Classifications

• • 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 electrode comprises I) an electrically conductive supporting substrate, 2) an intermediate, electrically conductive, barrier layer and 3) an electrocatalytically active, solid solution-type, outer coating.
• the barrier layer is selected from the group consisting of platinum-iridium alloys and oxides of cobalt, manganese, palladium, lead and platinum.
• Electrode development in this industry may be traced through the use of graphite and platinized titanium to the development of a recent composite type electrode which appears to be especially well suited for use in anodic applications.
• These electrodes consist of a valve metal substrate and a coating'which has variously been characterized as mixed oxide, mixed crystal, solid solution and ceramic semi-conductor, but which is characterized for the most part by being based upon a co-deposit of a valve metal oxide and a non-valve metal oxide.
• the solid solutiontype coating offers substantial advantage in terms of oxygen over-voltage which makes its use as an oxygen anode extremely appealing economically.
• the chlorine overvoltage remains substantially constant for long periods of time in use
• the same electrode is employed as an oxygen anode
• the oxygen overvoltage while initially low, steadily increases until, if carried to an extreme, the anode passivates completely, i.e., fails to pass any electrical current.
• the anode when employed as the anode in sulfuric acid, the anode will passivate to such an extent as to render further operation uneconomic within approximately 30 hours at a current density of 1.0 ampere per square inch of anode surface area. Therefore, if the advantage of the extremely low oxygen overvoltage available with the solid solution-type electrodes is to be obtained, a method of preventing the passivation of this coating during use in the evolution of oxygen must be provided. Once it was found that apparently the method of passivation involves the slow diffusion of oxygen through the solid solution-type coating into the supporting valve metal, attention was directed to some method of preventing such diffusion.
• a further object of the invention is to provide an improved electrolytic process involving the generation of oxygen at the anode, which process employs a passivation-resistant anode.
• a surprisingly effective electrode comprises 1) an electrically conductive supporting substrate; 2) a relatively thin, intermediate, electrically conductive, relatively oxygen-impermeable, barrier layer consisting essentially of a material selected from the group consisting of platinum-iridium alloys and oxides of cobalt, manganese, palladium, lead and platinum and, 3) an electrically-conductive, electro-catalytically active, electrolyte-resistant outer coating consisting essentially of a solid solution of a valve metal oxide with at least one non-valve metal oxide.
• Such an electrode not only exhibits an extremely low initial oxygen overvoltage, but retains that low overvoltage through extended periods of use. Furthermore, the wear-rate, that is, the physical loss of coating per unit time, is extremely low.
• a barrier layer of the type described apparently exhibits some sort of catalytic activity which assures that the mixture of materials subsequently applied will form a true solid solution, regardless of the substrate.
• the invention is defined broadly as relating to an electrode.
• the word electrode is intended to refer to either anodes or cathodes as it will be apparent that the composite electrodes of the present invention will carry current in either capacity.
• the primary advantage of the electrode is its resistance to passivation, which passivation usually occurs when oxygen is generated at or near the electrode surface, most applications of the electrode will be as an anode, especially as an oxygen anode. It will be understood, however, that if, as is suspected at this time, the method of failure of a chlorine anode also relates to oxygen passivation, the composite electrodes of this invention will also act as chlorine anodes of extended life.
• the invention is independent of the mechanical configuration of the substrate and hence may take any shape which will allow the application of the intermediate and other coatings by the techniques generally described hereinbelow.
• the electrodes may take the form of a wire, rod, cylinder, sheet and the like. Further, if the electrode is present in a sheet or plate form, it may be either solid or forarninous. Other configurations most useful in a particular application will be apparent to those skilled in the art.
• the first element of the composite electrode is the electrically conductive supporting substrate.
• the identity of the substrate is not as limited as it had been heretofore when a solid solutiontype coating was contemplated. While valve metal substrates, particularly titanium, will still be preferred for many applications because of their ability to heal themselves under corrosive cell conditions should a defeet in the coating arise, it is now possible to use most any material which has the desired combination of electrical conductivity and mechanical strength. Therefore graphite, steel, copper and the like are also quite useful for numerous applications of the present invention.
