EP0321711B1 - Process for manufacturing porous electrodes - Google Patents

Description

The invention relates to a process for the production of porous electrodes, in which a porous metal layer is formed on a scaffolding metallic carrier with an unevenness of the surface which promotes adhesion and provided with a galvanic metal deposit in the pores and, if appropriate, finally activated by lye treatment.

• ausreichende mechanische Festigkeit der Katalysatorschicht;
• wirtschaftliche Herstellung auch von großen Einheiten;
• Anwendbarkeit bei "zero-gap" Zellkonstruktionen (mit "Null-Abstand" zwischen Diaphragma und Elektrode);
• homogene Stromdichte-Verteilung bei "zero-gap"-Zellen; und
• verlustarme Übertragung der elektrischen Ladung zwischen dem Träger und dem Katalysator.

Active electrodes with only low overvoltages form one of the most important prerequisites for economical operation in electrochemical process technology. In alkaline electrolysis, such as alkali chloride electrolysis or water electrolysis, active electrodes based on Raney nickel are usually used. In addition to low overvoltages, other properties are required of such electrodes, namely:

• sufficient mechanical strength of the catalyst layer;
• economical production of large units;
• Applicability to "zero-gap" cell constructions (with "zero distance" between diaphragm and electrode);
• homogeneous current density distribution with "zero-gap"cells; and
• low-loss transmission of the electrical charge between the carrier and the catalyst.

Different methods for producing such electrodes are already known, in which essentially an activatable Ni / Al or Ni / Zn alloy is applied to an electrically conductive carrier, from which the soluble component (Al, Zn) is removed by subsequent lye treatment , which leaves a catalytically active Ni structure (Raney nickel). However, the electrodes obtained by the known methods are not completely satisfactory in one way or another:
According to E. Justi and A. Winsel ("Cold combustion", Franz Steiner Verlag, 1962, chapter 4.1), a sintered self-supporting catalyst electrode is produced by a pressing or rolling process with a coupled sintering process, but this is only an insufficient mechanical one with a thin layer thickness Has strength and can only be produced in relatively small dimensions.

Electrodes produced by means of galvanic suspension deposition (GB-PS 2 015 032; US-PS 4 302 322) can only be produced in smaller units, since the electrically conductive suspensions only allow regular deposition at low substrate heights. In addition, this technology cannot achieve a sufficiently high catalyst concentration.

Electrodes are obtained by intermetallic diffusion or galvanic deposition of Ni / Zn alloy (US Pat. No. 4,240,895; German Pat. No. 3,330,961), the structure of which is not very suitable for low-loss charge transfer.

Plasma spraying ("Hydrogen Energy Progress" V by T.N. Veziroglu and J.B. Taylor (Editors); Pergamon Press, New York, p. 933) hardly makes it possible to produce electrodes of a technically relevant size.

The process of reductive powder plating (DE-OS 28 29 901; Chem.-Ing.-Technik 5 (1980) 435) is the most technically mature, based on the following principle:
A spreadable paste of a powder mixture of Ni / Al and Ni in 50% alcohol and 1% methylcelulose is applied to a carrier plate and dried. The sheet thus coated is then rolled down to about 50% in a cold rolling mill, so that the catalytic powder layer is strongly compacted and mechanically adhered to or in the matrix. The powder is reductively welded by briefly annealing at 700 ° C in an H₂ atmosphere. This creates an activatable catalyst layer that adheres firmly to the electrically conductive, mechanically stable electrode carrier.

Although electrodes of this type have excellent catalytic activity and mechanical strength, because of the necessary deformation of the carrier sheet, only continuous ("full") smooth electrodes can be produced. However, such geometrical structures are difficult to use in gas-developing electrochemical reactions in the "zero-gap" configuration. As is well known, the geometric shape of a perforated plate or expanded metal is necessary for this purpose.

DE-PS 29 14 094 of the applicant finally describes a process in which a porous electrode layer is formed on a metal support, such as nickel or iron mesh, by sintering a suspension application of powder containing nickel powder or nickel alloy and pore-forming substances a nickel-zinc alloy is deposited electrolytically. Finally, zinc is removed from this galvanically coated sintered body by immersion in alkali, which can be done in situ if the electrodes are used.

Noticeable overvoltages are also measured with such electrodes.

The invention is therefore based on the object of providing an economical and technically feasible process for producing active electrodes which as far as possible meet the criteria mentioned above.

The process according to the invention of the type mentioned at the outset, which was developed for this purpose, is essentially characterized in that the carrier is coated on one or both sides with a dry-rolled layer of a mixed powder of (a) fine-particle carbonyl metal with low bulk density and high sliding resistance and (b) a catalytic Effective or activated by lye treatment powdery component in a: b ratio of 3: 1 to 1: 3 is provided, which is consolidated by galvanic coating with metal, whereupon, if necessary, finally activated.

