EP0113931B1 - Cathode for the electrolytic production of hydrogen, and its use - Google Patents

Abstract

Cathode for the electrolytic production of hydrogen, having an active surface which comprises a nickel substrate and a coating film of dendrites of nickel or cobalt. This cathode can be used in a cell for the electrolysis of sodium chloride brine.

Description

The invention relates to a method for the electrolytic production of hydrogen on a cathode, in particular in an alkaline solution.

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.

Various solutions have been proposed to reduce the overvoltage when hydrogen is released from the cathodes.

Thus, in US-A-4,105,516 (PPG INDUSTRIES, INC.), It is proposed to add a compound of a transition metal to the electrolyte in contact with a mild steel cathode, for example chloride nickel or cobalt. This known process results in an appreciable drop in the electrolysis voltage. On the other hand, it retains the disadvantage of using steel cathodes which are the seat of progressive corrosion during electrolysis.

According to patent BE-A-864 880 (OLIN CORPORATION), metal ions with low hydrogen overvoltage are introduced into the catholyte and the plating of these ions, in the metallic state in situ on the cathode, is carried out for 1 'electrolysis. In this known method, any metal ion with low hydrogen overvoltage can be used and the cathode can be made of copper, steel or any other suitable material; copper cathodes are however especially recommended, in association with metal plating ions selected from iron, nickel, chromium, molybdenum and vanadium. Copper cathodes used in accordance with the preferred embodiment of this known method, however, also have the disadvantage of undergoing progressive corrosion during electrolysis. In addition, the overvoltage on hydrogen evolution on copper cathodes is generally high and experience has shown that, despite the gain achieved on the overvoltage by the addition of plating ions to the electrolysis bath, the overall voltage remained abnormally high.

In patent application EP-A-35 837 (EI DU PONT DE NEMOURS AND COMPANY), an electrolysis process is described in which a cathode is used comprising a coating layer of alpha iron on a conductive steel substrate. soft, possibly coated with a layer of nickel. This known method has the disadvantage of being ill-suited to the electrolysis of aqueous solutions of sodium chloride in cells with a membrane with selective permeability, since the alpha iron coating of the cathode undergoes rapid corrosion there in contact with the catholytes to high content of sodium hydroxide. It follows that in practice, this known method allows only a small gain on the electrolysis voltage, at the cost of a significant consumption of alpha iron which risks contaminating the catholyte.

Also known are electrodes intended for electrochemical oxidation processes, the active surface of which has a dendritic structure. This is how, in document DE-A-2037968, an electrolysis cell is described, a positive electrode of which comprises nickel dendrites. Document FR-A-1427244 describes a porous electrode comprising, on a nickel substrate, a dendritic coating of active metal, this electrode being intended to be the seat of an electrochemical oxidation reaction in a fuel cell .

The invention aims to provide a method for the electrolytic production of hydrogen, which allows a gain on the electrolysis voltage, significantly greater than the gains which can be obtained with the cathodes and the known methods described above, and which does not have the disadvantages.

The invention therefore relates to a process for the electrolytic production of hydrogen, on a cathode, according to which an electrode is used for the cathode, the active surface of which comprises a nickel substrate and a coating layer of nickel dendrites. or cobalt.

In the method according to the invention, the dendrites of the coating layer of the cathode are single crystals of small dimensions, having a branched structure, very aerated, resulting from the interruption of the growth of crystalline germs (A. DE SY and J. VIDTS, "'Treaty of structural metallurgy", 1962, NICl and DUNOD, pages 38 and 39).

The nickel substrate can have any shape suitable for the destination of the cathode. It can be, for example, a solid or perforated plate, a wire, a trellis or a stack of beads. It can have a smooth surface state; a rough surface finish is preferred, however, since it generally lends itself to better adhesion of the dendrite layer. Although it may be formed from a unitary nickel block, the nickel substrate is preferably constituted by a nickel film applied to a support made of a material which is better conductive of electricity than nickel, for example copper or in aluminium. In this embodiment of the invention, the nickel film must be impermeable to electrolytes, when the material used for the underlying support is likely to be degraded on contact with these electrolytes. In the case of a support made of a material that is inert with respect to these electrolytes, the nickel film can be either impermeable or permeable, an impermeable film being however preferable in all cases. The thickness to be given to the nickel film depends on various parameters, and in particular on the nature and the surface condition of the underlying support, and it must be at least sufficient to resist being torn off under the effect of thermal expansion of the support or by erosion on contact with the electrolyte. In practice, in the case where the support is made of copper, good results have been obtained with nickel films with a thickness of between 5 and 100 microns, more especially between 10 and 75 microns.

