Regenerative fuel cells

Abstract

The present invention provides a regenerative fuel cell comprising an anionic membrane capable of selectively passing anions, wherein the pH of the anolyte and/or catholyte is at least 10. The present invention also relates to a method of operating a regenerative fuel cell comprising an anionic membrane capable of selectively passing anions, wherein the pH of the anolyte and/or catholyte is at least 10.

Claims

1. A regenerative fuel cell capable of operating in a power delivery mode in which it generates electrical power by the reaction of electrochemically active species at an anode and a cathode, and in an energy storage mode in which it consumes electrical power to generate said electrochemically active species, the cell comprising: a reversible anode in an anode compartment containing an anolyte; a reversible cathode in a cathode compartment containing a catholyte; an anionic membrane separating the anode compartment from the cathode compartment, which membrane is capable of selectively passing anions; and conduits configured, in said power delivery mode, for carrying electrochemically active species to the anode and to the cathode and, in an energy storage mode, for carrying generated electrochemically active species away from the anode and away from the cathode; wherein the pH of the anolyte and/or the catholyte is at least 10.

2. The regenerative fuel cell of claim 1, wherein the conduits are configured: to carry a liquid anolyte and a liquid catholyte containing respective electrochemically active species to the anode compartment and to the cathode compartment in the power delivery mode; and to carry a liquid anolyte and a liquid catholyte containing respective regenerated electrochemically active species away from the anode compartment and the cathode compartment in the energy storage mode.

3. The regenerative fuel cell of claim 1, wherein one of the anode or the cathode is a porous gas electrode and wherein one of said conduit is configured to supply a gaseous electrochemically active species to that electrode; and one of said conduit is configured to carry a liquid anolyte or a liquid catholyte containing electrochemically active species to the other of the anode or the cathode in said power delivery mode, and to carry a liquid anolyte or a liquid catholyte containing regenerated electrochemically active species away from the other of the anode or the cathode in said energy storage mode.

4. The regenerative fuel cell of claim 3, further comprising a pressurized gas source configured to contain the gaseous electrochemically active species, which gas source is connected, in the power delivery mode, to a conduit for supplying the gaseous electrochemically active species to the porous gas electrode.

5. The regenerative fuel cell of claim 1, further comprising: at least one first vessel configured to contain a liquid electrolyte containing electrochemically active species, which first vessel is connected, in the power delivery mode, to a conduit for supplying the electrolyte to the appropriate electrode compartment, and at least one second vessel configured to receive a liquid electrolyte, which second vessel is connected, in the energy storage mode, to a conduit for receiving the electrolyte containing regenerated electrochemically active species from the appropriate electrode compartment.

6. The regenerative fuel cell of claim 1, wherein a redox couple present in the anolyte is selected from:
2H.sub.2O+2e.sup.−H.sub.2+2OH.sup.−,
S.sub.(sol)+2e.sup.−S.sup.2−,
Se.sub.(sol)+2e.sup.−Se.sup.2−,
2Te.sub.(sol)+2e.sup.−Te.sub.2.sup.2−,
Sb.sup.VO.sub.3+2e.sup.−Sb.sup.IIIO.sub.2+2OH.sup.−,
Fe.sup.III-triethanolamine+e.sup.−Fe.sup.II-triethanolamine,
[Cr(NH.sub.3).sub.6].sup.3++e.sup.−[Cr(NH.sub.3).sub.6].sup.2+,
[Cr.sup.III-(picolinate).sub.3].sup.3++e.sup.−[Cr.sup.II-(picolinate).sub.3].sup.2+, and
[Cr.sup.III-(picolinate).sub.2(OH)].sub.2.sup.4++e.sup.−[Cr.sup.II-(picolinate).sub.2(OH)].sub.2.sup.2+.

