NO155702B - COATED METAL ELECTRODE FOR ELECTROLYSE PROCESSES AND PROCEDURES OF MANUFACTURING THEREOF. - 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

Technical area

The 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 or alloys containing one or more of these metals, and also silicon-iron alloys, coated with an electrocatalytic coating containing one or more metals from the platinum group or oxides of these metals possibly mixed with other oxides.

By "film-forming metal" is meant a metal which has

the characteristic that when it is connected as an anode in the electrolyte in which the coated anode is later to be used, a passivating oxide is quickly formed which protects the underlying metal against corrosion due to the electrolyte. These metals are also often referred to as "valve metals".

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

State of the art

Norwegian patent document 138907 relates to electrodes for electrolysis processes and generally describes electroplating of a thick top layer of e.g. platinum metal on a thermally cleavable substrate. This substrate is not incorporated into a barrier layer formed as a surface oxide film by the substrate (e.g. titanium) during formation of the film. The patent does not make any allusion regarding a separately formed Ti02_barrier layer, see e.g. the example on page 9, lines 5-7.

US patent document 4127468 relates to a completely different type of electrode, i.e. a finely divided, porous electrode consisting of a noble metal (metal from group VIII, Pt, Pd, Ir etc.) which is alloyed with another metal, e.g. . rhenium, which may optionally be supported on a titanium substrate (see column 2, line 52). The patent's description, however, concentrates on gas diffusion electrodes and the porous alloy and its production. There is no reference to a barrier layer.

According to previous proposals (see e.g. British patent documents 855107 and 869865) a titanium electrode with a coating of a metal from the platinum group was provided with an inert barrier layer of titanium oxide at the porous places of the coating,

and this barrier layer was preferably formed or reinforced by means of a heat treatment. According to a later proposal made in British patent document 925080, the inert barrier layer of titanium oxide was formed in advance by electrolytic treatment or heating of the titanium substrate in an oxidizing atmosphere before the platinum group metal was applied. This pre-formation of such a barrier layer was also proposed in British patent document 1147422 with the aim of improving the anchoring of an active coating consisting of or containing platinum group metal oxides.

The later development of coatings formed from mixed crystals or solid solutions of co-deposited oxides of film-forming metals and platinum group metals (see US Patent 3,632,498) provided commercially applicable electrodes that revolutionized the chlor-alkali industry and have been widely used for other purposes. With these electrodes, excellent results were achieved without the need for a reinforced or previously formed inert barrier or anchoring layer on the substrate, and it is generally accepted today that the previously formed or reinforced inert barrier layers are harmful to the results of use. Judging from the previous state of the art, the proposals regarding preformed or reinforced,

inert barrier layers to have been unsuccessful attempts to avoid defects that the previous coatings rather than the substrate were afflicted with.

Nevertheless, some proposals have still been made in an attempt to improve inert barrier layers,

e.g. by applying a titanium oxide barrier layer from a solution containing Ti 4+ ions. This has again been shown to be detrimental to the electrodes' performance properties.

Another attempt has been to provide a non-passivating barrier layer or intermediate layer under the active outer coating. Typical proposals have been doped tin dioxide substrates, thin substrates of one or more platinum metals, such as a platinum-iridium alloy, and substrates of cobalt oxide or lead oxide etc. Although various patents have claimed that these electrodes provide marginal improvements for particular applications, none of these proposals in practice led to any significant improvement or to extensive commercial application.

Description of the invention

The invention relates to an electrode for use in electrolysis processes, comprising a substrate of film-forming metal with a porous, electrocatalytic coating comprising at least one platinum group metal and/or its oxide optionally mixed with other metal oxides, in an amount of at least 2 g/m the platinum group metal or metals per projected surface area of the substrate, as the substrate under the coating has a pre-formed barrier layer consisting of a surface oxide film grown from the substrate, and the electrode is characterized by the fact that rhodium and/or iridium has been incorporated into the pre-formed barrier layer surface oxide film during the formation of this , in an amount of up to 1 g/m<2 > (calculated as metal) per projected surface area of the substrate.

