Electrolyte Regeneration for Organic Redox Flow Batteries Based on Water-Soluble Phenzaine-Based Compounds

Abstract

The present invention provides a process for the regeneration of an electrolyte solution of a redox-flow battery containing at least one (preferably substituted) phenazine compound, said process comprising at least one of the following steps (a), (b) and (c): (a) treatment of the electrolyte solution to be regenerated in order to convert organic degradation compounds contained therein to a (substituted) phenazine compound; (b) removal of precipitated material from the electrolyte solution and subsequent modification of the precipitated organic degradation compounds to obtain a (substituted) phenazine compound; and (c) separation of redox active compounds other than (substituted) phenazine compounds in particular inorganic electrolytes, from an electrolyte solution containing (substituted) phenazine compounds, and/or separation of (substituted) phenazine compounds from a solution containing redox active compounds other than (substituted) phenazine compounds.

Claims

1. A process for the regeneration of an electrolyte solution of a redox-flow battery containing at least one phenazine compound, the process comprising at least one of the following steps (a), (b).sub.1 and/or (c): (a) treatment of the electrolyte solution to be regenerated in order to convert organic degradation compounds contained therein to a phenazine compound; (b) removal of a precipitated material from the electrolyte solution and subsequent modification of precipitated organic degradation compounds to obtain the phenazine compound; and (c) separation of redox active compounds other than phenazine compounds from the electrolyte solution containing phenazine compounds, and/or separation of phenazine compounds from a solution containing redox active compounds other than phenazine compounds.

2. The process of claim 1, wherein the electrolyte solution is an aqueous solution.

3. The process of claim 1, wherein the at least one phenazine compound is selected from the following compounds of General Formulas (1)-(6): ##STR00016## ##STR00017## wherein, each R.sup.1-R.sup.8 in General Formula (1), each R.sup.1-R.sup.10 in General Formula (2), each R.sup.1-R.sup.4 in General Formula (3), each R.sup.1-R.sup.6 in General Formula (4), each R.sup.1-R.sup.6 in General Formula (5), and each R.sup.1-R.sup.8 in General Formula (6) is independently selected from: H, -Alkyl, -AlkylG.sup.a, -Aryl, SO.sub.3H, SO.sub.3.sup., PO.sub.3H.sub.2, OH, OG.sup.a, SH, -Amine, NH.sub.2, CHO, COOH, COOG.sup.a, CN, CONH.sub.2, CONHG.sup.a, CONG.sup.a.sub.2, -Heteroaryl, -Heterocycyl, NOG.sup.a, N.sup.+OG.sup.a, F, Cl, and Br, or are joined together to form a saturated or unsaturated carbocycle, more preferably from H, -Alkyl, -AlkylG.sup.a, SO.sub.3H/SO.sub.3.sup., OG.sup.a, and COOH; wherein each G.sup.a is independently selected from: H, -Alkyl, -AlkylG.sup.b, -Aryl, SO.sub.3H, SO.sub.3.sup., PO.sub.3H.sub.2, OH, OAlkyl, OOH, OOAlkyl, SH, SAlkyl, NH.sub.2, NHAlkyl, NAlkyl.sub.2, NAlkyl.sub.3.sup.+, NHG.sup.b, NG.sup.b.sub.2, NG.sup.b.sub.3.sup.+, CHO, COOH, COOAlkyl, CN, CONH.sub.2, CONHAlkyl, CONAlkyl.sub.2, -Heteroaryl, -Heterocycyl, NOG.sup.b, N.sup.+OAlkyl, F, Cl, and Br; wherein each G.sup.b is independently selected from: H, -Alkyl, -Aryl, SO.sub.3H, SO.sub.3.sup., PO.sub.3H.sub.2, OH, OAlkyl, OOH, OOAlkyl, SH, SAlkyl, NH.sub.2, NHAlkyl, NAlkyl.sub.2, NAlkyl.sub.3.sup.+, CHO, COOH, COOAlkyl, CN, CONH.sub.2, CONHAlkyl, CONAlkyl.sub.2, -Heteroaryl, -Heterocycyl, N.sup.+OAlkyl, F, Cl, and Br.

