Note: Descriptions are shown in the official language in which they were submitted.
CA 02859939 2014-06-19
WO 2013/099780
PCT/JP2012/083168
DESCRIPTION
Title of Invention
HIGH-LOAD DURABLE ANODE FOR OXYGEN GENERATION AND
MANUFACTURING METHOD FOR THE SAME
Technical Field
The present invention relates to an anode for oxygen generation used for
various industrial electrolyses and a manufacturing method for the same; more
in
detail, it relates to a high-load durable anode for oxygen generation and a
manufacturing method for the same used for industrial electrolyses including
manufacturing of electrolytic metal foils such as electrolytic copper foil,
aluminum
liquid contact, and continuously electrogalvanized steel plate, and metal
extraction,
having superior durability under high-load electrolysis conditions.
Background Art
In industrial electrolyses including manufacturing of electrolytic copper
foil,
aluminum liquid contact, continuously electrogalvanized steel plate and metal
extraction, oxygen generation is involved at the anode. For this reason, the
anode
which is coated chiefly with iridium oxide having durability to oxygen
generation,
as electrode catalyst, on the titanium metal substrate has been widely
applied.
Generally speaking, in this type of industrial electrolysis involving oxygen
generation at the anode, electrolysis is usually performed at a constant
electric
current in view of production efficiency, energy saving, etc. Current density
has
been in a range from several A/dm2 mainly applied in the industrial fields
including
metal extraction to 100A/dm2 at maximum for manufacturing electrolytic copper
foil.
However, nowadays, it is often seen that electrolysis is performed at a
current
1
CA 02859939 2014-06-19
WO 2013/099780
PCT/JP2012/083168
density of 300A/dm2 - 700A/dm2 or more for higher product quality or for
providing
special performance characteristics. Such high electric current is not
supplied to
all the anodes installed to the industrial electrolysis system, but rather, it
is
considered that an anode is installed as an auxiliary one at a specific point
where
high-load electrolysis condition is applied to provide special performance
characteristics to the product obtained from the electrolysis.
Under the electrolysis at such a high current density, the electrode catalyst
layer is highly loaded and electric current tends to be concentrated there,
causing
rapid consumption of the electrode catalyst layer. Moreover, organic
substances
or impurity elements added for stabilizing products cause various
electrochemical
and chemical reactions, the concentration of hydrogen ion increases resulting
from the oxygen generation reaction, lowering the pH value, and consumption of
electrode catalyst is expedited.
One solution to solve these problems may be to increase the surface area of
the electrode catalyst layer so as to decrease the actual electric current
load. For
instance, one solution is to apply a substrate of mesh or punched metal,
instead
of conventional plate substrates, to increase the surface area physically. Use
of
these substrates, however, involves undesirable extra processing costs.
Furthermore, actual current density decreased by physically increased surface
area of the substrate does not improve the electric current concentration at
the
electrode catalyst layer, resulting in little suppression effect on catalyst
consumption.
In the thermolysis formation method of the electrode catalyst layer by
repeating
coating and baking, if the amount of coating iridium per time is increased, it
is
simply considered that the formed catalyst layer is soft and fluffy; but by
this
method only, increase in the effective surface area of the catalyst layer of
the
electrode is limited and improvements in consumption of the catalyst layer
under
high-load conditions and in durability could not be observed clearly.
2
CA 02859939 2014-06-19
WO 2013/099780
PCT/JP2012/083168
As an electrode for this kind of electrolysis, electrode with a low oxygen
generation potential and a long service life is required. Conventionally, as
electrode of this type, an insoluble electrode comprising a conductive metal
substrate, such as titanium, covered with a catalyst layer containing precious
metal or precious metal oxide has been applied. For example, PTL 1 discloses
an
insoluble electrode prepared in such a manner that a catalyst layer containing
iridium oxide and valve metal oxide is coated on a substrate of conductive
metals,
such as titanium, heated in oxidizing atmosphere and baked at a temperature of
650 degrees Celsius - 850 degrees Celsius, to crystallize valve metal oxide
partially. This electrode, however, has the following drawbacks. Since the
electrode is baked at a temperature of 650 degrees Celsius or more, the metal
substrate, such as of titanium causes interfacial corrosion, and becomes poor
conductor, causing oxygen overvoltage to increase to an unserviceable degree
as
electrode. Moreover, the crystallite diameter of iridium oxide in the catalyst
layer
enlarges, resulting in decreased effective surface area of the catalyst layer,
leading to a poor catalytic activity.
PTL 2 discloses use of an anode for copper plating and copper foil
manufacturing prepared in such a manner that a catalyst layer comprising
amorphous iridium oxide and amorphous tantalum oxide in a mixed state is
provided on a substrate of conductive metal, such as titanium. This electrode,
however, features amorphous iridium oxide, and is insufficient in electrode
durability. The reason why durability decreases when amorphous iridium oxide
is
applied is that amorphous iridium oxide shows unstable bonding between iridium
and oxygen, compared with crystalline iridium oxide.
