Electrodes with multicomponent coatings
Franks et al.  
 Electronor Corporation, Panar City, Panama Filed: Apr. 19, 1973 Appl. No.: 352,499  
 Assignee:  
 U.S. Cl 204/290 F, 204/98 Int. Cl. B01K 3/04 Field of Search 204/290 F, 95  
 References Cited UNITED STATES PATENTS 9/1966 Messner 204/219 10/1971 Bianchi et al. 204/290 F 1 Apr. 1, 1975 3,627,669 12/1971 Entwisle et a]. 204/290 F 3,684,543 8/1972 DeNora et a1 204/290 F 3,725,223 4/1973 DeNora et al. 204/99 3,732,157 5/1973 Dewitt 204/95 3,751,296 8/1973 Beer 204/290 F 3,776,834 12/1973 OLeary 204/96 3,779,889 12/1973 Loftfield 204/95 3,793,164 2/1974 Kolb et al. 204/99 Primary ExaminerO. R. Vertiz Assistant Examiner-Wayne A. Langel Attorney, Agent, or Firml-lammond &amp; Littell [57] ABSTRACT Electrodes useful in a wide variety of electrolytic processes comprise a conductive substrate bearing on at least a portion of the surface. thereof a fourcomponent coating, said components being the oxides of tin, antimony, at least one platinum group metal, and a valve metal selected from the group titanium and tantalum.  
 14 Claims, No Drawings ELECTRODES WITH MULTICOMPONENT COATINGS BACKGROUND OF THE INVENTION Recent years have seen a proliferation of dimensionally stable electrodes, i.e., wear resistant conductive substrates bearing on the surface thereof an electrically conductive, electrocatalytically active coating. Among these have been electrodes coated with (1) an antimony oxide-doped tin oxide containing a platinum metal oxide as the electrocatalytic agent or (2) mixed crystals (solid solutions) of a valve metal oxide and a platinum metal oxide. While such electrodes are far superior to the previously employed graphite, particularly in the area of chlor-alkali electrolysis, understandable efforts have continued to extend the life of these electrodes (that is, reduce the platinum metal wear-rate per unit of product) and/or to reduce the tendency of the coatings to passivate (that is, increase in operating potential to a point at which further operation becomes impractical), especially under oxygen-evolving conditions. Further, owing to inherent limitations relating both to life and passivation tendencies, no single electrode coating system has been found applicable to use in a wide variety of electrochemical processes.  
 STATEMENT OF THE INVENTION Therefore, it is an object of the present invention to provide a coated electrode having a long coating life.  
  It is a further object of the present invention to provide an electrode, the coating of which is extremely resistant to passivation.  
  It is a still further object of the present invention to provide an electrode, the properties of which may be adapted for use in a variety of electrochemical processes.  
  These and further objects of the present invention will become apparent to those skilled in the art from the specification and claims that follow.  
  There has now been found an electrode comprising an electrically conductive supporting substrate bearing on at least a portion of the surface thereof a coating consisting essentially of from L to 10.0 percent antimony oxide, from 30 to 90 percent tin dioxide, from 1.0 to 50 percent of at least one platinum group metal oxide, and from 0.5 to 30 percent of a valve metal oxide selected from the group consisting of titanium and tantalum oxides, with the proviso that the mole ratio of tin to antimony oxides is between 95:5 and 85:15. Further, within the aforestated ranges, those coatings having high valve metal and platinum metal oxide concentrations are particularly useful as anodes at which oxygen is evolved. On the other hand, those coating compositions having low valve metal oxide concentrations and moderate concentrations of platinum metal oxides are particularly useful in chlor-alkali electrolysis.  
 DESCRIPTION OF THE PREFERRED EMBODIMENTS As stated, the invention lies in a combined coating of oxides of tin, antimony, at least one platinum group metal, and a valve metal selected from the group titanium and tantalum on a conductive substrate, useful as an electrode, especially as-an anode, in a variety of electrochemical processes including electrowinning of metals (e.g., copper, nickel, and zinc) from aqueous solution; chlor-alkali electrolysis including chlorine,  
 chlorate, or hypochlorite production; electroplating;  
 oxygen evolution from organic acidic solutions; ozone generation; cathodic protection; electrodialysis; and 5 the like.  