4. The process of claim 3, wherein 2 to 5 or 1 to 5 of R.sup.1-R.sup.8 in General Formula (1), R.sup.1-R.sup.10 in General Formula (2), R.sup.1-R.sup.4 in General Formula (3), R.sup.1-R.sup.6 in General Formula (4), R.sup.1-R.sup.6 in General Formula (5), and R.sup.1-R.sup.8 in General Formula (6) are independently selected from -Alkyl, -AlkylG.sup.a, -Aryl, SO.sub.3H, SO.sub.3.sup., PO.sub.3H.sub.2, OH, OG.sup.a, SH, -Amine, NH.sub.2, CHO, COOH, COOG.sup.a, CN, CONH.sub.2, CONHG.sup.a, CONG.sup.a.sub.2, -Heteroaryl, -Heterocycyl, NOG.sup.a, N.sup.+OG.sup.a, F, Cl, and Br, or are joined together to form a saturated or unsaturated carbocycle, more preferably from -Alkyl, -AlkylG.sup.a, SO.sub.3H/SO.sub.3.sup., OG.sup.a, and COOH; wherein each G.sup.a is independently selected from: H, -Alkyl, -AlkylG.sup.b, -Aryl, SO.sub.3H, SO.sub.3.sup., PO.sub.3H.sub.2, OH, OAlkyl, OOH, OOAlkyl, SH, SAlkyl, NH.sub.2, NHAlkyl, NAlkyl.sub.2, NAlkyl.sub.3*, NHG.sup.b, NG.sup.b.sub.2, NG.sup.b.sub.3.sup.+, CHO, COOH, COOAlkyl, CN, CONH.sub.2, CONHAlkyl, CONAlkyl.sub.2, -Heteroaryl, Heterocycyl, NOG.sup.b, N.sup.+OAlkyl, F, Cl, and Br; wherein each G.sup.b is independently selected from: H, -Alkyl, -Aryl, SO.sub.3H, SO.sub.3.sup., PO.sub.3H.sub.2, OH, OAlkyl, OOH, OOAlkyl, SH, SAlkyl, NH.sub.2, NHAlkyl, NAlkyl.sub.2, NAlkyl.sub.3.sup.+, CHO, COOH, COOAlkyl, CN, CONH.sub.2, CONHAlkyl, CONAlkyl.sub.2, -Heteroaryl, -Heterocycyl, N.sup.+OAlkyl, F, Cl, and Br.

5. The process of claim 3, wherein the at least one phenazine compound comprises at least one SO.sub.3H or SO.sub.3.sup. group.

6. The process of claim 3, wherein the at least one phenazine compound is selected from the following compounds: ##STR00018##

7. The process of claim 1, wherein the electrolyte solution further contains a base, wherein the base is sodium or potassium hydroxide.

8. The process of claim 1, wherein in step (a), the electrolyte solution is treated with an oxidizing agent.

9. The process of claim 8, wherein the oxidizing agent is O.sub.2 or H.sub.2O.sub.2.

10. The process of claim 1, wherein in step (b) the precipitated material is removed from the electrolyte solution by filtration or centrifugation.

11. The process of claim 1, wherein in step (b) the subsequent modification of the precipitated material involves alkylation, sulfonation, and/or hydroxylation of the precipitated material.

12. The process of claim 1, wherein in step (b) the subsequent modification of the precipitated material involves fragmentation of polymerized phenazine compounds.

13. The process of claim 1, wherein in step (c) the redox active compounds other than phenazine compounds are inorganic redox active compounds including transition metal ions and/or halogen ions, wherein the transition metal ions and/or halogen ions comprise VCl.sub.3/VCl.sub.2, Br/ClBr.sub.2, Cl.sub.2/Cl.sup., Fe.sup.2+/Fe.sup.3+, Cr.sup.3+/Cr.sup.2+, Ti.sup.3+/Ti.sup.2+, V.sup.3+/V.sup.2+, Zn/Zn.sup.2+, Br.sub.2/Br.sup., I.sup.3/I.sup., VBr.sub.3/VBr.sub.2, Ce.sup.3+/Ce.sup.4+, Mn.sup.2+/Mn.sup.3+, Ti.sup.3+/Ti.sup.4+, Cu/Cu.sup.+ and/or Cu.sup.+/Cu.sup.2+ based compounds.

14. The process of claim 1, wherein in step (c) the redox active compounds other than phenazine compounds are M.sub.3[Fe(CN).sub.6] and/or M.sub.4[Fe(CN).sub.6], wherein M is a cation, wherein the cation is sodium, potassium, or ammonium or mixtures thereof.

15. The process of claim 1, wherein in step (c) the phenazine compounds are separated from the electrolyte solution by decreasing the pH value of the solution.

16. The process of claim 15, wherein the pH value is decreased to a pH of 7 or lower.

17. The process of claim 15, wherein the pH value is decreased using inorganic or organic acids.

18. The process of claim 1, wherein the process comprises at least two of steps (a), (b) and/or (c).

19. The process of claim 1, wherein the process comprises all three steps (a), (b) and (c).

20. A rocess for the regeneration of an aqueous electrolyte solution of a redox-flow battery containing at least one inorganic redox active compound, the process comprising at least one of the following steps (a), (b), and/or (c): (a) treatment of the electrolyte solution in order reduce the at least one inorganic redox active compound to the reduced state; (b) removal of precipitated material from the electrolyte solution and subsequent modification of the precipitated material to obtain at least one water soluble inorganic redox active compound; and/or (c) separation of inorganic redox active compounds from phenazine compounds.

21. The process of claim 20, wherein the at least one inorganic redox active compound is selected from a metal ion complex, wherein the is an iron metal iron complex.

22. The process of claim 20, wherein in step (a) reducing the at least one inorganic redox active compound is carried out using a reducing agent, wherein the reducing agent is sodium sulfite, potassium sulfite, sodium dithionite, sodium formate, and/or ascorbic acid.

23. The process of claim 20, wherein in step L1 the precipitated material is removed from the electrolyte solution by filtration or centrifugation.

24. The process of claim 21, wherein in step M the subsequent modification of the precipitated material involves treatment of the precipitate with a cyanide, wherein the cyanide comprises KCN and/or NaCN.

25. The process of claim 20, wherein in step (c) the phenazine compounds are separated from the electrolyte solution by decreasing the pH value of the solution.

