Apparatus and method for producing vanadium sulfuric acid solution for vanadium redox flow batteries

Abstract

To obtain a vanadium sulfuric acid solution from vanadium pentoxide powder with high efficiency, without using a calcination furnace or reducing agent. [Solution] A cell stack comprising a plurality of unit cells, each comprising a positive electrode cell, an ion exchange membrane, a negative electrode cell, and a bipolar plate in order from the first end, wherein the bipolar plate comprises, in order from the first end, a negative electrode electrode which is a graphite molded plate containing 70% by mass or more of graphite and a positive electrode which is formed by coating a titanium plate with a platinum group metal oxide, and carbon felt is arranged in the negative electrode cell; a positive electrode tank connected to each of the positive electrode cells so as to be able to circulate fluid; a negative electrode tank connected to each of the negative electrode cells so as to be able to circulate fluid; and a reaction tank for reacting vanadium pentoxide powder with a vanadium sulfuric acid solution, is provided as an apparatus for producing a vanadium sulfuric acid solution for a vanadium redox flow battery.

Description

[Technical Field]

[0001] The present invention relates to an apparatus for producing a vanadium sulfuric acid solution for vanadium redox flow batteries and a method for producing a vanadium sulfuric acid solution for vanadium redox flow batteries using the apparatus. [Background technology]

[0002] In vanadium redox flow batteries, vanadium sulfuric acid solution is used for both the positive and negative electrodes. The vanadium redox flow battery charges and discharges as the valence of vanadium ions changes between divalent and trivalent at the negative electrode, and between tetravalent and pentavalent at the positive electrode. Therefore, by efficiently and inexpensively manufacturing vanadium sulfuric acid solution, the cost of vanadium redox flow batteries can be reduced.

[0003] Non-Patent Document 1 summarizes the prior art for manufacturing vanadium sulfuric acid solutions for vanadium redox flow batteries. Non-Patent Document 1 introduces four main methods for manufacturing vanadium sulfuric acid solutions: chemical reduction, electrolysis, solvent extraction, and ion exchange. According to Non-Patent Document 1, chemical reduction is the most common manufacturing method, and it has the advantages of being simple, highly efficient, fast, and easy to scale up. However, this method has the disadvantage of excessively high manufacturing costs because the cost of the reducing agent is incurred as a variable cost. Electrolysis avoids the use of reducing agents and toxic gases and shortens the manufacturing process, but it is deemed unsuitable for mass production due to difficulties in maintaining the suspension of vanadium pentoxide powder, resulting in low production efficiency and the need for a complex filtration system. Solvent extraction has the advantage of directly producing a vanadium sulfuric acid solution without going through vanadium pentoxide. However, it has the problem of emulsification and third-phase formation easily occurring during the extraction process, and a long process flow. Ion exchange methods offer similar advantages to solvent extraction methods, but their limited adsorption capacity makes them unsuitable for high-concentration vanadium-containing solutions. Furthermore, ion exchange resins are easily toxic and difficult to regenerate, resulting in low production efficiency and high manufacturing costs.

[0004] Non-Patent Document 2 also summarizes the prior art for manufacturing vanadium sulfuric acid solutions for vanadium redox flow batteries. According to Non-Patent Document 2, five methods for manufacturing vanadium sulfuric acid solutions are introduced: electrochemical reduction, chemical reduction, thermal reduction, catalytic reduction, and solvent extraction. Non-Patent Document 2 expresses a negative view on the mass production application of the electrochemical reduction method, citing the low solubility of vanadium pentoxide in acidic solutions, slow reaction rate, and complex experimental equipment that leads to energy waste and prolonged processing time. The chemical reduction method is said to be currently adopted at the industrial level due to its simple process, ease of operation, and high speed. Non-Patent Document 2 discloses that in the thermal reduction method, V(V) is reduced to V(IV) even in air at high temperatures of 800°C or higher, and that it is possible to reduce it to V(III) at around 600°C under a protective atmosphere such as N2. Furthermore, a method for producing a 3.5-valent vanadium sulfuric acid solution using this is introduced. However, this reduction process requires high temperatures, necessitating expensive furnaces with controlled atmospheres, resulting in high energy consumption, increased variable costs, and significant capital investment. Catalytic reduction is presented as an innovative technology combining hydrogen or formic acid with a Pt / C or alloyed Pt catalyst, and involves the synthesis of vanadium sulfuric acid solution from vanadium pentoxide or vanadium sulfate. However, there is no evidence of industrial prototypes being made, making even cost estimation difficult. Solvent extraction is a method for directly producing vanadium sulfuric acid solution without going through vanadium pentoxide. In the third stage of the extraction process, emulsification separates the hydrophilic and hydrophobic phases, and the desired V ions are extracted by back extraction. However, the degradation of the organic solvent generates a large amount of waste. Considering the disposal costs, mass production is difficult.

[0005] In summary, the manufacturing method for vanadium sulfuric acid solution requires the use of vanadium pentoxide powder as a raw material. However, vanadium pentoxide powder has almost no solubility in sulfuric acid solution. Therefore, vanadium is generally reduced from 5-valent to 4-valent or lower before dissolving in sulfuric acid solution. The vanadium in vanadium pentoxide powder is generally reduced by calcining the vanadium pentoxide powder at high temperatures or by using a reducing agent. However, calcining vanadium pentoxide powder at high temperatures requires a calcination furnace, resulting in high equipment costs and a significant increase in energy costs. Furthermore, using a reducing agent requires the same number of moles of reducing agent as vanadium, leading to increased running costs.

[0006] According to Non-Patent Document 1, chemical reduction is the most prevalent method, thus denying the possibility of mass production using electrochemical reduction. According to Non-Patent Document 2, solvent extraction is considered more promising, and electrochemical reduction is deemed unsuitable for mass production. In response to these claims, Patent Documents 1 and 2 propose improvements to the electrochemical method.

[0007] Patent Document 1 discloses a circulating electrolytic reactor comprising a V2O5 powder supply reactor, a sedimentation tank, and an electrolytic cell, as well as a method for producing a vanadium sulfuric acid solution for a vanadium flow battery using this apparatus. In this apparatus, a graphite plate is used for the negative electrode, while a DSA (Dimensionally Stable Anode) is used for the positive electrode, which is a titanium plate coated with iridium oxide or the like, in order to reduce the overpotential for oxygen generation to decompose water with sulfuric acid at the positive electrode. However, there is no description of the bipolar plates. Referring to the drawings, there is one inlet and one outlet, so the same liquid passes through both sides of the membrane, and a suspension of vanadium pentoxide is introduced. Patent Document 1 only describes the result of obtaining a trivalent vanadium sulfuric acid solution. The current density used in the example in Patent Document 1 is 1500 A / m². 2 That is the case.

