TECHNOLOGY FOCUS - ELECTRICAL POWER AND ENERGY - Preferred Catholyte: The catholyte further comprises a redox catalyst selected from an N-donor compound and/or from a polyoxometallate species. The catholyte solution is acidic, where the compound (I) is non-ionic or anionic in its oxidized form; or the catholyte solution is alkali, where the compound (I) is non-ionic or anionic in its reduced form. Preferred Fuel Cell: The redox fuel cell comprises unit for supplying an oxidant to the cathode region of the cell; the catholyte solution comprising at least one non-volatile catholyte component flowing in fluid communication with the cathode, the catholyte solution comprising a electron-transfer mediator of compound (I) which is at least partially reduced at the cathode in operation of the cell, and at least partially regenerated by, optionally indirect, reaction with the oxidant after such reduction at the cathode, and a redox catalyst catalyzing the regeneration of the mediator. In the redox fuel cell, the ion selective polymer electrode membrane is cation and/or proton selective, or anion selective.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Compound: The compound (I) is non-neutral in its reduced form, or non-neutral in its non-reduced form. The compound (I) is soluble in polar solvent or aqueous solvent. The compound (I) comprises region of electron delocalisation, optionally a planar conjugated region, extending beyond a single aromatic ring. In the compound (I), the functional groups are terminated, or capped, with at least one spacer element and/or further functional unit selected from alkyl, alkenyl, aryl, cycloalkyl, alkaryl, alkenaryl, aralkyl, aralkenyl, or heterocyclic units, in which any hydrocarbon chain may optionally be branched. The compound (I) is selected from 5 compounds as given in the specification e.g. compound of formulae (Ia)-(Ic). Preferred Definitions:    L=linking bond, sp2 or sp3 hybridized carbon, O, N, S, imino, sulfonyl, sulfinyl, alkylamino, protonated alkylamino, quaternary alkylammonium, carbonyl, ester, ether, amido, sulphonamide, phosphonate, phosphate, alkysulfonyl, alkenylsulfonyl , arylsulfonyl, alkylsulfinyl, alkenylsulfinyl, arysulfinyl, alkylthio, alkenylthio, arylthio, oxyester, oxyamido, aryl, cycloalkyl, heteroaryl, 2-5C alkyl, 2-5C alkenyl, 2-5C alkynyl, or amino acid;    X=-(Sp)-T1;    T1=phenyl (substituted at 2-6 positions by R9-R13, respectively);    Sp=straight or branched chain alkyl, straight or branched chain alkenyl, aryl, cycloalkyl, alkaryl alkenaryl, aralkyl, aralkenyl, heterocyclic cycloalkyl, alkaryl alkenaryl, aralkyl, or aralkenyl (all optionally substituted);    R1-R8 and R9-R13=alkyl, alkenyl, aryl, cycloalkyl, alkaryl alkenaryl, aralkyl, aralkenyl (substituted by at least one the G1), or G1;    G1=-F, -CHO, -COCH3, -COCH2CH3, -COCH2CH2COOH, -COOH, -(COOH)2, -NH2, -NH3+, -N(CH3)2, -NH(CH3)2+, -N(CH3)3+, -N(CH2CH3)2, -NH(CH2CH3)+, -N(CH2CH3)3+, -CH2N(CH3)2, -CH2NH(CH3)2+, -CH2N(CH3)3+, -N-(PhNH3+)2, -N-(PhCH2NH3+)2, -N-(PhC2H4NH3+)2, -N-(PhSO3)2, -N-(PhCH2SO3)2, -Ph-N-(PhNH3+)2, -Ph-N-(PhCH2NH3+)2, -Ph-N-(PhC2H4NH3+)2, -Ph-N-(PhSO3)2, -Ph-N-(PhCH2SO3)2, -Ph-N-(PhC2H4SO2)2, -OH, -CH2OH, -CH(OH)CH3, -OSO3-, -SO3-, CH2SO3-, -CH2OSO3-, -PO(OH)2, -OPO(OH)2, -CO-Gly-OH, -CO-Glu-OH or-CO-Asp-OH.    SPECIFIC COMPOUNDS - Use of 6 compounds are specifically claimed as the compound (I) e.g. 4,4',4'',4'''-(1,4-phenylenebis(azanetriyl))tetrabenzenesulfonic acid (I'), 4,4',4'',4'''-(biphenyl-4,4'-iylbis(azanetriyl))tetrabenzenesulfonic acid, N,N,N',N'-tetrakis-(4-dimethylaminomethyl-phenyl)-benzene-1,4-diamine, N1,N1'-(biphenyl-4,4'-diyl)bis(N1-(4-aminophenyl)naphthalene-1,4-diamine), and 4,4',4'',4'''-(4,4'-(4-(bis(4-sulfophenyl)amino)phenylazanediyl)bis(4,1-phenylene))bis(methanetriyl)tetrabenzenesulfonic acid.