Patent Publication Number: US-2022216495-A1

Title: Electrolytic reduction system and method of vanadium electrolyte

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefits of Taiwan application Serial No. 110100016, filed on Jan. 4, 2021 the disclosures of which are incorporated by references herein in its entirety. 
     TECHNICAL FIELD 
     The present invention relates to a vanadium electrolyte for a vanadium battery, and particularly provides an electrolytic reduction system and method of a vanadium electrolyte. 
     BACKGROUND 
     There are 46 patents for preparing vanadium electrolytes from industrial waste recovery. China is the main applicant country. The Chinese Academy of Sciences, Squirrel, Anshan Iron and Steel Group Corporation, Beijing Zhongkai, etc. are the main patentees. In only 1 of the 46 patents, V 2 O 5  powder is dissolved in an H 2 SO 4  solution and electrolyzed into 3.5-valent vanadium. The patent has been applied for in China, Japan, Canada and other countries. The patent publication numbers are as follows: CN 1502141 A, CA 2420014 A1, JP 2004519814(A), WO 02/15317 A1. 
     A traditional method is to mix vanadium pentoxide with a sulfuric acid solution, add a reducing agent, and prepare a sulfuric acid solution containing tetravalent vanadium. The tetravalent vanadium is electrolyzed into 3.5-valent vanadium. 3.5-valent vanadium is a mixed solution of tetravalent vanadium and trivalent vanadium. The solubility of pentavalent vanadium in sulfuric acid is very low, about 0.1 M to 0.2 M. Flow batteries require a vanadium ion concentration of 1.0 M to 3.0 M. The traditional method is to reduce pentavalent vanadium dissolved in sulfuric acid to tetravalent vanadium with a reducing agent. The solubility of tetravalent vanadium can reach 3.0 M or more. Part of the tetravalent vanadium is reduced to trivalent vanadium by electrolytic reduction, such that the average valence of vanadium ions is 3.5. 
     However, the dissolution properties of vanadium solid are so chemical additives and/or other additives are generally added to improve the dissolution properties. However, the additives tend to reduce the purity of the electrolyte and thus affect the performance of the vanadium battery. Therefore, the general method for producing a vanadium electrolyte still requires complicated processes to remove the additives. 
     In view of this, it is urgent to provide an electrolytic reduction system of a vanadium electrolyte and a method for producing the electrolyte to overcome the defects of the conventional electrolytic reduction system and method of a vanadium electrolyte. 
     SUMMARY 
     Therefore, one aspect of the present invention is to provide an electrolytic reduction system of a vanadium electrolyte. The electrolytic reduction system comprises a separating device and an electrolytic tank, and can effectively electrolytically reduce pentavalent vanadium ions in a sulfuric acid solution to tetravalent vanadium ions and trivalent vanadium ions, thereby preparing the vanadium electrolyte applicable to vanadium batteries. 
     Another aspect of the present invention is to provide a method for producing a vanadium electrolyte, comprising the following steps: performing a separating process on a sulfuric acid solution containing insoluble vanadium pentoxide solid to obtain a sulfuric acid solution in which pentavalent vanadium ions are dissolved, and reducing the pentavalent vanadium ions by an electrolytic reduction reaction, thereby preparing the vanadium electrolyte for vanadium batteries. 
     According to one aspect of the present invention, an electrolytic reduction system of a vanadium electrolyte is provided. The electrolytic reduction system comprises a separating device and an electrolytic tank. The separating device is configured to separate a mixture consisting of a vanadium pentoxide solid and a sulfuric acid solution, thereby obtaining a vanadium solution from a liquid discharging port of the separating device and a vanadium solid from a solid discharging port of the separating device. The vanadium solution comprises pentavalent vanadium ions. The electrolytic tank comprises a separating membrane, and the separating membrane separates the electrolytic tank into a vanadium solution sub-tank and a sulfuric acid sub-tank. The vanadium solution sub-tank connects to the liquid discharging port of the separating device to contain the vanadium solution. The electrolytic tank is configured to reduce the pentavalent vanadium ions in the vanadium solution to tetravalent vanadium ions and trivalent vanadium ions, and a molar ratio of the tetravalent vanadium ions to the trivalent vanadium ions is (1+/−0.1):(1+/−0.10). 
     According to an embodiment of the present invention, the separating device may be a cyclone separator. 
     According to another embodiment of the present invention, the electrolytic reduction system may optionally comprises a filtering device. The filtering device comprises a liquid inlet and a filtrate outlet, wherein the liquid inlet connects to the liquid discharging port of the separating device, and the filtrate outlet connects to the vanadium solution sub-tank of the electrolytic tank. The filtering device is configured to filter the vanadium solution. 
