U.S. patent application number 15/453157 was filed with the patent office on 2018-03-08 for co2 reduction catalyst, co2 reduction electrode and co2 reduction 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 Yuki KUDO, Satoshi MIKOSHIBA, Asahi MOTOSHIGE, Akihiko ONO, Yoshitsune SUGANO, Jun TAMURA, Arisa YAMADA.
Application Number | 20180066370 15/453157 |
Document ID | / |
Family ID | 61281644 |
Filed Date | 2018-03-08 |
United States Patent
Application |
20180066370 |
Kind Code |
A1 |
YAMADA; Arisa ; et
al. |
March 8, 2018 |
CO2 REDUCTION CATALYST, CO2 REDUCTION ELECTRODE AND CO2 REDUCTION
DEVICE
Abstract
The present embodiments provide a CO.sub.2 reduction catalyst
which is used for a reduction reaction of carbon dioxide and shows
a high efficiency in water, and a CO.sub.2 reduction electrode and
a CO.sub.2 reduction device, which contain the CO.sub.2 reduction
catalyst. This catalyst contains a conductive material and a
porphyrin complex which has a specific structure and is insoluble
in water. The porphyrin complex is insoluble in water because it
contains only a small number of hydrophilic groups in its
structure. The CO.sub.2 reduction electrode and the CO.sub.2
reduction device contain this catalyst.
Inventors: |
YAMADA; Arisa; (Kawasaki,
JP) ; MIKOSHIBA; Satoshi; (Yamato, JP) ; ONO;
Akihiko; (Kita, JP) ; KUDO; Yuki; (Yokohama,
JP) ; TAMURA; Jun; (Chuo, JP) ; SUGANO;
Yoshitsune; (Kawasaki, JP) ; MOTOSHIGE; Asahi;
(Yokohama, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KABUSHIKI KAISHA TOSHIBA |
Minato-ku |
|
JP |
|
|
Assignee: |
KABUSHIKI KAISHA TOSHIBA
Minato-ku
JP
|
Family ID: |
61281644 |
Appl. No.: |
15/453157 |
Filed: |
March 8, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C25B 11/04 20130101;
B01J 2531/842 20130101; C25B 11/0405 20130101; B01J 31/1815
20130101; B01J 31/00 20130101; C25B 3/04 20130101; B01J 2531/025
20130101; C25B 11/0415 20130101; C25B 11/0447 20130101 |
International
Class: |
C25B 11/04 20060101
C25B011/04; C25B 3/04 20060101 C25B003/04 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 2, 2016 |
JP |
2016-171975 |
Claims
1. A CO.sub.2 reduction catalyst comprising: a conductive material;
and a porphyrin complex which is carried on said conductive
material and represented by the following Formula (A): ##STR00004##
wherein, each R independently represents a group selected from the
group consisting of hydrogen, hydrocarbon groups having 1 to 12
carbon atoms, a hydroxyl group, an amino group, a carboxy group, a
sulfo group, a mercapto group and a formyl group and are optionally
the same or different, and adjacent R groups are optionally bound
with each other via a hydrocarbon chain having 1 to 12 carbon atoms
to form a cyclic structure; M represents a (2+n)-valent metal ion,
wherein n is a number of 0 or larger; X represents an n-valent
anion; and the total number of hydroxyl groups, amino groups, sulfo
groups and mercapto groups that are contained in one molecule of
said porphyrin complex is 10 or less.
2. The CO.sub.2 reduction catalyst according to claim 1, wherein
said metal ion is Fe ion.
3. The CO.sub.2 reduction catalyst according to claim 1, wherein
said total number of hydroxyl groups, amino groups, sulfo groups
and mercapto groups that are contained in one molecule of said
porphyrin complex is 8 or less.
4. The CO.sub.2 reduction catalyst according to claim 1, wherein
said porphyrin complex is represented by the following Formula
(A-1) or (A-2): ##STR00005## wherein, each R independently
represents a group selected from the group consisting of hydrogen,
hydrocarbon groups having 1 to 12 carbon atoms and are optionally
the same or different, and adjacent R groups are optionally bound
with each other via a hydrocarbon chain having 1 to 12 carbon atoms
to form a cyclic structure; and said hydrocarbon groups and
hydrocarbon chain are optionally substituted with a hydroxyl group,
an amino group, a carboxy group, a sulfo group, a mercapto group or
a formyl group, with a proviso that the total number of hydroxyl
groups, amino groups, sulfo groups and mercapto groups that are
contained in one molecule of said porphyrin complex is 10 or
less.
5. The CO.sub.2 reduction catalyst according to claim 1, further
comprising a surfactant.
6. A method of reducing carbon dioxide, said method comprising:
bringing a CO.sub.2 reduction electrode comprising the CO.sub.2
reduction catalyst according to claim 1 into contact with an
electrolyte solution; and introducing carbon dioxide to said
electrolyte solution and reducing the thus introduced carbon
dioxide by said electrode.
7. A CO.sub.2 reduction device comprising: an oxidation electrode;
a CO.sub.2 reduction electrode comprising the CO.sub.2 reduction
catalyst according to claim 1; and a power supply element connected
to said oxidation electrode and said CO.sub.2 reduction
electrode.
8. The device according to claim 7, wherein said power supply
element comprises a semiconductor layer which performs charge
separation with light energy.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is based upon and claims the benefit of
priority from the prior Japanese Patent Application No.
