Redox fuel cell

1. A redox fuel cell comprising:

an anode region comprising an anode and a cathode region comprising a cathode, said regions being separated by an ion selective polymer electrolyte membrane;

a fuel passage through which fuel is supplied to the anode region of the cell;

an oxidant inlet that supplies an oxidant to the cathode region of the cell;

an electrical circuit between the anode and the cathode; and

a non-volatile catholyte solution flowing fluid communication with the cathode, the catholyte solution comprising a polyoxometallate being at least partially reduced at the cathode in operation of the cell, and at least partially re-generated by reaction with the oxidant after such reduction at the cathode, the catholyte solution further comprising at least one counterion for the polyoxometallate, wherein the at least one counterion comprises one or more divalent ions and the polyoxometallate of the catholyte solution is not present in the fuel.

2. A redox fuel cell according to claim 1 wherein the one or more divalent ions are selected from Ca, Mg, Mn, Fe, Co, Ni, Cu, Zn, Sr, Ba, Be, Cr, Cd, Hg, Sn, ions from the 2 nd and 3 rd transition metal series, from the lanthanides, and from combinations of two or more thereof.

3. A redox fuel cell according to claim 2 wherein the one or more divalent ions are selected from Mg, Ca, Mn, Fe, Co, Ni, Cu, and Zn, and from combinations of two or more thereof.

4. A redox fuel cell according to claim 1 wherein the polyoxometallate and associated counterion is represented by the formula:

X a [Z b M c O d ]

wherein:

X is selected from the group consisting of: hydrogen, alkali metals, alkaline earth metals, ammonium, transition metal ions and combinations of two or more thereof, wherein at least one X is a divalent ion,

Z is selected from the group consisting of: B, P, S, As, Si, Ge, Ni, Rh, Sn, Al, Cu, I, Br, F, Fe, Co, Cr, Zn, H 2 , Te, Mn, Se, and combinations of two or more thereof,

M is a metal selected from the group consisting of: Mo, W, V, Nb, Ta, Mn, Fe, Co, Cr, Ni, Zn, Rh, Ru, Tl, Al, Ga, In, a metal selected from the 1st, 2nd, and 3rd transition metal series, a metal selected from the lanthanide series, and combinations of two or more thereof;

a is a number of X necessary to charge balance the [Z b M c O d ] anion;

b is from 0 to 20;

c is from 1 to 40; and

d is from 1 to 180.

5. A redox fuel according to claim 4 wherein the divalent ion is selected from Ca, Mg, Mn, Fe, Co, Ni, Cu, Zn, Sr, Ba, Be, Cr, Cd, Hg, Sn, ions from the 2nd and 3rd transition metal series, from the lanthanides, and from combinations of two or more thereof.

6. A redox fuel cell according to claim 5 wherein the divalent ion is selected from Ca, Mg, Mn, Fe, Co, Ni, Cu, Zn, and from combinations of two or more thereof.

7. A redox fuel cell according to claim 4 wherein b ranges from 0 to 2.

8. A redox fuel cell according to claim 4 wherein c ranges from 5 to 20.

9. A redox fuel cell according to claim 4 wherein d ranges from 30 to 70.

10. A redox fuel cell according to claim 4 wherein M is selected from the group consisting of: vanadium, molybdenum, and combinations thereof.

11. A redox fuel cell according to claim 4 wherein Z is phosphorus.

12. A redox fuel cell according to claim 4 wherein X comprises a combination of hydrogen and at least one of an alkali metal and an alkaline earth metal, wherein at least one X is a divalent ion.

13. A redox fuel cell according to claim 4 wherein at least one X is hydrogen.

14. A redox fuel cell according to claim 13 wherein X comprises at least one hydrogen and at least one other material selected from alkali metals, alkaline earth metals, ammonium, and combinations of two or more thereof, wherein at least one X is a divalent ion.

15. A redox fuel cell according to claim 1 comprising from 2 to 4 vanadium centres in the polyoxometallate.

16. A redox fuel cell according to claim 15 wherein the polyoxometallate comprises H 3 Na 2 PMo 10 V 2 O 40 , wherein the protons and/or sodium ions are at least partially replaced by one or more divalent ions.

17. A redox fuel cell according to claim 15 , wherein the polyoxometallate comprises H 3 Na 2 PMo 9 V 3 O 40 , wherein the protons and/or sodium ions are at least partially replaced by one or more divalent ions.

18. A redox fuel cell according to claim 15 wherein the polyoxometallate comprises H 3 Na 4 PMo 8 V 4 O 40 , wherein the protons and sodium ions are at least partially replaced by one or more divalent ions.

19. A redox fuel cell according to claim 1 wherein the catholyte solution comprises at least one ancillary redox species.

20. A redox fuel cell according to claim 19 wherein the ancillary redox species is selected from ligated transition metal complexes, further polyoxometallate species, and combinations thereof.

21. A redox fuel cell according to claim 20 wherein the transition metals in the transition metal complexes are selected from the group consisting of: 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 and molybdenum II-VI.

22. A redox fuel cell according to claim 1 wherein the catholyte solution is substantially free from any ancillary redox species.

23. A redox fuel cell according to claim 1 wherein the concentration of the polyoxometallate in the catholyte solution is over 0.075M.

24. A catholyte solution for use in a redox fuel cell according to claim 1 , the solution comprising a polyoxometallate and a counterion comprising one or more divalent ions.

25. A method of generating electricity by the redox fuel cell of claim 1 , comprising:

supplying fuel to the anode region;

supplying oxidant to the cathode region; and

generating electricity through the electrical circuit between the anode and the cathode.