Note: Descriptions are shown in the official language in which they were submitted.
CA 02529190 2008-11-10
1
Electrode with Electroconductive Titanium Oxide and Process for Manufacturing
Same
The present invention relates to an electrode, a process of manufacturing the
electrode, and the use thereof.
Background of the invention
Electrodes for use in industrial electrolysis, water electrolysis, and other
electrolytic processes such as a platinum group metal oxide coated electrode
usually
have a low electric resistance at high currents. However, such electrodes
usually have a
short durability.
US 4,568,568 discloses a method of plasma spray coating particles on an
electrode substrate involving heating the particles at -temperatures up to
6000 C, which
then collide with the substrate at a high speed, whereby the particles
partially melt and
produce a layer of even thickness on the substrate. The particles do not
Impart an
increased surface area to the obtained electrode.
- The present invention intends to solve the drawbacks of the prior art and to
provide a particle coated electrode having increased specific surface area,
stability and
performance, which finds a great number of applications. The Invention also
intends to
provide a convenient and reliable process of adhering particles to an
electrode in a cost-
effective way. A further Intention of the invention is to provide a process
which enables
adhering particles to an electrode without deforming the shape of the
particles.
The invention
The present Invention relates to a process for manufacturing an electrode
comprising depositing on an electrode substrate a binder dispersion comprising
a
precursor of a conductive or semiconductive oxide, forming a conductive or
semiconductive oxide coating from the precursor on the electrode substrate,
depositing
an electroconductive titanium oxide and electrode particles on the conductive
or
semiconductive oxide coating, adhering the electroconductive titanium oxide
and the
electrode particles to the formed conductive or semiconductive oxide coating.
By the term "dispersion" as used herein is comprised besides ordinary
dispersions, suspensions and slurries of particles, also solutions of e.g.
oxide forming
precursors.
According to one embodiment, the conductive or semiconductive oxide is
adhered by decomposing the precursor, preferably by thermally decomposing it.
However, the precursor can also be precipitated resulting in the formation of
an oxide
from the original precursor which may be e.g..a hydroxide or hydrated oxide of
titanium or
other suitable metal.
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The material of the electrode substrate may be of any conductive element which
can retain its physical integrity during the manufacturing and its subsequent
use in e.g. an
electrolytic cell and which preferably can resist alkaline and acidic
electrolytes. Suitable
electrode substrate materials include electrically conductive metals such as
copper,
nickel, valve metals such as titanium, tantalum, zirconium or niobium, and
alloys or
mixtures thereof, preferably titanium or alloys thereof.
The configuration of the electrode substrate used is not critical. A suitable
electrode substrate may, for example, take the form of a flat sheet or plate,
a curved
surface, a convoluted surface, a punched plate, a woven wire screen, an
expanded mesh
sheet, a rod, or a tube. However, the electrode substrate preferably has a
planar shape,
most preferably in the form of a sheet, mesh or plate.
The electrode substrate can be roughened by means of sand blasting, grit
blasting, chemical etching and the like. The use of chemical etchants is well
known and
such etchants include most strong inorganic acids, such as hydrochloric acid,
sulphuric
acid, nitric acid and phosphoric acid, but also organic acids such as oxalic
acid.
The precursor of the conducting or semiconducting oxide, which can be in the.
form of a dissolved salt or acid, can be dissolved in an acidic aqueous or
organic
dispersion or mixtures thereof. Preferred organic dispersions include alcohols
such as
iso-propanol, n-propanol, or butanol, or mixtures thereof. Organic salts or
acids are
preferably dissolved in an organic solvent, most preferably in an alcohol as
described
herein, whereas inorganic salts and acids preferably are dissolved in a
substantially
aqueous dispersion.
Preferably, the organic and/or aqueous binder dispersions have a pH from about
0.5 to about 4, most preferably from about 0.5 to about 2. Preferably, the
binder
dispersion has a metal concentration from about 10 to about 200, most
preferably from
about 20 to about 30 g metal /I.