It is desirable that the dendrite coating layer be substantially uniform on the nickel substrate and in an amount at least equal to 0.0005 g per dm ² of area of the substrate. The maximum admissible value for the thickness of the layer of dendrites depends on various factors and it is fixed in particular by the advantage of preserving a homogeneous active surface on the electrode and of avoiding a modification of the geometry of the cathode. A layer of dendrites of exaggerated thickness may indeed be torn locally from the substrate under the action of turbulence created by the release of hydrogen; in the case of openwork cathodes, it also risks causing poorly controlled obstruction of the cathode openings. For these reasons, it is desirable that the coating layer of dendrites does not exceed 25 g and preferably 15 g per dm ² of area of the substrate. Cathodes which have been found to be particularly advantageous are those in which the dendrite coating layer has a weight of between 0.001 and 10 g per dm ² of area of the substrate, the values of between 0.002 and 5 g and especially those at least equal. at 1 g per dm ² of substrate area generally leading to the best results.

In the method according to the invention, the dendrite coating layer of the cathode can be produced by any suitable means. In a preferred embodiment of the invention, a cathode is used whose coating layer of dendrites is an electrolytic deposit of nickel or cobalt, which has been produced in an electrolyte containing nickel or cobalt ions, where the cathode is the seat of a proton reduction. The electrolyte is preferably an aqueous electrolyte, more particularly water, or an aqueous solution of alkali metal chloride or hydroxide, containing nickel or cobalt ions. Good results have been obtained with aqueous solutions of alkali metal hydroxide, in particular sodium hydroxide, containing from 20 to 35% by weight of alkali metal hydroxide and, preferably, approximately 30% by weight of alkali metal hydroxide. The cathode is brought to an adequate potential to be the seat of a reduction of protons. The choice of cathode potential which should be imposed on the cathode depends on various parameters and in particular on the nature of the nickel layer (in particular its surface state, the state of its crystal lattice, the possible presence of impurities and, where appropriate, its porosity), the choice of electrolyte used and its concentration. It can be determined in each particular case by routine laboratory work. By way of example, in the case where the alkaline solution used is an aqueous solution containing approximately 30% by weight of sodium hydroxide, the cathodic potential must be fixed between -1.30 and -2 V, the most often between -1.55 and -1.65 V compared to a reference calomel electrode, with saturated potassium chloride solution. The quantity of nickel or cobalt ions to be used in the electrolyte depends on various parameters, in particular on the geometry of the cathode, the thickness or the weight desired for the coating layer of dendrites, the surface area nickel substrate, the nature of the electrolyte and its volume. As a general rule, it can be easily determined in each particular case by routine laboratory work. The nickel or cobalt ions can be introduced into the electrolyte all at once or either continuously or intermittently. They can be introduced into the electrolyte by any suitable means, for example by dissolution of a soluble nickel or cobalt compound, such as nickel or cobalt chloride, or by controlled corrosion of a structure (for example a wire, plate or lattice) made of nickel, cobalt or an alloy or compound of these metals, brought to an anode potential regulated in the electrolyte. An interesting means consists in dispersing in the electrolyte a powder of nickel or cobalt, or of a compound or alloy of these metals, the oxides being preferred. In this embodiment of the invention, it is desirable to use a powder as fine as possible. Generally, powders are used in which the average particle diameter is less than 50 microns and preferably does not exceed 35 microns. Powders which are generally well suited are those in which the average particle diameter is between 1 and 32 microns, the best results having been obtained with powders whose average particle diameter is less than 25 microns.