26. The process of claim 25, wherein the pH value is decreased to a pH of 7 or lower.

27. The process of claim 25, wherein the pH value is decreased using inorganic or organic acids.

28. The process of claim 20, wherein the process comprises at least two of steps (a), (b), and/or (c).

29. The process of claim 20, wherein the process comprises all three steps (a), (b), and (c).

30. The process of claim 21, wherein the metal iron complex is M.sub.3[Fe(CN).sub.6] or M.sub.4[Fe(CN).sub.6], wherein M is a cation, wherein the cation is sodium, potassium, or ammonium or mixtures thereof.

31. The process of claim 3, wherein 1, 3 or 4 or 3 to 4 of R.sup.1-R.sup.8 in General Formula (1), R.sup.1-R.sup.10 in General Formula (2), R.sup.1-R.sup.4 in General Formula (3), R.sup.1-R.sup.6 in General Formula (4), R.sup.1-R.sup.6 in General Formula (5), and R.sup.1-R.sup.8 in General Formula (6) are independently selected from -Alkyl, -AlkylG.sup.a, -Aryl, SO.sub.3H, SO.sub.3.sup., PO.sub.3H.sub.2, OH, OG.sup.a, SH, -Amine, NH.sub.2, CHO, COOH, COOG.sup.a, CN, CONH.sub.2, CONHG.sup.a, CONG.sup.a.sub.2, -Heteroaryl, -Heterocycyl, NOG.sup.a, N.sup.+OG.sup.a, F, Cl, and Br, or are joined together to form a saturated or unsaturated carbocycle, more preferably from -Alkyl, -AlkylG.sup.a, SO.sub.3H/SO.sub.3.sup., OG.sup.a, and COOH; wherein each G.sup.a is independently selected from: H, -Alkyl, -AlkylG.sup.b, -Aryl, SO.sub.3H, SO.sub.3.sup., PO.sub.3H.sub.2, OH, OAlkyl, OOH, OOAlkyl, SH, SAlkyl, NH.sub.2, NHAlkyl, NAlkyl.sub.2, NAlkyl.sub.3.sup., NHG.sup.b, NG.sup.b.sub.2, NG.sup.b.sub.3.sup.+, CHO, COOH, COOAlkyl, CN, CONH.sub.2, CONHAlkyl, CONAlkyl.sub.2, -Heteroaryl, Heterocycyl, NOG.sup.b, N.sup.+OAlkyl, F, Cl, and Br; wherein each G.sup.b is independently selected from: H, -Alkyl, -Aryl, SO.sub.3H, SO.sub.3.sup., PO.sub.3H.sub.2, OH, OAlkyl, OOH, OOAlkyl, SH, SAlkyl, NH.sub.2, NHAlkyl, NAlkyl.sub.2, NAlkyl.sub.3.sup.+, CHO, COOH, COOAlkyl, CN, CONH.sub.2, CONHAlkyl, CONAlkyl.sub.2, -Heteroaryl, -Heterocycyl, N.sup.+OAlkyl, F, Cl, and Br.

Description

EXAMPLES

[0200] In the following, the present invention is exemplified for the regeneration of 7,8-Dihydroxyphenazinesulfonic acid (DHPS) as the negolyte and a mixture of sodium and potassium ferrocyanide as the posolyte.

Example 1: Analysis of Degradation Products of DHPS

[0201] ##STR00004##

[0202] DHPS is soluble in base such as, sodium or potassium hydroxide and can be used as a negolyte in the organic half-cell (e.g. half-cell A) of an RFB. The DHPS is the oxidized and the DHPS-H.sub.2 the reduced form representing the discharged and charged state respectively. DHPS can undergo a variety of different chemical and electrochemical degradation reactions which typically occur as a result of extended charging/discharging cycling.

[0203] Regeneration of DHPS follows the degradation pathway observed and is characteristic for the DHPS degradation products resulting therefrom. The compounds identified as degradation products are Phenazine-type degradation products and/or over-reduced degradation products. Surprisingly, it has been found by the present inventors that the over-reduced compounds may be regenerated towards the phenazine-type products when applying slightly oxidative conditions.

[0204] Identification of Degradation Products:

[0205] The following degradation products have been identified via HPLC-MS: [0206] 1. Peak at 2.11 min: mass in ESI() is 295, in ESI(+) 297. Molar mass of M=296 corresponds to over-reduced DHPS, which is further called H.sub.4-DHPS. [0207] 2. Peaks at 3.87 and 4.29 min: both peaks have the same pattern in LC-MS: in ESI() MH=279, in ESI(+) M+H=281. Molar mass of M=280 corresponds to over-reduced MHPS (Monohydroxyphenazinesulfonic acid), which is designated as H.sub.4-MHPS. [0208] 3. Peaks at 5.54 and 6.02 min: both peaks have the same pattern in LC-MS: in ESI() MH=275, in ESI(+) M+H=277. Molar mass of M=276 corresponds to following isomers of MHPS:

##STR00005## [0209] 4. Peak at 6.97 min: mass in ESI() is 211, in ESI(+) 213. Molar mass M=212 corresponds to the following structure (DHP, Dihydroxyphenazine):

##STR00006## [0210] The structure was verified with a standard sample. [0211] 5. Peak at 8.04 min: mass in ESI() is 195, in ESI(+) 197. Molar mass M=196 corresponds to the following structure (MHP, Monohydroxyphenazine):

##STR00007## [0212] The structure of that phenazine degradation product was confirmed by the use of a standard sample as a reference.