The barrier layer's surface oxide film is made non-passivating by the incorporation of rhodium and/or iridium as metal or as a compound, usually the oxide or a partially oxidized compound.

The invention also relates to a method for the production of such an electrode and which is characterized by the fact that the barrier layer is formed by applying to the substrate one or more coatings of a highly diluted (as defined below) acidic solution containing a thermally cleavable compound of rhodium and/or iridium, after which the applied coating or each applied coating is dried and heated to form on the substrate a surface film of film-

forming metal oxide and at the same time at least partially cleaving the compounds, the applied coating or each applied coating of the highly diluted solution containing such an amount of the compound that it is absorbed substantially completely in the surface film formed during heating, and the number of applied coatings is such that the thus formed barrier layer is brought to contain up to 1.0 g/m 2 of rhodium and/or iridium per projected surface area of the substrate.

The paint or solution used will typically contain an organic solvent, such as isopropyl alcohol, an acid (especially HCl, HBr or HI) or another agent (eg NaF) which attacks the film-forming metal and facilitates the formation of film-forming metal oxide during the subsequent heat treatment, and one or more heat-cleavable salts of iridium and/or rhodium. This solution will usually be at least 5 times more diluted, preferably approx. 10 or more times more diluted (expressed by the content of precious metal) than the paint solution that can be used for the production of the outer, porous, electrocatalytic oxide coatings. This means that the amount of iridium and/or rhodium will be reduced, e.g. to 1/5 or 1/10 or even 1/100, in relation to the amount of the corresponding platinum group metal in the paint used to form the outer coating, with approximately the same amount of solvent and acid. The diluted paint will generally contain 1-15 g/l iridium and/or rhodium (calculated as metal).

The action of the acid or other agent which attacks or corrodes the film-forming metal and facilitates the formation of the oxide film during the subsequent heat treatment is very important. Without a suitable agent which provides this effect, the formation of the surface oxide film of the film-forming metal will be substantially prevented or strongly inhibited.

It has been found that when one coating of a given solvent/acid mixture is applied to a film-forming metal substrate that has previously been cleaned and etched in the usual way, after which heating is carried out to drive off the solvent, a given amount of film-forming metal oxide is formed. This method can be repeated a number of times (typically four or five times with 4 ml of HCl in 60 ml of isopropyl alcohol applied to a titanium substrate, dried and heated to 500°C for 10 minutes) before the growth of film-forming metal oxide during successive treatments is inhibited. The first layer of the integrating surface oxide film that is formed will be relatively porous. This enables the subsequently applied coating of the acidic paint to penetrate this porous first layer during the drying step, so that the acid will attack the underlying film-forming metal. Ions of the film-forming metal are thus provided by the substrate for conversion to oxide during the subsequent heating, and this oxide will partially form in the pores of the first layer. The porosity of the obtained oxide film is thus reduced after each coating cycle until no more film-forming metal from the substrate can be converted to oxide. An extremely stable, relatively compact and impermeable film of film-forming metal oxide can thus be formed by applying a limited number of coats of acidic paint followed by drying and heating.

In the production of barrier layers according to the invention, each applied paint coating contains such a small amount of the iridium and/or rhodium compound that the electrocatalyst formed by the thermal decomposition is completely incorporated into the integrating surface film of film-forming metal oxide that is formed each time. As a rule, each applied coating of the paint will contain a maximum of approx. 0.2 g/m<2 >iridium and/or rhodium per projected surface area of the substrate, usually far less. Moreover, the application of further layers of the diluted paint will be stopped after the number of coatings has been applied beyond which the growth of the surface oxide film on the film-forming metal ceases or is inhibited. The optimum amount of electrocatalytic agent in the diluted paint and the optimum number of coatings to be applied to achieve a satisfactory compact, impermeable barrier layer can thus be very easily determined for any particular substrate, solvent/acid and electrocatalytic material . In a number of cases, two or ten layers of the highly diluted paint will be applied, each application being followed by drying and heating at 400-600°C for 5-15 minutes, possibly with the exception of the last layer which can be heated for a longer time, perhaps for several hours or days at 450-600°C

in air or in a reducing atmosphere (e.g. ammonia/hydrogen).