7. The regenerative fuel cell of claim 1, wherein a redox couple present in the catholyte is selected from:
O.sub.2+4e.sup.−+2H.sub.2O4OH.sup.−,
[Fe.sup.III(CN).sub.6].sup.4−+e.sup.−[Fe.sup.II(CN).sub.6].sup.3−,
Te.sup.VIO.sub.4.sup.2−+H.sub.2O+2e.sup.−Te.sup.IVO.sub.3.sup.2−+2OH.sup.−,
I.sup.VIIO.sub.3.sup.−+6H.sup.++6e.sup.−I.sup.−+3H.sub.2O,
Pb.sup.IVO.sub.3.sup.2−+3H.sup.++2e.sup.−HPb.sup.IIO.sub.2.sup.−+H.sub.2O,
dehydroascorbate+2H.sub.2O+2e.sup.−ascorbate+2OH.sup.−,
ethanal+2H.sub.2O+2e−ethanol+2OH.sup.−,
propanal+2H.sub.2O+2e.sup.−propanol+2OH.sup.−,
glyoxal+2H.sub.2O+2e.sup.−glycoaldehyde+2OH.sup.−, and
pyruvate+2H.sub.2O+2e.sup.−lactate+2OH.sup.−.

8. The regenerative fuel cell of claim 1, wherein a pair of redox couples present in the anolyte and catholyte is selected from the following combinations:
2S.sub.(sol)+4e.sup.−2S.sup.2−
O.sub.2+4e.sup.−+2H.sub.2O4OH.sup.−,
2Se.sub.(sol)+4e.sup.−2Se.sup.2−
O.sub.2+2H.sub.2O+4e.sup.−4OH.sup.−,
4Te.sub.(sol)+4e.sup.−2Te.sub.2.sup.2−
O.sub.2+2H.sub.2O+4e.sup.−4OH.sup.−,
2Sb.sup.VO.sub.3+2H.sub.2O+4e.sup.−2Sb.sup.IIIO.sub.2+4OH.sup.−
O.sub.2+2H.sub.2O+4e.sup.−4OH.sup.−,
[Fe.sup.III(CN).sub.6].sup.4−+e.sup.−[Fe.sup.II(CN).sub.6].sup.3−
Fe.sup.III-triethanolamine+e.sup.−Fe.sup.II-triethanolamine,
Pb.sup.IVO.sub.2.sup.−+3H.sub.2O+2e.sup.−HPb.sup.IIO.sub.2.sup.−+3OH.sup.−
2H.sub.2O+2e.sup.−H.sub.2+2OH.sup.−,
2[Fe.sup.III(CN).sub.6].sup.4−+2e.sup.−2[Fe.sup.II(CN).sub.6].sup.3−
2H.sub.2O+2e.sup.−H.sub.2+2OH.sup.−,
Te.sup.VIO.sub.4.sup.2−+H.sub.2O+2e.sup.−Te.sup.IVO.sub.3.sup.2−+2OH.sup.−
2H.sub.2O+2e.sup.−H.sub.2+2OH.sup.−, and
I.sup.VIIO.sub.3.sup.−+3H.sub.2O+6e.sup.−I.sup.−+6OH.sup.−
6H.sub.2O+6e.sup.−3H.sub.2+6OH.sup.−.

9. The regenerative fuel cell claim 1, wherein the pH of the anolyte ranges from 12 to 15 throughout said energy storage mode and throughout said power delivery mode.

10. The regenerative fuel cell claim 1, wherein a redox couple present in the anolyte or catholyte comprises a gaseous species.

11. The regenerative fuel cell of claim 1, wherein a redox couple present in the catholyte is:
O.sub.2+2H.sub.2O+4e.sup.−4OH.sup.−.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIGS. 1 and 2 are schematic sectional views of two liquid/gas regenerative fuel cells (the terms “liquid” and “gas” denoting the phases of the electroactive material supplied to the two electrodes;

(2) FIG. 3 is a schematic view of a liquid/liquid regenerative fuel cell;

(3) FIG. 4 is a schematic view of an assembled stack of a plurality of cells of the present invention;

(4) FIG. 5 is an exploded view of the assembled stack shown in FIG. 4;

(5) FIG. 6 is an exploded view of each of the individual cells present in the assembled stack shown in FIG. 4; and

(6) FIG. 7 shows an I-V curve of a polysulphide-air alkaline regenerative fuel cell of the present invention.

