The present invention relates to a
redox fuel
cell comprising an
anode and a
cathode separated 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 electrical circuit between the
anode and the
cathode; a non-volatile catholyte solution flowing in fluid communication with the cathode, the catholyte solution comprising a polyoxometallate
redox couple being at least partially reduced at the cathode in operation of the cell, and at least partially re-generated by reaction with the oxidant in a regeneration zone after such reduction at the cathode, the catholyte solution further comprising one or more
vanadium species that result from the
speciation of the polyoxometallate at an elevated temperature and / or pressure, wherein the polyoxometallate is represented by the formula:Xa[ZbMcOd]wherein X is selected from
hydrogen, alkali metals, alkaline earth metals,
ammonium,
transition metal ions and combinations of two or more thereof; Z is selected from B, P, S, As, Si, Ge, Ni, Rh, Sn, Al, Cu, I, Br, F, Fe, Co, Cr, Zn, H2, Te, Mn and Se and combinations of two or more thereof; M comprises
vanadium and optionally one or more of Mo, W, Nb, Ta, Mn, Fe, Co, Cr, Ni, Zn Rh, Ru, TI, Al, Ga, In and other metals selected from the 1st, 2nd and 3rd
transition metal series and the
lanthanide series and combinations of two or more thereof; a is a number of X necessary to charge balance the [ZbMcOd] anion; b is from 0 to 20; c is from 1 to 40; d is from 1 to 180; X includes an amount of a non-
hydrogen cation and the
molar ratio of the non-
hydrogen cation to
vanadium is more than 0 and less than 1.