CA2824173A1 - Coating for metallic cell element materials of an electrolysis cell - Google Patents

Abstract

The invention relates to a cathode half-shell of an electrolytic cell, comprising metallic cell-element components, comprising ° a metallic support structure (2) welded to the rear wall of the cathode half shell (1), and at least one metallic spring element (4) arranged thereon in a plane-parallel manner, ° an oxygen-consuming cathode arranged on the at least one metallic spring element and comprising a perforated metallic carrier (5) and a catalyst strip (6), mechanically pressed thereonto, consisting of PTFE and silver oxide, the silver oxide being reduced in the electrolysis operation to obtain silver and therefore producing a uniform connection, characterised in that it is highly conductive, between the components of the oxygen-consuming cathode and the at least one spring element, at least one of the metallic components being provided with an electrically conductive coating comprising at least two layers, ° one first layer, which is applied directly to the cell-element materials, being selected from a group which contains Au, B-doped nickel, Ni-sulphide, and mixtures thereof, this first layer having a layer thickness of between 0.005 and 0.2 µm, and ° a second layer, applied to the first layer, consisting of silver and having a layer thickness of between 0.1 to 30 µm.

Description

COATING FOR METALLIC CELL ELEMENT MATERIALS OF AN
ELECTROLYSIS CELL
[0001] The present invention relates to a cathode half shell, wherein the metallic components have a specific electrically conductive coating as characterised in the preamble of claim 1.

[0002] In electrochemical processes, chemical reactions are controlled by an external electric current. Within the electrochemical cells, a conductive, stable, inexpensive conductor is needed to transport the electrons. Here, nickel has proved to be an ideal material for the electrodes. However, a disadvantage is the formation of poorly or non-conductive nickel surfaces when the electrodes are operated at potential ranges in which nickel oxide or nickel hydroxide species are formed. Due to these low-level potentials, oxide or hydroxide formation occurs in many processes.

[0003] Ohmic losses at the surface of the nickel compromise the efficiency of the whole system, such as, for example, zinc/air and nickel/metal hydride batteries, oxygen cathodes in chlor-alkali electrolysis or oxygen electrodes in alkaline fuel cells.

[0004] These poorly or non-conductive oxide or hydroxide layers are, for example, a hindrance when pure nickel is used as an oxygen evolution electrode in electrolysis. But even in systems where, as a conductive mesh, expanded metal or sheet, nickel comes into contact with catalytically active material, such as carbon, platinised carbon, etc., the isolating layer has a negative effect. For example, the oxide or hydroxide layers prevent an optimum current flow even with oxygen-depolarised electrodes, and therefore steps are required to improve or maintain conductivity in industrial electrolysis.

[0005] In the literature, numerous diagrams relating to electrochemical stability are documented in the "Atlas of Electrochemical Equilibria in Aqueous Solutions"
by Marcel Pourbaix (1974). Pourbaix's results show that under the pH conditions of 13-15 and cathodic potentials above approximately 0.4-0.6 V, measured against NHE, which exist during chlor-alkali electrolysis with oxygen-depolarised cathodes under electric load, the familiar formation of nickel oxide occurs in the form of passivation.

[0006] An additional problem results from the start-up and shutdown of chlor-alkali electrolysis cells, whereby soluble hydroxides may be formed when mid-range potentials of approximately 0.6 V, measured against NHE, are passed through. As the Pourbaix diagram does not allow statements regarding the kinetics to be made, it is not possible to predict the actual formation of these hydroxides due to decomposition reactions, i.e.
corrosion. Practical electrolysis tests are therefore required in order to ascertain the behaviour of nickel under oxidising conditions, such as in case of chlor-alkali electrolysis with oxygen-depolarised cathodes.

[0007] Various patent specifications, for example EP 1 033 419 B1 or EP 1 A1, describe electrolysis cells for chlor-alkali electrolysis with oxygen-depolarised cathodes, in which nickel is used as the material for the metallic components on the cathode side. Nothing is said about the corrosion stability of the nickel regarding the formation of non-conductive oxide or hydroxide compounds.

[0008] EP 1 041 176 A1 describes a method for an electrolysis cell with a gas-diffusion electrode with the purpose of minimising ohmic losses during current supply to the oxygen-depolarised cathodes (here referred to as gas diffusion electrodes) by means of the metallic components for current distribution. It already includes a description of a coating with excellent conductivity that is metallic in nature. No further details, especially with regard to its corrosion stability, are given.