[0008] Patent Document 2 proposes a manufacturing apparatus that separates the tank for dissolving vanadium pentoxide powder from the tank for electrolysis. Therefore, although not described, it is likely that there are two inlets and two outlets. Patent Document 1 only reduces vanadium to a valence of 3, but Patent Document 2 electrolyzes it to a valence of 2.3. The maximum current density is 200 mA / cm². 2 Preferably 120 mA / cm² 2 In Patent Document 2, the example shows that only electrolysis is performed in a single cell, and there is no description whatsoever regarding the bipolar plate. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Chinese Patent Application Publication Number: CN102011135 [Patent Document 2] Chinese Patent Application Publication Number: CN118497769 [Non-patent literature]

[0010] [Non-Patent Document 1] Yun Guo, Jie Huang, Jun-Kai Feng, “Research progress in preparation of electrolyte for all-vanadium redox flow battery”, Journal of Industrial and Engineering Chemistry, 118, (2023), 33-43 [Non-Patent Document 2] Muneeb Ul Hassan Naseer, Bo Pan, Shaona Wang, Yeqing Lyu, Biao Liu, Lanjie Li, Jian Qi, Hao Du, “A comprehensive review of advancements in vanadium electrolyte preparation for Vanadium Redox Flow Batteries”, Journal of Environmental Chemical Engineering, 13, (2025), 118402

Summary of the Invention

Problems to be Solved by the Invention

[0011] In the inventions disclosed in Patent Documents 1 and 2, it is possible to obtain a vanadium sulfate solution from vanadium pentoxide powder without using a high-cost firing furnace or a reducing agent that increases the running cost. However, it cannot be said that the inventions disclosed in Patent Documents 1 and 2 can obtain a vanadium sulfate solution with sufficient efficiency. For example, the current density actually adopted in the examples in Patent Document 1 is 1500 A / m 2 On the other hand, the current density actually adopted in the examples in Patent Document 2 is 100 mA / cm 2 , that is, 1000 A / m 2 . At these current densities, the equipment investment amount of the electrolysis device also becomes quite large, and it cannot be said to be a sufficient current density. If a higher current density can be applied, it is possible to obtain a vanadium sulfate solution from vanadium pentoxide powder in a shorter time. That is, if the available current density doubles, the amount of vanadium sulfate solution that can be produced with the same equipment doubles, so the equipment cost is halved. Also, by using a multi-layer cell structure, the investment efficiency of the electrolysis equipment increases as the number of cells increases, but for this, it is necessary to use a bipolar plate structure. However, there is no description about the bipolar plate in these Patent Documents 1 and 2.

[0012] The object of the present invention is to obtain a vanadium sulfate solution with high efficiency from vanadium pentoxide powder without using a firing furnace or a reducing agent.

Means for Solving the Problems

[0013] The inventors of the present invention have found that by making the cell stack have a specific configuration, it becomes possible to apply a larger current density to the cell stack, and by making it a more efficient multilayer structure, a very efficient cell stack can be made, and thus the present invention has been completed.

[0014] (1) A first aspect of the present invention is a cell stack including a plurality of unit cells each including a positive electrode cell, an ion exchange membrane, a negative electrode cell, and a bipolar plate in this order from a first end side, wherein the bipolar plate includes a negative electrode which is a graphite formed plate containing 70% by mass or more of graphite, and a positive electrode formed by coating a platinum group metal oxide on a titanium plate, and a carbon felt is disposed in the negative electrode cell, a positive electrode tank fluidly connected to be able to circulate fluid between each of the positive electrode cells, a negative electrode tank fluidly connected to be able to circulate fluid between each of the negative electrode cells, and a reaction tank for reacting vanadium pentoxide powder with a vanadium sulfate solution, and is a vanadium sulfate solution production apparatus for a vanadium redox flow battery.

[0015] (2) A second aspect of the present invention is a cell stack comprising a plurality of unit cells, each comprising a positive electrode cell, an ion exchange membrane, a negative electrode cell, and a bipolar plate in order from the first end, wherein the bipolar plate comprises, in order from the first end, a negative electrode electrode which is a graphite molded plate containing 70% by mass or more of graphite, and a positive electrode which is formed by coating a titanium plate with a platinum group metal oxide, and carbon felt is arranged in the negative electrode cell, a positive electrode tank connected to each of the positive electrode cells so as to be able to circulate fluid, and a negative electrode tank connected to each of the negative electrode cells so as to be able to circulate fluid, and a vanadium sulfuric acid solution production apparatus for vanadium redox flow batteries comprising a cell stack, a positive electrode tank connected to each of the negative electrode cells so as to be able to circulate fluid, and a vanadium sulfuric acid solution tetravalent or a first trivalent vanadium sulfuric acid solution circulating between the negative electrode tank and each of the negative electrode cells, a sulfuric acid solution circulating between the positive electrode tank and each of the positive electrode cells, and a flow rate of 6000 A / m² through the cell stack from the first end toward the second end opposite to the first end. 2 A method for producing a vanadium sulfuric acid solution for a vanadium redox flow battery, comprising the steps of passing the following current through each of the negative electrode cells to electrolyze the tetravalent vanadium sulfuric acid solution or the first trivalent vanadium sulfuric acid solution to produce a second trivalent vanadium sulfuric acid solution or a divalent vanadium sulfuric acid solution, wherein nitrogen gas is passed through the positive electrode tank and the negative electrode tank.

[0016] (3) A third aspect of the present invention is a cell stack comprising a plurality of unit cells, each having a positive electrode cell, an ion exchange membrane, a negative electrode cell, and a bipolar plate in order from the first end, wherein the bipolar plate comprises, in order from the first end, a negative electrode electrode which is a graphite molded plate containing 70% by mass or more of graphite and a positive electrode which is formed by coating a titanium plate with a platinum group metal oxide, and carbon felt is arranged in the negative electrode cell, a positive electrode tank connected to each of the positive electrode cells so as to be able to circulate fluid, a negative electrode tank connected to each of the negative electrode cells so as to be able to flow fluid, and a method for reacting vanadium pentoxide powder with a vanadium sulfuric acid solution. Using a vanadium redox flow battery vanadium sulfuric acid solution production apparatus equipped with a reaction tank, the steps are: reacting vanadium pentoxide powder with a first divalent vanadium sulfuric acid solution in the reaction tank to produce a tetravalent vanadium sulfuric acid solution or a first trivalent vanadium sulfuric acid solution; circulating the tetravalent vanadium sulfuric acid solution or the first trivalent vanadium sulfuric acid solution between the negative electrode tank and each of the negative electrode cells, circulating the sulfuric acid solution between the positive electrode tank and each of the positive electrode cells, and applying 6000 A / m to the cell stack from the first end towards the second end opposite to the first end. 2 A method for producing a vanadium sulfuric acid solution for a vanadium redox flow battery, comprising the steps of: electrolyzing the tetravalent vanadium sulfuric acid solution or the first trivalent vanadium sulfuric acid solution in each of the negative electrode cells by passing the following current to produce a second trivalent vanadium sulfuric acid solution or a second divalent vanadium sulfuric acid solution, wherein nitrogen gas is passed through the positive electrode tank and the negative electrode tank, but nitrogen gas is not passed through the reaction tank.

[0017] (4) A fourth aspect of the present invention is a cell stack including a plurality of unit cells each including a positive electrode cell, an ion exchange membrane, a negative electrode cell, and a bipolar plate in this order from the first end side, wherein the bipolar plate includes, in this order from the first end side, a negative electrode made of a graphite molded plate containing 70% by mass or more of graphite, and a positive electrode formed by coating a platinum group metal oxide on a titanium plate, and a carbon felt is disposed in the negative electrode cell, a positive electrode tank fluidly connected to be circulable between each of the positive electrode cells, a negative electrode tank fluidly connected to be circulable between each of the negative electrode cells, and a reaction tank for reacting vanadium pentoxide powder with a vanadium sulfate solution. Using a vanadium sulfate solution production apparatus for a vanadium redox flow battery, a first tetravalent vanadium sulfate solution or a first trivalent vanadium sulfate solution is circulated between the negative electrode tank and each of the negative electrode cells, a sulfuric acid solution is circulated between the positive electrode tank and each of the positive electrode cells, and from the first end side toward the second end side opposite to the first end side, 6000 A / m 2 The following current is passed through to electrolyze the first tetravalent vanadium sulfate solution or the first trivalent vanadium sulfate solution in each of the negative electrode cells to produce a first divalent vanadium sulfate solution. A first step, a second step of reacting vanadium pentoxide powder with the first divalent vanadium sulfate solution in the reaction tank to produce a second tetravalent vanadium sulfate solution or a second trivalent vanadium sulfate solution, circulating the second tetravalent vanadium sulfate solution or the second trivalent vanadium sulfate solution between the negative electrode tank and each of the negative electrode cells, circulating the sulfuric acid solution between the positive electrode tank and each of the positive electrode cells, and from the first end side toward the second end side, 6000 A / m 2 The following current is passed through to electrolyze the second tetravalent vanadium sulfate solution or the second trivalent vanadium sulfate solution in each of the negative electrodes to produce a third trivalent vanadium sulfate solution or a second divalent vanadium sulfate solution. A third step, and nitrogen gas is passed through the positive electrode tank and the negative electrode tank, and nitrogen gas is not passed through the reaction tank. This is a method for producing a vanadium sulfate solution for a vanadium redox flow battery.

