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
  • C25B11/075—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material consisting of a single catalytic element or catalytic compound
  • C25B11/081—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material consisting of a single catalytic element or catalytic compound the element being a noble metal
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T29/00—Metal working
  • Y10T29/49—Method of mechanical manufacture
  • Y10T29/49002—Electrical device making
  • Y10T29/49108—Electric battery cell making
  • Y10T29/49115—Electric battery cell making including coating or impregnating

Definitions

• the invention relates to a composite electrode for electrochemical purposes, a process for its production and its use for the anodic oxidation of inorganic and organic compounds and as an anode in galvanic baths.
• the composite electrode according to the invention is particularly suitable for the production of peroxo compounds such as peroxodisulfates, peroxomonosulfates, peroxodi- and monophosphates, peroxodicarbonates, perhalates, in particular perchlorates, and of the associated acids and their hydrolysis products.
• platinum is preferred as the anode material because of its chemical properties. It is often the only metal that can be used for such processes.
• Composite electrodes made of a base metal with a firmly adhering platinum coating.
• Composite electrodes are known in which the anode material platinum is attached as a relatively thin layer on a corrosion-resistant carrier metal that is as good a conductor as possible. So it is z. B. is known to generate a platinum coating by cathodic deposition from galvanic platinum baths or platinum salt melts.
• a platinum coating by cathodic deposition from galvanic platinum baths or platinum salt melts.
• electroplated platinum layer the coating adheres insufficiently to the carrier material when it is used as an anode for the electrolysis.
• insufficient service life can be achieved.
• the massive platinum metal used for the above anode processes is e.g. B. as 120 to 150 ⁇ m thick wires or as a rolled, 10 to 100 ⁇ m thick film.
• the electrical current is preferably conducted onto the platinum metal by metals which are anodically stable in the electrolyte in question or which are capable of forming a passive layer, so-called valve metals.
• the platinum itself is fastened to such carrier metals by means of different measures. Titanium, tantalum or zircon is usually used as the carrier metal.
• tantalum-coated silver wire with a diameter of 1 to 2 mm for anodic electrochemical processes, on which a long platinum wire is spirally attached by means of spot welding.
• Another type of anode are on a titanium rod with rungs protruding on both sides of the platinum wires is fixed by clamping or welding. This forms a flat anode covered with platinum wire.
• the invention is therefore based on the object of eliminating the disadvantages of the prior art described above and of providing a composite electrode which is particularly suitable for anodic oxidation, delivers a high current efficiency and is also distinguished by long service lives during operation. This object is achieved by the method defined in the claims.
• a composite electrode made of a valve metal base with a platinum foil overlay adhering to it can be obtained by hot isostatic pressing of the metal base and platinum foil between release agent layers, if one takes into account that release agent layer which comes into contact with the platinum foil during hot isostatic pressing metal not alloyed with platinum with a melting temperature of at least 100 ° C. above the applied hot pressing temperature or a metal foil provided with diffusion barriers.
• Diffusion barriers are barrier layers that prevent the penetration of foreign substances such as metal atoms or carbon into the platinum metal.
• Diffusion barriers made of metal nitrides, sulfides, carbides and carbonitrides, but preferably those made of metal oxides, are expediently used for the process according to the invention.
• a ceramic film can also be used as the release agent layer, which contains no carbon or carbon-releasing compounds.
• the release agent layer contains no carbon or carbon-releasing compounds.
• the platinum layer must be removed by at least 1 micron, preferably at least 2 microns to remove all incorporated material. It has been shown that mechanically incorporated particles, such as. B. ceramic fibers, which reduce current efficiency, although these are inert to the platinum metal.
• all ceramic films are used which do not release any platinum chemically contaminating substances under the process conditions.
• sheets or foils made of the separating agent, base metal and platinum as the supporting metal are layered one on top of the other to produce composite electrodes and these layers are hot isostatically pressed together.
• a valve metal is used as the base metal.
• individual layers are placed one above the other in the sequence release agent / base metal / platinum / release agent and to produce a composite electrode with double-sided support in the sequence release agent / platinum / base metal / platinum / release agent.
