U.S. patent application number 15/220839 was filed with the patent office on 2017-02-02 for method of forming composite catalyst layer, structure for electrochemical reaction device, and 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 | 20170029964 15/220839 |
Document ID | / |
Family ID | 57885855 |
Filed Date | 2017-02-02 |
United States Patent
Application |
20170029964 |
Kind Code |
A1 |
KITAGAWA; Ryota ; et
al. |
February 2, 2017 |
METHOD OF FORMING COMPOSITE CATALYST LAYER, STRUCTURE FOR
ELECTROCHEMICAL REACTION DEVICE, AND ELECTROCHEMICAL REACTION
DEVICE
Abstract
A method of forming a composite catalyst layer includes
repeating a first step of forming a first deposit part and a second
step of forming a second deposit part to alternately deposit the
first and second catalyst materials. At least one effective
thickness out of a first effective thickness calculated from a
growth rate of the first deposit part and a second effective
thickness calculated from a growth rate of the second deposit part
is not less than 0.02 nm nor more than 0.5 nm.
Inventors: |
KITAGAWA; Ryota; (Setagaya,
JP) ; Sugano; Yoshitsune; (Kawasaki, JP) ;
Yamagiwa; Masakazu; (Yokohama, JP) ; Tsutsumi;
Eishi; (Kawasaki, JP) ; Tamura; Jun; (Minato,
JP) ; Ono; Akihiko; (Kita, JP) ; Kudo;
Yuki; (Yokohama, JP) ; Mikoshiba; Satoshi;
(Yamato, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kabushiki Kaisha Toshiba |
Minato-ku |
|
JP |
|
|
Assignee: |
Kabushiki Kaisha Toshiba
Minato-ku
JP
|
Family ID: |
57885855 |
Appl. No.: |
15/220839 |
Filed: |
July 27, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C25B 11/0484 20130101;
C25B 9/08 20130101; Y02P 20/135 20151101; C25B 11/0478 20130101;
C25B 1/003 20130101; Y02P 20/133 20151101; C25B 11/0405
20130101 |
International
Class: |
C25B 11/04 20060101
C25B011/04; C25B 9/08 20060101 C25B009/08 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 29, 2015 |
JP |
2015-149357 |
Claims
1. A method of forming a composite catalyst layer comprising
repeating a first step of forming a first deposit part by
depositing a first catalyst material on a substrate and a second
step of forming a second deposit part by depositing a second
catalyst material in contact with a surface of the first deposit
part on the substrate, to alternately deposit the first and second
catalyst materials, wherein at least one effective thickness out of
a first effective thickness calculated from a growth rate of the
first deposit part and a second effective thickness calculated from
a growth rate of the second deposit part is not less than 0.02 nm
nor more than 0.5 nm.
2. The method of claim 1, wherein the first and second catalyst
materials are deposited using an atomic layer deposition
method.
3. The method of claim 1, wherein the first catalyst material
contains a first metal, an oxide of the first metal, or a nitride
of the first metal, and wherein the second catalyst material
contains a second metal, an oxide of the second metal, or a nitride
of the second metal.
4. The method of claim 1, wherein a deposit part having the
effective thickness of not less than 0.02 nm nor more than 0.5 nm
out of the first deposit part and the second deposit part has a
discontinuous structure including a gap part.
5. The method of claim 1, wherein, in a deposit part having the
effective thickness of not less than 0.02 nm nor more than 0.5 nm
out of the first deposit part and the second deposit part, a number
density of metal atoms of the catalyst material is not less than
1.0.times.10.sup.5 nor more than 1.0.times.10.sup.7 per unit area
of 1 micrometer.times.1 micrometer, the number density being
calculated from a three-dimension atom probe analysis result of the
composite catalyst layer.
6. The method of claim 1, wherein the substrate contains at least
one of a carbon material, a metal material, a metal oxide, and a
semiconductor material.
7. The method of claim 1, wherein the first and second deposit
parts is not heated at a temperature higher than a deposition
temperature of the first catalyst material and higher than a
deposition temperature of the second catalyst material after
repeating the first and second steps.
8. A structure for an electrochemical reaction device comprising: a
substrate containing at least one of a carbon material, a metal
material, a metal oxide, and a semiconductor material; and a
composite catalyst layer disposed on the substrate, wherein the
composite catalyst layer contains a first metal element selected
from the group consisting of transition metals and a second metal
element having 2.0 electronegativity or more.
9. The structure of claim 8, wherein the first metal element is Co,
Ni, Fe, or Mn, and wherein the second metal element is Mo, W, Ru,
Os, Rh, Ir, Pd, Pt, or Au.
10. An electrochemical reaction device comprising: an electrolytic
solution tank comprising a first storage part storing a first
electrolytic solution and a second storage part storing a second
electrolytic solution; a first catalyst layer immersed in the first
electrolytic solution to oxidize the first electrolytic solution;
and a second catalyst layer immersed in the second electrolytic
solution to reduce the second electrolytic solution, wherein the
first catalyst layer contains a first metal element selected from
the group consisting of transition metals and a second metal
element having 2.0 electronegativity or more.
11. The device of claim 10, wherein the first metal element is Co,
Ni, Fe, or Mn, and wherein the second metal element is Mo, W, Ru,
Os, Rh, Ir, Pd, Pt, or Au.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is based upon and claims the benefit of
priority from Japanese Patent Application No. 2015-149357, filed on
Jul. 29, 2015; the entire contents of which are incorporated herein
by reference.
FIELD
[0002] Embodiments disclosed herein generally relate to a method of
forming a composite catalyst layer, a structure for an
electrochemical reaction device, and an electrochemical reaction
device
BACKGROUND
[0003] The development of artificial photosynthesis technology that
replicates photosynthesis of plants to electrochemically convert
sunlight to a chemical substance has been recently progressing in
consideration of an energy problem and an environmental problem.
Converting sunlight to a chemical substance to store it in a
cylinder or a tank is advantages in that it costs lower for energy
storage and has a less storage loss than converting sunlight to
electricity to store it in a battery.
[0004] An electrochemical reaction device capable of artificial
photosynthesis includes, for example, a reduction electrode
immersed in a first electrolytic solution containing carbon
dioxide, an oxidation electrode immersed in a second electrolytic
solution containing water, and a photoelectric conversion layer
electrically connected to the reduction electrode and the oxidation
electrode. The reduction electrode and the oxidation electrode are
each formed through the deposition of a film containing a catalyst
on a substrate, for instance.
[0005] When light enters the photoelectric conversion layer, the
oxidation electrode oxidizes the water through an oxidation
reaction to produce oxygen. Further, the reduction electrode
reduces the carbon dioxide through a reduction reaction to produce
a carbon compound, or produce hydrogen or the like from water. The
electrochemical reaction device can thus produce a desired chemical
substance through the reduction reaction and the oxidation reaction
using light energy. To enhance efficiency of the aforesaid
electrochemical reaction, a used catalyst material preferably has
high catalytic activity.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] FIG. 1 is an explanatory schematic view of an example of a
method of forming a composite catalyst layer.
