FI68090B - BELAGD METALLELEKTROD MED FOERBAETTRAT SPAERRSKIKT - Google Patents

Abstract

An electrode for use in electrolytic processes comprises a substrate of film-forming metal such as titanium having a porous electrocatalytic coating comprising at least one platinum-group metal and/or oxide thereof possibly mixed with other metal oxides in an amount of at least about 2 g/m<2> of the platinum-group metal(s) per projected surface area of the substrate. Below the coating is a preformed barrier layer constituted by a surface oxide film grown up from the substrate. This preformed barrier layer has rhodium and/or iridium as metal or compound incorporated in the surface oxide film during formation thereof in an amount of up to 1 g/m<2> (as metal) per projected surface area of the substrate.

Description

This invention relates to electrodes for use in electrolysis processes of the type having a substrate of a film-forming metal such as titanium, tantalum, zirconium, niobium, tungsten, aluminum and alloys containing one or more of these metals. , as well as silicon-iron alloys, the substrate of which is coated with an electrocatalytic layer containing one or more platinum group metals or their oxides, optionally mixed with other oxides.

By "film-forming metal" is meant that having the property that when combined as an anode with an electrolyte in which the coated anode is then to act, a rapidly passivating oxide film is formed which protects the underlying metal from corrosion by the electrolyte. These metals are also often referred to as "valve metals".

More particularly, the invention relates to dimensionally stable electrodes provided with an improved barrier or intermediate layer between a film-forming metal substrate and an electrocatalytic outer layer.

In early proposals (see, e.g., British Patents 855,107 and 869,865), a titanium electrode having a platinum group metal coating was provided with an inert barrier layer of titanium oxide at porous locations in the coating, this barrier layer being preferably formed or reinforced by heat treatment. Subsequently, in British Patent 925,080, an inert barrier layer of titanium oxide was preformed by electrolytically treating or heating a titanium substrate in an oxidizing atmosphere before applying a platinum group metal. The preforming of such a barrier layer was also defended in British Patent 1,147,422 for the purpose of improving the anchoring of an active coating consisting of or containing platinum group metal oxides.

Subsequently, the development of coatings consisting of mixed crystalline or solid solutions of film-forming metals and co-precipitated oxides of platinum group metals (see U.S. Patent No. 3,632,498) resulted in commercially viable electrodes that revolutionized the chlor-alkali industry and have been widely used in other applications as well. These electrodes achieved excellent performance without the need for a reinforced or preformed inert barrier or anchoring layer for the substrate, and it is now generally accepted that preformed or reinforced inert barrier layers are detrimental to performance. In retrospect, early proposals for preformed or reinforced inert barrier layers appear to have been unsuccessful attempts to avoid defects that were inherent in previous coatings and not in the substrate.

In any case, some suggestions for trying to improve the inert barrier layers have still been made, for example, by applying a titanium oxide barrier layer from a solution containing Ti ions. Again, this has been found to degrade the performance of the electrodes.

Il 3 68090

Another approach has been to provide a non-passable barrier layer or interlayer under the active outer coating. Typical proposals have been brushed tin dioxide sublayers; thin lower layers of one or more Platinum metals, such as platinum-iridium alloy; cobalt oxide or lead oxide sublayers, etc. Although various patents have claimed marginal improvements to these electrodes in certain applications, virtually none of these proposals have resulted in a significant improvement or any widespread commercial use.

This invention relates to an electrode having a film-forming metal substrate having a porous electrocatalytic outer layer containing at least n.

2 g / m (as platinum group metal per projected surface area of the substrate) of at least one platinum group metal and / or its oxide, optionally mixed with other metal oxides, and an improved non-passivating barrier layer between the substrate and the coating.

According to the invention, this barrier layer is a preformed surface oxide film grown from a film-forming base, to which rhodium and / or iridium is added to the surface oxide film during its formation in an amount of up to 1 g / m (as metal) per projected surface area of the substrate.