[0009] 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 surfaces is significantly improved via subsequent chemical doping with alkali oxides at low temperature in the presence of hydrogen peroxide. This application is thus particularly suitable for the operating conditions in fuel cells, storage batteries and chlor-alkali electrolysis.

[0010] The process described in DE 10 2004 034 886 A1 was first used successfully for the operation of laboratory chlor-alkali electrolysis cells with oxygen depolarisation.
Oxygen-depolarised cathodes whose manufacture is described, for example, in EP

587 B1 or DE 37 10 168 A1 were used for this. These electrodes consist of an electrically conductive grid, usually a nickel wire grid, onto which has been rolled a catalyst strip made up of a silver/PTFE or silver oxide/PTFE mixture. The grid of the gas diffusion electrode is in electrical contact with the nickel current distributor, the conductivity of which has been improved as per the process described in DE 10 2004 034 886 A1.

During operation of these laboratory cells no voltage increase or signs of corrosion in the form of nickel decomposition were detected despite the cells being repeatedly switched off and it can therefore be assumed that DE 10 2004 034 886 A1describes an effective process for protecting the nickel from corrosion.

[0011] EP 1 601 817 A1 describes an electrolysis cell that is commercially exploited and used for conventional chlor-alkali electrolysis. US 7 670 472 B2 describes an electrolysis cell that represents a design configuration within the cathodic compartment which allows the electrolysis cell for chlor-alkali electrolysis to be operated with oxygen-depolarised cathodes.

[0012] The design of the electrolysis cell described in EP 1 601 817 A1 was altered on the basis of the technical features of US 7 670 472 B2 so as to allow the resulting electrolysis cell for chlor-alkali electrolysis to be operated with oxygen depolarisation. For this, an electrode consisting of a nickel grid onto which a catalyst strip made of silver oxide and PTFE has been rolled, as in principle described in DE 37 10 168 A1, was used as the oxygen-depolarised cathode. The current supply to the oxygen-depolarised cathode located in the cathode compartment was realised in such a manner that a lamella-type support structure positioned parallel to the back wall of the cathode has been inserted, said structure being electrically connected to the back wall via vertically arranged webs by means of welded joints. An elastic element is attached to this support structure so that when the cathode half shell and the anode half shell of the cell are screwed together, a press-fit is created with the wire grid of the oxygen-depolarised cathode, which ensures electrical contact and an even current distribution.
Such elastic elements are already described in various patent specifications, for example in EP 1 446 515 A2 and particularly EP 1 451 389 A2, and consist of various compressible layers made of metal wires, which, pressed together like a sandwich, ensure elasticity.

[0013] The process for treating the nickel oxide surfaces described in DE

034 886 A1 was used on the nickel components in order to ensure the conductivity of the nickel oxide surfaces resulting from passivation during operation.

[0014] In a test series 1 two such redesigned electrolysis cells having an active electrolysis surface of 2.7m2 with Flemion F8020 membranes were operated at a current density of 4I(A/m2, 88 C operating temperature, an NaCI anolyte concentration of 210 g/I, an NaOH catholyte discharge concentration of 32%w/w and with saturated moist oxygen in a stoichiometric excess of 20%. Fla. 1 shows the voltage curve of two electrolysis cells over the first 65 days of operation. Different symbols are used for each of the electrolysis cells (shaded diamonds and unshaded triangles).

[0015] In the first 30 days of operation, the electrolysis cells showed a stable cell voltage. On the 30th day of operation, the current to the two electrolysis cells was turned off. After it had been switched back on and a current density of 4kA/m2 had been reached, both cells showed an increased ohmic resistance in the form of a voltage increase of up to 100mV. After another 4 days of operation, the electrolysis cells were switched off again. After they had been turned back on and a current density of 4kA/m2 had been reached, the ohmic resistance had increased further, resulting in an additional voltage increase of approximately another 200nnV. After approximately another 30 days of operation, the two electrolysis cells were switched off and the components inspected.
This showed that the conductivity of the components made of nickel ¨ the support structure and elastic element ¨ had decreased considerably. The oxygen-depolarised cathodes used were checked in laboratory cells and compared with reference specimens.
During laboratory operation, this component also exhibited an increased voltage compared to the reference specimens, which could at least in part be attributed to reduced conductivity of the nickel wire grid due to oxidations. The protective effect of the process described in DE 10 2004 034 886 A1 was thus not effective under certain potential and operating conditions, which obviously occur when the electrolysis cells are switched off.