[0007] FIGS. 2A-2D are views each illustrating a relation between
an effective thickness of an FeOx deposit part and in-plane
distribution of Fe atoms in the FeOx deposit part.
[0008] FIG. 3 is a chart illustrating a relation between the
effective thickness of the FeOx deposit part and in-plane number
density of the Fe atoms in the FeOx deposit part.
[0009] FIG. 4 is a view illustrating an example of a TEM
observation image of the composite catalyst layer.
[0010] FIG. 5 is a schematic view illustrating a configuration
example of an electrochemical reaction device.
[0011] FIG. 6 is a chart illustrating a relation between
overvoltage and current density.
[0012] FIG. 7 is a chart illustrating a relation between
overvoltage and current density.
[0013] FIG. 8 is a chart illustrating a relation between a ratio of
the total effective thickness of the FeOx deposit part and
solar-to-hydrogen efficiency.
DETAILED DESCRIPTION
[0014] A method of forming a composite catalyst layer of an
embodiment includes:
repeating a first step of forming a first deposit part by
depositing a first catalyst material on a substrate and a second
step of forming a second deposit part by depositing a second
catalyst material in contact with a surface of the first deposit
part on the substrate, to alternately deposit the first and second
catalyst materials. At least one effective thickness out of a first
effective thickness calculated from a growth rate of the first
deposit part and a second effective thickness calculated from a
growth rate of the second deposit part is not less than 0.02 nm nor
more than 0.5 nm.
[0015] 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.
[0016] FIG. 1 is an explanatory view of an example of the method of
forming the composite catalyst layer of this embodiment. The
example of the method of forming the composite catalyst layer of
this embodiment includes: a step of forming a deposit part 302a by
depositing a first catalyst material on a substrate 301; and a step
of forming a deposit part 302b by depositing a second catalyst
material in contact with a surface of the deposit part 302a on the
substrate 301.
[0017] The example of the method of forming the composite catalyst
layer of this embodiment includes a step of repeating the step of
forming the deposit part 302a and the step of forming the deposit
part 302b to alternately deposit the first and second catalyst
materials. At this time, in the step of forming the deposit part
302a for the second time onward, the first catalyst material is
deposited so as to be in contact with the surface of the deposit
part 302b. Through the above-described steps, a composite catalyst
layer 302 is formed.
[0018] The first and second catalyst materials each contain, for
example, a metal, a metal oxide, or a metal nitride. Examples of a
method of depositing the first and second catalyst materials
include an atomic layer deposition (ALD) method. The ALD method is
capable of forming a thin film with, for example, 0.1 nm or less
and thus is suitable for forming a catalyst layer having a high
light transmitting property. Further, with the ALD method, a use
amount of the materials to be deposited can be minimized, and thus
it is suitable for forming a catalyst layer that is resource saving
and costs low. The method of depositing the first and second
catalyst materials is not limited to the ALD method, but may be a
chemical vapor deposition (CVD) method, a molecular beam epitaxy
(MBE) method, a vacuum deposition method, or a sputtering method,
for instance.
[0019] The ALD method includes: a step of introducing precursor
vapor or gas of a material to be deposited into a reaction chamber
set to a predetermined temperature and a predetermined degree of
vacuum and causing a substrate disposed in the reaction chamber to
adsorb the precursor of the material to be deposited; a step of
introducing inert gas such as nitrogen or argon into the reaction
chamber to purge the reaction chamber of the precursor vapor or gas
not having reacted; a step of introducing reactive vapor or gas
into the reaction chamber to cause the precursor adsorbed on the
surface of the substrate to react with the reactive vapor or gas;
and a step of introducing inert gas such as nitrogen or argon into
the reaction chamber to purge the reaction chamber of the reactive
vapor or gas not having reacted. The ALD method includes a step of
repeating one cycle of an operation in which the step of causing
the material to be deposited to be adsorbed and the step of purging
the reaction chamber of the precursor vapor or gas of the material
to be deposited are sequentially performed.
[0020] Examples of the reactive vapor or gas include materials
having oxidizing ability, such as water, ozone, oxygen, oxygen
plasma, and hydrogen peroxide, and materials having reducing
ability, such as gases of hydrogen, ammonia, and nitrogen and their
plasma.
[0021] The deposit part 302a has a discontinuous structure
including gap parts 320a. The deposit part 302b has a discontinuous
structure including gap parts 320b. The structure in FIG. 1 is not
restrictive, and it suffices if at least one of the deposit part
302a and the deposit part 302b has the discontinuous structure
including the gap parts.
[0022] The ALD method has a difficulty in depositing two kinds or
more of catalyst materials in a mixed state in the same step. A
possible method to form a composite catalyst layer by using the ALD
method is, for example, to form a first catalyst material layer on
a substrate, form a second catalyst material layer on the first
catalyst material layer, and thereafter perform heat treatment to
compound the first and second catalyst material layers. However, in
a case where the first and second catalyst material layers each
have a continuous structure without a gap part, their surfaces
dominantly contribute to catalytic activity, making it difficult to
compound the first and second catalyst materials. In addition, if
the substrate has poor heat resistance, the heat treatment may
deteriorate electrode and device characteristics, and thus a
substrate having poor heat resistance cannot be used
[0023] By forming at least one deposit part out of the deposit part
302a having a first discontinuous structure and the second deposit
part 302b having a second discontinuous structure, it is possible
to compound the first and second catalyst materials without heat
treatment. The catalyst layer formed of the composite of the first
and second catalyst materials has high catalytic activity and can
improve efficiency of, for example, an electrochemical
reaction.
[0024] When the deposit part 302a and the deposit part 302b are
alternately deposited, the plural deposit parts 302b come into
contact with each other in the gap part 320a, and the plural
deposit parts 302a come into contact with each other in the gap
part 320b. This enables the second catalyst material to diffuse to
the deposit part 302a and the first catalyst material to diffuse to
the deposit part 302b. At this time, in the step of forming the
deposit part 302b, the deposit part 302a when the second catalyst
material is deposited may contain the second catalyst material, and
the deposit part 302b when the second catalyst material is
deposited may contain the first catalyst material.
[0025] The method of forming the composite catalyst layer of this
embodiment does not require heat treatment at a temperature higher
than a deposition temperature of the deposit part 302a and higher
than a deposition temperature of the deposit part 302b. In other
words, the highest value of process temperatures in all the steps
may be equal to or lower than the deposition temperature of the
deposit part 302a and equal to or lower than the deposition
temperature of the deposit part 302b. This can facilitate forming
the composite catalyst layer having high catalytic activity and
allows the use of substrates of more kinds. Further, since the heat
treatment is not necessary, the deterioration of the device
characteristic can be prevented.
