FR2811338A1 - Electrocatalytic iridium oxide coating production on tantalum substrate, used in construction of metallic electrodes with high corrosion resistance and long life, comprises thermal decomposition of iridium tetrachloride on tantalum surface - Google Patents

Abstract

The formation of a tantalum coating of controlled roughness on a metallic base, to provide a suitable surface for the subsequent deposition of iridium oxide, comprises thermal decomposition of iridium tetrachloride (IrCl4) on the tantalum surface. IrCl4 is applied in an octanol solution on to the tantalum substrate and thermal decomposition is effected in a controlled oxidizing atmosphere.

Description

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  PROCESS FOR THE PREPARATION OF METAL MATERIALS FOR THEM

USE AS ELECTRODES

  The present invention relates to a new process for preparing metallic materials for their use

as electrodes.

  The preparation of electrode materials with improved performance is a constant concern of the industry. This concern responds to an ever greater need for material resistant to the constraints of new applications which arise from technological advances. In particular, the electrochemical industry, which implements electrolysis processes, uses electrodes immersed in an often acidic electrolyte and whose anode gives off, during the electrochemical reaction, a corrosive gas such as oxygen or chlorine. These corrosive conditions, in addition to severe operating parameters (high current densities) considerably reduce the service life of the materials used. It was therefore necessary to move towards materials allowing to resist such constraints

  of use in order to prolong its lifespan.

  In general, the electrodes commonly used for such applications are characterized by a stable geometry resulting from high chemical and electrochemical inertia and by a constant potential during very long periods of use reaching, or even exceeding, two to three years. These electrodes known as DSA (Dimensionally Stable Anodes) have already shown their

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  good electrochemical performance as an anode for the

release of oxygen.

  In the current manufacture of these anodes, a metallic compound such as Ti, Ta, Nb, Zr, Sn and their alloys is used, covered with a layer of an electrocatalytic material composed of oxides of precious metals such as IrO2, PtOx, Ru02, possibly mixed with

  oxides of valve metals such as SnO2, TiC2 or Ta2O5.

  Tantalum is particularly suitable for the manufacture of electrodes because of its excellent resistance to corrosion and its electrochemical stability. Tantalum also has the advantage of being particularly biocompatible in biomedical electrode applications. It is thus increasingly used tantalum for the manufacture of stimulators with electrical impulses, in particular for "pacemakers". Due to the cost and density of tantalum, one avoids making solid tantalum anodes and it is preferred to use a lower value metallic compound which is covered with a tantalum substrate. We then treat tantalum to receive a layer

  electrocatalytic composed of precious metal oxides.

  Patent FR 2 748 495, of which the Applicant is the holder, describes a process for the preparation of metallic materials of the M / Ta / IrO2 type by thermal decomposition of iridium tetrachloride IrCl4 on a tantalum substrate where M represents any metal. In this process, a layer of tantalum substrate is first deposited on any metallic compound and then a coating of IrO2 on this tantalum substrate after sandblasting and pickling of the tantalum substrate.

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  Sandblasting and pickling are considered essential steps to increase the reception area of the metal substrate. The purpose of sandblasting is also to give a certain roughness to the surface of the metal substrate, in order to allow the subsequent deposition of the electrocatalytic layers of oxides of

  precious metals adhere perfectly to it.

  These sandblasting and chemical stripping steps are therefore widely used by industry. These are described in particular in patents FR 2 735 386 and FR 2

  748,495 owned by the Applicant.

  Patent FR 2,748,495 recommends, for example, corundum sandblasting and pickling with hydrochloric acid.

or hydrofluoric.

  However, this preparation technique presents

many disadvantages.

  During the sanding step, an abrasion of the tantalum substrate layer is in fact carried out. However, the deposition of tantalum substrate layers for even a few μm on a metallic compound is a difficult operation, whether by electrochemistry, by vacuum deposition, by sputtering, by ion deposition (Ion Platting) or by deposition from a reactive atmosphere (CVD: Chemical Vapor Deposition). It is therefore particularly unfortunate to have to perform the abrasion of these tantalum layers which are also obtained with difficulty. In addition, sanding does not allow to consider all forms of electrodes. In fact, for the sandblasting operation to be effective, all of the

  tantalum surface is accessible to the abrasive material.