By visual observation or under a microscope, it can be determined that barrier layers produced in this way on an etched or unetched titanium substrate generally retain the same area of clear appearance as titanium oxide films produced in the same way, but which do not contain iridium and/or rhodium electrocatalyst, typically a light blue, yellow and/or red "interference" film color.

The diluted, acidic paint solution used to produce the barrier layer according to the invention preferably only contains a thermally cleavable iridium and/or rhodium compound as the film-forming metal oxide component is provided by the substrate. However, the diluted paint may contain small amounts of other constituents, such as other platinum group metals (ruthenium, palladium, platinum, osmium and especially ruthenium), gold, silver, tin, chromium, cobalt, antimony, molybdenum, iron, nickel, manganese, tungsten, vanadium, titanium, tantalum, zirconium, niobium, bismuth, lanthanum, tellurium, phosphorus, boron, beryllium, sodium, lithium, calcium, strontium, lead or copper compounds or mixtures thereof. If any small amount of a compound of a film-forming metal is used, this will usually be a metal other than the film-forming metal in the substrate to thereby cause the surface film to be doped. Excellent results have been obtained using iridium/ruthenium compounds in a weight ratio of approx.

2:1, calculated as metal. When such additives are contained in the diluted paint, they will of course

lie in an amount compatible with the small amount of the main electrocatalyst, i.e. an iridium and/

or rhodium compound, so that substantially the entire amount) of the main electrocatalyst and additive will be incorporated into the surface film of the film-forming metal oxide. In any case, the total amount of iridium and/or rhodium and other metals will be below 1 g/m 2 , and as a rule below 0.5 g/m <2>, and the additional amount of metal will be present in an amount less than rhodium and/or iridium. These iridium/rhodium compounds and other metal compounds may be thermally cleavable to form the metal or the oxide, but in neither case is complete cleavage required.

For example, barrier layers containing partially cleaved iridium chloride containing up to approx. 5% by weight of the original chloride, showed excellent properties. Barrier layer containing as little as 0.1-0.3 g/m (calculated as metal)

of iridium and/or rhodium oxide/chloride in the surface films,

gives excellent results. Experiments have shown that a barrier layer containing 0.5-0.6 g/m 2 (calculated as metal) of iridium,

provides an optimal effect expressed by increased lifespan for the coated electrodes. When the amount of iridium is increased beyond these values, no further increase in lifespan is obtained.

When a titanium substrate is used, it has been shown that the surface oxide film mainly consists of titanium oxide in rutile form. It is believed that the formation of rutile, e.g. at 400-500°C, is catalyzed by the rhodium and/or iridium in the diluted coating solution.

After the improved barrier layer has been formed as

is impermeable to electrolyte and to evolved oxygen, the porous, outer, electrocatalytic coating is applied using common methods, e.g. by applying a series of coatings of a relative

show concentrated solution containing a thermally cleavable compound of a platinum group metal, followed by heating. Each applied outer coating will contain at least 0.4 g/m 2 of the platinum group metal per projected area of the substrate, and the coating method is repeated to build up an effective outer coating containing at least approx. 2 g/m 2 of the platinum group metal or metals, usually in oxide form. The coating components can be selected so that a coating is obtained which mainly consists of a solid solution of at least one oxide of a platinum group metal, as described in US patent document 3632498. The coating is advantageously made up of a solid solution of ruthenium and titanium oxides with an atomic ratio between ruthenium and titanium of 1:1-1:4. In this case, the coating consists of several superimposed layers which typically have an appearance with microcracks and are quite porous. When an improved barrier layer is used according to the invention together with such a coating, the properties of the electrode are greatly improved in standard accelerated lifetime tests under oxygen-releasing conditions. It can be predicted that under the conditions that prevail in the normal technical production of chlorine, the improved electrode will have a significantly longer lifespan as it is known that one of the reasons why these electrodes fail after prolonged use in the production of chlorine is due to the influence of oxygen on the substrate. It will also be possible to achieve the same lifespan with a significant reduction in the thickness of the outer coating, thereby making it possible to save the amount of coating materials used and the work effort and energy consumption necessary for production.

