Anodes with extended service life and methods for their manufacturing

Abstract: Anodes for the generation of electrochemical processes, consisting of a support and an electrocatalytically active top-layer which contains iridium-oxide; a method for producing them and an electrochemical cell in which the anode is applied. The lifetime of such anodes may be prolonged substantially by choosing the oxygen/iridium-ratio between 2.1 and 2.9, the average particle seize of the iridium-oxide being between 3 and 100 nm. The lifetime may be prolonged even further by the presence of an intermediate layer, between support and top-layer, consisting of an oxide of Ta, Nb, Co of Pb or of a mixture thereof, if desired mixed with other metal-oxides. This intermediate layer affixes either directly to the support and the top-layer, or indirectly, via one or more additional intermediate layers.

Description: The invention concerns an anode for the generation of oxygen in an electro-chemical process comprising a support of titanium or a titanium alloy which carries an electro-chemically active porous top layer comprising iridium oxide. The invention in particular concerns the composition of electrodes, which are used as anodes in industrial electrochemical processes, in which the anodic process consists fully or partly of the generation of oxygen. It also concerns electrochemical cells, in which these anodes are used. Such processes take place for example, when the electrolyte contains water and furthermore corrosive components, such as alkali or acid, in particular sulfuric acid, whether or not mixed with other inorganic or organic components. It has been found that an anode according to the invention may also be used and even offers advantages over existing anodes, when oxygen generation is not the only anodic process or even not the principal anodic process or with other anodic processes. This is for instance the case, when the electrolyte contains sea water. An anode according to the invention has advantages for oxygen generation in sulfuric acid medium, but also in the use or co-use of other inorganic or organic acids in the electrolyte or in electrolytes with a neutral or acid composition. These anodes have the special property that they function for a longer period of time during the oxygen generation in sulfuric acid medium than the state-of-the-art industrial anodes. Moreover, they function for a longer period of time in a number of cases in other electrochemical processes and with electrolytes of other compositions. Processes, in which this extended functioning has been established, include water electrolysis, electroplating of tin, zinc, chromium, nickel and copper, in batch or in continuous processes, sea water electrolysis and cathodic protection. This invention is based on the demand for a longer service life of anodes than achieved in state-of-the-art anodes. This is necessary because of the heavier working conditions for anodes in advanced electrochemical cells and processes, like a very high current density. For each electrochemical process, the principle holds that an optimum in service life of the anode is obtained by a certain chemical and morphological composition, layer design and method for manufacturing the anode. In the literature, many compositions and methods for manufacturing industrial anodes have been described. However, in particular an oxygen-generating anode with an electrocatalytically active coating of an iridium oxide, whether or not mixed with tantalum oxide, has yielded a very stable anode for the use in sulfuric acid as an electrolyte. Such known anodes are applied onto an electrically conductive support, existing of titanium or another valve metal, such as Ta, Nb, Zr, Hf, Mo and/or W, as such or in an alloy. They have been described in the literature, like by Comninellis et al. and by Busse et al. in the Proceedings of the Symposium on Performance of Electrodes for Industrial and Electrochemical Processes, The Electrochemical Society, Pennington (NJ), Volume 89-10, 1989, p. 229 and 245 respectively. Such anodes may be prepared by dissolving tantalum chloride and/or iridium chloride in an alcohol, in the desired proportions. This solution is applied onto the substrate layerwise followed by a heattreatment at approximately 450°C. The resulting anodes yield an extended service life, in sulfuric acid as a test electrolyte, with respect to previously used anode compositions. In several studies, a mixed iridium oxide/ tantalum oxide coating (for example in 70/30 ratio) has been found to give the best results in service life under the applied conditions. Anodes with this composition and by the mentioned method show a good service life with respect to previously used coating compositions