     According to another embodiment of the present invention, the filtering device may optionally comprise a counterflow circuit, and the counterflow circuit comprises a counterflow feeding pipe and a counterflow discharging pipe. The counterflow feeding pipe connects to the liquid discharging port of the separating device and the filtrate outlet of the filtering device, and the counterflow discharging pipe connects to a feeding port of the separating device and the liquid inlet of the filtering device. 
     According to another embodiment of the present invention, the vanadium solution sub-tank may optionally comprise a voltage detector. The voltage detector is configured to measure a voltage of the vanadium solution to detect the molar ratio of the tetravalent vanadium ions to the trivalent vanadium ions. 
     According to another aspect of the present invention, a method for producing a vanadium electrolyte is provided. The producing method comprises the following steps: first mixing a vanadium pentoxide solid with first sulfuric acid liquid to form a mixture consisting of part of the vanadium pentoxide solid and the sulfuric acid solution, the sulfuric acid solution containing pentavalent vanadium ions; then, performing a separating process on the mixture to separate the part of the vanadium pentoxide solid from the sulfuric acid solution; and then, performing an electrolytic reduction process on the sulfuric acid solution to reduce the pentavalent vanadium ions to tetravalent vanadium ions and trivalent vanadium ions, wherein a molar ratio of the tetravalent vanadium ions to the trivalent vanadium ions is (1+/−0.1):(1+/−0.1). 
     According to an embodiment of the present invention, the separating process is performed by a cyclone separator. 
     According to another embodiment of the present invention, before the electrolytic reduction process is performed, the producing method may selectively perform a filtering process to filter the sulfuric acid solution. 
     According to another embodiment of the present invention, a feeding step may be performed before the electrolytic reduction process to introduce the sulfuric acid solution into the vanadium solution sub-tank of the electrolytic tank, and introduce second sulfuric acid liquid into the sulfuric acid sub-tank of the electrolytic tank. After the feeding step is performed, the sulfuric acid solution is subjected to an electrolytic reduction step so as to form a reduced solution from the sulfuric acid solution. Then, a detecting step is performed on the reduced solution to determine a vanadium ion composition of the reduced solution. If a molar number of the tetravalent vanadium ions is greater than a molar number of the trivalent vanadium ions, the electrolytic reduction step is performed. If the molar number of the tetravalent vanadium ions is less than the molar number of the trivalent vanadium ions, the feeding step is performed. Furthermore, if the molar number of the tetravalent vanadium ions is equal to the molar number of the trivalent vanadium ions, the vanadium electrolyte of the present invention is prepared. 
     According to another embodiment of the present invention, the detecting step is to determine the vanadium ion composition of the reduced solution by measuring a voltage of the reduced solution. 
     By the electrolytic reduction system of the vanadium electrolyte and the method for producing the electrolyte of the present invention, the mixture containing the vanadium pentoxide solid and the sulfuric acid solution may be effectively separated by the separating device, and further the pentavalent vanadium ions in the solution may be effectively reduced by the subsequent electrolytic reduction process. Accordingly, the present invention does not need to add additional chemical additives and/or other additives that can improve the dissolution properties of vanadium pentoxide, and does not require a purification step, so the process flow can be reduced, impurities in the sulfuric acid solution can be reduced, and the efficiency of the electrolytic reduction process can be improved. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       In order to have a more complete understanding of the embodiments and advantages of the present invention, reference is made to the following description and the corresponding drawings. It must be emphasized that the various features are not drawn to scale and are for illustration purposes only. The contents of the relevant diagrams are described as follows: 
         FIG. 1  is a schematic device diagram of an electrolytic reduction system of a vanadium electrolyte according to an embodiment of the present invention. 
         FIG. 2  is a schematic device diagram of an electrolytic reduction system of a vanadium electrolyte according to another embodiment of the present invention. 
         FIG. 3  is a schematic device diagram of an electrolytic reduction system of a vanadium electrolyte according to yet another embodiment of the present invention. 
         FIG. 4  is a schematic flow chart of a method for producing a vanadium electrolyte according to some embodiments of the present invention. 
         FIG. 5  is a schematic flow chart of a method for producing a vanadium electrolyte according to some embodiments of the present invention. 
         FIG. 6  shows the change in an open circuit voltage of a battery under different charging and discharging conditions 
     
