having
cavities being the storage parts. As the electrolytic solution tank
11, for example, a material transmitting light is used.
[0018] The storage part 111 stores an electrolytic solution 21
containing a substance to be reduced. The substance to be reduced
is a substance that is reduced by a reduction reaction. The
substance to be reduced contains, for example, carbon dioxide. The
substance to be reduced may further contain hydrogen ions. Changing
the amount of water and electrolytic solution components contained
in the electrolytic solution 21 can change the reactivity and
thereby change the selectivity of the substance to be reduced and
the ratio of a chemical substance to be produced.
[0019] The storage part 112 stores an electrolytic solution 22
containing a substance to be oxidized. The substance to be oxidized
is a substance that is oxidized by an oxidation reaction. The
substance to be oxidized is, for example, water, an organic matter
such as alcohol or amine, or an inorganic oxide such as iron oxide.
The electrolytic solution 22 may contain the same substance as that
in the electrolytic solution 21. In this case, the electrolytic
solution 21 and the electrolytic solution 22 may be recognized as
one electrolytic solution.
[0020] The pH of the electrolytic solution 22 is preferably higher
than the pH of the electrolytic solution 21. This facilitates
migration of hydrogen ions, hydroxide ions and the like. A liquid
junction potential due to the different in pH allows
oxidation-reduction reaction to effectively proceed.
[0021] The reduction electrode 31 is immersed in the electrolytic
solution 21. The reduction electrode 31 contains, for example, a
reduction catalyst for the substance to be reduced. A compound to
be produced by the reduction reaction differs depending on the kind
of the reduction catalyst or the like. The compound to be produced
by the reduction reaction is, for example, a carbon compound such
as carbon monoxide (CO), formic acid (HCOOH), methane (CH.sub.4),
methanol (CH.sub.3OH), ethane (C.sub.2H.sub.6), ethylene
(C.sub.2H.sub.4), ethanol (C.sub.2H.sub.5OH), formaldehyde (HCHO),
or ethylene glycol; or hydrogen. The compound produced by the
reduction reaction may be recovered through, for example, a product
flow path. In this event, the product flow path is connected, for
example, to the storage part 111. The compound produced by the
reduction reaction may be recovered through another flow path.
[0022] The reduction electrode 31 may have a structure of, for
example, a thin-film shape, a lattice shape, a granular shape, or a
wire shape. The reduction electrode 31 does not necessarily have to
be provided with the reduction catalyst. A reduction catalyst
provided outside the reduction electrode 31 may be electrically
connected to the reduction electrode 31.
[0023] The oxidation electrode 32 is immersed in the electrolytic
solution 22. The oxidation electrode 32 contains, for example, an
oxidation catalyst for the substance to be oxidized. A compound to
be produced by the oxidation reaction differs depending on the kind
of the oxidation catalyst or the like. The compound to be produced
by the oxidation reaction is, for example, hydrogen ions. The
compound produced by the oxidation reaction may be recovered
through, for example, a product flow path. In this event, the
product flow path is connected, for example, to the storage part
112. The compound produced by the oxidation reaction may be
recovered through another flow path.
[0024] The oxidation electrode 32 may have a structure of for
example, a thin-film shape, a lattice shape, a granular shape, or a
wire shape. The oxidation electrode 32 does not necessarily have to
be provided with the oxidation catalyst. An oxidation catalyst
provided other than the oxidation electrode 32 may be electrically
connected to the oxidation electrode 32.
[0025] In the case where the oxidation electrode 32 is stacked on
the photoelectric conversion body 33 and immersed in the
electrolytic solution 22 and the photoelectric conversion body 33
is irradiated with light via the oxidation electrode 32 to perform
the oxidation-reduction reaction, the oxidation electrode 32 needs
to have a light transmitting property. The light transmittance of
the oxidation electrode 32 is preferably, for example, at least 10%
or more of an irradiation amount of the irradiating light to the
oxidation electrode 32, and more preferably 30% or more thereof.
Not limited to this, but the photoelectric conversion body 33 may
be irradiated with light, for example, via the reduction electrode
31.
[0026] The photoelectric conversion body 33 has a surface 331
electrically connected to the reduction electrode 31 and a surface
332 electrically connected to the oxidation electrode 32. Note that
the photoelectric conversion body 33 does not necessarily have to
be provided. Another generator may be connected to the oxidation
electrode 32 and the reduction electrode 31. The generator is not
limited to the photoelectric conversion element having the
photoelectric conversion body. Examples of the generator include a
system power supply, a storage battery, or the renewable energy
such as the wind power, water power, and the geothermal power. The
reduction electrode 31, the oxidation electrode 32, and the
photoelectric conversion body 33 are stacked. The reduction
electrode 31 is in contact with the surface 331, and the oxidation
electrode 32 is in contact with the surface 332. In this case, a
stack including the reduction electrode 31, the oxidation electrode
32, and the photoelectric conversion body 33 is also referred to as
a photoelectric conversion cell. The photoelectric conversion cell
is immersed in the electrolytic solution 21 and the electrolytic
solution 22 through the ion exchange membrane 4.
[0027] The photoelectric conversion body 33 has a function of
performing charge separation by energy of irradiating light such as
sunlight. Electrons generated by the charge separation move to the
reduction electrode side and holes move to the oxidation electrode
side. This allows the photoelectric conversion body 33 to generate
electromotive force. As the photoelectric conversion body 33, a
photoelectric conversion body of a pn junction type or a pin
junction type can be used. The photoelectric conversion body 33 may
be fixed, for example, to the electrolytic solution tank 11. Note
that the photoelectric conversion body 33 may be formed by stacking
a plurality of photoelectric conversion layers. The sizes of the
reduction electrode 31, the oxidation electrode 32, and the
photoelectric conversion body 33 may be different from one
another.
[0028] The ion exchange membrane 4 is provided in a manner to
separate the storage part 111 and the storage part 112. As the ion
exchange membrane 4, for example, Neosepta (registered trademark)
of Astom Corporation, Selemion (registered trademark) or Aciplex
(registered trademark) of Asahi Glass Corporation, Ltd., Fumasep
(registered trademark) or fumapem (registered trademark) of
Fumatech Corporation, Nafion (registered trademark) of DuPont
Corporation being a fluorocarbon resin made by performing
sulfonation and polymerization on tetrafluoroethylene, lewabrane
(registered trademark) of LANXESS Corporation, IONSEP (registered
trademark) of IONTECH Corporation, Mustang (registered trademark)
of PALL Corporation, ralex (registered trademark) of mega
Corporation, Gore-Tex (registered trademark) of Gore-Tex
Corporation, or the like can be used. Besides, the ion exchange
membrane may be constituted of a membrane whose basic structure is
hydrocarbon, or a membrane having an amine group in anion exchange.
Note that the ion exchange membrane 4 does not necessarily have to
be provided.
[0029] When the ion exchange membrane 4 is a proton exchange
membrane, the hydrogen ions can migrate to the electrolytic
solution 21 side. Use of an ion exchange membrane being a solid
polymer membrane such as Nafion can increase the migration
efficiency of the ions. Note that the ion exchange membrane 4 does
not necessarily have to be provided, but a salt bridge such as agar
may be provided in place of the ion exchange membrane 4.
[0030] The porous body 6 is immersed in the electrolytic solution
21. The porous body 6 has pore portions. The pore size of the pore
portion is preferably, for example, 1 .mu.m or less. When it is 1
.mu.m or less, the influence of the gas exchange velocity between a
gas phase and a liquid phase can be reduced. Further, it is
possible to suppress movement of electrolytic solution components
into the porous body 6 due to surface tension, and efficiently
separate the gas phase and the liquid phase.
[0031] The porous body 6 is formed using a porous body of a resin
material such as polyolefin such as polyethylene (PE), polyethylene
terephthalate (PET), polybutylene terephthalate (PBT),
polypropylene (PP), polytetrafluoroethylene (PTFE) or the like.