PTL 3 discloses an electrode coated with a catalyst layer comprising a double
layer structure by a lower layer of crystalline iridium oxide and an upper
layer of
3
CA 02859939 2014-06-19
WO 2013/099780
PCT/JP2012/083168
amorphous iridium oxide, in order to suppress consumption of the catalyst
layer
and to enhance durability of the electrode. The electrode disclosed by PTL 3
is
insufficient in electrode durability because the upper layer of the catalyst
layer is
amorphous iridium oxide. Moreover, crystalline iridium oxide exists only in
the
lower layer, not uniformly distributed over the entire catalyst layer,
resulting in
insufficient electrode durability.
PTL 4 discloses an anode for zinc electrowinning in which a catalyst layer
containing amorphous iridium oxide as a prerequisite and crystalline iridium
oxide,
as a mixed state is provided on a substrate of conductive metal like titanium.
PTL
5 discloses an anode for cobalt electrowinning in which a catalyst layer
containing
amorphous iridium oxide as a prerequisite and crystalline iridium oxide, as a
mixed state is provided on a substrate of conductive metal like titanium.
However,
it is thought that electrode durability of these two electrodes is not enough
because they contain a large amount of amorphous iridium oxide, as
prerequisite.
To solve these problems, the inventors of the present invention have
developed,
aiming chiefly at decreasing oxygen generation overvoltage for the case that
the
amount of coating of iridium per time is 2g/m2 or less, (1) the baking method
to
form a catalyst layer in which crystalline iridium oxide and amorphous iridium
oxide coexist by low temperature baking (370 degrees Celsius - 400 degrees
Celsius) plus high temperature post-bake (520 degrees Celsius - 600 degrees
Celsius); and (2) the baking method to form a catalyst layer in which almost
complete crystalline iridium oxide only is contained by high temperature
baking
(410 degrees Celsius - 450 degrees Celsius) plus high temperature post-bake
(520 degrees Celsius - 560 degrees Celsius); and patent applications have been
made for these two methods as of the same date with the present application.
According to these two inventions, lead adhesion resistivity can be achieved
when the amount of iridium coating per time is 2g/m2 or less, in the
electrolysis
4
CA 02859939 2014-06-19
WO 2013/099780
PCT/JP2012/083168
condition of current density not more than 100A/dm2, and at the same time,
improvement of durability from increase of the effective area of catalyst
layer and
reduction of oxygen generation overvoltage can be achieved.
Recently, however, in order to enhance the quality of products or to provide
special performance characteristics to products, electrolysis at a current
density
of 300A/dm2 ¨ 700A/dm2 or more has been frequently conducted. Recent trend is
that such high electric current is not supplied to all the anodes installed to
the
industrial electrolysis system, but rather, an auxiliary anode is installed at
a
specific point where high-load electrolysis condition is applied to provide
special
performance characteristics to products obtained from the electrolysis.
Under the electrolysis at such a high current density, the electrode catalyst
layer is highly loaded and electric current tends to be concentrated there,
causing
rapid consumption of the electrode catalyst layer. Moreover, organic substance
or
impurity elements added for stabilizing product quality cause various
electrochemical and chemical reactions, the concentration of hydrogen ion
increases in concomitant with the oxygen generation reaction, lowering the pH
value, and consumption of electrode catalyst is further expedited. From these
phenomena, it became clear that the enhancement of durability by the increase
of
the effective area of catalyst layer and the reduction of oxygen generation
overvoltage may not always be achieved by the inventions relating to the
above-mentioned two patent applications by the inventors of the present
invention.
Citation List
Patent Literature
PTL 1: JP2002-275697A (JP3654204B)
PTL 2: JP2004-238697A (JP3914162B)
PTL 3: JP2007-146215A
5
CA 02859939 2014-06-19
WO 2013/099780
PCT/JP2012/083168
PTL 4: JP2009-293117A (JP4516617B)
PTL 5: JP2010-001556A (JP4516618B)
Summary of Invention
Technical Problem
In order to solve the above-mentioned problems, the present invention aims to
provide a high-load durable anode for oxygen generation and a manufacturing
method for the same, having a superior durability under the conditions of
high-load, which can improve current distribution to the electrode catalyst
layer,
suppress consumption of the electrode catalyst and improve durability of the
electrode catalyst by enlarging effective surface area of the electrode
catalyst
layer under the conditions of high-load.
Solution to Problem
As the first solution to achieve the above-mentioned purposes, the present
invention provides an anode for oxygen generation comprising a conductive
metal
substrate and a catalyst layer containing iridium oxide formed on the
conductive
metal substrate, wherein the amount of coating of iridium per time for the
catalyst
layer is 2g/m2 or more, the coating is baked in a relatively high temperature
region
of 430 degrees Celsius - 480 degrees Celsius to form the catalyst layer
containing
amorphous iridium oxide and the catalyst layer containing the amorphous
iridium
oxide is post-baked in a further high temperature region of 520 degrees
Celsius -
600 degrees Celsius to crystallize almost all amount of iridium oxide in the
catalyst layer.
