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.^[1]

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:

${\displaystyle p={\frac {2\ \gamma \cos \theta }{r}}}$

The gas pressure p is in relation to 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 between the solid and the liquid has 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 radii 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:

1. top layer of fine-grained material
2. layer from different groups
3. 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. This type of production was dropped for economic reasons because the electrodes were thick and heavy, with a common thickness of 2 mm, while the individual layers had to be very thin and without defects. The sales price was too high and the electrodes could not be produced continuously.

Since about 1970, PTFEs are used to produce an electrode having both hydrophilic and hydrophobic properties while chemically stable and which can be used as binders. This means that, in places with a high proportion of PTFE, no electrolyte can penetrate the pore system and vice versa. In that case the catalyst itself should be non-hydrophobic.^[2]

In acidic electrolytes the catalysts are usually precious metals like platinum, ruthenium, iridium and rhodium. In alkaline electrolytes, like zinc-air batteries and alkaline fuel cells, it is usual to use less expensive catalysts like carbon, manganese, silver, nickel foam or nickel mesh.

At first solid electrodes were used in the Grove cell, Francis Thomas Bacon was the first to use gas diffusion electrodes for the Bacon fuel cell,^[3] converting hydrogen and oxygen at high temperature into electricity. Over the years, gas diffusion electrodes have been adapted for various other processes like:

• Zinc-air battery since 1980
• Nickel-metal hydride battery since 1990
• Chlorine production by electrolysis of waste hydrochloric acid ^[4]
• Chloralkali process^[5]
• Electrochemical reduction of carbon dioxide

GDE is produced at all levels. It is not only used for research and development firms but for larger companies as well in the production of a membrane electrode assembly (MEA) that is in most cases used in a fuel cell or battery apparatus. Companies that specialize in high volume production of GDE include Johnson Matthey, Gore and Gaskatel. However, there are many companies which produce custom or low quantity GDE, allowing different shapes, catalysts and loadings to be evaluated as well, which include FuelCellStore, FuelCellsEtc, and many others.

• Anion exchange membrane
• Concentration cell
• Electrode potential
• Glossary of fuel cell terms
• Ion transport number
• Ion selective electrode
• Liquid junction potential