EP3597791B1 - Method for improving the performance of nickel electrodes - Google Patents

Description

The invention relates to a method for improving the performance of nickel electrodes, in particular precious metal-coated nickel electrodes for use in sodium chloride electrolysis.

The invention is based on the known use of nickel electrodes as hydrogen-generating electrodes in alkali chloride electrolysis and the improvement process known per se by coating nickel electrodes with noble metals or noble metal oxides.

Cathodes for sodium chloride electrolysis, at which hydrogen is generated in an alkaline solution, usually consist of iron or nickel. If nickel electrodes are used, they can consist entirely of nickel, or only nickel surfaces are used in which substrates made of other metals are superficially nickel-plated.

As in the Offenlegungsschrift EP 298 055 A1 mentioned, nickel electrodes can be coated with a metal from subgroup VIII, especially the platinum metals (including Pt, Ru, Rh, Os, Ir, Pd) of the periodic table of the elements or an oxide of such a metal or mixtures thereof.

The electrode produced in this way can, for. B. be used in sodium chloride electrolysis as a cathode to generate hydrogen. Many coating variants are known here, because the coating of metal oxides can be modified in various ways so that different compositions arise on the surface of the nickel electrode. According to the US-A-5 035 789 is z. B. used a ruthenium oxide-based coating on nickel substrates as the cathode.

When the nickel-based electrodes are in operation, the quality of the electrode decreases over time, in such a way that the cell voltage increases during sodium chloride electrolysis, so that a new coating of the electrode may be necessary. This is technically complex because the electrolysers have to be switched off and the electrodes have to be removed from the electrolysis cells.

It is therefore an object of the invention to find a simpler form of increasing or restoring output that does not require the removal of the electrolytic cells.

According to the teaching of the patent US-A-4,555,317 iron compounds or finely divided iron are added to the catholyte in order to lower the cell voltage during sodium chloride electrolysis. In the case of nickel electrodes coated with precious metal, the coating of the cathodes with iron can have a disruptive effect on the electrolysis and increase the cell voltage.

According to the published patent application which has become known further EP 1 487 747 A1 a 0.1 to 10% by weight platinum-containing compound is added to the sodium chloride electrolysis. Here, the solution of the platinum-containing compound is added to the water which forms the catholyte, 0.1 to 2 liters of the aqueous solution of the platinum-compound-containing solution being added per liter of water. the EP 1 487 747 A1 apart from the general reference to the use of the platinum compound during the electrolysis, does not disclose any information about the conditions used in the process - electrodes, electrode areas, current density, etc., which are necessary for a technical adjustment.

According to the publication JP 1011988 A To restore the activity of a deactivated cathode based on a Raney nickel structure with a low hydrogen overvoltage, a soluble compound of a metal of the platinum group is added to the sodium hydroxide solution during the operation of the sodium chloride electrolysis in the catholyte. For example, a sodium chloride electrolysis cell is operated with 32% strength by weight sodium hydroxide solution, a salt concentration of 200 g / l sodium chloride at 90 ° C. and a current density of 2.35 kA / m ² . The cathode is electrolessly nickel-plated for pretreatment and then nickel-plated in a nickel bath. For example, platinum chlorate was metered into the catholyte as an active compound, which led to a decrease in the cell voltage by 100 mV.

According to the US-A-4 105 516 During the electrolysis of alkali metal chlorides, metal compounds are added to the catholytes, which lower the hydrogen overvoltage and thus reduce the cell voltage. The one in the US-A-4 105 516 The examples listed in turn describe the dosage and effects that result from the addition of an iron compound that is added to the catholyte of a sodium chloride diaphragm laboratory cell. The cell has an anode made of expanded titanium metal coated with ruthenium and titanium oxide. The cathode consists of iron in the form of expanded metal. The examples show the use of cobalt or iron solution on the iron cathode. The disadvantages of iron compounds in the treatment of coated nickel electrodes have already been pointed out above.

According to the further known patent US-A-4,555,317 it is known that sodium chloride electrolysis can be started with a nickel-coated copper cathode. Here, an active coating of nickel and cobalt dendrites is applied, with a porous platinum layer previously deposited electrolytically as an intermediate layer for anchoring the dendrites became. An initial dosage under electrolysis conditions of the cell was carried out with hexachloroplatinic acid at a high current density of 3 kA / m ² in several steps.