[0026] Controlling a formation condition of the deposit part is
important to form the deposit part having the discontinuous
structure. The method of forming the composite catalyst layer of
this embodiment controls an effective thickness of the deposit part
to a predetermined value or less. The deposit part whose effective
thickness is controlled to the predetermined value or less has the
discontinuous structure.
[0027] The effective thickness is an apparent film thickness
calculated from the growth rate of the deposit part and may differ
from the actual thickness of the deposit part. In the ALD method,
the growth rate corresponds to a deposition thickness per cycle
(Growth Per Cycle: GPC). GPC is calculated as follows, for
instance. The thicknesses of three material layers or more
deposited in different numbers of (for example, 100, 200, 300)
cycles are measured with, for example, an atomic force microscope
(AFM), a spectral ellipsometer, or a transmission electron
microscope (TEM). A relation between the number of the deposition
cycles and the actually measured thickness of each of the samples
of the material layers is plotted. A gradient of linear
approximation of each of these plots corresponds to GPC.
[0028] FIGS. 2 are views each illustrating a relation between an
effective thickness of an FeOx deposit part and in-plane
distribution of Fe atoms (including FeOx molecules) in a 10 nm
diameter region of the FeOx deposit part, when the FeOx deposit
part containing iron oxide (FeOx) which is the second catalyst
material is deposited on a CoOx deposit part containing cobalt
oxide (CoOx) which is the first catalyst material. FIG. 2A
illustrates the in-plane distribution when the effective thickness
is 0.02 nm, FIG. 2B illustrates the in-plane distribution when the
effective thickness is 0.1 nm, and FIG. 2C illustrates the in-plane
distribution when the effective thickness is 0.3 nm, and FIG. 2D
illustrates the in-plane distribution when the effective thickness
is 0.5 nm.
[0029] As is seen in FIG. 2A to FIG. 2D, the FeOx deposit part has
a discontinuous structure when the effective thickness falls within
the range of not less than 0.02 nm nor more than 0.5 nm. This shows
that at least one effective thickness out of the first effective
thickness calculated from the growth rate of the deposit part 302a
and the second effective thickness calculated the growth rate of
the deposit part 302b is preferably not less than 0.02 nm nor more
than 0.5 nm (5 angstrom meters). As is also seen in FIG. 2A to FIG.
2D, the in-plane distribution of the Fe atoms is sparser as the
effective thickness of the FeOx deposit part is smaller.
[0030] FIG. 3 is a chart illustrating a relation between the
effective thickness of the FeOx deposit part and the in-plane
number density (normalized by the number density when the effective
thickness is 0.5 nm) of the Fe atoms (including the FeOx molecules)
in the FeOx deposit part, which in-plane number density is obtained
from 3-dimension atom probe (3DAP) analysis results, when the FeOx
deposit part is formed on the CoOx deposit part.
[0031] The in-plane number density of the Fe atoms in the FeOx
deposit part is analyzed as follows, for instance. A
depth-direction Fe concentration profile of a deposited layer is
obtained using the 3-dimension atom probe. In the obtained
concentration profile, a region with the peak being the center up
to a half value width of the peak is defined as an FeOx region.
Next, the number of the Fe atoms (including the FeOx molecules) in
the FeOx region seen in a normal direction of the layer is counted.
A value equal to the counted number divided by a value of the
measured area is the in-plane number density of the Fe atoms. In
counting the number of the Fe atoms in the FeOx region, if the Fe
atoms overlap with each other along the depth direction at a
specific position of the deposited layer, the number of the Fe
atoms at this position is counted as 1. Consequently, the in-plane
number density can be found, with the number density distribution
in the depth direction excluded, and the in-plane distribution
(coverage) can be taken into consideration.
[0032] As is seen in FIG. 3, the in-plane number density of the Fe
atoms substantially linearly decreases as the effective thickness
of the FeOx deposit part decreases. In the result in FIG. 3, the
in-plane number density of the Fe atoms in the FeOx deposit part is
within a range of not less than 6.8.times.10.sup.6 nor more than
6.5.times.10.sup.5 per unit area of 1 micrometer.times.1
micrometer. This shows that, in a deposit part whose effective
thickness is not less than 0.02 nm nor more than 0.5 nm, out of the
deposit part 302a and the deposit part 302b, the number density of
metal atoms calculated from the 3-dimension atom probe analysis
results is preferably not less than 1.0.times.10.sup.5 nor more
than 1.0.times.10.sup.7 per unit area of 1 micrometer.times.1
micrometer.
[0033] Another example of the analysis method for evaluating
continuity of the deposit part is TEM analysis. For example,
regarding a deposited layer, a layer pattern is observed or atoms
are mapped from an upper surface direction using TEM. The
continuity of the deposit part is evaluated by image analysis of an
obtained image. When a ratio of the deposit part region occupying
the whole image is not less than about 10% nor more than about 90%,
the deposit part can be determined as having a discontinuous
structure.
[0034] The substrate 301 is preferably a conductor or a
semiconductor, for instance. As the substrate 301, a carbon
material, a semiconductor material, a metal material, or a metal
oxide is usable, for instance. Examples of the carbon material
include carbon black, activated carbon, fullerene, carbon nanotube,
graphene, ketjen black, and diamond. Examples of the metal oxide
include indium tin oxide (ITO), zinc oxide (ZnO), fluorine-doped
tin oxide (FTO), aluminum-doped zinc oxide (AZO), and
antimony-doped tin oxide (ATO). TiO.sub.2, WO.sub.3, BiVO4, TaON,
or SrTiO.sub.3 may be used as the metal oxide, for instance.
Examples of the metal material include at least one metal out of
Cu, Al, Ti, Ni, Ag, W, Co, and Au, and an alloy containing any of
the aforesaid metals. A stack of the metal materials listed above
may be used. Examples of the semiconductor material include
silicon, germanium, silicon germanium, GaAs, GaP, GaInP, AlGaInP,
CdTe, and CuInGaSe.
[0035] In the method of forming the composite catalyst layer of
this embodiment, the heat treatment at a temperature higher than
the deposition temperature of the first catalyst material and at a
temperature higher than the deposition temperature of the second
catalyst material is not required after the deposit part 302b is
formed. This allows the use of a substrate whose heatproof
temperature is, for example, 200.degree. C. or lower, as the
substrate 301.
[0036] A material contained in each of the first and second
catalyst materials differs depending on required properties. For
example, in a case where oxygen is to be produced through the
oxidation of water, the first and second catalyst materials each
contain, for example, at least one metal out of Co, Fe, Ni, Mn, Ru,
and Ir, an oxide of any of the aforesaid metals, or a nitride of
any of the aforesaid metals. In a case where hydrogen is to be
produced through the reduction of water, the first and second
catalyst materials each contain, for example, at least one metal
out of Pt, Ni, Co, Mo, and Ir, an oxide of any of the aforesaid
metals, or a nitride of any of the aforesaid metals. In a case
where carbon monoxide, formic acid, other hydrocarbon, or the like
is to be produced through the reduction of carbon dioxide, the
first and second catalyst materials each contain, for example, at
least one metal out of Au, Ag, Zn, Cu, and In, an oxide of any of
the aforesaid metals, or a nitride of any of the aforesaid
metals.
