Electrode For A Redox Flow Battery, Redox Flow Battery And Hydrogen Generation With A Redox Flow Battery

Abstract

The present invention relates to the field of redox flow batteries and combines the conventional use of a redox flow battery for electrochemical energy storage with the production of hydrogen as additional energy storage system. Accordingly, the present invention provides an electrode for a redox flow battery, which is suitable for such dual use as well as a respective redox flow battery. The present invention also provides a method for generating hydrogen with a redox flow battery. Such a method is useful for energy storage during daily as well as seasonal fluctuations in energy production.

Claims

1. An electrode for a redox flow battery comprising: a substrate, and a coating applied to a surface of the substrate, wherein the coating comprises a conductive carbon material, a (semi-)conductive polymer and, optionally, an oxygen evolution reaction (OER) catalyst.

2. The electrode according to claim 1, wherein the coating comprises a conductive carbon material, a (semi-)conductive polymer and an oxygen evolution reaction (OER) catalyst.

3. The electrode according to claim 1, wherein the conductive carbon material is selected from graphite, carbon felt, carbon fiber, thermal and acid treated graphite, carbon-polymer composite materials, carbon nanotubes, carbon black, graphene, Ir-modified carbon felt and graphene-oxide nanoplatelets.

4. The electrode according to claim 1, wherein the conductive carbon material is carbon nanotubes.

5. The electrode according to claim 4, wherein the carbon nanotubes are unmodified carbon nanotubes or chemically or physically modified carbon nanotubes other than sulfonated carbon nanotubes.

6. The electrode according to claim 5, wherein the carbon nanotubes are surface modified by an adsorption layer or by chemical modification of their surface.

7. The electrode according to claim 1, wherein the (semi-) conductive polymer is selected from polyaniline, polyacetylene, polyphenylene vinylene, polypyrrole, polythiopene, poly(3,4-ethylenedioxythiophene), polyphenylene sulfide and a mixture thereof

8. The electrode according to claim 1, wherein the (semi-) conductive polymer is selected from polyacetylene, polyphenylene vinylene, polypyrrole, polythiopene, poly(3,4-ethylenedioxythiophene), polyphenylene sulfide and a mixture thereof.

9. The electrode according to claim 1, wherein the OER catalyst is a metal powder or a metal salt powder.

10. The electrode according to claim 1, wherein the OER catalyst is selected from Ru, Ir, Pd, Pt, Au, Ni, Fe, Os, Co, Mn, Zn and their alloys, oxides, respective mixed oxides and perovskites.

11. The electrode according to claim 9, wherein the OER catalyst is selected from metallic Ru, Ir, Pd, Pt, Au, Ni, Fe, Os, Co, Mn, Zn and their alloys.

12. The electrode according to claim 9, wherein the OER catalyst is selected from metallic Ru, Ir, Pd, Pt, Au, Fe, Os, Co, Mn, Zn and their alloys.

13. The electrode according to claim 9, wherein the OER catalyst is not a metal salt.

14. The electrode according to claim 1, wherein the OER catalyst is nickel on silica/alumina.

15. The electrode according to claim 1, wherein the weight ratio of the OER catalyst, the carbon material and the (semi-) conductive polymer in the coating is 50:10:40 to 80:4:16.

16. The electrode according to claim 1, wherein the substrate is carbon-based.

17. The electrode according to claim 1, wherein the substrate comprises graphite and, optionally, polypropylene.

18. An aqueous redox-flow battery comprising the electrode according to claim 1.

19. The aqueous redox-flow battery according to claim 18 comprising a flow cell comprising a positive electrode and a negative electrode, wherein the positive electrode comprises a substrate, and a coating applied to a surface of the substrate, wherein the coating comprises a conductive carbon material, a (semi-)conductive polymer and, optionally, an oxygen evolution reaction (OER) catalyst.

20. A method for operating an aqueous redox-flow battery comprising the following steps: (1) providing an aqueous redox-flow battery; (2) operating the redox-flow battery in a charging/discharging mode; (3) overcharging the redox-flow battery, thereby generating hydrogen gas; (4) terminating overcharging of the redox-flow battery and discharging the redox-flow battery; and (5) operating the redox-flow battery in a charging/discharging mode.

21. A method for generating hydrogen with an aqueous redox-flow battery comprising the following steps: (1) providing an aqueous redox-flow battery; (2) fully charging the redox-flow battery; (3) continuing charging of the redox-flow battery after the battery is fully charged, thereby generating hydrogen gas; (4) discharging the redox-flow battery; and (5) optionally, operating the redox-flow battery in the charging/discharging mode.