The surface oxide film of the barrier layer is rendered non-passivating by incorporating rhodium and / or iridium as a metal or compound, usually an oxide or a partially oxidized compound.

Another aspect of the invention is a method of making such an electrode, wherein the formation of the barrier layer comprises applying a very dilute acidic paint, i.e. one containing a small amount of a thermally decomposable iridium and / or rhodium compound which, during decomposition and concomitant film-forming metal oxide surface film formation is absorbed into this surface film, which dilute paint usually contains about 1-15 g / l of iridium and / or rhodium (as a metal).

The paint used typically contains an organic solvent such as isopropyl alcohol, an acid (especially HCl, HBr or HJ) or another substance (e.g. NaF) which corrodes the film-forming metal and favors the formation of the film-forming metal oxide during subsequent heat treatment and one or more thermally decomposable iridium and / or rhodium salt. Usually, this solution is at least five times more dilute, and preferably about 10 or more times more dilute (in terms of its precious metal content) than the paint solution that can be used to make porous electrocatalytic oxide outer layers; this means that the amount of iridium and / or rhodium is reduced, e.g., by about 1/5 or 1/10 or even 1/100 of the amount of the corresponding platinum group metal in the paint used to make the outer layer with approximately the same amount of solvent and acid.

The action of an acid or other substance which traps or corrodes the film-forming metal and promotes the formation of an oxide film during the subsequent heat treatment is very important; without a suitable substance which produces this effect, the formation of the surface oxide film of the film-forming metal is substantially prevented or retarded.

It has been found that applying a single layer of solvent / acid pose to a film-forming metal substrate previously subjected to conventional cleaning and pickling treatments, and then heating after drying to remove the solvent, gives a given amount of film-forming metal oxide. This procedure can be repeated several times (usually four or five times with 4 ml of HCl in 60 ml of isopropyl alcohol applied to a titanium support, dried and heated at 500 ° C for 10 minutes) before successive growths of the film-forming metal oxide. inhibited during treatments. The first layer of the formed integral surface oxide film is relatively porous. This allows the acidic paint for the next applied layer to penetrate this porous first layer during the drying step so that the acid adheres to the underlying film-forming metal. The base thus releases the film-forming metal ions into a convertible oxide during the subsequent heating, and the oxide is partially formed in the pores of the first layer. The porosity of the obtained oxide film thus decreases after each coating cycle until no more metal forming the film from the base material can be converted to oxide. Thus, a very stable, relatively dense and impermeable film of the metal oxide forming the film can be formed by applying a limited number of layers of acid paint and then drying and heating.

In order to prepare the barrier layers according to the invention, each applied paint layer contains such a small amount of iridium and / or rhodium compound that the electrocatalyst formed by thermal decomposition is completely associated with the integral surface film of the film-forming metal oxide which is formed each time. Usually, each paint layer contains at most about 0.2 g / m iridium and / or rhodium per projected surface area of the substrate, usually much less. In addition, the application of additional layers of 6,68090 dilute paint is stopped after the number of coats on one side of which the growth of the surface oxide film by the film-forming metal ceases or is inhibited. Thus, the optimum amount of electrocatalytic agent in dilute paint and the optimum number of layers to be applied to form a satisfactorily dense, impermeable barrier layer can be determined quite easily for any given substrate, solvent / acid and electrocatalytic material. In many cases, 2-10 layers of very dilute paint are applied, each followed by drying and heating at n-400-600 ° C for about 5-15 minutes, with the possible exception of the last layer, which can be heated for a longer time, possibly several hours or days 450- At 600 ° C in air or a reducing atmosphere (eg ammonia / hydrogen).

When viewed with the naked eye or microscope, the barrier layers thus formed on a pickled or unpainted titanium substrate usually retain the same range of distinct appearances as titanium oxide films prepared in the same manner and free of iridium and / or rhodium electrocatalyst, typically bright blue and yellow blue, yellow the color of the "interference" film.