[0037] The second catalyst material may contain a compound
different from that contained in the first catalyst material.
Examples of the combination of their materials include a
combination of a metal oxide and a metal, a combination of a metal
oxide and a metal nitride, and a combination of a metal and a
metal.
[0038] In a case where oxygen is to be produced through the
oxidation of water, a combination of the first catalyst material
containing a transition metal element (Co, Ni, Fe, Mn) and the
second catalyst material containing a metal element having 2.0
electronegativity or more exhibits high activity. Examples of the
metal element having 2.0 electronegativity or more include Mo, W,
Ru, Os, Rh, Ir, Pd, Pt, and Au. A ratio of the number of metal
atoms having 2.0 electronegativity or more to the total number of
metal atoms is preferably not less than 15% nor more than 70%.
[0039] The presence of the metal element having 2.0
electronegativity or more improves oxygen adsorptivity of the
surface of the transition metal and also enables the transition
metal element to be in a higher oxidized state, enabling an
improvement of catalytic activity. In a case where the deposit part
containing the transition metal is formed on top of the deposit
part of the metal having 2.0 electronegativity or more, the
thickness of the deposit part containing the transition metal has a
great influence on the catalytic activity, and the thinner this
deposit part (for example, less than 1 nm), the higher the
catalytic activity. However, as the deposit part is thinner, the
catalyst layer is likely to be less durable. The composite catalyst
layer formed by the formation method in this embodiment has high
catalytic activity because the first and second catalyst materials
are three dimensionally compounded on an atomic level. The
composite catalyst layer can also have high durability because it
is free from the aforesaid thickness restriction.
[0040] An example of a structure including a composite catalyst
layer that can be formed by the above-described method of forming
the composite catalyst layer will be described with reference to
FIG. 4. FIG. 4 is a view illustrating an example of a TEM
observation image of the composite catalyst layer. The composite
catalyst layer 302 illustrated in FIG. 4 is provided above a
substrate 301. In FIG. 4, an oxide layer 303 is between the
substrate 301 and the composite catalyst layer 302 and a protection
layer 304 is on the composite catalyst layer 302, but the oxide
layer 303 and the protection layer 304 do not necessarily have to
be provided.
[0041] The description of the substrate 301 can be assisted by the
description of the substrate 301 illustrated in FIG. 1 when
necessary. The composite catalyst layer 302 decreases activation
energy of a chemical reaction. For example, the composite catalyst
layer 302 accelerates an electrochemical oxidation-reduction
reaction. The composite catalyst layer 302 contains a first
catalyst material and a second catalyst material. The description
of the first catalyst material and the description of the second
catalyst material can be assisted by the previous description when
necessary.
[0042] The ALD method requires a longer deposition time than other
film formation methods and thus is likely to be poor in
productivity. So, the thickness of the composite catalyst layer 302
is preferably not less than 1 nm nor more than 100 nm, for
instance. In a case where the composite catalyst layer 302 is
disposed on a light receiving surface of an electrochemical
reaction device, the composite catalyst layer 302 preferably has a
light transmitting property. In this case, the thickness of the
composite catalyst layer 302 is more preferably not less than 1 nm
nor more than 30 nm. Light transmittance of the composite catalyst
layer 302 is preferably 50% or more, more preferably 70% or more of
an amount of irradiating light. The composite catalyst layer 302
may have a plurality of island-shaped catalyst layers.
[0043] In a case where light having a wide wavelength region, such
as sunlight, enters instead of light having a single wavelength,
the light transmittance of the composite catalyst layer 302 is
calculated as follows. After transmittances t (.lamda.) of light
having 300 nm to 1000 nm wavelengths (.lamda.) are measured with a
spectrophotometer, the light transmittance can be calculated
through calculation using the known spectrum I (.lamda.) of the
sunlight (sunlight transmittance
T=.SIGMA.t(.lamda.).times.I(.lamda.)/.SIGMA.I(.lamda.)).
[0044] An interface between the deposit part 302a and the deposit
part 302b cannot be observed in the composite catalyst layer 302
illustrated in FIG. 4. The deposit part having a 0.5 nm effective
thickness or less is very thin and if the first and second catalyst
materials are compounded as in the method of forming the composite
catalyst layer of this embodiment, the interface between the
deposit part 302a and the deposit part 302b is difficult to
observe. Accordingly, analyzing the composite catalyst layer 302
formed by the method of forming the composite catalyst layer of
this embodiment may be impractical.
[0045] The structure of this embodiment is usable as a structure
for an electrochemical reaction device such as an electrochemical
reaction device, for instance. The composite catalyst layer of this
embodiment is high in light transmitting property, catalytic
activity, durability, and other properties. Accordingly, its use as
a catalyst layer of, for example, an electrochemical reaction
device can enhance conversion efficiency, durability, and the other
properties.
Second Embodiment
[0046] FIG. 5 is a schematic view illustrating a configuration
example of an electrochemical reaction device. The electrochemical
reaction device illustrated in FIG. 5 includes an electrolytic
solution tank 1, a photoelectric conversion layer 31, a catalyst
layer 32, a conductive layer 33, an insulating layer 34, a
conductive layer 35, a catalyst layer 36, an insulating layer 37,
and a wiring line 38. A structure having the photoelectric
conversion layer 31, the catalyst layer 32, the conductive layer
35, and the catalyst layer 36 can be regarded as one photoelectric
conversion cell.
[0047] The electrolytic solution tank 1 has a storage part 11
storing an electrolytic solution 21 and a storage part 12 storing
an electrolytic solution 22. The electrolytic solution tank 1 is
not limited to a particular shape and may have any
three-dimensional shape having cavities serving as the storage
parts. For example, the electrolytic solution tank 1 may have a
cylindrical shape or a square shape.
[0048] The storage part 11 and the storage part 12 are separated
from each other by, for example, an ion exchange membrane 4. The
ion exchange membrane 4 is permeable only to specific ions and
separates a product of an oxidation reaction and a product of a
reduction reaction from each other. Examples of the ion exchange
membrane 4 include a cation exchange membrane permeable to hydrogen
ions.
[0049] The electrolytic solution 21 at least contains a substance
to be oxidized. The substance to be oxidized is a substance that is
to be oxidized through the oxidation reaction. Examples of the
substance to be oxidized include water. Other substances to be
oxidized include organic matters such as alcohol and amine.
[0050] The electrolytic solution 22 at least contains a substance
to be reduced. The substance to be reduced is a substance that is
to be reduced through the reduction reaction. Examples of the
substance to be reduced include carbon dioxide.