    
    
     DETAILED DESCRIPTION OF DISCLOSED EMBODIMENTS 
     The specific implementation of the present invention will be further described below with reference to the accompanying drawings and embodiments. The following embodiments are only used to illustrate the technical solutions of the present invention more clearly, and cannot be used to limit the protection scope of the present invention. 
     The “sulfuric acid liquid” referred to in the present invention refers to an aqueous sulfuric acid solution, and the “sulfuric acid solution” can refer to an aqueous sulfuric acid solution in which a vanadium pentoxide solid is dissolved, or an aqueous sulfuric acid solution in which a vanadium pentoxide solid and other substances are dissolved. 
     Please refer to  FIG. 1 .  FIG. 1  is a schematic device diagram of an electrolytic reduction system of a vanadium electrolyte according to an embodiment of the present invention. The electrolytic reduction system  100  includes a separating device  110  and an electrolytic tank  140 . The separating device  110  and the electrolytic tank  140  are coupled to each other. Specifically, the separating device  110  is coupled to the electrolytic tank  140  via a pipeline  113   a.    
     The separating device  110  includes a feeding port  111 , a liquid discharging port  113  and a solid discharging port  115 . The separating device  110  of the present invention is not particularly limited, and only needs to be used to separate liquid from an insoluble solid in a mixture consisting of the liquid and the insoluble solid to be separated. 
     In some specific embodiments, the separating device  110  of the present invention may be a cyclone separator. The mixture to be separated is introduced into the separating device  110  via the feeding port  111 , the separated insoluble solid is discharged from the solid discharging port  115 , and the separated liquid is discharged from the liquid discharging port  113 . 
     The mixture is prepared by adding vanadium pentoxide solid (V2O5) to sulfuric acid liquid. Because the vanadium pentoxide solid has poor dissolution properties, the vanadium pentoxide solid cannot be completely dissolved in the sulfuric acid liquid. In other words, the mixture is a sulfuric acid solution containing the vanadium pentoxide solid (that is, part of the vanadium pentoxide, this part is insoluble in the sulfuric acid liquid) and pentavalent vanadium ions (that is, the other part of vanadium pentoxide dissolved in the sulfuric acid liquid). The reaction mechanism of the vanadium pentoxide solid dissolved in the sulfuric acid liquid is shown in the following formula (I). 
       2H + +V 2 O 5 ↔2VO 2   + +H 2 O  (I)
 
     The sulfuric acid liquid can be stored in the sulfuric acid tank  120 . When the sulfuric acid liquid needs to be pumped to the separating device  110 , a valve  121  is opened, so the sulfuric acid liquid may be introduced into a pipeline  115   a  via a pipeline  120   a , and further pumped to the feeding port  111  via a pump  151  and a pipeline  115   b . At the same time, a valve  131  may be opened, and the vanadium pentoxide solid stored in a storage tank  130  may be added to the sulfuric acid liquid flowing through the pipeline  115   b  via a pipeline  130   a  to form the mixture. The mixture is pumped to the separating device  110  to be further separated by the separating device  110 . 
     It should be noted that the storage tank  130 , the pipeline  130   a  and the valve  131  shown in  FIG. 1  are merely illustrative. Those of ordinary skill in the technical field to which the present invention belongs can use well-known technical means or unit structures to feed the vanadium pentoxide solid into the sulfuric acid liquid to mix the two and prevent the sulfuric acid liquid in the pipeline  115   b  from flowing back into the storage tank  130 . 
     The separated vanadium pentoxide solid (that is, the part not dissolved in the sulfuric acid liquid) is discharged via the solid discharging port  115 . In order to further improve the utilization rate of the vanadium pentoxide solid, the discharged vanadium pentoxide solid is introduced into the pipeline  115   a  to be mixed with the sulfuric acid liquid pumped into the pipeline  115   a . Similarly, in the pipeline  115   a , the vanadium pentoxide solid may not be completely dissolved in the sulfuric acid liquid. In other embodiments, in the pipeline  115   a , the vanadium pentoxide solid may be completely dissolved in the sulfuric acid liquid. In addition, in other embodiments, the vanadium pentoxide solid discharged from the solid discharging port  115  may also be recovered, subjected to further treatment (such as a drying step), and fed into a storage tank  130  by means of automated equipment or manual transportation. 
     In some embodiments, the sulfuric acid liquid and the vanadium pentoxide solid may also be added to the electrolyte mixing tank. After mixing, the mixture is pumped to the feeding port  111  via a pipeline. In the embodiments, the vanadium pentoxide solid discharged from the solid discharging port  115  may be recovered and fed into the mixing tank and be mixed with the sulfuric acid liquid again. In other embodiments, the sulfuric acid tank  120  and the storage tank  130  may connect to the mixing tank, and the mixing tank further connects to the feeding port  111 . 
     The sulfuric acid solution containing pentavalent vanadium ions discharged from the liquid discharging port  113  may be pumped to a vanadium solution sub-tank  141  of the electrolytic tank  140  via the pipeline  113   a , the pump  153  and the pipeline  113   b . The electrolytic tank  140  includes a vanadium solution sub-tank  141 , a sulfuric acid sub-tank  143  and a separating membrane  140   a , and the separating membrane  140   a  separates the vanadium solution sub-tank  141  from the sulfuric acid sub-tank  143 . It may be understood that the separating membrane  140   a  is not particularly limited, and only needs to not allow vanadium ions in the sulfuric acid solution in the vanadium solution sub-tank  141  to pass through. In the electrolytic tank  140 , a reduction reaction is performed in the vanadium solution sub-tank  141 , and the pentavalent vanadium ions in the sulfuric acid solution may be reduced to tetravalent vanadium ions and trivalent vanadium ions. The reaction mechanism of reduction of the pentavalent vanadium ions to the tetravalent vanadium ions and the trivalent vanadium ions is shown in the following formulas (II) and (III). 
       VO 2   + +2H +   +e   − →VO 2+ +H 2 O  (II)
 
