Note: Descriptions are shown in the official language in which they were submitted.
~s~s~
DURABLE ELECTRODE FOR ELECTROLYSIS
AND PROCESS FOR PRODUCTION THEREOF
1FIELD OF THE IWVENTION
The present invention relates to electrodes for
electrolysis (hereinafter referred to as "electrolytic
electrodes") and a process for the production of the same.
5More particularly, the present invention relates to
electrolytic electrodes showing high durability, i.e., a
long service life, when used in electrochemical processes,
e.g., an aqueous solution in which the generation of
oxygen at the anode is involved, and a process for the
10production of the same.
BACKGROUND OF THE INVENTION
Heretofore, electrolytic electrodes comprising a
substrate of valve metals, e.g., titanium (Ti), have been
used as superior insoluble metal electrodes in the field
15of electrochemistry. In particular, they have been widely
used as anodes for the generation of chlorine in the salt
(sodium chloride) electrolytic industry. In addition to
Ti, tantalum ~Ta), niobium (Nb), zirconium (Zr), hafnium
(Hf), vanadium (V), molybdenum (Mo), tungsten (W), etc.
20are known as valve metals.
These metal electrodes are produced by coating
metallic titanium with various electrochemically active
13~
~g~52
1 substances such as platinum group metals and their oxides.
Examples of such platinum group metals and their oxides
are described in, e.g., U.S. Patent Nos. 3,632,498 and
3,711,385. These electrodes can maintain a low chlorine
overvoltage over a long period of time as electrodes for
the generation of chlorine.
Howèver, when the above metal electrodes are
used as anodes in electrolysis for the generation of
oxygen or electrolysis in which the generation of oxygen
is involved, the anode overvoltage gradually increases.
In extreme cases, the anode is passivated and thus it
becomes impossible to continue the electrolysis.
The phenomenon of passivation of the anode is
believed to be caused mainly by the formation of
electrically non-conductive titanium oxides that result
from (1) the oxidation of the titanium base material with
oxygen by the electrode coating-constituting oxide
substance itself; (2) oxygen diffusion-permeating through
the electrode coating; or (3) the electrolyte.
Formation of such electrically non-conductive
oxides in the interface between the base material and the
electrode coating causes the electrode coating to peel
off. This creates problems such as a breakdown of the
electrode.
Electrochemical processes in which the anode
~S90~
-- 3 --
1 product is oxygen, or where oxygen is generated at the
anode as a side reaction, include: (1) electrolysis using
a sulfuric acid bath, a nitric acid bath, an alkali bath
or the like; (2) electrolytic separation of chromium (Cr),
copper (Cu), zinc (Zn), or the like; (3) various types of
electroplating; (4) electrolysis of dilute salt water, sea
water, hydrochloric acid, or the like; and (5)
electrolysis for the production of chlorate, and so forth.
These processes are all industrially important. However,
the above-described problems have hindered metal
electrodes from being used in these processes.
U.S. Patent No. 3,775,284 discloses a technique
to overcome passivation of the electrode due to permeation
of oxygen. In this technique, a barrier layer of a
platinum (Pt)-iridium (Ir) alloy, or of an oxide of coba
lt (Co), manganese (Mn), lead (Pb), palladium (Pd), and Pt
is provided between the electrically-conductive substrate
and the electrode coating.
The substances forming the intermediate barrier
layer prevent the diffusion-permeation of oxygen during
electrolysis to some extent. However, these substances
are electrochemically very active and therefore, react
with the electrolyte passing through the electrode
coating. This produces electrolytic products, e.g., gas,
on the surface of the intermediate barrier layer which
~259052
gives rise to additional problems. For example, the
adhesion of the electrode coating is deteriorated due to
physical and chemical influences of the electrode coatiny
peeling off before the life of the substance of the
electrode coating is over. Another problem is the
corrosion resistance of the resulting electrodes is
poor. Thus, the method disclosed in U.S. Patent No.
3,775,284 fails to produce electrolytic electrodes which
have high durability.
Japanese Patent Application (OPI) No. 40381/76
(the term "OPI" used herein refers to a "Published
Unexamined Patent Application") of Hooker Chemical and
Plastics Corp. was published on April 5, 1976 and
discloses an intermediate coating layer comprising tin
oxide coped with antimony oxide for coating the anode.