The dilute acid paint solution used to make the barrier layer of the invention preferably contains only a thermally decomposable iridium and / or rhodium compound, as the substrate provides a film-forming metal oxide component for use. However, a dilute paint may contain small amounts of other components, such as other platinum group metals (ruthenium, palladium, platinum, osmium, especially ruthenium), gold, silver, tin, chromium, cobalt, antimony, molybdenum, iron , nickel, manganese, tungsten, vanadium, titanium, tantalum, zirconium, niobium, 7 68090 bismuth, lanthanum, teurium, phosphorus, boron, beryllium, sodium, lithium , calcium, strontium, lead and copper compounds and mixtures thereof. Usually, if a small amount of the film-forming metal compound is used, it is a different metal than the film-forming metal substrate in order to contribute to the lacquering of the pin film. Excellent results have been obtained with iridium / ruthenium compounds in a weight ratio of about 2: 1 calculated as metal. When such additives are included in a dilute paint mixture, their amount is such that it is compatible with a small amount of the main electrocatalyst, i.e. the iridium and / or rhodium compound, so that substantially all of the main electrocatalyst and additive adhere to the film-forming metal oxide surface film. In any case, the total amount of iridium and / or rhodium, n min and other metals is less than 1 g / m 2 and usually less than 0.5 g / m 2 and the excess metal is present in a smaller amount than rhodium and / or iridium. These iridium / rhodium compounds and other metal compounds may be thermally decomposable to form a metal or oxide, but in either case it is not necessary to proceed to complete decomposition. For example, barrier layers containing partially dispersed iridium chloride containing up to about 5% by weight of the original chlorine have shown excellent properties. Barrier layers containing

O

only 0.1-0.3 g / m (as metal) .iridium and / or rhodium oxide / chloride in their surface films give excellent results. Experiments have shown that the barrier layer 2 containing 0.5-0.6 g / m 2 (as metal) of iridium provides an optimum effect with regard to the extended life of the coated electrodes. Increasing the amount of iridium above these values does not prolong life.

3 68090

When a titanium substrate is used, the surface oxide film is found to be predominantly rutile titanium oxide; presumably the formation of rutile, e.g. at about 400-500 ° C, is catalyzed by rhodium and / or iridium in a dilute coating solution.

After formation of the improved barrier layer, which is impermeable to electrolyte and evolved oxygen, the porous electrocatalytic outer layer is applied using standard techniques, for example, by applying to the preformed barrier layer several layers of relatively concentrated solution containing thermally decomposable platinum group metal. Each coating layer contains at least 0.4 g / m platinum group metal per projected surface area of the substrate and the coating procedure is repeated to grow an effective outer layer containing 2 at least about 2 g / m platinum group metals, usually in oxide form. The coating components may be selected to provide a coating consisting essentially of a solid solution of at least one film-forming metal oxide and at least one platinum group metal oxide described in U.S. Patent 3,632,498. Preferably, the coating is a solid solution of ruthenium and titanium oxides in which ruthenium-titanium-atom ratio is 1: 1-1: 4. In this case, the coating consists of several overlapping layers, typically having a microcrack structure, and is quite porous. The use of the improved barrier layer of the invention in connection with such a coating greatly improves the performance of the electrode in standard accelerated lifetime tests under oxygen generating conditions. An electrode improved from predictable to normal chlorine under conditions of commercial production has a substantially longer lifespan, as it is known that one of the causes of damage to these electrodes

II

9 68090 after prolonged use in chlorine production is due to the effect of oxygen on the substrate. It is also possible to obtain the same service life by significantly reducing the thickness of the outer layer, which makes it possible to save on the amount of coating material used and the work and energy consumed in production.

