U.S. patent application number 12/073611 was filed with the patent office on 2008-09-11 for electrode for electrolysis and electrolysis unit.
This patent application is currently assigned to SANYO ELECTRIC CO., LTD.. Invention is credited to Mineo Ikematsu, Kazuhiro Kaneda, Katsura Kawata, Kenta Kitsuka.
Application Number | 20080217168 12/073611 |
Document ID | / |
Family ID | 39494629 |
Filed Date | 2008-09-11 |
United States Patent
Application |
20080217168 |
Kind Code |
A1 |
Kitsuka; Kenta ; et
al. |
September 11, 2008 |
Electrode for electrolysis and electrolysis unit
Abstract
There are disclosed an electrode for electrolysis capable of
efficiently forming ozone by electrolysis of an electrolytic
solution (e.g., water) at ordinary temperature with a low current
density, and an electrolysis unit using the electrode. The
electrode for electrolysis includes a substrate and a surface layer
formed on the surface of the substrate, and the surface layer is
made of an amorphous insulator, for example, a thin film of
amorphous tantalum oxide, amorphous tungsten oxide or amorphous
aluminum oxide.
Inventors: |
Kitsuka; Kenta; (Gunma,
JP) ; Kaneda; Kazuhiro; (Saitama, JP) ;
Ikematsu; Mineo; (Ibaraki, JP) ; Kawata; Katsura;
(Saitama, JP) |
Correspondence
Address: |
MCDERMOTT WILL & EMERY LLP
600 13TH STREET, N.W.
WASHINGTON
DC
20005-3096
US
|
Assignee: |
SANYO ELECTRIC CO., LTD.
|
Family ID: |
39494629 |
Appl. No.: |
12/073611 |
Filed: |
March 7, 2008 |
Current U.S.
Class: |
204/242 ;
204/292 |
Current CPC
Class: |
C25B 1/13 20130101; C25B
11/093 20210101 |
Class at
Publication: |
204/242 ;
204/292 |
International
Class: |
C25B 9/00 20060101
C25B009/00; C25B 11/04 20060101 C25B011/04 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 7, 2007 |
JP |
2007-057294 |
Feb 1, 2008 |
JP |
2008-022524 |
Claims
1. An electrode for electrolysis comprising a substrate and a
surface layer formed on the surface of the substrate, wherein the
surface layer is an amorphous insulator.
2. The electrode for electrolysis according to claim 1, wherein the
insulator is an oxide of a single metal or a composite metal
oxide.
3. The electrode for electrolysis according to claim 1 or 2,
wherein the insulator is tantalum oxide or tungsten oxide.
4. The electrode for electrolysis according to claim 1 or 2,
wherein the insulator is aluminum oxide.
5. The electrode for electrolysis according to claim 1, 2, 3 or 4,
wherein a thickness of the surface layer is in a range of 20 nm or
more to 2000 nm or less.
6. The electrode for electrolysis according to claim 1, 2, 3, 4 or
5, wherein the substrate is provided with an intermediate layer
positioned on an inner side of the surface layer and made of a
metal which is not easily oxidized on the surface of the
substrate.
7. An electrolysis unit in which an anode having water permeability
is constituted of the electrode for electrolysis according to claim
1, 2, 3, 4, 5 or 6 and in which the anode and a cathode having
water permeability are arranged on both surfaces of a cation
exchange film.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to an electrode for
electrolysis for use in an industrial or household electrolysis
process.
[0002] In general, ozone is a substance having a very strong
oxidizing power, and it is expected that water in which ozone is
dissolved, so-called ozone water is applied to a broad range of
cleaning sterilization treatment of water and sewage, food and the
like, and a cleaning treatment of a semiconductor device
manufacturing process. As methods for forming the ozone water,
there are known a method for dissolving, in water, ozone formed by
irradiation with an ultraviolet ray or electric discharge, a method
for forming ozone in water by electrolysis of the water, and the
like.
[0003] In Japanese Patent Application Laid-Open No. 11-77060
(Patent Document 1), an ozone water forming device is disclosed
which includes ozone forming means for forming an ozone gas with an
ultraviolet lamp and a tank to store water, whereby the formed
ozone gas is supplied to the water in the tank to form the ozone
water. Additionally, in Japanese Patent Application Laid-Open No.
11-333475 (Patent Document 2), an ozone water forming device is
disclosed which mixes an ozone gas formed by a discharge type ozone
gas forming device with water at a predetermined ratio by a mixing
pump, in order to efficiently dissolve the ozone gas in the
water.
[0004] However, in the above-mentioned ozone water forming method
for generating the ozone gas by the ultraviolet lamp or the
discharge system described above to dissolve this ozone gas in the
water, the ozone gas forming device, an operation for dissolving
the ozone gas in the water and the like are required, so that the
device is liable to become complicated. The method is a method for
dissolving the formed ozone gas in the water, and hence it has a
problem that it is difficult to efficiently form the ozone water
having a desired concentration.
[0005] In Japanese Patent Application Laid-Open No. 2002-80986
(Patent Document 3), as a method for solving the above-mentioned
problem, a method for forming ozone in water by the electrolysis of
the water is disclosed. In such a method, an electrode for forming
ozone is used which is constituted of an electrode substrate
material formed of a porous body or a mesh-like body, and an
electrode catalyst including an oxide of a platinum group element
or the like.
[0006] Moreover, in Japanese Patent Application Laid-Open No.
2007-016303 (Patent Document 4), it is disclosed that model tap
water as an electrolytic solution is electrolytically treated with
an electrode for electrolysis including a surface layer made of a
dielectric material such as tantalum oxide, to form ozone.
[0007] However, in the method disclosed in Patent Document 3
described above, diamond is used as an electrode substance, and
hence there is a problem that cost of the device itself soars.
[0008] Moreover, in the method for forming the ozone water by the
electrolysis of water as disclosed in Patent Document 3, the
platinum group element is a standard anode material, and has a
characteristic that the element is hardly dissolved in an aqueous
solution which does not include any organic substance. However, the
element as the electrode for forming ozone has a low ozone forming
efficiency, and it is difficult to efficiently form the ozone water
by an electrolysis process. In such ozone water formation by the
electrolysis process using the conventional electrode for forming
ozone, the electrolysis for the ozone formation requires a high
current density of 1 A/cm.sup.2 or more, and an electrolyte needs
to be set to a low temperature. This raises a problem that very
high energy is consumed. Furthermore, platinum is expensive. When
lead dioxide is used instead of platinum, there is a problem of
toxicity.
[0009] Furthermore, even when the electrode for electrolysis
disclosed in Patent Document 4 described above is used, ozone is
formed, but further improvement of an ozone forming current
efficiency has been demanded.
SUMMARY OF THE INVENTION
[0010] The present invention has been developed in order to solve a
conventional technical problem, and an object thereof is to provide
an electrode for electrolysis capable of efficiently forming ozone
by electrolysis of water with a low current density.
[0011] An electrode for electrolysis according to the invention of
a first aspect comprises a substrate and a surface layer formed on
the surface of the substrate, characterized in that the surface
layer is an amorphous insulator.
[0012] The electrode for electrolysis according to the invention of
a second aspect is characterized in that in the above invention,
the insulator is an oxide of a single metal or a composite metal
oxide.
[0013] The electrode for electrolysis according to the invention of
a third aspect is characterized in that in the above inventions,
the insulator is tantalum oxide or tungsten oxide.