According to the US-A-4 160 704 For example, metal ions which have a low hydrogen overvoltage can be added to catholytes of a membrane electrolysis cell of sodium chloride electrolysis in order to coat the cathode. The addition takes place during the electrolysis. However, only the addition of platinum oxide to improve an iron or copper cathode is given as an example.

In sodium chloride electrolysis, the cathode coatings usually consist of platinum metals, platinum metal oxides or their mixtures, e.g. a ruthenium - ruthenium oxide mixture. <Page 3a>

Like in the EP 129 374 The usable platinum metals include ruthenium, iridium, platinum, palladium and rhodium. The cathode coating is not long-term stable, especially not under conditions in which no electrolysis takes place or when electrolysis is interrupted, in which, for example, reverse electrical currents can occur. This means that the noble metal coating is damaged to a greater or lesser extent over the operating time of the electrolyzer. Likewise, impurities which, for example, get from the brine by diffusion into the lye, for example iron ions, can be deposited on the cathode or specifically on the active centers of the precious metal-containing coating and thereby deactivate it. The result is that the cell voltage increases again in the electrolysis operation, which increases the energy consumption for the production of chlorine, hydrogen and caustic soda and significantly worsens the economic efficiency of the electrolysis process. Likewise, only individual electrolysis cell elements of an electrolyzer can show damage to the cathode coating. It is technically highly complex and therefore not economical to switch off the entire electrolyser to replace the damaged cell elements and to remove the cell elements with the damaged coating, since this is associated with production losses and considerable costs.

It is yet another method of improving precious metal coated nickel electrodes from US Pat DE 102007003554A1 has become known, in which a hexachloroplatinate or sodium hexachloroplatinate solution is metered into the sodium hydroxide solution in which a cathode coated with ruthenium oxide is operated during operation of the sodium chloride electrolysis at a production ^{current density in the range of several kA / m 2.} The cell voltage should vary in a voltage range of 0 to 5 volts or 0.5 - 500mV and a frequency of 10-100 Hz with an amplitude of 20-100mV. The platinum compound is metered into the catholyte in particular in the inlet to the cathode chamber In the pamphlet EP 590 260 A1 describes a process for the catalytic activation of a cathode for alkaline water electrolysis with a platinum metal, in which a water-soluble platinum metal salt is added to the electrolyte in the electrolyzer and the platinum metal ions of the salt are galvanically deposited on the cathode when the electrolyzer is in operation. The current density should be set to 100 A / m ² or more. The deposition time here is preferably 1 to 3 days.
a cathode area of 2.7 m ² at a current density of 1 to 8 kA / m ² . The metering rate of the platinum-containing solution based on the platinum content per m ^{2 of} cathode area is between 0.001 g Pt / (h ^∗ m ² ) and 1 g Pt / (h ^∗ m ² ).

Disadvantage of the DE 102007003554A1 disclosed coating process is that when the electrolysis comes to a standstill, the positive effect that is initially achieved by the platinum doping described cannot be maintained. Since electrolysers fail or have to be exhibited for various technical reasons, a new platinum metering has to be carried out after each standstill, which introduces additional complexity into the operational process, which is disadvantageous for a production facility. Obviously, the coating of the electrode with platinum or platinum oxide that can be achieved with this process is not as durable as would be desirable for the production process. In addition, platinum from the platinum solution may not be completely deposited on the electrode surface and thus rare and expensive precious metal material is lost.

The object of the invention is to develop a special method for improving nickel electrodes that are coated with platinum metals, platinum metal oxides or mixtures thereof, or for nickel electrodes without a coating for use as cathodes in the electrolysis of sodium chloride, that can be used while the electrolysis is running , avoids a prolonged interruption of the electrode operation to restore the cathode activity and produces an improvement in the activity of the nickel electrodes, which is not immediately lost in the event of a standstill. In particular, the method should not impair the functioning of the operating system for the electrolysis.

