EP0131978B1 - Process for manufacturing an electrode for electrochemical processes, and cathode for the electrolytic production of hydrogen - Google Patents

Abstract

An electrically conductive substrate is coated with a material containing an unsintered powder of a metal active for electrochemical proton reduction and colloidal silica and the said material is heated on the substrate successively in an oxidizing atmosphere and then in a reducing atmosphere. The electrode may be employed as a cathode for electrolytic production of hydrogen in an alkaline medium.

Description

The present invention relates to a method of manufacturing an electrode for electrochemical processes.

In electrolysis processes, it is generally sought to reduce the potentials of electrochemical reactions at the electrodes to as low a value as possible. This is particularly the case in the electrolysis processes in which hydrogen gas is produced on the active surface of a cathode, such as the processes for the electrolysis of water, aqueous solutions of hydrochloric acid and aqueous solutions of sodium chloride.

The cathodes most commonly used hitherto for the electrolysis of water or aqueous solutions of sodium or potassium chloride have generally consisted of plates or lattices of mild steel. These known cathodes have the advantage of easy implementation and low cost. The overvoltage on the evolution of hydrogen on these known steel cathodes is however relatively high, which increases the cost of the electrolysis processes. Steel cathodes have the additional disadvantage of being the site of progressive corrosion on contact with concentrated aqueous solutions of sodium hydroxide, as they are generally obtained in electrolysis cells with selective permeability membranes.

To improve the energy efficiency of the electrolysis processes, cathodes have been proposed obtained by applying, on a steel or nickel support, a coating formed of a nickel powder mixed with a polysilicate and then subjecting the nickel powder sintering in a reducing atmosphere at a temperature above 760 ° C (Journal of the Electrochemical Society, vol. 128, No4, April 1981 - DE Hall: "Electrodes for alkaline water electrolysis", pages 740 to 746).

In patent US-A 4362647 (Agency of Industrial Science & Technology), cathodes obtained by subjecting a nickel plate or a nickel powder sintered on a conductive support have been proposed to two consecutive heat treatments, respectively an oxidation at a temperature above 800 ° C and a reduction at a temperature between 300 and 600 ° C. In this known process, the oxidation is usually carried out at a temperature between 900 and 1000 ° C.

Compared to the electrodes formed from steel or nickel plates as such, these known cathodes generally allow an improvement in the energy efficiency of the processes for the electrolysis of water or aqueous solutions of sodium chloride.

The invention aims to provide a method of manufacturing electrodes which, when used as cathodes in electrolysis processes where hydrogen is generated, make it possible to further improve the energy efficiency of the electrolysis process .

Consequently, the invention relates to a method of manufacturing an electrode for electrochemical processes, according to which an electrically conductive substrate is coated with a material containing a powder of at least one active metal for the electrochemical reduction of protons and said material is heated on the substrate, successively in an oxidizing atmosphere then in a reducing atmosphere; according to the invention, a material is used in which the aforementioned active metal is in the form of an unsintered powder, associated with colloidal silica.

In the process according to the invention, the active metal selected must be a metal which can be oxidized by heating in an oxidizing atmosphere, and whose oxide can be reduced to the solid metallic state by heating in a reducing atmosphere. The selection of the active metal also depends on the destination of the electrode. In the case where it is intended to serve as a cathode for the electrolytic production of hydrogen in an electrolysis process, it is advantageously selected from cobalt, iron, manganese and nickel.

The substrate can be made of any electrically conductive material compatible with the active metal and the oxidation and reduction treatments used. For example, in the case where the active metal is selected from cobalt, iron, manganese and nickel, it is advantageous to choose the material of the substrate from these metals and their alloys.

The substrate can have any suitable shape with the destination of the electrode. It can be, for example, a solid or perforated plate, a wire, a trellis or a stack of beads. It may have a smooth surface state, a rough surface state being however preferred. It may possibly be linked to an underlying support made of a different material, for example a material which is better conductive of electricity such as copper or aluminum.

Heating in an oxidizing atmosphere has the function of oxidizing the active metal. The choice of temperature, atmosphere and heating time depends on the active metal selected and must therefore be determined in each particular case by routine laboratory work. After heating in an oxidizing atmosphere, the active metal is in the state of metal oxide.

