TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: The polyoxometalate is of formula X1a(ZM112O40), and comprises (H3+ePMo12-eVeO40), (H3+ePW12-eVeO40), (HfX1gPMo12-eVeO40), and/or (HfX1gPW12-eVeO40), preferably (H3Na2PMo10V2O40), (H3Na2PMo9V3O40), or (H3Na4PMo8V4O40). The vanadium(IV) compound comprises at least one of vanadium oxide of formula (VO2) or (V2O4), vanadium oxide sulfate (VOSO4), vanadyl acetylacetonate, vanadium oxide boron tetrafluoride of formula (VO(BF4)2) and/or their hydrated versions. The redox fuel cell comprises combination of: (H3+ePMo12-eVeO40) or (H3+ePW12-eVeO40) with added vanadium oxide of formula (V2O4) and/or vanadium oxide sulfate (VOSO4); or (HfX1gPMo12-eVeO40) or (HfX1gPW12-eVeO40) with added vanadium oxide of formula (V2O4) and/or vanadium oxide sulfate (VOSO4). The concentration of the vanadium (IV) compound is at least 0.05M, at least 0.1M, at least 0.15M, at least 0.2M, at least 0.25M, or at least 0.3M. The molar ratio between the polyoxometalate and the vanadium(IV) compound is at least 1:10, at least 1.5:10, at least 2:10, at least 2.5:10, or at least 3:10. The redox fuel cell contains 2-4 vanadium centers in the polyoxometalate. The catholyte solution: comprises at least one ancillary redox species; or is substantially free from any ancillary redox species. The redox species comprises ligated transition metal complexes, and/or further polyoxometalate species, where the transition metal(s) in the transition metal complexes is (or are) manganese in oxidation states (II)-(V), iron (I)-(IV), copper (I)-(III), cobalt (I)-(III), nickel (I)-(III), chromium (II)-(VII), titanium (II)-(IV), tungsten (IV)-(VI), vanadium (II)-(V), or molybdenum (II)-(VI). The concentration of the polyoxometalate in the catholyte solution is at least 0.1 M, preferably at least 0.20M.e = 2-5; andf+g = 3+e. Preferred Definitions:    b = 0-2;    c = 5-20;    d = 30-70;    M1 = di-, tri-, tetra-, or penta-vanadium and/or Mo;    Z1 = P; and    X1 = Na and/or Li.