Patent Publication Number: US-2015075978-A1

Title: High-load durable anode for oxygen generation and manufacturing method for the same

Description:
TECHNICAL FIELD 
     The present invention relates to an anode for oxygen generation used for various industrial electrolyses and a manufacturing method for the same; more in detail, it relates to a high-load durable anode for oxygen generation and a manufacturing method for the same used for industrial electrolyses including manufacturing of electrolytic metal foils such as electrolytic copper foil, aluminum liquid contact, and continuously electrogalvanized steel plate, and metal extraction, having superior durability under high-load electrolysis conditions. 
     BACKGROUND ART 
     In industrial electrolyses including manufacturing of electrolytic copper foil, aluminum liquid contact, continuously electrogalvanized steel plate and metal extraction, oxygen generation is involved at the anode. For this reason, the anode which is coated chiefly with iridium oxide having durability to oxygen generation, as electrode catalyst, on the titanium metal substrate has been widely applied. Generally speaking, in this type of industrial electrolysis involving oxygen generation at the anode, electrolysis is usually performed at a constant electric current in view of production efficiency, energy saving, etc. Current density has been in a range from several A/dm 2  mainly applied in the industrial fields including metal extraction to 100 A/dm 2  at maximum for manufacturing electrolytic copper foil. 
     However, nowadays, it is often seen that electrolysis is performed at a current density of 300 A/dm 2 -700 A/dm 2  or more for higher product quality or for providing special performance characteristics. Such high electric current is not supplied to all the anodes installed to the industrial electrolysis system, but rather, it is considered that an anode is installed as an auxiliary one at a specific point where high-load electrolysis condition is applied to provide special performance characteristics to the product obtained from the electrolysis. 
     Under the electrolysis at such a high current density, the electrode catalyst layer is highly loaded and electric current tends to be concentrated there, causing rapid consumption of the electrode catalyst layer. Moreover, organic substances or impurity elements added for stabilizing products cause various electrochemical and chemical reactions, the concentration of hydrogen ion increases resulting from the oxygen generation reaction, lowering the pH value, and consumption of electrode catalyst is expedited. 
     One solution to solve these problems may be to increase the surface area of the electrode catalyst layer so as to decrease the actual electric current load. For instance, one solution is to apply a substrate of mesh or punched metal, instead of conventional plate substrates, to increase the surface area physically. Use of these substrates, however, involves undesirable extra processing costs. Furthermore, actual current density decreased by physically increased surface area of the substrate does not improve the electric current concentration at the electrode catalyst layer, resulting in little suppression effect on catalyst consumption. 
     In the thermolysis formation method of the electrode catalyst layer by repeating coating and baking, if the amount of coating iridium per time is increased, it is simply considered that the formed catalyst layer is soft and fluffy; but by this method only, increase in the effective surface area of the catalyst layer of the electrode is limited and improvements in consumption of the catalyst layer under high-load conditions and in durability could not be observed clearly. 
     As an electrode for this kind of electrolysis, electrode with a low oxygen generation potential and a long service life is required. Conventionally, as electrode of this type, an insoluble electrode comprising a conductive metal substrate, such as titanium, covered with a catalyst layer containing precious metal or precious metal oxide has been applied. For example, PTL 1 discloses an insoluble electrode prepared in such a manner that a catalyst layer containing iridium oxide and valve metal oxide is coated on a substrate of conductive metals, such as titanium, heated in oxidizing atmosphere and baked at a temperature of 650 degrees Celsius-850 degrees Celsius, to crystallize valve metal oxide partially. This electrode, however, has the following drawbacks. Since the electrode is baked at a temperature of 650 degrees Celsius or more, the metal substrate, such as of titanium causes interfacial corrosion, and becomes poor conductor, causing oxygen overvoltage to increase to an unserviceable degree as electrode. Moreover, the crystallite diameter of iridium oxide in the catalyst layer enlarges, resulting in decreased effective surface area of the catalyst layer, leading to a poor catalytic activity. 
     PTL 2 discloses use of an anode for copper plating and copper foil manufacturing prepared in such a manner that a catalyst layer comprising amorphous iridium oxide and amorphous tantalum oxide in a mixed state is provided on a substrate of conductive metal, such as titanium. This electrode, however, features amorphous iridium oxide, and is insufficient in electrode durability. The reason why durability decreases when amorphous iridium oxide is applied is that amorphous iridium oxide shows unstable bonding between iridium and oxygen, compared with crystalline iridium oxide. 
     PTL 3 discloses an electrode coated with a catalyst layer comprising a double layer structure by a lower layer of crystalline iridium oxide and an upper layer of amorphous iridium oxide, in order to suppress consumption of the catalyst layer and to enhance durability of the electrode. The electrode disclosed by PTL 3 is insufficient in electrode durability because the upper layer of the catalyst layer is amorphous iridium oxide. Moreover, crystalline iridium oxide exists only in the lower layer, not uniformly distributed over the entire catalyst layer, resulting in insufficient electrode durability. 