[0018] (5) A fifth aspect of the present invention is an electrode for a vanadium sulfuric acid solution production apparatus for a vanadium redox flow battery, comprising a graphite-molded plate containing graphite and a plate in which a platinum group metal oxide is coated on a titanium plate. [Effects of the Invention]

[0019] According to the present invention, it is possible to obtain a vanadium sulfuric acid solution from vanadium pentoxide powder with high efficiency without using a calcination furnace or reducing agent. [Brief explanation of the drawing]

[0020] [Figure 1] This figure shows a schematic of the cell stack included in the vanadium redox flow battery vanadium sulfuric acid solution production apparatus of the present invention. [Figure 2] This is a schematic diagram of the vanadium sulfuric acid solution production apparatus for vanadium redox flow batteries according to the present invention. [Modes for carrying out the invention]

[0021] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

[0022] (Cell stack 20) Figure 1 is a schematic diagram of the cell stack 20 provided in the vanadium sulfuric acid solution manufacturing apparatus 10 for vanadium redox flow batteries of the present invention. As shown in Figure 1, the cell stack 20 comprises a plurality of unit cells 30, each containing a positive electrode cell 40, an ion exchange membrane 50, a negative electrode cell 60, and a bipolar plate 80, in that order from left to right (first end side). The bipolar plate 80 includes, in that order from left to right, a negative electrode electrode 90 which is a graphite molded plate containing 70% by mass or more of graphite, and a positive electrode electrode 100 which is formed by coating a titanium plate with a platinum group metal oxide. The positive electrode 100 is located to the left of the leftmost unit cell 30, and a positive electrode current collector plate 110 is located to the left of the positive electrode 100. On the other hand, to the right of the rightmost unit cell 30, the positive electrode cell 40, ion exchange membrane 50, negative electrode cell 60, negative electrode 90, and negative electrode current collector plate 120 are located in that order from left to right. Each of the positive electrode cell 40, ion exchange membrane 50, negative electrode cell 60, bipolar plate 80, negative electrode 90, positive electrode 100, positive electrode current collector plate 110, and negative electrode current collector plate 120 has a planar structure in Figure 1 where the vertical direction of the paper is the vertical direction and the direction perpendicular to the paper is the horizontal direction. The positive electrode current collector plate 110 and the negative electrode current collector plate 120 are connected to a power source and are configured to allow electricity to flow from the positive electrode current collector plate 110 to the negative electrode current collector plate 120.

[0023] As described above, the cell stack 20 has multiple unit cells 30. Specifically, it is preferable to have 10 to 100 unit cells 30, and more preferably 10 to 40 unit cells 30.

[0024] (Ion exchange membrane 50) The ion exchange membrane 50 separates the positive electrode cell 40 and the negative electrode cell 60, and H + Or OH - These are so-called ion exchange membranes that allow ions to pass through. There are two types of ion exchange membranes: cation exchange membranes and anion exchange membranes. Anion exchange membranes are advantageous in that they do not allow vanadium ions to pass through, but fluorine-based cation exchange membranes are advantageous in terms of durability. Representative cation exchange membranes include the Nafion series developed by DuPont (now sold by Chemours). Alternatively, AGC's FORBLUE S series is also widely used.

[0025] (Negative electrode 90) The negative electrode 90 is a graphite molded plate containing 70% by mass or more of graphite. More preferably, the negative electrode 90 contains 80% to 95% by mass of graphite. The remainder is resin. For use as an electrode, it needs to have low electrical resistance, and therefore, the more graphite added, the better. However, increasing the amount of graphite added tends to make the molded body brittle and prevent it from achieving the required strength. It also tends to become less susceptible to plastic deformation. The negative electrode 90 preferably has a volume resistivity of 0.5 mΩm or less, and more preferably 0.1 mΩm or less. In this invention, since it is used in a state with very strong acidic oxidation-reduction potential, the usable resins are limited, and polyvinylidene fluoride (PVDF) or polypropylene (PP) are preferred. Polyolefins other than PP may also be added. Although the manufacturing method is different, phenolic resin can also be used.

[0026] (Positive electrode 100) The positive electrode 100 is a metal plate formed by coating a titanium plate with a platinum group metal oxide. Generally, anodes commonly sold as DSA (Dimensionally Stable Anode) can be used. DSA is also a trademark of Denora Permeric, and Denora Permeric has an overwhelmingly large market share. There are types for oxygen generation and chlorine generation, and both can be used, but in this invention, the oxygen generation type is naturally advantageous.

[0027] (Dynamo plate 80) As described above, the bipolar plate 80 includes a negative electrode 90 and a positive electrode 100. The negative electrode 90 and the positive electrode 100 can be integrated by fusion or crimping. Alternatively, the negative electrode 90 and the positive electrode 100 can simply be placed on top of each other, but it is preferable to press them together with a predetermined pressure. If the negative electrode 90 is a graphite plate manufactured by calcining coke, it has low resistance, which is a good characteristic, but it also has the characteristic of not undergoing any plastic deformation. Since the surface of the positive electrode 100 is not perfectly flat, the contact points with the positive electrode 100 will be localized in a graphite plate that does not undergo plastic deformation. Localized contact is extremely dangerous because the current density becomes extremely large locally when a large current is passed through it. In contrast, a graphite molded plate made by mixing graphite powder with resin is capable of plastic deformation and can be plastically deformed by applying pressure. The pressure required to plastically deform the negative electrode 90 described above is approximately 0.01 MPa.

[0028] (Positive electrode cell 40) The positive electrode cell 40 is a space formed between the plane of the positive electrode 100 and the plane of the ion exchange membrane 50 in Figure 1, and is configured to allow fluid to flow throughout this entire space. The positive electrode cell 40 is a space partitioned by the positive electrode 100, the ion exchange membrane 50, and a frame (not shown). The positive electrode cell 40 is equipped with a fluid inlet and a fluid outlet, and is configured so that fluid flowing in from the fluid inlet can flow through the positive electrode cell 40 and flow out to the fluid outlet.

[0029] (Negative electrode cell 60) The negative electrode cell 60 is a space formed between the plane of the ion exchange membrane 50 and the plane of the negative electrode 90 in Figure 1, and is configured to allow fluid to flow throughout this entire space. The negative electrode cell 60 is a space partitioned by the ion exchange membrane 50, the negative electrode 90, and a frame (not shown). Carbon felt 70 is placed inside the negative electrode cell 60. The negative electrode cell 60 is equipped with a fluid inlet and a fluid outlet, and is configured so that fluid flowing in from the fluid inlet can flow through the negative electrode cell 60 and flow out to the fluid outlet.

[0030] (Carbon Felt 70) The carbon felt 70 is a component formed from carbon fibers with sufficient conductivity into a felt-like material. Examples of carbon felt 70 suitable for use in the present invention include SGL GFD 4.65EA manufactured by SGL Carbon. In the present invention, the carbon felt 70 is placed in the negative electrode cell 60. The carbon felt 70 is in direct contact with the negative electrode 90.

[0031] By bringing the carbon felt 70 into direct contact with the negative electrode 90, the carbon felt 70 acts as part of the negative electrode 90. The graphite forming the negative electrode 90 and the carbon fibers forming the carbon felt 70 have high hydrogen overpotentials, and since the carbon fibers forming the carbon felt 70 are spread throughout the entire negative electrode cell 60, vanadium ions in the vanadium sulfuric acid solution can receive electrons from the carbon felt 70 before hydrogen ions in the vanadium sulfuric acid solution, making it possible to effectively reduce vanadium.

