Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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CATHODE FOR ELECTROLYTIC EVOLUTION OF HYDROGEN
FIELD OF THE INVENTION
The invention relates to an electrode, with particular reference to a metal
electrode for
use as a cathode for evolution of hydrogen in industrial electrolytic
processes and a
method for its production.
BACKGROUND OF THE INVENTION
The electrolysis of alkali brines for the simultaneous production of chlorine
and alkali
and the electrochemical processes of hypochlorite and chlorate generation are
the most
typical examples of industrial electrolytic applications with cathodic
evolution of
hydrogen, but the electrode is not limited to any particular use. In the
industry of the
electrolytic processes, competitiveness is associated to several factors, the
main of
which being the reduction of energy consumption, directly linked to process
voltage; this
justifies the many efforts to reduce the various components of the latter,
among which
cathodic overvoltage must be included. Cathodic overvoltages naturally
obtainable by
means of electrodes made of chemically resistant material (for example carbon
steel)
with no catalytic activity have been considered acceptable for a long time. In
the specific
case, the market nevertheless requires increasing concentrations of caustic
product,
which made the use of carbon steel cathodes unfeasible due to corrosion
problems; in
addition, the increase in the cost of energy has made advisable to use
catalysts for
facilitating the cathodic evolution of hydrogen. One possible solution is to
use nickel
substrates, chemically more resistant than carbon steel, and platinum-based
catalytic
coatings. Cathodes of this type are normally characterised by an acceptable
cathodic
overvoltage, presenting however limited useful lifetimes, probably due to poor
adhesion
of the coating to the substrate. A partial improvement in the adherence of the
catalytic
coating to the nickel substrate is obtainable by the addition of rare earths
to the
formulation of the catalytic layer, optionally as a porous external layer that
performs a
protective function against the underlying platinum-based catalytic layer;
this type of
cathode is sufficiently durable under normal operating conditions, being
liable however
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to suffer serious damages following the occasional current reversals
inevitably produced
in case of malfunctioning of industrial plants.
A partial improvement in the resistance to current reversals is obtainable by
activating
the nickel cathode substrate with a coating consisting of two distinct phases,
a first
platinum-based catalytic phase added with rhodium and a second phase
comprising
palladium having a protective function. This type of formulation, however,
requires high
loads of platinum and rhodium in the catalytic phase, such as to determine a
rather high
production cost.
A less expensive catalytic coating which presents high activity combined with
some
resistance to current reversals is obtained from mixtures of ruthenium and
rare earths,
for example praseodymium; the resistance of electrodes obtained according to
such a
formulation can be increased by interposing a platinum-based thin layer
between the
cathode substrate and the catalytic coating.
The above formulations made possible to obtain electrodes capable of
functioning for
sufficient times in correctly operated industrial electrolysers provided,
according to a
common practice in the industry, with polarisation devices actuated in case of
scheduled or sudden plant shut-downs by imposing a small residual voltage
which
serves to protect the cell components from corrosion. With these devices,
current
reversals can only occur during the short period of time that elapses between
the shut-
down of the electrical load and the onset of the residual voltage, during
which the
cathodes should not undergo any appreciable damage. However, the most recent
advancements in the design of industrial electrolysers, in particular of
electrolysers for
the production of chlorine and alkali from alkali brines consisting of
electrolytic cells with
the anodic and cathodic compartments separated by ion-exchange membranes,
provide
the use of materials and construction techniques which make possible to
dispense with
the polarisation devices, whose installation and management accounts for an
important
additional cost. The plant shut-down in an electrolyser free of polarising
device entails,
at least in an initial phase, cell voltage reversal phenomena caused by the
presence of
reaction product residues in the two compartments: in these conditions, the
electrolysis
cell can work for a short period as a battery, with the relevant cathodes
being subject to
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the passage of anodic current. This entails the need of providing cathodes
with a much
higher tolerance to current reversals, compared to the best prior art
formulations.
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 suitable for use as a
cathode in
electrolytic processes comprising a substrate made of metal, for example
nickel,
provided with a catalytic coating formed by at least three distinct layers: an
internal
layer, in direct contact with the substrate, containing platinum, at least one
intermediate
layer consisting of a mixture of oxides containing 40-60% by weight of rhodium
referred
to the elements and an external ruthenium oxide-based layer.
