A perfluorosulfonic acid ion exchange membrane for a flow battery, a preparation method thereof, and an application thereof

Abstract

The present invention relates to the field of preparation of ion exchange membranes, and discloses a perfluorosulfonic acid ion exchange membrane for a flow battery, a preparation method thereof, and an application thereof. By virtue of the different solubilities of different molecular weight components in a perfluorosulfonic acid resin in a solvent under different conditions, a perfluorosulfonic acid ion exchange membrane having two molecular weight components is prepared. The perfluorosulfonic acid ion exchange membrane prepared by the present invention has high ion selectivity and mechanical strength, and the assembled flow single cell has high coulombic efficiency and a low self-discharge rate, and is suitable for popularization and use in flow batteries.

Description

Technical Field

[0001] The present invention relates to the field of ion exchange membranes, and particularly to a perfluorosulfonic acid ion exchange membrane for a flow battery, its preparation method and application. Background Art

[0002] In the 1960s, DuPont of the United States developed a commercialized Nafion series of perfluorosulfonic acid ion exchange membranes. The main chain of its molecule is a polytetrafluoroethylene structure, and the side chain is a perfluorosulfonic acid ether structure. This structure has good corrosion resistance and electrochemical stability. It was initially widely used in the chlor-alkali industry. Later, with the development of technology, its application scope has expanded to fields such as hydrogen production by water electrolysis, fuel cells, and flow batteries.

[0003] In the field of flow batteries, perfluorosulfonic acid ion exchange membranes are still the most mainstream ion exchange membranes. The selective permeability of the ion membrane is a very important indicator, which mainly affects the coulombic efficiency and self-discharge rate of the flow battery. However, since the field of flow batteries is still an emerging energy storage battery field, the current perfluorosulfonic acid ion exchange membranes are not specifically designed for the flow battery field. In comparison, the problems of poor ion selective permeability and low coulombic efficiency need to be solved urgently. Therefore, designing an ion exchange membrane specifically applicable to the flow battery field is one of the research directions of researchers in this field. Summary of the Invention

[0004] In order to make up for the deficiencies of the prior art, the present invention provides a perfluorosulfonic acid ion exchange membrane for a flow battery, its preparation method and application, to further improve the ion selectivity of the membrane in the flow battery, improve the coulombic efficiency of the battery, and reduce the self-discharge rate.

[0005] The technical solution of the present invention is as follows:

[0006] A preparation method of a perfluorosulfonic acid ion exchange membrane, comprising the following steps:

[0007] (1) Adding a perfluorosulfonic acid resin raw material into a polar high-boiling solvent X, heating and refluxing under normal pressure to fully dissolve the soluble components in the perfluorosulfonic acid resin raw material, cooling to room temperature, filtering the obtained mixture, and taking the filtrate as a casting solution, which is called slurry A;

[0008] (2) After drying the solid matter filtered out in (1), completely dissolving it in a solvent Y, and the obtained casting solution is called slurry B;

[0009] (3) Preparing an ion exchange membrane A layer from slurry A by a conventional membrane preparation method, and then using the ion exchange membrane A layer as a base membrane, and preparing an ion exchange membrane B layer on one side of the ion exchange membrane A layer by a conventional membrane preparation method, so that the ion exchange membrane forms an AB structure.

[0010] Alternatively, the slurry B is prepared into the ion exchange membrane B layer by a conventional film-forming method, and then, using the ion exchange membrane B layer as the base membrane, the ion exchange membrane A layer is prepared on one side of the ion exchange membrane B layer by a conventional film-forming method, so that the ion exchange membrane forms an AB structure, and all are perfluorosulfonic acid ion exchange membranes prepared by the present invention.

