KR20140034138A - Coating for metallic cell- element materials of an electrolytic cell - Google Patents

Abstract

The present invention relates to a cathode half shell of an electrolysis cell comprising a metal cell element component, said metal cell element component being positioned plane-parallel on the metal support structure and the metal support structure welded to the rear wall of the cathode half shell. At least one metal elastic element, comprising:-an oxygen comprising opposing at least one metal elastic element, a perforated metal grid, and a catalyst strip of PTFE and silver oxide mechanically indented into the perforated metal grid A depolarized cathode, wherein the silver oxide is a uniform connection / coupling between the components of the oxygen-depolarized cathode and the at least one elastic element, resulting in a connection / bonding characterized by high conductivity While the operation of the electrolysis plant is reduced to silver, at least one of said metal parts comprises at least two layers. Comprising an electroconductive coating, the two layers being applied directly to the cell element material and selected from the group comprising Au, B-doped nickel, Ni sulfide and mixtures thereof, A cathode half shell of an electrolysis cell comprising a first layer having a thickness of 0.005 to 0.2 μm and a second layer applied to the first layer and made of silver and having a layer thickness of 0.1 to 30 μm. .

Description

COATING FOR METALLIC CELL-ELEMENT MATERIALS OF AN ELECTROLYTIC CELL}

The invention relates to a cathode half shell in which the metal part has a specific electroconductive coating, as characterized in the preamble of claim 1.

In electrochemical processes, chemical reactions are controlled by external currents. Inside electrochemical cells, conductive, stable and inexpensive conductors are needed to transfer electrons. Here, nickel proved to be the ideal material for the electrode. However, when the electrode is operated in the potential region where nickel oxide or nickel hydroxides are formed, there is a disadvantage that a poor or non-conductive nickel surface is formed. Due to this low level potential, oxides or hydroxides are formed in many processes.

The ohmic loss at the surface of nickel is, for example, of the entire system such as zinc / air and nickel / metal hydroxide batteries, oxygen cathodes in chlor-alkali electrolysis, or oxygen electrodes in basic fuel cells. Decreases the efficiency.

Poor or non-conductive oxide or hydroxide layers are a barrier, for example when pure nickel is used as the oxygen generating electrode in electrolysis. However, even in systems in which nickel is in contact with catalytically active materials such as conductive mesh, expanded metal, or sheet, nickel, carbon plated carbon, etc., the insulating layer has a negative effect. For example, the oxide or hydroxide layer prevents optimal current flow despite the oxygen-depolarized electrode, and therefore, a step of improving or maintaining conductivity in industrial electrolysis is required.

In the literature, numerous diagrams relating to electrochemical stability are recorded in Marcel Pourbaix's "Atlas of Electrochemical Equilibria in Aqueous Solutions" (1974). The results of Pourbaix show that the cathode potential measured for NHEs, which have a pH between 13 and 15, and which are present during chlor-alkali electrolysis with an oxygen-depolarized cathode at electrical load, is approximately 0.4 to Under conditions above 0.6 V, the familiar form of nickel oxide occurs in the immobilized form.

Further problems arise from the start and stop of the chlor-alkali electrolysis cell, whereby soluble hydroxide can form when a midrange potential of approximately 0.6 V measured for NHE passes. The Fubei plot is not allowed to describe kinematics, and it is impossible to predict the actual formation of such hydroxides due to decomposition reactions, ie corrosion. Therefore, in order to confirm the behavior of nickel under oxidizing conditions such as when chlorine-alkali electrolysis is performed with the oxygen-depolarized cathode, an actual electrolysis test is required.

Various patent specifications, for example EP 1 033 419 B1 or EP 1 092 789 A1, describe electrolysis cells in which chlorine-alkali electrolysis is carried out with an oxygen-depolarized cathode, with nickel being a metal part on the cathode side. It is used as a material of. There is no description of the corrosion stability of nickel with respect to the formation of nonconductive oxides or hydroxide compounds.

EP 1 041 176 A1 discloses an electrolysis cell with a gas-diffusion electrode for the purpose of minimizing the resistance loss during the current supply to the oxygen-depolarized cathode (herein referred to as the gas diffusion electrode) by the metal parts for current distribution. A method is described. A description is already included for coatings that are substantially metal and have good conductivity. In particular, further details regarding corrosion stability are not described.

