NOVELTY - As a redox catalyst and/or mediator in a fuel cell catholyte solution, a heterocyclic derivative (I) is used.    USE - As a redox catalyst and/or mediator in fuel cell catholyte solution, for redox fuel cells (claimed) as power sources for: portable products such as portable electronics products; for transport vehicles such as automobiles; auxiliary power for caravans and other recreational vehicles, boats etc; stationary uses such as uninterruptible power for mobile phone stations, hospitals, computer systems etc and combined heat and power for homes and businesses.    ADVANTAGE - In the catholyte solution, reoxidation of the redox mediator species and the catalyst occurs very rapidly, which allows the fuel cell to produce a higher sustainable current than with catholytes of the prior art.    DETAILED DESCRIPTION - As a redox catalyst and/or mediator in a fuel cell catholyte solution, a heterocyclic derivative of formula (I) is used.    X and R1-R8=H, halo, hydroxyl, amino, protonated amino, imino, nitro, cyano, acyl, acyloxy, sulphate, sulfonyl, sulfinyl, alkylamino, protonated alkylamino, quaternary alkyiammonium, carboxy, carboxylic acid, ester, ether, amido, sulfonate, sulfonic acid, sulphonamide, phosphonic acid, phosphonate, phosphate, alkylsulfonyl, arylsulfonyl, alkoxycarbonyl, alkylsulfinyl, arylsulfinyl, alkylthio, arylthio, alkyl, alkoxy, oxyester, oxyamido, aryl, fused-aryl, arylamino, aryloxy, heterocycloalkyl, heteroaryl, fused-heteroaryl, 2-5C alkenyl, 2-5C alkynyl, azido, phenylsulfonyloxy, or amino acid;or    R1+X, R5+X, R1+R2, R2+R3, R3+R4, R4+R8, R8+R7, R7+R6 and R6+R5=optionally substituted ring structure;    L=the optional presence of a linking bond or group between the two neighbouring aromatic rings of the structure, and when present may form an optionally substituted ring structure with one or both of R4 and R8.    At least one substituent group of the structure is a charge-modifying substituent. INDEPENDENT CLAIMS are included for the following:    (1) a catholyte for use in a redox fuel cell, comprising at least the compound (I);    (2) a redox fuel cell comprising: a membrane electrode assembly or stack of membrane electrode assemblies comprising an anode and a cathode separated by an ion selective polymer electrolyte membrane; an anode chamber adjacent the anode of the membrane electrode assembly; a cathode chamber adjacent the cathode of the membrane electrode assembly; unit for supplying a fuel to the anode chamber of the cell; unit for supplying an oxidant to the cell; unit for providing an electrical circuit between the anode and the cathode of the cell; and a catholyte solution comprising at least one non-volatile catholyte component, where the catholyte solution comprising the redox mediator species of compound (I); and    (3) a method of operating a redox fuel cell involving: providing a membrane electrode assembly comprising an anode and a cathode separated by an ion selective polymer electrolyte membrane; providing an anode chamber adjacent the anode of the membrane electrode assembly; providing a cathode chamber adjacent the cathode of the membrane electrode assembly; supplying a fuel to the anode chamber of the cell; supplying an oxidant to the cell; providing an electrical circuit between the anode and the cathode of the cell; and providing the catholyte solution.