The precursor may be any suitable organic and/or inorganic salt or acid.
Preferably, the precursor is a mixture of at least two organic and/or
inorganic salts or
acids of titanium, tantalum, tin, antimony, indium and tin salts, preferably
of titanium and
tantalum. Preferably, buthyl or ethyl titanate and buthyl or ethyl tantalite
are employed in
combination. According to one embodiment, buthyl titanate and buthyl tantalite
are
employed in combination. The molar ratio of titanium to tantalum suitably is
from about
9:1 to about 7:3, preferably from about 9:1 to about 8:2. Precursors of
organic salts
and/or acids are preferred, since their corresponding conductive or
semiconductive
oxides can be formed at a lower temperature. This is preferred because a low
heating
temperature renders the electroconductive titanium oxide particles less
oxidised resulting
in higher electroconductivity.
CA 02529190 2011-01-21
2a
In accordance with one aspect of the present invention, there is provided a
process for manufacturing an electrode comprising electroconductive titanium
oxide and
electrode particles having a particle size from 0.5 to 100 micrometers, said
process
comprising depositing on an electrode substrate a binder dispersion comprising
a
precursor of a conductive or semiconductive oxide, forming a conductive or
semiconductive oxide coating from said precursor on the electrode substrate,
depositing
an electroconductive titanium oxide and electrode particles on the conductive
or
semiconductive oxide coating, adhering the electroconductive titanium oxide
and the
electrode particles having a particle size from 0.5 to 100 micrometer to the
formed
conductive or semiconductive oxide.
In accordance with another aspect of the present invention, there is provided
an
electrode comprising an electrode substrate, a conductive or semiconductive
oxide
coating adhered to said electrode substrate, and electrode particles having a
particle
size from 0.5 to 100 micrometer and electroconductive titanium oxide adhered
to said
conductive or semiconductive oxide coating.
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According to one embodiment, electroconductive titanium oxide is suspended in
the binder dispersion. As a result, a conductive or semiconductive oxide
coating binding
an evenly dispersed electroconductive titanium oxide will be formed on the
electrode
substrate. This may be advantageous to better adhere subsequently deposited
electrode
particles, because the electroconductive titanium oxide particles, which
preferably are
smaller than the electrode particles, surround the electrode particles and
thus impart
better adhesion between the electrode substrate, the electroconductive
titanium oxide
particles and the electrode particles, due to an increased contact area.
According to one embodiment, the precursor is thermally decomposed at a
temperature from about 300 to about 600, more preferably from about 450 to
about 500
C. However, if the precursor is a colloidal solution, e.g. a slightly alkaline
alcohol solution
of alcoxy-titanium and tantalum in ammonia, the decomposition can be carried
out at a
temperature from about 300 to about 450 C. This lower temperature is possible
probably
due to the fact that colloidal solutions such as colloidal hydroxide or
hydrated oxides
solutions can be transformed to oxides by means of dehydration.
According to one embodiment, electroconductive titanium oxide and electrode
particles suspended in an aqueous or organic dispersion, preferably an aqueous
dispersion, are deposited on the formed conductive or semiconductive oxide
coating.
According to one embodiment, electroconductive titanium oxide and electrode
particles are suspended in the binder dispersion resulting in adhesion of
electrode
particles to the oxide coating formed from the precursor.
In order to get a thicker conductive or semiconductive oxide coating, the
deposition procedure can be repeated, preferably at least 2 times, most
preferably at
least 4 times. Preferably, the thickness of the oxide is from about 2 to about
4 m.
According to one embodiment, the electroconductive titanium oxide has a
particle size from about 0.1 to about 100, more preferably from about I to
about 20, even
more preferably from about 5 to about 20 m, and most preferably from about 5
to about
10 m.
The electroconductive titanium oxide preferably is substantially in magneli
phase
(including various oxides such as Ti407 and Ti5O9) and/or TO depending on
where the
electrode to be manufactured will be used.
