Redox systems

1. A catholyte comprising a composition, the composition comprising a polyoxometalate and an additive tolerant to the properties of the polyoxometalate, the properties of the polyoxometalate being maintained despite the presence of the additive, the composition being in the form of an aqueous solution, wherein the additive is, or comprises, one or more acid, or salt or derivative thereof, wherein the acid is a carboxylic acid or a sulphonic acid, and wherein the additive is effective to reduce the freezing point and/ or elevate the boiling point of the composition.

2. The catholyte of claim 1 , wherein the additive is present in an amount effective to reduce the freezing point and/or elevate the boiling point of the composition.

3. The catholyte of claim 2 , wherein the additive is present in an amount effective to reduce the freezing point and/or elevate the boiling point of the composition by at least about 1° C. compared to the freezing point or boiling point of the composition in the absence of the additive.

4. The catholyte as claimed in claim 1 , wherein the polyoxometalate is a redox-active polyoxometalate, the additive is tolerant to the oxidizing and/or reducing character of the polyoxometalate, and the redox activity of the polyoxometalate is maintained despite the presence of the additive.

5. The catholyte as claimed in claim 1 , wherein the composition has catalytic activity despite the presence of the additive.

6. The catholyte as claimed in claim 1 wherein the additive is tolerant of the acidity of the composition.

7. The catholyte as claimed in claim 6 wherein the composition has a pH of 3 or less.

8. The catholyte as claimed in claim 1 wherein the additive is effective to reduce the freezing point, at standard pressure, of the composition by 20° C. or more.

9. The catholyte as claimed in claim 1 where the polyoxometalate is based on the Keggin structure.

10. The catholyte as claimed in claim 1 wherein the polyoxometalate is of the formula:

X a [Z b M c O d ]

wherein:

X is selected from hydrogen, alkali metals, alkaline earth metals, ammonium or alkyl ammonium 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, H 2 , Te, Mn and Se and combinations of two or more thereof;

M is a metal selected from Mo, W, V, Nb, Ta, Mn, Fe, Co, Cr, Ni, Zn Rh, Ru, Tl, 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 [M c O d ] anion;

b is from 0 to 20;

c is from 1 to 40; and

d is from 1 to 180.

11. The catholyte as claimed in claim 1 wherein the counterions of the polyoxometalate comprise at least one divalent ion.

12. The catholyte as claimed in claim 11 wherein the polyoxometalate and associated counterion are represented by the formula:

X a [Z b M c O d ]

wherein:

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

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

M is a metal selected from Mo, W, V, Nb, Ta, Mn, Fe, Co, Cr, Ni, Zn Rh, Ru, Tl, 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 [M c O d ] anion;

b is from 0 to 20;

c is from 1 to 40; and

d is from 1 to 180.

13. The composition as claimed in claim 1 wherein the polyoxometalate is represented by the formula:

X a [Z b M c O d ]

wherein:

X is selected from hydrogen, alkali metals, alkaline earth metals, ammonium or alkyl ammonium 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, H 2 , Te, Mn and Se and combinations of two or more thereof;

M comprises at least one V atom, and M is a metal selected from Mo, W, V, Nb, Ta, Mn, Fe, Co, Cr, Ni, Zn Rh, Ru, Tl, 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 [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.

14. The composition as claimed in claim 13 further comprising a Vanadium (IV) compound.

15. The catholyte as claimed in claim 1 wherein the polyoxometalate is represented by the formula:

X a [Z b M c O d ]

wherein:

X is selected from hydrogen, alkali metals, alkaline earth metals, ammonium 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, H 2 , Te, Mn and Se and combinations of two or more thereof;

M comprises W and optionally one or more of Mo, V, Nb, Ta, Mn, Fe, Co, Cr, Ni, Zn Rh, Ru, Tl, Al, Ga, In and other metals selected from the 1st, 2nd and 3rd transition metal series and the lanthanide series;

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

b is from 0 to 5;

c is from 5 to 30; and

d is from 1 to 180.

16. The catholyte as claimed in claim 1 wherein the additive has a molecular weight of 200 or less.

17. The catholyte as claimed in claim 1 wherein the additive is fully dissociated.

18. The catholyte as claimed in claim 1 wherein the additive is more acidic than the polyoxometalate.

19. The catholyte as claimed in claim 1 in which the acid is a sulphonic acid.

20. The catholyte as claimed in claim 19 in which the sulphonic acid is a halogenated sulphonic acid.

21. The catholyte as claimed in claim 19 in which the sulphonic acid is methanesulphonic acid.

22. A fuel cell comprising the composition of claim 1 .