In a particular embodiment of the invention, a cathode is used, the active surface of which comprises, between the nickel substrate and the dendrite coating layer, a porous intermediate layer, intended to reinforce the attachment of the dendrites to the substrate or to improve the electrochemical properties of the cathode. The porous intermediate layer is advantageously made of an electrically conductive material, having good electrochemical properties, this material possibly being, for example, a platinum group metal or an oxidized metallic compound of the spinel type, such as those described in Patent EP-A-8476 (SOLVAY & Cie). Preferably, the porous intermediate layer is made of platinum or is obtained by spraying a nickel oxide powder in a plasma jet.

The cathode used in the method according to the invention can be prefabricated. However, in a preferred embodiment of the invention, the dendrite coating layer is formed in situ on the cathode mounted in the electrolysis cell for which it is intended. For this purpose, there is in the cell, the cathode provided with the nickel substrate and possibly with an intermediate layer. It may also be necessary to periodically regenerate the dendrite coating layer to account for a progressive destruction thereof, for example under the effect of erosion caused by the alkaline solution or the hydrogen gas produced. For this purpose, it suffices to add nickel or cobalt ions to the electrolyte at the appropriate time, each addition being able to be carried out during a temporary stoppage of the electrolysis or while maintaining the latter in activity. The frequency and extent of the regenerations depend on the speed at which the dendrite coating layer is eroded or torn from the cathode; this speed itself depends on a large number of parameters, including in particular the nature of the nickel substrate, the possible presence of a porous intermediate layer between the substrate and the dendrite coating layer, the turbulence and the viscosity of the alkaline solution and the flow of hydrogen produced. The frequency and extent of the regenerations must therefore be determined in each particular case, which can be easily done by routine laboratory work. As a variant, it is also possible to carry out a continuous addition of nickel or cobalt ions to the electrolyte throughout the period during which the cathode is in service.

The process according to the invention finds a particularly advantageous application for the electrolytic production of hydrogen in an alkaline solution, in particular in cells with a permeable diaphragm or a membrane with selective permeability for the electrolysis of sodium chloride brines, such as those described, by way of example, in patents FR-A-2,164,623, 2,223,083, 2,230,411, 2,248,335 and 2,387,897 (SOLVAY & Cie). It has been found that the use, in accordance with the invention, of a cathode associating a nickel substrate and a coating layer of dendrites of nickel or cobalt made it possible, all other things remaining equal, to achieve a significant gain on the electrolysis voltage not only compared to a process using the same cathode whose active layer consists of the nickel substrate alone, without the dendrite coating layer, but also compared to the processes using cathodes with nickel substrates carrying a porous active coating made of a material with a hydrogen overvoltage lower than cobalt or nickel, such as, for example, a porous platinum coating or a porous coating obtained by spraying a nickel oxide powder into a plasma jet.

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 g of sodium chloride per kg was electrolysed in a laboratory cell with vertical electrodes separated by a membrane with selective cation permeability NAFION NX 90107 (FROM THE PONT DE NEMOURS).

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 above-mentioned 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

In the test which will be described, there was used a cathode according to the invention, the active surface of which consisted of a nickel substrate and a coating layer of nickel dendrites. For this purpose, we first placed in the cell, a temporary cathode formed of a nickel disc; to form the layer of nickel dendrites on the disk used as substrate, the anode chamber and the cathode chamber were supplied respectively with the aqueous solution of sodium chloride and the dilute solution of sodium hydroxide and the electrolysis was started with the nickel disc serving as cathode, under the nominal current density of 3 kA / m ^2. The electrolysis voltage, measured between the anode and the cathode, stabilized at 3.65 V. We then added a nickel chloride solution to the catholyte, in a quantity adjusted so that it corresponds to an addition of 2 g of nickel. The electrolysis voltage fell to 3.43 V, following the formation of the layer of nickel dendrites. The gain compared to the original voltage, before the addition of nickel chloride, is therefore 220 mV.

Example 2

The procedure was as in Example 1, using an aqueous solution of nickel sulfocyanide in place of the nickel chloride solution. At the start of the cell, before the addition of the nickel sulfocyanide solution, the electrolysis voltage stabilized at 3.63 V. After the addition of the nickel sulfocyanide solution and the subsequent formation of the layer of nickel dendrites on the nickel substrate of the cathode, the electrolysis voltage fell to 3.38 V, which corresponds to a gain of 250 mV compared to the initial voltage.

