Description
Title of the Invention: ELECTRODE FOR WATER-SPLITTING REACTION AND
METHOD FOR PRODUCING THE SAME
Technical Field
[0001]
The present invention relates to an electrode for water-splitting reaction
comprising a photocatalyst, which is capable of producing hydrogen and/or oxygen by
conducting a water-splitting reaction utilizing sunlight.
[Background Art]
[0002]
Practical realization of high performance light energy conversion system
utilizing renewable energy such as solar energy rapidly increases the importance in
recent years from the standpoints of inhibition of global warming and the aim of
departure from dependency of fossil resources that are running out. Above all, the
technology of splitting water using solar energy to produce hydrogen is not only a
technology of the existing petroleum refining and raw material supply of ammonia and
methanol, but a technology required in the upcoming hydrogen energy society based on
a fuel cell.
[0003]
Water-splitting reaction by photocatalyst is widely studied from the 1970s
(Non-Patent Document 1). Many photocatalysts had the disadvantages that due to
large band gap, water splitting proceeds if in ultraviolet region, but visible light region
cannot be utilized, and even through the visible light region can be utilized, the catalyst
itself is unstable in water. However, on or after 2000, water can be split by light
energy in a visible light region, and photocatalyst stable in water, that is, visible light
photocatalyst, began to get published
For example, oxynitride, nitride, oxysulfide and sulfide are known as the
visible light photocatalyst (Non-Patent Document 1).
[0004]
Water splitting method by photocatalyst is greatly classified into two kinds.
One is a method of conducting water splitting reaction in a suspension, and another is a
1
method of conducting water splitting using an electrode comprising a conductive metal
substrate and photocatalyst deposited on the substrate in a thin film form, and a counter
electrode.
[0005]
The former is that photocatalyst capable of performing complete splitting of
water is currently limited, and additionally, hydrogen and oxygen that are products of
water splitting reaction are formed as a mixed gas, and this requires separation of
hydrogen and oxygen after recovering. On the other hand, the latter has the advantage
of having many selections of photocatalysts that can be used. For this reason, in recent
years, an example of forming visible light water splitting photocatalyst in a thin film
form and carrying out water splitting in high efficiency is reported (Non-Patent
Document 2).
[0006]
Generally, deposition of photocatalyst on a conductive metal support is
conducted by physical vapor deposition (PVD) methods represented by a vacuum
deposition method and a sputtering method and chemical vapor deposition (CVD)
methods (those methods are generically named dry process); coating methods
represented by spin coating and screen printing; sol-gel methods; and electrophoretic
deposition (EPD) methods (Non-Patent Document 3 and Patent Document 1).
[0007]
On the other hand, a method of covering the circumference of photocatalyst
particles with semiconductor or good conductor, having conductivity higher than that of
photocatalyst used, in order to improve electron conductivity between photocatalyst
particles and between photocatalyst and a support is proposed. For example, a method
of covering the circumference of iron oxide or tungsten oxide particles that are
photocatalyst with oxide semiconductor of titanium, aluminum, antimony, tin, zinc,
zirconium or the like to improve conductivity is proposed (Patent Document 2).
[0008]
However, where oxides of metal species different from photocatalyst, such as
titanium oxide are used as a conductor to iron oxide or tungsten oxide that is
photocatalyst, the respective energy levels greatly differ. This gave rise to the problem
2
that mismatch occurs in level, causing resistance, and conductivity is decreased.
[0009]
To solve the above problem, in an electrode for water-splitting reaction
comprising a support having deposited thereon at least one kind of photocatalyst
particles selected from the group consisting of oxynitride, nitride, oxysulfide and sulfide,
an electrode for water-splitting reaction having semiconductor or good conductor
between the photocatalyst particles and between the photocatalyst particles and the
support is disclosed (Patent Document 3). According to this disclosure, semiconductor
or good conductor is present around the photocatalyst particles, and as a result, internal
resistance of the electrode is decreased, and photoelectric conversion efficiency can be
improved.
Citation List
Patent Document
[0010]
Patent Document 1: JP-A-2006-297230
Patent Document 2: GB200416616A
Patent Document 3: JP-A-2011-131170
Non-Patent Document
[0011]
Non-Patent Document 1: Chem. Soc. Rev., 2009, 38, 253-278
Non-Patent Document 2: Thin Solid Films, 2010, 518, 5855-5859
Non-Patent Document 3: J. Phys. Chem. B 2001, 105, 10893-10899
Summary of the Invention
Problems that the Invention is to Solve
[0012]
Visible light photocatalyst has excellent performance in water splitting
performance in that longer wavelength light can be utilized in water photolysis. On
the other hand, the visible light photocatalyst is required to conduct nitridation reaction
and sulfidation reaction in producing the photocatalyst. Those reactions require
production processes at high temperature, and therefore have the problem that when the
photocatalyst is deposited on a metal support, it is difficult to apply to a production
3
method by the above-described dry process.
[0013]
When photocatalyst particles suspended in a solution are deposited as a starting
raw material on a metal support by a coating method, electrophoresis or the like, contact
face between support surface and photocatalyst particles and between photocatalyst
particles is small. This gave rise to the problem that resistance is generated and
electron conductivity in a photocatalyst film and electron conductivity between
photocatalyst and support surface are remarkably decreased.
[0014]
On the other hand, in the technique according to Patent Document 3, charge
transfer is further accelerated in order to further improve photoelectric conversion
efficiency. As a result, more semiconductors and good conductors must be introduced
in the circumference of photocatalyst particles to increase conductive path. However,
where many conductors are introduced, there was the problem that the conductors
absorb light or reflect light, thereby inhibiting light absorption by photocatalyst.
[0015]
The technique according to Patent Document 3 is the embodiment that
photocatalyst particles are fixed to a support and conductor is then introduced, and
therefore had the problem that semiconductor or good conductor is difficult to be
introduced between the photocatalyst particles and the support, and conduct path is
difficult to be formed between the photocatalyst particles and the support.
[0016]
The present invention has been made in view of the above problems, and has
an object to provide an electrode for water-splitting reaction capable of increasing
conductive path between a photocatalyst layer and a current collecting layer without
inhibiting light absorption by photocatalyst, and a method for producing the same.
Means for Solving the Problems
[0017]
As a result of intensive investigations to solve the above problems, the present
inventors have obtained the following findings.
(1) In an electrode for water-splitting reaction, conductive path is secured between
4
a photocatalyst layer and a current collecting layer by providing a contact layer
containing semiconductor or good conductor between the photocatalyst layer and the
current collecting layer.
(2) In an electrode for water-splitting reaction, when the photocatalyst layer
consists essentially of photocatalyst particles and the contact layer is provided along a
surface shape at a current collecting layer side of the photocatalyst layer, the
photocatalyst particles in the photocatalyst layer and the contact layer are directly
contacted to each other. As a result, conductive path between the photocatalyst layer
and the current collecting layer is increased.
(3) In an electrode for water-splitting reaction, the form that light receivable
photocatalyst particles come in contact with the contact layer is obtained by decreasing
the thickness of the photocatalyst layer (for example, the form that one to three layers of
the photocatalyst particles are laminated). As a result, photoelectric conversion
efficiency is improved.
(4) In an electrode for water-splitting reaction, absorption or reflection of light by
semiconductor or good conductor can be inhibited by containing semiconductor or good
conductor in the photocatalyst layer and eccentrically locating the semiconductor or the
good conductor in the photocatalyst layer at a contact layer side than an opposite side
(light-receiving face side) to the contact layer (in other words, amount of semiconductor
or good conductor present at the light-receiving face side is decreased) As a result,
photoelectric conversion efficiency is improved.
(5) The electrode for water-splitting reaction can be easily produced by forming a
photocatalyst layer, forming a contact layer on one face of the photocatalyst layer, and
then forming a current collecting layer on a face at a side opposite the photocatalyst
layer of the contact layer.
(6) Particularly, the electrode can be easily produced by providing a photocatalyst
layer on a substrate, depositing semiconductor or good conductor on a face at a side
opposite the substrate of the photocatalyst layer to form a contact layer along a face
shape of the photocatalyst layer, providing a current collecting layer on a face at a side
opposite the photocatalyst layer of the contact layer, and then removing the substrate.
(7) The contact layer and the current collecting layer can be easily formed by a
5
sputtering method.
(8) An electrode for water-splitting reaction having high photoelectric conversion
efficiency can be produced from a photocatalyst that has been difficult to exert the
photoelectric conversion efficiency by a sputtering method or the like, by forming the
photocatalyst layer with photocatalyst particles.
(9) By forming the photocatalyst layer with photocatalyst particles, production
efficiency is improved by sputtering, vapor deposition or the like, and an electrode
having large area can be produced.
[0018]
The present invention has been made based on the following findings.
A first invention is an electrode for water-splitting reaction comprising: a
photocatalyst layer; a current collecting layer; and a contact layer that contains
semiconductor or good conductor and is provided between the photocatalyst layer and
the current collecting layer, wherein the photocatalyst layer consists essentially of
photocatalyst particles, and the photocatalyst layer is provided along the surface shape
of the photocatalyst layer at the current collecting layer side of the photocatalyst layer.
[0019]
The term “consists essentially of photocatalyst particles” used herein means
that the photocatalyst layer consists of photocatalyst particles described hereinafter,
further contains semiconductor or good conductor, or contains other components such
as a metal oxide, a composite oxide, or a hydrophilic material that does not have
absorption in a visible light region in a range that the advantage of the present invention
is not impaired, and preferably means that the photocatalyst layer consists of the
photocatalyst particles described hereinafter, or further contains semiconductor or good
conductor.
[0020]
In the first invention, it is preferably that the number of stack of photocatalyst
particles the photocatalyst layer in a lamination direction of the photocatalyst layer and
the contact layer is from 1 to 3. The term “the number of stack of photocatalyst
particles in a lamination direction of the photocatalyst layer and the contact layer”
means the number of photocatalyst particles stacked vertically toward a light receiving
6
face from the contact layer, and can be specified by, for example, observing a
cross-section of an electrode for water-splitting reaction with SEM-EDX, or by an
optical microscope, a surface profiler or the like.