[0014] The electrode for electrolysis according to the invention of
a fourth aspect is characterized in that in the inventions of the
first and second aspects, the insulator is aluminum oxide.
[0015] The electrode for electrolysis according to the invention of
a fifth aspect is characterized in that in the above inventions, a
thickness of the surface layer is in a range of 20 nm or more to
2000 nm or less.
[0016] The electrode for electrolysis according to the invention of
a sixth aspect is characterized in that in the above inventions,
the substrate is provided with an intermediate layer positioned on
an inner side of the surface layer and formed of a metal which is
not easily oxidized on the surface of the substrate.
[0017] An electrolysis unit of the invention of a seventh aspect is
characterized in that an anode having water permeability is
constituted of the electrode for electrolysis according to the
above inventions, and the anode and a cathode having water
permeability are arranged on both surfaces of a cation exchange
film.
[0018] According to the invention of the first aspect, in the
electrode for electrolysis including the substrate and the surface
layer formed on the surface of the substrate, the surface layer is
the amorphous insulator, so that ozone can efficiently be formed by
the electrolysis of an electrolytic solution with a low current
density by use of the electrode as the anode.
[0019] In particular, unlike the conventional technology, the
temperature of the electrolytic solution does not have to be
especially set to a low temperature, and the high current density
is not required, so that power consumption required for the ozone
formation can be reduced.
[0020] According to the invention of the second aspect, in the
above invention, the insulator is the oxide of the single metal or
the composite metal oxide. In particular, as in the invention of
the third aspect, the insulator is tantalum oxide or tungsten
oxide. In consequence, an empty level around a bottom of a
conduction band at an energy level higher than Fermi level as much
as about a half of a band gap receives electrons from an
electrolyte, and owing to the electrons, an oxygen forming reaction
is suppressed as compared with a case where the surface layer is
made of a conductor, a crystallized metal oxide or the like.
Instead, an ozone forming reaction is more efficiently caused.
[0021] Therefore, the electrons move with a higher energy level,
whereby an ozone forming efficiency for causing the ozone forming
reaction can be raised.
[0022] According to the invention of the fourth aspect, in the
above inventions, the insulator is aluminum oxide, so that the
electrode for electrolysis according to the above inventions can be
made of a comparatively inexpensive material, and production cost
can be reduced. Moreover, any toxic substance such as lead dioxide
is not used, whereby an environmental load can be reduced.
[0023] According to the invention of the fifth aspect, in the above
inventions, the thickness of the surface layer is in a range of 20
nm or more to 2000 nm or less, so that the surface layer can be
made of a thin film, and the electrons can move in the electrode
via impurities of the surface layer or Fowler-Nordheim tunneling.
Therefore, owing to an electrode reaction in the anode, the empty
level around the bottom of the conduction band at the energy level
higher than Fermi level as much as about the half of the band gap
can receive the electrons from the electrolyte, and the movement of
the electrons is caused with the higher energy level, whereby the
electrolysis can be performed with the low current density, and
ozone can efficiently be formed.
[0024] According to the invention of the sixth aspect, in the above
inventions, the substrate is provided with the intermediate layer
positioned on the inner side of the surface layer and formed of the
metal which is not easily oxidized on the surface of the substrate.
Therefore, the electrode reaction can be caused with the high
energy level in the surface of the surface layer. In consequence,
ozone can efficiently be formed with a lower current density.
[0025] In particularly, according to such inventions, the
intermediate layer is formed of the metal which is not easily
oxidized on the surface of the substrate. Therefore, when the
electrolysis is performed with the electrode, it is possible to
avoid a disadvantage that the substrate surface is oxidized and
non-conducted. In consequence, durability of the electrode can be
improved. As compared with the whole substrate is made of the
material constituting the intermediate layer, the production cost
can be reduced. Even in such a case, ozone can similarly
efficiently be formed.
[0026] In the electrolysis unit according to the invention of the
seventh aspect, the anode having the water permeability is
constituted of the electrode for electrolysis according to the
above inventions, and the anode and the cathode having the water
permeability are arranged on both the surfaces of the cation
exchange film. Therefore, protons move in the cation exchange film,
whereby even when the electrolytic solution is pure water, ozone
can efficiently be formed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 is a cross sectional view of an electrode for
electrolysis according to the present invention (Examples 1,
3);
[0028] FIG. 2 is a flow chart of a manufacturing method of the
electrode for electrolysis according to the present invention
(Examples 1, 3);
[0029] FIG. 3 shows an X-ray diffraction pattern of the electrode
for electrolysis according to the present invention (Example
1);
[0030] FIG. 4 is a schematically explanatory view of an
electrolysis device according to the present invention;
[0031] FIG. 5 is a diagram showing an ozone forming current
efficiency in a case where the electrode for electrolysis prepared
on conditions is used (Example 1);
[0032] FIG. 6 is a flow chart of a manufacturing method of an
electrode for electrolysis according to another example (Example
2);
[0033] FIG. 7 is a cross sectional view of the electrolysis
according to the example (Example 2);
[0034] FIG. 8 is an X-ray diffraction pattern of the electrode for
electrolysis according to the present invention (Example 2);
[0035] FIG. 9 is an X-ray diffraction pattern of the electrode for
electrolysis according to the present invention (Example 2);
[0036] FIG. 10 is a diagram showing an ozone forming current
efficiency in a case where the electrode for electrolysis prepared
on conditions is used (Example 2);
[0037] FIG. 11 is an X-ray diffraction pattern of the electrode for
electrolysis according to the present invention (Example 3);
[0038] FIG. 12 is a diagram showing an ozone forming current
efficiency in a case where the electrode for electrolysis prepared
on conditions is used (Example 3); and
[0039] FIG. 13 is a schematic explanatory view of an electrolysis
unit to which the electrode for electrolysis according to the
present invention is applied.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0040] A preferable embodiment of an electrode for electrolysis
according to the present invention will hereinafter be described
with reference to the drawings. FIG. 1 is a cross sectional view of
an electrode 1 for electrolysis of the present invention. As shown
in FIG. 1, the electrode 1 for electrolysis is constituted of a
substrate 2, a close contact layer 3 formed on the surface of the
substrate 2, an intermediate layer 4 formed on the surface of the
close contact layer 3, and a surface layer 5 formed on the surface
of the intermediate layer 4. In the electrode 1 for electrolysis,
the substrate 2 is provided with a titanium plate 6 as an electric
conductor, and conduction can be realized between the titanium
plate 6 and the intermediate layer 4 via a silver paste 7 as a
conductive material. Furthermore, this silver paste 7 and the
titanium plate 6 are coated with a seal material 8, and this does
not contribute to electrolysis. It is to be noted that a way to
realize the conduction is not limited to this example.
[0041] In the present invention, the substrate 2 is made of a
conductive material of, for example, platinum (Pt), a valve metal
such as titanium (Ti), tantalum (Ta), zirconium (Zr) or niobium
(Nb), an alloy of two or more of these valve metals, silicon (Si)
or the like. In particular, Si having the surface thereof treated
so as to be flat is used in the substrate 2 for use in the present
embodiment.
[0042] The close contact layer 3 is formed on the surface of the
substrate 2 so as to improve a close contact property between the
substrate 2 and the intermediate layer 4 formed of, for example,
platinum on the surface of the close contact layer 3, and the close
contact layer is made of titanium oxide, titanium nitride or the
like. It is to be noted that in the present embodiment, titanium
oxide is used.
