REDOX FLOW CELL

Abstract

A method of operating a flow cell. The method comprises providing a flow cell suitable for generating electrical power from hydrogen and a metal electrolyte. Said flow cell comprises a precipitate of metal oxi and said metal oxide comprises vanadium or manganese. The method further comprises electrochemically generating a redox active precipitate removal species from a precursor species, wherein said redox active precipitate removal species is capable of converting said metal oxide. The method further comprises exposing said metal oxide to said redox active precipitate removal species to effect conversion of the metal oxide.

Claims

1. A method of operating a flow cell, the method comprising: providing a flow cell suitable for generating electrical power from hydrogen and a metal electrolyte, wherein said flow cell comprises a precipitate of metal oxide, and wherein said metal oxide comprises vanadium or manganese; electrochemically generating a redox active precipitate removal species from a precursor species, wherein said redox active precipitate removal species is capable of converting said metal oxide; and exposing said metal oxide to said redox active precipitate removal species to effect conversion of the metal oxide.

2. The method according to claim 1, wherein the precursor species comprises a metal.

3. The method according to claim 1, wherein the precursor species comprises titanium.

4. The method according to claim 1, wherein the precursor species comprises Ti.sup.4+, Al.sup.3+, Sn.sup.4+, Fe.sup.3+ species.

5. The method according to claim 1, wherein the redox active precipitate removal species comprises a metal selected from Ti.sup.3+, Al.sup.2+, Sn.sup.2+, Fe.sup.2+.

6. The method according to claim 1, wherein said flow cell comprises a catholyte chamber for said metal electrolyte and wherein said electrochemically generating is conducted in said catholyte chamber.

7. The method according to claim 1, wherein: said flow cell comprises a catholyte chamber for said metal electrolyte, said electrochemically generating is conducted in an electrochemical cell separate from said catholyte chamber, and said metal electrolyte is delivered into said separate electrochemical cell.

8. The method according to claim 1, wherein said electrochemically generating is conducted at or below a voltage sufficient to effect reduction of the precursor species.

9. The method according to claim 1, wherein said electrochemically generating is conducted at a voltage above about 0.0 V.

10. The method according to claim 1, wherein said electrochemically generating is conducted below a voltage at which oxidation of the precursor species occurs.

11. The method according to claim 1, wherein said electrochemically generating is conducted at a voltage up to about 0.5 V.

12. The method according to claim 1, wherein said electrochemically generating is conducted at a voltage up to about 0.1 V.

13. The method according to claim 1, further comprising supplying hydrogen during said electrochemically generating.

14. The method according to claim 13, wherein said hydrogen is generated by water electrolysis.

15. The method according to claim 1, wherein said flow cell comprises a catholyte chamber and wherein, prior to the step of electrochemically generating, said method comprises: providing depleted metal electrolyte and redox active precipitate removal species to said catholyte chamber; and charging the flow cell, thereby converting the depleted metal electrolyte into charged metal electrolyte and converting the redox active precipitate removal species into said precursor species.

16. An electrochemical apparatus comprising a first flow cell and a second flow cell: wherein the first flow cell comprises: a reversible hydrogen gas anode, in an anode chamber; and a reversible liquid catholyte cathode in a cathode chamber, the cathode chamber comprising a metal oxide precipitate; and wherein the second flow cell is configured to generate a redox active precipitate removal species from a precursor species, said second flow cell being in fluid communication with the first flow cell to enable passage of liquid catholyte between the second flow cell and cathode chamber of the first flow cell.

17. An electrochemical apparatus comprising a flow cell, the flow cell comprising: a reversible hydrogen gas anode, in an anolyte chamber; and a reversible cathode in a catholyte chamber, the catholyte chamber comprising a metal oxide precipitate; wherein the apparatus is configured to generate a redox active precipitate removal species in the catholyte chamber.

18. The electrochemical apparatus according to claim 17, further comprising the redox active precipitate removal species or a precursor species capable of generating the redox active precipitate removal species in the catholyte chamber.

