NOVELTY - A catholyte (c1) comprises redox mediator and redox catalyst. In the catholyte multidentate N-donor ligand complex is provided as the redox mediator and/or redox catalyst.    USE - In redox fuel cells (claimed) as power sources for portable products (such as portable electronics products), for transport vehicles (such as automobiles, both main and auxiliary), auxiliary power for e.g caravans and other recreational vehicles and boats, as uninterruptible power for hospitals computers and combined heat and power for homes and businesses.    ADVANTAGE - Due to the catholyte composition, reoxidation of the catalyst occurs, catalyzed by the transition metal complex of a multidentate N-donor redox catalyst, which 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. Also the use of a bi-membrane in the redox fuel cell maximizes the potential of the cell, by maintaining the potential due to a pH drop between the anode and catholyte solution.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) a redox fuel cell comprising an anode and a cathode separated by an ion selective polymer electrolyte membrane; device for supplying a fuel to the anode region of the cell; device for supplying an oxidant to the cathode region of the cell; device for providing an electrical circuit between the anode and the cathode; and catholyte solution having at least one non-volatile catholyte component flowing in fluid communication with the cathode. 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. The catholyte solution comprises a complexed multidentate N-donor ligand as the redox mediator and/or as a redox catalyst catalyzing the regeneration of the mediator;    (2) operating a proton exchange membrane fuel cell involving forming H+ ions at an anode situated adjacent to a proton exchange membrane; supplying the catholyte (c1) 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; and    (3) N-methyl-N,N',N'-tris(2-(4-chloro)-pyridylmethyl)ethane-1,2-diamine (trilen-Cb).