       VO 2+ +2H +   +e   − →V 3+ +H 2 O  (III)
 
     When the pentavalent vanadium ions are reduced to the tetravalent vanadium ions and the trivalent vanadium ions, the sulfuric acid solution (containing the tetravalent vanadium ions and the trivalent vanadium ions) in the vanadium solution sub-tank  141  is guided to the storage tank  145  via the pipeline  141   a , and may be used as the vanadium electrolyte of a vanadium battery. It may be understood that, in some embodiments, the vanadium solution sub-tank  141  may optionally include a detector to measure the molar numbers of the tetravalent vanadium ions and the trivalent vanadium ions in the sulfuric acid solution of the vanadium solution sub-tank  141 , thereby confirming whether the vanadium ion composition (that is, the content of the tetravalent vanadium ions and the trivalent vanadium ions) of the sulfuric acid solution meets the requirements of application. The detector can measure the potential of the sulfuric acid solution to know the composition of vanadium ions in the sulfuric acid solution. In some specific embodiments, according to application requirements, the molar ratio of the tetravalent vanadium ions to the trivalent vanadium ions in the sulfuric acid solution may be 1:0 to 0:1, preferably (1+/−0.5):(1+/−0.5), and more preferably, (1+/−0.1):(1+/−0.1). 
     Further, the sulfuric acid sub-tank  143  is used to store the sulfuric acid liquid required for the electrolysis, and the sulfuric acid liquid in the sulfuric acid sub-tank  143  may be circulated via the pipeline  143   a , the pump  155  and the pipeline  143   b . It may be understood that, according to the feeding method of the sulfuric acid sub-tank  143 , the pipeline  143   a  or the pipeline  143   b  may connect to the sulfuric acid storage tank (not shown), so that the sulfuric acid liquid in the sulfuric acid sub-tank  143  may be circulated. In some embodiments, the pipeline  143   a  or the pipeline  143   b  may connect to the sulfuric acid tank  120 , thereby reducing the number of storage tank units. The arrangement of the pipeline and the valve body is well known to those of ordinary skill in the technical field to which the present invention belongs, so it will not be repeated here. 
     Due to poor dissolution properties of a vanadium pentoxide solid (i.e. pentavalent vanadium ions), to increase the content of tetravalent vanadium ions and trivalent vanadium ions produced by electrolysis, the sulfuric acid liquid used to dissolve the vanadium pentoxide solid has a higher concentration to increase the dissolved amount of the vanadium pentoxide solid. According to the foregoing description, in the separating device  110  and the electrolytic tank  140 , the concentration of the sulfuric acid liquid is not changed. Therefore, the concentration of the sulfuric acid liquid in the sulfuric acid solution stored in the storage tank  145  (that is, the concentration of the sulfuric acid liquid in the prepared electrolyte) is equal to the concentration of the sulfuric acid liquid stored in the sulfuric acid tank  120 . 
     In some embodiments, to make the produced electrolyte meet the application requirements of vanadium batteries, the storage tank  145  may connect to a diluting device (not shown) to reduce the concentration of the sulfuric acid liquid in the electrolyte. In some specific embodiments, the diluting device may be an electrophoresis sulfuric acid separating tank in which sulfate (SO42−) in the electrolyte is replaced with hydroxide (OH—) through an ion exchange membrane, thereby reducing the concentration of the sulfuric acid liquid. In the specific embodiments, the concentration of the sulfuric acid liquid in the electrolyte may be 4 mol (M) to 6 mol (M), and after the dilution process of the electrophoresis sulfuric acid separating tank, the concentration of the sulfuric acid liquid may be 1 mol (M) to 3 mol (M). 
     It should be noted that the tetravalent vanadium ions and trivalent vanadium ions have good dissolution properties, so when the concentration of the sulfuric acid liquid in the electrolyte decreases, the tetravalent vanadium ions and trivalent vanadium ions will not precipitate out. 
     Based on the aforementioned description, it may be understood that the units and pipelines of the electrolytic reduction system  100  of the present invention are made of materials that can withstand acid corrosion. 
     In addition, according to the aforementioned description, the mixture of the present invention only consists of the sulfuric acid solution and the undissolved vanadium pentoxide solid, and does not contain other chemical additives or other additives that may be used to improve the solubility of the vanadium pentoxide solid. Therefore, the sulfuric acid solution after solid-liquid separation does not contain other chemical additives or other additives, and the electrolytic efficiency of the subsequent electrolytic reduction reaction may be further improved. 