[0043] The intermediate layer 4 is made of a metal which is not
easily oxidized, for example, platinum or gold (Au), a conductive
metal oxide such as iridium oxide, palladium oxide or ruthenium
oxide, or an oxide superconductor. Alternatively, the intermediate
layer is made of a metal which is oxidized but has conductivity,
for example, ruthenium (Ru), rhodium (Rh), palladium (Pd), iridium
(Ir) or silver (Ag) included in platinum group elements. It is to
be noted that the metal oxide is not limited to the oxide
beforehand constituting the intermediate layer 4, and may include a
metal oxide obtained by electrolytic oxidization.
[0044] However, when the intermediate layer 4 is made of the metal
oxide having the conductivity, for example, iridium oxide or the
like, the conductor is adversely affected by oxygen atoms
constituting the metal oxide. Therefore, it is preferable that the
intermediate layer 4 is made of the metal which is not easily
oxidized. In the present embodiment, the intermediate layer 4 is
made of platinum.
[0045] It is to be noted that when the substrate 2 is made of
platinum, needless to say, the surface of the substrate 2 is also
made of platinum, so that the intermediate layer 4 does not have to
be especially constituted. However, when the substrate 2 is made of
platinum in this manner, steep rise of cost is incurred. Therefore,
it is industrially preferable that the substrate 2 is made of an
inexpensive material, and the intermediate layer 4 made of a noble
metal or the like is formed on the surface of the substrate 2.
There is not any special restriction on the above constitution, as
long as the substrate 2 is made of a substance which does not have
any conductivity, for example, a glass plate and at least a contact
surface between the substrate 2 and the surface layer 5 described
later is coated with a material having the conductivity. This can
also suppress steep rise of cost required for the material for use
in constituting the substrate 2.
[0046] Moreover, the surface layer 5 is an amorphous (an infinite
form, non-crystalline) insulator provided together with the
intermediate layer 4 so as to coat the intermediate layer 4. In the
present embodiment, the insulator is made of tantalum oxide (TaOx),
tungsten oxide (WOx) or aluminum oxide (AlOx) in the form of a
layer on the surface of the substrate 2. This surface layer 5 is
formed into a thin film having a predetermined thickness above 0 to
1 mm or less, preferably 20 nm or 2000 nm in the present
embodiment.
[0047] It is to be noted that in the present embodiment, examples
of the insulator include amorphous tantalum oxide, tungsten oxide
and aluminum oxide, but the insulator is not limited to these
examples, and an amorphous oxide of a single metal as an insulator
may be used. Specific examples of the oxide include TiOx, NbOx,
HfOx, NaOx, MgOx, KOx, CaOx, ScOx, VOx, CrOx, MnOx, FeOx, CoOx,
NiOx, CuOx, ZnOx, GaOx, RbOx, SrOx, YOx, ZrOx, MoOx, InOx, SnOx,
SbOx, CsOx, BaOx, LaOx, CeOx, PrOx, NdOx, PmOx, SmOx, EuOx, GdOx,
TbOx, DyOx, HoOx, ErOx, TmOx, YbOx, LuOx, PbOx and BiOx.
Alternatively, an amorphous composite metal oxide as an insulator,
SiOx, GeOx or the like may be used.
EXAMPLE 1
[0048] Next, a manufacturing method of an electrode 1 for
electrolysis according to Example 1 of the present invention will
be described with reference to a flow chart of FIG. 2. First,
silicon (Si) constituting a substrate 2 is pretreated in step S1.
Here, it is preferable that phosphorous (P), boron (B) and the like
are introduced as impurities into Si to improve the conductivity.
Si having a very flat surface is used. It is to be noted that in
the present example, Si is used as the substrate 2, but a
conductive material may be used.
[0049] In the pretreatment, the substrate 2 of Si is treated with
5% of hydrofluoric acid to remove a native oxide film formed on the
surface of the substrate 2. In consequence, the surface of the
substrate 2 is further flattened. It is to be noted that the
pretreatment does not have to be performed. Afterward, the surface
of the substrate 2 is rinsed with pure water, and then in step S2,
the substrate is introduced into a chamber of an existing
sputtering system to form a film thereon.
[0050] In the step S2, a close contact layer 3 for improving a
close contact property of an intermediate layer 4 as described
above is formed on the surface of the substrate 2. The close
contact layer 3 is formed on the substrate 2 by a reactive
sputtering process. The close contact layer 3 is made of titanium
oxide, so that the film is formed at room temperature for ten
minutes on conditions that Ti is used as a first target, a supply
power is 6.17 W/cm.sup.2, an oxygen partial pressure is 52%
(Ar:O.sub.2 24:26) and a film forming pressure is 0.6 Pa. In
consequence, the close contact layer 23 of titanium oxide having a
thickness of about 50 nm is formed on the surface of the substrate
2. It is to be noted that in the present example, as a method for
forming a film of the close contact layer 3, the reactive
sputtering process is used, but the present invention is not
limited to this example. For example, a sputtering process, a CVD
process, an ion plating process, a plating process, or a
combination of one of these processes and thermal oxidation may be
used.
[0051] Subsequently, in step S3, the intermediate layer 4 is formed
on the surface of the substrate 2 provided with the close contact
layer 3. The intermediate layer 4 is formed on the surface of the
substrate 2 by a sputtering process. In the present example, the
intermediate layer 4 is made of platinum, so that the film is
formed at room temperature for about one minute and eleven seconds
on conditions that Pt (80 mm.phi.) is used as a first target, a
supply power is 4.63 W/cm.sup.2, and an Ar gas pressure is 0.7 Pa.
In consequence, the intermediate layer 4 having a thickness of
about 200 nm is formed on the surface of the substrate 2 provided
with the close contact layer 3. It is to be noted that in the
present example, as a method for forming a film of the intermediate
layer 4, the sputtering process is used, but the present invention
is not limited to this example. For example, a CVD process, an
evaporation process, an ion plating process, a plating process or
the like may be used.
[0052] Subsequently, a surface layer 5 is formed on the surface of
the substrate 2 provided with the intermediate layer 4. In the
present example, the surface layer 5 is formed using a spin coat
process, so that the surface of the substrate 2 provided with the
intermediate layer 4 is coated with an organic aluminum compound
solution as a surface layer constituting material. In the present
example, the surface layer 5 is made of aluminum oxide, so that an
organic aluminum compound is used in which a functional group such
as a hydroxyl group, an aldehyde group, an alkyl group, a carboxyl
group or an alkoxyl group is coordinated in aluminum having a
coordination number of 3. Moreover, it is preferable that aluminum
in this organic aluminum compound solution is in a range of about
0.4 wt % to 3 wt %. It is to be noted that in the present example,
the organic aluminum compound solution is used as the surface layer
constituting material, but the present invention is not limited to
this example. An aluminum-containing compound from which a
substance other than aluminum can be removed by calcinating, for
example, aluminum chloride, aluminum bromide, aluminum iodide or
the like may be used.
[0053] Then, in step S4, the surface constituting material is
dripped on the surface of the substrate 2 provided with an
intermediate layer 4 to form a thin film by a spin coat process. In
the present example, conditions of the spin coat process are set to
five seconds at 1000 rpm, 15 seconds at 3000 rpm. Afterward, the
surface of the substrate is dried in an environment at room
temperature and then 200.degree. C. for ten minutes (step S5). In
consequence, a surface layer 5 is formed of the surface layer
constituting material including the aluminum compound on the
surface of the intermediate layer 4 of the substrate 2.