Further, a hollow fiber membrane in a laminated structure having a
non-porous film and porous films provided to hold the non-porous
film in between may be used. The porous film using polyolefin or
the like is preferable because it has a high mechanical strength.
As the non-porous body, for example, a resin having high gas
permeability such as urethan is used. The above laminated structure
can increase the mechanical strength, increase separation between
the liquid phase and the gas phase, and accelerate dissolution of
carbon oxide in the electrolytic solution due to an increase in
contact area between the gas phase and the liquid phase.
[0032] The porous body 6 preferably has a hydrophobic property or
water repellency with respect to the electrolytic solution. The
porous body 6 having the hydrophobic property or water repellency
can prevent the electrolytic solution from flowing back to the flow
path 50 via the porous body 6. An example of the method of
imparting the hydrophobic property or water repellency is a method
of mixing a hydrophobic element such as fluorine into the
above-described material, a method of subjecting water repellent
treatment on the porous body of the above-described material or the
like.
[0033] The flow path 50 extends to connect the outside of the
electrolytic solution tank 11 and the porous body 6. The flow path
50 is, for example, a flow path for supplying, to the porous body
6, gas containing carbon dioxide supplied from a carbon dioxide
supply unit 7 provided outside the electrochemical reaction device.
The gas containing carbon dioxide is supplied to the electrolytic
solution 21 via the porous body 6. A part of the flow path 50 may
be embedded in the porous body 6. The shape of the flow path 50 is
not particularly limited as long as it is a shape having a cavity
allowing the gas containing carbon dioxide to flow, such as a
tube.
[0034] Next, an operation example of the electrochemical reaction
device illustrated in FIG. 1 will be described. When light enters
the photoelectric conversion body 33, the photoelectric conversion
body 33 generates photoexcited electrons and holes. In this event,
the photoexcited electrons gather at the reduction electrode 31 and
the holes gather at the oxidation electrode 32. This causes
electromotive force in the photoelectric conversion body 33. The
light is preferably sunlight, but light of a light-emitting diode
or an organic EL may be made to enter the photoelectric conversion
body 33.
[0035] A case of using electrolytic solutions containing water and
carbon dioxide as the electrolytic solution 21 and the electrolytic
solution 22 to produce carbon monoxide will be described. Around
the catalyst layer 32, as expressed by the following formula (1),
the oxidation reaction of water occurs to lose electrons and
produce oxygen and hydrogen ions. At least one of the produced
hydrogen ions migrates to the storage part 111 through the ion
exchange membrane 4.
2H.sub.2O.fwdarw.4H.sup.++O.sub.2+4e.sup.- (1)
[0036] Around the reduction electrode 31, as expressed by the
following formula (2), the reduction reaction of carbon dioxide
occurs in which hydrogen ions react with carbon dioxide while
receiving electrons to produce carbon monoxide and water. Further,
hydrogen ions receive electrons to produce hydrogen as expressed by
the following formula (3). At this time, the hydrogen may be
produced simultaneously with the carbon monoxide.
CO.sub.2+2H.sup.++2e.sup.-.fwdarw.CO+H.sub.2O (2)
2H.sup.++2e.sup.-.fwdarw.H.sub.2 (3)
[0037] The photoelectric conversion body 33 needs to have an
open-circuit voltage equal to or more than a potential difference
between a standard oxidation-reduction potential of the oxidation
reaction and a standard oxidation-reduction potential of the
reduction reaction. For example, the standard oxidation-reduction
potential of the oxidation reaction in Expression (1) and is 1.23
[V]. The standard oxidation-reduction potential of the reduction
reaction in Expression (2) and is 0.03 [V]. The standard
oxidation-reduction potential of the oxidation reaction in
Expression (3) and is 0 [V]. At this time, the open-circuit voltage
needs to be made 1.26 [V] or more in the reactions Expression (1)
and Expression (2).
[0038] The open-circuit voltage of the photoelectric conversion
body 33 is preferably set to be higher by a value of an overvoltage
than the potential difference between the standard
oxidation-reduction potential of the oxidation reaction and the
standard oxidation-reduction potential of the reduction reaction.
For example, the overvoltage in each of the oxidation reaction in
Expression (1) and the reduction reaction in Expression (2) is 0.2
[V]. In the reactions in Expression (1) and Expression (2), the
open-circuit voltage is preferably set to 1.66 [V] or more.
Similarly, in the reactions in Expression (1) and Expression (3),
the open-circuit voltage is preferably set to 1.63 [V] or more.
[0039] Of the raw materials relating to the reaction expressed in
Expression (2), only carbon dioxide is gas and the other is liquid.
Therefore, causing carbon dioxide to efficiently dissolve in liquid
is important to increase the reaction efficiency. Increased
dissolution velocity of carbon dioxide is advantageous to progress
of the reaction.
[0040] The reduction reaction of hydrogen ions and carbon dioxide
is the reaction consuming hydrogen ions. Accordingly, when the
amount of hydrogen ions is small, the efficiency of the reduction
reaction becomes worse. It is therefore preferable that the
concentration of the hydrogen ions is made different between the
electrolytic solution 21 and the electrolytic solution 22 to make
it easy for the hydrogen ions to mitigate due to the concentration
difference. The concentration of anions (for example, hydroxide
ions or the like) may be made different between the electrolytic
solution 21 and the electrolytic solution 22.
[0041] The reaction efficiency of Expression (2) varies depending
on the concentration of carbon dioxide dissolved in the electrolyte
solution. The reaction efficiency increases with an increase in
carbon dioxide concentration, and decreases with a decrease in
carbon dioxide concentration. The reaction efficiency of Expression
(2) varies also depending on the hydrogen carbonate ion or
carbonate ion concentration. However, the hydrogen carbonate ion
concentration or the carbonate ion concentration can be adjusted by
increasing the electrolytic solution concentration or adjusting the
pH, and is thus adjusted more easily than the carbon dioxide
concentration. Note that if an ion exchange membrane is provided
between the oxidation electrode and the reduction electrode,
complete prevention of decrease in performance is difficult because
carbon dioxide gas, carbonate ions, hydrogen carbonate ions and the
like pass through the ion exchange membrane 4.
[0042] The electrochemical reaction device in this embodiment
includes a porous body immersed in the electrolytic solution
containing carbon dioxide, and supplies gas containing carbon
dioxide from the outside of the electrolytic solution tank via the
porous body. The porous body increases the contact area between the
gas containing carbon dioxide being the gas phase and the
electrolytic solution being the liquid phase. This facilitates
supply of the gas containing carbon dioxide to the electrolytic
solution. Accordingly, the dissolution efficiency of the carbon
dioxide to the electrolytic solution improves and the reduction
efficiency can be increased. Further, imparting the hydrophobic
property or water repellency to the porous body can increase
separation between the gas phase and the liquid phase by the
surface tension.
[0043] It is difficult to keep the gas containing carbon dioxide
flowing and keep a pressurized state during stop of the reaction.
Therefore, it is preferable to stop the supply of the gas
containing carbon dioxide during stop of the reaction. In this
event, when the porous body has no hydrophobic property or water
repellency, carbon dioxide is dissolved in the electrolytic
solution to reduce the pressure in the flow path for supplying the
gas containing carbon dioxide. This causes the electrolytic
solution to easily flow back into the flow path. If the
electrolytic solution flows back, the electrolytic solution
evaporates due to the gas containing carbon dioxide and the
electrolytic solution components precipitate. When the electrolytic
solution components precipitate, the pore portions of the porous
body and the inside of the flow path become more likely to be
clogged. In contrast to this, imparting the hydrophobic property or
water repellency to the porous body can increase separation between
the gas phase and the liquid phase by the surface tension to
thereby suppress back flow. In the electrochemical reaction device
with the above structure, the dissolution efficiency can be
increased, the device can be downsized, and the electrolytic
solution containing carbon dioxide with high concentration can be
produced with a simple system, so that the whole efficiency
improves.
