EP1125005B1 - Cathode for electrolysing aqueous solutions - Google Patents

Abstract

A cathode comprising an electrically conductive substrate made of titanium, nickel, tantalum, zirconium, niobium, iron or alloys thereof, coated with an intermediate layer of oxides of titanium and of a precious metal, and with an outer layer comprising metal oxides of titanium, zirconium and a precious metal, can be used for the electrolysis of solutions, in particular for the electrolysis of aqueous solutions of alkali metal chlorides.

Description

The present invention relates to a cathode which can be used for electrolysis of aqueous solutions in which a reaction occurs water reduction.

More particularly, the present invention relates to a cathode activated for the electrolysis of alkaline aqueous solutions of chlorides of alkali metals, and especially for the preparation chlorine and sodium hydroxide.

Thus, industrially, chlorine and sodium hydroxide are manufactured in electrolytic cells, each of them comprising several mild steel cathodes and several coated titanium anodes of a mixture of titanium oxides and ruthenium. They are usually fed with electrolytic solution of about 200 to 300 g / l sodium chloride.

However, these mild steel cathodes have a surge relatively high in absolute value as reduction cathodes of water and also have resistance to corrosion by chlorine dissolved insufficient.

By overvoltage is meant the difference between the thermodynamic potential of the redox couple concerned (H ₂ O / H ₂ ) with respect to a reference cathode and the potential actually measured in the medium concerned, with respect to the same reference electrode. By convention we use the term overvoltage to designate the absolute value of the cathode overvoltage.

In order to overcome these disadvantages, it has been proposed to many cathodes.

Thus, in the French patent application FR2311108, describes a cathode whose substrate is a plate made of titanium, zirconium, niobium or alloy essentially consisting of an association of these metals and on which a layer of metal oxide is applied, essentially consisting of an oxide of one or more selected metals among ruthenium, rhodium, palladium, osmium, iridium and platinum and optionally an oxide of one or more metals selected from calcium, magnesium, strontium, barium, zinc, chromium, molybdenum, tungsten, selenium and tellurium.

US Pat. No. 4,100,049 discloses a cathode comprising a substrate of iron, nickel, cobalt or an alloy of these metals and a coating of palladium oxide and zirconium oxide.

In the European patent application EP 209427, it is proposed to cathode consisting of an electrically conductive nickel substrate, stainless steel or mild steel with a coating consisting of plurality of metal oxide layers, the surface layer being constituted by a valve metal oxide, that is to say a metal chosen from groups 4b, 5b and 6b of the Periodic Table of Elements and the intermediate layer consisting of a precious metal oxide of the group VIII, i.e., ruthenium, rhodium, palladium, osmium, iridium and platinum.

The intermediate and superficial layers can be formed by the oxide of the only metal concerned or by a mixed oxide of metal concerned and the second metal in a small proportion.

Although these cathodes have a satisfactory overvoltage, the plaintiff has found during the evaluation of the said cathodes a modification of the polarization curve after the first scanning evidence of damage to the superficial layer, which is detrimental to a good protection of the substrate and therefore leads to limited life of said electrodes.

We have now found a cathode to reduce the overvoltage of the water reduction reaction in an alkaline medium, characterized in that it is constituted by an electroconductive substrate coated with an intermediate layer of oxides based on titanium and a metal Group VIII of the Periodic Table of Elements and an outer layer of metal oxides comprising titanium, zirconium and a precious metal of group VIII of the classification periodic element, the precious metal in the outer layer representing a molar amount of between 30% and 50% relative to the metals used in the composition of this layer.

By precious metal of group VIII of the Periodic Table elements, ruthenium, rhodium, palladium, osmium, iridium or platinum. Preferably, we will use the ruthenium or iridium and especially ruthenium.

Advantageously, the intermediate layer contains oxides of titanium and ruthenium.

Preferably the outer layer of metal oxides contains oxides of titanium, zirconium and ruthenium.