[0051] The electrolytic solution 21 and the electrolytic solution
22 may contain the same substance. In this case, the electrolytic
solution 21 and the electrolytic solution 22 may be regarded as one
electrolytic solution.
[0052] The photoelectric conversion layer 31 has a function of
separating electric charges using energy of irradiating light such
as sunlight. As the photoelectric conversion layer 31, a
pn-junction or pin-junction photoelectric conversion layer is
usable, for instance.
[0053] The photoelectric conversion layer 31 has a face 311
electrically connected to the catalyst layer 32 and a face 312
electrically connected to the catalyst layer 36. The photoelectric
conversion layer 31 is immersed in the electrolytic solution 21.
The photoelectric conversion layer 31 corresponds to the substrate
301 in the first embodiment.
[0054] The catalyst layer 32 is in contact with the face 311. The
catalyst layer 32 is immersed in the electrolytic solution 21. The
catalyst layer 32 contains an oxidation catalyst for the substance
to be oxidized. 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 is recovered through, for example, a recovery
path. At this time, the recovery path is connected to, for example,
the storage part 11.
[0055] The catalyst layer 32 has a light transmitting property. As
the catalyst layer 32, the composite catalyst layer 302 in the
first embodiment is usable, for instance. The catalyst layer 32 can
be regarded as an oxidation electrode layer. The oxidation
electrode layer oxidizes the electrolytic solution 21.
[0056] The conductive layer 33 is in contact with the face 312. The
conductive layer 33 is immersed in the electrolytic solution 21.
The conductive layer 35 is immersed in the electrolytic solution
22.
[0057] The insulating layer 34 covers the side surface of the
photoelectric conversion layer 31, the side surface of the catalyst
layer 32, and the side surface of the conductive layer 33. The
insulating layer 34 covers the upper surface of the conductive
layer 33.
[0058] The insulating layer 34 is immersed in the electrolytic
solution 21. The presence of the insulating layer 34 can hinder a
leakage current of the photoelectric conversion layer 31 and can
also hinder the erosion of the photoelectric conversion layer 31 by
the electrolytic solution 21.
[0059] The catalyst layer 36 is in contact with the conductive
layer 35. The catalyst layer 36 is immersed in the electrolytic
solution 22. The catalyst layer 36 contains, for example, a
reduction catalyst for the substance to be reduced. A compound
produced by the reduction reaction differs depending on, for
example, the kind of the reduction catalyst. Examples of the
compound produced by the reduction reaction include: carbon
compounds such as carbon oxide (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), and ethylene glycol; and hydrogen. The compound produced by
the reduction reaction is recovered through, for example, a
recovery path. At this time, the recovery path is connected to, for
example, the storage part 12.
[0060] The insulating layer 37 covers the side surface of the
conductive layer 35 and the side surface of the catalyst layer 32.
The insulating layer 37 covers the lower surface of the conductive
layer 35. The insulating layer 37 is immersed in the electrolytic
solution 22. The presence of the insulating layer 37 can hinder a
leakage current and also hinder the erosion of the conductive layer
35 by the electrolytic solution 22.
[0061] The wiring line 38 electrically connects the conductive
layer 33 and the conductive layer 35. The wiring line 38 passes
through the insulating layer 34 and the insulating layer 37, for
instance.
[0062] An operation example of the electrochemical reaction device
illustrated in FIG. 5 will be described. When light enters the
photoelectric conversion layer 31, for example, through the
catalyst layer 32, the photoelectric conversion layer 31 generates
electrons and holes. The holes migrate toward the face 311 and the
electrons migrate toward the face 312. Consequently, the
photoelectric conversion layer 31 is capable of generating an
electromotive force. The light is preferably sunlight, but the
light entering the photoelectric conversion layer 31 may be light
of a light-emitting diode or an organic EL.
[0063] 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 catalyst layer 32, as expressed by the
following equation (1), the water undergoes the 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 12 through the ion exchange membrane 4.
2H.sub.2O.fwdarw.4H.sup.++O.sub.2+4e.sup.- (1)
[0064] Around the catalyst layer 36, as expressed by the following
equation (2), the carbon dioxide undergoes the reduction reaction
and the hydrogen ions react with the carbon dioxide while receiving
the electrons, so that carbon monoxide is produced. Further, in
addition to the carbon monoxide, hydrogen may be produced by the
hydrogen ions receiving the electrons. At this time, the hydrogen
may be produced simultaneously with the carbon monoxide.
2CO.sub.2+4H.sup.++4e.sup.-.fwdarw.2CO+H.sub.2O (2)
[0065] The photoelectric conversion layer 31 needs to have an
open-circuit voltage equal to or larger 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 equation (1) is 1.23
[V/vs. NHE]. The standard oxidation-reduction potential of the
reduction reaction in the equation (2) is -0.1 [V/vs. NHE]. At this
time, in the reactions of the equation (1) and the equation (2),
the open-circuit voltage needs to be 1.33 [V] or higher.
[0066] The open-circuit voltage of the photoelectric conversion
layer 31 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 overvoltage or more. For example,
the overvoltages of the oxidation reaction in the equation (1) and
the reduction reaction in the equation (2) are both 0.2 [V]. In the
reactions of the equation (1) and the equation (2), the
open-circuit voltage is preferably 1.73 [V] or higher.
[0067] The electrochemical reaction device of this embodiment is
not limited to the structure illustrated in FIG. 5. For example,
the composite catalyst layer 302 in the first embodiment may be
used as the catalyst layer 36 instead of as the catalyst layer 32.
That is, a first catalyst layer that causes the first electrolytic
solution to undergo one of oxidation and reduction or a second
catalyst layer that causes the second electrolytic solution to
undergo the other of oxidation and reduction has the composite
catalyst layer 302 in the first embodiment. Further, the
photoelectric conversion layer 31 may be irradiated with light
through the conductive layer 33.
[0068] The composite catalyst layer in the first embodiment is
usable not only as a catalyst for the electrochemical reaction
device of this embodiment but also as a catalyst for an existing
electrochemical reaction device such as a battery or an
electrolysis cell. Examples of the electrolysis cell include a
water electrolysis cell and a CO.sub.2 electrolysis cell. Similarly
to an alkaline water electrolysis cell, the electrolysis cell may
have an anode and a cathode, which are immersed in an electrolytic
tank and are separated by a diaphragm. Similarly to a solid polymer
electrolyte cell, the electrolysis cell may have a membrane
electrode assembly (MEA) structure that is a stack of an anode, a
solid polymer membrane, and a cathode. These cells are driven by a
system power source, or by an external power source of renewable
energy such as sunlight, wind power, or heat of the earth. In a
case where sunlight is used, the structure in which the
photoelectric conversion layer is outside the electrolysis cell is
different from the structure of the electrochemical reaction
device.