(7) FIG. 8 shows a cyclic voltammogram for the redox couple [Cr(NH.sub.3).sub.6].sup.3++e.sup.−[Cr(NH.sub.3).sub.6].sup.2+ at pH 10, 0.1M NaNO.sub.3 and 0.045M [Cr(NH.sub.3).sub.6].sup.3+ using a glassy carbon electrode. Reversible peaks around 0.4V vs RHE (−0.2 vs SHE) are seen.

DESCRIPTION OF EMBODIMENTS AND EXAMPLES

(8) FIG. 1 shows a schematic of a regenerative fuel cell in which the electrochemically active materials used to generate power are (a) a gas (supplied to the cathode) and (b) dissolved ions in a liquid anolyte (supplied to the anode).

(9) In the power delivery mode, the liquid anolyte containing the electrochemically active species S.sup.2− is pumped from a compartment of an anolyte storage container (1), through a conduit (2) and into the anolyte compartment (3), where it is oxidised at an anode (4) according to the half reaction:
S.sup.2−.fwdarw.S.sub.(sol)+2e.sup.−

(10) The anolyte containing the spent electrolyte species sulphur (S) is then carried away from the anolyte compartment through a second conduit (5) to the anolyte storage container (1), where it is stored in a compartment separate from the fresh anolyte compartment. The cathode and at least part of the catholyte compartment (8) are formed by a porous gas flow electrode and gaseous air (containing oxygen as the electrochemically active material) is pumped by an air compressor (6) through a conduit (7), to the cathode/cathode compartment (8), where the oxygen present in the air is reduced to hydroxide ions (OH.sup.−) according to the half reaction:
O.sub.2+4e.sup.−+2H.sub.2O.fwdarw.4OH.sup.−
and the current is collected by a current collector (9).

(11) An alkaline membrane (10) separates the anolyte and catholyte compartments (3 & 8) and selectively passes the hydroxide ions from the catholyte to the anolyte side on the membrane (10) to balance the charge, thereby completing the electrical circuit. The oxygen depleted air is carried away from the catholyte compartment (8) by a second conduit (11) and discharged to the atmosphere.

(12) In the energy storage mode, the system is reversed so that the electrochemically active species sulphur (S) is pumped from the anolyte storage container (1), through the conduit (5) to the anolyte compartment (3), where the spent electrolyte species S.sub.sol is reduced at the anode (4) to form the electrochemically active species S.sup.2−. The resulting regenerated electrolyte is transferred away from the anolyte container (3) by the pump, through the second conduit (2) to the anolyte storage container (1). Meanwhile, hydroxide ions at the catholyte side of the alkaline membrane (10) are catalysed oxidised at the porous gas cathode (9) to water and oxygen (O.sub.2); the oxygen is then transferred away from the porous cathode (8) through the conduit (11) and discharged into the atmosphere.

(13) It will be appreciated that any of the other liquid anolyte alkaline redox couples mentioned above can be substituted for the sulphur redox couple.

(14) FIG. 2 shows a schematic sectional view of a liquid/gas regenerative fuel cell similar to that set out in FIG. 1 but is less schematic in that it shows the anode and cathode compartments being narrower and shows the electrodes in the form of flow channel plates combined with current collectors.

(15) FIG. 3 shows a schematic of a regenerative fuel cell in which the electrochemically active materials supplied respectively to the anode and to the cathode are both dissolved in an aqueous liquid electrolyte.

(16) In the power delivery mode, the liquid anolyte containing the electrochemically active species A.sup.(X+n)− (where A is the electrochemically active material supplied to the anode and (X+n) is the number of negative charges that A contains) is pumped from a compartment of an anolyte storage container (21), through a conduit (22) and into the anolyte compartment (23), where it is oxidised at the anode (24) according to the half reaction:
A.sup.(X+n)−.fwdarw.A.sup.X+ne.sup.−

(17) The anolyte containing the spent electrolyte species A.sup.X− is then carried away from the anolyte compartment through a second conduit (25) to the anolyte storage container (21), where it is stored in a compartment separate from the fresh anolyte compartment.

(18) The liquid catholyte containing the electrochemically active species C.sup.Y is pumped from a compartment of a catholyte storage container (26), through a conduit (27) and into the catholyte compartment (28), where it is reduced at the cathode (29) according to the half reaction:
C.sup.Y+ne.sup.−.fwdarw.C.sup.(Y−n)

(19) The catholyte containing the spent catholyte species C.sup.(Y−n) is then carried away from the catholyte compartment through a second conduit (30) to the catholyte storage container (26), where it is stored in a compartment separate from the fresh catholyte compartment.

