HK1065647B - Method for producing gas diffusion electrodes - Google Patents

Description

The present invention relates to a method for the manufacture of porous gas diffusion electrodes of the type described in claim 1, such a gas diffusion electrode may be based, for example, on a catalytically active silver or silver alloys for use in electrochemical cells, particularly chlorine-alkali electrolysis, or alkaline fuel cells.

In electrochemical cells, the reduction of oxygen to platinum, silver, or even carbon is performed. Platinum is usable in both acidic and alkaline environments, whereas silver and carbon are corrosion stable only in alkaline electrolytes. However, in the case of the silver catalyst, a rapid deviation also occurs in alkaline media, which is explained by the overlay of the oxidized surface of the silver. (Texas Instruments, US 35 05 120).

In addition to stabilization, the process for producing an active silver catalyst must also ensure that the active surface of the silver is sufficiently large, i.e. the grain size of the silver is as small as possible.

In addition to controlling pH, temperature and saturation, crystallization germs play an important role in producing the smallest silver particles. A known process (EP 0 115 845) is to reduce a mixture of silver nitrate and mercury nitrate on a PTFE dispersion by adding potassium oxide, thus producing a silver amalgam with the smallest particle diameter.

In order to produce gas diffusion electrodes from these catalysts, as required in fuel cells or in chlor-alkali electrolysis, the powders must be processed into a homogeneous, flat electrode. This electrode must be electrically conductive and allow both electrolyte and gas to enter. So areas of the electrode must be able to be moistened, while other areas must be protected from moisture. A solution to this problem has been shown with a bi-porous pore structure. The electrolyte can initially penetrate both the small and large pores effortlessly.

Therefore, an attempt was made to build a biporous pore system using the material properties. This means that you need hydrophilic and hydrophobic materials. Suitable hydrophobic materials are some thermoplastics - such as polytetrafluoroethylene. The mentioned catalysts and the silver are always hydrophilic. So if you mix silver and PTFE together and form a flat electrode, these different areas have hydrophilic and hydrophobic properties.

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Two methods are known in which a thin, homogeneous gas diffusion electrode is rolled from such hydrophobic/hydrophilic materials. The method (EP 0 144 002, US 4 696 872) involves mixing the catalyst particles and the PTFE in a special mixer so that a fine mesh hydrophobic net system is deposited on the catalyst. In a powder roll, the loosely poured mass is rolled into a film of about 0.2 mm thick. This method has been proven for mixtures of PTFE and carbon, or PTFE and Raney nickel. It is also possible to see how an RTF silver foil alloy containing 80% aluminium can be rolled into electrically conductive foils.

However, in order to produce silver electrodes, a silver oxide/PTFE mixture is first processed in the powder roller and then electrochemically reduced (DE 37 10 168). The silver oxide is stable enough to withstand the impact of the rollers. In addition, the volume decreases when the silver oxide is converted into silver, so that additional pores are created in the gas diffusion electrode. The parameters used to reduce the particle size can be adjusted quite well. The disadvantage of this method is that it is not yet known how silver alloys with catalytic properties can be electrochemically reduced.

The purpose of the present invention is to provide a method for the manufacture of a gas diffusion electrode which not only avoids the disadvantages of the state of the art but in particular produces reproducible results in the process product.

A process of the type described at the outset solves this problem according to the invention by: The silver catalyst pore system is filled with a wetting fluid,a dimensionally stable solid with a grain size above that of the silver catalyst is mixed under the silver catalyst,thus this compression stable mass is formed into a homogeneous catalyst band in a calender and,in a second calender step,an electrically conductive by-product material is embedded in the catalyst band.

The special feature of the method of the present invention is that the inner pore system of the ductile materials is now filled with a liquid. Since this liquid cannot be compacted and is also firmly bound to the pore system by capillary forces, even at the present pressure of up to 600 kg/cm2 the liquid cannot be removed from the micropores. A further addition of some carbon powder or the volatile ammonium carbonate can continue to absorb the mechanical pressure of the powder rollers. These rough additions of typically 10-100 μm core diameter protect the porcelain system with a greater porosity, providing a greater degree of sealing.