More preferably, the outer layer consists essentially of ZrTiO ₄ accompanied by RuO ₂ and optionally ZrO ₂ and / or TiO ₂ .

The material constituting the substrate may be chosen from electrically conductive materials. We will choose it advantageously in the group consisting of titanium, nickel, tantalum, zirconium, nobium, iron and their alloys.

Preferably, titanium, nickel, iron or their alloys.

The molar ratio of precious metal / titanium in the layer intermediate is preferably between 0.4 and 2.4.

The zirconium / titanium molar ratio in the outer layer is generally between 0.25 and 9 and preferably between 0.5 and 2.

The precious metal in the outer layer is between 30% and 50% molar with respect to metals used in the composition of this layer.

a) prétraitement d'un substrat pour conférer des caractéristiques de rugosité à la surface,

b) revêtement du substrat prétraité à l'aide d'une solution A contenant essentiellement du titane et un métal précieux, suivi de séchage, puis calcination du substrat ainsi revêtu,

c) revêtement du substrat obtenu en (b) à l'aide d'une solution B comprenant du titane, du zirconium et un métal précieux, suivi de séchage, et de la calcination du substrat ainsi revêtu.

The cathode according to the present invention can be prepared according to a method which consists in performing the following steps:

a) pretreatment of a substrate to impart roughness characteristics to the surface,

b) coating the pretreated substrate with a solution A containing essentially titanium and a precious metal, followed by drying and then calcining the substrate thus coated,

c) coating the substrate obtained in (b) with a solution B comprising titanium, zirconium and a precious metal, followed by drying, and calcining the substrate thus coated.

Pretreatment generally involves subjecting the substrate, either to sandblasting followed by acid washing or stripping with an aqueous solution of oxalic acid, hydrofluoric acid, a mixture of hydrofluoric acid and nitric acid, a mixture of hydrofluoric acid and glycerol, a mixture of hydrofluoric acid, nitric acid and glycerol or a mixture of hydrofluoric acid, nitric acid and hydrogen peroxide, followed by one or more washing (s) with degassed demineralized water.

The substrate can be in the form of solid plate, plate perforated, expanded metal or cathode basket made from metal deployed or perforated.

Solution A is usually prepared by contacting at at room temperature and with stirring, essentially a mineral salt or organic titanium and a precious metal with water or in a organic solvent, optionally in the presence of a chelating agent. The temperature can be raised above the ambient to facilitate the dissolution of salts.

Advantageously, a mineral or organic salt is brought into contact titanium and a precious metal with water or in a solvent organic, optionally in the presence of a chelating agent.

Titanium and the precious metal are preferably present in the solution A at a concentration at most equal to 10 mol / l.

Solution B is usually prepared by contacting, at at room temperature and with stirring, a mineral or organic salt of titanium, zirconium and a precious metal with water or in a organic solvent, optionally in the presence of a chelating agent. When the contacting is exothermic, an ice bath is used to cool the reaction medium.

Advantageously, a mineral or organic salt is brought into contact of titanium, zirconium and a precious metal with water or in a organic solvent, optionally in the presence of a chelating agent.

The titanium salts and preferred precious metals are the chlorides, oxychlorides, nitrates, oxynitrates, sulphates and alkoxides. Advantageously, the chlorides of the precious metals, the ruthenium chlorides, titanium chlorides and titanium oxychlorides are used.

As zirconium salts, it is possible to use the chlorides, sulfates, zirconyl chloride, zirconyl nitrate, alkoxides such as than butyl zirconate.

Zirconium and zirconyl chlorides are particularly preferred.

As an organic solvent, mention may be made of the light alcohols of preferably isopropanol and ethanol, and more preferably isopropanol and absolute ethanol.

Although it is possible to use either water or a solvent organic preparation to prepare solution B, it is however preferred to use a organic solvent when the metal salts are solid at temperature room.

Thus when the metal salt is zirconium chloride, solvent is absolute ethanol or absolute isopropanol.

