U.S. patent application number 15/261095 was filed with the patent office on 2017-08-24 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, Eishi TSUTSUMI, Masakazu YAMAGIWA.
Application Number | 20170241026 15/261095 |
Document ID | / |
Family ID | 59629690 |
Filed Date | 2017-08-24 |
United States Patent
Application |
20170241026 |
Kind Code |
A1 |
ONO; Akihiko ; et
al. |
August 24, 2017 |
ELECTROCHEMICAL REACTION DEVICE
Abstract
An electrochemical reaction device includes: a first
electrolytic solution tank including first and second storage parts
storing first and second electrolytic solutions containing carbon
dioxide and water respectively; reduction and oxidation electrodes
immersed in the first and second electrolytic solutions
respectively; a generator connected to the reduction and oxidation
electrodes; a second electrolytic solution tank including a third
storage part storing a third electrolytic solution containing
carbon dioxide; and a flow path connecting the first and third
storage parts. The third electrolytic solution is lower in
temperature than the first electrolytic solution.
Inventors: |
ONO; Akihiko; (Kita, JP)
; MIKOSHIBA; Satoshi; (Yamato, JP) ; KUDO;
Yuki; (Yokohama, JP) ; KITAGAWA; Ryota;
(Setagaya, JP) ; TAMURA; Jun; (Chuo, JP) ;
SUGANO; Yoshitsune; (Kawasaki, JP) ; TSUTSUMI;
Eishi; (Kawasaki, JP) ; YAMAGIWA; Masakazu;
(Yokohama, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KABUSHIKI KAISHA TOSHIBA |
Minato-ku |
|
JP |
|
|
Assignee: |
KABUSHIKI KAISHA TOSHIBA
Minato-ku
JP
|
Family ID: |
59629690 |
Appl. No.: |
15/261095 |
Filed: |
September 9, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C25B 1/10 20130101; Y02E
60/36 20130101; C25B 1/003 20130101; Y02E 60/366 20130101; C25B
15/08 20130101; C25B 9/10 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 |
Feb 23, 2016 |
JP |
2016-032480 |
Claims
1. An electrochemical reaction device comprising: a first
electrolytic solution tank including a first storage part storing a
first electrolytic solution containing carbon dioxide and a second
storage part storing a second electrolytic solution containing
water; a reduction electrode immersed in the first electrolytic
solution; an oxidation electrode immersed in the second
electrolytic solution; a generator connected to the reduction
electrode and the oxidation electrode; a second electrolytic
solution tank including a third storage part storing a third
electrolytic solution containing carbon dioxide; and a flow path
connecting the first storage part and the third storage part,
wherein a temperature of the third electrolytic solution is lower
than a temperature of the first electrolytic solution.
2. The device of claim 1, further comprising: a first separation
tank including a fourth storage part storing a fourth electrolytic
solution containing carbon dioxide and a first gas-liquid
separation membrane dividing the fourth storage part into a
plurality of regions; a second separation tank including a fifth
storage part storing a fifth electrolytic solution containing water
and a second gas-liquid separation membrane dividing the fifth
storage part into a plurality of regions; a second flow path
connecting the first storage part and the fourth storage part; a
third flow path connecting the third storage part and the fourth
storage part; and a fourth flow path connecting the second storage
part and the fifth storage part.
3. The device of claim 2, further comprising: a carbon dioxide
generation source containing carbon dioxide having a higher
temperature than the temperature of the first electrolytic
solution; a reduction reaction device reducing a product produced
by a reduction reaction of the carbon dioxide; a distiller disposed
on the third storage part; a fifth flow path connecting the first
storage part and the carbon dioxide generation source; and a sixth
flow path connecting the third storage part and the reduction
reaction device.
4. The device of claim 3, further comprising: a first cooler
disposed at the third storage part; a first heater disposed at the
second flow path; a second cooler disposed at the fourth flow path;
and a second heater disposed at the fifth flow path.
5. The device of claim 3, wherein the device pertains at least one
of heat exchange between the second electrolytic solution tank and
the second separation tank, heat exchange between the first
separation tank and the second separation tank, heat exchange
between the reduction reaction device and the second electrolytic
solution tank, heat exchange between the carbon dioxide generation
source and the distiller, or heat exchange between the reduction
reaction device and the distiller.
6. The electrochemical reaction device of claim 3, further
comprising at least one of a heat transfer member connecting
between the second electrolytic solution tank and the second
separation tank, a heat transfer member connecting between the
first separation tank and the second separation tank, a heat
transfer member connecting between the reduction reaction device
and the second electrolytic solution tank, a heat transfer member
connecting between the carbon dioxide generation source and the
distiller, or a heat transfer member connecting between the
reduction reaction device and the distiller.
7. The device of claim 1, wherein the generator includes a
photoelectric conversion body having a first face connected to the
reduction electrode and a second face connected to the oxidation
electrode.
8. The electrochemical reaction device of claim 1, further
comprising an ion exchange membrane disposed between the first
storage part and the second storage part.
9. The device of claim 1, wherein a pressure applied to the third
electrolytic solution is higher than a pressure applied to the
first electrolytic solution.
10. An electrochemical reaction device comprising: a first
electrolytic solution tank including a first storage part storing a
first electrolytic solution containing carbon dioxide and a second
storage part storing a second electrolytic solution containing
water; a reduction electrode immersed in the first electrolytic
solution; an oxidation electrode immersed in the second
electrolytic solution; a generator connected to the reduction
electrode and the oxidation electrode; a second electrolytic
solution tank including a third storage part storing a third
electrolytic solution containing carbon dioxide; and a flow path
connecting the first storage part and the third storage part,
wherein a pressure applied to the third electrolytic solution is
higher than a pressure applied to the first electrolytic
solution.
11. The device of claim 10, further comprising: a first separation
tank including a fourth storage part storing a fourth electrolytic
solution containing carbon dioxide and a first gas-liquid
separation membrane dividing the fourth storage part into a
plurality of regions; a second separation tank including a fifth
storage part storing a fifth electrolytic solution containing water
and a second gas-liquid separation membrane dividing the fifth
storage part into a plurality of regions; a second flow path
connecting the first storage part and the fourth storage part; a
third flow path connecting the third storage part and the fourth
storage part; and a fourth flow path connecting the second storage
part and the fifth storage part.
12. The device of claim 11, further comprising: a carbon dioxide
generation source containing carbon dioxide having a higher
temperature than the temperature of the first electrolytic
solution; a reduction reaction device reducing a product produced
by a reduction reaction of the carbon dioxide; a distiller disposed
on the third storage part; a fifth flow path connecting the first
storage part and the carbon dioxide generation source; and a sixth
flow path connecting the third storage part and the reduction
reaction device.
13. The device of claim 10, wherein the generator includes a
photoelectric conversion body having a first face connected to the
reduction electrode and a second face connected to the oxidation
electrode.
14. The electrochemical reaction device of claim 10, further
comprising an ion exchange membrane disposed 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 Application No. 2016-032480, filed on
Feb. 23, 2016; the entire contents of which are incorporated herein
by reference.
FIELD
[0002] Embodiments described herein relate generally to an
electrochemical reaction device.
BACKGROUND
[0003] Artificial photosynthesis technology of electrochemically
converting sunlight into a chemical substance in imitation of
photosynthesis of plants is under development from viewpoints of
energy problem and environmental problem. This is because, for
example, this technology makes it possible to obtain sufficient
energy even if a chemical substance produced by the conversion from
sunlight in a land which is of low utility value and not used for
the production of plants, such as, for example, a desert is
transported to a distant place. Converting sunlight to a chemical
substance to store it in a cylinder or a tank is advantageous in
that it costs lower for energy storage and has a less storage loss
than converting sunlight to electricity to store it in storage
batteries.
[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), these electrodes being
immersed in water in which carbon dioxide is dissolved. In this
case, the electrodes are electrically connected to each other via
an electric wire or the like. The electrode having the oxidation
catalyst oxidizes H.sub.2O using light energy to produce oxygen
(1/2O.sub.2) and obtains a potential. The electrode having the
reduction catalyst obtains the potential from the electrode that
causes the oxidation reaction, thereby reducing the carbon dioxide
to produce formic acid (HCOOH) or the like. Such two-stage
excitation for obtaining the reduction potential of the carbon
dioxide makes the two-electrode type device low in conversion
efficiency from the sunlight to the chemical energy.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] FIG. 1 is a schematic view illustrating a structure example
of an electrochemical reaction device.
