Note: Descriptions are shown in the official language in which they were submitted.
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DUAL-LAYERED ELECTRODE FOR ELECTROLYSIS
The present invention relates to an electrode, a. method of preparing the
electrode, and the use thereof.
Background of the invention
Electrodes coated with titanium oxide, iridium oxide, and ruthenium oxide are
commercially used today in electrolytic cells. Such electrodes can be prepared
in
accordance with EP 715 002 61 disclosing a method wherein an anhydrous solvent
comprising precursors of mixed metal oxides is deposited on a substrate to
form an
electrocatalytic oxide coating. However, electrodes produced by this method
have a fairly
low activity resulting in ohmic losses and high cell voltages in electrolytic
cells, which
leads to increased electric energy consumption. A further problem of
conventional
electrodes is their relatively short service life. The present invention
intends to solve
these problems.
The invention
The present invention relates. to a method of preparing an electrode
comprising
providing an, electrode substrate, depositing on said electrode substrate a
first
substantially aqueous coating solution comprising precursors of a valve metal
oxide and
of at least two platinum group metal oxides, treating the first coating
solution to provide a
first metal oxide coating layer on the electrode substrate, depositing on said
first coating
layer a second substantially organic coating solution comprising precursors of
a valve
metal oxide and of at least one platinum group metal oxide, wherein at least
one of the
precursors is in organic form, treating said second coating solution to
provide a second
metal oxide coating layer on the first coating layer.
The electrode substrate can be any, valve metal or valve metal surfaced
substrate such as titanium, tantalum, zirconium, niobium, tungsten, silicon or
alloys
thereof, preferably titanium. Valve metals are known as film-forming metals
having the
property, when connected as an electrode in the electrolyte in which the
coated electrode
is expected to operate, of rapidly forming a passivating oxide film which
protects the
underlying metal from corrosion by the electrolyte. The substrate can have any
suitable
shape such as a rod, tube, woven or knitted wire, perforated or non-perforated
plate,
louver, or mesh, e.g. an expanded mesh. Titanium or other valve metal clad on
a
conducting metal core or substrate can also be' used. Preferably, the
electrode substrate
is perforated or has the shape of a mesh having openings with a diameter from
about I to
about 10, more preferably from about 2 to about 5 mm. Preferably, the
electrode
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substrate is roughened using chemicals means such as etching, e.g. acid
etching, or
mechanical such as blasting, e.g. sand blasting, grit blasting by means of
e.g. aluminium
oxide grits. It is preferred that the substrate surface has a roughness Ra
from about 2 to
about 12, more preferably from about 3 to about 6, and most preferably from
about 4 to
about 5 m as measured using the SURFTEST 212 surface roughness tester
(Mitutoyo,
Japan). After -the surface of the substrate has been roughened, it may be
subjected to
thermal oxidation by heating the substrate surface at an elevated temperature
in an
oxygen containing atmosphere for about 1 to about 3 hours. The temperature of
such
treatment is preferably from about 350 to about 600, more preferably from
about 400 to
about 500 C.
The precursors of the platinum group metal oxides dissolved in the first
coating
solution preferably comprise at least two water-soluble compounds of platinum,
iridium,
palladium, rhodium, osmium, and ruthenium, more preferably ruthenium and at
least one
of iridium, palladium, platinum, rhodium, and osmium, and most preferably
ruthenium and
iridium. Suitable precursors include .e:g. RuC13i HzRuCIs, IrCl3, and mixtures
thereof.
Preferably, the precursors are soluble also in acidified aqueous solutions.
Suitable valve
metal oxide precursors include - water-soluble compounds of aluminium,
zirconium,
bismuth, tungsten, niobium, titanium, silicon and tantalum, preferably
titanium, e.g. TiC14.
Preferably, the first coating solution is acidified, suitably by hydrochloric
acid
and/or other mineral acids to a pH of from about 0 to about 5, more preferably
from about
0 to about 2.
