A proton exchange membrane and a preparation method and application thereof

Abstract

The application discloses a proton exchange membrane and a preparation method and application thereof. The application provides a perfluorosulfonic acid membrane with sulfonic acid groups arranged in a horizontal direction or a vertical direction. The perfluorosulfonic acid membrane is heated to enter a glass transition state, and an electric field is applied to the area or the vertical direction of the membrane respectively to arrange the sulfonic acid groups, so that the proton transfer channel of the membrane is shortened, and the proton conductivity is improved. In the application, the electric field is applied to the perfluorosulfonic acid membrane in the glass state, the arrangement of the sulfonic acid groups in the membrane is promoted, the aggregation of the sulfonic acid groups is reduced, the size of the ion channel is increased, the crystallinity of the microstructure of the membrane is improved, the stability of the material is improved, and the hydrogen ion conduction of the proton membrane is improved.

Description

Technical Field

[0001] This application belongs to the field of fuel cells, specifically relating to a proton exchange membrane, its preparation method, and its application. Background Technology

[0002] Proton exchange membrane fuel cells (PEMFCs) have attracted widespread attention due to their advantages such as high efficiency, environmental friendliness, high reliability, and rapid start-up at room temperature. However, performance, lifespan, and cost remain key issues restricting their large-scale application. Among these, the proton exchange membrane (PEM) is the core component of the PEMFC and plays a crucial role in its performance. It acts as a gas barrier and a proton conductor, and its proton conduction capacity is closely related to the proton transport channels within the membrane.

[0003] Among proton exchange membranes, perfluorosulfonic acid membranes are the most widely used. To improve membrane conductivity, researchers have modified them using various methods. The Journal of Materials Chemistry A 6, 20836 (2018) discloses a method of applying voltage to the membrane during Nafion membrane casting, which improves membrane crystallinity and proton conductivity. However, applying an electric field during casting can easily introduce other influencing factors, such as uneven membrane thickness, leading to inconsistent electric field effects and poor experimental repeatability. Furthermore, patent application number 201710960419.5 proposes a method using perfluorosulfonic acid resin and acetone as raw materials to prepare a colloid suitable for electrophoresis, and then fabricating a perfluorosulfonic acid proton exchange membrane via electrophoresis. The membrane thickness, deposition rate, and density are controlled by applying an electric field.

[0004] Patent application CN201610330517.6 proposes a method for preparing SGO / Nafion composite membranes by doping sulfonated graphene (SGO) into perfluorosulfonic acid (Nafion) membranes to improve membrane ionic conductivity. Furthermore, Journal of Membrane Science 653(2022)120516 discloses a method for preparing SGO / Nafion composite membranes in a strong magnetic field via a self-assembled sol-gel method. However, this method requires the presence of magnetic nanoparticles, introducing an additional step. Moreover, magnetic nanoparticles cannot be directly mixed with polymers to form a material with a sufficiently stable structure, which may reduce the uniformity and stability of the membrane. Summary of the Invention

[0005] Based on the above background technology, the present invention provides a method for directly applying voltage to commercial Nafion membranes, thereby improving the proton transport channel by oriented alignment of sulfonate groups through an electric field, thereby increasing the proton conductivity of Nafion, avoiding the influence of other factors during membrane casting, and eliminating the need to introduce additional nanoparticles.

[0006] This application provides a method for preparing a perfluorosulfonic acid membrane, comprising the following steps:

[0007] (1) Heat the perfluorosulfonic acid film to induce its glass transition state;

[0008] (2) The perfluorosulfonic acid membrane after step (1) is placed in an electric field and treated under low electric field strength and high electric field strength in sequence; the treatment under low electric field strength is to switch the electric field direction every 1-5 minutes.

[0009] Based on the above technical solution, preferably, the sulfonate groups in the perfluorosulfonic acid membrane prepared in step (2) are oriented. The oriented arrangement of sulfonate groups is achieved by placing the glass-transformed membrane in an electric field in the area direction or perpendicular direction. First, the side chains containing sulfonate groups are activated by multiple electric field direction changes under low electric field strength, and then the sulfonate group arrangement direction is fixed under high electric field without changing the electric field direction.