[0006] FIG. 2 is a schematic view illustrating another structure
example of the electrochemical reaction device.
[0007] FIG. 3 is a schematic view illustrating a structure example
of a photoelectric conversion cell.
[0008] FIG. 4 is a schematic view illustrating another structure
example of the electrochemical reaction device.
[0009] FIG. 5 is a schematic view illustrating another structure
example of the electrochemical reaction device.
[0010] FIG. 6 is a schematic view illustrating another structure
example of the electrochemical reaction device.
[0011] FIG. 7 is a schematic view illustrating another structure
example of the electrochemical reaction device.
DETAILED DESCRIPTION
[0012] An electrochemical reaction device of an embodiment
includes: a first electrolytic solution tank including a first
storage part storing a first electrolytic solution containing
carbon dioxide and a second storage part storing a second
electrolytic solution containing water; a reduction electrode
immersed in the first electrolytic solution; an oxidation electrode
immersed in the second electrolytic solution; a generator connected
to the reduction electrode and the oxidation electrode; a second
electrolytic solution tank including a third storage part storing a
third electrolytic solution containing carbon dioxide; and a flow
path connecting the first storage part and the third storage part.
A temperature of the third electrolytic solution is lower than a
temperature of the first electrolytic solution.
[0013] Embodiments will be hereinafter described with reference to
the drawings. The drawings are schematic, and for example, the
sizes such as the thickness and width of each constituent element
may differ from the actual sizes of the constituent element. In the
embodiments, substantially the same constituent elements are
denoted by the same reference signs and a description thereof will
be omitted in some case. In this specification, the term "connect"
not only means "directly connect" but also may include the meaning
of "indirectly connect".
[0014] FIG. 1 is a schematic view illustrating a structure example
of an electrochemical reaction device. As illustrated in FIG. 1,
the electrochemical reaction device includes an electrolytic
solution tank 11, an electrolytic solution tank 12, a reduction
electrode 31, an oxidation electrode 32, a photoelectric conversion
body 33, an ion exchange membrane 4, a flow path 51, and a flow
path 52.
[0015] The electrolytic solution tank 11 has a storage part 111 and
a storage part 112. The electrolytic solution tank 11 is not
limited to have a particular shape and may have any
three-dimensional shape having a cavity serving as the storage
part.
[0016] The storage part 111 stores an electrolytic solution 21
containing a substance to be reduced. The substance to be reduced
is a substance that undergoes a reduction reaction to be reduced.
The substance to be reduced contains, for example, carbon dioxide.
Further, the substance to be reduced may contain hydrogen ions.
Changing an amount of water contained in the electrolytic solution
21 or changing electrolytic solution components can change
reactivity to change selectivity of the substance to be reduced and
a ratio of a produced chemical substance.
[0017] The storage part 112 stores an electrolytic solution 22
containing a substance to be oxidized. The substance to be oxidized
is a substance that undergoes an oxidation reaction to be oxidized.
The substance to be oxidized is, for example, water, or 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 contained in the electrolytic solution 21. In this case,
the electrolytic solution 21 and the electrolytic solution 22 may
be regarded as one electrolytic solution.
[0018] The electrolytic solution 22 preferably has higher pH than
pH of the electrolytic solution 21. This facilitates the migration
of hydrogen ions, hydroxide ions, and the like. Further, al quid
junction potential due to the difference in pH enables effective
progress of an oxidation-reduction reaction.
[0019] The electrolytic solution tank 12 has a storage part 113
storing an electrolytic solution 23. The electrolytic solution 23
contains carbon dioxide, for instance. The electrolytic solution
tank 12 has a function as a reduction catalyst absorber. The
temperature of the electrolytic solution 23 is lower than the
temperature of the electrolytic solution 21.
[0020] The reduction electrode 31 is immersed in the electrolytic
solution 21. The reduction electrode 31 contains a reduction
catalyst for the substance to be reduced, for instance. A compound
produced by the reduction reaction differs depending on, for
example, the kind of the reduction catalyst. For example, the
compound produced by the reduction reaction is: 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 a product flow
path, for instance. In this case, the product flow path is
connected to, for example, the storage part 111. The compound
produced by the reduction reaction may be recovered through another
flow path.
[0021] The reduction electrode 31 may have a structure in a thin
film form, a lattice form, a granular form or a wire form, for
instance. The reduction electrode 31 does not necessarily contain
the reduction catalyst. A reduction catalyst provided separately
from the reduction electrode 31 may be electrically connected to
the reduction electrode 31.
[0022] The oxidation electrode 32 is immersed in the electrolytic
solution 22. The oxidation electrode 32 contains an oxidation
catalyst for the substance to be oxidized, for instance. A compound
produced by the oxidation reaction differs depending on, for
example, the kind of the oxidation catalyst. Examples of the
compound produced by the oxidation reaction include hydrogen ions.
The compound produced by the oxidation reaction may be recovered
through a product flow path, for instance. In this case, the
product flow path is connected to, for example, the storage part
112. The compound produced by the oxidation reaction may be
recovered through another flow path.
[0023] The oxidation electrode 32 may have a structure in a thin
film form, a lattice form, a granular form, or a wire form, for
instance. The oxidation electrode 32 does not necessarily contain
the oxidation catalyst. An oxidation catalyst provided separately
from the oxidation electrode 32 may be electrically connected to
the oxidation electrode 32.
[0024] In a case where the oxidation electrode 32 is stacked and
immersed in the electrolytic solution 22, and where light is
radiated to the photoelectric conversion body 33 through the
oxidation electrode 32 to cause the oxidation-reduction reaction,
the oxidation electrode 32 needs to have a light transmitting
property. Light transmittance of the oxidation electrode 32 is
preferably, for example, at least 10% or more, more preferably 30%
or more of an irradiation amount of the light irradiating the
oxidation electrode 32.
[0025] This is not restrictive, and the photoelectric conversion
body 33 may be irradiated with the light through the reduction
electrode 31, for instance.
[0026] The photoelectric conversion body 33 has a face 331
electrically connected to the reduction electrode 31 and a face 332
electrically connected to the oxidation electrode 32. In FIG. 1,
the face 331 and the reduction electrode 31, and the face 332 and
the oxidation electrode 32 are connected by heat transfer members
such as wiring lines having a heat transfer property, for instance.
Connecting the photoelectric conversion body to the reduction
electrode or the oxidation electrode by the wiring line or the like
is advantageous as a system, since constituent elements are
separated according to the function. The photoelectric conversion
body 33 may be disposed outside the electrolytic solution tank 11.
Incidentally, 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.
[0027] The photoelectric conversion body 33 has a function of
separating electric charges when given energy of the irradiating
light such as sunlight. Electrons and holes generated by the charge
separation migrate to the reduction electrode side and the
oxidation electrode side respectively. Consequently, the
photoelectric conversion body 33 can generate an electromotive
force. As the photoelectric conversion body 33, a pn-junction or
pin-junction photoelectric conversion body is usable, for instance.
The photoelectric conversion body 33 may be fixed to the
electrolytic solution tank 11, for instance. Incidentally, the
photoelectric conversion body 33 may be composed of a stack of a
plurality of photoelectric conversion layers.
[0028] The reduction electrode 31, the oxidation electrode 32, and
the photoelectric conversion body 33 may be different in size.
[0029] The ion exchange membrane 4 is disposed so as to separate
the storage part 111 and the storage part 112. Examples of the ion
exchange membrane 4 include Neosepta (registered trademark)
manufactured by ASTOM Corporation, Selemion (registered trademark)
and Aciplex (registered trademark) manufactured by Asahi Glass Co.
Ltd., fumasep (registered trademark) and fumapem (registered
trademark) manufactured by Fumatech GmbH, Nafion (registered
trademark), which is a fluorocarbon resin produced through
polymerization of sulfonated tetrafluoroethylene, manufactured by
Du Pont, Lewabrane (registered trademark) manufactured by LANXESS,
IONSEP (registered trademark) manufactured by IONTECH, Mustang
(registered trademark) manufactured by Pall Corporation, ralex
(registered trademark) manufactured by MEGA a.s., and Gore-Tex
(registered trademark) manufactured by W. L. Gore & Associates.
The ion exchange membrane 4 may be formed of a film having a
hydrocarbon basic skeleton or for anion exchange, may be formed of
a film having an amine group. Incidentally, the ion exchange
membrane 4 does not necessarily have to be provided.