and methods, also at the high current densities (for example at 30,000 A/m 2 ), such as those which are applied in modern, rapid, electroplating techniques. Under these conditions, high demands are put on the anodes. Since the prolonged anode service life leads to greater reliability of operation and lower operating costs, achieving an extended service life is an important advantage for the electrochemical industry. The explication below shall reveal the invention and it compares the invention with existing knowledge and technology with respect to service life of such anodes. Service life of anodes: the roles of crystallinity and adhesion As described in the literature, various mechanisms exist for the de-activation of such anodes. In practice, this de-activation becomes notable by an increased applied voltage at constant current. Several types of problems are mostly pointed out as responsible for this de-activation, i.e. the growth of an oxygen skin on the substrate (underneath the coating), which acts insulating and detaching on the coating. Also, a loss of electrocatalytic activity of the coating by loss of noble metal and/or its activity have been mentioned. We shall give below insights on these practical problems for a better understanding of the solutions to them, without claiming the completeness or even correctness of the arguments. It is observed experimentally that during anode manufacturing, by extreme heat treatment in an oxygen atmosphere of the conductive metal support, in particular titanium, or upon oxygen generation by a working anode, an insulating layer is formed of titanium oxide, which approaches the formula TiO 2 in its composition. However, otherwise this oxide may have any composition TiO x , in which 0 < x < 2. This titanium oxide may occur in various structures, among which are the crystal forms rutile and anatase or an amorphous form. We have observed experimentally with the aid of Raman spectroscopy that in our supports and the anodes, made from them according to the invention, the oxide of the Ti support has the rutile structure. This does not exclude however, that anatase and/or amorphous Ti oxides occur after some anode manufacturing methods and/or in some parts of the oxide skin, which occurs on the Ti metal. Crystalline iridium dioxide IrO 2 occurs in the same crystal form (rutile) with crystal lattice distances, which are not too different from rutile TiO 2 . We believe therefore that this agreement in crystal structures may be a major reason for our observation that IrO 2 and TiO 2 layers show good adhesion. This may be one explanation, why iridium oxide coatings, according to the invention, have such a good service life. The layered structure of the coating and the heat treatment after each application of a layer promotes the good adhesion between the titanium support oxide and the deposited iridium oxide, also between the successive iridium oxide deposits. However, the titanium oxide, which is formed during the anode action does not undergo such an adhering heat treatment. Moreover support oxide growth leads to a support volume increase. Therefore, this oxide growth may lead to detachment of the coating from the support and it may limit the service life of the anode. Also, it is true that the electrical conduction of the anode is restricted by conversion of titanium oxide TiO 2-x , grown on the Ti support as oxygen deficient and therefore electrically conductive oxide (an n-type semi-conductor), into the stoichiometric, non-conductive TiO 2 . This conversion is effectuated by the highly oxidising environment, which is created by the anode during oxygen generation. To prevent these two effects of formation of an insulating titanium oxide layer and detachment of the coating, the use of an intermediate layer (type A) has been proposed. This layer prevents the penetration of corrosive electrolyte to the underlying metal support. Thereby it is prevented that electrocatalytic processes are started at the support with the mentioned damaging effects, also by intermediates in these processes. An example is the German Patent Application DE 3219003, which describes an electrode for oxygen generation, consisting of a Ti support, an intermediate layer of a conductive oxide of tantalum and/or of niobium in quantities between 0.001 and 2 g/m 2 , with an electrode coating of Ta 2 O 5 and/or IrO 2 . This intermediate layer is found to be very effective for protection of the support and for extending the service life of the anode. However, that invention uses stoichiometric iridium dioxide and ditantalum pentoxide. According to the invention it has been found that the service life of an anode may be prolonged by using two intermediate layers, viz. one of type A and one of