[0010] Based on the above technical solutions, preferably, the low electric field strength is 1.0-3.0 V / cm; and the high electric field strength is 7.0-10.0 V / cm.

[0011] Based on the above technical solutions, preferably, the electric field direction is switched 5-10 times under low electric field strength; the treatment under high electric field strength does not change the electric field direction, and the electric field is applied for 1-2 hours.

[0012] Based on the above technical solutions, preferably, the electric field direction includes an area direction and a vertical direction; the area direction is parallel to the plane of the perfluorosulfonic acid film, and the vertical direction is perpendicular to the area of ​​the perfluorosulfonic acid film.

[0013] Based on the above technical solutions, preferably, when using an area-direction electric field, the film is fixed on the surface of a non-conductive substrate with electrode plates connected to both ends. The electrode plates at both ends of the non-conductive substrate are connected to a DC power supply, forming a capacitor in the area direction, and the film is located in the electric field of the area-direction capacitor. When using a vertical direction electric field, the film is sandwiched between two non-conductive substrates to form a "sandwich" structure. The upper and lower surfaces of the "sandwich" are connected to electrode plates, and the electrode plates on the upper and lower surfaces are connected to a DC power supply, forming a capacitor in the vertical direction, and the film is located in the electric field of the vertical direction capacitor.

[0014] Based on the above technical solutions, preferably, the non-conductive substrate is selected from one of glass plate, polytetrafluoroethylene plate, or bakelite board.

[0015] Based on the above technical solution, preferably, the heating of the perfluorosulfonic acid film to bring it into the glass transition state involves fixing the film on the surface of a substrate and heating the substrate to bring the film to the glass transition temperature; the temperature is 125-150℃.

[0016] Based on the above technical solutions, preferably, the substrate is selected from one of glass plate, polytetrafluoroethylene plate, or bakelite board.

[0017] Based on the above technical solutions, preferably, the EW value of the perfluorosulfonic acid membrane is in the range of 700-1200 g / mol.

[0018] The present invention provides a proton exchange membrane comprising a perfluorosulfonic acid membrane obtained by the above preparation method.

[0019] The present invention also provides the application of the above-mentioned proton exchange membrane in the field of fuel cells.

[0020] Beneficial effects

[0021] 1. This application can promote the directional arrangement of sulfonate ions in the membrane and reduce the aggregation of sulfonate ions by applying an electric field to the perfluorosulfonic acid membrane in the glassy state, thereby increasing the size of the ion channel, improving the crystallinity of the membrane microstructure, improving the stability of the material, and facilitating the conduction of hydrogen ions by the proton exchange membrane.

[0022] 2. In this application, the side chains containing sulfonate groups are activated by multiple electric field direction changes under low electric field intensity in the glassy state. This allows the side chains containing sulfonate groups to be rotated multiple times, making them more flexible and controllable. Then, electric field curing is performed, which is more conducive to achieving the directional arrangement of sulfonate groups, thereby improving the conductivity.

[0023] 3. In this application, the prepared perfluorosulfonic acid membrane is subjected to electric field treatment in a glassy state. Compared with applying an electric field during the preparation of the perfluorosulfonic acid membrane, the effect of the electric field on the membrane is more stable and the membrane thickness is more uniform. Attached Figure Description

[0024] Figure 1 These are TEM images of Comparative Example 1(a), Example 2(b), and Example 1(c);

[0025] Figure 2 The FTIR plots of Examples 1 and 2 and Comparative Example 1 are shown below;

[0026] Figure 3 The conductivity test results are for Examples 1 and 2 and Comparative Example 1;

[0027] Figure 4 The conductivity test results are for Example 3 and Comparative Examples 2-5;

[0028] Example 1

[0029] Take a 10cm*10cm perfluorosulfonic acid membrane with an EW value of 1100g / mol (thickness 50 micrometers), lay it flat on the surface of a glass plate, place the glass plate on the surface of a heating stage, and heat the glass plate to make the surface temperature of the membrane reach 125℃, so that the membrane enters the glass transition state.