     Please refer to  FIG. 2 .  FIG. 2  is a schematic device diagram of an electrolytic reduction system of a vanadium electrolyte according to another embodiment of the present invention. The unit configuration and connection relationship of the electrolytic reduction system  200  are substantially the electrolyte as those of the electrolytic reduction system  100 . The difference between the two electrolytic reduction systems is that the electrolytic reduction system  200  includes a filtering device  260  and a voltage detector  270 . 
     The filtering device  260  may be arranged between the liquid discharging port  213  and the pump  253  to further filter the sulfuric acid solution discharged from the liquid discharging port  213 , thereby ensuring that the sulfuric acid solution does not contain a fine vanadium pentoxide solid, and avoiding reducing the electrolytic efficiency of the electrolytic tank  240 . The liquid discharging port  213  may connect to a liquid inlet  261  of the filtering device  260  via a pipeline  213   a , and a filtrate outlet  263  of the filtering device  260  may connect to a vanadium solution sub-tank  241  of the electrolytic tank  240  via a pipeline  260   a , a pump  253 , and a pipeline  260   b.    
     As the flow of the sulfuric acid solution through the filtering device  260  increases, the vanadium pentoxide solid remaining in the filtering device  260  easily blocks filtering holes, thereby reducing the filtering efficiency of the filtering device  260 . Therefore, in some embodiments, the filtering device  260  may optionally include a removal circuit used to remove the vanadium pentoxide solid remaining in the filtering holes, thereby extending the service life of the filtering device  260 , and improving the efficiency of the filtering device  260 . 
     The removal circuit removes the vanadium pentoxide solid from the filtering device  260  in a counterflow manner. A removal liquid (for example, a sulfuric acid liquid) may be introduced from the filtrate outlet  263  of the filtering device  260 , and the removal liquid and the removed vanadium pentoxide solid may be discharged from the liquid inlet  261 . Since the amount of the vanadium pentoxide solid in the filtering device  260  is very small, the removed vanadium pentoxide solid may generally be dissolved in the removal liquid. 
     The voltage detector  270  is arranged between the vanadium solution sub-tank  241  and the storage tank  245  of the electrolytic tank  240  to detect the vanadium ion composition of the sulfuric acid solution after electrolysis. Specifically, the voltage detector  270  is configured to measure the voltage of the vanadium solution to detect the molar ratio of the tetravalent vanadium ions to the trivalent vanadium ions in the sulfuric acid solution. 
     As shown in  FIG. 2 , although the voltage detector  270  connects to the vanadium solution sub-tank  241  via the pipeline  241   a , the present invention is not limited to this. In some embodiments, a measuring terminal of the voltage detector  270  may directly measure the vanadium ion composition of the sulfuric acid solution in the vanadium solution sub-tank  241 . 
     In other embodiments, the measuring terminal of the voltage detector  270  may be used to measure the vanadium ion composition of the sulfuric acid solution in the pipeline  241   a , and the voltage detector  270  may signally connect to a valve (not shown) arranged in the pipeline  270   a . According to the detection result of the voltage detector  270 , if the vanadium ion composition of the sulfuric acid solution meets the application requirements, the voltage detector  270  may transmit an “open” signal to the valve arranged in the pipeline  270   a  to allow the sulfuric acid solution to flow into the storage tank  245  via the pipeline  270   a.    
     On the contrary, if the vanadium ion composition of the sulfuric acid solution does not meet the application requirements, the voltage detector  270  does not transmit a signal to the valve arranged in the pipeline  270   a . It may be understood that the voltage detector  270  measures the vanadium ion composition of the sulfuric acid solution in the vanadium solution sub-tank  241  by a technical means well known to those of ordinary skill in the technical field to which the present invention belongs, so it will not be repeated here. 