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  The sandblasting operation is for example not really suitable for the preparation of grid-shaped electrodes,

  although this form is widely used by industry.

  Finally, the sandblasting step requires subsequent cleaning in an ultrasonic bath so as to remove traces of abrasive material. There is always a risk of persistence of such materials which can be truly embedded in the tantalum substrate. During the etching step, the use of hydrofluoric acid is always a significant risk.

because of its high toxicity.

  It is also necessary to carry out cleaning in order to remove all traces of chemical compounds which may interact with the reaction medium during the commissioning of the electrodes. But it seems obvious that chemical cleaning is not really suitable when these electrodes are intended to be used in the human body as a pacemaker for example. The risks of permanence of fluorinated or chlorinated residues on these electrodes, as well as the risks of persistence of abrasive materials (possible encrustation during the sanding step), are disadvantages

  major in vivo implantation of these electrodes.

  The inventors, at the cost of many efforts, have resolved many of these major drawbacks by proposing a new method for preparing electrode material of the M / Ta / IrO2 type where M represents a compound

any metallic.

  The object of the invention is therefore to propose a new process for the preparation of anode material of the M / Ta / IrO2 type which does not include the sanding and pickling steps considered essential by the prior art. Another object pursued by the invention is

  improving the service life of these electrodes.

  The inventors have surprisingly discovered that tantalum can be directly deposited in a roughness layer controlled by electrochemistry in a medium.

containing molten salts.

  The process according to the invention therefore relates to the manufacture of electrode materials comprising at least one metallic compound and a tantalum substrate, and is characterized in that the deposition of a substrate layer is carried out directly roughness tantalum controlled on the metallic compound, by electrochemistry in a medium containing molten salts and under

inert atmosphere.

  t5 By metallic compound is meant any metal commonly used in this type of application. This metal can in particular be chosen from the group comprising copper, nickel, titanium, tantalum, their alloys,

steel or stainless steel.

  The tantalum substrate is here a layer of tantalum which is deposited electrochemically in a medium

containing molten salts.

  The cathodic current density of the electrochemical reaction of deposition of tantalum is between 50 and 150 mA.cm-2, preferably is

between 50 and 100 mA.cm-2.

  Due to the elimination of the sanding and pickling steps used by the prior art in the process developed by the inventors, it is now possible to deposit only a thin layer of tantalum on the metal compound. Indeed, the methods of the prior art had to provide a larger layer of tantalum in

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  knowing that part of the thickness of this layer would be reduced by abrasion. Thus, the tantalum substrate layer of controlled roughness has a thickness greater than or equal to about 2 μm, preferably a thickness between 20 and 50 μm. The mean roughness or mean deviation of the roughness Ra of the tantalum layer deposited in the process according to the invention is for its part between approximately 0.5 and

  1.5 pm and preferably is equal to 1 pm.

  The roughness measurement is carried out using a computerized profilometer of the TENCOR brand. This device is capable of measuring lengths of the order of a pm which characterize the shape of the profile of the rough layer of tantalum. The acquisition of the data is done mechanically by the horizontal displacement of a point on a part of the sample which will thus reproduce the profile of the tantalum deposit. This data acquisition is illustrated in FIG. 1 in which, Rt represents the maximum deviation between the peaks of the roughness, Ra represents the average deviation of the roughness, L represents the total distance over which the measurement is carried out and hi are as many measurements of deviations from the average necessary to characterize the rough profile. The average deviation of the roughness Ra is calculated according to the following formula: 1L jhi + j- jh: + ... + Ihn Ra = --fhildx = Lo no Ra is expressed in pm, L represents the total distance over which the measurement is performed, hi, represents the deviation from the mean for measurement i and n the

number of measurements performed.

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  So that the results can be considered reproducible, it is necessary on the one hand to choose a length L quite large compared to the size of the sample and on the other hand to adjust the speed of displacement of the point on the deposit according to the profile studied. L is usually set at 2mm and the speed

  of displacement of the point on the deposit is 20 pm.s-.

