TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Composition: The catholyte solution comprises at least one ancillary redox species selected from ligated transition metal complexes (preferably manganese in oxidation states II-V, Iron in oxidation states I-IV, copper in oxidation states I-III, cobalt in oxidation states I-III, nickel in oxidation states I-III, chromium in oxidation states (II-VII), titanium in oxidation states II-IV, tungsten in oxidation states IV-VI, vanadium in oxidation states II-V or molybdenum in oxidation states II-VI), and/or further a poiyoxometallate species; or is substantially free from any ancillary redox species. The catholyte solution contains poiyoxometallate (at least 0.075 to more than 0.1M). The polyoxometallate is of formula Xa(ZbMcOd) (I).X = H, alkali metals, alkaline earth metals and/or ammonium (preferably H and alkali metal and/or alkaline earth metal ions);Z = B, P, S, As, Si, Ge, Ni, Rh, Sn, Al, Cu, I, Br, F, Fe, Co, Cr, Zn, H2, Te, Mn and/or Se (preferably P);M = Mo, W, V, Nb, Ta, Mn, Fe, Co, Cr, Ni, Zn Rh, Ru, Ti, Al, Ga, In or metals selected from the 1st, 2nd or 3rd transition metal series and/or lanthanide series (preferably Mo and/or V);a = number of X necessary to charge balance the (McOd) anion;b = 0-20 (preferably 0-2);c = 1-40 (preferably 5-20);andd = 1-180 (preferably 30-70).The polyoxometallate (I) contains 2 to 4 vanadium centered in the polyoxometallate; and at least one X is hydrogen. SPECIFIC COMPOUNDS - H3Na2PMo10V2040, H3Na2PMo9V3O40, and H3Na4PMo8V4O40 are specifically claimed polyoxometallate (I).    EXAMPLE - A catholyte solutions (test) containing H3Na2PMo10V2O40 (INV 1) was prepared according to US 4396687. Another catholyte solution was prepared by a mediator i.e. 0.8M vanadium (IV) sulfate and a catalyst including two vanadium atoms i.e. 0.055M H5Mo12V2O40, and oxygen was bubbled through it for two hours to oxidise the vanadium in the mediator to a +5 oxidation state. The performances of the catholytes were compared using a redox regenerative cathode and a hydrogen anode. A commercial anode was used with a platinised gas diffusion layer, one half MEA using a 125 mm Nafion (RTM: perfluorocarboxylic acid resin membrane). To allow direct comparison between the systems, the same flow rates of hydrogen and oxygen were used. The catholyte and cell temperature was also kept at the same temperature for all the measurements. A reticulated vitreous carbon (RVC) electrode was used for the cathode. The catholyte solutions were pumped through this electrode before passing to a heated reservoir where oxygen was bubbled through the solution to allow reoxidation to occur. The total liquid volume was 500 cm3. The cathode was assembled. The membrane assembly was placed on top of the cathode assembly and anode arrangement (similar to the cathode arrangement) was placed on the other side of the membrane to complete the cell. By applying varying loads across the cell, it was possible to establish the maximum current that could be drawn from the cell, and achieving a sufficient rate of regeneration to maintain cell operation indefinitely. The maximum current densities were determined. The maximum current density (mA/cm2) of test/control was found to be 122.9/85.7.