Redox systems

Abstract

A composition comprising a polyoxometalate and an additive tolerant to the properties of the polyoxometalate, wherein the properties of the polyoxometalate are maintained despite the presence of the additive, and wherein the additive is effective to reduce the freezing point and/or elevate the boiling point of the composition. Such a composition may be used in a fuel cell.

Claims

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.sub.a[Z.sub.bM.sub.cO.sub.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.sub.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.sub.cO.sub.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.sub.a[Z.sub.bM.sub.cO.sub.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.sub.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.sub.cO.sub.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.sub.a[Z.sub.bM.sub.cO.sub.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.sub.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.sub.bM.sub.cO.sub.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.sub.a[Z.sub.bM.sub.cO.sub.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.sub.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.sub.bM.sub.cO.sub.d].sup.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.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will now be described in further non-limiting detail and with reference to the Figures in which:

(2) FIG. 1 shows P31 NMR spectra of a polyoxometalate system with an additive of the present invention;

(3) FIG. 2 shows P31 NMR spectra of a polyoxometalate system without an additive of the present invention;

(4) FIG. 3 shows a mass spectrometry comparison between a polyoxometalate system with and without an additive of the present invention, together with a computer generated simulation of an equivalent system showing the expected isotopic distribution for the POM in question;

(5) FIG. 4 shows a cyclic voltammogram of three polyoxometalate systems with, respectively, no additive, one additive and two additives of the present invention;

(6) FIG. 5 shows a steady state comparison at 1.5 A/cm.sup.2 between a polyoxometalate system with and without an additive of the present invention;

(7) FIG. 6 shows an IV curve comparing a polyoxometalate system with and without an additive of the present invention; and

(8) FIG. 7 shows the effect of sparged air in regenerating (i.e. oxidizing after reduction) a polyoxometalate system with and without an additive of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(9) Solutions containing methanesulphonic acid or its salts were made by dissolving the required amount of additive into 0.3M polyoxometalate (POM) solution and then adjusting the volume back to its original value by evaporating off the required volume of water.

(10) Sodium methanesulfonate (SMS) and methanesulfonic acid were purchased from Sigma Aldrich and used as received. Lithium methanesulfonate (LMS) was made by neutralising methanesulfonic acid with LiOH and drying under vacuum.

(11) The influence on solution freezing point of added lithium methane sulfonate (LMS) to 0.3M H.sub.7PO.sub.40Mo.sub.8V.sub.4 in aqueous solution (HV4) was as follows.

(12) TABLE-US-00001 LMS Approximate Concentration T.sub.f 2M 19 C. 3M 31 C. 4M 40 C. 5M <50 C.

(13) The influence upon solution freezing point of addition of 1M and 2M sodium methanesulfonate (SMS) or lithium methanesulfonate (LMS) to 0.3 M H.sub.3Na.sub.4PO.sub.40Mo.sub.8V.sub.4 (NaV4), 0.3M H.sub.7PO.sub.40Mo.sub.8V.sub.4 (HV4) and 0.3M H.sub.10P.sub.2O.sub.44Mo.sub.8V.sub.4 (FC6) was as follows.

(14) TABLE-US-00002 Sodium Series Freeze Point, C. Lithium Series Freeze Point, C. 1M in NaV4 9.0 1M in NaV4 10.2 2M in NaV4 17.5 2M in NaV4 20.4 1M in HV4 10.2 1M in HV4 12.8 2M in HV4 19.3 2M in HV4 24.7 1M in FC6 10.5 1M in FC6 12.2 2M in FC6 21.0 2M in FC6 24.1

(15) These values were measured by analysing their cooling curves and recording phase changes indicated by a change in cooling rate.

(16) Evidence of Keggin type structures still being present was provided by NMR analysis. The phosphorus 31 NMR spectrum was compared to that of standard HV4 and shown to be similar (see FIGS. 1 and 2), indicating that major speciation changes had not taken place.

(17) Further evidence of Keggin structures still being present after treatment with antifreeze additives is given by the Mass Spectrometry data shown in FIG. 3. The electrospray mass spectra were recorded in the negative mode with a cone voltage of 10 eV and a probe temperature of 450 C., with ultrapure water as the eluent. The upper section of FIG. 3 shows the spectra recorded with the antifreeze additive which is similar to that of untreated POM solution (middle section) and similar to a computer generated simulation of the expected POM formula (lower section).

(18) Cyclic Voltammetry carried out at 30 C. showed all the expected redox processes as shown in FIG. 4. This figure shows a comparison between HV4 and a solution of HV4 into which 1M NaOMs had been added and also a solution of HV4 into which 1M HOMs and 1M NaOMs had been added. Changes in solution pH, conductivity and viscosity account for the observed differences. However, major changes in the redox behaviour of the POM system were not observed.

(19) The fuel cell performance, at 110 C. and 3 bar absolute pressure, of antifreeze catholytes was evaluated and the results are presented in FIGS. 5, 6 and 7. FIG. 5 shows a comparison between an antifreeze catholyte in a fuel cell operating at a constant current density of 1.5 A cm.sup.2 and an untreated catholyte under similar conditions. Periodically, the load was switched off and the cell was allowed to reach open circuit potential for 30 seconds in order to monitor the redox state of the catholyte. FIG. 5 shows that the antifreeze catholyte sustains the current throughout the duration of the test and the open circuit potential is unchanged. This indicates that the regeneration of reduced species is in equilibrium with the electrochemical reduction.

(20) FIG. 6 shows a comparison between the polarisation curves, recorded at a sweep rate of 500 mA sec.sup.1, between a catholyte with added antifreeze additive and an untreated catholyte. After an initial drop, the antifreeze catholyte shows a polarisation curve with very similar gradient to that of the untreated catholyte.

(21) FIG. 7 shows the effect on the open circuit potential of the catholyte of sparging air through a reduced solution. The antifreeze catholyte regenerates at a comparable rate to the untreated catholyte and after 30 minutes has reached a higher open circuit potential than the untreated version.