Method for reducing vanadium ions and method for manufacturing electrolyte for vanadium redox battery using same

Abstract

The present invention relates to a method for reducing vanadium ions and a method for manufacturing an electrolyte for a vanadium redox battery using same, wherein in the reduction of vanadium ions, (A) a first reducing agent is added to a first vanadium compound solution having a first oxidation state, followed by reaction, thereby preparing a second vanadium compound solution having a second oxidation state lower than the first oxidation state; and (B) a second reducing agent and a catalyst are added to the second vanadium compound solution having the second oxidation state, followed by reaction, thereby preparing a third vanadium compound solution having a third oxidation state lower than the second oxidation state, wherein the first oxidation state is 4.5 to 5, the second oxidation state is 4.1 to 4.4, and the third oxidation state is 3 to 4 (exclusive).

Description

Method for reducing vanadium ions and method for producing an electrolyte for a vanadium redox battery using the same

[0001] The present invention relates to a method for reducing vanadium ions and a method for producing an electrolyte for a vanadium redox battery using the method.

[0002] As existing power generation systems, such as thermal power plants that use fossil fuels and generate massive greenhouse gas emissions and environmental pollution, and nuclear power plants with inherent stability and waste disposal issues, reveal various limitations, research into developing more environmentally friendly and highly efficient energy sources and power supply systems utilizing these sources is rapidly increasing. In particular, power storage technologies enable a wider and more diverse use of renewable energy, which is highly susceptible to external conditions, and can further enhance the efficiency of power utilization. Therefore, development is focusing on these technologies, and among these, interest and R&D in secondary batteries are growing significantly.

[0003] These demands are accelerating the development of redox batteries (VRBs), which are the most industrially viable option for the ESS market, along with lithium-ion batteries.

[0004] The above redox battery refers to an oxidation / reduction battery that can directly convert the chemical energy of an active material into electrical energy. It is an energy storage system that can store renewable energy sources, such as solar and wind power, whose output fluctuates greatly depending on external conditions, and convert them into high-quality electricity. Specifically, in a redox battery, an electrolyte containing an active material that undergoes oxidation / reduction reactions circulates between the electrodes and a storage tank, allowing charging and discharging to occur.

[0005] In addition, the types of redox batteries vary depending on the active material used in the electrolyte, among which vanadium redox batteries and zinc / bromine redox batteries are known. In the vanadium redox battery, the electrolyte accounts for the highest price proportion among the battery components, so lowering the price of the electrolyte is essential to securing the price competitiveness of the vanadium redox battery.

[0006] Meanwhile, in the currently used vanadium redox battery system, an electrolyte containing vanadium with an oxidation number of 3.5 is applied simultaneously to the positive and negative electrodes of the cell, and through a pre-charging process, the positive electrolyte is converted from vanadium with an oxidation number of 3.5 to vanadium with an oxidation number of 4, and the negative electrolyte is converted from vanadium with an oxidation number of 3.5 to vanadium with an oxidation number of 3.

[0007] In this pre-charging process, the same current is applied to the positive and negative electrodes, so if a vanadium electrolyte with a volume ratio other than 1:1 of trivalent vanadium and tetravalent vanadium is injected, less conversion occurs at one electrode and more conversion occurs at the other electrode, which destroys the oxidation state balance and causes capacity unevenness, resulting in reduced long-term performance. Therefore, the industry prefers a vanadium electrolyte with an oxidation state of exactly 3.5.

[0008] Accordingly, to obtain the above-mentioned 3.5-valent vanadium electrolyte, there have been conventional chemical reduction methods or battery-driven methods. While the chemical reduction method allows for a short manufacturing time, it has the disadvantage of leaving unreacted residues (such as transition metals or expensive reducing agents) in the manufactured electrolyte. The battery-driven method, on the other hand, presents challenges such as battery-driven costs, differences in reaction times depending on battery specifications, and the need for precise battery charge control to achieve the correct oxidation number.

