Method for preparing proton conducting membrane and proton conducting membrane

Abstract

The invention discloses a method for preparing a proton conducting membrane and the proton conducting membrane. The method comprises the following steps: providing a membrane casting solution containing polyvinylidene fluoride, a water-soluble nucleating agent and polyvinylpyrrolidone; casting the membrane casting liquid on the surface of the support body to form a membrane casting liquid thin layer; heating the film casting liquid thin layer to enable crystal nucleuses to grow and remove the organic solvent in the crystal nucleuses to prepare a thin film; placing the film in deionized water to remove the water-soluble nucleating agent so as to prepare a porous film; and placing the porous membrane in an initiation system, so that polyvinylpyrrolidone contained in the porous membrane is subjected to a cross-linking reaction, and the proton conducting membrane is prepared. According to the method provided by the embodiment of the invention, polyvinylidene fluoride, the water-soluble nucleating agent and polyvinylpyrrolidone are used for preparing the proton conducting membrane, so that the physical and chemical stability, selectivity and conductivity of the proton conducting membrane are improved.

Description

Technical Field

[0001] This application belongs to the technical field of proton-conducting membranes, and particularly relates to a method for preparing a proton-conducting membrane and a proton-conducting membrane. Background Art

[0002] Energy storage systems can effectively suppress the intermittency and volatility brought by the grid connection of renewable energy power generation, maintain grid stability, and ensure the safety of the power system. The vanadium redox flow battery (VRFB) is an electrochemical energy storage technology. Its core is to use the multiple valence states of vanadium to carry out redox reactions between the positive and negative electrodes, thereby realizing the storage and release of electrical energy, and having advantages such as decoupling of power and capacity, flexible operation, and long cycle life.

[0003] The proton-conducting membrane is one of the core components of the vanadium redox flow battery. The proton-conducting membrane isolates the positive and negative electrode electrolytes when flowing through the stack, avoiding self-discharge caused by the penetration of vanadium ions with different valence states and resulting in capacity loss, while allowing proton migration to ensure charge balance. The performance and structure of the proton-conducting membrane determine the efficiency and cycle stability of the vanadium battery. The proton-conducting membrane required for the vanadium redox flow battery should have the following characteristics: ① low vanadium ion permeability, small cross-contamination, reducing battery self-discharge and improving energy efficiency. ② high hydrogen proton permeability, small membrane resistance, and improving voltage efficiency. ③ having a certain mechanical strength, being resistant to chemical corrosion and electrochemical oxidation, and ensuring a long cycle life. ④ small water penetration during battery charge and discharge, maintaining the water balance of the positive and negative electrode electrolytes.

[0004] The proton-conducting membranes of vanadium redox flow batteries are mainly divided into ion exchange membranes and nanoporous membranes. Due to the insufficient chemical stability of ion exchange groups in strong acid and strong oxidation environments, ion exchange membranes usually have poor stability and high costs; nanoporous membranes usually have low selectivity for hydrogen ions and vanadium ions and cannot meet the Coulomb efficiency requirements of the battery. Therefore, improvement is urgently needed. Summary of the Invention

[0005] The embodiments of this application provide a method for preparing a proton-conducting membrane and a proton-conducting membrane. The method of the embodiments of this application uses polyvinylidene fluoride, a water-soluble nucleating agent, and polyvinylpyrrolidone to prepare a proton-conducting membrane, improving the physical, chemical stability, selectivity, and conductivity of the proton-conducting membrane.

[0006] In a first aspect, the embodiments of this application provide a method for preparing a proton-conducting membrane, including:

[0007] Providing a casting solution containing polyvinylidene fluoride, a water-soluble nucleating agent, and polyvinylpyrrolidone;

[0008] Making the casting solution flow on the surface of a support to form a thin layer of the casting solution;

[0009] Heat the thin layer of the casting solution to grow crystal nuclei and remove the organic solvents therein to obtain a film.

[0010] Place the film in deionized water to remove the water-soluble nucleating agent to obtain a porous membrane.

[0011] Place the porous membrane in an initiation system to cause the polyvinylpyrrolidone contained in the porous membrane to undergo a cross-linking reaction to obtain a proton-conducting membrane.