type B, which are described below. These layers may be applied with either different composition or with different methods or with both. A possible explanation is that one of these intermediate layers, further called type A, is so chosen that it provides an optimum protection of the underlying support and the native oxide layer on it against penetration of the electrolyte and of the products or intermediates of the electrochemical process of oxygen generation and/or of the other electrochemical processes, occurring in the electrolyte. This layer A serves to prevent the further growth of an oxide skin on the underlying support. This oxide growth is a main cause for de-activation of an anode upon prolonged use, in particular during oxygen generation and even more in particular with an electrolyte, containing sulfuric acid. In a preferred embodiment of an anode according to the invention, a longer service life is realised by combining a top layer of iridium oxide or iridium oxide, mixed with tantalum oxide, with an intermediate layer of type A, consisting preferably of tantalum oxide with 0-49 mole percent of iridium oxide, cobalt oxide or lead oxide, or a mixture of two or more of these three metal oxides. The other intermediate layer, type B, may serve in this explanation in particular for promoting the adhesion between the layers above and under it. Also it will retard the de-activation of the anode by detachment of adjacent layers. In the following the correctness of this explanation is assumed for better describing the improved action of intermediate layers on the service life of anodes in the invention. The two-in-one function (protection and adhesion) of one intermediate layer is split up in two intermediate layers with different functions and different possibilities for composition and manufacturing. This splitting of the two functions is schematic and possibly over-simplified. However, it has led to a realisation of an increased service life with respect to corresponding anodes with one intermediate layer, which is ascribed to a separate optimisation of the specific functions of each intermediate layer. It shall be clear that in the layered structure of an anode according to the invention the second intermediate layer may be built-in in two ways, types 1 and 2, which are illustrated in the following scheme. Schematic structure of anodes according to the invention, with a support, two intermediate layers and a top layer. Type 1: Support I intermediate layer type A I intermediate layer type B I top layer. Type 2: Support I intermediate layer type B I intermediate layer type A I top layer. Also it shall be clear that this structure may be extended as desired to three intermediate layers, in which the intermediate layer type A is provided on both sides by an intermediate layer type B, both layers being applied with equal or different compositions and/or methods. This leads schematically to an anode structure, type 3. Type 3: Support I intermediate layer type B I intermediate layer type A I intermediate layer type B I top layer. The description, given here, of the action of the intermediate layers is only meant to explain the service life-extending action of intermediate layers. It does not in any way restrict the scope of application of the anodes in question. In Figure 1 through 3, the anode structure according to types 1 through 3, respectively, are illustrated, for the sake of simplicity on a flat support. It shall be clear that anodes with these layer structures, according to the invention, may exist also with other support forms, like cylinders or in gauze. The choice of materials and manufacturing methods for the anode structure with two types of intermediate layers, according to the invention, leads to the following description. MATERIALS CHOICE Intermediate layer type B Upon use of the preferred support, titanium or an alloy of titanium with one or more valve metals, like Ta, Nb, Hf or Zr, an oxide skin may be formed on the titanium upon application of a method according to the invention, the oxide having a rutile or rutile-like structure. Also the iridium oxide in the top layer has a rutile-like structure. The intermediate layer B shall therefore be preferably composed of one or more metal oxides with a rutile (-like) structure, schematically represented by the chemical formula MO 2 . This formula also includes those oxides, which are often described as MO 2 , but which are actually characterised by small deviations of stoichiometry, e.g. with deviations in the oxygen stoichiometry of no more than 0.1, or by lattice defects. Metal oxides with rutile structure have been described, e.g. by Rogers et al. in Inorganic Chemistry 8 (1969) 841. They occur among other metals in single or