Method for non-destructive recovery of electrode activity in a vanadium flow battery

Abstract

The present application relates to the technical field of charge-discharge battery for energy storage, in particular to a method for restoring the electrode activity of a full vanadium flow battery stack without damage, which overcomes the problem that the existing method for restoring the electrode activity of a stack without damage and disassembly still shortens the service life of the stack and reduces the energy storage capacity of the electrolyte, and has the following process: (1) closing the full vanadium flow battery, and discharging the electrolyte in the stack; (2) connecting the positive electrolyte inlet and outlet and the negative electrolyte inlet and outlet of the full vanadium flow battery stack to the storage container containing the recovery liquid, and making the recovery liquid in the storage container circulate through the positive electrolyte chamber of the stack and the negative electrolyte chamber of the stack to realize the non-damage restoration of the positive and negative electrode activity; the recovery liquid contains a main recovery agent, and the main recovery agent is an acid with a concentration of 3-10 mol / L; the temperature of the recovery liquid is 30-50 DEG C.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of energy storage charge-discharge battery, in particular to a method for restoring the electrode activity of a full vanadium redox flow battery stack without damage. BACKGROUND

[0002] The full vanadium redox flow battery (VRFB) is considered as an ideal choice for long-term energy storage due to its high safety, ultra-long charge-discharge cycle life (>20 years), renewable electrolyte, and flexible capacity expansion. However, the VRFB system equipment in the electrolyte and the stack also has the pain points of difficult operation and maintenance, high operation and maintenance cost, and complex operation and maintenance methods. It is still necessary to achieve breakthroughs through technological innovation. The problem of renewable electrolyte can be solved by various methods such as full mixing, water replenishment, addition of stabilizers, chemical reduction, and electrochemical reduction. The operation and maintenance of the stack is particularly prominent due to the aging and failure of the internal core materials, which leads to battery performance degradation. The reduction of reaction activity caused by the aging of the stack electrode, the excessive internal resistance of the battery, and the reduction of voltage efficiency, which leads to the reduction of battery discharge capacity and the increase of thermal loss, has a huge impact on the whole battery equipment temperature control system and overall energy efficiency.

[0003] Currently, there are mainly two types of methods to solve the problem of electrode activity failure in the stack:

[0004] The first type is to replace the stack or disassemble the stack to replace the electrode. Obviously, both of these two replacement methods have great disadvantages. Replacing the stack not only has a huge cost, but also causes material waste. The failure time of the key materials in the old stack is inconsistent, which causes waste of materials such as ion membranes, bipolar plates, liquid flow frames, and sealing pads, etc. in addition to the electrodes. Replacing the electrode by disassembling the stack not only damages the stack structure, but also increases the risk of internal and external leakage of the stack.

[0005] The second type is not disassembled electrode, that is, the method of realizing electrode activity recovery. For example, Chinese invention patents CN107565151A, CN108023107A and CN111509278A are all activity recovery treatment methods after the electrode runs for a long time. The representative scheme of this type of method is to first mix the electrolyte, then exchange the positive and negative terminals of the battery (reverse the electrodes), perform 1-10 charge-discharge cycles, and then exchange the terminals back. This electrode exchange method uses the oxygen evolution side reaction to increase the oxygen-containing functional groups of the negative electrode felt, thereby achieving the purpose of recovering the activity of the electrode felt. Although this method can recover the activity of the electrode felt, the activity recovery is at the expense of the balance of the electrolyte. The oxygen-containing functional groups increased by the electrode activity recovery come from the electrolytic water reaction in the electrolyte. Although this method is simple to operate, it has some disadvantages. The hydrogen evolution side reaction leads to water loss, disrupting the internal balance of the electrolyte. In the normal charge-discharge process, the hydrogen evolution and oxygen evolution side reactions also exist in the later stage of the battery operation, which will also reduce the water content in the electrolyte. When the total vanadium concentration and sulfate concentration in the electrolyte are too high, the vanadium ions of various valence states will be deposited and precipitated, affecting the service life of the electrode and the energy storage capacity of the electrolyte.