Magneli phase titanium oxide is preferably used for manufacturing electrodes
for
use in strongly acidic electrolytes such as sulphuric or nitric acid, due to
its capability of
resisting corrosive environments. TiO is preferably used in electrodes for use
in
electrolytes with a pH above about 1.5.
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Electroconductive titanium oxide can be prepared from conventional sintering
mixtures of nonconductive titanium oxide (Ti02) in commercially available
rutile or
anatase phase and titanium metal at a temperature of 1000 to 1500 C in
vacuum.
Electroconductive titanium oxide may also be prepared by mixing pulverised
TiO 2 in rutile phase and agate mortar followed by sintering. The obtained
electroconductive titanium oxide powder contains a mixture of Ti3O5, Ti407
and/or Ti509.
The term "electrode particles" as used herein means are electroconductive and
have a catalytic activity. The material may be diamond, e.g. boron doped
diamond,
titanium oxide such as titanium oxide in magneli phase (EbonexTM), tin
dioxide, magnetite
(Fe304), Ni-ferrite, R-lead dioxide (a-Pb02), BN, WC, SiC, and/or mixtures
thereof,
preferably diamond. Suitably, the electrode particles have a size from about
0.5 to about
100, preferably from about 1 to about 20, and most preferably from about 5 to
about 10
Diamond particles may be obtained from conventional diamond synthetic
processes at high temperature and high pressure.
According to one preferred embodiment, two different layers are applied on the
conductive or semiconductive oxide coating to provide an under layer suitably
comprising
electroconductive titanium oxide and a top layer of electrode particles to
increase the
stability of the electrode and more firmly adhere the electrode particles.
According to a preferred embodiment, a roughened, blasted and pickled
electrode substrate is painted with a binder dispersion comprising a precursor
of a
semiconducting oxide of a titanium oxide which is subsequently decomposed at a
temperature of from about 500 to about 600 C to form a conductive oxide
before
depositing a slurry of electroconductive titanium oxide having a titanium
content of about
3 to about 20 times of the metal content of the binder dispersion followed by
thermal
treatment at 400 to 500 C for 10 min. Subsequently, in a second step, a
dispersion
comprising about 50 wt% electrode particles and about 50 wt% electroconductive
titanium
oxide is deposited on the oxide coating and thermally treated to adhere the
electroconductive titanium oxide and the electrode particles to the formed
titanium oxide
coating. According to one embodiment, the second step is repeated at least 2
times,
preferably at least 3 times.
The obtained electrode can be further stabilised in vacuum or inert
atmosphere,
e.g. in argon gas at a temperature from about 500 to about 600 C.
The invention further relates to an electrode obtainable from the process as
described herein.
The invention further relates to an electrode comprising an electrode
substrate, a
conductive or semiconductive oxide adhered to the electrode substrate, and
electrode
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particles and electroconductive titanium oxide adhered to the conductive or
semiconductive oxide coating. The electrode substrate, the conductive or
semiconductive
oxide, the electroconductive titanium oxide, and the electrode particles
preferably are as
described herein.
5 According to one embodiment of the invention, the conductive or
semiconductive
oxide may contain several oxide layers, preferably two oxide layers.
According to one embodiment, a first layer of oxide coating comprises
electroconductive titanium oxide and electrode particles. The oxide coating of
the first
layer may contain from about 10 to about 70, preferably from about 40 to about
60 wt%
electrode particles. The first layer may contain from about 20 to about 80,
preferably from
about 30 to about 60 wt% electroconductive titanium oxide. Preferably, a
second layer
suitably comprises from about 30 to about 80, preferably from about 50 to
about 70 w%
electrode particles. Preferably, the remaining part of the second layer is
covered with
electroconductive titanium oxide. According to one embodiment, the content of
electroconductive titanium oxide is from about 20 to about 70, preferably from
about 30 to
about 50 wt% based on the weight of the oxide coating. Preferably, the
deposition of
electrode particles is from about 10 to about 500, more preferably from about
50 to about
100 g/m2 electrode substrate area. Preferably, the deposition of
electroconductive
titanium oxide is from about 5 to about 200, more preferably from about 10 to
about 100
g/m2 electrode substrate area.