DE 10 2004 034 886 A1 describes a process for the production of electrically conductive nickel oxide surfaces. Here, the poor conductivity of the nickel oxide surface is significantly improved through subsequent chemical doping with basic oxides at low temperatures in the presence of hydrogen peroxide. Thus, the present application is particularly suitable for operating conditions in fuel cells, storage batteries, and chlor-alkali electrolysis.

The process described in DE 10 2004 034 886 A1 was successfully used for the first time for the operation of an experimental chlor-alkali electrolysis cell with oxygen depolarization. For example, oxygen-depolarized cathodes whose preparation is shown in EP 1 402 587 B1 or DE 37 10 168 A1 have been used for this purpose. These electrodes consist of an electrically conductive grid, usually a nickel wire grid, wound around a catalyst strip of silver / PTFE or silver oxide / PTFE mixture. The grid of the gas diffusion electrode is in electrical contact with a nickel current distributor whose conductivity is improved according to the process described in DE 10 2004 034 886 A1. During operation of this experimental cell, although the cell was repeatedly switched off, no indication of corrosion in the form of voltage increase or nickel decomposition was detected, and accordingly DE 10 2004 034 886 A1 corroded nickel. It can be assumed that an effective process has been described for protecting against

EP 1 601 817 A1 describes an electrolysis cell commercially used and utilized for conventional chlor-alkali electrolysis. US 7 670 472 B2 describes an electrolysis cell which represents the design configuration inside the cathode compartment which allows the electrolysis cell for chlorine-alkali electrolysis to be operated with an oxygen-depolarized cathode.

The design of the electrolysis cell described in EP 1 601 817 A1 was changed on the basis of the technical features of US 7 670 472 B2, allowing the resulting electrolysis cell for chlor-alkali electrolysis to work with oxygen depolarization. For this purpose, according to the principle described in DE 37 10 168 A1, an electrode consisting of a nickel grid wound with a catalyst strip consisting of silver oxide and PTFE was used as the oxygen-depolarized cathode. The current supply to the oxygen-depolarized cathode located in the cathode compartment is inserted with a thin support structure positioned parallel to the rear wall of the cathode, via the webs arranged vertically by the welded joint. It is realized in an electrically connected manner to the rear wall. As the elastic element is attached to this support structure, the cathode half shell and anode half shell of the cell are screwed together, with a wire grid of oxygen-depolarized cathode that ensures electrical contact and even current distribution. A press-fit is formed. Such elastic elements are already described in various patent specifications, for example EP 1 446 515 A2 and in particular EP 1 451 389 A2, consisting of various compressible layers of metal wires pressed together like a sandwich to ensure elasticity. do.

In order to ensure the conductivity of the nickel oxide surface caused by passivation during operation, a process for treating the nickel oxide surface described in DE 10 2004 034 886 A1 was used for nickel components.

In test series 1, two such redesigned electrolysis cells with an active electrolysis surface of 2.7 m ² with a Flemion F8020 membrane, a current density of ⁴ kA / m ² , an operating temperature of 88 ° C., 210 g At a NaCl anolyte concentration of / l, NaOH catholyte exhaust concentration of 32% w / w, it was operated with 20% stoichiometric excess saturated saturated oxygen. 1 shows the voltage curves of two electrolysis cells in the first 65 days of operation. Different symbols are used for each of the electrolysis cells (shaded diamonds and unshaded triangles).

In the first 30 days of operation, the electrolysis cell showed a stable cell voltage. At 30 days of operation, the current to the two electrolysis cells was off. After the electrolysis cell was switched back on and a current density of ⁴ kA / m ² was reached, both cells showed increased ohmic resistance in the form of a voltage increase up to 100 mV. After four more days of operation, the electrolysis cell was switched back off. After the electrolysis cell is on again and a current density of ⁴ kA / m ² is reached, the ohmic resistance further increases, resulting in an additional voltage increase of approximately 200 mV. After approximately thirty more days of operation, the two electrolysis cells were switched off and the parts were inspected. This showed that the conductivity of parts consisting of nickel—support structures and elastic elements—has been significantly reduced. The oxygen-depolarized cathode used was checked in the experimental cell and compared to the reference sample. During the experimental run, the part also exhibited an increased voltage compared to the reference sample, which can be seen as a result of the reduced conductivity of the nickel wire grid due at least in part to oxidation. Thus, the protective effect of the process described in DE 10 2004 034 886 A1 was not effective under certain potential and operating conditions which apparently occur when the electrolysis cell is switched off.

