Abstract
This invention provides 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 after such reduction at the cathode, the catholyte solution further comprising vanadium species that result from the speciation of the polyoxometallate at an elevated temperature and/or pressure.
Claims
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.sub.7PMo.sub.8V.sub.4O.sub.40, H.sub.12P.sub.3Mo.sub.18V.sub.7O.sub.85, H.sub.10P.sub.2V.sub.4Mo.sub.8O.sub.44, or H.sub.10.75P.sub.2.25Mo.sub.8V.sub.4O.sub.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.
Description
(1) Various aspects of the present invention will now be more particularly described with reference to the following figures which illustrate embodiments of the present invention:
(2) FIG. 1 illustrates a schematic view of the cathode compartment of a fuel cell in accordance with the present invention;
(3) FIG. 2 illustrates charts demonstrating the performance of a fuel cell in accordance with the present invention over the entire test period;
(4) FIG. 3 illustrates a chart demonstrating the change in rate of regeneration of the polyoxometallate with changing temperature and pressure in the regeneration zone in a fuel cell in accordance with the present invention;
(5) FIG. 4 illustrates a chart demonstrating the change in rate of regeneration with changing temperature in the regeneration zone in a fuel cell in to accordance with the present invention;
(6) FIG. 5 illustrates charts showing the steady state performance of a fuel cell in accordance with the present invention at 600 mA/cm.sup.2 as a function of anode/cathode region temperature;
(7) FIG. 6 illustrates charts showing the steady state performance of a fuel cell in accordance with the present invention at 600 mA/cm.sup.2 as a function of the temperature of the regeneration zone;
(8) FIG. 7 illustrates the current/voltage data for Na.sub.4H.sub.3PV.sub.4Mo.sub.8O.sub.40 at 0.3M and H.sub.10P.sub.2V.sub.4Mo.sub.8O.sub.44 (POM6) at 0.45M in a standard fuel cell at 80 C. and 1 atm pressure; and
(9) FIG. 8 illustrates the steady-state single cell performance data of Na.sub.4H.sub.3PV.sub.4Mo.sub.8O.sub.40 at 0.3M compared with H.sub.10P.sub.2V.sub.4Mo.sub.8O.sub.44 (POM6) at 0.45M and H.sub.10.75P.sub.2.25Mo.sub.8V.sub.4O.sub.45 (POM6.2) at 0.3M at an elevated temperature (110 C.) and pressure (3 atm).
(10) Referring to FIG. 1, there is shown the cathode side of fuel cell 1 in accordance with the invention comprising a polymer electrolyte membrane 2 separating an anode (not shown) from cathode 3. Cathode 3 comprises in this diagram reticulated carbon and is therefore porous. However, other cathodic materials such as platinum may be used. Polymer electrolyte membrane 2 comprises cation selective Nafion 112 membrane through which protons generated by the (optionally catalytic) oxidation of fuel gas (in this case hydrogen) in the anode chamber pass in operation of the cell. Electrons generated at the anode by the oxidation of fuel gas flow in an electrical circuit (not shown) and are returned to cathode 3. Fuel gas (in this case hydrogen) is supplied to the fuel gas passage of the anode chamber (not shown), while the oxidant (in this case air) is supplied to oxidant inlet 4 of cathode gas reaction chamber 5. Cathode gas reaction chamber 5 (the catalyst reoxidation zone) is provided with exhaust 6, through which the by-products of the fuel cell reaction (eg water and heat) can be discharged.
(11) A catholyte solution comprising the oxidised form of the polyoxometallate redox catalyst is supplied in operation of the cell from catholyte reservoir 7 into the cathode inlet channel 8. The catholyte passes into reticulated carbon cathode 3, which is situated adjacent membrane 2. As the catholyte passes through cathode 3, the polyoxometallate catalyst is reduced and is then returned to cathode gas reaction chamber 5 via cathode outlet channel 9.
(12) Due to the advantageous composition of the catholyte of the present invention, reoxidation of the catalyst occurs very rapidly, which allows the fuel cell to produce a higher sustainable current than with catholytes of the prior art.
(13) FIGS. 2 to 6 were all created under the same experimental conditions. A fuel cell with a 25 cm.sup.2 active area was used in conjunction with a polyoxometallate of structure H.sub.7PMo.sub.8V.sub.4O.sub.40. The cell comprised an anode gas diffusion layer 34BC, which was supplied by SGL. An anode-membrane assembly with a 25-m thick membrane, supplied by W.L. Gore Ltd, was used. The cathode electrode was GFD2.5 graphite felt, supplied by SGL, compressed to about 1.15 mm thick. 650 ml of H.sub.7PMo.sub.8V.sub.4O.sub.40 catholyte was charged to the regenerator, a flow of 2 l/min of air was used during regeneration and 2 l/min of N.sub.2 was used when holding the cell at a specific oxidation state.
