METHOD FOR MANUFACTURING POSITIVE ELECTRODE ELECTROLYTE FOR REDOX FLOW BATTERY AND REDOX FLOW BATTERY

Abstract

The present disclosure relate to a method for preparing a cathode electrolyte for redox flow batteries including the steps of: forming a first cathode electrolyte by reducing vanadium pentoxide (V.sub.2O.sub.5) in an acidic solution in the presence of a specific reducing compound; forming a second cathode electrolyte by reducing vanadium pentoxide (V.sub.2O.sub.5) in an acidic solution in the presence of a linear or branched aliphatic alcohol having 2 to 10 carbon atoms; and mixing the first cathode electrolyte and the second cathode electrolyte, and to a redox flow battery including the cathode electrolyte obtained by the preparation method.

Claims

1. A method for preparing a cathode electrolyte for redox flow batteries comprising the steps of: forming a first cathode electrolyte by reducing vanadium pentoxide (V.sub.2O.sub.5) in an acidic solution in the presence of at least one reducing compound selected from the group consisting of dicarboxylic acid containing a linear or branched alkylene group having 0 to 6 carbon atoms, hydrazine, and L-ascorbic acid; forming a second cathode electrolyte by reducing vanadium pentoxide (V.sub.2O.sub.5) in an acidic solution in the presence of a linear or branched aliphatic alcohol having 2 to 10 carbon atoms; and mixing the first cathode electrolyte and the second cathode electrolyte.

2. The method for preparing a cathode electrolyte for redox flow batteries according to claim 1, wherein the first cathode electrolyte and the second cathode electrolyte are mixed in a volume ratio of 12:1 to 1:1.

3. The method for preparing a cathode electrolyte for redox flow batteries according to claim 1, wherein a concentration of the acidic solution in the first cathode electrolyte and the second cathode electrolyte is 0.1 M to 6 M.

4. The method for preparing a cathode electrolyte for redox flow batteries according to claim 1, wherein the acidic solutions in the first cathode electrolyte and the second cathode electrolyte contain sulfuric acid, respectively.

5. The method for preparing a cathode electrolyte for redox flow batteries according to claim 1, wherein a concentration of the vanadium pentoxide in the first cathode electrolyte and the second cathode electrolyte is 0.3 M to 3 M, respectively.

6. The method for preparing a cathode electrolyte for redox flow batteries according to claim 1, wherein a concentration of the reducing compound in the first cathode electrolyte is 0.3 M to 3 M, and a concentration of the aliphatic alcohol in the second cathode electrolyte is 0.3 M to 3 M.

7. The method for preparing a cathode electrolyte for redox flow batteries according to claim 1, wherein a ratio of the concentration of the reducing compound relative to the concentration of vanadium pentoxide in the first cathode electrolyte is 0.8 to 1.2.

8. The method for preparing a cathode electrolyte for redox flow batteries according to claim 1, wherein a ratio of the concentration of the aliphatic alcohol relative to the concentration of vanadium pentoxide in the second cathode electrolyte is 0.8 to 1.2.

9. A redox flow battery, comprising: a cathode electrolyte for redox flow batteries prepared by the method of claim 1; metal ions containing a V.sup.2+/V.sup.3+ redox couple; and an anode electrolyte containing a sulfuric acid solution.

10. The redox flow battery according to claim 9, wherein the redox flow battery includes at least one unit cell, the unit cell comprising: a separation membrane through which ions pass; a pair of electrodes opposite to the center of the separation membrane; and the cathode electrolyte and anode electrolyte which are present respectively in a cathode cell and an anode cell partitioned by the separation membrane.

11. The redox flow battery according to claim 9, wherein the unit cell further includes a pair of flow frames attached to face each other on respective sides of the separation membrane.

12. The redox flow battery according to claim 9, wherein the unit cell further includes a cell frame formed on the outer surface of the electrodes.

13. The redox flow battery according to claim 9, further comprising: a cathode electrolyte tank for storing the cathode electrolyte; a cathode electrolyte pump for circulating the cathode electrolyte from the cathode electrolyte tank to the cathode cell of the unit cell during charge and discharge; an anode electrolyte tank for storing the anode electrolyte; and an anode electrolyte pump for circulating the anode electrolyte from the anode electrolyte tank to the anode cell of the unit cell during charge and discharge.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0055] FIG. 1 is a graph showing discharge capacities of redox flow batteries of Example 1 and Comparative Examples 1 and 3 according to operation cycles.

[0056] FIG. 2 is a graph showing energy efficiencies of redox flow batteries of Example 1 and Comparative Examples 1 and 3 according to operation cycles.

[0057] FIG. 3 is a graph showing discharge capacities of redox flow batteries of Example 2 and Comparative Examples 2 and 4 according to operation cycles.