(20) The alkaline membrane (31) separating the anolyte and catholyte compartment (23, 28) selectively passes hydroxide ions from the catholyte to the anolyte side of the membrane (11) to balance the charge and thereby completing the electrical circuit.

(21) In the energy storage mode, the system is reversed so that the spent A.sup.X− is pumped from the anolyte storage container (21), through the conduit (25) to the anolyte compartment (23), where it is reduced at the anode (24) to the regenerated electrochemically active species A.sup.(X−n)−. The resulting regenerated electrolyte is transferred away from the anolyte container (23) by the pump through the conduit (22) to the anolyte storage container (21). Meanwhile, spent C.sup.(Y−n) is pumped from the catholyte storage container (26), through the conduit (30) to the catholyte compartment (28), where it is oxidised at the cathode (29) to form electrochemically active species C.sup.Y. The resulting regenerated catholyte is transferred away from the catholyte compartment (28) by the pump through the conduit (27) to the catholyte storage container (26).

(22) It will be appreciated that any of the liquid anolyte or catholyte alkaline redox couples mentioned above can be used in the set up shown in FIG. 3.

(23) FIG. 4 shows a schematic of an assembled stack of a plurality of cells of the present invention. A single anode current collector and a single cathode collector are connected to the current collectors 24, 29 of the individual cells. The stacking of cells and the feeding of electroactive species to the anode and to the cathode compartments is already well-known in the field of regenerative fuel cells and so further description will be omitted.

(24) FIG. 5 shows an exploded view of the assembled stack shown in FIG. 4. One of the cells towards the centre of the stack is shown in exploded view, which is shown in further detail in FIG. 6.

(25) FIG. 6 shows an exploded view of the components of a single cell present in the assembled stack shown in FIG. 4. The flow channel plates are two-sided and each provides electrochemically active material to the two cells in the stack on either side of it; thus each plate has flow channels on both of its sides for providing electrochemically reactive material to a cathode located on the left hand side of the plate and different electrochemically reactive material to an anode located on the right hand side of the plate. Likewise, a flow channel is provided on the other side of the same plate for providing electrochemically reactive material to an anode located on that side of the plate. The flow channels are serpentine in shape.

(26) In the power delivery mode, a fluid (liquid or gas) that contains electrochemically reducible material enters the flow channel of the cathode side (I) of the plate (the left hand side of the right hand plate, as seen in FIG. 6) through a catholyte inlet (IV). Similarly, a liquid anolyte containing an electrochemically oxidisable material enters an anolyte inlet (II) on the anode side of the flow channel plate (III) (the right hand side of the left hand plate as seen in FIG. 6) and passes via a separate flow channel into the anode compartment. The anolyte and the catholyte flow channels on the outsides of the two plates (as seen in FIG. 6) service the adjacent cells of the stack. The two fluids simultaneously diffuse through porous diffusion electrodes (VIII) where the electrochemically active species in the anolyte is oxidised and the electrochemically active species in the catholyte is reduced at catalytic layers adjacent to an anion exchange membrane (VII), thereby generating electrical current that passes through the current collectors (4,9,24,29) of the cells (see FIGS. 1 and 3). The spent fluids exit the flow channels through the anolyte outlet (V) and catholyte outlet (VI) respectively. Anions (which will generally be hydroxide ions) are transported though the membrane (IX) between the anode and cathode sides of the flow channel plates (I and III). The membrane itself is electrically insulating. The flexible sealing gaskets (VII) ensure hermetic sealing between the anolyte and catholyte sides of the flow channel plates (I and III) when the stack of cells is clamped together via the screws shown in FIGS. 4 and 5.

(27) In power storage mode, the direction of the flow of material in the plates is reversed. The electrochemically active species are regenerated by reduction and oxidation of the spent materials present in the anolyte and catholyte at the catalytic layers adjacent to the anion exchange membrane (IX) respectively.