     PTL 4 discloses an anode for zinc electrowinning in which a catalyst layer containing amorphous iridium oxide as a prerequisite and crystalline iridium oxide, as a mixed state is provided on a substrate of conductive metal like titanium. PTL 5 discloses an anode for cobalt electrowinning in which a catalyst layer containing amorphous iridium oxide as a prerequisite and crystalline iridium oxide, as a mixed state is provided on a substrate of conductive metal like titanium. However, it is thought that electrode durability of these two electrodes is not enough because they contain a large amount of amorphous iridium oxide, as prerequisite. 
     To solve these problems, the inventors of the present invention have developed, aiming chiefly at decreasing oxygen generation overvoltage for the case that the amount of coating of iridium per time is 2 g/m 2  or less, (1) the baking method to form a catalyst layer in which crystalline iridium oxide and amorphous iridium oxide coexist by low temperature baking (370 degrees Celsius-400 degrees Celsius) plus high temperature post-bake (520 degrees Celsius-600 degrees Celsius); and (2) the baking method to form a catalyst layer in which almost complete crystalline iridium oxide only is contained by high temperature baking (410 degrees Celsius-450 degrees Celsius) plus high temperature post-bake (520 degrees Celsius-560 degrees Celsius); and patent applications have been made for these two methods as of the same date with the present application. 
     According to these two inventions, lead adhesion resistivity can be achieved when the amount of iridium coating per time is 2 g/m 2  or less, in the electrolysis condition of current density not more than 100 A/dm 2 , and at the same time, improvement of durability from increase of the effective area of catalyst layer and reduction of oxygen generation overvoltage can be achieved. 
     Recently, however, in order to enhance the quality of products or to provide special performance characteristics to products, electrolysis at a current density of 300 A/dm 2 -700 A/dm 2  or more has been frequently conducted. Recent trend is that such high electric current is not supplied to all the anodes installed to the industrial electrolysis system, but rather, an auxiliary anode is installed at a specific point where high-load electrolysis condition is applied to provide special performance characteristics to products obtained from the electrolysis. 
     Under the electrolysis at such a high current density, the electrode catalyst layer is highly loaded and electric current tends to be concentrated there, causing rapid consumption of the electrode catalyst layer. Moreover, organic substance or impurity elements added for stabilizing product quality cause various electrochemical and chemical reactions, the concentration of hydrogen ion increases in concomitant with the oxygen generation reaction, lowering the pH value, and consumption of electrode catalyst is further expedited. From these phenomena, it became clear that the enhancement of durability by the increase of the effective area of catalyst layer and the reduction of oxygen generation overvoltage may not always be achieved by the inventions relating to the above-mentioned two patent applications by the inventors of the present invention. 
     CITATION LIST 
     Patent Literature 
     PTL 1: JP2002-275697A (JP3654204B) 
     PTL 2: JP2004-238697A (JP3914162B) 
     PTL 3: JP2007-146215A 
     PTL 4: JP2009-293117A (JP4516617B) 
     PTL 5: JP2010-001556A (JP4516618B) 
     SUMMARY OF INVENTION 
     Technical Problem 
     In order to solve the above-mentioned problems, the present invention aims to provide a high-load durable anode for oxygen generation and a manufacturing method for the same, having a superior durability under the conditions of high-load, which can improve current distribution to the electrode catalyst layer, suppress consumption of the electrode catalyst and improve durability of the electrode catalyst by enlarging effective surface area of the electrode catalyst layer under the conditions of high-load. 
     Solution to Problem 
     As the first solution to achieve the above-mentioned purposes, the present invention provides an anode for oxygen generation comprising a conductive metal substrate and a catalyst layer containing iridium oxide formed on the conductive metal substrate, wherein the amount of coating of iridium per time for the catalyst layer is 2 g/m 2  or more, the coating is baked in a relatively high temperature region of 430 degrees Celsius-480 degrees Celsius to form the catalyst layer containing amorphous iridium oxide and the catalyst layer containing the amorphous iridium oxide is post-baked in a further high temperature region of 520 degrees Celsius-600 degrees Celsius to crystallize almost all amount of iridium oxide in the catalyst layer. 
     As the second solution to achieve the above-mentioned purposes, the present invention provides an anode for oxygen generation comprising a conductive metal substrate and a catalyst layer containing iridium oxide formed on the conductive metal substrate, wherein the amount of coating of iridium per time for the catalyst layer is 2 g/m 2  or more and the degree of crystallinity of iridium oxide in the catalyst layer after the post-baking is made to be 80% or more. 
     As the third solution to achieve the above-mentioned purposes, the present invention provides an anode for oxygen generation comprising a conductive metal substrate and a catalyst layer containing iridium oxide formed on the conductive metal substrate wherein the amount of coating of iridium per time for the catalyst layer is 2 g/m 2  or more and the crystallite diameter of iridium oxide in the catalyst layer is 9.0 nm or less. 
     As the fourth solution to achieve the above-mentioned purposes, the present invention provides an anode for oxygen generation comprising a conductive metal substrate and a catalyst layer containing iridium oxide formed on the conductive metal substrate, wherein a base layer containing tantalum and titanium ingredients is formed by the arc ion plating (hereafter called AIP) process on the conductive metal substrate before the formation of the catalyst layer. 
