Gas diffusion electrode for polymer electrolyte membrane fuel ce

Details Gas diffusion electrode for polymer electrolyte membrane fuel ce Gas diffusion electrode for polymer electrolyte membrane fuel ce

1998-10-05

1999-12-07

Bell, Bruce F.

Chemistry: electrical current producing apparatus, product, and

Having earth feature

429 41, 429 40, 204283, 521 27, 427115, 427 77, 427123, 4273722, 4273831, 427384, H01M 486

Patent

active

059980575

DESCRIPTION:

BRIEF SUMMARY
The invention relates to polymer electrolyte membrane fuel cells, in particular gas diffusion electrodes for fuel cells or electrolysis cells, a method of making a gas diffusion electrode for fuel cells or electrolysis cells, a method of coating gas diffusion electrodes with a catalytically active layer, and a method of making a membrane and electrode unit.
In polymer electrolyte membrane fuel cells, a gas diffusion mat is used as electrode between polymer electrolyte membrane and current collectors, such as e.g. bi-polar plates. This mat has the function of dissipating the current produced in the membrane, and it has to allow diffusion of the reaction gasses to the catalytic layer. Moreover, the gas diffusion electrode has to be hydrophobic at least in the layer facing the membrane, in order to prevent that water formed in the reaction process floods the pores of the gas diffusion electrode. For many applications, for example aerospace, it is important furthermore that the materials employed for building the cell stacks are of light weight and consume little space. An as inexpensive as possible fabrication of the materials is always of interest.
So far, mats of graphitized fabric are used for such gas diffusion electrodes, which are available from a density of 116 g/m.sup.2. The gas diffusion mats of graphitized fabric often do not permit sufficiently good diffusion of oxygen, in particular O.sub.2 from the air under low pressure, and moreover they are relatively heavy. Fabrication thereof necessitates high temperatures, resulting in a correspondingly high consumption of energy and high prices.


SUMMARY OF THE INVENTION

It is the object of the invention to make available a gas diffusion electrode which is inexpensive to manufacture and of light weight and which permits good diffusion of oxygen, in particular from the air under a slight pressure above atmospheric, and which furthermore displays the required high electrical conductivity and is hydrophobic.
A further object of the invention is to make available a polymer electrolyte membrane fuel cell comprising such a gas diffusion electrode.
An additional object of the invention consists in indicating a method of making such a gas diffusion electrode.
Another object of the invention is to make available a method of coating a gas diffusion electrode with a catalytically active layer.
A still further object of the invention consists in indicating a method of making a membrane electrode unit.
These objects are met by the gas diffusion electrode of the present as set forth below.


BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a polymer electrolyte membrane fuel cell.


DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The gas diffusion electrodes according to the invention are suitable for fuel cells, in particular polymer electrolyte membrane fuel cells, and polymer electrolyte membrane electrolysis cells. In polymer electrolyte fuel cells, the gas diffusion electrodes according to the invention can be utilized both as anode and as cathode, whereas in electrolysis cells they can be employed only on the hydrogen side, as oxidation would take place on the oxygen side. The gas diffusion electrodes according to the invention can be used in especially advantageous manner in polymer electrolyte membrane fuel cells using hydrogen as fuel gas and air as oxidant and operated at low pressure of less than 0.5 bar, preferably less than 0.1 bar. Particularly preferred are operating pressure differences in the order of magnitude of about 10 mbar.
As starting material for the gas diffusion electrodes according to the invention, a very lightweight carbon fiber nonwoven fabric, preferably of carbonized fibers is used. Particularly suitable are carbonized carbon fiber nonwoven fabrics having mass-area ratios of up to 60 g/m.sup.2, typically 30 g/m.sup.2. Carbonized carbon fibers can be produced with much lower expenses than graphitized fibers, since their manufacture requires considerably lower temperatures.
For fabrication of a gas diffusion electrode according to the inv

REFERENCES:
patent: 4518705 (1985-05-01), Soloman et al.
patent: 5217821 (1993-06-01), Landsman et al.
patent: 5441823 (1995-08-01), Naimer et al.
patent: 5702839 (1997-12-01), Frost et al.
patent: 5716437 (1998-02-01), Denton et al.

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