The invention relates to a process for improving the performance of nickel electrodes that are uncoated or that have a coating based on platinum metals, platinum metal oxides or a mixture of platinum metals and platinum metal oxides, and are used in sodium chloride electrolysis by the membrane process as electrodes that generate hydrogen, in which in the electrolysis of sodium chloride a water-soluble or sodium hydroxide-soluble platinum compound, in particular hexachloroplatinic acid or particularly preferably a sodium platinum, particularly preferably sodium hexachloroplatinate (Na ₂ PtCl ₆ ) and / or sodium hexahydroxyplatinate (Na ₂ Pt (OH) ₆ ) is added to the catholyte , characterized in that the addition in the electroysis operation at a current density of 0.2 A / m ² to 50 A / m ² , at a temperature of the catholyte in the range of 40 ° C to 95 ° C, with an amount of platinum per m ^{2 of} electrode area from 0.3 g / m ² to 10 g / m ² , preferably from 0.35 g / m ² to 8 g / m ² , particularly preferably from 0.4 g / m ² to 5 g / m ² , the reduced current density being maintained for a total of 2 to 200 minutes from the start of metering.

The amount of platinum in the context of the invention means the content of platinum metal in the added platinum compound.

Electrode area here means in particular the entire active electrode area wetted by the catholyte. For the sake of simplification, the electrode area preferably relates to the geometric dimensions of the active electrode area wetted by the catholyte.

In particular, either the sodium hexachloroplatinate can be dosed to the catholyte as an aqueous solution or in an alkaline solution, or the hexachloroplatinic acid is dosed directly into the catholyte, in particular the sodium hydroxide solution, in which case a reaction with the alkali to form sodium chloroplatinate takes place.

To avoid the precipitation of platinum metal particles in the catholyte at a high current density, as in the processes known from the prior art, the addition of the platinum compound is carried out according to the invention with ongoing electrolysis under greatly reduced load, ie the current density for the platinum metering is at most 95 A / m ² set.

Another preferred embodiment of the addition of platinum is that the temperature of the catholyte is 60 to 90 ° C., preferably 75 to 90 ° C., when the platinum compound is added.

In a preferred embodiment of the invention, the electrode coating is in the form of platinum metals and / or platinum metal oxides on the coated nickel electrodes, the platinum metals / platinum metal oxides being based on one or more metals of the series: ruthenium, iridium, palladium, platinum, rhodium and osmium, in particular preferably on those of the series: ruthenium, iridium and platinum.

Another preferred embodiment of the new method consists in the fact that, in addition to the above-mentioned soluble platinum compound, other further water-soluble compounds of the noble metals of the 8th subgroup of the periodic table of the elements, in particular compounds of palladium, iridium, rhodium, osmium or ruthenium, preferably palladium or Ruthenium are added to the catholyte. These are used in particular in the form of water-soluble salts or complex acids.

In a preferred new process, the noble metal content of the other water-soluble compounds of the noble metals of subgroup 8, based on the platinum metal of the soluble platinum compound, is 1 to 50% by weight.

A preferred variant of the new method is characterized in that the proportion of platinum in the platinum compound in the catholyte after the addition is 0.01 to 310 mg / L, preferably 0.02 to 250 mg / L, particularly preferably 0.03 to 160 mg / L.

In a preferred variant of the new process, the volume flow of the catholyte during the metering in is from 0.1 to 10 L / min, preferably from 0.2 to 5 L / min.

To avoid unnecessary losses of platinum metal, in a particularly preferred embodiment of the new method, the concentration of platinum metal in the catholyte emerging from the electrolysis cell is monitored continuously or discontinuously.

The sodium chloride electrolysis by the membrane process is typically carried out as follows, by way of example. A solution containing sodium chloride is fed to an anode chamber with an anode, and a sodium hydroxide solution is fed to a cathode chamber with a cathode. The two chambers are separated by an ion exchange membrane. Several of these anode and cathode chambers are combined to form an electrolyzer. In addition to the chlorine formed, a less concentrated sodium chloride-containing solution leaves the anode chamber than was added. In addition to hydrogen, a more highly concentrated sodium hydroxide solution leaves the cathode chamber than was supplied. The production current density is, for example, 4 kA / m ² . The geometrically projected cathode area is 2.7m2, which corresponds to the membrane area. The cathode consists of an expanded nickel metal, which is provided with a special coating (also called coating here) (manufacturer e.g. Industrie De Nora) in order to reduce the hydrogen overvoltage.