Suitably, at least about 70, preferably at least about 90, and most preferably
at
least about 95 volume percent of the solvent in the substantially aqueous
coating solution
is comprised of water.
The first coating solution is suitably deposited on the substrate by applying
the
solution on the electrode substrate, preferably until the total loading of the
first layer is
from about 0.5 to about 10, more preferably from about 1 to about 6, and most
preferably
from about 1.5 to about 3 g metal /m2. The process of depositing the coating
solution can
be repeated in order to obtain a thicker layer having the desired metal oxide
content. It is
desirable to let the coating air dry after each repetition at a temperature
from about 20 to
about 70, preferably from about 20 to about 50 C. The drying can take from
about 10 to
about 20 minutes. The coating solution can then be heat treated at a
temperature from
about 300 to about 600, preferably from about 450 to about 550 C for suitably
about 10
to about 30, minutes in order to convert the precursors to their corresponding
metal
oxides.
Suitable platinum group oxide precursors of the second coating solution
include
organic compounds, such as organic salts and acids of ruthenium, osmium,
rhodium,
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iridium, palladium, and platinum, and mixtures thereof, preferably ruthenium
and
optionally at least one of iridium, palladium, rhodium, and osmium, and most
preferably
ruthenium and iridium. Suitable valve metal oxide precursors can include e.g.
organic
compounds such as organic salts and acids thereof include water-soluble
compounds of
aluminium, zirconium, bismuth, tungsten, niobium, titanium, silicon and
tantalum,
preferably titanium. However, it is sufficient that at least one of the
precursor compounds
is present in its organic form, i.e. includes organic compounds such as
organic metal salts
or acids such as e.g. titanium alcoxide, tetrabuthyl titanate, and/or
tetrapentyl titanate.
It has been found that coating solutions for providing the second or outermost
coating layer containing at least one precursor in organic form in a
substantially organic
coating solution results in an electrode having increased activity when
deposited on the
first coating layer.
Suitably, at least about 70, preferably at least about 90, and most preferably
at
least about 95 volume percent of the solvent in the substantially organic
coating solution
is comprised of organic solvent. ' -
Preferred organic solvents of the second coating. solution include alcohols,
preferably lower alcohols, more preferably acidified anhydrous, lower alkyl
alcohols
having from about 3 to about 5 carbons atoms, such as 1-buthanol, 1-propanol,
2-
propanol, 1-pentanol and 2-pentanol and 3-methyl-2-butanol. The second coating
solutions preferably include a concentrated acid, such as a mineral acid, e.g.
hydrochloric
acid adjusting the pH to from about -1 to about 5, preferably from about -1 to
about 2.
The second coating solution is suitably applied to the obtained first coating
layer
until the total metal loading of the second layer is from about 1 to about 10,
preferably
from about 1.5 to about 3.5 g metaUm2: The deposition process can be repeated
in order
to obtain a thicker second coating layer or a further coating layer on the
second coating
layer. In industrial use, the loading of the second coating solution is
preferably from about
I to about 10, most preferably from about 1.5 to about 3.5 g metal/m2.
Preferably, the
second coating solution is air dried and heat treated in the same way as the
first coating
solution so as to form the second coating layer.
According to one preferred embodiment, precursors of the two platinum metal
oxides are dissolved in the first coating, solution in a mola ratio of about
1:2 to about 2:1,
preferably from about 2:3 to about 3:2. According to one preferred embodiment,
at least
two precursors of platinum metal oxides are dissolved in the second coating
solution in
the same mole ratio as in the first coating solution. According to one
preferred
embodiment, precursors of the platinum and valve metal oxides are dissolved in
the
coating solutions in a mole ratio of valve metal to platinum metal(s) of about
1:2 to about
2:1, preferably from about 4:5 to about 1:1.
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According to one preferred embodiment, precursors of iridium and ruthenium
oxides are dissolved in at Ieast one of the first and/or the second coating
solutions in a
mole ratio of about 1:2 to about 2:1, preferably from about 2:3 to about 3:2.