[0030] The flow path 51 and the flow path 52 have a function as
electrolytic solution flow paths to distribute the electrolytic
solutions. Their function is not limited to this, and the
electrolytic solutions and the products by the oxidation-reduction
reaction may be distributed through the flow path 51 and the flow
path 52. For the electrolytic solution tanks 11, 12 and the flow
paths 51, 52, materials that transmit light may be used, for
instance.
[0031] The flow path 51 connects the storage part 111 and the
storage part 113. The ions and other substances contained in the
electrolytic solution 21 can move to the electrolytic solution tank
12 through the flow path 51.
[0032] The flow path 52 connects the storage part 111 and the
storage part 113. Ions and other substances contained in the
electrolytic solution 23 can move to the electrolytic solution tank
11 through the flow path 52.
[0033] The shape of the flow path 51 and the flow path 52 is not
limited to a particular shape, provided that they have a shape
having a cavity allowing the electrolytic solutions to flow
therethrough, such as a pipe shape. The electrolytic solution of at
least one of the flow path 51 and the flow path 52 may be
circulated by a circulation pump. At least part of the electrolytic
solution 21 moves to the storage part 113 through the flow path 51,
for instance. At least part of the electrolytic solution 23 moves
to the storage part 111 through the flow path 52, for instance. The
arrows illustrated in FIG. 1 indicate circulation directions of the
electrolytic solutions.
[0034] Next, an operation example of the electrochemical reaction
device illustrated in FIG. 1 will be described. When light is
incident on the photoelectric conversion body 33, the photoelectric
conversion body 33 generates photoexcited electrons and holes. At
this time, the photoexcited electrons gather to the reduction
electrode 31 and the holes gather to the oxidation electrode 32.
Consequently, the electromotive force is generated in the
photoelectric conversion body 33. As the light, sunlight is
preferable, but light of a light emitting diode, an organic EL, or
the like may be incident on the photoelectric conversion body
33.
[0035] The following describes a case where electrolytic solutions
containing water and carbon dioxide are used as the electrolytic
solution 21 and the electrolytic solution 22 and carbon monoxide is
produced. Around the oxidation electrode 32, as expressed by the
following formula (1), the water undergoes an oxidation reaction
and loses electrons, so that oxygen and hydrogen ions are produced.
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 carbon dioxide undergoes a reduction
reaction and the hydrogen ions react with the carbon dioxide while
receiving the electrons, so that carbon monoxide and water are
produced. Further, in addition to the carbon monoxide, hydrogen is
produced by the hydrogen ions receiving the electrons 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 the formula (1) is 1.23 [V].
The standard oxidation-reduction potential of the reduction
reaction in the formula (2) is -0.03 [V]. The standard
oxidation-reduction potential of the reaction in the formula (3) is
0 V. In this case, the open-circuit voltage needs to be 1.26 [V] or
more in the reactions of the formula (1) and the formula (2).
[0038] The open-circuit voltage of the photoelectric conversion
body 33 is preferably higher than the potential difference between
the standard oxidation-reduction potential of the oxidation
reaction and the standard oxidation-reduction potential of the
reduction reaction by a value of overvoltages or more. For example,
the overvoltages of the oxidation reaction in the formula (1) and
the reduction reaction in the formula (2) are both 0.2 [V]. The
open-circuit voltage is preferably 1.66 [V] or more in the
reactions of the formula (1) and the formula (2). Similarly, the
open-circuit voltage is preferably 1.63 [V] or more in the
reactions of the formula (1) and the formula (3).
[0039] The reduction reactions of hydrogen ions and carbon dioxide
are reactions consuming hydrogen ions. This means that a small
amount of the hydrogen ions results in low efficiency of the
reduction reaction. So, the electrolytic solution 21 and the
electrolytic solution 22 preferably have different hydrogen ion
concentrations so that the concentration difference facilitates the
migration of the hydrogen ions. The concentration of anions (for
example, hydroxide ions) may be made different between the
electrolytic solution 21 and the electrolytic solution 22.
[0040] Reaction efficiency of the formula (2) varies depending on
the concentration of the carbon dioxide dissolved in the
electrolytic solution. The higher the concentration of the carbon
dioxide, the higher the reaction efficiency, and as the former is
lower, the latter is lower. Since solubility of the carbon dioxide
is low, it is difficult to increase the concentration of the carbon
dioxide in the electrolytic solution. The reaction efficiency of
the formula (2) also varies depending on the concentration of
hydrogen carbonate ions or carbonate ions. However, the
concentration of hydrogen carbonate ions or the concentration of
carbonate ions can be adjusted by an increase of the electrolytic
solution concentration or the adjustment of pH and thus is more
easily adjusted than the carbon dioxide concentration.
Incidentally, even if the ion exchange membrane is provided between
the oxidation electrode and the reduction electrode, carbon dioxide
gas, carbonate ions, hydrogen carbonate ions, and so on pass
through the ion exchange membrane 4 and thus it is difficult to
completely prevent performance deterioration.
[0041] A possible method to increase the carbon dioxide
concentration may be, for example, a method of blowing the carbon
dioxide directly to the electrolytic solution tank 11. However, in
a case where the reduction product is gaseous carbon monoxide or
the like, the carbon dioxide gas and the carbon monoxide gas need
to be separated. This results in a cost increase due to the
complication of the device, and an energy loss due to the need for
energy for the separation.
[0042] The electrochemical reaction device of this embodiment
includes the first electrolytic solution tank used for the
oxidation-reduction reaction and the second electrolytic solution
tank connected to the first electrolytic solution tank. The
temperature of the electrolytic solution stored in the storage part
of the second electrolytic solution tank is lower than the
temperature of the electrolytic solution stored in the storage part
of the first electrolytic solution tank. For example, cooling the
storage part in the second electrolytic solution tank can make the
temperature of the electrolytic solution stored in the second
electrolytic solution tank lower than the temperature of the
electrolytic solution stored in the storage part of the first
electrolytic solution tank. Solubility of the carbon dioxide in the
second electrolytic solution tank is higher than solubility of the
carbon dioxide in the first electrolytic solution tank.
[0043] It is possible to increase the carbon dioxide concentration
in the second electrolytic solution tank also by making a pressure
applied to the electrolytic solution 23 higher than a pressure
applied to the electrolytic solution 21. In this case, the pressure
of the storage part of the second electrolytic solution tank may be
set higher than the pressure of the storage part of the first
electrolytic solution tank. Further, a pressure regulator may be
provided in the flow path 52.
[0044] By supplying the first electrolytic solution tank with the
electrolytic solution whose carbon dioxide concentration has been
adjusted high in the second electrolytic solution tank, it is
possible to increase the carbon dioxide concentration of the
electrolytic solution stored in the first electrolytic solution
tank. This can improve efficiency of the reduction reaction.
[0045] If the storage parts of the first electrolytic solution tank
are cooled, the reactions by the catalysts deteriorate and
accordingly reaction efficiency tends to lower. If the pressure is
applied to the storage parts of the first electrolytic solution
tank, pressure resistance of the electrolytic solution tank needs
to be increased, leading to an increased cost and a complicated
structure. Further, the increase of the pressure resistance worsens
maintainability, for example, making the change of the electrodes
troublesome.
[0046] For a reduction of a supply amount of the carbon dioxide and
efficient absorption of the carbon dioxide in the electrolytic
solution, an interval between bubbles of the carbon dioxide passing
through the electrolytic solution needs to be wide. However, the
interval of the bubbles becomes short when the carbon dioxide
concentration is increased, allowing the downsizing of the
electrolytic solution. A cooling temperature is preferably equal to
or lower than the temperature of the electrolytic solution in the
first electrolytic solution tank, for instance. If the temperature
of the electrolytic solution is increased by the
oxidation-reduction reaction, the cooling temperature is preferably
not lower than the room temperature nor higher than the temperature
of the electrolytic solution of the first electrolytic solution
tank. The cooling temperature is more preferably not lower than a
temperature at which the electrolytic solution freezes nor more
than the temperature of the electrolytic solution.
