Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
  • C25C7/00—Constructional parts, or assemblies thereof, of cells; Servicing or operating of cells
  • C25C7/02—Electrodes; Connections thereof
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23F—NON-MECHANICAL REMOVAL OF METALLIC MATERIAL FROM SURFACE; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL; MULTI-STEP PROCESSES FOR SURFACE TREATMENT OF METALLIC MATERIAL INVOLVING AT LEAST ONE PROCESS PROVIDED FOR IN CLASS C23 AND AT LEAST ONE PROCESS COVERED BY SUBCLASS C21D OR C22F OR CLASS C25
  • C23F13/00—Inhibiting corrosion of metals by anodic or cathodic protection
  • C23F13/02—Inhibiting corrosion of metals by anodic or cathodic protection cathodic; Selection of conditions, parameters or procedures for cathodic protection, e.g. of electrical conditions
  • C23F13/06—Constructional parts, or assemblies of cathodic-protection apparatus
  • C23F13/08—Electrodes specially adapted for inhibiting corrosion by cathodic protection; Manufacture thereof; Conducting electric current thereto
  • C23F13/12—Electrodes characterised by the material
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
  • C25B11/051—Electrodes formed of electrocatalysts on a substrate or carrier
  • C25B11/073—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material
  • C25B11/091—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material consisting of at least one catalytic element and at least one catalytic compound; consisting of two or more catalytic elements or catalytic compounds
  • C25B11/093—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material consisting of at least one catalytic element and at least one catalytic compound; consisting of two or more catalytic elements or catalytic compounds at least one noble metal or noble metal oxide and at least one non-noble metal oxide

Definitions

• the invention concerns industrial electrodes with increased service life, methods for their manufacturing, an electrochemical cell, in which these electrodes are applied and electrochemical processes, in which oxygen gas is generated as main or side product.
• the invention in particular concerns the composition of and methods for the manufacturing 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 processes, in which these anodes are used.
• 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.
• corrosive components such as alkali or acid, in particular sulfuric acid, whether or not mixed with other inorganic or organic components.
• 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.
• 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 processes, 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.
• 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 heat-treatment 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.
• 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/m2), 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.
• an insulating layer is formed of titanium oxide, which approaches the formula TiO2 in its composition.
• 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.
• 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 IrO2 occurs in the same crystal form (rutile) with crystal lattice distances, which are not too different from rutile TiO2. We believe therefore that this agreement in crystal structures may be a major reason for our observation that IrO2 and TiO2 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.
• the titanium oxide which is formed during the anode action does not undergo such an adhering heat treatment.
• 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.
• an intermediate layer (type A)
• 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/m2, with an electrode coating of Ta2O5 and/or IrO2.
• This intermediate layer is found to be very effective for protection of the support and for extending the service life of the anode.
• that invention uses stoichiometric iridium dioxide and ditantalum pentoxide.
• the composition IrO2 has electrocatalytic properties, which are far inferior to a special form of so-called 'hydrous iridium oxide', iridium hydroxy-oxide, which contains many hydroxyl groups, especially at the electrocatalytically active surface, e.g. represented by the approximate structural formula IrO(OH)2.
• iridium hydroxy-oxide hereafter called iridium oxide
• the oxygen/iridium ratio is 3, whereas in IrO2 this ratio is 2.
• Hydroxy-oxides IrO2+xH y are characterised by a molar excess x of oxygen and a molar amount y of hydrogen.
• the iridium oxide must be stable and conduct the electrical current well during the entire anode service life.
• the composition IrO2 is well-sinted.
• Anodes contain iridium oxides, in which the O/Ir ratio is larger than 2, e.g. 2.5. They are prepared according to specific procedures and they show variations to larger and smaller x in IrO2+x, depending on the chosen manufacturing conditions.
• This mixed oxide/hydroxide of iridium shows both a long-lived good electrical conduction and a high electrocatalytic activity, resulting in material with a surprisingly long-lived electrocatalytic stability. It differs considerably in chemical composition from stoichiometric IrO2, but also, according to crystallographic/morphological analysis with X-ray powder diffractometry (XRPD) and high-resolution transmission electron microscopy (HR-TEM) it has a crystal structure, which may best be described as 'rutile-like', a modified IrO2 structure.
