TECHNOLOGY FOCUS - ELECTRONICS - Preferred Solution: The catholyte solution comprises the complexed multidentate N-donor ligand as the redox mediator, and redox catalyst; or the redox mediator and the complexed multidentate N-donor ligand as redox catalyst; or the complexed multidentate N-donor ligand as redox mediator and a further material as redox catalyst. The catholyte solution is an aqueous solution.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: The complexed multidentate N-donor ligand comprises a transition metal complex of the multidentate N-donor ligand; and at least one (preferably at least two) pyridine substituent. The multidentate ligand is complexed to a transition metal via 3 to 6 nitrogen atoms, where one to five of the nitrogen atoms are contained in at least one optionally substituted aromatic heterocycles. The multidentate N-donor ligand is selected from N,N-bis(pyridine-2-yl-methyl)-bis(pyridine-2-yl)methylamine (N4Py) or their derivatives, from 1,9-bis(2-pyridyl)-2,5,8-triazanonane (pydien), N-methyl-N,N',N'-tri(2-pyridylmethyl)ethane-1,2-diamine (trilen), N,N,N',N'-tetrakis(2-pyridylmethyl)ethane-1,2-diamine (tpen) or their derivatives.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: The oxidant is reduced in the catholyte solution by the redox catalyst, where the resulting oxidized redox catalyst is effective at least partially to oxidize the mediator to regenerate the mediator after its reduction at the cathode. The N-donor ligand is of formula R4-CH2-N(C(R1)(R2)(R3))-CH2-R5 (I). The multidentate N-donor ligand is of formula R1a-N(R2a)-(CH2)n-N(R3a)-R4a (II) or R1b-N(R2b)-(CH2)n-NH-(CH2)m-N(R3b)-R4b (III).R1-R5=2-5 organic groups which each include at least one N donor atom;n and m=1-10;R1a-R4a=1 to 4 organic groups which each include at least one N donor atom;R1b to R4b=one to three organic groups which each include at least one N donor atom.The organic groups which include at least one N donor atom are optionally substituted aromatic heterocycles (preferably pyridine or its derivatives). The redox mediator comprises a modified ferrocene species.    TECHNOLOGY FOCUS - POLYMERS - Preferred Components: The ion selective polymer electrolyte membrane is a cation selective membrane which is selective in favor of protons versus other cations, where the catholyte is acidic; or is an anion selective membrane, where the catholyte is alkaline. The ion selective polymer electrolyte membrane is a bi-membrane. EXAMPLE - A catholyte solution comprised of 1,1'-bis(methylsulfonato)-ferrocene disodium salt (10 mM) and Fe(trilen-(SO3Na)3) (1 mM) in a buffer solution containing Na2SO4 (0.05 M) and NaHSO4 (0.05 M) at pH 2. A performance of the obtained catholyte solution was assessed using a redox cathode and a hydrogen anode. A commercial anode was used with a platinized gas diffusion layer, Nafion (RTM: one half membrane electrode assembly using a 0.125 mm). A reticulated vitreous carbon (RVC) electrode was used for the cathode. The catholyte solution was pumped through this electrode before passing to a reservoir from where it was recirculated. The total liquid volume was 25 cm3. The catholyte was partially oxidized by bubbling oxygen through the solution at 65degrees C for 30 minutes where the absorbance at 650 nm was measured as 0.24. This resulting solution was flowed through the fuel cell whilst a polarization curve was recorded. The solution was partially reduced in the fuel cell by drawing a current over a period of several minutes. The ability of the catholyte to regenerate was then assessed by monitoring the recovery of the open circuit potential whilst bubbling oxygen through the catholyte. The recovery of the open circuit potential over 100, 300 and approximately 480 seconds was found to be 0.659, 0.73 and 0.75 (unit not defined) respectively.