[0044] Structural examples of the components in the electrochemical
reaction device will be further described. As the electrolytic
solution containing water applicable to the electrolytic solution,
for example, an aqueous solution containing an arbitrary
electrolyte can be used. This solution is preferably an aqueous
solution accelerating an oxidization reaction of water. Examples of
the aqueous solution containing an electrolyte include aqueous
solutions containing phosphoric acid ions (PO.sub.4.sup.2-), boric
acid ions (BO.sub.3.sup.3-), sodium ions (Na.sup.+), potassium ions
(K.sup.+), calcium ions (Ca.sup.2+), lithium ions (Li.sup.+),
cesium ions (Cs.sup.+), magnesium ions (Mg.sup.2+), chloride ions
(Cl.sup.-), hydrogen carbonate ions (HCO.sup.3-) and so on.
[0045] Examples of the electrolytic solution containing carbon
dioxide applicable to the electrolytic solution include aqueous
solutions containing LiHCO.sub.3, NaHCO.sub.3, KHCO.sub.3,
CsHCO.sub.3, phosphoric acid, boric acid and the like. The
electrolytic solution containing carbon dioxide may contain
alcohols such as methanol, ethanol, and acetone. The electrolytic
solution containing water may be the same as the electrolytic
solution containing carbon dioxide. However, preferably, the
absorption amount of carbon dioxide in the electrolytic solution
containing carbon dioxide is high. Accordingly, as the electrolytic
solution containing carbon dioxide, a solution different from the
electrolytic solution containing water may be used. The
electrolytic solution containing carbon dioxide is preferably an
electrolytic solution that decreases the reduction potential of
carbon dioxide, has high ion conductivity, and contains a carbon
dioxide absorbent that absorbs carbon dioxide.
[0046] As the above-described electrolytic solution, for example,
an ionic liquid which is made of a salt of cations such as an
imidazolium ion or a pyridinium ion 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 can be used.
Other examples of the electrolytic solution include amine solutions
of ethanolamine, imidazole, and pyridine, or aqueous solutions
thereof. Examples of amine include primary amine, secondary amine,
and tertiary amine. These electrolytic solutions may have high ion
conductivity, have a property of absorbing carbon dioxide, and have
characteristics of decreasing the reduction energy.
[0047] Examples of the primary amine include methylamine,
ethylamine, propylamine, butylamine, pentylamine, hexylamine, and
the like. Hydrocarbons of the amine may be substituted by alcohol,
halogen, or the like. Examples of the amine whose hydrocarbons are
substituted include methanolamine, ethanolamine, chloromethyl
amine, and so on. Further, an unsaturated bond may exist. These
hydrocarbons are the same in the secondary amine and the tertiary
amine.
[0048] Examples of the secondary amine include dimethylamine,
diethylamine, dipropylamine, dibutylamine, dipentylamine,
dihexylamine, dimethanolamine, diethanolamine, dipropanolamine, and
so on. The substituted hydrocarbons may be different. This also
applies to the tertiary amine. Examples in which the hydrocarbons
are different include methylethylamine, methylpropylamine, and so
on.
[0049] Examples of the tertiary amine include trimethylamine,
triethylamine, tripropylamine, tributylamine, trihexylamine,
trimethanolamine, triethanolamine, tripropanolamine,
tributanolamine, triexanolamine, methyl diethylamine,
methyldipropylamine, and so on.
[0050] Examples of the cation of the ionic liquid include a
1-ethyl-3-methylimidazolium ion, a 1-methyl-3-propylimidazolium
ion, a 1-butyl-3-methylimidazole ion, a
1-methyl-3-pentylimidazolium ion, a 1-hexyl-3-methylimidazolium
ion, and so on.
[0051] Note that a second place of the imidazolium ion may be
substituted. Examples of the cation having the imidazolium ion in
which second place is substituted include a
1-ethyl-2,3-dimethylimidazolium ion, a
1-2-dimethyl-3-propylimidazolium ion, a
1-butyl-2,3-dimethylimidazolium ion, a
1,2-dimethyl-3-pentylimidazolium ion, a
1-hexyl-2,3-dimethylimidazolium ion, and so on.
[0052] Examples of the pyridinium ion include methylpyridinium,
ethylpyridinium, propylpyridinium, butylpyridinium,
pentylpyridinium, hexylpyridinium, and so on. In both of the
imidazolium ion and the pyridinium ion, an alkyl group may be
substituted, or an unsaturated bond may exist.
[0053] Examples of the anion include a fluoride ion, a chloride
ion, a bromide ion, an iodide ion, BF.sub.4.sup.-, PF.sub.6.sup.-,
CF.sub.3COO.sup.-, CF.sub.3SO.sub.3.sup.-, NO.sub.3.sup.-,
SCN.sup.-, (CF.sub.3SO.sub.2).sub.3C.sup.-,
bis(trifluoromethoxysulfonyl)imide,
bis(perfluoroethylsulfonyl)imide, and so on. A dipolar ion in which
the cation and the anion of the ionic liquid are coupled by
hydrocarbons may be used. Note that a buffer solution such as a
potassium phosphate solution may be supplied to the storage parts
111, 112.
[0054] FIG. 2 is a schematic sectional view illustrating a
structural example of a photoelectric conversion cell. The
photoelectric conversion cell illustrated in FIG. 2 includes a
conductive substrate 30, the reduction electrode 31, the oxidation
electrode 32, the photoelectric conversion body 33, a light
reflector 34, a metal oxide body 35, and a metal oxide body 36.
[0055] The conductive substrate 30 is provided to be in contact
with the reduction electrode 31. Note that the conductive substrate
30 may be regarded as a part of the reduction electrode. An example
of the conductive substrate 30 is a substrate containing at least
one or a plurality of Cu, Al, Ti, Ni, Fe, and Ag. For example, a
stainless substrate including a stainless steel such as SUS may be
used. The conductive substrate 30 is not limited thereto, and may
be constituted using a conductive resin. Besides, the conductive
substrate 30 may be constituted using a semiconductor substrate
such as Si or Ge. Further, a resin film or the like may be used as
the conductive substrate 30. For example, a membrane applicable to
the ion exchange membrane 4 may be used as the conductive substrate
30.
[0056] The conductive substrate 30 has a function as a supporter.
The conductive substrate 30 may be provided so as to separate the
storage part 111 and the storage part 112. The provision of the
conductive substrate 30 can improve the mechanical strength of the
photoelectric conversion cell. Besides, the conductive substrate 30
may be regarded as a part of the reduction electrode 31. Further,
the conductive substrate 30 does not necessarily have to be
provided.
[0057] The reduction electrode 31 preferably contains a reduction
catalyst. The reduction electrode 31 may contain both a conductive
material and the reduction catalyst. Examples of the reduction
catalyst include materials decreasing activation energy to reduce
the hydrogen ions and carbon dioxide. In other words, the examples
include materials which lower overvoltage when hydrogen and carbon
compounds are generated by the reduction reaction of the hydrogen
ions and carbon dioxide. For example, a metal material or a carbon
material can be used. As the metal material, for example, a metal
such as platinum, nickel, or an alloy containing the metal can be
used in the case of hydrogen. In the reduction reaction of carbon
dioxide, a metal such as gold, aluminum, copper, silver, platinum,
palladium, or nickel, or an alloy containing the metal can be used.
As the carbon material, for example, graphene, carbon nanotube
(CNT), fullerene, ketjen black, or the like can be used. Note that
the reduction catalyst is not limited thereto, and, for example, a
metal complex such as a Ru complex or a Re complex, or an organic
molecule having an imidazole skeleton or a pyridine skeleton may be
used as the reduction catalyst. Besides, a plurality of materials
may be mixed.