[0009] Accordingly, in patent document 1 and non-patent document 1, when preparing a vanadium electrolyte having an oxidation number of 3.5, a spontaneous oxidation reaction is carried out (reaction temperature 70 to 80°C) by using an auxiliary reducing agent (formic acid, HCOOH) in the presence of a platinum catalyst in a vanadium compound solution having an oxidation number of 4, so that the electrons generated thereby are V 4+ → V 3+ Reduction to V using the optimal formic acid concentration 4+ : V 3+ , 50:50 ratio (V 3.5+ ) is disclosed, and in this case, no additional external electrical device is required, and not only does relatively inexpensive formic acid serve as an organic reducing agent, but the organic reducing agent is completely decomposed through the high reactivity of a platinum catalyst, leaving no residue in the electrolyte.

[0010] However, the conventional method involved a two-step process, specifically, when reducing a vanadium compound solution with an oxidation number of 5 to a vanadium compound solution with an oxidation number of 4, oxalic acid was used, and then, when reducing a vanadium compound solution with an oxidation number of 4 to a vanadium compound solution with an oxidation number of 3.5, formic acid and a platinum catalyst were used.

[0011] Accordingly, a method of reducing the two-step process to a one-step process has recently been developed (domestic patent registration number, 10-2408365). Specifically, this has the advantages of simplifying the process and reducing the time by directly reducing a vanadium solution with an oxidation number of 5 to a vanadium solution with an oxidation number of 3.5 in the presence of a noble metal catalyst such as platinum (Pt) or iridium (Ir). However, in this case, there is a possibility that a problem with the durability of the catalyst may occur. For example, since the standard reduction potential of vanadium (5) (1.0 V) and the standard reduction potential of platinum (1.2 V) are close to each other, the durability of the catalyst may be significantly reduced due to the ionization of Pt particles.

[0012] Therefore, there is a need for research on a method for reducing vanadium ions, which shortens the manufacturing process time of the above two-step process, does not deteriorate the durability of the platinum catalyst, and enables reuse, and a method for manufacturing an electrolyte for a redox battery using the method.

[0013] [Prior Art Literature]

[0014] [Patent Document]

[0015] (Patent Document 1) Korean Patent Publication No. 10-2019-0102532

[0016] [Non-patent literature]

[0017] (Non-patent literature 1) Nature Communications volume 10, Article number: 4412 (2019) / Jihyun Heo et al.

[0018] The present invention provides a method for reducing vanadium ions, which shortens the manufacturing time and enables reuse of the catalyst, and a method for manufacturing an electrolyte for a vanadium redox battery using the method.

[0019] One embodiment of the present invention is a method for reducing vanadium ions,

[0020] (A) A first reducing agent is added to a solution of a first vanadium compound having a first oxidation number, and a reaction is performed to produce a solution of a second vanadium compound having a second oxidation number lower than the first oxidation number,

[0021] (B) A second reducing agent and a catalyst are added to a solution of a second vanadium compound having a second oxidation number and reacted to produce a solution of a third vanadium compound having a third oxidation number lower than the second oxidation number.

[0022] A method for reducing a vanadium ion is provided, wherein the first oxidation number is 4.5 to 5, the second oxidation number is 4.1 to 4.4, and the third oxidation number is 3 or more and less than 4.

[0023] In addition, another embodiment of the present invention provides a method for producing an electrolyte for a vanadium redox battery using the method for reducing vanadium ions.

[0024] The method for reducing vanadium ions according to the present invention and the method for producing an electrolyte for a vanadium redox battery using the same are methods in which a reduction process is performed to a specific valence using a first reducing agent, and then a second reducing agent and a catalyst are added to perform additional reduction, so that the durability of the catalyst may not be reduced.

[0025] In addition, since sodium sulfite is used as the first reducing agent, and this reduction process is performed only up to a certain number of hydrides without deteriorating catalyst durability, the amount of sodium sulfite used as the reducing agent can be reduced while shortening the manufacturing time compared to the conventional two-step reduction method.

[0026] Figure 1 shows V using Na2SO3 as a reducing agent according to Experimental Example 1. 5+ → V 4+ This is a graph of the results of ICP analysis of Pt concentration in the electrolyte in the reduction section.

[0027] Figure 2 is V according to Experimental Example 1-1 5+ → V 4+ This is a graph of the results of ICP analysis of Pt concentration in the electrolyte in the reduction section.