(14) FIG. 2 demonstrates the durability of the cell over the entire test period (between 6 Jun. 2012 and 15 Jun. 2012). FIG. 2a shows the polarisation curves over the test period from various open circuit voltages (OCPs). The shape of the slopes in FIG. 2a remains almost constant throughout the test period with a small improvement seen in the slope of the linear portion improving between 6 Jun. 2012 and 11 Jun. 2012. This continuity indicates that the performance of the cell did not degrade during the test period. FIG. 2b shows the membrane and cell resistivity values obtained by Galvano-electrochemical impedance spectroscopy (GEIS) over the test period. The membrane resistivity (HFR) remains almost constant throughout the test period but there is a larger shift in the overall (LFR) resistance of the cell. Any change in resistance could be due to the OCP of the catholyte which can lead to shifts in its dynamic equilibrium and could also lead to a subsequent interaction with vanadium species and the membrane. It is therefore concluded that there is no loss in cell performance during the test period.
(15) FIG. 3 demonstrates the regeneration curves where the temperature of the anode and cathode regions (Tc) is between 80 and 85 C., the temperature of the regeneration zone (Tr) varies between 80 and 120 C. and the pressure in the regeneration zone (Pr) varies between 0 and 2 barG (gauge pressure i.e. relative to ambient pressure). In order to separate anode/cathode region performance from the regeneration zone performance, testing of the two components was separated. The regeneration zone was tested by first reducing the catholyte in the fuel cell to an OCP of less than 0.8V. The N.sub.2 sparge (at 2 l/min) was used under these conditions to ensure adequate mixing within the regeneration zone. The regeneration rate at various regeneration zone temperatures (Tr) was then measured at an open circuit voltage of between 0.8V and 0.88V with 2 l/min air feed to the regeneration zone when it is either at 0-0.3 barG or 1.95-2.05 barG pressure. When testing the regeneration zone, the temperature of the anode and the cathode regions (Tc) is maintained at 80 C.7 C. The anode and cathode regions were tested by measuring the steady state voltage at 0.6A/cm.sup.2 over a 1 hour period whilst varying the temperature of the anode and cathode regions (Tc) and maintaining the Tr at 110 C. (with 2 l/min air).
(16) Under normal cell operation conditions (Tc=80 C./Tr=80 C./Pr=0 barG) it takes more than 4000s for the POM to regenerate from an OCP of 0.80V to an OCP of 0.88V. Once pressure is applied to the system the rate is much improved and reaches 0.88V within 1500s. FIG. 3 shows that at 2 barG pressure, subsequent improvements are seen with increased Tr. Once Tr reaches 120 C., it is possible to reach an OCP of 0.88V within 300s. Further, increasing the Tc at a Tr of 110 C. also results in a slightly improved performance. However, the increase is not as great as with increases in Tr.
(17) FIG. 4 demonstrates the time taken to reach an OCP of 0.88V from 0.8V where Tc is 80 C., Tr varies between 80 and 120 C. and Pr is maintained at 2 barG. Again, FIG. 4 shows that at 2 barG pressure, subsequent improvements are seen with increased Tr. Once Tr reaches 120 C., it is possible to reach an OCP of 0.88V within 300s.
(18) FIGS. 5 and 6 demonstrate the steady state performance at 600 mA/cm.sup.2 as a function of Tc and Tr respectively. FIGS. 5a and 6a show that in general, steady state performance increases as a function of both Tc and Tr. Tcin is defined as the catholyte temperature just before it flows into the cell. The result obtained for 140612B could be due to dehydration of the POM which could have dented the performance. The concentration of the POM had gone down by around 90% during system operation according to the optical level; this level was corrected before 150612A was performed. FIGS. 5b and 6b do not indicate a rise in OCP during steady state operation with elevated temperature, which is not what is expected from the regeneration curves. This phenomenon is due to the fact that the OCP at a fixed oxidation state rises as the POM catholyte temperature falls.
(19) FIG. 7 shows the current/voltage data for a known polyoxometallate species (Na.sub.4H.sub.3PV.sub.4Mo.sub.8O.sub.40 at 0.3M) and a preferred polyoxometallate species of the present invention POM6 (H.sub.10P.sub.2V.sub.4Mo.sub.8O.sub.44 at 0.45M) in a liquid flow cathode PEM fuel cell at 80 C. and 1 atm pressure. Under these conditions, the preferred polyoxometallate species of the present invention demonstrates a relatively poor performance compared to the known polyoxometallate species, particularly at higher current densities.
(20) FIG. 8 shows the steady-state single cell performance data of a known polyoxometallate species (Na.sub.4H.sub.3PV.sub.4Mo.sub.8O.sub.40 at 0.3M) compared with preferred polyoxometallate species of the present invention, POM6 (H.sub.10P.sub.2V.sub.4Mo.sub.8O.sub.44 at 0.45M) and POM6.2 (H.sub.10.75P.sub.2.25Mo.sub.8V.sub.4O.sub.45 at 0.3M) at an elevated temperature (110 C.) and pressure (3 atm). The data shows a further improvement in continuous operation with the application of only mildly increased temperature and pressure.
