Sulphonated polyimides, membranes and fuel cell

Details Sulphonated polyimides, membranes and fuel cell Sulphonated polyimides, membranes and fuel cell

1999-05-25

2001-06-12

Woodward, Ana (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

From carboxylic acid or derivative thereof

C525S432000, C525S436000, C528S350000, C528S352000

Reexamination Certificate

active

06245881

ABSTRACT:

This invention relates to sulphonated polyimides, which find application particularly in the preparation of ion exchange membranes notably for the manufacture of fuel cells.
The use of solid polymer electrolytes was proposed in the 1950s and applied notably in the construction of fuel cells which were intended particularly to supply space craft with energy.
The interest in fuel cells is now progressing beyond the generation of power for space craft and the automobile industry has interest in them for at least two reasons.
the first rests on the concern to avoid pollution caused by internal combustion engines. In effect it is clear that it will be difficult to prevent all discharges of nitrogen oxides, unburnt hydrocarbons and oxygenated compounds by means of all the improvements that one can expect through better control of combustion.
the second reason, for the longer term, is to research motors that use a fuel other than the fossil fuels that it is known will not last for ever.
Any system based on hydrogen can respond to the concerns mentioned above. The source of supply is potentially inexhaustible and electrochemical combustion only produces water.
The schematic assembly of a fuel cell that permits at the same time the production of electrical energy and incidentally the synthesis of water for the needs of the crew of a space vehicle, is represented in part in
FIG. 1
appended.
The ion exchange type of membrane formed from a solid polymer electrolyte (
1
), is used to separate the anode compartment (
2
) where oxidation occurs of the fuel, such as hydrogen H
2
(
4
) according to the equation:
2H
2
→4H
+
+4e
−
;
from the cathode compartment (
3
) where the oxidant such as oxygen O
2
is reduced according to the equation:
O
2
+4H
+
+4e
−
→2H
2
O
with production of water (
6
) while the anode and the cathode are connected through an external circuit (
10
).
The anode (
7
) and the cathode (
8
) are essentially constituted by a porous support, for example made of carbon, on which particles of a noble metal such as platinum are deposited.
The membrane and electrode assembly is a very thin assembly with a thickness of the order of a millimetre and each electrode is supplied from the rear with the gases using a fluted plate.
One very important point is to properly maintain the membrane in an optimum moisturised state so as to ensure maximum conductivity.
The membrane has a double role. On the one hand it acts as an ionic polymer permitting the transfer (
9
) of hydrated protons H
3
O
+
from the anode to the cathode, and on the other hand it keeps each of the gases oxygen and hydrogen in their compartments.
The polymer constituting the membrane must therefore fulfil a certain number of conditions relating to its mechanical, physico-chemical and electrical properties.
First of all, the polymer must be able to give thin films, between 50 and 100 micrometres thick, which are dense and without defects. The mechanical properties, rupture stress modulus, ductility, must make it compatible with the assembly operations which include, for example, being clamped between metal frames.
The properties must be conserved when it passes from a dry to a moist state.
The polymer must have good thermal stability to hydrolysis and exhibit good resistance to reduction and oxidation up to 100° C. This stability shows itself in terms of variation in ionic resistance and in terms of variation in mechanical properties.
Finally, the polymer must have high ionic conductivity, this conductivity is provided by strongly acidic groups such as phosphoric acid groups, but above all by sulphonic groups linked to the polymer chain. Because of this these polymers will generally be specified by their equivalent mass, that is to say, the weight of polymer in grams per acid equivalent.
By way of example, the best systems developed at present are capable of supplying a specific power of 1 W.cm
−2
, or a current density of 4 A.cm
−2
for 0.5 Volts.
Since 1950, numerous families of polymers or sulphonated polycondensates have been tested as membranes without it being at present possible to establish with certainty the relationships between chemical structure, film morphology and performance.
At first, sulphonated phenolic type resins prepared by sulphonation of polycondensed products such as phenol-formaldehyde resins were used.
The membranes prepared with these products are low cost, but they do not have sufficient stability to hydrogen at 50-60° C. for applications of long duration.
Next one turned towards sulphonated polystyrene derivatives which have greater stability compared with that of the sulphonated phenolic resins but cannot be used at more than 50-60° C.
At the present time, the best results are obtained with copolymers, the linear main chain of which is perfluorinated and the side chain of which carries a sulphonic acid group.
These copolymers are commercially available under the trademark NAFION® from the Du Pont Company or ACIPLEX-S® from the Asahi Chemical Company. Others are experimental, products by the DOW Company for the manufacture of the membrane named “XUS”.
These products have been the subject of numerous developments and conserve their properties for several thousands of hours between 80 and 100° C. with current densities that depend on the partial pressures of the gases and the temperature. The current density is typically 1 A.cm
−2
at 0.7 Volts for Nafion® 112 with a thickness of 50 &mgr;m.
The polymers of the Nafion® type are obtained by co-polymerisation of two fluorinated monomers, one of which carries the SO
3
H group. A second route for obtaining perfluorinated membranes has been explored in documents by G. G. Scherer: Chimia, 48 (1994), p. 127-137; and by T. Monose et al., patent U.S. Pat. No. 4,605,685. It involves the grafting of styrene or fluorinated styrene monomers onto fluorinated polymers which are subsequently sulphonated. These membranes however have properties close to those of fluorinated co-polymers.
If one tries to draw lessons from the teachings of the prior art, it is apparent that the best chemical structure for a polymer that can be used in the form of a membrane for the exchange of protons corresponds to the following criteria:
a main chain totally perfluorinated
branches bearing a sulphonic acid group
equivalent weight between 800 and 1200.
In the documents by W. Grot; Chem. Ing. Tech., 50, 299 (1978) and by G. G. Scherer: Phys. Chem., 94, 1008-1024 (1990) they claim for these structures “very good thermal stabilities”; however, it should be taken into consideration that the notion of thermal stability has to be taken here as the ability to resist acid hydrolysis at a temperature between 60 and 100° C. over a period of several thousands of hours and that therefore the information from these documents must be considered prudently.
To that, it would be proper to add resistance to oxidation in contact with oxygen in the cathode compartment and resistance to reduction in the presence of H
2
.
On the other hand, from the viewpoint of the development of fuel cells that can be used for automobile traction, another important problem that will henceforth be clearly identified by the experts is the cost of the membrane.
In 1995, the cost of membranes produced or under development was of the order of 3000 to 3500 French francs per square metre and one might estimate that it would be necessary to divide this cost by 10 or indeed 20 in order for it to play a part in the industrial development of fuel cells for the automobile industry.
With a view to lowering the costs, poly 1,4-(diphenyl-2,6)-phenyl ethers, sulphonated on the main chain, the polyether-sulphones and polyether-ketones have been synthesised and tested without really holding their own against the fluorinated membranes with regard to their immediate performance and their durability.
In effect, the rigidity of the chains makes these products insoluble and it becomes difficult to obtain the thin films necessary for the creation of the membranes.
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