As the second solution to achieve the above-mentioned purposes, the present
invention provides an anode for oxygen generation comprising a conductive
metal
substrate and a catalyst layer containing iridium oxide formed on the
conductive
6
CA 02859939 2014-06-19
WO 2013/099780
PCT/JP2012/083168
metal substrate, wherein the amount of coating of iridium per time for the
catalyst
layer is 2g/m2 or more and the degree of crystallinity of iridium oxide in the
catalyst layer after the post-baking is made to be 80% or more.
As the third solution to achieve the above-mentioned purposes, the present
invention provides an anode for oxygen generation comprising a conductive
metal
substrate and a catalyst layer containing iridium oxide formed on the
conductive
metal substrate wherein the amount of coating of iridium per time for the
catalyst
layer is 2g/m2 or more and the crystallite diameter of iridium oxide in the
catalyst
layer is 9.0nm or less.
As the fourth solution to achieve the above-mentioned purposes, the present
invention provides an anode for oxygen generation comprising a conductive
metal
substrate and a catalyst layer containing iridium oxide formed on the
conductive
metal substrate, wherein a base layer containing tantalum and titanium
ingredients is formed by the arc ion plating (hereafter called AIP) process on
the
conductive metal substrate before the formation of the catalyst layer.
As the fifth solution to achieve the above-mentioned purposes, the present
invention provides a manufacturing method for an anode for oxygen generation,
wherein the amount of coating of iridium per time for a catalyst layer is 2
g/m2 or
more and the catalyst layer containing amorphous iridium oxide is formed by
baking in a relatively high temperature region of 430 degrees Celsius - 480
degrees Celsius and the catalyst layer containing amorphous iridium oxide is
post-baked in a further high temperature region of 520 degrees Celsius - 600
degrees Celsius to crystallize almost all amount of iridium oxide in the
catalyst
layer.
As the sixth solution to achieve the above-mentioned purposes, the present
7
CA 02859939 2014-06-19
WO 2013/099780
PCT/JP2012/083168
invention provides a manufacturing method for an anode for oxygen generation,
wherein the amount of coating of iridium per time for a catalyst layer is
2g/m2 or
more and the catalyst layer containing amorphous iridium oxide is formed by
baking in a relatively high temperature region of 430 degrees Celsius - 480
degrees Celsius and the catalyst layer containing amorphous iridium oxide is
post-baked in a further high temperature region of 520 degrees Celsius - 600
degrees Celsius to make the degree of crystallinity of iridium oxide in the
catalyst
layer to be 80% or more.
As the seventh solution to achieve the above-mentioned purposes, the present
invention provides a manufacturing method for an anode for oxygen generation,
wherein the amount of coating of iridium per time for a catalyst layer is
2g/m2 or
more and the catalyst layer containing amorphous iridium oxide is formed by
baking in a relatively high temperature region of 430 degrees Celsius - 480
degrees Celsius and the catalyst layer containing amorphous iridium oxide is
post-baked in a further high temperature region of 520 degrees Celsius - 600
degrees Celsius to make the crystallite diameter of iridium oxide in the
catalyst
layer to be 9.0nm or less.
As the eighth solution to achieve the above-mentioned purposes, the present
invention provides a manufacturing method for an anode for oxygen generation
comprising a conductive metal substrate and a catalyst layer containing
iridium
oxide formed on the conductive metal substrate, wherein an AIP base layer
containing tantalum and titanium ingredients is formed by the AIP process on
the
conductive metal substrate before the formation of the catalyst layer.
Advantageous Effects of Invention
In the formation for the electrode catalyst layer containing iridium oxide by
the
present invention, the amount of coating of iridium per time of the catalyst
layer is
8
CA 02859939 2014-06-19
WO 2013/099780 PCT/JP2012/083168
2g/m2 or more, baking is conducted, instead of the conventional repeated
baking
operations at 500 degrees Celsius or more, which are the perfect crystal
deposition temperature, by two steps: baking in a relatively high temperature
region of 430 degrees Celsius - 480 degrees Celsius to form a catalyst layer
containing amorphous iridium oxide and post-baking in a further high
temperature
region of 520 degrees Celsius - 600 degrees Celsius to suppress the
crystallite
diameter of iridium oxide in the electrode catalyst layer preferably to 9.0nm
or less
and to crystallize most of the iridium oxide preferably to 80% or more in
crystallinity. Thus, the growth of crystallite diameter of iridium oxide was
able to
be suppressed and the effective surface area of the catalyst layer was able to
be
increased. Thus, according to the present invention, the growth of crystallite
diameter of iridium oxide can be suppressed. As the reasons, the following are
considered. The baking is conducted by two stages: first, coating and baking
is
repeated in a relatively high temperature region of 430 degrees Celsius - 480
degrees Celsius and then post-baking in a further high temperature of 520
degrees
Celsius - 600 degrees Celsius. Compared with the baking at a high temperature
from the beginning by the conventional method, crystallite diameter under the
present invention will not enlarge beyond a certain degree. If the growth of
crystallite diameter of iridium oxide is suppressed, the smaller the
crystallite =
diameter is, the larger the effective surface area of the catalyst layer will
be. Then,
the oxygen generation overvoltage of the electrode can be decreased, oxygen
generation is promoted, and the reaction to form Pb02 from lead ion can be
suppressed. In this way, Pb02 attachment and covering on the electrode were
suppressed.
