EP0142638A1 - Electrodes based on nickel, cobalt, iron, with active coating, and process for their manufacture - Google Patents

Abstract

A process for the manufacture of thin activated electrodes including a base metal, which can be iron, cobalt or nickel, with a good adhering active surface layer. Both the base metal and the surface layer are galvanically sequentially deposited as separate layers on a removable electrically-conductive carrier. The surface layer is deposited as an activatable alloy including the base metal and a leachable metal, particularly zinc. The galvanizing deposited layers are subsequently separated as a unit from the carrier. The alloy is activated by leaching the leachable metal therefrom before, during or after separation of the carrier. Two activatable layers of alloy can be deposited on the carrier with a layer of the base metal deposited between the alloy layers.

Description

The invention relates to a method for producing activated electrodes from base metal on the basis of iron, cobalt, nickel with active coating by means of galvanic deposition and activation, and to electrodes obtainable thereafter.

Some technical electrolysis processes such as alkaline water electrolysis or chlor-alkali electrolysis require an electrode with a low hydrogen overvoltage as the cathode. Furthermore, an anode is required for the alkaline water electrolysis, which enables a surge-free development of oxygen. For this reason, catalytically active electrodes are used.

The best known catalyst for hydrogen evolution is platinum. Because of its high price, however, only very thin Pt coatings are provided on the electrodes, which are typically below 1 mg Pt / cm ² , which is at the expense of effectiveness, which is then no longer fully satisfactory. As can be seen from comparative experiments by Teledyne Energy Systems (Hydrogen Energy Progress-IV, Editors TN Veziroglu, WD Van Vorst, JH Kelley, Pergamon Press, Oxford, 1982 pages 151-158), catalysts based on non-noble metals are generally cheaper .

As an example of this, Teledyne reviewed nickel / molybdenum catalysts developed by the BP Research Center in Middlesex (EP patent application 79 301 963.9). Furthermore, nickel boride catalysts (DE-PS 2 307 852), which, however, gave higher hydrogen overvoltages compared to platinum coatings (Hydrogen Energy Progress III-Editors TN Veziroglu, K. Fueki., T.Ohta, Pergamon Press, Oxford, 1981 pages 15- 27). Nickel sulfides are known as further catalysts (BE-PS 864 275) and various coatings with mixed transition metal oxides (Seminar on Hydrogen as an Energy Vector, EEC Report Eur 6085, 1978, pages 166-180) or transition metals (GB-PS 1 510 099 or U.S. Patent 4,152,240).

The hydrogen overvoltage is reduced very effectively by using Raney nickel electrodes (E. Justi, A. Winsel, "Cold Combustion, Fuel Cells", Franz Steiner Verlag, Mainz, 1962). However, these electrodes, which were originally produced using a pressing process, can only be produced with great difficulty and uneconomically in the usual technical dimensions. LURGI has therefore developed an improved rolling process (J. Müller, K. Lohrberg, H. Wüllenweber, Chem.-Ing.-Techn. 52 (1980) pp. 435-36), which enables the electrode work surface to be easily enlarged. Then nickel or steel sheet is plated with Raney nickel powder and activated in the usual way by treatment with KOH. In addition to a low overvoltage, the excellent long-term behavior of the electrodes produced in this way is remarkable: after an operating time of 1 year at a current density of 2 kA / m ² and an operating temperature of 92 ° C, the electrode potential was unchanged.

Similar long-term behavior is also shown by electrodes coated with Raney nickel, which can be obtained particularly simply by activation of an electrodeposited Ni / Zn layer (J. Divisek, H. Schmitz, J. Mergel: Chem.-Ing.-Techn. 52 (1980) p. 465): From a solution containing Ni ^{2 +} and Zn ^{2 +} ions, an Ni / Zn alloy is electrodeposited on an electrode matrix and then activated with KOH solution in the usual way to give _R aney nickel. The electrode thus obtained can be used as Serve cathode in alkaline hydrogen evolution or as a cathode and / or anode in alkaline water electrolysis. The matrix used must have good electrical conductivity and can have different geometric shapes, so that one is not bound in this regard when designing an electrolyser. Preferred geometric shapes of such an electrode matrix are wire mesh, expanded metal or perforated sheet. The latter form is particularly important because it enables the so-called "sandwich construction" in an electrolytic cell for alkaline water electrolysis, in which the two electrodes are pressed directly onto the gas separator (diaphragm, ion conductor) with "zero spacing" so that the electrode spacing is minimized and the ohmic voltage drops become negligible.