[0011] Furthermore, the perfluorosulfonic acid resin raw material in step (1) is a commercially available perfluorosulfonic acid resin or a homogeneous perfluorosulfonic acid ion exchange membrane and its edge and corner wastes, or a used perfluorosulfonic acid ion exchange membrane and its edge and corner wastes after impurity treatment;

[0012] Furthermore, the polar high-boiling solvent X in step (1) is one or a mixture of more of N,N'-dimethylformamide (DMF), N,N'-dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), and N-methylpyrrolidone (NMP);

[0013] Furthermore, the solvent Y in step (2) is the polar high-boiling solvent X or an alcohol / water mixture, and the alcohol / water mixture includes, but is not limited to, methanol / water, ethanol / water, n-propanol / water, and isopropanol / water mixtures;

[0014] Furthermore, the pressure during the dissolution of the solid substance in step (2) is higher than normal pressure or higher than standard atmospheric pressure. Usually, the pressure is greater than 0.5 MPa, and the specific pressure range is adjusted according to the type of solvent used and is not limited herein. This is because when the system pressure increases, the boiling point of the solvent will increase, enabling substances that are difficult to dissolve at the normal boiling point to dissolve under pressure.

[0015] Furthermore, the conventional film-forming method is the steel belt casting film-forming method or the solution casting film-forming method. The steel belt casting film-forming method is suitable for systems where the viscosity of the casting solution / slurry is relatively large and the fluidity on a smooth horizontal surface is relatively small. Its basic principle is to scrape the viscous casting solution / slurry onto a smooth horizontal steel belt and then dry the solvent by a program to obtain the ion exchange membrane; the solution casting film-forming method is suitable for systems where the viscosity of the casting solution / slurry is relatively small and the fluidity on a smooth horizontal surface is relatively large. Its basic principle is to pour the solution with a relatively small viscosity into a smooth horizontal glass tank, limit the further diffusion and flow of the casting solution through the tank edge, and then dry the solvent by a program to obtain the ion exchange membrane. Select a suitable film-forming method according to the actual situation.

[0016] Furthermore, the concentrations of the slurry A and the slurry B are both 5-25 wt%. If the concentration of the casting solution is too low, it will cause waste of the solvent and prolong the film-forming time; if the concentration of the casting solution is too high, it needs to be concentrated, which will increase the process. If the initial feeding ratio of the perfluorosulfonic acid resin raw material in step (1) is higher, a mixture with a relatively large viscosity will be obtained, which will make filtration difficult and a slurry A with a relatively high concentration cannot be obtained.

[0017] To make this technical solution easier to understand, the following further elaboration is provided:

[0018] Currently, due to the relatively complex synthesis process of perfluorosulfonic acid resin raw materials, the controllability of the molecular weight during their preparation is poor, resulting in a very wide molecular weight range, generally ranging from several thousand to several hundred thousand. In the process of preparing homogeneous ion exchange membranes using these perfluorosulfonic acid resin raw materials, a high-pressure reaction kettle is required. Under high temperature and high pressure, all molecular weight components in the perfluorosulfonic acid resin raw materials can be completely dissolved to prepare the ion exchange membrane. Therefore, in the homogeneous membrane obtained by the current membrane preparation method, the distribution of molecules in different molecular weight segments in the membrane is random and relatively uniform, as Figure 1 shown. As a result, it will limit the degree of crystallization of the molecules in the membrane, making the arrangement between the membrane polymer molecules relatively loose, not very dense, with relatively large "pores", which reduces the ion selectivity. To solve this problem, in the present invention, in step (1), the perfluorosulfonic acid resin raw materials are dissolved in a polar high-boiling solvent under normal pressure, and the components with lower molecular weights are preferentially dissolved in the solvent. The components with higher molecular weights are insoluble under these conditions. Then, filtration is carried out, and slurry A is used to prepare the membrane to obtain a perfluorosulfonic acid ion exchange membrane A layer with a relatively low average molecular weight. Slurry B is prepared using component B, and then a perfluorosulfonic acid ion exchange membrane B layer with a relatively high average molecular weight is prepared to obtain an ion exchange membrane with an AB structure, as Figure 2 shown.