• this material consists of sintered crystallites with an individual particle size mainly between 5 and 15 nm for a number of very durable anode samples.
• anodes with smaller and larger iridium oxide particles (3-100 nm) a very high electrocatalytic activity and stability of the anodes has been found with respect to anodes, prepared by previously described procedures.
• this iridium oxide, whether or not mixed with tantalum oxide is both very active and stable for a long time in anodes, according to the invention.
• anode is obtained, according to the invention, of which the electrocatalytically active top layer signifies an improvement with respect to known top layers, like the ones described in the German patent DE 3219003.
• iridium oxide is present as such, or in mixtures with other metal oxides, in a very finely divided crystalline form, as established by XRPD and HR-TEM.
• tantalum oxide is present in a non-crystalline, amorphous phase. Therefore, no mixed-crystal material of iridium oxide and tantalum oxide has been obtained.
• iridium oxide occurs in a so-called nanocrystalline form with an usual particle size distribution far below 100 nm. Also the dimensions of the crystalline unit cell (measured according to the rutile structure) deviate significantly from those of pure rutile IrO2, as known from the ASTM-Powder Data File and as found by Comninellis and Vercesi.
• Iridium oxide as used in the invention may be converted by heating at 800°C or above into well-crystallised and well-characterised rutile-IrO2. However, it loses its excellent properties as long-lived, electrocatalytically very active top layer by this treatment.
• an electrocatalytic top layer consisting of mixed oxides of iridium oxide and cobalt oxide, or iridium oxide and lead oxide, or iridium oxide with a mixture of these two oxides.
• Such a top layer is manufactured by a similar method as described in the invention for mixed iridium/tantalum oxides.
• an anode, according to the invention with a top layer of iridium/tantalum/cobalt oxide, containing preferably at least 50 mole percent of iridium oxide, gives an improved service life with respect to known anodes.
• An anode contains therefore in the electrocatalytic top layer a special form of iridium oxide, having the formula IrO2+x H y, characterised by a O/Ir ratio (2+x) higher than 2, viz. between 2.1 and 2.9 and preferably with and O/Ir ratio around 2.5.
• the special composition and morphology of the iridium oxide, present in anodes, according to the invention may be obtained by one of the methods, according to the invention and possibly by other methods, which are not described in the invention.
• An anode has an electrocatalytic coating (top layer), consisting of iridium oxide and cobalt, lead and/or tantalum oxide, preferably with at least 50 mole percent iridium oxide and no more than 20 mole percent tantalum oxide, which top layer is directly deposited onto the support or through one or more intermediate layers.
• the intermediate layers serve to protect the support from the corrosive action of the electrolyte and the (intermediate) products of the electrochemical process or the process itself. They also serve for adhesion between support and either an intermediate layer or a top layer, or between an intermediate layer and top layer, or a combination of these layers.
• support also includes for this purpose the oxide skin, which is present on it before it undergoes a method according to the invention or the oxide skin, which is formed on the substrate as a consequence of a method for manufacturing an anode according to the invention, in an electrochemical cell according to the invention or as a consequence of an electrochemical process, using an electrochemical cell, 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 tantalum oxide in the intermediate layer is fully or partly replaced by iridium oxide with similar service life.
• the intermediate layer has a different composition, type B, containing electrically conductive titanium oxide or tin oxide.
• This embodiment may be realised as described hereafter, for example by manufacturing tin oxide with the approximate chemical formula SnO2, mixed with indium oxide to give a solid solution of indium tin oxide (ITO) or other mixture, or by manufacturing titanium oxide with approximate chemical formula TiO2, mixed with tantalum oxide, in particular with ditantalum pentoxide Ta2O5, whether or not in a solid solution.
• ITO indium tin oxide
• TiO2 titanium oxide with approximate chemical formula TiO2 mixed with tantalum oxide, in particular with ditantalum pentoxide Ta2O5, whether or not in a solid solution.