However, the anode used is an anode intended for the
generation of chlorine, and hence an electrode provided
with an intermediate coating forming substance disclosed
in the above publication does not show the generation of
oxygen.
U.S. Patent No. 3,773,555 discloses an electrode
in which a layer of an oxide of, e.g., Ti, and a layer of
a platinum group metal or an oxide thereof are laminated
and coated on the electrode. However, this electrode has
the problem that when it is used in electrolysis in which
the generation of o~ygen is involved, passivation occurs.
,,
., ~ .
,
.
~2~9~5~:
-- 5 --
S UMMARY OF THE I NVENT I ON
The present invention provides the ability to
overcome the above-described problems. ~ore specifically,
an object of the present invention is to provide
electrolytic electrodes which are especially suitable for
use in electrolysis in which the generation of oxygen is
involved, i.e., which resist passivation and have high
durability.
Another object of the present invention is to
provide a process for producing such electrolytic
electrodes.
The above described objects are met by:
(I) An elecrtrolytic electrode comprising
(a) an electrode substrate of an electrically-
conductive metal;
(b3 an electrode coating of an electrode active
substance; and
(c3 an intermediate layer provided between the
electrode substrate and the electrode coating, wherein the
intermediate layer (c) comprises a mixed oxide of
(i) an oxide of at least one member
selected from the group consisting of titanium (Ti) and
tin (Sn), each having a valence number of 4, and
(ii) an oxide of at least one member
selected from the group consisting of aluminum (Al),
gallium (Ga), iron (~e), cobalt (Co), nickel (Ni) and
~90~ii2
1 thallium (Tl), each having a valence number of 2 or 3, and
platinum (Pt~ dispersed in the mixed oxide; and
(II) A process for producing an electrolytic electrode,
comprising the steps of:
(1) coating an electrode substrate of an
electrically conductive metal with a solution containing
(i) salt(s) of Ti and/or Sn,
(ii) salt(s) of at least one metal selected
from the ~roup consisting of Al, Ga, Fe, Co, Ni and Tl,
and
(iii) a salt of Pt to provide a coated
substrate;
(2) heating in an oxidizing atmosphere the
electrode substrate coated with said solution in step (1),
thereby forming on the electrode substrate an intermediate
layer comprising a mixed oxide of
(i) an oxide of at least one member
selectecl from the group consisting of Ti and Sn, and
(ii) an oxide of at least one member
selectecl from the group consisting of Al, Ca, Fe, Co, Ni,
Tl, and Pt dispersed in the mixed oxide, and
(3) subsequently coating the inter~ediate layer
with a layer of an electrode active substance.
DETAI~E_D_DESCRIPTION OF THE INVENTION
The present invention is based on the discovery
, ~ . :' .
. . .
-~ -52
1 that the provision of the intermediate layer between the
substra-te and the electrode coating enables one to obtain
an electrode which can be used with sufficient durability
as an anode for electrochemical processes in which the
generation of oxygen is involved.
The intermediate layer of the present invention
is corrosion-resistant and is electrochemically inactive.
A unction of the intermediate layer is to protect the
electrode substrate, e.g., Ti, so as to prevent
passivation of the electrode without reducing its
electrical conductivity. At the same time, the
intermediate layer acts to enhance the adhesion or bonding
between the base material and the electrode coating~
Accordingly, the present invention provides
electrolytic electrodes which have sufficient durability
when used in electrolysis for the generation of oxygen or
electrolysis in which oxygen is generated as a side
reaction. Such processes have heretofore been considered
difficult to perform with conventional electrodes.
The present invention is explained in greater
detail below.
In the production of the electrode substrate of
the present invention, corrosion-resistant, electrically-
conductive metals, e.g., Ti, Ta, Nb, and Zr, and their
base alloys can be used. Suitable examples are metallic
1 Ti, and Ti-base alloys, e.g., Ti-Ta-Nb and Ti-Pd, which
have heretofore been commonly used. The electrode base
material can be in any suitable form such as in the form
of a plate, a perforated plate, a rodr or a net-like
member.
The electrode substrate of the present invention
may be of a type coated with a platinum group metal such
as Pt or a valve metal such as Ta and Nb in order to
increase corrosion resistance or enhance the bonding
between the substrate and the intermediate layer.