Titanium and zirconium are usually present in the solution B at a concentration ranging from 0.5 to 5 mol / l. The concentration of metal valuable in solution B is usually between 0.05 and 10 mol / l and preferably between 0.1 and 5 mol / l.

Solution A can be deposited on the pretreated substrate using different techniques such as sol-gel, electrochemical deposition, electroplating, spraying or coating. advantageously the pretreated substrate is coated with solution A, for example with the aid of a brush. The substrate thus coated is then dried in air and / or in a oven at a temperature below 150 ° C. After drying, the substrate is calcined under air or under inert gases such as nitrogen, argon or still under inert gases enriched with oxygen at a temperature at less than 300 ° C and preferably between 450 ° C and 550 ° C for a period ranging from 10 minutes to 2 hours.

For step (c) of the process, the same techniques can be used deposition and the same operating conditions for drying and calcination as step (b) except that the deposition is carried out with solution B.

Other techniques such as chemical vapor deposition (CVD), physical vapor deposition (PVD), plasma projection, are also suitable for coating the pretreated substrate with intermediate layer and an outer layer.

Solution A can be deposited on one of the faces of substrate pretreated only on both sides. We can also drop the solution B on both sides of the substrate coated with the layer intermediate.

Depending on the thickness of the desired intermediate layer, repeat step (b) several times. Similarly, we can repeat several times step (c) of the process.

In the intermediate layer, the mass of deposited product is at least equal to 2 g / m ² , generally between 10 g / m ² and 60 g / m ² and, preferably, between 20 g / m ² and 35 g. / m ² related to the geometrical surface of the substrate.

The concentration of solution A is judiciously chosen from so that this preferred deposited mass can be obtained in repeating step (b) in a reasonable number of times and preferably between 1 and 10 times.

In the outer layer, the mass of deposited product is at least equal to 5 g / m ² , generally between 5 g / m ² and 70 g / m ² and, preferably, between 25 g / m ² and 50 g. / m ² , related to the geometrical surface of the substrate. In general, solution B is prepared so that its concentration makes it possible to obtain a preferred deposited mass by repeating at least 1 time step (c) and preferably between 2 and 10 times.

The cathode of the present invention is particularly suitable electrolysis of aqueous solutions of alkali metal chlorides and especially aqueous solutions of NaCl.

The use of the cathode in combination with an anode allows to electrolytically synthesize the chlorine and hydroxide of a metal alkaline with a high Faraday yield.

As anode, the DSA anodes (Dimensionally Stable Anode) consisting of a titanium substrate coated with an oxide layer titanium and ruthenium. The molar ratio ruthenium / titanium in this layer is advantageously between 0.4 and 2.4.

The cathode of the present invention has the advantage of having a overvoltage lower than the cathodes of the prior art during a operation in electrolysis.

In addition, the cathode of the present invention does not suffer from modification in the first characterization cycles and presents a greater chemical stability against aggressive alkaline media.

The following examples illustrate the invention without limiting it.

EXAMPLES 1. PREPARATION OF A CATHODE (according to the invention) 1.1 PRETREATMENT AND DEPOSITION OF THE INTERMEDIATE LAYER

We sand with particles of corundum a titanium plate thickness of 2 mm and dimensions 4 cm x 1 cm, to which welded a round rod of current supply.

A solution A, containing ruthenium and titanium in an equimolar quantity, is then prepared by mixing at room temperature with stirring 2.45 g of RuCl ₃ , of purity greater than or equal to 98%, 3.64 cm ³ of TiOCl ₂ , 2HCl at 127 g / l in Ti and 2.5 cm ³ of absolute isopropanol.

The end of one of the faces of the plate is then coated pretreated, representing a surface of 1 cm x 4 cm, with solution A to with a brush, then let it dry for 30 minutes in the open air and ambient temperature. The coated plate is then dried additionally in an oven at 120 ° C for 30 minutes, then calcined in an oven under air at 500 ° C for 30 min.

These operations (coating, drying and calcination) are repeated two more times and at the end of these three coatings the mass of Ru and Ti oxides deposited is equal to 18 g / m ^{2, based} on the geometric surface of the plate.