[0028] Figure 3 is V according to Experimental Example 1-1 4+ → V 3+ This is a graph of the results of ICP analysis of Pt concentration in the electrolyte in the reduction section.

[0029] Figure 4 is V according to Experimental Example 1-1 5+ → V 4+ → V 3.5+ This is a photograph of the HR-TEM mapping analysis results of the Pt / C shape before and after the reaction.

[0030] Figure 5 is V according to Experimental Example 2 5+ → V 4+ This is a graph showing the results of ICP analysis of Pt concentration by vanadium oxidation number in the electrolyte in the reduction section.

[0031] Figure 6 is V according to Experimental Example 3 5+ , V 3.5+ This is a graph showing the results of ICP analysis of the degree of carbon oxidation after Pt / C impregnation in the electrolyte.

[0032] Figure 7 is V according to Experimental Example 3 5+ This is a TEM catalyst morphology analysis photo before and after Pt / C impregnation in the electrolyte.

[0033] Fig. 8 is V 5+ → V 3.5+ This is a graph evaluating the durability of Pt / C through repeated manufacturing of electrolyte.

[0034] Figure 9 is a graph showing the results of ICP analysis according to Pt / C catalyst particle size.

[0035] Figure 10 is a graph showing UV-vis analysis results according to Pt / C catalyst particle size.

[0036] When a part in this specification is said to "include" a certain component, this does not mean that it excludes other components, but rather that it may include other components, unless otherwise specifically stated.

[0037] Hereinafter, the present invention will be described in detail.

[0038] One embodiment of the present invention is a method for reducing vanadium ions,

[0039] (A) A first reducing agent is added to a solution of a first vanadium compound having a first oxidation number, and a reaction is performed to produce a solution of a second vanadium compound having a second oxidation number lower than the first oxidation number,

[0040] (B) A second reducing agent and a catalyst are added to a solution of a second vanadium compound having a second oxidation number and reacted to produce a solution of a third vanadium compound having a third oxidation number lower than the second oxidation number.

[0041] A method for reducing vanadium ions is provided, wherein the first oxidation number is 4.5 to 5, the second oxidation number is 4.1 to 4.4, and the third oxidation number is 3 or more and less than 4.

[0042] Here, the first oxidation number may be 4.5 to 5, the second oxidation number may be 4.2 to 4.4, and the third oxidation number may be 3 or more and less than 4.

[0043] The oxidation number range indicated above refers to a range encompassing intermediate valencies of the stated valencies. For example, '4.5 to 5' can refer not only to 4.5 and 5, but also to 4.6, 4.7, 4.8, and 4.9.

[0044] Specifically, the first oxidation number may be 5, the second oxidation number may be 4.4, and the third oxidation number may be 3.5, with an error range of ±0.01.

[0045] According to research conducted by the inventors of the present application, it was confirmed that when vanadium is reduced using a first reducing agent until it has a specific valence as described above, and then a catalytic reduction process is performed thereafter, the reaction time can be shortened while the catalyst can be infinitely reused.

[0046] Below, the manufacturing method is described in detail.

[0047] First, in process (A), the first vanadium compound solution can be prepared by mixing a vanadium precursor and an acidic solution. Here, the vanadium precursor can be at least one selected from the group consisting of V2O5, VOSO4, NH4VO3, and V2O4.

[0048] Here, the vanadium oxidation number of the first vanadium compound solution prepared from the vanadium precursor may be 4.5 to 5, and more specifically, 5.

[0049] It is preferable that the above acidic solution be at least one selected from the group consisting of sulfuric acid, hydrochloric acid, nitric acid, and phosphoric acid, but any strong acid may be used without limitation.

[0050] In the above first vanadium compound solution, the vanadium concentration may be in the range of 0.5 M to 2.5 M, and the concentration of the acidic solution may be in the range of 0.1 M to 10 M. Hereinafter, since no additional addition, etc. is performed in the second vanadium compound solution and the third vanadium compound solution, they also have the concentrations in the above range.

[0051] Thereafter, a first reducing agent is added to the first vanadium compound solution and reacted to produce a second vanadium compound solution having a second oxidation number lower than the first oxidation number.