[0016] Based on the thermodynamic equilibrium reactions for noble metals such as silver and gold, described in the 'Atlas of Electrochemical Equilibria in Aqueous Solutions' (1974), the electrochemical stability diagrams for chlor-alkali electrolysis operating conditions at 85 C were recalculated in order to obtain a detailed overview of the electrochemical conditions:

[0017] For nickel, the result for 10-6 mol/kg at 85 C against an NHE
(normal hydrogen electrode) is, in simplified form, the stability diagram shown in Fig. 2. Here, environment A is characterised by passivation, environments B and C by corrosion and environment D by immunity. According to this, at 85 C the corrosive environment for hydroxide formation is always passed through in the start-up environment (load and potential increase) and shutdown environment (load and potential decrease) and these therefore represent the critical operating conditions.

[0018] For gold, the result, for 10-6 mol/kg at 85 C against an NHE (normal hydrogen electrode) is, in simplified form, the stability diagram shown in Fiq. 3.
Here, environment A
is characterised by passivation, environment B by corrosion and environment D
by immunity.

[0019] Similar to nickel in Fig. 2, the diagram shows a possible corrosive environment in mid-range potentials, where hydroxide compounds may be formed.
However, tests using gold in strongly alkaline caustic soda solution show hardly any signs of decomposition. It can thus be concluded that there is a kinetic hindrance and gold can be regarded as a stable metal for chlor-alkali electrolysis under oxidising conditions.

[0020] For silver, the result, for 10-6 mol/kg at 85 C against an NHE
(normal hydrogen electrode) is, in simplified form, the stability diagram shown in Fig. 4. Here, environment A is characterised by passivation, environment B by corrosion and environment D by immunity.

[0021] From Fig. 4 it is clear that silver also has a narrow corrosion environment albeit in the sour pH range. In the alkaline and in particular under oxidising conditions silver tends towards passivation through the formation of oxidising species.
Corrosion stability would thus be given, the question of conductivity under chlor-alkali electrolysis conditions with oxygen-depolarised cathodes would need to be tested.

[0022] WO 01/57290 A1 "Electrolysis cell provided with gas diffusion electrodes"
describes an electrolysis cell with gas diffusion electrodes, in which attention is drawn to the protective function of silver coatings under oxidising conditions. In particular, it describes a metal current conductor with openings, said conductor being made of silver, stainless steel or nickel, although nickel should preferably be coated with silver.

[0023] As literature and the experience of various operators confirm the stability of silver on nickel, the nickel components of the electrolysis cells were electro-silver-plated.
For this, a coating thickness of approximately lOpm was applied to the nickel.

[0024] In a test series 2, two electrolysis cells were tested in continuous operation in a similar manner to test series 1. Both cells have an active electrolysis surface of 2.7m2 and are equipped with Flemion F8020 membranes. The continuous current density was 4kA/m2, the operating temperature 88 C, the NaCI anolyte concentration 210 g/I, the NaOH catholyte discharge concentration 32%w/w and the stoichiometric excess of saturated moist oxygen was again 20%. Fig. 5 shows the voltage curve over 80 days of operation. Different symbols are used for each of the electrolysis cells (shaded diamonds and unshaded triangles).

[0025] The results of test series 2 according to Fig. 5 again show a voltage increase.
This time it was continuous. The start-up and shutdown procedures which occurred repeatedly during the operating period had no perceptible effect on the cell voltage, unlike the observations made for test series 1 on the basis of Fig. 1.

[0026] The cell elements were inspected after 80 days of operation and the condition of the metallic support structure and the metallic elastic element analysed.
By way of example, transverse micrographs of silver-plated nickel monofilaments of the elastic element are shown on a scale of 100:1 in Figure 6. The micorgraph clearly shows the silver spalling off in the filament sample from the bottom while the sample from the top exhibits a loosened silver coating and a reduction of approximately 50% in the coating thickness.

[0027] Material comparisons between samples from the upper and lower part of the cell element also show a transfer of decomposed silver, which dissolves at the top due to corrosion and is deposited again on the bottom part of the cell (data not shown). It can thus be seen that simply electroplating the nickel with a layer of silver under oxidising electrolysis conditions under no circumstances suffices to form an electrochemically stable connection.