[0054] Afterward, in step S6, the substrate 2 provided with the
intermediate layer 4 and the surface layer 5 is calcinated
(annealed) at 400.degree. C. to 900.degree. C. in a muffle furnace,
at 600.degree. C. in atmospheric air for ten minutes to obtain the
electrode 1 for electrolysis. In consequence, the surface layer
constituting material applied to the surface of the intermediate
layer 4 is uniformly applied aluminum oxide. In the present
example, the present film forming operation is performed once,
whereby the calcinated and formed surface layer 5 of aluminum oxide
has a thickness of about 25 nm. It is to be noted that the film
forming operation may be repeated as much as a plurality of times
to set the thickness of the surface layer 5 to about 20 nm to 2000
nm.
[0055] The surface layer 5 of the electrode 1 for electrolysis
obtained as described above is all aluminum oxide. That is, the
surface layer constituting material includes an aluminum-containing
compound, for example, an organic aluminum compound in which a
plurality of functional groups are coordinated in addition to
aluminum. Alternatively, the material includes aluminum chloride,
aluminum bromide, aluminum iodide or the like. The material is
calcinated, whereby substances other than aluminum, that is,
functional groups of organic substances, chloride, bromine and the
like are removed. On the other hand, aluminum reacts with oxygen in
the atmosphere to form aluminum oxide.
[0056] FIG. 3 shows an X-ray diffraction pattern of the surface
layer 5 of the electrode 1 for electrolysis obtained as described
above. It is to be noted that in FIG. 3, the electrode 1 for
electrolysis is constituted on conditions that a calcinating
temperature in constituting the surface layer 5 are 700.degree. C.,
750.degree. C., 800.degree. C., 850.degree. C. and 900.degree. C.
In general, X-ray diffraction (XRD) is used in analysis of a
crystal structure, whereby the crystal structure of aluminum oxide
constituting the surface layer 5 can be analyzed. In the present
example, the structure was observed using an X-ray diffraction
apparatus (D8 Discover manufactured by Bruker AXS Co.).
[0057] According to the observation, diffraction peaks (2.theta.)
shown in an X-ray diffraction pattern of the surface layer 5 of the
electrode 1 for electrolysis obtained in the present example were
about 36.1.degree., about 38.degree. and about 39.6.degree.
regardless of the calcinating temperature at which the electrode 1
was constituted. In general, as the crystal structure of aluminum
oxide, hexagonal system of .alpha.-alumina, .beta.-alumina or the
like is known, but diffraction peaks (2.theta.) of 35.15.degree.,
57.50.degree. and 43.36.degree. inherent in aluminum oxide
(Al.sub.2O.sub.3) were not present in any X-ray diffraction pattern
of the surface layer 5 of the electrode 1 for electrolysis.
Therefore, it is seen that aluminum oxide forming the surface layer
5 of the electrode 1 for electrolysis by the above method does not
have any crystal structure, has an infinite form, and is, so-called
amorphous. It is to be noted that in this case, the diffraction
peak around 36.1.degree. is a peak of titanium oxide (101)
constituting a close contact layer 3, and the diffraction peak
around 39.6.degree. is a peak of platinum (111) constituting the
intermediate layer 4.
[0058] It is to be noted that in the present example, the surface
layer 5 is formed of amorphous aluminum oxide by coating the
surface of the substrate (the surface of the intermediate layer 4
in the present example) with a surface layer constituting material
including an aluminum compound by a spin coat process to calcinate
the material at a predetermined temperature, but the method
constituting the surface layer 5 with amorphous aluminum oxide is
not limited to this example.
[0059] As another method, there is a method for forming the surface
layer 5 by a thermal CVD process. In this thermal CVD process, the
close contact layer 3 and the intermediate layer 4 are successively
formed on the surface of the substrate 2 in the same manner as in
the above example. Afterward, the organic aluminum compound as the
surface layer constituting material is vaporized, and guided to a
reaction tube by use of an appropriate carrier gas to perform a
chemical reaction on the surface of the substrate 2 heated to a
high temperature of, for example, 500.degree. C. to 900.degree. C.,
preferably 600.degree. C. to 800.degree. C.
[0060] In consequence, with regard to the substances excluding
aluminum in the organic aluminum compound as the surface layer
constituting material, for example, an organic substance is removed
from the surface of the substrate 2 heated to the high temperature,
and only aluminum reacts with oxygen in the atmosphere to form
aluminum oxide on the surface of the substrate 2. Aluminum oxide
formed on the surface of the substrate 2 (in actual, the surface of
the intermediate layer 4) constitutes an amorphous thin film (an
aluminum oxide film).
[0061] It is to be noted that in addition to this method, examples
of the method for constituting the surface layer 5 of amorphous
aluminum oxide include a dip process.
[0062] It is to be noted that in the present example, the close
contact layer 3 made of titanium oxide is formed on the surface of
the substrate 2 made of Si. Therefore, platinum constituting the
intermediate layer 4 is directly diffused in the substrate 2 to
form platinum silicide, and it can be prevented that the substrate
surface is oxidized and non-conducted during electrolysis. The
close contact layer 3 of titanium oxide can improve a close contact
property between platinum constituting the intermediate layer 4 and
the substrate 2. In consequence, durability of the electrode 1 can
be improved.
(Electrolysis Method by use of Electrode for Electrolysis and
Evaluation of Electrode)
[0063] Next, formation of ozone by electrolysis using the electrode
1 for electrolysis manufactured as described above will be
described with reference to FIGS. 4 and 5. FIG. 4 is a
schematically explanatory view of an electrolysis device 10 to
which the electrode 1 for electrolysis is applied, and FIG. 5 is a
diagram showing an ozone forming current efficiency in a case where
the electrode for electrolysis prepared on conditions is used.
[0064] The electrolysis device 10 is constituted of a treatment
tank 11, the electrode 1 for electrolysis as an anode, an electrode
12 as a cathode, and a power source 15 which applies a direct
current to the electrodes 1, 12. Then, a cation exchange film (a
diaphragm: Nafion (trade name) manufactured by Dupont) 14 is
provided so as to be positioned between these electrodes 1 and 12,
and divides the inside of the treatment tank 11 into one region
where the electrode 1 is present and the other region where the
electrode 12 is present. Moreover, a stirring device 16 is provided
in a region in which the electrode 1 for electrolysis as the anode
is immersed.
[0065] Furthermore, model tap water 13 as an electrolytic solution
is received in this treatment tank 11. It is to be noted that in an
experiment of the present example, the model tap water is used as
the electrolytic solution, but the cation exchange film is
provided, whereby even in a case where pure water is treated, a
substantially similar effect is obtained. It is to be noted that
the electrolytic solution for use in the experiment is an aqueous
solution model tap water, and a component composition of this model
tap water 13 includes 5.75 ppm of Na.sup.+, 10.02 ppm of Ca.sup.2+,
6.08 ppm of Mg.sup.2+, 0.98 ppm of K.sup.+, 17.75 ppm of Cl.sup.-,
24.5 ppm of SO.sub.4.sup.2- and 16.5 ppm of CO.sub.3.sup.2-.