• Zuführung einer Natriumchlorid-haltigen wässrigen Lösung zu einer Anodenkammer mit einer Anode und Zuführung von Natronlauge zu einer Kathodenkammer mit einer Kathode, wobei Anoden- und Kathodenkammer über eine Ionenaustauschermembran voneinander getrennt vorliegen;
• Einstellen einer Produktionsstromdichte von mindestens 1 kA/m² bezogen auf die Elektrodenfläche;
• Ableitung der Natriumchlorid-haltigen Lösung aus der Anodenkammer zusammen mit dem an der Anode gebildeten Chlorgas und Abtrennung des Chlors von der flüssigen Phase;
• Zufuhr des abgetrennten Chlors zu einer geeigneten Aufbereitung insbesondere mindestens umfassend die Trocknung, Aufreinigung und gegebenenfalls Kompression des Chlorgases;
• Zufuhr der aus dem Anodenraum abgeführten Natriumchlorid-haltigen Lösung zu einer Aufkonzentrierung und Reinigung, wobei die Aufkonzentrierung und Reinigung insbesondere mindestens die folgenden Schritte umfasst: Zerstörung von Chlorat-Nebenprodukten, Entchlorung, Konzentrationserhöhung durch Zugabe von Natriumchlorid, Reinigung durch Fällungsreagenzien, Filtration und Ionenaustausch zur Entfernung unerwünschter Kationen,
• anschließend Wiedereinleitung der Natriumchlorid-haltigen Lösung in die Anodenkammer;
• Ableitung der Natronlauge aus der Kathodenkammer zusammen mit dem an der Kathode gebildeten Wasserstoff und Abtrennung des Wasserstoffs von der flüssigen Phase;
• gegebenenfalls Zufuhr des abgetrennten Wasserstoffs zu einer geeigneten Aufbereitung und Reinigung;
• Zufuhr der aus dem Kathodenraum abgeführten Natronlauge zu einem Sammelbehälter und ggf. zu einer weiteren geeigneten Aufbereitung und Reinigung;
• Verdünnung einer Teilmenge der aus dem Kathodenraum abgeführten Natronlauge mit Wasser und Wiedereinleitung in den Kathodenraum;

das dadurch gekennzeichnet, dass zur Erniedrigung der Elektrolysespannung bei Erreichung eines vorgegebenen mittleren Höchstspannungswertes im Elektrolysebetrieb die Stromdichte auf einen Wert von unter 100 A/m² aber mindestens 0,2 A/m² herabgesetzt wird, dann das Verfahren nach einem der Ansprüche 1 bis 8 durchgeführt wird und anschließend die Stromdichte auf die Produktionsstromdichte wieder angehoben und die Produktion fortgesetzt wird.The invention also relates to a process for the production of chlorine, sodium hydroxide solution and hydrogen according to the principle of membrane electrolysis on a production scale using nickel electrodes or coated nickel electrodes as the cathode with the following steps:

• Feeding an aqueous solution containing sodium chloride to an anode chamber with an anode and feeding sodium hydroxide solution to a cathode chamber with a cathode, the anode and cathode chambers being separated from one another by an ion exchange membrane;
• Setting a production current density of at least 1 kA / m ² based on the electrode area;
• Discharge of the sodium chloride-containing solution from the anode chamber together with the chlorine gas formed at the anode and separation of the chlorine from the liquid phase;
• Supply of the separated chlorine to a suitable treatment, in particular at least including the drying, purification and, if necessary, compression of the chlorine gas;
• The sodium chloride-containing solution discharged from the anode compartment is fed to a concentration and purification, the concentration and purification in particular includes at least the following steps: destruction of chlorate by-products, dechlorination, increase in concentration by adding sodium chloride, purification using precipitation reagents, filtration and ion exchange to remove undesired cations,
• then reintroduction of the sodium chloride-containing solution into the anode chamber;
• Discharge of the caustic soda from the cathode chamber together with the hydrogen formed at the cathode and separation of the hydrogen from the liquid phase;
• if necessary, supply of the separated hydrogen to a suitable treatment and purification facility;
• The sodium hydroxide solution discharged from the cathode compartment is fed to a collecting container and, if necessary, to further suitable processing and cleaning;
• Dilution of a portion of the sodium hydroxide solution discharged from the cathode compartment with water and reintroduction into the cathode compartment;