[0047] The temperature of the electrolytic solution in the first
electrolytic solution tank is preferably higher than the freezing
point. For example, in a case where the electrolytic solution
contains ions such as potassium ions or sodium ions for the purpose
of increasing an absorption amount of carbon dioxide, increasing
the concentrations of carbon dioxide ions and HCO.sub.3 ions, and
increasing solution resistance of the electrolytic solution, the
electrolytic solution does not freeze at .degree. C. However,
extreme cooling requires a large cooler, leading to a cost increase
and an energy loss, and thus the temperature of the electrolytic
solution is preferably 0.degree. C. or higher in some case.
Further, 5.degree. C. or higher or 10.degree. C. or higher is
preferable in some case because of a concern about an energy loss
of the whole electrochemical reaction device and reaction
deterioration due to the extreme cooling of the electrolytic
solution.
[0048] Temperature regulators may be provided in the electrolytic
solution tanks 11 12 or the flow paths 51, 52 to impede the
deterioration of reaction efficiency due to a temperature decrease
of the electrolytic solution. Adjusting the temperature by the
temperature regulator 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 Further since even a
temperature difference of several .degree. C. can produce the
effect, irradiating the electrolytic solution flow path between the
first electrolytic solution tank and the second electrolytic
solution tank or irradiating the electrolytic solution tank with
sunlight to heat it is efficient owing to the use of natural
energy. Further, in a later-described case where the primary
reaction is caused by electric energy generated by the conversion
from sunlight, heat energy and light energy of the sunlight can be
efficiently used, resulting in further improvement of
efficiency.
[0049] Structure examples of the constituent elements in the
electrochemical reaction device will be further described. As a
water-containing electrolytic solution usable as the electrolytic
solution, an aqueous solution containing a desired electrolyte is
usable, for instance. This solution is preferably an aqueous
solution that promotes the oxidation reaction of water. Examples of
the aqueous solution containing the 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.-), or hydrogen carbonate ions (HCO.sub.3.sup.-).
[0050] Examples of an electrolytic solution containing carbon
dioxide usable as the electrolytic solution include aqueous
solutions containing LiHCO.sub.3, NaHCO.sub.3, KHCO.sub.3,
CsHCO.sub.3, phosphoric acid, or boric acid. The electrolytic
solution containing carbon dioxide may contain alcohol such as
methanol, ethanol, or acetone. The electrolytic solution containing
water may be the same as the electrolytic solution containing
carbon dioxide. However, an absorption amount of carbon dioxide in
the electrolytic solution containing carbon dioxide is preferably
high. So, 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 lowers a reduction
potential of carbon dioxide, has high ion conductivity, and
contains a carbon dioxide absorbent that absorbs carbon
dioxide.
[0051] As the aforesaid electrolytic solution, an ionic liquid that
contains salts of cations such as imidazolium ions or pyridinium
ions and anions such as BF.sub.4.sup.- or PF.sub.6.sup.- and is in
a liquid state in a wide temperature range, or its aqueous solution
is usable, for instance. Other examples of the electrolytic
solution include solutions of amine such as ethanolamine,
imidazole, and pyridine, and aqueous solutions thereof. Examples of
the amine include primary amine, secondary amine, and tertiary
amine. These electrolytic solutions may be high in ion
conductivity, have a property of absorbing carbon dioxide, and have
a property of lowering reduction energy.
[0052] Examples of the primary amine include methylamine,
ethylamine, propylamine, butylamine, pentylamine, and hexylamine.
Hydrocarbons of the amine may be substituted by alcohol, halogen,
or the like. Examples of the amine whose hydrocarbons are
substituted include methanolamine, ethanolamine, and chloromethyl
amine. Further, an unsaturated bond may exist. The same thing can
be said for hydrocarbons of the secondary amine and the tertiary
amine.
[0053] Examples of the secondary amine include dimethylamine,
diethylamine, dipropylamine, dibutylamine, dipentylamine,
dihexylamine, dimethanolamine, diethanolamine, and dipropanolamine.
The substituted hydrocarbons may be different. This also applies to
the tertiary amine. Examples of the amine having different
hydrocarbons include methylethylamine and methylpropylamine.
[0054] Examples of the tertiary amine include trimethylamine,
triethylamine, tripropylamine, tributylamine, trihexylamine,
trimethanolamine, triethanolamine, tripropanolamine,
tributanolamine, tripropanolamine, triexanolamine,
methyldiethylamine, and methyldipropylamine.
[0055] Examples of the cations of the ionic liquid include [0056]
1-ethyl-3-methylimidazolium ions, 1-methyl-3-propylimidazolium
ions, 1-butyl-3-methylimidazole ions, 1-methyl-3-pentylimidazolium
ions, and 1-hexyl-3-methylimidazolium ions.
[0057] A second place of imidazolium ions may be substituted.
Examples of the cations in which the second place of the
imidazolium ions is substituted include [0058]
1-ethyl-2,3-dimethylimidazolium ions,
1-2-dimethyl-3-propylimidazolium ions,
1-butyl-2,3-dimethylimidazolium ions,
1,2-dimethyl-3-pentylimidazolium ions, and
1-hexyl-2,3-dimethylimidazolium ions.
[0059] Examples of pyridinium ions include methylpyridinium,
ethylpyridinium, propylpyridinium, butylpyridinium,
pentylpyridinium, and hexylpyridinium. In both of the imidazolium
ions and the pyridinium ions, an alkyl group may be substituted, or
an unsaturated bond may exist.
[0060] Examples of the anions include fluoride ions, chloride ions,
bromide ions, iodide ions, 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(trifluoromethoxysulfonyl)imide, and
bis(perfluoroethylsulfonyl)imide. Dipolar ions in which the cations
and the anions of the ionic liquid are coupled by hydrocarbons may
be used. Incidentally, a buffer solution such as a potassium
phosphate solution may be supplied to the storage parts 111,
112.
[0061] FIG. 2 is a view illustrating another example of the
electrochemical reaction device. The electrochemical reaction
device illustrated in FIG. 2 is different from the electrochemical
reaction device illustrated in FIG. 1 in that 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 face 331 and the oxidation electrode 32 is in
contact with the face 332. In this case, a stack including the
reduction electrode 31, the oxidation electrode 32, and the
photoelectric conversion body 33 is also called a photoelectric
conversion cell. The photoelectric conversion cell penetrates
through the ion exchange membrane 4 and is immersed in the
electrolytic solution 21 and the electrolytic solution 22.
[0062] FIG. 3 is a schematic cross-sectional view illustrating a
structure example of the photoelectric conversion cell. The
photoelectric conversion cell illustrated in FIG. 3 includes a
conductive substrate 30, the reduction electrode 31, the oxidation
electrode 32, the photoelectric conversion body 33, a light
reflective body 34, a metal oxide body 35, and a metal oxide body
36.
[0063] The conductive substrate 30 is in contact with the reduction
electrode 31. The conductive substrate 30 may be regarded as part
of the reduction electrode. Examples of the conductive substrate 30
include a substrate containing at least one or more of Cu, Al, Ti,
Ni, Fe, and Ag. For example, a stainless steel substrate containing
stainless steel such as SUS may be used. The conductive substrate
30 is not limited to the above and may be formed of a conductive
resin. Alternatively, the conductive substrate 30 may be
constituted by a substrate of a semiconductor such as Si or Ge.
Further, a resin film or the like may be used as the conductive
substrate 30. For example, the film usable as the ion exchange
membrane 4 may be used as the conductive substrate 30.
[0064] The conductive substrate 30 has a function as a support. The
conductive substrate 30 may be disposed so as to separate the
storage part 111 and the storage part 112. The presence of the
conductive substrate 30 can improve mechanical strength of the
photoelectric conversion cell. Further, the conductive substrate 30
may be regarded as part of the reduction electrode 31. Further, the
conductive substrate 30 does not necessarily have to be
provided.
[0065] 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 a material that reduces activation energy for
reducing hydrogen ions or carbon dioxide. In other words, a
material that lowers the overvoltages when hydrogen and a carbon
compound are produced by the reduction reactions of hydrogen ions
and carbon dioxide is usable. For example, a metal material or a
carbon material is usable. For example, in the production of
hydrogen, a metal such as platinum or nickel, or an alloy
containing this metal is usable as the metal material. In the
reduction reaction of carbon dioxide, a metal such as gold,
aluminum, copper, silver, platinum, palladium, or nickel, or an
alloy containing this metal is usable. As the carbon material,
graphene, carbon nanotube (CNT), fullerene, or ketjen black is
usable, for instance. The reduction catalyst is not limited to
these, and may be, 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, or may be a mixture of a plurality
of materials.