Heating in a reducing atmosphere has the function of reducing this metal oxide to the state of metal. The choice of heating conditions also depends on the active metal selected.

According to the invention, the material of the electrode which is subjected to heating in an oxidizing atmosphere contains the active metal powder in the unsintered state, associated with colloidal silica. Thus, according to a first characteristic of the method according to the invention, it is expressly avoided to sinter the active metal powder before heating the material in an oxidizing atmosphere. It is desirable use an active metal powder as fine as possible. As a general rule, a powder is used in which the average particle diameter does not exceed 50 microns and preferably 30 microns. Powders which are generally well suited are those in which the average particle diameter is between 1 and 25 microns, more especially those with an average diameter of less than 20 microns.

According to the second characteristic of the method according to the invention, the active metal powder is associated with colloidal silica. The optimum amount of colloidal silica to be used depends on various factors, in particular the nature of the active metal and its particle size. In general, a relative amount by weight of colloidal silica in the material is used, comprised between 0.5 and 10% of the weight of active metal, the amounts comprised between 0.8 and 4% of this weight being preferred.

In the process according to the invention, the colloidal silica can be used in the form of a gel which is mixed as it is with the active metal powder to form the aforementioned material.

According to a particularly advantageous embodiment of the method according to the invention, the active metal powder is dispersed in a colloidal silica solution, preferably aqueous, to form the aforementioned material which is then applied as it is, to the state of a liquid suspension, on the substrate by any suitable means, for example by soaking the substrate in the suspension, by coating with a brush or roller or by spraying. In this embodiment of the invention, the maximum admissible concentration of silica in the suspension is imposed by the need to produce a stable colloidal silica solution. It depends on various factors, in particular on the concentration of the active metal suspension and on the possible presence of additives such as stabilizers of the colloidal solution or thickeners. As a general rule, in the case of aqueous solutions, the silica content of the colloidal solution should not exceed 30% by weight, values between 3 and 28% and more especially between 10 and 25% being desired.

The active metal powder can be dispersed in the colloidal silica solution as it is and the resulting suspension applied to the substrate. In general, it is desirable to dilute the colloidal silica solution with water before dispersing the active metal powder therein, so as to facilitate this dispersion and to give the suspension a viscosity compatible with good coating of the substrate. . The optimum amount of dilution water is variable depending on the particle size of the active metal powder, the relative amount of active metal added to the colloidal silica solution and the viscosity desired. In practice, good results are obtained by using an amount of dilution water such that the weight content of active metal in the resulting aqueous suspension is between 10 and 80%, preferably 15 and 60%, the contents included. between 20 and 50% being especially advantageous.

According to a variant of the embodiment of the invention, which has just been described, it is advantageous to subject the aforementioned material to drying on the substrate, before heating it in an oxidizing atmosphere, the drying having the function of drain at least most of the water from the colloidal solution. In this variant of the invention, the drying is advantageously regulated so that, at the end of the latter, the water content of the material does not exceed 10%, preferably 5% of the weight of the material. During drying, sintering of the active metal powder should be avoided.

In another variant of the embodiment of the invention, which has just been described, a colloidal silica solution is used which additionally contains lithium ions as stabilizing agent. In this variant of the invention, the lithium ions can be introduced by any suitable means into the colloidal silica solution, preferably in the form of lithium hydroxide. The content of lithium ions in the colloidal solution is preferably adjusted so as to achieve therein a molar ratio Si0 ₂ : Li0 ₂ of between 3 and 25, preferably 4 and 10. Solutions of colloidal silica especially suitable in the context of the invention are those described in patent US-A 2,668,149 (Du Pont).

In the process according to the invention, the heating in an oxidizing atmosphere and the heating in a reducing atmosphere are preferably carried out at temperatures for which no melting or sintering of the metal powder is caused.

For example, in the case where the active metal is selected from cobalt, iron, manganese and nickel, heating in an oxidizing atmosphere can be carried out in air, preferably at a temperature not exceeding not 850 ° C and heating in a reducing atmosphere may be carried out in hydrogen at a temperature not exceeding 600 ° C. Suitable working temperatures are those between 600 and 800 ° C, and more particularly between 700 and 760 ° C, for heating in an oxidizing atmosphere and those between 300 and 500 ° C, and more particularly between 350 and 450 ° C, for heating in a reducing atmosphere.