[0021]
In the first invention, the photocatalyst layer may contain semiconductor or
good conductor. In this case, it is preferred that the semiconductor or the good
conductor is eccentrically located in the photocatalyst layer at the contact layer side than
the side opposite the contact layer. The term “eccentrically located at the contact layer
side than the side opposite the contact layer” means that semiconductor or good
conductor is contained in relatively large amount at a contact layer side than a light
receiving face side of the photocatalyst layer.
[0022]
The electrode for water-splitting reaction according to the first invention is
preferably that a photocurrent density at measurement potential of 1 V (vs. RHE) is 350
μA/cm2 or more.
[0023]
A second invention is a method for producing an electrode for water-splitting
reaction, comprising: a photocatalyst layer formation step of forming a photocatalyst
layer; a contact layer formation step of covering one face of the photocatalyst layer with
semiconductor or good conductor to form a contact layer; and a current collecting layer
formation step of forming a current collecting layer on a face of the contact layer at the
side opposite the photocatalyst layer of the contact layer.
[0024]
In the contact layer formation step according to the second invention, it is
preferred that the contact layer is formed on one face of the photocatalyst layer by
sputtering the semiconductor or the good conductor.
[0025]
In the current collecting layer formation step according to the second invention,
it is preferred that the current collecting layer is formed by sputtering.
[0026]
In the second invention, it is more preferred that the method further comprises
7
a non-contact photocatalyst removal step of removing the photocatalyst particles that
are not come in contact with the contact layer.
[0027]
In the photocatalyst layer formation step according to the second invention, it
is preferred that the photocatalyst layer is formed by laminating photocatalyst on one
face of a first substrate. In this case, it is preferred in the non-contact photocatalyst
removal step that the first substrate is removed.
[0028]
In the second invention, it is preferred that the method further comprises a
reinforcing substrate formation step of providing a second substrate at a side opposite
the contact layer of the current collecting layer after the current collecting layer
formation step.
[0029]
A third invention is a method for producing an electrode for water-splitting
reaction, comprising a photocatalyst layer formation step of laminating photocatalyst
particles on one face of a first substrate; a contact layer formation step of forming a
contact layer by covering a face of the photocatalyst layer at the side opposite the first
substrate side of the photocatalyst layer with semiconductor or good conductor; a
current collecting layer formation step of forming a current collecting layer on a face of
the contact layer at the side opposite the photocatalyst layer of the contact layer; a
reinforcing substrate formation step of laminating a second substrate on a face of the
current collecting layer at a side opposite the contact layer of the current collecting
layer; and a substrate peeling step of peeling the first substrate.
[0030]
In the third invention, it is preferred that the contact layer is formed by
sputtering in the contact layer formation step.
[0031]
A fourth invention is a method for producing hydrogen, comprising irradiating
an electrode for water-splitting reaction dipped in water or an electrolyte aqueous
solution with light to conduct a water photolysis, wherein the electrode for
water-splitting reaction is the electrode for water-splitting reaction according to the first
8
invention.
[0032]
A fifth invention is a method for producing hydrogen, comprising irradiating an
electrode for water-splitting reaction dipped in water or an electrolyte aqueous solution
with light to conduct a water photolysis, wherein the electrode for water-splitting
reaction is the electrode for water-splitting reaction obtained by the second invention or
the third invention.
Advantage of the Invention
[0033]
According to the present invention, the contact layer containing semiconductor
or good conductor is provided between the photocatalyst layer and the current collecting
layer, the photocatalyst layer consists essentially of photocatalyst particles, and the
contact layer is provided along a surface shape at a current collecting layer side (side
opposite a light receiving face) of the photocatalyst layer. This makes it possible to
increase conductive path between the photocatalyst layer and the current collecting
layer without inhibiting light absorption by photocatalyst, and can improve
photoelectric conversion efficiency.
[0034]
According to the present invention, of an electron and a hole generated in
photocatalyst by light irradiation, the hole is guided in the contact layer when the
photocatalyst is a photocatalyst for hydrogen production, and the electron is guided in
the contact layer when the photocatalyst is a photocatalyst for oxygen production. As
a result, recoupling is inhibited, and photoelectric conversion efficiency can be
improved.
[0035]
Furthermore, according to the present invention, even in case of using a
photocatalyst that is required to be supported in a particle form, an electrode having
high photoelectric conversion efficiency can be obtained. Since such photocatalyst,
for example, is produced through a high temperature reaction such as nitridation
reaction or sulfidation reaction, the photocatalyst is difficult to be formed in a thin film
form, such as a visible light photocatalyst containing at least two kinds of metals.
9
[0036]
Water splitting activity of photocatalyst greatly depends on crystallinity of
photocatalyst, and activity tends to become high as the crystallinity is high. Visible
light photocatalyst generally has high crystallinity, and a method of obtaining in the
form of powdery particles is established as the production method. Therefore,
utilizing the production technique of a visible light photocatalyst without any change, a
photocatalyst electrode having high efficiency can be produced using powdery
photocatalyst particles obtained.
Furthermore, according to the present invention, the photocatalyst layer is not
required to be formed by sputtering or the like in which time required in the step is
relatively long. As a result, production time can be shortened.
Brief Description of the Drawings
[0037]
Fig. 1 is a conceptual view of a water splitting reaction apparatus (two
electrode system) that is one application example of the electrode for water-splitting
reaction according to the present invention.
Fig. 2 is a view schematically showing an electrode (100) for water-splitting
reaction of the present invention according to one embodiment.
Fig. 3 is a view for explaining a production method (S10) of the electrode for
water-splitting reaction of the present invention according to one embodiment.
Fig. 4A and Fig. 4B are a view for explaining a production step (S1) of the
electrode for water-splitting reaction of the present invention according to one
embodiment.
Fig. 5A and Fig. 5B are a view for explaining production steps (S2 and S3) of
the electrode for water-splitting reaction of the present invention according to one
embodiment.
Fig. 6 is a view for explaining a production step (S4a) of the electrode for
water-splitting reaction of the present invention according to one embodiment.
Fig. 7 is a view for explaining a production step (S4b) of the electrode for
water-splitting reaction of the present invention according to one embodiment.
Fig. 8 is a view schematically showing an electrode for water-splitting reaction
10
obtained through the production method (S10) of the electrode for water-splitting
reaction of the present invention according to one embodiment.
Fig. 9A and Fig. 9B are SEM images showing cross-sectional shapes of
electrodes for water-splitting reaction according to Examples.
Fig. 10 is SEM image showing a cross-sectional shape of an electrode for
water-splitting reaction according to Comparative Example.
Fig. 11A to Fig. 11D are photographs and element mapping view, by
SEM-EDS in cross-sections of electrodes for water-splitting reaction according to
Examples.
Mode for Carrying out the Invention
[0038]

Schematic view of a water splitting reaction apparatus (two electrode system)
is shown in Fig. 1. In Fig. 1, an anode 61 (electrode at oxidation side) and a cathode
63 (electrode at reduction side) are connected by a conductor 64, and those are dipped in
water or an electrolyte aqueous solution 65. The electrolyte solution in the anode 61
and the cathode 63 are separated by a diaphragm 62.
[0039]
In Fig. 1, the electrode for water-splitting reaction of the present invention is
used as the anode 61. However, the electrode for water-splitting reaction of the present
invention may be used as the cathode 63, and the electrode for water-splitting reaction
of the present invention may be used as the anode 61 and the cathode 63.
[0040]
When the electrode 61 for water-splitting reaction of the present invention is
used as a cathode, sunlight is inserted from a cathode side, and when the electrode for
water-splitting reaction of the present invention is used as an anode and a cathode,
sunlight is inserted from both sides of the anode and the cathode.
[0041]
Generally, in a water splitting reaction apparatus that recovers hydrogen gas
and oxygen gas separately, a two electrode system constituted of an electrode for
water-splitting reaction and a counter electrode is used. In water splitting reaction in
11
the two electrode system, the following reaction proceeds on each electrode in an acidic
solution.
(Anode) H2O+2h+ → 1/2O2+2H+
(Cathode) 2H++2e− → H2
wherein h+ is a hole, and e− is an electron.
[0042]
In a neutral solution, the following reaction proceeds on each electrode.
(Anode) H2O+2h+ → 1/2O2+2H+
(Cathode) 2H2O+2e− → H2+ 2OH−
[0043]
In a basic solution the following reaction proceeds on each electrode.
(Anode) 2OH−+2h+ → 1/2O2+H2O
(Cathode) 2H2O+2e− → H2+2OH−
[0044]
In this case, it is important how an electron (e−) or a hole (h+) on a catalyst
surface light-excited and a reaction material participate efficiently, and how recoupling
between an electron and a hole is inhibited and electrons generated guide in a cathode
side efficiently. Particularly, resistance between photocatalyst particles and resistance
between photocatalyst particles and a current collector inhibit electron conduction.
Therefore, this must be reduced as possible.
[0045]
The electrode for water-splitting reaction of the present invention that can
increase conductive path between a photocatalyst layer and a current collecting layer
without inhibiting light absorption by a photocatalyst, can decrease resistance between
photocatalyst particles and resistance between photocatalyst particles and a current
collector, and can improve photoelectric conversion efficiency is described below.
[0046]

The electrode 100 for water-splitting reaction of the present invention
according to one embodiment is schematically shown in Fig. 2. The electrode 100 for
water-splitting reaction comprises a photocatalyst layer 10, a current collecting layer 20,
12
and a contact layer 20 containing semiconductor or good conductor provided between
the photocatalyst layer 10 and the contact layer 30, wherein the photocatalyst layer 10
consists essentially of photocatalyst particles 11, and the contact layer 20 is provided
along a shape at a current collecting layer 30 side of the photocatalyst layer 10. In the
electrode 100 for water-splitting reaction, electrons generated by light irradiation in the
photocatalyst layer 10 flow to the current collecting layer 30 through the contact layer
20.
