Note: Descriptions are shown in the official language in which they were submitted.
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ELECTRODE FOR EVOLUTION OF GASEOUS PRODUCTS AND METHOD OF
MANUFACTURING THEREOF
FIELD OF THE INVENTION
The invention relates to an electrode suitable for functioning as anode in
electrolysis cells, for
instance as oxygen-evolving anode in electrolysis cells used in
electrometallurgical processes,
as chlorine-evolving anode either in chlor-alkali cells or as anode for
hypochlorite generation in
undivided cells.
BACKGROUND OF THE INVENTION
Substoichiometric compositions of titanium oxides of formula Tix02x-1, with x
ranging from 4 to
10, also known as titanium Magneli phases, are obtained by high temperature
reduction of
titanium dioxide under a hydrogen atmosphere. These suboxides are corrosion-
resistant
ceramic materials comparable to graphite in terms of electrical conductivity.
In light of such
corrosion resistance and conductivity characteristics these materials, which
are produced both
in massive and in powder form, may be used as protective coatings of metal
substrates for
electrochemical applications. There is also known the possibility of adding
small amounts of
doping agents to these ceramic materials, such as for instance tin oxide, in
order to slightly
increase their conductivity, stability and resistance to corrosion. In
general, the deposition of
these ceramic materials as metal substrate protectors is carried out starting
from the material in
powder form in accordance with known techniques, such as, hot flame spraying,
plasma
spraying or detonation thermal spraying. All of these techniques share the
common feature of
requiring a high operative temperature (>400 C) in order to obtain an
acceptable adhesion
between sprayed powder particles and metal substrate. Furthermore, the good
adhesion of
deposited powder particles to the substrate also depends on the reciprocal
nature of the
substrate and the powder.
The above mentioned spraying techniques allow depositing very compact layers
of ceramic
material on the surface of a metal substrate. Such compactness is in fact
required for an
efficient anticorrosion function. More precisely, it is generally accepted in
the art that the
apparent density of the deposited ceramic layer must not be lower than 95% of
the overall
theoretical density in order to obtain an efficient material.
These ceramic materials may also be used as catalyst supports. In the
manufacturing of an
electrode starting from a metal substrate, the catalyst is applied in a step
subsequent to the
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deposition of the titanium Magneli phase onto such substrate, generally by
thermal
decomposition of precursors. This mode of application, however, has the
drawback of leading to
the formation of ceramic layers wherein a major fraction of the catalyst
applied turns out to be
scarcely accessible to the electrolyte, the final product thus being hardly
efficient in terms of
activity and lifetime. Usually, in order to obtain electrodes of suitable
performances for an
industrial application, the loading of the Magneli phase-supported catalyst
must be not lower
than 20-30 g/m2.
Moreover, the use of the above mentioned powder deposition techniques on metal
substrates is
not advisable whenever such powders also comprise noble metal oxides as
catalysts, because
such oxides are not stable to temperatures above 400 C and tend to decompose,
thereby
hindering an appropriate deposition. The preparation of titanium suboxide and
noble metal
oxide mixtures to be subsequently deposited onto a substrate by means of the
above
mentioned techniques is hence not easy to practise.
The inventors surprisingly found out a method for manufacturing electrodes
comprising a valve
metal-based substrate coated with at least one layer of noble metals or oxides
thereof
supported on titanium suboxides overcoming the inconveniences of the prior
art.
SUMMARY OF THE INVENTION
Various aspects of the invention are set out in the accompanying claims.
Under one aspect, the invention relates to an electrode for evolution of
gaseous products in
electrolytic cells comprising a valve metal substrate whereto at least one
layer of a coating
having an interconnected porosity is attached, the layer consisting of at
least one catalyst
containing noble metals or oxides thereof taken alone or in admixture,
supported on titanium
suboxide species expressed by the formula Tix02x-1, with x ranging from 4 to
10, the specific
catalyst loading being comprised between 0.1 and 25 g/m2.
The term interconnected is used herein to mean a porosity mostly consisting of
a network of
pores in mutual fluid communication and not isolated. In order to obtain a
layer having an
interconnected porosity, it is normally considered that the apparent density
of such layer must
be lower than 95% of the overall theoretical density which a compact layer
with no porosity at all
having an equivalent composition would exhibit.
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Under another aspect, the invention relates to an electrode for evolution of
gaseous products in
electrolytic cells consisting of a valve metal substrate and at least a
coating layer having an
interconnected porosity bound thereto, said at least one layer comprising at
least one catalyst
consisting of noble metals or oxides thereof taken alone or in admixture,
supported on a mixture
of titanium suboxides of formula Tix02x-1, with x ranging from 4 to 10, said
at least one layer
being deposited onto said substrate by cold gas spray technique. The term cold
gas spray is
used herein to mean a deposition technique of solid particles onto substrates
supposedly known
to a person skilled in the art, based on accelerating powder particles
transported by a
compressed carrier gas. During their trajectory, the carrier gas and the
particles are split into
two different paths so that the time of residence of powders inside the hot
gas phase is limited,
thereby preventing powders to be heated above 200 C.
