Anode for oxygen evolution
United States Patent De Nora et al.  
 ANODE FOR OXYGEN EVOLUTION Inventors: Oronzio De Nora; Giuseppe Bianchi; Antonio Nidola; Giovanni Trisoglio, all of Milan, Italy Assignee: Electronor Corporation, Panama City Filed: May 17, 1973 Appl. No.: 361,022  
 Foreign Application Priority Data References Cited UNITED STATES PATENTS 10/1971 Dewitt 204/290 F 14 1 Apr. 15, 1975 3,632,498 1/1972 Beer 204/290 F 3,711,385 1/1973 Beer..... 204/59 3,751,296 8/1973 Bccr 117/230 FOREIGN PATENTS OR APPLICATIONS 1.147.442 4/1969 United Kingdom 117 230 Primary Examine&#39;r.1ohn H. Mack Assistant Examiner-Aaron Weisstuch Attorney, Agent, or Firm-Hammond &amp; Littell [57] ABSTRACT Novel electrode for oxygen evolution comprising an electroconductive base provided with an outer coating containing a mixed material of tantalum oxide and iridium oxide and preparation and use thereof.  
 10 Claims, No Drawings- ANODE FOR OXYGEN EVOLUTION STATE OF THE ART In various electrochemical process such as, for example, in the production of chlorine and other halogens, the production of chlorates, the electrolysis of other salts which undergo decomposition under electrolysis conditions and other electrolysis processes, it has recently become commercially possible to use dimensionally stable electrodes in place of graphite. These dimensionally stable electrodes usually have a film forming valve metal base such as titanium, tantalum, zirconium, aluminum, niobium and tungsten, which has the capacity to conduct current in the cathodic direction and to resist the passage of current in the anodic direction and are sufficiently resistant to the electrolyte and conditions used within an electrolytic cell, for example, in the production of chlorine and caustic soda, to be used as electrodes in electrolytic processes. In the anodic direction, however, the resistance of the valve metals to the passage of current goes up rapidly, due to the formation of an oxide layer thereon, so that it is no longer possible to conduct current to the electrolyte in any substantial amount without substantial increase in voltage which makes continued use of uncoated valve metal electrodes in an electrolytic process uneconomical.  
  It is, therefore, customary to apply electrically conductive electrocatalytic coatings to these dimensionally stable valve metal electrode bases. The electrode coatings must have the capacity to continue to conduct current to the electrolyte over long periods of time without becoming passivated, and in chlorine production, must have the capacity to catalyze the formation of chlorine molecules from the chloride ions at an anode. They must be electroconductive and electrocatalytic and must adhere firmly to the valve metal base over long periods of time under cell operating conditions.  
  The commercially available coatings contain. a catalytic metal or oxide from the platinum group metals, i.e., platinum, palladium, iridium, ruthenium, rhodium, osmium and a binding or protective agent such as titanium, dioxide, tantalum pentoxide and other valve metal oxides in sufficient amount to protect the platinum group metal or oxide from being removed from the electrode in the electrolysis process and to bind the platinum group metal or oxide to the electrode base. The binding and protective metal oxide is usually in excess of the platinum group metal or oxide. Anodes of this nature have been described in British Pat. No. 1,231,280.  
  In anodes for the recovering of metals by electrowin&#39; ning, a continual source of difficulty has been the selection of a suitable material for the anode. The requirements are insolubility, resistance to the mechanical and chemical effects of oxygen liberated on its surface, low oxygen overvoltage, and resistance to breakage in handling. Lead anodes containing 6 to percent antimony have been used in&#39; most plants. Such anodes are attacked by chloride if present in the electrolyte. This is the case at the huge plant at Chuquicamata, Chile, where it is necessary to remove cupric chloride dissolved from the ore by passing the solution over cement copper, reducing the cupric to insoluble cuprous chloride. At this plant there was also developed an anode of a copper-silicon alloy, called the Chilex anode, used in a portion of the tank-room. It has a longer life but raises the power consumption because of greater resistance and greater oxygen overvoltage.  
