EP0198752A1 - Cathode for electrolysis and process for manufacturing said cathode - Google Patents

Abstract

The cathodes consist of an electrically conductive substrate carrying a coating based on an oxide of a metal of the platinum group in combination with an oxide of a nonprecious metal of group VIII of the Periodic Classification. …<??>The cathodes are particularly suitable for use in cells for the electrolysis of water or of aqueous solutions of alkali metal halides, cells employing the membrane technology.

Description

The present invention relates to a new cathode usable in electrolysis. It also relates to a method of manufacturing this cathode. It relates very particularly to a cathode which can be used in the electrolysis of aqueous alkali metal halide solution which is remarkable in particular for the low value of its working potential and for the stability over time of its electrochemical performance.

This cathode belongs to the family of metallic cathodes, activated, obtained by coating a cathode substrate by means of various activation materials, the aim being essentially to reduce the hydrogen overvoltage in an alkaline medium. One of the techniques that can be used, commonly called "large area nickel", consists of forming a coating of microporous nickel on a ferrous substrate by first depositing a nickel-zinc alloy and then removing the zinc. Other techniques consist in depositing certain metal alloys on the substrate, such as nickel-molybdenum alloys (English patent 992350).

European patent application 0129374 describes cathodes carrying a coating consisting of a mixture of at least one metal of the platinum group and at least one oxide of a metal of the metal group, the platinum of the platinum group representing 2 to 30 % of the weight of said mixture.

Japanese patent application published under No. 5713189 describes a nickel or nickel alloy cathode bearing a coating consisting of a platinum group metal or an oxide of said metal.

British patent 1511719 describes a cathode comprising a metal substrate, a cobalt coating and a second ruthenium coating.

American patent 4100049 describes a cathode comprising a substrate and a coating constituted by a mixture of precious metal oxide and valve metal oxide, in particular zirconium oxide.

Japanese patent application published under the number 54090080 describes a technique for manufacturing a cathode consisting in treating a ferrous substrate with perchloric acid and then in coating this cathode by sintering with active substances comprising ruthenium, iridium, iron and nickel in metallic form or as a metal compound.

A deposition technique on a substrate, for example made of nickel, of a coating made of a nickel-palladium alloy is also described in American patent 3216919: according to this patent, an alloy layer is applied to the substrate. of powder then proceeds to sintering said alloy powder.

It has also been proposed (Russian patent 264096) the coating of an electrode by electrodeposition of a ruthenium-nickel alloy.

The Japanese patent application published under the number 54110983 (American patent n ° 4465580) describes a cathode carrying a coating constituted by a dispersion of particles of nickel or a nickel alloy and an activator constituted by platinum, ruthenium, iridium , rhodium, palladium or osmium or an oxide of these metals.

Japanese patent application published under number 53010036 describes 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 minus a platinum group metal.

European patent application 0129734 describes a cathode manufacturing technique, by depositing on an electroconductive substrate a coating solution comprising a metal oxide precursor and, necessarily, a pickling agent in order to dissolve the most soluble in the substrate and / or a coating layer previously deposited. This technique further comprises an operation of removing the most volatile part of the coating solution, said part containing the solubilized fractions of the substrate (op. Cit. Page 14).

The invention proposes a new cathode, usable in particular in the electrolysis of aqueous solutions of alkali metal halides, said cathode being constituted by an electrically conductive substrate carrying a coating based on a metal oxide of the platinum group , this cathode being characterized in that it carries a coating consisting of at least two metal oxides from group VIII of the periodic table of the elements, chosen respectively among the precious metals and among the non-precious metals of said group VIII.

The invention relates most particularly to cathodes comprising an electrically conductive substrate and a coating, the said coating consisting of ruthenium oxide (Ru 0 ₂ ) associated with one or more oxides of iron, cobalt or nickel and, optionally with one or several other Group VIII precious metal oxides.

Among such cathodes, the invention especially relates to those in the coating of which the ruthenium oxide has a microcrystalline structure and the oxides of non-precious metals a crystal structure.

The invention particularly relates to cathodes in the coating of which all or part of the above-mentioned oxides are in the form of scales.