22. The method according to claim 20, wherein in step (3) the potential is increased until a maximum potential is reached and, thereafter, the current flow is continued and the voltage remains at about the maximum potential until the end of step (3).

23. The method according to claim 20, wherein a positive electrolyte of the redox-flow battery is used in excess.

24. The method according to claim 20, wherein the aqueous redox-flow battery does not comprise additional catalytic beds.

25. The method according to claim 20, wherein at least one of a redox active species is an organic compound.

26. The method according to claim 20, wherein the redox-flow battery is an organic redox-flow battery.

27. The method according to claim 20, wherein electrodes of the redox-flow battery are carbon electrodes.

28. The method according to claim 20, wherein the aqueous redox-flow battery comprises an electrode comprising a substrate, and a coating applied to a surface of the substrate, wherein the coating comprises a conductive carbon material, a (semi-)conductive polymer and, optionally, an oxygen evolution reaction (OER) catalyst.

29. The method according to claim 28, wherein the electrode is a positive electrode of the aqueous redox-flow battery.

30. A method for operating the aqueous redox-flow battery according to claim 18, wherein the aqueous redox-flow battery is operated in a charging/discharging mode and in electrolyzer mode for production of gaseous hydrogen in an alternating manner.

31. The method according to claim 20, wherein the hydrogen gas produced in said method is removed from the aqueous redox-flow battery and stored separately from the liquid electrolytes of the aqueous redox-flow battery.

32. The method according to claim 31, wherein the hydrogen gas produced in step (3) is stored in geological underground.

33. The method according to claim 31, wherein the hydrogen gas produced in step (3) is stored in a salt cavern.

34. (canceled)

35. (canceled)

Description

BRIEF DESCRIPTION OF THE FIGURES

[0072] In the following a brief description of the appended figures will be given. The figures are intended to illustrate the present invention in more detail. However, they are not intended to limit the subject matter of the invention in any way.

[0073] FIG. 1 shows for Example 1 the development of voltage (U), current (I) and electric charge (Q) of the exemplified Redox-Flow-Battery for a cycling experiment (a) (cycling between fully charged (1.7 V) and discharged (1.0 V) state; (b) overcharging and hydrogen gas generation; (c) cycling between fully charged (1.7 V) and discharged (1.0 V) state).

[0074] FIG. 2 shows for Example 2 the voltage curve of the cycling experiments of electrode A.

[0075] FIG. 3 shows for Example 2 the battery cell polarization experiments before (dots) and after (crosses) water electrolysis (grey) and depiction of the respective power densities (black) before (dots) and after (crosses) water electrolysis of electrode A.

[0076] FIG. 4 shows for Example 2 the voltage curve of the cycling experiments of electrode B.

[0077] FIG. 5 shows for Example 2 the battery cell polarization experiments before (dots) and after (crosses) water electrolysis (grey) and depiction of the respective power densities (black) before (dots) and after (crosses) water electrolysis of electrode B.

[0078] FIG. 6 shows for Example 2 the voltage curve of the cycling experiments of electrode C.

[0079] FIG. 7 shows for Example 2 the voltage curve of the cycling experiments of electrode D.

[0080] FIG. 8 shows for Example 2 the battery cell polarization experiments before (dots) and after (crosses) water electrolysis (grey) and depiction of the respective power densities (black) before (dots) and after (crosses) water electrolysis of electrode D.

EXAMPLES

Example 1

Generation of Hydrogen with an Aqueous Redox-Flow Battery

[0081] An organic aqueous redox-flow-battery cell assembly was used. Pressed carbon electrodes served as both, the positive and negative electrode. 7,8-Dihydroxyphenazine-2-sulfonic acid (0.5 M) was used as a redox-active species for the negative aqueous electrolyte (negolyte) solution and an aqueous mixture of potassium and sodium ferrocyanide (0.4 M) was used as redox-active species for the positive electrolyte (posolyte) solution. The posolyte redox-active species was used in slight excess. Electrolyte solutions were pumped by peristaltic pumps (Drifton BT100-1L, Cole Farmer Ismatec MCP and BVP Process IP 65) at a rate of 48 mL/min to the corresponding electrodes, respectively. In the redox-flow cell, the positive and negative electrolyte solutions were separated by a cation exchange membrane (630K, supplier: fumatech). The gap between electrode surface and membrane was 0.5 mm on each side of the cell. According to Faraday's law, a maximum capacity of 536 mAh was achievable by the applied redox-flow battery cell setup.