[0012] According to an embodiment of one aspect of the present application, the mass fraction of polyvinylidene fluoride in the casting solution is 10% to 25%.

[0013] According to an embodiment of one aspect of the present application, the water-soluble nucleating agent is a polymer monomer having a hydrophilic group and a carbon-carbon double bond.

[0014] According to an embodiment of one aspect of the present application, the water-soluble nucleating agent includes one or more of vinyl sulfonate, styrene sulfonate, and allyl sulfonate.

[0015] According to an embodiment of one aspect of the present application, the mass fraction of the water-soluble nucleating agent in the casting solution is 1% to 10%.

[0016] According to an embodiment of one aspect of the present application, the polyvinylpyrrolidone includes one or more of K15, K30, K60, and K90 in terms of the average molecular weight.

[0017] According to an embodiment of one aspect of the present application, the mass fraction of polyvinylpyrrolidone in the casting solution is 0.5% to 10%.

[0018] According to an embodiment of one aspect of the present application, the organic solvents include one or more of N,N-dimethylacetamide, N,N-dimethylformamide, N-methylpyrrolidone, and dimethyl sulfoxide.

[0019] According to an embodiment of one aspect of the present application, the thickness of the thin layer of the casting solution is 100 - 500 microns.

[0020] According to an embodiment of one aspect of the present application, the heating temperature is 60 - 150 °C; the heating time is 1 - 6 h.

[0021] According to an embodiment of one aspect of the present application, the initiation system includes 0.5% to 8% initiator by mass percentage.

[0022] According to an embodiment of one aspect of the present application, the initiator includes one or more of persulfate, hydrogen peroxide, and azobisisobutyronitrile.

[0023] According to an embodiment of one aspect of the present application, the solvent of the initiation system is water.

[0024] In a second aspect, an embodiment of the present application provides a proton-conducting membrane prepared by the method of the first aspect.

[0025] The embodiments of the present application have at least the following beneficial effects:

[0026] The method provided by the embodiments of the present application adopts the blending technology of polyvinylpyrrolidone and polyvinylidene fluoride, enabling the two polymer chains to intertwine with each other, improving the dimensional stability of the proton-conducting membrane, slowing down the water solubility of polyvinylpyrrolidone during the preparation process, and the incorporation of polyvinylpyrrolidone does not affect the growth of crystal nuclei. The pore size of the proton-conducting membrane is controlled by heating; in addition, polyvinylpyrrolidone undergoes a cross-linking reaction in the initiation system to form cross-linked polyvinylpyrrolidone, improving the physical and chemical stability of the proton-conducting membrane, thereby avoiding the dissolution of hydrophilic polyvinylpyrrolidone contained therein when the proton-conducting membrane is in use. The blending introduction and cross-linking of polyvinylpyrrolidone by heating increase the crystal grain size of polyvinylidene fluoride to a certain extent, but do not affect the pore structure of the polyvinylidene fluoride porous membrane. Therefore, the obtained proton-conducting membrane has abundant pores, ensuring a smooth proton transport channel.

[0027] In the proton-conducting membrane provided by the embodiments of the present application, the polyvinylidene fluoride chains contained therein are intertwined with polyvinylpyrrolidone. Polyvinylpyrrolidone is protonated in the acidic environment of the electrolyte, causing the membrane surface to carry a positive charge, thereby repelling vanadium ions through the Donnan effect and improving the selectivity of the proton-conducting membrane; at the same time, the protonated amino groups of cross-linked polyvinylpyrrolidone are beneficial to the rapid transfer of protons, improving the conductivity of the proton-conducting membrane. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings required for use in the embodiments of the present application will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0029] Figure 1 Shows a flowchart of the method for preparing a proton-conducting membrane according to an embodiment of the present application;

[0030] Figure 2 Shows the surface electron microscope images of the proton-conducting membrane before and after self-crosslinking in Embodiment 1 of the present application;

[0031] Figure 3 Shows the surface electron microscope images of the proton-conducting membrane before and after self-crosslinking in Embodiment 2 of the present application;

[0032] Figure 4 Shows the surface electron microscope images of the proton-conducting membrane before and after self-crosslinking in Embodiment 3 of the present application;

[0033] Figure 5 The figure shows the performance comparison diagram of the proton conduction membrane of Example 1 of the present application and the proton conduction membrane of Comparative Example 1 when used in a vanadium redox flow battery. Detailed implementation manners

[0034] In order to make the invention purpose, technical solution and beneficial technical effects of the present application clearer, the present application will be described in detail below in conjunction with specific embodiments. It should be understood that the embodiments described in this specification are only for explaining the present application and not for limiting the present application.

