FR2583781A1 - CATHODE FOR ELECTROLYSIS AND METHOD FOR MANUFACTURING THE SAME CATHODE - Google Patents

Abstract

THE INVENTION RELATES TO CATHODES. THESE CATHODES ARE CONSTITUTED BY AN ELECTRICALLY CONDUCTIVE SUBSTRATE CARRYING A COATING CONSISTING OF A PLURALITY OF METAL OXIDE LAYERS, THE SURFACE LAYER CONSTITUTING OF A VALVE METAL OXIDE AND THE OR ONE OF THE METAL OXIDE LAYERS AND THE INTERMEDIATE CONSTITUENT LAYER 'A PRECIOUS METAL FROM THE GROUP VIII. THE CATHODES ARE PARTICULARLY USED IN CELLS FOR THE ELECTROLYSIS OF WATER OR AQUEOUS SOLUTIONS OF ALKALINE METAL HALOGENIDES, CELLS USING MEMBRANE TECHNOLOGY.

Description

CATHODE FOR ELECTROLYSIS

  AND A METHOD OF MANUFACTURING THE CATHODE DITE

  The present invention relates to a new cathode used

  sand in electrolysis. It also relates to a manufacturing process

  cation of this cathode. It particularly concerns a

  cathode for use in the electrolysis of aqueous halogen

  the remarkable value of its working potential and the stability over time of its

electrochemical performance.

  This cathode belongs to the family of activated metal cathodes, obtained by coating a cathode substrate by means of

  various activation materials, the aim pursued being essential-

  to reduce the hydrogen overvoltage in an alkaline medium.

  British Patent 1,511,719 discloses a cathode comprising a metal substrate, a cobalt coating and a second coating

ruthenium.

  US Patent 4,100,049 discloses a cathode comprising a substrate and a coating consisting of a mixture of precious metal oxide and valve metal oxide, in particular zirconium oxide. A deposition technique on a substrate, for example made of nickel, of a nickel-palladium alloy coating is also described in US Pat. No. 3,216,919: according to this patent, an alloy layer is applied to the substrate. form

  of powder then sintering said alloy powder.

  Japanese Patent Application Publication No. 54-110983 (US Patent No. 4,465,580) discloses a cathode having a coating consisting of a dispersion of particles of nickel or a nickel alloy and a platinum activator, ruthenium, iridium, rhodium, palladium or osmium or an oxide of

these metals.

  Japanese Patent Application Publication No. 53-010036 discloses a cathode having a valve metal substrate and a coating of an alloy of at least one platinum group metal and a valve metal and optionally a surface coating of 'at least one

platinum group metal.

  The invention proposes a novel cathode, which can be used in particular in the electrolysis of aqueous solutions of metal halides.

  alkaline, the cathode being an electrical substrate

  electrically conductive coating having a platinum group metal oxide coating, said cathode being characterized in that it carries a coating consisting of a plurality of metal oxide layers, the surface layer being substantially formed by a a metal oxide valve and the intermediate layer or at least one of the intermediate layers being substantially constituted by a precious metal oxide of group VIII of the periodic table of elements. The term "substantially" is used herein, in connection with the surface and intermediate layers, to indicate that said layers may be constituted by the oxide of only the metal concerned or by a mixed oxide of the metal in question and the second metal in a small proportion by example in a molar ratio

not exceeding 1/10.

  In the invention, the term "valve metal" is used in its usual meaning, that is to say, it refers to the metals of groups 4b, 5b and 6b of the periodic table, with the exception of

however, being made of chromium.