[0030] Electrodes (designated A and B), each 20 cm long, were placed at both ends of a glass plate, with a 15 cm gap between them. A DC power supply was used to apply a 45 V voltage to both electrodes (A as positive, B as negative) for 4 minutes. Then, the voltage direction was switched (B as positive, A as negative), and this process was repeated for another 4 minutes. This cycle was repeated 8 times to complete the activation process. The voltage was then adjusted to 150 V (A as positive, B as negative) and maintained for 1.5 hours to complete the directional alignment of sulfonate groups.

[0031] Example 2

[0032] Take a 10cm*10cm perfluorosulfonic acid membrane with an EW value of 1100g / mol (thickness 50 micrometers), sandwich it between two glass plates with a single glass plate thickness of 0.6cm, place it on the surface of the heating stage, and heat the glass plates to make the membrane surface temperature reach 125℃, so that the membrane enters the glass transition state.

[0033] Stainless steel plates were placed over the top and bottom of a glass plate. Two electrodes (A and B) of a DC power supply were connected to these plates, with the electrode spacing equal to the thickness of the glass mold plus the film thickness (1.25 cm). A DC voltage of 3.5V (A as positive, B as negative) was applied to the electrodes for 4 minutes. The voltage direction was then reversed (B as positive, A as negative), and this process was repeated for another 4 minutes. This was repeated a total of 8 times to complete the activation process. The voltage was then adjusted to 12.5V (A as positive, B as negative) and maintained for 1.5 hours to complete the directional alignment of sulfonate groups.

[0034] Comparative Example 1

[0035] Similar to the first step of Implementation 1, using the same glass plate and film, the film is heated to 125°C and held for 2.1 hours before stopping.

[0036] The membrane materials from Examples 1 and 2, and Comparative Example 1, were immersed in a 2 wt% Pb(Ac)₂ aqueous solution for 3 days to stain the Nafion ion clusters. They were then washed with ultrapure water and dried at room temperature. For testing, the stained membranes were embedded in epoxy resin, and cross-sections were observed using transmission electron microscopy. The results are as follows: Figure 1 As shown. The dark spots in the figure (circled in the middle) are related to Pb. 2+ The brighter areas are aggregates of Nafion fluorocarbon framework, while the darker areas are aggregates of sulfonate ions. The darkness of the ion clusters in the TEM image depends on the aggregation density and depth of the sulfonate ions. The higher the aggregation density of sulfonate ions, the darker the ion aggregation sites appear in the TEM image. As can be seen, the dark area in Comparative Example 1, which was heat-treated but without an applied electric field film, is smaller, and almost no obvious dark area is visible. Figure 1 a) This indicates that the density of sulfonate groups in the membrane decreases, resulting in relatively smaller ion channel sizes. In Examples 1 and 2 of this application, the dark area of ​​the membrane increases after electric field treatment and is uniformly distributed in the gray area. This suggests that the electric field treatment after heating causes the sulfonate groups to rearrange and aggregate, forming denser hydrophilic clusters that are uniformly and densely distributed in the fluorocarbon matrix, resulting in larger ion channel sizes.

[0037] The membranes of Examples 1 and 2 and Comparative Example 1 were subjected to FT-IR testing to study the internal structure of the membranes, such as... Figure 2 As shown, the increased intensity of the S–O stretching vibration peak in the film after electric field treatment is due to the increased surface area of ​​the clusters after treatment, at 960 cm⁻¹. -1 An increase in the intensity of the ether bond (C–O–C) peak indicates a rearrangement of the side chains.