The outer layer may also be formed of one or more platinum group metals, for example a platinum-iridium alloy, which is useful in chlorate production and, to a limited extent, in diaphragm or membrane cells for chlorine production. With conventional Pt / Ir-coated electrodes, the coatings must be relatively thick (at least about 5 g / m2) to avoid passivation problems. In connection with the improved barrier layer of the invention, thinner and more porous layers of platinum metals can be used without the problems or disadvantages of substrate oxidation associated with previously known passive titanium oxide barrier layers.

It is also possible to apply the outer layer by plasma spraying a solid solution of the film-forming metal oxide and the platinum group metal oxide. For example, a solid solution powder can be prepared by air spraying as disclosed in U.S. Patent No. 3,677,975, and this powder is then plasma sprayed onto a substrate. Alternatively, the coating is applied by plasma spraying at least one film-forming metal oxide onto the preformed barrier layer and then attaching the platinum group metals and / or their oxides to the plasma-sprayed film-forming metal oxide, e.g., according to the procedure of U.S. Patent No. 4,140,813. Again, the improved barrier layer extends the service life and makes it possible to reduce the precious metal content of the coating to 10,68090.

In the preferred method of mass production of electrodes, a series of electrode substrates are subjected to a series of pretreatments, including the formation of a pickling and barrier layer, by permanently coating the substrate in said dilute solution and heating the substrate and then applying the electrocatalytic outer layer to the substrates. This procedure eliminates the disadvantage associated with the "bottleneck" between the pickling bath and the coating line in commercial electrode coating plants. In a conventional pulping process, a set of substrates is pretreated by sandblasting, followed by pickling, rinsing and drying, and these substrates are then individually coated in a coating / drying line. Thus, it has been necessary to synchronize pickling with coating / drying, as pickled substrates cannot be left for long periods (more than about 2 days) without adversely affecting electrode performance due to oxidation of the substrate in air prior to coating, especially if dust or dirt adheres to the thin oxide film. . Pre-coated sets of substrates with an improved barrier layer immediately after pickling avoid this bottleneck effect and surface-treated substrates can be stored without the risk of further oxidation. Any dust or dirt that may settle on the barrier layer can be easily blown off before coating, as it will not stick to the film.

Moreover, the process of permanently coating a series of stacked substrates is satisfactory for forming an improved barrier layer oxide film growing from the substrate. A similar treatment is not satisfactory for the application of conventional coatings, where the additional thickness of each applied layer must be increased on and over the metal substrate forming the film n 68090 and its very thin surface oxide film.

The electrode substrate can be any film-forming metal plate, with titanium being preferred for cost reasons. Rods, tubes and stretched nets of titanium or other film-forming metals can also be surface-treated by the method of the invention. A coating of titanium or other film-forming metal on the conductive core can also be used. For most applications, the substrate is coated prior to surface treatment to provide a rough surface that provides good adhesion to the electrocatalytic coating to be applied later. It is also possible to surface treat porous sintered or plasma sprayed titanium with dilute paint solutions in the same way, but preferably the porous titanium is only a surface layer on a pore-free substrate.

Electrodes with an improved barrier layer according to the present invention are excellently suitable as anodes for chlor-alkali electrolysis. These electrodes have also shown excellent performance when used for electrolytic recovery in a chloride-sulfate mixed electrolyte that produces a mixture of chlorine and oxygen.

The present invention is further illustrated by the following examples.

Example I

Titanium coupons measuring 7.5 x 2 cm and available under the tradename "Contimet 30" were degreased, rinsed in water, dried and pickled for 1/2 hour in oxalic acid. A paint solution, 68090 12 containing 6 ml of n-propanol, 0.4 ml of HCl (concentrated) and 0.1 g of iridium and / or rhodium chloride, was then applied with a brush to both sides of the coupons in four thin layers. The coupons were dried to evaporate the solvent and then heated in air to 500 ° C for 10 minutes after each of the first three layers and for 30 minutes after the last layer. This gives a barrier layer of rhodium and / or iridium content (calculated as metal) of about 0.2 to 0.3 g / m, depending on the amount of solution in each applied layer, determined by weight measurement.