• Each sequence forms an element that creates a composite electrode.
• a stack is usually formed from several such elements.
• the height of the stack and the area of the foils and sheets are only limited by the size of the autoclave furnace in which the hot isostatic pressing is carried out.
• the elements are stacked in a rectangular or square tin can, which is preferably made of stainless steel. However, other materials can also be used, provided they are stable under the specified process conditions.
• a film of release material is placed on the top of the stack.
• the open top, preferably rectangular or square can is then tightly welded to a lid made of the same material as the can.
• a thin tube is welded into the lid or the side walls of the can, through which a vacuum is applied inside the can.
• the pipe stub is then clamped off and welded closed in a vacuum-tight manner.
• diffusion welding in an autoclave is carried out at a gas pressure of 100 to 1200 bar, in particular at 200 to 1000 bar and a temperature of 650 to 900 ° C during a holding time of at least 0.5 hours. It is preferably pressed at a temperature of 700 to 850 ° C and a holding time of 0.5 to 5 hours, preferably from 0.5 to 3 hours.
• release agents are made from fabrics of ceramic fibers, such as, for. B. are available for commercially available refractory linings used.
• a ceramic fabric foil or a ceramic paper with a thickness of at most 1 mm is preferably used.
• Such a release agent layer called a release film, prevents the metals lying on both sides from welding. According to the invention, however, only such ceramic separating material is used which does not give off any substances which impair the electrochemical properties of the surface metal, in particular no substances which chemically contaminate platinum.
• the commercially available separating fabric contains small amounts of organic compounds which, when heated in an autoclave to over 600 ° C., emit organic or carbon-containing vapors, from which carbon is deposited on the platinum surface and is alloyed into the platinum lattice.
• the ceramic separating fabric is therefore freed from oxidizable carbon compounds and from carbon itself in a separate operation by annealing in a pure oxygen or oxygen-containing atmosphere, in particular in air at 600 to 700 ° C.
• a Post-treatment z. B. can be eliminated with an alkali melt of KOH or a KOH / NaOH mixture.
• a metal foil instead of a ceramic fabric or paper.
• metals can be used that do not alloy (or only slightly) with the base or the supporting metal under the conditions of hot isostatic pressing.
• Small, microscopically thin alloy layers created by diffusion on the adjacent foils or sheets of platinum and separating metal during hot pressing must be removed again mechanically, chemically or anodically after the metal composite has been completed.
• Usual chemical post-treatments are carried out, for example, by etching, e.g. B. with aqua regia or by anodic etching.
• metal foils which contain a diffusion barrier can be produced by forming an oxide layer in a pure oxygen or oxygen-containing atmosphere, preferably in air at high temperatures.
• the oxide layers are preferably produced by heating the metal foils to 400 to 800 ° C., in particular to 450 to 650 ° C.
• a molybdenum foil is preferably used as the release agent, which is preferably provided completely with an oxide layer in air by a thermal pretreatment at 450 to 600 ° C.
• a molybdenum foil provided with a diffusion barrier does not adhere to platinum or titanium after hot pressing.
• metals are also used as release agents which have a diffusion barrier on their surface which consists of a nitride, sulfide, carbide or a carbonitride layer. Such layers are obtained by customary reactions of the release agent with the respective reagents.
• metal foils such as. B. from iron, nickel, tungsten, zirconium, niobium, tantalum, titanium or alloy steel foils, in particular low-carbon steel foils such as AISI / 1010, which are provided with appropriate diffusion barriers, used as a release agent.
• the diffusion barriers are preferably created by oxidation of the metals in air or oxygen.
• metal foils e.g. B. made of molybdenum or tungsten
• a diffusion barrier ie without using an oxidizing pretreatment.
• the firmly adhering film must then be removed chemically or electrochemically.
• Untreated metal foils such as. B. iron or nickel is used, after roughening a roughened platinum surface is obtained, which has a smooth surface only after prolonged electrolysis or after chemical or mechanical treatment.
• the use of firmly adhering but chemically removable metal foils has the advantage that the platinum coating is protected when the platinum / valve metal composite is processed into the finished electrode. So it is z. B.
• oxidized metal foils can be easily lifted off the surfaces of the composite electrodes and are then reusable for the inventive method.
• An electrode with good electrolysis properties can be achieved by particularly smooth and shiny electrode surfaces, such as those obtained by using an oxidized molybdenum foil in the process according to the invention.