[0213] During the cycling in the RFB half-cell DHPS is reduced to DHPS-H.sub.2. This species may eliminate water to form both isomers of MHPS, depending on which hydroxy group is eliminated. Further, DHPS-H.sub.2 may be reduced to H.sub.4-DHPS, as detected by HPLC. Finally, overreduction of MHPS leads to two isomers of H.sub.4-MHPS. In total, one or more of the above degradation species may be observed as a result of continuous operation of a redox flow battery based on the negolyte DHPS.

[0214] In addition, degradation species resulting from the loss of the sulfonic acid group SO.sub.3H as a substituent of the substituted phenazine compound employed as electrolyte compound were observed: DHP and MHP.

[0215] In summary, DHPS loses one or more of its substituents, e.g. one or both sulfonic acid and/or one or more hydroxy group(s) as a result of a larger number of charge/discharge cycles under operation conditions.

[0216] The inventors of the present invention observed that the samples of over-reduced species are accessible for oxidation to MHPS and DHPS (H.sub.4-MHPS and H.sub.4-DHPS) by storing them under air. Also, oxidation was achieved under experimental conditions by chemically reducing DHPS with sodium dithionite to H.sub.4-DHPS and H.sub.4-MHPS. Such a reference sample, left under air for several hours, allowed for conversion of H.sub.4-DHPS and H.sub.4-MHPS to DHPS and MHPS. Accordingly, over-reduced phenazine species may be readily regenerated under (e.g. mildly) oxidative conditions.

[0217] Solubility and Electrochemical Properties of the Degradation Species.

[0218] Various phenazine species (mentioned above and derivable from DHPS) are all electrochemically active, as shown by independent species synthesis. MHP and DHP were synthesized and cyclized in a redox flow cell. Both electrolytes have OCV values of approx. 1,4 V. A solution of DHPS containing approx. 15-20% of MHPS and DHP (as DHPS degraded species) was tested in a RFB cell: The capacity observed during cycling corresponds to the overall concentration of phenazines in solution (DHPS, MHPS and DHP). Thus, MHPS and DHP are also electrochemically active. Their formation (as degradation products of DHPS charge/discharge cycling) was hence found not to decrease the charge capacity in a flow cell.

[0219] However, the various degradated species exhibit individual solubilities, depending on the substitution pattern. The solubility of DHPS in a 1:1 mixture of NaOH and KOH with 0.5M free base concentration is about 1.4-1.6M. Loss of a single hydroxy group was found not to significantly decrease DHPS's solubility: both MHPS isomers are well soluble in a 1:1 mixture of NaOH and KOH. In contrast, phenazines without sulfonic acid group were found to be significantly less soluble. For example, solubility of DHP in a 1:1 mixture of NaOH and KOH is reduced to a value of about 0.2-0.4 M, depending on the amount of the free base concentration. Solubility of MHP under such conditions is even lower, i.e. 0.1-0.3M. Surprisingly, the solubility of both DHP and MHP in a DHPS solution up to 0.5 M does not change significantly. Therefore, the phenazine-type degradation species, especially the phenazine-type degradation species DHP and MHP without the sulfonic acid substituent, precipitate without interfering with or impairing the solubility of the solution's DHPS.

[0220] In summary, degradation of DHPS after an extended cycling period leads to electrochemically active phenazine degradation species being devoid of one or more substituents and over-reduced species. The inventors determined (i) that over-reduced DHPS species are amenable oxidative to regeneration of reduced phenazine-type compounds even under slightly oxidative conditions. Other phenazine-type degradation species were found to accumulate over time until their solubility limit is reached such that they start to precipitate. MHP and DHP were found to represent degradation species exhibiting the lowest solubility precipitating first.

[0221] Based on the data collected by the degradation experiments, three degradation genera were identified as shown in FIG. 1: [0222] 1. The over-reduced species H.sub.4-MHPS and H.sub.4-DHPS, that are e.g. formed under battery overcharging conditions. They may be converted to redox-activity H.sub.2-MHPS and H.sub.2-DHPS, such thatby straight-forward oxidationthe battery function may be restored, described in more detail as Regeneration A below. [0223] 2. MHPS isomers were found to be sufficiently soluble and electrochemically active with their OCV being comparable to DHPS. Thus, such degradation species were found not to require regeneration efforts. [0224] 3. Degradation species resulting from an extended cycling period, DHP and MHP. Though they are still electrochemically sufficiently active, they precipitate when reaching their solubility limit upon accumulation in the electrolyte solution. These degradation species have to be separated (e.g. filtered off) and re-converted to more soluble electrolytically active phenazines, described as Regeneration B below.

Example 2: Phenazine Treatment in Solution (Regeneration A)

Example 2.a

[0225] Oxygen was bubbled through a solution containing the over-reduced species H.sub.4-MHPS and H.sub.4-DHPS. Upon oxidation treatment, HPLC analysis confirmed that peaks representing H.sub.4-MHPS and H.sub.4-DHPS disappeared, whereas peaks of the oxidized species MHPS and MHP increased.