[0047]
(Photocatalyst layer 10)
The photocatalyst layer 10 is a layer consisting essentially of the photocatalyst
particles 11, and cocatalysts 12 are optionally supported on the photocatalyst particles
11.
[0048]
The photocatalyst constituting the photocatalyst particle 11 can be any
compound so long as the following requirements are satisfied.
(1) Potential of electrons generated by light irradiation is negative than potential that
can reduce hydrogen ion or water molecule to hydrogen molecule, or potential of holes
generated by light irradiation is positive than potential that can oxidize water or hydride
ion into oxygen molecule.
(2) Even though the photocatalyst is irradiated with light in an aqueous solution and
water splitting reaction proceeds, the compound is stable.
[0049]
The shape of the photocatalyst particles 11 is preferably a powder shape. The
term “powder shape” used herein means an aggregate of solids in which bonding
between photocatalyst particles is mild, and having a certain degree of fluidity. The
term “powder shape” preferably means a state that when the photocatalyst particles 11
have formed the photocatalyst layer 10, the photocatalyst particles 11 are bonded to
each other by electrostatic force.
[0050]
Examples of the photocatalyst at a hydrogen generation side, that reduces
hydrogen ion or water specifically include:
13
oxides such as SrTiO3, SrTiO3 doped with Cr, Sb, Ta, Rh, Na, Ga, K, La or the
like, LaTi2O7, SnNb2O6, CuBi2O4, LaTi2O7 doped with Cr, Fe or the like, and SnNb2O6;
oxynitride compounds such as LaTiO2N, CaTaO2N, SrTaO2N, BaTaO2N,
LaTaO2N, Y2Ta2O5N2, Zn1+xGeN2Ox and Ga1-xZnxN1-xOx (x is a numerical value of
from 0 to 1, hereinafter the same);
nitride compounds such as TaON, Ta3N5, GaN, Ta3N5, GaN doped with Mg,
and Ge3N4;
sulfide compounds such as ZnS, ZnS doped with Cu, Ni or Pb, CdS doped with
Ag, CdxZn1-xS, CuInS2, CuIn5S8, CuGaS2, CuGa3S5, CuGa5S8, AgGaS2, AgGa3S5,
AgGa5S8, AgGa0.9In0.1S2, AgIn5S8, NaInS2, AgInZn7S9, CuInGaS2, Cu0.09In0.09Zn1.82S2,
Cu0.25Ag0.25In0.5ZnS2, and Cu2ZnSnS4;
oxysulfide compounds such as Sm2Ti2O5S2, La5Ti2CuS5O7, La5Ti2AgS5O7, and
La5Ti2AgO5S7;
oxysulfide compounds containing La and In (described in Chemistry Letters
2007, 36, 854-855);
selenide compounds such as CuGaSe2, CuGa3Se5, CuGa5Se8, AgxCu1-xGaSe2,
AgxCu1-xGa3Se5, AgxCu1-xGa5Se8, AgGaSe2, AgGa3Se5, AgGa5Se8, and CuInGaSe2; and
oxyselenide compounds such as La5Ti2CuSe5O7, La5Ti2AgSe5O7;
chalcogenide compounds in which S and Se are partially mixed in optional
proportions, such as La5Ti2Cu(Sx, Se1-x)5O7, and La5Ti2Ag(Sx,Se1-x)5O7.
However, the photocatalyst is not limited to the exemplified materials.
[0051]
Examples of the photocatalyst at an oxygen generation side, that oxidizes water
molecule or hydroxide ion into oxygen molecule specifically include:
oxides such as TiO2 doped with Cr, Ni, Sb, Nb, Th, Sb or the like, WO3,
Bi2WO6, Bi2MoO6, In2O3(ZnO)3, PbBi2Nb2O9, BiVO4, Ag3VO4, AgLi1/3Ti2/3O2, and
AgLi1/3Sn2/3O2;
oxynitride compounds such as LaTiO2N, CaNbO2N, BaNbO2N, SrNbO2N,
LaNbO2N, TaON, CaTaO2N, SrTaO2N, BaTaO2N, LaTaO2N, Y2Ta2O5N2, Zn1+xGeN2Ox,
Ga1-xZnxN1-xOx;
nitride compounds such as Ta3N5, Ta3N5 doped with Mg and Zr, GaN, GaN
14
doped with Mg, and Ge3N4;
oxysulfide compounds such as Sm2Ti2O5S2, and La5Ti2AgS5O7;
oxyselenide compounds such as La5Ti2AgSe5O7; and
chalcogenide compounds in which S and Se are partially mixed in optional
proportions, such as La5Ti2Cu(Sx, Se1-x)5O7, and La5Ti2Ag(Sx,Se1-x)5O7.
However, the photocatalyst is not limited to the exemplified materials.
[0052]
In the present invention, the photocatalyst is preferably at least one compound
selected from the group consisting of oxynitride, nitride, oxysulfide, sulfide, selenide
and oxyselenite, and more preferably at least one compound selected from the group
consisting of oxynitride, nitride, oxysulfide and selenide. Above all, compounds
containing at least two kinds of metals are preferred, and of those, visible light
photocatalysts are more preferred.
[0053]
Those photocatalysts are generally obtained by conducting nitridation reaction
or sulfidation reaction under high temperature conditions. Therefore, it may be
difficult to support those photocatalysts on a substrate by a method such as sputtering or
the like. When those photocatalysts consist essentially of photocatalyst particles, the
performance of those photocatalysts is exerted, and an electrode having high
photoelectric conversion efficiency can be obtained.
[0054]
Oxynitride and nitride are further preferred from the standpoint of high
photoelectric conversion efficiency. Use of LaTiO2N, Ta3N5 and Ta3N5 doped with Mg
and Zr is preferred from that photocatalyst activity is high, abundance on the ground is
rich, and costs are low, and LaTiO2N is most preferred.
[0055]
The above-described photocatalysts can be synthesized by the conventional
methods (for example, a method disclosed in J. Phys. Chem. B 2003, 107, 791-797, a
method disclosed in Journal of Flux Growth, 2010, 5, 81, and a method disclosed in
Catalysis Today 2003, 78, 555-560).
[0056]
15
The average particle size of primary particles of the photocatalysts 11 is not
particularly limited so long as its performance is exerted. The lower limit is preferably
1 nm or more, more preferably 10 nm or more, and still more preferably 50 nm or more.
The upper limit is preferably 500 μm or less, more preferably 300 μm or less, still more
preferably 200 μm or less, particularly 100 μm or less.
[0057]
The average particle size of secondary particles of the photocatalysts 11 can be
a size that can achieve sufficient light absorption while contacting the contact layer 20.
The lower limit is 10 nm or more, preferably 20 nm or more, more preferably 50 nm or
more, still more preferably 100 nm or more, and particularly preferably 500 nm or more.
The upper limit is 5000 μm or less, preferably 1000 μm or less, more preferably 500 μm
or less, still more preferably 300 μm or less, particularly preferably 200 μm or less, and
most preferably 100 μm or less.
[0058]
The term “primary particles” used herein means particles of minimum unit
constituting a powder, and the term “secondary particles” used herein means particles in
the state that the primary particles are aggregated. The term “particle size” used herein
means tangent diameter in a constant direction (Feret’s diameter), and the particle size
of the primary particles and the secondary particles can be measured by the
conventional means such as XRD, TEM or SEM.
[0059]
If necessary, the cocatalysts 12 are supported on the photocatalyst particles 11.
Examples of the cocatalysts 12 of the photocatalyst at a hydrogen generation
side specifically include Pt, Pd, Rh, Ru, Ni, Au, Fe, NiO, RuO2, and Cr-Rh oxide.
[0060]
Examples of the cocatalysts 12 of the photocatalyst at an oxygen generation
side specifically include IrO2, Pt, Co, Fe, C, Ni, Ag, and spinel compounds containing
Ni, Fe, Co or Mn, such as MnO, MnO2, Mn2O3, Mn3O4, CoO, Co3O4, NiCo2O4,
NiFe2O4, CoFe2O4 and MnCo2O4.
However, the cocatalysts are not limited to those materials.
[0061]
16
The photocatalyst layer 10 may contain other components in a range that the
advantages of the present invention are not impaired. Examples of the components
that can be contained include hydrophilic materials that do not have absorption in a
visible light region, for example, metal oxides such as Al2O3, Bi2O3, BeO, CeO2, Ga2O3,
GeO, GeO2, La2O3, MgO, Nb2O3, Sb2O3, Sb2O5, Sc2O3, SiO2, Sm2O3, SnO2, TiO2, ZnO,
ZrO2, Y2O3 and WO3; composite oxides such as SiO2-Al2O3 and ZrO2-Al2O3; zeolite;
and heteropolyacids.
[0062]
The photocatalyst layer 10 preferably has smaller thickness. Particularly, it is
preferred that the number of stack of the photocatalyst particles 11 in a lamination
direction of the photocatalyst layer 10 and the contact layer 20 is from 1 to 3. By this,
many light-receivable photocatalyst particles 11 come in contact with the contact layer
20, the thickness of the photocatalyst layer 10 is decreased, and resistance between the
photocatalyst particles 11 can be decreased. As a result, photoelectric conversion
efficiency can be improved.
[0063]
The term “the number of stack of the photocatalyst particles in a lamination
direction of the photocatalyst layer and the contact layer” means the number of stack of
the photocatalyst particles 11 acting as photocatalyst. The photocatalyst particles 11
may directly come in contact with the contact layer 20 and may directly come in contact
with the photocatalyst particles 11 embedded in the contact layer 20, but the
photocatalyst particles 11 embedded in the contact layer 20 are not included in the
number of stack.