2016-171975, filed on Sep. 2, 2016, the entire contents of which
are incorporated herein by reference.
FIELD Embodiments of the present invention relate to a CO.sub.2
reduction catalyst, a CO.sub.2 reduction electrode and a CO.sub.2
reduction device.
BACKGROUND
[0002] In recent years, from the standpoints of energy problems and
environmental issues, artificial photosynthesis technology which
mimics plant photosynthesis and electrochemically converts solar
energy into chemical energy has been developed. As compared to
converting sunlight into electricity and storing the electricity in
a storage battery, it is advantageous to convert solar energy into
chemical energy, entrap the chemical energy in a chemical substance
(high-energy substance) and store the chemical substance in a
cylinder or a tank in that the energy storage cost can be reduced
and storage loss is smaller.
[0003] Until now, technologies for extracting hydrogen, which is a
high-energy substance (primarily a chemical fuel), from water have
been gradually established. As a technology which utilizes light
energy, photoelectrochemical reaction devices that comprise a
laminate (e.g., a silicon solar cell) in which a photovoltaic layer
is sandwiched by a pair of electrodes have been studied. On the
electrode in the light-irradiated side of such a device, a reaction
which oxidizes water (2H.sub.2O) with light energy and yields
oxygen (O.sub.2) and hydrogen ions (4H.sup.+) takes place. On the
other electrode, a reaction which utilizes the hydrogen ions
(4H.sup.+) generated on the electrode in the light-irradiated side
and an electric potential (e.sup.-) generated in the photovoltaic
layer to produce chemical substances such as hydrogen (2H.sub.2)
takes place. Further, photoelectrochemical reaction devices in
which silicon solar cells are laminated are also known. However, in
these methods, although sunlight is converted into chemical energy
with high efficiency, it is not easy to store and transport the
thus produced hydrogen. Considering the energy problems and
environmental issues, it is preferred to allow an easily storable
and transportable carbon compound other than hydrogen to entrap the
chemical energy.
[0004] Incidentally, a technology which highly efficiently converts
CO.sub.2 existing in a large amount in the atmosphere or the like
into a chemical substance or the like useful as a chemical fuel has
not been established. Still, at the laboratory level,
photoelectrochemical reaction devices utilizing light energy have
been examined. For example, there is known a device of a
two-electrode system which comprises an electrode containing a
reduction catalyst for reduction of carbon dioxide (CO.sub.2) and
an electrode containing an oxidation catalyst for oxidation of
water (H.sub.2O), wherein the electrodes are immersed in
CO.sub.2-dissolved water. In this device, the electrodes are
electrically connected via an electric wire or the like. On the
electrode containing an oxidation catalyst, as in the case of
extracting hydrogen from water, H.sub.2O is oxidized by light
energy and oxygen (1/2O.sub.2) is thereby produced and, at the same
time, an electric potential is generated. The electrode containing
a reduction catalyst reduces CO.sub.2 by acquiring the electric
potential from the electrode eliciting the oxidation reaction,
whereby formic acid (HCOOH) and the like are produced. There are
several reports on such a device.
[0005] According to the investigation by the present inventors, it
is also known to utilize a porphyrin complex in these technologies.
In such a known technology, CO.sub.2 is reduced to CO by utilizing
a porphyrin complex dissolved in an organic solvent as a CO.sub.2
reduction catalyst. However, in this technology, although the
complex is required to be dissolved or dispersed, since porphyrin
is insoluble in water and it is thus difficult to dissolve
porphyrin in water, the reaction hardly takes place in water. In
addition, since the reaction is not an electrode reaction, a
sacrificial reagent is necessary.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] FIG. 1 is a schematic cross-sectional view of a CO.sub.2
reduction electrode according to one embodiment; and
[0007] FIG. 2 is a schematic view of a CO.sub.2 reduction device
according to one embodiment.
DETAILED DESCRIPTION
[0008] Embodiments will now be explained with reference to the
accompanying drawings.
[0009] The CO.sub.2 reduction catalyst according to the present
embodiment comprises:
[0010] a conductive material; and
[0011] a porphyrin complex which is carried on the conductive
material and represented by the following Formula (A):
##STR00001##
[0012] (wherein, Rs each represent a group selected from the group
consisting of hydrogen, hydrocarbon groups having 1 to 12 carbon
atoms, a hydroxyl group, an amino group, a carboxy group, a sulfo
group, a mercapto group and a formyl group and are optionally the
same or different, and adjacent Rs are optionally bound with each
other via a hydrocarbon chain having 1 to 12 carbon atoms to form a
cyclic structure;
[0013] M represents a (2+n)-valent metal ion, wherein n is a number
of 0 or larger;
[0014] X represents an n-valent anion; and
[0015] the total number of hydroxyl groups, amino groups, sulfo
groups and mercapto groups that are contained in one molecule of a
porphyrin complex is 10 or less).
[0016] The method of reducing carbon dioxide according to the
present embodiment comprises the steps of:
[0017] bringing a CO.sub.2 reduction electrode comprising the
above-described CO.sub.2 reduction catalyst into contact with an
electrolyte solution; and
[0018] introducing carbon dioxide to the electrolyte solution and
reducing the thus introduced carbon dioxide by the above-described
electrode.