[0052] The first reducing agent may be sodium sulfite (Na2SO3).

[0053] Traditionally, oxalic acid was primarily used to reduce pentavalent vanadium ions to tetravalent vanadium ions. However, in the case of oxalic acid, the reduction reaction proceeds at high temperatures, approximately 80°C or higher, for approximately 6 hours or more, making it difficult to shorten the process time.

[0054] On the other hand, when using the sodium sulfite, the reduction reaction was carried out by adding sodium sulfite in a ratio of V2O5 1M:SO2 1M into a vanadium compound solution containing 1M V2O5 based on a 4M sulfuric acid aqueous solution, and it was confirmed that vanadium having a 5-valent valence was converted to vanadium having a 4-valent valence within 30 minutes. Therefore, when using the first reducing agent according to the present invention, a process that is more than 6 times faster than the conventional oxalic acid is possible.

[0055] Meanwhile, according to the present invention, since the reduction process of process (A) using the sodium sulfite as described above is carried out only until the oxidation number of vanadium becomes the second oxidation number, i.e., 4.1 to 4.4, the time can be further shortened to about 20 to 25 minutes.

[0056] Therefore, according to the present invention, the reaction time of the above process (A) may be 20 to 60 minutes.

[0057] In addition, when the sodium sulfite is used as the first reducing agent, the reaction temperature of the reduction reaction may be 20°C to 80°C, specifically 20°C to 60°C, and more specifically 20°C to 30°C.

[0058] That is, when sodium sulfite is used, the reduction reaction is easily performed even at room temperature, so there is no need to increase the reaction temperature. This has the effect of further increasing process efficiency compared to when the reaction is performed at about 80℃ or higher using existing oxalic acid.

[0059] This first reducing agent should be added at a concentration that can reduce all vanadium ions of the first vanadium compound solution to have the second oxidation number, and specifically, it can be added in a slightly more generous range, and thus, it can be added at 1 to 1.5 times (M) the vanadium concentration (M) of the first vanadium compound solution.

[0060] Meanwhile, the reason why process (A) is carried out until the second oxidation state, that is, the point where the oxidation state becomes 4.1 to 4.4, is to improve the durability of the catalyst used in the reduction process carried out in the subsequent process (B) to enable reuse, while reducing the amount of sodium sulfite used.

[0061] After preparing a second vanadium compound solution having a second oxidation number using sodium sulfite as described above, a third vanadium compound solution is prepared by adding a second reducing agent and a catalyst and reacting them.

[0062] At this time, it is preferable that the second reducing agent be at least one selected from the group consisting of formic acid, formaldehyde, methanol, ethanol, oxalic acid, and ammonium hydroxide, but any substance that does not leave impurities other than in a gaseous form may be used without limitation thereto.

[0063] The second reducing agent should also be added at a concentration that allows all vanadium ions of the second vanadium compound solution to be reduced to the third oxidation number, and more specifically, it can be added in a slightly more generous range, and therefore, it can be added at 0.5 times (M) to 0.65 times (M) the vanadium concentration (M) of the second vanadium compound solution. The electrolyte finally manufactured through the addition concentration of the second reducing agent is V 3.5+ It can be precisely controlled by oxidation number.

[0064] In addition, the catalyst introduced together with the second reducing agent may be introduced in a state in which platinum is supported on at least one carrier selected from a group of various structures composed of carbon, and in detail, may be the Pt / C.

[0065] As described above, in process (B), formic acid or the like is used as a second reducing agent, and for reduction by formic acid, the use of a catalyst is required. However, as confirmed by the inventors of the present application, the durability of the catalyst is lowered depending on the oxidation number of vanadium, and considering that the catalyst mainly uses expensive precious metals, if the durability is lowered and the number of reuses is reduced, there is a problem that the efficiency of the process cost is drastically reduced.

[0066] Accordingly, the inventors of the present application evaluated the durability of Pt / C used as a catalyst during the reduction process and studied in depth a method for infinite reuse of the catalyst, and as a result, confirmed that the durability of the catalyst did not deteriorate when the oxidation number of the vanadium ion fell below 4.4, and were able to set the time point of process (B) in which the catalyst was used, which will be described below with experimental examples.