[0064]
“The number of stack of the photocatalyst particles in a lamination direction
of the photocatalyst layer and the contact layer” is obtained by observing a cross-section
in a lamination direction of the electrode for water-splitting reaction with SEM
(Scanning Electron Microscope) and SEM-EDX (Scanning Electron
Microscope/Energy Dispersive X-ray Spectroscopy).
[0065]
The specific measurement method is that a cross-section of a sample
17
(hereinafter sometimes simply referred to as a “cross-section”) in which a central part of
the electrode for water splitting reaction to be measured has been cut in a lamination
direction, is observed with SEM, and the number of photocatalyst particles stacked on
the outermost surface is visually measured. It confirms by SEM-EDX that the
particles stacked on the outermost surface are the component of the photocatalyst
supported. The measurement with SEM is conducted by selecting 3 to 5 optional
regions each having 10 μm × 10 μm in the cross-section. The regions are not selected
at the electrode edge, but at the central part of the electrode, in which unevenness in
thickness of the photocatalyst layer is small.
[0066]
The photocatalyst layer 10 may contain semiconductor or good conductor
described hereinafter. In this case, the semiconductor or the good conductor is
preferably eccentrically located at a contact layer 20 side than a side (light receiving
face side) opposite the contact layer 20, of the photocatalyst layer 10 (that is, the
amount of the semiconductor or the good conductor present at the light receiving face
side is relatively small than the contact layer 20 side). This can inhibit absorption or
reflection of light by the semiconductor or the good conductor, thereby improving
photoelectric conversion efficiency.
[0067]
(Contact layer 20)
The contact layer 20 is a layer containing semiconductor or good conductor.
Materials showing good electroconductivity and not catalyzing reverse reaction of water
splitting reaction and reaction pairing the water splitting reaction of photocatalyst can be
used as the semiconductor or the good conductor.
[0068]
Examples of the materials specifically include Au, C, Cu, Cd, Co, Cr, Fe, Ga,
Ge, Hg, Ir, In, Mn, Mo, Nb, Pb, Ru, Re, Rh, Sn, Sb, Ta, Ti, V, W, TiN, TiO2, Ta3N5,
TaON, ZnO, SnO2, Indium Tin Oxide (ITO), SnO, TiO2(:Nb), SrTiO3(:Nb),
fluorine-doped tin oxide (FTO), CuAlO2, CuGaO2, CuInO2, ZnO, ZnO(:Al), ZnO(:Ga),
ZnO(:In), GaN, GaN(:C), GaN(:Si), GaN(:Sn), and their alloys and mixtures.
However, the materials are not limited to the materials exemplified above. When the
18
contact layer 20 is formed by sputtering as described after, the contact layer is
preferably a layer comprising a metal such as Au, Nb, Ta, Ti or Zr.
[0069]
In the present specification, the description of “α(:β)” means that α is doped
with β. For example, TiO2(:Nb) means that TiO2 is doped with Nb.
[0070]
The contact layer 20 is provided along a surface shape at a current collecting
layer 30 side of the photocatalyst layer 10. That is, the contact layer 20 has the form of
covering the side opposite the light receiving face of the photocatalyst layer 10. This
form achieves that the photocatalyst particles 11 in the photocatalyst layer 10 are
directly contacted with the contact layer 20. As a result, conductive path between the
photocatalyst layer 10 and the current collecting layer 30 is increased, thereby
improving photoelectric conversion efficiency. Furthermore, the photocatalyst layer
10 and the contact layer 20 are strongly bonded to each other, and this can inhibit that
the photocatalyst particles 11 easily drop out of the contact layer 20.
[0071]
The photocatalyst layer 10 may contain the same semiconductor or good
conductor contained in the contact layer 20, or similar semiconductor or good conductor,
as described above. It is preferred that the contact layer 20 is not provided at a light
receiving face side of the photocatalyst layer 10. This can inhibit absorption or
reflection of light by semiconductor or good conductor, thereby improving photoelectric
conversion efficiency.
[0072]
The thickness of the contact layer 20 is not particularly limited so long as the
thickness is the degree of capable of covering the side opposite of the light receiving
face of the photocatalyst layer 10. For example, the lower limit is 0.3 nm or more,
preferably 1 nm or more, and more preferably 10 nm or more, and the upper limit is
generally 1 mm or less.
[0073]
In the present invention, strength of the contact layer 20 is increased as the
thickness of the contact layer 20 is increased. For example, if the contact layer 20 has
19
strength to such an extent that the contact layer 20 can be independently handled by
increasing the thickness of the contact layer 20, the electrode for water-splitting reaction
can be constituted without providing the current collecting layer 30 described
hereinafter. In other words, the contact layer 20 and the current collecting layer 30
may be an integrated layer constituted of the same material.
[0074]
(Current collecting layer 30)
The current collecting layer 30 is a layer containing a metal or non-metal.
More specifically, the current collecting layer 30 comprises a metal or a conductive
material formed by a non-metal conductive material such as carbon (graphite), oxide
conductor, or a conductive film containing a conductive material. Above all, the
current collecting layer 30 preferably comprises a single substance of an element
showing good electroconductivity or an alloy thereof.
[0075]
Examples of a single substance of an element specifically include Al, Au, Cu,
Mo, Ni, Pd, Pt, Ti and W. On the other hand, examples of an alloy specifically include
stainless steel, carbon steel titanium alloy and aluminum alloy. However, the current
collecting layer is not limited to the materials exemplified above. The current
collecting layer preferably comprises titanium or titanium alloy from that the layer is
stable in a wide pH range in an electrolyte aqueous solution and shows high
electroconductivity, and from the standpoints of costs of material and easy availability.
[0076]
In the electrode 100 for water-splitting reaction, the current collecting layer 30
has the form covering the side opposite the photocatalyst layer 10 of the contact layer
20. The thickness of the current collecting layer 30 is not particularly limited so long
as the thickness is the degree capable of functioning as an electrode for water-splitting
reaction. From the standpoints of formation method of a material and costs, the lower
limit is 0.1 μm or more, and preferably 1 μm or more, and the upper limit is 10 mm or
less, and preferably 5 mm or less.
[0077]
Thus, the electrode 100 for water-splitting reaction can increase conductive
20
path between the photocatalyst layer and the current collecting layer without inhibiting
light absorption by photocatalyst, by providing the contact layer 20 containing
semiconductor or good conductor between the photocatalyst layer 10 and the current
collecting layer 30 and providing the contact layer 20 along a surface shape at the
current collecting layer 30 side of the photocatalyst layer 10, and can improve
photoelectric conversion efficiency.
[0078]
The electrode for water-splitting reaction according to the present invention has
excellent photoelectric conversion performance that has not been obtained in the
conventional electrode for water-splitting reaction. Specifically, photocurrent density
in measurement potential 1V (vs. RHE) is 350 μA/cm2 or more, preferably 400 μA/cm2
or more, and more preferably 500 μA/cm2 or more. The electrode for water-splitting
reaction having such excellent photoelectric conversion efficiency can be easily
produced as follows.
[0079]

The order of each step of a production method S10 of the electrode for
water-splitting reaction according to one embodiment is shown in Fig. 3. Schematic
views for explaining the production process of the electrode for water-splitting reaction
of the present invention according to one embodiment are shown in Figs. 4 to 8. The
production method S10 is described below with reference to Fig. 3 and Figs. 4 to 8 as
appropriate.
[0080]
As shown in Fig.3, the production method S10 comprises a photocatalyst layer
formation step S1 of forming a photocatalyst layer, a contact layer formation step S2 of
covering one face of the photocatalyst layer with semiconductor or good conductor to
form a contact layer, and a current collecting layer formation step S3 of forming a
current collecting layer on a face at a side opposite the photocatalyst layer of the contact
layer. The production method S10 optionally further comprises a non-contact
photocatalyst removal step S4. The non-contact photocatalyst removal step S4
preferably comprises a reinforcing substrate formation step S4a or a cleaning step S4c,
21
as described hereinafter.
[0081]

The step S1 is a step of forming a photocatalyst layer in the form of a thin layer.
Method for decreasing thickness of the photocatalyst layer is not particularly limited.
Kneading with a binder, decreasing a thickness by pressure molding, decreasing a
thickness by laminating photocatalyst on a first substrate, and the like can be applied to
the method. In particular, it is preferred to form a photocatalyst layer in the form of a
thin layer by laminating the photocatalyst on a first substrate from that a strong thin film
can be formed without using a binder and impurities are difficult to incorporate between
the photocatalyst and the contact layer.
[0082]
The reason is that conductive path of an electron may be restricted by that a
binder and impurities remain between the photocatalyst particles and the contact layer,
and photoelectric conversion efficiency of the electrode for water-splitting reaction
obtained may be decreased.
[0083]
In other words, in the photocatalyst layer formation step S1, the photocatalyst
particles 11, and the photocatalyst particles 11 and a substrate 50a are preferably
adhered by electrostatic force of the photocatalyst particles. By this, the photocatalyst
particles 11 having the cocatalyst 12 supported thereon are laminated on the first
substrate 50a as shown in Fig. 4A and Fig. 4B.
[0084]
A material that is inactive to a reaction with photocatalyst and has excellent
chemical stability and heat resistance is selected as the first substrate 50a. Glass plate,
Ti plate and Cu plate are particularly preferably used.
[0085]
Stack of the photocatalyst particles 11 on the first substrate 50a can be
conducted by, for example, dispersing the photocatalyst particles 11 in a solvent to
obtain a suspension, and applying the suspension to the substrate 50a having been
subjected to polishing and cleaning, followed by drying.
22
[0086]
Examples of the solvent used include water; alcohols such as methanol and
ethanol; ketones such as acetone; and aromatics such as benzene, toluene and xylene.
When the photocatalyst particles 11 are dispersed in the solvent, the photocatalyst
particles 11 can be uniformly dispersed in the solvent by performing ultrasonic
treatment.