[0019] In the perfluorosulfonic acid ion exchange membrane described in the present invention, since the average molecular weight of the ion exchange membrane A layer is relatively low, its molecular chains are relatively easy to move during the membrane preparation process, resulting in a higher degree of crystallization and smaller pores between molecules, which is more conducive to improving the selective permeability of the ion membrane. The average molecular weight of the ion exchange membrane B layer is relatively large, its molecular chains are longer, the entanglement between the molecular chains is more chaotic, and the molecular chains are not easy to slip relative to each other, resulting in a higher mechanical strength. As for the specific molecular weight ranges of the A layer and the B layer, they are related to the molecular weight range of the perfluorosulfonic acid resin raw materials selected themselves and the type of polar high-boiling solvent X selected in step (1), and no specific limitations are made.

[0020] Although the perfluorosulfonic acid ion exchange membrane described in the present invention has the AB layer structure, since its essence is all perfluorosulfonic acid resin and the structural units in its polymer molecules are completely the same, there is good compatibility between the A layer and the B layer, and delamination is not likely to occur during the operation of the flow battery.

[0021] Another object of the present invention is to protect the perfluorosulfonic acid ion exchange membrane prepared by the above method.

[0022] Further, in the ion exchange membrane of the present invention, the A layer accounts for 40-60% of the total thickness. This is because if the thickness of the A layer is too large, the thickness of the B layer will be too small, which will affect the mechanical strength of the membrane itself; if the thickness of the A layer is too small, the thickness of the B layer will be too large, which will affect the ion selectivity of the membrane itself and thus reduce the Coulomb efficiency of the battery.

[0023] The third object of the present invention is to protect the application of the perfluorosulfonic acid ion exchange membrane prepared by the above method in a flow battery.

[0024] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0025] The present invention provides a perfluorosulfonic acid ion exchange membrane for a flow battery. The membrane preparation method uses a traditional perfluorosulfonic acid resin as the film-forming raw material. By simply adjusting the process path of membrane preparation, the high-molecular-weight part and the low-molecular-weight part in the resin can be separated and used to prepare the membrane synergistically. Through the different solubilities of different molecular weight components in the perfluorosulfonic acid resin in a solvent under different conditions, a perfluorosulfonic acid ion exchange membrane with two molecular weight components is prepared. This ion exchange membrane has high ion selectivity, higher Coulomb efficiency, a low self-discharge rate, and moreover, this ion exchange membrane has higher mechanical strength and meets the requirements during the actual assembly of a flow battery stack. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The attached Figure 2 drawing of the present invention Figure 1 is a schematic structural diagram of a conventional perfluorosulfonic acid ion exchange membrane in the prior art;

[0027] The attached Figure 2 drawing is a schematic structural diagram of the perfluorosulfonic acid ion exchange membrane prepared by the present invention.

[0028] Among them: the A layer is the region with relatively small molecular weight, and the B layer is the region with relatively large molecular weight. DETAILED DESCRIPTION OF THE INVENTION

[0029] To better understand the present invention, the content of the present invention will be further clarified below in conjunction with embodiments. However, the content of the present invention is not limited to the following embodiments only.

[0030] Unless otherwise specified, the experimental methods used in the present invention are all conventional methods, and the experimental equipment, materials, reagents, etc. used can all be obtained from commercial channels.

[0031] In the present invention, the thickness of the ion exchange membrane is measured by a Fisher thickness tester, and 50 values are measured at different positions for each sample and averaged;

[0032] The tensile strength and elongation at break of the ion membrane were tested with reference to the standard GB / T 1040.3-2006 "Determination of tensile properties of plastics - Part 3: Test conditions for films and sheets". The membrane was cut into strips with a width of 10 mm and an initial clamp spacing of 50 mm, and the experiment was carried out at a tensile rate of 200 mm / min.

[0033] For the flow battery of the present invention, the all-vanadium flow battery with the most mature technical process was selected for the example description, but it does not limit the specific type of the flow battery. The application of the perfluorosulfonic acid ion exchange membrane described in the present invention in other types of flow batteries is also within the protection scope of the present invention.

[0034] The test method for the vanadium ion permeability of the ion membrane was carried out with reference to the standard NB / T 42080-2016 "Test method for ion conductive membranes for all-vanadium flow batteries".

