Method for lossless recovery of all-vanadium redox flow pile electrode activity

Abstract

The invention relates to the technical field of charge-discharge batteries for energy storage, in particular to a method for losslessly recovering the electrode activity of an all-vanadium redox flow electric pile, which overcomes the problems that the service life of the electric pile is shortened and the energy storage capacity of electrolyte is reduced in the conventional method for recovering the electrode activity in a lossless manner without disassembling the electric pile, and has the following processes: (1) closing an all-vanadium redox flow battery, discharging the electrolyte in the electric pile; (2) communicating a positive electrode electrolyte inlet and outlet and a negative electrode electrolyte inlet and outlet of the all-vanadium redox flow battery stack to a liquid storage container in which recovery liquid is stored, and enabling the recovery liquid in the liquid storage container to circularly flow through a positive electrode liquid chamber of the stack and circularly flow through a negative electrode liquid chamber of the stack so as to realize lossless recovery of the activity of a positive electrode and a negative electrode; the recovery liquid contains a main recovery agent, and the main recovery agent is an acid with the concentration of 3-10 mol / L; the temperature of the recovery liquid is 30-50 DEG C.

Description

Technical Field

[0001] The present invention relates to the technical field of charge-discharge batteries for energy storage, and in particular to a method for losslessly recovering the activity of electrodes of an all-vanadium liquid flow battery stack used in an all-vanadium liquid flow battery. Background Art

[0002] Vanadium redox flow batteries (VRFBs) are considered an ideal choice for long-term energy storage due to their high safety, ultra-long charge and discharge cycle life (>20 years), electrolyte regeneration, and flexible capacity expansion. However, VRFB system equipment also has pain points in both the electrolyte and the stack, such as difficult operation and maintenance, high operation and maintenance costs, and complex operation and maintenance methods. Breakthroughs are still needed through technological innovation. The problem of electrolyte regeneration can be solved through various methods such as full mixing, water replenishment, stabilizer addition, drug chemical reduction, and electrochemical reduction. The operation and maintenance of the stack is particularly prominent due to the aging and failure of internal core materials, resulting in battery performance degradation. Among them, the aging of the stack electrodes leads to reduced reaction activity, excessive battery internal resistance, and reduced voltage efficiency, which in turn leads to reduced battery discharge capacity and increased heat loss, which has a huge impact on the temperature control system and overall energy efficiency of the entire battery equipment.

[0003] There are currently two main methods to solve the problem of electrode activity failure in battery stacks: The first type is to replace the fuel cell stack or dismantle the stack to replace the electrodes. Obviously, both replacement methods have very big disadvantages. Replacing the fuel cell stack is not only costly, but also causes waste of materials. The failure time of key materials in the old fuel cell stack is inconsistent, which results in waste of materials other than electrodes, such as ion membranes, bipolar plates, liquid flow frames and sealing gaskets; dismantling the stack to replace electrodes not only destroys the fuel cell stack structure, but also increases the risk of internal and external leakage of the fuel cell stack.

[0004] The second type is a method for restoring electrode activity without disassembling the battery stack. For example, Chinese invention patents CN107565151A, CN108023107A and CN111509278A are all methods for restoring the activity of the battery stack after it has been running for a long time. A representative solution of this type of method is to first fully mix the electrolyte, then exchange the positive and negative terminals of the battery (reversal), perform 1-10 charge and 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 restoring the activity of the electrode felt. Although this method can restore the activity of the electrode felt, this activity recovery comes at the expense of the balance of the electrolyte. The oxygen-containing functional groups that increase the electrode activity recovery come from the water electrolysis reaction in the electrolyte. Although this method is simple to operate, it also has shortcomings: the hydrogen evolution side reaction causes water loss and destroys the internal balance of the electrolyte. In the later stage of battery operation, the hydrogen evolution and oxygen evolution side reactions continue to exist during normal charging and discharging, which will also reduce the water content in the electrolyte. When the total vanadium concentration and sulfate concentration in the electrolyte are too high, it will cause vanadium ions of various valence states to precipitate and precipitate, affecting the life of the battery stack and the energy storage capacity of the electrolyte.