The electrode obtained after heating in a reducing atmosphere can generally, after cooling, be used as it is, in the electrochemical process for which it is intended.

However, it is preferred, according to a particular embodiment of the invention, to subject the electrode to an oxidation treatment after heating in a reducing atmosphere. This oxidation treatment can be carried out in ambient air and it is preferably carried out at a temperature above ambient temperature but not exceeding the maximum temperature of heating in a reducing atmosphere. A practical way to realize it consists in cooling the electrode in the presence of air at the end of the heating in a reducing atmosphere.

In another embodiment of the method according to the invention, a coating containing a metal selected from chromium, molybdenum, cobalt, nickel, is applied to the electrode, after heating in a reducing atmosphere, ruthenium, rhodium, palladium, osmium, iridium, platinum, lanthanum and rare earth elements.

All other things remaining equal, this embodiment of the invention allows an additional gain in electrical voltage in the electrochemical processes, and especially in the electrolysis processes.

In the implementation of this embodiment of the invention, the metal of the coating can be applied to the electrode by any suitable means, for example by a technique of projection in a plasma jet. In the case where the metal of the coating is selected from chromium, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium and platinum, it is useful to carry out a deposition by process electrolytic. To this end, it has been found to be especially advantageous to carry out the electrolytic deposition of the selected metal, in an electrolyte containing ions of said metal, where the electrode is the site of an electrolytic reduction of protons.

According to an advantageous variant of this embodiment of the invention, to apply the metal of the coating to the electrode, a layer of a heat-decomposable compound of said metal is first deposited there, then said compound is subjected to a treatment thermal decomposition to release an oxide from said metal, and the oxide is then heated in a reducing atmosphere. This variant of this embodiment of the method according to the invention finds a particularly advantageous application in the case where the metal of the coating is selected from lanthanum and rare earth elements. In this variant, the thermally decomposable compound can be any compound which, by heating in a controlled atmosphere, releases an oxide of the metal selected for the coating. It can be, for example, a nitrate, a sulfate, a phosphate, a chloride, a salt of carboxylic acid such as a formate, an acetate, a propionate or an oxalate. It can be used in the solid state, for example in the form of a powder, or in the liquid state, for example in the form of a molten salt, of a suspension or of a solution. The heating temperature and the controlled atmosphere must be chosen according to the metal selected and the thermally decomposable compound used. Under certain specific conditions (especially when the thermally decomposable compound is a nitrate or an oxalate), the heating can be carried out in an inert atmosphere (for example in a nitrogen or argon atmosphere). In practice, however, it is preferred to carry out the heating in an oxidizing atmosphere, generally in air, at a temperature below 1000 ° C, preferably not exceeding 850 ° C; temperatures between 100 and 800 ° C and more especially those not exceeding 750 ° C are preferred. Heating in a reducing atmosphere can generally be carried out in a hydrogen atmosphere, at a temperature not exceeding 600 ° C, usually between 200 and 500 ° C, depending on the metal selected for coating the electrode.

The electrodes obtained by the process according to the invention find applications in various electrochemical processes, such as, for example, cathodic protection, electrolysis and fuel cells. Their use is especially recommended as cathodes for the electrolytic production of hydrogen by electrochemical reduction of protons in aqueous alkaline medium. Such use is especially advantageous in electrolysis cells for the production of aqueous solutions of alkali metal hypochlorite, as well as in permeable diaphragm cells and with membrane with selective permeability for the electrolysis of aqueous chloride solutions. sodium, such as those described in patents FR-A 2164623, 2 223 083, 2230411, 2 248 335 and 2 387 897 (Solvay & Cie).

The advantage of the invention will become apparent from the description of the following application examples.

In each of the following examples, an aqueous brine containing 255 sodium chloride per kg was electrolysed in a laboratory cell with vertical electrodes, separated by a membrane with selective catalytic permeability Nafion (Du Pont of Ne- mours).

