Fuel cell membranes

Details Fuel cell membranes Fuel cell membranes

2001-02-20

2004-05-11

Kalafut, Stephen (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S047000, C521S028000, C526S243000

Reexamination Certificate

active

06733914

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to perfluorinated ion exchange membranes for use in electrochemical devices, in particular fuel cells using either hydrogen gas or methanol as a fuel. Such membranes were previously made from extruded films of a precursor polymer, containing sulfonyl halide functional groups, by contacting the extruded films with a solution of alkali hydroxide, such as sodium or potassium hydroxide, followed by acid exchange with a strong acid such as hydrochloric or nitric acid. The precursor polymer is commonly made by the copolymerisation of tetrafluoroethylene with a comonomer containing a polymerizable vinyl group and one or more sulfonyl fluoride groups. Examples of such precursor polymers are those having the general formula:
wherein X is from 0 to 2. Products made from of these precursor polymers are commercially available under the tradenames NAFION AND FLEMION, from DuPont and Asahi, respectively. Other precursor polymers which have been used in the past are those prepared from short branch polymers of the same formula, but wherein X=0, having a repeating unit of the general formula:
For the purposes of the present invention, short branch polymers are preferred.
The composition of such polymers can be described in terms of the comonomer ratio n, or, more commonly, in terms of the equivalent weight (EW) of the final polymer in the sulfonic acid form, that is, after the acid exchange described above. Previously known fuel cell membranes, such as those described above, can exhibit excellent performance. However, continuing effort has been directed to achieving optimum EW from such membranes. From the standpoint of maximum ionic conductivity, it is desirable to choose the lowest possible EW, preferably lower than 900. However, when methanol is used as a fuel, membranes made of such low EW polymers exhibit excessive crossover of methanol. When either hydrogen or methanol is used as a fuel, the membranes exhibit excessive water absorption and resulting swelling and loss of mechanical strength under high humidity conditions. Accordingly, a continuing need exists for such membranes which exhibit minimum cross over of methanol, and more constant water uptake under variable humidity conditions than membranes of the prior art.
SUMMARY OF THE INVENTION
The present invention provides ion exchange membranes which exhibit excellent ionic conductivity and more uniform water absorption, particularly when used in fuel cells.
Specifically, the instant invention provides, in a perfluorinated polymeric ion exchange membrane comparing SO
2
F moieties, the improvement wherein about from 0.1 to 30 wt % of the SO
2
F moieties are converted to SO
2
NH
2
moieties. The invention further provides membrane electrode assemblies and fuel cells incorporating these membranes.


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