[0067] Experimental Example 1

[0068] V of 0.75 to 2.1M 5+ To prepare the electrolyte, V2O5 was dispersed in a 4M sulfuric acid solution, and then SO3 of Na2SO3, a reducing agent, was added. 2- After adding 0.75 M to 2.1 M of Na2SO3, the mixture was stirred at 25 °C. After adding Na2SO3, samples were sampled at regular intervals and analyzed using an ICP-AES (inductively coupled plasma analyzer, Optima 8300). The ICP-AES analysis was performed according to the manual of the analysis equipment.

[0069] Figure 1 shows V using Na2SO3 as a reducing agent according to Experimental Example 1. 5+ → V 4+ This is a graph of the results of ICP analysis of Pt concentration in the electrolyte in the reduction section. Specifically, after adding Na2SO3 at room temperature (25 ℃) and 10 minutes of reaction time, V2O5 was V 4+ V is reduced to 4+ It was confirmed that the concentration was increased. After 20 minutes of reaction, 1M V2O5 was 1M V 4+ It was confirmed that all of them were reduced to .

[0070] <Experimental Example 1-1>

[0071] As described above, the conventional one-step vanadium ion reduction method has a problem in that the durability of the catalyst is significantly reduced. At this time, since the catalyst is affected by the vanadium oxidation number, in order to confirm the degree of Pt dissolution according to the change in vanadium oxidation number, the dissolution of Pt particles was confirmed in an atmosphere in which vanadium with an oxidation number of 5 was reduced to vanadium with an oxidation number of 4.

[0072] Specifically, the experiment in an atmosphere reducing vanadium with oxidation number 5 to vanadium with oxidation number 4 was 1.55 MV 5+ 50 mg of Pt / C was added to 50 ml of electrolyte (based on 2.2 M sulfuric acid aqueous solution), and a reduction reaction was performed at 80°C in 2.284 ml of formic acid. In each reduction process, electrolyte samples were collected at each time point during the reduction reaction, and the Pt concentration was confirmed through ICP analysis, and the results are shown in Fig. 2 below.

[0073] In addition, we confirmed whether Pt particles were eluting in an atmosphere that reduces vanadium with an oxidation number of 4 to vanadium with an oxidation number of 3, and specifically, in the above experimental method, 1.55 MV 4+ The reduction reaction was performed in the same manner except that 50 ml of electrolyte (based on 2.2 M sulfuric acid aqueous solution) was used as the standard, and ICP analysis was performed accordingly, and the results are shown in Figure 3 below.

[0074] Referring to Figures 2 and 3 below, V 5+ → V 4+ As a result of checking the Pt concentration in the electrolyte over time in the reduction section, it can be confirmed that the Pt concentration in the electrolyte increases as the reaction time progresses. However, V 4+ → V 3+ The Pt concentration in the electrolyte decreased rapidly at the moment of transition to the reduction section. Based on the close proximity of the theoretically known vanadium standard reduction potential and the Pt ionization potential through this experiment, the Pt / C catalyst showed V5+ In V 4+ It can be seen that Pt ionization proceeds within the reduced section.

[0075] Additionally, V 5+ → V 4+ → V 3+ HR-TEM images were taken before and after the reaction for the reduction section, and the results are shown in Figure 4 below.

[0076] Referring to Figure 4, it can be seen that the Pt particles are clumped together as they are re-precipitated after ionization. That is, V 5+ → V 4+ As ionization of Pt occurs in V 4+ → V 3+ As the reduction zone progresses, the Pt ionization voltage range is exceeded. Consequently, some of the ionized Pt is reduced again, resulting in re-precipitation. This loss of Pt particles and the agglomeration of particles through re-precipitation inevitably leads to a decline in catalyst performance and a reduction in the number of re-use cycles.

[0077] Experimental Example 2

[0078] According to the above experimental example 1, the dissolution of Pt in the electrolyte is V 5+ → V 4+ It was confirmed that this occurred in the reduction section, and the following experiment was performed to confirm the critical vanadium oxidation number for the elution of this catalyst.