[0005] In addition, when we were re-studying new methods for non-destructive electrode activity recovery without disassembling the battery stack, 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: Chinese invention patents CN116505002A and CN1141566493A are both pre-treatment methods before stacking, also known as front-end treatment. In both cases, graphite felt electrodes are impregnated with an active liquid and then calcined at high temperature with an activated gas or inert gas to obtain graphite felt electrodes with high electrochemical activity. Although this method produces highly active electrodes, it is a front-end graphite felt electrode pretreatment and is not suitable for recovering the activity of aged electrodes after long-term cycling of the stack. The Journal of Shenyang Jianzhu University published an article titled "Effects of Acid and Heat Treatment Methods of Graphite Felt Electrodes on the Performance of All-vanadium Liquid Flow Batteries," which reported two methods for recovering aging or inactivated graphite felt electrodes. One method is to soak the inactivated graphite felt electrodes in 98% concentrated sulfuric acid for 5 hours to restore the electrochemical activity of the graphite felt electrodes. The other method is to calcine them at 400 degrees for 30 hours to restore the electrochemical activity of the graphite felt electrodes. Although both methods can restore the activity of the graphite felt electrodes, given the internal structure of the fuel cell stack, the viscosity of 98% concentrated sulfuric acid is relatively high, which will damage the ion membrane and make it difficult to drive flow, so it is not suitable for in-stack activity recovery of graphite felt electrodes. The 400-degree high-temperature calcination method is even less feasible. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a method for non-destructive restoration of electrode activity of an all-vanadium liquid flow battery stack, thereby overcoming the problem that the existing non-destructive method for restoring electrode activity without disassembling the battery stack still shortens the battery stack life and reduces the electrolyte energy storage capacity.

[0007] The technical solution adopted by the present invention to solve the technical problem is: a method for non-destructively restoring the activity of the electrodes of an all-vanadium liquid flow battery, comprising the following steps: (1) Shut down the all-vanadium redox flow battery and drain the electrolyte from the battery stack; (2) connecting the cathode electrolyte inlet and outlet and the anode electrolyte inlet and outlet of the all-vanadium redox flow battery stack to a liquid storage container containing a recovery liquid, and circulating the recovery liquid in the liquid storage container through the cathode liquid chamber and the anode liquid chamber of the stack to achieve lossless recovery of the activity of the positive and negative electrodes; The recovery solution contains a main recovery agent, which is an acid with a concentration of 3-10 mol / L; The temperature of the recovery solution is 25-50°C.

[0008] Given that the commonly used all-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 can not only minimize the impact of the recovery solution on the battery system and the stack, but also play a role in activity recovery. The mechanism by which acid, as the main recovery agent, increases activity is that acid has the effect of increasing oxygen-containing functional groups on the electrode, that is, the hydroxyl (-OH) and carboxyl (-COOH) groups inside the electrode increase, and the oxygen-containing functional groups have a catalytic effect on the electrochemical reaction of the vanadium battery, thereby improving the performance of the vanadium battery; the reason why higher concentrations of acid are not selected is that as the treatment time increases, the strong oxidizing property of the concentrated acid will continue to oxidize the beneficial functional groups on the electrode surface, turning them into deep oxides CO or CO2 that are eventually emitted, which in turn reduces the performance of the vanadium battery. This situation has been verified by experiments.

[0009] Specifically, the time for the recovery liquid to circulate through the positive electrode liquid chamber of the fuel cell stack and the time for the recovery liquid to circulate through the negative electrode liquid chamber of the fuel cell stack are both ≥5h.

[0010] Specifically, the main restoration agent is one or more of sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, oxalic acid and citric acid.

[0011] Specifically, the recovery solution contains an auxiliary recovery agent at a weight content of ≤1%, and the auxiliary recovery agent is one or more of hydrogen peroxide, persulfate, dichromate, permanganate, and chlorate solutions. The use of auxiliary recovery agents is as follows: auxiliary recovery agents are oxides, providing more oxygen-containing functional groups to help restore electrode activity. For example, after hydrogen peroxide circulates within the electrode, it undergoes subsequent charge and discharge processes accompanied by electron and proton transfer, forming various intermediates on the electrode surface, such as adsorbed OH, O, OOH, and other oxygen-containing functional groups, providing more reactive species on the carbon felt electrode surface.