[0019] Further, the CO.sub.2 reduction device according to the
present embodiment comprises:
[0020] an oxidation electrode;
[0021] a CO.sub.2 reduction electrode comprising the
above-described CO.sub.2 reduction catalyst; and
[0022] a power supply element connected to the oxidation electrode
and the CO.sub.2 reduction electrode.
[0023] The "CO.sub.2 reduction catalyst" (hereinafter, for
convenience, may be simply referred to as "catalyst") according to
the present embodiment has a function of inducing the generation of
a carbon compound by a CO.sub.2 reduction reaction. In the present
embodiment, the term "CO.sub.2 reduction catalyst" does not mean a
compound which has a function of inducing or promoting a CO.sub.2
reduction reaction but means such a compound which is integrated
with a conductive carrier carrying the compound.
[0024] The catalyst according to the present embodiment comprises a
conductive material and a porphyrin complex having a specific
structure. In this catalyst, since the porphyrin complex serving as
the active center of the reaction is fixed by the conductive
material and thus unlikely to elute into an electrolyte, a
sacrificial reagent or the like is not required for reduction
reaction.
[0025] In the present embodiment, the porphyrin complex is a
material which has a function of inducing or promoting a CO.sub.2
reduction reaction by lowering the activation energy for the
reduction of CO.sub.2. In other words, the porphyrin complex is a
material which reduces an overvoltage occurring during the
generation of a carbon compound by a CO.sub.2 reduction reaction.
In the present embodiment, as such a material, a porphyrin complex
represented by the following Formula (A) is used:
##STR00002##
[0026] (wherein, Rs each represent a group selected from the group
consisting of hydrogen, hydrocarbon groups having 1 to 12 carbon
atoms, a hydroxyl group (--OH), an amino group (--NH.sub.2), a
carboxy group (--C(.dbd.O)OH), a sulfo group (--SO.sub.3H), a
mercapto group (--SH) and a formyl group (--C(.dbd.O)H), and are
optionally the same or different, and adjacent Rs are optionally
bound with each other via a hydrocarbon chain having 1 to 12 carbon
atoms to form a cyclic structure;
[0027] M represents a (2+n)-valent metal ion, wherein n is a number
of 0 or larger;
[0028] X represents an n-valent anion; and
[0029] the total number of hydroxyl groups, amino groups, sulfo
groups and mercapto groups that are contained in one molecule of a
porphyrin complex is 10 or less).
[0030] The above-described hydrocarbon groups may be saturated or
unsaturated and linear or branched. In addition, the hydrocarbon
groups may have a cyclic structure. Specific examples of such
hydrocarbon groups include a methyl group, an ethyl group, an
n-propyl group, an i-propyl group, an n-butyl group, an n-pentyl
group, an n-hexyl group, an octyl group, a cyclohexyl group, a
cyclooctyl group, a cyclohexylmethyl group, a phenyl group, a tolyl
group, a naphthyl group and a benzyl group. These hydrocarbon
groups may further have a substituent(s) such as a hydroxyl group,
an amino group and/or a carboxyl group. Moreover, adjacent Rs may
be bound with each other via a hydrocarbon chain to form a cyclic
structure. In this case, the hydrocarbon chain may be saturated or
unsaturated. Examples of such a structure include a phthalocyanine
structure in which the Rs at 2- and 3-positions, 7- and
8-positions, 12- and 13-positions and 17- and 18-positions are
bound via unsaturated hydrocarbon chains and form aromatic
rings.
[0031] The above-described amino group may be substituted with one
or two hydrocarbon groups having 1 to 12 carbon atoms. Specific
examples of such an amino group include a methylamino group, a
dimethylamino group and a methylethylamino group.
[0032] The above-described metal ion is an ion of an element
selected from the group consisting of Groups 8, 9 and 10 elements,
among which Fe, Co, Ni, Ru, Rh, Pd, Mn and Co ions are preferred, a
Fe ion is more preferred, and a trivalent Fe ion is particularly
preferred. Since these metal ions can each take a plurality of
valences, carbon dioxide coordinated with these metals is
reduced.
[0033] The above-described anion neutralizes an electric charge
when the valence of the metal ion is higher than 2. Accordingly,
the anion does not exist when the metal ion is divalent. When the
metal ion is trivalent, n is 1 and the metal ion M is bound with a
single monovalent anion X. When the metal ion M is tetravalent, n
is 2 and the metal ion is bound with two monovalent anions X or a
single divalent anion X. Examples of the anion X include halogen
ions such as chlorine and fluorine ions, a hydroxide ion, a
bicarbonate ion, a carbonate ion, a bisulfate ion and a sulfate
ion.
[0034] It is preferred that the porphyrin complex of the present
embodiment does not dissolve in water. As described below, the
reason for this is to fix the porphyrin complex on a carrier so as
to not only obtain the catalytic effect of the porphyrin complex on
the electrode surface but also inhibit elution of the porphyrin
complex into a reaction medium such as an electrolyte solution.
Therefore, it is preferred that the amount of a hydrophilic
group(s) contained in the porphyrin complex used in the present
embodiment be small. Specifically, the total number of hydroxyl
groups, amino groups, sulfo groups and mercapto groups that are
contained in one molecule of the porphyrin complex is required to
be 10 or less, preferably 8 or less, more preferably 6 or less,
most preferably 0. It is noted here that these hydrophilic groups
are contained in a porphyrin ligand and an anion binding to the
metal ion M is not included in these hydrophilic groups.