[0082] Prior to each experimental test, the membrane was conditioned in 0.5 M KOH/NaOH (50/50) for at least 150 h. The electrolyte solution reservoir was purged with N.sub.2 gas for 1 h before start of charging. During the experiments the inert atmosphere was maintained.

[0083] Electrochemical testing was performed on a BaSyTec (BaSyTec GmbH, 89176 Asselfingen, Germany) battery test system. The redox-flow battery cell was cyclized between 1.0 and 1.7 V at 10 mA/cm.sup.2. For cycling, the cell was charged at a current density of 10 mA/cm.sup.2 up to 1.7 V (fully charged) and discharged at the same current density down to a 1.0 V cut-off. In some tests, at least one further cycle of charging/discharging (between 1.0 and 1.7 V) was performed.

[0084] Before generating hydrogen gas, the cell was fully charged (to 1.7 V). Once an applied voltage of 1.7 V had been reached, charging was continued until the current dropped below 1.5 mA/cm.sup.2 (full charging of the limiting negative electrolyte). The overcharging potential was then set to 2.7 V and the current flow (charging) was continued at 10 mA/cm.sup.2 to the overcharging voltage of 2.7 V. The potential rose, as the excess of the posolyte was used for oxidation while on the negative electrode hydrogen gas started being produced. Accordingly, hydrogen gas formation was observed at the negative electrode. At about 2.6 V, the ferrocyanide redox-active species was observed to be oxidized to ferricyanide. A plateau was reached, where the voltage remained essentially constant at a current density of 10 mA/cm.sup.2. Hydrogen gas was observed to be continuously produced.

[0085] After about 17 h (about 1280 mAh) of overcharging and thus producing hydrogen gas, the overcharging mode was manually terminated, followed by battery cell discharging to 1.0 V. Thereafter, cycling of the battery cell (charging/discharging mode) between 1.0 to 1.7 V as described above was carried out. It was found that the redox-flow battery waswithout any loss of functionagain usable (in the in the redox-flow battery charging/discharging mode for default battery cell cycling) after being operated for an extended period of time in the overcharging operation mode for hydrogen gas production. Thus, the redox flow battery may be switch from its default cell cycling mode to the overcharging operation mode and again back to the default cell cycling mode.

[0086] FIG. 1 shows the voltage (U), current (I) and electric charge (Q) of the exemplified Redox-Flow-Battery as a function of time in the course of the experiment described above (a) (cycling between fully charged (1.7 V) and discharged (1.0 V) state; (b) overcharging and hydrogen gas generation; (c) cycling between fully charged (1.7 V) and discharged (1.0 V) state).

[0087] The produced hydrogen was collected. It was calculated that about 1 Ah allowed to produce approx. 0.42 l of hydrogen gas. At the plateau of 2.6 V, 4.8 Wh were required per liter hydrogen gas. This corresponds to 48% electrical energy efficiency. Further details are provided in Table 1 below:

TABLE-US-00001 TABLE 1 Example Redox-Flow-Battery Hydrogen production: Battery power 500.000 kW 500 MW Battery storage duration 6 h Battery capacity 3.000.000 kWh 3.000 MWh Hydrogen production capability 80.769.231 L/h 80.769 m.sup.3/h 6.793 t/h Heat value 2,995 kWh/m.sup.3 Heat value production 241.904 kWh/h Efficiency 48%

[0088] The applied Redox-Flow-Battery according to Example 1 is able to store 3.000 MWh of electrical energy, e.g. adapted for balancing daily fluctuations (representing short term energy storage).

[0089] The hydrogen gas may be stored in a salt cavern. In combination with salt cavern storage, the Redox-Flow-Battery of the present Example is able to store 172 GWh of hydrogen gas for balancing seasonal fluctuations (representing long term energy storage). The storage would be sufficient for 714 hours or 30 days of hydrogen production. Details of an exemplified salt cavern, e.g. for storage of hydrogen produced by the (exemplified) Redox-Flow-Battery, are provided below in Table 2:

TABLE-US-00002 TABLE 2 Exemplified salt-cavern: Cavern Volume 500.000 m.sup.3 Position below underground 1000 m Max pressure 180 bar Min pressure 60 bar Utilizable hydrogen 4.850 T Leakage 0.0015% Hydrogen Gas Storage duration 714 h 30 d Energy capacity 172.175.000 kWh 172 GWh

Example 2

Comparison of Differently Coated Electrodes

[0090] Next, redox flow batteries (RFBs) with differently coated electrodes were tested (i) in conventional cycling and polarization of the RFB; and (ii) in hydrogen production.