A preferred implementation of this process is shown as follows: First, silver or a silver alloy is produced by a precipitation process. It is advantageous to perform the precipitation on a PTFE dispersion. The best experience is made with a mixture of 15% Teflon and 85% silver. By adding formaldehyde during precipitation, the silver hydroxide is immediately converted into a silver crystal in the alkaline environment. The precipitate is washed and dried. Subsequent tempering at 200°C improves the electrical contact between the silver particles and drives the remaining liquids.

The powder is mixed with a liquid of about 5%-40%, but preferably 8% of the liquid, which can penetrate into the pore system of PTFE and silver. Due to the hydrophobic nature of PTFE, only isopropanol, ethanol and methanol are suitable. When the powder is moistened and filled with such solvents, an exchange of liquids can follow.

Another type of wetting agent is the so-called surfactants, which penetrate the pore system and also cover the surface of the catalyst, thus reducing its surface roughness. This lower surface roughness now causes the silver catalyst to escape from the compression zone during the rolling process, while other powder components which have not been treated remain in the compression zone, thus forming the electrode bond in which the silver catalyst is embedded (Figure 4).

In a second pair of rollers, a metal supporting frame in the form of woven mesh or stretch metals can be rolled in, thus improving mechanical stability and electrical conductivity. After this process, the gas diffusion electrode is dried. After that, the electrode has a silver coating between 0.2 kg/m2 and 1.5 kg/m2.

This method can of course also be combined with other methods, which eliminate the harmful formaldehyde in the precipitation process and reduce it by electrochemical means after the production of the GDE.

The final gas diffusion electrode can be modified to allow better transport of the resulting sodium salt. For example, it is recommended to insert a coarse drainage system. This is possible by pressing a net onto the finished electrode and then pulling it back. The negative print of the net forms channels in which the electrolyte can later drain parallel to the electrode surface.

The following illustrations show further features, details and advantages of the invention. Fig. 1 (Fig. 1) a functional diagram of a system according to the invention,Fig. 2 a microscopic image of a silver electrode before use,Fig. 3 a silver electrode after use in the same representation,Fig. 4 a PTFE frame embedded in a silver catalyst,Fig. 5 a current/voltage diagram of a chlor-alkali electrolysis and inFig. 6 the same curve according to the invention.

Claims (6)

1. A process for the production of a gas diffusion electrode from a silver catalyst on a PTFE substrate, characterised in that

   - the pore system of the silver catalyst is filled with a wetting liquid,

   - a dimensionally stable solid body with a grain size above that of the silver catalyst is mixed among the silver catalyst,

   - that mass which is thus stable in respect of compression is shaped in a calender to form a homogeneous catalyst strip, and

   - in a second calendering step an electrically conductive discharge conductor material is impressed into the catalyst strip.
2. A process according to claim 1 characterised in that 5% isopropanol is used as the wetting liquid and 30% ammonium carbonate or ammonium hydrogen carbonate is used as the solid and those two fillers are expelled after electrode production by a heat treatment step at preferably 110°C.
3. A process according to claim 1 characterised in that a tenside - preferably 5% Triton X 100 - is used as the wetting liquid, which both penetrates into the pore system of the catalyst but also reduces surface friction so that the silver catalyst can slide out of the compacting zone and the dimensionally stable ammonium carbonate and the PTFE binder absorb the rolling pressure.
4. A process in particular according to claim 1 characterised in that in the first calender step a homogeneous catalyst strip of a thickness of 0.2 - 0.5 mm is produced.
5. A process according to one of the preceding claims characterised in that the rolling gap is set to 350 µm and the rolling advance is set to about 2 metres per minute.
6. A process according to one of the preceding claims characterised in that a silver-plated nickel wire mesh of a wire thickness of 0.15 mm and a mesh width of 0.45 mm with a silver covering of about 10 µm in thickness is used as the electrical discharge conductor material.