• the layer thickness of an intermediate layer (measured as the quantity of metal coverage in g/m2) may be as large as that of the top layer, but in many investigated anodes it is smaller.
• the coverage by iridium oxide in the top layer is approximately 10 g/m2, calculated as the noble metal.
• the extension of service life by compositions and methods according to the invention has been found also for anodes with a lighter or heavier coverage with iridium oxide.
• An anode according to the invention is schematically built up as follows, in its most simple form (type 1).
• Type 1 Support
• anode type 2 The schematic built-up of an anode with one intermediate layer of type A (called anode type 2) is as follows:
• Type 2 Support
• intermediate layer type A top layer.
• 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.
• 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.
• the second intermediate layer may be built-in in two ways, types 3 and 4, 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 3 Support
• Type 4 Support
• 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.
• Type 5 Support
• 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.
• 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 MO2. This formula also includes those oxides, which are often described as MO2, 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 MO2 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.
• those metal oxides MO2 with a rutile structure are applied in the invention, which effectively conduct electrical current.
• IrO2, RuO2 or PtO2 metallic conduction themselves
• those metal oxides MO2 which may become n- or p-type semi-conducting oxides by a small modification.
• IrO2+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 TiO2 and SnO2.
• a well-known example of conductive SnO2 is iridium-tin oxide, which has been successfully applied in the invention.
• titanium oxide the modification may be carried out in two ways.
• the first is the one, in which titanium oxide has the formula TiO2-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 TiO2 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 TiO2-x are useful for application in adhering intermediate layers. However, these oxides TiO2-x have a tendency to oxidise in the oxidising environment during anode operation, thereby reducing the oxygen deficiency x.
• These oxides may both be obtained separately (ex-situ) as well as on the substrate (in-situ).
• the literature gives various methods, for example in the references of the cited article by Millot et al.
• the manufacturing of titanium suboxides TiO2-x is preferably carried out in situ, by heating of a suitable, commercially available titanium-containing precursor, such as TiCl4 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.
• a suitable, commercially available titanium-containing precursor such as TiCl4 or tetrabutyl titanate
• 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 Ti1-h (M h ) O2+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 TiO2-x. Therefore, they are more stable in anodes as intermediate layer B than the corresponding anodes with oxygen-deficient oxides.
• 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.
• 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-chloride 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.
• 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.
• 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.
• an electrode In that electrode, a large amount of the expensive noble metal iridium is used in the lower layer.
• the lower layer contains less iridium oxide, namely 0-39 metal mole percent, yielding a surprisingly good service life.
• a top layer 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 distabilisation of the intermediate layer and also because it increases the costs of the intermediate layer.
• 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 IrO2and of the tantalum oxide as Ta2O5.
• another composition of iridium oxide is used in the top layer, namely IrO2+xH y , calculated as IrO2, with less than 20 mole percent of tantalum oxide, calculated as Ta2O5.
• 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.
• 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.
• 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.
• 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 TaCl5 and titanium tetrachloride TiCl4, also the chloroiridic acid H2IrCl6 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 tetra-ethoxy and tetra-butoxy-titanium and penta-ethoxy- and penta-butoxy-tantalum; metal b-diketonates, like the acetylacetonates, dipivaloylmethanates (also called tetramethyloctanedionates) and tri- or hexafluoroacetyl-acetonates of iridium, tantalum, cobalt or titanium.
• metal alkoxides among which are mentioned tetra-al
• 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.
• 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.
• 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.
• the electrolyte has an acid composition, like when it contains sulfuric acid besides other components, like water.
• 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.
• compositions of oxide mixtures have been calculated with the number of moles of the stoichiometric compounds, for instance iridium oxide as IrO2 and tantalum oxide as Ta2O5. 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.
• Anodes 1-3 were prepared according to the invention by dissolving a mixture of chloroiridic acid H2IrCl6 and tantalum pentachloride TaCl5 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 cm2. 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/m2 noble metal was applied on every anode.
• These anodes were connected by a titanium holder to a measuring equipment.