[0028] These tests show that there is a further need to provide coatings that result in electrochemically stable connections in the form of stable conductivities of the metallic components of the cathode half shell without the above disadvantages occurring.

[0029] The aim of the present invention is thus to achieve the following objectives:
¨ Provision of an alternative corrosion protection coating for the metallic cell element components of the cathode half shell of an electrolysis cell ¨ Guaranteeing an increased adhesive strength of the coating on the surfaces of the cell element components so that no non-conductive oxide layers can be formed ¨ Stable operation of an electrolysis cell in respect of the cell voltage being as constant as possible over a longer period of time at a given current load despite an arbitrary number of start-ups and shutdowns and thus a longer life cycle ¨ Minimisation of the ohmic and thus conductivity losses during current conduction from the metallic components to the metallic grid of the oxygen-depolarisation cathode [0030] The objective is achieved by a cathode half shell of an electrolysis cell comprising metallic cell element components comprising - a metallic support structure welded to the back wall of the cathode half shell and at least one metallic elastic element positioned plane-parallel thereon, - an oxygen-depolarised cathode which lies against at least one metallic elastic element, said oxygen-depolarised cathode comprising a perforated metallic grid and a catalyst strip made up of PTFE and silver oxide mechanically press-fitted thereto, wherein the silver oxide is reduced to silver during operation of the electrolysis plant and in so being generates a uniform connection/bond between the components of the oxygen-depolarised cathode and the at least one elastic element, said connection/bond being characterised by high conductivity, wherein at least one of the metallic components is provided with an electrically conductive coating comprising at least two layers, wherein - a first layer, which is applied directly to the cell element materials, is selected from a group which contains Au, B-doped nickel, Ni sulphides and mixtures thereof, this first layer having a layer thickness of 0.005 to 0.2 pm, and - a second layer, which is applied to the first layer, is made of silver, this second layer having a layer thickness of 0.1 to 30 pm.

[0031] The present invention also claims that all cell element components which are contained in the cathode half shell and conduct an electric current are coated. Here, preferably those cell element components of the cathode half shell of the electrolysis cell which are in contact with caustic soda solution have the inventive coating.

[0032] Also claimed is the use of the inventive cathode half shell of the electrolysis cell in chlor-alkali electrolysis.

[0033] The following figures are used to describe the invention in greater detail.

Fig. 1: Electrolysis cell voltage for test series 1: Shown is the voltage curve over the first 65 days of operation of an electrolysis cell using an electrode as described in DE 10 2004 034 886 A1.
Fig. 2: Simplified stability diagram for Ni-H20 at 85 C against NHE
Fig. 3: Simplified stability diagram for Au-H20 at 85 C against NHE
Fig. 4: Simplified stability diagram for Ag-H20 at 85 C against NHE
Fig. 5: Electrolysis cell voltage for test series 2: shown is the voltage curve over 80 days of operation of an electrolysis cell using metallic cell element components that are provided with a 10 pm thick layer of silver.
Fig. 6: Transverse micrograph of a silver-plated nickel wire from test series 2 on a scale of 100:1 Fig. 7: Electrolysis cell voltage for test series 3: shown is the voltage curve over 240 days of operation of an electrolysis cell using metallic cell element components that are coated with a 0.15 pm thin layer of gold and a 25 pm thick layer of silver.
Fig. 8: Transverse micrograph of the silver-plated nickel wire with an intermediate layer of gold from test series 3 on a scale of 25:1 Fig. 9: Transverse micrograph of the silver-plated nickel wire with an intermediate layer of gold from test series 3 on a scale of 500:1 Fig. 10: SEM image of Ni-Ag binder layer with light layer of gold Fig. 11: Basic configuration of the metallic cell element components in a cathode half shell which are provided with the inventive coating [0034] From materials chemistry it is known that nickel and silver do not bond. Even above melting point, these metals cannot be mixed, they merely form a monotectic system. As this behaviour does not apply to mixtures of nickel/gold and gold/silver, . .
. , coating tests of 3-layer systems were commenced. As a result the nickel components were first coated with a thin 0.15 pm layer of gold followed by a 25 pm-layer of silver. The nickel components which were thus prepared were installed in newly manufactured chlor-alkali electrolysis cells with oxygen-depolarised cathodes and subjected to a continuous load test in test series 3.