[0006] In addition, in the process of re-researching new electrode activity recovery methods without disassembling the electrode, we found that some existing technologies for improving the activity of carbon-based electrodes cannot be applied to the non-destructive recovery of aged electrode activity. The specific situation is as follows:

[0007] Chinese invention patent CN116505002A and Chinese invention patent CN1141566493A are both pre-treatments before assembly, also known as front-end treatment. Both of them immerse the graphite felt electrode in an active liquid, and then calcine it at high temperature in an activation gas or inert gas to obtain a graphite felt electrode with high electrochemical activity. Although this method obtains an electrode with high activity, it belongs to pre-treatment of front-end graphite felt electrode and is not suitable for recovery of the activity of aged electrodes after long-term cycling of the electrode stack.

[0008] The article "Influence of Acid and Heat Treatment Methods on the Performance of Graphite Felt Electrode for Vanadium Redox Flow Battery" in Journal of Shenyang Jianzhu University reports two methods for recovering the activity of aged or deactivated graphite felt electrodes. One method is to immerse the deactivated graphite felt electrode in 98% concentrated sulfuric acid for 5 hours to recover the electrochemical activity of the graphite felt electrode. The other method is to calcine the graphite felt electrode at 400 degrees Celsius for 30 hours to recover the electrochemical activity of the graphite felt electrode. Although both methods can recover the activity of the graphite felt electrode, considering the internal structure of the electrode stack, 98% concentrated sulfuric acid has a high viscosity, which can damage the ion membrane and is not convenient to drive the flow, so it is not suitable for recovering the activity of the graphite felt electrode in the stack. The 400-degree high-temperature calcination method is even more impractical. SUMMARY

[0009] The technical problem solved by the present application is to provide a method for non-destructively recovering the electrode activity of a full vanadium redox flow battery, so as to overcome the problem that the existing method for non-destructively recovering the electrode activity without disassembling the battery still shortens the service life of the battery and reduces the energy storage capacity of the electrolyte.

[0010] The technical solution adopted by the present application to solve the technical problem is a method for non-destructively recovering the electrode activity of a full vanadium redox flow battery, which has the following processes,

[0011] (I) Turn off the full vanadium redox flow battery, and discharge the electrolyte in the battery;

[0012] (II) Connect the positive electrolyte inlet and outlet and the negative electrolyte inlet and outlet of the full vanadium redox flow battery to a storage container containing a recovery liquid, and make the recovery liquid in the storage container circulate through the positive electrolyte chamber of the battery and circulate through the negative electrolyte chamber of the battery, so as to non-destructively recover the positive and negative electrode activities;

[0013] The recovery liquid contains a main recovery agent, and the main recovery agent is an acid with a concentration of 3-10 mol / L;

[0014] The temperature of the recovery liquid is 25-50 DEG C.

[0015] Since the commonly used full vanadium electrolyte is a 1.7 mol / L total vanadium and 4.5 mol / L sulfuric acid solution, the use of 3-10 mol / L acid minimizes the impact of the recovery liquid on the battery system and the battery, and also plays a role in activity recovery. The mechanism by which the acid as the main recovery agent increases activity is that the acid has the effect of increasing oxygen-containing functional groups on the electrode, i.e., the number of hydroxyl groups (-OH) and carboxyl groups (-COOH) inside the electrode increases. Oxygen-containing functional groups have a catalytic effect on the electrochemical reaction of the vanadium battery, thereby improving the performance of the vanadium battery. A high-concentration acid is not selected because as the treatment time is prolonged, the strong oxidizing property of the concentrated acid will continue to oxidize the beneficial functional groups on the electrode surface, causing them to become deep oxides CO or CO2, which are ultimately discharged, thereby reducing the performance of the vanadium battery. This situation has been verified by experiments.

[0016] Specifically, the recovery liquid circulates through the positive electrolyte chamber of the battery and circulates through the negative electrolyte chamber of the battery for ≥5 h.

[0017] Specifically, the main recovery agent is one or more of sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, oxalic acid, and citric acid.