  Suitable substrates include generally any metal of sufficient electrical conductivity and mechanical and chemical resistance to the cell environment in which it is to be employed. For example, these materials may include nickel, steel, stainless steel, titanium, niobium, zirconium, and tantalum. Especially preferred for most applications are titanium, niobium, or tantalum substrates. Of course, those substrates bearing an exterior coating, such as copper or aluminum-cored titanium or a platinum or other conductive metal layer over a titanium substrate, are contemplated. Generally, prior to deposition of the coating and in order to provide a base to which the coating may be satisfactorily anchored, an etching or other cleaning operation is employed.  
  The configuration of the electrode will vary considerably with the application intended but may generally be in the form of a rod or a sheet, either continuous or foraminous, of the appropriate material.  
  What may be considered the first of the components in the coating composition is tin dioxide, preferably present in the form of crystalline SnO and employed within the range of from 30 to 90 percent by weight of the total coating composition on an oxide basis, especially 30 to 50 percent for oxygen applications and 60 to 90 percent for chlorine.  
  The antimony oxide component enters into the tin oxide crystal lattice, rendering same more electrically conductive. Although the antimony is present in an indeterminate oxide form owing to its entrance into the tin oxide crystal lattice, it may be expressed for convenience sake as Sb O Thus, on this basis, the antimony oxide is present within the range of from 1.0 to 10, preferably 4.0 to l0, percent by weight.  
  The foregoing ranges of tin and antimony oxides are further qualified by the proviso that they be present, respectively, in the range, on a mole ratio basis as the oxides, of 95:5 to 85:15, especially 90:10. In this fashion there is obtained the desired doping effect of the antimony on the tin oxide without the presence of an excess separate phase of antimony oxides.  
  The third component of the coating is at least one &#34;platinum group metal oxide, by which term it is intended to include the oxides of platinum, palladium, ruthenium, iridium, rhodium, and osmium, preferably ruthenium, iridium, rhodium, and palladium, and especially mixtures of ruthenium with iridium, rhodium, or palladium oxides/These platinum group metal oxides are present in their most highly oxidized form and within the range of from 1.0 to 50 percent by weight. When the electrode is being fabricated for use as an anode at which oxygen is evolved, primarily or as a co- Ta O it is understood that mixtures of tantalum oxides may in fact be present. The amounts of valve metal oxides employed are generally within the range of from 0.5 to 30 percent by weight, especially 15 to 25, for  
 trichloride or pentachloride, and stannic chloride or dibutyl tin dichloride.  
  It will be understood by those skilled in the art that it is possible to use a number of combinations of preoxygen-evolving applications and 0.5 to 3.0 for chlor- 5 formed oxides of the various component metals and alkali electrolysis. Further, for a chlor-alkali applicasalts of the remaining materials, although it is generally tion, titanium is preferred as the valve metal whereas believed that preformed valve metal oxides should not in oxygen-evolving applications the preferred valve be employed nor should separately preformed tin and metal is tantalum, although they are interchangeable in antimony oxides be used. Further, if thermal decompomany instances. Generally speaking, the use of small l sition is incomplete, small amounts of salts may remain amounts of the valve metal oxide acts to extend the life without detrimental effect in the coating, for example, of the electrode coating while the incorporation of small amounts of chloride in the primarily oxide coatlarger amounts adds resistance to passivation. g-  
