Alkaline manganese redox flow battery using an inhibitor

Abstract

The redox flow battery includes a redox flow cell and a supply and storage system external to the redox flow cell. The supply and storage system includes first and second electrolytes for circulating through the redox flow cell. The first electrolyte is a liquid electrolyte having electrochemically active manganese species having multiple reversible oxidation states in the redox flow cell. The electrochemically active manganese species may undergo a reaction that results in the precipitation of manganese oxide solids. The first electrolyte includes an inhibitor that limits the self-discharge reaction. The inhibitor includes an oxoanion compound.

Description

Technical Field

[0001] (Cross - Reference to Related Applications) This disclosure claims priority to U.S. Patent Application No. 17 / 326,518, filed May 21, 2021.

Background Art

[0002] Flow batteries, also known as redox flow batteries or redox flow cells, are designed to convert electrical energy into chemical energy, which can be stored and later released when there is a demand. As an example, flow batteries can be used with renewable energy systems such as wind power generation systems to store energy in excess of consumer demand and later release that energy when there is a greater demand.

[0003] A typical flow battery includes a redox flow cell having a negative electrode and a positive electrode separated by an electrolyte layer, which may include a separator such as an ion - exchange membrane. To drive a reversible redox reaction between redox couples, a negative fluid electrolyte (sometimes referred to as an anode solution) is supplied to the negative electrode and a positive fluid electrolyte (sometimes referred to as a cathode solution) is supplied to the positive electrode. During charging, the supplied electrical energy causes a reduction reaction in one electrolyte and an oxidation reaction in the other electrolyte. The separator prevents the electrolytes from freely and rapidly mixing but allows selected ions to pass through to complete the redox reaction. During discharge, the chemical energy contained in the liquid electrolyte is released in a reverse reaction, and electrical energy can be taken out from the electrodes.

Summary of the Invention

Means for Solving the Problems

[0004] A redox flow battery according to an example of the present disclosure includes a redox flow cell and a supply and storage system external to the redox flow cell. The supply and storage system includes first and second electrolytes for circulating through the redox flow cell. The first electrolyte is a liquid electrolyte having electrochemically active manganese species having a plurality of reversible oxidation states in the redox flow cell. The electrochemically active manganese species may undergo reactions that cause precipitation of manganese oxide solids. The first electrolyte further includes an inhibitor that limits self-discharge reactions, and the inhibitor includes an oxoanion compound.

[0005] In a further embodiment of any of the foregoing embodiments, the inhibitor is selected from the group consisting of phosphates (PO 4 3- ), nitrates (NO 3 - ), and sulfates (SO 4 2- ), or any compound that decomposes in the electrolyte to form phosphate, nitrate, or sulfate species, and mixtures thereof.

[0006] In a further embodiment of any of the foregoing embodiments, the inhibitor is selected from the group consisting of phosphates, and mixtures thereof.

[0007] In a further embodiment of any of the foregoing embodiments, the inhibitor is selected from the group consisting of nitrates, and mixtures thereof.

[0008] In a further embodiment of any of the foregoing embodiments, the inhibitor is selected from the group consisting of sulfates, and mixtures thereof.

[0009] In a further embodiment of any of the foregoing embodiments, the inhibitor is a compound that decomposes during operation of the redox flow battery to produce phosphate, nitrate, or sulfate species in solution.

[0010] In a further embodiment of any of the foregoing embodiments, the first electrolyte has a composition in which there is 0.0001 mol to 0.1 mol of inhibitor per 1 mol of manganese ions.

[0011] In a further embodiment of any of the foregoing embodiments, the first electrolyte has a composition in which there is 0.001 mol to 0.01 mol of inhibitor per 1 mol of manganate.

[0012] In a further embodiment of any of the foregoing embodiments, the electrochemically active manganese species is MnO 4 - , and MnO 4 2- .

[0013] The electrolyte for a redox flow battery according to an example of the present disclosure includes a supporting electrolyte containing a solvent and a supporting salt, and electrochemically active manganese species dissolved in the supporting electrolyte. The electrochemically active manganese species has a plurality of reversible oxidation states in the supporting electrolyte. The inhibitor dissolves in the supporting electrolyte and limits the reaction of the electrochemically active manganese species that causes precipitation of manganese oxide solids. The inhibitor includes an oxoanion compound.