[0058] FIG. 4 is a graph showing energy efficiencies of redox flow batteries of Example 2 and Comparative Examples 2 and 4 according to operation cycles.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0059] Hereinafter, the present invention will be described in more detail with reference to the following examples. However, these examples are set forth only to illustrate the present invention, and the scope of the present invention is not limited thereto.

EXAMPLES AND COMPARATIVE EXAMPLES

Preparation and Operation of Redox Flow Battery

[0060] Single cells were assembled by using the components shown in Table 1 below, and a redox flow battery was operated according to the charging/discharging conditions shown in Table 1 below while injecting the cathode electrolyte and anode electrolyte prepared in examples and comparative examples at about 100 ml.

TABLE-US-00001 TABLE 1 Single cell components and charging/discharging conditions Items Single cell Electrode Maker SGL (GFD 3) components (product name) Separation Maker GEFC (104) membrane (product name) Bipolar plate Thickness (mm) 3 Effective area (cm.sup.2) 35 Charging/ Charging/discharging voltage 1.0 V-1.6 V discharging conditions Charging/discharging current 50 mA/cm.sup.2

[0061] 2. Electrolytes of examples and comparative examples were prepared by the following method.

Example 1

(1) Preparation of First Cathode Electrolyte

[0062] 1.8 mol of oxalic acid anhydride was injected into 0.5 L of a 10 M sulfuric acid solution, and was completely dissolved until the solution became a clear liquid at about 60° C. Then, a small amount (1.8 mol) of vanadium pentoxide having purity of 98% or more was injected to perform a stepwise redox reaction. After the completion of the reaction, distilled water was added to 1 L, and residual suspended materials were removed by filtration under reduced pressure to prepare a first cathode electrolyte.

(2) Preparation of Second Cathode Electrolyte

[0063] 5 mol of vanadium pentoxide having purity of 98% or more was slowly dissolved in 5 mol of 95% sulfuric acid to form a slurry, and 0.5 L of a 0.72 M aqueous ethanol solution was slowly injected at 60° C-100° C. to perform a stepwise redox reaction. Then, residual ethanol was volatilized at 120° C. or higher and distilled water was added and diluted to 1 L. The diluted reaction product was filtered under reduced pressure to prepare a second cathode electrolyte.

Example 2 and Comparative Examples 1 to 4

[0064] Cathode electrolytes were prepared in the same manner as in Example 1, except that a redox couple and a reducing compound or alcohol was added to a sulfuric acid solution according to the conditions shown in Tables 2 and 3 below.

Preparation of Anode Electrolyte

[0065] The same amount of electrolyte V(IV) was injected into both electrodes of an electrochemical cell, which was then subjected to an electrochemical reaction in which the electrochemical cell was charged with a current of 50 mA/cm.sup.2 to 1.6 V in a first step, charged with a current of 20 mA/cm.sup.2 to 1.6 V in a second step, and charged with a current of 8 mA/cm.sup.2 or less to 1.7 V in a third step, thereby preparing an anode electrolyte containing pure trivalent vanadium ions and sulfuric acid in the anode of this cell.

[0066] This anode electrolyte was injected into the battery of Table 1 above, and the operation of this battery was performed.

TABLE-US-00002 TABLE 2 Preparation of cathode electrolytes of Examples 1 and 2 Classification Example 1 Example 2 First Second Second cathode cathode First cathode cathode electrolyte electrolyte electrolyte electrolyte Cathode Concentration [M] of 1.8 1.8 1.5 1.5 electrolyte V.sup.4+/V.sup.5+ redox couple Concentration [M] of 5 5 5 5 sulfuric acid Reducing compound or Oxalic acid Ethanol 1.8M Oxalic acid Ethanol 1.5M alcohol 1.8M 1.5M Volume ratio of first cathode 7:3 9:1 electrolyte to second cathode electrolyte

TABLE-US-00003 TABLE 3 Preparation of cathode electrolytes of Comparative Examples 1 to 4 Classification Comparative Comparative Comparative Comparative Example 1 Example 2 Example 3 Example 4 Cathode Concentration [M] of 1.8 1.5 1.8 1.5 electrolyte V.sup.4+/V.sup.5+ redox couple Concentration [M] of 5 5 5 5 sulfuric acid Reducing compound Oxalic acid Oxalic acid Ethanol 0.72M Ethanol 0.6M 1.8M 1.5M

3. Operation Results of Redox Flow Batteries of Examples and Comparative Examples

[0067] The results of operating a redox flow battery using the electrolytes obtained in Example 1 and 2 and Comparative Examples 1 to 4 respectively are shown in Tables 4 to 9 below. Further, the operation results thereof are compared and shown in FIGS. 1 to 4.

[0068] (1) Energy efficiency (EE)=[discharge energy (Wh)/charge energy (Wh)]*100

[0069] (2) Charge efficiency (CE)=[discharge capacity (Wh)/charge capacity (Wh)]*100

[0070] (3) Voltage efficiency (VE)=[energy efficiency/charge efficiency]* 100

[0071] (4) V utility rate (AhL/mol): vanadium (V) utility rate was calculated by the following General Formula 1.