     As the fifth solution to achieve the above-mentioned purposes, the present invention provides a manufacturing method for an anode for oxygen generation, wherein the amount of coating of iridium per time for a catalyst layer is 2 g/m 2  or more and the catalyst layer containing amorphous iridium oxide is formed by baking in a relatively high temperature region of 430 degrees Celsius-480 degrees Celsius and the catalyst layer containing amorphous iridium oxide is post-baked in a further high temperature region of 520 degrees Celsius-600 degrees Celsius to crystallize almost all amount of iridium oxide in the catalyst layer. 
     As the sixth solution to achieve the above-mentioned purposes, the present invention provides a manufacturing method for an anode for oxygen generation, wherein the amount of coating of iridium per time for a catalyst layer is 2 g/m 2  or more and the catalyst layer containing amorphous iridium oxide is formed by baking in a relatively high temperature region of 430 degrees Celsius-480 degrees Celsius and the catalyst layer containing amorphous iridium oxide is post-baked in a further high temperature region of 520 degrees Celsius-600 degrees Celsius to make the degree of crystallinity of iridium oxide in the catalyst layer to be 80% or more. 
     As the seventh solution to achieve the above-mentioned purposes, the present invention provides a manufacturing method for an anode for oxygen generation, wherein the amount of coating of iridium per time for a catalyst layer is 2 g/m 2  or more and the catalyst layer containing amorphous iridium oxide is formed by baking in a relatively high temperature region of 430 degrees Celsius-480 degrees Celsius and the catalyst layer containing amorphous iridium oxide is post-baked in a further high temperature region of 520 degrees Celsius-600 degrees Celsius to make the crystallite diameter of iridium oxide in the catalyst layer to be 9.0 nm or less. 
     As the eighth solution to achieve the above-mentioned purposes, the present invention provides a manufacturing method for an anode for oxygen generation comprising a conductive metal substrate and a catalyst layer containing iridium oxide formed on the conductive metal substrate, wherein an AIP base layer containing tantalum and titanium ingredients is formed by the AIP process on the conductive metal substrate before the formation of the catalyst layer. 
     Advantageous Effects of Invention 
     In the formation for the electrode catalyst layer containing iridium oxide by the present invention, the amount of coating of iridium per time of the catalyst layer is 2 g/m 2  or more, baking is conducted, instead of the conventional repeated baking operations at 500 degrees Celsius or more, which are the perfect crystal deposition temperature, by two steps: baking in a relatively high temperature region of 430 degrees Celsius-480 degrees Celsius to form a catalyst layer containing amorphous iridium oxide and post-baking in a further high temperature region of 520 degrees Celsius-600 degrees Celsius to suppress the crystallite diameter of iridium oxide in the electrode catalyst layer preferably to 9.0 nm or less and to crystallize most of the iridium oxide preferably to 80% or more in crystallinity. Thus, the growth of crystallite diameter of iridium oxide was able to be suppressed and the effective surface area of the catalyst layer was able to be increased. Thus, according to the present invention, the growth of crystallite diameter of iridium oxide can be suppressed. As the reasons, the following are considered. The baking is conducted by two stages: first, coating and baking is repeated in a relatively high temperature region of 430 degrees Celsius-480 degrees Celsius and then post-baking in a further high temperature of 520 degrees Celsius-600 degrees. Celsius. Compared with the baking at a high temperature from the beginning by the conventional method, crystallite diameter under the present invention will not enlarge beyond a certain degree. If the growth of crystallite diameter of iridium oxide is suppressed, the smaller the crystallite diameter is, the larger the effective surface area of the catalyst layer will be. Then, the oxygen generation overvoltage of the electrode can be decreased, oxygen generation is promoted, and the reaction to form PbO 2  from lead ion can be suppressed. In this way, PbO 2  attachment and covering on the electrode were suppressed. 
     Further, according to the present invention, simultaneously with increase in the effective surface area of catalyst layer, electric current is evenly distributed, that is, the concentration of electric current is suppressed, and consumption of the catalyst layer by electrolysis is reduced, which leads to improvement of electrode durability. 
     Furthermore, according to the present invention, improved quality of products and provision of special performance characteristics to products are achieved by controlling the amount of coating of iridium to 2 g/m 2  or more per time. When electrolysis is performed at a current density of 300 A/dm 2 -700 A/dm 2  or more, or also an auxiliary anode is provided at a specified spot under a high load electrolysis conditions to give special performance characteristics to products obtained from electrolysis, load to the electrode catalyst layer can be lessened, electric current concentration can be prevented and consumption of electrode catalyst layer can be suppressed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a graph indicating the change of degree of crystallinity of iridium oxide (IrO 2 ) of the catalyst layer by baking temperature and post-bake temperature. 
         FIG. 2  is a graph indicating the change of crystallite diameter of iridium oxide (IrO 2 ) of the catalyst layer by baking temperature and post-bake temperature. 
         FIG. 3  is a graph indicating the change of the electrostatic capacity of the electrode by baking temperature and post-bake temperature. 
         FIG. 4  is a graph indicating the dependence of oxygen overvoltage on baking conditions. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The following explains embodiments of the present invention, in detail, in reference to the figures. In the present invention, it is found that if the effective surface area of the electrode catalyst layer is increased to suppress adhesive reaction of lead oxide to the electrode surface, oxygen generation overvoltage can be reduced and then, oxygen generation is promoted and at the same time the adhesive reaction of lead oxide can be suppressed. In addition, the present invention has been completed from the idea that it is necessary that iridium oxide of the catalyst layer is mainly crystalline in order to improve the electrode durability at the same time, and experiments were repeated. 