[0014] In a further embodiment of any of the foregoing embodiments, the inhibitor is selected from the group consisting of phosphates (PO 4 3- ), nitrates (NO 3 - ), and sulfates (SO 4 2- ), or any compound that decomposes in the electrolyte to form phosphate, nitrate, or sulfate species, and mixtures thereof.

[0015] In a further embodiment of any of the foregoing embodiments, the inhibitor is selected from the group consisting of phosphates and mixtures thereof.

[0016] In a further embodiment of any of the foregoing embodiments, the inhibitor is selected from the group consisting of nitrates and mixtures thereof.

[0017] In a further embodiment of any of the foregoing embodiments, the inhibitor is selected from the group consisting of sulfates and mixtures thereof.

[0018] In a further embodiment of any of the foregoing embodiments, the inhibitor is a compound that decomposes in an electrolyte to produce phosphate, nitrate, or sulfate species in solution.

[0019] A further embodiment of any of the foregoing embodiments includes a composition in which there is from 0.0001 to 0.1 mole of inhibitor per mole of manganate.

[0020] In a further embodiment of any of the foregoing embodiments, the electrochemically active manganese species is MnO 4 - , and MnO 4 2- is.

[0021] The various features and advantages of the present disclosure will become apparent to those skilled in the art from the following detailed description. The drawings attached to the detailed description can be briefly described as follows.

Brief Description of the Drawings

[0022]

Figure 1

Mode for Carrying Out the Invention

[0023] FIG. 1 schematically shows a portion of an exemplary redox flow battery 20 for selectively storing and discharging electrical energy. As an example, the flow battery 20 can be used to convert electrical energy generated in a renewable energy system into chemical energy, which can be stored until a time when there is a greater demand, at which point the flow battery 20 can be used to convert the chemical energy back into electrical energy. The flow battery 20 can supply electrical energy to, for example, a power grid.

[0024] The flow battery 20 includes a first electrolyte 22 having electrochemically active species 24 that function as a redox pair with respect to an additional second electrolyte 26 having electrochemically active species 28. At least the first electrolyte 22 is liquid, although most commonly the second electrolyte 26 is also liquid. The electrolytes 22 / 26 are included in a supply / storage system 30 that includes a first container 32 and a second container 34.

[0025] For example, the electrochemically active species 24 are manganese oxyanions, namely, MnO having oxidation states of +6 and +7, respectively. 4 - and MnO 4 2- is based on. Manganese in the oxyanion has multiple reversible oxidation states in a selected liquid solution, namely, in a supporting electrolyte that includes, but is not limited to, a basic salt aqueous solution. The electrochemically active species 28 of the second electrolyte 26 are not particularly limited. For example, the electrochemically active species 28 are selected from vanadium, iron, chromium, zinc, molybdenum, sulfur, cerium, lead, tin, titanium, germanium, and combinations thereof, halogens such as bromine, chlorine, and combinations thereof, or organic molecules containing groups that undergo electrochemically reversible reactions, such as quinones, or nitrogen-containing organics such as quinoxaline or pyrazine, or sulfur-containing organics such as phenothiazine.

[0026] As an example, the manganese / sulfur-based half-cell reactions are shown below. Since these reactions are well-known like those of other systems, they will not be discussed further. Cathode: 2NaMnO 4 +2Na + +2e - ←→2Na 2 MnO 4 E 0 =0.551 vs. SHE Anode: 2Na 2 S 2 ←→Na 2 S 4 +2Na + +2e -E 0 = -0.45 V vs. SHE Electrolytes 22 and 26 are circulated by pump 35 through respective supply lines 38 to at least one redox flow cell 36 of flow battery 20 and returned from cell 36 through return line 40 to vessels 32 and 34. Of course, additional pumps 35 and valves (not shown) at the inlets / outlets of the components of flow battery 20 can also be used to control the flow if necessary. A plurality of cells 36 can be provided as a stack within the loop of the flow circuit.

[0027] Each cell(s) 36 has a first electrode 42, a second electrode 44 spaced apart from the first electrode 42, and an electrolyte separator layer 46 disposed between the first electrode 42 and the second electrode 44. For example, electrodes 42 and 44 are porous carbon structures such as carbon paper or felt. Generally, cell(s) 36 can include bipolar plates, manifolds, etc. for sending electrolytes 22 and 26 through flow field channels to electrodes 42 and 44. However, it is of course possible to use other configurations. For example, cell(s) 36 can alternatively be configured for flow-through operation, in which case the fluid electrolytes 22 and 26 are pumped directly into electrodes 42 and 44 without using flow field channels.