V utility rate (AhL/mol)=discharge capacity (Ah) of the cycle/molar concentration (mol/L) of vanadium   [General Formula 1]

TABLE-US-00004 TABLE 4 Operation results of redox flow battery using the electrolyte of Comparative Example 1 Charge Discharge Operation capacity capacity EE CE VE V utility rate cycle (Ah) (Ah) (%) (%) (%) (AhL/mol) 3.71 3.57 87 96 90 1.98 10 3.75 3.63 86 97 89 2.02 20 3.75 3.61 86 96 89 2.01 40 3.52 3.37 86 96 89 1.87 70 2.95 2.84 84 96 88 1.57 100 2.70 2.59 84 96 87 1.48

TABLE-US-00005 TABLE 5 Operation results of redox flow battery using the electrolyte of Example 1 Charge Discharge Operation capacity capacity EE CE VE V utility rate cycle (Ah) (Ah) (%) (%) (%) (AhL/mol) 3.97 3.71 85 93 91 2.06 5 3.91 3.74 87 96 91 2.08 10 4.12 3.85 87 95 91 2.14 15 4.17 3.89 86 95 91 2.16 20 4.14 3.85 86 95 90 2.14 25 4.03 3.74 85 95 90 2.08 30 3.90 3.62 85 95 90 2.01 40 3.67 3.40 85 95 90 1.89

TABLE-US-00006 TABLE 6 Operation results of redox flow battery using the electrolyte of Comparative Example 2 Charge Discharge Operation capacity capacity EE CE VE V utility rate cycle (Ah) (Ah) (%) (%) (%) (AhL/mol) 3.09 2.93 86 95 90 1.94 10 3.10 2.97 86 96 90 1.98 20 3.12 2.98 86 96 90 1.99 40 2.99 2.86 85 96 89 1.91 70 2.80 2.68 84 96 88 1.79 100 2.67 2.56 84 96 88 1.70

TABLE-US-00007 TABLE 7 Operation results of redox flow battery using the electrolyte of Example 2 Discharge Charge capacity capacity EE CE VE V utility rate Cycle (Ah) (Ah) (%) (%) (%) (AhL/mol) 3.24 3.10 85 93 91 2.07 5 3.16 3.10 87 96 91 2.07 10 3.13 3.08 87 96 91 2.07 15 3.01 2.97 86 96 90 1.98 20 2.97 2.90 86 95 90 1.93

TABLE-US-00008 TABLE 8 Operation results of redox flow battery using the electrolyte of Comparative Example 3 Discharge Charge capacity capacity EE CE VE V utility rate Cycle (Ah) (Ah) (%) (%) (%) (AhL/mol) 3.37 3.22 83 96 87 1.79 10 3.46 3.37 85 97 87 1.87 20 3.52 3.40 84 97 87 1.89 40 3.22 3.12 83 97 86 1.73 70 2.87 2.78 82 97 85 1.55 100 2.70 2.61 82 97 85 1.45

TABLE-US-00009 TABLE 9 Operation results of redox flow battery using the electrolyte of Comparative Example 4 Discharge Charge capacity capacity EE CE VE V utility rate Cycle (Ah) (Ah) (%) (%) (%) (AhL/mol) 2.98 2.78 82 93 87 1.81 10 2.91 2.80 83 96 86 1.86 20 2.87 2.76 82 96 86 1.84 40 2.60 2.50 81 96 85 1.67 70 2.38 2.28 81 96 85 1.52 100 2.14 2.05 81 96 85 1.36

[0072] As shown in the results of Tables 4 to 9 and FIGS. 1 to 4, it was found that the redox flow batteries of Examples 1 and 2 had higher discharge capacity and energy efficiency as compared to the redox flow batteries of Comparative Examples 1 to 4, respectively, and the efficiency and performance of the redox flow batteries of Examples 1 and 2 did not greatly deteriorate even if the operation cycle was increased, as compared to the redox flow batteries of Comparative Examples 1 to 4.

[0073] (6) Measurement of Internal Resistance of Redox Flow Battery

[0074] Internal resistances of the redox flow batteries using the electrolytes obtained in the examples and the comparative examples, respectively, were measured using HIOKI BT3563.

[0075] The measurement results thereof are shown in Table 10 below.

TABLE-US-00010 TABLE 10 Results of measurement of internal resistance Classification Internal resistance (Ω * cm.sup.2) Comparative Example 1 1.40 Example 1 1.09 Comparative Example 2 1.33 Example 2 1.12 Comparative Example 3 1.75 Comparative Example 4 1.68

[0076] As given in Table 10 above, it was found that the redox flow batteries using the electrolytes obtained in Examples 1 and 2 had lower internal resistances as compared to the redox flow batteries using the electrolytes obtained in Comparative Examples 1 to 4.