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Concept
Gas diffusion electrode
Summary
Gas diffusion electrodes (GDE) are electrodes with a conjunction of a solid, liquid and gaseous interface, and an electrical conducting catalyst supporting an electrochemical reaction between the liquid and the gaseous phase.
GDEs are used in fuel cells, where oxygen and hydrogen react at the gas diffusion electrodes, to form water, while converting the chemical bond energy into electrical energy. Usually the catalyst is fixed in a porous foil, so that the liquid and the gas can interact. Besides these wetting characteristics, the gas diffusion electrode must, of course, offer an optimal electric conductivity, in order to enable an electron transport with low ohmic resistance.
An important prerequisite for the operation of gas diffusion electrodes is that both the liquid and the gaseous phase coexist in the pore system of the electrodes which can be demonstrated with the Young–Laplace equation:
The gas pressure p is in relation with the liquid in the pore system over the pore radius r, the surface tension γ of the liquid and the contact angle Θ. This equation is to be taken as a guide for determination because there are too many unknown, or difficult to achieve, parameters. When the surface tension is considered, the difference in surface tension of the solid and the liquid have to be taken into account. But the surface tension of catalysts such as platinum on carbon or silver are hardly measurable. The contact angle on a flat surface can be determined with a microscope. A single pore, however, cannot be examined so it is necessary to determine the pore system of an entire electrode. Thus in order to create an electrode area for liquid and gas, the path can be chosen to create different pore radius r, or to create different wetting angles Θ.
In this image of a sintered electrode it can be seen that three different grain sizes were used. The different layers were:
top layer of fine-grained material
layer from different groups
gas distribution layer of coarse-grained material
Most of the electrodes that were manufactured from 1950 to 1970 with the sintered method were for use in fuel cells.
Official source
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