  The average roughness values Ra presented are an average of five measurements made on different

  sections of the rough surface of the same tantalum deposit.

  The molten salt electrolyte which can be used in the process according to the invention is at a temperature of between 700 and 800 ° C. under an inert atmosphere and contains the mixture LiF-NaF-K2TaF7 or K2TaF7 represents from 10% to 30% by mass of the composition. , preferably represents 20% by mass of the composition. As an inert atmosphere, we can

  in particular use argon or nitrogen.

  The process according to the invention provides numerous

  advantages over the methods of the prior art.

  In addition to the fact of eliminating two steps in the method of manufacturing the electrodes, the method according to the invention makes it possible to be free from the constraints of

  form imposed by the sandblasting method.

  Furthermore, the electrodes produced in the process according to the invention do not require pickling with HF or HCl. The additional chemical cleaning step after the pickling step to remove acid residues is also eliminated. The toxicological risk due to the persistence of acid residues or chemical cleaning products or encrustation

  abrasive material on the electrodes is thus discarded.

  This then represents a major advantage for their

  use as biocompatible material.

  The inventors have naturally also sought to improve the service life of the electrodes according to the invention. They therefore had the merit of highlighting the conditions for producing the electrocatalytic coating of iridium oxide so that the duration of

  normalized electrode life is increased.

  In practice, in order to measure the service life of the Io electrodes, an accelerated test is carried out to measure the service life of the anodes. This consists of carrying out the electrolysis of a concentrated sulfuric acid solution using the anode to be tested, under a current density 25 to 50 times greater than the current density applied in industrial processes . The lifetimes of the anodes under these conditions are therefore shorter than the lifetimes under normal operating conditions, which facilitates the comparative study in view

  optimize the anode preparation conditions.

  In practice, in order to validly compare the anodes prepared with areal masses of different electrocatalytic oxide, the normalized service life t is defined by the following equation: operational life t = areal mass of Ir02 The areal mass of Ir02 is defined as the mass of electrocatalytic oxide IrO2 deposited on the coating

per unit area of substrate.

  The normalized service life t is thus expressed in h.m2.g-1. Jo The deposition of the rough tantalum substrate according to the process of the invention is followed by a coating with a

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  electrocatalytic coating of iridium oxide by thermal decomposition on the tantalum substrate, of iridium tetrachloride IrCl4 under a controlled oxidizing atmosphere, the iridium tetrachloride being in solution in an organic solvent and being previously applied

on the tantalum substrate.

  Concretely, once the deposition of the tantalum substrate of controlled roughness has been carried out, the iridium tetrachloride IrCl4, in solution in an organic solvent, o10 is applied, for example by brush, to the rough surface of the tantalum substrate. The organic solvent which can be used can be chosen from alcoholic solvents, in particular from methanol, ethanol, propanol, isopropanol, butanol, octanol or mixtures thereof, preferably the alcoholic solvent is a mixture of ethanol / isopropanol and even more preferably the

alcoholic solvent is octanol.

  Octanol is particularly suitable for the decomposition of iridium tetrachloride because its use allows the temperature of this decomposition to be lowered to a point where tantalum is not oxidized. In fact, this type of heat treatment also affects the tantalum substrate layer. The tantalum substrate then risks oxidation when the temperatures of thermal decomposition are too high. However, the layer of iridium oxide which is applied must precisely prevent such passivation of the tantalum substrate. Thus, an excessively high decomposition temperature of iridium tetrachloride, would oxidize the tantalum substrate, which would reduce the lifespan

standardized electrodes.

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  The choice of a controlled oxidizing atmosphere also makes it possible to promote the deposition of iridium oxide, which contributes to the increase in the normalized service life of the electrodes. In the process according to the invention, the thermal decomposition of IrCl4 is carried out under a controlled atmosphere comprising from 70% to 100% of

  dioxygen, preferably comprising 100% of oxygen.

  The process according to the invention is therefore also applicable to the production of an electrocatalytic coating of iridium oxide by thermal decomposition of iridium tetrachloride IrCl4 on a tantalum substrate, characterized in that the iridium tetrachloride is previously applied in solution in octanol on the tantalum substrate and that said thermal decomposition is carried out under an atmosphere

controlled oxidant.

