| http://www.w3.org/ns/prov#value | - In the present invention, the first layer of a fluoropolymer having carboxylic acid groups (--COOM wherein M is a hydrogen atom or an alkali metal) and facing the cathode side, is preferably made of a hydrolyzed copolymer of tetrafluoroethylene with ##STR1## wherein m is 0 or 1, n is from 1 to 5 and R is an alkyl group having from 1 to 10 carbon atoms, which preferably has an ion exchange capacity
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