Example 1

(28) Synthesis of an Alkaline Anion Exchange Membrane

(29) An Alkaline Anion Exchange Membrane (AAEM) which is useful in the present invention may be synthesised according to the method set out in Varcoe et al. (Chem. Mater. 2007, vol. 19, 2686-2693), U.S. patent application Ser. No. 12/523,533 or European Patent Application 04103145.1.

(30) Preparation of an Alkaline Regenerative Fuel Cell

(31) Example of an alkaline regenerative fuel cell (ARFC) based on polysulphides-oxygen and an RFC as described in connection with FIGS. 4 to 6.

(32) The ARFC was built with standard fuel cell (FC) parts such as endplates, graphite flow channels, catalytic gas diffusion electrodes and an alkaline membrane with active area of 5 cm.sup.2. Endplates and inlet/outlet hose barbs were made from plastic (specifically PVC) and are stable both in alkaline and polysulfide solutions. Gold coated current collector plates were introduced between endplates and graphite flow channel plates to collect the current. A membrane electrode assembly (MEA) comprising an alkaline membrane and platinum catalysed gas diffusion electrodes (Pt loading 1 mg/cm.sup.2, acquired from Electrochem Solutions Inc. of 10000 Wehrle Drive, Clarence N.Y. 14031, USA on both sides of the MEA, was used in the present example. This MEA was prepared in a similar way to an electron-beam-grafted ETFE alkaline anion-exchange membrane as used in the metal-cation-free solid-state alkaline fuel cells described by John R. Varcoe et al (Electrochemistry Communications; vol. 8 (2006) 839-843). The MEA was sandwiched between graphite flow channel plates. In order to prevent gas and electrolyte leakage, silicone gaskets were situated according to FIG. 6.

(33) The polysulphide electrolyte was prepared by dissolving elemental sulphur in an aqueous sodium sulphide_solution; NaOH was added to adjust the pH. The total concentrations of the components in the mixture were: 3M NaOH, 1.5M Na.sub.2S and 3M of S. This alkaline (pH˜14) polysulphide solution was circulated through a filter (in order to prevent any insoluble substances from blocking the flow channels) to the anode side of the cell while air was pumped to the cathode side at constant flow rate in the range of 200-500 ml/min. This high rate was required to prevent catalyst poisoning at the cathode side. Both energy storage and power delivery modes were carried out at a current density of 2 mA/cm.sup.2 for 20 minutes with a short period of rest where the open circuit voltage (OCV) was measured. The I-V curve of this polysulphide-air ARFC is set out in FIG. 7.

Example 2

(34) Improvement of the Performance of the Regenerative Fuel Cell

(35) In regenerative fuel cells, some of the ions of the redox couple present in the anolyte and/or catholyte may cross over the membrane which separates the anolyte compartment and the catholyte compartment into the catholyte and/or anolyte, respectively. This is called cross contamination and may cause a reduction in the overall operational efficiency of the regenerative fuel cell. For example, contaminants may react with the electrode catalyst and consequently reduce the activity of the catalyst (known as “catalyst poisoning”).

(36) Improvements in both the selectivity of the anion exchange membrane (thereby reducing the amount of crossover contamination) and the resistance of the catalysts towards poisoning may increase the performance of the regenerative fuel cell.

(37) An experiment was carried out to simulate an improved membrane in an alkaline regenerative fuel cell (ARFC) based on polysulphides-oxygen redox couples. In this test, the membrane was “swept” by passing flows of electrolyte over the two sides of the membrane, thereby removing cross contaminants. The use of such a swept membrane resulted in the open circuit potential of the regenerative fuel cell being >100 mV higher than the open circuit potential of the regenerative fuel cell without the swept membrane. This experiment demonstrates that it is possible to increase the performance of the regenerative fuel cell by reducing the amount of crossover contamination which occurs during operation by improving the selectivity of the membrane. Reducing the amount of crossover contamination may also reduce the amount of catalyst poisoning.

(38) It has been found that even small amounts of cross contamination (for example, by sulphides) may be enough to reduce the activity of catalysts such as platinum. However, we have found that, even after poisoning, such catalysts can remain active (for example, for oxygen reduction reactions) although the rate of catalysis may be reduced.