[0018] Specifically, the recovery liquid has a weight content of ≤1% of an auxiliary recovery agent, and the auxiliary recovery agent is one or more of hydrogen peroxide, persulfate, dichromate, permanganate and chlorate solution. The role of the auxiliary recovery agent is that the auxiliary recovery agent is an oxidant, provides more oxygen-containing functional groups, and assists in recovering the electrode activity. For example, hydrogen peroxide, after circulating in the electrode, can form various intermediates such as adsorbed OH, O and OOH on the electrode surface through subsequent charge and discharge processes accompanied by electron and proton transfer, thereby providing more reactive substances on the surface of the carbon felt electrode.

[0019] Specifically, the main recovery agent and the auxiliary recovery agent are mixed and stirred for at least 1h to be uniformly mixed.

[0020] Specifically, the recovery liquid has a weight content of ≤1% of a side reaction inhibiting additive, and the side reaction inhibiting additive is one or more of Pb 2+ , Cd 2+ , Zn 2+ , As 3+ , Sb 3+ , Bi 3+ , In 3+ , Mn 4+ and Sn 2+ ion compounds. The role of the side reaction inhibiting additive is that, similar to the additives of the electrolyte, the addition of these metal ions can increase the hydrogen evolution potential on the electrode, thereby achieving the inhibition of the generation of side reactions.

[0021] Specifically, the circulating recovery liquid is filtered in process (two). The electrode felt or electrolyte precipitate is filtered to avoid contamination of the recovery liquid and affect the subsequent recycling.

[0022] Specifically, process (three) is performed, in which the stopped recovery liquid is pumped out, filtered and then pumped back into the storage container.

[0023] Specifically, the mesh number of the filter screen for filtering the recovery liquid is ≥1600 mesh.

[0024] Specifically, if the heat released by the battery stack itself cannot heat the recovery liquid to the required temperature, the recovery liquid in the storage container is heated in process (two).

[0025] The beneficial effects of the present application are:

[0026] 1. The electrode has electrocatalytic activity comparable to that of the initial electrode material, which can greatly prolong the service life of the all-vanadium battery and has no negative effect on the comprehensive performance and cycle life of the all-vanadium redox flow battery.

[0027] 2. The recovery liquid has no effect on the ion exchange membrane in the stack.

[0028] 3、acid base recovery solution treatment graphite felt electrode makes its surface oxygen-containing functional groups increase, has catalytic effect on vanadium battery electrochemical reaction;

[0029] 4、The electrode activity recovery method of the application, wherein the electrode activity recovery solution can be recycled, and is environment-friendly and energy-saving. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 is the ability of the ion exchange membrane to prevent vanadium after flowing through the positive and negative electrode liquid chamber with different concentrations of sulfuric acid base recovery solution. DETAILED DESCRIPTION

[0031] The technical features of the application are further illustrated by the following examples in conjunction with the accompanying drawings, but the protection scope of the application is not limited to the following examples.

[0032] The electrode targeted by the application is generally a carbon electrode, such as carbon felt, graphite felt, modified graphite felt, and modified carbon felt.

[0033] Example 1

[0034] Carbon felt is used as the electrode for full vanadium flow battery charging and discharging test, the positive and negative electrolyte is 4.5 mol / L H2SO4 solution with a total vanadium concentration of 1.7 mol / L, each 60 mL, the power and capacity ratio of the whole battery is 1:4, the charging and discharging test is carried out at a current density of 160 mA / cm 2 The average value after 5 cycles is calculated, and the coulombic efficiency, voltage efficiency and energy efficiency of the new electrode felt, the electrode felt after 1000 cycles and the regenerated felt after recovery solution treatment are recorded, as shown in Table 1 below. Compared with the first cycle, the voltage efficiency of the full vanadium flow battery in this embodiment decreases from 83% to 72.3% after 1000 cycles, then the battery test module is stopped, the positive and negative circulation pumps are reversed, the positive and negative electrolyte in the module cavity is emptied, then the inlet and outlet of the single battery positive and negative electrodes are switched to the recovery solution channel, the recovery solution is continuously introduced into the stack at 35℃, the recovery solution is a mixture of 6 mol / L sulfuric acid, 1% hydrogen peroxide based on the weight of the whole recovery solution and 0.5% bismuth nitrate Bi(NO3)3 based on the weight of the whole recovery solution, and the continuous circulation is maintained for 5 hours, finally the recovery solution in the module is emptied, and the inlet and outlet of the stack are switched back to the original electrolyte tank connected to the positive and negative electrodes, and the charging and discharging test is carried out at a current density of 160 mA / cm 2

[0035] Table 1: Coulombic efficiency (CE), voltage efficiency (VE) and energy efficiency (EE) of the single battery using activated carbon felt as the electrode in Example 1 ​

[0036]

[0037] In addition, the activated carbon felt in Example 1 was treated with the recovery liquid. The cyclic voltammetry test results are shown in Table 2.