19. The electrochemical apparatus according to claim 16, wherein the precursor species comprises titanium.

20. The electrochemical apparatus according to claim 16, wherein the redox active precipitate removal species comprises a metal selected from Ti.sup.3+, Al.sup.2+, Sn.sup.2+, and Fe.sup.2+.

21-25. (canceled)

Description

BRIEF DESCRIPTION OF FIGURES

[0090] The application will now be further described, by way of example only, with reference to the accompanying drawings, in which:

[0091] FIG. 1 is a schematic sectional view of a liquid/gas redox flow battery of the disclosure (the terms “liquid” and “gas” denoting the phases of the organic redox active species supplied to the cathode and anode respectively).

[0092] FIG. 2 is a schematic sectional view of a second embodiment of a liquid/gas redox flow battery according to the present disclosure.

[0093] FIG. 3 is a graph showing voltage against time for two power cycles.

[0094] FIG. 4 shows a series of graphs (a)-(d): (a) depicting efficiency performance of a cell over 10 cycles, (b) depicting capacity loss over time with cell overcharging, (c) depicting current changes over time during a step of electrochemically generating a redox active Ti.sup.3+ precipitate removal species, and (d) depicting efficiency performance of the cell over 10 cycles after precipitate removal.

FIGURES AND EXAMPLES

[0095] FIGS. 1 and 2 depict redox flow batteries according to the present disclosure. The operation of the batteries in both FIGS. 1 and 2 is similar and the same reference numerals are adopted to describe the function of components performing the same function, below. Differences between the function of the two batteries are discussed below.

[0096] In the power delivery mode, the liquid catholyte containing a power delivery/energy storage species is pumped by a pump (11) from a chamber of a catholyte storage container (12A), through a conduit (12B) and into the catholyte chamber (9), where it is reduced at a cathode (2) according to the half reaction:

[00008]${\mathrm{Mn}}^{n+1}+e^{-}\rightleftharpoons {\mathrm{Mn}}^n$

[0097] The catholyte containing the spent electrolyte species is then carried away from the catholyte chamber through a second conduit (1) to the catholyte storage container (12A), where it is stored in a chamber separate from the fresh catholyte chamber.

[0098] The anode and at least part of the anolyte chamber (8) are formed by a porous gas flow electrode (4) and hydrogen is supplied from a pressurised gas source vessel (7) through a conduit (13), to the anode/anode chamber (8), where the hydrogen is oxidised to protons (H.sup.+) according to the half reaction:

[00009]$H_2\rightleftharpoons 2H^{+}+2e^{-}$

[0099] and the current is collected by a current collector (also labelled 4). A proton exchange membrane (3) separates the anolyte and catholyte chambers (8 & 9) and selectively passes the protons from the anolyte to the catholyte side of the membrane (3) to balance the charge, thereby completing the electrical circuit. Any unreacted hydrogen is carried away from the anolyte chamber (8) by a second conduit (5) and returned to the pressurised gas source vessel (7) via compressor (6).

[0100] In the energy storage mode, the system is reversed so that the power delivery/energy storage species X.sup.n is pumped from the catholyte storage container (12A), through the conduit (1) to the catholyte chamber (9), where the spent electrolyte species X.sup.n is oxidised at the cathode (2) to form the redox active species X.sup.n+2. The resulting regenerated electrolyte is transferred away from the catholyte container (9) by the pump (11), through the second conduit (12B) to the catholyte storage container (12A). Meanwhile, protons at the anolyte side of the proton exchange membrane (3) are catalytically reduced at the porous gas anode (4) to hydrogen gas; the hydrogen is transferred away from the porous anode (4) through the conduit (5) and compressed by the compressor (6) before being stored in the pressurised gas source vessel (7).

[0101] It will be appreciated that the above system is illustrated with a power delivery/energy storage species that undergoes a two-electron reduction (X.sup.n+2+2e.sup.−.fwdarw.X.sup.n). However, the power delivery/energy storage species could be one which undergoes a single-electron reduction). Moreover, although the discussion above is formulated in the context of a manganese power delivery/energy storage species, it will be appreciated that the procedure is analogous for a flow cell employing a vanadium power delivery/energy storage species and electrolyte comprising same.

