JP2019160469A - Electrolytic solution for redox flow battery, and redox flow battery - Google Patents

Abstract

To provide: an electrolytic solution for a redox flow battery, which enables high input and output in charge and discharge; and a redox flow battery.SOLUTION: An electrolytic solution for a redox flow battery comprises a transition metal element which is one of Group IV to VIII elements in the periodic table as a battery active material and further, hydroxy acid and haloid ions so that its conductivity becomes 0.4 Scmor more. A redox flow battery comprises: a positive electrode 11; a negative electrode 21; and a permeable membrane 3 provided between the positive electrode 11 and the negative electrode 21. At least one of the positive electrode 11 and the negative electrode 21 includes an electrolyte solution circulation type electrode for circulating the electrolytic solution for the redox flow battery in the electrode.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an electrolyte for a redox flow battery and a redox flow battery. More specifically, the present invention enables high input / output at the time of charging / discharging and suitably generates halogen gas even when charging / discharging at a large current density. The present invention relates to a redox flow battery electrolyte that can be prevented and a redox flow battery.

  Non-Patent Document 1 discloses that an electrolyte containing vanadium as a battery active material contains chloride ions.

E. Sum, M. Rychcik and M. Skyllas-Kazacos, J. Power source, 16 (1985) 85-95

  However, the technique of Non-Patent Document 1 has found room for further improvement from the viewpoint of realizing high input / output at the time of charging / discharging of the redox flow battery.

  Then, the subject of this invention is providing the electrolyte solution for redox flow batteries and a redox flow battery which can perform high input / output at the time of charging / discharging.

  Other problems of the present invention will become apparent from the following description.

  The above problems are solved by the following inventions.

(Claim 1)
It contains a transition metal element which is a group 4 element to a group 8 element in the periodic table as a battery active material, and further contains an oxyacid and a halide ion so that the conductivity is 0.4 Scm ⁻¹ or more. Electrolyte for redox flow battery.
(Claim 2)
2. The redox flow battery electrolyte solution according to claim 1, wherein the oxyacid is sulfuric acid.
(Claim 3)
3. The redox flow battery electrolyte according to claim 1, wherein the halide ions are chloride ions or bromide ions.
(Claim 4)
The said transition metal element is a group 4 element or a group 5 element in a periodic table, The electrolyte solution for redox flow batteries in any one of Claims 1-3 characterized by the above-mentioned.
(Claim 5)
5. The redox flow battery electrolyte according to claim 4, wherein the transition metal element is vanadium.
(Claim 6)
A positive electrode, a negative electrode, and a diaphragm provided between the positive electrode and the negative electrode;
At least one of the positive electrode and the negative electrode is an electrolyte solution flow type electrode that distributes the redox flow battery electrolyte solution according to any one of claims 1 to 6 in the electrode,
The electrolyte flow type electrode is composed of conductive carbon felt and one or more kinds of graphite having a diameter of 1 μm or less selected from fibrous, granular, and planar shapes mixed in the conductive carbon felt, The apparent thickness in the normal direction to the electrolyte flow direction in the electrolyte flow type electrode is 3 mm or less,
The diaphragm has a thickness of 150 μm or less;
A redox flow battery having a charge / discharge area resistivity of 0.8 Ωcm ² or less.
(Claim 7)
The redox flow battery according to claim 6, wherein the charge / discharge area resistivity is 0.7 Ωcm ² or less.
(Claim 8)
The redox flow battery according to claim 6, wherein the charge / discharge area resistivity is 0.6 Ωcm ² or less.
(Claim 9)
The redox flow battery according to claim 6, wherein the charge / discharge area resistivity is 0.5 Ωcm ² or less.

  ADVANTAGE OF THE INVENTION According to this invention, the electrolyte solution for redox flow batteries and redox flow battery which can perform high input / output at the time of charging / discharging can be provided.

The figure which illustrates notionally an example of the redox flow battery of this invention The figure explaining the normal line direction to the electrolyte distribution direction in the electrolyte distribution type electrode The figure explaining the result of an Example and a comparative example

  Below, the form for implementing this invention is demonstrated in detail.

  First, the electrolyte solution for redox flow batteries of the present invention will be described, and then the redox flow battery of the present invention using the electrolyte solution will be described.

[Electrolytic solution for redox flow battery]
The electrolyte solution for a redox flow battery of the present invention contains a transition metal element which is a group 4 element to a group 8 element in the periodic table as a battery active material, and further has a conductivity of 0.4 Scm ⁻¹ or more at 17 ° C. As such, it contains oxyacid and halide ions.