• the intermediate barrier layer has been said to be selected from the group consisting of platinum-iridium alloys and oxides of cobalt, manganese, palladium, lead and platinum. These materials may be applied in relatively thin layers, that is, as low as 0.1 micron, to form an electrically-conductive layer which appears to prevent the diffusion of oxygen through the relatively porous outer coating to the underlying substrate.
• any barrier layer meeting the other criteria herein, especially electroconductivity, which is less permeable to oxygen diffusion than the covering solid solution-type layer, will theoretically result in improved resistance to passivation.
• Cobalt, manganese, lead and palladium oxides may be provided directly, and in the proper crystalline form, on the electrically conductive substrate by electrolytic deposition using techniques well-known to those skilled in the art.
• Platinum oxide is not deposited directly but rather a metallic coating of platinum is first electrodeposited, followed by a brief heat treatment which appears to convert a substantial portion of the platinum to the oxide form. It appears at this time that the heat treatment is critical since it has been found that the solid solution coating will not adhere to the untreated metal itself and other methods of forming the platinum oxide have proven unsuccessful for the same reason.
• the efi'ect of the heat treatment in converting the platinum to the proper form may be evidenced by the distinct color change from the original metallic finish.
• platinum-iridium alloys appear to remain substantially in the metallic form even following thermal application of the solid solution. These alloys generally, but not necessarily, contain 20-50% iridium, typically 30%, and are applied by any of the known methods. such as thermochemical deposition from mixed salt solution, which result in an adherent, relatively non-porous layer.
• This coating consists of a solid solution of a valve metal oxide with at least one non-valve metal oxide.
• valve metal in this context has its usual significance, that is, it relates to metals such as titanium, tantalum, zirconium, niobium and the like.
• the non-valve metal oxide is chosen to be such that the desired solid solution is formed, that is, it must have the proper crystal size to mesh with the crystal lattice of the valve metal oxide, usually be substitution of one atom of non-valve metal for one atom of valve metal, thus providing electrical conductivity in a normal nonconductive material.
• non-valve metal oxides useful in the practice of the present invention including platinum, palladium, iridium, ruthenium, rhodium, osmium, molybdenum, tin, tungsten, vanadium, chromium, rhenium, manganese and the like.
• EXAMPLE 1 A piece of 0.16 inch thick solid titanium sheet (A.S.T.M. B 265 581 Grade 2) is degreased with acetone and etched 10 minutes at C. in 20% l-lCl. This sheet is made the cathode in a 2% solution of chloroplatinic acid in 2.0 N HCl. Platinum is deposited for 10 minutes at room temperature and a current density of 6.2 amperes per square foot (a.s.f.). The electrodeposited platinum metal-coated titanium substrate thus obtained is then heated in air for 7 minutes at 450C. A solution consisting of 1 gram RuCl -XH O (0.4 gram Ru metal), 6.2 ml. n-butyl alcohol, 3.0 ml.
• This electrode when employed as the anodeis a l N NaOH solution at 80C. and an applied current density of 2.0 amperes per square inch (a.s.i.), exhibits an oxygen overvoltage of 0.29 volts.
• the electrode prepared above When the electrode prepared above is operated as an anode in a 100 grams per liter aqueous solution of sulfuric acid at 20C. and a current density of 4 a.s.i., it continues to generate oxygen for 110 hours.
• the barrier layer very significantly extends the useful life of a solid solution-type electrode.
• the life of the anode at 4 a.s.i. is equivalent to several months of operation at normal commercial current densities of from. 30-40 a.s.f.
• EXAMPLE 2 A piece of 0.060 inch expanded titanium mesh is pretreated as in Example 1 and made the anode in a solution containing 291 grams Co(NO '6H O. Cobalt oxide is deposited for minutes at a temperature of 60C; and a current density of 4.8 a.s.f. Six coats of the ruthenium-titanium solution are applied as in Example 1. Oxygen overvoltage is again measured at 0.29 volts and 47 hours is required for the anode to passivate (H 80 at 3 a.s.i.
• EXAMPLE 3 A piece of .060 expanded titanium is pretreated as in Example 1 and made the anode in a solution containing 3.3 grams per liter palladium nitrate. Palladium oxide is deposited for 1 hour at a temperature of 50C. and a current density of 2.4 a.s.f. After application of the solid solution-type coating as before, an electrode is obtained which exhibits an oxygen over-voltage of 0.29 and a life of 37 hours at 3 a.s.i.
