TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: The redox mediator comprises a modified ferrocene species.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: The catholyte solution comprises: the complexed multidentate N-donor ligand as the redox mediator, and as the redox catalyst; a redox mediator and the complexed multidentate N-donor ligand as redox catalyst; and the complexed multidentate N-donor ligand as redox mediator and a further material as redox catalyst. In the operation of (I), 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 catholyte solution is an aqueous solution. The complexed multidentate N-donor ligand comprises a transition metal complex of the multidentate N-donor ligand. The macrocyclic N-donor ligand is or comprises a molecular component containing a ring of seven or more atoms, preferably a polycyclic ring in which seven or more atoms are present in the polycyclic ring structure whilst one or more individual rings of the polycycle may comprise less than six or fewer atoms. The N-donor ligand is stable under acidic pH conditions and as a whole has macrocyclic structure. The N-donor ligand comprises a number of different parts, at least one of which has a macrocyclic structure. The N-donor ligand comprises a multiplicity of N-donor atoms, one, some or all of which are contained within the macrocyclic structure, where not all the N-donor atoms are contained within the macrocyclic structure, one or more other N-donor atoms are incorporated into substituent or pendent arms of the macrocyclic structure or of other parts of the N-donor ligand. The number of N-donor atoms in the N-donor ligand is 1-8. The N-donor atoms within the N-donor ligand are contained within an aliphatic environment and/or within an optionally substituted aromatic or non-aromatic heterocycle. The N-donor atoms within the macrocyclic structure: carry one or more substituent groups, which are H, optionally substituted alkyl, alkenyl, alkynyl, aryl, cycloalkyl, heterocyclic, heteroaryl, halo, hydroxy, amino, imino, nitro, cyano, acyl, acyloxy, sulfate, sulfonyl, sulfinyl, carbonyl, carboxy, carboxylic acid, ester, ether, amido, sulfonate, sulfonic acid, sulfonamide, phosphonate, phosphonyl, phosphonic acid, alkoxycarbonyl, thio, alkoxy, oxyester, oxyamido, aryloxy, heterocycloalkyl, azido and alkyl, alkaryl, alkenaryl, aralkyl or aralkenyl groups substituted with one or more of the above functional groups; and are connected together by any suitable connecting group, which is optionally substituted 1-6C aliphatic carbon chains, alkene, alkyne, aryl, alkylaryl, carbocyclic groups, heteroaryl groups, amide, ether, ester, ketone, carbonyl containing groups, or alkyl, alkenyl, aryl, cycloalkyl, alkaryl, alkenaryl, aralkyl or aralkenyl groups substituted with one or more of the above functional groups. The macrocyclic structure carries one or more suitable substituents at any suitable position on the ring. The N-donor ligand is bridged and/or cross-linked to form a more rigid structure and/or a cage like system. The N-donor ligand comprises a bridging unit linking between carbon atoms and/or nitrogen atoms contained within the macrocyclic structure, where the bridging unit contains one or more further N-donor atoms (preferably 1-3 further N-donor atoms). The further N-donor atoms(s) is or are contained within an optionally substituted aromatic or aliphatic environment. The length of the bridging unit is 3-11 atoms long, where the bridging unit is an alkylheteroaryl, alkylamine unit, 1-4C alkyl and/or contains alkyne, aryl, alkylaryl, carbonyl, carboxylate, ketone, ether and/or ester groups. The N-donor ligand is a cyclic diamine compound of formula (II). The multidenate N-donor ligand is macrocyclic amine compound of formula (III), (IV), (V), (VI) or (VII). The catholyte is acidic or alkaline.R, R1a, R2a, R2, R3 = H or any substituent group (alkyl(optionally substituted), alkenyl, alkynyl, cycloalkyl, (hetero)aryl, heterocyclic, halo, OH, amino, imino, nitro, CN, (acyl)oxy, sulfate, sulfonyl, sulfinyl, carbonyl, carboxy, carboxylic acid, ester, ether, amido, sulfonate, sulfonic acid, sulfonamide, phosphonate, phosphonyl, phosphonic acid, alkoxycarbonyl, thio, alkoxy, oxyester, oxyamido, aryloxy, heterocycloalkyl, azido and alkyl, alkaryl, alkenaryl, aralkyl or aralkenyl groups substituted with one or more of the above functional groups);X = non-N containing linking group;X1 = non N-containing bridging group;X2 = bridging substituent optionally including a coordinating atom;a-d, m = 1-5;n = 0-4; andn1 = 1-5.    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 or an anion selective membrane. The ion selective polymer electrolyte membrane is a bimembrane.