[0066] The oxidation electrode 32 preferably contains an oxidation
catalyst. The oxidation electrode 32 may contain both a conductive
material and the oxidation catalyst. Examples of the oxidation
catalyst include a material that reduces activation energy for
oxidizing water. In other words, a material that lowers the
overvoltage when oxygen and hydrogen ions are produced by the
oxidation reaction of water is usable. Examples thereof include
iridium, platinum, cobalt, and manganese. Further, as the oxidation
catalyst, a binary metal oxide, a ternary metal oxide, or a
quaternary metal oxide is usable, for instance. 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), and ruthenium
oxide (Ru--O). Examples of the ternary metal oxide include
Ni--Co--O, La--Co--O, Ni--La--O, and Sr--Fe--O. Examples of the
quaternary metal oxide include Pb--Ru--Ir--O and La--Sr--Co--O. The
oxidation catalyst is not limited to these, and may be a metal
complex such as a Ru complex or a Fe complex, or a mixture of a
plurality of materials.
[0067] At least one of the reduction electrode 31 and the oxidation
electrode 32 may have a porous structure. Examples of a material
usable for the electrode having the porous structure include, in
addition to the above-listed materials, carbon black such as ketjen
black and VULCAN XC-72, activated carbon, and metal fine powder.
The porous structure can increase the area of an active surface
contributing to the oxidation-reduction reaction and thus can
increase conversion efficiency.
[0068] In a case where an electrode reaction with a low current
density is caused using relatively low irradiation energy of light,
the catalyst material can be selected from a wide range of options.
Accordingly, it is easy to cause the reaction using, for example, a
ubiquitous metal, and it is also relatively easy to obtain
selectivity of the reaction. On the other hand, in a case where the
photoelectric conversion body 33 is not disposed in the
electrolytic solution tank 11 and is electrically connected to at
least one of the reduction electrode 31 and the oxidation electrode
32 by, for example, a wiring line, the electrode area is usually
decreased due to a reason such as the downsizing of the
electrolytic solution tank, and the reaction is sometimes caused
with a high current density. In this case, a noble metal is
preferably used as the catalyst.
[0069] The photoelectric conversion body 33 has a stacked structure
of a photoelectric conversion layer 33x, a photoelectric conversion
layer 33y, and a photoelectric conversion layer 33z. The number of
the stacked photoelectric conversion layers is not limited to that
in FIG. 3.
[0070] The photoelectric conversion layer 33x has, 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 microcrystalline silicon. The i-type
semiconductor layer 331i is a layer that absorbs light in a short
wavelength range including 400 nm, for instance. Accordingly, in
the photoelectric conversion layer 33x, charge separation is caused
by energy of light in the short wavelength range.
[0071] The photoelectric conversion layer 33y has, 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 microcrystalline silicon. The i-type
semiconductor layer 332i is a layer that absorbs light in an
intermediate wavelength range including 600 nm, for instance.
Accordingly, in the photoelectric conversion layer 33y, charge
separation is caused by energy of light in the intermediate
wavelength range.
[0072] The photoelectric conversion layer 33z has, 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 microcrystalline silicon. The i-type semiconductor layer
333i is a layer that absorbs light in a long wavelength range
including 700 nm, for instance. Accordingly, in the photoelectric
conversion layer 33z, charge separation is caused by energy of
light in the long wavelength range.
[0073] The p-type semiconductor layers or the n-type semiconductor
layers each can be formed of, for example, a semiconductor material
to which an element that is to be a donor or an acceptor is added.
Incidentally, in the photoelectric conversion layer, as the
semiconductor layers, the semiconductor layers containing silicon,
germanium, or the like are used, but the semiconductor layers are
not limited to these, and maybe compound semiconductor layers, for
instance. As the compound semiconductor layers, semiconductor
layers containing, for example, GaAs, GaInP, AlGaInP, CdTe, or
CuInGaSe are usable, for instance. Further, layers containing a
material such as TiO.sub.2 or WO.sub.3 may be used, provided that
photoelectric conversion is possible. Further, the semiconductor
layers each may be monocrystalline, polycrystalline, or amorphous.
Further, the photoelectric conversion layer may include a zinc
oxide layer.
[0074] The light reflective body 34 is between the conductive
substrate 30 and the photoelectric conversion body 33. Examples of
the light reflective body 34 include a distributed Bragg reflection
layer composed of a stack of metal layers or semiconductor layers,
for instance. Owing to the presence of the light reflective body
34, light that cannot be absorbed by the photoelectric conversion
body 33 can be reflected to enter one of the photoelectric
conversion layer 33x to the photoelectric conversion layer 33z,
enabling to enhance conversion efficiency from light to a chemical
substance. As the light reflective body 34, a layer of a meal such
as Ag, Au, Al, or Cu or an alloy containing at least one of these
metals is usable, for instance.
[0075] The metal oxide body 35 is between the light reflective body
34 and the photoelectric conversion body 33. The metal oxide body
35 has a function of enhancing light reflectivity by adjusting an
optical distance, for instance. For the metal oxide body 35, a
material capable of ohmic contact with the n-type semiconductor
layer 331n is preferable used. As the metal oxide body 35, a layer
of a light-transmissive metal oxide such as, for example, indium
tin oxide (ITO), zinc oxide (ZnO), fluorine-doped tin oxide (FTO),
aluminum-doped zinc oxide (AZO), or antimony-doped tin oxide (ATO)
is usable.
[0076] The metal oxide body 36 is between the oxidation electrode
32 and the photoelectric conversion body 33. The metal oxide body
36 may be disposed on a surface of the photoelectric conversion
body 33. The metal oxide body 36 has a function as a protective
layer preventing the photoelectric conversion cell from being
broken by the oxidation reaction. The presence of the metal oxide
body 36 can prevent the corrosion of the photoelectric conversion
body 33 to extend the life of the photoelectric conversion cell.
Incidentally, the metal oxide body 36 does not necessarily have to
be provided.
[0077] As the metal oxide body 36, a dielectric thin film of
TiO.sub.2, ZrO.sub.2, Al.sub.2O.sub.3, SiO.sub.2, or HfO.sub.2 is
usable, for instance. The metal oxide body 36 preferably has a
thickness of 10 nm or less, further 5 nm or less. This is intended
to obtain electrical conductivity by a tunnel effect. As the metal
oxide body 36, a layer of a light transmissive metal oxide such as,
for example, 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.
[0078] The metal oxide body 36 may have, for example, a stacked
structure of a metal and a transparent conductive oxide, a
composite structure of a metal and another conductive material, or
a composite structure of a transparent conductive oxide and another
conductive material. The above structure can decrease the number of
parts, decrease the weight, and facilitate the manufacture,
enabling cost reduction. The metal oxide body 36 may have functions
as a protective layer, a conductive layer, and a catalyst
layer.
[0079] In the photoelectric conversion cell illustrated in FIG. 3,
a face of the n-type semiconductor layer 331n opposite to its
contact surface with the i-type semiconductor layer 331i is a first
face of the photoelectric conversion body 33, and a face of the
p-type semiconductor layer 333p opposite to its contact surface
with the i-type semiconductor layer 333i is a second face. The
photoelectric conversion cell illustrated in FIG. 3 has the stacked
structure of the photoelectric conversion layer 33x to the
photoelectric conversion layer 33z as described above and thus is
capable of absorbing lights in a wide wavelength range of sunlight,
enabling more efficient use of energy of sunlight. In this case, a
high voltage can be obtained owing to the series connection of the
photoelectric conversion bodies.
[0080] In FIG. 3, electrons and holes having undergone the charge
separation can be used as they are in the oxidation-reduction
reaction, since the electrodes are stacked on the photoelectric
conversion body 33. Further, the photoelectric conversion body 33
and the electrodes need not be electrically connected by wiring
lines or the like. This enables a high-efficiency
oxidation-reduction reaction.
[0081] The plural photoelectric conversion bodies may be
electrically connected in parallel. A dual junction or single-layer
photoelectric conversion body may be used. A stack of two
photoelectric conversion bodies, or four photoelectric conversion
bodies or more may be used. A single-layer photoelectric conversion
layer may be used instead of the stack of the plural photoelectric
conversion layers.
[0082] The electrochemical reaction device of this embodiment is a
simplified system with a reduced number of parts owing to the
integration of the reduction electrode, the oxidation electrode,
and the photoelectric conversion body. This facilitates at least
one of, for example, manufacture, installation, and maintenance.