According to
one preferred embodiment, precursors of titanium, iridium and/or ruthenium are
dissolved
in the coating solutions in a mole ratio of titariium to iridium and ruthenium
of about 1:2 to
about 2:1, preferably from about 4:5 to about 1:1.
Each coating solution is suitably deposited by immersion of the electrode
substrate in the coating solution or by means of other suitable methods such
as spraying,
e.g. electrostatic spraying, rolling or brush painting. Even though a process
providing two
layers (with the defined coatings) is preferred, further layers may also be
adhered.
The invention also relates to an electrode obtainable by the method as
disclosed
herein.
The invention also relates to an electrode comprising an electrode substrate,
a,
first coating layer having a charge/projected area from about 10 to about 200,
preferably
from about 25 to about 200, and most preferably from about 25 to about 190
mC/cm2
(mCo.ulomb/cm2), said first coating layer comprising a valve metal oxide and
at least two
platinum group metal oxides.deposited on said electrode substrate, and a
second coating
layer having a charge/projected area from about 210 to about 1000, more
preferably from
about 250 to about 1000, and most preferably from about 300 to about 800
mC/cm2, said
second layer comprising a valve metal oxide and at least one platinum group
metal oxide
deposited on the first coating layer.
The charge/projected area was measured by an electro-double layer
measurement with cyclic voltammograms in sulphuric acid. The measuring
condition of
the cyclic voltammograms was 5OmV/second at a sweep rate in the range of 0.3
to 1.1V
(vs. RHE (Reversible Hydrogen Electrode)) in 0.5M sulphuric acid. The measured
values
in mC/cm2 are proportional to the active specific surface area of the
electrodes. More
information about this method can be found in L.D. Burke et al, Electroanal.
Chem.
96(1976) 19-27 and R.F. Savinell et al, J. Electrochem. Soc. 137(1990) 489-
494.
It has been found that an electrode according to the invention shows a
superior
activity while providing higher stability and longer service life in view of
existing
electrodes.
Preferably, the electrode substrate is as described herein. Particularly, the
electrode substrate is suitably perforated or has the shape of a mesh having
openings
with a diameter from about 1 to about 10, more preferably from about 2 to
about 5 mm. It
has been found that the electrodes with openings within the defined ranges
when
immersed in an operated cell produce small bubbles of evolved gas, which in
turn results
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in an increased homogeneous current distribution and lower ohmic loss,
particularly in a
membrane cell.
The coating layers of the electrode may comprise platinu.m group metal oxides,
such as oxides of iridium, palladium, rhodium, osmium, and ruthenium,
preferably oxides
5 of ruthenium and at least one of 'iridium, rhodium, osmium, more preferably
oxides of
ruthenium and iridium. The coating layers also comprise at least one valve
metal oxide
such as an oxide of titanium, tantalum, zirconium, niobium, tungsten, and
silicon,
preferably titanium.
Preferably, the roughness Ra of the electrode is from about 2 to about 12,
more
preferably from about 3 to about 6, and most preferably from about 4 to about
5}Lm.
The metal oxide layers preferably contain from about 40 to about 70 mole
percent ,counted as valve metal, preferably as tantalum and/or titanium, from
about 20 to
about 30 mole percent of ruthenium oxide counted as ruthenium, and from about
10 to
about 30 mole percent of another platinum group metal oxide counted as metal.
The
oxide coating on the electrode substrate is also effective in increasing the
service life of
the electrode by retarding the corrosion of the platinum group metals.
Even though a process providing two layers (with the defined coatings) is
preferred,
further layers optionally with same or similar chemical composition may also
be adhered.
The invention also relates to the use of the electrode in an electrolytic
cell.
Preferably, the electrode is used as an anode, preferably as a dimensionally
stable
anode, particularly in an ion membrane cell for the production of e.g. alkali
metal
hydroxide, particularly sodium hyd'roxide.