  The invention is particularly aimed at cathodes comprising an electrically conductive substrate and a coating, said coating consisting of one or more layers of ruthenium oxide (Ru 02) associated with one or more layers of oxides of titanium and / or zirconium. It relates especially to cathodes whose coating comprises Ru 02 and Ti 02. Among such cathodes, mention is made especially of

  cathodes whose coating comprises from the electrostatic substrate

  conductive one or more layers of Ru 02 followed by one or more layers of Ti 02 is a succession of one or more layers Ti 02 / one or more layers Ru 02 / one or more layers Ti 02. We would not go out of of the invention by interposing in this succession of layers other oxides of precious or non-precious metals or by replacing all or part of Ru 02 and / or Ti 02 by (Ru x Ti 1x) 2 with x increasing between

  electroconductive substrate and the surface in contact with the electri-

  trolyte. For the purposes of the invention, the term "surface layer" refers precisely to the oxide layer whose surface is in contact

  with the electrolyte, the term interlayer

  coating any layer disposed between the electroconductive substrate and said surface layer. The invention is particularly aimed at cathodes in the coating of which all or part of the oxides

  mentioned above are in the form of scales.

  For the purposes of the invention, the term "shell" denotes a film

  plane, cylinder or sphere portion or the combination of

  form, the thickness of which is less than one tenth of the average of the two dimensions of the quadrilateral in which the

  said scale may be inscribed, the average value of the said dimensions

  can be between 1 and 100 microns and more accurate

between 3 and 30 microns.

  As indicated, the coating consists entirely of

  or partially by at least one oxide of a precious metal, that is to say ruthenium, rhodium, palladium, osmium, iridium and platinum. Ruthenium oxide or a combination of said oxide with one or more is preferred in the invention.

other precious metal oxides.

  In the coating of the cathodes according to the invention, the molar ratio of the oxides of precious metals and metal valve generally between 10/1 and 1/10 and preferably between 1/5

and 5/1.

  The material constituting the substrate may be chosen from electrically conductive materials. It will be chosen advantageously from the group consisting of nickel, stainless steel and

  mild steel without this enumeration being limiting.

  The substrate may be in the form of a plate, sheet, with or without a number of orifices or perforations, mesh, wire mesh or expanded metal, grids, said materials may have a flat shape, cylindrical or any other

  shape according to the technology used.

  The invention also relates to a method of manufacturing these cathodes. This process consists essentially in depositing on the substrate, possibly subjected to appropriate prior treatment, the

  salt layers of the metals and then subject the whole to a

  thermal effect leading to the oxidized form.

  The pretreatment of the substrate advantageously consists of a degreasing - if necessary - followed by stripping, mechanical t / or

  chemical, following techniques now well known.

  The technique consists essentially of depositing on the substrate

  successively the layers of metal salt solutions.

  In the case of mixed oxide deposition, the same technology can be used or the layers containing the two metal salts concerned can be deposited directly. In general, the metal salts are deposited in the form of a solution or suspension. Depending on the nature of the salt, the solvent or the diluent may consist of water, mineral or organic acid or organic solvent. We

  preferably uses an organic solvent such as dimethyl

  formamide, an alcohol and especially ethanol or 2-ethyl hexanol.

  In general, the atomic concentration of metal is between 3 × 10 -2 and 3 moles / liter and preferably between 1 and

2 moles / liter.

  The metal salts which can be used in the invention are generally

  usually constituted by the inorganic or organic salts of metals, such as, for example, halides, nitrates, carbonates, sulphates, or even acetates, acetylacetonates. In the case

  salts leading to platinum and ruthenium oxides are used.

  is advantageously hexachloroplatinic acid hexahydrate and the

ruthenium chloride hydrate.

  The deposition of the aforementioned salt layers may be carried out according to conventional techniques: immersion of the substrates in the solution or suspensions or solutions, coating by means of a brush, brush

  or assimilated, electrostatic projection.

  The preparation of the solutions or suspensions and the deposition are generally at room temperature and in air. Naturally, it is possible, if necessary, to raise the temperature in particular to facilitate the dissolution of certain salts, and / or to work under

  nitrogen atmosphere or other inert gas vis-à-vis said salts.

  The conversion of metal salts into oxides is generally

  usually by heat treatment. This treatment is advantageously preceded by air baking to completely or partially remove the solvent or diluent. This baking can be carried out at a temperature up to 200 C, the temperature range from 100 to 150 C being particularly recommended. The duration

  of this treatment is usually a few tens of minutes.

  The actual treatment is generally carried out under air at a

  temperature varying, according to the salts used, between 200 and 1000 C.