[0087]
A method for applying the suspension to the substrate 50a is not particularly
limited. Examples the method include a spraying method, a dipping method, a
squeeze method, a doctor blade method, a spin coating method, a screen coating method,
a roll coating method and an inkjet method. Also, the examples also include a method
of disposing a glass plate on the botton of container containing the photocatalyst
suspension, and withdrawing the solvent from the container after the photocatalyst
settles out. Drying conditions after the application are that a temperature higher than
the melting point of a solvent is maintained or heating is conducted at a temperature to
such an extent that a solvent evaporates in a short period of time (for example, about 15
to 200°C). In this case, it is preferred to control vaporization rate such that the coating
liquid does not flocculate. By this, the photocatalyst particles 11 can be appropriately
laminated on the substrate 50a.
[0088]
(Contact layer formation step S2)
The step S2 is a step of covering one face of the photocatalyst layer with
semiconductor or good conductor to form the contact layer. A method of covering
with semiconductor or good conductor is not particularly limited. Wet processes such
as a spraying method, a spin coating method, an electrophoresis method and a plating
method, and dry processes such as a vacuum deposition method and sputtering method
can be used as the method.
[0089]
Of those methods, a vacuum deposition method and a sputtering method in
which impurities are difficult to be incorporated between the photocatalyst and the
contact layer are preferred, and a sputtering method is particularly preferred in that a
23
contact layer having strong bonding to the photocatalyst particles and being difficult to
be peeled can be formed, and the method is easily applied to high melting point metal.
[0090]
When the sputtering method is used, the thickness of the contact layer can be
easily controlled by adjusting sputtering rate and treatment time. By this, the contact
layer 20 can be formed on one face of the photocatalyst layer along the shape thereof as
shown in Fig. 5A.
[0091]
Although the optimum temperature varies depending on the kind of the
photocatalyst, semiconductor or good conductor used, the photocatalyst temperature in
the contact layer formation step S2 is preferably from about 30 to 500°C. The contact
layer formation step may be carried out at low temperature, and after the film formation,
heating may be conducted at from 30 to 500°C.
[0092]
(Current collecting layer formation step S3)
The step S3 is a step of forming a current collecting layer on a face at a side
opposite the photocatalyst layer of the contact layer. A method of forming the current
collecting layer on the contact layer is not particularly limited. For example, a
sputtering method can be used. In this case, the thickness of the current collecting
layer can be easily adjusted by adjusting sputtering rate and treatment time.
[0093]
Alternatively, the current collecting layer may be formed by applying a
conductive adhesive (for example, silver paste or carbon paste) to the contact layer and
then adhering a metal or non-metal support becoming the current collecting layer
thereto. In this case, the shape of the current collecting layer is not limited to a plate
shape, and materials having punching metal shape, mesh shape or lattice shape or a
porous body having penetrated fine holes may be used.
[0094]
By this, the current collecting layer 30 can be formed on one face of the contact
layer 20 as shown in Fig. 5B.
[0095]
24
The contact layer 20 and the current correcting layer 30 may be an integrated
layer constituted of the same material as described above. That is, the contact layer
having sufficient thickness and capable of functioning as a current collecting layer can
be formed by conducting sputtering for a long period of time in the step S2. In this
case, the step S3 is included in the step S2.
[0096]
(Non-contact photocatalyst removal step S4)
The step S4 is a step of removing photocatalyst particles that do not come in
contact with the contact layer. The removal method is not particularly limited, and for
example, a cleaning step S4c such as ultrasonic cleaning treatment can be applied.
[0097]
Examples of a cleaning liquid used include water; electrolyte aqueous solution;
alcohols such as methanol, ethanol and propanol; aliphatic hydrocarbons such as
pentane and hexane; aromatic hydrocarbons such as toluene and xylene; ketones such as
acetone and methyl ethyl ketone; esters such as ethyl acetate; halogenides such as
fluorocarbon; ethers such as diethyl ether and tetrahydrofuran; sulfoxides such as
dimethylsufoxide; and nitrogen-containing compounds such as dimethyl formamide.
Water and soluble compounds such as methanol, ethanol and tetrahydrofuran are
preferably used.
[0098]
The current collecting layer has small thickness. Therefore, where breakage
or the like of the electrode for water-splitting reaction is a concern in the step S4, it is
preferred that the electrode is subjected to the cleaning step S4c through the reinforcing
substrate formation step S4a of providing a second substrate on a face opposite the
contact layer of the current collecting layer.
[0099]
The method of providing the second substrate is not particularly limited.
Example of the method includes a method of adhering the current collecting layer and
the second substrate using an adhesive such as epoxy adhesive. Specifically, a second
substrate 50b can be attached to a face opposite the contact layer 20 of the current
collecting layer 30 through an adhesive layer 40, as shown in Fig. 6.
25
[0100]
When the photocatalyst particles 11 are laminate on the first substrate 50a in
the step S1, the photocatalyst particles that do not come in contact with the contact layer
20 are removed by the cleaning step S4c or the like after passing a substrate removal
step S4b that removes the first substrate 50a, preferably after the substrate removal step
S4b subsequent to the reinforcing substrate formation step S4a, as shown in Fig. 7.
[0101]
The removal method of the first substrate 50a is not particularly limited.
Examples of the removal method that can be used include a method of mechanically
peeling the first substrate 50a, a method of removing a substrate by dipping in water to
wet a photocatalyst particle stack part, thereby weakening the bonding between the
photocatalyst particles, a method of removing a substrate by dissolving the substrate in
a chemical such as an acid or an alkali, and a method of removing a substrate by
physically destroying the substrate. A method of peeling a substrate is preferred in that
the possibility of damage of the photocatalyst layer is low (this case is called a substrate
peeling step). The contact layer 20 and a part of the non-contact photocatalyst
particles 11 can be physically removed together with the first substrate 50a by the
substrate removal step S4b.
[0102]
On the other hand, the photocatalyst particles 11 that come in contact with the
contact layer 20 are physically bonded to a certain extent to the contact layer 20.
Therefore, when removing the first substrate 50a, the photocatalyst particles 11 remain
at a contact layer 20 side without dropping out. In this case, the non-contact
photocatalyst particles that have not been removed in the substrate removal step S4b are
preferably further subjected to removal treatment by the cleaning step S4c or the like.
[0103]
The electrode for water-splitting reaction according to the present invention
can be easily produced by passing the above steps S1 to S3 and optionally the step S4.
For example, as shown in Fig. 8, the electrode 100 for water-splitting reaction of the
present invention according to one embodiment can be provided on the second substrate
50b through an adhesive layer 40.
26
[0104]
Thus, the production method S10 of an electrode for water-splitting reaction
according to the present invention is characterized by forming a photocatalyst layer,
forming a contact layer and then forming a current collecting layer. By this
constitution, the contact layer can be provided along a face shape of the photocatalyst
layer, and the photocatalyst particles in the photocatalyst layer and the contact layer are
directly contacted to each other. As a result, conductive path between the
photocatalyst layer and the current collecting layer can be increased.
[0105]
Furthermore, according to the production method S10 of an electrode for
water-splitting reaction according to the present invention, the semiconductor or good
conductor contained in the photocatalyst layer can be eccentrically located at a contact
layer side than a side (light receiving face side) opposite the contact layer of the
photocatalyst layer (that is, the amount of the semiconductor or good conductor present
at the light receiving face side is decreased). By this, absorption or reflection of light
by semiconductor or good conductor can be inhibited, light absorption by the
photocatalyst is not disturbed, and photoelectric conversion efficiency can be improved.
[0106]
Furthermore, according to the production method S10 of an electrode for
water-splitting reaction according to the present invention, the thickness of the
photocatalyst layer can be decreased (for example, a form that only one to three layers
of photocatalyst particles are laminated), and light receivable photocatalyst particles
come in contact with the contact layer. As a result, resistance between photocatalyst
particles can be decreased, and photoelectric conversion efficiency can be further
improved.
[0107]
The surface of electrode for water-splitting reaction obtained by the above
method can be further modified with TiO2, Al2O3 or the like. The modification method
is not particularly limited, but, for example, an electron layer deposition (ALD) method
can be utilized.
[0108]
27

Production method of hydrogen according to the present invention is described
below.
The production method of hydrogen according to the present invention is
characterized that the electrode for water-splitting reaction of the present invention
dipped in water or an electrolyte aqueous solution is irradiated with light to perform
water-splitting, thereby producing hydrogen.
[0109]
The method for producing hydrogen by water-splitting is not particularly
limited, but there are a method of recovering hydrogen and oxygen generated together
or separately, and then separating those, and a method of generating hydrogen and
oxygen separately and recovering those individually. The method of generating
hydrogen and oxygen separately and recovering those individually is generally used.
[0110]
Specifically, hydrogen is produced using a two electrode system water-splitting
reaction apparatus having an anode and a cathode, at least one of those being the
electrode for water-splitting reaction of the present invention. The two electrode
system water-splitting reaction apparatus is preferred in that it is not necessary to
separate hydrogen and oxygen, since hydrogen and oxygen are generated in two
water-splitting tanks partitioned by a diaphragm, respectively.
The production method by the two electrode system water-splitting reaction
apparatus shown in Fig. 1 is described below as an example.
[0111]
Fig. 1 is an example of an apparatus in the case of using the electrode for
water-splitting reaction of the present invention as the anode 61.
In the water-splitting reaction apparatus, when providing the explanation in the
case of using an acidic aqueous solution as the water or the electrolyte aqueous solution
65 as a example, by irradiating the anode 61 with light such as sunlight entered through
a transmissive window (not shown) provided on a water-splitting tank having water or
the electrolyte aqueous solution 65 therein, charge separation of photocatalyst occurs,
thereby an electron (e-) and a hole (h+) are formed. The hole generated oxidizes water
28
to form an oxygen molecule and a proton. The oxygen molecule formed is recovered
through an upper gas recovery port by gas-liquid separation. The proton formed is
conveyed to the cathode 63 side through the diaphragm 62.