     In the present invention, a two-step baking is performed, first, in a relatively high temperature region of 430 degrees Celsius-480 degrees Celsius to form a catalyst layer containing amorphous IrO 2  in the baking, then, in a further high temperature region of 520 degrees Celsius-600 degrees Celsius to post-bake, through which the iridium oxide of the catalyst layer is almost completely crystallized. 
     Through the experiments conducted by inventors of the present invention, it has been proved that the catalyst layer containing amorphous iridium oxide, which can greatly increase the effective surface area, consumes amorphous iridium oxide quite rapidly by electrolysis and durability is reduced relatively. In other words, it is considered that the electrode durability cannot be improved unless iridium oxide of the catalyst layer is crystallized. Therefore, in order to achieve the purpose of the present invention that the effective surface area of the electrode catalyst layer is increased and the overvoltage of the electrode is reduced, the present invention applies two-step baking: high temperature baking plus high temperature post-baking in order to control the crystallite diameter of iridium oxide of the catalyst layer, through which iridium oxide crystal, smaller in size than the conventional product precipitates, resulting in increased effective surface area of the electrode catalyst layer and reduced overvoltage. 
     In the present invention, a catalyst layer containing amorphous iridium oxide is formed on the surface of the conductive metal substrate by baking in a relatively high temperature region of 430 degrees Celsius-480 degrees Celsius; thereafter, the catalyst layer of amorphous iridium oxide is post-baked in a further high temperature region of 520 degrees Celsius-600 degrees Celsius to crystallize the iridium oxide in the catalyst layer almost completely. 
     According to the present invention, improved quality of products and provision of special performance characteristics to products are achieved by controlling the amount of coating of iridium to 2 g/m 2  or more per time. When electrolysis is performed at a current density of 300 A/dm 2 -700 A/dm 2  or more, or also an auxiliary anode is provided at a specified spot under a high load electrolysis conditions to give special performance characteristics to products obtained from electrolysis, load to the electrode catalyst layer can be lessened, electric current concentration can be prevented and consumption of electrode catalyst layer can be suppressed. 
     The baking temperature in a relatively high temperature region of 430 degrees Celsius-480 degrees Celsius and the post-baking temperature in a further high temperature region of 520 degrees Celsius-600 degrees Celsius are determined by the crystal particle size and the degree of crystallinity of iridium oxide to be formed in the catalyst layer, and the catalyst layer with a low oxygen overvoltage and a high corrosion resistance is formed in the above-mentioned temperature region. 
     In the present invention, the growth of the crystallite diameter of iridium oxide was able to be suppressed and the effective surface area of the catalyst layer was able to be increased by controlling the crystallite diameter of the iridium oxide in the electrode catalyst layer to a small number, preferably equal to or less than 9.0 nm and most of the iridium oxide was crystallized, preferably, to the degree of crystallinity equal to or more than 80%. 
     Prior to forming the catalyst layer, if the AIP base layer containing tantalum and titanium components is provided on the conductive metal substrate, it is possible to prevent further interfacial corrosion of the metal substrate. 
     The base layer consisting of TiTaO x  oxide layer may be applied instead of the AIP base layer. 
     The catalyst layer was formed in such a manner that hydrochloric acid aqueous solution of IrCl 3 /Ta 2 Cl 5  as a coating liquid was coated on the AIP coated titanium substrate at 3 g-Ir/m 2  per time and baked at a temperature by which part of IrO 2  crystallizes (430-480 degrees Celsius). After repeating the coating and baking process until the necessary support amount of the catalyst was obtained, one hour post-bake was conducted at a further high temperature (520 degrees Celsius-600 degrees Celsius). In this way, the electrode sample was prepared. The prepared sample was measured for IrO 2  crystalline of the catalyst layer by X-ray diffraction, oxygen generation overvoltage, electrostatic capacity of electrode, etc. and evaluated for sulfuric acid electrolysis and gelatin-added sulfuric acid electrolysis and lead adherence test. 
     As a result, it has been found that most of the IrO 2  of the formed catalyst layer was crystalline, the crystallite diameter became smaller, and the electrode effective surface area increased. Accelerated life evaluation was carried out and found that, as to be described later, sulfuric acid electrolysis life was about 1.4 times that of the conventional product, and gelatin-added sulfuric acid electrolysis life was about 1.5 times that of the conventional product, proving improvement in durability. 
     The experimental conditions and methods by the present invention are as follows. 
     In order to investigate formation temperature of amorphous iridium oxide and the range of post-bake temperature for successive crystallization, a sample shown in Table 1 was manufactured and subjected to measurements of X-ray diffraction, cyclic voltammetry, oxygen overvoltage, etc. 
     The surface of titanium plate (JIS-I) was subjected to the dry blast with iron grit (G120 size), followed by pickling in an aqueous solution of concentrated hydrochloric acid for 10 minutes at the boiling point for cleaning treatment of the metal substrate of the electrode. The cleaned metal substrate of the electrode was set to the AIP unit applying Ti—Ta alloy target as a vapor source and a coating of tantalum and titanium alloy was applied as the base layer on the surface of the metal substrate of the electrode. Coating condition is shown in Table 1. 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
               