     Please refer to  FIG. 3 .  FIG. 3  is a schematic device diagram of an electrolytic reduction system of a vanadium electrolyte according to another embodiment of the present invention. The unit configuration and connection relationship of the electrolytic reduction system  300  are substantially the electrolyte as those of the electrolytic reduction system  200 . The difference between the two electrolytic reduction systems is that the filtering device  360  of the electrolytic reduction system  300  further includes a removal circuit (not shown in the figure). The removal circuit substantially includes a valve  365 , a valve  367 , a pipeline  360   c , a pipeline  360   d , and a valve  369 . 
     The removal circuit is used to clean the filtering device  360  to remove the vanadium pentoxide solid remaining in the filtering device  360 , thereby extending the service life of the filtering device  360 , and improving the utilization rate of the vanadium pentoxide solid. 
     In the electrolytic reduction system  300 , generally, when the sulfuric acid solution containing pentavalent vanadium ions and a small amount of insoluble vanadium pentoxide solid is discharged from the liquid discharging port  313 , the sulfuric acid solution is first introduced into the pipeline  313   a , the valve  367  is closed and the valve  365  is open, so the sulfuric acid solution may further flow through the filtering device  360  and flow out from the pipeline  360   a  to be pumped to the vanadium solution sub-tank  341  of the electrolytic tank  340 . 
     When the filtering device  360  needs to be cleaned, the valve  365  is closed, and the valve  367  is open, so that the sulfuric acid liquid discharged from the liquid discharging port  313  may flow into the pipeline  360   c , and flow into the filtering device  360  from the filtrate outlet  363  of the filtering device  360  in a counterflow manner to remove the vanadium pentoxide solid remaining on a filtering unit (for example, a filtering plate). Then, the valve  369  is open, so that the removed vanadium pentoxide solid and sulfuric acid liquid may flow back into the pipeline  315   b  via the pipeline  360   d , and the utilization rate of the vanadium pentoxide solid may be improved. Through the design of the internal flow passage of the filtering device  360 , or the addition of a valve at the inlet end (that is, the position where the filtering device  360  is connected) of the pipeline  360   a , the sulfuric acid liquid flowing back into the filtering device  360  cannot flow out of the pipeline  360   a , so as to avoid decrease in the effect of removing the vanadium pentoxide solid. 
     In some embodiments, the sulfuric acid solution (that is, the sulfuric acid liquid in which the removed vanadium pentoxide solid is dissolved) flowing out of the liquid inlet  361  of the filtering device  360  may also be introduced to an additional storage tank or a mixing tank for mixing the sulfuric acid liquid and the vanadium pentoxide solid. 
     Please refer to  FIG. 1  and  FIG. 4  at the same time.  FIG. 4  is a schematic flow chart of a method for producing a vanadium electrolyte according to some embodiments of the present invention. In the method  400 , the sulfuric acid liquid stored in the sulfuric acid tank  120  and the vanadium pentoxide solid in the storage tank  130  are mixed first to form a mixture, as shown in step  410 . Due to the poor dissolution properties of the vanadium pentoxide solid, only part of the vanadium pentoxide solid may be dissolved in the sulfuric acid liquid. In other words, the mixture contains insoluble vanadium pentoxide solid and a sulfuric acid solution, and the sulfuric acid solution contains pentavalent vanadium ions. 
     Then, as shown in step  420 , the mixture is introduced into the separating device  110  through the feeding port  111  of the separating device  110  for being subjected to the separating process, thereby obtaining the separated vanadium pentoxide solid from the solid discharging port  115  of the separating device  110 , and obtaining the separated sulfuric acid solution from the liquid discharging port  113 . Then, the sulfuric acid solution may be pumped to the vanadium solution sub-tank  141  of the electrolytic tank  140  via the pipeline  113   a , the pump  153  and the pipeline  113   b , and further the electrolytic reduction process may be performed to reduce the pentavalent vanadium ions in the sulfuric acid solution to tetravalent vanadium ions and trivalent vanadium ions, and produce the vanadium electrolyte of the present invention, as shown in step  430  and step  440 . 
     In the electrolytic process, vanadium ions are electrolytically reduced from pentavalent ions (VO2+) to tetravalent ions (VO2+), trivalent ions (V3+), and divalent ions (V2+) in sequence. This is because the standard redox potentials from pentavalence to tetravalence, from tetravalence to trivalence, and from trivalence to divalence are 1.0 V, 0.34 V, and −0.26 V, respectively, as shown below. 
       VO2+(aq)+2H+(aq)+ e −↔VO2+(aq)+H2O(l) φ=1.00 V
 