Example 3

In this test, a cathode according to the invention was used, the active surface of which consisted of a nickel substrate and a coating layer of cobalt dendrites. For this purpose, the procedure was as in Example 1, except that the aqueous solution of nickel chloride was replaced by an aqueous solution of cobalt acetate, in a quantity adjusted so that it corresponded to a addition of 1 g of cobalt.

At the start of the cell using the nickel disc as a temporary cathode, the electrolysis voltage was fixed at 3.70 V. After the formation of a coating layer of cobalt dendrites on the nickel disc, consecutively on addition of the cobalt acetate solution to the catholyte, the electrolysis voltage fell to 3.46 V, which corresponds to a voltage gain of 240 mV.

Example 4

The procedure was as in Example 3, the only differences being that the cobalt acetate solution was replaced by an aqueous cobalt chloride solution and that the latter was added to the catholyte in a controlled amount. so that it corresponds to an addition of 2 mg of cobalt. At the start of the cell with the temporary cathode, the electrolysis voltage was established at 3.67 V. After the addition of the cobalt chloride solution, the electrolysis voltage dropped to 3.58 V, which corresponds to a gain of 90 mV on the original voltage.

Example 5

The test of Example 4 was continued, with an additional addition of cobalt chloride solution in a quantity adjusted so that it corresponds to an additional addition of 2 mg of cobalt. The electrolysis voltage fell to 3.46 V, resulting in a total gain of 210 mV compared to the original voltage.

Example 6

The procedure was as in Example 3, but substituting a cobalt oxide powder for the cobalt acetate solution. Cobalt oxide powder had an average particle diameter of less than 20 microns.

When the cell started with the temporary cathode, the electrolysis voltage was established at 3.68 V. The cobalt oxide powder was then dispersed in the catholyte, in two fractions of equal weight each corresponding to 1 g of cobalt. The electrolysis voltage went successively to 3.44 V and then to 3.36 V, thus resulting in a gain of 320 mV compared to the original voltage.

Example 7

In this test, a cathode according to the invention was used, the active surface of which consisted of a nickel substrate and a coating layer of nickel dendrites. To make the cathode, a provisional cathode consisting of a mild steel disc carrying an impermeable coating of 30 microns of nickel, obtained by electrolytic deposition, was first placed in the cell. This coating is intended to constitute the above-mentioned substrate. . A layer of nickel dendrites was then deposited on the substrate and, for this purpose, a nickel oxide powder was dispersed in the catholyte, in a quantity adjusted so that it corresponds to 4 g of nickel. The particle size of the nickel oxide powder was characterized by an average particle diameter of less than 20 microns; it was added to the catholyte in four successive fractions of equal weight. The electrolysis conditions are given in table 1. The total gain on the electrolysis voltage is approximately 300 mV.

Example 8

• - deux fractions d'une poudre d'oxyde de nickel ayant un diamètre moyen de particules inférieur à 20 microns, chaque fraction étant réglée pour qu'il y corresponde une addition de 1 g de nickel;
• - deux fractions d'une poudre d'oxyde de cobalt ayant un diamètre moyen de particules compris entre 2 et 32 microns, chaque fraction étant réglée pour qu'il y corresponde une addition de 1 g de cobalt.

The procedure was as in the test of Example 7, using, for the temporary cathode, a copper disc covered with a film of 16 to 64 microns of nickel, applied by spraying a nickel powder into a plasma jet. When the cell started with this temporary cathode, the electrolysis voltage was established at 3.50 V. An electrolytic deposition of a porous layer of platinum was then carried out on the substrate. To this end, while keeping the cell in activity, three consecutive additions of a hexachloroplatinic acid solution were carried out, the three additions being adjusted so that they correspond respectively to 2, 3 and 20 mg of platinum. After formation of the porous platinum layer, the electrolysis voltage fell to 3.28 V. The catholyte was then added in succession:

• - two fractions of a nickel oxide powder having an average particle diameter of less than 20 microns, each fraction being adjusted so that there corresponds an addition of 1 g of nickel;
• - two fractions of a cobalt oxide powder having an average particle diameter of between 2 and 32 microns, each fraction being adjusted so that it corresponds to an addition of 1 g of cobalt.