The outer coating can also be; formed from one or more metals from the platinum group, e.g. a platinum-iridium alloy useful for the production of chlorate and to a limited extent in diaphragm or membrane cells for chlorine production. For ordinary Pt/Ir-coated electrodes, the coatings must be relatively thick (at least approx. 5 g/m 2 ) to avoid passivation problems. When the improved barrier layer is used according to the invention, thinner and more porous layers of the platinum metals can be used without problems arising due to oxidation of the substrate or without the disadvantages associated with the previously known passive barrier layers of titanium oxide.

It is also possible to apply the outer coating by plasma spraying of a solid solution of an oxide of a film-forming metal and an oxide of a platinum group metal. For example, a powder in the form of a solid solution can be produced by flame spraying as described in US patent 3677975, and this powder is then plasma sprayed onto the substrate. Alternatively, the coating is applied by plasma spraying of at least one oxide of a film-forming metal over the previously formed barrier layer, after which the platinum group metal or metals and/or their oxides are incorporated into the plasma-sprayed oxide of the film-forming metal, e.g. by the method described in US patent 4140813. Again, the improved barrier layer increases the lifespan and makes it possible to reduce the content of precious metal in the coating.

According to a preferred method for mass production of the electrodes, a set of electrode substrates is subjected together to a series of pre-treatments, including etching and formation of a barrier layer by dipping the substrate set in the diluted solution and heating the substrate set, after which the outer, electrocatalytic coating is applied to the substrates, one at a time. This method avoids the disadvantage of commercial electrode coating systems which are associated with a "bottleneck" between the etching bath and the coating line. In the usual mass production method, a set of substrates is pre-treated by sandblasting followed by etching, rinsing and drying, and these substrates are then individually coated in a coating/firing line.

It has thus been necessary to synchronize the etching with the coating/firing because the etched substrates cannot remain for a long time (longer than approx. 2 days) without this affecting the electrode's performance characteristics due to air oxidation of the substrate before coating, especially if dust or dirt becomes embedded in the thin oxide film. When the sets of substrates are pre-coated with an improved barrier layer immediately after etching, this bottleneck effect is avoided, and the surface-treated substrates can be stored without the risk of further oxidation. Any dust or dirt that may be deposited on the barrier layer can be easily blown away before coating as it is not anchored in the film.

Also, the dip coating method for a set of substrates stacked against each other is satisfactory for producing the improved barrier layer oxide film grown from the substrate. A similar handling is not satisfactory for the application of the usual coatings, where an additional thickness of each applied coating must

is built up over and on top of the film-forming metal base and its very thin surface oxide film.

The electrode base can be made of a sheet of any film-forming metal, titanium being preferred for economic reasons. Rods, tubes and expanded sheets of titanium or other film-forming metals can also be surface-treated using the present method. Titanium or another film-forming metal which has been applied by cladding to a conductive core can also be used. For most applications, the base will be etched before the surface treatment so that a rough surface is obtained which provides good anchorage for the later applied electrocatalytic coating. It is also possible to surface treat porous, sintered or plasma-sprayed titanium with the diluted paint solutions in the same way, but the porous titanium will preferably only form a surface layer on a non-porous base.

The electrodes with an improved barrier layer applied according to the invention are excellently suitable as anodes for chlorine-alkali electrolysis. These electrodes have also shown outstanding performance characteristics when used for electrorecovery in a mixed chloride-sulphate electrolyte which produces a mixed emission of chlorine and oxygen.

The best embodiments of the invention

The invention will be further described in the following examples.