  In summary, an example of a preferred anode for In order that those skilled in the art may more readily oxygen-evolving applications is a coating of from 30 t 15 understand the present invention and certain preferred er t S O 4 O t 8,0 r t 51 0 20 to 40 embodiments by which it may be carried into effect, percent platinum metal oxide, and to 25 percent following Specific examples are afforded valve metal oxide on a titanium, tantalum, or niobium substrate. EXAMPLE 1 On the other hand, an example of a preferred chlo- A series of electrodes is prepared and evaluated as fine anode is a Coating 0f 60 i0 90 Percenl z, 4 t0 anodes as follows. In each instance, the quantity of 10 percent Sb O l .0 to percent platinum metal dithermally decomposable salt set forth in Table l is disoxide, and 0.5 to 3.0 percent titanium or tantalum solved in 45 ml of ethanol with stirring. The resultant oxide on a titanium substrate. 25 solution is brushed onto an expanded titanium mesh While many of the variety of methods known for prosubstrate, previously cleaned by etching for minutes ducing mixed metal oxide coatings may be employed, in boiling (l8%) aqueous hydrochloric acid. The soluthe preferred method of preparing the multicomponent tion is applied to the mesh by brushing, followed by coating composition of the substrate is by deposition drying the anode for 3 minutes at 1 10 C and firing in from a solution of the appropriate thermochemically 30 air at 500 C and 7 minutes. This brushing, drying, and decomposable salts. For example, it is desirable to baking procedure is repeated untilacoating containing paint or brush an acidified alcoholic solution of said 1.7 grams of ruthenium per square foot of anode sursalts onto the substrate followed by drying at l00l40 face is obtained (usually 6-10 coats). Following the C for from 3 to 10, especially 5, minutes and finally by final baking, the electrodes are evaluated as anodes in baking in an oxidizing atmosphere, e.g., air, at 450 to 3 a 150 g/l sulfuric acid solution at 3 amperes per square 520 C, espically 500 C, for from 5 to l0, especially inch opposite a titanium mesh cathode and at an elecabout 7, minutes. This procedure may then be repeated trode gap of 2 inches. The test is continued until the anany number of times until the desired coating thickness odes have passivated, i.e., a voltage of 8.0 volts or is obtained, for example, 6 to 10 coats. The preferred greater is obtained. The lifetime of the anode, that is, solvents for the thermally decomposable salts are the the number of hours of successful operation until paslower alkanols, such as ethanol, propanol, amyl alcosivation occurs, is reported in the following Table 1.  
 TABLE I RuCl .xH O SnCl .5H O sno sbcl sb o, (38%) R60 Tact, Ta O,, Lifetime Anode g 71 g 71 g 7! g &#39;4 hrs.  
 hol, and especially n-butyl alcohol, although other sol- From this it is apparent that Anodes 4 and 5, accordvents including water, may be employed, to which ing to the present invention, are greatly superior to eithere is generally added from 0 to percent by volther an anode combining the valve metal and platinum ume of an acid, such as concentrated hydrochloric acid group metal (anode l) or the platinum metal- (36%). The concentration of the salts from which the y- System (Anode Further, Anode 3illl1S- coating composition is derived is such as to give a metal trates that the range of components of the present incontent in solution within the range of 50 to 200 grams vention is critical to obtaining an anode having a long per liter. The salts employed are generally any ther- ,0 lif mally decomposable inorganic or organic salt or organic ester of the metals in question such as the chlo- EXAMPLE 2 rides, nitrates, alkoxides, alkoxy halides, resinates, Four electrodes were prepared from the f ll i amines, and the like. Specific and illustrative examples l i include potassium hexachlororuthenate, hexachloroi-. Anode 6 50 ml n-butanol, 12.5 g SnCl -5H O, 0.9  
 ridic acid, ruthenium trichloride or tribromide, or-&#39; thobutyl titanate, tantalum pentachloride, antimony g SbCl and 1.] g RuClg-xH o (38% Ru). Anode 7 45 ml ethanol, 5.0 g orthobutyl titanate,  
 1.1 g SbCl 15.1 g SnCl -5H O, and 7.6 g RuCl -x- H (38% Ru).  
 Anode 8 50 ml n-butanol, 12.5 g SnCL&#39;SH O, 0.91 g SbCl 7.0 g orthobutyl titanate, and 1.1 g RuCl -xl-l O (38% Ru).  
 Anode 9 45 ml ethanol, 4.5 g TaCl,=,, 1.1 g SbCl 15.1 g SnCl -5H O, and 7.6 g RuCl -xH O (38% Ru).  
  Each anode is prepared by applying six coats of the solution by brush, with heating in air between each coat first at 1 C for 3 minutes followed by 7 minutes at 500 C.  