[0035] For simplicity, only some numerical ranges are explicitly disclosed herein. However, any lower limit can be combined with any upper limit to form a range not explicitly recorded; and any lower limit can be combined with other lower limits to form a range not explicitly recorded, and similarly any upper limit can be combined with any other upper limit to form a range not explicitly recorded. In addition, although not explicitly recorded, each point or single value between the range endpoints is included in this range. Thus, each point or single value can be used as its own lower limit or upper limit and combined with any other point or single value or combined with other lower limits or upper limits to form a range not explicitly recorded.

[0036] In the description herein, it should be noted that unless otherwise specified, "above" and "below" include the present number, and the meaning of "several" in "one or several" is two or more.

[0037] The above-mentioned inventive content of the present application does not intend to describe every disclosed embodiment or every implementation manner in the present application. The following description more specifically gives examples of exemplary embodiments. Throughout the application, guidance is provided through a series of embodiments, which can be used in various combinations. In each instance, the listings are only representative groups and should not be construed as exhaustive.

[0038] In a first aspect, an embodiment of the present application provides a method for preparing a proton conduction membrane, including:

[0039] Providing a casting solution containing polyvinylidene fluoride, a water-soluble nucleating agent, and polyvinylpyrrolidone;

[0040] Making the casting solution flow on the surface of a support to form a thin layer of the casting solution;

[0041] Heating the thin layer of the casting solution to enable crystal nuclei to grow and remove the organic solvent therein to obtain a thin film;

[0042] Placing the thin film in deionized water to remove the water-soluble nucleating agent to obtain a porous membrane;

[0043] Place the porous membrane in an initiation system to cause the polyvinylpyrrolidone contained in the porous membrane to undergo a cross-linking reaction, thereby obtaining a proton-conducting membrane.

[0044] During the polymer crystallization process, a crystal nucleus refers to a seed or starting point for forming an ordered structure among polymer molecules. This process involves rearranging the polymer molecules dissolved in a solvent into an ordered structure to form solid polymer crystals. Generally, polymer molecules exceeding the saturation concentration begin to aggregate into small clusters in the liquid to form crystal nuclei. In the embodiments of the present application, a water-soluble nucleating agent is added to promote the formation of crystal nuclei.

[0045] According to the method of the embodiments of the present application, by adding a water-soluble nucleating agent to the casting solution and controlling the heating temperature and time, "nucleation - controllable growth" of the proton-conducting membrane containing polyvinylidene fluoride and polyvinylpyrrolidone is achieved. Utilizing the "space" between polyvinylidene fluoride grains to penetrate each other, a porous membrane with nano-scale pore diameters is formed; breaking through the traditional phase separation method for preparing porous membranes and its pore size limitations, making the proton-conducting membrane more suitable for the all-vanadium redox flow battery system.

[0046] According to the method of the embodiments of the present application, by adopting the blending technology of polyvinylpyrrolidone and polyvinylidene fluoride, the two polymer chains are intertwined with each other, improving the dimensional stability and slowing down the water solubility of polyvinylpyrrolidone; and the incorporation of polyvinylpyrrolidone does not affect the growth of crystal nuclei, achieving the control of the pore size of the proton-conducting membrane.

[0047] According to the method of the embodiments of the present application, place the porous membrane in an initiation system to cause the polyvinylpyrrolidone contained in the porous membrane to undergo a cross-linking reaction to form cross-linked polyvinylpyrrolidone, improving the physical and chemical stability of the proton-conducting membrane, thereby reducing or avoiding the dissolution of hydrophilic polyvinylpyrrolidone in the electrolyte during the use of the proton-conducting membrane. It is very necessary for polyvinylpyrrolidone to undergo a self-crosslinking reaction. On the one hand, it enables the proton-conducting membrane to have good selectivity and ionic conductivity, and also avoids the rapid attenuation of the battery capacity due to the dissolution of polyvinylpyrrolidone in the electrolyte during the charge and discharge process of the proton-conducting membrane.