[0035] In test series 3, two electrolysis cells were tested in continuous operation in a similar manner to test series 2. Both cells have an active electrolysis surface of 2.7m2 and are equipped with Flemion F8020 membranes. The continuous current density was 4kA/m2, the operating temperature 88 C, the NaCI anolyte concentration 210 WI, the NaOH catholyte discharge concentration 32%w/w and the stoichiometric excess of saturated moist oxygen was again 20%. Fig. 7 shows the voltage curve for test series 3 over 240 days of operation. Different symbols are used for each of the electrolysis cells (shaded diamonds and unshaded triangles).

[0036] The results of test series 3 according to Fia. 7 show a slight voltage increase at the beginning which can be attributed to the characteristics of the oxygen-depolarised cathode used. This is then followed by a stable phase over more than 200 days of operation. A plurality of start-ups and shutdowns have no perceptible influence on the cell voltage.

[0037] After completing the test, the 3 metallic components ¨ the support structure, the elastic element and the oxygen-depolarised cathode including the wire grid ¨ were inspected and the condition verified via micrographs. This is shown in Figures 8 and 9.
Significant loosening of the layers or spalling off was not observed. The nickel support structure is evenly electro-silver-plated, the surfaces are slightly roughened.

[0038] Surprisingly, a physically uniform bond between the oxygen-depolarised cathode and the elastic element positioned plane-parallel below it was also found during the inspection. The silver oxide rolled into the catalyst strip of the oxygen-depolarised cathode is reduced to silver during the first start-up of the electrolysis cell. In the process, a physically extremely uniform bond results from the silver strip that forms, the metallic grid of the oxygen-depolarised cathode and the at least one elastic element that has the inventive coating, said bond being very difficult to separate during disassembly as the silver layers of the components have at least partially formed a chemical bond. This type of bond leads to low ohmic losses during current transport through the electrolysis cell and a low and stable cell voltage that is not affected by start-ups and shutdowns is thus achieved during long-term operation.

[0039] Fig. 10 shows the condition of the intermediate gold layer which forms the binder layer between the nickel and silver. There is no noticeable corrosion here either.

[0040] Finally, Fiq. 11 shows a basic configuration of the metallic cell element components provided with the inventive coating. The base is the cathode half shell (1).
Metallic webs (2) that are welded to both the back wall as well as the current distributor (3) component, are arranged parallel to the narrow side wall. The elastic element (4) component is press-fitted between the current distributor (3) and the oxygen-depolarised cathode. The oxygen-depolarised cathode positioned plane-parallel thereto consists of a perforated metallic grid, or rather wire grid (5), onto which a catalyst strip (6) is rolled, which, during operation of the electrolysis cell for the intended purpose, forms a connection/bond with the metallic grid (5) and the elastic element (4), said connection/bond being characterised by high conductivity and thus a low ohmic resistance.

Claims (4)

1. Cathode half shell of an electrolysis cell comprising metallic cell element components comprising .circle. a metallic support structure welded to the back wall of the cathode half shell and at least one metallic elastic element positioned plane-parallel thereon, .circle. an oxygen-depolarised cathode which lies against at least one metallic elastic element, said oxygen-depolarised cathode comprising a perforated metallic grid and a catalyst strip made up of PTFE and silver oxide mechanically press-fitted thereto, wherein the silver oxide is reduced to silver during operation of the electrolysis plant and in so being generates a uniform connection/bond between the components of the oxygen-depolarised cathode and the at least one elastic element, said connection/bond being characterised by high conductivity wherein at least one of the metallic components is provided with an electrically conductive coating comprising at least two layers, wherein ~ a first layer, which is applied directly to the cell element materials, is selected from a group which contains Au, B-doped nickel, Ni sulphides and mixtures thereof, this first layer having a layer thickness of 0.005 to 0.2 µm, and ~ a second layer, which is applied to the first layer, is made of silver, this second layer having a layer thickness of 0.1 to 30 µm.

2. Cathode half shell of an electrolysis cell according to claim 1, characterised in that all cell element components of the electrolysis cell that conduct an electric current are coated.

3. Cathode half shell of an electrolysis cell according to claims 1 or 2, characterised in that all cell element components of the electrolysis cell that are in contact with caustic soda are coated.

4. Use of the cathode half shell of the electrolysis cell according to one of the preceding claims, characterised in that the electrolysis cell is used for chlor-alkali electrolysis.