which is characterized in that to lower the electrolysis voltage when a predetermined average maximum voltage value is reached in electrolysis operation, the current density is reduced to a value of below 100 A / m ² but at least 0.2 A / m ² , then the method according to one of claims 1 to 8 is carried out and then the current density is increased again to the production current density and production is continued.

Production current density is understood here to mean, in particular, a current density of at least 1 kA / m ^2.

The production scale here is in particular the conversion of at least 5 kg / h sodium chloride to chlorine and caustic soda per electrolysis cell.

In the case of individual cells, the maximum voltage value is in particular the maximum electrolysis voltage across the individual cell, which is to be regarded as tolerable in terms of the energy efficiency of the electrolysis process. This threshold value is typically around 80 mV above the best mean voltage value after the cell has been put into operation.

In the case of electrolysers with a large number of individual cells, the mean value of the measured voltages is used as a comparison value for the sake of simplicity.

In a preferred embodiment of the new electrolysis process, the concentration of the sodium chloride-containing solution is at least 150 g / L.

In a further preferred embodiment of the new electrolysis process, the NaOH content in the sodium hydroxide solution is at least 25 percent by weight.

Sodium chloride-containing solution and sodium hydroxide solution are preferably heated to at least 60 ° C. before introduction.

In a further preferred embodiment of the new electrolysis process, the sodium chloride-containing solution is brought to a pH value below 6.

Examples

The following test examples were carried out on technical electrolysers each with 144 elements (single electrolysis cells), the nickel cathodes of which were provided with a coating based on a mixture of ruthenium / ruthenium oxide from Denora.

The average voltage for each electrolyser was calculated from the mean value of the 144 elements. To compare the voltages or voltage changes of the electrolysis, the voltage values at a current density in electrolysis operation of 4.5 kA / m ^{2 were} used.

In the event that no voltage values were available at this current density because the electrolyser was not operated at this current density at the time called up, the measured voltage was converted to the voltage corresponding to ^{the current density of 4.5 kA / m 2.} The conversion was carried out using a linear regression of the current-voltage data in the range from 3 to 5 kA / m ² . In this current range, the current-voltage characteristic of an electrolyzer is linear.

example 1

A technical electrolyser was operated at an average voltage of 3.27 V and a current density of 4.5 kA / m ² .

The following procedure was carried out:
Within 30 minutes, the current density was ^{reduced from 4.5 kA / m 2} to a current density of 11.8 A / m ² and kept constant at this value. After 10 min, 8 L of a solution of hexachloroplatinate solution (25 g Pt / L) were metered into the sodium hydroxide solution (32%) at 0.8 L / min over the course of 10 min. The proportion of platinum of the platinum compound in the sodium hydroxide solution rose to up to 16 mg / L. The current density remained at the constant value of 11.8 A / m ² and was held at this value for a further 30 minutes after the addition. Overall, the time during which the current density was kept at 11.8 A / m ² since the start of the addition was 40 minutes, after which the current density was increased again to 4.5 kA / m ² within 45 minutes.

The temperature of the caustic soda over the entire procedure varied in the range from 76 to 90 ° C.

The volume flow of sodium hydroxide solution during the metering time was 3.6 L / min per element.

Thus, 200 g of platinum got onto the surface of 144 cathodes (surface of a cathode: 2.7 m ² ). This corresponds to an amount of platinum of 0.51 g / m ² .

The mean voltage at 4.5 kA / m ² fell after the addition from the initial value of 3.27 V to 3.10 V. This corresponds to a voltage decrease of 170 mV.