[0032] (Positive electrode current collector plate 110 and negative electrode current collector plate 120) The positive electrode current collector plate 110 and the negative electrode current collector plate 120 are components for conducting current in the cell stack 20, with the positive electrode current collector plate 110 on the positive side and the negative electrode current collector plate 120 on the negative side. It is preferable that the positive electrode current collector plate 110 and the negative electrode current collector plate 120 are good conductors. Therefore, it is preferable that the positive electrode current collector plate 110 and the negative electrode current collector plate 120 are copper plates. Gold plating may also be applied to the surface to minimize contact resistance.

[0033] (Vanadium sulfuric acid solution production apparatus 10 for vanadium redox flow batteries) Figure 2 is a schematic diagram of the vanadium sulfuric acid solution production apparatus 10 for vanadium redox flow batteries according to the present invention. The vanadium sulfuric acid solution production apparatus 10 for vanadium redox flow batteries comprises a cell stack 20, a positive electrode tank 130, and a negative electrode tank 140.

[0034] The fluid outlet of the positive electrode tank 130 is connected via piping 301 to the fluid inlet of each positive electrode cell 40 of the cell stack 20, allowing fluid to flow. A pump 201 is provided in piping 301, and valves 405 and 406 are provided upstream and downstream of the pump 201, respectively, along the flow path of piping 301. The fluid outlet of each positive electrode cell 40 is connected via piping 303 to the first fluid inlet of the positive electrode tank 130, allowing fluid to flow. Preferably, a gas-liquid separator 210 is provided along the flow path of piping 303, before the positive electrode tank 130. The pump 201 provided in piping 301 allows the fluid to circulate between the positive electrode tank 130 and each positive electrode cell 40. Preferably, the positive electrode tank 130 has a sealed structure, and separate nitrogen inlet and exhaust passages are provided.

[0035] The fluid outlet of the negative electrode tank 140 is connected via piping 302 to the fluid inlet of each negative electrode cell 60 of the cell stack 20, allowing fluid to flow through it. A pump 202 is provided in piping 302, and valves 401 and 402 are provided upstream and downstream of the pump 202, respectively, along the flow path of piping 302. Downstream of valve 402, piping 306 is connected along the flow path of piping 302, forming a branching path with this connection point as the branching point. The fluid flowing through piping 302 can branch off to piping 306 from this connection point. Downstream of the connection point with piping 306, piping 302 is provided with valve 403. Downstream of valve 403, piping 302 is connected to the fluid inlet of each negative electrode cell 60. A valve 404 is provided along the flow path of piping 306, and downstream of valve 404, piping 306 is connected to the first fluid inlet of the reaction tank 150, which will be described later, allowing fluid to flow through it. The fluid outlet of each negative electrode cell 60 is connected to the first fluid inlet of the negative electrode tank 140 via piping 304, allowing fluid to flow through it. A pump 202 provided in piping 302 allows the fluid to circulate between the negative electrode tank 140 and each negative electrode cell 60. The negative electrode tank 140 is preferably a sealed structure, with separate nitrogen inlet and exhaust passages.

[0036] The vanadium sulfuric acid solution production apparatus 10 for vanadium redox flow batteries includes a reaction tank 150 separate from the tank used for electrolysis. The fluid outlet of the reaction tank 150 is connected to the second fluid inlet of the negative electrode tank 140 via piping 305, allowing fluid to flow through it. A pump 203 is provided in piping 305, and valves 407 and 408 are provided upstream and downstream of the pump 203, respectively, along the flow path of piping 305. Furthermore, piping 350 is connected downstream of valve 408 along the flow path of piping 305, forming a branching path with this connection as the branching point. The fluid flowing through piping 305 can branch off to piping 350 from this branching point. Valves 410 and 411 are provided in that order downstream of the branching point with piping 350 along the flow path of piping 305. Downstream of valve 411 along the flow path of piping 305 is connected to the second fluid inlet of the negative electrode tank 140. A valve 409 is provided downstream of the point where pipe 350 is connected to pipe 305, and further downstream is a flow path for discharging the fluid flowing through pipe 350 as a product. A feeder 160 is provided above the reaction tank 150 for introducing vanadium pentoxide powder into the reaction tank 150. Since it is impossible to make the reaction tank 150 a sealed structure, it is used in an open state. However, measures are necessary to prevent the scattering of vanadium pentoxide powder. An agitator 460 is provided inside the reaction tank 150, and this agitator 460 can be driven by a motor 450 provided on top of the reaction tank 150. By driving the agitator 460 with the motor 450, the fluid in the reaction tank 150 can be stirred.

[0037] The vanadium sulfuric acid solution production apparatus 10 for vanadium redox flow batteries includes a first sulfuric acid tank 180 which serves as the source of concentrated sulfuric acid. Concentrated sulfuric acid is stored in the first sulfuric acid tank 180. One end of a fluid-flowable pipe 307 is inserted into the first sulfuric acid tank 180. The other end of the pipe 307 is connected to the first fluid inlet of the second sulfuric acid tank 170 so that fluid can flow through it. A metering pump 205 is provided in the pipe 307, which can inject a predetermined amount of concentrated sulfuric acid from the first sulfuric acid tank 180 into the second sulfuric acid tank 170.

[0038] The vanadium sulfuric acid solution production apparatus 10 for vanadium redox flow batteries includes a second sulfuric acid tank 170 for adjusting the concentration of the sulfuric acid solution to a predetermined concentration. A stirring device 480 is provided in the second sulfuric acid tank 170, and this stirring device 480 is drivable by a motor 470 mounted on the second sulfuric acid tank 170. By driving the stirring device 480 with the motor 470, the fluid in the second sulfuric acid tank 170 can be stirred. The fluid outlet of the second sulfuric acid tank 170 is connected to the second fluid inlet of the reaction tank 150 via piping 308, allowing fluid to flow through it. A pump 204 is provided in piping 308. Valves 412 and 413 are provided upstream and downstream of the pump 204 in the flow path of piping 308, respectively. Downstream of valve 413 in the flow path of piping 308 is connected to the second fluid inlet of the reaction tank 150. Pipe 308 is connected to pipe 309 downstream of pump 204 and upstream of valve 413, on the flow path of pipe 308, so that the fluid flowing through pipe 308 can be diverted to pipe 309 at the connection point. As described above, one end of pipe 309 is connected to pipe 308, and the other end is connected to the second fluid inlet of the positive electrode tank 130 so that fluid can flow through it. Valves 414 and 415 are provided in this order on the flow path of pipe 309.

[0039] The positive electrode tank 130, negative electrode tank 140, and second sulfuric acid tank 170 of the vanadium redox flow battery vanadium sulfuric acid solution production apparatus 10 are connected to a pure water source. One end of piping 310 is connected to the pure water source so that fluid can flow through it. The other end of piping 310 is connected to the third fluid inlet of the negative electrode tank 140 so that fluid can flow through it. Valves 416 and 418 are provided on piping 310 in that order. On the flow path of piping 310, downstream of valve 416 and upstream of valve 418, piping 312 is connected, so that the fluid flowing through piping 310 can be diverted to piping 312 at this connection point. On the flow path of piping 310, downstream of the connection point of piping 312 and upstream of valve 418, piping 311 is connected, so that the fluid flowing through piping 310 can be diverted to piping 311 at this connection point. As described above, one end of pipe 312 is connected to pipe 310, and the other end is connected to the third fluid inlet of the positive electrode tank 130 so that fluid can flow through it. A valve 419 is provided in the flow path of pipe 312. As described above, one end of pipe 311 is connected to pipe 310, and the other end is connected to the second fluid inlet of the second sulfuric acid tank 170 so that fluid can flow through it. A valve 417 is provided in the flow path of pipe 311.

[0040] (Embodiment 1) Embodiment 1 is a method for producing a vanadium sulfuric acid solution for a vanadium redox flow battery according to the first embodiment of the present invention. Embodiment 1 uses the cell stack 20, positive electrode tank 130, and negative electrode tank 140 shown in Figures 1 and 2.