[0048] It has been found through research that the introduction and self-crosslinking of polyvinylpyrrolidone increase the grain size to a certain extent, but do not affect the pore structure of the porous membrane.

[0049] In addition, the method according to the embodiments of the present application has the advantages of simple film-making method and low production cost, and is easy to scale up industrially and carry out continuous film-making.

[0050] In some alternative embodiments, the mass fraction of polyvinylidene fluoride in the casting solution is 10% - 25%.

[0051] Optionally, the mass fraction of polyvinylidene fluoride in the casting solution can be any value among 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25% or a range composed thereof.

[0052] According to the embodiments of the present application, when the mass content of polyvinylidene fluoride in the casting solution is within the above range, the pores of the prepared membrane are appropriate. If the mass content of polyvinylidene fluoride in the casting solution is less than 10%, it is not conducive to casting into a film, increasing the casting difficulty and other parameters need to be regulated. If the mass content of polyvinylidene fluoride in the casting solution is greater than 25%, the pore density is increased to a certain extent, which has a certain impact on the internal resistance of the proton conduction membrane.

[0053] In some alternative embodiments, the water-soluble nucleating agent is a polymer monomer having a hydrophilic group and a carbon-carbon double bond.

[0054] According to the embodiments of the present application, the water-soluble nucleating agent having the above hydrophilic group is beneficial to its water solubility and is also beneficial to being removed in water during the preparation process. The water-soluble nucleating agent has a carbon-carbon double bond, which is beneficial to increasing the crystal nuclei in the mixture, beneficial to increasing the number of crystal nuclei and promoting the growth of crystal nuclei, so that the pores of the prepared proton conduction membrane have appropriate sizes.

[0055] In some alternative embodiments, the water-soluble nucleating agent includes one or more of vinyl sulfonate, styrene sulfonate, and allyl sulfonate. The water-soluble nucleating agents of the above types can be used as crystal nuclei in the system to promote the crystallization of the system to generate pores with appropriate sizes.

[0056] In some alternative embodiments, the mass fraction of the water-soluble nucleating agent in the casting solution is 1% - 10%.

[0057] Optionally, the mass fraction of the water-soluble nucleating agent in the casting solution can be any value among 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% or a range composed thereof.

[0058] According to the embodiments of the present application, by controlling the mass fraction of the water-soluble nucleating agent in the casting solution within the above range, it is beneficial to provide an appropriate amount of crystal nuclei, beneficial to controlling the pores of the proton conduction membrane to have appropriate sizes.

[0059] In some alternative embodiments, polyvinylpyrrolidone includes one or more of K15, K30, K60, and K90 in terms of average molecular weight. K15, K30, K60, and K90 are grades (models) used based on the average molecular weight of polyvinylpyrrolidone. Using polyvinylpyrrolidone with the above molecular weights is beneficial to controlling the selectivity and ionic conductivity of the proton conduction membrane.

[0060] In some alternative embodiments, the mass fraction of polyvinylpyrrolidone in the casting solution is 0.5% to 10%.

[0061] Optionally, the mass fraction of polyvinylpyrrolidone in the casting solution can be any value among 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, 10% or a range composed thereof.

[0062] According to the embodiments of the present application, when the mass fraction of polyvinylpyrrolidone in the casting solution is within the above range, the prepared proton-conducting membrane has good selectivity and ionic conductivity. If the mass fraction of polyvinylpyrrolidone in the casting solution is less than 0.5%, the improvement of the selectivity and ionic conductivity of the proton-conducting membrane is not obvious; if the mass fraction of polyvinylpyrrolidone in the casting solution is greater than 10%, the hydrophilic characteristics of polyvinylpyrrolidone cannot be effectively utilized, and the water migration of the proton-conducting membrane is affected to a certain extent.

[0063] In some alternative embodiments, the organic solvent includes one or more of N,N-dimethylacetamide, N,N-dimethylformamide, N-methylpyrrolidone, and dimethyl sulfoxide.

[0064] According to the embodiments of the present application, polyvinylidene fluoride, water-soluble nucleating agent, and polyvinylpyrrolidone can be better dispersed and dissolved in the above-mentioned types of solvents, which is also convenient for subsequent removal.