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. While the
examples here below provide more specific details of the reactions, the
following general
principles may here be disclosed. The following example will further
illustrate how the
described invention may be performed without limiting the scope of it.
All parts and percentages refer to part and percent by weight, if not
otherwise
stated.
Examgle I
A titanium expanded mesh having a thickness of 1 mm and length and width of
80 and 24 mm respectively was used as electrode substrate after having been
degreased
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and pickled in boiling hydrochloric acid. A first coating solution was
deposited on the
substrate having a molar ratio of Ti:Ru:lr of 2:1:1, in which the total lr+Ru
concentration
was 50 g/I. The solution was prepared by dissolving ruthenium trichloride,
iridium
trichloride, and titanium tetrachloride in a hydrochloric acid~ based
solution. The solution
was then dried at 60 C followed by thermal decomposition at 460 C for 10
minutes. This
deposition step was repeated three times. A second coating solution was then
prepared
by mixing hexachloro ruthenic acid and hexachloro iridic acid into a titanium
solution
comprising tetrabuthyl ortho titanate in n-propyl alcohol. 10 volume percent
of HCI was
added to the alcohol solution. The molar ratio of Ti:Ru:lr was 2:1:1. The
total lr+Ru
concentration was 30 g/l. The deposition and thermal decomposition of the
second
coating solution on the substrate was made in the same way as the first
coating solution.
The obtained electrode sample was then stabilised at 520 C for 60 minutes.
The chlorine
evolution potential at 90 C in a 300 g/1 NaCl solution was tested at pH 2 for
the electrode
(used as anode) and for a comparative electrode produced in the same way as
the first
coating layer but with six repetitions instead of three. The current density
was 40 A/dm2.
Table 1 below shows the difference between the two electrodes. An accelerated
life test
was also performed in a Na2SO4*10H20 250 g/1 electrolyte at 60 C at a pH of
2. The
current density was 50A/dm2. The electrodouble layer measurement by cyclic
voltammograms was performed in 0.5M sulphuric acid. Measuring conditions were
0.3 to
1.1V vs. RHE at a sweep rate of 50mV/second.
Table 1
Sample CI2 evolution Accelerated life Cyclic voltammograms
potential (V vs NHE) (hours at 50A/dm2) the second (top) coatin
layer (mC/cm2)
Electrode according 1.36 285 410
to invention
Comparative 1.38 195 190
electrode
As can be seen from table 1, a lower C12 evolution potential is obtained for
the electrode
according to the invention, which means lower cell voltage, and lower electric
energy
consumption. As can be further seen, the service life of the electrode of the
invention is
far better than the comparative electrode. The charge/projected area of the
electrode of
the invention was shown to be far larger than the comparative electrode, which
results in
increased service life and lower CI2 evolution (higher activity).
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Example 2
A second coating solution was prepared -by mixing ruthenium chloride-into a
titanium
solution comprising tetrabuthyl ortho titanate in n-butyl alcohol. 10 volume
percent of HCI
was added to the alcohol solution. The molar ratio of Ti:Ru was 2:1. The total
Ru
concentration was 40 g/I. An electrode with a first oxide layer prepared
according to
Example I was then coated with this second coating solution. The deposition
and thermal
decomposition was made in the same way as in Example 1. Chlorine potential and
electrodouble layer measurements, according to Example 1, were then performed
on the
obtained electrode. Table 2 below shows the results of these measurements.
Table 2
Sample CI2 evolution Cyclic voltammograms of
potential (V vs NHE) the second (top) coating
layer (mC/cm2)
Electrode according 1.35 570
to the invention
As can be seen from a comparison of tables I and 2, a substantially lower CI2
evolution
potential is obtained for the electrode according to the invention with only
one piatinum
group metal oxide in the second layer, which again means lower energy
consumption.
The charge/projected area of the obtained electrode is also substantially
higher than the
comparative electrode.