Example 2.b

[0226] The solution sample containing over-reduced species H.sub.4-MHPS and H.sub.4-DHPS was treated by addition of hydrogen peroxide (30 wt % solution in water). HPLC analysis of the reaction mixture proved disappearance of the peaks representing H.sub.4-MHPS and H.sub.4-DHPS and emergence of an MHPS peak.

[0227] In summary, it could be demonstrated that over-reduced phenazine species may be readily regenerated under oxidative conditions.

Example 3: Treatment of Precipitated DHPS Degradation Species (Regeneration B)

[0228] The degradation of DHPS was found to generate a variety of degradation species, which precipitate due to their lower solubility. Their solubility decreases as a result of loss of substituents (functional groups other than hydrogen) of DHPS or by polymerization phenomena. Unsubstituted phenazine itself is barely soluble in water. The detected degradation species exhibiting the lowest solubility are DHP and MHP. According to the invention, these species may and are advantageously removed, e.g. be filtered off. They were found to be amenable to chemical regeneration.

Example 3.a Chemical Hydroxylation of MHP to DHP and Trihydroxy Phenazine

[0229] ##STR00008##

[0230] Phenazine-2-ol (an isomer of MHP) was converted to 5,10-dioxo-5.sup.5,10.sup.5-phenazine-2-ol upon treatment by hydrogen peroxide in glacial acetic acid or with 3-chlorobenzoic acid at between 20-100 C., preferably between 30-80 C. most preferably between 40-60 C. 5,10-dioxo-5.sup.5,10.sup.5-phenazine-2-ol was then nitrated by nitric acid in combination with or without nitrous acid under cooling to at least 0 C., at least 10 C., at least 20 C. under room temperature, or under heating to at least at 30 C., at least 40 C., at least 50 C. 3-nitro-5,10-dioxo-5.sup.5,10.sup.5-phenazine-2-ol was converted to 5,10-dioxo-5.sup.5,10.sup.5-phenazine-2,3-diol by treatment with a base such as potassium hydroxide, potassium carbonate, sodium carbonate or sodium hydroxide at temperatures from 20 C.-120 C., preferably 40 C.-100 C. Finally, 5,10-dioxo-5.sup.5,10.sup.5-phenazine-2,3-diol was reduced to phenazine-2,3-diol by treatment with reagents such as but not limited to trifluoroacetic anhydride and sodium iodide in acetonitrile at rt, or titanium(IV)-chloride and tin(II)-chloride in acetonitrile at rt, or titanium(IV)-chloride and sodium iodide in acetonitrile at 30 C., or aqueous sodium hydrosulfite and sodium hydroxide at rt, or zinc in aqueous sodium hydroxide solution, or tin(II)-chloride in hydrochloric acid, or by catalytic reduction with sodium hydrophosphite over palladium on carbon (5%) in THF/water at rt, or by hydrogenation with catalytic palladium on charcoal (10% Pd) or Raney nickel (2-10%) under hydrogen (1-5 bar) in EtOH or MeOH.

##STR00009##

[0231] Phenazine-2-ol was converted to 5,10-dioxo-5.sup.5,10.sup.5-phenazine-2-ol by treatment with hydrogen peroxide in glacial acetic acid or with 3-chlorobenzoic acid at between 20-100 C., preferably between 30-80 C. most preferably between 40-60 C. 5,10-dioxo-5.sup.5,10.sup.5-phenazine-2-ol was then nitrated with nitric acid in combination with or without nitrous acid at room temperature, or under heating to at least at 30 C., at least 40 C., at least 50 C., at least 60 C., at least 70 C., at least 80 C., at least 90 C., at least 100 C., at least 110 C., at least 120 C., at least 130 C., at least 140 C., at least 150 C. The mixture of 3,8-dinitro-5,10-dioxo-5.sup.5,10.sup.5-phenazin-2-ol and 3,7-dinitro-5,10-dioxo-5.sup.5,10.sup.5-phenazine-2-ol was then converted to 5,10-dioxo-5.sup.5,10.sup.5-phenazine-2,3,7-triol by treatment with a base such as potassium hydroxide, potassium carbonate, sodium carbonate or sodium hydroxide at temperatures from 20 C.-150 C., preferably 40 C.-120 C. Finally, 5,10-dioxo-5.sup.5,10.sup.5-phenazine-2,3,7-triol was reduced to phenazine-2,3,7-triol by treatment with reagents such as trifluoroacetic anhydride and sodium iodide in acetonitrile at rt, or titanium(IV)-chloride and tin(II)-chloride in acetonitrile at rt, or titanium(IV)-chloride and sodium iodide in acetonitrile at 30 C., or aqueous sodium hydrosulfite and sodium hydroxide at rt, or zinc in aqueous sodium hydroxide solution, or tin(II)-chloride in hydrochloric acid, or by catalytic reduction with sodium hydrophosphite over palladium on carbon (5%) in THF/water at rt, or by hydrogenation with catalytic palladium on charcoal (10% Pd) or Raney nickel (2-10%) under hydrogen (1-5 bar) in EtOH or MeOH.