Operation of chlor-alkali electrolysis at 85 ° C. according to thermodynamic equilibrium reactions for precious metals such as silver and gold, described in “Atlas of Electrochemical Equilibria in Aqueous Solutions” (1974). The electrochemical stability charts for the conditions have been recalculated to get a detailed overview of the electrochemical conditions.

For nickel, the result of 10 ⁻⁶ mol / kg at 85 ° C. for NHE (normal hydrogen electrode) is the stability plot shown in FIG. 2, in simplified form. Here, Environment A is characterized by passivation, Environment B and Environment C are characterized by corrosion, and Environment D is characterized by immunity. According to this, the corrosive environment which forms the hydroxide at 85 ° C. is always passed in the starting environment (load and potential increase) and in the stop environment (load and potential decrease), thus these environments represent critical operating conditions.

For gold, the result of ^10-6 mol / kg at 85 ° C. for NHE (standard hydrogen electrode) is the stability plot shown in FIG. 3, in simplified form. Here, Environment A is characterized by passivation, Environment B is characterized by corrosion, and Environment D is characterized by immunity.

Similar to the nickel of FIG. 2, the diagram shows the possible corrosion environment at midrange potentials at which hydroxide compounds can be formed. However, tests using gold in strong basic caustic soda solutions show little indication of any degradation. Therefore, it is possible to determine that there is a kinetic disturbance factor and that gold can be regarded as a stable metal for chlor-alkali electrolysis under oxidizing conditions.

For silver, the result of 10 ⁻⁶ mol / kg at 85 ° C. for NHE (standard hydrogen electrode) is the stability plot shown in FIG. 4, in simplified form. Here, Environment A is characterized by passivation, Environment B is characterized by corrosion, and Environment D is characterized by immunity.

From Figure 4 it is clear that although in the acidic pH range, silver also has a narrow corrosive environment. In basicity, especially under oxidizing conditions, silver tends to passivate through the formation of oxidative species. Thus, given corrosion stability, the problem of conductivity would need to be tested under chlor-alkali electrolysis conditions with oxygen-depolarized cathodes.

WO 01/57290 A1 "Electrolysis cell provided with gas diffusion electrodes" describes an electrolysis cell comprising a gas diffusion electrode, wherein the protective function of the silver coating under oxidizing conditions is described. Get attention. In particular, a metal current conductor with openings is described and nickel should preferably be coated with silver, but the conductor consists of silver, stainless steel, or nickel.

As a result of the literature and experience of various operators confirmed the stability of the silver on the nickel, the nickel component of the electrolysis cell was electro-silver-plated. To this end, a coating thickness of approximately 10 μm was applied to the nickel.

In test series 2, two electrolysis cells were tested in continuous operation, in a manner similar to test series 1. Both cells have an active electrolysis surface of 2.7 m ² and are equipped with the Flemion F8020 membrane. The continuous current density was 4 kA / m ² , the operating temperature was 88 ° C., the NaCl anolyte concentration was 210 g / l, the NaOH catholyte discharge concentration was 32% w / w, and the stoichiometric excess of saturated wet oxygen was again 20 Was%. 5 shows the voltage curve after 80 days of operation. Different symbols are used for each of the electrolysis cells (shaded diamonds and unshaded triangles).

The results of test series 2 according to FIG. 5 again show an increase in voltage. This time was continuous. The start and stop procedures that occur repeatedly during the operation period have no appreciable effect on the cell voltage, unlike the observations made in test series 1 based on FIG. 1.

After 80 days of operation the cell elements were inspected and the conditions of the metal support structure and the metal elastic elements were analyzed. As an example, a transverse micrograph of a nickel monofilament plated with silver of an elastic element is shown in FIG. 6 on a scale of 100: 1. While the sample from the top shows a loose silver coating and a reduction of approximately 50% of the coating thickness, the micrograph clearly shows the silver spalling from the bottom in the filament sample.

In addition, the material comparison between the samples from the top and bottom of the cell element shows the transfer of degraded silver, which dissolves at the top due to corrosion and precipitates back on the bottom portion of the cell (data not shown). Thus, it can be seen that, under no circumstances, simply electroplating nickel with a silver layer under oxidative electrolysis conditions is sufficient to form an electrochemically stable connection.