The intermediate layer is provided on the above-
described electrode substrate and comprises a composite
having Pt dispersed in a mixed oxide of an oxide of Ti
and/or Sn having a valence number of 4 and an oxide of at
least one member selected from the group consisting of Al,
Ga, Fe, Co, Ni and Tl having a valence number of 2 or 3.
An electrolytic electrode comprising an
electrode substrate of an electrically conductive metal
such as Ti and an electrode coating of a metal oxide,
wherein an intermediate layer of a mixed oxide of an oxide
of Ti and/or Sn and an oxide of Ta and/or Nb is provided
between the substrate and the electrode coating is
disclosed in U.S. Patent Nos. 4,471,006 and 4,484,999.
This electrode is resistant to passivation and excels in
durability. The intermediate layer used in the electrode
: ~.
~s~0~2
1 exhibits good conductivity as an N-type semiconductor.
However, since the intermediate layer has limited carrier
concentration, further improvement with respect to
conductivity was desired.
Due to the concept of providing an intermediate
layer possessing much higher conductivity -than the
intermediate layer of the electrode of these patents, the
present invention has made it possible to produce an
electrode which eliminates the drawback suffered by the
electrode of these patents and offers still higher
conductivity and durability.
As the substance to make up the intermediate
layer contemplated by this invention, a composite having
Pt dispersed in a mixed oxide of an oxide of Ti and/or Sn
and an oxide of at least one member selected from the
group consisting of Al, Ga, Fe, Co, Ni and Tl has been
demonstrated to suit the purpose of this invention and
provide an outstanding effect. The substance of the
intermediate layer provides excellent resistance to
corrosion, exhibits no electrochemical activity, and
possesses ample conductivity. The term "oxide" or "mixed
oxide" is meant to embrace solid solutions of metal oxides
and metal oxides which are nonstoichiometric or have
lattice defects. As used in this invention, the
expression "TiO2"~ "Sno2 ~ "A123 ' Ga2O3 , FeO ,
-- 10 --
1 ~Fe2O3~, "cOOn, "Co2O3n, "NiOn, nT12O3n, etc. and the term
~mixed oxide" embrace solld solutions of such metal oxides
and those metal oxides nonstoichiometric or having lattice
defects, for the sake of convenience.
The substance of the intermediate layer, as
described above, is any combination of Pt substantially in
a metallic form, an oxide of a metal having a valence of 4
~Ti or Sn), and an oxide of a metal having a valence of 2
or 3 (Al, Ga, Fe, Co, Ni and Tl).
Specifically, any of the mixed oxides TiO2-
A1205~ TiO2-Ga203, SnO2-FeO, SnO2-CoO, TiO2-SnO2-Co203,
TiO2-SnO2-NiO, TiO2-Al2O3-Tl2O30 SnO2-Ga2O3 Fe23 and
TiO2-SnO2-A12O3-Ga2O3 can be used advantageously to
achieve an ample effect when combined with Pt dispersed
therein.
The proportions of the component oxides of the
mixed oxide are not specificaly defined and a wide range
of proportions may be used. For prolonged retention of
durability and conductivity of the electrode, it is
desirable for the ratio of the oxide of the tetravalent
metal to the oxide of the divalent or trivalent metal to
be in the range of about 95:5 to about 10:90 by the mol of
metal. The amount of Pt to be dispersed in the mixed
oxide desirably falls in the range of about 1 to 20 mol%
based on the total amount of substance making up the
intermediate layer.
.~,~ ..
~59Qt~2
1 The formation of the intermediate layer in the
electrode can be advantaqeously effected by the thermal
decomposition method which comprises the steps of applying
a mixed solution containing chlorides or other salts of
component metals destined to make up the aforementioned
intermediate layer to the metal substrate and then heating
the coated substrate under an atmosphere of an oxidizing
gas at temperatures of about 350 to 600C thereby
producing a mixed oxide. Other methods may be adopted if
desired so long as the method is capable of forming a
homogeneous, compact coating having Pt dispersed in an
electroconductin~ mixed oxide. By the afore-mentioned
thermal decomposition method, Ti, Sn, Al, Ga, Fe, Co, Ni
and Tl are readily converted into their corresponding
oxides while Pt compound is merely decomposed thermally
into metallic platinum and is not converted into an oxide at
all.