The direct current 4-terminal method or the impedance method can be used for measuring the conductivity. The conductivity (Scm ⁻¹ ) corresponds to the reciprocal of the volume resistivity (Ωcm), and an increase in the conductivity (Scm ⁻¹ ) means a decrease in the volume resistivity (Ωcm).

  Transition metal elements that suitably function as battery active materials in redox flow batteries are Group 4 to Group 8 elements in the periodic table. Examples of such transition metal elements include Ti, V, Cr, Mo, W, Mn, Fe etc. are mentioned. The transition metal element is more preferably a group 4 element to a group 5 element. These elements can be used as ions having a valence suitable for the battery.

  An oxyacid (also referred to as an oxo acid) is an acid containing an oxygen atom, and for example, sulfuric acid, phosphoric acid, and the like can be used. Moreover, the transition metal oxyacid, titanic acid, vanadic acid, chromic acid, molybdic acid, tungstic acid, manganic acid, iron (tetravalent) acid, etc. which were mentioned above can also be used.

  It is preferable that the oxyacid can form an oxymetalate together with a transition metal element used as a battery active material. When vanadium is used as a battery active material, phosphoric acid, perchloric acid, nitric acid, etc. can be used in addition to sulfuric acid as the oxyacid that forms (poly) oxyvanadate. In the case of titanium and manganese, in addition to sulfuric acid, etc., titanic acid and manganic acid themselves become oxyacids.

  The transition metal element present as an oxymetalate in the electrolytic solution can perform a battery reaction with an electrode composed of carbon in particular. However, in this case, there is room for further improvement in realizing a good electrode reaction because the viscosity of the electrolytic solution is too large and the conductivity is not sufficient.

In this regard, in the present invention, by adding oxyacid and halide ions so that the electrical conductivity is 0.4 Scm ⁻¹ or more, the function of reducing the viscosity of the electrolyte and improving the electrical conductivity by the halide ions are achieved. Pull out and realize good electrode reaction. Thus, it is not suggested at all from the above-mentioned Non-Patent Document 1 to extract the function of halide ions. According to the electrolyte solution for a redox flow battery of the present invention, high input / output at the time of charging / discharging of the redox flow battery can be realized.

  The halide ions are preferably chloride ions or bromide ions. Halide ions are preferably added to the electrolyte as a halogen acid hydroacid. Specifically, it is preferable to contain chloride ions in the electrolytic solution, for example, by adding hydrogen chloride, and bromide ions, for example, by adding hydrogen bromide. Hydrochloric acid-hydrobromic acid mixed solution or the like may be added so that chloride ions and bromide ions coexist.

[Redox Flow Battery]
FIG. 1 is a diagram conceptually illustrating an example of the redox flow battery of the present invention.

  In FIG. 1, 1 is a positive electrode cell, 2 is a negative electrode cell, and 3 is a diaphragm that separates the positive electrode cell 1 and the negative electrode cell 2. A spacer 4 forms a gap corresponding to each of the positive electrode cell 1 and the negative electrode cell 2.

  A positive electrode 11 is provided in the positive electrode cell 1, and a negative electrode 21 is provided in the negative electrode cell 2.

  At least one of the positive electrode 11 and the negative electrode 21 (both in the present embodiment) is an electrolytic solution flow type electrode that allows an electrolytic solution (also referred to as an active material solution) to flow through the electrode, and the redox flow of the present invention described above as the electrolytic solution. A battery electrolyte is used.

  The electrolyte flow type electrode is composed of a conductive carbon felt and one or more types of graphite having a diameter of 1 μm or less selected from fibrous, granular, and planar shapes mixed in the conductive carbon felt. Thereby, a stable large steady-state current density is obtained by multi-dimensional diffusion exceeding two-dimensional diffusion in a plane. When the above graphite is mixed in the conductive carbon felt, it can be supported stably by preferably entanglement.

  One or more types of graphite having a diameter of 1 μm or less selected from fibrous, granular, and planar shapes may be nanographene such as carbon nanotubes and fullerenes.

  As shown in FIG. 2, the electrolyte flow type electrode constituting the positive electrode 11 has an apparent thickness in the normal direction to the electrolyte flow direction of the electrolyte flow type electrode of 3 mm or less. Thereby, it is possible to prevent the voltage drop (current × resistance) in the electrode chamber from becoming larger than necessary. The same applies to the electrolyte flow type electrode constituting the negative electrode 21.