The cylindrical cell included an anode formed of a circular titanium plate, pierced with vertical slits and coated with an active material of mixed crystals, consisting of 50% by weight of ruthenium dioxide and 50% by weight of titanium dioxide.

The cathode consisted of a non-perforated disc whose constitution is defined in each example.

The overall surface of each electrode of the cell was 102 cm ² , and the distance between the anode and the cathode was fixed at 6 mm, the membrane being placed at equal distance from the anode and the cathode.

During the electrolysis, the anode chamber was continuously supplied with the abovementioned aqueous brine and the cathode chamber with a dilute aqueous solution of sodium hydroxide, the concentration of which was adjusted to maintain, in the catholyte, a concentration of about 32% by weight of sodium hydroxide. The temperature was continuously maintained at 90 ° C in the cell. In all the tests, the density of the electrolysis current was maintained at the fixed value of 3 kA per m ² of area of the cathode. This produced chlorine at the anode and hydrogen at the cathode.

First series of tests (in accordance with the invention) Example 1

(a) Composition of the cathode: An electrode obtained in the manner described below, in accordance with the invention, was used for the cathode.

A coating composition was prepared, by mixing the following constituents:

The nickel powder used in this coating composition had a particle size such that its specific surface was approximately equal to 0.6 _m 2 / g.

For the colloidal silica solution, a colloidal solution containing approximately 20% by weight of silica and 2.1% by weight of lithium oxide was used.

As a thickener, a polysaccharide was used.

On a nickel plate serving as a substrate, six consecutive layers of this coating composition were applied, the plate being subjected to drying for half an hour in an oven at 70 ° C. after the application of each layer. The thickness of the coating material thus formed on the substrate, after the application of the six layers, was approximately 100 microns and it weighed approximately 400 g per m ² of area.

The substrate and its coating were then heated in an oven at 750 ° C for 5 hours, in the presence of air, so as to oxidize practically all of the nickel in the coating. After being cooled, they were treated at 450 ° C for one hour in an oven swept by a stream of hydrogen, then cooled to room temperature, while maintaining the atmosphere of hydrogen in the oven.

(b) Electrolysis results: The electrode obtained at the end of the process which has just been described has been mounted as such as a cathode in the electrolysis cell. During electrolysis, the voltage across the cell was established at 3.29 V.

Example 2

• (a) Composition of the cathode: An electrode was produced by proceeding in the manner described in Example 1, except for the final cooling: this, which followed the treatment at 450 ° C. in an atmosphere of hydrogen, was carried out in air, so as to cause a partial re-oxidation of the nickel.
• (b) Electrolysis results: The voltage at the terminals of the electrolysis cell using the electrode thus obtained as a cathode has stabilized at 3.16 V. The potential of the cathode has also been measured by means of the Luggin capillary measurement method, connected to a KCI saturated calomel reference electrode (ECS) (Modern Electrochemistry, Bockris and Reddy, Ple- num Press, 1970, vol. 2, pages 890 and 891). The cathodic potential quickly stabilized at around -1.18 V.

Example 3

• (a) Constitution of the Cathode: To manufacture the cathode, the procedure was first carried out as described in Example 1. The electrode thus obtained underwent five consecutive coatings with an aqueous solution of a water-soluble lanthanum compound , so that it corresponds to a total weight of about 50 g of lanthanum per m ² of area of the electrode. At the end of each of the five coatings, the electrode was subjected to drying for half an hour in an oven at 70 ° C., then to an oxidative heating in an oven at 750 ° C. for 5 minutes in the presence of air and then to a reduction treatment for one hour in an oven at 450 ° C swept by a stream of hydrogen.
• (b) Electrolysis results: The voltage across the cell using this cathode was 3.19 V.

Example 4

• (a) Constitution of the cathode: The operations of the process described in Example 3 were repeated, with the following modifications:
  • - The solution of the water-soluble lanthanum compound has been replaced by an aqueous solution of a water-soluble molybdenum compound;
  • - The quantity of solution used in the five coatings has been adjusted so that there corresponds a supply, on the electrode, of approximately 150 g of molybdenum per m ² of area of the electrode;
  • - the oxidative heating was carried out at 400 ° C, for 1 hour.
• (b) Electrolysis results: The voltage across the electrolysis cell was fixed at 3.13V.