• EXAMPLE 5 A piece of unimpregnated Union Carbide graphite Grade YAV is ground down to expose a fresh surface. MnO is then deposited on this surface as in Example 3, and six coats of the solid solution coating are applied as in Example 1. This sample again gave the low oxygen overvoltage of 0.29. The anode is operated for 16 hours without change, it being apparent that passivation will not occur absent the film-forming metal substrate.
• Example 6 A piece of .016 inch titanium sheet is pretreated as in Example 1 and made the anode in a solution containing 300 grams per liter Pb(NO 2 grams per liter Cu(NO -H O, and 1 gram per liter non-ionic wetting agent. An undetermined amount of lead dioxide is deposited. Six coats of the titanium-ruthenium solution are applied as before, with the exception that the bake temperature is reduced to 300C. owing to the low'decomposition temperature of lead dioxide. X-ray analysis establishes the presence of the usual solid solution structure and a 12 hour test demonstrates that no passivation occurs.
• EXAMPLE 7 A titanium metal sheet is provided, by thermochemical deposition, with a 70% platinum iridium alloy layer amounting to 4.5 milligrams per square inch. Six' applications of the solid solution coating are then made as in Example 1. The resultant electrode continues to operate for 305 hours at 4 a.s.i. in 100 g/l. H2804.
• An electrode comprising:
• a relatively thin, intermediate, electrically conductive, relatively oxygen-impermeable, barrier layer consisting essentially of one oxide of the group consisting of oxides of cobalt and lead and c. an electrically-conductive, electrocatalytically active, electrolyte-resistant, solid solution-type outer coating consisting of at least one valve metal oxide and at least one oxide of metal selected from the group consisting of platinum, palladium, iridium, ruthenium, rhodium, osmium, molybdenum, tin, tungsten, vanadium, chromium, rhenium, and manganese.
• An electrode as in claim 1 wherein the outer coating is a solid solution of a valve metal oxide and a nonvalve metal oxide selected from the group consisting of platinum, palladium, iridium, rhodium and ruthenium.
• An electrode of claim 3 wherein the outer coating is a solid solution of titanium dioxide and ruthenium oxide.
• a composite electrode comprising an electrically conductive supporting substrate and an electrically conductive, electro-catalytically active solid solution-type outer coating consisting of at least one valve metal oxide and at least one oxide of a metal selected from the group consisting of platinum, palladium, iridium, ruthenium, rhodium, osmium, molybdenum, tin, tungsten, vanadium, chromium, rhenium and manganese, the improvement which comprises providing said electrode with a relatively thin, intermediate, electrically-conductive, relatively oxygen-impermeable, barrier layer selected from the group consisting of oxides of cobalt and lead.
• thermochemical co-deposition and decomposition of a mixture comprising a valve metal consisting of oxides of cobalt and lead and an electrically-conductive, electrocatalytically active, electrolyte resistant, solid solution-type outer coating consisting of at least one valve metal oxide and at least one oxide of a metal selected from the group consisting of platinum, palladium, iridium, ruthenium, rhodium, osmium, molybdenum, tin. tungsten, vanadium, chromium, rhenium and manganese.
• column 1 On the cover sheet, column 1, after line 6, insert Assignee ELECTRONOR CORPORATION, Panama City, Panama
• column 2 line 8 "Heissenbrger” should read Neissenberger Signed and sealed this 18th day of June 197b,.

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Citations (8)

• 1971
  • 1971-01-28 US US00110775A patent/US3775284A/en not_active Expired - Lifetime
  • 1971-03-19 SE SE7103558A patent/SE371373B/xx unknown
  • 1971-03-22 LU LU62825D patent/LU62825A1/xx unknown
  • 1971-03-22 DE DE2113795A patent/DE2113795C3/de not_active Expired
  • 1971-03-22 IL IL36457A patent/IL36457A/en unknown
  • 1971-03-22 BE BE764623A patent/BE764623A/fr not_active IP Right Cessation
  • 1971-03-22 FR FR7109920A patent/FR2083493B1/fr not_active Expired
  • 1971-03-23 NL NL7103893A patent/NL7103893A/xx unknown
  • 1971-03-23 JP JP46016245A patent/JPS5119429B1/ja active Pending
  • 1971-04-19 GB GB2488471*A patent/GB1344540A/en not_active Expired

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