[0041] In Embodiment 1, a tetravalent or trivalent vanadium sulfuric acid solution is prepared in the negative electrode tank 140. The tetravalent or trivalent vanadium sulfuric acid solution may be injected via piping 305. After that, valve 411 is closed. The sulfuric acid solution is then injected into the positive electrode tank 130 from piping 309. The sulfuric acid solution can be prepared by the following method: A predetermined amount of concentrated sulfuric acid stored in the first sulfuric acid tank 180 is injected into the second sulfuric acid tank 170 via piping 307 using a metering pump 205. Also, with valves 416 and 417 open and valves 418 and 419 closed, a predetermined amount of pure water is injected into the second sulfuric acid tank 170 from a pure water source via piping 310 and 311. The agitator 480 is driven by motor 470 to agitate the mixed solution of concentrated sulfuric acid and pure water in the second sulfuric acid tank 170 to obtain a predetermined amount of sulfuric acid solution of a predetermined concentration. After injecting the sulfuric acid solution into the positive electrode tank 130 via piping 309, valve 415 is closed. Then, pump 202 circulates the tetravalent or trivalent vanadium sulfuric acid solution between the negative electrode tank 140 and each negative electrode cell 60. The tetravalent or trivalent vanadium sulfuric acid solution circulates through piping 302 and piping 304 in the direction indicated by the arrows in Figure 2. Additionally, pump 201 circulates the sulfuric acid solution between the positive electrode tank 130 and each positive electrode cell 40. The sulfuric acid solution circulates through piping 301 and piping 303 in the direction indicated by the arrows in Figure 2. Furthermore, current is passed through the cell stack 20 from the positive electrode current collector plate 110 toward the negative electrode current collector plate 120 to electrolyze the tetravalent or trivalent vanadium sulfuric acid solution in each negative electrode cell 60. When the tetravalent vanadium sulfuric acid solution is electrolyzed, a trivalent or divalent vanadium sulfuric acid solution is produced, and when the trivalent vanadium sulfuric acid solution is electrolyzed, a divalent vanadium sulfuric acid solution is produced. At this time, valves 404, 411, 415, 418 and 419 are kept closed, and valves 401, 402, 403, 405 and 406 are kept open. By keeping each valve in this state, the sulfuric acid solution circulation system and the vanadium sulfuric acid solution circulation system can be kept in a sealed state. At this time, nitrogen is flowed through the positive electrode tank 130 and the negative electrode tank 140. Furthermore, the oxygen gas generated in the positive electrode cell 40 flowing through the piping 303 is separated by the gas-liquid separator 210, and the separated mixture of oxygen and nitrogen gas is exhausted.

[0042] The obtained trivalent or divalent vanadium sulfuric acid solution, for example, the trivalent vanadium sulfuric acid solution, is moved from the negative electrode tank 140 to the reaction tank 150 by opening valves 401, 402, and 404 and closing valve 403, and operating pump 202. Next, the trivalent vanadium sulfuric acid solution can be discharged as a product through piping 350 by opening valves 407, 408, and 409 and closing valve 410, and operating pump 203. The obtained divalent vanadium sulfuric acid solution is used to react with vanadium pentoxide powder. Alternatively, the obtained trivalent vanadium sulfuric acid solution may be used to circulate between the negative electrode tank 140 and each negative electrode cell 60.

[0043] The current density applied to cell stack 20 is 7000 A / m². 2 It is preferable to keep it below 2000 A / m 2 ~6000A / m 2 It is preferable to do so. 7000 A / m 2 When the voltage exceeds a certain level, the electrolytic voltage increases, power efficiency decreases, and heat generation increases, posing a risk of the temperature exceeding 80°C. On the other hand, 2000 A / m 2 If the value is less than the required amount, the electrolysis process will take a long time, reducing the amount that can be produced from the same equipment and increasing costs.

[0044] The reaction that occurs in the negative electrode cell 60 at this time is as follows: V 4+ +e - →V 3+ , and / or, V 4+ +2e - →V 2+ Furthermore, the V generated by the electrolysis described above 3+ Further electrolysis can lead to the next reaction. V 3+ +e - →V 2+

[0045] Furthermore, the reaction that occurs in the positive electrode cell 40 at this time is as follows: 2H₂O → O₂ + 4H + +4e -

[0046] (Embodiment 2) Embodiment 2 is a method for producing a vanadium sulfuric acid solution for a second vanadium redox flow battery according to the present invention.

[0047] In Embodiment 2, a divalent vanadium sulfuric acid solution is prepared in the reaction tank 150. Then, vanadium pentoxide powder is introduced into the reaction tank 150 from the feeder 160, and the vanadium pentoxide powder is reacted with the divalent vanadium sulfuric acid solution in the reaction tank 150 to produce a tetravalent or trivalent vanadium sulfuric acid solution. The reaction that takes place in the reaction tank at this time is as follows. V2O5(s)+4VSO4(aq)+5H2SO4(aq)→3V2(SO4)3(aq)+5H2O(l), V2O5(s)+2VSO4(aq)+3H2SO4(aq)→4VOSO4(aq)+3H2O(l)

[0048] By injecting sulfuric acid from the second sulfuric acid tank 170 via piping 308, the sulfuric acid concentration and amount of the resulting tetravalent or trivalent vanadium sulfuric acid solution in the reaction tank 150 can be adjusted. Furthermore, some or all of the resulting tetravalent or trivalent vanadium sulfuric acid solution can be discharged as a product from the reaction tank 150 via piping 305 and 350 by closing valve 410, opening valves 407, 408, and 409, and operating pump 203.

[0049] Next, valve 409 is closed, and valves 407, 408, 410, and 411 are opened to operate pump 203, transferring the portion of the obtained tetravalent or trivalent vanadium sulfuric acid solution that was not discharged as a product from reaction tank 150 to negative electrode tank 140. After that, valve 411 is closed. At this time, the sulfuric acid concentration and amount of the tetravalent or trivalent vanadium sulfuric acid solution in the negative electrode tank 140 can be adjusted by injecting pure water into the negative electrode tank 140 via piping 310. Also, the sulfuric acid solution is injected into the positive electrode tank 130 via piping 309. After that, valve 415 is closed. At this time, the sulfuric acid concentration and amount of the sulfuric acid solution in the positive electrode tank 130 can be adjusted by injecting sulfuric acid from the second sulfuric acid tank 170 via piping 308 and 309, and / or pure water via piping 310 and 312 into the positive electrode tank 130. Subsequently, pump 202 circulates a tetravalent or trivalent vanadium sulfuric acid solution between the negative electrode tank 140 and each negative electrode cell 60. The tetravalent or trivalent vanadium sulfuric acid solution circulates through pipes 302 and 304 in the direction indicated by the arrows in Figure 2. Pump 201 also circulates a sulfuric acid solution between the positive electrode tank 130 and each positive electrode cell 40. The sulfuric acid solution circulates through pipes 301 and 303 in the direction indicated by the arrows in Figure 2. Furthermore, current is passed through the cell stack 20 from the positive electrode current collector plate 110 toward the negative electrode current collector plate 120 to electrolyze the tetravalent or trivalent vanadium sulfuric acid solution in each negative electrode cell 60. If the tetravalent vanadium sulfuric acid solution is electrolyzed, a trivalent or divalent vanadium sulfuric acid solution is produced. If the trivalent vanadium sulfuric acid solution is electrolyzed, a divalent vanadium sulfuric acid solution is produced. The current density applied to the cell stack 20 is the same as in Embodiment 1. In this state, valves 404, 411, 415, 418, and 419 are closed, and valves 401, 402, 403, 405, and 406 are open. By setting each valve in this state, the sulfuric acid solution circulation system and the vanadium sulfuric acid solution circulation system can be kept in a sealed state. In addition, nitrogen is flowed through the positive electrode tank 130 and the negative electrode tank 140. Furthermore, the oxygen gas generated in the positive electrode cell 40 flowing through the piping 303 is separated by the gas-liquid separator 210, and the separated oxygen-nitrogen mixture gas is exhausted.