[0112]
On the other hand, the electron excited in a conduction band migrates into the
cathode 63 by conducting through a conductor 64. The electron and proton migrated
into the cathode 63 are reacted with each other by the cathode 63 having hydrogen
generation catalytic ability, and a hydrogen molecule is formed. The hydrogen
molecule formed is recovered through an upper gas recovery port by gas-liquid
separation.
[0113]
In the water-splitting reaction apparatus, hydrogen can be also produced using
the electrode for water-splitting reaction of the present invention in the cathode 63. In
this case, by irradiating the cathode 63 with light entered through the transmissive
window (not shown) provided at a cathode 63 side, charge separation of the
photocatalyst occurs, and an electron and a hole are formed. The electron excited
reduces a proton in an aqueous solution to form a hydrogen molecule. The hydrogen
molecule is recovered through an upper gas recovery port by gas-liquid separation.
[0114]
On the other hand, to compensate the hole formed in the photocatalyst of the
cathode 63, electron migration occurs from the anode 61 to the cathode 63 through the
conductor 64. As a result, oxidation reaction of water occurs in the anode 61, an
oxygen molecule and a proton are formed, and the proton formed migrates to a cathode
63 side through the diaphragm 62. The oxygen molecule formed is recovered through
an upper gas recover port.
[0115]
The electrode for water-splitting reaction of the present invention may be used
in both the anode 61and the cathode 63. In this case, by irradiating both the anode
61and the cathode 63 with light, the above reaction occurs, and hydrogen can be
produced.
[0116]
29
The anode 61 is an electrode having oxidation function. The electrode in the
case that the electrode for water-splitting reaction of the present invention is not used in
the anode 61 is not particularly limited. A catalyst having ability capable of oxidizing
water or a hydroxide ion into an oxygen molecule is generally used. Specifically, the
examples include IrO2, Pt, Co, Fe, C, Ni, Ag, and spinel compounds containing Ni, Fe,
Co or Mn, such as MnO, MnO2, Mn2O3, Mn3O4, CoO, Co3O4, NiCo2O4, NiFe2O4,
CoFe2O4 and MnCo2O4. Those catalysts are the same as exemplified as the cocatalysts
at an oxygen generation side among the above-mentioned cocatalysts 12.
[0117]
The anode 61 can use a photocatalyst that is not the electrode for
water-splitting reaction of the present invention. In other words, the anode 61 can use
a photocatalyst in which a potential of a hole formed by irradiation with light is more
positive than a potential that can oxidize a hydroxide ion into an oxygen molecule.
[0118]
The cathode 63 is an electrode having reduction function. An electrode in the
case that the electrode for water-splitting reaction of the present invention is not used in
the cathode 63 is not particularly limited. A catalyst having ability capable of reducing
water or a proton into a hydrogen molecule is generally used. Specifically, Pt, Pd, Rh,
Ru, Ni, Au, Fe, NiO, RuO2 and Cr-Rh oxide can be exemplified. Those catalysts are
the same as exemplified as cocatalysts at a hydrogen generation side among the
above-mentioned cocatalysts 12.
[0119]
The cathode 63 can use a photocatalyst that is not the electrode for
water-splitting reaction of the present invention. In other words, the cathode 63 can
use a photocatalyst in which a potential of an electron formed by irradiation with light is
more negative than a potential that can reduce a water molecule or a hydrogen ion into a
hydrogen molecule.
[0120]
The anode 61 and/or the cathode 63 may directly use the above-mentioned
materials. However, the materials can be used by using metals such as Al, Au, Cu, Mo,
Ni, Pd, Pt, Ti and W; alloys such as stainless steel, carbon steel, titanium alloy and
30
aluminum alloy; high conductive carbon materials such as glassy carbon and graphite;
and the like as a support, and fixing the materials to those supports. Above all, from
that it is stable in a wide pH range and shows high electroconductivity, in an electrolyte
aqueous solution, it is preferred to use metallic titanium, titanium alloy, glassy carbon,
graphite and the like as a support.
[0121]
The diaphragm 62 is not particularly limited so long as it can partition an anode
and a cathode and migration of a proton generated at an anode side is possible. For
example, an ion-exchange membrane, silica membrane, a zeolite membrane, a salt
bridge and the like can be used, and an ion exchange membrane and a salt bridge are
generally used.
[0122]
The conductor 64 is not particularly limited so long as it is constituted of a
member capable of conducting an electron. Good conductors such as copper, gold,
and graphite are generally used.
[0123]
Water and the electrolyte aqueous solution are not particularly limited so long
as it can be decomposed by irradiation with light to generate hydrogen and oxygen.
Pure water, and an aqueous solution containing a cation such as alkali metal ion or
alkaline earth metal ion, or an anion such as chloride ion, sulfate ion or nitrate ion are
exemplified. Of those anions and cations, it is preferred to use sodium sulfate Na2SO4
as an electrolyte from the standpoint that the aqueous solution is near neutral and does
not form a by-product such as chlorine gas.
[0124]
The pH of water or the electrolyte aqueous solution 65 is not particularly
limited. However, the pH is preferably a range of from 2 to 14 from, for example, that
the damage to the inside of the water-splitting reaction apparatus, for example, the
water-splitting tank and the electrode for water-splitting reaction, is small while
maintaining stability of the photocatalyst used in the electrode for water-splitting
reaction. The temperature of water and the electrolyte aqueous solution 65 is not
particularly limited. Low temperature is preferred for high reaction efficiency. The
31
temperature is generally from 0 to 100°C.
[0125]
The light that irradiates the electrode for water-splitting reaction in water or the
electrolyte aqueous solution 65 is not particularly limited, and light capable of causing
water-splitting reaction can be used. Specifically, visible light such as sunlight,
ultraviolet light, infrared light and the like can be utilized. Light containing visible
light is preferred from that energy is large, and above all, sunlight that its amount is
inexhaustible is more preferred.
The wavelength of the irradiation light is not particularly limited. The
wavelength is generally from 200 nm to 1,000 nm.
Example
[0126]
The present invention is described more specifically below by reference to
examples, but the invention is not limited by the following examples and various
modifications can be made in a range within the scope thereof.
[0127]
(Method of obtaining the number of stack of photocatalyst particles of photocatalyst
layer)
An electrode to be evaluated is cut in a lamination direction so as to pass
through a central part of the electrode, and a cross-section of the electrode laminated is
observed. It confirms that surface particles are photocatalyst particles by the
measurement with the following SEM-EDX. Three to five regions each having 10 μm
× 10 μm on the cross-section are selected, and observed with the following SEM, and
the number of stack of photocatalyst particles in the photocatalyst layer is visually
measured. An average value of the measurement results is used as the number of
stack.
(SEM)
Apparatus: S-4700, manufactured by Hitachi, Ltd.
Accelerating voltage: 10 kV or 20 kV
[0128]
(SEM-EDX)
32
Apparatus: (SEM) JSM-7600F, manufactured by JASCO Corporation
(EDX) Manufactured by Thermo Fisher Scientific
SEM accelerating voltage: 15 kV
EDX measurement range: 10 mm2
[0129]

(Preparation of LaTiO2N)
LaTiO2N was prepared according to the method described in Journal of Flux
Growth, 2010, 5, 81.
La2O3 and TiO2 (all 99.99%) were mixed in a molar ratio of 1:1, and the
resulting mixture was mixed with NaCl as a flux. The resulting mixture was heated at
1,423K for 5 hours using an alumina crucible, and then cooled to 1,023K in a rate of
1K/min. NaCl was removed from a white bulk obtained, by dissolution with distilled
water, thereby obtaining La2TiO7 becoming a precursor. La2TiO7 obtained was heated
at 1,223K for 3 hours in NH3 stream of 250 mL/min to conduct nitridation treatment,
thereby obtaining LaTiO2N. As a result of confirmation with XRD, formation of
LaTiO2N was confirmed.
[0130]
(Preparation of IrO2/LaTiO2N)
LaTiO2N (0.1 g) obtained above was stirred for 3 hours in a solution containing
IrO2 colloid (5.0 mass% of IrO2 to LaTiO2N, pH was adjusted to 5.0 using NaOH
aqueous solution) to support IrO2 on LaTiO2N.
[0131]
The IrO2 colloid solution was prepared according to the method described in J.
Phys. Chem. Lett., 2011, 2, 402. Specifically, Na2[IrCl6]⋅6H2O (0.056 g) was
dissolved in 50 mL distilled water, and pH was adjusted to 12.0. The resulting solution
was heated in a water bath at 80°C for 20 minutes to obtain a light blue solution. The
solution was iced, and 1 mL of an iced 3M HNO3 aqueous solution was added dropwise
thereto. Stirring was conducted for 80 minutes while icing, and a deep blue IrO2
colloid solution was obtained.
[0132]
33
(Preparation of Ta3N5)
Ta2O5 (99.9%, manufactured by Rare Metallic Co., Ltd.) was heated to 1,123K
in a rate of 10K/min in NH3 stream of 500 mL/min, maintained at the temperature for
15 hours, and then cooled to room temperature, thereby obtaining Ta3N5. As a result
of confirmation with XRD and EDX, the formation of Ta3N5 was confirmed.
[0133]
(Preparation of Ta3N5:Mg, Zr)
Ta2O5, Mg(NO3)2⋅6H2O and Zr(NO3)2⋅2H2O were mixed in a molar ratio of
1:2/9:4/9, a small amount of ethanol was added thereto, and the resulting mixture was
ground and mixed until the mixture dries. The same operation was again conducted to
perform mixing. The mixture was burned at 923K for 1 hour in air atmosphere to
obtain a composite oxide precursor of Ta, Mg and Zr. Na2CO3 was added to the
precursor obtained so as to achieve a molar ratio of (Ta+Mg+Zr):Na=1:1, and the
resulting mixture was ground and mixed in a mortar. The resulting mixture was heated
to 1,173K at a rate of 10K/min in NH3 stream of 200 mL/min, and maintained at the
temperature for 20 hours. The mixture was cooled to room temperature to obtain
Mg/Zr-doped Ta3N5 (Ta3N5:Mg, Zr). As a result of confirmation with XRD and EDX,
formation of Ta3N5 was confirmed.