1.2 DEPOSIT OF THE EXTERNAL LAYER General procedure

Chloride or oxychloride is mixed with stirring. zirconium, ruthenium chloride and chloride or oxychloride titanium with absolute ethanol. In the case of chlorides, solution B is prepared cold and kept cold by a water / ice bath, under stirring until used.

In the case of oxychlorides, solution B is prepared at 60 ° C. and maintained at this temperature, with stirring, until use.

The plate coated with 1.1 is then coated with solution B at using a paintbrush. The coated plate is initially dried 30 minutes in the open air and at room temperature, then in a second time dried additionally in an oven at 120 ° C for 30 minutes, and finally calcined in an oven under air at 500 ° C for 30 minutes.

These operations (coating, drying and calcination) are repeated several times until a mass of deposited oxides ranging from 30 g / m ² to 45 g / m ² relative to the geometric surface of the plate is obtained.

2. EVALUATION OF THE CATHODE - PROCEDURE:

The performances of the cathode, vis-à-vis the reduction of the water, are evaluated from a polarization curve, carried out in a 1M sodium hydroxide solution and at a temperature of between 20 ° C and 25 ° C (room temperature).

By polarization curve is meant the curve of variation of the measured cathodic potential relative to a reference electrode, by example a saturated calomel electrode (ECS), depending on the density current.

The experimental setup consists of the cathode to be evaluated, a platinum counter-electrode (surface 5 cm ² ) and an ECS reference electrode elongated with a capillary, which is placed in close proximity to the cathode.

The whole is immersed in the electrolytic solution (1M NaOH) stirred by magnetic stirring.

The three electrodes are connected to the terminals of a potentiostat. The potential of the cathode is imposed by the apparatus and one raises after the equilibrium of the system the value of the current passing through said system.

This potential is varied from -0 mV / ECS to -1500 mV / ECS.

EXAMPLE 1 (ACCORDING TO THE INVENTION)

Solution B is prepared by mixing with stirring in a glass flask 1.07 g of RuCl ₃ , 2.59 g of ZrOCl ₂ , 8H ₂ O, 1.55 ml of TiOCl ₂ , 2HCl in 7 ml of absolute ethanol. or a total molar composition of 0.3 Ru-0.7 (Ti, 2Zr).

The coated plate of the intermediate layer is then coated with solution B thus prepared, and then dried and calcined in air as indicated in the general procedure. These operations are repeated 8 times and at the end of the last calcination, the deposited mass is 39 g / m ² relative to the geometric surface of the plate.

The cathode thus prepared was evaluated using the procedure described previously. The cathodic potential is -1.375 V / ECS for a current density of -2 kA / m ² .

By way of comparison, the cathodic potential of a cathode of nickel is -1.475 V / ECS under the same conditions.

EXAMPLE 2 (ACCORDING TO THE INVENTION)

Solution B is prepared by mixing with stirring in a glass flask 2.49 g RuCl ₃ , 2.59 g of ZrOCl ₂ , 8H ₂ O, 1.55 ml of TiOCl ₂ , 2HCl in 10 ml of absolute ethanol is an overall molar composition of 0.5 Ru-0.5 (Ti, 2Zr).

The coated plate of the intermediate layer is then coated with solution B thus prepared, and then dried and calcined in air as indicated in the general procedure. These operations are repeated 8 times and at the end of the last calcination, the external deposited mass is 41 g / m ² relative to the geometric surface of the plate.

The cathode thus prepared was evaluated using the procedure described previously. The cathode potential is -1.195 V / ECS for a current density of -2 kA / m ² .

EXAMPLE 3 (ACCORDING TO THE INVENTION)

Solution B is prepared by mixing with stirring in a glass vessel, cooled with an ice bath, 2.49 g of RuCl ₃ , 2.80 g of ZrCl ₄ , 1.32 ml of TiCl _4. in 10 ml of absolute ethanol, ie a total molar composition of 0.5 Ru-0.5 (Ti, Zr).