[0035] Among such porphyrin complexes, those having aromatic groups
at the 5-, 10-, 15- and 20-positions and those having saturated
hydrocarbon groups at the 2-, 3-, 7-, 8-, 12-, 13-, 17- and
18-positions are preferred because of the ease of synthesis and the
availability. Among these porphyrin complexes, ones represented by
the following Formula (A-1) or (A-2) are particularly
preferred.
##STR00003##
[0036] In these Formulae, R's each represent a group selected from
the group consisting of hydrogen, hydrocarbon groups having 1 to 12
carbon atoms and are optionally the same or different, and adjacent
R's are optionally bound with each other via a hydrocarbon chain
having 1 to 12 carbon atoms to form a cyclic structure; and the
above-described hydrocarbon groups and hydrocarbon chain are
optionally substituted with a hydroxyl group, an amino group, a
carboxy group, a sulfo group, a mercapto group or a formyl group,
with a proviso that the total number of hydroxyl groups, amino
groups, sulfo groups and mercapto groups that are contained in one
molecule of the above-described porphyrin complex is 10 or less,
preferably 8 or less, more preferably 6 or less, most preferably
0.
[0037] Examples of such a porphyrin complex include
5,10,15,20-tetraphenyl-21H,23H-porphyrin iron (III) chloride and
2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin iron (III)
chloride.
[0038] In the present embodiment, a porphyrin complex layer may
contain two or more types of the above-described porphyrin
complexes and may additionally contain a different metal, a metal
compound, a metal complex, an organic compound or the like which
exhibits a CO.sub.2-reducing action.
[0039] As described above, the porphyrin complex layer has a
function of inducing the generation of a carbon compound by a
CO.sub.2 reduction reaction. The carbon compound generated by the
reduction reaction varies depending on the type and the like of the
porphyrin complex. Examples of the carbon compound include carbon
monoxide (CO), formic acid (HCOOH), methane (CH.sub.4), methanol
(CH.sub.3OH), ethane (C.sub.2H.sub.6), .sup.ethylene
(C.sub.2H.sub.4), ethanol (C.sub.2H.sub.5OH), formaldehyde (HCHO),
acetaldehyde (CH.sub.3CHO), acetic acid (CH.sub.3COOH), ethylene
glycol (HOCH.sub.2CH.sub.2OH), 1-propanol
(CH.sub.3CH.sub.2CH.sub.2OH) and isopropanol
(CH.sub.3CHOHCH.sub.3).
[0040] Such a porphyrin complex can be obtained at a lower cost
than conventional noble metals; therefore, there is an advantage
that the catalyst can be produced inexpensively.
[0041] The conductive material used in combination with the
porphyrin complex is not particularly restricted as long as it is
capable of coming into contact with the porphyrin complex such that
an electrical continuity can be established therebetween, and the
conductive material is preferably one which contains a carbon
material such as a carbon black, carbon nanotubes (CNT), graphene
or fullerene.
[0042] A variety of carbon blacks that differ in particle size,
particle shape and particle structure are known, and any of these
carbon blacks can be used. Examples thereof include
[0043] Ketjen Black, Acetylene Black, Norit and Vulcan (all of
which are registered trademarks). Alternatively, a metal material
can be used as the conductive material. Examples of a metal
material that can be used include metals such as Au, Ag, Cu, Al,
Pt, Ni, Zn, Sn, Bi and Pd, and alloy materials containing a
plurality of these metals, such as SUS. Moreover, for example, a
translucent metal oxide such as ITO (indium tin oxide), ZnO (zinc
oxide), FTO (fluorine-containing tin oxide: fluorine-doped tin
oxide), AZO (aluminum-containing zinc oxide: aluminum-doped zinc
oxide) or ATO (antimony-containing tin oxide: antimony-doped tin
oxide) may also be used as the conductive material.
[0044] These various conductive materials can be used in a
combination of two or more thereof.
[0045] The conductive material may be in the form of a plate, a
rod, a thin film, a wire, a lattice or the like and used as a
carrier which carries the porphyrin complex on the surface or the
like, or the conductive material may be mixed as a powder material
with the porphyrin complex and the resulting mixture may be used as
a material to be molded by compression or the like. Particularly,
by using a carrier made of a metal or a metal oxide, the mechanical
strength of the CO.sub.2 reduction catalyst or the CO.sub.2
reduction electrode can be improved.
[0046] As the conductive material, a composite of a carbon material
and a conductive resin, a conductive ion exchange resin or the like
may also be used. Further, a resin material such as an ionomer may
be used as well.
[0047] When the conductive material is a carrier, the carrier
preferably comprises a porous part. Typically, it is preferred that
the carrier be entirely porous; however, the carrier may be
partially non-porous, in other words, the carrier may contain a
compact part.
[0048] It is desired that the porous part have a pore distribution
whose peak is in a range of 5 nm to 20 .mu.m. By this pore
distribution, the catalytic activity can be improved. The term
"pore" used herein means a void observed in a cross-section of the
porous part. Further, the term "pore distribution" means the
distribution of pore sizes (void widths) per unit length on the
porous outermost surface observed in a cross-section of the porous
part. The pore distribution can be determined by, for example,
measurement by a gas adsorption method, measurement by mercury
intrusion porosimetry, particle size distribution measurement by a
laser diffraction-scattering method, dry density measurement by a
constant-volume expansion method, measurement using an AFM (atomic
force microscope), or image processing of a TEM image.