In the activation of thin nickel electrodes, especially when perforated sheets now, however, difficulties arise: Useful deposits of Raney alloys, particularly Ni Zn alloys of a Ni ^{2 +} / ^-, and Zn ²⁺ ions electrolyte containing require cathode current densities of 4 -7 _{A /} dm ² or more. At the same time, the potential across the entire surface of the electrode matrix to be coated with an activatable alloy must be as uniform as possible, ie, for uniform, controlled deposition, the potential differences within the cathode caused by ohmic voltage drop must not exceed 40 mV. With small sheet thicknesses of the electrode matrix of approximately 0.2-0.5 mm, as are preferred for alkaline water electrolysis because of the lower price and technological advantages compared to sheet thicknesses of 1 mm and higher, such potential differences within the cathode surface can be expected : For example, with the electroplated Ni / Zn coating of a technically dimensioned electrode Matrix for the alkaline water electrolysis of 2 - 4 m ² , during the electrodeposition electrical currents in the order of 1000 - 3000 A. In order to be able to carry out the electrodeposition at such currents with a potential difference in the electrode of less than 40 mV, the ohmic resistance in the sheet must be kept correspondingly small. Either you increase the sheet thickness of the electrodes and accept considerable technological disadvantages and increased costs, or you keep the current path through the electrodes correspondingly short during the electrodeposition. In the case of larger electrode areas, this is only possible through a large number of contact points or lines. Only then can the galvanic deposition become uniform and reproducible and, after subsequent treatment with KOH, can be placed in an activated layer (Raney nickel layer) of an electrode for chloralkali electrolysis or alkaline water electrolysis being transformed.

Within the electrolyser for alkaline electrolysis, multiple contacts across the electrode surface are also often required, but these do not usually coincide with the ones described above, so that the contacts used for the coating have to be removed again and new ones have to be attached. This creates uniformity on the electrode surface and thus errors that interfere with performance. If, on the other hand, one provides pressure contacts for the current distribution during the electrodeposition of the nickel alloy, then uncoated areas arise at the pressure points, which later also interfere with the electrolysis.

A uniform, error-free and controlled galvanic Ni / Zn coating of thin electrodes with technical dimensions can therefore not be achieved in the usual way. The same naturally also applies to electrodes based on Co and Fe, which are coated with an alloy of an electrode base metal and a metal such as tin or zinc which can be removed by lye treatment and then removed this component can be activated.

The invention is therefore based on the object of providing a method with which even the thinnest electrodes with a perfect catalytically active coating can be produced in a simple manner.

The process according to the invention developed for this purpose is characterized in that both the activatable alloy required for forming the active layer (s) from the base metal and metal which can be removed by lye treatment, in particular zinc, as well as the base metal in a removable electrically conductive carrier deposits the sequence required for the electrode to be produced one after the other and activates the activatable alloy before, during or in particular after removal of the carrier by lye treatment.

Although the galvanic production of electrode sheets and also the activation of such sheets by galvanic deposition of activatable alloys and their activation by alkali treatment are known, so far both steps (sheet production and galvanic deposition of activatable alloy) have been carried out separately without exception, that is, the already finished sheet is galvanically provided with activatable alloy in a separate process. This procedure has the disadvantages mentioned above, in particular in the production of thin electrodes, with additional adhesion problems of the alloy layer on the electrode sheet due to surface changes which are difficult to detect due to drying, storage, shipping and manipulation of the sheets.

According to the invention, electrode base metal, such as, in particular, pure nickel, is electrodeposited as a supporting core layer and activatable alloy, in particular Ni / Zn alloy (with a uniform or changing Zn concentration), in a different, expedient order on an electrically well-conductive base. Such removable carriers for the galvanic production of metal sheets and components are known in the art. To produce an electrode that is active on both sides, for example, an activatable alloy (for example NiZn) is first deposited on a carrier and then pure base metal (for example nickel), after which an activatable alloy layer (for example again NiZn) is applied again. However, it is also possible to produce double layers which can only be activated on one side or, for example, to deposit powder particles, for example nickel powder, together with the base metal, which leads to roughening of the core layer.

The alloy layer is activated in the usual way by lye treatment, which is carried out before the multilayer deposition is detached from the support or simultaneously with it. In particular, however, the galvanic deposition is first removed from the carrier and then activated by lye treatment.

According to the invention, self-supporting activated electrodes with a thickness of less than 1 mm are obtained, in particular by a rapid process in which the layers are deposited with a current strength of approximately 10-20 A / dm ² .

The thickness of the core layer is preferably about 0.1-0.5 mm, in particular between 0.1 and 0.3 mm, and that of the activatable alloy is about ^10-100 μm.

The core layer (as well as the activation layers) are made in the desired form, e.g. produced continuously or as a perforated plate or the like, which is easily possible by metting or by using photoresist in the manner in which printed circuits are produced.

The layers of the electrodes to be activated can also be provided with a "hole pattern" different from the core layer, so that the finished electrode has bright metallic areas which then makes pressure contacting of the electrodes possible. For example, the first activatable layer formed on the removable base can be formed with a hole pattern corresponding to the later points of contact of the electrode on a base such as a checker plate, after which the core layer is applied including these areas. Such hole pattern formation only in the activation layer can be achieved according to known technology with the help of two-layer photomasks or two single-layer masks. At such bare metal areas of the activated electrode, the contact resistance is practically negligible when the electrodes are loosely springy in contact with a metallic conductive "base".