[0050] The obtained trivalent or divalent vanadium sulfuric acid solution, for example, the trivalent vanadium sulfuric acid solution, is moved from the negative electrode tank 140 to the reaction tank 150 by opening valves 401, 402, and 404 and closing valve 403, and operating pump 202. Next, the trivalent vanadium sulfuric acid solution can be discharged as a product through piping 350 by opening valves 407, 408, and 409 and closing valve 410, and operating pump 203. The obtained divalent vanadium sulfuric acid solution is used to react with vanadium pentoxide powder. The obtained trivalent vanadium sulfuric acid solution may be used to circulate between the negative electrode tank 140 and each negative electrode cell 60.

[0051] (Embodiment 3) Embodiment 3 is a method for producing a vanadium sulfuric acid solution for a vanadium redox flow battery according to the third embodiment of the present invention.

[0052] In Embodiment 3, a tetravalent or trivalent vanadium sulfuric acid solution is prepared in the negative electrode tank 140. This tetravalent or trivalent vanadium sulfuric acid solution may be injected via piping 305. Then, valve 411 is closed. In addition, the sulfuric acid solution is injected into the positive electrode tank 130 from piping 309. Then, valve 415 is closed. This sulfuric acid solution can be prepared in the same manner as in Embodiment 1. Subsequently, pump 202 circulates the tetravalent or trivalent vanadium sulfuric acid solution between the negative electrode tank 140 and each negative electrode cell 60. The tetravalent or trivalent vanadium sulfuric acid solution circulates in the directions indicated by the arrows in Figure 2 through piping 302 and piping 304. In addition, pump 201 circulates the sulfuric acid solution between the positive electrode tank 130 and each positive electrode cell 40. The sulfuric acid solution circulates in the directions indicated by the arrows in Figure 2 through piping 301 and piping 303. Furthermore, current is passed through the cell stack 20 from the positive electrode current collector plate 110 toward the negative electrode current collector plate 120 to electrolyze the tetravalent or trivalent vanadium sulfuric acid solution in each negative electrode cell 60, thereby generating a divalent vanadium sulfuric acid solution. The current density applied to the cell stack 20 is the same as in Embodiment 1. At this time, valves 404, 411, 415, 418, and 419 are kept closed, while valves 401, 402, 403, 405, and 406 are kept open. By setting each valve in this state, the sulfuric acid solution circulation system and the vanadium sulfuric acid solution circulation system can be kept in a sealed state. At this time, nitrogen is flowed through the positive electrode tank 130 and the negative electrode tank 140. In addition, the oxygen gas generated in the positive electrode cell 40 flowing through the piping 303 is separated by the gas-liquid separator 210, and the separated oxygen-nitrogen mixture gas is exhausted.

[0053] Next, valves 401, 402, and 404 are opened, valve 403 is closed, and pump 202 is operated to inject the obtained divalent vanadium sulfuric acid solution into reaction tank 150. At this time, valve 407 is kept closed. After injecting the divalent vanadium sulfuric acid solution into reaction tank 150, valve 404 is closed. At this time, valves 412 and 413 may be opened, valve 414 may be closed, and pump 204 may be operated to inject the sulfuric acid solution from piping 308 into reaction tank 150 to adjust the divalent vanadium sulfuric acid solution in reaction tank 150 to a predetermined concentration and volume. After that, vanadium pentoxide powder is added to reaction tank 150 from feeder 160. In reaction tank 150, the vanadium pentoxide powder is reacted with the divalent vanadium sulfuric acid solution to produce a tetravalent or trivalent vanadium sulfuric acid solution. In this case, the sulfuric acid solution in the positive electrode tank 130 and each positive electrode cell 40 can be reused, and as long as the liquid level has not changed, it can be used again as is. Vanadium ions may be introduced through the ion exchange membrane 50, but this does not interfere with operation.

[0054] The concentration and amount of sulfuric acid in the resulting tetravalent or trivalent vanadium sulfuric acid solution can be adjusted in the reaction tank 150 by injecting sulfuric acid from the second sulfuric acid tank 170 via piping 308. Furthermore, some or all of the resulting tetravalent or trivalent vanadium sulfuric acid solution can be discharged as a product from the reaction tank 150 via piping 305 and 350 by closing valve 410, opening valves 407, 408, and 409, and operating pump 203.

[0055] Next, valves 407, 408, 410, and 411 are opened, and valve 409 is closed, and pump 203 is operated to transfer the obtained tetravalent or trivalent vanadium sulfuric acid solution from reaction tank 150 to negative electrode tank 140. After that, valve 411 is closed. At this time, the sulfuric acid concentration and amount of the tetravalent or trivalent vanadium sulfuric acid solution in the negative electrode tank 140 can be adjusted by injecting pure water into the negative electrode tank 140 via piping 310. Then, pump 202 circulates the tetravalent or trivalent vanadium sulfuric acid solution between the negative electrode tank 140 and each negative electrode cell 60. The tetravalent or trivalent vanadium sulfuric acid solution circulates through piping 302 and piping 304 in the direction indicated by the arrows in Figure 2. Pump 201 also circulates the sulfuric acid solution between the positive electrode tank 130 and each positive electrode cell 40. The sulfuric acid solution circulates through pipes 301 and 303 in the direction indicated by the arrows in Figure 2. Furthermore, current is passed through the cell stack 20 from the positive electrode current collector plate 110 toward the negative electrode current collector plate 120 to electrolyze the tetravalent or trivalent vanadium sulfuric acid solution in each negative electrode cell 60. When the tetravalent vanadium sulfuric acid solution is electrolyzed, a trivalent or divalent vanadium sulfuric acid solution is produced, and when the trivalent vanadium sulfuric acid solution is electrolyzed, a divalent vanadium sulfuric acid solution is produced. The current density applied to the cell stack 20 is the same as in Embodiment 1. At this time, valves 404, 411, 415, 418 and 419 are closed, and valves 401, 402, 403, 405 and 406 are open. By keeping each valve in this state, the sulfuric acid solution circulation system and the vanadium sulfuric acid solution circulation system can be kept in a sealed state. In this process, it is preferable to keep nitrogen flowing through the positive electrode tank 130 and the negative electrode tank 140. Furthermore, the oxygen gas generated in the positive electrode cell 40 flowing through the piping 303 is separated by the gas-liquid separator 210, and the separated oxygen-nitrogen mixture is exhausted.

[0056] The obtained trivalent or divalent vanadium sulfuric acid solution, for example, the trivalent vanadium sulfuric acid solution, is moved from the negative electrode tank 140 to the reaction tank 150 by opening valves 401, 402, and 404 and closing valve 403, and operating pump 202. Next, the trivalent vanadium sulfuric acid solution can be discharged as a product through piping 350 by opening valves 407, 408, and 409 and closing valve 410, and operating pump 203. The obtained divalent vanadium sulfuric acid solution is used to react with vanadium pentoxide powder. The obtained trivalent vanadium sulfuric acid solution may be used to circulate between the negative electrode tank 140 and each negative electrode cell 60. [Examples]

[0057] The present invention will be described in more detail by the following examples.