The coated plate of the intermediate layer is then coated with solution B thus prepared, and then dried and calcined in air as indicated in the general procedure. These operations are repeated 8 times and at the end of the last calcination, the deposited mass is 45 g / m ² of the plate, relative to the geometric surface of the plate. The cathode thus prepared was evaluated using the procedure described previously. The cathodic potential is -1.190 V / ECS for a current density of -2 kA / m ² in a 1M NaOH solution.

EXAMPLE 4 (NOT COMPLYING WITH THE INVENTION)

A cathode is prepared according to the patent application EP 209 427 and we evaluate it.

The substrate consists of a plate of 4 x 1 x 0.2 cm, which has been welded a round rod of current supply. We perform a surface treatment with corundum.

A solution of 2 g of RuCl ₃ in 2 ml of ethanol is prepared at ambient temperature. The control plate is coated with this solution. Then, the plate is dried in air at 120 ° C for 30 minutes, followed by calcination in air (500 ° C, 30 minutes). A deposit of 16 mg / m ² of RuO ₂ is obtained.

A solution in 2 cm3 of ethanol, 2.6 ml of TiOCl ₂ , HCl at 2.5 mol / l of Ti is prepared at ambient temperature. The same treatments are applied coating / steaming / calcination under air. 8.5 g / m ² of TiO _{2 are} thus deposited.

The cathode potential of this electrode is -1.240 V / ECS for a current density of -2 kA / m ² evaluated according to the procedure described above.

Although this potential is satisfactory, there is, however, a important modification of the polarization curve after the first sweeping and the appearance of solid particles in the solution, which is characteristic of a modification of the surface layer and its damage, which is prohibitive for the long-term use of this cathode.

Claims (14)

1. Cathode for the electrolysis of aqueous solutions, consisting of an electrically conductive substrate coated with an intermediate layer of oxides based on titanium and on a precious metal from group VIII of the Periodic Table of the Elements and with an outer layer of metal oxides comprising titanium, zirconium and a precious metal from group VIII of the Periodic Table of the Elements, characterized in that the precious metal in the outer layer represents a molar amount of between 30% and 50% relative to the metals forming part of the composition of this layer.
2. Cathode according to Claim 1, characterized in that the substrate is chosen from the group consisting of titanium, nickel, tantalum, zirconium, niobium, iron and alloys thereof.
3. Cathode according to Claim 2, characterized in that the substrate is made of titanium, iron or nickel.
4. Cathode according to one of Claims 1 to 3, characterized in that the precious metal from group VIII of the Periodic Table of the Elements is ruthenium, rhodium, palladium, osmium, iridium or platinum.
5. Cathode according to Claim 4, characterized in that the precious metal is ruthenium or iridium.
6. Cathode according to one of Claims 1 to 5, characterized in that the intermediate layer consists of titanium oxide and ruthenium oxide.
7. Cathode according to one of Claims 1 to 6, characterized in that the outer layer of metal oxides contains zirconium oxide, titanium oxide and ruthenium oxide.
8. Cathode according to Claim 7, characterized in that it consists essentially of ZrTiO₄ accompanied by RuO₂. and optionally ZrO₂ and/or TiO₂.
9. Cathode according to one of Claims 1 to 8, characterized in that the precious metal/titanium molar ratio in the intermediate layer is between 0.4 and 2.4.
10. Cathode according to one of Claims 1 to 8, characterized in that the zirconium/titanium molar ratio in the outer layer is between 0.25 and 9.
11. Cathode according to Claim 10, characterized in that the zirconium/titanium molar ratio is between 0.5 and 2.
12. Use of a cathode according to one of Claims 1 to 11, for the electrolysis of aqueous solutions of alkali metal chlorides.
13. Use according to Claim 12, characterized in that the aqueous solutions of alkali metal chlorides are aqueous NaCl solutions.
14. Process for manufacturing chlorine and alkali metal hydroxide by electrolysis of the corresponding chloride using a cathode according to one of Claims 1 to 11.