NOVELTY - Redox fuel cell (1) (I) comprises: an anode and a cathode (3) separated by an ion selective polymer electrolyte membrane (2); means for supplying a fuel to the anode region of the cell; means for supplying an oxidant to the cathode region of the cell; means for providing an electrical circuit between the anode and the cathode; and a catholyte solution comprising at least one non-volatile catholyte component flowing in fluid communication with the cathode.    USE - (I) is useful as: power sources for (portable products such as portable electronics products, transport vehicles such as automobiles, both main and auxiliary, auxiliary power for caravans and other recreational vehicles and boats); uninterruptible power for hospitals and computers; and combined heat and power for homes and businesses.    ADVANTAGE - (I): are efficient; inexpensive and/or inexpensive to assemble; uses inexpensive and/or environmentally friendly materials; yields adequate and/or sufficient maintainable current densities and/or cell potentials; too small in their construction; operates at low temperature; does not produce unwanted by-products and/or pollutants and/or noxious materials; and has practical and commercial utility in portable applications such as automotive and portable electronics. The catholyte allows greater flexibility in designing catholyte systems for the fuel cell to produce for higher potential and/or a higher sustainable current than with catholytes of the prior art.    DETAILED DESCRIPTION - Redox fuel cell (1) (I) comprises: an anode and a cathode (3) separated by an ion selective polymer electrolyte membrane (2); means for supplying a fuel to the anode region of the cell; means for supplying an oxidant to the cathode region of the cell; means for providing an electrical circuit between the anode and the cathode; and a catholyte solution comprising at least one non-volatile catholyte component flowing in fluid communication with the cathode, where the catholyte solution comprises a redox mediator 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 comprises a complexed multidentate macrocyclic N-donor ligand as the redox mediator and/or as a redox catalyst catalyzing the regeneration of the mediator. INDEPENDENT CLAIMS are included for:    (1) a catholyte for use in a fuel cell, comprising a redox mediator and a redox catalyst, where a multidentate N-donor ligand complex is provided in the catholyte as the redox mediator and/or as the redox catalyst; and    (2) a method of operating a proton exchange membrane fuel cell comprising: forming H+ ions at an anode situated adjacent to a proton exchange membrane; supplying the catholyte with its redox mediator in an oxidized state and its redox catalyst in a reduced state to a cathode situated oppositely adjacent to the proton exchange membrane; and allowing the mediator to become reduced upon contact with the cathode concomitantly with H+ ions passing through the membrane to balance charge.    DESCRIPTION OF DRAWING(S) - The figure illustrates a schematic view of the cathode compartment of a fuel cell.    Fuel cell (1)    Polymer electrolyte membrane (2)    Cathode (3)    Oxidant inlet (4)    Cathode gas reaction chamber (5)