                   
               
             
            
               
                 Target(vapor source) 
                 Alloy disk comprising Ta:Ti = 60 wt %:40 
               
               
                   
                 wt % (back surface cooling) 
               
               
                 Vacuum pressure 
                 1.5 × 10 −2  Pa or less 
               
               
                 Metal substrate temperature 
                 500 degrees Celsius or less 
               
               
                 Coating pressure 
                 3.0 × 10 −1 ~4.0 × 10 −1  Pa 
               
               
                 Vapor source charge power 
                 20~30 V, 140~160 A 
               
               
                 Coating time 
                 15~20 minutes 
               
               
                 Coating thickness 
                 2 micuron (weight increase conversion) 
               
               
                   
               
            
           
         
       
     
     The coated metal substrate was heat-treated at 530 degrees Celsius in an electric furnace of air circulation type for 180 minutes. 
     Then, the coating solution prepared by dissolving iridium tetrachloride and tantalum pentachloride in concentrated hydrochloric acid was applied on the coated metal substrate. After drying, the thermal decomposition coating was conducted for 15 minutes in the electric furnace of air circulation type at a temperature shown in Table 2 to form an electrode catalyst layer comprising mixture oxides of iridium oxide and tantalum oxide. The amount of coating solution was determined so that the thickness of coating per time of the coating solution corresponds to approx. 3.0 g/m 2 , as iridium metal. This coating-baking operation was repeated nine times to obtain the electrode catalyst layer of approx. 27.0 g/m 2 , as converted for metal iridium. 
     Then, the coated sample with catalyst layer was subjected to the post bake in the electric furnace of air circulation type for one hour at a temperature shown in Table 2 to manufacture an electrode for electrolysis. In addition, a sample not subjected to post-bake was manufactured for comparison purpose. 
     Baking temperature and post-bake temperature of each sample are shown in Table 2. 
     Experimental Items for Evaluation 
     (1) Degree of crystallinity and measurement of crystallite diameter 
     IrO 2  crystallinity and crystallite diameter of the catalyst layer were measured by X-rays diffractometry. 
     The degree of crystallinity was estimated from the diffraction peak intensity. 
     (2) Electrostatic capacity of electrode 
     Method: cyclic voltammetry 
     Electrolyte: 150 g/L H 2 SO 4  aq. 
     Electrolysis temperature: 60 degrees Celsius 
     Electrolysis area: 10×10 mm 2    
     Counter electrode: Zr plate (20 mm×70 mm) 
     Reference electrode: Mercurous sulphate electrode (SSE) 
     (3) Measurement of oxygen overvoltage 
     Method: current interrupt method 
     Electrolyte: 150 g/L H 2 SO 4  aq. 
     Electrolysis temperature: 60 degrees Celsius 
     Electrolysis area: 10×10 mm 2    
     Counter electrode: Zr plate (20 mm×70 mm) 
     Reference electrode: Mercurous sulphate electrode (SSE) 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                   
                   
                   
                   
                 Oxygen 
               
               
                   
                   
                   
                   
                   
                   
                 generation 
               
               
                   
                 Baking 
                 Post-bake 
                 Degree of 
                 Crystallite 
                 Electrostatic 
                 overvoltage 
               
               
                   
                 temperature 
                 temperature 
                 crystallinity 
                 diameter 
                 capacity 
                 (V vs. SSE 
               
               
                 Sample No. 
                 (° C.) 
                 (° C.) 
                 (%) 
                 (nm) 
                 (C/m 2 ) 
                 @100 A/dm 2 ) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 1 
                 430 
                 none 
                 0 
                 0 
                 88.8 
                 0.851 
               
               
                 2 
                   
                 520 
                 100 
                 7.7 
                 21.6 
                 0.963 
               
               
                 3 
                   
                 560 
                 100 
                 7.8 
                 15.4 
                 0.987 
               
               
                 4 
                   
                 600 
                 100 
                 7.7 
                 11.6 
                 1.021 
               
               
                 5 
                 480 
                 none 
                 72 
                 9.3 
                 13.7 
                 0.983 
               