Further, this structure eliminates a need for wiring lines
connecting the photoelectric conversion body to the reduction
electrode and the oxidation electrode, achieving an increased light
transmittance and an increased light-receiving area.
[0083] The photoelectric conversion body 33 is in contact with the
electrolytic solution, which may lead to its corrosion and the
dissolving of corrosive products in the electrolytic solution to
deteriorate the electrolytic solution. A possible measure to
prevent the corrosion may be to provide a protective layer.
However, components of the protective layer may dissolve in the
electrolytic solution. Here, providing a filter such as a metal ion
filter in the flow path or the electrolytic solution tank hinders
the deterioration of the electrolytic solution.
[0084] The electrochemical reaction device of this embodiment is an
art suitable as a measure for surplus power and is required to make
good use of solar energy. In a case where illuminance of sunlight
is high, when there is no surplus power, energy is obtained as much
as possible, and when there is surplus energy, the energy is
consumed by being used for circulating the electrolytic solution.
This enables efficient energy mix to increase the total energy
utilization ratio. In a case where a buffer solution is used as the
electrolytic solution, a small reaction amount also results in a
small pH change caused by the reaction. So, during a non-reaction
period, the electrolytic solution is circulated to keep the
electrolytic solution components uniform, and during the reaction,
the supply of the electrolytic solution is restricted or stopped.
This can prevent a decrease of total efficiency and reduce the
cost. For example, preferably, the electrolytic solution is
circulated using nighttime wind power or low-cost surplus power,
and in the daytime, the oxidation-reaction reaction is caused, with
the circulation of the electrolytic solution being stopped or with
the minimum supply amount of the electrolytic solution.
[0085] A structure example of the electrochemical reaction device
is not limited to that in FIG. 1. FIG. 4 is a schematic view
illustrating another example of the electrochemical reaction
device. The electrochemical reaction device illustrated in FIG. 4
is different from the electrochemical reaction device illustrated
in FIG. 1 at least in that it further includes a separation tank
13, a separation tank 14, a flow path 53 to a flow path 55.
[0086] The separation tank 13 has a storage part 114a storing an
electrolytic solution 24 and a gas-liquid separation membrane 114b
dividing the storage part 114a into a plurality of regions. The
gas-liquid separation membrane 114b includes, for example, a hollow
fiber membrane and so on. The hollow fiber membrane contains, for
example, a silicone resin, a fluorine-based resin
(perfluoroalkoxyalkane (PFA), a perfluoroethylene propene copolymer
(F E P), polytetrafluoroethylene (PTFE), an
ethylene-tetrafluoroethylene copolymer (ETFE), polyvinylidene
fluoride (PVDF), polychlorotrifluoroethylene (PCTFE), an
ethylene-chlorotrifluoroethylene copolymer (ECTFE)), or the
like.
[0087] In the electrochemical reaction device illustrated in FIG.
4, part of the reduction product in the electrolytic solution tank
11 is extracted in the separation tank 13. An outer side of the
gas-liquid separation membrane 114b (its surface side opposite to
its contact surface with the electrolytic solution 24) is
pressure-reduced and the electrolytic solution 24 containing a
gaseous product passes through the gas-liquid separation membrane
114b, enabling the efficient separation of the gaseous product and
carbon dioxide. In a case where the product is, for example, carbon
monoxide, only carbon monoxide gas can be separated by the
gas-liquid separation in the separation tank 13.
[0088] The flow path 51 is connected to the storage part 114a. The
flow path 53 connects the storage part 113 and the storage part
114a. The separation tank 14 has a storage part 115a storing an
electrolytic solution 25 and a gas-liquid separation membrane 115b
dividing the storage part 115a into a plurality of regions. The
flow path 54 connects the storage part 112 and the storage part
115a. The flow path 55 connects the storage part 112 and the
storage part 115a. At least part of the electrolytic solution 22 is
supplied to the storage part 115a through the flow path 54. At
least part of the electrolytic solution 25 is supplied to the
storage part 112 through the flow path 55. Circulation pumps or the
like may be provided in the flow path 54 and the flow path 55.
[0089] An outer side of the gas-liquid separation membrane 115b
(its surface side opposite to its contact surface with the
electrolytic solution 25) is pressure-reduced and the electrolytic
solution containing a gaseous product passes through the gas-liquid
separation membrane 115b, so that oxygen gas and dissolved oxygen
can be separated similarly to the carbon dioxide. It can be
conceived to directly recover and use the oxygen gas generated in
the electrolytic solution tank 11, but since the oxygen gas is
dissolved in the electrolytic solution 22, it is difficult to
completely recover the oxygen gas. Since the dissolved oxygen
deteriorates performance of the oxide electrode, the dissolved
oxygen is desirably recovered in the form of gas. Unlike the gas
separation in the electrolytic solution tank 11, it is possible to
recover gases generated in a plurality of cells at a time.
Accordingly, the total flow path length for the gas recovery is
shortened, enabling a simplified system. In this case, by providing
temperature regulators in the separation tank 14 or the flow path
54 and the flow path 55 as in the electrolytic solution tank 12 in
order to efficiently recover the oxygen gas, it is possible to
efficiently separate oxygen from the electrolytic solution.
[0090] By providing a temperature regulator in the separation tank
13 or the flow path 51, it is possible to enhance separation
efficiency of the product. For the complete gas separation, the
dissolved gas in the electrolytic solution is preferably removed as
much as possible. An agitator is preferably provided in the
separation tank 13 to enhance efficiency of removing the dissolved
gas by temperature distribution or the like.
[0091] A 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 not less than -10.degree. C. nor more than 10.degree. C. Too
high a temperature of the electrolytic solution 24 in the
separation tank 13 is likely to decrease the gas concentration of
the product due to the vaporization of carbon dioxide dissolved in
the electrolytic solution 24. Excessive heating leads to efficiency
deterioration because of a large energy loss by the heating.
[0092] In a case where the product is a water-soluble liquid
substance such as methanol or ethanol, a separation method in the
separation tank 13 may be distillation or membrane separation, for
instance. In this case, a temperature regulator is desirably
provided to improve separation efficiency. The separation membrane
may be zeolite, for instance. Heat especially on an upstream side
is large and thus is likely to deteriorate the total efficiency. To
cope with this, providing a heat insulator in the separation tank
13 can prevent the efficiency deterioration.
[0093] In a case where an ion exchange membrane or a flow path is
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. By the above structure, it is possible to easily
separate and extract oxygen being the reaction product in the
oxidation side.
[0094] A suitable electrolytic solution differs depending on each
catalyst, and by making the electrolytic solutions in contact with
the catalyst layers different, it is possible to improve
efficiency. Furthermore, making pH on the oxidation side larger
than that on the reduction side is advantageous in that a liquid
junction potential caused by the pH difference can compensate for
an insufficient potential of the reaction.
[0095] An electrochemical reaction device illustrated in FIG. 5
includes the structure of the electrochemical reaction device
illustrated in FIG. 4, a flow path 56, a cooler 61a, a cooler 61b,
a heater 62a, a heater 62b, a pump 71, and a pressure valve 72.
[0096] The flow path 56 is connected to the storage part of the
electrolytic solution tank 12. For example, the flow path 56 is
connected to a carbon dioxide generation source 80.
[0097] The cooler 61a has a function of cooling the electrolytic
solution flowing in the flow path 56. The cooler 61a may be
disposed inside or outside the flow path 56, for instance.
[0098] The cooler 61b has a function of cooling the electrolytic
solution 23. The cooler 61b may be disposed inside or outside the
storage part 113, for instance.
[0099] The heater 62a has a function of heating the electrolytic
solution 25. The heater 62a may be disposed inside or outside the
storage part 115a, for instance.
[0100] The heater 62b has a function of heating the electrolytic
solution flowing in the flow path 54. The heater 62b may be
disposed inside or outside the flow path 54, for instance.
[0101] The pump 71 has a function of promoting the supply of the
electrolytic solution from the storage part 114a to the storage
part 113. The pump 71 is disposed inside or outside the flow path
53, for instance. The pump 71 does not necessarily have to be
provided.
[0102] The pressure valve 72 has a function of promoting the supply
of the electrolytic solution from the storage part 113 to the
storage part 111. The pressure valve 72 is disposed inside or
outside the flow path 52, for instance. Examples of the pressure
valve 72 include an orifice valve and a pulse valve. The pressure
valve 72 does not necessarily have to be provided.