Within the meaning of the invention the term tortoiseshell designates a film of planar shape, of portion of cylinder or sphere or the combination of said shapes, the thickness of which is less than one tenth of the average of the two dimensions of the quadrilateral in which said tortoiseshell can be entered, the average value of said dimensions can be between 1 and 100 microns and more precisely between 3 and 30 microns.

As indicated, the coating consists wholly or partially of at least one oxide of a precious metal, that is to say ruthenium, rhodium, palladium, osmium, iridium and platinum. Preference is given in the invention to ruthenium oxide or to a combination of said oxide with one or more other precious metal oxides.

In the coating of cathodes according to the invention, the molar ratio of the oxides of precious and non-precious metals is generally between 10/1 and 1/10 and preferably between 1/5 and 5 / l.

The material constituting the substrate can be chosen from electrically conductive materials. It will advantageously be chosen from the group consisting of nickel, stainless steel and mild steel without this list being limiting.

The substrate may be in the form of a plate, sheet, with or without a certain number of orifices or perforations, lattice, metallic canvas or expanded metal, grids, the said materials being able to have a planar, cylindrical shape or any other shape according to the technology employed.

The invention also relates to a method of manufacturing these cathodes.

This process essentially consists in depositing on the substrate, possibly subjected to an appropriate preliminary treatment, one or more layers of one or more salts of metals of each category (precious metals - non-precious metals) of group VIII, then subjecting the together with a heat treatment leading to the oxidized form.

The preliminary treatment of the substrate advantageously consists of a degreasing - if necessary - followed by a pickling, mechanical and / or chemical, according to techniques now well known.

It is possible to deposit on this substrate one or more layers of a solution or suspension containing all of the metal salts (or precursors of the oxides); these precursors can also be deposited separately in the form of successive layers. One can also deposit one or more layers of a part of the precursors, cause after each layer or only after the last decomposition of the precursor and then repeat the same operation with the other part of the oxide precursors. The foregoing description is voluntarily schematic for the purpose of simplification, but it is easily understood that all combinations of precursors are possible and that in particular the same precursor may be present in several layers, either alone or associated with the same precursor in the different layers or to different precursors from one layer to another.

In general, the above-mentioned precursors are deposited in the form of a solution or suspension. Depending on the nature of the precursor, the solvent or diluent may consist of water, mineral or organic acid or even an organic solvent. An organic solvent is preferably used such as dimethylformamide, an alcohol and in particular ethanol or 2-ethyl hexanol. In general, the atomic concentration of metal is between 3.10 and 3 mole / liter and preferably between 1 and 2 mole / liter.

The oxide precursors which can be used in the invention generally consist of the inorganic or organic salts of metals, such as for example halides, nitrates, carbonates, sulfates, or acetates, acetylacetonates. In the case of precursors of platinum and ruthenium oxides, hexachloroplatinic acid hexahydrate and ruthenium chloride hydrate will advantageously be used.

The aforementioned layers of precursors can be deposited using conventional techniques: immersion of the substrates in the solution (s), coating with a brush, brush or the like, electrostatic spraying.

The preparation of solutions and the deposition of said solutions are generally done at room temperature and in air. Naturally, it is possible, if necessary, to raise the temperature in particular to facilitate the dissolution of certain precursors, and / or to work under an atmosphere of nitrogen or other inert gas with respect to the precursors.

The transformation of precursors into oxides is generally done by heat treatment. This treatment is advantageously preceded by an air drying intended to totally or partially remove the solvent or diluent. This steaming can be carried out at a temperature of up to 200 ° C, the temperature range from 100 to 150 ° C being particularly recommended. The duration of this treatment is generally a few tens of minutes. The actual treatment is generally carried out in air at a temperature varying, depending on the precursors used, between 200 and 1000 ° C. Preferably one operates at a temperature between 400 and 750 ° C. The duration of this heat treatment is generally between 15 min and 1 h per layer. This heat treatment can be carried out after each baking or after the last baking in the case of the deposition of several layers.