               
                 6 
                   
                 520 
                 85 
                 8.5 
                 18.1 
                 1.011 
               
               
                 7 
                   
                 560 
                 82 
                 8.5 
                 14.4 
                 1.031 
               
               
                 8 
                   
                 600 
                 98 
                 8.7 
                 14.5 
                 1.035 
               
               
                 9 
                 500-520 
                 none 
                 100 
                 9.1 
                 7.6 
                 1.051 
               
               
                 (Conventional 
               
               
                 product) 
               
               
                   
               
            
           
         
       
     
     The changes of IrO 2  crystal characteristics by the baking temperature and the post-bake temperature were as follows. 
     As for the estimation of degree of crystallinity, the intensity of the crystal diffraction peak (θ=28 degrees) of each sample is expressed as a ratio when compared with the intensity of the crystal diffraction peak (θ=28 degrees) of the conventional product which is assumed as 100. The results are given in Table 2. In addition,  FIG. 1  is a graph showing the degree of crystallinity based on the data in Table 2. 
     As is clear from Table 2 and  FIG. 1 , the degree of crystallinity of iridium oxide after post-bake of Samples 2-4 and Samples 6-8 of the example by the present invention, which had been subjected to baking in a relatively high temperature region of 430 degrees Celsius-480 degrees Celsius plus post-bake in a further high temperature region of 520 degrees Celsius-600 degrees Celsius was 80% or more. On the other hand, iridium oxide attributable to the electrode catalyst layer treated by the baking at 430 degrees Celsius without post-bake (Sample 1) did not show a clear peak, proving that the catalyst layer of this sample comprises amorphous iridium oxide. The degree of crystallinity of the electrode catalyst layer baked at 480 degrees Celsius without post-bake (Sample 5) was 72% with a lot of remaining amorphous iridium oxide. In addition, Sample 9, which is a conventional product was fully crystallized, showing the degree of crystallinity being 100%, but the crystallite diameter increases to 9.1 nm, resulting in a low value of the electrostatic capacity of electrode at 7.6 with small effective surface area. 
     In other words, as the change of the degree of crystallinity by a high temperature post-bake, clear peak of IrO 2  attributable to the electrode catalyst layer was observed after baking at 430 degrees Celsius and post-bake in a further high temperature, showing that amorphous IrO 2  of the catalyst layer had changed to crystalline by a high temperature post-bake. In addition, it was found that the peak intensity was similar to that of the conventional product at any post-bake temperatures, showing that amorphous IrO 2  did not remain. On the other hand, the products treated by the baking at 480 degrees Celsius showed a further high degree of crystallinity by a high temperature post-bake. However, it was found that a small amount of amorphous IrO 2  still existed after post-bake at 520 degrees Celsius and 560 degrees Celsius. By contrast, the degree of crystallinity of IrO 2  after the post-bake at 600 degrees Celsius was almost equivalent to the conventional product, showing full crystallization. 
     Then, the crystallite diameter was calculated from X-ray diffraction. The results are shown in Table 2.  FIG. 2  was prepared based on the data in Table 2 relating to the crystallite diameter. 
     The crystal diameter of the amorphous IrO 2  formed by the baking at 430 degrees Celsius without post-bake is indicated as “0”. It was found that if post-bake is applied, amorphous IrO 2  was crystallized, but the crystallite diameter of the formed crystal became smaller than that of the conventional product. In addition, there is little mutual dependence observed between the post-bake temperature and the crystallite diameter of IrO 2 . 
     On the other hand, the crystallite diameter of the baked product in 480 degrees Celsius followed by post-bake gave a smaller one than the conventional product, regardless of the post-bake temperature. In other words, crystallinity of IrO 2  of the catalyst layer formed in a low temperature baking increased by post-bake, but the increasing of IrO 2  crystallite diameter was able to be suppressed. 
     As is evident from the data on the crystallite diameter in Table 2 and  FIG. 2 , the crystallite diameter of iridium oxide after post-bake of Samples 2-4 and Samples 6-8 of the examples by the present invention, which was subjected to baking in a relatively high temperature region of 430 degrees Celsius-480 degrees Celsius plus post-bake in a further high temperature region of 520 degrees Celsius-600 degrees Celsius was 9.0 nm or less. On the other hand, iridium oxide attributable to the electrode catalyst layer treated by the baking at 430 degrees Celsius without post-bake (Sample 1) did not show a clear peak, proving that the catalyst layer of this sample comprises amorphous iridium oxide. The crystallite diameter of the electrode catalyst layer baked at 480 degrees Celsius without post-bake (Sample 5) was large to 9.3 nm. The crystallite diameter of iridium oxide of Sample 9, which is the conventional product, was as large as 9.1 nm. 
     Then, measurements were made about the change of effective surface area of the electrode catalyst layer prepared by high temperature baking in a relatively high temperature region of 430 degrees Celsius-480 degrees Celsius plus post-bake in a further high temperature region of 520 degrees Celsius-600 degrees Celsius. 
     Electrostatic capacity of the electrode calculated by the cyclic voltammetry method is shown in Table 2. Electrostatic capacity of the electrode is proportional to the effective surface area of electrode, and it may be right to say that the higher the capacity, the higher the effective surface area also is.  FIG. 3  shows the relationship between the electrostatic capacity and the baking conditions of the catalyst layer, based on the data in Table 2. 