[0069] As the 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 accelerates the oxidation reaction of water.
Examples of the aqueous solution containing the electrolyte include
aqueous solutions containing phosphate 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).
[0070] Examples of the carbon dioxide-containing electrolytic
solution 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 carbon
dioxide-containing electrolytic solution may contain alcohol such
as methanol, ethanol, or acetone. The water-containing electrolytic
solution may be the same as the carbon dioxide-containing
electrolytic solution. However, an absorption amount of carbon
dioxide in the carbon dioxide-containing electrolytic solution is
preferably high. So, a solution different from the water-containing
electrolytic solution may be used as the carbon dioxide-containing
electrolytic solution. The carbon dioxide-containing electrolytic
solution 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.
[0071] As the aforesaid electrolytic solution, an ionic liquid that
contains salt 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 characteristic of lowering reduction energy.
[0072] Examples of the primary amine include methylamine,
ethylamine, propylamine, butylamine, pentylamine, and hexylamine.
Hydrocarbon of the amine may be replaced with, for example, alcohol
or halogen. Examples of the amine whose hydrocarbon is replaced
include methanolamine, ethanolamine, and chloromethyl amine.
Further, an unsaturated bond may be present. The same thing can be
said for hydrocarbons of the secondary amine and the tertiary
amine.
[0073] Examples of the secondary amine include dimethylamine,
diethylamine, dipropylamine, dibutylamine, dipentylamine,
dihexylamine, dimethanolamine, diethanolamine, and dipropanolamine.
The replaced hydrocarbons may be different. This is also the same
for the tertiary amine. Examples of the amine having different
hydrocarbons include methylethylamine and methylpropylamine.
[0074] Examples of the tertiary amine include trimethylamine,
triethylamine, tripropylamine, tributylamine, trihexylamine,
trimethanolamine, triethanolamine, tripropanolamine,
tributanolamine, tripropanolamine, triexanolamine,
methyldiethylamine, and methyldipropylamine.
[0075] Examples of the cation of the ionic liquid include a
1-ethyl-3-methylimidazolium ion, a 1-methyl-3-propylimidazolium
ion, a 1-butyl-3-methylimidazole ion, a
1-methyl-3-pentylimidazolium ion, and a 1-hexyl-3-methylimidazolium
ion
[0076] The position 2 of the imidazolium ion may be replaced.
Examples of the cation which is the imidazolium ion having the
replaced position 2 include a 1-ethyl-2,3-dimethylimidazolium ion,
a 1,2-dimethyl-3-propylimidazolium ion, a
1-butyl-2,3-dimethylimidazolium ion, a
1,2-dimethyl-3-pentylimidazolium ion, and a
1-hexyl-2,3-dimethylimidazolium ion.
[0077] Examples of the pyridinium ion include methylpyridinium,
ethylpyridinium, propylpyridinium, butylpyridinium,
pentylpyridinium, and hexylpyridinium. In the imidazolium ion and
the pyridinium ion, an alkyl group may be replaced, and an
unsaturated bond may be present.
[0078] Examples of the anion include a fluoride ion, a chloride
ion, a bromide ion, an iodide ion, BF.sub.4.sup.-, PF.sub.6.sup.-,
CF.sub.3COO.sup.-, CF.sub.3SO.sub.3.sup.-, NO.sub.3.sup.-,
SCN.sup.-, (CF.sub.3SO.sub.2).sub.3C.sup.-,
bis(trifluoromethoxysulfonyl)imide,
bis(trifluoromethoxysulfonyl)imide, and
bis(perfluoroethylsulfonyl)imide. It may be a dipolar ion in which
the cations and the anions of the ionic liquid are coupled by
hydrocarbons. Incidentally, a buffer solution such as a potassium
phosphate solution may be supplied to the storage parts 11, 12.
[0079] The photoelectric conversion layer 31 has a semiconductor
layer of, for example, silicon, germanium, silicon germanium, GaAs,
GaInP, AlGaInP, CdTe, or CuInGaSe. The semiconductor may have, for
example, a monocrystalline, polycrystalline, or amorphous
structure. Further, the photoelectric conversion layer 31 is more
preferably of a multijunction type in order to obtain a high
open-circuit voltage. Between the catalyst layer 32 and the
photoelectric conversion layer 31, a transparent conductive film
of, for example, ITO may be disposed.
[0080] The photoelectric conversion layer 31 need not be of a pn
junction type or a pin junction type. The photoelectric conversion
layer 31 may be a p-type or n-type semiconductor. In this case, it
is possible to cause an oxidation-reduction reaction by light
irradiation, using a barrier formed on an interface between the
electrolytic solution and the semiconductor. If the open-circuit
voltage high enough to cause the oxidation-reduction reaction
cannot be obtained, an auxiliary power source may be connected to
the wiring line 38 to compensate for a deficient voltage.
[0081] The catalyst layer 32 contains an oxidation catalyst. As the
oxidation catalyst, a material that decreases activation energy for
oxidizing water is usable. In other words, a material that lowers
overvoltage when oxygen and hydrogen ions are produced by the
oxidation reaction of water is usable.
[0082] In a case where the composite catalyst layer 302 in the
first embodiment is used as the catalyst layer 32 and oxygen is
produced by the catalyst layer 32, the catalyst layer 32 preferably
contains two or more metal elements selected from Co, Fe, Ni, Mn,
Ru, and Ir. The above metal elements may exist in a state of an
oxide or a nitride. Alternatively, the catalyst layer 32 may
contain a metal element selected from transition metals such as Co,
Ni, Fe, and Mn and a metal element having 2.0 electronegativity or
more, such as Mo, W, Ru, Os, Rh, Ir, Pd, Pt, or Au.
[0083] The catalyst layer 36 contains a reduction catalyst. As the
reduction catalyst, a material that decreases activation energy for
hydrogen evolution or reducing carbon dioxide is usable. In other
words, a material that lowers overvoltage when a carbon compound is
produced by the reduction reaction of carbon dioxide is usable. For
example, a metal material is usable. For example, a metal such as
gold, aluminum, copper, silver, platinum, palladium, or nickel, or
an alloy containing this metal is usable as the metal material.
[0084] In a case where the composite catalyst layer 302 in the
first embodiment is used as the catalyst layer 36 and hydrogen is
produced by the catalyst layer 36, the catalyst layer 36 preferably
contains two or more metal elements selected from Pt, Ni, Co, Mo,
and Ir. In a case where the composite catalyst layer 302 in the
first embodiment is used as the catalyst layer 36 and a carbon
compound such as carbon monoxide, formic acid, or hydrocarbon is
produced by the catalyst layer 36, the catalyst layer 36 preferably
contains two or more metal elements selected from Au, Ag, Zn, Cu,
and In. The above metal elements may exist in a state of an oxide
or a nitride
[0085] The conductive layer 33 and the conductive layer 35 each
contain, for example, at least one metal out of Cu, Al, Ti, Ni, Ag,
W, Co, and Au, an alloy containing any of the aforesaid metals, a
transparent conductive oxide such as ITO, ZnO, FTO, AZO, or ATO, or
a carbon material such as carbon black, activated carbon,
fullerene, carbon nanotube, graphene, ketjen black, or diamond. The
conductive layer 33 and the conductive layer 35 each may have a
film stack of the aforesaid materials.