  The thermal decomposition is thus carried out in an oven at a temperature between 200 and 540 C, preferably between 310 and 4500C, and more

  preferably still at a temperature equal to 400 C.

  The electrocatalytic coating of Ir02 can for example be carried out according to the following steps: (a) deposition, on the rough tantalum substrate, of a solution containing precursors of IrCl4 coating, (b) steaming to partially evaporate the solvent, (c) heat treatment at a temperature T1 of between 200 and 400 C, (d) repeating the last three steps (a), (b) and (c), as often as necessary to achieve the number of first layers of coating desired catalytic, Il 2811338 (e) deposit, on the metal substrate, of a solution containing the precursor IrCl4, (f) steaming to completely evaporate the solvent, (g) heat treatment at a temperature T2 between 320 and 540, C, (h) resumption of these last three operations (e), (f) and (g) until the desired surface mass of iridium oxide, O10 is obtained. In more detail, the method according to the invention can comprise the following steps: (i) Deposit of direct from a rough tantalum substrate on a metallic compound by electrochemistry in a bath containing molten salts, under an inert atmosphere, (j) deposition, on the rough tantalum substrate, of an organic solution containing the coating precursor IrCl4, (k) steaming to partially evaporate the solvent for approximately 2 to 5 minutes, (1) heat treatment at a temperature T1 of between 200 and 400 ° C., for approximately 5 minutes, (m) resumption of the last three stages (j), (k) and (1), as often as necessary to produce the number of first desired layers of catalytic coating, (n) deposition, on the metal substrate, of a solution containing the precursor IrCl4, (o) baking to completely evaporate the solvent for about 10 minutes,

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  (p) heat treatment at a temperature T2 between 320 and 540 C for approximately 10 minutes, (q) resumption of these last three operations (n), (o) and (p) until the areal mass is obtained of desired iridium oxide, (r) final calcination at a temperature T3 of between 320 and 540 ° C. for approximately 90 minutes. The present invention will now be described more

  in detail using the following comparative examples.

EXAMPLE 1

  1 5 The deposition of a tantalum substrate is carried out by electrolysis in the medium of molten fluorides according to the process

  of the invention. The electrolyte is a LiF-NaF- mixture

  K2TaF-, of composition 20% by mass in K2TaF7. The temperature is between 700 and 800 C. The electrolysis is carried out under an argon atmosphere. The anode is made of tantalum and the electrode is formed by the metal support. The influence of the current density on the roughness of the deposited tantalum layer is studied. This produces the removal of a tantalum substrate at a current density which is varied from 0 to 300 mA.cm-2. The results

are shown in Figure 2.

  Figure 2 illustrates the influence of the current density on the average roughness of the tantalum layer 3o deposited on the metallic compound. The abscissa axis represents the current density in mA.cm2, the axis of

  ordinates represent the average roughness of tantalum in pm.

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EXAMPLE 2

  Four deposits of electrocatalyst coatings of Iridium IrO2 oxide are carried out, under the same conditions and in accordance with the process of the invention, on four tantalum substrates using four different solvents for the IrCl4 precursors so as to study the influence on the standard service life of the electrodes. tO For the first deposit, methanol is used as

  coating precursor solvent IrCl4.

  For the second deposit, ethanol is used as

  coating precursor solvent IrCl4.

  For the third deposit, we choose butanol and finally for the fourth electrode, we take octanol

  as a solvent for the coating precursor IrCl4.

  The decomposition temperatures are Tl: 400 C, T2: 430 C, the final calcination temperature is T3: 450 C. The atmosphere of the oven in which the

thermal decomposition is air.

  Table 1 and Figure 3 show the results

of this study.

  Am / S represents the surface mass (in g.m-2) on the electrode of the electrocatalytic coating deposit

of IrO2.

  In Figure 3, the abscissa axis represents the number of carbon atoms present in the alcoholic solvent molecule used. The ordinate axis represents the normalized lifetime of the anodes produced with the

different alcohols.