After a further 126 days of operation, the mean voltage at 4.1 kA / m ^{2 was} 3.07 V. Converted to a current density of 4.5 kA / m ^{2, this} corresponds to a mean voltage of 3.13 V. The voltage drop is still 140 mV .

After a standstill and a total of 129 operating days after the metering, the mean voltage at 4.5 kA / m ^{2 was} 3.16 V. The voltage drop is still 110 mV.

After a further standstill and a total of 133 operating days after the metering, the mean voltage at 4.5 kA / m ^{2 was} 3.17 V. The voltage drop is still 100 mV.

Example 2

Comparative example:
A technical electrolyser was operated at an average voltage of 3.15 V and a current density of 4.2 kA / m ² . Converted to a current density of 4.5 kA / m ^2, this results in a voltage of 3.19 V.

The following procedure was carried out:
During operation, 6 L of a solution of hexachloroplatinate solution (7.1 g Pt / L) were metered into the sodium hydroxide solution (32%, 90 ° C.) at 1 L / h over the course of 6 h. The current density varied in the range from 4.3 to 4.7 kA / m ² .

This resulted in 43 g of platinum on the surface of 144 cathodes (surface of a cathode: 2.7 m ² ). This corresponds to an amount of platinum of 0.11 g / m ² .

After all of the hexachloroplatinate solution had been added, an average voltage of 3.17 V was obtained ^{at a current density of 4.7 kA / m 2.} The conversion to a current density of 4.5 kA / m ² results in an average voltage of 3.14 V. This corresponds to a voltage reduction of 50 mV.

After 5 days of operation, an average voltage of 3.16 V was measured ^{at 4.5 kA / m 2.} The voltage drop is therefore only 20 mV.

After a total of 8 operating days after the dosing, the electrolyzer was switched off. After the shutdown, an average voltage of 3.17 V was measured at 4.4 kA / m ^2. Converted to 4.5 kA / m ^2, this results in an average voltage of 3.18 V. This almost completely eliminates the voltage reduction originally achieved.

Example 3 Another comparative example

A technical electrolyser was operated at an average voltage of 3.17 V and a current density of 4.3 kA / m ² . Converted to a current density of 4.5 kA / m ^2, this results in a voltage of 3.2 V.

The following procedure was carried out:
Within 30 minutes, the current density was ^{reduced from 4.3 kA / m 2} to a current density of 11.8 A / m ² and kept constant at this value. After 10 minutes, 8 L of a solution of hexachloroplatinate solution (6.25 g Pt / L) were dosed into the sodium hydroxide solution at 0.8 L / h over the course of 10 minutes. The current density remained at the constant value of 11.8 A / m ² and was held at this value for a further 30 minutes after the addition. Overall, the time during which the current density was kept at 11.8 A / m ² was 40 minutes since the start of the addition, after which the current density was increased to 3.8 kA / m ² within 45 minutes.

The temperature of the caustic soda over the entire procedure varied in the range from 76 to 90 ° C.

50 g of platinum thus got onto the surface of 144 cathodes (surface of a cathode: 2.7 m ² ). This corresponds to an amount of platinum of 0.13 g / m ² .

After the addition, an average voltage of 3.0 V was determined at a current density of 3.8 kA / m ^2. Converted to a current density of 4.5 kA / m ^2, this results in an average voltage of 3.1 V. The voltage decrease is accordingly 100 mV.

After a total of 8 operating days after the metering and a shutdown, an average voltage of 3.19 V was measured ^{at a current density of 4.5 kA / m 2.} The voltage drop is therefore only 10 mV and is therefore almost completely eliminated.

Claims (9)