[0012] Specifically, the main restoration agent and the auxiliary restoration agent are mixed and stirred for at least 1 hour until they are uniformly mixed.

[0013] Specifically, the recovery solution contains an additive that inhibits side reactions at a weight content of ≤1%, and the additive that inhibits side reactions is Pb 2+ 、Cd 2+ 、Zn 2+ 、As 3+ 、Sb 3+ 、Bi 3+ 、In 3+ 、Mn 4+ and Sn 2+ One or more ionic compounds. The purpose of using additives to inhibit side reactions is: similar to electrolyte additives, the addition of these metal ions can increase the hydrogen evolution potential on the electrode, thereby inhibiting the occurrence of side reactions.

[0014] Specifically, in process (2), the circulating recovery fluid is filtered. The electrode felt or electrolyte sediment is filtered to prevent the recovery fluid from being contaminated and affecting subsequent recycling.

[0015] Specifically, the process includes step (3): extracting the recovery fluid that has stopped being used, filtering it, and then pumping it back into the liquid storage container.

[0016] Specifically, the mesh number of the filter screen for filtering the recovery liquid is ≥1600 mesh.

[0017] Specifically, if the heat released by the battery stack itself cannot heat the recovery liquid to the required temperature, then in process (2), the recovery liquid in the liquid storage container is heated.

[0018] The beneficial effects of the present invention are: 1. It has an electrocatalytic activity equivalent to that of the initial electrode material, which can greatly extend the service life of the all-vanadium battery and has no negative impact on the overall performance and cycle life of the all-vanadium redox flow battery; 2. The recovery solution has no effect on the ion exchange membrane in the fuel cell stack; 3. The acid-based recovery solution is used to treat the graphite felt electrode to increase the oxygen-containing functional groups on its surface, which has a catalytic effect on the electrochemical reaction of the vanadium battery; 4. The electrode activity regeneration method adopted by the present invention, wherein the electrode activity recovery liquid can be recycled, is environmentally friendly and energy-saving. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is a diagram of the vanadium resistance of the ion membrane after different concentrations of sulfate-based recovery solutions flow through the positive and negative electrode liquid chambers. DETAILED DESCRIPTION

[0020] The present invention further illustrates the technical features with the following embodiments in conjunction with the accompanying drawings, but the protection scope of the present invention is not limited to the following embodiments.

[0021] The electrodes targeted by the present invention are generally carbon electrodes, such as carbon felt, graphite felt, modified graphite felt, and modified carbon felt.

[0022] Example 1 The carbon felt was used as the electrode for the all-vanadium redox flow battery charge and discharge test. The positive and negative electrolytes were 60 mL each of the common commercial 1.7 mol / L total vanadium concentration 4.5 mol / L H2SO4 solution. The power and capacity ratio of the whole battery was 1:4. At 160 mA / cm 2 The charge and discharge cycles were tested at a current density of 500 nm, and the average values ​​after 5 cycles were 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 treatment with the recovery solution were recorded respectively, as shown in Table 1 below. Compared with the first cycle, the voltage efficiency of the all-vanadium liquid flow battery in this embodiment decreased from 83% to 72.3% after 1000 cycles. Subsequently, the battery test module was stopped, the positive and negative circulation pumps were reversed, and the positive and negative electrolytes in the module cavity were drained. Then, the positive and negative inlets and outlets of the single cell were switched to the recovery solution channel, and the 35°C recovery solution was continuously passed into the stack. The recovery solution was a mixture of 6 mol / L sulfuric acid, 1% of the weight of the total recovery solution, and 0.5% of the weight of the bismuth nitrate Bi(NO3)3. The cycle was maintained for 5 hours. Finally, the recovery solution in the module was drained, and the inlet and outlet of the stack were switched back to the electrolyte tank originally connected to the positive and negative electrodes. At 160 mA / cm 2 The battery was charged and discharged for 5 cycles at a current density of 100 nm. At this time, the voltage efficiency increased from 72.3% to 83.2%, and the battery's charge and discharge efficiency was restored.