The inventors have surprisingly observed that the deposition via cold gas
spray technique of a
Magneli phase-type ceramic powder, for example consisting of a titanium
Magneli phase
powder previously catalysed with noble metal oxides by thermal decomposition
of precursors,
onto a substrate made of a valve metal such as titanium, tantalum, zirconium
or niobium, leads
to a structure of surprisingly enhanced duration even at very low catalyst
loadings. In particular,
the lifetime of an electrode obtained as hereinbefore described in common
industrial
electrochemical applications is much higher compared to the one of an
electrode having the
same nominal content of catalyst but prepared by traditional thermal
decomposition.
In one embodiment, the valve metal of choice for the substrate is titanium.
In one particular embodiment, the coating layer has an interconnected porosity
with an apparent
density ranging higher than 75% and lower than 95% of the overall theoretical
density.
In another embodiment, the electrode has a coating layer containing a specific
catalyst loading
of 0.1 to 10 g/m2.
In yet another embodiment, the noble metal oxide-based catalyst consists of
iridium oxide.
Under another aspect, the invention relates to a method for manufacturing an
electrode
according to the invention comprising the steps of: preparing a titanium
suboxide powder
expressed by the formula Tix02x-1, with x ranging between 4 and 10;
impregnating said powder
with a precursor solution of a noble metal oxide-based catalyst with
subsequent thermal
decomposition; depositing the obtained powder on a valve metal substrate by
cold gas spray
technique.
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Under yet another aspect, the invention relates to an electrolysis cell
comprising a cathodic
compartment containing a cathode and an anodic compartment containing an
anode, wherein
said anode of said anodic compartment is an electrode as hereinbefore
described.
Under yet another aspect the invention relates to an industrial
electrochemical process
comprising the anodic evolution of a gas from an electrolytic bath on an
electrode as
hereinbefore described.
The following examples are included to demonstrate particular embodiments of
the invention,
whose practicability has been largely verified in the claimed range of values.
It should be
appreciated by those of skill in the art that the compositions and techniques
disclosed in the
examples which follow represent compositions and techniques discovered by the
inventors to
function well in the practice of the invention; however, those of skill in the
art should, in light of
the present disclosure, appreciate that many changes can be made in the
specific embodiments
which are disclosed and still obtain a like or similar result without
departing from the scope of
the invention.
EXAMPLE 1
An appropriate volume of titanium Magneli phase powder in admixture with
iridium oxide was
sprayed onto a titanium grade 1 sheet of 10 cm x 10 cm x 0.2 cm size,
previously sandblasted
with corundum grit #36 and etched in boiling hydrochloric acid in order to
obtain a rough surface
free of titanium oxide species. Such powder was obtained by mixing a suitable
mass of titanium
Magneli phase powder¨ previously sieved to a size range of 100 to 400 pm ¨ to
an acidic
solution containing a soluble precursor of iridium, namely iridium trichloride
in aqueous HCI.
Such mixture was then calcined in oxidising atmosphere in a rotary oven.
The spraying parameters selected for cold gas spray technique application were
the following:
Nozzle-to-sheet gap: 20 mm
Primary gas: nitrogen
(Primary) gas pressure: 30 bar
Gas flow-rate: 6 m3/h
Feeder gas flow-rate: 4 %
Throat size: 1 mm
Scan rate: 50 mm/s
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As a final target of the cold gas spraying process, a homogeneous coating
containing 10 g/m2 of
iridium was obtained.
The thus obtained electrode was identified as sample #1.
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EXAMPLE 2
An appropriate volume of titanium Magneli phase powder in admixture with
ruthenium oxide
was sprayed onto a titanium grade 1 sheet of 10 cm x 10 cm x 0.2 cm size,
previously
sandblasted with corundum grit #36 and etched in boiling hydrochloric acid in
order to obtain a
rough surface free of titanium oxide species. Such powder was obtained by
mixing a suitable
mass of titanium Magneli phase powder¨ previously sieved to a size range of
100 to 400 pm ¨
to an acidic solution containing a soluble precursor of ruthenium, namely
ruthenium trichloride in
aqueous HCI. Such mixture was then calcined in oxidising atmosphere in a
rotary oven.
The spraying parameters selected for cold gas spray technique application were
the following:
Nozzle-to-sheet gap: 20 mm
Primary gas: nitrogen
(Primary) gas pressure: 30 bar
Gas flow-rate: 6 m3/h
Feeder gas flow-rate: 4 %
Throat size: 1 mm
Scan rate: 50 mm/s
As a final target of the cold gas spraying process, a homogeneous coating
containing 10 g/m2 of
ruthenium was obtained.
The thus obtained electrode was identified as sample #2.