[0079] Specifically, 1.55 MV 5+ Add 50 mg of Pt / C to 50 ml of electrolyte (based on 2.2 M sulfuric acid aqueous solution), and perform reduction reaction at 80°C with 2.284 ml of formic acid, and V 5+ In V 4+ During the reduction process, the vanadium oxidation number point was confirmed, and after collecting a sample, ICP analysis was performed, and the results are shown in Figure 5 below.

[0080] Referring to Figure 5, V5+ When reduced, the oxidation number of vanadium is V 4+ It can be seen that the ionization of Pt particles increases rapidly as V decreases. 5+ In V 4.4+ It can be seen that Pt is ionized and eluted within the Pt / C catalyst during the reduction process, and Pt ionization does not appear at all when the vanadium oxidation number is lower than 4.4. In other words, it means that the durability of the Pt / C catalyst is guaranteed only when the vanadium oxidation number is lower than 4.4. This evaluation result, in other words, is a conventional one-step vanadium electrolyte manufacturing process (V 5+ → V 3.5+ ) it can be seen that the durability of expensive Pt / C is significantly reduced, which causes problems in process efficiency.

[0081] Experimental Example 3

[0082] Furthermore, since the catalyst comprises Pt incorporated into a carbon support, the durability of the carbon support is also important. Accordingly, in order to evaluate the durability of the carbon support, the catalyst was recovered after the following experiments and subjected to GC-MS and ICP analysis. The results are shown in Table 1 and Figure 6.

[0083] -Sample 1: 1.55 MV 5+ Add 15 mg of Pt / C to 15 ml of electrolyte (based on 2.2 M sulfuric acid aqueous solution), and soak at 80°C for 65 hours.

[0084] -Sample 2: 1.55 MV 3.5+ Add 15 mg of Pt / C to 15 ml of electrolyte (based on 2.2 M sulfuric acid aqueous solution), and soak at 80°C for 65 hours.

[0085] -Sample 3: 1.55 MV 5+ Wait for 65 hours at 80℃ without adding Pt / C to 15 ml of electrolyte (based on 2.2 M sulfuric acid aqueous solution).

[0086] SampleV5+ with Pt / CV 3.5+ with Pt / CV 5+ without Pt / CCO2mol / mol (%)220.640.11

[0087] Referring to Table 1 above, the GC analysis results V 5+ It was confirmed that carbon was oxidized during 65 hours of impregnation of Pt / C in the electrolyte. On the other hand, V 3.5+ There was almost no oxidation of carbon in the electrolyte.

[0088] Also, referring to Figure 6 below, the ICP analysis results show that V impregnated with Pt / C 5+ It can be confirmed that Pt has been eluted by detecting 88 ppm of Pt in the electrolyte.

[0089] On the other hand, V 3.5+ In the electrolyte, almost no Pt was detected. In this experiment, carbon was V 5+ It can be seen that oxidation occurs even when exposed to electrolyte, and accordingly V 5+ It can be seen that the electrolyte solution causes structural collapse of carbon supporting Pt particles and causes detachment and agglomeration of Pt particles, which has a significant impact on the performance and durability of the Pt / C catalyst.

[0090] Additionally, the catalyst morphology was analyzed by TEM analysis before and after the reaction in Sample 1, and is shown in Figure 7 below.

[0091] Referring to Figure 7, the TEM analysis results confirmed that many morphologies had changed from the existing catalyst form. First, the detachment phenomenon of Pt particles was confirmed, then the partial agglomeration phenomenon of Pt particles was confirmed, and finally, the oxidation of carbon was confirmed to appear at the edge.

[0092] Experimental Example 4

[0093] To simulate the step 1 vanadium electrolyte manufacturing process and verify the durability of the actual Pt / C catalyst, a 1 L volume filter reactor was used to conduct a Pt / C continuous reuse evaluation.

[0094] Specifically V 5+ → V 3.5+ 830 ml per serving, 1.7 MV 3.5+ Manufacturing standard, reaction time 1 hour, Pt / C 0.83 g added, formic acid 40.33 ml added, after 1 hour of reaction, Pt / C catalyst and V were filtered at the bottom of the reactor. 3.5+ The electrolyte was separated and the Pt / C catalyst in the filter was reused in the next reaction. The results are shown in Figure 8 below.