A solid solution of titanium oxide and ruthenium oxide with a titanium: ruthenium atomic ratio of about 2: 1 was then applied by brushing a solution of 6 ml of n-propanol, 0.4 ml of HCl (concentrated), 3 ml of butyl titanate and 1 g of RuCl 2. a and heating in air at 400 ° C for 5 minutes. (Note: this solution is 10 times more concentrated noble metal: in terms of propanol solvent than the dilute solution used to make the barrier layer). This procedure was repeated until the coating was present at a thickness of about 10 g / m 2 (i.e., about 4 g / m Ru metal).

The electrodes thus prepared are subjected to comparative electrochemical experiments with similar electrodes with (a) a TiO 2 blocking layer prepared by the same procedure but with the paint consisting only of 6 ml of n-propanol and 0.4 ml of HCl (concentrated), and (b) there is no barrier layer at all. The first results show that the electrode according to the invention has a clearly superior service life in accelerated service life tests as anodes under oxygen-generating conditions and in the chlor-alkali electrolyte it should have many times longer service life than the comparable anode (a) and considerably longer than the reference anode (b).

13 68090

Example II

The titanium coupon was degreased, rinsed in water, dried, pickled and then surface treated as in Example I with a paint solution containing iridium and ruthenium chlorides in a weight ratio of 2: 1 (as metal). The treatment was repeated four times until the titanium dioxide film formed contained 0.2 g / m 2 of Ir and 0.1 g / m 2 of Ru, both calculated as metals. The heat treatment was performed at 400 ° C for 10 minutes after each applied layer. The TiO 2 * RuCl 2 outer layer was then applied as in Example I. The same comparative electrochemical experiments have given the same promising first results as in Example I.

Example III

The titanium coupons were degreased, rinsed in water, dried and pickled as in Example I and treated with a similar iridium chloride solution as in Example I. The solution was applied in four thin layers and the coupons were dried to evaporate the solvent and then heated to 480 ° C for 7 minutes on each coating. The iridium concentration was varied to obtain an iridium content (calculated as metal) of 0.3, 0.6 and 0.8 g / m 2 in the barrier layer.

A solid solution of titanium dioxide and ruthenium dioxide was then applied as in Example I except that the thickness of the coating corresponded to 20 g / m (approximately 8 g / m of Ru metal). These electrodes were subjected to accelerated lifetime experiments under oxygen generating conditions.

The maximum lifetime was observed with a coupon with barrier layer 2 containing 0.6 g / m Ir. This represented a 10.3-fold increase in lifetime compared to a similar electrode without a barrier layer (or a TiO 2 barrier layer without iridium). For comparison, a similar coated electrode without a barrier layer but with the addition of 0.6 g of iridium dispersed in the coating shows only a marginal prolongation of the service life.

Example IV

The electrodes were prepared in the same manner as in Example I, but using a dilute paint containing chlorides of various platinum group metals such as palladium alone, platinum and ruthenium as well as rhodium and iridium as described above to prepare the barrier layer. These electrodes were subjected to comparative lifetime experiments as oxygen evolution anodes. Only electrodes with a barrier layer containing Rh and / or Ir showed a significant increase in lifespan in this experiment? Combinations of Rh and / or Ir with minor amounts of other platinum group metals or their compounds, especially Ru and Pd, also produced significant improvements.

Example V

Titanium coupons were provided with barrier layers containing approximately 0.2 g / m iridium and / or rhodium after the procedure of Example I. They were then painted with a solution of 0.5 g of iridium chloride and 1 g of platinum chloride in 10 ml of isopropyl alcohol and 10 ml of linalool and heated in an oven to 350 ° C. The ammonia / hydrogen mixture was then passed for about 30 seconds to form a layer containing 70% Pt and 30% Ir. The coating procedure was repeated to grow n coatings containing 4 g / m 2 Pt / Ir alloy. For similar electrodes coated with less than 7 g / m 2 Pt / Ir alloy, but without an improved barrier layer, it has been reported that operation at elevated current density causes passivation and at least 7 g / m 2 must be applied to achieve satisfactory operation over long periods of time. This problem of tl 15 68090 is apparently overcome by an electrode according to the invention which works satisfactorily with a layer of 4 g / m 2.