Platinum in the internal layer is present predominantly in metallic form,
especially in
operating conditions under cathodic hydrogen evolution, however, is not
excluded,
especially prior to the first use, that platinum or a fraction thereof may be
present in form
of oxide.
In one embodiment, the internal layer consists of a layer of platinum alone.
In one embodiment, the external layer consists of a layer of ruthenium oxide
alone. In
the present context, the term ruthenium oxide indicates that such element is
present,
after the preparation of the electrode, mainly in oxide form; it is not
excluded, especially
in operating conditions under cathodic hydrogen evolution, that such oxide can
be
partially reduced to ruthenium metal.
In one embodiment, the mixture of oxides of the intermediate layer further
contains,
besides rhodium, 10-30% by weight palladium and 20-40% by weight of rare
earths; in
one embodiment, the rare earth content consists entirely of praseodymium. In
the
present context, the term mixture of oxides indicates that the elements of the
relative
formulation are present, after the preparation of the electrode, mainly in
form of oxides;
is not excluded, especially in operating conditions under cathodic hydrogen
evolution,
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that a fraction of such oxides can be reduced to metal or even form hydrides,
as in the
case of palladium.
The inventors have surprisingly observed that formulations of this type impart
a
resistance to current reversals several times higher than the closest prior
art
formulations at substantially reduced specific loading of noble metal.
In one embodiment, the specific loading of platinum in the internal layer is
between 0.3
and 1.5 g/m2, the sum of the specific loading of rhodium, palladium and rare
earths in
the intermediate layer is between 1 and 3 g/m2 and the specific loading of
ruthenium in
the external layer is between 2 and 5 g/m2. The inventors have in fact
observed that, in
the case of the above formulations, so reduced noble metal loadings are more
than
sufficient to impart a high catalytic activity combined with a resistance to
current
reversals unprecedented in the prior art.
Under another aspect, the invention relates to a method for the preparation of
an
electrode which comprises the application in one or more coats of an acetic
solution of
Pt(NH3)2(NO3)2 (platinum diamino dinitrate) to a metallic substrate, with
subsequent
drying at 80-100 C, thermal decomposition at 450-600 C and optional repetition
of the
cycle until the desired loading is achieved (e.g., 0.3-1.5 g/m2 of Pt as
metal); the
application in one or more coats of an acetic solution containing a rhodium
nitrate and
optionally nitrates of palladium and rare earths to the internal catalytic
layer thus
obtained, with subsequent drying at 80-100 C, thermal decomposition at 450-
600 C
and optional repetition of the cycle until the desired loading is achieved
(e.g., 1-3 g/m2
as the sum of Rh, Pd and rare earths); the application in one or more coats of
an acetic
solution of Ru nitrosyl nitrate to the intermediate catalytic layer thus
obtained, with
subsequent drying at 80-100 C, thermal decomposition at 450-600 C and optional
repetition of the cycle until the desired loading is achieved (for example, 2-
5 g/m2 of Ru
as metal).
As it is well known, Ru nitrosyl nitrate designates a commercially available
compound
expressed by the formula Ru(N0)(NO3)3, sometimes written as Ru(N0)(NO3)x to
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indicate that the average oxidation state of ruthenium can slightly deviate
from the value
of 3.
The above application of the solutions may be carried out by brushing,
spraying,
5 dipping, or other known technique.
The inventors have observed that the use of the specified precursors in the
adopted
preparation conditions favours the formation of catalysts with a particularly
ordered
crystal lattice, with a positive impact in terms of activity, durability and
resistance to
current reversals.
The best results were obtained by adjusting the thermal decomposition
temperature of
the various solutions in the range between 480 and 520 C.
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
An amount of Pt diamino dinitrate, Pt(NH3)2(NO3)2 corresponding to 40 g of Pt
was
dissolved in 160 ml of glacial acetic acid. The solution was stirred for 3
hours while
maintaining the temperature at 50 C, and then brought to the volume of one
litre with
10% by weight acetic acid (platinum solution).
An amount of Ru(N0)(NO3)3 corresponding to 200 g of Ru was dissolved in 600 ml
of
glacial acetic acid with addition of a few ml of concentrated nitric acid. The
solution was
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stirred for three hours while maintaining the temperature at 50 C. The
solution was
then brought to a volume of 1 I with 10% by weight acetic acid (ruthenium
solution).