[0066] The electrode 1 for electrolysis is provided by the
above-mentioned manufacturing method, a thickness of the surface
layer 5 of the electrode 1 for electrolysis is about 25 nm, and a
calcinating temperature in forming the surface layer 5 is
600.degree. C. For comparison, there are used an electrode for
electrolysis (formed of AlOx by sputtering) in which the surface
layer 5 is formed of aluminum oxide by a sputtering process instead
of the spin coat process, an electrode for electrolysis
(spin-coated with TaOx) in which the surface layer 5 is formed of
tantalum oxide (TaOx) by the spin coat process, and an electrode
for electrolysis (spin-coated with TiOx) in which the surface layer
5 is formed of titanium oxide (TiOx) by the spin coat process.
[0067] In the electrode for electrolysis formed of AlOx by the
sputtering, the surface layer 5 is formed on the surface of the
intermediate layer 4 formed in the same manner as in the above
example, so that a target is the surface layer constituting
material of Al, an rf power is set to 100 W, an Ar gas pressure is
set to 0.9 Pa, and a distance between the substrate 2 and the
target is set to 60 mm, to execute film formation at room
temperature. Afterward, the substrate 2 provided with the surface
layer 5 is obtained by executing thermal oxidation at 600.degree.
C. in a muffle furnace in the atmospheric air for 30 minutes.
[0068] In the electrode for electrolysis spin-coated with TaOx, the
surface layer 5 of tantalum oxide is formed on the surface of the
intermediate layer 4 formed by a method similar to the present
example, by the spin coat process on similar conditions. Then, the
electrode is obtained by calcinating the surface layer at a
temperature of 600.degree. C. in the atmospheric air for ten
minutes. It is to be noted that a thickness of the surface layer 5
is about 25 nm.
[0069] In the electrode for electrolysis spin-coated with TiOx, the
surface layer 5 of titanium oxide is similarly formed on the
surface of the intermediate layer 4 by the spin coat process on the
similar conditions. Then, the electrode is obtained by calcinating
the surface layer at a temperature of 600.degree. C. in the
atmospheric air for ten minutes. It is to be noted that a thickness
of the surface layer 5 is about 50 nm. The surface layer 5 formed
on the conditions is made of titanium oxide having an anatase type
crystal structure.
[0070] It is to be noted that the film thicknesses of the surface
layers of the above electrodes for electrolysis are obtained by
conversion substrated on carried amounts of Al, Ta and Ti acquired
by evaluation with a X-ray fluorescence analysis device (JSX-3220ZS
Element Analyzer manufactured by JEOL Ltd.).
[0071] On the other hand, platinum is used in the electrode 12 as
the cathode. Alternatively, the electrode may be constituted of an
insoluble electrode in which platinum is calcinated on the surface
of a titanium substrate, a platinum-iridium-substrated electrode
for electrolysis, a carbon electrode or the like.
[0072] According to the above constitution, 150 ml of model tap
water 13 is received in each region of the treatment tank 11, and
the electrode 1 for electrolysis and the electrode 12 are immersed
in the model tap water, respectively. It is to be noted that a
distance between the electrodes is 10 mm. Then, the power source 15
applies a constant current with a current density of about 25
mA/cm.sup.2 to the electrode 1 for electrolysis and the electrode
12. Moreover, a temperature of the model tap water 13 is
+15.degree. C.
[0073] In the present example, to evaluate an amount of ozone to be
formed by each electrode for electrolysis, an amount of ozone
formed in the model tap water 13 after the electrolysis for five
minutes on the above conditions is measured by an indigo process
(DR4000 manufactured by HACH Co.), and a ratio of a charge which
has contributed to the ozone formation with respect to the total
amount of the supplied charge, that is, an ozone forming current
efficiency is calculated.
[0074] As shown in FIG. 5, in the experiment, in a case where the
electrode 1 for electrolysis prepared in the present example (the
surface layer 5 was made of AlOx by the spin coat process) was
used, the ozone forming current efficiency was about 5.64%. On the
other hand, when the surface layer 5 of AlOx was constituted by the
sputtering process, the ozone forming current efficiency was about
4.0%. In consequence, it has been seen that the ozone forming
current efficiency is high in a case where the surface layer 5 is
constituted by the spin coat process as compared with a case where
the surface layer 5 is constituted by the sputtering process.
[0075] Moreover, when the surface layer 5 was made of another
material such as TaOx by the spin coat process (in the experiment,
the surface layer of the electrode for electrolysis was made of
crystallized tantalum oxide), the ozone forming current efficiency
was about 1.5%. When the layer was made of TiOx (in the experiment,
the surface layer of the electrode for electrolysis was made of
titanium oxide having the anatase type crystal structure), the
ozone forming current efficiency was about 0.3%. It has been seen
that even in a case where the surface layer 5 is formed into a
substantially equal film thickness by a similar method, when the
surface layer 5 is made of AlOx, the ozone forming current
efficiency is remarkably high as compared with a case where the
surface layer is made of TaOx (crystallized tantalum oxide in the
experiment) or TiOx (crystallized titanium oxide in the
experiment).
[0076] It is seen from the above experiment result that ozone can
be formed in the electrolytic solution even by the electrolysis of
the electrolytic solution by use of each electrode for electrolysis
as the anode. However, in a case where the electrode 1 for
electrolysis having the surface layer 5 of AlOx formed by the spin
coat process of the present example is used, the ozone forming
current efficiency is remarkably high as compared with a case where
the surface layer 5 is formed of another material by another
process. This is supposedly because the thin-film surface layer 5
of amorphous aluminum oxide is formed on the surface of the
substrate 2 (actually the intermediate layer 4) by the spin coat
process on the conditions.
[0077] In particular, a thin film of amorphous aluminum oxide is
formed into a thickness of 20 nm to 2000 um, so that electrons move
to the intermediate layer 4 made of a conductive material via an
impurity level in the surface layer 5 or Fowler-Nordheim
tunneling.
[0078] Usually, when a metal electrode is used as the electrode for
electrolysis, an empty level right above Fermi level receives the
electrons from an electrolyte, whereby an electrode reaction in the
anode preferentially causes an oxygen forming reaction. When the
surface layer 5 is made of the crystallized metal oxide, a metal
segregates in a grain boundary between crystals, and a current
flows. Even in this case, the empty level right above the Fermi
level receives the electrons from the electrolyte, whereby the
oxygen forming reaction is preferentially caused by the electrode
reaction in the anode.
[0079] On the other hand, in a case where the electrode 1 for
electrolysis provided with the surface layer 5 is used as in the
example, the surface layer 5 is made of amorphous aluminum oxide,
so that an empty level around a bottom of a conduction band having
an energy level higher as much as an about half of a band gap than
the Fermi level receives the electrons from the electrolyte. Owing
to the electrons, the oxygen forming reaction is suppressed unlike
the above case, and instead an ozone forming reaction is more
efficiently caused.
[0080] Therefore, in a case where the electrode 1 for electrolysis
according to the present invention is used, it is supposed that the
electrons move at a higher energy level to cause the ozone forming
reaction, and an ozone forming efficiency rises as compared with a
case where the electrode for electrolysis of platinum or the like,
or the electrode for electrolysis provided with the surface layer
of crystallized tantalum oxide or titanium oxide is used.
[0081] In consequence, a current having a predetermined low current
density of 0.1 mA/cm.sup.2 to 2000 mA/cm.sup.2, preferably 1
mA/cm.sup.2 to 1000 mA/cm.sup.2 is applied to the electrode 1 for
electrolysis, whereby ozone can efficiently be formed. Even when
the temperature of the electrolytic solution is not especially set
to a low temperature and is set to ordinary temperature of
+15.degree. C. as in the present example, ozone can efficiently be
formed. Therefore, power consumption required for the ozone
formation can be reduced.