Example VI

Titanium coupons were provided with barrier layers 2 containing about 0.2 g / m iridium and / or rhodium after the procedure of Example 9. A layer of approximately 400 g / m titanium dioxide was then plasma sprayed onto the barrier layer using standard techniques. The plasma-sprayed titanium oxide layer was then coated with layers containing 2 g / m 2 (as metal) of ruthenium oxide and / or iridium oxide in various proportions by painting with a solution of 6 ml of propanol and 1 g of RuCl 2 and / or IrCl 2 and by heating in air to 500 ° C for 10 minutes, after each coating. Preliminary electrochemical testing indicates that these electrodes should have a very long lifespan as anodes in mercury-chlor-alkali cells operating at high current densities. The results disclosed in U.S. Patent No. 4,140,813 show that the electrodes of the present invention achieve the same excellent service life with only one-fifth of its precious metal addition.

Example VII

Titanium coupons were provided with barrier layers containing about 0.3 g / m 2 iridium, rhodium, and iridium / ruthenium in a weight ratio of 2: 1 after the procedure of Example I (except that in some cases the final heating was extended to several hours).

An aqueous solution containing iridium chloride and tantalum chloride (equal amounts of Ir and Ta metals) was applied with a brush to both sides of the coupons in 5, 10 and 15 layers. Each applied layer si ie 68090 contained about 0.5 g / m iridium. After each layer, the coupons were dried and heated in the air to 450 ° C for 10 minutes and for one hour after the last layer. The coating obtained was a solid solution of iridium and tantalum oxides containing about 2.5, 5 and 7.5 g / m 2 of iridium. The electrodes were tested as anodes in 10% sulfuric acid at 60 ° C with a current density of 1.2 kA / m 2 and the power was turned off for 15 minutes in each 24 hour period without removing the electrodes from the acid bath. The first results showed excellent performance compared to the corresponding electrodes formed on a standard titanium substrate and a titanium-palladium alloy substrate containing 0.15% palladium. The titanium substrate with the barrier layer according to the invention is, of course, much cheaper than this titanium palladium alloy and provides a greatly improved resistance to cell stoppage and the passivating effect of oxygen evolution. Based on preliminary observations, the electrodes of the invention with a small iridium addition (2.5 g / m + 0.3 g / m in the barrier layer) should have an excellent service life compared to similar electrodes without a barrier layer.

Claims (22)