  These electrodes are evaluated as anodes in a horizontal mercury cell spaced 0.14 inch above and parallel to a mercury cathode flowing at a rate of 450 ml/minute. The electrolyte is a 310 g/l brine solution having a pH within the range of 3-6 and a temperature of about 70 C. To establish the wear-rate of the anodes, electrolysis is conducted at 6 amperes per square inch for 500 hours, the loss being determined by weight differential. Results, together with the composition of each anode coating calculated on an oxide basis, appear in Table 2.  
 weight basis, from to 90 percent SnO from 1.0 to 50 percent of at least one platinum group metal oxide, and from 0.5 to 30 percent of a valve metal oxide selected from the group consisting of titanium and tanta lum oxides, with the proviso that the mole ratio of tin to antimony oxides is between 95:5 and 85:15.  
  2. An electrode as in claim 1 wherein the supporting substrate is selected from the group consisting of nickel, steel, stainless steel, titanium, niobium, zirconium, and tantalum.  
  3. An electrode as in claim 1 wherein the platinum metal oxide is RuO 4. An electrode as in claim 1 wherein the valve metal oxide is TiO 5. An electrode as in claim 1 wherein the valve metal oxide is amorphous tantalum oxide.  
  6. An electrode as in claim 1 wherein the ratio of tin to antimony oxides is about 90:10.  
  7. An anode for use in oxygen-evolving applications, which anode comprises an electrically conductive supporting substrate bearing on at least a portion of the surface thereof a coating consisting essentially of from 4.0 to 8.0 percent antimony oxide, calculated as Sb O Anode Wear-Rate g/ton C 1 EXAMPLE 3 An anode coating solution if prepared from 45 ml ethanol, 4.5 g TaCI 1.1 g SbCl 15.1 g SnCl -5H O, and 7.6 g RuCl -xH O (38% Ru). An etched titanium mesh substrate is coated by brushing, drying at 1 10 C for 3 minutes, and baking in air at 500 C for 7 minutes. The coating procedure is repeated until a coating having a platinum group metal content of 1 gram per square foot is obtained. This is labeled Anode l0.  
  Anode l l is prepared in an identical fashion but substituting 0.92 g of lrCl and 6.54 g RuCl &#39;xH O for the ruthenium content of Anode l0. Anode 12 is likewise similar with the exception that 1.28 g of RhCl -3H O and 6.65 g RuCl -xH O comprise the platinum group metal content.  
  When evaluated according to the lifetime test described in Example 1 above, Anodes l0, l1, and 12 have lifetimes, respectively, of 185, 250, and 350 hours. This indicates the substantial improvement possible employing a mixture of platinum metal oxides in the coating.  
 We claim:  
  1. An electrode comprising an electrically conductive supporting substrate bearing on at least a portion of the surface thereof a coating consisting essentially of from 1.0 to 10 percent antimony oxide, as Sb O on a on a weight basis, from 30 to 50 percent SnO from 20 to 40 percent of at least one platinum metal oxide, and from 15 to 25 percent of a valve metal oxide selected from the group consisting of titanium and tantalum oxides, with the proviso that the mole ratio of tin to antimony oxides is between 95:5 and :15.  
  8. An anode as in claim 7 wherein the substrate is selected from the group consisting of nickel, steel, stainless steel, titanium, niobium, zirconium, and tantalum.  
  9. An anode as in claim 7 wherein the platinum metal oxide is a combination of RuO and lrO 10. An anode as in claim 7 wherein the platinum metal oxide is a combination of ruthenium and rhodium oxides.  
  11. An anode as in claim 7 wherein the valve metal oxide is amorphous tantalum oxide.  
  12. An anode for use in chlor-alkali electrolysis, which anode comprises a valve metal substrate selected from the group consisting of titanium, niobium, zirconium, and tantalum bearing on at least a portion of the surface thereof a coating consisting essentially of from 4.0 to 10 percent antimony oxide, calculated as Sb O on a weight basis, from 60 to percent SnO from 1.0 to 25 percent of at least one platinum group metal oxide, and from 0.5 to 3.0 percent of a valve metal oxide selected from the group consisting of titanium and tan-&#39; talum oxides, with the proviso that the mole ratio of tin to antimony oxides is between :5 and 85:15.  
  13. An anode as in claim 12 wherein the substrate is titanium.  
  14. An anode as in claim 12 wherein the valve metal oxide is TiO