[0058] (Example 1) <Device Configuration> The following apparatus was prepared in accordance with the vanadium sulfuric acid solution production apparatus for vanadium redox flow batteries shown in Figures 1 and 2. A cell stack was formed by arranging a positive electrode current collector plate, a positive electrode, 10 unit cells, a positive electrode cell, an ion exchange membrane, a negative electrode cell, a negative electrode, and a negative electrode current collector plate in order from the first end to the second end opposite the first end. Each unit cell contains a positive electrode cell, an ion exchange membrane, a negative electrode cell, and a bipolar plate in order from the first end, and the bipolar plate contains a negative electrode and a positive electrode in order from the first end. Each positive electrode cell and each negative electrode cell is formed so that fluid can flow throughout the entire plane perpendicular to the direction from the first end to the second end within each cell. The effective electrode area after arrangement on the frame was 500 cm2, and the thickness of each positive electrode cell and negative electrode cell (distance from the first end to the second end of the cell) was 4.2 mm. The frame has four 25mm diameter manifolds formed at positions away from where the electrodes are located. The first and second manifolds are connected to each positive electrode cell so that fluid can flow through them, while the third and fourth manifolds are connected to each negative electrode cell so that fluid can flow through them. Available for distributionIt is connected to the first and third manifolds, which are fluid inlets, and the second and fourth manifolds, which are fluid outlets. The first and second current collector plates, located at both ends of the frame, are gold-plated copper plates. The negative electrode is a DSA electrode (Dimensionally Stable Anode), and the positive electrode is a graphite molded plate (SGL SIGRACELL TF6) containing 70% or more by mass of graphite. Nafion N117 (183um) was used as the ion exchange membrane. A 4.6mm thick carbon felt (SGL GFD 4.65EA) was placed in each negative electrode cell, compressed to 4.2mm in the direction from the first end to the second end. Two 50L heat-resistant PVC tanks were prepared; one was used as the positive electrode tank and connected to the first and second manifolds. The other was used as the negative electrode tank and connected to the third and fourth manifolds. Two magnetic pumps were used, circulating vanadium sulfuric acid solution at the negative electrode and 4.5M sulfuric acid at the positive electrode. Each tank was connected to a sealed nitrogen piping system, with a constant flow rate of 10 L / min, and the exhaust gas was discharged outside.

[0059] <1st process> 30 L of a 0.7 mol / L vanadium sulfuric acid solution with a valency of 4.02 was added to the negative electrode tank, and 200 A (4000 A / m) was applied. 2 The ion was electrolyzed to a valency of 2.0 with a current of 34 minutes, with an average voltage of 27V and an energy input of 3.06kWh. The Coulomb efficiency was over 99%, and no H2 was generated except for the last few seconds. The temperature at the end was 53°C.

[0060] 30 L of the obtained 0.7 mol / L divalent vanadium sulfuric acid solution was transferred to another 50 L heat-resistant PVC tank, and 3822 g of vanadium pentoxide powder was added to it in air while stirring. The bulk density of the powder was 1.3 g / cm³. 3The angle of repose was approximately 45 degrees. A table feeder with a hopper was placed on a scale, and the material was dispensed in as consistent a volume as possible. Dispensing was done at a rate of approximately 255 g / min, and all dispensing was completed in 15 minutes. The temperature at the start of dispensing was 40°C, and the temperature rose during and after dispensing, reaching a maximum temperature of 73°C about 1 hour after dispensing began. After 1.5 hours, a sample was taken and analyzed, and it was found that 30 L of a 2.1 mol / L 3.99 valent vanadium sulfuric acid solution was obtained. 4.5 mol / L sulfuric acid was added to the obtained vanadium sulfuric acid solution to obtain 37 L of a 1.7 mol / L tetravalent vanadium sulfuric acid solution. Of this, 15 L was used as the 1.7 mol / L tetravalent vanadium sulfuric acid product.

[0061] <Second process> To the remaining 22 L of 1.7 mol / L tetravalent vanadium sulfuric acid solution from the first step, 8 L of 4.5 mol / L sulfuric acid was added to make 30 L of 1.25 mol / L vanadium sulfuric acid solution. This 30 L of 1.25 mol / L vanadium sulfuric acid solution was poured into the negative electrode tank of the electrolytic cell. 200 A (4000 A / m 2 Electrolysis was performed to obtain 30 L of a 1.25 mol / L divalent vanadium sulfuric acid solution. The process took 60 minutes, the average voltage was 27 V, and the energy input was 5.40 kWh. The Coulomb efficiency was over 99%, and no H2 was generated except for the last few seconds. The temperature at the end was 49°C.

[0062] 30 L of the obtained 1.25 mol / L divalent vanadium sulfuric acid solution was transferred to another 50 L heat-resistant PVC tank, and 1706 g of vanadium pentoxide powder was added while stirring. The addition was carried out at a rate of approximately 114 g / min, and all addition was completed in 15 minutes. The temperature at the start of addition was 38°C, and the temperature continued to rise during and after addition, reaching a maximum temperature of 61°C 53 minutes after the start of addition. After 1.5 hours, a sample was taken and analyzed, yielding 30 L of 1.88 M 3.01 valent vanadium sulfuric acid solution. 4.5 mol / L sulfuric acid was added to this to obtain 33.1 L of 1.7 mol / L trivalent vanadium sulfuric acid solution. Of this, 20.7 L was used as the 1.7 mol / L trivalent vanadium sulfuric acid product.

[0063] <3rd process> In the second step, 4.5 mol / L sulfuric acid was added to the remaining 12.4 L of 1.7 mol / L trivalent vanadium sulfuric acid solution to obtain 30 L of 0.7 mol / L trivalent vanadium sulfuric acid solution. This solution was then electrolytically reduced to 2 valency to obtain 30 L of 0.7 mol / L divalent vanadium sulfuric acid solution. By repeating the above steps, it was possible to produce vanadium sulfuric acid solutions of predetermined concentrations, both trivalent and tetravalent, from vanadium pentoxide powder.

[0064] (Comparative Example 1) Air cylinders were connected to each tank instead of N2. Without any current flowing, the positive electrode would be filled with 100% oxygen, which is dangerous, and the negative electrode would also be dangerous when H2 was generated. Electrolysis proceeded successfully up to the trivalent state. Beyond that point, it took longer than planned, and the valency did not drop below 2.3. The current efficiency when the 2.3 valency was reached was approximately 40%.

[0065] A comparison of Example 1 and Comparative Example 1 shows that electrolysis proceeds more smoothly when N2 is flowed through each tank compared to when air is flowed through each tank.

[0066] (Comparative Example 2) The electrolytic current density was increased to 7000 A / m² using the same process as in Example 1. 2 We attempted electrolysis of a tetravalent vanadium sulfuric acid solution. The generated oxygen could not be completely removed and remained inside, causing the cell resistance to become unstable. The voltage gradually increased and continued to rise indefinitely, so we stopped the electrolysis.

[0067] From a comparison between Example 1 and Comparative Example 2, the electrolytic current density was 7000 A / m². 2 If the value is less than this, it can be seen that electrolysis proceeds smoothly.

[0068] (Comparative Example 3) Instead of using a graphite molded plate containing 70% or more graphite by mass as the negative electrode, a graphite plate cut from a block of artificial graphite was used and superimposed on the positive electrode. The graphite plate in the third cell cracked, causing leakage and preventing current flow. After replacing the cracked graphite plate and using a soft, thick gasket for sealing with the frame, there was no leakage. However, when current was applied, the temperature of the sixth cell rose abnormally, causing frame distortion. This distortion led to leakage and further loss of current flow.

[0069] From a comparison between Example 1 and Comparative Example 3, a graphite molded sheet containing 70% or more by mass of graphite was found to be... Negative electrode When used in this way, electrolysis proceeds more smoothly compared to when a graphite plate cut from a block of artificial graphite is used as the negative electrode.

[0070] (Comparative Example 4) In each negative electrode cell, the carbon felt was omitted. In this case, current flow was possible. However, in Example 1, the current density was 4000 A / m². 2 Even in that case, the voltage per cell was 2.7V, whereas in Comparative Example 2, the current density was 1000 A / m². 2 Even under these conditions, the voltage rose to approximately 4.6V per cell and continued to rise while electrolysis was ongoing. Moreover, the current density was 1200 A / m². 2 This was the limit; exceeding this limit resulted in a violent generation of hydrogen. Current density: 500 A / m 2 Even at relatively low currents, hydrogen generation became intense once the valence dropped below trivalent, and the current efficiency decreased drastically.