       VO2+(aq)+2H+(aq)+ e −↔V3+(aq)+H2O(l) φ=0.34 V
 
       V3+(aq)+ e −↔V2+(aq) φ=−0.26 V
 
     The electrolytic tank  140  includes a vanadium solution sub-tank  141  and a sulfuric acid sub-tank  143  separated by a separating membrane  140   a . It may be understood that the type of the separating membrane  140   a  is not particularly limited, but the separating membrane  140   a  may effectively block the passage of sulfuric acid molecules and vanadium ions (that is, pentavalent vanadium ions and reduced tetravalent vanadium ions and trivalent vanadium ions) in the sulfuric acid solution in the vanadium solution sub-tank  141 . 
     When step  430  is performed, when the pentavalent vanadium ions are reduced to the tetravalent vanadium ions and the trivalent vanadium ions, the sulfuric acid solution containing the tetravalent vanadium ions and the trivalent vanadium ions may flow into the storage tank  145  via the pipeline  141   a  and be used in a vanadium battery. 
     Please refer to  FIG. 2  and  FIG. 5  at the same time.  FIG. 5  is a schematic flow chart of a method for producing a vanadium electrolyte according to some embodiments of the present invention. The process of the method  500  is substantially the electrolyte as that of the method  400 . The difference between the two methods is that after the separating process (that is, step  520 ), the obtained sulfuric acid solution may be subjected to a filtering process (as shown in step  530 ), and the electrolytic reduction process  540  of the method  500  includes a plurality of operation steps and determination steps, and the steps are described in detail below. 
     When the filtering process is performed, the sulfuric acid solution obtained from the liquid discharging port  213  is introduced into the filtering device  260  via the pipeline  213   a  to further filter out fine insoluble vanadium pentoxide solids in the sulfuric acid solution, thereby improving the efficiency of the subsequent electrolytic reduction process and increasing the utilization rate of vanadium pentoxide. Similarly, via the pipeline  260   a , the pump  253  and the pipeline  260   b , the sulfuric acid solution filtered by the filtering device  260  may be pumped to the vanadium solution sub-tank  241  for being subjected to the subsequent electrolytic reduction process  540 . 
     When the electrolytic reduction process  540  is performed, the feeding step  541  is performed first, so that the sulfuric acid solution may be pumped into the vanadium solution sub-tank  241  via the pipeline  260   b , and the sulfuric acid liquid may be introduced into the sulfuric acid sub-tank  243  via a circulating pipeline. The circulating pipeline includes a pipeline  243   a , a pump  255  and a pipeline  243   b . In some embodiments, the circulating pipeline may include an additional sulfuric acid liquid storage tank to maintain the concentration of the sulfuric acid liquid in the sulfuric acid sub-tank  243  by circulation. 
     Then, electrodes of the electrolytic tank  240  are immersed in the sulfuric acid solution in the vanadium solution sub-tank  241  and the sulfuric acid liquid in the sulfuric acid sub-tank  243 , respectively, and direct current is applied to perform the electrolysis step, thereby reducing the pentavalent vanadium ions in the sulfuric acid solution in the vanadium liquid sub-tank  241  to obtain tetravalent vanadium ions and trivalent vanadium ions, as shown in step  543 . During electrolysis, the valence of vanadium ions gradually changes from pentavalent vanadium ions to tetravalent and trivalent vanadium ions. If the electrolysis continues, the molar ratio of the tetravalent vanadium ions to the trivalent vanadium ions gradually changes from 1:0 to 0:1, and even divalent vanadium ions are produced. 
     When the pentavalent vanadium ions in the sulfuric acid solution are reduced to tetravalent vanadium ions and trivalent vanadium ions, to determine whether the composition desired to be controlled (the ideal composition is tetravalent vanadium ion:trivalent vanadium ion=1:1) is reached in the electrolytic reduction step, the voltage detector  270  performs a detecting step on the electrolytically reduced sulfuric acid solution to determine whether the molar ratio of the tetravalent vanadium ions to the trivalent vanadium ions is 1:1, as shown in step  545  and step  547 . If the molar ratio of the tetravalent vanadium ions to the trivalent vanadium ions in the sulfuric acid solution after the electrolytic reduction is (1+/−0.1):(1+/−0.1), the vanadium electrolyte of the present invention is produced, as shown in step  550 . If the molar ratio of the tetravalent vanadium ions to the trivalent vanadium ions in the sulfuric acid solution after the electrolytic reduction is not (1+/−0.1):(1+/−0.1), whether the molar number of the tetravalent vanadium ions is greater than the molar number of the trivalent vanadium ions is further determined, as shown in step  549 . 
     Since the standard redox potentials from pentavalence to tetravalence, from tetravalence to trivalence, and from trivalence to divalence are 1.0 V, 0.34 V, and −0.26 V, respectively, the determination may be performed by measuring the voltage of a measuring electrode to a reference electrode. 
     If the molar number of the tetravalent vanadium ions is greater than the molar number of the trivalent vanadium ions, the electrolytic reduction step is continued to be performed on the sulfuric acid solution in the vanadium solution sub-tank  241  to reduce part of the tetravalent vanadium ions to trivalent vanadium ions. On the contrary, if the molar number of the tetravalent vanadium ions is less than the molar number of the trivalent vanadium ions, the filtered sulfuric acid solution is introduced into the vanadium solution sub-tank  241  to electrolytically reduce the introduced pentavalent vanadium ions to tetravalent vanadium ions, so that the molar ratio of the tetravalent vanadium ions to the trivalent vanadium ions in the sulfuric acid solution in the vanadium solution sub-tank  241  meets 1:0 to 0:1. 
     In one embodiment, the electrolytic reduction system of the vanadium electrolyte and the method for producing the vanadium electrolyte of the present invention do not need to add chemical additives or other additives to increase the dissolution properties of the vanadium pentoxide solid, and pentavalent vanadium ions in the sulfuric acid solution may be reduced to tetravalent vanadium ions and trivalent vanadium ions by an electrolytic reduction process, thereby producing the vanadium electrolyte meeting the application requirements of vanadium batteries. 
     The following embodiments are used to illustrate the application of the present invention, but are not intended to limit the present invention. Anyone familiar with the art can make various variations and modifications without departing from the spirit and scope of the present invention. 
       FIG. 6  shows the measured open circuit voltage (OCV) of an all-vanadium flow battery under different battery state of charge (SOC) levels. In the process of charge and discharge of batteries, the composition of an electrolyte gradually may change with the charge and discharge time. The measured open circuit voltage of the battery may also change. The principle may be expressed by the following equation. 
     