Example 3.b Enzymatic Hydroxylation of MHP to DHP

[0232] ##STR00010##

[0233] Phenazine-2-ol was converted to phenazine-2,3-diol by treatment with hydrogen peroxide or NAD(P)H/oxygen in the presence of a enzyme, such as hydroxylase, or monooxygenase, or PhzA from Pseudomonas aureofaciens, Pseudomonas aeruginosa or Pseudomonas fluorescens.

Example 3.c Sulfonation of MHP

[0234] ##STR00011##

[0235] Phenazine-2-ol was converted to 5,10-dioxo-5.sup.5,10.sup.5-phenazine-2-ol by treatment with hydrogen peroxide in glacial acetic acid or with 3-chlorobenzoic acid at between 20-100 C., preferably between 30-80 C. most preferably between 40-60 C. 5,10-dioxo-5.sup.5,10.sup.5-phenazine-2-ol was then sulfonated with sulfuric acid in combination with or without sulfur trioxide (20-40% SO.sub.3) under cooling to at least at 0 C., at least 10 C., at least 20 C. under room temperature, or under heating to at least at 30 C., at least 40 C., at least 50 C., at least 60 C., at least 70 C., at least 80 C., at least 90 C., at least 100 C., at least 110 C., at least 120 C., at least 130 C., at least 140 C., at least 150 C. 3-hydroxy-5,10-dioxo-5.sup.5,10.sup.5-phenazine-2-sulfonic acid and 3-hydroxy-5,10-dioxo-5.sup.5,10.sup.5-phenazine-2,7-disulfonic acid were then reduced to 3-hydroxyphenazine-2-sulfonic acid and 3-hydroxyphenazine-2,7-disulfonic acid, correspondingly, by treatment with reagents such as trifluoroacetic anhydride and sodium iodide in acetonitrile at rt, or titanium(IV)-chloride and tin(II)-chloride in acetonitrile at rt, or titanium(IV)-chloride and sodium iodide in acetonitrile at 30 C., or aqueous sodium hydrosulfite and sodium hydroxide at rt, or zinc in aqueous sodium hydroxide solution, or tin(II)-chloride in hydrochloric acid, or by catalytic reduction with sodium hydrophosphite over palladium on carbon (5%) in THF/water at rt, or by hydrogenation with catalytic palladium on charcoal (10% Pd) or Raney nickel (2-10%) under hydrogen (1-5 bar) in EtOH or MeOH.

Example 3.d Alkylation of DHP

[0236] ##STR00012##

[0237] Phenazine-2,3-diol was reacted with different alkylating reagents (Bitte die verwendeten Alkylierungsmittel angeben) in the presence of a base such as sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, triethyl amine or sodium methylate at 0-120 C., preferably between 20-80 C. to yield 2-hydroxy-3-[(3-hydroxyphenazine-2-yl)oxy]-N,N,N-trimethylpropan-1-aminium chloride, or 3-[(3-hydroxyphenazine-2-yl)oxy]-N,N,N-trimethylpropan-1-aminium bromide, or 3-[(3-hydroxyphenazine-2-yl)oxy]propanoic acid.

Example 3.e Sulfonation of DHP to DHPS

[0238] ##STR00013##

[0239] Phenazine-2,3-diol was converted to 7,8-dihydroxyphenazine-2-sulfonic acid by treatment with sulfuric acid in combination with or without sulfur trioxide (20-40% SO.sub.3) under cooling to at least at 0 C., at least 10 C., at least 20 C. under room temperature, or under heating to at least at 30 C., at least 40 C., at least 50 C., at least 60 C., at least 70 C., at least 80 C., at least 90 C., at least 100 C., at least 110 C., at least 120 C., at least 130 C., at least 140 C., at least 150 C.

Example 3.f Fragmentation of Polymerized Phenazine

[0240] Polymeric active material that precipitates could be fragmented and used as raw material for the electrolyte production. The process was either a chemical depolymerization, optionally in the presence of a catalyst, optionally under oxidative or reductive conditions, optionally under pressure and optionally at high temperatures, or a biological depolymerization in the presence of an enzyme or, optionally, in the presence of an organism.

[0241] In summary it has been shown that phenazine compounds may be converted to a variety of soluble species by various measures.

Example 4: Regeneration of Ferrocyanide in Solution

[0242] ##STR00014##

[0243] In the present example, a mixture of potassium and sodium ferrocyanide is used as a posolyte for an RFB. The ferrocyanide is the reduced and the ferricyanide the oxidized form representing the discharged and charged state, respectively.

Example 4.a Ferrocyanide Treatment in Solution

[0244] The required amount of regeneration reagent (reducing agent) was added to a solution containing 347 mM of sodium/potassium hexacyanoferrate (II) and 257 mM sodium/potassium hexacyanoferrate (III) (SOC 43%) and 0.49 mM base (1:1 mixture of KOH and NaOH). The mixture was stirred for a given time at a given temperature (see Table below). The solution was analyzed by UV-Vis, and the base concentration was determined by titration.