[0082] Moreover, the surface layer 5 of the electrode 1 capable of
realizing the efficient ozone formation can be formed by the spin
coat process as described above, so that productivity can be
improved as compared with a case where the layer is formed by a
conventional sputtering process. Moreover, the electrode for
electrolysis can be manufactured with low manufacturing cost, and
an inexpensive equipment can be realized. The surface layer 5 is
formed by the thermal CVD process as described above, whereby
satisfactory stability and high production efficiency can be
realized. Furthermore, any toxic substance such as lead dioxide is
not used, whereby an environmental load can be reduced.
EXAMPLE 2
[0083] Next, a manufacturing method of an electrode 21 for
electrolysis according to Example 2 of the present invention will
be described with reference to a flow chart of FIG. 6. It is to be
noted that FIG. 7 is a schematic constitution diagram of the
electrode 21 for electrolysis obtained by the example. First, in
step S11, Si constituting a substrate 22 is pretreated in the same
manner as in the above example. A material of the substrate 22 is
similar to that of the above example, so that description thereof
is omitted. Subsequently, in step S12, the substrate is introduced
into a chamber of an existing sputtering device to form a film.
[0084] In the step S12, a close contact layer 23 for improving a
close contact property of an intermediate layer 24 is formed on the
surface of the substrate 22 as described above. The close contact
layer 23 is formed on the substrate 22 by a reactive sputtering
process in the same manner as in the above example. The close
contact layer 23 is made of titanium oxide, so that the film is
formed at room temperature for ten minutes on conditions that Ti is
used as a first target, a supply power is 6.17 W/cm.sup.2, an
oxygen partial pressure is 52% (Ar:O.sub.2 24:26) and a film
forming pressure is 0.6 Pa. In consequence, the close contact layer
23 made of titanium oxide having a thickness of about 50 nm is
formed on the surface of the substrate 22.
[0085] Subsequently, in step S13, the intermediate layer 24 is
formed on the surface of the substrate 22 provided with the close
contact layer 23 in the same manner as in the above example. The
intermediate layer 24 is formed on the substrate 22 by a sputtering
process. In the present example, the intermediate layer 24 is made
of platinum, so that the film is formed at room temperature for
about one minute and eleven seconds on conditions that Pt (80
mm.phi.) is used as a first target, a supply power is 4.63
W/cm.sup.2, and an Ar gas pressure is 0.7 Pa. In consequence, the
intermediate layer 24 having a thickness of about 200 nm is formed
on the surface of the substrate 22 provided with the close contact
layer 23.
[0086] Subsequently, a surface layer 25 is formed on the surface of
the substrate 22 provided with the intermediate layer 24. In the
present example, the surface layer 25 is formed by a sputtering
process. In a case where the surface layer is made of tantalum
oxide, the film is formed at room temperature for five to 180
minutes on conditions that the target is changed to Ta as a surface
layer constituting material, an rf power is 100 W, an Ar gas
pressure is 0.9 Pa and a distance between the substrate 22 and the
target is 60 mm (step S14). In consequence, the surface layer 25
having a thickness of about 7 nm to 1000 nm is formed on the
surface of the intermediate layer 24 of the substrate 22. It is to
be noted that the film thicknesses of the intermediate layer 24 and
the surface layer 25 are obtained by conversion substrated on
carried amounts of Pt and Ta acquired by evaluation with a X-ray
fluorescence.
[0087] Afterward, in step S15, the substrate 22 provided with the
surface layer 25 is thermally oxidized at temperatures of
300.degree. C., 400.degree. C., 500.degree. C. and 600.degree. C.
in a muffle furnace in the atmospheric air for 30 minutes, to
obtain the electrode 21 for electrolysis. In consequence, a
tantalum metal constituting the surface layer 25 formed on the
surface of the intermediate layer 24 is uniformly oxidized. It is
to be noted that the tantalum metal is thermally oxidized to
constitute tantalum oxide, so that a thickness of the surface layer
25 is about 14 nm to 2000 nm.
[0088] It is to be noted that here, Ta is an example of the
material constituting the surface layer 25. However, this material
may be changed to W, whereby a tungsten metal constituting the
surface layer 25 is thermally oxidized to constitute tungsten
oxide.
[0089] FIG. 8 shows an X-ray diffraction pattern of the electrode
21 for electrolysis (the surface layer 25 is made of tantalum
oxide) obtained as described above, and FIG. 9 shows an X-ray
diffraction pattern of the electrode 21 for electrolysis (the
surface layer 25 is made of tungsten oxide) obtained as described
above. X-ray diffraction is used in the same manner as in the above
example, whereby a crystal structure of tantalum oxide (tungsten
oxide) constituting the surface layer 25 can be analyzed. Even in
such an example, the structure was observed using an X-ray
diffraction apparatus (D8 Discover manufactured by Bruker AXS
Co.).
[0090] FIG. 8 shows the X-ray diffraction patterns of the electrode
21 oxidized at 600.degree. C., 500.degree. C., 400.degree. C. and
300.degree. C. in order from the upside. It is to be noted that for
comparison, an X-ray diffraction pattern of an electrode (having
the surface only of Pt) which is not provided with the surface
layer 25 is shown in the bottom. In consequence, in the electrode
21 oxidized at a temperature of 600.degree. C., a diffraction peak
(a peak shown by a solid circle in FIG. 8) inherent in tantalum
oxide (Ta.sub.2O.sub.5) and a diffraction peak (a peak shown by *
in FIG. 8) inherent in platinum constituting the intermediate layer
24 are recognized. Therefore, it is seen that the surface layer 25
of crystalline tantalum oxide (Ta.sub.2O.sub.5) is formed on the
conditions.
[0091] On the other hand, in the electrode 21 oxidized at a
temperature of 400.degree. C., a diffraction peak (a peak shown by
a open circle in FIG. 8) inherent in tantalum oxide (TaO) and a
diffraction peak inherent in platinum are recognized. Therefore, it
is seen that the surface layer 25 of crystalline tantalum oxide
(TaO) is formed on the conditions.
[0092] Moreover, in the electrode 21 oxidized at a temperature of
300.degree. C., a diffraction peak (a peak shown by a black
triangle in FIG. 8) inherent in tantalum (Ta) and a diffraction
peak inherent in platinum are recognized. Therefore, it is seen
that a part of the surface layer 25 remains as tantalum on the
conditions.
[0093] On the other hand, in the electrode 21 oxidized at a
temperature of 500.degree. C., any diffraction peak inherent in
tantalum oxide or tantalum described above is not recognized, and
the diffraction peak inherent in platinum and a halo indicating an
amorphous state (a non-crystalline state) are recognized.
Therefore, it is seen that the surface layer 25 of amorphous
tantalum oxide is formed on the conditions. It is to be noted that
even in comparison with the X-ray diffraction pattern of the
platinum electrode shown for comparison, it is easily seen that the
amorphous state is present in the electrode on the conditions.
