Process and Apparatus for Continuously Coating a Solid Electrolyte With a Catalytically Active Metal

1. Field of the Invention

This invention relates to a process for continuously coating a solid electrolyte with a catalytically active metal, as well as an apparatus for implementing the method.

2. Description of the Prior Art

In aqueous electrolysis, in particular in the production of hydrogen by decomposition of water, solid electrolytes based on organic synthetic materials are increasingly used. It proves necessary to coat solid electrolytes of this type, which as a rule are in the form of sheets, with a suitable electro-catalyst which in most cases is a platinum metal. A coating process of this type, which is based on electrolytic deposition of the metal in question, and a corresponding apparatus are known from German Offenlegungsschrift No. 2,821,271. According to the latter, the plastic sheet which is to be coated is first impregnated with an aqueous metal salt solution and then inserted into a sandwich electrolytic cell and subjected to an electrolysis, the metal being deposited on the cathode side.

The process and apparatus of the type described are quite satisfactory for laboratory purposes and usually have relatively small dimensions. Difficulties arise in the coating of large-area sheets and of large quantities, above all with respect to maintaining the bath temperature and the uniformity of the electrolysis process. In addition, such a method would not be very economical. There is therefore a requirement for improving the process and for suggesting a more suitable apparatus.

Accordingly, one object of this invention is to provide a novel process and apparatus for continuously coating a solid electrolyte with a catalytically active metal, whereby a perfect and uniform deposition even on large-area sheets is ensured in a simple and economical manner.

A further object, in particular, is to make industrial large-scale production possible.

These and other objects are achieved by providing a new and improved process and apparatus for continuously coating a solid electrolyte with a catalytically active metal wherein the solid electrolyte in the form of a sheet is dried, is treated in a solution which contains the metal as a salt, then is rinsed in distilled water and finally is subjected to an electrolysis step in a cell which is fed with distilled water of constant temperature and to which a constant current density is applied. The solid electrolyte is passed through, in a water bath between two rolls formed as electrodes, wherein a felt of carbon is interposed on the cathode side.

A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1 is a diagrammatic cross-sectional view through one embodiment of the apparatus of the invention.

FIG. 2 is a semi-diagrammatic longitudinal cross-sectional view through one embodiment of the equipment; and

FIG. 3 is a diagrammatic cross-sectional view through another embodiment of the equipment.

Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, and more particularly to FIG. 1 thereof, a first type of equipment suitable for carrying out the process is shown diagrammatically. In a rectangular trough 1 which can consist, for example, of polypropylene, there is a water bath 2 (distilled or demineralized water). Within the trough 1 are provided two rolls 4 and 5 which advantageously are arranged with horizontal axes and serve as the electrodes. The lower roll 4 which serves as the anode and preferably consists of platinized titanium is completely submerged in the water bath 2, whilst the upper roll 5 which acts as the cathode and advantageously consists of corrosion-resistant steel is immersed in the water only in the electrolysis zone. The solid electrolyte 3 which is to be coated and is in the form of a sheet is located between the two rolls 4 and 5, a felt 6 of carbon being interposed between the two rolls 4 and 5. The felt 6 is arranged on the cathode side and, in the present example, coaxially envelops the roll 5. It is rigidly bonded to the latter, for example by means of a synthetic adhesive (epoxy resin). The contact force is indicated by a vertical arrow F.

FIG. 2 represents a semi-diagrammatic longitudinal cross-sectional view through an embodiment of the apparatus of the invention. In a rectangular trough 1 of polypropylene, a frame 7 of the roll support is fitted, wherein a lower roll 4, which serves as the anode, of platinized titanium in a stationary mounting is driven by a drive motor 10. An upper roll 5 of stainless steel, which serves as the cathode, and has a felt 6 of carbon as a sheathing, which is firmly bonded thereto by means of synthetic resin, is mounted for vertical displacement in insulated bearings 8 which are in turn supported via springs 9 on the frame 7. The contact pressure of the roll 5 can be adapted in an optimum manner to the particular conditions by selecting the spring pre-tension which can be adjusted via the fixing screws. The spring force is indicated by vertically downward-pointing arrows F. The rolls 4, 5 are provided with current leads 11, 12. Reference numeral 3 represents the cross-section of the solid electrolyte, which is to be coated, in the form of a sheet. The level of the water bath 2 (distilled or demineralized water) is not important. The water can partially or wholly cover the upper roll. The drawing of the apparatus is not true to scale. In particular, the roll width can be many times that in the illustration and depends on the width of the sheet which is to be coated.

FIG. 3 shows a diagrammatic cross-sectional view through another embodiment of the apparatus of the invention. The trough 1, the water bath 2, the solid electrolyte 3 in the form of a sheet and the lower roll 4 serving as the anode (positive pole) largely correspond to the arrangement according to FIG. 1. The upper roll 5 acting as the cathode, however, does not carry a sheathing, but the felt 6 of carbon is merely inserted loosely on the cathode side between the solid electrolyte 3 and the roll 5 and, in the zone of the rolls, is passed through plane-parallel with the solid electrolyte 3. This arrangement enables the felt 6 to be rapidly exchanged, if required by circumstances (different dimensions, for example thickness, physiochemical nature, wear, and the like).

The invention is not restricted to the embodiments shown in the figures. The principle of the rolls can also be put into practice by different arrangements. For example, the rolls can also be arranged with their axes in a vertical position or inclined position (at an angle to the horizontal).

See FIG. 1 and FIG. 2.

A dried sheet of solid electrolyte 3 (base: perfluorinated sulphonic acid: trade name "Nafion" from DuPont de Nemours) having a square area of 250 mm side length and a thickness of 0.2 mm was impregnated for 30 minutes in a solution of 0.5% by weight of Pt(NH.sub.3).sub.2 (NO.sub.2).sub.2 (diamminoplatinum dinitrite) in distilled water at 90.degree. C. After the impregnation, the sheet was removed from the solution and rinsed with distilled water. The sheet was then subjected in the apparatus according to FIG. 2 to an electrolysis process by passing it through between the rolls 4 and 5. The deposition of the metallic platinum layer on the cathode side was carried out under the following conditions:

Total contact force 2F of the springs: 220 N

Current intensity: 30 A

Speed of rotation of the lower roll: 0.17/minute

Corresponding speed of advance of the sheet: 1.6 cm/minute

Temperature of the water bath: 25.degree. C.

After the end of the disposition process, the solid electrolyte sheet was boiled up in 1 N hydrochloric acid in order to remove the unconverted platinum complex salt. The coating thickness of the deposited platinum layer was about 0.5 mg/cm.sup.2.

Of course, the process is not restricted to the above example. In principle, any catalytically active metals can be continuously deposited from suitable salt solutions onto the surface of a solid electrolyte in this manner. As a variation, the process which can be realized with the equipment according to FIG. 3 can here also be used.

The process according to the invention and the apparatus according to the invention enable solid electrolytes to be continuously coated over a large area with catalytically active metals, in particular noble metals, firmly adhering surface layers of high uniformity being obtained.

Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.