  The operation is preferably carried out at a temperature of between 400 and 750 ° C. The duration of this heat treatment is generally between 15 minutes and 1 hour per layer. We can do this treatment

  after each steaming or after the last steaming.

  The cathode according to the invention is characterized by the excel-

  slow adhesion of the electroactive coatings on the substrate.

  The cathode of the invention is suitable for use in electrolysis cells in which the water or aqueous solution is electrolyzed with hydrogen production by electrolysis, released at

  the cathode. The cathode is particularly suitable for electro-

  lysis of aqueous solutions of alkali metal chlorides and

  aqueous solutions of sodium chlorides and to the elec-

  trolysis of water, for example in the electrolysis of aqueous solutions of potassium hydroxide. In the electrolysis cells, it is possible to use microporous diaphragms as separators, but the cathodes according to the invention are of particular interest in

membrane technology.

  The following examples illustrate the invention.

EXAMPLE 1

  The substrate consists of a nickel plate of

x 10 x 0.6 mm.

  Surface treatment with corundum

  (average diameter of the beads 250 Pm).

  a) is prepared at 23 C a solution in 2 cm3 of ethanol, 2 g of Ru C13, xHCl, yH20, containing about 38% by weight of ruthenium metal. The nickel plate is coated with this solution. Steaming was carried out under air (120 ° C., 30 minutes), followed by heat treatment in air (500 ° C., 30 minutes). After cooling, the coating / steaming / heat treatment sequence is repeated.

  A deposit of 1.4 mg / cm 2 of Ru 2 O 2 is obtained in the form of

  the average size between 3 and 30 pm, and presenting

  in X-ray the structure of Ru 02.

  b) A solution in 2 cm3 of ethanol of 2.6 cm3 of Ti OC12, 2HCl at 2.5 mol / l of Ti is prepared at 23 ° C. The same treatments (2 layers) coating / steaming / heat treatment are carried out

  under a. 0.8 mg / cm 2 of TiO 2 are thus deposited.

  c) This cathode, tested in sodium hydroxide at 450 g / l, at 85 ° C. and at 50 A / dm 2, has a working potential of -1225 mV per

  compared to the saturated calomel electrode (E.C.S.).

  d) A disc of 80 mm in diameter, consisting of a rolled and rolled nickel mesh, coated with Ru 02 / Ti 02 following the process described above, is used as the cathode of a cell

  electrolysis of aqueous solution of sodium chloride - techno-

membrane.

  The operating conditions are: - intensity = 30 A / dm2 temperature = 85 C

- soda 32% by weight.

  It is observed that: - the voltage at the terminals of this cell has, with respect to the voltage at the terminals of a cell in which the

  cathode consists of single nickel uncoated gain of 300 mV.

  - this gain is constant at 300 mV after 6 days of operation

continuously.

EXAMPLE 2

  A nickel substrate having been treated with

  surface under the conditions of Example 1.

  Two solutions are prepared at 230 ° C: Solution A: the solution in 2 cm 3 of ethanol of 2 g of

Ru C13, xHCl, yH20 from Example 1.

  Solution B: a solution in 1 cm 3 of ethanol of 1.3 cm 3 of Ti 0 Cl 2, 2 HCl at 2.5 mol / l of Ti is deposited on the nickel substrate two layers of solution B, according to the coating sequence / baking / heat treatment of Example 1, then after cooling two layers of the solution A also following the sequence coating / baking / heat treatment and again two layers of the solution B followed by the same treatments. The total deposit of metal oxides is 1.75

mg / cm2 of which 0.6 mg / cm 2 Ru 02.

  This cathode carrying a triple coating comprising Ti 025 Ru 02 and Ti 02 is tested in sodium hydroxide, as in Example 1: the working potential is - 1240 mV with respect to E.C.S. After

  hours the potential is - 1210 mV.

EXAMPLE 3

  a) A substrate of Ni having undergone a treatment of

  surface under the conditions of Example 1.

  b) Two solutions are prepared at 23 ° C.: Solution A: the solution in 2 cm 3 of ethanol of 2 g of

Ru C13, xHCl, yH2O from Example 1.