General Cell Test Setup:

[0091] In all experiments 25 ml of 7,8-Dihydroxyphenazine-2-sulfonic acid (0.5 M) was used as a redox-active species for the negative aqueous electrolyte (negolyte) solution and 45 ml of an aqueous mixture of potassium and sodium ferrocyanide (0.467M) was used as redox-active species for the positive electrolyte (posolyte) solution. Electrolyte solutions were pumped by peristaltic pumps (Drifton BT100-1L, Cole Parmer Ismatec MCP and BVP Process IP 65) at a rate of 72 mL/min to the corresponding electrodes, respectively. In the redox-flow cell, the positive and negative electrolyte solutions were separated by a cation exchange membrane (e.g.: 620PE from fumatech). The gap between electrode surface and membrane was 1.5 mm on each side of the cell.

[0092] Prior to each experimental test, the membrane was conditioned in 0.5 M KOH/NaOH (50/50) for at least 72 h. The electrolyte solution reservoir was purged with N2 gas for 1 h before start of charging. During the experiments, the inert gas atmosphere was maintained at a pressure of 30 to 40 mbar.

[0093] Electrochemical testing was performed on a Biologic battery test system. The redox-flow battery cell was cyclized between 1.0 and 1.6 V at 20 mA/cm.sup.2. For cycling, the cell was charged at a current density of 20 mA/cm.sup.2 up to 1.6 V (fully charged) and discharged at the same current density down to a 1.0 V cut-off.

Cycling Experiments

[0094] In all tests prior to the hydrogen evolution, the batteries were cycled and polarized to obtain a reference cycle and polarization of the battery. After polarization, the hydrogen production was performed followed by a full cycle and a polarization. For comparison, the battery was cycled again after the polarization. Accordingly, the following test plan was used: [0095] 1. Galvanostatic charging of the battery and holding U as it reached the potential of 1.6 V until the battery's current I was below 9 mA; [0096] 2. Galvanostatic discharging of the battery and holding U as it reached the potential of 1.0 V until the battery's current I was above minus 9 mA; [0097] 3. Repeating step 1; [0098] 4. Polarization of the battery: testing the battery's characteristics while discharging with increasing currents every 30 seconds; [0099] 5. Repeating step 1 to compensate lost charge during the polarization; [0100] 6. Hydrogen production: continuing to charge the battery over its capacity with 13.33 mA/cm.sup.2 for 5 hours, with a cut off potential of 5 V. The lower current was chosen to keep the overall power of the system under the highest possible power of 0.4 W/cm.sup.2 (a normal cyclization has a power of 0.18 W/cm.sup.2); [0101] 7. After the hydrogen production, the battery was paused for 1 hour before the battery was discharged again (without holding the potential at 1 V); [0102] 8. Repeating step 1; [0103] 9. Polarization; and [0104] 10. Full cycle of charging and discharging according to steps 1 and 2.

[0105] In order to measure and prove the hydrogen evolution, a hydrogen sensor was attached to the gas outlet of the negolyte. In all experiments the sensor measured hydrogen. In some experiments the hydrogen was collected to calculate the efficiency of the hydrogen production. To confirm the tightness of the membrane, a hydrogen sensor was also attached to the gas outlet of the posolyte. In none of the experiments hydrogen was measured on the posolyte's gas outlet.

Electrodes Tested

[0106] The compound material for all electrodes was made of a mixture of 80% graphite and 20 polypropylene, while the coatings of the positive electrodes varied. The coatings were pressed onto the compound material of the electrodes in two steps. In a first step, the coating powders were pressed onto the base electrode by applying 5 metric tons for 10 seconds at 120 C. After the first pressing, excess coating was blown off with compressed air and the electrode had its shape of a 4 cm4.2 cm rectangle. Subsequently, a micro/macro embossing was pressed onto the side with the coating material. The structure of the electrode surface was achieved by applying 4 metric tons for 10 seconds at 120 C. The employed embossing was applied in the center of the electrodes on a 2.3 cm1.9 cm area and features 24 large coinages with a height of 1.4 mmarranged in 4 rows next to each otherand 255 small coinages with a height of 0.33 mm around those. For all tests, the negative electrode was a conventional electrode with standard coating (200 mg Cabot Carbon PBX135). Table 3 shows the different coatings used for the positive electrodes:

TABLE-US-00003 TABLE 3 Active layer composition of electrodes employed as positive electrodes in the electrochemical set-up. Electrode Description Material of the electrode active layers A Carbon only Multiwalled Carbon Nano 200 mg Tubes B Carbon + polymer Polyaniline 130 mg Multiwalled Carbon Nano 70 mg Tubes C OER catalyst only Nickel on Silica/Alumina 400 mg (65% wt.) D Carbon + polymer + Nickel on Silica/Alumina 270 mg OER catalyst Polyaniline 80 mg Multiwalled Carbon Nano 20 mg Tubes