It has been found that the obtained electrodes can remain stable even in
corrosive atmosphere under high potentials of more than 2V vs NHE and high
currents.
This may be due to the fact that the oxide formed from the binder dispersion
adheres
particles of electroconductive titanium oxide, which in turn, possibly in
combination with
the oxide coating formed from the binder solution, adhere the electrode
particles.
According to one embodiment, the electrode has a second layer comprising
electroconductive electrode particles of diamond, tin dioxide, magnetite
(Fe304), nickel
ferrite, R-lead dioxide, titanium oxide, BN, WC, SiC, Si3N4 or mixtures
thereof, preferably
of titanium oxide and/or diamond, and most preferably diamond.
The electrode can take any shape. However, a planar electrode will be
preferred
for most applications. Preferably, the electrode does not comprise bi-metal
spinel in any
of its layers. Preferably, the electrode does not comprise any platinum group
metals or
oxides thereof since this may lead to passivity problems.
The invention also relates to the use of an electrode in an electrolytic cell,
for
electrolytic processes in water treatment, secondary battery, such as in redox
flow cells,
and electrolytic ozone generation.
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Particularly, electrodes provided with electrode particles of boron doped
diamond
can be used as anodes for generation of oxygen, ozone, hydrogen peroxide,
hydroxyl
radicals; in water electrolysis, water treatment, and electroorganic synthesis
due to its
good electric conductivity as p-type semiconductor. As a cathode, the
electrode is
preferably used for electroorganic synthesis, formation of OH radicals,
various oxidation
processes, redox flow battery for power storage, and normalization of power
consumption.
The invention being thus described, it will be obvious that the same may be
varied in many ways. Such variations are not to be regarded as a departure
from the gist
and scope of the present invention, and all such modifications as would be
obvious to
one skilled in the art are intended to be included within the scope of the
claims. The
following examples will further illustrate how the described invention may be
performed
without limiting the scope of it.
Example 1
A titanium plate with a thickness of 1 mm was grit-blasted to a surface
roughness of Ra=5 gm, and pickled with sulphuric acid in order to prepare an
electrode
substrate. A binder solution comprising TiCl4 and TaCl5i dissolved in a 10 wt%
HCI
solution, was applied on the electrode substrate and heated at 540 C for 10
min. The
coating and heating steps were repeated 4 times resulting in an oxide film of
0.2 m on
the electrode substrate of tantalum and titanium oxides in a molar ratio of Ta
to Ti of 1 to
9. A slurry was prepared by suspending an electroconductive titanium oxide
powder in a
HCI solution of penta-butyl tantalite and tetra-butyl titanate with a molar
ratio of Ti to Ta of
8 to 2. The weight ratio of electroconductive titanium oxide to the total Ti
and Ta metal
content in the binder dispersion was 20:1. The dispersion was stirred and
painted on the
oxide film. After drying, the electrode was first heated at 60 C for 10 min,
then heated at
450 C for 10 min. A porous oxide coating of 60 g/m2 was obtained having a
specific
surface area of I0m2/m2 projected substrate area. On the porous oxide coating
a slurry
prepared from 50 wt% electroconductive titanium oxide and 50 wt% boron doped
electroconductive diamond powder with an average particle size of 7 to 10 m
was
applied. The slurry was subsequently dried and heated at 450 C for 10 min.
The
deposition of the slurry was repeated once followed by the same heat
treatment. The
obtained electrode showed to work well in a continuous electrolysis process at
a current
density of 1A/dm2.