composite oxides for Ti, Sn, Si, Ge, Mn, Cr, V, Rh, Ru, Ir, Pt, Re, Os, Mo, W, Ta, Nb or Pb. The dioxides MO 2 of these metals may therefore in principle be used for improvement of the adhesion of two layers of an anode, e.g. on a Ti basis, thus extending the service life. In the intermediate layer or layers B, those metal oxides MO 2 with a rutile structure are applied in the invention, which effectively conduct electrical current. This is in particular the case for those rutile-metal oxides MO 2 , which show metallic conduction themselves, such as IrO 2 , RuO 2 or PtO 2 , and for those metal oxides MO 2 , which may become n- or p-type semi-conducting oxides by a small modification. For a non-stoichiometric iridium oxide IrO 2+x we expect p-type conduction. This p-type behaviour is well known to be supported by doping with small amounts of lower-valent metals, which may be present in amounts, varying from parts per hundred-thousand to over one percent. The presence of such amounts of these metals may improve the service life of anodes. N-type behaviour is found for example in modified TiO 2 and SnO 2 . A well-known example of conductive SnO 2 is iridium-tin oxide, which has been successfully applied in the invention. For titanium oxide the modification may be carried out in two ways. The first is the one, in which titanium oxide has the formula TiO 2-x , with x any value between 0.001 and 0.6. For small x-values up to approximately 0.008, the original rutile structure of TiO 2 is kept. For larger values of x, so-called Magneli phases are formed with modified rutile structures, like those described by Millot and others in Progress in Solid State Chemistry 17 (1987) 263-293. Both forms of oxygen-deficient TiO 2-x are useful for application in adhering intermediate layers. However, these oxides TiO 2-x have a tendency to oxidise in the oxidising environment during anode operation, thereby reducing the oxygen deficiency x. Such an oxidation may lead upon prolonged continuation to the formation of non-conductive TiO 2 , therefore to anode de-activation. For this reason, the application of this way to make the titanium oxide conductive is preferably done by using at least one Magneli-phase material, represented by the molecular formula T n iO 2n-1 , with n = 3-8, i.e. with O/Ti ratios between 1.66 and 1.875, therefore with x-values between 0.125 and 0.34. These oxides may both be obtained separately (ex-situ) as well as on the substrate (in-situ). For that purpose the literature gives various methods, for example in the references of the cited article by Millot et al. In the invention, the manufacturing of titanium suboxides TiO 2-x is preferably carried out in situ, by heating of a suitable, commercially available titanium-containing precursor, such as TiCl 4 or tetrabutyl titanate, in a suitable atmosphere (vacuum or another inert atmosphere, low in oxygen gas pressure). Also and in particular, plasma spraying of titanium oxide particles, will yield a good result, in vacuum or in air atmosphere. These stable titanium oxide intermediate layers prolong the anode service life according to the invention. The second approach for obtaining conductive titanium oxide consists of introducing higher-valent metal ions for substitution of the tetravalent titanium ion in the oxide. A doped titanium oxide Ti 1-h (M h ) O 2+k is applied, in which a small part h of the Ti is replaced by pentavalent ions, like Ta or Nb, or hexavalent ions as Mo or W, with a possible simultaneous increase of the oxygen stoichiometry by an amount k. These doped titanium oxides have been found not to be subject to oxidation to the same extent as TiO 2-x . Therefore, they are more stable in anodes as intermediate layer B than the corresponding anodes with oxygen-deficient oxides. Therefore, an anode according to the invention preferably contains titanium oxide, homogeneously doped with niobium oxide and/or tantalum oxide, the iridium and/or tantalum contents of the titanium oxide being minimally 0.0010 mole percent, but preferably over 0.0025 mole percent with an upper limit not exceeding 10 mole percent. At equal doping levels, the anode with titanium oxide, containing tantalum oxide as intermediate layer B, is more stable than the one with titanium and niobium oxide. Therefore the titanium oxide/tantalum oxide intermediate layer B is preferred. The solubility of tantalum oxide in solid titanium oxide is some weight percents with the materials and methods of the invention. Additional non-dissolved tantalum oxide is present as a second phase. It does not disturb the good action of the mixed oxide in the intermediate layer up to 70 mole percent of tantalum. An intermediate layer of type B in an anode according to the invention in this