[0245] Reaction Equations with Different Regeneration Reagents: [0246] Sodium sulfite: 0.5 eq Na.sub.2SO.sub.3 is required for the reduction of 1 eq Na.sub.3[Fe(CN).sub.6]

2Na.sub.3[Fe(CN).sub.6]+Na.sub.2SO.sub.3+2NaOH.fwdarw.2Na.sub.4[Fe(CN).sub.6]+Na.sub.2SO.sub.4.sup.+H.sub.2O [0247] Sodium dithionite: 0.16 eq Na.sub.2S.sub.2O.sub.4 is required for the reduction of 1 eq Na.sub.3[Fe(CN).sub.6]

6Na.sub.3[Fe(CN).sub.6]+Na.sub.2S.sub.2O.sub.4+8NaOH.fwdarw.6Na.sub.4[Fe(CN).sub.6]+2Na.sub.2SO.sub.4+4H.sub.2O [0248] Sodium formate: 0.5 eq HCOONa is required for the reduction of 1 eq Na.sub.3[Fe(CN).sub.6]

2Na.sub.3[Fe(CN).sub.6]+HCOONa+3NaOH.fwdarw.2Na.sub.4[Fe(CN).sub.6]+Na.sub.2CO.sub.3+2H.sub.2O

[0249] Results:

TABLE-US-00001 Regeneration Reaction SOC and hexacyanoferrate reagent Amount conditions concentration Sodium sulfite 125 mM Room SOC 2%, c(Fe(CN).sub.6.sup.2) = Na.sub.2SO.sub.3 temperature, 2 h 591 mM Potassium sulfite 125 mM Room SOC 0%, c(Fe(CN).sub.6.sup.2) = K.sub.2SO.sub.3 temperature, 2 h 587 mM Sodium dithionite 50 mM Room SOC 1%, c(Fe(CN).sub.6.sup.2) = Na.sub.2S.sub.2O.sub.4 temperature, 2 h 594 mM Sodium formate 125 mM 40 C., 43 h SOC 8%, c(Fe(CN).sub.6.sup.2) = HCOONa 538 mM Ascorbic acid (as 300 mM Room SOC 0%, c(Fe(CN).sub.6.sup.2) = 0.5M solution in 1M temperature, 3 h 415 mM KOH/NaOH)

[0250] In summary it was shown that the concentration of sodium/potassium hexacyanoferrate (II) in the solution treated was dramatically increased.

Example 5: Treatment of an Iron Complex Precipitate Resulting from an Iron Complex Electrolyte

[0251] Ferrocyanide is prone to degradation due to external factors such as light, pH, electrochemical reactions, chemical reactions or physical reactions over time. As a result of exposure to such conditions, ferrocyanide changes its chemical or physical properties, such as solubility, electrochemical potential or activity. Reduction of solubility may also be involved such that degradation species may precipitate. Precipitated material may be filtered off and used as an iron source for the production or regeneration of ferrocyanide.

Example 5.a

[0252] ##STR00015##

[0253] The precipitate of iron (III) hydroxide was treated with a mixture of sodium cyanide (3 eq) and potassium cyanide (3 eq) at 0-120 C., preferably between 20-80 C. to yield sodium/potassium hexacyanoferrate (III). Sodium/potassium hexacyanoferrate (III) was then reduced to sodium/potassium hexacyanoferrate (II) by treatment with a reducing reagent such as sodium sulfite, or sodium dithionite, or sodium formate.

Example 6: Recovery of Negolytes and Posolytes from their Respective Solutions

[0254] In the following example the separation of negolytes from a posolyte solution or, vice versa, the separation of posolytes from a negolyte solution is shown: In the following example, the procedure is described for the recovery of (i) DHPS and (ii) potassium/sodium ferrocyanide by treating solutions (as they may occur upon an extended period of cycling in either solution of half-cell A or half-cell B) containing both of (i) and (ii), respectively.

[0255] This procedure involves the following steps: [0256] a) DHPS was precipitated as acid or as salt from a 1:1 (v/v) mixture of DHPS (0.5 M DHPS in 0.5 M base (NaOH/KOH n/n=1/1)). Potassium/sodium ferrocyanide (0.65 M iron(II) hexacyanide in 0.23 M base (NaOH/KOH n/n=1/1)) by addition of an acid to the electrolyte solution. The precipitated phenazine was separated and further purified. The purified DHPS could be used for preparing an electrolyte solution. [0257] b) The remaining acidic solution contained the iron hexacyanide. Simple addition of base converted the acidic solution to the basic electrolyte. Further separation and purification of the iron hexacyanide was achieved by crystallization from the solution at lower temperatures, preferably below 20 C., or precipitation by addition of inorganic or organic salts that lower the solubility of the hexacyanide. The purified iron hexacyanide can be re-used for preparing an electrolyte solution.

Example 6a: Impact of the pH Value on the Phenazine Purity and Recovery Yield

[0258] Hydrochloric acid (37%) was added at room temperature to a mixture of 5 mL DHPS (0.5 M DHPS in 0.5 M base (NaOH/KOH n/n=1/1)) and 5 mL potassium/sodium ferrocyanide (0.65 M iron(II) hexacyanide in 0.23 M base (NaOH/KOH n/n=1/1)) to adjust the solution to different pH values. The precipitated electrolyte mixture was dissolved in 2 M potassium hydroxide to a volume on 10 mL. The electrolyte concentrations were determined by HPLC and are summarized in Table 1.