Table 1

  Alcohol Tl T2 T3 Am / S Lifetime (oC) (C) (C) (gm-2) normalized (h.m2. G-1) Methanol 430 450 40 12.03 8.13 0.4 Etharnol | 400 430,450 11.81. 8, 65 0.4 Ethanol. 0 3 Butanol 400 430 450 11.66 10.9 0.6 Octanol 400 430 1 450 11.82 11.5 0.6 It appeared from this test that the normalized lifetime is optimal when the precursor IrC14 is

  applied in solution in octanol.

EXAMPLE 3

  The thermal decomposition of IrCl4 is carried out on eight samples of tantalum substrate, in accordance with the method according to the invention and at a fixed temperature Tl, equal to 380 C and at identical temperatures T2 and T3 which vary for these different samples at

  optimize the decomposition temperature.

  The solvent chosen for the IrCl4 coating precursor is the ethanol / isopropanol mixture. The atmosphere of

oven is air.

  Table 2 and Figure 4 show the results

of this study.

  In Figure 4, the abscissa axis represents the temperature T2 (or T3) used for the thermal decomposition (in C) of IrCl4 on the tantalum substrate of the electrodes. The ordinate axis represents the lifetime

  normalized (in h.m2.g-1) of the electrodes thus treated.

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Table 2

  N T1 (C) T2 (C) T3 (C) Am / SDample duration (g.m-2) normalized life 2 1 (h.m2.g-)

1,380 3,340 340 9.8 0.3

2,380 380 I 380, 10 0.4

3,380 420,420 10.4 3.9 0.1

4 3 380 440 440 10.3 5.8 0.5

380 460 460 10.6 16.7 1.7

6 380 480 480 10.9 11.4 1.0

7 380 500 500 11.8 9.8 1.0

I

8 [380 540 540 25.6 0.1

  It emerges from this study that the preferential decomposition temperature range in the ethanol / isopropanol mixture to obtain an optimized normalized service life is 440 to 500 ° C. and that a too high thermal decomposition temperature decreases

the normalized service life.

EXAMPLE 4

  We carry out the same study as that carried out in

  Example 3. The solvent used here is octanol.

  The results of this study are reported in the

Table 3 and Figure 5.

  In Figure 5, the abscissa axis represents the thermal decomposition temperature of IrCl4. The ordinate axis represents the normalized service life (in h.m2.g

1) electrodes thus treated.

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Table 3

  i Tl (C) T2 (C) T3 (C) Am / S Duration of Sample (g.m- ') normalized life (h.m2.g-) i 200 320 320 11.5 0.4

2,200 360 360 11.71 1.7

3,200 380 380 11.71 11.4 1.2

4J 1200 400 400 0 11.08 27.9 2.9

200 i 420 420 11.66 18.4 2

6,200,440 4,440 11.64 14.9 1.6

__ 7 2,200 460 460 12.16 14.4 1.6

8 200 500 500 13 0.2

  It emerges from this study that the preferred decomposition temperature range in octanol to obtain an optimized normalized service life is 380 to 460aC and that a thermal decomposition temperature

  too high decreases the normalized service life.

EXAMPLE 5

  The thermal decomposition of IrCl4 is carried out on six samples of tantalum substrate, in accordance with the method according to the invention and under atmospheres containing 0, 10, 20, 40, 70 and 100% of oxygen for these different samples so as to study the influence of the gas composition on decomposition and

  so as to improve the standardized service life of the electrodes.

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  The solvent chosen for the IrCl4 coating precursor is the ethanol / isopropanol mixture. The decomposition temperatures are T1 = 400 C and T2 = 430 C. The final calcination temperature is T3 =

S 450 C.

  The results of this study are reported in the

  Table 4 and on the curve of Figure 6.

  In Figure 6, the percentage of oxygen is represented by the abscissa axis and the normalized life o10 in h.m2.g-1 is represented by the ordinate axis.

Table 4

  % O2 T1 (C) T2 (C) T3 (C) Am / S Duration of (g.m-2) normalized life (h.m2.g-1)

O 400 430 450 11.92 0

400 430 450 12.25 8.4

400 430 450 11.17 7.7

400 430 450 11 8.6

400 430 450 12.42 12.24

400 '430 4 450 11.33 17.4

  This study shows that the normalized lifetime of the electrodes is optimized when decomposition

  thermal reaction of the precursor IrCl4 in the ethanol / iso- mixture

  propanol takes place under an atmosphere composed of 100% dioxygen.

