Redox flow cell for storage of electric energy, containing a reaction cell having two polarity-specific chambers (1, 2) for catholyte and anolyte, which are each connected to a store for liquid and are separated by a membrane for ion exchange and which are equipped with electrodes (10, 11), wherein the chambers (1, 2) are each filled with or are containing redox-active components present undiluted, dissolved, emulgated or dispersed in an electrolyte solvent, wherein the membrane is a size-exclusion membrane having a selectivity of at least 400 g/mol and both redox-active components have a molar mass of at least 400 g/mol and contain a structural unit of formula I and of formula II or of formula I and of formula II and of formula III or of formula IV wherein PG1, PG2, PG3 and PG4 independently of one another are backbones of organic polymers which are independently from each other derived from monomers for the ring-opening polymerization, for the oxidative coupling, for the polycondensation, for the die ring-opening metathesis-polymerization, for the radical polymerization or for the ionic polymerization selected from the group of monomers M1 to M59 wherein the corrugated bonds represent the covalent bonds LI1 to LI5 or the links to the bridging groups LI1 to LI5, or wherein PG1 is a framework of a hyper-branched organic polymer, that is derived from the structures P1 to P9 and that comprises groups LI1 with at least one of the structures L1 to L21 hereinafter defined or wherein LI1 represents a covalent bond, in which P1 is a hyper-branched polyethyleneimine of the following structure P2 is a hyper-branched polyglycerol of the following structure P3 to P7 are dendrimers of the following structures P3 P4 P5 P6 with a molar ratio of phthalic acid to succinic acid from 10:90 to 30:70, preferably preferred 20:80 P7 is a hydrogentated P6-structure with a molar ratio of hexahydrophthalic acid to succinic acid from 10:90 to 30:70, preferably preferred 20:80 P8 is a hyper-branched polypropyleneimine- or polypropyleneiminedendrimer dendrimers of the following structure and P9 is a hyper-branched polyester of the following structure RAE1 represents a monovalent redox-active group, LI1, LI2 and LI3 independently of one another are covalent bonds between atoms of PG1 and RAE1, PG2 and COM1 or PG3 and COM2 or are divalent organic bridge groups selected from the hereinafter defined residues L1 to L26, COM1 and COM2 independently of one another are hydrogen, hydroxyl, an amino group, a sulfonic acid group or different monovalent organic side groups which are selected from the residues of the alkyl group of up to 20 carbon atoms, the carboxyl group, the alkylamino group with up to 20 carbon atoms in the alkyl residue, the dialkylamino group with up to 20 carbon atoms in the alkyl residues, the carboxylic amide group, the N-alkylcarboxylic amide group with up to 20 carbon atoms in the alkyl residue, the N,N-dialkylcarboxylic amide group with up to 20 carbon atoms in the alkyl residues, or which derive from acrylic acid, methacrylic acid, alkyl acrylates, alkyl methacrylates, oligoethylene-glycol acrylates and -methacrylates, styrene, styrene sulfonate and its salts, (vinylbenzyl)trimethylammoniumchloride and comparable salts, butadiene, isoprene, ethyleneoxide, propyleneoxide, vinylmethylether, vinylethylether, thiophene, 3-hexylthiophene, 3,4-ethylenedioxythiophene and pyrrol, RAE2 prepresents a divalent redox-active group, LI4 represents a covalent bond between atoms of two RAE2 groups or between atoms of RAE2 and PG4 or is a divalent organic bridge group selected from the residues L1 to L26 defined hereinafter, LI5 represents a covalent bond between atoms of two PG4 groups or between atoms of RAE2 and PG4 or a divalent organic bridge group chosen from residues L1 to L16 defined hereinafter, the residues of formulae L1 to L26 have the following meaning wherein the corrugated bonds represent the connections to the groups PG1 and RAE1, PG2 and COM1, PG3 and COM2, PG4 and RAE2 and two RAE2 groups, x represents an integer of >=1, y represents an integer of >=0, z represents an integer of >=0, a represents an integer of >=1, and b represents an integer of >= 0, and the potential difference between the redox reactions of the redox-active components taking place each in the anolyte and in the catholyte is between greater than 0 and 4.0 V, with the proviso that poly(4,4'-bipyridine-co-poly(ethyleneglycol) is excluded as redox-active component in the anolyte.