• the method according to the invention gives electrodes which are inexpensive and stable and whose use is not restricted to certain electrolysis current densities by those welding or contact points which limit the current flow, since the current supply takes place over the entire pressed surface and also the thickness the base or substrate metal is freely selectable. Contact overheating, electrical flashovers or a high voltage drop, such as occurs on the thin solid platinum wire electrodes, is therefore excluded. With the method according to the invention, even large-area electrodes for current densities of more than 10 or even more than 100 kA / m 2 can be produced with at the same time low use of circuit boards and high stability.
• the electrodes produced according to the invention have a high current efficiency in the anodic oxidation.
• potassium persulfate by direct electrolysis z. B.
• electrodes produced by the method according to the invention under the Using annealed ceramic separating agent layers 15 minutes after the start of electrolysis gives a current efficiency of 25 to 40% and using oxidized molybdenum foils as separating agent layers achieves a current efficiency of 80% (as on solid platinum).
• only current yields between 0 and 25% can be achieved with electrodes which have been produced by hot isostatic pressing with carbon-containing ceramic release agents.
• a square box with a base area of 50 x 50 cm and a height of 8 cm is made from a stainless steel sheet (WST.Nr. 1.4571) of 2 mm thickness by bending and welding.
• WST.Nr. 1.4571 a stainless steel sheet
• 20 elements with the layer sequence ceramic paper from 95% Al2O3, which was previously annealed in air at 700 ° C for one hour
• manufacturer DMF-Fasertechnik, Düsseldorf, type DK-Flex 16, 1 mm / titanium 3 mm / platinum foil 50 ⁇ m stacked on top and covered with 1 mm ceramic paper on the top.
• the lid and side walls of the can are welded together.
• a vacuum is applied to the sealed and welded can via the evacuation device (stainless steel tubes with a diameter of 5 mm and a length of 50 mm and a wall thickness of 2 mm). After a leak test, the tube is closed by squeezing and welding.
• the tightly sealed can thus prepared for hot isostatic pressing is placed in an autoclave oven. This is pressurized with 275 bar argon and heated to 700 ° C over a period of 0.5 hours. The pressure rises to 980 bar. This condition is maintained for 2 hours and then the oven is turned off. The overpressure is then released. The cooling and relaxation phase lasts about an hour.
• the cooled can is cut open and the contents removed.
• one-sided composite electrodes are obtained, the mechanical, z. B. by polishing or chemical post-treatment by etching with aqua regia or anodic etching which give the same target current yields and voltages as solid Pt anodes in the persulfate or perchlorate electrolysis.
• Example 1 The procedure for producing titanium sheets covered on both sides with platinum foil is as described in Example 1, but commercial molybdenum foil of 50 ⁇ m thickness is used as the release agent. Elements are formed from layers in the following order: titanium sheet 2 mm / platinum foil (50 ⁇ m) / molybdenum foil 50 ⁇ m / ceramic paper 1 mm. A platinum foil is used that is smaller than the titanium sheet. This leaves a margin several millimeters wide. Then, as described in Example 1, hot isostatic pressing is carried out at 700 ° C. and at 1000 bar. In the metal composite obtained in this way, the molybdenum foil adheres to the titanium as well on the platinum and is anodically removed with dilute sulfuric acid. In this way, a high-gloss platinum surface free of impurities is obtained. It can be seen that no discernible diffusion zone is formed between molybdenum and platinum in the process parameters used.
• Example 2 is repeated using a 50 ⁇ m thick steel foil AISI 1010 instead of a molybdenum foil. Under the process parameters used, a diffusion zone between iron and platinum with a thickness of about 1 ⁇ m is formed.
• the titanium / platinum / iron composite thus obtained is shaped into a tube analogous to DE-PS 16 71 425 and welded with electrolyte inlet and outlet heads to form a finished anode.
• the iron layer is removed anodically with H2SO4 and the platinum surface is etched with aqua regia or mechanically polished.
• a carefully degreased, 50 ⁇ m thick molybdenum foil is heated in an oven in air at 550 ° C for 15 minutes.
• a matt gray thin oxide layer is formed from very fine crystals.
• a layer of ceramic paper / titanium / platinum / molybdenum foil / platinum / titanium / ceramic paper is produced from this metal foil provided with a diffusion barrier.
• the foils and sheets used correspond to those from Examples 1 and 2.
• hot isostatic pressing is carried out at 700 ° C. and at 1000 bar in an autoclave, as described in Example 1.
• the platinum-titanium composite sheets obtained in this way can easily be separated from the oxidized molybdenum foil. In this way, an electrode with a matt, glossy platinum surface is obtained which, in the case of persulfate electrolysis, immediately provides current yields, like solid platinum sheet.
• the molybdenum foil can be used again after oxidation.