• Top layer composition Tantalum compound Noble metal coverage (g/m2) Service life (days) 1 IrO x /TaO x (85/15) Ta(OEt)5 10.4 2.5 2 IrO x /TaO x (85/15) Ta(OEt)5 11.5 2.5 3 IrO x /TaO x (85/15) TaCl5 10.4 3
• tantalum pentachloride and tantalum penta-ethoxide are both suitable precursors for the manufacturing of top layers of mixed iridium oxide/tantalum oxide. See also Claims 2, 32-35, 41, 42 and 45.
• Anodes 4-6 were manufactured according to the invention, with an electrocatalytic top layer of iridium oxide or of mixed iridium oxide/tantalum oxide, according to the method of Example 1.
• anodes 7-9 were manufactured with iridium oxide and with iridium oxide, mixed with tantalum oxide, by a method which differs from that of Example 1 and also anode 10 was manufactured, again by a different method.
• Anodes 4-10 were investigated with X-ray diffractometry XRD, HR-TEM and the IrO x top layer in elemental analysis. The analyses demonstrate that anodes 4-6 contain the iridium oxide in a very finely divided nanocrystalline form with individual particle size around 10 nm.
• the O/Ir ratio in anodes 4 and 5 is around 2.5.
• the tantalum oxide-containing anode 6 the tantalum oxide is present as an amorphous phase, next to rutile-like iridium oxide particles.
• Anodes 7-9 show crystalline IrO2 in the rutile structure with particle sizes greater than 100 nm and O/Ir ratios in anodes 7 and 8, which are close to 2.
• the tantalum oxide in anode 9 is present in separate crystalline phases of Ta2O5.
• the analyses of anode 10 show that no visible crystallinity is present (individual particle size smaller than 3 nm) and that the O/Ir ratio is high, around 3.0.
• Table 2 Analyses and service lifes of anodes on the basis of various types of iridium oxide or of mixed iridium oxide/tantalum oxide Anode nr. Composition top layer (mole%) O/Ir ratio Particle size (nm)* Service life (days) 4 IrOx 2.4 11 13 5 IrOx 2.6 10 12 6 IrOx/TaOx (85/15) 8 32 7 IrOx 2.0 > 100 5 8 IrOx 2.05 > 100 6 9 IrOx/TaOx (85/15) > 100 9 10 IrOx 3.1 ⁇ 3 6 * Average value.
• iridium oxide or mixed iridium oxide tantalum oxide top layers a form of iridium oxide is present which is very suitable for application in oxygen-generating anodes with a long service life, when these top layers are deposited by a method according to Example 1.
• This iridium oxide consists of very small crystalline particles of about 10 nm with a rutile-like structure. The O/Ir ratio in these particles is around 2.5, which is significantly higher than has been found for well-crystallised IrO2 with a rutile structure and particle size above 100 nm. No indications have been found for the formation of a so-called mixed-crystal material of iridium oxide and tantalum oxide under these conditions.
• iridium oxide/tantalum oxide anode after heating by a method according to Example 1, an amorphous tantalum oxide phase is present and in the well-crystallised IrO2 also crystalline Ta2O5. These crystalline compounds with big IrO2 particles do not give mixed-crystal material here. They also show a short service life as a top layer in an anode. The service life of an anode may also be increased by addition of tantalum oxide to the top layer of iridium oxide. See also Claims 1, 26 and 30.
• Flat plate anodes 11-17 were manufactured, according to Example 1, having dimensions of 2.5 x 2.5 cm2, at a different heating temperature of 420 °C, however and at various ratios between the amounts of iridium oxide and tantalum oxide, at a noble metal coverage of 10 g/m2. Service lifes were measured as recorded in Table 3, at a current density of 10,000 A/m2 and an equal cell temperature for all cells, amounting to 30-35 °C during testing. Table 3 Service life of anodes with various compositions of the top layer Anode nr.
• Top layer composition (mole%) Service life (days) 11 IrO x /TaO x (60/40) 14.5 12 IrO x /TaO x (70/30) 21.5 13 IrO x /TaO x (75/25) 89 14 IrO x /TaO x (80/20) 124 15 IrO x /TaO x (85/15) 283 16 IrO x /TaO x (90/10) 186 17 IrO x 112
• Anodes 18-24 were manufactured and tested with an iridium oxide/lead oxide top layer, according to Example 2, however with a cell temperature of 50 °C.