An electrode for use in electrolytic processes, which electrode comprises a film-forming metal substrate having a porous electrocatalytic coating comprising at least one platinum group metal and / or oxide thereof, optionally mixed with other metal oxides, in an amount of at least about 2 g / ml. m of platinum group metal (s) per projected surface area of the substrate, and which substrate during the coating has a preformed barrier layer which is a surface oxide film grown from the substrate, characterized in that the preformed barrier layer has rhodium and / or iridium incorporated into the surface oxide film during formation thereof in an amount of up to 1 g / m 2 (as metal) per projected surface area of the substrate. 2. An electrode according to claim 1, characterized in that the porous electrocatalytic coating is supported by a plurality of layers arranged on top of each other with microcracked configuration. 3. An electrode according to claim 2, characterized in that the porous electrocatalytic coating consists predominantly of a solid solution of at least one oxide of film-forming metal and at least one platinum group metal oxide. 4. An electrode according to claim 3, characterized in that the porous electrocatalytic coating is a solid solution of ruthenium and titanium oxides with an atomic ratio of ruthenium: titanium of 1: 1 to 1: 4. 5. An electrode according to claim 1, characterized in that the porous electrocatalytic coating consists predominantly of one or more platinum group metals. 6. An electrode according to claim 5, characterized in that the porous electrocatalytic coating is a platinum-iridium alloy. ti 23 6 80 90 » 7. An electrode according to claim 1, characterized in that the porous electrocatalytic coating is a plasma-sprayed layer of at least one film-forming metal oxide, incorporating the platinum group metal (s) and / or oxides thereof. Electrode according to any of the preceding claims, characterized in that the surface oxide film of the barrier layer contains at least one extra added metal in addition to rhodium and / or iridium, but in a smaller amount than the rhodium and / or iridium, the total metal of the barrier layer. - 2 content is up to 1 g / m. 9. An electrode according to claim 8, characterized in that said film contains up to 0.5 g / m 2 of iridium and ruthenium in a weight ratio of about 2: 1. 10. An electrode according to any of the preceding claims, characterized in that the substrate is titanium and the surface oxide film is predominantly rutile titanium dioxide. 11. A method of preparing an electrode for use in electrolytic processes, which method comprises forming a barrier layer on a film-forming metal substrate and applying over the barrier layer a porous outer electrocatalytic coating which includes at least one platinum group metal and / or oxide. thereof, optionally mixed with other oxides, in an amount of at least about 2 g / m 2 of platinum group metal (s) per projected surface area of the substrate, characterized in that the barrier layer is formed by applying one or more coatings of a highly dilute acid solution containing a thermally decomposable compound of rhodium and / or iridium, drying and heating the coating or any coating to form on the substrate a surface film of oxide of film-forming metal and at the same time to etherstone partially decomposing compounds, wherein the applied coating or any applied coating of said highly dilute solution ng contains an amount of said compound which is substantially completely absorbed in the surface film formed during heating, and that the number of coatings applied is such that the thus formed barrier layer contains up to 2 to 1.0 g / m of rhodium and / or iridium per projected surface area of the substrate. 12. A method according to claim 11, characterized in that the applied coating or any applied coating of the solution contains up to 0.2 g / m of rhodium and / or iridium metal per projected surface area of the substrate. A method of manufacturing an electrode for use in electrolytic processes, which method involves forming a barrier layer on a film-forming metal substrate and applying over the barrier layer a porous outer electrocatalytic coating comprising at least one platinum group metal and / or oxides. thereof, optionally mixed with other oxides, in an amount of at least about 2 g / m contains up to 0.2 g / m 2 (as metal per projected surface area of the substrate) of a thermally decomposable compound of rhodium and / or iridium in a solution which attacks the substrate of film-forming metal, and after drying heating each coating to to form an oxide barrier layer of the film-forming metal containing up to 1.0 g / m of iridium and / or rhodium. 14. A method according to any one of claims 11-13, characterized in that 2-5 coatings of the diluted solution are applied, each coating being followed by heating to between about 300 ° C and 600 ° C. for about 5-15 minutes, the final coating possibly being heated for a longer period. 15. A method according to any one of claims 11-14, characterized in that the heating is carried out to completely decompose said compound. 16. A method according to any one of claims 11-15, characterized in that the porous outer electrocatalytic coating is formed by applying a plurality of coatings of a relatively concentrated solution containing a thermally decomposable platinum group metal compound over the preformed barrier layer. 17. A method according to claim 16, characterized in that each applied outer coating contains at least 0.4 g / m 2 of platinum group metal per projected area of the substrate base. 18. A method according to any of claims 11 to 15, characterized in that the porous outer electrocatalytic coating is applied by plasma spraying. 19. A method according to any one of claims 11-15, k other than that, wherein the porous outer electrocatalytic coating is applied by plasma spraying at least one oxide of film-forming metal over the preformed barrier layer (s) and then incorporating the platinum group metal (s) and / or oxides thereof in the plasma-sprayed oxide of film-forming metal. 20. A method according to any one of claims 11 to 19, characterized in that a set of electrode substrates is subjected together to a series of pre-treatment.