1. Method for improving the performance of nickel electrodes which are uncoated or have a coating based on platinum metals, platinum metal oxides or a mixture of platinum metals and platinum metal oxides and are used as hydrogen-evolving electrodes in sodium chloride electrolysis by the membrane process, where a platinum compound which is water-soluble or soluble in sodium hydroxide solution, in particular hexachloroplatinic acid or a sodium platinate, particularly preferably Na₂PtCl₆ and/or Na₂Pt(OH)₆ is metered into the catholyte during the electrolysis of sodium chloride, characterized in that the metered addition is carried out during electrolysis operation at a reduced current density of from 0.2 A/m² to 50 A/m², at a temperature of the catholyte in the range from 40°C to 95°C, using an amount of platinum per m² of electrode area of from 0.3 g/m² to 10 g/m², preferably from 0.35 g/m² to 8 g/m², particularly preferably from 0.4 g/m² to 5 g/m², with the decreased current density being maintained from the commencement of the metered addition for a total of from 2 to 200 minutes.
2. Method according to Claim 1, characterized in that to the platinum compound are added further other water-soluble compounds of the noble metals of transition group 8 of the Periodic Table of the Elements, in particular compounds of the platinum group, particularly preferably of palladium, iridium, rhodium, osmium or ruthenium, preferably palladium or ruthenium.
3. Method according to Claim 2, characterized in that the proportion of noble metal of the further water-soluble compounds of the noble metals of transition group 8 is from 1 to 50% by weight, based on the platinum metal of the soluble platinum compound.
4. Method according to at least one of Claims 1 to 3, characterized in that the temperature of the catholyte at which the metered addition of the platinum compound is carried out is in the range from 60 to 90°C, preferably from 75 to 90°C.
5. Method according to at least one of Claims 1 to 4, characterized in that the proportion of platinum of the platinum compound in the catholyte after the metered addition is from 0.01 to 310 mg/l, preferably from 0.02 to 250 mg/l, particularly preferably from 0.03 to 160 mg/l.
6. Method according to at least one of Claims 1 to 5, characterized in that the catholyte is passed as a volume flow over the electrode surface and said volume flow of the catholyte during the contact time of the electrode surface with the catholyte containing the platinum compound is from 0.1 to 10 1/min, preferably from 0.2 to 5 1/min.
7. Method according to Claim 6, characterized in that the concentration of platinum metal in the catholyte exiting from the electrolysis cell is continuously or discontinuously monitored.
8. Method according to at least one of Claims 1 to 7, characterized in that the method is carried out on coated nickel electrodes, with the coating comprising platinum metal/platinum metal oxide based on one or more metals from the group consisting of: ruthenium, iridium, palladium, platinum, rhodium and osmium, preferably from the group consisting of: ruthenium, iridium and platinum.
9. Process for producing chlorine, sodium hydroxide solution and hydrogen according to the principle of membrane electrolysis on a production scale using nickel electrodes or coated nickel electrodes as cathode, comprising the steps of:

   - introduction of an aqueous solution containing sodium chloride into an anode chamber having an anode and introduction of sodium hydroxide solution into a cathode chamber having a cathode, where anode chamber and cathode chamber are separated from one another by an ion-exchange membrane;

   - setting of a production current density of at least 1 kA/m² based on the electrode area;

   - discharge of the sodium chloride-containing solution from the anode chamber together with the chlorine gas formed at the anode and separation of the chlorine from the liquid phase;

   - feeding of the chlorine which has been separated off to a suitable treatment, in particular comprising at least drying, purification and optionally compression of the chlorine gas;

   - feeding of the sodium chloride-containing solution discharged from the anode space to concentration and purification, in particular comprising at least the steps of: destruction of chlorate by-products, dechlorination, increasing of the concentration by addition of sodium chloride, purification by means of precipitation reagents, filtration and ion exchange to remove undesirable cations,

   - subsequent reintroduction of the sodium chloride-containing solution into the anode chamber;

   - discharge of the sodium hydroxide solution from the cathode chamber together with the hydrogen formed at the cathode and separation of the hydrogen from the liquid phase;

   - optionally feeding of the hydrogen which has been separated off to a suitable treatment and purification;

   - feeding of the sodium hydroxide solution discharged from the cathode space to a collection vessel and optionally to a further suitable treatment and purification;

   - dilution of a partial amount of the sodium hydroxide solution discharged from the cathode space with water and reintroduction into the cathode space;
   characterized in that the current density is reduced to a value of less than 100 A/m² but at least 0.2 A/m² in order to lower the electrolysis voltage on attainment of a prescribed average maximum voltage value during electrolysis operation, the method according to any of Claims 1 to 8 is carried out and the current density is subsequently increased again to the production current density and production is continued.