[0065] In some embodiments, the support has a smooth plane. The support includes flat glass, stainless steel, etc.

[0066] In some alternative embodiments, the thickness of the thin layer of the casting solution is 100 - 500 microns.

[0067] According to the embodiments of the present application, when the thickness of the thin layer of the casting solution is within the above range, the proton-conducting membrane has appropriate internal resistance and appropriate selectivity. If the thickness of the thin layer of the casting solution is too small, its selectivity is affected to a certain extent. If the thickness of the thin layer of the casting solution is too large, its internal resistance is affected to a certain extent.

[0068] In some alternative embodiments, the heating temperature is 60 - 150 °C; the heating time is 1 - 6 h.

[0069] According to the embodiments of the present application, when the heating temperature is within the above range, the internal resistance and selectivity of the prepared membrane are moderate. Controlling the heating temperature and time is beneficial to controlling the pore size and total pore volume of the proton-conducting membrane.

[0070] Those skilled in the art can use a suitable system to use polyvinylpyrrolidone. According to the amount of polyvinylpyrrolidone, a suitable dosage of initiator can be selected in the system to promote the cross-linking reaction of polyvinylpyrrolidone. In some alternative embodiments, the initiator system includes 0.5% to 8% initiator by mass percentage.

[0071] In some alternative embodiments, the initiator includes one or more of persulfate, hydrogen peroxide, and azobisisobutyronitrile. The above types of initiators can promote the cross-linking reaction of polyvinylpyrrolidone, and are also beneficial for subsequent removal or direct complete reaction without affecting the performance of the final product.

[0072] In some alternative embodiments, the solvent of the initiator system is water.

[0073] According to the embodiments of the present application, since the cross-linking reaction time of polyvinylpyrrolidone is about 1 to 6 hours and the temperature is about 90 to 120 °C, using water as the solvent of the initiator system is beneficial to control the reaction rate and the balance of the reaction, and reduce its influence on the pore size and performance of the proton conduction membrane.

[0074] In some embodiments, the cross-linking reaction time of polyvinylpyrrolidone is 1 to 6 hours, and the cross-linking reaction temperature of polyvinylpyrrolidone is 90 to 120 °C.

[0075] According to the embodiments of the present application, controlling the cross-linking reaction time of polyvinylpyrrolidone to be 1 to 6 hours and the temperature to be 90 to 120 °C, and using water as the solvent of the initiator system is beneficial to control the reaction rate and the balance of the reaction, and reduce its influence on the pore size and performance of the proton conduction membrane.

[0076] In a second aspect, the embodiments of the present application provide a proton conduction membrane prepared by the method of the first aspect.

[0077] According to the embodiments of the present application, the proton conduction membrane has high stability, high selectivity, and high ionic conductivity, and the size of its pore diameter is adjustable, and it can be applied in different scenarios.

[0078] Embodiment

[0079] The following examples more specifically describe the content disclosed in the present application. These examples are only for illustrative purposes, because various modifications and changes within the scope of the present application disclosure are obvious to those skilled in the art. Unless otherwise stated, all parts, percentages, and ratios reported in the following examples are based on weight, and all reagents used in the examples are commercially available or synthesized according to conventional methods, and can be used directly without further treatment, and the instruments used in the examples are all commercially available.

[0080] Example 1

[0081] An embodiment of the present application provides a method for preparing a proton-conducting membrane, including:

[0082] (1) Dissolve polyvinylidene fluoride powder, polyvinylpyrrolidone K60, and sodium allylsulfonate in dimethyl sulfoxide solvent, so that the mass concentration of polyvinylidene fluoride is 15%, the molecular weight of polyvinylidene fluoride is 680,000, the mass concentration of polyvinylpyrrolidone K60 is 2%, and the mass concentration of sodium allylsulfonate is 5%; make a casting solution,

[0083] (2) Cast a thin layer of the casting solution with a thickness of 300 microns on the surface of a flat glass, and place it in an 80 ℃ oven. After the solvent evaporates, a thin film is formed;

[0084] (3) Place the obtained thin film in deionized water. After removing the nucleating agent, a porous membrane is obtained;

[0085] (4) Place the obtained porous membrane in an aqueous solution containing 5% ammonium persulfate as an initiator, and perform self-crosslinking of the polyvinylpyrrolidone in the porous membrane. The self-crosslinking temperature is 90 ℃ , and the self-crosslinking time is 2 h. After the self-crosslinking is completed, a proton-conducting membrane is obtained.

