Polymer electrolyte membrane and process for the production...

Details Polymer electrolyte membrane and process for the production... Polymer electrolyte membrane and process for the production...

1997-10-06

2001-04-10

Berman, Susan W. (Department: 1711)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S006000, C429S047000, C204S242000, C522S162000, C522S164000, C522S904000, C522S905000, C528S125000, C528S128000, C528S175000, C528S226000, C528S229000

Reexamination Certificate

active

06214488

ABSTRACT:

The invention relates to polymer electrolyte membranes based on a sulfonated aromatic polyether ketone.
Cation exchanger membranes are employed, inter alia, in electrochemical cells which use, instead of a liquid electrolyte, a polymeric solid electrolyte as ion conductor. Examples of these are water electrolyzers and hydrogen/oxygen fuel cells. Membranes for this application must satisfy stringent demands with respect to chemical, electrochemical and mechanical stability and proton conductivity. For this reason, hitherto only fluorinated membranes which principally contain sulfonic exchanger functions, have successfully been employed in long-term operation, for example in chlor-alkali electrolysis.
Although the use of fluorinated exchanger membranes constitutes the state of the art, they have disadvantages for use in solid electrolyte applications. In addition to the high cost, such materials having the above-required properties are, in membrane form, only available with defined parameters (thickness, exchanger capacity) and can be processed neither thermoplastically nor as solutions. However, it is precisely applications as a polymeric solid electrolyte in fuel cells/electrolysis that require membranes having modifiable properties, enabling optimum matching of the membrane properties to the requirements in the cell.
The modifiable properties include variation of the membrane thickness, since, in particular at high current densities, the resistance, which is proportional to the membrane thickness, makes up a considerable proportion of the electrical losses of the cell. Commercial perfluorinated membranes typically have a thickness of from 170 to 180 &mgr;m; thicknesses of less than 0.1 mm are desirable. Polymers which allow thermoplastic or solution processing enable membranes to be produced in any desired thickness.
The modifiable properties include the degree of cross-linking of the membrane. The low membrane resistance required causes a high ion exchange capacity of the membrane. However, all chemically uncrosslinked membranes (these also include commercial perfluorinated membranes) have a limited ion exchange capacity in practice since the membrane swells considerably with increasing value, in particular at elevated temperatures, and its mechanical properties become inadequate. However, polymer materials which, after conversion into a membrane, are in principle chemically crosslinkable offer the opportunity of restricting swelling.
Although polymers typically used for cation exchanger membranes, such as, for example, sulfonated polystyrenes, can be prepared from liquid monomers and can be polymerized in membrane form of any desired thickness after addition of crosslinker molecules, the hydrogen atoms on the main aliphatic chain mean, however, that they do not have the long-term chemical stability which is required.
Further properties which distinguish a good cation exchanger membrane are insensitivity during operating interruptions, resistance to delamination of a support film and (in the case of alkali metal chloride electrolysis) insensitivity to brine impurities.
The object was therefore to provide ion-conductive membranes which are suitable for use as polymeric solid electrolytes, have adequate chemical stability and can be produced from polymers which are soluble in suitable solvents. It should preferably be possible to make these membranes more stable by subsequent treatment.
This object is achieved by a process for the production of a polymer electrolyte membrane from sulfonated, aromatic polyether ketone, in which an aromatic polyether ketone of the formula (I)
in which
Ar is a phenylene ring having p- and/or m-bonds,
Ar′ is a phenylene, naphthylene, biphenylylene, anthrylene or another divalent aromatic unit,
X, N and M, independently of one another, are 0 or 1,
Y is 0, 1, 2 or 3,
p is 1, 2, 3 or 4,
is sulfonated, the sulfonic acid is isolated and dissolved in an organic solvent, and the solution is converted into a film. This process comprises converting at least 5% of the sulfonic groups in the sulfonic acid into the sulfonyl chloride groups, reacting the sulfonyl chloride groups with an amine containing at least one crosslinkable substituent or a further functional group, where from 5% to 25% of the original sulfonic groups are converted into sulfonamide groups, subsequently hydrolyzing unreacted sulfonyl chloride groups, isolating the resultant aromatic sulfonamide and dissolving it in an organic solvent, converting the solution into a film, and then crosslinking the crosslinkable substituents in the film.
Asymmetrical membranes derived from a sulfonated, aromatic polyether ketone are the subject-matter of EP-A-182 506. However, the membranes described therein contain no crosslinkable or crosslinked groups.
The sulfonation of the polyether ketone of the formula (I) is preferably carried out by dissolving it in from 94 to 97% strength by weight sulfuric acid, adding a sulfonating agent to the resultant solution until the concentration of sulfuric acid is from 98 to 99.5% by weight, and working up the reaction batch as soon as the desired degree of sulfonation has been reached. It is favorable to work under conditions under which sulfonation is substantially suppressed or under which sulfonation does not yet occur.
The aromatic polyether ketones indicated in the formula (I) are readily accessible. They can in principle be built up by electrophilic polycondensation by the Friedel-Crafts method, in which an aromatic diacid dihalide is reacted with an aromatic ether.
In the polymers of the formula I, the indices are preferably matched in such a way that P=2−(1−X)·M.
The polymer where P=1, X=0, M=1, Y=0 and N=0 is commercially available under the name VICTREX Polymers in which N=1 or Y=3 or P=4 or X=1 can preferably be prepared by a nucleophilic process.
It is preferred for all the divalent aromatic radicals —Ar— in the polymer to be sulfonated to comprise phenylene, preferably 1,4-phenylene. The sulfonating agent, which serves to increase the sulfuric acid concentration and for sulfonation, is preferably fuming sulfuric acid, chlorosulfonic acid or sulfur trioxide.
The concentration of the sulfuric acid used for the dissolution is preferably from 96 to 96.5%. The dissolution temperature depends on the ratio between the number of ether bridges and carbonyl bridges. With increasing proportion of ether groups relative to carbonyl groups, the reactivity of the polyether ketone main chain for electrophilic substitution (for example sulfonation) increases.
The number of sulfonic groups which can be introduced depends on the number of aromatic rings bridged by oxygen atoms. Only O-phenyl-O units are sulfonated under the stated conditions, while O-phenyl-CO groups remain unsulfonated. In general, the temperature during dissolution of the polymer is between 10 and 60° C., in particular between 20 and 60° C., preferably between 30 and 50° C. During this dissolution process, sulfonation of the main chain is substantially suppressed. Our own NMR studies have shown that no degradation occurs during sulfonation.
After complete dissolution of the sample, the concentration of the sulfuric acid is increased, for example by adding oleum, until the H
2
SO
4
concentration is from 98 to 99.9% by weight, in particular from 98 to 99.5% by weight, preferably from 98.2 to 99.5% by weight. The reaction temperature during the actual sulfonation can be higher than during the dissolution process. In general, the sulfonation is carried out at from 10 to 100° C., in particular at from 30 to 90° C., preferably at from 30 to 80° C. Both an increase in the temperature and an extension of the reaction time increase the degree of sulfonation of the polymer. Typical reaction times are between 0.5 and 10 hours, in particular between 1 and 8 hours, preferably between 1.5 and 3 hours. Reaction times of longer than 10 hours only increase the degree of sulfonation to an insignificant extent. An increase in the tempe

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