According to the invention, therefore, a catalytically active or activatable powder, one component (a) of which has adhesion-promoting, "matting" properties, such as those in particular in carbonyl nickel with an average particle size (according to Fisher) of 2.2 to 3.0 μm, a bulk density of 0.5 to 0.65 g / cm³, a specific surface area of 0.68 m² / g and an angle of repose of 70 ° (INCO 255) can be found, cold-rolled on one or both sides on a framework-forming, metallic conductive support with an adhesion-promoting surface, creating a manageable body that is consolidated by galvanic metal deposition and, if necessary, finally activated by leaching.

The carrier used is a fine-meshed metal mesh, in particular steel or nickel mesh with a small mesh size of approximately 200 to 600 μm, which prevents a dry-rolled powder layer from falling through from a mixed powder of the abovementioned properties, or in particular a perforated plate with a roughened surface which, for. B. is obtained by sandblasting, flame spraying or chemical treatment. Particularly preferred is a perforated nickel plate roughened by galvanic deposition of carbonyl-nickel powder (e.g. 1 -5 mg / cm²; in a nickel plating bath), to which dry-rolled layers adhere excellently, but easily to the perforations by slight vibration (tapping) can be removed.

Component (a) of the mixed powder is preferably carbonyl iron or carbonyl nickel powder and in particular carbonyl nickel with a grain size of about 2 to 3 μm and a bulk density of 0.5 to 0.7 g / cm³.

Component (b) is a catalytically active material or one which can be activated by alkali treatment, such as, in particular, nickel sulfide, molybdenum sulfide and molybdenum or nickel alloy with aluminum, zinc, tin, etc. Components a and b are in a ratio of 3: 1 to 1: 3 , in particular 2: 1 to 1: 2 but preferably in a ratio of 1: 1 (in weight) and approximately similar grain size, component (b) may also be somewhat coarser and may have grain sizes in the range from 10 to 100 μm.

In addition, the mixed powder can contain 5 to 20% by weight (based on the mixture (a) and (b)) of a detachable or sublimable filler, such as. B. KCl, NaCl, ammonium carbaminate, ammonium carbonate, naphthalene, etc.

The thickness of the dry rolling layer on one or both sides is in particular 50 to 400 μm, corresponding to a powder mixture application of approximately 30 to 160 mg / cm², in particular approximately 40 to 90 mg / cm².

The metal powder is rolled onto the carrier under a relatively low pressure application, in particular of 0.5 to 10 bar.

The galvanic consolidation is carried out by metal deposition at a current density which is preferably selected in the range from 0.1 to 10 A / dm². Nickel or nickel alloy with a soluble component is preferably deposited.

The thorough consolidation of the dry rolled layer through galvanic metal deposition is particularly important and is influenced by different techniques, such as. B. by appropriate choice of the contact pressure with a view to the formation of an optimal (coarse-pored) porosity (the dry layer), which also makes the areas near the carrier accessible in the galvanic deposition of consolidating metal, or by increasing the current density during the galvanic consolidation or by generating a coarse-pored structure of the dry-rolled layer by using a removable filler, which is removed again before galvanic consolidation, or finally by changing the electrical conductivity of the mixed powder during galvanic consolidation, in which oxidation of the surface of the dry-rolled layer decreases towards the carrier Powder particles at the beginning of the electrodeposition ensure that metal deposition initially takes place in areas close to the carrier, while as the electrodeposition progresses in the nickel bath, the oxide layer dissolves t takes place, so that finally the outermost areas are also included in the galvanic consolidation. Such anoxidation of the surface is achieved in particular by pretreating the powder in air at about 200 ° C.

The depth grading of the superficial oxidation of the powder of the dry roll layer can, for. B. can be achieved in that for the production of the dry roll layer first sieved and oxidized powder on a flat surface subsequently increasingly oxide-free powder is applied, whereupon after the support (in particular perforated plate) has been placed on it, compression is carried out by rolling.

The invention is explained below using exemplary embodiments:

Example 1:

A perforated nickel plate of 0.5 mm thickness with 35% transparency and 1 mm hole diameter was roughened on both sides by galvanic fixation of suspended INCO carbonyl nickel powder (with small particle size, irregular particle shape and high surface activity).

A dry mixture of Ni-Al and carbonyl-nickel (1: 1) was rolled onto the roughened layers thus obtained with a pressure of 5 bar in a layer thickness of approx. 200 µm each. This dry mixture has the property that it sticks relatively firmly in the roughened matrix, while the transparent areas (holes) remain free. Perforated sheet obtained in this way and provided with an activatable powder mixture can be freely moved without risk and immersed in an electrolyte (Wattsches bath). The final mechanical fixation of the metal powder by electrolytically deposited nickel then took place in this. The electrolysis time was 1 hour at a bath temperature of 30 ° C and a current density of 1 A / dm². The electrode body obtained can be activated and is generally activated in situ immediately when used.