embodiment contains preferably a mixture of titanium oxide and tantalum oxide, with minimally 0.0010 and preferably minimally 0.0025 mole percent of tantalum and maximally 70 mole percent. This intermediate layer is preferably obtained by dissolving titanium and tantalum compounds, preferably titanium tetrachloride or tetrabutyl titanate and tantalum pentachloride or tantalum penta-ethoxide or -butoxide, in an alcohol, for example n-butanol, and by applying the solution by the same method as the other layers, which are applied before or after this intermediate layer, whether or not with addition of hydrochloric acid. Furthermore, service life-extending action has been obtained by admixture in the intermediate layer type B of the following rutile-type metal oxides, which have not been mentioned before: SiO 2 , CeO 2 , MnO 2 , CrO 2 , VO 2 (tetragonal or monoclinic), RhO 2 , alpha-ReO 2 , OsO 2 , MoO 2 , WO 2 , TaO 2 , NbO 2 and PbO 2 . This includes also those metal oxides MO 2+x with a rutile or rutile-like structure, in which the stoichiometry deviates from the ideal value 2 by an amount x (0<x<0.5). The materials choices and/or the layer sequences in the anodes according to the invention differ from those of known anodes. In British Patent GB 2239260, an electrode is described with long service life for oxygen generation, in which two types of layers are applied interchangingly. This electrode may be manufactured with one lower layer and one upper layer, so that it shows agreement with an electrode (anode) according to the invention. However, that electrode possesses a lower layer, which is composed of 40-79.9 metal mole percent of iridium oxide and 60-20.1 metal mole percent of tantalum oxide, plus an upper layer, which is composed of 80-99.9 metal mole percent of iridium oxide and 20-0.1 metal mole percent of tantalum oxide. In that electrode, a large amount of the expensive noble metal iridium is used in the lower layer. In an electrode (anode) according to the invention, having one intermediate layer of tantalum oxide with iridium oxide and a top layer of iridium oxide with tantalum oxide, the lower layer contains less iridium oxide, namely 0-49 metal mole percent, yielding a surprisingly good service life. In Japanese Patent JP 2-190491 a top layer is described, containing 5-95 mole percent of iridium oxide and additional tantalum oxide, with an intermediate layer of a platinum dispersion in tantalum oxide, whether or not mixed with iridium oxide, up to 20 mole percent. This platinum dispersion is not used for anodes according to the invention, because it may lead to dissolution of the noble metal upon prolonged contact with the electrolyte, therefore to destabilisation of the intermediate layer and also because it increases the costs of the intermediate layer. In publication JP 2-61083 a top layer of 30-80 mole percent of iridium oxide is described, containing 70-20 mole percent of tantalum oxide, with an intermediate layer of 85-95 mole percent of tantalum oxide and furthermore iridium oxide. The formula of the iridium oxide is given as IrO 2 and of the tantalum oxide as Ta 2 O 5 . In a preferred composition according to the invention another composition of iridium oxide is used in the top layer, namely IrO 2+x H y , calculated as IrO 2 , with less than 20 mole percent of tantalum oxide, calculated as Ta 2 O 5 . In publication JP-63-235493 the top layer consists of iridum oxide. The intermediate layer contains maximally 50 mole percent Ta in the form of tantalum oxide, furthermore iridium oxide. In the preferred composition of the intermediate layer according to the invention, it contains 60-100 mole percent tantalum as tantalum oxide, plus iridium oxide or another oxide in 39-0 mole percent. In publication JP-60-22074 a top layer of iridium oxide or iridium oxide with tantalum oxide is mentioned, together with an intermediate layer of tantalum oxide with titanium oxide and/or tin oxide. In the published German Patent Application DE 3401952 an intermediate layer is used of tantalum oxide, titanium oxide and/or tin oxide, moreover containing a platinum dispersion, in combination with a top layer of iridium oxide or iridium oxide with tantalum oxide. Anodes, which are manufactured according to these published methods and with the mentioned compositions, deviate in essential points from anodes according to the invention. For the latter anodes, a longer service life is found at the same or even lower costs, or the extended service life more than compensates for higher costs. Manufacturing methods A method for manufacturing anodes according to the invention consists of cleaning a support and preferably by etching it, to remove undesired surface oxides and other surface components, among which