These tests show that it is further necessary to provide a coating which results in an electrochemically stable connection in the form of a stable conductivity of the metal part of the cathode half shell without the occurrence of the above mentioned disadvantages.

Accordingly, an object of the present invention is to achieve the following objects.

-Providing an alternative corrosion protection coating for the metal cell element parts of the cathode half shell of the electrolysis cell

Ensuring an increased adhesive strength of the coating on the surface of the cell element part such that a non-conductive oxide layer cannot be formed

Stable operation of the electrolysis cell at a constant number of start and stops, and for as long as possible the cell voltage over a longer period of time at a given current load, despite longer life cycles.

Minimizing the resistive losses and thus the conduction losses during current conduction from the metal parts to the metal grid of the oxygen-depolarized cathode

The above object is a cathode half shell of an electrolysis cell comprising a metal cell element component, the metal cell element component comprising:

At least one metal resilient element positioned plane-parallel on the metal support structure and the metal support structure welded to the rear wall of the cathode half shell,

An oxygen-depolarized cathode disposed against the at least one metal elastic element and comprising a perforated metal grid and a catalyst strip made of PTFE and silver oxide mechanically indented into the perforated metal grid,

The silver oxide, during the operation of the electrolysis plant, generates a connection / coupling characterized by high conductivity, with a uniform connection / coupling between the components of the oxygen-depolarized cathode and the at least one elastic element. , Reduced to silver,

At least one of the metal parts has an electroconductive coating comprising at least two layers, the two layers being:

A first layer applied directly to the cell element material and selected from the group comprising Au, B-doped nickel, Ni sulfide and mixtures thereof, having a layer thickness of 0.005 to 0.2 °;

It is achieved by the cathode half shell of the electrolysis cell, which is applied to the first layer, consists of silver and consists of a second layer having a layer thickness of 0.1 to 30 ?.

The invention also claims that all cell element components contained within the cathode half shell and conducting current are coated. Here, preferably, the cell element parts of the cathode half shell of the electrolysis cell in contact with the caustic soda solution have the coating of the present invention.

Also claimed is the use of the inventive cathode half shell of an electrolysis cell in chlor-alkali electrolysis.

The following figures are used to explain the invention in more detail.
FIG. 1 shows the voltage curves in the first 65 days of operation of an electrolysis cell with electrodes, as shown in DE 10 2004 034 886 A1, with electrolysis cell voltage in test series 1. FIG.
2 is a simplified stability plot of Ni—H ₂ O at 85 ° C. for NHE.
3 is a simplified stability plot of Au-H ₂ O at 85 ° C. for NHE.
4 is a simplified stability plot of Ag-H ₂ O at 85 ° C. for NHE.
5 is the electrolysis cell voltage in test series 2, 10? The voltage curves for 80 days of operation of an electrolysis cell using a metal cell element component with a thick layer of silver are shown.
FIG. 6 is an overlaid micrograph of nickel wire plated with silver from Test Series 2 with a scale of 100: 1.
FIG. 7 shows the voltage curves for 240 days of operation of an electrolysis cell using a metal cell element component coated with a 0.15 μm thick gold layer and a 25 μm thick silver layer with electrolysis cell voltage in test series 3. FIG. .
FIG. 8 is an overlaid micrograph of nickel plated silver wire with an interlayer of gold from Test Series 3 with a scale of 25: 1.
FIG. 9 is an overlaid micrograph of nickel plated silver wire with an intermediate layer of gold from Test Series 3 with a scale of 500: 1.
10 is an SEM image of a Ni—Ag binder layer with a lightweight layer of gold.
Figure 11 shows the basic configuration of a metal cell element component in a cathode half shell with a coating of the present invention.

It is known from material chemistry that nickel and silver do not bond. Even when the melting point is exceeded, these metals cannot be mixed and only form a monotectic system. This behavior did not apply to mixtures of nickel / gold and gold / silver, which initiated a coating test of a three-layer system. As a result, the nickel component was first coated with a thin 0.15 μm gold layer followed by a 25 μm silver layer. The nickel component thus produced was installed in a freshly prepared chlor-alkali electrolysis cell with an oxygen-depolarized cathode and subjected to continuous load testing in test series 3.