The production process is explained in more detail below using examples:

example 1

A 50 µm thick layer of Ni / Zn alloy (Ni / Zn weight ratio 60:40) was first electroplated onto a cathodically polarized base made of polished chrome-plated steel. The electrolyte contained NiCl ₂ , ZnCl ₂ and H ₃ B0 ₃ . Electrolysis was carried out at 60 ° C. and a cathodic current density of 5 _{A /} dm ² . A 150 μm thick layer of pure nickel was then deposited. Electrolysis was carried out with a current density of 5 _{A /} dm ² and bath temperature of 50 ° C. in an electrolyte which contained _N iS0 ₄ , _N aCl and H _3B 0 ₃ . A third 50 µm thick Ni / Zn layer of the same composition as the first was then galvanically produced.

• Kathodisch: - 1005 mV
• Anodisch: + 490 mV

The resulting sheet of three layers was mechanically removed from the stainless steel electrode (Fig. 1) and then treated with hot KOH, so that Zn dissolved and a porous nickel layer was left behind. In this way, one Obtained catalytically coated electrode on both sides with Raney nickel, which was used as an anode and cathode for an alkaline water electrolysis: The following potential values were measured against an HgO reference electrode in 10 _M KOH at 100 ° C. and a current density of 200 mA / cm ² :

• Cathodic: - 1005 mV
• Anodic: + 490 mV

The sum of both potentials is 1495 mV. Water electrolysis with these electrodes and a NiO diaphragm worked at a cell voltage of 1.53 V. Both are extremely good characteristics.

Example 2

chrome-plated On a polished / steel base, a paint mask was created photographically using the technique common to printed circuits, the pattern of which enabled the galvanic deposition of a perforated plate (hole diameter 4 mm, free surface 41%). As in Example 1, three layers were again electrodeposited (first Ni / Zn, second Ni, third Ni / Zn), the perforated plate formed in this way was mechanically removed from the stainless steel base and activated with KOH.

The obtained active electrodes were fitted together with a _N i _O -Diaphragma in an electrolysis cell for alkaline water electrolysis, and performing the electrolysis. A cell voltage of 1.52 V was measured in _K OH solution at 100 ^° C. and 200 mA / cm ² . The entire current-voltage curve is shown in FIG. 2.

The above two examples clearly show that active electrodes for alkaline electrolysis can be obtained very easily according to the invention. The coating and electrode shapes can be easily specified and varied similarly to the printed circuits.

The electrodes produced according to the invention are not only characterized by high effectiveness, but also by easy manufacture and lower material consumption, and are therefore very inexpensive. Very thin, large-area, active electrodes can only be produced galvanically in this way: Starting from a given thin nickel sheet of 150 µm, for example, larger electrodes could not be activated in a uniform and controlled manner by Ni / Zn deposition and subsequent alkali treatment because of the contact difficulties mentioned above .

Example 3

The procedure of Example 1 was repeated, but the nickel layer was deposited from an electrolyte with carbonyl nickel powder whirled up therein, so that powder particles were galvanically fixed together with the nickel layer which formed. In this way, a roughening could be built into the Ni intermediate layer, which is useful for some purposes, e.g. to reduce potential differences by increasing the geometric surface. This electrode also showed the same excellent properties in water electrolysis after activation as the electrode according to Example 1.

_M an can of course also produce electrodes that are based on only two layers (a Ni layer and an activatable Ni-Zn alloy layer). During the deposition of the alloy, a changing Ni: Zn ratio can also be provided by varying the current density, and in this way not only three discrete layers but also a multi-layer material can be produced. The latter procedure is useful for certain special purposes.

Claims (8)

1. A process for the production of activated electrodes made of base metal on the basis of iron, cobalt, nickel with active coating by electrodeposition and activation, characterized in that both the formation of the active layer (s) required on a removable electrically conductive carrier as well as the _G around metal in the time required for the produced electrode order successively deposited before and activatable alloy, during, or especially after removal of the carrier activated activatable alloy of the base metal and by alkali treatment leachable metal, in particular zinc by alkali treatment. 2. The method according to claim 1, characterized in that the removable, electrically conductive by metting or _d e carrier prior to the galvanization by metting or photoresist is provided with an insulating lacquer pattern corresponding to the desired electrode structure. 3. The method according to claim 1 or 2, characterized by a double deposition of activatable alloy with a base metal layer deposited in between. 4. The method according to any one of the preceding claims, characterized in that together with the base metal separation alkali-resistant metal powder, in particular nickel powder, is fixed galvanically. ₅ . Method according to one of the preceding claims, characterized in that at least one of the activatable layers is produced with a hole pattern required for the subsequent pressure contacting of the electrode on exposed base metal. 6. The method according to any one of the preceding claims, characterized in that the activatable alloy is deposited in a thickness of 10 to 100 pm and the base metal itself in a thickness of 0.1 to 0.5 mm, in particular 0.1 to 0.3 mm becomes. 7. The method according to any one of the preceding claims, characterized in that the deposition takes place with a current density of 10 to 20 A / dm ² . 8. Activated electrodes with a thickness of <1 mm obtainable according to one of the preceding claims.

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