[0071] A comparison between Example 1 and Comparative Example 4 shows that using carbon felt in the negative electrode cell results in better electrolysis compared to when carbon felt is not used.

[0072] (Comparative Example 5) In Example 1, a hole was made in the lid of the tank into which the vanadium pentoxide powder was added, and a powder inlet and stirring rod were inserted. However, to minimize the gap in the hole in the lid, a hose nipple was attached to the lid, and a large amount of N2 was introduced from there at a rate of 30 L / min, causing the N2 to spray out through the gap in the hole. This was an attempt to seal in N2 to prevent oxidation of the divalent vanadium sulfuric acid solution. There was no difference in the reaction behavior of the vanadium pentoxide powder with and without the introduction of N2.

[0073] A comparison between Example 1 and Comparative Example 5 shows that the reaction tank does not need to be filled with nitrogen. [Explanation of symbols]

[0074] 10. Vanadium sulfuric acid solution production apparatus for vanadium redox flow batteries 20-cell stack 30 unit cells 40 Positive Electrode Cells 50 Ion exchange membrane 60 Negative Electrode Cells 70 Carbon Felt 80 bipolar plates 90 Negative electrode 100 Positive electrode 110 Positive electrode current collector plate 120 Negative electrode current collector plate 130 Positive electrode tank 140 Negative electrode tank 150 reaction tanks 160 feeders 170 Second sulfuric acid tank 180 First sulfuric acid tank

Claims

1. A cell stack comprising a plurality of unit cells, each having a positive electrode cell, an ion exchange membrane, a negative electrode cell, and a bipolar plate in that order from the first end, wherein the bipolar plate includes, in that order from the first end, a negative electrode which is a graphite molded plate containing 70% by mass or more of graphite, and a positive electrode which is formed by coating a titanium plate with a platinum group metal oxide, and the negative electrode cell has carbon felt arranged in it. A positive electrode tank, connected to each of the positive electrode cells in a way that allows fluid to circulate between them, A negative electrode tank, to which fluid can circulate between each of the negative electrode cells, and A reaction tank for reacting vanadium pentoxide powder with vanadium sulfuric acid solution, Equipment for producing vanadium sulfuric acid solution for vanadium redox flow batteries.

2. A cell stack comprising a plurality of unit cells, each having a positive electrode cell, an ion exchange membrane, a negative electrode cell, and a bipolar plate in order from the first end, The bipolar plate comprises, in order from the first end, a negative electrode which is a graphite molded plate containing 70% by mass or more of graphite, and a positive electrode which is formed by coating a titanium plate with a platinum group metal oxide, and a cell stack in which carbon felt is arranged in the negative electrode cell. A positive electrode tank, and, connected to each of the positive electrode cells in a way that allows fluid to circulate between them, Using a vanadium sulfuric acid solution production apparatus for vanadium redox flow batteries, which includes a negative electrode tank to which a fluid can be circulated between each of the negative electrode cells, A tetravalent vanadium sulfuric acid solution or a first trivalent vanadium sulfuric acid solution is circulated between the negative electrode tank and each negative electrode cell, a sulfuric acid solution is circulated between the positive electrode tank and each positive electrode cell, and 6000 A / m is applied to the cell stack from the first end towards the second end opposite to the first end. 2 The process includes the step of electrolyzing the tetravalent vanadium sulfuric acid solution or the first trivalent vanadium sulfuric acid solution in each of the negative electrode cells by passing the following current to produce a second trivalent vanadium sulfuric acid solution or a divalent vanadium sulfuric acid solution, A method for producing a vanadium sulfuric acid solution for a vanadium redox flow battery, wherein nitrogen gas is passed through the positive electrode tank and the negative electrode tank.

3. A cell stack comprising a plurality of unit cells, each having a positive electrode cell, an ion exchange membrane, a negative electrode cell, and a bipolar plate in order from the first end, The bipolar plate comprises, in order from the first end, a negative electrode which is a graphite molded plate containing 70% by mass or more of graphite, and a positive electrode which is formed by coating a titanium plate with a platinum group metal oxide, and a cell stack in which carbon felt is arranged in the negative electrode cell. A positive electrode tank, connected to each of the positive electrode cells in a way that allows fluid to circulate between them, A negative electrode tank, which is connected to each of the negative electrode cells in a way that allows fluid to flow between them, and Using a vanadium redox flow battery vanadium sulfuric acid solution production apparatus equipped with a reaction tank for reacting vanadium pentoxide powder with vanadium sulfuric acid solution, The steps include reacting vanadium pentoxide powder with a first divalent vanadium sulfuric acid solution in the reaction tank to produce a tetravalent vanadium sulfuric acid solution or a first trivalent vanadium sulfuric acid solution, The tetravalent vanadium sulfuric acid solution or the first trivalent vanadium sulfuric acid solution is circulated between the negative electrode tank and each negative electrode cell, the sulfuric acid solution is circulated between the positive electrode tank and each positive electrode cell, and 6000 A / m is applied to the cell stack from the first end towards the second end opposite to the first end. 2 The process includes the step of electrolyzing the tetravalent vanadium sulfuric acid solution or the first trivalent vanadium sulfuric acid solution in each of the negative electrode cells by passing the following current to produce a second trivalent vanadium sulfuric acid solution or a second divalent vanadium sulfuric acid solution, A method for producing a vanadium sulfuric acid solution for a vanadium redox flow battery, wherein nitrogen gas is passed through the positive electrode tank and the negative electrode tank, but nitrogen gas is not passed through the reaction tank.

4. A cell stack comprising a plurality of unit cells, each having a positive electrode cell, an ion exchange membrane, a negative electrode cell, and a bipolar plate in order from the first end, The bipolar plate comprises, in order from the first end, a negative electrode which is a graphite molded plate containing 70% by mass or more of graphite, and a positive electrode which is formed by coating a titanium plate with a platinum group metal oxide, and a cell stack in which carbon felt is arranged in the negative electrode cell. A positive electrode tank, connected to each of the positive electrode cells in a way that allows fluid to circulate between them, A negative electrode tank, to which fluid can circulate between each of the negative electrode cells, and Using a vanadium redox flow battery vanadium sulfuric acid solution production apparatus equipped with a reaction tank for reacting vanadium pentoxide powder with vanadium sulfuric acid solution, A first tetravalent vanadium sulfuric acid solution or a first trivalent vanadium sulfuric acid solution is circulated between the negative electrode tank and each negative electrode cell, a sulfuric acid solution is circulated between the positive electrode tank and each positive electrode cell, and 6000 A / m is applied to the cell stack from the first end towards the second end opposite to the first end. 2 The first step involves passing the following current to electrolyze the first tetravalent vanadium sulfuric acid solution or the first trivalent vanadium sulfuric acid solution in each of the negative electrode cells to produce a first divalent vanadium sulfuric acid solution. The second step involves reacting vanadium pentoxide powder with the first divalent vanadium sulfuric acid solution in the reaction tank to produce a second tetravalent vanadium sulfuric acid solution or a second trivalent vanadium sulfuric acid solution. A tetravalent vanadium sulfuric acid solution or the second trivalent vanadium sulfuric acid solution is circulated between the negative electrode tank and each negative electrode cell, the sulfuric acid solution is circulated between the positive electrode tank and each positive electrode cell, and 6000 A / m is applied to the cell stack from the first end towards the second end. 2 The third step includes passing the following current to electrolyze the second tetravalent vanadium sulfuric acid solution or the second trivalent vanadium sulfuric acid solution in each of the negative electrodes to produce a third trivalent vanadium sulfuric acid solution or a second divalent vanadium sulfuric acid solution, A method for producing a vanadium sulfuric acid solution for a vanadium redox flow battery, wherein nitrogen gas is passed through the positive electrode tank and the negative electrode tank, but nitrogen gas is not passed through the reaction tank.

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