  As shown in FIG. 2, the flow direction of the electrolytic solution is preferably parallel to the electrode surface of the electrolytic solution flow-type electrode (for example, a surface disposed in parallel with the diaphragm). As a result, the flow resistance can be reduced and the cell resistance can also be reduced, especially in contrast to the case where the electrolyte is circulated in the direction perpendicular to the electrode surface (normal line).

  That is, when the electrolyte solution is circulated in a direction perpendicular to the electrode surface (normal line), a member provided with a groove (electrolyte flow channel) for evenly distributing the electrolyte solution to the electrolyte solution flow type electrode is required. This groove becomes a resistance component. Moreover, in this case, it is necessary to make the electrolyte flow type electrode high in density, and the pressure when the electrolyte is fed increases. On the other hand, if the flow direction of the electrolyte solution is parallel to the electrode surface of the electrolyte flow type electrode, these can be improved, and the high input / output at the time of charging / discharging of the redox flow battery is more suitable. realizable.

The ratio of the peak height in the vicinity of 1320 cm ⁻¹ to the peak height in the vicinity of Raman shift 1580 cm ⁻¹ of the surface of the carbon fiber constituting the conductive carbon felt is preferably 50% or less in the Raman spectrum. Further, the peak height near 1320 cm ⁻¹ with respect to the peak height near 1580 cm ⁻¹ of Raman shift of one or more kinds of graphite having a diameter of 1 μm or less selected from fibrous, granular and planar shapes mixed in the conductive carbon felt. The proportion is preferably 15% or less. Thereby, the degree of graphitization becomes a sufficiently high value of 15% or more, and high electrode reactivity is obtained.

  The thickness of the diaphragm 3 is 150 μm or less. Thereby, the voltage drop in the diaphragm 3 can be made small. As the diaphragm 3, an ion exchange membrane, a microporous membrane such as an NF membrane (nanofiltration membrane) or an MF membrane (microfiltration membrane) can be used.

The diaphragm 3 preferably has a sheet resistivity of 0.3 Ωcm ² or less in a 4N HCl aqueous solution at 25 ° C. By using the diaphragm 3 having such high conductivity (low sheet resistivity), the charge / discharge area resistivity of the redox flow battery described later in detail can be made 0.8 Ωcm ² or less.

  The positive electrode active material liquid stored in the positive electrode active material liquid tank 12 is configured to flow into the positive electrode cell 1 by driving the pump 13. Further, the positive electrode active material liquid that has flowed out of the positive electrode cell 1 is configured to be returned to the positive electrode active material liquid tank 12. In this way, a circulation system for circulating and supplying the positive electrode active material liquid from the positive electrode active material liquid tank 12 into the positive electrode cell 1 is configured.

  The negative electrode active material liquid stored in the negative electrode active material liquid tank 22 is configured to flow into the negative electrode cell 2 when the pump 23 is driven. The negative electrode active material liquid that has flowed out of the negative electrode cell 2 is returned to the negative electrode active material liquid tank 22. In this way, a circulation system for circulating and supplying the negative electrode active material liquid from the negative electrode active material liquid tank 22 into the negative electrode cell 2 is configured.

  In the illustrated example, conductive sheets 5 and 5 are provided so as to be in contact with each of the positive electrode 11 and the negative electrode 21, and the entire cell is sandwiched by the pressing plates 6 and 6 from the outside of the conductive sheets 5 and 5. The positive electrode 11 and the negative electrode 21 can be connected to an external circuit via the conductive sheets 5 and 5, respectively. Although not shown, it is also preferable to form a stacked structure (cell stack) in which a plurality of single cells each composed of a positive electrode cell and a negative electrode cell are stacked by using a bipolar plate or the like as the conductive sheets 5 and 5.

  The redox flow battery circulates and supplies the negative electrode active material liquid and the positive electrode active material liquid to the positive electrode cell 1 and the negative electrode cell 2, respectively, and accompanies the electrode reaction (oxidation reduction reaction) of the active material in the active material liquid at both electrodes. Charge and discharge.

By using the redox flow battery electrolyte of the present invention having a conductivity of 0.4 Scm ⁻¹ or more for the negative electrode and / or positive electrode active material liquid of such a redox flow battery, the charge / discharge area of the redox flow battery is The resistivity can be 0.8 Ωcm ² or less, preferably 0.5 Ωcm ² or less, and most preferably 0.4 Ωcm ² or less. The charge / discharge area resistivity is a value obtained by dividing the area resistivity obtained from the open circuit voltage and the operating voltage at the time of output and the average current density of the positive and negative electrodes by the number of serial cells (single cells) stacked in the cell stack. The charge / discharge area resistivity is a value measured at 20 ° C.