[0049] In the present embodiment, the pore distribution of the
carrier preferably has a plurality of peaks in the above-described
range. By this, an increase in the surface area, an improvement in
the diffusibility of ions and reactants and a high
electroconductivity can all be realized at the same time.
[0050] A carrier having such a porous structure can be prepared by
compressing a powder-form conductive material, or by etching a
pore-free compact material and thereby forming pores.
[0051] The carrier may also have through-holes. Such a structure
can also be formed by, for example, removing a part of the
above-described carrier by etching or the like.
[0052] In this manner, by allowing the carrier to have a porous
structure or providing the carrier with through-holes, the
diffusibility of ions and reactants can be improved with pores and
through-holes while maintaining a high electroconductivity and
large active surface area of the catalyst. As the surface area of
the catalyst is increased and the mass of reactants is thereby
increased, the supply of products and raw material substances are
limited by substance diffusion; however, problems associated
therewith can also be solved by the porous structure or
through-holes at the same time.
[0053] The catalyst according to one embodiment comprises a
porphyrin complex layer on such a carrier.
[0054] The porphyrin complex layer may exist on a part of the
carrier surface and is not required to cover the whole carrier. It
is preferred that the carrier and this porphyrin complex layer have
an electrical continuity with each other.
[0055] FIG. 1 is a conceptual drawing that schematically shows a
CO.sub.2 reduction catalyst 100 according to one embodiment. In the
catalyst 100 shown in FIG. 1, a porphyrin complex layer 102 is
laminated on a conductive material 101, and the conductive material
101 and the porphyrin complex layer 102 have an electrical
continuity. The porphyrin complex layer may consist of only a
porphyrin complex or have a structure in which a porphyrin complex
is dissolved or dispersed in a solid conductive medium such as a
conductive resin.
[0056] The structure of the CO.sub.2 reduction electrode of this
embodiment is not restricted to the one shown in FIG. 1, as long as
the conductive material and the porphyrin complex layer are in an
electrically continuous state. Further, in the CO.sub.2 reduction
electrode, the form of the conductive material is not particularly
restricted and, for example, the conductive material may be take
any form of a thin film, a lattice, particles and a wire.
[0057] In the catalyst of another embodiment, the conductive
material and the porphyrin complex may be integrated. For example,
the catalyst may be obtained by forming a single composition layer
containing both the conductive material and the porphyrin complex
on a substrate. Alternatively, the catalyst may be obtained by
molding a single composition containing both the conductive
material and the porphyrin complex into a plate shape, a rod shape
or the like.
[0058] In yet another embodiment, the catalyst can be obtained by
molding a mixture, in which a powder-form conductive material and a
powder-form porphyrin complex are mixed, by compression or the
like. In this case, a conductive resin or the like may also be used
in combination as a binder.
[0059] These catalysts can each be directly used as a CO.sub.2
reduction electrode; however, they can also each be arranged on a
support made of a metal or the like and used as a CO.sub.2
reduction electrode.
[0060] Further, it is preferred that the catalyst according to
these embodiments contain a surfactant in the porphyrin complex
layer or the like. The use of a surfactant makes the catalyst more
likely to desorb a gas generated by a reduction reaction. As a
result, a large contact area can be maintained between the catalyst
and an electrolyte solution, so that the reduction reaction can be
further promoted.
[0061] As the surfactant, for example, hydrophilic group-containing
vinyl compounds such as polyvinylpyrrolidone and polyvinyl
alcohols, derivatives thereof and polymers can be used. Other
material may also be used as long as it has a function equivalent
to that of the above-described compounds and the like.
[0062] Further, it is preferred that the catalysts according to
these embodiments contain an ion exchange resin. By using an ion
exchange resin such as Nafion (registered trademark), for example,
adsorption of ions contributing to the reaction can be controlled.
In addition, depending on the intended use, the type of the ion
exchange resin is not restricted, and a material which has a
function comparable to that of an ion exchange resin may be used as
well.
[0063] FIG. 2 is a schematic drawing that shows one example of the
structure of CO.sub.2 reduction device 200 according to the present
embodiment. The CO.sub.2 reduction device 200 shown in FIG. 2
comprises: a container 201; a CO.sub.2 reduction electrode 202; an
oxidation electrode 203 which oxidizes water; a power supply
element 204 which is electrically connected to the CO.sub.2
reduction electrode 202 and the oxidation electrode 203; and an
electrolyte solution 205 which is retained in the container 201 and
is in contact with the CO.sub.2 reduction electrode 202 and the
oxidation electrode 203. The CO.sub.2 reduction electrode 202 is
the CO.sub.2 reduction electrode according to the present
embodiment and, as the CO.sub.2 reduction electrode 202, the
catalyst according to the present embodiment may be arranged on the
surface of an electrode made of a metal or the like, or the
catalyst according to the present embodiment can be used as is. The
porphyrin complex layer of the CO.sub.2 reduction electrode is
required to be in contact with the electrolyte solution.