[0095] Referring to Figure 8, V is continuously 5+ → V 3.5+ The durability evaluation of Pt / C catalyst through manufacturing was V from 36 times. 5+ In V 3.5+ It was confirmed that the reduction was not complete. As a result of manufacturing the 36th vanadium electrolyte, the oxidation number of vanadium was V 3.526+ It was found that the activity of the Pt / C catalyst decreased. From this, it can be concluded that replacing the expensive Pt / C catalyst with a new one after using it for only 35 times is quite inefficient for process operation. This experiment was conducted in the previous V 5+ In V 4+ This result confirms the catalytic instability of Pt / C once again when the reduction reaction proceeds.

[0096] Combining the above experimental examples 1 to 4, V through the conventional one-step reaction 3.5+ Although the electrolyte manufacturing method may appear to be a fairly efficient process, it can be seen that it shows a problem of significantly low durability of the main element, Pt / C.

[0097] Therefore, in order to create a process that theoretically allows permanent reuse of expensive Pt / C, V 4+ → V 3.5+ Using the above catalyst in the manufacturing, V 5+ → V 4+ The reaction is preferably a two-step manufacturing process using a reducing agent. However, in this manufacturing process, V 5+ → V 4+ Considering that the use of oxalic acid in the reaction has low efficiency due to high process temperature and long process time, V is used as much as possible. 5+ → V 4+ It is desirable to shorten the reduction reaction time, and accordingly, as confirmed by the inventors of the present application, the process time can be shortened by a factor of six by using sodium sulfite as a reducing agent in the above step.

[0098] Meanwhile, in order to reduce the amount of sodium sulfite used while shortening the process time and ensuring catalyst durability to enable permanent reuse, the catalyst should be substantially V as derived from Experimental Example 2. 4.4+ Based on the fact that it is not eluted below, the above effect can be achieved by performing a reduction reaction using sodium sulfite up to an oxidation number of around 4.4, and performing reduction using a Pt / C catalyst at an oxidation number of 4.4 or lower.

[0099] That is, according to the present invention, it can be seen that the process time can be shortened, the amount of sodium sulfite reducing agent used can be reduced, and the problem of deterioration in catalyst durability can also be solved.

[0100] Meanwhile, the platinum of the catalyst may be introduced in the form of particles, and at this time, the platinum may have a particle size of 1 nm to 20 nm. Here, the particle size may refer to a value obtained by directly measuring 100 particles identified in the HR-TEM image and then averaging them.

[0101] The size of the catalyst is the average size of the particles that make up the catalyst. For example, if the particles of the synthesized catalyst exist in multiple sizes, the average size of the particles can be defined as the size of the catalyst. In addition, the size of the catalyst can be measured by obtaining an image through a high-magnification scanning electron microscope (HR-TEM) and utilizing the function equipped on the analysis device. Specifically, the size of the catalyst is measured based on the size of the roughly spherical particles confirmed in the high-magnification scanning electron microscope image based on the scale bar recorded in the image. If there are multiple sizes of particles confirmed in the high-magnification scanning electron microscope image, multiple spherical particles are extracted from them, and the median value of the sizes of the particles measured in comparison with the scale bar can be the size of the catalyst.

[0102] Beyond the above range, if the size is too small, not only is manufacturing difficult, but the reaction area increases, which may increase the possibility of dissolution, and if it is too large, the reaction area decreases without affecting the lifespan, which increases the reaction time, and in addition, the manufacturing cost increases due to the increased use of Pt, which is difficult to manufacture, making it inefficient.

[0103] Therefore, if the above range is satisfied, the dissolution problem is not affected regardless of the particle size. Therefore, to confirm this, the following Experimental Example 5 is provided.

[0104] Experimental Example 5

[0105] V 5+ → V 4+ The degree of ionization of Pt particles during the reduction reaction was evaluated according to particle size. Specifically, 1.55 MV 5+50 mg of Pt / C (particle size (D50): 5 nm, 10 nm) was added to 50 ml of electrolyte (based on 2.2 M sulfuric acid aqueous solution), and a reduction reaction was performed at 80°C in 2.284 ml of formic acid. Samples were obtained every 5 minutes and the degree of Pt dissolution and the degree of vanadium reduction were confirmed through the ICP analysis and UV-vis analysis, and the results are shown in Figures 9 and 10 below.