[0038] Table 2: Cyclic voltammetry test results of the activated carbon felt treated with the recovery liquid

[0039]

[0040] E in Table 2 pa E represents the electrode potential value of the oxidation peak, E pc E represents the electrode potential value of the reduction peak, ΔE P E represents the electrode potential difference of the redox couple doublet (the smaller the difference, the higher the electrode activity), the electrode felt treated with the recovery liquid has a redox potential difference of 0.08 V smaller than that of the electrode felt after 1000 cycles, indicating that the activity of the electrode felt is restored.

[0041] Example 2

[0042] The carbon felt was used as an electrode for the full vanadium redox flow battery to perform charge and discharge tests. The positive and negative electrolytes were both 4.5 mol / L H2SO4 solution with a total vanadium concentration of 1.7 mol / L, 60 mL each, and the power and capacity ratio of the entire battery was 1:1. The charge and discharge tests were performed at a current density of 160 mA / cm 2 The average values after 5 cycles were calculated, and the Coulomb efficiency, voltage efficiency and energy efficiency of the new electrode felt, the electrode felt after 1000 cycles and the regenerated felt treated with the recovery liquid were recorded, as shown in Table 3 below. Compared with the first cycle, the voltage efficiency of the full vanadium redox flow battery in this embodiment decreased from 82.1% to 80.3% after 1000 cycles. Subsequently, the battery test module was stopped, the positive and negative circulation pumps were reversed, the positive and negative electrolytes in the module cavity were emptied, and then the positive and negative inlet and outlet of the single battery were switched to the recovery liquid channel. The recovery liquid was continuously introduced into the stack at 45°C, and the recovery liquid was a mixture of 7 mol / L sulfuric acid and 0.5% hydrogen peroxide based on the weight of the entire recovery liquid. The continuous circulation was maintained for 5 hours, and finally the recovery liquid in the module was emptied and the inlet and outlet of the stack were switched back to the original electrolyte tanks connected to the positive and negative electrodes. The charge and discharge tests were performed at a current density of 160 mA / cm 2

[0043] Table 3: Coulomb efficiency (CE), voltage efficiency (VE) and energy efficiency (EE) of the single battery using the activated carbon felt treated with the recovery liquid as the electrode in Example 2

[0044] ​

[0045] Example 3

[0046] The graphite felt was used as the electrode for the full vanadium flow battery to perform the charge and discharge test. The positive and negative electrolyte was the common commercial 1.7 mol / L total vanadium concentration 4.5 mol / L H2SO4 solution of 60 mL each. The power and capacity ratio of the whole battery was 1:1. The charge and discharge test was performed at the current density of 160 mA / cm 2 The average value after 5 cycles was calculated. The coulombic efficiency, voltage efficiency and energy efficiency of the new electrode felt, the electrode felt after 1000 cycles and the regenerated felt after the recovery liquid treatment were recorded respectively. As shown in Table 4, compared with the first cycle, the voltage efficiency of the full vanadium flow battery in this embodiment decreased from 80.6% to 70.9% after 1000 cycles. Then, the battery test module was stopped. The positive and negative circulation pumps were reversed. The positive and negative electrolyte in the module cavity was emptied. Then, the inlet and outlet of the single battery positive and negative electrodes were switched to the recovery liquid channel. The mixed acid recovery liquid composed of 8 mol / L sulfuric acid and 8 mol / L phosphoric acid at 35°C was continuously introduced into the stack. The weight ratio of sulfuric acid and phosphoric acid was 50% respectively. The continuous circulation was maintained for 5 hours. Finally, the recovery liquid in the module was emptied. The inlet and outlet of the stack were switched back to the original electrolyte tank connected to the positive and negative electrodes. The charge and discharge test was performed at the current density of 160 mA / cm 2 The voltage efficiency increased from 70.9% to 75.8%. The charge and discharge efficiency of the battery was restored.