When the conductive carbon felt is used as an electrolyte (active material liquid) flow type electrode, when charging / discharging at a large current density (for example, 1 A / cm ² or more), the DC resistance component of the internal resistance is It largely depends on the resistance (volume resistivity). In an electrolytic solution (active material solution) having a conductivity smaller than 0.4 Scm ⁻¹ , the voltage that rises during charging (current density × electrolytic solution resistance component when the thickness of the conductive carbon felt is 3 mm) Since the halide ion exceeds the potential at which it is oxidized, the risk of the halogen gas being released from the electrolyte increases. On the other hand, if the internal resistance (charge / discharge area resistivity) of the battery is larger than 0.8 Ωcm ² , the potential of the positive electrode during charging becomes 1.7 V (potential to hydrogen electrode) or more, and the halogen overvoltage such as chlorine increases. Even with the conductive carbon felt, it is difficult to prevent the generation of halogen gas.

On the other hand, in the redox flow battery of the present invention, the conductivity of the electrolytic solution is 0.4 or more, and the charge / discharge area resistivity (internal resistance) of the battery is 0.8 Ωcm ² or less, preferably 0.7 Ωcm ² or less. More preferably, it is 0.6 Ωcm ² or less, and most preferably 0.5 Ωcm ² or less. Thereby, high input / output is possible at the time of charging / discharging, and generation of halogen gas can be suitably prevented even when charging / discharging at a large current density. According to the present invention, it has been confirmed that the charge / discharge area resistivity can be reduced to 0.4 Ωcm ² or less.

Since the redox flow battery is capable of high input / output, for example, several times the rated input / output density, specifically, the apparent input / output density is 0.5 W / cm ² or more, and the apparent current density is 0.1. It can be set to 5 A / cm ² or more. As a result, the cell stack, which is the battery body, can be reduced in size, and the amount of each component used can be reduced, so that the battery stack can be used for a wide range of applications in terms of cost. For example, the sheet resistivity of the single cell portion 0.8Omucm ² below by the present invention, preferably 0.7Omucm ² or less, more preferably 0.6Omucm ² or less, most preferably if the 0.5Omucm ² below, for example, the apparent When the current density is 0.5 A / cm ² and 1.0 A / cm ² , the input voltage rise or output voltage drop is 0.25 V / cell and 0.5 V / cell, respectively. When the voltage is 1.2 V, the voltage efficiency can be reduced to about 50%. The battery of conventional active materials electrolyte communication type is about ² internal resistance 1 .OMEGA.cm, the current density of 1A / cm ² in this was a state where almost no output takes lauch.

  In the above description, the case where both the positive electrode and the negative electrode of the redox flow battery are constituted by the electrolyte flow type electrodes has been mainly shown, but the present invention is not limited to this. Only one of the positive electrode and the negative electrode may be constituted by the electrolyte flow type electrode. For example, the negative electrode can be constituted by an electrolyte flow type electrode, and the positive electrode can be an oxygen electrode.

  Examples of the present invention will be described below, but the present invention is not limited to these examples.

1. Preparation of Electrolyte Solutions Electrolytes 1 to 7 were prepared by containing hydrogen chloride (HCl), hydrogen bromide (HBr), sulfuric acid (H ₂ SO ₄ ), and vanadyl sulfate (VOSO ₄ ) in water at the concentrations shown in Table 1. did.

2. Test Method (1) Conductivity of Electrolyte Solution Conductivities were measured for electrolyte solutions 1-7. When two types of methods (DC four-terminal method and impedance method) were used for the measurement of conductivity, it was confirmed that the results of both methods almost coincided. The measurement temperature is 17 ° C. The measured values are shown in Table 1.

(2) Battery charge / discharge area resistivity (internal resistance)
A. Preparation of Cathode Solution and Cathode Solution Each of the electrolyte solutions 1 to 7 is divided into two solutions, one of which is a cathode solution (a solution containing tetravalent and pentavalent vanadium ions) and the other is a cathode solution (a divalent and a divalent solution). The vanadium ions were oxidized and reduced so as to be a liquid containing trivalent vanadium ions) to obtain a positive electrode solution and a negative electrode solution.