  Attempts to use mixed oxide coatings such as RuO TiO for oxygen evolution have not been satisfactory in commercial use because passivation takes place after 200 to 1,000 hours of operation at a current density of 1.2 KA per m The use of a Ta O Ru0 mixed oxide coating improves the electrocatalytic activity and the life of the anode somewhat but not enough for commercial use. The use of a TiO -IrO coating has lower electrocatalytic activity.  
 OBJECTS OF THE INVENTION It is an object of the invention to provide a novel anode for oxygen evolution having an outer coating containing a mixed material containing tantalum oxide and iridium oxide.  
  It is an additional object of the invention to provide a novel electrode with an outer coating of tantalum oxide and iridium oxide doped to improve the catalytic activity for oxygen evolution.  
  It is another object of the invention to provide novel electrodes having a coating of a mixed material of Ta O -IrO on a valve metal alloy base having improved mechanical stability.  
  It is a further object of the invention to provide a novel process for the electrowinning of metals.  
  These and other objects and advantages of the invention will become obvious from the following detail description.  
 THE INVENTION The novel electrodes of the invention are comprised of an electroconductive base provided with a coating over at least a portion of its outer surface of a mixed material of tantalum oxide and iridium oxide. The coating may be as little as 5% of the outer surface of the electrode but preferably covers 50 to of the active face of the electrode. The preferred ratio of tantalum to iridium calculated in percent of metal is 1:1 to 0.34:1.  
  The electrode base may be made of any electroconductive material such as iron, nickel, lead, copper, etc. or alloys thereof but is preferably a valve metal such as tungsten, titanium, tantalum, niobium, aluminum or zirconium or alloys of two or more of said metals. The valve metals bases may be provided with an intermediate layer such as an oxide of the valve metal or a coating of another metal such as platinum group metals. The base may be a valve metal and at least either one metal having a low hydrogen overvoltage such as alloy of titanium with iron, cobalt, nickel, palladium, vandadium or molybdenum, or mixtures of two or more of said metals; or one metal suitable to form with titanium a protective oxide film even in acid solution such as an alloy of titanium with niobium, tantalum, zirconium or mixtures of two or more of said metals.  
  In a preferred embodiment of the invention, the electroconductive base is an alloy of a valve metal with a platinum group metal which has an improved corrosion resistance to acid electrolytes encountered in the use of the electrodes such as 5 to 15% sulfuric acid or 1 to 5% hydrochloric acid. A particularly useful alloy is titanium containing 0.1 to 0.20% by weight of palladium. This corrosion resistance of the support of the coating prevents chipping off of the coating even if the anode is immersed for a few hours in an acid electrolyte without anodic polarization.  
  In a modification of the invention, the coating containing tantalum oxide and iridium oxide can be doped with an oxide of a metal with a valence of less than +4 to increase the catalytic activity for oxygen evolution without adversely effecting the mechanical properties of the coatings.  
  Without wishing to be limited to the following theoretical discussion, it is believed that the semiconductivity of the Ta O -IrO- system is of the n type and that the addition of the doping metal oxide reverses the type of conductivity from n-type to p-type which improves the anodic process by producing electronic holes.  
  The doping metal oxide may be present in the coating in amounts ranging from 0.5 to 5.0% preferably 1.5 to 3.0% by weight of the said system calculated as metal. Examples of suitable doping metal oxides are alkaline earth metals such as calcium, magnesium, barium and members of Groups VIII, VI B and VII B of the periodic Table such as cobalt, iron and nickel, chromium, molybdenum, manganese, etc.  
  The increase in the catalytic activity of the doped coatings is shown by the lower anode potential of doped anodes as compared to undoped anodes after 8,000 hours of operation of the anodes under identical working conditions. The doping seems to have no adverse effect on the mechanical properties of the coatings as there is no coating loss in either instance even after 8,000 hours operation.  
  The electrodes of the invention are particularly useful for electrolytic processes such as cathodic protection, electroflotation, organic electrosynthesis such as hydrodimerization of acrylonitrile and most particularly the electrowinning of metals. The said electrodes have a high electrocatalytic activity and a very low passivation rate of a few millivolts per month at a current density of 1.2 to 2.0 KA per m and a negligible weight loss if kept under anodic polarization.  