[0086] Example 2

[0087] The difference between this example and Example 1 is that: the mass content of 15% polyvinylidene fluoride in the casting solution is 15%; the mass content of polyvinylpyrrolidone K60 in the casting solution is 3%; the mass content of sodium allylsulfonate in the casting solution is 5%.

[0088] Example 3

[0089] The difference between this example and Example 1 is that: the molecular weight of polyvinylpyrrolidone uses polyvinylpyrrolidone K30. The mass content of polyvinylpyrrolidone K60 in the casting solution is 5%.

[0090] Example 4

[0091] The difference between this example and Example 1 is that: the molecular weight of polyvinylpyrrolidone uses polyvinylpyrrolidone K90. The mass content of polyvinylpyrrolidone K90 in the casting solution is 1%.

[0092] Comparative Example 1

[0093] The difference between this comparative example and Example 1 is that: dissolve polyvinylidene fluoride powder and sodium allylsulfonate in dimethyl sulfoxide solvent, so that the mass concentration of polyvinylidene fluoride is 15%, and the mass concentration of sodium allylsulfonate is 5%; make a casting solution.

[0094] Test section

[0095] 1) Electron microscopy observation of the proton-conducting membrane: The products during the preparation of the proton-conducting membrane in the examples were observed using a JSM7900F field emission scanning electron microscope from JEOL Ltd., Japan. It was found that for the proton-conducting membranes doped with polyvinylpyrrolidone of different molecular weights, the surface morphologies before and after crosslinking are as Figures 2 to 4 shown. Among them, the proton-conducting membranes of Examples 1-3 are successively as Figures 2 to 4 shown. In the figures, the left pictures are those before the crosslinking of polyvinylpyrrolidone, and the right pictures are those after crosslinking. It can be seen from the figures that for the porous membranes containing polyvinylpyrrolidone prepared based on the "nucleation-controlled growth" principle of the raw materials, the surface presents a rich pore structure with interlaced grains. After the self-crosslinking reaction of polyvinylpyrrolidone, the grain volume increases to a certain extent, compressing the "space" between the grains, reducing the pore size of the membrane, and the amount of increase in the grain volume is related to the molecular weight and doping amount of polyvinylpyrrolidone. Through research, it was found that a pore size within an appropriate range in the proton-conducting membrane is beneficial to the improvement of selectivity.

[0096] 2) Performance detection of the proton-conducting membrane:

[0097] The proton-conducting membrane was placed in a flow battery, with graphite felt as the electrode, the positive electrolyte being 1.7M VO 2+ / 3M H2SO4, and the negative electrolyte being 1.7M V 3+ / 3M H2SO4. Constant current charge and discharge were carried out at a current density of 150 mA cm -2 , with the charging cut-off voltage being 1.65V and the discharging cut-off voltage being 0.8V.

[0098] The detection was carried out at room temperature. The average Coulomb efficiency (CE) was calculated by taking 20 charge and discharge cycles. The voltage efficiency (VE) was measured based on the discharging mid-voltage / charging mid-voltage, and the energy efficiency (EE) was calculated according to CE×VE.

[0099] The charge and discharge test results of the proton-conducting membranes in Examples 1-3 and the proton-conducting membrane in Comparative Example 1 for a vanadium redox flow battery are shown in Table 1 (the current density is set to 150 mA cm -2 ).