[0102] During power delivery mode, MnO.sub.2 builds up over time as described herein. The redox flow battery can then be operated in a precipitate removal mode to remove the oxide build-up.

[0103] The RFB fixture is purchased from Scribner Associates. The cell comprises two POCO graphite bipolar plates with a machined flow field in contact with gold-plated copper current collectors that are held together utilizing anodized aluminum end plates. Commercially available 0.32 mm thick untreated carbon paper (SGL group, Germany, Sigracet SGL 10AA, typically 3 layers) or 4.6 mm thick untreated graphite felt (SGL group, Germany, Sigracell GFD4,6 EA) was used as the positive electrode. The hydrogen negative electrode was obtained from Fuel Cell Store, 0.4 mgPt cm-2 loading on Carbon Paper or 0.03 mgPt cm−2 loading on Carbon Cloth). The membrane was Nafion 212 (nominal thickness 52 μm). A peristaltic pump (for example, Masterflex easy-load, Cole-Palmer) and a platinum-cured silicone tubing (L/S 14, 25 ft) (for example, Masterflex platinum-cured silicone tubing) were used to pump the manganese electrolyte through the cell at flow rate of 25-100 mL min−1. Hydrogen was provided by a fuel cell test station (850e, Scribner Associates), passing through the negative side at a flow rate of 35-150 mL min−1. Due to the current range, polarization curves were recorded using a fuel cell test station (850e, Scribner Associates) whereas galvanostatic charge and charge experiments were conducted with a Gamry potentiostat 3000.

[0104] In-Situ Generation of Redox Active Species

[0105] In the first embodiment shown in FIG. 1, precipitate removal is achieved with generation of redox active precipitate removal species in-situ in the catholyte chamber (9).

[0106] This embodiment employs a catholyte comprising manganese and Ti.sup.4+ species in sulphuric acid solution. The manganese species functions as the power delivery/energy storage species while the titanium species functions as the precursor species for conversion into a redox active species.

[0107] The catholyte was prepared by initially adding sulphuric acid to a solution of Ti(SO.sub.4).sub.2 or TiOSO.sub.4. A corresponding amount of MnCO.sub.3 or MnSO.sub.4 was then slowly added. Effervescence of CO.sub.2 was observed as a result, facilitating metal solubility.

[0108] The catholyte was exposed to a cell voltage between 0 and 0.1 V to effect generation of Ti.sup.3+ redox active precipitate removal species from the Ti.sup.4+ precursor species. Reduction of the precursor species was achieved with no power input.

[0109] Precipitate removal mode involves reducing the oxide precipitate with the Ti.sup.3+ redox active precipitate removal species.

[0110] The redox active precipitate removal species/oxide precipitate reduction reaction may proceed as follows:

[00010]$2{\mathrm{Ti}}^{\left(\mathrm{III}\right)}+{\mathrm{Mn}}^{\left(\mathrm{IV}\right)}\rightleftharpoons 2{\mathrm{Ti}}^{\left(\mathrm{IV}\right)}+{\mathrm{Mn}}^{\left(\mathrm{II}\right)}$

[0111] Mn(II), such as Mn.sup.2+, is soluble in aqueous electrolyte and hence the reduction reaction solubilises the precipitate.

[0112] After precipitate removal, the catholyte was exposed to a cell voltage between 0 and 0.1 V again to re-generate Ti.sup.3+ redox active precipitate removal species for further precipitate removal, as required.

[0113] Independent Generation of Redox Active Species

[0114] In the second embodiment shown in FIG. 2, the redox flow battery comprises an independent electrochemical stack (14) and conduits (15) fluidly connecting the electrochemical stack (14) to the catholyte chamber (9). The electrochemical stack (14) includes a liquid catholyte chamber with associated cathode and a gaseous (hydrogen) anode chamber and associated anode (not labelled or illustrated). The function of these components is similar to that described above and will not be explained in detail.