[0094] FIG. 9 shows the X-ray diffraction patterns of the electrode
21 oxidized at 600.degree. C., 500.degree. C., 400.degree. C. and
300.degree. C. in order from the upside. It is to be noted that for
comparison, an X-ray diffraction pattern of an electrode (having
the surface only of Pt) which is not provided with the surface
layer 25 is shown in the bottom in the same manner as described
above. According to the patterns, in the electrode 21 oxidized at a
temperature of 600.degree. C., 500.degree. C. or 400.degree. C., a
diffraction peak (a peak shown by a open circle in FIG. 9) inherent
in tungsten oxide (WO.sub.3) and a diffraction peak (a peak shown
by * in FIG. 9) inherent in platinum constituting the intermediate
layer 24 are recognized. Therefore, it is seen that the surface
layer 25 of crystalline tungsten oxide (WO.sub.3) is formed on the
conditions.
[0095] On the other hand, in the electrode 21 oxidized at a
temperature of 300.degree. C., the above-mentioned diffraction peak
inherent in tungsten oxide (WO.sub.3) is not recognized, and the
diffraction peak inherent in platinum only is recognized.
Therefore, it is seen that the surface layer 25 of amorphous
tungsten oxide is formed on the conditions.
(Electrolysis Method by use of Electrode for Electrolysis and
Evaluation of Electrode)
[0096] Next, formation of ozone by electrolysis using the electrode
21 for electrolysis manufactured as described above will be
described with reference to FIG. 10. FIG. 10 is a diagram showing
an ozone forming current efficiency in a case where the electrode
for electrolysis prepared on the conditions is used. In the
drawing, a solid circle shows an ozone forming current efficiency
in a case where the surface layer 25 is made of tantalum oxide, and
a open circle shows an ozone forming current efficiency in a case
where the surface layer 25 is made of tungsten oxide. It is to be
noted that experiment results are obtained using an electrolysis
device 10 of the above example, and a constitution of the device
and experiment conditions are similar to those described above, so
that description thereof is omitted.
[0097] According to this experiment, in a case where the surface
layer 25 of tantalum oxide was constituted, when the oxidizing
temperature was 300.degree. C., the ozone forming current
efficiency was about 3.6%. The ozone forming current efficiencies
were about 6.6% at an oxidizing temperature of 400.degree. C.,
about 7.2% at an oxidizing temperature of 500.degree. C., and about
2.4% at an oxidizing temperature of 600.degree. C. Here, at the
oxidizing temperature of 300.degree. C., 400.degree. C. or
600.degree. C., the surface layer has a crystal structure of
tantalum oxide or tantalum. On the other hand, at the oxidizing
temperature of 500.degree. C., amorphous tantalum oxide which does
not have any crystal structure is formed as the surface layer
25.
[0098] According to such a result, it is seen that in a case where
the surface layer 25 of tantalum oxide is formed and the surface
layer of amorphous tantalum oxide which does not have any crystal
structure is formed, the ozone forming current efficiency is
highest.
[0099] Moreover, in a case where the surface layer 25 of tungsten
oxide was constituted, when the oxidizing temperature was
300.degree. C., the ozone forming current efficiency was about
6.1%. The ozone forming current efficiencies were about 2.4% at an
oxidizing temperature of 400.degree. C., about 3.6% at an oxidizing
temperature of 500.degree. C., and about 4.2% at an oxidizing
temperature of 600.degree. C. Here, at the oxidizing temperature of
400.degree. C., 500.degree. C. or 600.degree. C., the surface layer
has a crystal structure of tungsten oxide. On the other hand, at
the oxidizing temperature of 300.degree. C., amorphous tungsten
oxide which does not have any crystal structure is formed as the
surface layer 25.
[0100] According to such a result, it is seen that in a case where
the surface layer 25 of tungsten oxide is formed and the surface
layer of amorphous tungsten oxide which does not have any crystal
structure is formed, the ozone forming current efficiency is
highest.
EXAMPLE 3
[0101] Next, an electrode for electrolysis according to Example 3
of the present invention will be described. It is to be noted that
a manufacturing method of an electrode 31 for electrolysis obtained
according to such an example is similar to that shown in the flow
chart of FIG. 2 in Example 1, and a schematic constitution diagram
is substantially similar to FIG. 1, so that detailed description of
the manufacturing method is omitted.
[0102] That is, in the electrode for electrolysis according to the
example, a close contact layer 3 of titanium oxide is formed on the
surface of Si constituting a substrate by a sputtering process as
described above, and an intermediate layer 4 of platinum is formed
on the surface of the close contact layer 3 by the sputtering
process.
[0103] Subsequently, a surface layer 5 is formed on the surface of
a substrate 2 provided with the intermediate layer 4. In such an
example, the surface layer 5 is formed by a spin coat process, so
that the surface of the substrate 2 provided with the intermediate
layer 4 is coated with an organic tantalum compound solution as a
surface layer constituting material. In the present embodiment, the
surface layer 5 of tantalum oxide is formed using a Ta(OEt).sub.5
solution in the present example. It is to be noted that in the
present example, ethyl acetate is used as a solvent of the
Ta(OEt).sub.5 solution. It is to be noted that in the present
example, the Ta(OEt).sub.5 solution is used as the surface layer
constituting material, but the present invention is not limited to
this example. There is not any special restriction on the material,
as long as the material is a tantalum-containing compound which can
be calcinated to remove a substance other than tantalum therefrom,
thereby forming a film of tantalum oxide. In the present example,
ethyl acetate is used as the solvent, but the present invention is
not limited to this example, and another solvent such as an
alcohol-substrated solvent may be used.
[0104] Then, the surface layer constituting material is dripped on
the surface of the substrate 2 provided with the intermediate layer
4 to form a thin film by the spin coat process. Conditions in the
spin coat process according to such an example are five seconds
with 1000 rpm and 15 seconds at 3000 rpm in the same manner as in
Example 1. Afterward, the film is dried in an environment at room
temperature for ten minutes and then at 200.degree. C. for ten
minutes.
[0105] Afterward, the substrate 2 provided with the intermediate
layer 4 and the surface layer 5 is calcinated (annealed) at
400.degree. C. to 700.degree. C. in a muffle furnace in the
atmospheric air for ten minutes, to obtain the electrode for
electrolysis. In consequence, the surface of the intermediate layer
4 is uniformly coated with tantalum oxide as the surface layer
constituting material.
[0106] The surface layer 5 of the electrode 1 for electrolysis
obtained as described above is all tantalum oxide. That is, the
surface layer constituting material is a tantalum-containing
compound which is calcinated as described above to remove therefrom
substances other than tantalum, that is, functional groups of
organic substances and the like. On the other hand, tantalum reacts
with oxygen in the atmosphere to constitute tantalum oxide.
[0107] FIG. 11 shows an X-ray diffraction pattern of the electrode
1 for electrolysis (the surface layer 5 is made of tantalum oxide)
obtained as described above. X-ray diffraction is used in the same
manner as in the above examples, whereby a crystal structure of
tantalum oxide constituting the surface layer 5 can be analyzed.
Even in such an example, the structure was observed using an X-ray
diffraction apparatus (D8 Discover manufactured by Bruker AXS
Co.).
[0108] FIG. 11 shows the X-ray diffraction patterns of the
electrode 1 calcinated at 700.degree. C., 600.degree. C.,
500.degree. C. and 400.degree. C. in order from the upside.
According to the patterns, in the electrode 1 calcinated at a
temperature of 700.degree. C. or 600.degree. C., a diffraction peak
(a peak shown by a solid circle in FIG. 11) inherent in tantalum
oxide (Ta.sub.2O.sub.5) is recognized. Therefore, it is seen that
the surface layer 5 of crystalline tantalum oxide (Ta.sub.2O.sub.5)
is formed on the conditions.