[0086] The insulating layer 34 and the insulating layer 37 each
contain, for example, a resin material such as epoxy resin,
fluorocarbon resin, or cycloolefin resin, a metal oxide, nitride,
or oxynitride containing Ti, Zr, Al, Si, or Hf, or a glass material
whose main component is silica, boric acid, or phosphoric acid.
[0087] The electrochemical reaction device of this embodiment can
have higher efficiency of the conversion from light to a chemical
substance by including the composite catalyst layer of the first
embodiment.
EXAMPLES
Example 1
[0088] In this example, composite catalyst layers of cobalt oxide
(CoOx) and iron oxide (FeOx) were formed by an ALD method and their
catalytic activities were evaluated.
[0089] A 200 nm thick glass substrate with an ITO film was
introduced into a reaction chamber of a deposition apparatus,
CoO.sub.x was deposited on the substrate to a 1 nm effective
thickness by the ALD method at a 150.degree. C. temperature to form
a first deposit part. Next, without the substrate taken out of the
reaction chamber, FeOx was deposited to a 0.1 nm effective
thickness by the ALD method at a 150.degree. C. temperature to form
a second deposit part, whereby a sample of an example 1-1 was
fabricated. Further, the deposition condition was changed so that
the effective thickness of FeOx became 0.25 nm, 0.5 nm, 0.75 nm,
and 1.0 nm, and under each of the conditions, the above step was
separately performed, whereby samples of an example 1-2 to an
example 1-5 were fabricated.
[0090] A 200 nm thick glass substrate with an ITO film was
introduced into the reaction chamber of the deposition apparatus,
and only CoOx was deposited to a 1 nm effective thickness by the
ALD method at a 150.degree. C. temperature, whereby a sample of a
comparative example 1-1 including a catalyst layer was
fabricated.
[0091] A 200 nm thick glass substrate with an ITO film was
introduced into the reaction chamber of the deposition apparatus,
and only FeOx was deposited to a 1 nm effective thickness by the
ALD method at a 150.degree. C. temperature, whereby a sample of a
comparative example 1-2 including a catalyst layer was
fabricated.
[0092] Only a prescribed area (8 mm diameter) of each of the
samples of the example 1-1 to the example 1-5 and the comparative
examples 1-1, 1-2 was exposed using a Kapton tape. Working
electrodes including the respective samples were each placed in one
compartment of an H-cell, a counter electrode of a Pt wire was
placed in the other compartment, and a glass filter was disposed
between the two compartments. As a reference electrode, Ag/AgCl was
used.
[0093] Catalyst properties were evaluated from steady-state
polarization curves which are obtained by varying the potential in
30 mV increments or 50 mV increments and reading a current value at
each potential five minutes later. Series resistance components R
such as a solution resistance and a substrate resistance were
measured by AC impedance, and an effective potential
(E.sub.appl-IR) applied to each of the electrodes was calculated.
Overvoltage .eta. of an oxygen production reaction was estimated by
the following equation.
.eta. = ( E appl - IR ) - E 0 + E ref = ( E appl - IR ) - 1.23 +
0.059 .times. pH + 0.199 ( 1 ) ##EQU00001##
[0094] FIG. 6 illustrates current density-overvoltage curves of the
samples of the example 1-1 to the example 1-5 and the comparative
examples 1-1, 1-2. As is seen in FIG. 6, the samples whose FeOx
deposit part on the CoOx deposit part has an effective thickness of
0.1-0.5 nm can have higher activity than CoOx and FeOx. When the
film thickness of FeOx is 0.5 nm or less, FeOx can have a
discontinuous structure as described above, and accordingly the
surface portion is thought to have a state in which CoOx and FeOx
are in a homogeneous state. Further, it is seen that, as FeOx is
thicker, the composite catalyst layer has the catalytic activity
closer to that of FeOx (comparative example 2) and thus has a state
close to the continuous structure, and has a stacked structure of
CoOx and FeOx, instead of a composite state of these. It is also
seen that catalytic activity improves even without heat treatment
for homogenization performed after the formation of the catalyst
layer.
Example 2
[0095] In this example, composite catalyst layers of CoOx and Ru
were formed by ALD, and their catalytic activities were
evaluated.
[0096] A 200 nm thick glass substrate with an ITO film was
introduced into a reaction chamber of a deposition apparatus,
CoO.sub.x was deposited on the substrate to a 0.4 nm effective
thickness by the ALD method at a 150.degree. C. temperature to form
a first deposit part. Next, without the substrate taken out of the
reaction chamber, Ru was deposited to a 0.14 nm effective thickness
by the ALD method at a 150.degree. C. temperature. This was
repeated a plurality of times, whereby a sample of an example 2-1
having a 5 nm thick catalyst layer was fabricated. A ratio of the
total effective thickness of a Ru deposit part to the thickness of
the obtained catalyst layer was 25%.
[0097] A 200 nm thick glass substrate with an ITO film was
introduced into the reaction chamber of the deposition apparatus,
CoOx was deposited on the substrate to a 0.4 nm effective thickness
by the ALD method at a 150.degree. C. temperature. Next, without
the substrate taken out of the reaction chamber, Ru was deposited
to a 0.42 nm effective thickness by the ALD method at a 150.degree.
C. temperature. This was repeated a plurality of times, whereby a
sample of an example 2-2 having a 5 nm thick catalyst layer was
fabricated. A ratio of the total effective thickness of a Ru
deposit part to the thickness of the obtained catalyst layer was
50%.
[0098] A 200 nm thick glass substrate with an ITO film was
introduced into the reaction chamber of the deposition apparatus,
and CoOx was deposited on the substrate by the ALD method, whereby
a sample of a comparative example 2-1 having a 5 nm thick catalyst
layer was fabricated.
[0099] A 200 nm thick glass substrate with an ITO film was
introduced into the reaction chamber of the deposition apparatus,
and CoOx was deposited on the substrate by the ALD method, whereby
a sample of a comparative example 2-2 having a 5 nm thick catalyst
layer was fabricated.
[0100] Their catalytic activities were evaluated as in the example
1. As an electrolytic solution, a boric acid buffer solution (about
pH9.2) was used. FIG. 7 is a chart illustrating current
density-overvoltage curves of the samples of the examples 2-1, 2-2
and the comparative examples 2-1, 2-2. As a result of the
measurement, it was confirmed that the catalysts of the examples
2-1, 2-2 can have higher activity than those of the comparative
examples 2-1, 2-2. This shows that a composite catalyst including a
layer containing a transition metal and a layer containing a high
electronegativity metal can synergistically have higher activity
than when these layers are each used alone.