surface contaminants. The support may be subsequently kept clean in a non-oxidising environment or it may form a natural oxide skin in an oxidising environment, for example air. Subsequently, solutions of the desired metal compounds and compositions are applied to the support, layerwise, with suitable application methods, like those which are usual in the paint industry, like by painting with a brush or roller. These solutions onto the support are subsequently dried at ambient or higher temperature. These solutions, which are called 'paint' in the production, are preferably prepared from metal compounds, which are soluble in alcohols with the chemical formula C n H 2n+1 OH, preferably with nvalues ranging from 1 to 5, more in particular with n = 4 and especially with normal butanol. In these solutions water may be present by accident or on purpose as a minor or major component. The metal compounds must be soluble in the alcohol or the aqueous alcohol or in water. In the latter case they will remain in solution by dispersing well or by addition of the alcohol. Suitable precursors are metal chlorides, like tantalum pentachloride TaCl 5 and titanium tetrachloride TiCl 4 , also the chloroiridic acid H 2 IrCl 6 is taken in this category, in all occurring forms, including hydrates and aqueous solutions; metal alkoxides, among which are mentioned tetra-alkoxy-titanium and penta-alkoxy-tantalum compounds, like tetraethoxy and tetra-butoxy-titanium and penta-ethoxy- and penta-butoxy-tantalum; metal β-diketonates, like the acetylacetonates, dipivaloylmethanates (also called tetramethyloctanedionates) and tri- or hexafluoroacetyl-acetonates of iridium, tantalum, cobalt or titanium. The concentrations of the metal compounds may vary per applied layer. Per 'paint' layer a heat treatment may be given or per complete functional layer according to anode types 1 through 5. By repeated 'painting', the layer will obtain the desired layer thickness. For each subsequent layer another method may be applied. Upon heat treatment of a layer, the support and all previously applied layers will undergo this heat treatment. The temperature may be increased to the desired end temperature, preferably between 400°C and 650°C, very rapidly, more slowly, or stepwise. All factors mentioned above have an influence on quality and service life of the anode. For application of intermediate layers, other methods are also very suitable. For instance, during anode manufacturing, one or more adhering and/or protecting intermediate layers have been made in a vacuum oven or in ovens with an inert (= non-oxygen-containing or oxidising) gas atmosphere, at temperatures between 400°C and 1000°C, resulting in an anode with long service life. Upon application of an intermediate layer, consisting of tantalum oxide and/or niobium oxide, whether or not mixed with titanium oxide, by plasma spraying, the resulting anode has a long service life. An anode according to the invention, therefore with extended service life, especially distinguishes itself physically from known anodes by an electrocatalyst, which is based on iridium oxide, including mixtures of iridium oxide with other metal oxides, having a composition and morphology, which deviate from those of known anodes for oxygen generation, based on iridium oxide. Both the materials choice and the manufacturing method are of importance for this result. Extended service life of anodes has been achieved also by application of one or more protecting and/or adhering intermediate layers of different compositions and by various methods. Also, it has been found that an electrochemical cell, in which oxygen is generated, besides possibly other anodic products, has a longer service life, when it contains an anode according to the invention. In particular this is the case when the electrolyte has an acid composition, like when it contains sulfuric acid besides other components, like water. However, when the electrolyte has a basic composition or when it contains sea water or other sodium chloride or other metal chloride-containing components, the cell with an anode according to the invention demonstrates an extended service life with respect to corresponding cells with a known anode. Therefore it can be said that an oxygen-generating electrochemical process is preferably carried out in an electrochemical cell, containing an anode according to the invention. EXAMPLES The invention shall be demonstrated with some examples. In these examples the indicated compositions of oxide mixtures have been calculated with the number of moles of the stoichiometric compounds, for instance iridium oxide as IrO 2 and tantalum oxide as Ta 2 O 5 . Furthermore, it is noted that only within one Example anodes have been compared, as