Example 5

• (a) Composition of the cathode: We started by manufacturing an electrode as described in Example 1. The electrode thus obtained was placed as a temporary cathode in the electrolysis cell described above, and we started electrolysis as described. As soon as the voltage at the terminals of the cell has stabilized, a solution of hexachloroplatinic acid is introduced into the catholyte, in a quantity adjusted so that there corresponds an electrolytic deposit of approximately 2 g of platinum per m ² d area of the temporary cathode.
• (b) Electrolysis results: after adding the hexachloroplatinic acid solution as described above, the voltage across the cell decreased and stabilized at around 3.13 V.

Second series of tests (reference tests) Example 6

In this test, the cathode of the cell consisted of a plate of sandblasted nickel as is the. During electrolysis, the voltage across the cell stabilized at 3.36 V.

Example 7

(a) Composition of the cathode: The coating composition described in Example 1 was used, which was applied in five successive layers on a nickel plate, the plate being subjected to drying by half × hr in an oven at 70 ° _C. after applying each layer. The thickness of the coating material thus formed on the nickel plate was about 100 microns and it weighed about 400 g per m ² of area.

The plate and its coating were then heated for 30 minutes in an oven at 750 ° C swept by a stream of hydrogen, so as to cause sintering of the nickel powder.

(b) Electrolysis results: The voltage across the cell during electrolysis was 3.33 V.

Example 8

• (a) Constitution of the cathode: The operation was carried out as for the electrode of example 2, but in addition carrying out a sintering of the nickel powder before heating in an oxidizing atmosphere. To carry out the sintering, the substrate and its coating were heated in an oven under a hydrogen atmosphere, at 750 ° C. for 30 minutes.
• (b) Electrolysis results: The potential of the cathode, measured as in Example 2, stabilized at -1.20 V.

A comparison of the electrolysis results obtained in the tests of Examples 1 - 5 with those of Examples 6 and 8 shows that the cathodes obtained by the method according to the invention generally allow a substantial voltage gain.

A comparison of Examples 2 and 8 further shows that the absence of sintering before heating in an oxidizing atmosphere does not harm the cathodic potential.

Claims (9)

1. Process for the manufacture of an electrode for electrochemical processes, according to which an electrically conductive substrate is coated with a material containing a powder of at least one metal which is active for electrochemical proton reduction and the said material is heated on the substrate, successively in an oxidizing atmosphere and then in a reducing atmosphere, characterized in that a material is employed in which the abovementioned active metal is in the form of an unsintered powder, associated with colloidal silica.

2. Process according to Claim 1, characterized in that use is made of a material in which the quantity of colloidal silica is between 0.8 and 4% by weight of active metal.

3. Process according to Claim 1 or 2, characterized in that a suspension of the active metal powder in a colloidal silica solution is used as the material.

4. Process according to Claim 3, characterized in that use is made of a suspension containing from 20 to 50% by weight of active metal in an aqueous solution of colloidal silica containing a weight of silica of between 0.8 and 4% by weight of active metal.

5. Process according to Claim 3 or4, characterized in that use is made of a colloidal silica solution which contains lithium ions in a quantity controlled so as to produce a molar S'0₂:Lio₂ ratio of between 4 and 10.

6. Process according to any one of claims 1 to 5, characterized in that use is made of a substrate made of a material chosen from cobalt, iron, manganese, nickel and alloys of these metals, and an active metal chosen from cobalt, iron, manganese and nickel.

7. Process according to any one of claims 1 to 6, characterized in that the material is heated at a temperature between 600 and 800°C in an oxidizing atmosphere and at a temperature between 300 and 500°C in a reducing atmosphere.

8. Process according to any one of Claims 1 to 7, characterized in that at the end of the heating in a reducing atmosphere, there is applied on the electrode a coating containing a metal chosen from chromium, molybdenum, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium, platinum, lanthanum and rare earth elements.

9. Process according to Claim 8, characterized in that, to apply the coating metal on the electrode, a layer of a heat-labile compound of the said metal is first deposited on it, then the said compound is subjected to a thermal decomposition treatment so as to liberate an oxide of the said metal and the oxide is then heated in a reducing atmosphere.