Further, according to the present invention, simultaneously with increase in
the
effective surface area of catalyst layer, electric current is evenly
distributed, that is,
the concentration of electric current is suppressed, and consumption of the
catalyst
layer by electrolysis is reduced, which leads to improvement of electrode
durability.
Furthermore, according to the present invention, improved quality of products
9
CA 02859939 2014-06-19
WO 2013/099780
PCT/JP2012/083168
and provision of special performance characteristics to products are achieved
by
controlling the amount of coating of iridium to 2g/m2 or more per time. When
electrolysis is performed at a current density of 300A/dm2 - 700A/dm2 or more,
or
also an auxiliary anode is provided at a specified spot under a high load
electrolysis conditions to give special performance characteristics to
products
obtained from electrolysis, load to the electrode catalyst layer can be
lessened,
electric current concentration can be prevented and consumption of electrode
catalyst layer can be suppressed.
Brief Description of Drawings
[Fig. 1] Fig. 1 is a graph indicating the change of degree of crystallinity of
iridium
oxide (1r02) of the catalyst layer by baking temperature and post-bake
temperature.
[Fig. 2] Fig. 2 is a graph indicating the change of crystallite diameter of
iridium
oxide (Ir02) of the catalyst layer by baking temperature and post-bake
temperature.
[Fig. 3] Fig. 3 is a graph indicating the change of the electrostatic capacity
of the
electrode by baking temperature and post-bake temperature.
[Fig. 4] Fig. 4 is a graph indicating the dependence of oxygen overvoltage on
baking conditions.
Description of Embodiments
The following explains embodiments of the present invention, in detail, in
reference to the figures. In the present invention, it is found that if the
effective
surface area of the electrode catalyst layer is increased to suppress adhesive
reaction of lead oxide to the electrode surface, oxygen generation overvoltage
can be reduced and then, oxygen generation is promoted and at the same time
the adhesive reaction of lead oxide can be suppressed. In addition, the
present
invention has been completed from the idea that it is necessary that iridium
oxide
CA 02859939 2014-06-19
WO 2013/099780
PCT/JP2012/083168
of the catalyst layer is mainly crystalline in order to improve the electrode
durability at the same time, and experiments were repeated.
In the present invention, a two-step baking is performed, first, in a
relatively high
temperature region of 430 degrees Celsius - 480 degrees Celsius to form a
catalyst layer containing amorphous Ir02 in the baking, then, in a further
high
temperature region of 520 degrees Celsius - 600 degrees Celsius to post-bake,
through which the iridium oxide of the catalyst layer is almost completely
crystallized.
Through the experiments conducted by inventors of the present invention, it
has
been proved that the catalyst layer containing amorphous iridium oxide, which
can
greatly increase the effective surface area, consumes amorphous iridium oxide
quite rapidly by electrolysis and durability is reduced relatively. In other
words, it is
considered that the electrode durability cannot be improved unless iridium
oxide of
the catalyst layer is crystallized. Therefore, in order to achieve the purpose
of the
present invention that the effective surface area of the electrode catalyst
layer is
increased and the overvoltage of the electrode is reduced, the present
invention
applies two-step baking: high temperature baking plus high temperature
post-baking in order to control the crystallite diameter of iridium oxide of
the
catalyst layer, through which iridium oxide crystal, smaller in size than the
conventional product precipitates, resulting in increased effective surface
area of
the electrode catalyst layer and reduced overvoltage.
In the present invention, a catalyst layer containing amorphous iridium oxide
is
formed on the surface of the conductive metal substrate by baking in a
relatively
high temperature region of 430 degrees Celsius - 480 degrees Celsius;
thereafter,
the catalyst layer of amorphous iridium oxide is post-baked in a further high
temperature region of 520 degrees Celsius - 600 degrees Celsius to crystallize
the
iridium oxide in the catalyst layer almost completely.
11
CA 02859939 2014-06-19
WO 2013/099780
PCT/JP2012/083168
According to the present invention, improved quality of products and provision
of
special performance characteristics to products are achieved by controlling
the
amount of coating of iridium to 2g/m2 or more per time. When electrolysis is
performed at a current density of 300A/dm2 - 700A/dm2 or more, or also an
auxiliary anode is provided at a specified spot under a high load electrolysis
conditions to give special performance characteristics to products obtained
from
electrolysis, load to the electrode catalyst layer can be lessened, electric
current
concentration can be prevented and consumption of electrode catalyst layer can
be suppressed.
The baking temperature in a relatively high temperature region of 430 degrees
Celsius - 480 degrees Celsius and the post-baking temperature in a further
high
temperature region of 520 degrees Celsius - 600 degrees Celsius are determined
by the crystal particle size and the degree of crystallinity of iridium oxide
to be
formed in the catalyst layer, and the catalyst layer with a low oxygen
overvoltage
and a high corrosion resistance is formed in the above-mentioned temperature
region.
In the present invention, the growth of the crystallite diameter of iridium
oxide
was able to be suppressed and the effective surface area of the catalyst layer
was
able to be increased by controlling the crystallite diameter of the iridium
oxide in
the electrode catalyst layer to a small number, preferably equal to or less
than
9.0nm and most of the iridium oxide was crystallized, preferably, to the
degree of
crystallinity equal to or more than 80%.