       
         
           
             E 
             = 
             
               
                 ( 
                 
                   
                     E 
                     
                       0 
                       , 
                       + 
                     
                   
                   - 
                   
                     E 
                     
                       0 
                       , 
                       - 
                     
                   
                 
                 ) 
               
               + 
               
                 
                   
                     R 
                     ⁢ 
                     T 
                   
                   
                     n 
                     ⁢ 
                     F 
                   
                 
                 ⁢ 
                 ln 
                 ⁢ 
                 
                   
                     
                       
                         
                           [ 
                           
                             VO 
                             2 
                             + 
                           
                           ] 
                         
                         ⁡ 
                         
                           [ 
                           
                             H 
                             + 
                           
                           ] 
                         
                       
                       2 
                     
                     ⁡ 
                     
                       [ 
                       
                         V 
                         
                           2 
                           + 
                         
                       
                       ] 
                     
                   
                   
                     
                       [ 
                       
                         VO 
                         
                           2 
                           + 
                         
                       
                       ] 
                     
                     ⁡ 
                     
                       [ 
                       
                         V 
                         
                           3 
                           + 
                         
                       
                       ] 
                     
                   
                 
               
             
           
         
       
     
     In the above equation, E0,+ and E0,− are the positive electrode potential and the negative electrode potential of a battery, respectively. In the process of charge and discharge of the battery, the components in the battery gradually change, and the components include pentavalent vanadium ions (VO2+), tetravalent vanadium ions (VO2+), trivalent vanadium ions (V3+), divalent vanadium ions (V2+), and hydrogen ions (H+). 
     According to the aforementioned description, the electrolytic reduction system of the vanadium electrolyte and the method for producing the electrolyte of the present invention can effectively separate the mixture containing a vanadium pentoxide solid and a sulfuric acid liquid by the separating device to obtain a sulfuric acid solution in which pentavalent vanadium ions are dissolved. Then, the sulfuric acid solution obtained by the separation is pumped to an electrolytic tank to be subjected to an electrolytic reduction process, thereby effectively reducing the pentavalent vanadium ions in the sulfuric acid solution to tetravalent vanadium ions and trivalent vanadium ions. When the molar numbers of the tetravalent vanadium ions and the trivalent vanadium ions are the same, the vanadium electrolyte is obtained. 
     In summary, the electrolytic reduction system of the vanadium electrolyte and the method for producing the electrolyte of the present invention do not require additional chemical additives and other additives to improve the dissolution properties of the vanadium pentoxide solid, so that impurities in the sulfuric acid solution are effectively reduced and a complicated purification step is not required, thereby improving the electrolytic efficiency of the electrolytic reduction process, and producing the vanadium electrolyte meeting the application requirements of vanadium batteries. 
     Although the present invention has been disclosed in the above embodiments, it is not intended to limit the present invention. Anyone of ordinary skill in the technical field to which the present invention belongs can make various variations and modifications without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention shall be subject to the scope of the attached claims.