The cathode of the invention is suitable for use in electrolysis cells in which water or aqueous solution is electrolyzed with production of hydrogen by electrolysis, released at the cathode. The cathode is particularly suitable for the electrolysis of aqueous solutions of alkali metal chlorides and in particular aqueous solutions of sodium chloride and for the electrolysis of water, for example in the electrolysis of aqueous solutions of potassium hydroxide . In electrolysis cells, microporous diaphragms can be used 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 200 x 10 x 1 mm.

A surface treatment is carried out using corundum (average diameter of the beads 250 μm).

• . de 1 g de Ru C1₃, xHCL, yH₂0, contenant environ 38% en poids de ruthénium métal,
• . et 2 g de Ni(NO₃)₂, ^6H ₂ ^0.

A solution in 2 cm ³ of ethanol is prepared at 23 °

• . 1 g of Ru C1 ₃ , xHCL, yH ₂ 0, containing about 38% by weight of ruthenium metal,
• . and 2 g of Ni (NO ₃ ) ₂ , ^6H ₂ ^0.

The nickel plate is coated with this solution. Steaming is carried out in air (120 ° C, 30 min), followed by a heat treatment in air (500 ° C, 30 min). After cooling, the coating / baking / heat treatment sequence is repeated twice.

A deposit of 1.7 mg / cm ² is obtained of a mixture essentially in the form of scales of average dimensions of between 3 and 30 μm, and having in X-ray radiography the structures of Ru 0 ₂ and NiO. Ruthenium oxide has a microcrystalline structure and nickel oxide has a crystalline structure.

This cathode, tested in soda at 450 g / 1, at 85 ° C and under 50 A / dm ² has a working potential of - 1160 mV compared to the saturated calomel electrode (ECS).

By way of comparison, a coating consisting solely of Ru 0 ₂ (at the rate of 3 mg / cm ² ) is deposited under the same conditions. We observe that the work potential is - 1300 mV compared to DHW

In addition, an 80 mm diameter disc, consisting of a mesh of expanded and rolled nickel, coated with Ru O ₂ / NiO using the process described above, is used as the cathode of a solution electrolysis cell aqueous sodium chloride - membrane technology.

• - intensité = 30 A/d_m ²
• - température = 85°C
• - soude 32% en poids.

The operating conditions are:

• - intensity = 30 A / d _m ²
• - temperature = 85 ° C
• - soda 32% by weight.

• - que la tension aux bornes de cette cellule présente, par rapport à la tension aux bornes d'une cellule dans laquelle la cathode est constituée du seul nickel non revêtu un gain de 350 mV.
• - que ce gain est constant à 350 mV après 30 jours de fonctionnement continu.

We observe :

• - that the voltage across this cell has, compared to the voltage across a cell in which the cathode is made of uncoated nickel alone, a gain of 350 mV.
• - that this gain is constant at 350 mV after 30 days of continuous operation.

EXAMPLE 2

A nickel substrate is used which has undergone a surface treatment under the conditions of Example 1.

• - solution A : la solution dans 1 cm³ d'éthanol d'1 g de Ru Cl₃, xHCL, yH₂O de l'exemple 1.
• - solution B : une solution dans 1 cm³ d'éthanol de 1g de Ni(NO₃)₂, 6^H ₂ ^0.

Two solutions are prepared at 23 ° C:

• solution A: the solution in 1 cm ³ of ethanol of 1 g of Ru Cl ₃ , xHCL, yH ₂ O from Example 1.
• - solution B: a solution in 1 cm ³ of ethanol 1 g of Ni (NO _{3) 2,} ^6H ₂ ^0.

2 layers of solution B are deposited on the nickel substrate, according to the coating / stoving / heat treatment sequence of Example 1, then after cooling 1 layer of solution A, also following the coating / stoving / heat treatment sequence.

This cathode carrying a double coating comprising NiO and Ru 0 ₂ (X-ray) is tested in sodium hydroxide, as in Example 1: the working potential is - 1170 mV relative to DHW

EXAMPLE 3

The nickel substrate and the two solutions A and B of Example 2 are used.

A first layer of solution A is deposited on the nickel (coating / baking / heat treatment sequence of Example 1) and then, after cooling, 2 layers of solution B (also coating / baking / heat treatment sequence of Example 1). 'example 1).