• a steel foil AISI 1010 is heated in air at 500 ° C for 10 minutes. A violet-gray oxide layer is formed.
• the oxidized steel foil is used instead of the molybdenum foil to produce a composite, as described in Example 4. After hot isostatic pressing, the workpieces can be easily separated. This results in a black roughened platinum surface, which is stained with aqua regia before use.
• Example 3 is repeated using a nickel foil instead of a steel foil.
• a composite is obtained which has a roughened platinum surface and which, after etching in aqua regia, gives an electrode which has yields like solid platinum in the persulfate electrolysis.
• a carefully degreased molybdenum foil is heated in air at 500 ° C for 15 minutes.
• This molybdenum foil a stack of elements consisting of layers in the order of titanium / platinum / molybdenum / Al2O3 paper is produced.
• hot isostatic pressing is carried out.
• the metal composite thus obtained has a matt glossy platinum surface and can be used for the electrolysis without further pretreatment.
• Example 2 As described in Example 1, a stack of layers of titanium 2mm / tantalum 100 ⁇ m / platinum 50 ⁇ m / Al2O3 paper 1 mm is formed and the whole is hot isostatically pressed at 850 ° C and 1000 bar. In this way, a platinum tantalum electrode is obtained, which is reinforced with cheap titanium.
• the following examples illustrate the use of the electrodes according to the invention in an electrolysis apparatus.
• An undivided cell is used to determine the anode behavior in potassium or sodium persulfate electrolytes
• a divided electrolysis cell is used to determine the anode behavior in sodium perchlorate electrolysis and in the production of ammonium persulfate.
• the electrolytic cells consist of a PVC frame with inflow and outflow, in which the anode is fixed on one side and the cathode on the other side via seals in such a way that an electrode spacing of 2 to 10 mm is achieved, which is technical Electrolysis corresponds.
• stainless steel cathodes are used which, like the anodes, have a rectangular area of 2 x 3 cm2.
• 2 PVC frames are used, between which a separator is clamped using seals.
• the electrolyte flows through the entire electrolysis room with the aid of suitable pumps (such as Heidolph Krp 30). If split cells are used, the electrolyte is passed through both the cathode and the anode space. In this way, a residence time of the electrolyte in the electrode gap of approximately 0.4 seconds is achieved.
• the mixture of gas and electrolyte produced at the electrodes is conveyed upwards by the pumping action and separated in a gas separator located above. From the outlet of the separator, the electrolyte is then fed back into the suction port of the pump.
• the current yield is determined in the usual way by titrimetric determination of the anodically formed compounds or by gas analysis of the cell gas.
• Cells are used for technical electrolysis as used in DE-PS 16 71 425 for potassium or sodium persulfate electrolysis.
• a tubular electrode is produced from a metal composite produced according to Example 4 with a platinum surface of 550 x 260 mm. This electrode is used at a cell current of 1000 A for a precipitation electrolysis for the production of potassium persulfate.
• a current efficiency of 75% is achieved at a current density of 9 KA / m2 .
• This yield corresponds to that which until now could only be achieved with solid platinum foil anodes in the first half of their term. No corrosion can be found at the platinum-titanium transition point that is exposed during electrolysis.
• an electrode with a surface area of 6 cm 2 is produced and used for the electrolysis of an electrolyte composed of 3.1 m H2SO4 and 2.8 m Na2SO4 and an addition of rhodanide for the production of sodium persulfate.
• the electrolysis is carried out in a cell at 20 ° C and 5.4 A cell current (9 kA / m2).
• the same electrolyte is electrolyzed on a massive platinum sheet electrode under the same conditions.
• the yields are then determined by titration using known analysis methods. It can be seen that a persulfate yield of 65% is achieved with the anode produced according to Example 4 as well as with the platinum sheet anode.
• Ammonium persulfate electrolysis is carried out with a metal composite electrode produced according to Example 4 with an anode area of 20 cm 2.
• a metal composite electrode produced according to Example 4 with an anode area of 20 cm 2.
• an electrolyte composition of 0.1 m H2SO4, 2.6 m (NH4) 2SO4, 0.9 m (NH4) 2S2O8 and an addition of rhodanide to destroy caroate at an electrolysis temperature of 40 ° C a yield of 82% is achieved.
• the same yield is achieved with a comparison cell which is equipped with a solid platinum foil as an anode.
• the yields of the electrolytic NaClO4 formation from NaClO3 on composite electrodes produced according to Example 4 are compared with electrodes made of solid platinum foil.
• the current densities are 7 kA / m2 each.
• yields of 85% are achieved in both cases.
• the same current yields are achieved with the composite electrodes according to the invention as would otherwise only be achieved with solid platinum electrodes.