  - Solution B: a solution in ethanol (2.4 cm3) / HCl (1 cm3

1 N) of 1 g of ZrOC12, 8H20.

  c) Two layers of solution B are deposited on the substrate

  according to the coating / steaming / heat treatment sequence of the

  Then, after cooling, two layers of solution A, also following the coating / steaming / heat treatment sequence and again two layers of solution B, followed by the same treatments. (d) The total amount of metal oxides is 1.8 mg / cm2, of which 0.7 mg / cm 2 RuO 2 This cathode, with a triple coating Zr 029 Ru 02, Zr 02

  is tested in soda as in Example 1.

  The working potential is -1210 mV compared to E.C.S .;

  after 16 hours, the potential is -1200 mV.

EXAMPLE 4

  a) The nickel substrate and the solutions A and B of

Example 3

  b) Deposit on the substrate two layers of solution A then two layers of solution B following the process and conditions

  coating / steaming / heat treatment of Example 1.

  (c) The total amount of metal oxides is 1.2 mg / cm2

0.7 mg / cm 2 of Ru 2 O.

  This cathode, bearing a double coating Ru 02 / Zr 02 is

  tested in soda as in Example 1.

  The working potential is -1210 mV compared to E.C.S.

  After 16 hours, the potential is unchanged.

R E V E N D I CATIONS

  1 / Cathode usable in an electrolysis cell, constituting

  of an electrically conductive substrate having a coating based on a platinum group metal oxide, said cathode being characterized in that it carries a coating consisting of a plurality of layers of metal oxides, the surface layer being substantially constituted by a valve metal oxide and the intermediate layer or at least one of the intermediate layers being substantially constituted by a precious metal oxide of

  Group VIII of the Periodic Table of Elements.

  2 / cathode according to claim 1, characterized in that the surface layer and / or the intermediate layer or layers are

  consisting of the only metals of interest.

  3 / cathode according to claim 1, characterized in that the surface layer and / or the intermediate layer or at least one of the intermediate layers is constituted by a mixed oxide of

  metal concerned and the second metal in a small proportion.

  4 / cathode according to any one of claims 1 to 3,

  characterized in that the coating consists of one or more

  ruthenium oxide (Ru 02) layers associated with one or more

  several layers of titanium and / or zirconium oxides.

  Cathode according to one of Claims 1 to 4,

  characterized in that the coating comprises, from the substrate one or more layers of Ru 02 and then one or more layers of Ti 02 and / or Zr 02 'or a succession of one or more layers of Ti 02 and / or Zr 02 / one or more layers of Ru 02 / one or more

layers of Ti 02 and / or Zr 02.

  6 / cathode according to any one of claims 1 to 5,

  characterized in that, in the coating, all or part of the

  Oxides are in the form of scales.

  7 / cathode according to any one of claims 1 to 6,

  characterized in that the substrate is selected from the group consisting of

  killed by nickel, stainless steel and mild steel.

  8 / Method for manufacturing cathodes according to any one

  Claims 1 to 7, characterized in that it consists in depositing

  on the substrate, possibly subjected to a preliminary treatment

  metal salt layers, then submit the

  seems to heat treatment leading to the oxidized form.

  9 / A method according to claim 8, in which one deposits

  successively on the substrate the layers of solutions or suspen-

salt of metals.

  Process for manufacturing cathodes according to claim 3, characterized in that the metal salts intended to form the mixed oxide are deposited in the form of one or more layers.

of the same solution of said salts.

  11 / A method according to any one of Claims 8 to 10,

  characterized in that the metal salts are selected from salts

  inorganic or organic metals, such as, for example, halogens

  nitrates, carbonates, sulphates, or even

acetates, acetylacetonates.

  12 / A method according to any one of Claims 8 to 11,

  characterized in that the heat treatment is carried out at a

  temperature between 200 and 1000 C.

  13 / A method according to claim 12, characterized in that the heat treatment is preceded by a baking to completely or partially remove the solvent or diluent metal salts, said baking being carried out at a temperature of

reach 200 C.