  The novel method of the invention for the preparation of the electrodes of the invention comprises applying to an electroconductive electrode base a solution of a thermally decomposible compound of tantalum and a thermally decomposible compound ofiridium, drying the coated electrode base by evaporation of the solvent and then heating the dried electrode base in the presence of an oxygen containing gas such as to form the desired electrode.  
  The heating step is preferably effected at temperatures of 350 to 600C, the optinum temperature being 500 to 550C. At temperatures below 350C, the oxidation is not completed or requires too long heating time and at temperatures above 600C, the electrode base is likely to be subjected to distortions and/or destruction by the high temperatures.  
  The preliminary drying step is preferably effected by gentle heating in air to evaporate the solvent and codeposit the metal compounds. However, any convenient procedure may be used to remove the solvent such as standing under reduced pressure.  
  In a preferred embodiment of the process, the coating is applied in multiple coats with short periods of intermediate heating such as 500 to 550C for 5 to minutes with a longer final heating after the last coat such as 500 to 550C for 45 minutes to l 1% hours. The  
 coating obtained thereby is very adherent and quite uniform.  
  The electrodes of this invention are particularly useful for electrowinning process used in the production of various metals because they do not add impurities to the bath which deposit on the cathode, .with the metals being won, and thereby contaminate the refined metal, as do anodes of for example lead containing antimony and bismuth which give impure cathode refined metals. Moreover, their resistance to the acid solutions and oxygen evolution and their excellent anode potential makes them desirable for this use.  
  In the following examples there are described several preferred embodiments to illustrate the invention. However, it should be understood that the invention is not intended to be limited to the specific embodiments.  
 EXAMPLE I 24 Titanium plates 10 mm by 10 mm were etched in boiling 20% hydrochoric acid for 60 minutes and were then thoroughly washed with water. The plates were then coated with an aqueous solution of the compositions of Table I in 12 to 15 coats. After the application of each coat, the plates were dried and then heated for 10 minutes at 450C to 600C in an oven with forced air circulation and then allowed to air cool. After the last coat, the plates were heated in the oven at the same temperature for 1 hour and were then air cooled. The values of Table I are calculated as weight of free metal. The tantalum chloride was used as a solution in 20% hydrochloric acid.  
  The anode potential for each anode was then determined by electrolysis of 10% by weight sulfuric acid at 60C and a current density of 1.2 KA/m The initial anode potential (against NHE) and the anode potential after 3,000 and 6,000 hours was determined and the coating loss&#39; was then determined. The values are reported in Table II.  
 TABLE IV Anode potential (NHE) in Volts after coating loss initial 600 h i000 h 1200 h in mg/cm anode potential and the anode potential after 600, 1,000 or 1,200 hours are reported in Table IV. The final loss of the coating was determined at the end of 5 the test.  
  60C at a current density of 1.2 KA/m The initial Sample TABLE II Coating weight Anode Potential V(NHE) loss initial after after value 3000 hs. 6000 hs. mgjcni Sample Final heating in C The results of Table IV show that RuO -TiO coated electrodes become passivated after only 1,000 hours EXAMPLE Ill 10 plates of titanium containing 0.l5% of palladium 10 X 10 mm) were sandblasted and then etched in refluxing hydrochloric acid for 60 minutes. The  
 TABLE V Coating compositions in mg of Sample No. free metal plates were then coated with the compositions of Table V. The compositions were applied in 15 to 20 coats 2 and the Ta O -RuO coated electrodes are only slightly improved and the TiO -lrO coated electrodes are no better.  
 with intermediate heating at 450 C for 10 minutes In an oven with forced air circulation and cooling in air. The final heating was effected at the temperatures in Table V for 1 hour followed by air cooling.  