[0134]

(Example 1)
IrO2/LaTiO2N (30 mg) obtained above was suspended in 1 mL of 2-propanol,
the suspension (20 μL) was added dropwise on a first glass substrate (soda lime glass),
and drying was repeated three times to form 10 mm × 10 mm photocatalyst layer.
Nb becoming a contact layer was laminated by a sputtering method. An
apparatus used was a mini-sputtering equipment (MNS-2000-RF) manufactured by
ULVAC, Inc., treatment time was 5 minutes at 150W, and substrate temperature was
623K. As a result of observation with SEM, the thickness of the contact layer obtained
under the conditions was 120 nm.
Ti becoming a current collecting layer was laminated by a sputtering method.
Treatment time was 3 hours at 150W. An electrode having Ti laminated thereon was
34
taken out, and a second glass substrate (soda lime glass) was adhered to the current
collecting layer using an epoxy resin. Finally, the first glass substrate was peeled, and
ultrasonic cleaning was conducted in pure water for 10 minutes. Thus,
(IrO2/LaTiO2N)/Nb/Ti electrode was obtained.
Using the electrode obtained, photocurrent density was measured according to
“Evaluation of electrode” described hereinafter. The results obtained are shown in
Table 1.
[0135]
(Examples 2 and 3)
(IrO2/LaTiO2N)/Ta/Ti electrode was obtained in the same manner as in
Example 1 except that Ta was laminated in place of Nb becoming a contact layer and Ti
was then laminated. In Example 2, pH of the electrolyte solution at the measurement
of photocurrent density was 5.9, and in Example 3, the pH thereof was 13.5. The
results obtained are shown in Table 1.
[0136]
(Example 4)
(IrO2/LaTiO2N)/Ti/Ti electrode was obtained in the same manner as in
Example 1 except that Ti was laminated in place of Nb becoming a contact layer
(contact layer and current collecting layer were integrated in one boy), and photocurrent
density was measured. The result obtained is shown in Table 1.
[0137]
(Example 5)
(IrO2/LaTiO2N)/Zr/Ti electrode was obtained in the same manner as in
Example 1 except that Zr was laminated in place of Nb becoming a contact layer. The
result obtained is shown in Table 1.
[0138]
(Example 6)
(IrO2/LaTiO2N)/Au/Ti electrode was obtained in the same manner as in
Example 1 except that Au was laminated in place of Nb becoming a contact layer. The
result obtained is shown in Table 1.
[0139]
35
(Example 7)
The Mg/Zr-doped Ta3N5 (50 mg) described above was added to 45 mL of
toluene and 5 mL of 1-dodecanol, and the resulting suspension was refluxed at 120°C in
nitrogen atmosphere to hydrophobize particles. The particles were arranged on a Cu
plate in the form of single particle using Langmuir Blodgett method, and treated for 1
minute in a distance to UV light source of 1 cm using UV ozone cleaning modification
experimental equipment (ASM401N, manufactured by Asumi Giken, Limited). Ti was
deposited by a sputtering method to form a contact layer. Ta3N5 particles were
transferred to a Ti plate having a conductive adhesive (Pyro-Duct 599 (graphite base),
manufactured by AREMUCO; hereinafter referred to as “PD599”) spin-coated thereon
from the Cu plate. Lead line was connected to the Ti plate, and unnecessary portion
was covered with an epoxy resin. Thus, Ta3N5/Ti/PD599-Ti electrode was prepared.
Using the electrode obtained, photocurrent density was measured according to
“Evaluation of electrode” described hereinafter. The result obtained is shown in Table
1.
[0140]
(Example 8)
Ta3N5 (30 mg) obtained above was suspended in 1 mL of 2-propanol, 20 μL of
the resulting suspension was added dropwise to a first glass substrate (soda lime glass),
and drying was repeated three times to form a photocatalyst layer of 10 mm × 10 mm.
Nb constituting a contact layer was laminated by a sputtering method. An
apparatus used was a mini-sputtering equipment (MNS-2000-RF) manufactured by
ULVAC, Inc., treatment time was 5 minutes at 100W, and substrate temperature was
573K.
Ti constituting a current collecting layer was laminated by a sputtering method.
Treatment time was 3 hours at 200W. An electrode having Ti laminated thereon was
taken out, and a second glass substrate (soda lime glass) was adhered to the current
collecting layer using an epoxy resin. Finally, the first substrate was peeled, and
ultrasonic cleaning was conducted in pure water for 10 minutes. Thus, Ta3O5/Nb/Ti
electrode was obtained.
Subsequently, 0.1M ammonia aqueous solution was added dropwise to 0.01M
36
cobalt nitrate aqueous solution until reaching pH=8.5 to prepare a solution having cobalt
ammine complex formed therein. The Ta3O5/Nb/Ti electrode was dipped in the
solution for one hour to support CoOx as a cocatalyst thereon. Thus, an electrode was
obtained.
Using the electrode obtained, photocurrent density was measured according to
described hereinafter. The results are shown in Table 2.
[0141]
(Example 9)
In Example 8, the surface of the Ta3O5/Nb/Ti electrode before supporting CoCx
thereon was modified with TiO2 by an atomic layer decomposition (ALD) method
(apparatus used: SAL100H, manufactured by Kan Manufactory Co., Ltd.). Titanium
isopropoxide (Japan Advanced Chemicals) and water were used as precursors, and film
formation was performed at a sample temperature of 373K. Thereafter, CoCx as a
cocatalyst was supported in the same manner as in Example 8. Thus, an electrode was
obtained.
Using the electrode obtained, photocurrent density was measured according to
described hereinafter. The results are shown in Table 2.
[0142]
(Comparative Example 1)
A suspension of the above-described LaTiO2N powder, acetone and iodine was
prepared, FTO (fluorine-doped tin oxide) plate was dipped in the suspension, and a
voltage of 100 V was applied for 10 second using the FTO plate as a counter electrode,
thereby preparing LaTiO2N/FTO electrode. 30 μL of 20 mM TiCl4/methanol solution
was added dropwise to the LaTiO2N/FTO electrode obtained, followed by drying at
250°C for 1 minute in air atmosphere. The dipping treatment and drying treatment
were repeated several times to adsorb TiCl4 on the surface of the LaTiO2N/FTO
electrode. The electrode having TiCl4 adsorbed thereon was subjected to nitridation
treatment at 500°C for 1 hour in NH3 stream of 50 mL/min to obtain LaTiO2N/TiN/FTO
electrode having TiN introduced in a photocatalyst layer. The LaTiO2N/TiN/FTO
electrode was dipped in a solution containing the above-described IrO2 colloid for 3
hours while stirring to obtain (IrO2/LaTiO2N)/TiN/FTO electrode supporting 4 wt% of
37
IrO2.
Using the electrode obtained, photocurrent density was measured according to
“Evaluation of electrode” described hereinafter. The result obtained is shown in Table
1.
[0143]
(Comparative Example 2)
Ta3N5:Mg,Zr/PD599-Ti electrode was obtained in the same manner as in
Example 7 except that Ti becoming a contact layer was not laminated, and photocurrent
density was measured. The result obtained is shown in Table 1.
[0144]

(Photocurrent density)
Evaluation of an electrode prepared was performed by current-potential
measurement in three electrode system using a potentiostat. A separable flask having a
planar window was used as an electrochemical cell, Ag/AgCl electrode was used as a
reference electrode, and Pt wire was used as a counter electrode. Na2SO4 (99.0%,
manufactured by Wako Pure Chemical Industries, Ltd.) 0.1M aqueous solution having
pH adjusted to 5.9 to 13.5 by NaOH (special grade, Taisei Kagaku) was used as an
electrolyte solution. Oxygen and carbon dioxide were removed by filling the inside of
the electrochemical cell with argon, and sufficiently conducting bubbling before the
measurement. In the electrochemical measurement, 300W xenon lamp equipped with
a gold mirror and a cutoff filter (L-42, manufactured by Hoya Corporation), or a solar
simulator (AM1.5) was used as a light source, and white light having a wavelength of
420 to 750 nm was emitted from the planar window of the electrochemical cell.
Photocurrent density (μA/cm2) in measurement potential 1V (vs. RHE) was measured in
the respective electrodes. The results obtained are shown in Table 1 below.
38
[0145]
Table 1
Photosemiconductor
material Cocatalsyt Number of stack of
Photocatalyst layer
Contact layer Current
collecting layer pH
Photocurrent
density*
(μA/cm2)
Light source
Material Thickness
(nm) Material
Example 1 LaTiO2N IrO2 1 Nb 120 Ti 5.9 1300 AM1.5
Example 2 LaTiO2N IrO2 1 Ta 190 Ti 5.9 1800 AM1.5
Example 3 LaTiO2N IrO2 1 Ta 190 Ti 13.5 5500 AM1.5
Example 4 LaTiO2N IrO2 1 Ti 160 Ti 5.9 1100 AM1.5
Example 5 LaTiO2N IrO2 1 Zr 260 Ti 5.9 1100 AM1.5
Example 6 LaTiO2N IrO2 1 Au 3000 Au 5.9 2000 AM1.5
Example 7 Ta3N5:Mg, Zr None 1 Ti 2200 PD599-Ti 6 360 300W Xe
(λ≥420 nm)
Comparative Example 1 LaTiO2N IrO2 8 TiN - FTO 5.9 300 AM1.5
Comparative Example 2 Ta3N5:Mg, Zr None 1 None 0 PD599-Ti 6 1.2 300W Xe
(λ≥420 nm)
(*) Photocurrent density is measurement value in measurement potential 1V (vs. RHE)
39
[0146]
As is apparent from the results shown in Table 1, the electrodes obtained in
Examples 1 to 6 had photocurrent density larger than that of the electrode obtained in
Comparative Example 1. In particular, the electrodes obtained in Examples 1 to 6
have the photocurrent density of 1,000 μA/cm2 or more, and the photocurrent density of
3 to 7 times the photocurrent density of the electrode obtained in Comparative Example
1 was obtained in the electrodes obtained in Examples 1 to 6.