[0109] On the other hand, in the electrode 1 calcinated at a
temperature of 500.degree. C. or 400.degree. C., a diffraction peak
inherent in tantalum oxide (Ta.sub.2O.sub.5) described above is not
recognized, and a halo indicating an amorphous state (a
noncrystalline state) is recognized. Therefore, it is seen that the
surface layer 5 of amorphous tantalum oxide is formed on the
conditions.
(Electrolysis Method by use of Electrode for Electrolysis and
Evaluation of Electrode)
[0110] Next, formation of ozone by electrolysis using the electrode
1 for electrolysis manufactured as described above will be
described with reference to FIG. 12. FIG. 12 is a diagram showing
an ozone forming current efficiency in a case where the electrode
for electrolysis prepared on the conditions is used. It is to be
noted that experiment results are obtained using an electrolysis
device 10 of the above example, and a constitution of the device
and experiment conditions are similar to those described above, so
that description thereof is omitted.
[0111] According to this experiment, when the calcinating
temperature was 400.degree. C., the ozone forming current
efficiency was about 7.0%. The ozone forming current efficiencies
were about 12.0% at a calcinating temperature of 500.degree. C.,
about 6.1% at a calcinating temperature of 600.degree. C., and
about 4.6% at a calcinating temperature of 700.degree. C. Here, at
the calcinating temperature of 600.degree. C. or 700.degree. C.,
the surface layer has a crystal structure of tantalum oxide. On the
other hand, at the calcinating temperature of 500.degree. C. or
400.degree. C., amorphous tantalum oxide which does not have any
crystal structure is formed as the surface layer 5.
[0112] According to such a result, it is seen that in a case where
the surface layer of tantalum oxide is formed and the surface layer
of amorphous tantalum oxide which does not have any crystal
structure is formed, the ozone forming current efficiency is high
as compared with a case where the surface layer 5 of tantalum oxide
having a crystal structure is formed.
[0113] It is seen from the experiment results of Examples 2 and 3
that an electrolytic solution may be electrolyzed using either
electrode for electrolysis as an anode to form ozone in the
electrolytic solution. However, in a case where the surface layer 5
(25) of amorphous tantalum oxide or amorphous tungsten oxide is
formed, an ozone forming efficiency is high as compared with a case
where the surface layer of crystalline tantalum oxide or tungsten
oxide is formed.
[0114] This is supposedly because a thin film of amorphous tantalum
oxide or tungsten oxide is formed, so that electrons move to an
intermediate layer made of a conductive material via impurities in
the surface layer or Fowler-Nordheim tunneling.
[0115] Moreover, usually, when a metal electrode is used as the
electrode for electrolysis, an empty level right above Fermi level
receives the electrons from an electrolyte, whereby an electrode
reaction in the anode preferentially causes an oxygen forming
reaction. When the surface layer is made of the crystallized metal
oxide, a metal segregates in a grain boundary between crystals, and
a current flows. Even in this case, the empty level right above the
Fermi level receives the electrons from the electrolyte, whereby
the oxygen forming reaction is preferentially caused by the
electrode reaction in the anode.
[0116] On the other hand, in a case where the electrode for
electrolysis provided with the surface layer as in the above
examples is used, the surface layer is made of an amorphous metal
oxide such as amorphous tantalum oxide or tungsten oxide, so that
an empty level around a bottom of a conduction band having an
energy level higher as much as an about half of a band gap than the
Fermi level receives the electrons from the electrolyte. Owing to
the electrons, the oxygen forming reaction is suppressed unlike the
above case, and instead an ozone forming reaction is more
efficiently caused.
[0117] Therefore, in a case where the electrode for electrolysis
according to the above examples is used as the anode, it is
supposed that the electrons move at a higher energy level to cause
the ozone forming reaction, and an ozone forming efficiency rises
as compared with a case where the electrode for electrolysis of
platinum or the like, or the electrode for electrolysis provided
with the surface layer of crystallized tantalum oxide (the
crystallized metal oxide) is used.
[0118] In consequence, a current having a predetermined low current
density of 0.1 mA/cm.sup.2 to 2000 mA/cm.sup.2, preferably 1
mA/cm.sup.2 to 1000 mA/cm.sup.2 is applied to the electrode 1 for
electrolysis, whereby ozone can efficiently be formed. Even when
the temperature of the electrolytic solution is not especially set
to a low temperature and is set to ordinary temperature of
+15.degree. C. as in the present example, ozone can efficiently be
formed. Therefore, power consumption required for the ozone
formation can be reduced.
[0119] Moreover, the surface layer 5 of the electrode 1 capable of
realizing the efficient ozone formation can be formed by not only
the sputtering process but also the spin coat process as described
above, so that productivity can be improved. Moreover, the
electrode for electrolysis can be manufactured with low
manufacturing cost, and an inexpensive equipment can be
realized.
[0120] Furthermore, as in the above examples, the substrate 2 of Si
is provided with the intermediate layer 4 including at least a
metal which is not easily oxidized, a metal oxide having
conductivity or a metal having conductivity even when oxidized, and
the surface layer 5 is further formed on the surface of the
intermediate layer 4 as described above, so that the electrons can
effectively move in the surface layer 5. Therefore, the electrode
reaction can be caused with a high energy level in the surface of
the surface layer 5, and ozone can efficiently be formed with a
lower current density.
[0121] It is to be noted that in a case where the substrate 2 is
made of a material similar to that of the intermediate layer 4,
that is, a material including at least a metal which is not easily
oxidized, a metal oxide having conductivity or a metal having
conductivity even when oxidized, it is possible to constitute an
electrode capable of similarly efficiently forming ozone without
being especially provided with the intermediate layer 4. However,
the substrate 2 is coated with the intermediate layer 4 made of the
above material as in the present invention, whereby it is possible
to realize with low production cost the electrode 1 capable of
similarly efficiently forming ozone.
[0122] Moreover, the electrode for electrolysis according to the
examples of the present invention is not limited to the electrode
shown in the electrolysis device 10, and may be used as, for
example, an anode for an electrolysis unit 26 shown in FIG. 13.
[0123] That is, the electrolysis unit 26 shown in FIG. 13 is
constituted of the electrode 1 or 21 for electrolysis constituting
the anode according to the above examples, an electrode 28
constituting the cathode, and a cation exchange film 29.
[0124] The electrode 1 (or 21 as the anode) and the electrode 28
(the cathode) are provided with a plurality of water permeable
holes 27A and 28A for securing water permeability, respectively.
Then, the electrodes 1 and 28 are arranged on both surfaces of the
cation exchange film (Nafion (trade name) manufactured by Dupont
Co. was used in the present example) 29, to constitute the
electrolysis unit 26,
[0125] According to such a constitution, the electrolysis unit 26
is immersed in a treatment tank in which an electrolytic solution
is received, and a constant current with a predetermined current
density is applied between both the electrodes 1, 28. In
consequence, an appropriate zero gap is maintained between the
electrode 1 and the cation exchange film 29 and electrode 28, and
protons move in the cation exchange film 29, whereby ozone can
efficiently be formed even when the electrolytic solution is pure
water. The water permeable holes 27A and 28A allow a formed gas to
flow therethrough, whereby stable ozone formation can be
realized.
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