 EXAMPLE II For comparative purposes, electrodes were prepared as follows. Titanium plates 10 mm by 10 mm were The results of Table 11 show that the electrodes of the invention have high electrocatalytic activity and a very low passivation rate and that the weight loss of the coating is negligible when within the limits of the invention. It should be noted that the ratio of Ta to lr for samples F to F is about 034. Optimum values are obtained in the heating range of 500550C.  
 etched in boiling 20% hydrochloric acid for minutes and were then thoroughly washed with water. The  
  OOOOOOOOWO 05050505 5 5555555555 0 0 m nmmmaeem s. r nmnmnmnmnm AB DEF H1 ABmDE mH 0 5 4 4 plates were then coated with an aqueous solution of the compositions of Table III in 12 to 15 coats. After the application of each coat, the plates were dried and then heated for 10 minutes at 450 to 550C in an oven with forced air circulation and then allowed to air cool. After the last coat, the plates were heated in the oven Coating weight loss in mg/cm after 2000 hs.  
 TABLE VI after The anode potentials and coating weight loss were so determined as in Example 1 and the results are reported Anode Potential in V(NHE) initial value The results of Table VI show that the electrodes of the invention with a titanium palladium alloy base in Table VI.  
 Sample No.  
 heating temp in &#34;C 5 .0 6O69696O6Q6 TABLE Ill Coating composition Sample No. in mg (metal) The anode potential for each anode was then determined by electrolysis of 10% by weight sulfuric acid at at the same temperature for 1 hour and where then air cooled. The values of Table III are calculated as weight of free metal.  
 have excellent electrocatalytic activity and low passivation rates.  
 EXAMPLE IV To demonstrate the improved corrosion resistance of a titanium palladium alloy, 10 plates made of titanium containing 0.15% by weight of palladium (10 X 10 mm) were sand-blasted and then etched in refluxing 20% hydrochloric acid for 60 minutes. The plates were then coated with the compositions of Table V using the procedure of Example 111. The anode potential was determined for each electrode by electrolysis of 10% sulfuric acid at 60C and a current density of 1.2 KA/m The initial anode potential and the value after 1,000 and 2,000 hours and the coating weight loss after 2,000 hours was determined. Moreover, the current was halted for minutes in each 24 hour period without removing the electrode from the acid bath. The results are reported in Table V11.  
  The results of Table V11 show that the electrodes of the invention having a titanium palladium alloy base have excellent electrocatalytic activity and low passivation rates and the coating does not chip off even without anodic polarization.  
 EXAMPLE V 10 titanium plates X 20 mm) were etched in refluxing 20% hydrochloric acid for 60 minutes and after being thoroughly washed with water, the plates were coated with an aqueous solution containing 2.01 mg (as free metal) of TaCl 3.2 mg (as free metal) oflrC1 and 0.0394 ml of hydrochloric acid. The solution was applied in 12 coats-with intermediate heating and cooling and a final heating as described in Example I.  
  The coated titanium plates were used as anodes in cells for the recovery of zinc from an aqueous electrolyte containing 100 g/liter of Zn S0 (as free metal), 10% sulfuric acid and 10 to 50 ppm of glue. The cathode was a pure aluminum sheet with a smooth surface and the electrolyte gap was 10 mm. The current density was 500 A/m and the electrolyte temperature was 35C. The anode potential, loss of coating, zinc thickness on the cathode and the morphology of the zinc deposit are reported in Table V111 The cathodic current efficiency was found to be 92-95% in all cases and the purity of the zinc deposit was 99.9999%.  
 EXAMPLE V1 Using the procedure of Example V, five titanium plates (20 X 20 cm) were coated with the composition of Example V. The coated plates were used as anodes in ace for recovery of copper from an aqueous electrolyte containing 100 g/liter (as free metal) of CuSO and 10 g/liter of sulfuric acid and the cathode was a smooth steel plate. The electrolyte gap was 15 mm and the bath temperature was C. The current density was 500 A/m The anode potential, loss of coating and copper thickness and morphology of the copper deposit are reported in Table 1X.  
 TABLE 1X Test Anode potential Coating Cu deposit Cu deposit No. V (NHE) weight loss thickness morphology in mm 1 1.47 0 4. smoo The cathodic current efficiency was found to be in all cases and the purity of the copper was 99.9999%.&#39;  
 EXAMPLE V11 TABLE x Specimen Liquid Coating per gm of noble metal/m each titanium sheet coupon Specimen Liquid Coating per gm of noble metal/m No. each titanium sheet coupon 3.2 do. do. 1r l6 C.1Cl. ..6H ,O 0.21 do. do. C11  
  0.0374 mls. 4.4A.4B.4C Tt1Cl,-, 1.87 mg. Tu  
 lrCl 3.2 do. do. lr 1(1 CaCl2.6HgO 0.26 do. do. Ca HCl 0.0362 mls.  