[0064] The electric power supplied by the power supply element 204
may be an electric power obtained from a system; an electric power
obtained from conversion of kinetic energy, potential energy,
thermal energy or the like into electrical energy; an electric
power obtained from conversion of light energy by a solar cell or
the like; an electric power obtained from conversion of chemical
energy of a fuel cell, a storage battery or the like; or an
electric power obtained from conversion of sound vibration or the
like. In the present embodiment, the electric power is preferably
one obtained from conversion of natural energy, particularly solar
energy. In order to allow a reduction reaction by the catalyst to
take place, it is required that the electric power be not smaller
than the difference between the redox potential generated by
oxidation of water and the CO.sub.2 reduction potential, and an
electromotive force of not less than 1.06 V is necessary for the
conversion of CO.sub.2 into methane while an electromotive force of
1.2 V is necessary for the conversion of CO.sub.2 into methanol and
an electromotive force of not less than 1.33 V is necessary for the
conversion of CO.sub.2 into CO. Therefore, the electromotive force
is preferably not less than 1.0 V, more preferably a larger voltage
to include an overvoltage, still more preferably not less than 1.3
V.
[0065] The electrolyte solution 205 is stored in, for example, a
container such as an electrolyte solution tank. It is also possible
to replenish the electrolyte solution 205 through a supply flow
path. In this case, a heater and a temperature sensor may be
arranged in a part of the supply flow path. Further, the inside of
the container may be filled with vaporized components of the
electrolyte solution 205.
[0066] The electrolyte solution 205 contains water (H.sub.2O) and
carbon dioxide (CO.sub.2). Examples of the electrolyte solution 205
include aqueous solutions containing phosphate ions
(PO.sub.4.sup.2-), borate 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.-), bicarbonate ions
(HCO.sub.3-) and/or the like. For example, as the electrolyte
solution 205, an aqueous solution containing LiHCO.sub.3,
NaHCO.sub.3, KHCO.sub.3, CsHCO.sub.3 or the like can be used. The
electrolyte solution 205 may also contain an alcohol such as
methanol, ethanol or acetone. Further, different electrolyte
solutions may be used as the electrolyte solution in which the
oxidation electrode 203 is immersed and the electrolyte solution in
which the CO.sub.2 reduction electrode 202 is immersed. In this
case, it is preferred that the electrolyte solution in which the
oxidation electrode 203 is immersed contain at least water and the
electrolyte solution in which the CO.sub.2 reduction electrode 202
is immersed contain at least carbon dioxide. In addition, the
production ratio of carbon compounds can be changed by adjusting
the amount of water contained in the electrolyte solution in which
the CO.sub.2 reduction electrode 202 is immersed. Moreover, carbon
dioxide may be blown into the electrolyte solution 205 by bubbling
or the like. For this purpose, the device according to the present
embodiment can be provided with a carbon dioxide introduction pipe
206.
[0067] As the electrolyte solution 205, an ionic liquid which
contains a salt of a cation such as an imidazolium ion or a
pyridinium ion and an anion such as BF.sub.4.sup.- or
PF.sub.6.sup.- and is in a liquid state over a wide temperature
range, or an aqueous solution thereof can be used. Examples of
other electrolyte solution include amine solutions of ethanolamine,
imidazole, pyridine or the like, and aqueous solutions thereof.
Examples of the amine include primary amines, secondary amines and
tertiary amines.
[0068] Examples of the primary amines include methylamine,
ethylamine, propylamine, butylamine, pentylamine and hexylamine.
The hydrocarbons of these amines may be substituted with an
alcohol, a halogen or the like. Examples of an amine whose
hydrocarbon is substituted include methanolamine, ethanolamine and
chloromethylamine. The hydrocarbons may also contain an unsaturated
bond. Such hydrocarbons are also applicable to secondary amines and
tertiary amines.
[0069] Examples of the secondary amines include dimethylamine,
diethylamine, dipropylamine, dibutylamine, dipentylamine,
dihexylamine, dimethanolamine, diethanolamine and dipropanolamine.
The substituted hydrocarbons may be different, and this is also
applicable to tertiary amines. Examples of amines with different
hydrocarbons include methylethylamine and methylpropylamine.
[0070] Examples of the tertiary amines include trimethylamine,
triethylamine, tripropylamine, tributylamine, trihexylamine,
trimethanolamine, triethanolamine, tripropanolamine,
tributanolamine, tripropanolamine, trihexanolamine,
methyldiethylamine and methyldipropylamine.
[0071] Examples of the cation in 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 and a 1-hexyl-3-methylimidazolium
ion.
[0072] These imidazolium ions may be substituted at the 2-position.
Examples of a cation which is an imidazolium ion substituted at the
2-position include a 1-ethyl-2,3-dimethylimidazolium ion, a
1,2-dimethyl-3-propylimidazolium ion, a
1-butyl-2,3-dimethylimidazolium ion,
1,2-dimethyl-3-pentylimidazolium ion and a
1-hexyl-2,3-dimethylimidazolium ion.
[0073] Examples of the pyridinium ion include methylpyridinium,
ethylpyridinium, propylpyridinium, butylpyridinium,
pentylpyridinium and hexylpyridinium ions. These imidazolium ions
and pyridinium ions may be substituted at an alkyl group and may
contain an unsaturated bond.
[0074] 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.-, a
bis(trifluoromethoxysulfonyl)imide anion and a
bis(perfluoroethylsulfonyl)imide anion. A dipolar ion in which a
cation and an anion of an ionic liquid are bound via a hydrocarbon
may also be used.