[0058] The oxidation electrode 32 preferably contains an oxidation
catalyst. The oxidation electrode 32 may contain both a conductive
material and the reduction catalyst. Examples of the oxidation
catalyst include materials decreasing activation energy to oxidize
water. In other words, the examples include materials which lower
overvoltage when oxygen and hydrogen ions are generated by the
oxidation reaction of water. The examples include iridium, iron,
platinum, cobalt, manganese, and the like. Besides, as the
oxidation catalyst, a binary metal oxide, a ternary metal oxide, a
quaternary metal oxide, or the like can be used. Examples of the
binary metal oxide include manganese oxide (Mn--O), iridium oxide
(Ir--O), nickel oxide (Ni--O), cobalt oxide (Co--O), iron oxide
(Fe--O), tin oxide (Sn--O), indium oxide (In--O), ruthenium oxide
(Ru--O), and so on. Examples of the ternary metal oxide include
Ni--Co--O, La--Co--O, Ni--La--O, Ni--Fe--O, Sr--Fe--O, and so on.
Examples of the quaternary metal oxide include Pb--Ru--Ir--O,
La--Sr--Co--O, and so on. Note that the oxidation catalyst is not
limited thereto, and a metal complex such as a Ru complex or a Fe
complex can also be used as the oxidation catalyst. Besides, a
plurality of materials may be mixed.
[0059] At least one of the reduction electrode 31 and the oxidation
electrode 32 may have a porous structure. Examples of the material
applicable to the electrode having the porous structure include a
carbon black such as ketjen black and VULCAN XC-72, activated
carbon, metal fine powder, and so on in addition to the
above-described materials. The area of an activation surface which
contributes to the oxidation-reduction reaction can be made large
by having the porous structure, and therefore, the conversion
efficiency can be increased.
[0060] The porous structure preferably has a fine pore distribution
of 5 nm or more and 100 nm or less. With the fine pore
distribution, the catalyst activity can be increased. Furthermore,
the porous structure preferably has a plurality of fine pore
distribution peaks. This can realize all of the increase in surface
area, the improvement in dispersion of ions and reactant, and high
conductivity at the same time. For example, the reduction electrode
31 may be constructed, for example, by stacking a reduction
catalyst layer containing particles (particulate reduction
catalyst) of a metal or an alloy applicable to the reduction
catalyst of 100 nm or less on a conductive layer of the
above-described material having a fine pore distribution of 5 .mu.m
or more and 10 .mu.m or less. In this case, the particle may have
the porous structure, but does not always need to have the porous
structure from the conductive property or the relationship between
the reaction site and the material diffusion. Besides, the
particles may be supported by another material.
[0061] The reduction electrode 31 may have, a stacked structure of
a porous conductive layer and a porous catalyst layer containing
the reduction catalyst. For example, a mixture of Nafion, and
conductive particles such as ketjen black or the like can be used
as the porous conductive layer, and a gold catalyst can be used as
the porous catalyst layer. Further, formation of projections and
recesses of 5 .mu.m or less on the surface of the porous catalyst
layer can increase the reaction efficiency. Further, the surface of
the porous catalyst layer is oxidized by application of a high
frequency, and then subjected to electrochemical reduction, whereby
the reduction electrode 31 having a nanoparticle structure can be
formed. Other than gold, metal such as copper, palladium, silver,
zinc, tin, bismuth, or lead is preferable. Besides, the porous
conductive layer may further have a stacked structure in which
layers have different pore sizes. The stacked structure having the
different pore sizes makes it possible to adjust the difference in
reaction due to the product concentration near the electrode or the
difference in pH, by the pore sizes to improve the efficiency.
[0062] When an electrode reaction with low current density is
performed by using relatively low light irradiation energy, there
is a wide range of options in catalyst material. Accordingly, for
example, it is easy to perform a reaction by using a ubiquitous
metal or the like, and it is also relatively easy to obtain
selectivity of the reaction. On the other hand, when the
photoelectric conversion body 33 is not provided in the
electrolytic solution tank 11, but the photoelectric conversion
body 33 is electrically connected to at least one of the reduction
electrode 31 and the oxidation electrode 32 by a wire or the like,
an electrode area generally becomes small for the reason of
miniaturizing the electrolytic solution tank or the like, and the
reaction is performed with high current density in some cases. In
this case, it is preferable to use a noble metal as the
catalyst.
[0063] The photoelectric conversion body 33 has a stacked structure
including a photoelectric conversion layer 33x, a photoelectric
conversion layer 33y, and a photoelectric conversion layer 33z. The
number of stacked photoelectric conversion layers is not limited to
that illustrated in FIG. 2.
[0064] The photoelectric conversion layer 33x includes, for
example, an n-type semiconductor layer 331n containing n-type
amorphous silicon, an i-type semiconductor layer 331i containing
intrinsic amorphous silicon germanium, and a p-type semiconductor
layer 331p containing p-type microcrystal silicon. The i-type
semiconductor layer 331i is a layer which absorbs light in a short
wavelength region including, for example, 400 nm. Accordingly,
charge separation occurs at the photoelectric conversion layer 33x
due to the light energy in the short wavelength region.
[0065] The photoelectric conversion layer 33y includes, for
example, an n-type semiconductor layer 332n containing n-type
amorphous silicon, an i-type semiconductor layer 332i containing
intrinsic amorphous silicon germanium, and a p-type semiconductor
layer 332p containing p-type microcrystal silicon. The i-type
semiconductor layer 332i is, for example, a layer which absorbs
light in an intermediate wavelength region including 600 nm.
Accordingly, the charge separation occurs at the photoelectric
conversion layer 33y due to the light energy in the intermediate
wavelength region.
[0066] The photoelectric conversion layer 33z includes, for
example, an n-type semiconductor layer 333n containing n-type
amorphous silicon, an i-type semiconductor layer 333i containing
intrinsic amorphous silicon, and a p-type semiconductor layer 333p
containing p-type microcrystal silicon. The i-type semiconductor
layer 333i is, for example, a layer which absorbs light in a long
wavelength region including 700 nm. Accordingly, the charge
separation occurs at the photoelectric conversion layer 33z due to
the light energy in the long wavelength region.
[0067] The p-type semiconductor layer or the n-type semiconductor
layer can be formed by, for example, adding an element to be donor
or acceptor into the semiconductor material. Note that the
semiconductor layer containing silicon, germanium, or the like is
used as the semiconductor layer in the photoelectric conversion
layer, but is not limited thereto, and for example, a compound
semiconductor layer or the like can be used. As the compound
semiconductor layer, for example, a semiconductor layer containing
GaAs, GaInP, AlGaInP, CdTe, CuInGaSe, or the like can be used.
Besides, a layer containing a material such as TiO.sub.2 or
WO.sub.3 may be used as long as it can perform the photoelectric
conversion. Further, each semiconductor layer may be single
crystalline, polycrystalline, or amorphous. Besides, a zinc oxide
layer may be provided in the photoelectric conversion layer.
[0068] The light reflector 34 is provided between the conductive
substrate 30 and the photoelectric conversion body 33. An example
of the light reflector 34 is a distribution Bragg reflector
composed of, for example, a stack of metal layers or semiconductor
layers. The provision of the light reflector 34 makes it possible
to reflect the light which could not be absorbed by the
photoelectric conversion body 33, and cause the light to enter any
of the photoelectric conversion layer 33x to the photoelectric
conversion layer 33z, thereby increasing the conversion efficiency
from light to chemical substances. As the light reflector 34, for
example, a layer of a metal such as Ag, Au, Al, Cu, an alloy
containing at least one of these metals, or the like can be
used.
[0069] The metal oxide body 35 is provided between the light
reflector 34 and the photoelectric conversion body 33. The metal
oxide body 35 has a function of, for example, adjusting an optical
distance to increase the light reflectivity. As the metal oxide
body 35, it is preferable to use a material which can come into
ohmic-contact with the n-type semiconductor layer 331n. As the
metal oxide body 35, for example, a layer of light-transmissive
metal oxide such as an indium tin oxide (ITO), zinc oxide (ZnO),
fluorine-doped tin oxide (FTO), aluminum-doped zinc oxide (AZO), or
antimony-doped tin oxide (ATO) can be used.