I. Battery Charge / Discharge Test A redox flow battery similar to that shown in FIG. 1 was prepared. The electrolyte flow type electrode used for the positive electrode and the negative electrode is composed of conductive carbon felt (PAN-based, fired at 1500 ° C., bulk density of 10.5 kg / m ² ), and a fibrous diameter mixed in the conductive carbon felt. It is composed of graphite (carbon nanotubes) of 1 μm or less, and the apparent thickness in the normal direction to the electrolyte flow direction in the electrolyte flow type electrode is 3 mm. As the diaphragm, an ion exchange membrane having a thickness of 50 μm (“Nafion (registered trademark) 212” manufactured by DuPont) was used.

The positive electrode solution and the negative electrode solution obtained by the above-mentioned “a. Preparation of positive electrode solution and negative electrode solution” were circulated at a positive electrode and a negative electrode of the redox flow battery at a flow rate of 5 mL / min. The charge / discharge area resistivity at a current density of 0.5 A / cm ² was measured. The measured values are shown in Table 1.

3. Evaluation From Table 1, the electrolytic solutions 2 to 5 and 7 have a conductivity of the electrolytic solution of 0.4 Scm ⁻¹ or more, which satisfies the conditions of the present invention. As shown in FIG. 3 in which the conductivity in Table 1 is plotted against the halide ion concentration, a correlation is found between the concentration of sulfuric acid and halide ions (hydrogen halide) and the conductivity of the electrolyte. It was done. Therefore, it can be seen that by adjusting the concentration of sulfuric acid and halide ions, the conductivity of the electrolytic solution can be increased to 0.4 Scm ⁻¹ or more.

Further, by using the electrolytic solutions 2 to 5 and 7 of the present invention in which the electrical conductivity of the electrolytic solution is 0.4 Scm ⁻¹ or more, the charge / discharge area resistivity (internal resistance) of the battery is 0.8 Ωcm ² or less, preferably Is 0.7Ωcm ² or less, more preferably 0.6Ωcm ² or less, and most preferably 0.5Ωcm ² or less.

  In addition, when an additional test omits the inclusion of one or more kinds of graphite (here, carbon nanotubes) having a diameter of 1 μm or less selected from fibrous, granular, and planar shapes into the conductive carbon felt, or an electrolytic solution It was confirmed that when the apparent thickness in the normal direction with respect to the flow direction of the electrolyte in the flow-through electrode exceeds 3 mm, the increase in charge / discharge area resistivity becomes significant. It was also confirmed that the charge / discharge area resistivity was remarkably increased when the thickness of the diaphragm exceeded 150 μm.

1: Positive electrode cell 11: Positive electrode 12: Positive electrode active material liquid tank 13: Pump 2: Negative electrode cell 21: Negative electrode 22: Negative electrode active material liquid tank 23: Pump 3: Separator 4: Spacer 5: Conductive sheet 6: Press plate

Claims (9)

It contains a transition metal element which is a group 4 element to a group 8 element in the periodic table as a battery active material, and further contains an oxyacid and a halide ion so that the conductivity is 0.4 Scm ⁻¹ or more. Electrolyte for redox flow battery.   2. The redox flow battery electrolyte solution according to claim 1, wherein the oxyacid is sulfuric acid.   3. The redox flow battery electrolyte according to claim 1, wherein the halide ions are chloride ions or bromide ions.   The said transition metal element is a group 4 element or a group 5 element in a periodic table, The electrolyte solution for redox flow batteries in any one of Claims 1-3 characterized by the above-mentioned.   5. The redox flow battery electrolyte according to claim 4, wherein the transition metal element is vanadium. A positive electrode, a negative electrode, and a diaphragm provided between the positive electrode and the negative electrode;
At least one of the positive electrode and the negative electrode is an electrolyte solution flow type electrode that distributes the redox flow battery electrolyte solution according to any one of claims 1 to 6 in the electrode,
The electrolyte flow type electrode is composed of conductive carbon felt and one or more kinds of graphite having a diameter of 1 μm or less selected from fibrous, granular, and planar shapes mixed in the conductive carbon felt, The apparent thickness in the normal direction to the electrolyte flow direction in the electrolyte flow type electrode is 3 mm or less,
The diaphragm has a thickness of 150 μm or less;
A redox flow battery having a charge / discharge area resistivity of 0.8 Ωcm ² or less. The redox flow battery according to claim 6, wherein the charge / discharge area resistivity is 0.7 Ωcm ² or less. The redox flow battery according to claim 6, wherein the charge / discharge area resistivity is 0.6 Ωcm ² or less. The redox flow battery according to claim 6, wherein the charge / discharge area resistivity is 0.5 Ωcm ² or less.

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