Coating processes – Electrical product produced – Fuel cell part
Reexamination Certificate
1999-05-06
2003-02-04
Ryan, Patrick (Department: 1753)
Coating processes
Electrical product produced
Fuel cell part
C427S335000, C427S372200, C427S385500
Reexamination Certificate
active
06514561
ABSTRACT:
The invention relates to an electrolytic membrane comprising an ion-conducting polymer which is fixed in pores of a polyalkene membrane whose porosity is 30-90% by volume. Such membranes are used, for example, in fuel cells, electrolysis cells, primary and secondary batteries. In particular, such membranes are used at points where a high ion conductivity is desired in combination with a high mechanical strength. Although a high ion conductivity can be obtained by using a membrane having a small thickness, a small thickness is generally achieved at the expense of the strength of the membrane.
Such membranes are disclosed in U.S. Pat. No. 4,849,311. U.S. Pat. No. 4,849,311 describes an electrolytic membrane which contains an ion-conducting polymer which is fixed in the pores of a porous polyethylene membrane. The porosity of the membrane described in U.S. Pat. No. 4,865,930 is between 40 and 90%. The pores have a mean size between 0.001 &mgr;m and 0.1 &mgr;m. According to the teaching of U.S. Pat. No. 4,849,311, pores having a mean diameter greater than 0.1 &mgr;m are difficult to fill and, once filled, the electrolyte easily leaks out again. The membrane is preferably made of polyethylene having a weight-average molar mass of at least 500,000 g/mol. Nafion® is described as ion-conducting polymer; a perfluorocarbon compound containing a sulphonic acid group.
A disadvantage of such a membrane is that the ion conductivity is relatively low.
The object of the invention is to provide an electrolytic membrane having a higher ion conductivity.
According to the invention, this object is achieved in that the membrane is stretched in at least one direction and has a mean pore size between 0.1 and 5 &mgr;M.
The membrane according to the invention is found to have a higher ion conductivity than a known membrane having a smaller pore size and a comparable membrane thickness, porosity and strength.
An advantage of the membrane according to the invention is that it is gastight, as a result of which the membrane is very suitable to be used in a solid-polymer fuel cell.
The membrane according to the invention contains an ion-conducting polymer. Ion-conducting polymers which can be used are known and a few are even commercially available. Suitable polymers are described in Patent Specifications U.S. Pat. Nos. 4,849,311 and 4,865,930. Ion-conducting polymers which are preferably used are polymers based on perfluorosulphonic acid and copolymers of tetrafluoroethylene with perfluorosulphonyl ethoxyvinyl ether are very suitable, the sulphonyl groups being converted into sulphonic acid groups. Such polymers are commercially available under the brand names Nafion and Aciplex. Other examples of suitable ion-conducting polymers are complexes of alkali-metal or alkaline-earth-metal salts with a polar polymer. Examples of these are polyethylene glycol ethers. Complexes of the abovementioned polymers with an ion-donating acid can also be used.
In the membrane according to the invention, the ion-conducting polymer is fixed in the pores of a polyalkene membrane.
Suitable as porous polyalkene membrane are, in particular, porous membranes of polyethylene, polypropylene and ethylene-propylene copolymers. The porosity of the polyalkene membrane according to the invention is between 30 and 90% by volume. This means that the volume of the pores accounts for 30-90% by volume of the total volume of the total membrane.
It has been found that, at a porosity lower than 30%, the ion conductivity of the membrane decreases, while, at a porosity greater than 90%, the mechanical strength decreases undesirably.
The best results are obtained with a polyalkene membrane having a porosity of 60 to 85%.
The membrane according to the invention is stretched in at least one direction and has a mean pore size between 0.1 and 5 &mgr;m. In J. Electroanal. Chem. 235 (1987), 299-315, J. Leddy and N. E. Vanderborgh describe how the transport through a Nafion phase, and therefore also the ion conductivity, decreases at pore diameters greater than 0.05 &mgr;m. Surprisingly it was found that, indeed, a membrane stretched in at least one direction having a pore diameter greater than 0.05 &mgr;m has a better ion conductivity than a membrane having a pore diameter smaller than 0.05 &mgr;m.
With a mean pore diameter greater than 5.0 &mgr;m, the ion-conducting polymer can no longer be fixed in the pores because the pores are then too large. The best results are obtained with a pore diameter of 0.15 to 2.5 &mgr;m.
The electrolytic membrane according to the invention has a good mechanical strength, as a result of which no cracks occur even in the case of relatively thin membranes if the membrane is processed for its application. The electrolytic membrane according to the invention preferably has a tensile strength of at least 15 MPa, while the thickness of the membrane may vary from 15 to 150 &mgr;m, preferably from 20 to 60 &mgr;m. With such a thickness, the membrane according to the invention has an ion conductivity of at least 0.0004 S/cm, but the ion conductivity is preferably considerably higher, i.e. at least 0.0008 S/cm, while a particularly suitable membrane has an ion conductivity of 0.002 to 0.08 S/cm.
The invention also relates to a method of manufacturing the electrolytic membrane according to the invention.
Such a method is disclosed in U.S. Pat. No. 4,849,311. In it, the pores of a porous membrane, preferably polyalkene membrane, are filled by means of capillary condensation with a solution of an ion-conducting polymer.
A disadvantage of the method described in U.S. Pat. No. 4,849,311 is that it is difficult to manufacture an electrolytic polyalkene membrane whose pores have a mean diameter of more than 0.1 &mgr;m. According to the said patent specification, the mean pore diameter should preferably be even less than 0.025 &mgr;m. The reason given for this is that the ion-conducting polymer leaks away from pores greater than 0.1 &mgr;m so that such a membrane is unstable.
The object of the invention is to eliminate said disadvantage completely or partially.
According to the invention said object is achieved by a method which comprises the following steps:
a) dissolving an ion-conducting polymer in a solvent, at least 25% by weight of which is composed of a component having a boiling point higher than 125° C.,
b) applying an amount of the solution prepared under (a) to a horizontal polyalkene membrane stretched in at least one direction and having a pore volume of 30-90% of the total volume of the membrane, the amount of the solution being chosen in such a way that the volume of the ion-conducting polymer present therein is more than 60% of the pore volume, and the membrane being sealed at the underside,
c) evaporating the solvent at a temperature which is at least 80° C. and which is lower than the melting point of the polyalkene membrane.
A stable electrolytic membrane having an ion-conducting polymer fixed in the pores of a polyalkene membrane having a mean pore size between 0.1 and 5.0 &mgr;m is manufactured by the method according to the invention.
An advantage of the method according to the invention is that the ion-conducting polymer which is present in the solution on the horizontal stretched membrane becomes preferentially concentrated at the polyalkene surface. Since most of the surface is in the pores, the ion-conducting polymer becomes concentrated in the pores of the membrane. A method in which partially filled pores need to be refilled with a fresh solution can thereby be avoided. The disadvantage of refilling pores is that air inclusions are easily produced. The refilling of partially filled pores is therefore only possible in the presence of solvents which have a low wetting angle with the polyalkene. Such a limitation does not apply in the case of the method according to the invention.
A further advantage of the method according to the invention is that a porous membrane having a pore size above 0.1 &mgr;m is easy to impregnate, as a result of which virtually no air is enclosed in the membrane.
In the method according
Calis Gijsbertus H. M.
de Bruijn Frank A.
Mallant Ronald K. A. M.
Cantelano Gregg
DSM N.V.
Ryan Patrick
LandOfFree
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