EXAMPLE 6

  Thermogravimetric analyzes are carried out of four solutions of IrCl4 precursor in methanol, ethanol, butanol and octanol at 20 C / min. The results of this analysis are reported on the

Figure 7.

  In Figure 7, the abscissa axis represents the temperature (in C). And the y-axis represents the

  loss of mass as a percentage of alcoholic solutions.

  This Figure also shows the decomposition temperatures of IrCl4 first in IrO2 and then in Ir. The curve referenced by the triangles represents the thermogravimetric analysis of the solution of precursor IrC14 in methanol. The curve referenced by the circles is that of the analysis of the thermogravimetric analysis of the solution of precursor IrC14 in ethanol. The curve referenced by the squares is that of the thermogravimetric analysis of the solution of precursor IrCl4 in butanol. Finally, the curve referenced by the diamonds is that of the analysis of the thermogravimetric analysis of the solution of precursor IrCl4 in octanol. These temperatures are included in the

table 5.

Table 5

  Solvent solution precursor Decomposition temperature (C) Methanol 790 Ethanol 600 Butanol 410 Octanol 310

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  This study shows that the thermal decomposition temperature of IrCl4 is lower when

  IrCl4 is in solution in octanol, compared to thermal decomposition in other solvents whose length of the carbon chain is shorter.

Claims (5)

  1. Method for producing an electrocatalytic coating of iridium oxide by thermal decomposition of iridium tetrachloride IrCl4 on a tantalum substrate, characterized in that the iridium tetrachloride is previously applied in solution in the octanol on the tantalum substrate and that said thermal decomposition is carried out under an atmosphere controlled oxidant.   2. Method according to claim 1, characterized in that the thermal decomposition is carried out at a temperature between 200 and 540 C, preferably between 310 and 450 C, and more preferably   still at a temperature equal to 400 C.   3. Method according to any one of the claims   1 or 2, characterized in that the thermal decomposition of IrCl4 is carried out under a controlled atmosphere comprising from 70% to 100% of oxygen,   preferably comprising 100% dioxygen.   4. Method according to any one of the claims   1 to 3, characterized in that the electrocatalytic coating of Ir02 is carried out according to the following steps: (a) deposition, on the metallic compound and on the rough tantalum substrate, of a solution containing the coating precursor IrCl4, (b) parboiling to partially evaporate the solvent, (c) heat treatment at a temperature T1 between 200 and 400 C, (d) repeating the last three steps (a), (b) and (c), also often as necessary to 2 1 2811338   perform the number of first desired layers of catalytic coating, (e) depositing a solution containing the precursor IrCl4 on the tantalum substrate, (f) baking to completely evaporate the solvent, (g) heat treatment at a temperature T2 between 320 and 540 C, (h) resumption of these last three operations (e), (f) and (g) until mass is obtained   surface area of iridium oxide desired.   5. Method according to any one of the claims   1 to 4, characterized in that it comprises the following stages: (i) direct deposition of a rough tantalum substrate on a metallic compound by electrochemistry in a bath containing molten salts, under an inert atmosphere, (j) deposition , on the metallic compound and on the rough tantalum substrate, of an organic solution containing precursors of IrCl4 coating, (k) steaming to partially evaporate the solvent for approximately 2 to 5 minutes, (1) heat treatment at a temperature Tl between 200 and 400 C, for approximately 5 minutes, (m) repeating the last three steps (j), (k) and (1), as often as necessary to achieve the number of first desired layers of catalytic coating, 22 2811338   (n) depositing a solution containing the precursor IrCl4 on the rough tantalum substrate, (o) baking to completely evaporate the solvent for approximately 10 minutes, (p) heat treatment at a temperature T2 between 320 and 540 C, ( q) resumption of these last three steps (n), (o) and (p) until the desired surface mass of iridium oxide is obtained, (r) final calcination at a temperature T3 between 320 and 540 C.