Electrode EvaluationElectrode A

[0107] Electrode A was coated with a conductive carbon material, namely, multiwalled carbon nano tubes (CNT). The overcharging potential of electrode A reached a plateau at 2.3 V. The evaluation of the polarization shows an increase of the ohmic resistance from 4.469 /cm.sup.2 to 5.569 /cm.sup.2, as well as a maximum power density decrease from 111.886 mW/cm.sup.2 to 96.478 mW/cm.sup.2 (86.23% of the initial power). Hence, the battery could not charge and discharge with the entire initial performance. After the experiment, a swelling and disintegration of the active coating was observed. The respective data are depicted in FIG. 2 and FIG. 3.

Electrode EvaluationElectrode B

[0108] To prevent the swelling and subsequent detachment of the active coating observed with electrode A, conductive polymers (e.g. polyaniline emerald salt) were identified as additives for the carbon nano tubes and the base coating which was used in electrode B. A similar plateau at just under 2.3 V was reached while overcharging. While the evaluation of the polarizations showed a slightly lower initial power and higher ohmic resistance, the power loss was 7.21% lower than with a pure CNT coating (electrode A) as the initial maximum power decreased from 104.449 mW/cm.sup.2 to 97.597 mW/cm.sup.2 resulting in 93.44% of the initial power. The ohmic resistance increased from 4.883 /cm.sup.2 to 5.374 /cm.sup.2. The respective data are shown in FIG. 4 and FIG. 5.

Electrode EvaluationElectrode C

[0109] To get hold of the electrode corrosion due to oxygen evolution during electrolysis, OER-catalyst materials were used as coatings for the positive electrode C. However, the selected OER catalyst (silica/alumina supported nickel) did not show sufficient activity in the initial regular flow battery mode. To fully charge the battery took 21 hours, which is more than double of a normal RFB charging process with standard electrodes. The polarization resulted in a maximum power of 35.5938 mW/cm.sup.2 and an ohmic resistance of 10.7 /cm.sup.2. Surprisingly, the maximum power increased to 42.6744 mW/cm.sup.2 and the resistance decreased to 9.8 /cm.sup.2 after the electrolysis of water. However, the coating resulted in an overall low performance for a flow battery. The data for electrode C are shown in FIG. 6.

Electrode EvaluationElectrode D

[0110] For electrode C, the small surface of metal coatings compared to carbon coating is problematic for a redox flow battery and results in little to no chemical activity with the electrolyte during the regular flow battery mode. Another challenge is to firmly adhere and combine the metal powder (OER catalyst) with the conductive carbon material and with the compound of the electrode. These problems were solved with electrode D comprising a coating of an OER catalyst (nickel on silica/alumina powder), a polymer (polyaniline) as binding material and a conductive carbon material (multiwalled carbon nano tubes). With electrode D a stable overcharging potential at 2.25 V was hold for the 5 hours of hydrogen production. The polarization shows a rather low initial performance of the electrode compared to electrode A. After hydrogen production, however, the performance improved. The evaluation of the polarization shows a decrease of the ohmic resistance from 6.527 /cm.sup.2 to 5.803 /cm.sup.2, and a maximum power density increase from 60.271 mW/cm.sup.2 to 85.433 mW/cm.sup.2. Therefore, the power density increased 141.75% after the water electrolysis. A test without overcharging showed that normal cyclization does not result in a comparable effect. The production of hydrogen with this kind of electrode improves the functionality of the electrode. The data are shown in FIG. 7 and FIG. 8.

[0111] Table 4 shows a summary of electrode performances:

TABLE-US-00004 TABLE 4 Key performance indicators for the employed electrodes in the set-up before and after electrolysis of water. Polarisation before Electrolysis Polarisation after electrolysis Maximum Ohmic resistance Maximum Power Ohmic resistance Electrode Power density per cm.sup.2 density per cm.sup.2 A 111.886 mW/cm.sup.2 4.469 /cm.sup.2 96.478 mW/cm.sup.2 5.569 /cm.sup.2 B 104.449 mW/cm.sup.2 4.883 /cm.sup.2 97.597 mW/cm.sup.2 5.374 /cm.sup.2 C 35.5938 mW/cm.sup.2 10.7 /cm.sup.2 42.6744 mW/cm.sup.2 9.8 /cm.sup.2 D 60.271 mW/cm.sup.2 6.527 /cm.sup.2 85.433 mW/cm.sup.2 5.803 /cm.sup.2