This cathode, carrying a double coating comprising Ru 0 ₂ and N10 has a working potential of −1190 relative to ECS (test in the soda of example 1).

COMPARATIVE EXAMPLE

On the surface-treated nickel substrate according to Example 1, 3 layers of solution B are deposited according to the coating / steaming / heat treatment sequence.

This cathode, which has a NiO coating (2.2 mg / cm ² ), has a working potential (test in soda of example 1) of - 1430 mV compared to DHW

EXAMPLE 4

A nickel substrate treated as in Example 1 is used.

• - 1 g de Ru Cl₃, xHCL, yH₂0 de l'exemple 1
• - 2 g de Fe (NO₃)₃, 9 ^H ₂ ⁰

A solution in 2 cm ³ of ethanol of:

• - 1 g of Ru Cl ₃ , xHCL, yH ₂ 0 from Example 1
• - 2 g of Fe (NO ₃ ) ₃ , 9 ^H ₂ ⁰

Three layers of this solution are deposited on the nickel substrate following the coating / baking / heat treatment sequence of Example 1.

A deposit of 2.2 mg / cm ² of a mixture is obtained which exhibits the structures of Ru O ₂ and Fe ₂ 0 _{3 on} X-ray.

This cathode, tested in sodium hydroxide as in Example 1, has a working potential of - 1180 mV compared to E.C.S.

EXAMPLE 5

A nickel substrate treated as in Example 1 is used.

• - 1 g de Ru Cl₃, xHCL, yH₂0 de l'exemple 1
• - 2 g de Co(NO₃)₂, 6 H₂O

A solution in 2 cm ³ of ethanol of:

• - 1 g of Ru Cl ₃ , xHCL, yH ₂ 0 from Example 1
• - 2 g of Co (NO ₃ ) ₂ , 6 H ₂ O

Three layers of this solution are deposited on the nickel substrate following the coating / baking / heat treatment sequence of Example 1.

A deposit of 2.3 mg / cm ² is obtained of a mixture exhibiting on X-ray the structures of Ru O ₂ and Co ₃ O ₄ .

This cathode, tested in sodium hydroxide as in Example 1, has a working potential of - 1180 mV compared to E.C.S.

Claims (10)

1 - Cathode usable in an electrolysis cell, consisting of an electrically conductive substrate carrying a coating based on a metal oxide of the platinum group, this cathode being characterized in that it carries a coating consisting of at at least two oxides of metals from group VIII of the periodic table of the elements, chosen respectively from precious metals and from non-precious metals from said group VIII, all or part of the oxides constituting the coating in the form of scales. 2 - Cathode according to claim 1, characterized in that the coating consists of ruthenium oxide (Ru 0 2) associated with one or more oxides of iron, cobalt or nickel and, optionally with one or more other oxides of precious metals of group VIII. 3 - Cathode according to claim 2, characterized in that, in the coating, the ruthenium oxide has a microcrystalline structure and the oxides of non-precious metals a crystal structure. 4 - Cathode according to any one of claims 1 to 3, characterized in that the substrate is chosen from the group consisting of nickel, stainless steel and mild steel. 5 - A method of manufacturing cathodes according to any one of claims 1 to 4, characterized in that it consists in depositing on the substrate, optionally subjected to an appropriate preliminary treatment, one or more layers of one or more salts of metals (precious and non-precious) of each category of group VIII, then to subject the whole to a heat treatment leading to the oxidized form. 6 - Process according to claim 5, wherein all the metal salts are deposited simultaneously on the substrate in the form of one or more layers of a solution or suspension containing the said salts. 7 - Process according to claim 5, characterized in that the metal salts are deposited in the form of successive layers. 8 - Method according to any one of claims 5 to 7, characterized in that the metal salts are chosen from mineral or organic salts of metals, such as for example halides, nitrates, carbonates, sulfates, or still acetates, acetylacetonates. 9 - Method according to any one of claims 5 to 8, characterized in that the heat treatment is carried out at a temperature between 200 and 1000 ° C. 10 - Process according to claim 9, characterized in that the heat treatment is preceded by a steaming intended to completely or partially remove the solvent or diluent of the metal salts, the said steaming being carried out at a temperature up to 200 ° C.