[0070] The metal oxide body 36 is provided between the oxidation
electrode 32 and the photoelectric conversion body 33. The metal
oxide body 36 may be provided at a surface of the photoelectric
conversion body 33. The metal oxide body 36 has a function as a
protective layer which suppresses breakage of the photoelectric
conversion cell due to the oxidation reaction. The provision of the
metal oxide body 36 makes it possible to suppress corrosion of the
photoelectric conversion body 33, and elongate an operating life of
the photoelectric conversion cell. Note that the metal oxide body
36 does not necessarily have to be provided.
[0071] As the metal oxide body 36, for example, a dielectric thin
film such as TiO.sub.2, ZrO.sub.2, Al.sub.2O.sub.3, SiO.sub.2, or
HfO.sub.2 can be used The thickness of the metal oxide body 36 is
10 nm or less, and 5 nm or less. This is to obtain the conductive
property by the tunnel effect. As the metal oxide body 36, for
example, a layer of a light transmissive metal oxide such as an
indium tin oxide (ITO), zinc oxide (ZnO), fluorine-doped tin oxide
(FTO), aluminum-doped zinc oxide (AZO), or antimony-doped tin oxide
(ATO) may be used.
[0072] The metal oxide body 36 may have, for example, a structure
where a metal and a transparent conductive oxide are stacked, a
structure where a metal and another conductive material are
complexed, or a structure where a transparent conductive oxide and
another conductive material are complexed. The above structure
makes it possible to reduce the number of parts and weight, make it
easy to manufacture, and reduce the cost. The metal oxide body 36
may have functions as a protective layer, a conductive layer, and a
catalyst layer.
[0073] In the photoelectric conversion cell illustrated in FIG. 2,
a surface of the n-type semiconductor layer 331n opposite to a
contact surface with the i-type semiconductor layer 331i is the
first surface of the photoelectric conversion body 33, and a
surface of the p-type semiconductor layer 333p opposite to a
contact surface with the i-type semiconductor layer 333i is the
second surface. As described above, by stacking the photoelectric
conversion layer 33x to the photoelectric conversion layer 33z, the
photoelectric conversion cell illustrated in FIG. 2 can absorb the
light in a wide wavelength range of the sunlight and more
effectively utilize the solar energy. At this time, respective
photoelectric conversion layers are connected in series, and
therefore high voltage can be obtained.
[0074] In FIG. 2, the electrodes are stacked on the photoelectric
conversion body 33, and therefore the charge-separated electrons
and holes can be utilized as they are for the oxidation-reduction
reaction. Besides, it is unnecessary to electrically connect the
photoelectric conversion body 33 and the electrodes by the wire or
the like. It is therefore possible to perform the
oxidation-reduction reaction with high efficiency.
[0075] A plurality of photoelectric conversion bodies may be
electrically connected in parallel connection. A two-junction type,
single-layer type photoelectric conversion body may be used. A
stack of two or four or more photoelectric conversion bodies may be
provided. A single photoelectric conversion layer may be used
instead of the stack of the plurality of photoelectric conversion
layers.
[0076] The electrochemical reaction device in this embodiment is a
simplified system, in which the reduction electrode, the oxidation
electrode, and the photoelectric conversion body are integrated to
reduce the number of parts. Accordingly, for example, at least any
one of manufacture, installation, and maintenance becomes easy.
Further, the wires or the like connecting the photoelectric
conversion body with the reduction electrode and the oxidation
electrode become unnecessary, and therefore it is possible to
increase the light transmittance, and enlarge the light receiving
area.
[0077] There is a case where the photoelectric conversion body 33
is corroded because it is in contact with the electrolytic
solution, and a corrosion product is dissolved in the electrolytic
solution to cause deterioration of the electrolytic solution. To
prevent the corrosion, provision of a protective layer can be
considered. However, there is a case where a protective layer
component is dissolved in the electrolytic solution. Hence, a
filter such as a metal ion filter is provided in the flow path or
the electrolytic solution tank to suppress the deterioration of the
electrolytic solution.
[0078] The photo-electrochemical reaction device of the embodiment
is a technology suitable for measures for excess power, and it is
required to make use of the solar energy. When the illuminance of
sunlight is strong, energy is obtained as much as possible in the
case where there is no excess power and the energy is used for the
electrolytic solution circulation or the like for consumption in
the case where there is excess power. This effectively implements
energy mix, and can increase an energy utilization ratio as a
whole. In the case where a buffer solution is used for the
electrolytic solution, when the reaction amount is small, a change
in pH due to the reaction is also small. Hence, by circulating the
electrolytic solution when the reaction is not performed to keep
the electrolytic solution components uniform and by limiting or
stopping the supply of the electrolytic solution in the reaction
time, it is possible to suppress the decrease in total efficiency
and cost. For example, the oxidation-reduction reaction is
preferably performed by circulating the electrolytic solution using
wind power at night or excess power at low cost, and stopping the
electrolytic solution circulation or causing reaction at a minimum
supply amount in daytime.
[0079] The structural example of the electrochemical reaction
device is not limited to that in FIG. 1. FIG. 3 to FIG. 9 are
schematic views illustrating other examples of the electrochemical
reaction device. In the electrochemical reaction device illustrated
in FIG. 3, the photoelectric conversion body 33 is provided on the
outside of the electrolytic solution tank 11. In the
electrochemical reaction device illustrated in FIG. 4, the
photoelectric conversion body 33 is immersed in the electrolytic
solution 21 on the storage part 111 side. In the electrochemical
reaction device illustrated in FIG. 5, the photoelectric conversion
body 33 is immersed in the electrolytic solution 22 on the storage
part 112 side. Either the surface 331 and the reduction electrode
31 or the surface 332 and the oxidation electrode 32 are connected
by a conductive member such as a wire or the like. The case of
connecting the photoelectric conversion body and the reduction
electrode or the oxidation electrode by the wire or the like, is
advantageous in terms of a system because the components are
separated for each function.
[0080] The electrochemical reaction device illustrated in FIG. 6
further includes an electrolytic solution tank 12, a separation
tank 13, an electrolytic solution tank 14, a flow path 51 to a flow
path 56 in addition to the configuration illustrated in FIG. 1.
[0081] The electrolytic solution tank 12 includes a storage part
113a that stores an electrolytic solution 23, and a gas-liquid
separation membrane 113b provided to separate the storage part 113a
into a plurality of regions, for example, a region including liquid
and a region including gas. In the electrochemical reaction device
illustrated in FIG. 6, the porous body 6 is immersed in the
electrolytic solution 23. Further, the flow path 50 extends to
connect the outside of the electrolytic solution tank 12 and the
porous body 6. The description of the flow path 50 illustrated in
FIG. 1 can be appropriately quoted to the other description of the
flow path 50.
[0082] The electrolytic solution 23 contains, for example, carbon
dioxide with a higher concentration than that of the electrolytic
solution 21. The electrolytic solution 23 may contain a material
applicable to the electrolytic solution 21. The electrolytic
solution tank 12 has a function as a carbon dioxide absorber. An
example of a considerable method of increasing the concentration of
carbon dioxide in the electrolytic solution 23 is a method of
making the temperature of the electrolytic solution 23 lower than
the temperature of the electrolytic solution 21. For example, a
cooler may be provided which cools the inside of the storage part
113a. The shape of the electrolytic solution tank 12 is not limited
in particular as long as it is a three-dimensional shape having a
cavity being the storage part. As the electrolytic solution tank
12, for example, a material transmitting light may be used.
[0083] Another example of the considerable method of increasing the
concentration of carbon dioxide in the electrolytic solution 23 is
a method of making the pressure applied to the electrolytic
solution 23 higher than the pressure applied to the electrolytic
solution 21. For example, a pressure regulator may be provided
which makes the pressure in the storage part 113 higher than the
pressure in the storage part 111.
[0084] Supply of the electrolytic solution having the high carbon
dioxide concentration adjusted in the electrolytic solution tank 12
to the electrolytic solution tank 11 can increase the carbon
dioxide concentration in the electrolytic solution stored in the
electrolytic solution tank 11. This can improve the efficiency of
the reduction reaction.