[0035] Performance test conditions of the ion membrane for the all-vanadium flow battery: Charge-discharge experiments were carried out under the condition of a current density of 80 mA / cm 2 until charged to 1.55 V and discharged to 1.00 V. Graphite carbon felt produced by Liaoyang Jingu Carbon Materials Co., Ltd. was used as the reaction electrode, and the effective working area of the electrode was 48 cm 2 , and the positive and negative electrode electrolytes were VO 2+ / VO2 + and V 2+ / V 3+ sulfuric acid solutions, and the battery operating temperature was 37 °C.

[0036] The test method for the self-discharge duration was as follows: The single cell was charged to 1.55 V, the liquid path between the single cell and the electrolyte storage tank was disconnected, and the OCV of the battery was measured under the shelf condition. When the OCV dropped to 1.20 V, the time of the whole process was calculated, which was the self-discharge duration.

[0037] Example 1

[0038] (1) 100 g of Dongyue DHS103 perfluorosulfonic acid resin (—SO3H type, ion exchange capacity of 1.03 mmol / g) was added to 600 mL of DMF, and heated under reflux at 150 - 155 °C under normal pressure to fully dissolve the soluble components in the perfluorosulfonic acid resin. After cooling to room temperature, the obtained mixture was filtered, and the filtrate was taken, called casting solution A. After testing, its concentration was about 7 wt%.

[0039] (2) After drying the solid matter filtered out in (1), it was heated to 210 °C at 1.3 MPa in a high-pressure reactor to completely dissolve it into DMF to obtain casting solution / slurry B, and its concentration was about 10 wt%.

[0040] (3) Prepare ion exchange membrane A (with a thickness of 20 μm) from casting solution A by the solution casting method. Then, using ion exchange membrane A as the base membrane, prepare an ion exchange membrane B layer on one side of ion exchange membrane A by the solution casting method, so that the ion exchange membrane forms an AB structure with a total thickness of about 50 μm. This is the perfluorosulfonic acid ion exchange membrane prepared in this example.

[0041] Example 2

[0042] Change the membrane preparation sequence in step (3) of Example 1 to first prepare layer B (with a thickness of about 30 μm), and then prepare layer A. Other steps are the same as in Example 1, and the film preparation process for each layer is also consistent with that in Example 1, to prepare a perfluorosulfonic acid ion exchange membrane with a thickness of 50 μm.

[0043] Example 3

[0044] Evaporate and concentrate casting solution A in Example 1 to 15 wt% to make casting slurry A, and concentrate casting solution B to 13.5 wt% to make casting slurry B. Use the steel belt casting method to first prepare a 20-μm-thick layer A film, and then prepare a layer B film on layer A film to obtain a perfluorosulfonic acid ion exchange membrane with a total thickness of 50 μm.

[0045] Example 4

[0046] Change the thickness of layer A in Example 1 to 26 μm, and keep other conditions the same as in Example 1, to obtain a perfluorosulfonic acid ion exchange membrane with a total thickness of 50 μm.

[0047] Example 5

[0048] Change the thickness of layer A in Example 1 to 30 μm, and keep other conditions the same as in Example 1, to obtain a perfluorosulfonic acid ion exchange membrane with a total thickness of 50 μm.

[0049] Example 6

[0050] (1) Cut the membrane scraps of 100 g Nafion212 ion exchange membrane into small pieces with a size less than 2 cm * 2 cm and add them to 800 mL of DMAc. Heat and reflux at 164 - 166 °C under normal pressure to fully dissolve the soluble components in the membrane resin. Cool to room temperature, filter the resulting mixture, and take the filtrate, which is called casting solution A. After testing, its concentration is about 8 wt%;

[0051] (2) After drying the solid matter filtered out in (1), heat it to 160 °C under 1.15 MPa in a high-pressure reactor to completely dissolve it into an ethanol / water mixed solvent (mass ratio of 50 / 50) to prepare casting slurry B, with a concentration of about 13.5 wt%;

[0052] (3) The casting solution A was prepared into an ion exchange membrane A (with a thickness of 25 μm) by the solution casting method. Then, using the ion exchange membrane A as the base membrane, an ion exchange membrane B layer was prepared on one side of the ion exchange membrane A by the steel belt casting method, forming an AB structure for the ion exchange membrane with a total thickness of approximately 50 μm. This is the perfluorosulfonic acid ion exchange membrane prepared in this example.