[0028] Electrolyte separator layer 46 can be an ion exchange membrane, a microporous polymer membrane, or a material that prevents free and rapid mixing of electrolytes 22 and 26 but allows selected ions to pass through while electrically separating electrodes 42 and 44 to complete the redox reaction, such as an electrically insulating microporous matrix of silicon carbide (SiC), but is not limited thereto. In this regard, the flow circuits of electrolytes 22 / 26 are insulated from each other during normal operation such as charging, discharging, and the stopped state.

[0029] Electrolytes 22, 26 are sent to cell(s) 36 during an active charge / discharge mode to convert electrical energy to chemical energy or, in a reverse reaction, to convert chemical energy released into electrical energy that circulates through cell 36. Electrical energy is transmitted to and from cell(s) 36 via an electrical circuit 48 electrically coupled to electrodes 42, 44.

[0030] Also, flow battery 20 can be transitioned from an active charge / discharge mode to a complete stop mode in which neither electrolyte 22 nor 26 circulates through cell 36. For example, in the complete stop mode, electrolyte 22, 26 is emptied from cell 36 or left in cell 36 but not circulated. In this regard, flow battery 20 can also include a controller having hardware such as a microprocessor, software, or both, configured to control operation of flow battery 20, including stopping from an active charge / discharge mode and starting from an inactive stopped state.

[0031] At high pH levels such as about 14 or greater, manganese species in electrolyte 22, particularly those in which manganese has oxidation states of +6 and +7, are prone to reactions that cause precipitation of manganese oxide solids. Without wishing to be bound by any particular theory, it is hypothesized that manganese in the +6 and +7 oxidation states is involved in self-discharge, disproportionation, and / or decomposition reactions shown below to produce solid precipitates of MnO 2 The manganese oxide precipitate has low solubility in the electrolyte, and thus the reaction effectively serves as a removal mechanism for electrochemically active permanganate or manganate from the electrolyte. Further, since precipitation can occur within just a few days, effective removal of manganese by precipitation represents a decrease in durability and thus an obstacle to realizing other performance and cost advantages that make manganese electrolytes attractive.

[0032] Permanganate / Manganate Reactions: Self-Discharge: 4NaMnO 4+4NaOH ←→ 4Na 2 MnO 4 +2H 2 O + O 2 (g) Disproportionation: 3Na 2 MnO 4 +2H 2 O ←→ MnO 2 (s) + 2NaMnO 4 +4NaOH Decomposition: 2Na 2 MnO 4 +2H 2 O ←→ MnO 2 (s) + O 2 (g) + 4NaOH Regarding this, in order to limit such reactions of manganese in the +6 and +7 oxidation states and thereby enhance the durability of the flow battery 20, the electrolyte 22 contains an inhibitor 50. The inhibitor 50 inhibits the reaction and thus prevents the formation of manganese oxide precipitates from manganese in the +6 and +7 oxidation states. Here too, without wishing to be bound by a particular theory, it is thought that these manganese ions react to form clustered manganese oxide complexes of Mn x O y z and then these clusters are thought to catalyze further reactions of manganate by one or both of two mechanisms. One mechanism is thought to be the catalysis of oxygen generation by the clusters, and the other mechanism is thought to be the driving of cluster growth and the associated disproportionation and decomposition reactions. Under these concepts, the inhibitor 50 serves to strongly bind to the clusters, thereby blocking the catalytic reaction sites for oxygen generation and / or cluster growth.

[0033] The inhibitor 50 contains an oxoanion compound. The oxoanion, also called an oxyanion, is an ion having the general formula A x O z- y (wherein A is a chemical element and O is an oxygen atom). For example, the inhibitor 50 is a phosphate (PO 4 3- ), a nitrate (NO 3 -) and a sulfate (SO 4 2- ), or is selected from the group consisting of any compound that decomposes in the electrolyte to form a phosphate, nitrate, or sulfate species, and mixtures thereof.

[0034] Generally, the electrolyte 22 has a composition in which 0.0001 mol to 0.1 mol of the inhibitor 50 is present per 1 mol of the manganate. The amount at the lower limit of this range can be used due to a relatively lower inhibitory effect, while the amount at the upper limit of this range can be used due to a relatively higher inhibitory effect. In a further example, 0.001 mol to 0.01 mol of the inhibitor is present per 1 mol of manganese.