[0075] The pH of the electrolyte solution in which the CO.sub.2
reduction electrode 202 is immersed is preferably lower than the pH
of the electrolyte solution in which the oxidation electrode is
immersed. This allows hydrogen ions, hydroxide ions and the like to
move easily. In addition, a liquid junction potential based on the
pH difference can be effectively used in redox reaction.
[0076] The electrolyte solution in which the electrode 202 is
immersed and the electrolyte solution in which the oxidation
electrode 203 is immersed can be separated using an ion exchange
membrane. The ion exchange membrane has a function of allowing some
of the ions contained in the electrolyte solutions in which each
electrode is immersed to permeate therethrough, that is, a function
of blocking one or more ions contained in either of the electrolyte
solutions. As a result, for example, a difference in pH or ionic
strength can be created between the two electrolyte solutions. By
such a constitution, the CO.sub.2 reduction reaction can be
promoted.
[0077] Examples of the ion exchange membrane include cation
exchange membranes such as Nafion (registered trademark) and
Flemion (registered trademark), and anion exchange membranes such
as Neosepta (registered trademark) and Selemion (registered
trademark). A bipolar membrane in which cation exchange membrane
and anion exchange membrane are layered such as Neosepta
(registered trademark) can be used. The bipolar membrane is
preferably employed if the difference of pH between the electrolyte
solution in which the anode is immersed and the electrolyte
solution in which the cathode is immersed is large, for example,
the alkaline electrolyte and the acidic electrolyte are used. In
cases where the movement of ions between the two electrolyte
solutions does not have to be controlled, it is not necessary to
arrange such an ion exchange membrane.
[0078] The CO.sub.2 reduction catalyst can be recycled from a
degraded state by cleaning based on electrochemical redox, a
treatment with a compound having a cleaning effect, or cleaning
with heat, light or the like. It is preferred that the CO.sub.2
reduction device be made usable for or tolerable such recycle of
the catalyst by adjusting the voltage applied to the electrodes. It
is also preferred that the CO.sub.2 reduction device according to
the present embodiment have such a function of recycling the
catalyst.
[0079] In order to accelerate the supply of ions or substances to
the electrode surface, the CO.sub.2 reduction device may further
comprise a stirrer.
[0080] Further, the CO.sub.2 reduction device may also comprise a
measuring instrument(s) such as a thermometer, a pH sensor, a
conductivity meter, an electrolyte solution analyzer and a gas
analyzer, and it is preferred that parameters in the CO.sub.2
reduction device be measured by these measuring instruments and the
CO.sub.2 reduction device be controllable based on the measured
values.
[0081] The CO.sub.2 reduction device may be a batch-type reactor or
a flow-type reactor. In the case of a flow-type reactor, it is
desired that a supply flow path and a discharge flow path of an
electrolyte solution be secured. If the batch-type reactor is
employed, it is preferred that the reactor is operated when surplus
power is generated and is stopped when the power demand is large
and surplus power is not generated.
[0082] Next, an operation example of the CO.sub.2 reduction device
will be described. Here, as an example, a case of producing carbon
monoxide from carbon dioxide using an iron (III) chloride complex
as the porphyrin complex will be described.
[0083] First, electrons gather on the CO.sub.2 reduction electrode
from the power supply element, and the Fe ion contained in the
porphyrin complex is reduced. CO.sub.2 dissolving in the
electrolyte solution is coordinated with this Fe ion. Meanwhile, a
CO.sub.2 reduction reaction consequently takes place as represented
by the following Formula (1), wherein CO.sub.2 reacts with hydrogen
ions to generate carbon monoxide, which is a carbon compound, and
water (hydroxide ions when the electrolyte solution is alkaline).
The thus generated carbon monoxide dissolves in the electrolyte
solution at an arbitrary ratio. The area of the part where the
CO.sub.2 reduction reaction takes place is larger in a CO.sub.2
reduction electrode having a porous structure than in a CO.sub.2
reduction electrode having a non-porous structure. A recovery flow
path may also be arranged in the container of the electrolyte
solution so as to recover the generated carbon compound
therethrough.
2CO.sub.2+4H.sup.++4e.sup.-.fwdarw.2CO+2H.sub.2O
(2CO.sub.2+2H.sub.2O+4e.sup.-.fwdarw.2CO+4OH.sup.-) Formula (1)
[0084] Meanwhile, on the oxidation electrode, an oxidation reaction
of water takes place as represented by the following Formula (2),
whereby oxygen and hydrogen ions (water when the electrolyte
solution is alkaline) are generated and electrons flow to the power
supply element.
2H.sub.2O.fwdarw.4H.sup.++O.sub.2+4e.sup.-
(4OH.sup.-.fwdarw.2H.sub.2O+O.sub.2+4e.sup.-) Formula (2)
[0085] The hydrogen ions (water) generated by the oxidation
reaction migrate to the CO.sub.2 reduction electrode.
[0086] The embodiments described herein are presented for the
illustration purpose only, and the scope of the present invention
is not restricted thereto.
EXAMPLES
Example 1 Production Example of CO.sub.2 Reduction Electrode
[0087] In a 200-ml flask, 5,10,15,20-tetraphenyl-21H,23H-porphyrin
iron (III) chloride was dissolved in chloroform, and Ketjen Black
was added thereto. Subsequently, the solvent was removed using an
evaporator, and the complex was allowed to adsorb to Ketjen
Black.