Prior to forming the catalyst layer, if the AIP base layer containing tantalum
and
titanium components is provided on the conductive metal substrate, it is
possible
to prevent further interfacial corrosion of the metal substrate.
The base layer consisting of TiTa0x oxide layer may be applied instead of the
12
CA 02859939 2014-06-19
WO 2013/099780
PCT/JP2012/083168
AIP base layer.
The catalyst layer was formed in such a manner that hydrochloric acid aqueous
solution of IrC13/Ta2C15 as a coating liquid was coated on the AIP coated
titanium
substrate at 3g-Ir/m2 per time and baked at a temperature by which part of
Ir02
crystallizes (430 - 480 degrees Celsius). After repeating the coating and
baking
process until the necessary support amount of the catalyst was obtained, one
hour
post-bake was conducted at a further high temperature (520 degrees Celsius -
600
degrees Celsius). In this way, the electrode sample was prepared. The prepared
sample was measured for Ir02 crystalline of the catalyst layer by X-ray
diffraction,
oxygen generation overvoltage, electrostatic capacity of electrode, etc. and
evaluated for sulfuric acid electrolysis and gelatin-added sulfuric acid
electrolysis
and lead adherence test.
As a result, it has been found that most of the Ir02 of the formed catalyst
layer
was crystalline, the crystallite diameter became smaller, and the electrode
effective surface area increased. Accelerated life evaluation was carried out
and
found that, as to be described later, sulfuric acid electrolysis life was
about 1.4
times that of the conventional product, and gelatin-added sulfuric acid
electrolysis
life was about 1.5 times that of the conventional product, proving improvement
in
durability.
The experimental conditions and methods by the present invention are as
follows.
In order to investigate formation temperature of amorphous iridium oxide and
the range of post-bake temperature for successive crystallization, a sample
shown in Table 1 was manufactured and subjected to measurements of X-ray
diffraction, cyclic voltammetry, oxygen overvoltage, etc.
The surface of titanium plate (JIS-I) was subjected to the dry blast with iron
grit
(G120 size), followed by pickling in an aqueous solution of concentrated
13
CA 02859939 2014-06-19
WO 2013/099780
PCT/JP2012/083168
hydrochloric acid for 10 minutes at the boiling point for cleaning treatment
of the
metal substrate of the electrode. The cleaned metal substrate of the electrode
was set to the AIP unit applying Ti-Ta alloy target as a vapor source and a
coating
of tantalum and titanium alloy was applied as the base layer on the surface of
the
[Table 1]
Alloy disk comprising Ta:Ti=60wt%:40wt%
Target(vapor source)
(back surface cooling)
Vacuum pressure 1.5x10-2 Pa or less
Metal substrate temperature 500 degrees Celsius or less
Coating pressure 3.0x10-1-4.0x10-1Pa
Vapor source charge power 20-30V, 140-160A
Coating time 15-20 minutes
Coating thickness 2micuron(weight increase conversion)
The coated metal substrate was heat-treated at 530 degrees Celsius in an
Then, the coating solution prepared by dissolving iridium ,tetrachloride and
tantalum pentachloride in concentrated hydrochloric acid was applied on the
coated metal substrate. After drying, the thermal decomposition coating was
conducted for 15 minutes in the electric furnace of air circulation type at a
Then, the coated sample with catalyst layer was subjected to the post bake in
14
CA 02859939 2014-06-19
WO 2013/099780 PCT/JP2012/083168
the electric furnace of air circulation type for one hour at a temperature
shown in
Table 2 to manufacture an electrode for electrolysis. In addition, a sample
not
subjected to post-bake was manufactured for comparison purpose.
Baking temperature and post-bake temperature of each sample are shown in
Table 2.
Experimental items for evaluation
(1) Degree of crystallinity and measurement of crystallite diameter
Ir02 crystallinity and crystallite diameter of the catalyst layer were
measured by
X-rays diffractometry.
The degree of crystallinity was estimated from the diffraction peak intensity.
(2) Electrostatic capacity of electrode
Method: cyclic voltammetry
Electrolyte : 150 g/L H2SO4 aq.
Electrolysis temperature : 60 degrees Celsius
Electrolysis area : 10x10 mm2
Counter electrode : Zr plate (20 mmx70 mm)
Reference electrode : Mercurous sulphate electrode (SSE)
(3) Measurement of oxygen overvoltage
Method: current interrupt method
Electrolyte : 150 g/L H2SO4 aq.
Electrolysis temperature : 60 degrees Celsius
Electrolysis area : 10x10 mm2
Counter electrode : Zr plate (20 mmx70 mm)
Reference electrode : Mercurous sulphate electrode (SSE)
CA 02859939 2014-06-19
WO 2013/099780
PCT/JP2012/083168
[Table 2]
Oxygen
Baking Post-bake Degree of Crystallite Electrostatic
generation
temperature temperature crystallinity diameter capacity
Sample No. overvoltage
(V vs. SSE
( C) ( C) (%) (nm) (C/m2)
@100A/dm2)
1 none 0 0 88.8 0.851
2 520 100 7.7 21.6 0.963
430
3 560 100 7.8 15.4 0.987
4 600 100 7.7 11.6 1.021
none . 72 9.3 13.7 0.983
6 520 85 8.5 18.1 1.011
480
7 560 82 8.5 14.4 1.031
a 600 98 8.7 14.5 1.035
9
(Conventional 500-520 none 100 9.1 7.6 1.051
product)
The changes of Ir02 crystal characteristics by the baking temperature and the
post-bake temperature were as follows.