[0053] Example 7

[0054] (1) The scraps of Nafion 212 ion exchange membrane with a size less than 2 cm × 2 cm were cut from 100 g of Nafion 212 ion exchange membrane and added to 800 mL of DMSO. The mixture was heated under reflux at 185 - 190 °C under normal pressure to fully dissolve the soluble components in the membrane resin. After cooling to room temperature, the resulting mixture was filtered, and the filtrate was taken and called the casting solution A. After testing, its concentration was approximately 10 wt%.

[0055] (2) After drying the solid matter filtered out in (1), it was heated to 160 °C under 1.46 MPa in a high-pressure reactor to completely dissolve it in an isopropanol / water mixed solvent (mass ratio 60 / 40) to obtain a casting slurry B with a concentration of approximately 15.3 wt%.

[0056] (3) The casting solution A was prepared into an ion exchange membrane A (with a thickness of 25 μm) by the solution casting method. Then, using the ion exchange membrane A as the base membrane, an ion exchange membrane B layer was prepared on one side of the ion exchange membrane A by the steel belt casting method, forming an AB structure for the ion exchange membrane with a total thickness of approximately 50 μm. This is the perfluorosulfonic acid ion exchange membrane prepared in this example.

[0057] Example 7

[0058] (1) The waste Nafion 212 ion exchange membrane after 580 charge-discharge cycles of a vanadium redox flow battery was subjected to impurity removal treatment and then cut into small pieces with a size less than 2 cm × 2 cm, replacing the membrane scraps cut from the Nafion 212 ion exchange membrane in Example 6. The rest was the same as in Example 6 to prepare the perfluorosulfonic acid ion exchange membrane prepared in this example.

[0059] Comparative Example 1

[0060] Dongyue DHS103 perfluorosulfonic acid resin (—SO3H type, ion exchange capacity 1.03 mmol / g) was added to DMF. It was heated to 210 °C under 1.3 MPa in a high-pressure reactor to completely dissolve it in DMF to prepare a DMF solution of perfluorosulfonic acid resin with a concentration of 6 wt%. A 50-μm-thick perfluorosulfonic acid ion exchange membrane was prepared by the solution casting method.

[0061] Comparative Example 2

[0062] The thickness of the A layer in Example 1 was changed to 16 μm, and the others were kept the same as in Example 1, obtaining a perfluorosulfonic acid ion exchange membrane with a total thickness of 50 μm.

[0063] Comparative Example 3

[0064] The thickness of the A layer in Example 1 was changed to 37 μm, and the others were kept the same as in Example 1, obtaining a perfluorosulfonic acid ion exchange membrane with a total thickness of 50 μm.

[0065] Table 1 Test data of Examples 1-7, Comparative Examples 1-3 and Nafion 212 membrane

[0066]

[0067] In Table 1, it can be seen from the two groups of comparisons between Examples 1-5 and Comparative Example 1 or Examples 6 and Nafion 212 membrane that the perfluorosulfonic acid ion exchange membrane prepared by the present invention has higher tensile strength, higher vanadium resistance efficiency (lower vanadium ion permeability), longer self-discharge time and higher coulomb efficiency compared with the homogeneous perfluorosulfonic acid ion exchange membrane prepared from the same materials. It can be seen from Examples 1-3 that for ion exchange membranes with the same material composition, the film-forming processes of each step basically do not affect the performance of the ion membrane itself; it can be seen from Examples 1, 4, 5 and Comparative Examples 2 and 3 that when the total thickness of the membrane is the same and the composition materials are the same, the higher the proportion of the A layer (i.e., the thicker the thickness of the A layer), the better the vanadium resistance effect (or the better the ion selectivity) of the membrane, the longer the self-discharge time, and the higher the coulomb efficiency. However, at the same time, the voltage efficiency and mechanical strength (tensile strength) of the membrane will decrease. In particular, in Comparative Example 2, the proportion of the A layer is less than 40%. Compared with the homogeneous membrane of Comparative Example 1, its mechanical strength is very high, but the vanadium resistance performance is poor, the self-discharge time is short, and the coulomb efficiency is low; in Comparative Example 3, the proportion of the A layer is higher than 60%. Compared with the homogeneous membrane of Comparative Example 1, although its vanadium resistance performance is good, the self-discharge time is long, and the coulomb efficiency is high, but the mechanical strength is very low. Therefore, when preparing the ion exchange membrane, the proportion and thickness of the A layer should be reasonably selected to make the membrane have the best comprehensive performance. In addition, it should be noted that during the long-term operation of the battery, the ion exchange membranes prepared in the above examples did not show the phenomenon of foaming and delamination, indicating that there is good compatibility between different molecular weight components.