Example 1

Titanium specimens measuring 7.5 x 2 cm available under the trade name "Contimet 30" were degreased, rinsed in water, dried and etched for 0.5 hour in oxalic acid. A paint solution consisting of 6 ml of n-propanol,

0.4 ml of concentrated HCl and 0.1 g of iridium and/or rhodium chloride were then applied by brushing on both sides of the test pieces in the form of four thin coatings. The specimens were dried to evaporate the solvent and then heated in air to 500°C for 10 minutes after each of the first three coatings had been applied, and for 30 minutes after the final coating had been applied. This gives a content of 0.2-0.3 g/m 2 of rhodium and/or iridium (calculated as metal) in the barrier layer depending on the amount of solution that was applied for each coating, determined by weight measurement.

A solid solution of titanium oxide-ruthenium oxide with an atomic ratio between titanium and ruthenium of approx. 2:1 was then applied by brushing on a solution consisting of 6 ml of n-propanol, 0.4 ml of concentrated HCl, 3 ml of butyl titanate and 1 g of RuCl^, followed by heating in air at 400°C for 5 minutes (it should it is noted that this solution is 10 times more concentrated expressed by the ratio, noble metal:propanol than the diluted solution used for the production of the barrier layer). This method was repeated until the coating was present in a thickness of approx. 10 g/m 2 ' (ie approx. 4 g/m 2 Ru metal).

Electrodes which had been prepared in this way are subjected to comparable electrochemical experiments with similar electrodes (a) with a TiC^ barrier layer formed by the same method, but with a paint consisting exclusively of 6 ml of n-propanol and 0.4 ml of concentrated HC1, and

(b) with no barrier layer. The first results suggest

that the electrode according to the invention has a greatly superior lifetime in accelerated lifetime tests as anodes under oxygen-releasing conditions, and that in chlorine-alkali electrolysis they should have a lifetime that is several times longer than the comparison anode (a) and considerably longer than the comparison anode (b).

Example 2

A titanium specimen was degreased, rinsed in water, dried, etched and then surface treated as in Example 1 with a paint solution containing iridium and ruthenium chlorides in a 2:1 weight ratio (calculated as metal). The treatment was repeated four times until the formed titanium dioxide film contained 0.2 g/m <2> Ir and 0.1 g/m <2> Ru, both calculated as metal. The heat treatment was carried out at 400°C for 10 minutes after each applied coating. An outer coating of TiC>2.Ru02 was then applied as in Example 1. The same comparable electrochemical experiments have given the same initial promising results as in Example 1.

Example 3

Titanium specimens were degreased, rinsed in water, dried and etched as in Example 1 and treated with an iridium chloride solution similar to that of Example 1. The solution was applied in the form of four thin coatings, and the specimens were dried to evaporate the solvent. and then heated to 480°C for 7 minutes at the end of each coating application. The iridium concentration was varied so that a content of 0.3, 0.6 and 0.8 g/m 2 iridium (calculated as metal) was achieved in the barrier layer.

A solid solution of titanium dioxide-ruthenium dioxide was then applied as in example 1, but with the change that the coating thickness equaled 20 g/m 2 (approx. 8 g/m 2 Ru metal). These electrodes were subjected to accelerated lifetime tests under oxygen-releasing conditions. The maximum lifetime was observed for the test piece with a barrier layer containing 0.6 g/m 2 Ir. This gave a lifetime increase by a factor of 10.3 compared to the lifetime of a similar electrode without a barrier layer (or with a barrier layer of TiO 2 without iridium). In comparison, a similarly coated electrode with no barrier layer, but with the addition of 0.6 g of iridium dispersed in the coating, showed only a marginal lifetime increase.

Example 4

Electrodes were prepared in a similar manner to example 1, but using a diluted paint containing chlorides of various platinum group metals, including palladium, platinum and ruthenium alone, and also rhodium and iridium as described above, for the preparation of the barrier layer. These electrodes were subjected to comparable lifetime tests in the form of oxygen-releasing anodes. Only the electrodes that had a barrier layer containing Rh and/or Ir showed a marked increase in lifespan in this experiment. Combinations of Rh and/or Ir with smaller amounts of the other platinum group metals or their compounds, especially Ru and Pd, also gave significant improvements.