On a répertorié au Tableau III ci-dessous les résultats obtenus dans chacun des essais précédents.

Seconde série d'essais (essais de comparaison)The conditions of the electrolysis are recorded in Table II below. It is observed there that a first gain on the electrolysis voltage, relative to its value at the start of the cell, was realized after the formation of the platinum coating and that a second gain was still realized after the deposition of 'a layer of nickel and cobalt dendrites resulting from the addition of nickel and cobalt powders.

The results obtained in each of the preceding tests are listed in Table III below.

Second series of tests (comparison tests)

Example 9

In this example, the procedure was described in patent EP-A-35,837 cited above. To this end, a cathode consisting of a solid mild steel disc was mounted in the cell and electrolysis was started under the same conditions as in the previous tests. The electrolysis voltage was established at 3.64 V. 2 g of alpha iron were then added to the catholyte. The electrolysis voltage remained unchanged.

Example 10

In this test, the procedure was as described in patent BE-A-864,880 cited above. For this purpose, a cathode formed from a solid copper disc was used in the cell and electrolysis was started. The electrolysis voltage was established at 4 V. A nickel oxide powder was then dispersed in the catholyte in a quantity adjusted so that it corresponds to a weight of 2 g of nickel. The nickel oxide powder had a particle size characterized by an average particle diameter of less than 20 microns. It was scattered in the catholyte in two equal weight fractions. After the addition of the nickel oxide powder, the electrolysis voltage dropped to 3.80 V.

Example 11

In this test, the procedure was as described in US Pat. No. 4,105,516 cited above. For this purpose, a mild steel disc was used for the cathode and electrolysis was started. The electrolysis voltage was established at approximately 3.91 V. A nickel oxide powder was then dispersed in the catholyte in a quantity adjusted so that it corresponds to a weight of 2 g of nickel. The average grain diameter of the powder was less than 20 microns. The powder was added to the catholyte in two separate fractions, of equal weight, which resulted in the electrolysis voltage falling to 3.78 V.

A comparison of the electrolysis voltages reached in the tests of Examples 1 to 8, according to the invention, with those reached in the tests of Examples 9, 10 and 11 immediately shows the advantage of the invention.

Claims (10)

1. Process for the electrolytic production of hydrogen on a cathode, characterized in that an electrode whose active surface comprises a nickel substrate and a coating layer of dendrites of nickel or cobalt is used as the cathode.

2. Process according to Claim 1, characterized in that the dendrite coating layer of the electrode used has a weight of between 0.002 and 5 g per dm³ of substrate.

3. Process according to Claim 1 or 2, characterized in that the nickel substrate of the electrode used is an impervious nickel film on a support made of an electrically conductive material.

4. Process according to any one of Claims 1 to 3, characterized in that the electrode used comprises, between the nickel substrate and the dendrite coating layer, a porous intermediate layer made of an electrically conductive material.

5. Process according to Claim 4, characterized in that the porous intermediate layer of the electrode used is obtained by spattering a nickel oxide powder in a plasma jet onto the substrate.

6. Process according to any one of Claims 1 to 5, characterized in that the dendrite coating layer of the electrode used is an electrolytic deposit of nickel or cobalt produced in an electrolyte containing nickel ions or cobalt ions, in which the electrode is the seat of electrolytic proton reduction.

7. Process according to Claim 6, characterized in that the dendrite coating layer of the electrode used is an electrolytic deposit produced in an aqueous solution of an alkali metal hydroxide.

8. Process according to Claim 6 or 7, characterized in that the dendrite coating layer of the electrode used is an electrolytic deposit produced from nickel ions or cobalt ions, introduced in the form of a powder of nickel oxide or cobalt oxide.

9. Process according to Claim 6 or 7, characterized in that the dendrite coating layer of the electrode used is an electrolytic deposit produced from nickel ions or cobalt ions, introduced in the form of an aqueous solution of nickel chloride or cobalt chloride.

10. Process according to any one of Claims 1 to 9, characterized in that it is used in a cell for the electrolysis of sodium chloride brine.