[0103] Heat exchange between the separation tank 13 and the
separation tank 14 may be performed. The heat exchange is possible
by providing a heat transfer member 91 connecting, for example, the
separation tank 13 and the separation tank 14. The heat transfer
member 91 may be provided so as to connect the storage part 114a
and the storage part 115a, for instance. Alternatively, a heat
exchanger or the like may be separately connected.
[0104] An electrochemical reaction device illustrated in FIG. 6
further includes a cooler 61c in addition to the structure
illustrated in FIG. 5, and does not include the separation tank
13.
[0105] The flow path 53 connects the storage part 111 and the
storage part 113. The flow path 56 is connected to the storage part
111. The flow path 56 connects, for example, the storage part 111
and the carbon dioxide generation source 80. The carbon dioxide
generation source 80 may be disposed inside or outside the
electrochemical reaction device.
[0106] Heat exchange between the electrolytic solution tank 12 and
the separation tank 14 may be performed. The heat exchange is
possible by providing a heat transfer member 92 connecting, for
example, the electrolytic solution tank 12 and the separation tank
14.
[0107] The heat transfer member 92 may be provided so as to connect
the flow path 53 and the flow path 54, for instance. Alternatively,
a heat exchanger or the like may be separately connected.
[0108] The cooler 61c has a function of cooling the electrolytic
solution flowing in the flow path 53. The cooler 61c is disposed
inside or outside the flow path 53, for instance.
[0109] The pump 71 has a function of promoting the supply of the
electrolytic solution from the storage part 113 to the storage part
111. The pump 71 is disposed in the flow path 52, for instance.
[0110] The pressure valve 72 has a function of promoting the supply
of the electrolytic solution from the storage part 111 to the
storage part 113. The pressure valve 72 is disposed inside or
outside the flow path 52, for instance. Examples of the pressure
valve 72 include an orifice valve and a pulse valve. Incidentally,
the pressure valve 72 does not necessarily have to be provided.
[0111] In the electrochemical reaction devices illustrated in FIG.
5 and FIG. 6, the use of the coolers can facilitate lowing the
temperature of the electrolytic solution on the reduction side.
Further, the use of the heaters can facilitate raising the
temperature of the electrolytic solution on the oxidation side.
This can enhance reaction efficiency.
[0112] High-temperature carbon dioxide is generated in power
plants, incinerators, and the like. The direct supply of the
high-temperature carbon dioxide to the electrolytic solution tank
11 causes a temperature increase. The temperature increase is
preferably reduced by providing the cooler in the flow path 56
between the carbon dioxide generation source 80 and the
electrolytic solution tank 11. A cooler which cools the flow path
by, for example, the atmospheric air, seawater, river water, or the
like can also produce a sufficient effect.
[0113] It is possible to reduce an energy loss by supplying carbon
dioxide pressurized in the carbon dioxide generation source 80 such
as the power plant or the incinerator to the electrolytic solution
tank 11 or the electrolytic solution tank 12 through the flow path
without using a pump or the like. A pressure regulator may be
provided for pressure stabilization. Owing to the pressure
regulator, carbon dioxide with a stable pressure can be absorbed in
the electrolytic solution. This can enhance stability of the whole
device. Further, by improving efficiency by performing voltage
control across the reduction electrode and the oxidation electrode
and temperature control and pressure control of the electrochemical
reaction device according to a supply amount and the temperature of
carbon dioxide from the electrolytic solution tank 11 and an
operation signal of a carbon dioxide supply device, it is possible
to make the best use of performance of the device to improve the
efficiency.
[0114] In a case where the separation tank 13 is heated, the use of
heat of the carbon dioxide generation source or the like for the
heating reduces an energy loss to improve efficiency. On the other
hand, the use of heat of the high-temperature carbon dioxide gas
supplied from the carbon dioxide generation source lowers the
temperature of the carbon dioxide gas supplied to the electrolytic
solution tank 12 to improve efficiency.
[0115] An electrochemical reaction device illustrated in FIG. 7
further includes, in addition to the structure of the
electrochemical reaction device illustrated in FIG. 6, a distiller
81a, a reduction reaction device 81b, and a flow path 57 connecting
the storage part 113 and the reduction reaction device 81b. The
electrochemical reaction device further includes a cooler 61d
instead of the cooler 61c. Incidentally, it may include both the
cooler 61c and the cooler 61d.
[0116] The cooler 61d has a function of cooling the electrolytic
solution flowing in the flow path 52. The cooler 61d is disposed
inside or outside the flow path 52, for instance.
[0117] The distiller 81a has a function of distilling the product
in the storage part 113. The distiller 81a is connected to the
storage part 113. The distiller 81a is disposed on the electrolytic
solution tank 12, for instance. In the electrochemical reaction
device illustrated in FIG. 7, efficiency can be improved since heat
deprived of by the distillation in the distiller 81a and the
high-temperature carbon dioxide gas from the carbon dioxide
generation source can be efficiently used. However, since an
efficient heat exchanger leads to a cost increase, a simple heat
exchange method such as connecting pipes or the like by a heat
transfer member can also produce the effect. It is also possible to
exchange the heat of the high-temperature carbon dioxide gas
supplied from the carbon dioxide generation source 80 between the
carbon dioxide generation source 80 and the separation tank 13.
[0118] The reduction reaction device 81b has a function of reducing
the product in the storage part 113. In the reduction reaction
device 81b, a catalyst in which Al.sub.2O.sub.3 or the like carries
a metal such as an oxide of copper, palladium, or silver, or
Cu--ZnO, Pd--ZnO, or Cu--Zn--Cr is used, for instance, and methanol
can be mainly manufactured when hydrogen and CO gas which are raw
materials are made to flow at, for example, 150 to 300.degree. C.
under pressurization. Methanol can also be produced by a liquid
phase method that passes the hydrogen and the CO gas in a slurry of
the aforesaid catalyst under pressurization. The reduction reaction
device 81b includes a heat exchanger for removing heat generated by
the reaction, for instance. Further, the reduction reaction device
81b may be a device that produces ethanol or nickel by using
rhodium or the like, or produces methane by using ruthenium.
[0119] Examples of the product by the reduction reaction in the
reduction reaction device 81b include hydrocarbons such as methane,
methanol, ethanol, acetic acid, dimethyl ether, wax, olefin,
naphtha, and light oil. A heat source is not only the carbon
dioxide from the carbon dioxide generation source but also may
include at least part of the heat of the reaction between the
reduction product of carbon dioxide and hydrogen, for instance. For
example, the mutual heat utilization of using part of the reaction
heat obtained when methanol is produced by the reaction of carbon
monoxide and hydrogen in the reduction reaction device 81b improves
efficiency.
[0120] Heat exchange may take place between the carbon dioxide
generation source 80 and the electrolytic solution tank 12. The
heat exchange is possible by providing a heat transfer member 93
connecting, for example, the carbon dioxide generation source 80
and the electrolytic solution tank 12. The heat transfer member 93
may be provided so as to connect the flow path 56 and the distiller
81a, for instance. Alternatively, a heat exchanger or the like may
be separately connected.
[0121] Heat exchange may take place between the reduction reaction
device 81b and the distiller 81a. The heat exchange is possible by
providing a heat transfer member 94 connecting, for example, the
reduction reaction device 81b and the distiller 81a. Further, a
heat exchanger or the like may be separately connected.
[0122] In the electrochemical reaction device illustrated in FIG.
7, the heat exchange between the flow path 56 and the distiller 81a
and the heat exchange between the distiller 81a and the reduction
reaction device 81b make it possible to efficiently use and remove
the heat of the heat source.
[0123] Incidentally, the electrochemical reaction device
illustrated in FIG. 7 may include the separation tank 14, the flow
path 54, and the flow path 55 illustrated in FIG. 4 and so on.
Further, an agitator may be provided in an oxygen gas separator to
enhance efficiency of separating dissolved gas by temperature
distribution or the like. In this case, the use of the carbon
dioxide generation source 80, the high-temperature carbon dioxide
gas obtained from the carbon dioxide generation source 80, the heat
generated in the reduction reaction device 81b, or the like as the
heat source can improve efficiency. The combination of these heats
may be any, and an operation method for the heat exchange with any
of them can improve efficiency. Further, connecting the flow paths
or the like by the heat transfer member in order to mutually use
these heats can improve efficiency. The storage part 114a may be
connected to at least one of the storage part 112 and the storage
part 115a via a heat transfer member, for instance.
