Gas diffusion electrode

Gas diffusion electrodes are electrodes in which the three states of matter - solid, liquid and gaseous - are in contact and the solid, electron conducting catalyst catalyzes an electrochemical reaction between the liquid and the gaseous phase. The solid catalyst is usually compressed to form a porous film having a thickness of 200 microns.

Particularly well known is their use in fuel cells, which arises from the gases hydrogen and oxygen in a kind of cold combustion water and electrical energy.

Pore system

An important prerequisite for the operation of gas diffusion electrodes that both the liquid and the gaseous phase may be present in the pore system of the electrodes at the same time. As is to be realized, it can be seen on the Young-Laplace equation:

The gas pressure P is therefore to the liquid in the pore system relative to the pore radius r on the surface tension σ of the liquid and the contact angle Θ. This equation is to be understood only as a guide because too many parameters are unknown or difficult to determine:

• In the surface tension, the difference of the surface tension of the solid and the liquid must be considered. The surface tension of catalysts such as Pt on carbon or silver but hardly measurable.
• The contact angle on a flat surface can still be determined with a microscope. A single pore, however, can not be examined. It always determines the pore system of an entire electrode.

Thus, in order to create space for liquid and gas in an electrode, one can follow the path to generate different pore radii r or different wetting angle Θ. The next two chapters explain how this was achieved.

Sintered electrodes

In the image of the sintered electrode is to be seen that three different grain sizes were used, which were different layers:

In this way many electrodes between 1950 and 1970 were manufactured for use in fuel cells. For this purpose, stood at VARTA, Siemens and Others pilot production ready. This type of production was dropped, however, for economic reasons, because

• The electrodes were thick and heavy - were common thicknesses of 2 mm;
• The individual layers had to be very thin, but error-free;
• The metal price for this type electrodes was too high;
• The electrodes could not be produced continuously.

In the picture " principle of gas diffusion electrode" whose structure is shown again. In the center of the electrode is the so-called gas conduction. With only a small gas pressure of the electrolyte is displaced from this pore system. A small flow resistance ensures that the gas can freely propagate along the electrode. In a somewhat elevated gas pressure and the electrolyte is displaced in the pore system of the work shift, even if only partially. The top layer itself is so fine-pored chosen such that even when pressure peaks gas can pass through the electrode into the electrolyte.

Such electrodes were prepared by spreading and subsequent sintering or hot pressing. To create multi-layered electrodes, ie, a fine-grained material was first dispersed and smoothed into a die. Then the other materials were applied over one another in layers and then pressed. The production was not only error-prone but also to automate time-consuming and difficult.

Plastic-bonded electrodes

Therefore, since about 1970, a different approach is to produce an electrode having both hydrophilic and hydrophobic regions, because with the introduction of PTFE, a material that 's available, which

• Is chemically very stable,
• Can be used as a binder,
• Hydrophobically acts.

The pore system, this means that at the points with a high proportion of PTFE can not enter the electrolyte, however, it. At locations with low PTFE content Of course, the catalyst may be used in this case, even not also have hydrophobic character.

There are two technical variants to produce such PTFE catalyst mixtures:

• Dispersion of water, PTFE, catalyst and emulsifiers, thickeners ...
• Dry mixture of PTFE powder and catalyst powder

The dispersion route is chosen primarily for electrodes with polymeric electrolytes - so, for example, successfully introduced in the PEM fuel cell PEM or the HCL membrane electrolysis. When used in the liquid electrolyte, the dry method is suitable. While it can be dispensed with a mechanical compression in the dispersion route by evaporating the water and sintering the PTFE 's at 340 ° C.. Thus, these electrodes are very porous. But on the other hand, can quickly lead to cracks in the electrode can penetrate through the liquid electrolyte in the wrong drying conditions. Therefore, the dry mixing method has gained acceptance for applications with liquid electrolytes, such as the zinc -air battery or alkaline fuel cell AFC.

In addition to these wetting characteristics, the electrode must of course have an optimum electronic conductivity, so that the electrons can be transported with as little ohmic resistance.

Catalysts

Ultimately, the proper choice of catalyst is crucial. For the catalysis in acidic electrolytes mainly noble metal catalysts such as platinum, ruthenium, iridium and rhenium have prevailed. In alkaline systems such as the zinc -air battery inexpensive catalysts such as carbon, manganese or silver come into question.

Technical Parameters

To quantify the characteristics of the gas diffusion electrode of the following parameters are specified:

• Thickness [ microns ]
• Surface resistance [ Ω · cm ²]
• Permeability by Gurley Precision Instruments [s ] or permeation of gases permeation
• Overvoltage ( electrochemistry ) [V ]

Due to the small thickness of the gas diffusion electrode, the sheet resistance is also very low. In these geometries, the resistors can not be determined by the four -point method. Therefore Gasdiffusionslektrode between two gold plated punches is measured under a high pressure in order to minimize the contact resistance. Actual gas transport in the electrodes by diffusion and not by convection. Therefore, should not be set too high in the Pemeabilitätsmessung the pressure. In Gurley, this is set to 1296 [Pa ] (or 12 [mbar ] ). The overvoltage represents the most complex measurement test dar. for their measurement stable power supplies potentiostat, a stable reference electrode and a special test cell can be used. Due to the high current density at gas diffusion electrodes must pay special attention to a constant field lines. For this purpose a large distance to the counter electrode, and a tube- shaped electrolyte space is recommended. The Haber- Luggin capillary must not disturb the field pattern.

Use

First, the gas diffusion electrodes for use in fuel cells have been developed. Tendencies hydrogen was still working until 1950 at the Bacon cell to generate electricity from coal at high temperatures, so came in the years 1950 to generate electricity by gases, in particular, of course, because of the high reactivity. Over time, however, various applications have shown in other applications:

• Zinc -air battery since 1980
• Nickel - metal hydride battery since 1990
• Hydrochloric acid electrolysis since 2003
• Chlor-alkali electrolysis since 2011

• Electrode ( electrochemistry )
• Fuel cell technology