• the lead precursor was dibutyllead diacetate.
• the measured service lifes are given in Table 4.
• Table 4 Service lifes of anodes with a top layer of iridium oxide/lead oxide Anode nr.
• Top layer composition (mole%) Service life (days) 18 IrO x 5.0 19 IrO x /PbO x (95/5) 8.4 20 IrO x /PbO x (90/10) 14.1 21 IrO x /PbO x (85/15) 13.1 22 IrO x /PbO x (80/20) 13.3 23 IrO x /PbO x (75/25) 14.3 24 IrO x /PbO x (70/30) 8.5
• Anodes 25-27 were manufactured according to Example 1, with a top layer and no or one intermediate layer.
• the intermediate layer was manufactured from the metal chlorides, following a method as described in Example 1 for the top layer. The testing, however, differed from Example 1 by the current density, 30,000 A/m2, and a cell temperature of 33 °C.
• the noble metal coverage for the top layer was 10 g/m2, as was the total metal coverage for the intermediate layer.
• Service lifes are given in Table 5. Table 5 Service lifes of IrO x /TaO x anodes with or without intermediate layer Anode nr. Top layer composition (mole%) Intermediate layer composition (mole%) Service life (days) 25 IrO x /TaO x (85/15) no intermed.
• Anodes 28-30 were manufactured according to Example 1, with an iridium oxide/cobalt oxide top layer and a noble metal coverage of 10 g/m2, having an intermediate layer of tantalum oxide/iridium oxide with a total metal coverage of 10 g/m2. They were tested for service lifes at a current density of 30,000 A/m2. The results are given in Table 6. Table 6 Service lifes of iridium oxide/cobalt oxide anodes Anode nr.
• An anode (nr. 31) according to the invention was manufactured on a flat Ti plate.
• rutile titanium dioxide
• a second intermediate layer of tantalum oxide (3 g/m2 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/m2, 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/m2 and a cell temperature of 40 °C, which compares favourably with the service life of a comparable anode (nr. 32) 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 O2n-1. See Claims 10-14 and 22.
• Anodes 33-39 were manufactured and tested for service life, according to Example 5, with no, one, two or three intermediate layer(s).
• 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/m2.
• the intermediate layers were applied with a total metal coverage per layer of 5 g/m2, according to the method for the top layer of Example 1, using metal chlorides, metal alkoxides or metal-b-dikatonates with similar results. The results are given in Table 7.
• Table 7 Service lifes of anodes with various numbers and kinds of intermediate layers Anode nr.
• Anodes 40-46 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 TiO2, SnO2 and a mixture of the two oxides), there is also additional advantage in service life, see anodes 43-46.
• type B rutile oxides TiO2, SnO2 and a mixture of the two oxides
• Service lifes were measured at a current density of 30,000 A/m2, at 50 °C.
• Table 8 Service lifes of anodes with a tantalum oxide intermediate layer (1), whether or not with an extra intermediate layer (2) and a top layer of iridium oxide, whether or not mixed with tantalum oxide, on a titanium support Anode nr.
• Composition top layer (mole%) Composition intermediate layers 1 and 2 (mole%) * Metal coverage (layer) (g/m2) Service life (days) 40 IrO x TaO x (top): 10 10 (1): 3 41 IrO x /TaO x (85/15) TaO x top: 10 25 (1): 3 42 IrO x /TaO x (85/15) TaO x /IrO x (1) (75/25) (top): 10 34 (1): 3 TaO x (2) (2): 3 43 IrO x /TaO x (85/15) TaO x (1) (top): 10 31 TaO x /IrO x (2) (75/25) (1): 3 (2): 3 44 IrO x /TaO x (85/15) TaO x (1) (top): 10 30 TiO x (2) (1): 3 (2): 3 45 IrO x /TaO x (85/15) TaO x (1) (top): 10 48 SnO
• 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.
• the service life of the anode increases by adding tantalum oxide to the iridium oxide top layer. See Claims 10, 12, 21, 22 and 26.

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• 1991
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