5 As for the estimation of degree of crystallinity, the intensity of the
crystal
diffraction peak (0=28 degrees) of each sample is expressed as a ratio when
compared with the intensity of the crystal diffraction peak (0=28 degrees) of
the
conventional product which is assumed as 100. The results are given in Table
2.
In addition, Fig. 1 is a graph showing the degree of crystallinity based on
the data
in Table 2.
As is clear from Table 2 and Fig. 1, the degree of crystallinity of iridium
oxide
after post-bake of Samples 2-4 and Samples 6-8 of the example by the present
invention, which had been subjected to baking in a relatively high temperature
16
CA 02859939 2014-06-19
WO 2013/099780 PCT/JP2012/083168
region of 430 degrees Celsius - 480 degrees Celsius plus post-bake in a
further
high temperature region of 520 degrees Celsius - 600 degrees Celsius was 80%
or more. On the other hand, iridium oxide attributable to the electrode
catalyst
layer treated by the baking at 430 degrees Celsius without post-bake (Sample
1)
did not show a clear peak, proving that the catalyst layer of this sample
comprises
amorphous iridium oxide. The degree of crystallinity of the electrode catalyst
layer
baked at 480 degrees Celsius without post-bake (Sample 5) was 72% with a lot
of
remaining amorphous iridium oxide. In addition, Sample 9, which is a
conventional product was fully crystallized, showing the degree of
crystallinity
being 100%, but the crystallite diameter increases to 9.1nm, resulting in a
low
value of the electrostatic capacity of electrode at 7.6 with small effective
surface
area.
In other words, as the change of the degree of crystallinity by a high
temperature post-bake, clear peak of Ir02 attributable to the electrode
catalyst
layer was observed after baking at 430 degrees Celsius and post-bake in a
further
high temperature, showing that amorphous Ir02 of the catalyst layer had
changed
to crystalline by a high temperature post-bake. In addition, it was found that
the
peak intensity was similar to that of the conventional product at any post-
bake
temperatures, showing that amorphous Ir02 did not remain. On the other hand,
the products treated by the baking at 480 degrees Celsius showed a further
high
degree of crystallinity by a high temperature post-bake. However, it was found
that a small amount of amorphous Ir02 still existed after post-bake at 520
degrees
Celsius and 560 degrees Celsius. By contrast, the degree of crystallinity of
Ir02
after the post-bake at 600 degrees Celsius was almost equivalent to the
conventional product, showing full crystallization.
Then, the crystallite diameter was calculated from X-ray diffraction. The
results
are shown in Table 2. Fig. 2 was prepared based on the data in Table 2
relating to
the crystallite diameter.
17
CA 02859939 2014-06-19
WO 2013/099780
PCT/JP2012/083168
The crystal diameter of the amorphous Ir02 formed by the baking at 430
degrees Celsius without post-bake is indicated as "0". It was found that if
post-bake is applied, amorphous Ir02 was crystallized, but the crystallite
diameter
of the formed crystal became smaller than that of the conventional product. In
addition, there is little mutual dependence observed between the post-bake
temperature and the crystallite diameter of Ir02.
On the other hand, the crystallite diameter of the baked product in 480
degrees
Celsius followed by post-bake gave a smaller one than the conventional
product,
regardless of the post-bake temperature. In other words, crystallinity of Ir02
of the
catalyst layer formed in a low temperature baking increased by post-bake, but
the
increasing of Ir02 crystallite diameter was able to be suppressed.
As is evident from the data on the crystallite diameter in Table 2 and Fig. 2,
the
crystallite diameter of iridium oxide after post-bake of Samples 2-4 and
Samples
6-8 of the examples by the present invention, which was subjected to baking in
a
relatively high temperature region of 430 degrees Celsius - 480 degrees
Celsius
plus post-bake in a further high temperature region of 520 degrees Celsius -
600
degrees Celsius was 9.0nm or less. On the other hand, iridium oxide
attributable
to the electrode catalyst layer treated by the baking at 430 degrees Celsius
without post-bake (Sample 1) did not show a clear peak, proving that the
catalyst
layer of this sample comprises amorphous iridium oxide. The crystallite
diameter
of the electrode catalyst layer baked at 480 degrees Celsius without post-bake
(Sample 5) was large to 9.3nm. The crystallite diameter of iridium oxide of
Sample
9, which is the conventional product, was as large as 9.1nm.
Then, measurements were made about the change of effective surface area of
the electrode catalyst layer prepared by high temperature baking in a
relatively
high temperature region of 430 degrees Celsius - 480 degrees Celsius plus
post-bake in a further high temperature region of 520 degrees Celsius - 600
18
CA 02859939 2014-06-19
WO 2013/099780
PCT/JP2012/083168
degrees Celsius.