     As is clear from Table 2 and  FIG. 3 , the electrostatic capacity of the electrode of Samples 2-4 and Samples 6-8 of the example by the present invention, which were subjected to baking in a relatively high temperature region of 430 degrees Celsius-480 degrees Celsius plus post-bake in a further high temperature region of 520 degrees Celsius-600 degrees Celsius increased to a high point of 11.6 or more. On the other hand, IrO 2  of the catalyst layer formed by baking at 430 degrees Celsius without post-bake (Sample 1) showed the largest effective surface area (the electrolytic capacity of the electrode), since it is amorphous. After conducting post-bake, the effective surface area (the electrolytic capacity of the electrode) decreased since IrO 2  was crystallized, but it was still higher compared with the conventional product. This may be because the formed crystallite diameter was smaller than the conventional product. In addition, it was observed that the electrode effective surface area (the electrolytic capacity of the electrode) tended to decrease with the increasing of post-bake temperature. 
     Also, it has been found that if post-bake is conducted after the baking at 480 degrees Celsius (Samples 5-8), the effective surface areas (the electrolytic capacity of the electrode) are almost the same regardless of the post-bake temperature, meanwhile they doubled compared with the conventional product. This is probably due to a smaller IrO 2  crystallite diameter compared with the conventional product and also a small amount of amorphous IrO 2  remaining. Moreover, even if the post-bake temperature is increased, there was no change in the electrode effective surface area (the electrolytic capacity of the electrode). 
     The oxygen generation overvoltage (V vs. SSE @ 100 A/dm 2 ) of each sample was measured. The results are shown in Table 2. In addition, the dependence of the oxygen generation overvoltage on baking conditions is shown in  FIG. 4 . The trend of changing in the graph of  FIG. 4  was reverse to that of  FIG. 3 . With increase of the electrode effective surface area, the oxygen generation overvoltage of the samples tended to decrease. As the reason, it is considered that increased electrode effective surface area contributed to dispersion of electric current distribution, lowering the actual electric current. 
     The product with the largest effective surface area baked at 430 degrees Celsius without post-bake showed the lowest oxygen overvoltage, but oxygen overvoltage increased as a result of decreased effective surface area by post-bake. Similar trend was observed with the product baked at 480 degrees Celsius in dependence of oxygen overvoltage on the post-bake temperature. In addition, the oxygen overvoltage of these samples was found to be higher than that of the conventional product. This seems to be because the surface area increased compared with the conventional product. 
     In Table 2 and  FIG. 4 , it is indicated oxygen overvoltage of Samples 2-4 and Samples 6-8 of the examples by the present invention, which were subjected to baking in a relatively high temperature region of 430 degrees Celsius-480 degrees Celsius plus post-bake in a further high temperature region of 520 degrees Celsius-600 degrees Celsius decreased. 
     As mentioned above, the electrode manufactured by the baking means of baking in a relatively high temperature region of 430 degrees Celsius-480 degrees Celsius plus post-bake in a further high temperature region of 520 degrees Celsius-600 degrees Celsius, features to have a smaller IrO 2  crystal of the catalyst layer compared with the conventional product and an increased electrode surface area. In these samples, electric current distribution can be dispersed under a high-load condition and actual electric current load was decreased, from which such effects as suppression of catalyst consumption and improvement in durability can be expected. 
     EXAMPLES 
     The following describes examples by the present invention; provided, however, the present invention is not limited to these examples. 
     Example 1 
     The surface of titanium plate (JIS-I) was subjected to the dry blast with iron grit (G120 size), followed by pickling in an aqueous solution of concentrated hydrochloric acid for 10 minutes at the boiling point for cleaning treatment of the metal substrate of the electrode. The cleaned metal substrate of the electrode is set to the AIP unit applying Ti—Ta alloy target as a vapor source and a coating of tantalum and titanium alloy was applied as the AIP base layer on the surface of the metal substrate of the electrode. Coating condition is shown in Table 1. 
     The coated metal substrate was treated at 530 degrees Celsius in an electric furnace of air circulation type for 180 minutes. 
     Then, the coating solution prepared by dissolving iridium tetrachloride and tantalum pentachloride in concentrated hydrochloric acid is applied on the coated metal substrate. After drying, the thermolysis coating was conducted for 15 minutes in the electric furnace of air circulation type at 480 degrees Celsius to form an electrode catalyst layer comprising mixture oxides of iridium oxide and tantalum oxide. The amount of coating solution was determined so that the thickness of coating per time of the coating solution corresponds to approx. 3.0 g/m 2 , as iridium metal. This coating-baking operation was repeated nine times to obtain the electrode catalyst layer of approx. 27.0 g/m 2 , converted for metal iridium. 
     The X-ray diffraction was carried out for this sample. A clear peak of iridium oxide attributable to the electrode catalyst layer was observed, but the intensity of the peak was lower than that of Comparative Example 1, indicating that crystalline IrO 2  had been partially precipitated. 
     Next, an electrode for electrolysis was manufactured in such a manner that the sample coated with the catalyst layer is post-baked in an electric furnace of air circulation type at 520 degrees Celsius for one hour. 
     The X-ray diffraction was carried out for the sample with post-baking. A clear peak of iridium oxide attributable to the electrode catalyst layer was observed, but the intensity of the peak was still lower than that of Comparative Example 1, though was higher than before the post-bake. From this, it has been known that the degree of crystallinity of the catalyst layer formed by the low temperature baking, before the post-bake, has increased, but amorphous IrO 2  still remains partially. 
     About the electrode for electrolysis prepared in the above-mentioned manner, two types of life evaluation test were conducted for: Pure sulfuric acid solution and sulfuric acid solution with gelatin. Results are shown in Table 4. When compared with Comparative Example 1 (Conventional Product) in Table 4, the life for sulfuric acid electrolysis was 1.7 times and the life of gelatin-added sulfuric acid electrolysis was 1.1 times, identifying that durability to both sulfuric acid and organic additive has improved. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Sulfuric acid 
                 Gelatin-added 
               