[0023] Table 1: Coulombic efficiency (CE), voltage efficiency (VE) and energy efficiency (EE) of the single cell using the activated carbon felt treated with the recovery solution as the electrode in Example 1 In addition, the cyclic voltammetry curve test results of Example 1 using the recovery solution to treat the activated carbon felt are shown in Table 2.

[0024] Table 2: Cyclic voltammetry test results of activated carbon felt treated with recovery solution E in Table 2 pa / V represents the electrode potential value of the oxidation peak, E pc / V represents the electrode potential value of the reduction peak, ΔE P / V represents the double-peak electrode potential difference of the redox couple (the smaller the difference, the higher the electrode activity). The redox peak potential difference of the electrode felt after treatment with the recovery solution is 0.08V smaller than that of the electrode felt after 1000 cycles, indicating that the activity of the electrode felt has been restored.

[0025] Example 2 The carbon felt was used as the electrode for the all-vanadium redox flow battery charge and discharge test. The positive and negative electrolytes were 60 mL each of the common commercial 1.7 mol / L total vanadium concentration 4.5 mol / L H2SO4 solution. The power and capacity ratio of the whole battery was 1:1. At 160 mA / cm 2 The charge and discharge cycles were tested at a current density of 500 nm, and the average values ​​after 5 cycles were 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 treatment with the recovery solution were recorded respectively, as shown in Table 3 below. Compared with the first cycle, the voltage efficiency of the all-vanadium liquid 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, and the positive and negative electrolytes in the module cavity were drained. Then, the positive and negative inlets and outlets of the single cell were switched to the recovery solution channel, and the 45°C recovery solution was continuously passed into the stack. The recovery solution was a mixture of 7 mol / L sulfuric acid and 0.5% hydrogen peroxide by weight of the entire recovery solution. The continuous circulation was maintained for 5 hours. Finally, the recovery solution in the module was drained, and the inlet and outlet of the stack were switched back to the electrolyte tank originally connected to the positive and negative electrodes. At 160 mA / cm 2 The battery was charged and discharged for 5 cycles at a current density of 100 nm. At this time, the voltage efficiency increased from 80.3% to 82.9%, and the battery's charge and discharge efficiency was restored.

[0026] Table 3: Coulombic efficiency (CE), voltage efficiency (VE) and energy efficiency (EE) of the single cell using the activated carbon felt treated with the recovery solution as the electrode in Example 2 Example 3 Graphite felt was used as an electrode for the all-vanadium redox flow battery charge and discharge test. The positive and negative electrolytes were both 60 mL each of a common commercial 1.7 mol / L total vanadium concentration 4.5 mol / L H2SO4 solution. The power and capacity ratio of the entire battery was 1:1. At 160 mA / cm 2 The charge and discharge cycles were tested at a current density of 500 nm for 5 cycles, and the average values ​​after 5 cycles were 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 treatment with the recovery solution were recorded respectively, as shown in Table 4 below. Compared with the first cycle, the voltage efficiency of the all-vanadium liquid flow battery in this embodiment decreased from 80.6% to 70.9% after 1000 cycles. Subsequently, the battery test module was stopped, the positive and negative circulation pumps were reversed, and the positive and negative electrolytes in the module cavity were drained. Then, the positive and negative inlets and outlets of the single cell were switched to the recovery solution channel, and a mixed acid recovery solution consisting of 8 mol / L sulfuric acid and 8 mol / L phosphoric acid at 35°C was continuously passed into the stack, with the weight ratio of sulfuric acid to phosphoric acid being 50% each. The continuous circulation was maintained for 5 hours, and finally the recovery solution in the module was drained, and the inlet and outlet of the stack were switched back to the electrolyte tank originally connected to the positive and negative electrodes. At 160 mA / cm 2 The battery was charged and discharged for 5 cycles at a current density of 100 nm. At this time, the voltage efficiency increased from 70.9% to 75.8%, and the battery's charge and discharge efficiency was restored.

