Fuel cells for use at elevated temperatures and pressures

1. A redox fuel cell comprising an anode and a cathode separated by an ion selective polymer electrolyte membrane; a fuel supply to supply fuel to the anode; an oxidant supply to supply oxidant to the cathode; an electrical circuit provided 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 an elevated pressure, wherein the polyoxometallate comprises at least one of H 7 PMo 8 V 4 O 40 , H 12 P 3 Mo 18 V 7 O 85 , H 10 P 2 V 4 Mo 8 O 44 , or H 10.75 P 2.25 Mo 8 V 4 O 45 .

2. The redox fuel cell according to claim 1 wherein the speciation of the polyoxometallate occurs in situ.

3. The redox fuel cell according to claim 1 wherein the elevated temperature is no more than 120° C.

4. The redox fuel cell according to claim 1 wherein the elevated temperature is below the boiling point for the catholyte solution.

5. The redox fuel cell according to claim 1 wherein the elevated temperature is in the regeneration zone and/or in the cathode/anode region of the cell.

6. The redox fuel cell according to claim 5 wherein the elevated temperature is in the regeneration zone.

7. The redox fuel cell according to claim 1 wherein the elevated pressure is above ambient pressure.

8. The redox fuel cell according to claim 7 wherein the elevated pressure is around 2 barG pressure.

9. The redox fuel cell according to claim 1 wherein the elevated pressure is in the regeneration zone and/or in the cathode/anode region of the cell.

10. The redox fuel cell according to claim 9 wherein the elevated pressure is in the regeneration zone.

11. The redox fuel cell according to claim 1 wherein catholyte solution comprises at least one ancillary redox species.

12. The redox fuel cell according to claim 11 wherein the ancillary redox species is selected from ligated transition metal complexes, further polyoxometallate species, and combinations thereof.

13. The redox fuel cell according to claim 12 wherein the transition metal(s) in the transition metal complexes are selected from 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.

14. The redox fuel cell according to claim 1 wherein the catholyte solution is substantially free from any ancillary redox species, other than the vanadium species.

15. A catholyte solution for use in a redox fuel cell according to claim 1 .

16. The redox fuel cell according to claim 1 wherein the elevated temperature is in the regeneration zone and/or in the cathode/anode region of the cell.

17. The redox fuel cell according to claim 16 wherein the elevated temperature is in the regeneration zone.

18. The redox fuel cell according to claim 1 wherein the elevated pressure is above ambient pressure.

19. The redox fuel cell according to claim 18 wherein the elevated pressure is around 2 barG pressure.

20. The redox fuel cell according to claim 1 wherein the elevated pressure is in the regeneration zone and/or in the cathode/anode region of the cell.

21. The redox fuel cell according to claim 20 wherein the elevated pressure is in the regeneration zone.

22. The redox fuel cell according to claim 1 , wherein the elevated temperature is above 80° C.

23. The redox fuel cell according to claim 1 , wherein the elevated temperature is above 100° C.

24. The redox fuel cell according to claim 1 , wherein both the elevated temperature and the elevated pressure are used.