EXAMPLE
Example 1
[0124] An electrochemical reaction device having a structure was
fabricated. The structure includes a three-junction photoelectric
conversion body with a 500 nm thickness, a 300 nm thick ZnO layer
provided on a first face of the three-junction photoelectric
conversion body, a 200 nm thick Ag layer provided on the ZnO layer,
a 1.5 mm thick SUS substrate provided on the Ag layer, and a 100 nm
thick ITO layer provided on a second face of the three-junction
photoelectric conversion body.
[0125] The three-junction photoelectric conversion body has a first
photoelectric conversion layer that absorbs light in a short
wavelength range, a second photoelectric conversion layer that
absorbs light in an intermediate wavelength range, and a third
photoelectric conversion layer that absorbs light in a long
wavelength range. The first photoelectric conversion layer has a
p-type microcrystalline silicon layer, an i-type amorphous silicon
layer, and an n-type amorphous silicon layer. The second
photoelectric conversion layer has 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 has a p-type microcrystalline silicon layer, an i-type
amorphous silicon layer, and an n-type amorphous silicon layer.
[0126] An open-circuit voltage when the structure was irradiated
with light using a solar simulator (AM1.5, 1000 W/cm.sup.2) was
measured. The open-circuit voltage was 2.1 V.
[0127] A Ni(OH).sub.2 layer with a 200 nm thickness was formed as
an oxidation catalyst on the ITO layer on the structure of the
three-junction photoelectric conversion body by an
electrodeposition method using nickel nitrate. A 500 nm thick gold
nanoparticle layer carried by carbon was formed as a reduction
catalyst on the SUS substrate.
[0128] The above structure was cut into a square shape and its edge
portions were sealed with a thermosetting epoxy resin. The
periphery of the structure was surrounded by an ion exchange
membrane (Nafion (registered trademark)), whereby a single
sheet-shaped structure was formed. A 10 cm square unit was
fabricated from the combination of the ion exchange membrane and a
plurality of cells, and ten pieces of the units were arranged in
each of the vertical and lateral directions to fabricate a 100 cm
square photoelectrochemical reaction unit. The sheet-shaped
structure may be formed by, for example, embedding photoelectric
conversion cells in a plurality of holes of one ion exchange
membrane having the plural holes. The sheet-shaped structure may be
formed by arranging a plurality of structures in each of which a
photoelectric conversion cell is embedded in a hole of an ion
exchange membrane having one hole. Ion exchange membranes may be
embedded in holes of photoelectric conversion cells each having a
hole.
[0129] This sheet-shaped photoelectrochemical reaction unit is
sandwiched by a pair of 3 cm thick frames each having a hollow
portion with 100 cm length.times.100 cm width, and a silicone resin
layer was formed between the pair of frames. A window formed of
non-reflective glass for solar cell was fabricated to cover the
hollow portion of one of the pair of frames. An acrylic resin plate
was formed to cover the hollow portion of the other of the pair of
frames. Consequently, a sealed body encapsulating the
photoelectrochemical reaction unit was fabricated. Flow paths were
provided on the Ni(OH).sub.2 layer side and the gold nanoparticle
layer side of the photoelectrochemical reaction unit respectively.
As an electrolytic solution, a 0.5 M aqueous potassium hydrogen
phosphate solution containing saturated carbon dioxide gas was
used. A gas recovery flow path for capturing produced gas was
provided in part of an electrolytic solution tank. Through the
above, a photoelectrochemical reaction module was fabricated. An
acrylic vessel with an internal volume of 30 cm.times.3 cm.times.3
cm was connected as a mixing tank to the gold nanoparticle layer
side of the module.
[0130] This module was immersed in an electrolytic solution tank
which was a cylindrical glass vessel with a 30 cc volume, and 50
cc/min CO.sub.2 gas was blown to the electrolytic solution tank to
be dissolved in the electrolytic solution. This electrolytic
solution was supplied to the reduction electrode side of the module
at a 0.1 cc/min flow rate to be circulated. Further, a potassium
borate buffer solution on the oxidation electrode side was
circulated at a 0.1 cc/min flow rate via a buffer tank, which was a
cylindrical vessel with a 30 cc volume, without blowing
CO.sub.2.
[0131] In the module of the example 1, when A.M.1.5 pseudo sunlight
was radiated from the oxidation electrode side to cause a 0.5 hour
reaction, a current value was approximately 1 mA/cm.sup.2 at an
initial stage, but decreased to 0.4 mA/cm.sup.2.
[0132] In the module of the example 1, when the electrolytic
solution tank was put in ice water to be cooled after the 0.5 hour
reaction was caused by the radiation of the A.M.1.5 pseudo sunlight
from the oxidation electrode side, the current value recovered to
approximately 0.7 mA/cm.sup.2. From this, it is seen that cooling
the electrolytic solution containing carbon dioxide can improve
reaction efficiency.
[0133] In the module of the example 1, when the A.M.1.5 pseudo
sunlight was radiated from the oxidation electrode side and the
flow rate was set to 0.2 cc/min, it was possible to make the
current decrease time about 1.7 times. From this, it is seen that
increasing the circulation flow rate can impede the decrease of the
current.
Example 2
[0134] A composite substrate (4 cm square) having a 1.5 mm thick
SUS substrate connected to a generator via a lead and a
gold-carrying carbon film provided on the SUS substrate and
carrying 0.25 mg/cm.sup.2 gold, and a platinum foil (4 cm square)
were prepared. The generator is a simulation device of a solar
cell. A flow path and a gas flow path were formed on each of an
oxidation electrode side and a reduction electrode side of a 5 cm
square acrylic frame with a 1 cm thickness. The composite substrate
and the platinum foil were enclosed in the frame, an ion exchange
membrane (Nafion 117, 6 cm square) was provided between the
composite substrate and the platinum foil, and a silicon rubber
sheet and an acrylic plate (7 cm length.times.7 cm width.times.3 mm
thickness) were provided on each of an outer side of the composite
substrate and an outer side of the platinum foil, whereby a module
sandwiched by these was fabricated. A potassium phosphate buffer
solution with pH7 was supplied into the module. The composite
substrate was used as a reduction electrode, the platinum foil was
used as an oxidation electrode, and a silver-silver chloride
electrode was used as a reference electrode. Carbon dioxide was
decomposed by passing a current under a 37 mA: 2.3 mA/cm.sup.2
condition using a galvanostat. This module was immersed in an
electrolytic solution tank which was a cylindrical glass vessel
with a 30 cc volume, and CO.sub.2 gas at 50 cc/min was blown into
the electrolytic solution tank to be dissolved in the electrolytic
solution. This electrolytic solution was supplied to the reduction
electrode side of the module at a 0.1 cc/min flow rate to be
circulated. A potassium borate buffer solution on the oxidation
electrode side was circulated at a 0.1 cc/min flow rate via a
buffer tank, which was a 30 cc cylindrical vessel, without blowing
CO.sub.2.
[0135] In the module of the example 2, when a 0.5 hour reaction was
caused under a 37 mA current and a 0.1 cc/min circulation flow
rate, a potential was approximately -1 V at an initial stage, but
decreased to -1.4 V.
[0136] In the module of the example 2, when the electrolytic
solution tank was put in ice water to be cooled after the 0.5 hour
reaction was caused under the 37 mA current and the 0.1 cc/min
circulation flow rate, the potential recovered to approximately
-0.8 V. From this, it is seen that cooling the electrolytic
solution containing carbon dioxide can improve reaction
efficiency.
[0137] In the module of the example 2, when the flow rate was
changed to 0.2 cc/min after the 0.5 hour reaction was caused under
the 37 mA current and the 0.1 cc/min circulation flow rate, it was
possible to make the potential decrease time about twice. From
this, it is seen that increasing the circulation flow rate can
impede the decrease of the potential
[0138] While certain embodiments have been described, these
embodiments have been presented by way of example only, and are not
intended to limit the scope of the inventions. Indeed, the novel
embodiments described herein may be embodied in a variety of other
forms; furthermore, various omissions, substitutions and changes in
the form of the embodiments described herein may be made without
departing from the spirit of the inventions. The accompanying
claims and their equivalents are intended to cover such forms or
modifications as would fall within the scope and spirit of the
inventions.
* * * * *