 The samples were then tested in 10% sulfuric acid at TABLE XIII-Continued 2 60 C w an anodlc current. density of i KA/m to Speci- Temp. Co content Anode Potential Coating -determ1ne the anode potential and coatmg loss after men fi l heat initial ft weight 2,500 hours. The results are shown in Table XI. treatmen value 8000 hs. loss 15 V(NHE) mglcm TABLE X1 18 550C do. 1.52 1.57 do. 1C do. do. 1.52 1.57 do. 2 500C 2.5 1.52 1.53 do. Specimen Temp. Ca Anode Potential We1ght 2A do. do. 1.52 1.53 do. No. final heat content initial after loss 28 550C do. 1.52 1.54 do. treatment b.w.t. value 2500 hs. mg/cm 2() 2C do. do. 1.52 1.54 do. V(NHE) 3 500C 4 1.52 1.56 do. 3A do. do. 1.52 1.56 do. 5 a .5 L 5 0 3B 550C do. 1.52 1.56 do. 1,, C 1 1 t 2 do 3C do. do. 152 1.56 do. 1; 4 500C 5 1.52 1.56 do. 1B 550 C l 1.51 1.56 do.  
  4A do. do. 1.52 1.57 do. lC do. 1.51 V 1.56 do. 7 a 4B 550 C do. 1.52 1.56 do. 500 C 2 5 1.50 1.51 do. 4C d d 1 l 57 d 2A do. 1.51 1.51 do. 213 550C 2 5 1.50 1.52 do. I I 2C do. 1.50 1.52 do. Various mod1ficat1ons of the electrodes are processes 3 500C 4 0 of the invention may be made without departing from 3A do. 1.52 1.58 do. 3B 550C 4.0 155 the sp1r1t or scope thereof and it should be understood 2C 3106C 5 O g that the invention is to be limited only as defined in the O. 4A do. 1.52 1.60 do. appended, clams- 4B 550C 5.0 1.52 1.65 do. We claim: 4C 1. An electrode comprising an electroconductive base provided with a coating over at least a portion of its outer surface of a mixed material of tantalum oxide EXAMPLE V111 Using the procedure of Example V11, 20 X 20mm titanium coupons were coated with the composition of Table X11 with the same heatings.  
  The anode potentials and the coating losses after 8,000 hours in 10% sulfuric acid at 60 C with an anodic current density of 1.2 KA/m was determined as in Example V11 and the results are reported in Table X111.  
 TABLE X111 Speci- Temp. Co content Anode Potential Coating men final heat 7: b.w.t. initial after weight treatment value 8000 hs. loss V(NHE) mg/cm l 500C 1 1.52 1.56 0 1A do. do. 1.52 1.56 do.  
 and iridium oxide, in which the ratio of tantalum to iridium calculated as metal is 1:1 to 0.34 to l.  
  2. The electrode of claim 1 wherein the said base is a valve metal.  
  3. The electrode of claim 1 wherein the said base is an alloy of a valve metal and at least one of the platinum group metals.  
  4. The electrode of claim 2 wherein the valve metal is titanium.  
  5. The electrode of claim 1 wherein the said base is an alloy containing at least two valve metals.  
 6. The electrode of claim 1 wherein the said base is an alloy containing at least two valve metals and at least 0.34 to l.  
  9. An electrode comprising an electroconductive base provided with a coating over at least a portion of its outer surface of a mixed material of tantalum oxide and iridium oxide, in which the ratio of the tantalum to the iridium calculated as metal is 1:1 to 0.34 to 1, said coating further contains 0.1 to 5.0% by weight of an oxide of a metal selected from the group consisting of alkaline earth metals, cobalt, iron, nickel, chromium, molybdenum and manganese.  
  10. An electrode of claim 9 wherein the metal is selected from the group consisting of cobalt and an alkaline earth metal.