[0068] The above are typical examples and comparative examples of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and concept of the present invention, makes equivalent substitutions or changes, and should be covered within the protection scope of the present invention.

Claims

1. A preparation method of a perfluorosulfonic acid ion exchange membrane, characterized in that, It includes the following steps: (1) Preparation of slurry A: Add the perfluorosulfonic acid resin raw material into the polar high-boiling solvent X, heat and reflux under normal pressure to fully dissolve the soluble components in the perfluorosulfonic acid resin raw material, cool to room temperature, filter the obtained mixture, and take the filtrate as the casting solution, which is called slurry A; (2) Preparation of slurry B: After drying the solid matter filtered out in (1), completely dissolve it in the solvent Y, and the obtained casting solution is called slurry B; (3) Membrane preparation: Prepare the ion exchange membrane A layer from slurry A, and then use the ion exchange membrane A layer as the base membrane to prepare the ion exchange membrane B layer on one side of the ion exchange membrane A layer with slurry B; or prepare the ion exchange membrane B layer from slurry B, and then use the ion exchange membrane B layer as the base membrane to prepare the ion exchange membrane A layer on one side of the ion exchange membrane B layer with slurry A, so that the ion exchange membrane forms an AB structure; The ion exchange membrane A layer accounts for 40-60% of the total thickness.

2. The preparation method of a perfluorosulfonic acid ion exchange membrane according to claim 1, characterized in that, The perfluorosulfonic acid resin raw material described in step (1) is a commercially available perfluorosulfonic acid resin or a homogeneous perfluorosulfonic acid ion exchange membrane and its corner waste or a used perfluorosulfonic acid ion exchange membrane and its corner waste after impurity treatment.

3. The preparation method of a perfluorosulfonic acid ion exchange membrane according to claim 1, characterized in that, The polar high-boiling solvent X in step (1) is one or a mixture of N,N'-dimethylformamide (DMF), N,N'-dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), and N-methylpyrrolidone (NMP).

4. The preparation method of a perfluorosulfonic acid ion exchange membrane according to claim 1, characterized in that, The solvent Y in step (2) is the polar high-boiling solvent X or an alcohol / water mixture, and the alcohol / water mixture includes methanol / water, ethanol / water, n-propanol / water, and isopropanol / water mixtures.

5. The preparation method of a perfluorosulfonic acid ion exchange membrane according to claim 1, characterized in that, The pressure during the dissolution of the solid matter in step (2) is higher than normal pressure or higher than standard atmospheric pressure.

6. The preparation method of a perfluorosulfonic acid ion exchange membrane according to claim 1, characterized in that, The method for membrane preparation described in step (3) uses the steel belt casting method or the solution casting method.

7. The preparation method of a perfluorosulfonic acid ion exchange membrane according to claim 1, characterized in that, The concentrations of the slurry A and the slurry B are both 5-25 wt%.

8. A perfluorosulfonic acid ion exchange membrane, characterized in that, Prepared according to the preparation method described in any one of claims 1-7.

9. Application of a perfluorosulfonic acid ion exchange membrane, characterized in that, Application of the perfluorosulfonic acid ion exchange membrane in a flow battery.

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