[0047] Table 4: Coulombic efficiency (CE), voltage efficiency (VE) and energy efficiency (EE) of the single battery using the activated carbon felt as the electrode in Example 3

[0048]

[0049] In addition, the influence of the recovery liquid on the ion exchange membrane in the stack needs to be considered. After the relevant test, the 35°C recovery liquid also flowed through the ion exchange membrane for 5 hours. The vanadium blocking performance of the ion exchange membrane in the stack was determined (there is a corresponding test method in the standard). The concentration of the permeated vanadium ions represented the vanadium blocking capacity of the ion exchange membrane. The smaller the concentration of the permeated vanadium ions, the stronger the vanadium blocking capacity of the membrane. It was also proved that the influence of the recovery liquid on the vanadium blocking capacity of the membrane was smaller after the recovery liquid treatment. As shown in Table 5, after 3 hours of vanadium ion permeation, the preferred recovery liquid with the sulfuric acid concentration of 6 mol / L, 7 mol / L and 8 mol / L had little influence on the vanadium ion permeation of the ion exchange membrane. Figure 1

[0050] Finally, the recovery liquid was also filtered. The filter device could be arranged in the recovery system pipeline or filtered by the separate filter device outside the system pipeline.​

[0051] The above-described embodiments according to the present application are intended to be illustrative only. Changes can be made by those skilled in the art, without departing from the scope of the present application, which is defined by the following claims. The technical scope of the present application is not limited to the above-described embodiments. The technical scope of the present application must be determined based on the scope of the claims.

Claims

1. A method for non-destructive recovery of electrode activity in a vanadium redox flow battery, characterized by: The process comprises the following steps: (I) closing the all-vanadium redox flow battery and discharging electrolyte in the stack; (II) connecting the inlet and outlet of the positive electrolyte and the inlet and outlet of the negative electrolyte of the all-vanadium redox flow battery stack to the storage container containing the recovery liquid, and circulating the recovery liquid in the storage container through the positive electrolyte chamber of the stack and the negative electrolyte chamber of the stack to realize the lossless recovery of the positive and negative electrode activities; The recovery liquid contains a main recovery agent, and the main recovery agent is an acid with a concentration of 3-10 mol / L; The temperature of the recovery liquid is 25-50℃; The main recovery agent is one or more of sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, oxalic acid and citric acid; The recovery liquid contains an auxiliary recovery agent with a weight content of ≤1%, and the auxiliary recovery agent is one or more of hydrogen peroxide, persulfate, dichromate, permanganate and chlorate solution; The recovery solution has a weight content of ≤1% of a side reaction inhibiting additive, which is one or more of the following ion compounds: Pb 2+ , Cd 2+ , Zn 2+ , As 3+ , Sb 3+ , Bi 3+ , In 3+ , Mn 4+ and Sn 2+ .

2. The method of losslessly restoring full vanadium redox flow battery electrode activity of claim 1, wherein: The recovery liquid is circulated through the positive electrolyte chamber of the stack and the negative electrolyte chamber of the stack for ≥5h.

3. The method of losslessly restoring full vanadium redox flow battery electrode activity of claim 1, wherein: The main recovery agent and the auxiliary recovery agent are mixed and stirred for at least 1h to be uniformly mixed.

4. The method of losslessly restoring full vanadium redox flow battery electrode activity of claim 1, wherein: The circulating recovery liquid is filtered in process (II).

5. The method of losslessly restoring full vanadium redox flow battery electrode activity of claim 1, wherein: Process (III) is provided for filtering the recovery liquid.

6. The method of losslessly restoring full vanadium redox flow battery electrode activity according to claim 4 or 5, characterized in that: The mesh number of the filter screen for filtering the recovery liquid is ≥1600 mesh.

7. The method of losslessly restoring full vanadium redox flow battery electrode activity of claim 1, wherein: The recovery liquid in the storage container is heated in process (II).

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