[0085] The separation tank 13 has a storage part 114a that stores
an electrolytic solution 24, and a gas-liquid separation membrane
114b provided to separate the storage part 114a into a plurality of
regions, for example, a region including liquid and a region
including gas. The shape of the separation tank 13 is not limited
in particular as long as it is a three-dimensional shape having a
cavity being the storage part. As the separation tank 13, for
example, a material transmitting light is used.
[0086] The electrolytic solution tank 14 has a storage part 115a
that stores an electrolytic solution 25, and a gas-liquid
separation membrane 115b provided to separate the storage part 115a
into a plurality of regions. The electrolytic solution 25 may
contain a material applicable to the electrolytic solution 22. The
shape of the separation tank 14 is not limited in particular as
long as it is a three-dimensional shape having a cavity being the
storage part. As the separation tank 14, for example, a material
transmitting light may be used.
[0087] The shapes of the storage part 113a, storage part 114a, and
storage part 115a are not limited in particular, and may have the
same structure as that of the storage part 111 or the storage part
112. The gas-liquid separation membranes 113b, 114b, and 115b
include, for example, a hollow fiber membrane and so on. The hollow
fiber membrane includes, for example, a silicone resin or a
fluorine-based resin (perfluoro alkoxy alkane (PFA), perfluoro
ethylenepropene copolymer (FEP), polytetrafluoroethylene (PTFE),
ethylene tetrafluoroethylene copolymer (ETFE), polyvinylidene
fluoride (PVDF), polychlorotrifluoroethylene (PCTFE),
ethylene-chlorotrifluoroethylene copolymer (ECTFE)) or the
like.
[0088] The flow path 51 to the flow path 56 have a function as an
electrolytic solution flow path for circulating the electrolytic
solution. The flow path 51 connects the storage part 111 and the
storage part 114a. The flow path 52 connects the storage part 111
and the storage part 113a. The flow path 53 connects the storage
part 113a and the storage part 114a. The ions and other substances
contained in the electrolytic solution 21 can move to the
separation tank 13 via the flow path 51. The ions and other
substances contained in the electrolytic solution 23 can move to
the storage part 111 via the flow path 52. Not limited to the
above, the electrolytic solution and the product by the
oxidation-reduction reaction may be circulated through the flow
path 51 and the flow path 52.
[0089] The flow path 54 connects the storage part 112 and the
storage part 115a. The flow path 55 connects the storage part 12
and the storage part 115a. At least a part of the electrolytic
solution 22 is supplied to the storage part 115a via the flow path
54. The ions and other substances contained in the electrolytic
solution 22 can move to the electrolytic solution tank 14 via the
flow path 54. The ions and other substances contained in the
electrolytic solution 25 can move to the storage part 112 via the
flow path 55.
[0090] The shapes of the flow path 51 to the flow path 55 are not
particularly limited as long as they each have a cavity allowing
the electrolytic solution to flow, such as a pipe. The electrolytic
solution in at least one flow path of the flow path 51 to the flow
path 55 may be circulated by a circulating pump.
[0091] In the electrochemical reaction device illustrated in FIG.
6, a partial product of the reduction product in the electrolytic
solution tank 11 is extracted in the separation tank 13. Reduction
of the pressure on the outside of the gas-liquid separation
membrane 114b (the opposite side to the contact surface with the
electrolytic solution 24) and passage of the electrolytic solution
24 containing a gaseous product through the gas-liquid separation
membrane 114b makes it possible to efficiently separate the gaseous
product and carbon dioxide. In the case where the product is, for
example, carbon monoxide, only carbon monoxide gas can be separated
by gas-liquid separation in the separation tank 13.
[0092] In the case of dissolving carbon dioxide in the electrolytic
solution 21, excessive undissolved carbon dioxide floats as gas.
This gas is separated in the separation tank 13 and an electrolytic
solution to be obtained is supplied to the storage part 111,
whereby the concentration of the reduction product can be
increased.
[0093] In the electrochemical reaction device illustrated in FIG.
6, a partial product of the reduction product in the electrolytic
solution tank 11 is extracted in the separation tank 12. Reduction
of the pressure on the outside of the gas-liquid separation
membrane 113b (the opposite side to the contact surface with the
electrolytic solution 23) and passage of the electrolytic solution
23 containing a gaseous product through the gas-liquid separation
membrane 113b makes it possible to efficiently separate the gaseous
product and carbon dioxide. In the case where the product is, for
example, carbon monoxide, only carbon monoxide gas can be separated
by gas-liquid separation in the separation tank 12.
[0094] In the electrochemical reaction device illustrated in FIG.
6, reduction of the pressure on the outside of the gas-liquid
separation membrane 115b (the opposite side to the contact surface
with the electrolytic solution 25) and passage of the electrolytic
solution containing a gaseous product through the gas-liquid
separation membrane 115b makes it possible to efficiently separate
an oxygen gas and dissolved oxygen like carbon dioxide. It is
conceivable to directly recover and use an oxygen gas produced in
the electrolytic solution tank 11, but it is difficult to
completely recover the oxygen gas because the oxygen gas dissolves
in the electrolytic solution 22. This leads to a decrease in
performance of the oxidation electrode, and therefore the dissolved
oxygen is preferably recovered in a gaseous state. Unlike the gas
separation in the electrolytic solution tank 11, it is possible to
recover gas produced in a plurality of cells at a time. This can
shorten the total flow path length for gas recover to simplify the
system. In this case, for efficient recovery of the oxygen gas,
temperature regulators can be provided in the electrolytic solution
tank 14, the flow path 54, and the flow path 55 as in the
electrolytic solution tank 12, leading to efficient separation of
oxygen from the electrolytic solution.
[0095] Use of the gas-liquid separation membrane makes it possible
to obtain oxygen from the electrolytic solution and remove carbon
dioxide moving from the electrolytic solution on the reduction side
to the electrolytic solution on the oxidation side. Removal of
carbon dioxide from the electrolytic solution on the oxidation side
enables use of an arbitrary electrolytic solution to widen the
selectivity of the oxidation catalyst. The oxidation catalyst
differs in activity depending on the electrolytic solution
component, and therefore can be prevented from being deteriorated
in characteristics.
[0096] Use of cobalt as the oxidation catalyst for water is an
effective method because it affects the lifetime in the
electrolytic solution and characteristics. Besides, the movement of
carbon dioxide to the electrolytic solution 22 changes the pH of
the electrolytic solution. A shift of the value of the pH by 1
causes oxidation of 56 mV and an electromotive force at the
electrode on the reduction side.
[0097] The electromotive force can be used when performing an
electrolytic reaction. Because the pH changes with the reaction,
the potential caused by the difference in pH cannot be continuously
obtained. However, it is possible to continue the reaction with the
pH being maintained by continuously blowing carbon dioxide into the
electrolytic solution on the reduction side or circulating an
electrolytic solution with a high pH to the electrolytic solution
on the oxidation side. Further, it is possible to continue the
reaction utilizing the potential due to the difference in pH
without feeding energy from the outside also by using and
circulating a solution with a high pH existing in nature on the
oxidation side and an electrolytic solution with a low pH on the
reduction side. In this event, the potential obtained due to the
difference in pH caused from the movement of carbon dioxide lowers,
and therefore the removal of carbon dioxide from the electrolytic
solution on the oxidation side contributes to efficiently causing
the reaction.
[0098] The provision of the temperature regulator in the separation
tank 13 or the flow path 51 can increase the separation efficiency
of the product. For complete gas separation, it is preferable to
remove the dissolved gas in the electrolytic solution as much as
possible. To increase the efficiency of removing the dissolved gas
by temperature distribution or the like, the separation tank 13 is
preferably provided with a stirring means.
[0099] The difference between the temperature of the electrolytic
solution 24 in the separation tank 13 and the temperature of the
electrolytic solution 21 in the electrolytic solution tank 11 may
be -10.degree. C. or more and 10.degree. C. or less. When the
temperature of the electrolytic solution 24 in the separation tank
13 is too high, the dissolved carbon dioxide evaporates and the gas
concentration of the product is apt to decrease. Since the energy
loss due to heating is large, excessive heating causes a decrease
in efficiency.