               
                   
                 electrolysis 
                 sulfuric acid electrolysis 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Current density 
                 500 A/dm 2   
                 300 A/dm 2   
               
               
                 Electrolyte 
                 150 g/L 
                 150 g/L of H 2 SO 4  aq. + 50 ppm 
               
               
                   
                 of H 2 SO 4  aq. 
                 gelatin 
               
            
           
           
               
               
            
               
                 Electrolysis  
                 60° C. 
               
               
                 temperature 
               
               
                 Counter electrode 
                 Zr plate 
               
               
                 Criterion of 
                 At the time when a cell voltage increased  
               
               
                 electrolysis life 
                 1.0 V than an initial cell voltage. 
               
               
                   
               
            
           
         
       
     
     Example 2 
     The electrode for evaluation was manufactured in the same manner as with Example 1 except that post-bake was conducted in an electric furnace of air circulation type for one hour at 560 degrees Celsius and the same electrolysis evaluation was performed. 
     The X-ray diffraction performed after post-bake showed the degree of IrO 2  crystallinity and crystallite diameter of the catalyst layer equivalent to Example 1. 
     As shown in Table 4, when compared with Comparative Example 1 (Conventional Product) in Table 4, the life of sulfuric acid electrolysis was 1.5 times and the life of gelatin-added sulfuric acid electrolysis was 1.3 times, identifying that durability to both sulfuric acid and organic additive has improved. 
     Comparative Example 1 
     The electrode catalyst layer comprising the mixture oxide of iridium oxide and tantalum oxide was formed as with Example 1, but changing the baking temperature in the electric furnace of circulation air type to 520 degrees Celsius and the baking time to fifteen minutes. The electrode thus manufactured without post-bake was evaluated for electrolysis by the X-ray diffraction as with Example 1. 
     The X-ray diffraction was performed on this sample, from which a clear peak of iridium oxide attributable to the electrode catalyst layer was observed, verifying that IrO 2  in the catalyst layer is crystalline. 
     Life evaluation was made as with Example 1. From the results shown in Table 4, it has been made clear that the method of the low temperature baking plus high temperature post-bake, as suggested in the present invention, improves durability in electrolysis under high-load conditions. 
     Comparative Example 2 
     In the same manner as with Example 1 except that post-bake was carried out, the electrode for evaluation was manufactured and electrolysis evaluation was carried out in the same manner with Example 1. 
     As shown in Table 4, lives of the electrode baked at 480 degrees Celsius without post-bake for sulfuric acid electrolysis and gelatin-added sulfuric acid electrolysis were equivalent to that of the conventional product, proving no improvement in durability. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                 Baking 
                 Post-bake 
                 Life of sulfuric 
                 Life of gelatin-added 
               
               
                   
                 temperature 
                 temperature 
                 acid electrolysis 
                 sulfuric acid electrolysis 
               
               
                   
                 (° C.) 
                 (° C.) 
                 (hr) 
                 (hr) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Example 
                 1 
                 480 
                 520 
                 4182 
                 1084 
               
               
                   
                 2 
                 480 
                 560 
                 3665 
                 1304 
               
               
                 Comparative 
                 1 
                 520 
                 — 
                 2508 
                 978 
               
               
                 Example 
                 2 
                 480 
                 — 
                 2604 
                 1073 
               
               
                   
               
            
           
         
       
     
     INDUSTRIAL APPLICABILITY 
     The present invention relates to an anode for oxygen generation used for various industrial electrolyses and a manufacturing method for the same; more in detail, it is applicable to a high-load durable anode for oxygen generation used for industrial electrolyses including manufacturing of electrolytic metal foils such as electrolytic copper foil, aluminum liquid contact, continuously electrogalvanized steel plate and metal extraction, having superior durability under high-load electrolysis conditions.