[0101] Pattern observation of the example 2-1 and the example 2-2
was performed with a transmission electron microscope by EDS
element analysis, and the observation of an about 1 nm space showed
that CoOx and Ru in the composite catalyst were in a uniform
dispersion state without segregation.
[0102] Table 1 presents composition analysis results by X-ray
photoelectron spectroscopy of the example 2-1 and the example 2-2.
A ratio of the number of Ru atoms to the total number of Co atoms
and the Ru atoms was about 15% and about 70% respectively.
TABLE-US-00001 TABLE 1 Co Ru O Ru/(Ru + Co) (atomic %) (atomic %)
(atomic %) (%) Example 29.8 5.5 64.6 15.6 2-1 Example 19.3 42.3
38.4 68.7 2-2
Example 3
[0103] In this example, photoelectrochemical reactivities of
electrochemical reaction devices each including a composite
catalyst layer of cobalt oxide (CoOx) and iron oxide (FeOx) were
evaluated.
[0104] Three-junction photoelectric conversion layers each having a
first photoelectric conversion layer which absorbs light in a short
wavelength region, a second photoelectric conversion layer which
absorbs light in a mid wavelength region, and a third photoelectric
conversion layer which absorbs light in a long wavelength region
were prepared. 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 germanium layer, an
i-type amorphous silicon layer, and an n-type amorphous silicon
layer.
[0105] A ZnO layer was formed on a first face of the three-junction
photoelectric conversion layer, an Ag layer was formed on the ZnO
layer, and a SUS substrate was formed on the Ag layer. An ITO layer
was formed on a second face of the three-junction photoelectric
conversion layer. CoOx was deposited on the ITO layer to a 0.15 nm
effective thickness by the ALD method, and thereafter FeOx was
deposited to a 0.04 nm effective thickness. This was repeated
twelve times, and CoOx was finally deposited to a 0.15 nm effective
thickness, whereby a sample of an example 3-1 was fabricated. A
ratio of the total effective thickness of an FeOx deposit part to
the thickness of the obtained catalyst layer was 24%.
[0106] A ZnO layer was formed on a first face of the three-junction
photoelectric conversion layer, an Ag layer was formed on the ZnO
layer, and a SUS substrate was formed on the Ag layer. An ITO layer
was formed on a second face of the three-junction photoelectric
conversion layer. CoOx was deposited on the ITO layer to a 0.15 nm
effective thickness by the ALD method, and thereafter FeOx was
deposited to a 0.15 nm effective thickness. This was repeated eight
times, and CoOx was finally deposited to a 0.15 nm effective
thickness, whereby a sample of an example 3-2 was fabricated. A
ratio of the total effective thickness of an FeOx deposit part to
the thickness of the obtained catalyst layer was 47%.
[0107] A ZnO layer was formed on a first face of the three-junction
photoelectric conversion layer, an Ag layer was formed on the ZnO
layer, and a SUS substrate was formed on the Ag layer. An ITO layer
was formed on a second face of the three-junction photoelectric
conversion layer. CoOx was deposited on the ITO layer to a 0.15 nm
effective thickness by the ALD method, and thereafter FeOx was
deposited to a 0.625 nm effective thickness. This was repeated
eight times, and CoOx was finally deposited to a 0.15 nm effective
thickness, whereby a sample of an example 3-3 was fabricated. A
ratio of the total effective thickness of an FeOx deposit part to
the thickness of the obtained catalyst layer was 76%.
[0108] A ZnO layer was formed on a first face of the three-junction
photoelectric conversion layer, an Ag layer was formed on the ZnO
layer, and a SUS substrate was formed on the Ag layer. An ITO layer
was formed on a second face of the three-junction photoelectric
conversion layer. Only CoOx was deposited on the ITO layer to a
0.25 nm effective thickness, whereby a sample of a comparative
example 3-1 including a catalyst layer was fabricated.
[0109] A ZnO layer was formed on a first face of the three-junction
photoelectric conversion layer, an Ag layer was formed on the ZnO
layer, and a SUS substrate was formed on the Ag layer. An ITO layer
was formed on a second face of the three-junction photoelectric
conversion layer. Only FeOx was deposited on the ITO layer to a
0.25 nm effective thickness, whereby a sample of a comparative
example 3-2 including a catalyst layer was fabricated.
[0110] Stainless steel surfaces of the fabricated samples were each
electrically connected to a conducting wire using a copper tape.
The conducting wire was passed to a glass tube with a 6 mm
diameter, and a space between the sample and the glass tube was
filled with epoxy resin for sealing. Next, a peripheral portion of
the front surface and the entire rear surface of each of the
samples were encapsulated with epoxy resin, whereby electrodes were
formed.
[0111] Anodes of the electrodes having the respective samples with
a 1 cm.sup.2 area and cathodes of Pt wires were immersed in a boric
acid buffer electrolytic solution (pH9.2). The catalyst layer sides
of the sample surfaces were irradiated with light using a solar
simulator (AM1.5, 1000 W/m.sup.2). A value of a current passing
between the anode and the cathode was measured under the light
irradiation in the absence of the application of a bias across the
both electrodes. The measured current value corresponds to a
reaction amount of water in an oxidation-reduction reaction. From
the obtained current density J (A/cm.sup.2), solar-to-hydrogen
efficiency (STH) .eta..sub.STH (%) was calculated, assuming that
Faraday's efficiency of hydrogen production is 100%. The
solar-to-hydrogen efficiency .eta..sub.STH is calculated by the
following equation (2). In the equation (2), P.sub.1sun represents
radiant power of sunlight, and E.sup.0 represents standard voltage
of the hydrogen production reaction.
[ Equation 1 ] .eta. STH ( % ) = J ( mA / cm 2 ) .times. E 0 ( V )
.times. F ( % ) P 1 sun ( mW / cm 2 ) = J ( mA / cm 2 ) .times.
1.23 ( V ) .times. 1.00 100 ( mW / cm 2 ) ( 2 ) ##EQU00002##
[0112] FIG. 8 is a chart illustrating a relation between the ratio
of the total effective thickness of the FeOx deposit part and the
solar-to-hydrogen efficiency. As is seen in FIG. 8, a CoOx-FeOx
composite catalyst in which the ratio of the effective thickness of
FeOx is 75% can produce high photoelectrochemical reactivity due to
CoOx and FeOx. From this, it is thought that the composite of CoOx
and FeOx can have appropriate catalytic activity and a high light
transmitting property, and as a result, can produce higher
efficiency than CoOx and than FeOx.
[0113] As described hitherto, the electrochemical reaction device
including the composite catalyst layer fabricated by the method of
forming the composite catalyst layer of the above-described
embodiment has high photoelectrochemical reactivity.
[0114] 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.
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