[0088] This Ketjen Black was added to a 2.5-wt % Nafion solution
and dispersed by ultrasonication, and the resulting dispersion was
spray-coated on a carbon paper (GDL10BA) to prepare a CO.sub.2
reduction electrode.
Comparative Example 1
[0089] Ketjen Black carrying 30 wt % of gold thereon was added to a
2.5-wt % Nafion solution and dispersed by ultrasonication, and the
resulting dispersion was spray-coated on a carbon paper (GDL10BA)
to prepare an electrode.
Example 2
[0090] After adding 15 ml of ethanol to 1.5 g of Ketjen Black, 500
.mu.l of triethoxy-3-(2-imidazolin-1-yl)propyl silane was further
added. Then, 2.5 mL of pure water was added thereto, and the
resultant was allowed to react at 60.degree. C. for 1 hour. After
the reaction, the resulting solution was filtered, washed with
ethanol and water, and the dried under reduced pressure.
[0091] A porphyrin complex was added to the thus obtained Ketjen
Black in an amount of 36 wt %, and the resultant was dissolved in
chloroform. After removing the solvent using an evaporator, the
complex was allowed to adsorb to Ketjen Black.
[0092] This Ketjen Black was added to a 2.5-wt % Nafion solution
and dispersed by ultrasonication, and then resulting dispersion was
spray-coated on a carbon paper (GDL10BA) to prepare an
imidazoline-modified CO.sub.2 reduction electrode.
Comparative Example 2
[0093] After adding 15 ml of ethanol to 1.5 g of the gold-carrying
Ketjen Black used in Comparative Example 1, 500 .mu.l of
triethoxy-3-(2-imidazolin-1-yl)propyl silane was further added.
Then, 2.5 mL of pure water was added thereto, and the resultant was
allowed to react at 60.degree. C. for 1 hour. After the reaction,
the resulting solution was filtered, washed with ethanol and water,
and then dried under reduced pressure.
[0094] This Ketjen Black was added to a 2.5-wt % Nafion solution
and dispersed by ultrasonication, and then resulting dispersion was
spray-coated on a carbon paper (GDL10BA) to prepare an
imidazoline-modified gold catalyst electrode.
[0095] The thus prepared electrodes were each measured under the
following conditions using an electrochemical analyzer.
[0096] The resistance component was determined by measuring the
impedance under the following conditions: 0.95 V vs Ag/AgCl,
amplitude=10 my, frequency=100 to 0.1 Hz. In linear sweep
voltammetry (LSV), an H-type cell was used at 1 mV/sec in a range
of 0 to -0.6 V along with Selemion (registered trademark) as an
electrolyte membrane. The measurement was performed using a
platinum foil as a counter electrode, an Ag/AgCl electrode as a
reference electrode, and a 0.5 M aqueous K.sub.2CO.sub.3 solution
as an electrolyte solution.
Comparative Example 3
[0097] Further, as a control, an electrode prepared in accordance
with Y. Tian, et al., The Journal of Physical Chemistry B vol. 110.
pp. 23478 (2006) was used. Specifically, an electrode which was
prepared by immersing a conductive material in an aqueous solution
in which perchloric acid (0.1 M) and chloroauric acid (4 mM) were
dissolved and applying a voltage of -0.08 V to an Ag/AgCl
(saturated KCl) reference electrode was used.
[Evaluation]
[Electrochemical Measurement]
[0098] When the electrode obtained in Example 1 was evaluated at a
size of 4-cm square, a reduction current of 2.5 mA/cm.sup.2 was
observed at a constant voltage of -0.6 V (vs RHE), and the Faraday
efficiency of CO in this case was found to be 50%. Further, for the
electrode of Example 2, a reduction current of 1 mA/cm.sup.2 was
observed, and the Faraday efficiency of CO in this case was found
to be 50%.
[0099] Meanwhile, for the electrode of Comparative Example 1, a
reduction current of 4.7 mA/cm.sup.2 was observed, and the Faraday
efficiency of CO in this case was found to be 35%. For the
electrode of Comparative Example 2, a reduction current of 4.2
mA/cm.sup.2 was observed, and the Faraday efficiency of CO in this
case was found to be 30%.
[0100] Each electrode was also evaluated in the same manner at a
size of 1-cm square using a 0.25 M aqueous K.sub.2CO.sub.3 solution
as an electrolyte solution.
[0101] For the electrode obtained in Example 1, reduction currents
of 4.2, 2.0 and 0.8 mA/cm.sup.2 were observed at constant voltages
of -0.6, -0.5 and -0.4 V (vs RHE), respectively, and the Faraday
efficiency of CO was found to be 53, 60 and 45% in these cases,
respectively. For the electrode of Comparative Example 3, a
reduction current of 2.0 mA/cm.sup.2 was observed at a constant
voltage of -0.5 V (vs RHE), and the Faraday efficiency in this case
was found to be 55%.
[0102] It was found that the catalysts of Examples have a high CO
selectivity and a performance equivalent or superior to those of
Comparative Examples.
[0103] From these results, it is seen that, as compared to
conventional electrodes using a noble metal, the CO.sub.2 reduction
electrode according to the present embodiment can be produced at a
lower cost and has an equivalent or superior performance.
[0104] 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
methods and systems described herein may be embodied in a variety
of other forms; furthermore, various omissions, substitutions and
changes in the form of the methods and systems 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 fail within the scope and
spirit of the invention.
* * * * *