[0106] Referring to Figures 9 and 10, it can be confirmed that even if the particle size increases, it does not have a significant effect on Pt dissolution. Therefore, within the above range, there is no need to increase the particle size of Pt.

[0107] In addition, the second vanadium compound solution may be included in an amount of 0.001 g to 0.1 g, specifically 0.002 g to 0.05 g, and more specifically 0.0025 g to 0.006 g, based on 1 ml of the solution.

[0108] If it is included outside the above range and too little, the oxidation reaction of the second reducing agent becomes slow, so oxidation does not occur within a short period of time, and the reaction for producing the third vanadium compound solution may take a long time or may not occur sufficiently, so the desired electrolyte cannot be produced, which is not preferable.

[0109] The reaction of process (B) carried out using a catalyst in this way may be carried out at a reaction temperature of 60°C to 90°C, and more specifically, may be carried out at a reaction temperature of 60°C to 80°C.

[0110] Beyond the above range, the reaction may not occur or may be too slow, and if it is too high, the concentration may change as the solvent or reducing agent evaporates, which is not desirable.

[0111] Meanwhile, according to the present invention, a method for producing an electrolyte for a vanadium redox battery using the method for reducing vanadium ions is provided.

[0112] The above electrolyte for the vanadium redox battery uses the reduction method of the vanadium ion to produce V 3.5+ The vanadium compound solution is prepared and used, and other details are as known in the art.

Claims

1. (A) A first reducing agent is added to a solution of a first vanadium compound having a first oxidation number and reacted to produce a solution of a second vanadium compound having a second oxidation number lower than the first oxidation number, (B) A second reducing agent and a catalyst are added to a solution of a second vanadium compound having a second oxidation number and reacted to produce a solution of a third vanadium compound having a third oxidation number lower than the second oxidation number. A method for reducing vanadium ions, wherein the first oxidation number is 4.5 to 5, the second oxidation number is 4.1 to 4.4, and the third oxidation number is 3 or more and less than 4.

2. In claim 1, The first reducing agent is sodium sulfite (Na 2 SO 3 ) A method for reducing vanadium ions.

3. In claim 2, A method for reducing a vanadium ion whose first oxidation number is 5, whose second oxidation number is 4.4, and whose third oxidation number is 3.

5.

4. In claim 1, A method for reducing vanadium ions, wherein the vanadium concentrations of the first vanadium compound solution, the second vanadium compound solution, and the third vanadium compound solution are 0.5 M to 2.5 M.

5. In claim 1, A method for reducing vanadium ions, wherein the first reducing agent is added in an amount of 1 to 1.5 times (M) the vanadium concentration (M) of the first vanadium compound solution.

6. In claim 1, A method for reducing vanadium ions, characterized in that the second reducing agent is at least one selected from the group consisting of sodium sulfite, formic acid, formaldehyde, methanol, ethanol, oxalic acid, and ammonium hydroxide.

7. In claim 1, A method for reducing vanadium ions, wherein the second reducing agent is added in an amount of 0.5 times (M) to 0.65 times (M) the vanadium concentration (M) of the second vanadium compound solution.

8. In claim 1, The above catalyst is a method for reducing vanadium ions, wherein platinum is supported on at least one carrier selected from a group of various structures composed of carbon.

9. In claim 8, The above platinum is a method for reducing vanadium ions having a particle size of 1 nm to 20 nm.

10. In claim 1, A method for reducing vanadium ions, wherein the catalyst is added in an amount of 0.001 g to 0.1 g based on 1 ml of a second vanadium compound solution.

11. In claim 1, A method for reducing vanadium ions, wherein the reaction temperature of the above process (A) is 20°C to 80°C and the reaction time is 20 to 60 minutes.

12. In claim 1, A method for reducing vanadium ions, wherein the reaction temperature of the above process (B) is 60°C to 90°C and the reaction time is 5 to 60 minutes.

13. A method for producing an electrolyte for a vanadium redox battery using a method for reducing vanadium ions according to any one of claims 1 to 12.

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