[0115] The electrochemical stack comprises Ti.sup.4+ redox active precipitate removal species in the liquid catholyte side thereof. Spent catholyte from the catholyte chamber (9) is pumped to the electrochemical stack (14) and is mixed with the Ti.sup.4+ species. Energy input to the electrochemical stack (14) produces Ti.sup.3+ and O.sub.2 according to:

[00011]$\mathrm{Gas}\mathrm{side}\text{:}{H}_{2}O\rightleftharpoons O_2+4H^{+}+4e^{-}$$E_0=1.23V$$\mathrm{Mn}\mathrm{containing}\mathrm{side}\text{:}{\mathrm{Ti}}^{\left(\mathrm{IV}\right)}+e^{-}\rightleftharpoons {\mathrm{Ti}}^{\left(\mathrm{III}\right)}$$E_0=0.1$

[0116] The gas side reaction used an IrO.sub.2 metal catalyst and runs at a stack cell voltage of 1.6-1.7 V.

[0117] Once produced, the catholyte containing redox active Ti.sup.3+ species was pumped back to the catholyte chamber (9) and oxide precipitate was reduced to effect removal thereof, in a similar manner to that described in the first embodiment.

[0118] Capacity Loss

[0119] Capacity loss and amount of precipitate (e.g. MnO.sub.2) which is produced can be calculated by comparison of discharge time (RFB Capacity) during the first cycle with discharge time of subsequent cycles, as follows (e.g. with reference to FIG. 3):

[00012]$\mathrm{Capacity}\left(A\times s\right)=\mathrm{current}\left(A\right)\times \mathrm{time}\left(s\right)$$\mathrm{Capacity}\mathrm{cycle}1-\mathrm{capacity}\mathrm{cycle}2=\mathrm{capacity}\mathrm{Loss}\left(\mathrm{As}=\mathrm{Coulomb}\right)$$\mathrm{Capacity}\mathrm{loss}\left(C\right)\text{/}\mathrm{faraday}\mathrm{constant}\left(C{\mathrm{mol}}^{-1}\right)=\mathrm{mol}.\mathrm{of}\mathrm{electron}$$\mathrm{Mol}.e^{-}\times 0.5\mathrm{mol}{\mathrm{MnO}}_2\mathrm{formation}=\mathrm{mol}.{\mathrm{MnO}}_2\mathrm{produced}$

[0120] In general terms, cycles 1 and 2 may not necessarily be consecutive cycles.

[0121] Precipitate removal is based on operation of the system below 0.1V until the charge measured (which is associated to Ti(III) production) is equal to the capacity loss calculated above.

Example 1

[0122] A 5 cm.sup.2 cell, using graphite felt with thickness of 4.6 mm as its liquid electrode, standard hydrogen electrode with Pt loading of 0.4 mg/cm.sup.2 and 30% PTFE as gas half-cell, and Nafion 117 as proton exchange membrane was tested initially following the conditions below: [0123] 1. Electrolyte with 1M Mn and 1M Ti in 5M H.sub.2SO.sub.4 was used. [0124] 2. Electrolyte was supplied at 50 ml/min throughout the whole experiment. [0125] 3. Hydrogen gas (99.99% purity) was supplied at the rate of 100 ml/min.

[0126] The protocol was used to carry out the following experiments: [0127] 1. The cell was galvanostatic charged and discharged at 100 mA/cm.sup.2 for 10 cycles where its performance evaluation indexes (Energy efficiency (EE), Voltage efficiency (VE) and Coulombic efficiency (CE)) was calculated (shown in FIG. 4(a)). [0128] 2. The cell was charged at constant voltage of 1.8V until current density dropped to 10 mA/cm.sup.2 (shown in FIG. 4(b)). [0129] 3. A discharge cycle was attempted at 100 mA/cm.sup.2, however the cell immediately reached cut off voltage (0.65) which indicates that all the Mn.sup.3+ active species have precipitated by producing MnO.sub.2 (Mn.sup.4+). [0130] 4. To regenerate the electrolyte and remove the precipitate, Ti.sup.4+ was reduced to Ti.sup.3+. In order to achieve this electrochemical reaction, cell was potentiostaticly discharged at constant potential of 0.1V, until the current density dropped to 10 mA/cm.sup.2 (shown in FIG. 4(c)). [0131] 5. After regenerating the electrolyte and removing the precipitate, similar testing to step 1 was carried out and results are reported (shown in FIG. 4(d)).