This invention provides a redox fuel cell comprising an anode and a cathode sepd. by an ion selective polymer electrolyte membrane; 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 elec. circuit between the anode and the cathode; a non-volatile catholyte soln. flowing in fluid communication with the cathode, the catholyte soln. comprising 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 redn. at the cathode, and a transition metal complex of a multidentate N-donor ligand as a redox catalyst catalyzing the regeneration of the mediator the multi-dentate N-donor ligand comprising at least one heterocyclic substituent selected from pyrrole, imidazole, 1,2,3-triazole, 1,2,4-triazole, pyrazole, pyridazine, pyrimidine, pyrazine, indole, tetrazole, quinoline, isoquinoline and from alkyl, alkenyl, aryl, cycloalkyl, alkaryl, alkenaryl, aralkyl, aralkenyl groups substituted with one or more of the aforesaid heterocyclic groups. [on SciFinder(R)]