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Example 2
An electrode was prepared by depositing electroconductive titanium oxide on
the
titanium electrode substrate prepared in the same way as in example 1. An
electroconductive titanium oxide powder was suspended in a binder dispersion
containing
titanium trichloride and penta-butyl tantalite having a molar ratio of
titanium to tantalum of
9 to 1. The weight ratio of electroconductive titanium oxide to the total Ti
and Ta metal
content in the binder dispersion was 20 to 1. The binder dispersion was
applied on the
electrode substrate which was subsequently dried in air at room temperature
followed by
drying at 60 C and heat treatment at 500 C. The application of the binder
solution was
repeated three times. An electroconductive titanium oxide layer (substantially
as Ti407)
was formed under the same conditions as in example 1, in which the coating
amount was
60 g/m2 substrate area. Then, an electroconductive titanium oxide layer was
formed from
magneli phase titanium oxide particles having a size of 5 to 10 m, which were
suspended in a slurry, and then coated and heat treated at 450 C for 10
minutes as in
example 1. This procedure was repeated three times resulting in a total
deposition of 50 g
titanium metal /m2 substrate area. The electric conductivity of the electrode
was
somewhat higher than the electrode of example 1 due to the electrode
materials. The
active surface area was increased to 20 m2/m2 electrode substrate area. Then,
continuous electrolysis was performed at a current density of 2 A/dm2.
Example 3
An electrode according to example 2 was prepared, except for the electrode
particles which were of tin oxide and antimony oxide in rutile phase in a
molar ratio of tin
to antimony of 9:1. The electrode was tested in sulphuric acid electrolyte
containing 100
ppm phenol and showed to work since decomposition of phenol could be observed.
Example 4
An electrode was prepared in accordance with example 1 except for the
diamond particles which were replaced by TiO particles. Continuous
electrolysis was
performed in a H2SO4 solution at a current density of 3A/dm2.
Example 5
An electrode substrate was prepared as shown in example 1. The binder
dispersion was prepared by mixing acidic solution of tetra buthoxi-titanate
and penta
buthoxi tantalite in a molar ratio of 8 to 2 which then was neutralized with
ammonia. The
solution turned hazy white and colloidal precipitation was detected. Then,
butyl alcohol
was added to the hazy liquid containing hydrated titanium-tantalum co-oxide to
adjust the
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total metal content of the liquid to 15 g/I. The obtained liquid had a
viscosity of 10 to 20 c-
poise. Then, electroconductive titanium oxide was mixed into the dispersion
which
subsequently was applied with a brush to the electrode substrate. After
drying, the
substrate was heat treated at 300 C in air atmosphere resulting in a
deposition of 50 g
electroconductive titanium oxide/ m2 substrate area. Then 70 wt% of
electroconductive
titanium oxide and 30 wt% (3-PbO2 particles, whose average particle size was
10 to 12
gm, was applied onto the oxide coated substrate. The substrate was then dried
and heat
treated. Then, 20 g R-lead dioxide/m2 was deposited. The obtained electrode
had a
surface area of 8 m2/m2 electrode substrate, and could be used as anode in
continuous
electrolysis at a current density of 10A/dm2.
Example 6
A tin dioxide particle electrode was prepared by the same process as in
example
5, but where (3-lead dioxide was replaced by tin dioxide. The tin dioxide was
obtained by
co-precipitation of 90 mol% of tin tetra-chloride (SnC14) and antimony-penta-
chloride in
ethyl alcohol by neutralization with ammonia. About I mol% of iridium chloride
was then
added to the dispersion. Then, the dispersion was dried followed by heat
treatment at 400
C for 30 minutes in air. A black coloured electroconductive tin dioxide was
obtained.
Then, this tin dioxide was crushed and ground with agate mortar. The obtained
tin dioxide
powder was co-deposited with electroconductive titanium oxide on the electrode
substrate. The surface area of this electrode was 7 to 8 m2/m2 electrode
substrate. The
electrode was then used at a current density of 2 A/m2 and showed to work
well.