they have been prepared and tested in the same way, unless the Example teaches otherwise. Example 1 An anode is prepared by dissolving a mixture of chloroiridic acid H 2 IrCl 6 and tantalum pentachloride TaCl 5 or tantalum penta-ethoxide Ta(OEt) 5 in n-butanol in the desired proportions, until a total concentration of the metals was reached of 7 weight percent. This 'paint' was layerwise applied with a brush onto a cleaned and etched titanium cylinder with a diameter of 3 mm and a surface area for submersion into the electrolyte of 1 cm 2 . After application of each layer, the anode was placed in an oven, which was heated to 450°C and kept there for 20 minutes in an air atmosphere. In total, appr. 10 g/m 2 noble metal was applied on every anode. These anodes were connected by a titanium holder to a measuring equipment. Example 2 An anode (nr. 1) according to the invention was manufactured on a flat Ti plate. First an electrically well-conducting intermediate layer was deposited by plasma spraying of titanium dioxide (rutile) in vacuum, resulting in a layer thickness of 30 µm. Next, a second intermediate layer of tantalum oxide (3 g/m 2 of tantalum) was applied and on top of that a top layer was deposited, consisting of iridium oxide/ tantalum oxide (mole ratio 85/15) to give a noble metal coverage of 10 g/m 2 , using a method, according to Example 1, however at a heating temperature of 550 °C. The service life of this anode was 36 days at a current density of 30,000 A/m 2 and a cell temperature of 40 °C, which compares favourably with the service life of a comparable anode (nr. 2) without the titanium oxide intermediate layer, 19 days. The composition of the titanium oxide intermediate layer was verified after plasma spraying with the aid of XRD. It mainly consisted of so-called Magneli phases with chemical composition Ti n O 2n-1 . Example 3 Anodes 3-9 were manufactured and tested for service life with no, one, two or three intermediate layer(s). The service life test in this Example and all following Examples was carried out by submersing the anodes in 37 percent sulfuric acid as electrolyte. The cell further consisted of two counter electrodes, made of graphite plate. The current density was 30,000 A/m 2 . This current density was kept constant during the service life test by regulating the voltage. The cell temperature was 33 °C. As the end of the service life the time was chosen, where the applied voltage was twice the value of the initial voltage. Measured service lifes are given in Table 1. The top layer was a mixture of iridium oxide and tantalum oxide in a 85/15 mole percent ratio and a noble metal coverage of 10 g/m 2 . The intermediate layers were applied with a total metal coverage per layer of 5 g/m 2 , according to the method for the top layer of Example 1, using metal chlorides, metal alkoxides or metal-β-diketonates with similar results. The results are given in table 1. From this Table it follows that, at identical top layer, the service life of anodes increases with the number of intermediate layers, one of these in this Example consistently is tantalum oxide. Application of a titanium oxide layer on one side and even better on both sides of this protecting layer increases the service life of the anode. It is favourable for the service life when the TiO x -layer/-layers is/ are deposited by vacuum plasma spraying VPS, or if the TiO x is deposited together with some TaO x , according to a method of Example 1. Example 3 Anodes 10-16 were manufactured according to Example 1, with a Ti support, on which a tantalum oxide layer (1) was deposited and then an electrocatalytic top layer, consisting of iridium oxide or iridium oxide/tantalum oxide. When another intermediate layer (2) was applied between these two layers, consisting of tantalum oxide, mixed with iridium oxide, the service life of the anode was increased, see Table 8. With some lower layers (2) of type B (rutile oxides TiO 2 , SnO 2 and a mixture of the two oxides), there is also additional advantage in service life, see anodes 13-16. Service lifes were measured at a current density of 30,000 A/m 2 , at 50 °C. From this Table, it is concluded that the service life of an anode increases at identical top layer and tantalum oxide intermediate layer by inserting a second layer between either the support and the first intermediate layer or between that layer and the top layer. This second intermediate layer may be either a tantalum oxide layer, containing iridium oxide, or a layer, consisting of titanium oxide, tin oxide or a mixture of the two oxides. Also the result shows that with identical intermediate layer (tantalum oxide), the service life of the anode increases by adding tantalum oxide to the iridium oxide top layer.

