Compositions – Electrolytes for electrical devices
Patent
1994-12-22
1997-01-21
Bell, Bruce F.
Compositions
Electrolytes for electrical devices
252500, 521 27, 521 28, 521 31, 429 27, 429 30, 429 33, 429 41, 429 46, 429192, 429199, 204415, 204421, H01G 9022
Patent
active
055956769
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to a fluoroorganic polymeric material, to an ion-exchange membrane prepared therefrom, particularly to an ion-exchange membrane which is useful in an electrochemical device, and to a method for the preparation thereof.
BACKGROUND OF THE INVENTION
Ion-exchange membranes made from fluoroorganic polymeric materials having ion-exchange properties are used in a wide variety of applications. Such polymeric materials and membranes may contain fixed anionic groups and associated cations and be capable of exchanging cations, or they may contain fixed cationic groups and associated anions and thus be capable of exchanging anions, or the polymeric materials and the membranes may contain both fixed anionic groups and fixed cationic groups.
Ion-exchange membranes which are essentially hydraulically impermeable but which are permeable to solvated cations or anions, or both, are finding increasing applications in electrochemical devices, for example in fuel cells, in electrolytic cells in which an electrolyte is electrolysed and in electrochemical cells in which electrosynthesis is carried out. In recent years a major development has been in the use of cation-exchange membranes in chlor-alkali cells in which chlorine and an aqueous metal hydroxide are produced by the electrolysis of aqueous alkali metal chloride solution.
Although many fluoroorganic polymeric materials have been proposed for use as membranes for such chlor-alkali cells, in recent years perfluoroorganic polymers containing ion-exchange groups, particularly fixed sulphonic and carboxylic groups, have found favour in chlor-alkali cells on account of the resistance of such perfluoroorganic polymers to chemical degradation in the cell.
As examples of such perfluoroorganic polymers containing ion-exchange groups may be mentioned inter alia the perfluoroorganic polymeric materials containing sulphonic groups described in GB patent 1034197 and the perfluoroorganic polymeric materials containing carboxylic groups described in GB patents 1516048 and 1518387.
Fluoroorganic polymeric materials comprising a copolymer derived from a substantially fluorinated alkene and a perfluorocarbon vinyl ether bearing an ion-exchange group or group convertible thereto are known, for example copolymers of tetrafluoroethylene (TFE) and a perfluorocarbon vinyl ether sulphonyl fluoride, eg CF.sub.2 .dbd.CFOCF.sub.2 C(CF.sub.3)FOCF.sub.2 CF.sub.2 SO.sub.2 F wherein the moiety OCF.sub.2 C(CF.sub.3)FOCF.sub.2 CF.sub.2 SO.sub.2 F forms a pendent group from the polymer backbone. Such copolymers, after appropriate hydrolysis of the sulphonyl group in the pendent group to the free sulphonic acid, have been suggested for use as inter alia ion-exchange membranes for use in, for example, diffusion dialysis, electrolytic cells and fuel cells.
Recently, it has been shown in U.S. Pat. No. 4,358,545 that an improved membrane for use as an ion-exchange membrane in a chlor-alkali cell can be obtained by using a fluoroorganic polymeric material comprising a copolymer as described above wherein the pendent group is relatively short, eg in a copolymer derived from TFE and CF.sub.2 .dbd.CFOCF.sub.2 CF.sub.2 SO.sub.2 F. It has been suggested that this improvement is the result of less water being absorbed per sulphonic group at a given concentration of functional group and allows the use of a polymer of lower equivalent weight (EW) in chlor-alkali cells.
More recently, U.S. Pat. No. 4,940,525 has confirmed the advantages of such copolymers with relatively short pendent groups and furthermore suggested that for certain applications, for example in fuel cells, eg so-called solid polymer electrolyte fuel cells (SPFC), such copolymers should have a low EW and a certain maximum Hydration Product.
"Hydration Product" is defined in U.S. Pat. No. 4,358,545 as the mathematical product of the hydration of the copolymer, expressed as moles of adsorbed water per functional group, and the EW weight of the copolymer.
In our EP 0,331,321 it has been shown that the ion-e
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Barnes David J.
Gilday John P.
Bell Bruce F.
Imperial Chemical Industries plc
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