The amount of the substance of the intermediate
layer to be applied to the substrate preferably exceeds
about 5 x 10 3 mol/m calculated as metal. If the amount
is less than about 5 x 10 3 mol/m2 mentioned above, the
intermediate layer consequently formed does not provide
sufficient effects.
The thus-formed intermediate layer i5 then
,
~25~
1 coated with an electrode active substance which is
electrochemically active to produce the desired product.
Suitable examples of such electrode active substances are
metals, metal oxides or mixtures thereof, which have
superlor electrochemical characteristics and durability.
The type of the active substance can be determined
appropriately depending on the electrolytic reaction in
which the electrode is to be used. Active substances
particularly suitable for the above-described electrolytic
processes in which the generation of oxygen is involved
include: platinum group metal oxides, and mixed oxides of
platinum group metal oxides and valve metal oxides.
Typical examples include: Ir oxide, Ir oxide-Ru oxide, Ir
oxide-Ti oxide, Ir oxide-Ta oxide, Ru oxide-Ti oxide, Ir
oxide-Ru oxide-Ta oxide, and Ru oxide-Ir oxide-Ti oxide.
The electrode coating can be formed in any
suitable manner, e.g., by thermal decomposition,
electrochemical oxidation, or powder sintering. A
particularly suitable technique is the thermal
decomposition method as described in detail in U.S. Patent
Nos. 3,711,385 and 3,632,498.
The exact reason why the provision of the
intermediate layer, i.e., that layer of the mixed oxide of
4-valent and 2- or 3-valent metals and Pt dispersed
therein~ between the metal electrode substrate and the
- 13 -
~S9(~1SZ
1 electrode active coating produces the above-described
results is not well understood. However, while not
desiring to be bound the reason is believed as follows.
Crystallographically, it is confirmed that Al,
Ga, Fe, Co, Ni and Tl are in substantially of 6-
coordination state and the ionic radii of these metals in
a 6-coordinatLon state vary within the range between the
value by about 10% larger than and the value by about 10%
smaller than that of Ti or Sn. This indicates that the
mixed oxides of the metals form a layer of a uniform,
dense solid solution or mixed oxide composed mainly of a
rutile type crystal phase. Since such an intermediate
layer comprising a composite of Pt dispersed in such a
mixed oxide has a high resistance to corrosion, the
surface of the substrate covered with the dense metal
mixed oxide intermediate layer is protected from
oxidation, and hence passivation of the substrate is
prevented.
In the intermediate layer, the 4-valent and 2-or
3-valent metals are present simultaneously as oxides and
Pt is dispersed in the mixed oxides. Therefore, according
to generally known principles of Controlled Valency, the
intermediate layer becomes a p-type semi-conductor having
a very high electrical conductivity. Moreover, the Pt
dispersed in the mixed oxide confers high electron
conductivity to the mixed oxide.
- 14 -
~:S9052
1 Also, since Pt is a substance which offers
extremely high resistance to corrosion and has very high
potential for the generation of oxygen, it is deficient in
electrochemical activity and generally does not react with
the electrode and, thus functions to enhance the
durability of the electrode. Where metallic Ti, for
example, is used as a substrate, even when electrically
non-conductive Ti oxides are formed on the surface of the
substrate during the production of the electrode or during
the use of the electrode in electrolysis, the 2- or 3~
valent metal in the intermediate layer diffuses and
renders the Ti oxides semi-conductors. Accordingly, the
electrical conductivity of the electrode is maintained and
passivation is prevented.
In addition, the intermediate layer substance
which is composed mainly of rutile type oxides having
dispersed therein Pt enhances the adhesion or bonding
between the substrate of, e.g., metallic Ti, and the
electrode active coating of, e.g., platinum group metal
oxides and valve metal oxides, and hence increases the
durability of the electrode.
The present invention is described in greater
detail by reference to the following examples which are in
no way intended to limit the present invention. Unless
- 15 -
1otherwise indicated herein, all parts, percents, ratios
and the like are by weight.
EXAMPLE 1
A commercially available Ti plate having a
5thickness of 1.5 mm and a size of 50 mm x 50 mm was
degreased with acetone. Thereafter, the plate was
subjected to àn etching treatment using a 20~ aqueous
hydroc~loric acid solution maintained at 105C. The thus
treated Ti plate was used as an electrode substrate.