[0100] In the case where the product is a water-soluble liquid
substance such as methanol or ethanol, the separation method of the
separation tank 13 may be, for example, distillation or membrane
separation. In this case, the temperature regulator is preferably
provided for improvement of the separation efficiency. The
separation membrane may be, for example, zeolite. In particular,
the heat is large at the upstream, and therefore the whole
efficiency is apt to decrease. Thus, a heat insulating material is
provided in the separation tank 13, whereby the decrease in the
efficiency can be prevented.
[0101] In the case of directly blowing carbon dioxide into the
electrolytic solution tank 11, if the reduction product is gaseous
carbon monoxide or the like, it is necessary to separate a carbon
dioxide gas and a carbon monoxide gas. This may cause an increase
in cost due to complication of the device and an energy loss
because energy is required for the separation.
[0102] The electrochemical reaction device in this embodiment
includes a porous body immersed in an electrolytic solution
containing carbon dioxide in a second electrolytic solution tank,
and supplies gas containing carbon dioxide via the porous body from
the outside of the second electrolytic solution tank. The porous
body increases the contact area between the gas containing carbon
dioxide being the gas phase and the electrolytic solution being the
liquid phase. This facilitates supply of the gas containing carbon
dioxide to the electrolytic solution. Therefore, the efficiency of
dissolving carbon dioxide with respect to the electrolytic solution
can improve to increase the reduction efficiency. Further,
imparting to the porous body the hydrophobic property or water
repellency can increase the separation between the gas phase and
the liquid phase due to surface tension.
[0103] In the case of cooling the storage part 111, the reaction
efficiency is apt to decrease because the reaction by the catalyst
decreases. Besides, in the case of pressurizing the storage part
111, the cost increases and the structure becomes complicated
because of the need to increase the pressure resistance of the
electrolytic solution tank 11. Further, the increase in the
pressure resistance deteriorates the maintainability such as
complication of exchange of electrodes.
[0104] To reduce the supply amount of carbon dioxide or to causes
the electrolytic solution to efficiently absorb carbon dioxide, the
interval between bubbles of carbon dioxide passing through the
electrolytic solution needs to be large. However, increasing the
concentration of carbon dioxide decreases the interval between
bubbles, so that the electrolytic solution tank can be made
smaller. The cooling temperature is preferably, for example, equal
to or lower than the temperature of the electrolytic solution in
the first electrolytic solution tank. When the temperature of the
electrolytic solution increases by the oxidation-reduction
reaction, the cooling temperature is preferably equal to or higher
than room temperature and equal to or lower than the temperature of
the electrolytic solution in the first electrolytic solution tank.
The cooling temperature is more preferably equal to or higher than
the temperature at which the electrolytic solution does not freeze
and equal to or lower than the electrolytic solution
temperature.
[0105] In the case where an ion exchange membrane and a flow path
are provided between the oxidation electrode and the reduction
electrode in the electrolytic solution tank, the electrolytic
solution in contact with the oxidation electrode may be different
from the electrolytic solution in contact with the reduction
electrode. With the above configuration, oxygen being the reaction
product on the oxidation side can be easily separated and taken
out.
[0106] There is an electrolytic solution suitable for each
catalyst, and by changing the electrolytic solution in contact with
each catalyst layer, the efficiency can be improved. Furthermore,
there is an advantage, in the case where the pH is made larger on
the oxidation side as compared between the oxidation side and the
reduction side, that the potential of insufficient reaction can be
supplemented with the liquid junction potential resulting from a
difference in pH.
[0107] The temperature of the electrolytic solution 21 in the
electrolytic solution tank 11 is preferably higher than the
freezing temperature. For example, in the case where the
electrolytic solution contains ions such as potassium ion or sodium
ion in order to improve the amount of absorbing carbon dioxide, to
improve the carbon dioxide concentration and the HCO.sub.3 ion
concentration, and to improve the solution resistance of the
electrolytic solution, the electrolytic solution does not freeze at
0.degree. C. However, to excessively cool the electrolytic
solution, a large-size freezer is required, leading to cost and
energy loss, and therefore there is a case where the temperature is
preferably 0.degree. C. or higher. Besides, there may be an energy
loss in the whole electrochemical reaction device and a reaction
decrease due to excessive cooling of the electrolytic solution, and
therefore the temperature may be preferably 5.degree. C. or higher
and 10.degree. C. or higher.
[0108] The temperature regulators may be provided in the
electrolytic solution tanks 11, 12 and the flow path 51 to the flow
path 55 in order to suppress a decrease in reaction efficiency due
to a decrease in the electrolytic solution temperature. Regulation
of temperature by the temperature regulators improves the reaction
efficiency. For example, a cooler may be provided in the flow path
51, and a heater may be provided in the flow path 52. Since the
effect can be obtained even by a temperature difference of several
degrees Celsius, warming by sunlight irradiation of the
electrolytic solution flow path between the first electrolytic
solution tank and the second electrolytic solution tank and the
electrolytic solution tanks is efficient because natural energy can
be used. Further, in the case of performing a later-described main
reaction by converting the sunlight into electric energy, the
efficiency further improves because the heat energy and light
energy of the sunlight can be efficiently used.
[0109] An electrochemical reaction device illustrated in FIG. 7 has
a configuration which does not have the separation tank 13 and the
flow path 53 in the electrochemical reaction device illustrated in
FIG. 6, and further includes a porous body 6a, a porous body 6b, a
flow path 56, a flow path 57, pumps 71a to 71c, a pressure valve
72a, and a pressure valve 72b. The flow path 51 connects the
storage part 111 and the storage part 113a.
[0110] The porous body 6a is immersed in the electrolytic solution
23. The porous body 6b is immersed in the electrolytic solution 25.
The description of the porous body 6 can be appropriately quoted to
the other description of the porous body 6a and the porous body
6b.
[0111] The flow path 56 connects the porous body 6a and the pump
71c. The flow path 56 extends from the outside of the electrolytic
solution tank 12 to connect to the porous body 6a. The flow path 56
is a flow path for supplying gas containing carbon dioxide to the
porous body 6a. The flow path 57 connects the porous body 6b and
the pump 71c. The flow path 57 extends from the outside of the
electrolytic solution tank 14 to connect to the porous body 6b. The
flow path 57 is a flow path for recovering gas containing oxygen
from the electrolytic solution 25 via the porous body 6b. For the
shapes or materials of the flow path 56 and the flow path 57, the
shapes or materials applicable to the flow paths 51 to 55 are
used.
[0112] The pump 71a has a function of promoting supply of the
electrolytic solution from the storage part 113a to the storage
part 111. The pump 71a is provided, for example, inside or outside
the flow path 52. The pump 71a does not necessarily have to be
provided.
[0113] The pump 71b has a function of promoting supply of the
electrolytic solution from the storage part 115a to the storage
part 112. The pump 71b is provided, for example, inside or outside
the flow path 55. The pump 71b does not necessarily have to be
provided.
[0114] The pump 71c has a function of promoting supply of the gas
containing carbon dioxide to the storage part 113a. The pump 71c is
provided, for example, inside or outside the flow path 56. In this
case, a pressure regulator which increases the pressure in the
storage part 113a or the flow path 56 is preferably provided.
Further, a pressure regulator which increases the pressure in the
storage part 113a or the flow path 56 and reduces the pressure in
the flow path 57 may be provided. The pressure regulator may be
composed of, for example, a pressurizer and a pressure reducer.
[0115] The pressure valve 72a has a function of promoting supply of
the electrolytic solution from the storage part 111 to the storage
part 113 The pressure valve 72a is provided, for example, inside or
outside the flow path 51. The pressure valve 72b has a function of
promoting supply of the electrolytic solution from the storage part
112 to the storage part 115a The pressure valve 72b is provided,
for example, inside or outside the flow path 54. Examples of the
pressure valve 72a and the pressure valve 72b include an orifice
valve, a pulse valve and so on. Note that the pressure valve 72a
and the pressure valve 72b do not necessarily have to be
provided.