TABLE-US-00002 TABLE 1 Recovery of phenazine (sulfonic acid and sulfonate) from a ferrocyanide/DHPS mixture by addition of hydrochloric acid. Iron hexacyanide Phenazine Phenazine Phenazine pH concentration of the concentration of the recovery purity value precipitate [mM] precipitate [mM] [%] [%] 7.3 44.4 68.2 27 61 5.9 85.3 185.6 74 69 3.2 138.2 243.6 97 64 2.2 127.7 251.7 100 66 1.5 97.2 244.3 98 72

[0259] As can be taken from this table, by lowering the pH to less than 6, the recovery yield was significantly improved.

Example 6b: Influence of the pH Value and an Additional Washing of the Precipitate with Diluted Hydrochloric Acid on the Phenazine Purity and Recovery Yield

[0260] Hydrochloric acid (37%) was added at room temperature to a mixture of 5 mL DHPS (0.5 M DHPS in 0.5 M base (NaOH/KOH n/n=1/1)) and 5 mL potassium/sodium ferrocyanide (0.65 M iron(II) hexacyanide in 0.23 M base (NaOH/KOH n/n=1/1)) to adjust the solution to different pH values. The precipitated electrolyte mixture was washed with 1.2 M hydrochloric acid and dissolved in 2 M potassium hydroxide to a volume on 10 mL. The electrolyte concentrations were determined by HPLC and are summarized in Table 2.

TABLE-US-00003 TABLE 2 Recovery of phenazine (sulfonic acid and sulfonate) from a ferrocyanide/DHPS mixture by addition of hydrochloric acid and an additional washing of the precipitate with diluted hydrochloric acid. ferrocyanide Phenazine Phenazine Phenazine pH concentration of the concentration of the recovery purity value precipitate [mM] precipitate [mM] [%] [%] 2.2 21 250 100 92 0.5 22 226 90 91

[0261] In summary, it was that it is possible to separate mixed electrolytes by varying the pH value.

[0262] Technical Supportings:

[0263] All chemicals and solvents were used as bought.

[0264] Electrochemical Tests:

[0265] For electrochemical characterization, a small laboratory cell was used. A graphite felt (with an area of 6 cm.sup.2, 6 mm in thickness, supplier: SGL GFA 6EA) was employed as both the positive and negative electrode, and a cation exchange membrane (630K or 620PE, supplier: fumatech) was used to separate the positive and negative electrolytes. The membrane was conditioned in 0.5 M KOH/NaOH (50/50) for at least 150 h prior to each test. Electrolyte volumes range from 12 to 50 mL. The electrolytes were pumped by peristaltic pumps (Drifton BT100-1 L, Cole Parmer Ismatec MCP and BVP Process IP 65) at a rate of 24 mL/min to the corresponding electrodes, respectively. Electrochemical testing was performed on a BaSyTec (BaSyTec GmbH, 89176 Asselfingen, Germany) or a Bio-Logic (Bio-Logic Science Instruments, Seyssinet-Pariset 38170, France) battery test system by polarization curves, which were recorded in the charged state by galvanostatic holds and constant-current charge-discharge cycles. For cycling, the cell was charged at a current density of 25 mA/cm.sup.2 up to 1.7 V and discharged at the same current density down to 0.8 V cut-off.

[0266] Analytical Methods:

[0267] UV-VIS Spectroscopy [0268] Parameter for UV-Vis measurement: [0269] Device: PerkinElmer Lambda25 [0270] Layer thickness cuvette: 10 mm [0271] Temperature: 22.5 C.2.5 C. [0272] Detection: 200-700 nm [0273] Scan speed: 480 nm/min [0274] Slit: 1 nm [0275] Solvent for measurement: H.sub.3PO.sub.4 (25 mM)

[0276] An aqueous solution of the substance (0.1 M) was diluted with phosphorus acid (25 mM) to a final substance concentration of 2 M. A Hellma Makro UV-6030 cuvette was used for the measurement.

[0277] Infrared Spectroscopy [0278] Parameter for IR measurement: [0279] Device: Bruker Vector 22 [0280] Temperature: 22.5 C.2.5 C. [0281] Range: 550-4000 cm.sup.1

[0282] A small amount of the substance was applied to the crystal of the ATR unit.

[0283] High-Performance Liquid Chromatography (HPLC) [0284] Device: Hitachi Chromaster [0285] Column: Merck Chromolith HighResolution RP-18e 4.6100 mm [0286] Temperature: 40 C. [0287] Detection: 250/280 nm [0288] Solvent sample: Ammonium acetate 0.2 M [0289] Concentration [0290] sample: <0.5 mg/ml [0291] Inj. Volume: 2 l

TABLE-US-00004 Time H.sub.2O H.sub.3PO.sub.4 (0.5M) Acetonitrile Flow [min] % % % [ml/min] Gradient: 0.00 95.0 5.0 0.0 2,000 0.50 95.0 5.0 0.0 2,000 6.50 75.0 5.0 25.0 2,000 7.00 5.0 5.0 90.0 2,000 7.50 5.0 5.0 90.0 2,000 8.00 95.0 5.0 0.0 2,500 10.00 95.0 5.0 0.0 2,500

[0292] Mass Spectrometry [0293] Parameter for MS measurement: [0294] Device: Waters micromass triple quad [0295] Detection: 50-1000 m/z [0296] Ionization mode: ESI