COUNTEREXAMPLE 1
An appropriate volume of titanium Magneli phase powder in admixture with
iridium oxide was
plasma-sprayed onto a titanium grade 1 sheet of 10 cm x 10 cm x 0.2 cm size,
previously
sandblasted with corundum grit #36 and etched in boiling hydrochloric acid in
order to obtain a
rough surface free of titanium oxide species. Such powder was obtained by
mixing a suitable
mass of titanium Magneli phase powder¨ previously sieved to a size range of
100 to 400 pm ¨
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to an acidic solution containing a soluble precursor of iridium, namely
iridium trichloride in
aqueous HCI. Such mixture was then calcined in oxidising atmosphere in a
rotary oven.
The following spraying parameters were applied:
Nozzle-to-sheet gap: 90 mm
Primary gas: argon
(Primary) gas pressure: 60 bar
Throat size: 5 mm
Scan rate: 200 mm/s
As a final target of the plasma-spraying process, a homogeneous coating
containing 10 g/m2 of
iridium was obtained.
Due to the high temperature reached by the powder during the plasma spraying
process, it was
observed that Magneli phase-supported iridium oxide was partially converted to
iridium metal.
The thus obtained electrode was identified as sample #C1.
COUNTEREXAMPLE 2
An appropriate volume of titanium Magneli phase powder, previously sieved to a
size range of
100 to 400 pm, was plasma-sprayed onto a titanium grade 1 sheet of 10 cm x 10
cm x 0.2 cm
size, previously sandblasted with corundum grit #36 and etched in boiling
hydrochloric acid in
order to obtain a rough surface free of titanium oxide species.
The following spraying parameters were applied:
Nozzle-to-sheet gap: 90 mm
Primary gas: argon
(Primary) gas pressure: 60 bar
Throat size: 5 mm
Scan rate: 200 mm/s
An acidic solution was subsequently prepared containing ruthenium trichloride
and iridium
trichloride in suitable concentration and stoichiometric ratio. The above
plasma-sprayed
titanium sheet was dipped in such solution for 15 seconds, allowed to dry
slowly and finally
placed in a batch furnace at 450 C in oxidising atmosphere. In order to
obtain the required
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noble metal loading (5 g Ru/m2 and 2 g Ir/m2) the dipping and thermal
decomposition cycle was
repeated 4 times.
The thus obtained electrode was identified as sample #C2.
COUNTEREXAMPLE 3
A known volume of acidic solution containing a soluble precursor of ruthenium,
namely highly
concentrated ruthenium trichloride, was applied by electrostatic spraying onto
a titanium grade 1
sheet of 10 cm x 10 cm x 0.2 cm size, previously sandblasted with corundum
grit #36 and
etched in boiling hydrochloric acid in order to obtain a rough surface free of
titanium oxide
species. The solution was allowed to dry slowly and then decomposed in a batch
furnace at 450
C in oxidising atmosphere.
In order to obtain the required noble metal loading (24 g Ru/m2) the
electrostatic spraying and
thermal decomposition cycle was repeated 18 times.
The thus obtained electrode was identified as sample #C3.
The samples obtained in the above examples and counterexamples were subjected
to
electrolysis tests, as reported in Table 1 below:
TABLE 1
Hypochlorite Service life
production in Catalyst
Resistivity (c2
Sample ID faradaic accelerated loading
m)
efficiency* test** (garr12)
(0/0) (hours)
1 5 exp -6 73 1600 10 (Ir)
2 5 exp -6 75 1550 10 (Ru)
C1 5 exp -3 39 240 10 (Ir)
C2 5 exp -6 71 290 5 + 2
(Ru + Ir)
C3 5 exp -6 78 150 24 (Ru)
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*Hypochlorite production faradaic efficiency: measure of faradaic efficiency
by titration of active
chlorine present in an electrolyte sample obtained starting from an aqueous
NaCI solution at 30
g/I subjected to electrolysis for 10 minutes, at 25 C and at a current
density of 2 kA/m2. The
sample under test is the working anode, while the counterelectrode consists of
a titanium sheet.
**Accelerated test: electrolysis carried out in a solution of 5 g/I NaCI and
50 g/I Na2SO4, 30 C, 1
kA/m2. Anode and cathode are made of the same material. Electrode polarity is
reversed every
2 minutes.
The previous description shall not be intended as limiting the invention,
which may be used
according to different embodiments without departing from the scopes thereof,
and whose
extent is solely defined by the appended claims.
Throughout the description and claims of the present application, the term
"comprise" and
variations thereof such as "comprising" and "comprises" are not intended to
exclude the
presence of other elements, components or additional process steps.
The discussion of documents, acts, materials, devices, articles and the like
is included in this
specification solely for the purpose of providing a context for the present
invention. It is not
suggested or represented that any or all of these matters formed part of the
prior art base or
were common general knowledge in the field relevant to the present invention
before the priority
date of each claim of this application.