Landscapes

• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Electrodes For Compound Or Non-Metal Manufacture (AREA)
• Battery Electrode And Active Subsutance (AREA)
• Electrical Discharge Machining, Electrochemical Machining, And Combined Machining (AREA)
• Inert Electrodes (AREA)
• Other Surface Treatments For Metallic Materials (AREA)

Priority Applications (1)

Applications Claiming Priority (2)

Publications (2)

Family

ID=6358595

Family Applications (1)

Country Status (8)

Cited By (1)

Families Citing this family (14)

Family Cites Families (9)

• 1988
  • 1988-07-13 DE DE3823760A patent/DE3823760A1/de not_active Withdrawn
• 1989
  • 1989-07-12 ES ES89112763T patent/ES2050737T3/es not_active Expired - Lifetime
  • 1989-07-12 EP EP89112763A patent/EP0350895B1/de not_active Expired - Lifetime
  • 1989-07-12 BR BR898903423A patent/BR8903423A/pt not_active IP Right Cessation
  • 1989-07-12 DE DE89112763T patent/DE58907158D1/de not_active Expired - Fee Related
  • 1989-07-12 AT AT89112763T patent/ATE102663T1/de not_active IP Right Cessation
  • 1989-07-13 US US07/380,155 patent/US4995550A/en not_active Expired - Lifetime
  • 1989-07-13 JP JP1179215A patent/JP2991724B2/ja not_active Expired - Fee Related
  • 1989-07-13 CA CA000605568A patent/CA1339212C/en not_active Expired - Fee Related

Also Published As

Similar Documents

Legal Events