[0147]
In other words, it says that the form that a contact layer was provided between
a photocatalyst layer and a current collecting layer by the production method of the
present invention is a form excellent in photoelectric conversion efficiency as compared
with the form that a conductive material is simply introduced in a photocatalyst layer by
the conventional production method.
[0148]
As is apparent from the comparison between Example 7 and Comparative
Example 2, there was the difference of about 300 times in the photocurrent density
between the case of introducing the contact layer by sputtering and the case of directly
adhering a photocatalyst and a current collecting layer with a conductive adhesive. It
was seen from this fact that it is preferred to form a contact layer using an appropriate
conductor by a technique capable of forming close contact, such as a sputtering method
or a vacuum deposition method.
[0149]
(Observation of form of electrode)
Cross-sections of the electrodes obtained in Example 1 and Comparative
Example 1 were observed with SEM. The results obtained are shown in Figs. 9 and 10.
Fig. 9A and Fig. 9B are the cross-section (accelerating voltage: 20 kV) of the electrode
obtained in Example 1, and Fig. 10 is the cross-section (accelerating voltage: 10 kV) of
the electrode obtained in Comparative Example 1.
[0150]
As is apparent from Fig. 9A and Fig. 9B, the electrode obtained in Example 1
is that one or two layers, at a maximum three layers, of LaTiO2N particles were
40
stacked on a light receiving face side of the contact layer, and the average was one layer.
Almost all of the LaTiO2N particles came in contact with the contact layer.
[0151]
On the other hand, as is apparent from Fig. 10, in the electrode obtained in
Comparative Example 1, 5 or more layers of the LaTiO2N particles were stacked on a
light receiving face side of the current collecting layer, and the average stack number is
eight. The photocatalyst at the light receiving face side did not come in contact with
the current collecting layer.
[0152]
Thus, in the electrodes obtained in the Examples, the stack number of the
photocatalyst layer is small (the form that one or two layers, at a maximum three layers,
of photocatalyst particles are stacked, and the average stack number is one), and light
receivable photocatalyst particles come in contact with the contact layer. Therefore, it
is considered that there becomes no influence of resistance between photocatalyst
particles and photoelectric conversion efficiency was improved.
[0153]
The cross-section of the electrode obtained in Example 1 was subjected to
elemental mapping by SEM-EDS. The result obtained is shown in Fig. 11A to Fig.
11D. Fig. 11A is SEM image, Fig. 11B is the result of oxygen mapping corresponding
to the photocatalyst layer, Fig. 11C is the result of Nb mapping corresponding to the
contact layer, and Fig. 11D is the result of Ti mapping corresponding to the current
collecting layer.
[0154]
As is apparent from Fig. 11A to Fig. 11D, the electrode obtained in Example 1
is that the contact layer is formed along a shape at a current collecting layer side of the
photocatalyst layer. By this constitution, the photocatalyst particles in the
photocatalyst layer are directly contacted with the contact layer, and as a result, it is
considered that conductive path between the photocatalyst layer and the current
collecting layer was increased and photoelectric conversion efficiency was improved.
[0155]
As is apparent from Fig. 11C, in the photocatalyst layer, Nb was eccentrically
41
located at a contact layer side than a side (light receiving face side) opposite the contact
layer, of the photocatalyst layer (that is, the amount of Nb present at a light receiving
face side of the photocatalyst layer was relatively smaller than that at a contact layer
side). This can inhibit absorption or reflection of light by Nb, and it is considered that
light absorption by photocatalyst is not inhibited and photoelectric conversion efficiency
was further improved.
42
[0156]
Table 2
Photosemiconductor
material
Cocatalyst Number of
stack of
photocatalyst
layer
Contact layer Current
collecting layer
pH Photocurrent
density*
(μA/cm2)
Light
source
Surface
treatment
Material Thickness
(nm)
Material
Example 8 Ta3N5 CoOx 1 Nb 140 Ti 13 3000 300W Xe
(λ≥420 nm)
None
Example 9 Ta3N5 CoOx 1 Nb 140 Ti 13 5000 300W Xe
(λ≥420 nm)
TiO2
43
[0157]
As is apparent from Table 2, it was seen that by subjecting the surface
treatment with TiO2 to the electrode obtained, photocurrent density is further improved.
[0158]
The present invention has been described above by reference to the
embodiment that seems to be most practical and preferably at this time. However, the
present invention is not limited to the embodiment disclosed in the present specification,
and various modifications or changes can be made without departing the spirit and
scope of the present readable from the claims and the entire description. It should be
understood that the electrode for water-splitting reaction involving the modifications or
changes and the production method of the same are included in the technical scope of
the present invention.
[0159]
While the invention has been described in detail and with reference to
specific embodiments thereof, it will be apparent to one skilled in the art that various
changes and modifications can be made therein without departing from the spirit and
scope thereof. This application is based on Japanese patent application filed on March
8, 2012 (Application No. 2012-052248), the contents thereof being incorporated herein
by reference.
Industrial Applicability
[0160]
The present invention provides new possibility to production technique of an
electrode for water-splitting reaction. Furthermore, by utilizing the electrode of the
present invention, a system that efficiently promotes water splitting by applying
potential to photocatalyst can be constructed. By using the electrode for
water-splitting reaction according to the present invention, hydrogen production
technology by effective water splitting can be provided.
Description of Reference Numerals
[0161]
10 Photocatalyst layer
11 Photocatalyst particles
44
12 Cocatalyst
20 Contact layer
30 Current collecting layer
40 Adhesive layer
50a First Substrate
50b Second Substrate
61 Anode
62 Diaphragm
63 Cathode
64 Conductor
65 Water or electrolyte aqueous solution
100 Electrode for water-splitting reaction
45
Claims
1. An electrode for water-splitting reaction comprising: a photocatalyst
layer; a current collecting layer; and a contact layer that contains semiconductor or good
conductor and is provided between the photocatalyst layer and the current collecting
layer,
wherein the photocatalyst layer consists essentially of photocatalyst particles,
and
the contact layer is provided along the surface shape of the photocatalyst layer
at the current collecting layer side of the photocatalyst layer.
2. The electrode for water-splitting reaction according to claim 1,
wherein in the photocatalyst layer, the number of stack of photocatalyst particles in a
lamination direction of the photocatalyst layer and the contact layer is from 1 to 3.
3. The electrode for water-splitting reaction according to claim 1 or 2,
wherein the photocatalyst layer further comprises semiconductor or good conductor, and
the semiconductor or the good conductor is eccentrically located in the photocatalyst
layer at the contact layer side, than the side opposite the contact layer.
4. The electrode for water-splitting reaction according to any one of
claims 1 to 3, wherein a photocurrent density at measurement potential of 1 V (vs. RHE)
is 350 μA/cm2 or more.
5. A method for producing an electrode for water-splitting reaction,
comprising:
a photocatalyst layer formation step of forming a photocatalyst layer;
a contact layer formation step of covering one face of the photocatalyst layer
with semiconductor or good conductor to form a contact layer; and
a current collecting layer formation step of forming a current collecting layer
on a face of the contact layer at the side opposite the photocatalyst layer of the contact
layer.
6. The method for producing an electrode for water-splitting reaction
according to claim 5, wherein in the contact layer formation step, the contact layer is
formed on one face of the photocatalyst layer by sputtering the semiconductor or the
good conductor.
46
7. The method for producing an electrode for water-splitting reaction
according to claim 5 or 6, wherein in the current collecting layer formation step, the
current collecting layer is formed by sputtering.
8. The method for producing an electrode for water-splitting reaction
according to any one of claims 5 to 7, which further comprises a non-contact
photocatalyst removal step of removing the photocatalyst particles that are not come in
contact with the contact layer.
9. The method for producing an electrode for water-splitting reaction
according to claim 8, wherein in the photocatalyst layer formation step, the
photocatalyst layer is formed by laminating photocatalyst on one face of a first substrate,
and in the non-contact photocatalyst removal step, the first substrate is removed.
10. The method for producing an electrode for water-splitting reaction
according to any one of claims 5 to 9, which further comprises a reinforcing substrate
formation step of forming a second substrate at the side opposite the contact layer of the
current collecting layer after the current collecting layer formation step.
11. A method for producing an electrode for water-splitting reaction,
comprising:
a photocatalyst layer formation step of laminating photocatalyst particles on
one face of a first substrate;
a contact layer formation step of forming a contact layer by covering a face of
the photocatalyst layer at the side opposite the first substrate side of the photocatalyst
layer with semiconductor or good conductor;
a current collecting layer formation step of forming a current collecting layer
on a face of the contact layer at the side opposite the photocatalyst layer of the contact
layer;
a reinforcing substrate formation step of laminating a second substrate on a
face of the current collecting layer at the side opposite the contact layer of the current
collecting layer, and
a substrate peeling step of peeling the first substrate.
12. The method for producing an electrode for water-splitting reaction
according to claim 11, wherein the contact layer is formed by sputtering in the contact
47
layer formation step.
13 A method for producing hydrogen, comprising irradiating an electrode
for water-splitting reaction dipped in water or an electrolyte aqueous solution with light
to conduct a water photolysis, wherein the electrode for water-splitting reaction is the
electrode for water-splitting reaction according to any one of claims 1 to 4.
14. A method for producing hydrogen, comprising irradiating an electrode
for water-splitting reaction dipped in water or an electrolyte aqueous solution, with light
to conduct water photolysis, wherein the electrode for water-splitting reaction is the
electrode for water-splitting reaction obtained by the production method according to
any one of claims 5 to 12.