[0100] Table 1 Performance comparison table of the proton-conducting membranes in Examples 1-3 and Comparative Example 1

[0101]

[0102]

[0103] To compare the performance of the proton-conducting membrane in Example 1 and the proton-conducting membrane in Comparative Example 1 when used in an all-vanadium redox flow battery, charge-discharge tests were conducted. The charging cut-off voltage was set at 1.65 V, and the discharging cut-off voltage was set at 0.8 V. At room temperature, a current density of 150 mA cm -2 was controlled. After constant-current charging to the cut-off voltage, constant-current discharging was performed at the same current density until the discharging cut-off voltage was reached. The results are as Figure 5 shown. The proton-conducting membrane in Comparative Example 1 had better selectivity, with an average Coulombic efficiency of 98.1%. However, its surface resistance was high, and the voltage efficiency was only 71.0%. Therefore, the final energy efficiency was only 69.7%. The measured average Coulombic efficiency of the battery using the proton-conducting membrane in Example 1 was 96.1%, the voltage efficiency was 82.5%, and the energy efficiency was 79.3%. Both the voltage efficiency and the energy efficiency of the battery were significantly improved, indicating that the introduction of polyvinylpyrrolidone and self-crosslinking can effectively improve the performance of the proton-conducting membrane.

[0104] As can be seen from Table 1 and Figure 5 it can be seen that, compared with Comparative Example 1, the proton-conducting membranes in Examples 1-3 had higher voltage efficiency and energy efficiency. That is, when the proton-conducting membrane containing polyvinylpyrrolidone was used in an all-vanadium redox flow battery, both the voltage efficiency and the total energy efficiency of the battery were significantly improved. The proton-conducting membranes in Examples 1-3 had better performance.

[0105] In summary, the proton-conducting membrane of the embodiment of the present application has the advantages of high ion selectivity and high ionic conductivity. Moreover, the proton-conducting membrane of the present application also has the advantage of a simple preparation method, which is easy to scale up industrially, providing a new way to prepare a proton-conducting membrane with low cost and high performance.

[0106] It should also be noted that the exemplary embodiments mentioned in the present application describe some methods or systems based on a series of steps or devices. However, the present application is not limited to the order of the above steps. That is, the steps can be executed in the order mentioned in the embodiments, can be different from the order in the embodiments, or several steps can be executed simultaneously.

[0107] The above is only the specific implementation manner of the present application, but the protection scope of the present application is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present application can easily think of various equivalent modifications or substitutions, and these modifications or substitutions should all be covered within the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1. A method for preparing a proton-conducting membrane, comprising: providing a casting solution comprising polyvinylidene fluoride, a water-soluble nucleating agent, and polyvinylpyrrolidone; casting the casting solution on the surface of a support to form a thin layer of the casting solution; heating the thin layer of the casting solution to allow crystal nuclei to grow and remove the organic solvent therein to obtain a thin film; placing the thin film in deionized water to remove the water-soluble nucleating agent to obtain a porous membrane; placing the porous membrane in an initiation system to cause a crosslinking reaction of the polyvinylpyrrolidone contained in the porous membrane to obtain a proton-conducting membrane.

2. The method according to claim 1, wherein The mass fraction of the polyvinylidene fluoride in the casting solution is 10% to 25%.

3. The method according to claim 1, characterized in that, The water-soluble nucleating agent satisfies at least one of the following conditions: 1) The water-soluble nucleating agent is a polymerizable monomer having a hydrophilic group and a carbon-carbon double bond; 2) The water-soluble nucleating agent comprises one or more of vinyl sulfonate, styrene sulfonate, and allyl sulfonate; 3) The mass fraction of the water-soluble nucleating agent in the casting solution is 1% to 10%.

4. The method according to claim 1, wherein The polyvinylpyrrolidone includes one or more of K15, K30, K60, and K90 in terms of average molecular weight.

5. The method according to claim 1, wherein The mass fraction of the polyvinylpyrrolidone in the casting solution is 0.5% to 10%.

6. The method according to claim 1, characterized in that, The organic solvent comprises one or more of N,N-dimethylacetamide, N,N-dimethylformamide, N-methylpyrrolidone, and dimethyl sulfoxide.

7. The method according to claim 1, wherein The thickness of the thin layer of the casting solution is 100 - 500 microns.

8. The method according to claim 1, characterized in that The heating temperature is 60 - 150 °C; the heating time is 1 - 6 h.

9. The method according to claim 1, wherein The initiation system satisfies at least one of the following conditions: 1) The initiation system comprises 0.5% to 8% initiator by mass percentage; 2) The initiator comprises one or more of persulfate, hydrogen peroxide, and azobisisobutyronitrile; 3) The solvent of the initiation system is water.

10. A proton-conducting membrane, characterized in that, Obtained by the method according to any one of claims 1 to 9.

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