Electrostatic capacity of the electrode calculated by the cyclic voltammetry
method is shown in Table 2. Electrostatic capacity of the electrode is
proportional
to the effective surface area of electrode, and it may be right to say that
the higher
the capacity, the higher the effective surface area also is. Fig. 3 shows the
relationship between the electrostatic capacity and the baking conditions of
the
catalyst layer, based on the data in Table 2.
As is clear from Table 2 and Fig. 3, the electrostatic capacity of the
electrode of
Samples 2-4 and Samples 6-8 of the example by the present invention, which
were subjected to baking in a relatively high temperature region of 430
degrees
Celsius - 480 degrees Celsius plus post-bake in a further high temperature
region
of 520 degrees Celsius - 600 degrees Celsius increased to a high point of 11.6
or
more. On the other hand, 1102 of the catalyst layer formed by baking at 430
degrees Celsius without post-bake (Sample 1) showed the largest effective
surface area (the electrolytic capacity of the electrode), since it is
amorphous.
After conducting post-bake, the effective surface area (the electrolytic
capacity of
the electrode) decreased since Ir02 was crystallized, but it was still higher
compared with the conventional product. This may be because the formed
crystallite diameter was smaller than the conventional product. In addition,
it was
observed that the electrode effective surface area (the electrolytic capacity
of the
electrode) tended to decrease with the increasing of post-bake temperature.
Also, it has been found that if post-bake is conducted after the baking at 480
degrees Celsius (Samples 5-8), the effective surface areas (the electrolytic
capacity of the electrode) are almost the same regardless of the post-bake
temperature, meanwhile they doubled compared with the conventional product.
This is probably due to a smaller 1r02 crystallite diameter compared with the
conventional product and also a small amount of amorphous 1r02 remaining.
Moreover, even if the post-bake temperature is increased, there was no change
in
the electrode effective surface area (the electrolytic capacity of the
electrode).
19
CA 02859939 2014-06-19
WO 2013/099780 PCT/JP2012/083168
The oxygen generation overvoltage (V vs. SSE @ 100A/dm2) of each sample
was measured. The results are shown in Table 2. In addition, the dependence of
the oxygen generation overvoltage on baking conditions is shown in Fig. 4. The
trend of changing in the graph of Fig. 4 was reverse to that of Fig. 3. With
increase
of the electrode effective surface area, the oxygen generation overvoltage of
the
samples tended to decrease. As the reason, it is considered that increased
electrode effective surface area contributed to dispersion of electric current
distribution, lowering the actual electric current.
The product with the largest effective surface area baked at 430 degrees
Celsius without post-bake showed the lowest oxygen overvoltage, but oxygen
overvoltage increased as a result of decreased effective surface area by
post-bake. Similar trend was observed with the product baked at 480 degrees
Celsius in dependence of oxygen overvoltage on the post-bake temperature. In
addition, the oxygen overvoltage of these samples was found to be higher than
that of the conventional product. This seems to be because the surface area
increased compared with the conventional product.
In Table 2 and Fig. 4, it is indicated oxygen overvoltage of Samples 2-4 and
Samples 6-8 of the examples by the present invention, which were subjected to
baking in a relatively high temperature region of 430 degrees Celsius - 480
degrees Celsius plus post-bake in a further high temperature region of 520
degrees Celsius - 600 degrees Celsius decreased.
As mentioned above, the electrode manufactured by the baking means of
baking in a relatively high temperature region of 430 degrees Celsius - 480
degrees Celsius plus post-bake in a further high temperature region of 520
degrees Celsius - 600 degrees Celsius features to have a smaller Ir02 crystal
of
the catalyst layer compared with the conventional product and an increased
electrode surface area. In these samples, electric current distribution can be
CA 02859939 2014-06-19
WO 2013/099780 PCT/JP2012/083168
dispersed under a high-load condition and actual electric current load was
decreased, from which such effects as suppression of catalyst consumption and
improvement in durability can be expected.
Examples
The following describes examples by the present invention; provided, however,
the present invention is not limited to these examples.
<Example 1>
The surface of titanium plate (JIS-I) was subjected to the dry blast with iron
grit
(G120 size), followed by pickling in an aqueous solution of concentrated
hydrochloric acid for 10 minutes at the boiling point for cleaning treatment
of the
metal substrate of the electrode. The cleaned metal substrate of the electrode
is
set to the AIP unit applying Ti-Ta alloy target as a vapor source and a
coating of
tantalum and titanium alloy was applied as the AIP base layer on the surface
of
the metal substrate of the electrode. Coating condition is shown in Table 1.
The coated metal substrate was treated at 530 degrees Celsius in an electric
furnace of air circulation type for 180 minutes.