In test series 3, two electrolysis cells were tested in continuous operation, in a manner similar to test series 2. Both cells have an active electrolysis surface of 2.7 m ² and are equipped with the Flemion F8020 membrane. The continuous current density was 4 kA / m ² , the operating temperature was 88 ° C., the NaCl anolyte concentration was 210 g / l, the NaOH catholyte discharge concentration was 32% w / w, and the stoichiometric excess of saturated wet oxygen was again 20 It was%. 7 shows the voltage curve of test series 3 after 240 days of operation. Different symbols are used for each of the electrolysis cells (shaded diamonds and unshaded triangles).

The results of test series 3 according to FIG. 7 show an initial slight voltage increase which can be seen as a result of the characteristics of the oxygen-depolarized cathode used. A stable phase is then followed over more than 200 working days. Multiple start and stop have no appreciable effect on cell voltage.

After completing the test, three metal parts-oxygen-depolarized cathodes, including support structures, elastic elements, and wire grids-were examined and the conditions confirmed through micrographs. This is shown in FIGS. 8 and 9. No significant loosening or spalling off of the layers was observed. The nickel support structure is uniformly electro-silver-plated and the surface is slightly roughened.

Surprisingly, a physically uniform bond between the oxygen-depolarized cathode and the elastic element located plane-parallel below the oxygen-depolarized cathode was also found during the inspection. The silver oxide wound into the catalyst strip of the oxygen-depolarized cathode is reduced to silver during the initial start of the electrolysis cell. In this process, a physically extremely uniform bond is achieved from a metal strip of oxygen-depolarized cathode and a silver strip forming at least one elastic element having the coating of the present invention, wherein the silver layer of the component is at least partially formed of a chemical bond By forming the bond is very difficult to separate in the process of decomposition. This type of coupling leads to a low resistance loss in the current transfer process through the electrolysis cell, thereby achieving a low and stable cell voltage which is not affected by start and stop during long term operation.

10 shows the conditions of the intermediate gold layer forming a binder layer between nickel and silver. There is no obvious corrosion here.

Finally, Figure 11 shows the basic configuration of a metal cell element component with a coating of the present invention. The base is a cathode half shell 1. The parts of the current divider 3 as well as the metal webs 2 welded to the rear wall are arranged in parallel with the narrow side wall. A component of elastic element 4 is press fit between the current distributor 3 and the oxygen-depolarized cathode. Oxygen-depolarized cathodes, which are located plane-parallel to the elastic element, form a catalyst strip, which forms a connection / bonding with the metal grid 5 and the elastic element 4 during the operation of the electrolysis cell for the intended purpose. 6) consists of a wound perforated metal grid, or precisely a wire grid 5, wherein the connection / coupling is characterized by high conductivity and hence low ohmic resistance.

Claims (4)

As a cathode half shell of an electrolysis cell comprising a metal cell element component. The metal cell element component,
A metal support structure welded to the rear wall of the cathode half shell and at least one metal elastic element positioned plane-parallel on the metal support structure,
An oxygen-depolarized cathode placed against said at least one metal elastic element and comprising a perforated metal grid and a catalyst strip made of PTFE and silver oxide mechanically indented into said perforated metal grid,
The silver oxide, during the operation of the electrolysis plant, generates a connection / coupling characterized by high conductivity, with a uniform connection / coupling between the components of the oxygen-depolarized cathode and the at least one elastic element. , Reduced to silver,
At least one of the metal parts has an electroconductive coating comprising at least two layers, the two layers being:
A first layer applied directly to the cell element material and selected from the group comprising Au, B-doped nickel, Ni sulfide and mixtures thereof, having a layer thickness of 0.005 to 0.2 °;
-Cathode half shell of an electrolysis cell, applied to said first layer, consisting of silver, consisting of a second layer having a layer thickness of 0.1 to 30 ?. The method of claim 1,
Cathode half shell of an electrolysis cell, wherein all cell element components of the electrolysis cell conducting current are coated. 3. The method according to claim 1 or 2,
A cathode half shell of an electrolysis cell, wherein all cell element components of the electrolysis cell in contact with caustic soda are coated. In the use of the cathode half shell of an electrolysis cell according to one of the claims,
And said electrolysis cell is used for chlor-alkali electrolysis.

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