Example 5

Titanium test pieces were provided with a barrier layer containing approx. 0.2 g/m 2 iridium and/or rhodium, using the method of example 1. They were then ground with a solution containing 0.5 g iridium chloride and 1 g platinum chloride in 10 ml isopropyl alcohol and 10 ml linalool, and heated in an oven to 350°C. An ammonia/hydrogen mixture was then introduced for approx. 30 s for the production of a coating containing 70% Pt and 30% Ir. The coating method was repeated to build up a coating containing 4 g/m 2 of the Pt/Ir alloy. For similar electrodes coated with less than 7 g/m 2 of the Pt/Ir alloy, but without the improved barrier layer, it has been reported that use at elevated current density results in passivation and that at least 7 g/m 2 must be applied to achieve satisfactory operation for a longer period of time. This problem is apparently overcome by the electrode according to the invention which provides satisfactory operation with a coating on it

4 g/m .

Example 6

Titanium test pieces were provided with a barrier layer containing approx. 0.2 g/m 2 iridium and/or rhodium, using the method according to example 1. A layer of approx. 400 g/m 2 titanium oxide was then plasma sprayed onto the barrier layer using conventional methods. The plasma-sprayed titanium oxide layer was then coated with coatings containing 2 g/m (calculated as metal) ruthenium oxide and/or iridium oxide in various ratios, by painting with a solution of 6 ml of propanol and 1 g of RuCl3 and/or IrCl3 and heating in air to 500 C for 10 minutes after each coating. Preliminary electrochemical investigation suggests that these electrodes should have an extremely long lifetime used as anodes in mercury-chlor-alkali cells operated at high current densities. Based on the data published in US Patent 4140813, it appears that the electrode according to the invention will achieve the same excellent life span with as little as 1/5 of the amount of precious metal.

Example 7

Titanium test pieces were provided with a barrier layer containing approx. 0.3 g/m 2 iridium, rhodium and iridium/ruthenium in a weight ratio of 2:1, using the method according to example 1 (with the exception that in some cases the final heating was extended for several hours).

An aqueous solution containing iridium chloride and tantalum chloride (with Ir and Ta metal in equal weight ratio) was applied by brushing on both sides of the test pieces in the form of 5, 10 and 15 coats. Each applied coating contained approx. 0.5 g/m 2 iridium. After each coating application, the test pieces were dried and heated in air for 10 minutes at 450°C and for 1 hour after the intended coating application. The overall coating obtained consisted of a solid solution of iridium and tantalum oxides containing approx. 2.5, 5 and 7.5 g/m 2 iridium. The electrodes were examined as anodes in 10% sulfuric acid at 60°C and a current density of 1.2 kA/m<2>, the current supply being stopped for 15 minutes for each period of 24 hours without the electrodes being removed from the acid bath. The original results suggest a superior performance compared to similar electrodes on a pure titanium substrate and on a titanium-palladium alloy substrate containing 0.15% palladium. The titanium substrate with a barrier layer according to the invention is of course far less expensive than this titanium-palladium alloy and provides a greatly improved resistance to the necessity of shutting down the cell and to the passivating effect of oxygen release. Based on the preliminary indications, the electrodes according to the invention with a low amount of iridium (2.5 g/m 2 + 0.3 g/m 2 in the barrier layer)

have an outstanding lifetime compared to similar electrodes without the barrier layer.

Claims (19)

