Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Ion-exchange polymer or process of preparing
Reexamination Certificate
1998-12-22
2001-04-10
Pezzuto, Helen L. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Ion-exchange polymer or process of preparing
Reexamination Certificate
active
06214891
ABSTRACT:
The invention relates to a method for preparing a cation exchange membrane, in particular for electrochemical cells.
Cation exchange membranes having a catalytically active surface layer can be used as proton conductor membranes in fuel cells. For this purpose, the catalyst for the electrochemical reactions to be carried out in the fuel cell is used in finely disperse form on conductive activated carbon. The activated carbon serves to establish electrical contact with the catalyst particles.
Conventional methods for preparing cation exchange membranes which are coated with platinum and platinum metals and which can be used as solid proton conductors in low-temperature fuel cells are the so-called “ink” methods. These involve an electrically conductive carbon material (e.g. platinum/activated carbon having a platinum percentage by weight of from 20 to 40%) coated with a suitable catalyst being suspended in the solution of a sulfonated fluoropolymer and the suspension thus obtained being applied to a suitable membrane (U.S. Pat. Nos. 5,211,984, 5,272,017).
In various variations on the “ink” method, the suspension used for coating is further admixed with a hydrophobic material; for example polytetrafluoroethylene (PTFE) or fluorinated graphite (EP-A-0 483 085, EP-A-0 560 295, U.S. Pat. No. 5,272,017). The hydrophobization thus effected of the catalytically active layer manifests itself in membrane fuel cells, especially on the cathode side (also referred to as “oxygen side”), by increased effectivity of the catalytically active layer.
It is also known to use ruthenium, ruthenium oxide, iridium oxide, molybdenum carbide and tungsten carbide to optimize the efficiency of the catalytically active layer (U.S. Pat. No. 4,876,115, EP-A-0 560 295, K. Ledjeff et al., Int. J. Hydrogen Energy 19, 453-455 (1994)).
One method for preparing porous, catalytically active top layers is based on the use of mixtures of activated carbon, PTFE, platinum/carbon and activated carbon impregnated with a cation exchanger. These mixtures are applied to proton conductor membranes (EP-A-0 577 291). Usually, the material used for the catalytically active coating of the cation exchange membranes comprises polymers having perfluorinated carbon backbones which are laterally linked to ionic groups, usually sulfonic acid groups (for example ®Nafion TM). This also applies to the solutions of cation exchange polymers which are applied to membranes in accordance with said methods. While these polymers are chemically highly stable this stability (e.g. with respect to chlorine and alkalis), which is not even needed to its full extent in membrane fuel cells, does not by any means make up for their high price and the difficulties in processing them which are due to their poor solubility in conventional solvents.
Only one of the abovementioned methods (EP-A-0 577 291) employs an option of enlarging the specific surface area of the catalytically active top layer of the membrane and thus to increase the contact area between the fuel gases of a fuel cell and the catalyst. The process carried out to this end is at the expense, however, of the continuity of the proton conductor phase in the catalytically active top layer.
It is therefore an object of the present invention to provide a technically and economically favorable alternative to current coating methods of ion exchange membranes for electrochemical cells. It is a further object of the invention to provide a cation exchange membrane, in particular for a membrane/electrode unit, which allows the fuel gases of a membrane fuel cell free access to as large a catalytically active membrane surface area as possible.
This object is achieved according to the invention by a method for preparing a cation exchange membrane, comprising the introduction of an organic polymer having sulfonic acid groups and of finely disperse electrically conductive particles of a catalyst material into a liquid phase, the resulting suspension being used to coat a foil of a cation exchange material on at least one side. The organic polymer having sulfonic acid groups is soluble in an aprotic polar solvent and contains units of the formulae (Ar
1
X) and (Ar
2
Y) which are at least partially substituted by sulfonic acid groups, Ar
1
and Ar
2
being identical or different bivalent arylene radicals, X being oxygen or sulfur and Y being a carbonyl radical, sulfoxide radical or sulfonyl radical. The organic polymer is dissolved in a solvent, a finely disperse electrically conductive catalyst material is suspended in the solution and this suspension is used to coat a foil which contains a polymeric cation exchanger having sulfonic acid groups. The coating which still contains solvent is treated with a liquid which is miscible with the solvent, but in which the dissolved cation exchange material is insoluble, so that pores are formed in the top layer of the membrane.
The polymer may also contain a plurality of different units of the formula (Ar
1
X) and a plurality of different units of the formula (Ar
2
Y). The polymer may further contain bivalent radicals of the formula Ar
3
—C(CH
3
)
2
—, Ar
3
—C(CF
3
)
2
—, Ar
3
—C-(phenyl)
2
—, the radical Ar
3
-cyclohexylene or the radical —Ar-fluorene, Ar
3
being an aromatic unit.
The arylene radicals Ar
1
and Ar
2
are bivalent aromatic units, for example the phenylene, biphenylene, naphthylene or anthrylene radical. Preferably, Ar
1
and Ar
2
are the phenylene radical, in particular the 1,4-phenylene radical. Preferred aromatic units are aromatic polyetherketones, polyethersulfones, poly(arylene sulfides), for example of the formulae I to V, or polybenzimidazoles
The polymer of the cation exchange material may further contain bivalent N,N′-pyromellitic diimide units, phthalimide units and/or benzimidazole units.
Sulfonation affords polymers which carry a sulfonic acid group —SO
3
H on all or some of the aromatic units. Used in particular are sulfonation products of polyaryletherketones (I, II), polyarylethersulfones (III, IV) and polyarylthioethers (V), which have an ion exchange equivalent of from 0.3 mmol of H
+
/g to 2 mmol of H
+
/g. It is precisely these polymers which, owing to their chemical structure, are particularly stable under the conditions prevailing in a fuel cell.
The preparation of the polymers, the sulfonated polymers and the preparation of membranes from these polymers is disclosed, for example, by the literature mentioned below, which is incorporated herein by reference: EP-A-0 008 895; EP-A-O 575 807; DE-A-4 242 692; K. Ledjeff et al., J. Membrane Sci. 83, 211-220 (1993); B. C. Johnson et al., J. Polym. Sci., Polym Chem. Ed., 22, 721-737; A. Noshay, L. M. Robeson, J. Appl. Polym. Sci. 20, 1885-1903 (1976). Equally, mixtures of these sulfonated polymers with one another and mixtures of the sulfonated polymers with other polymers can be used which preferably are likewise soluble in aprotic polar solvents.
The method according to the invention comprises the following process steps:
1. Coating of a foil made of a cation exchange material by casting, spraying or immersion using a suspension containing the following components: a liquid solvent or suspension medium; a dissolved polymer electrolyte and an electrically conductive catalyst material, for example a conductive carbon material impregnated with a catalytically active metal. Optionally, further polymers may be present in the suspension.
2. Drying of the membrane, if necessary.
In order for the catalytically active layer thus applied to be rendered porous and thus have its specific surface area enlarged, one or more of the following process steps can then be carried out according to the invention:
3. Swelling of the membrane in a liquid which is a solvent for the polymer electrolyte present in the catalytically active layer.
4. Bringing the membrane obtained in step 3 into contact with a liquid which is miscible with the solvent, but is a non-solvent for the polymer electrolyte mentioned in step 3.
5. Drying of the membrane.
Preferably, prior to coating the foil is roughen
Schneller Arnold
Witteler Helmut
Frommer & Lawrence & Haug LLP
Hoechst Research & Technology Deutschland GmbH & Co. KG
Pezzuto Helen L.
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