Claims: Anode for the generation of oxygen in an electro-chemical process comprising a support of titanium or a titanium alloy which carries an electro-chemically active porous top layer comprising iridium oxide characterized in that the top layer is separated by at least a first intermediate layer comprising tantalum oxide which is void of any intentionally added iridium oxide wherein said layer comprises tantalum oxide for at least circa 51 mole percent, calculated as metal, and in that the first intermediate layer is stacked with a at least a second intermediate layer comprising at least one metal oxide taken from a group comprising tantalum oxide, titanium oxide, tin oxide and iridium oxide. Anode according to one of claim 1 characterised in that the first intermediate layer further comprises niobium oxide. Anode according to claim 2 characterised in that the first intermediate layer further comprises cobalt oxide. Anode according to claim 1, 2 or 3 characterised in that the first intermediate layer is stacked at both sides with a second intermediate layer either or not of the same composition. Anode according to Claim 1, 2, 3 or 4 characterised in that the second intermediate layer comprises titanium dioxide TiO 2 mixed with ditantalum pentoxide Ta 2 O 5 . Anode according to Claim 4 or 5, characterised in that said metal oxide in the second intermediate layer does not have the stoichiometric composition MO 2 , but a deviating composition MO 2±x , in which x is between 0 and 0.5. Anode according to claim 4, 5 or 6 characterized in that the second intermediate layer comprises tin dioxide. Anode according to Claim 7 characterised in that the tin dioxide has a lowered oxygen stoichiometry, in which x is between 0 and 0.5. Anode according to Claim 6, characterised in that the second intermediate layer contains titanium dioxide with a lowered oxygen stoichiometry, in which the non-stoichiometry x is between 0.125 and 0.34, and said titanium oxide has the structure of one or more so-called Magneli phases. Anode according to one of the preceding caims characterised in that the electrocatalytically active top layer consists of more than 80 mole percent iridium oxide, calculated as IrO 2 and less than 20 mole percent tantalum oxide, calculated as Ta 2 O 5 . Anode according to one of the Claims 1-9 characterised in that the electrocatalytically active top layer consists between 50-100 mole percent of iridium oxide and between 50-0 mole percent of cobalt oxide. Anode according to one of the claims 1-9 characterised in that the electrocatalytically active top layer consists between 50-100 mole percent of iridium oxide and between 50-0 mole percent of lead oxide. Anode according to one of the claims 1-9, characterised in that the electrocatalytically active top layer consists between 50-100 mole percent of iridium oxide and further of a combination of at least two of the oxides of tantalum, cobalt and/or lead. Anode according to any of the preceding claims characterised in that the support comprises a titanium alloy with a valve metal taken from a group of Ta, Nb, Zr, Hf, Mo and W Electrochemical cell with an electrolyte, characterised by an anode, according to one of the preceding claims. Electrochemical cell according to claim 15, characterised by the electrolyte being sulfuric acid, an alkaline composition or sea water.

Inventors: JANSEN HERMANUS JOHANNES, MACKOR ADRIANUS, JANSEN, HERMANUS JOHANNES, MACKOR, ADRIANUS
Applicants: MAGNETO CHEMIE BV, MAGNETO-CHEMIE B.V.
Application year: 1992
Main classification: C25B11
IPC codes: C23F13, C25C7, C25B11