An electrochemical device comprising (i) at least one polymer having a fully fluorinated polymer backbone chain and, pending therefrom, one or more side chains, comprising a plurality of sulfonic acid groups of the general formula -SO 3 - X (I) or a plurality of -SO 3 - X precursor groups of the general formula -SO 2 Y (II) in which X represents H+ or M, M represents a monovalent metal cation or -NH 4 + or Y represents a group that can be converted by treatment with the base M[OH - ] into -SO 3 - M; (ii) at least one metal salt comprising an F-containing anion selected from the group consisting of F - , FSO 3 - ; FPO 3 2- , HFPO 3 - , [M'F 4 ] - , [M'F 4 ] 2- , [M'F 6 ] 2- , [M'F 7 ] 2- in which M' represents one or more metal cations, NH 4 + or a combination thereof and has a valence such that the complex anion carries the negative charge as indicated, and wherein the side chains have the general formula -Rp-SO 3 - X (III) in which Rp links to the polymer backbone and represents a linear, branched or cyclic perfluorinated alkyl, alkoxy, polyoxyalkyl or aryl, and wherein the polymer has a backbone comprising repeating units derived from tetrafluoroethylene (TFE), hexafluoropropylene (HFP), or a perfluorinated vinyl or ally ethers corresponding to the formula: CF 2 =CF-(CF 2 ) l -O(R a f O) n (R b f O) m R c f (IV) or a combination thereof, wherein in formula (IV) R a f and R b f are different or identical linear or branched perfluoroalkylene groups of 1 to 6 carbon atoms, in particular 2 to 6 carbon atoms, m and n are independently 0 to 10, l is either 1 or 0, and R c f is a perfluoroalkyl group of 1 to 6 carbon atoms. (i) at least one polymer having a fully fluorinated polymer backbone chain and, pending therefrom, one or more side chains, comprising a plurality of sulfonic acid groups of the general formula -SO 3 - X (I) or a plurality of -SO 3 - X precursor groups of the general formula -SO 2 Y (II) in which X represents H+ or M, M represents a monovalent metal cation or -NH 4 + or Y represents a group that can be converted by treatment with the base M[OH - ] into -SO 3 - M; in which X represents H+ or M, M represents a monovalent metal cation or -NH 4 + or Y represents a group that can be converted by treatment with the base M[OH - ] into -SO 3 - M; (ii) at least one metal salt comprising an F-containing anion selected from the group consisting of F - , FSO 3 - ; FPO 3 2- , HFPO 3 - , [M'F 4 ] - , [M'F 4 ] 2- , [M'F 6 ] 2- , [M'F 7 ] 2- in which M' represents one or more metal cations, NH 4 + or a combination thereof and has a valence such that the complex anion carries the negative charge as indicated, and wherein the side chains have the general formula -Rp-SO 3 - X (III) in which Rp links to the polymer backbone and represents a linear, branched or cyclic perfluorinated alkyl, alkoxy, polyoxyalkyl or aryl, and wherein the polymer has a backbone comprising repeating units derived from tetrafluoroethylene (TFE), hexafluoropropylene (HFP), or a perfluorinated vinyl or ally ethers corresponding to the formula: CF 2 =CF-(CF 2 ) l -O(R a f O) n (R b f O) m R c f (IV) or a combination thereof, wherein in formula (IV) R a f and R b f are different or identical linear or branched perfluoroalkylene groups of 1 to 6 carbon atoms, in particular 2 to 6 carbon atoms, m and n are independently 0 to 10, l is either 1 or 0, and R c f is a perfluoroalkyl group of 1 to 6 carbon atoms. <EOS>