Separately, amounts of Rh(NO3)3, Pd(NO3)2 and Pr(NO3)3.6H20 corresponding to
4.25 g
of Rh, 1.7 g of Pd and 25.5 g of Pr expressed as metals were mixed under
stirring
(rhodium solution).
A mesh of nickel 200 of 100 mm x 100 mm x 0.89 mm size was subjected to a
process
of blasting with corundum, etching in 20% HCI at 85 C for 2 minutes and
thermal
annealing at 500 C for 1 hour.
The platinum solution was applied by brushing in a single cycle, carrying out
a drying
treatment for 10 minutes at 80-90 C and a thermal decomposition for 10 minutes
at
500 C, obtaining a specific loading of 0.8 g/m2 of Pt.
The rhodium solution was then applied by brushing in three coats carrying out
a drying
treatment for 10 minutes at 80-90 C and a thermal decomposition for 10 minutes
at
500 C after each coat, obtaining a specific loading of 1.4 g/m2 of Rh, 0.6
g/m2 of Pd and
0.84 g/m2 of Pr.
The ruthenium solution was then applied by brushing in four coats carrying out
a drying
treatment for 10 minutes at 80-90 C and a thermal decomposition for 10 minutes
at
500 C after each coat, obtaining a specific loading of 3 g/m2 of Ru.
The sample was subjected to a performance test, showing an ohmic drop-
corrected
initial cathodic potential of -930 mV/NHE at 3 kA/m2 under hydrogen evolution
in 33%
NaOH, at a temperature of 90 C.
The same sample was then subjected to cyclic voltammetry in the range from -1
to +0.5
V/NHE at a 10 mV/s scan rate; after 25 cycles, the cathodic potential was -935
mV /
NHE, which indicates a resistance current reversal perfectly suitable for
operation in
industrial electrolysers free of polarisation devices.
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COUNTEREXAMPLE
An amount of Pt diamino dinitrate, Pt(NH3)2(NO3)2 corresponding to 40 g of Pt
was
dissolved in 160 ml of glacial acetic acid. The solution was stirred for 3
hours while
maintaining the temperature at 50 C, and then brought to the volume of one
litre with
10% by weight acetic acid (platinum solution).
An amount of Ru(N0)(NO3)3 corresponding to 200 g of Ru was dissolved in 600 ml
of
glacial acetic acid with addition of a few ml of concentrated nitric acid. The
solution was
stirred for three hours while maintaining the temperature at 50 C. The
solution was
then brought to a volume of 1 I with 10% by weight acetic acid (ruthenium
solution).
Separately, an amount of Pr(NO3)2 corresponding to 200 g of Pr was dissolved
in 600
ml of glacial acetic acid with addition of a few ml of concentrated nitric
acid. The solution
was stirred for three hours while maintaining the temperature at 50 C. The
solution was
then brought to a volume of 1 I with10`)/0 by weight acetic acid (rare earth
solution). 480
ml of ruthenium solution were blended with 120 ml of rare earth solution and
left under
stirring for five minutes. The solution thus obtained was brought to 1 litre
with 10% by
weight acetic acid (ruthenium and praseodymium solution).
A mesh of nickel 200 of 100 mm x 100 mm x 0.89 mm size was subjected to a
process
of blasting with corundum, etching in 20% HCI at 85 C for 2 minutes and
thermal
annealing at 500 C for 1 hour.
The platinum solution was applied by brushing in a single cycle, carrying out
a drying
treatment for 10 minutes at 80-90 C and a thermal decomposition for 10 minutes
at
500 C, obtaining a specific loading of 1 g/m2 of Pt.
The ruthenium and praseodymium solution was then applied by brushing in 4
successive coats, carrying out a drying treatment for 10 minutes at 80-90 C
and a
thermal decomposition for 10 minutes at 500 C after each coat, until obtaining
the
deposition of 4 g/m2 of Ru and 1 g/m2 Pr
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The sample was subjected to a performance test, showing an ohmic drop-
corrected
initial cathodic potential of -930 mV/NHE at 3 kA/m2 under hydrogen evolution
in 33%
NaOH, at a temperature of 90 C.
The same sample was then subjected to cyclic voltammetry in the range from -1
to +0.5
V/NHE at a 10 mV/s scan rate; after 25 cycles, the cathodic potential was -975
mV /
NHE, which indicates a resistance current reversal suitable for operation in
industrial
electrolysers only if equipped with polarisation devices.
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.