Example 2:

Nickel mesh of 0.2 mm wire thickness and 0.5 mm mesh size was coated with a dry, binder-free mixture of Ni-Al / Mo / carbonyl nickel 0.45: 0.05: 0.5 on both sides by rolling as in Example 1 with each approx. 200 µm coated. The powder mixture remains firmly adhered to the net so that it can be handled and immersed in an electrolyte without special precautions. Since no binders were used that could possibly interfere with the subsequent electrolysis, galvanic coating in a conventional Watts nickel plating bath is possible. The final galvanic fixation or consolidation of the powder mixture on the network was then carried out in this under electrolysis conditions as in Example 1.

Example 3:

A perforated sheet of nickel roughened on the surface by deposition of carbonyl nickel powder, as in Example 1, was provided on both sides with a dry-rolled mixed powder layer of Ni-Al and carbonyl nickel (1: 1) with 10% addition of NaCl with a grain size of 50 to 100 μm. The rest of the procedure was as in Example 1, but dissolved out with water before the electrolysis in Watts Bad NaCl.

By using NaCl to create the dry-rolled layer, which is then removed before the electrolysis, the dry-rolled layer is given a "loosened up" structure which enables thorough galvanic consolidation of the layer by deposited nickel.

Example 4:

The procedure was again as in Example 1, but instead of the Ni-Al which can be activated by lye treatment, a catalytically active non-metallic powder of MoS₂ was used.

Example 5:

The procedure was again as in Example 1, but the dry powder mixture of Ni-Al and carbonyl nickel was half-oxidized for two hours at 200 ° C. in air before rolling, whereby the surface of the powder particles was provided with a thin oxide layer. The two powder halves were successively spread out on a flat surface with the oxidized material underneath and then connected to the roughened perforated plate by dry rolling.

In the subsequent galvanic fixation, the metal deposition then begins in the inner areas of the dry-rolled layer and, in the course of the electrolysis, asserts itself towards the surface with the gradual dissolution of the oxide skins in the outer area in the acidic electrolyte.

This technique ensures good consolidation, also of the inner areas.

Example 6:

The electrodes produced according to Examples 1 to 3 were activated in the usual manner by treatment in hot KOH solution and then as electrodes (anode and cathode) used in alkaline water electrolysis. At a current density of 400 mA / cm² and electrolyte temperature of 100 ° C, overvoltages of less than 80 mV were reached cathodically, anodically less than 250 mV. These values demonstrate an excellent catalytic effect of the electrodes obtained according to Examples 1 to 3.

Example 7:

The electrode produced according to Example 4 with molybdenum sulfide was used directly as a cathode in an alkaline water electrolysis operated at 100 ° C. and current densities of 400 mA / cm². An overvoltage of 140 mV was reached.

Claims (12)

1. Process for the production of porous electrodes wherein a porous metal layer is formed on a framework-providing metallic carrier with adhesion-promoting surface irregularity, and is provided with an electrodeposited metal deposit in the pores, and is possibly finally activated by treatment with alkaline solution, characterised in that the carrier is provided at one or both sides with a layer, rolled-on in the dry state, of a mixed powder of a) finely divided carbonyl metal of low bulk density and high sliding resistance and b) a pulverulent component catalytically effective or activatable by alkaline solution treatment, in the ratio a b of 3 : 1 to 1 : 3 (by weight), which is consolidated by coating with metal by electrode-position, whereupon finally activation is carried out if appropriate.
2. Process according to claim 1, characterised in that fine-mesh metal gauze or perforated metal sheet roughened by powder deposition is used as carrier.
3. Process according to claim 2, characterised in that fine-mesh nickel gauze or perforated nickel sheet roughened by deposition of carbonyl nickel powder is used.
4. Process according to one of the preceding claims, characterised in that carbonyl nickel powder having a particle size of 2 to 3 µm and a bulk density of 0.5 to 0.7 g/cm³ is used as component (a) of the mixed powder.
5. Process according to one of the preceding claims, characterised in that the mixed powder consists in roughly equal proportions of carbonyl nickel and Raney nickel alloy.
6. Process according to one of the preceding claims, characterised in that component (b) has a particle size of 10 to 100 µm.
7. Process according to one of the preceding claims, characterised in that the powder layer rolled on in the dry state has a thickness of 50 to 400 µm.
8. Process according to one of the preceding claims, characterised in that the consolidation using electrode-position of the cold-rolled-on mixed powder layer is carried out with a current density of 0.1 to 10 A/dm².
9. Process according to claim 8, characterised in that the electrodeposition consolidation is carried out by depositing nickel and/or nickel alloy with a soluble component, especially nickel-zinc or nickel-tin.
10. Process according to one of the preceding claims, characterised in that the rolling of the powder layer or layers on to the carrier is carried out with a pressure of 0.5 to 10 bars.
11. Process according to one of the preceding claims, characterised in that the mixed powder for rolling-on has additionally a removable filler in quantities of 5 to 20 % by weight (relatively to the mixed powder a + b).
12. Process according to one of the preceding claims, characterised in that a dry rolled layer of mixed powder is applied to the carrier at one or both sides, the powder particles of the said layer having a decreasingly oxidised surface from the outside towards the carrier.