[0116] In the electrochemical reaction device illustrated in FIG.
7, supply of the gas containing carbon dioxide pressurized using
the pump 71c to the storage part 113a makes it possible to increase
the efficiency of dissolving carbon dioxide. It is also possible to
suppress a decrease in circulation amount of carbon dioxide due to
pressure loss when passing through the porous body 6a.
[0117] Further, use of the pressure-reducing mechanism of the pump
71c makes it possible to take out oxygen and carbon dioxide
contained in the electrolytic solution via the porous body 6b.
Further, physical motive power of a motor or the like used in the
pressure pump is used for the pressure-reducing pump to make the
power source common, whereby the friction in drive or the like is
reduced and devices to be controlled can be reduced, resulting in
effect in reduction in efficiency and cost. Further, the efficiency
of separating carbon dioxide to be supplied and oxygen to be
recovered can be increased.
[0118] An electrochemical reaction device illustrated in FIG. 8 has
a configuration which does not have the porous body 6b and the pump
71c in the configuration illustrated in FIG. 7. In the
electrochemical reaction device illustrated in FIG. 8, oxygen
obtained in the electrolytic solution tank 14 is supplied to a
carbon dioxide generation source 8. In the carbon dioxide
generation source 8, carbon dioxide is generated using the supplied
oxygen and supplied to the electrolytic solution tank 12. The
oxygen recovered as describe above is supplied to the carbon
dioxide generation source, thereby making it possible to improve
the efficiency of the carbon dioxide generation source 8 and
improve the efficiency of the whole.
[0119] An electrochemical reaction device illustrated in FIG. 9 has
a configuration which does not have the flow path 57 and the pump
71c illustrated in FIG. 8. The flow path 56 connects a carbon
capture storage device 9 and the storage part 113a. The carbon
capture storage device 9 is connected to the carbon dioxide
generation source 8 via the flow path 58.
[0120] The carbon capture storage device 9 is provided from the
viewpoint of a reduction in exhaust amount of carbon dioxide. In
the carbon capture storage device 9, for example, the exhausted
carbon dioxide is absorbed into an amine solution or zeolite.
Re-emission of the absorbed carbon dioxide by heat or the like can
increase the concentration and purity of carbon dioxide. Thus
obtained carbon dioxide is, for example, buried in the earth or
used for extraction of a natural gas or a shale gas, whereby an
increase in carbon dioxide concentration in the air can be
suppressed. Supply of the carbon dioxide with high concentration
obtained by the carbon capture storage device 9 to the porous body
6a can increase the efficiency of dissolving carbon dioxide. The
use of the carbon capture storage device 9 as described above can
reduce the exhaust amount of carbon dioxide, improve the whole
system efficiency through use of oxygen, and realize a system
capable of obtaining valuable resources.
EXAMPLE
Example 1
[0121] An electrochemical reaction device including a structure was
fabricated. The structure includes a three-junction type
photoelectric conversion body with a thickness of 500 nm, a ZnO
layer with a thickness of 300 nm provided on a first surface of the
three-junction type photoelectric conversion body, an Ag layer with
a thickness of 200 nm provided on the ZnO layer, a SUS substrate
with a thickness of 1.5 mm provided on the Ag layer, and an ITO
layer with a thickness of 100 nm provided on a second surface of
the three-junction type photoelectric conversion body. Note that
each layer on the SUS substrate has a texture structure of a
submicron order for obtaining the light confinement effect.
[0122] The three-junction type photoelectric conversion body
includes a first photoelectric conversion layer which absorbs light
in the short wavelength region, a second photoelectric conversion
layer which absorbs light in the intermediate wavelength region,
and a third photoelectric conversion layer which absorbs light in
the long wavelength region. The first photoelectric conversion
layer includes a p-type microcrystalline silicon layer, an i-type
amorphous silicon layer, and an n-type amorphous silicon layer. The
second photoelectric conversion layer includes a p-type
microcrystalline silicon layer, an i-type amorphous silicon
germanium layer, and an n-type amorphous silicon layer. The third
photoelectric conversion layer includes a p-type microcrystalline
silicon germanium layer, an i-type amorphous silicon layer, and an
n-type amorphous silicon layer.
[0123] Next, a Ni catalyst layer with a thickness of 5 nm was
formed as an oxidation catalyst on the ITO layer by an atomic layer
deposition method. Further, a conducting wire was connected to the
rear surface of the SUS substrate. A composite substrate (4 cm
square) having a SUS substrate with a thickness of 1.5 mm and a
gold-bearing carbon film with a bearing amount of 0.25 mg/cm.sup.2
on the SUS substrate which were connected through the conducting
wire was prepared. An ion exchange membrane (Nafion 117, 6 cm
square) was provided between the composite substrate and the
structure, and a potassium carbonate solution was supplied into the
module. The composite substrate was used as a reduction electrode,
the oxidation catalyst side of the structure was used as an oxygen
electrode, and a silver-silver chloride electrode was used as a
reference electrode. A galvanostat was used to pass current under a
condition of 2.3 mA/cm.sup.2 to reduce carbon dioxide to thereby
produce carbon monoxide. In this event, carbon dioxide was supplied
via a porous body made by bundling tubular porous bodies each
having a urethane resin sandwiched in between polyethylene with a
pore size of 0.5 .mu.m or less.
[0124] A production efficiency .eta. that is the production
efficiency of carbon monoxide measured when the structure was
irradiated with light using a solar simulator (AM1.5, 1000
W/m.sup.2) is obtained by the following expression.
.eta. ( % ) = R ( CO ) .times. .DELTA. G.degree. P .times. S [ Math
1 ] ##EQU00001##
[0125] In the formula, R(CO) is a production rate (mol/s) of carbon
monoxide. .DELTA.G.degree. is standard Gibbs energy of combustion
of carbon monoxide. The Gibbs energy was set to 257.2 kJ/mol at
298K. P is irradiation energy of sunlight. The irradiation energy
was set to 0.1 J/scm.sup.2. S is a light receiving area of
sunlight. The result is listed in Table 1.
[0126] Recovery of gas was performed above the reduction electrode
to sample evaporated gas and identify and determine the quantity of
the gas by gas chromatography. The result is listed in Table 1.
Comparative Example 1
[0127] A module was produced by the same method as that in Example
1 except that carbon dioxide was supplied to the electrolytic
solution tank via a glass frit with a pore size of about 10 .mu.m,
and subjected to measurement. The result is listed in Table 1.
Example 2
[0128] A urethane resin of 0.1 .mu.m was sandwiched in between a
polyethylene porous film with a pore size of 0.5 .mu.m or less into
a tubular shape in the electrolytic solution on the oxidation
electrode side. The pressure in the tubular porous film was
reduced, and reaction and measurement were carried out by the same
means as that in Example 1. The result is listed in Table 1.
[0129] It is found, from the results of Examples 1, 2 and
Comparative example 1, that the provision of the porous body and
the supply of the gas containing carbon dioxide to the electrolytic
solution via the porous body increases the efficiency of dissolving
carbon dioxide and increases the efficiency of producing carbon
monoxide. It is also found that the provision of the porous body in
the electrolytic solution tank connected to the storage part on the
oxygen side and the recovery of the gas containing oxygen can
further increase the efficiency of producing carbon monoxide.
TABLE-US-00001 TABLE 1 Production Production CO content ratio of
efficiency efficiency [%] recovered gas [%] after 3 hours [%]
Example 1 3 95 2.6 Comparative 2 5 -- example 1 Example 2 3 95
3.8
[0130] The above embodiments have been presented by way of example
only, and are not intended to limit the scope of the inventions.
The above embodiments may be embodied in a variety of other forms,
and various omissions, substitutions and changes may be made
without departing from the spirit of the inventions. The above
embodiments and modifications thereof are included in the scope and
spirit of the inventions and included in the inventions described
in the claims and their equivalents.
* * * * *