Then, the coating solution prepared by dissolving iridium tetrachloride and
tantalum pentachloride in concentrated hydrochloric acid is applied on the
coated
metal substrate. After drying, the thermolysis coating was conducted for 15
minutes in the electric furnace of air circulation type at 480 degrees Celsius
to form
an electrode catalyst layer comprising mixture oxides of iridium oxide and
tantalum
oxide. The amount of coating solution was determined so that the thickness of
coating per time of the coating solution corresponds to approx. 3.0g/m2, as
iridium
metal. This coating-baking operation was repeated nine times to obtain the
electrode catalyst layer of approx. 27.0g/m2, converted for metal iridium.
The X-ray diffraction was carried out for this sample. A clear peak of iridium
oxide attributable to the electrode catalyst layer was observed, but the
intensity of
21
CA 02859939 2014-06-19
WO 2013/099780 PCT/JP2012/083168
the peak was lower than that of Comparative Example 1, indicating that
crystalline
Ir02 had been partially precipitated.
Next, an electrode for electrolysis was manufactured in such a manner that the
sample coated with the catalyst layer is post-baked in an electric furnace of
air
circulation type at 520 degrees Celsius for one hour.
The X-ray diffraction was carried out for the sample with post-baking. A clear
peak of iridium oxide attributable to the electrode catalyst layer was
observed, but
the intensity of the peak was still lower than that of Comparative Example 1,
though was higher than before the post-bake. From this, it has been known that
the degree of crystallinity of the catalyst layer formed by the low
temperature
baking, before the post-bake, has increased, but amorphous Ir02 still remains
partially.
About the electrode for electrolysis prepared in the above-mentioned manner,
two types of life evaluation test were conducted for: Pure sulfuric acid
solution and
sulfuric acid solution with gelatin. Results are shown in Table 4. When
compared
with Comparative Example 1 (Conventional Product) in Table 4, the life for
sulfuric
acid electrolysis was 1.7 times and the life of gelatin-added sulfuric acid
electrolysis was 1.1 times, identifying that durability to both sulfuric acid
and
organic additive has improved.
22
CA 02859939 2014-06-19
WO 2013/099780 PCT/JP2012/083168
[Table 3]
Gelatin-added
Sulfuric acid electrolysis
sulfuric acid electrolysis
Current density 500 A/dm2 300 A/dm2
150 g/L of H2SO4 aq. + 50 ppm
Electrolyte 150 g/L of H2SO4 aq.
gelatin
Electrolysis temperature 60 C
Counter electrode Zr plate
Criterion of electrolysis At the time when a cell voltage increased 1.0 V
than an initial
life cell voltage.
<Example 2>
The electrode for evaluation was manufactured in the same manner as with
Example 1 except that post-bake was conducted in an electric furnace of air
circulation type for one hour at 560 degrees Celsius and the same electrolysis
evaluation was performed.
The X-ray diffraction performed after post-bake showed the degree of Ir02
crystallinity and crystallite diameter of the catalyst layer equivalent to
Example 1.
As shown in Table 4, when compared with Comparative Example 1
(Conventional Product) in Table 4, the life of sulfuric acid electrolysis was
1.5 times
and the life of gelatin-added sulfuric acid electrolysis was 1.3 times,
identifying that
durability to both sulfuric acid and organic additive has improved.
<Comparative Example 1>
The electrode catalyst layer comprising the mixture oxide of iridium oxide and
tantalum oxide was formed as with Example 1, but changing the baking
temperature in the electric furnace of circulation air type to 520 degrees
Celsius
and the baking time to fifteen minutes. The electrode thus manufactured
without
post-bake was evaluated for electrolysis by the X-ray diffraction as with
Example
23
CA 02859939 2014-06-19
WO 2013/099780 PCT/JP2012/083168
1.
The X-ray diffraction was performed on this sample, from which a clear peak of
iridium oxide attributable to the electrode catalyst layer was observed,
verifying
that Ir02 in the catalyst layer is crystalline.
Life evaluation was made as with Example 1. From the results shown in Table 4,
it has been made clear that the method of the low temperature baking plus high
temperature post-bake, as suggested in the present invention, improves
durability
in electrolysis under high-load conditions.
<Comparative Example 2>
In the same manner as with Example 1 except that post-bake was carried out,
the electrode for evaluation was manufactured and electrolysis evaluation was
carried out in the same manner with Example 1.
As shown in Table 4, lives of the electrode baked at 480 degrees Celsius
without post-bake for sulfuric acid electrolysis and gelatin-added sulfuric
acid
electrolysis were equivalent to that of the conventional product, proving no
improvement in durability.
[Table 4]
Baking Post-bake Life of sulfuric Life of gelatin-
added
temperature temperature acid electrolysis
sulfuric acid electrolysis
( C) ( C) (hr) (hr)
1 480 520 4182 1084
Example
2 480 560 3665 1304
Comparative 1 520 2508 978
Example 2 480 2604 1073
Industrial Applicability
24
CA 02859939 2014-06-19
WO 2013/099780 PCT/JP2012/083168
The present invention relates to an anode for oxygen generation used for
various industrial electrolyses and a manufacturing method for the same; more
in
detail, it is applicable to a high-load durable anode for oxygen generation
used for
industrial electrolyses including manufacturing of electrolytic metal foils
such as
electrolytic copper foil, aluminum liquid contact, continuously
electrogalvanized
steel plate and metal extraction, having superior durability under high-load
electrolysis conditions.