[0027] Table 4: Coulombic efficiency (CE), voltage efficiency (VE) and energy efficiency (EE) of the single cell using the activated carbon felt treated with the recovery solution as the electrode in Example 3 In addition, it is necessary to consider the impact of the recovery solution on the ion exchange membrane in the fuel cell stack. After the relevant tests, the 35°C recovery solution also flows through the ion exchange membrane for 5 hours, and then the vanadium resistance performance of the ion exchange membrane in the fuel cell stack is measured (the standard has the corresponding test method). The concentration of vanadium ions permeated represents the vanadium resistance of the ion exchange membrane. The lower the concentration of permeated vanadium ions, the stronger the vanadium resistance of the membrane. It also proves that after the membrane is treated with the recovery solution, the recovery solution has less impact on the vanadium resistance of the membrane. Figure 1 As shown in , after 3 hours of vanadium ion penetration, the preferred recovery solution sulfate concentrations of 6 mol / L, 7 mol / L, and 8 mol / L have little effect on the penetration of vanadium ions through the ion exchange membrane.

[0028] Finally, the recovery fluid is filtered. A filtering device may be provided in the recovery system pipeline, or a separate filtering device may be provided outside the system pipeline.

[0029] With the above-described preferred embodiments of the present invention as a guide, and with reference to the above description, relevant personnel are fully capable of making various changes and modifications without departing from the technical scope of this invention. The technical scope of this invention is not limited to the contents of the specification and must be determined according to the scope of the claims.

Claims

1. A method for non-destructively restoring the activity of an all-vanadium liquid flow battery electrode, characterized by: The process is as follows: (1) Shut down the all-vanadium redox flow battery and drain the electrolyte from the battery stack; (2) connecting the cathode electrolyte inlet and outlet and the anode electrolyte inlet and outlet of the all-vanadium redox flow battery stack to a liquid storage container containing a recovery liquid, and circulating the recovery liquid in the liquid storage container through the cathode liquid chamber and the anode liquid chamber of the stack to achieve lossless recovery of the activity of the positive and negative electrodes; The recovery solution contains a main recovery agent, which is an acid with a concentration of 3-10 mol / L; The temperature of the recovery solution is 25-50°C; The main restoration 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.

2. The method for losslessly restoring the activity of an all-vanadium liquid flow battery electrode according to claim 1, characterized in that: The time for the recovery liquid to circulate through the positive electrode liquid chamber of the battery stack and the time for the recovery liquid to circulate through the negative electrode liquid chamber of the battery stack are both ≥5h.

3. The method for losslessly restoring the activity of an all-vanadium liquid flow battery electrode according to claim 1, characterized in that: The main restoration agent and the auxiliary restoration agent are mixed and stirred for at least 1 hour until they are uniformly mixed.

4. The method for losslessly restoring the activity of an all-vanadium liquid flow battery electrode according to claim 1 or 3, characterized in that: The recovery solution contains an additive that inhibits side reactions with a weight content of ≤1%, and the additive that inhibits side reactions is Pb 2+ 、Cd 2+ 、Zn 2 + 、As 3+ 、Sb 3+ 、Bi 3+ 、In 3+ 、Mn 4+ and Sn 2+ One or more ionic compounds.

5. The method for losslessly restoring the activity of an all-vanadium liquid flow battery electrode according to claim 1, characterized in that: In process (2), the circulating recovery fluid is filtered.

6. The method for losslessly restoring the activity of an all-vanadium liquid flow battery electrode according to claim 1, characterized in that: The process (3) is to extract the recovery fluid that has stopped being used, filter it, and then pump it back into the storage container.

7. The method for losslessly restoring the activity of an all-vanadium liquid flow battery electrode according to claim 5 or 6, characterized in that: The mesh number of the filter for filtering the recovery liquid should be ≥1600 mesh.

8. The method for losslessly restoring the activity of an all-vanadium liquid flow battery electrode according to claim 1, characterized in that: In process (2), the recovery liquid in the liquid storage container is heated.

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