1. Electrode for use in electrolysis processes, comprising a substrate of film-forming metal with a porous, electrocatalytic coating comprising at least one platinum group metal and/or its oxide optionally mixed with other metal oxides, in an amount of at least 2 g/m of the platinum group metal or metals per projected surface area of the substrate, the substrate under the coating having a previously formed barrier layer consisting of a surface oxide film grown from the substrate, characterized in that rhodium and/or iridium has been incorporated into the surface film of the previously formed barrier layer during its formation, in an amount of up to 1 g/m 2 (calculated as metal) per projected surface area of the substrate. 2. Electrode according to claim 1, characterized in that the porous, electrocatalytic coating consists of a series of superimposed layers with a structure that exhibits microcracks. 3. Electrode according to claim 1 or 2, characterized in that the porous, electrocatalytic coating mainly consists of a solid solution of at least one film-forming metal oxide and at least one oxide of a platinum group metal. 4. Electrode according to claim 3, characterized in that the porous, electrocatalytic coating consists of a solid solution of ruthenium and titanium oxides with an atomic ratio between ruthenium and titanium of 1:1-1:4. 5. Electrode according to claim 1, characterized in that the porous, electrocatalytic coating mainly consists of one or more platinum group metals. 6. Electrode according to claim 5, characterized in that the porous, electrocatalytic coating consists of a platinum-iridium alloy. 7. Electrode according to claim 1, characterized in that the porous, electrocatalytic coating consists of a plasma-sprayed layer of at least one film-forming metal oxide containing the platinum group metal or metals and/or oxides thereof. 8. Electrode according to claims 1-7, characterized in that the barrier layer's surface oxide film contains at least one additional added metal besides rhodium and/or iridium, but in a smaller amount than the rhodium and/or iridium, the barrier layer's total metal content being up to 1 g/m 2. 9. Electrode according to claim 8, characterized in that the film contains up to 0.5 g/m 2 iridium and ruthenium in a weight ratio of approx. 2:1. 10. Electrode according to claims 1-9, characterized in that the substrate consists of titanium and that the surface oxide film mainly consists of titanium dioxide in rutile form. 11. Method for the production of an electrode for use in electrolysis processes, where a barrier layer is formed on a substrate of film-forming metal and a porous, external, electrocatalytic coating is applied over the barrier layer which comprises at least one platinum group metal and/or its oxide optionally mixed with other oxides, in an amount of at least 2 g/m 2 of the platinum group metal or metals per projected surface area of the substrate, characterized in that the barrier layer is formed by applying one or more coatings of a highly diluted (as hereinafter defined) acidic solution containing a thermally cleavable compound of rhodium and/or iridium to the substrate, after which the applied coating or each applied coating is dried and is heated to form on the substrate a surface film of film-forming metal oxide and at the same time at least partially cleave the compounds, the applied coating or each applied coating of the highly diluted solution containing such an amount of the compound that it is substantially completely absorbed into the surface film which is formed during heating, and as the number of applied coatings is such that the barrier layer thus formed is brought to contain up to 1.0 g/m 2 of rhodium and/or iridium per projected surface area of the substrate. 12. Method according to claim 11, characterized in that the barrier layer is formed by applying to the substrate several coatings each containing up to 0.2 g/m<2> (calculated as metal per projected surface area of the substrate) of the thermally cleavable compound of rhodium and/or iridium. 13. Method according to claim 11 or 12, characterized in that from 2 to 5 coats of the diluted solution are applied, followed by heating to 300-600°C for 5-15 minutes after application of each coat, with the final coat possibly being heated for longer time. 14 . Method according to claims 11-13, characterized in that the heating is carried out so that the compound is incompletely split. 15. Method according to claims 11-14, characterized in that the porous, outer, electrocatalytic coating is formed by applying to the previously formed barrier layer a series of coatings of a relatively concentrated solution containing a thermally cleavable compound of a platinum group metal, followed by heating . 16. Method according to claim 15, characterized in that each outer coating is applied with a content of at least 0.4 g/m 2 platinum group metal per projected area of the substrate base. 17. Method according to claims 11-14, characterized in that the porous, outer, electrocatalytic coating is applied by plasma spraying. 18. Method according to claims 11-14, characterized in that the porous, outer, electrocatalytic coating is applied by plasma spraying of at least one film-forming metal oxide over the previously formed barrier layer, after which the platinum group metal or metals and/or oxides thereof are incorporated in the plasma-sprayed film-forming metal oxide. 19. Method according to claims 11-18., characterized in that a set of electrode substrates are together subjected to a series of pre-treatments which include etching and formation of the barrier layer by dip-coating the set of substrates in the diluted solution and heating the set of substrates, after which the outer, electrocatalytic coating is applied to one of the substrates at a time.