A mixture of 10% hydrochloric acid mixed
solution of cobalt chloride, containing 10 g/l of Co,
titanium chloride containing 10.4 g/l of Ti and a 10~
hydrochloric acid solution of chloroplatinic acid
containing 10 9/l of Pt, was coated on the Ti plate
15electrode substrate and dried. Thereafter, the plate was
heated for 10 minutes in a muffle furnace maintained at
500C. This procedure was repeated four times to form an
intermediate layer of a TiO2-Co2O3 mixed oxide ~molar
ratio of Ti to Co = 80:20) containing o 5 g~m2 of Pt
dispersed therein on the Ti ~ubs~a~e.
A butanol solution of iridium chloride
containing 50 g/l of Ir was coated on the above-formed
intermediate layer and heated for 10 minutes in a muffle
furnace maintained at 520C. .This procedure was repeated
25three times to produce an electrode with Ir oxide,
~ , .
`
- 16 -
~ S~ ~ 2
1 containing 3.0 y/m of Ir, as an electrode active
substance.
With the thus-produced eiectrode as an anode and
a graphite plate as a cathode, accelerated electrolytic
testing was performed in a 150 9/1 sulfuric acid
electrolyte at 60C, and at a current density of
100 A/dm2. The results demonstrated that this electrode
could be used in a stable manner for 420 hours.
For comparison, an electrode was produced in the
same manner as above except that the intermediate layer
did not contain Pt. This electrode was also tested in the
same manner as above. The results demonstrated that this
electrode was passivated in 280 hours and could no longer
be used.
- EXAMPLE 2
Electrodes were prepared by following the
procedure of Example 1, except that the substance for the
intermediate layer and that for the active coat of
electrode were varied as indicated in Table 1 below. The
thus prepared electrodes were subjected to accelerated
electroLysis testing for performance. The electrolysis
was conducted in an aqueous 150 g/liter sulfuric acid
solution as the electrolyte at a temperature of 80Ct and
at a current density of 250 A/dm2, with a platinum plate
as the cathode. The results obtained are shown in Table 1
below.
.J
- 17 -
~2Si9~5;~
1 Table 1
Intermediate Electrode Active Service
Run No. SubstateLayer Substance Life
(hours)
1 Ti Pt-TiO2-Al2o3 IrO2 75
(75:25)
2 TiPt-Tio2o3-Fe2o3 IrO2 80
(80:20)
3 Ti Pt-Tio2-co2o3- IrO2 80
SnO2
- (40:50:10)
4 Ti Pt-TiO2-Al2o3- RuO2-IrO2 45
Ga23 (50:50)
(80:10:10)
Ti Pt-Tio2-Tl2o3 RUo2-Iro2 38
(70:30) (50:50)
6 Ti Pt-Tio2-Al2o3- RuO2-IrO2 55
Fe23 (30:70)
(30:40:30)
7 Ti TiO2-Al2O3 RUo2-Iro2 10
(comparison) (80:20) (50:50)
Note: The numerical values given in parentheses
represent mole ratios of component metals
excluding Pt. The amount of Pt in the
intermediate layer was 0.5 g/m2 for each
electrode. The amount of the electrode
active substance was invariably 3 g/m2
as metal component.
From the results in Table 1, it can be seen that
the electrodes of this invention incorporating a Pt-
containing intermediate layer had a decisively longer
service life and exhibited higher durability than the
electrode (comparison) incorporating a conventional layer
which did not contain any Pt.
- 18 -
3~59~
1 EXAMPLE 3
An electrode was prepared by following the
procedure of Example 1, except that a mixed oxide of SnO2-
NiO having Pt dispersed therein (Sn 80:Ni 20 by metal mole
ratio, with Pt dispersed at a ratio of 1.3 g/m2) was used
as the intermedite layer and similar testing was
conducted. The electrolysis testing was carried out in an
aqueous 12N NaOH solution at a temperature of 95C and at
a current density of 250 A/dm with a platinum plate used
as the cathode.
This electrode had a service life of 38 hours.
Another electrode was prepared for comparison by repeating
the same procedure, except that the Pt was omitted from
the intermediate layer. This electrode for comparison had
a service life of 22 hours. Thus, the electrode of this
invention was demonstrated to have very high durability as
compared with the other electrode.
While the present invention has been described
in detail and with reference to specific embodiments
thereof, it will be apparent to one skilled in the art
that various changes and modifications can be made therein
without departing from the spirit and scope thereof.