[0035] As a further example, the electrolyte 22 has the following composition. 75% by weight of a supporting electrolyte containing 66% by weight of a solvent and 9% by weight of a supporting salt, 25% by weight of an electrochemically active manganese species dissolved in the supporting electrolyte, and Less than 1% by weight of the inhibitor 50 dissolved in the supporting electrolyte.

[0036] Further non-limiting examples of the inhibitor 50 for use in the above amounts are shown in the following table.

[0037]

Table 1

[0038] Although combinations of features are shown in the illustrated examples, it is not necessary to combine all of them to realize the benefits of the various embodiments of the present disclosure. In other words, a system designed in accordance with an embodiment of the present disclosure does not necessarily include all of the features shown in any one of the drawings, or all of the portions schematically shown in the drawings. Further, selected features of one exemplary embodiment can be combined with selected features of other exemplary embodiments.

[0039] The foregoing description is illustrative rather than restrictive. Variations and modifications to the disclosed examples that do not necessarily depart from the essence of the present disclosure may be apparent to those skilled in the art. Accordingly, the scope of legal protection given to the present disclosure can be determined only by examining the following claims.

Claims

1. A redox flow battery comprising: a redox flow cell; an external supply and storage system for the redox flow cell, the supply and storage system including first and second electrolytes for circulation through the redox flow cell, the first electrolyte being a liquid electrolyte having electrochemically active manganese species having a plurality of reversible oxidation states in the redox flow cell, the electrochemically active manganese species being capable of undergoing reactions that cause precipitation of manganese oxide solids, the supply and storage system; and the first electrolyte further comprising an inhibitor that limits a self-discharge reaction, the inhibitor including an oxoanion compound; the redox flow battery, wherein the electrochemically active manganese species are MnO₄⁻ and MnO₄²⁻.

2. wherein the inhibitor is selected from the group consisting of phosphate (PO ４ ３－ ), nitrate (NO ３ － ), and sulfate (SO ４ ２－ ), or any compound that decomposes in the electrolyte to form species of phosphate, nitrate, or sulfate, and mixtures thereof, the redox flow battery according to claim 1.

3. The redox flow battery according to claim 2, wherein the inhibitor is selected from the group consisting of phosphates and mixtures thereof.

4. The redox flow battery according to claim 2, wherein the inhibitor is selected from the group consisting of nitrates and mixtures thereof.

5. The redox flow battery according to claim 2, wherein the inhibitor is selected from the group consisting of sulfates and mixtures thereof.

6. The redox flow battery according to claim 2, wherein the inhibitor is a compound that decomposes during operation of the redox flow battery to produce species of phosphate, nitrate, or sulfate in solution.

7. The redox flow battery according to claim 2, wherein the first electrolyte has a composition in which there is present from 0.0001 mol to 0.1 mol of inhibitor per mol of manganese ions.

8. The redox flow battery according to claim 7, wherein the first electrolyte has a composition in which there is present from 0.001 mol to 0.01 mol of inhibitor per mol of manganate.

9. A redox flow battery electrolyte comprising: a supporting electrolyte including a solvent and a supporting salt; electrochemically active manganese species dissolved in the supporting electrolyte, the electrochemically active manganese species having a plurality of reversible oxidation states in the supporting electrolyte; an inhibitor dissolved in the supporting electrolyte that limits reactions of the electrochemically active manganese species that cause precipitation of manganese oxide solids, the inhibitor including an oxoanion compound; and. The electrolyte for the redox flow battery, wherein the electrochemically active manganese species are MnO₄⁻ and MnO₄²⁻.

10. wherein the inhibitor is selected from the group consisting of phosphate (PO ４ ３－ ), nitrate (NO ３ － ), and sulfate (SO ４ ２－ ), or any compound that decomposes in the electrolyte to produce species of phosphate, nitrate, or sulfate, and mixtures thereof, the electrolyte according to claim 9.

11. The electrolyte according to claim 10, wherein the inhibitor is selected from the group consisting of phosphates and mixtures thereof.

12. The electrolyte according to claim 10, wherein the inhibitor is selected from the group consisting of nitrates and mixtures thereof.

13. The electrolyte according to claim 10, wherein the inhibitor is selected from the group consisting of sulfates and mixtures thereof.

14. The electrolyte according to claim 10, wherein the inhibitor is a compound that decomposes in the electrolyte to form species of phosphate, nitrate or sulfate in the solution.

15. The electrolyte according to claim 10, having a composition in which 0.0001 mol to 0.1 mol of inhibitor is present per mole of manganate.

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