[0038] Figure 3 The results show the conductivity test results of the films in Examples 1 and 2 and Comparative Example 1. The conductivity, from smallest to largest, is as follows: the film in Comparative Example 1 without an applied electric field, the film in Example 2 activated and cured with an electric field applied in the vertical direction, and the film in Example 1 activated and cured with an electric field applied in the area direction. This demonstrates that applying an electric field for activation followed by an electric field for curing in this invention effectively improves proton conductivity.

[0039] Example 3

[0040] Take a 18cm*18cm perfluorosulfonic acid membrane (15 micrometers) with an EW value of 850g / mol, lay it flat and fix it on the glass surface, place the glass plate on the heating stage, and heat the glass plate to make the membrane surface temperature reach 150℃, so that the membrane enters the glass transition state.

[0041] Electrodes (named A and B), each 20 cm long, were placed at both ends of a glass plate, with a 20 cm gap between them. A DC power supply was used to apply a 40V voltage to both electrodes (A as positive, B as negative) for 2 minutes. Then, the voltage direction was switched (B as positive, A as negative), and this process was repeated for another 2 minutes. This was repeated six times to complete the "activation process." The voltage was then adjusted to 200V (A as positive, B as negative) and maintained for 1 hour to complete the treatment of sulfonate group orientation.

[0042] Comparative Example 2

[0043] Similar to the first step of implementation 3, the same glass plate and membrane are used. After heating the membrane to 150°C, without going through the activation process, a voltage of 200V is directly applied to the two electrodes (A is the positive electrode and B is the negative electrode) and maintained for 1.0 hour to complete the treatment of sulfonate directional alignment.

[0044] Comparative Example 3

[0045] Similar to the first step of implementation 3, the same glass plate and film are used. After heating the film to 150°C, no electric field treatment is performed, and the process is stopped after 74 minutes.

[0046] Comparative Example 4

[0047] The same steps as in step 3 are followed, except that the activation voltage is changed to 10V and the other steps remain the same, to complete the treatment of sulfonate directional alignment.

[0048] Comparative Example 5

[0049] Take a 18cm*18cm perfluorosulfonic acid membrane (15 micrometers) with an EW value of 850g / mol, lay it flat and fix it on the glass surface, place the glass plate on the heating stage, and heat the glass plate to make the membrane surface temperature reach 150℃, so that the membrane enters the glass transition state.

[0050] Electrodes (named A and B), each 20 cm long, were placed at both ends of a glass plate, with a 20 cm gap between them. A DC power supply was used to apply a voltage of 40 V to the two electrodes (A as the positive electrode and B as the negative electrode) without changing the direction of the electric field. This process was continued for 14 minutes to complete the "activation process." Then, the voltage was adjusted to 200 V (A as the positive electrode and B as the negative electrode) and maintained for 1 hour to complete the treatment of sulfonate ion orientation.

[0051] Figure 4The results show the conductivity test results of the membranes in Examples 3 and Comparative Examples 2-5. The conductivity, from smallest to largest, is as follows: Comparative Example 3 (membrane without applied electric field), Comparative Example 2 (membrane with electric field applied directly without activation), Comparative Example 5 (membrane activated without changing the electric field direction), Comparative Example 4 (membrane activated at 10V with switched electric field direction), and Example 3 (membrane activated at 40V with switched electric field direction before applying an electric field along its area). Comparisons of Examples 3 and 3 demonstrate that applying an electric field significantly improves the proton conductivity of the membrane; comparisons of Examples 3 and 2 demonstrate that the activation process before applying the electric field significantly improves conductivity; comparisons of Examples 3 and 4 demonstrate that the activation voltage range proposed in this invention is more effective in improving conductivity; and comparisons of Examples 3 and 5 demonstrate that the proposed switching of the electric field direction is more effective in improving conductivity. Furthermore, activating by switching the electric field direction before applying the electric field for curing is even more effective in improving proton conductivity.

Claims

1. A method for preparing a perfluorosulfonic acid membrane, characterized in that, Includes the following steps: (1) Heat the perfluorosulfonic acid film to induce its glass transition state; (2) The perfluorosulfonic acid membrane after step (1) is placed in an electric field and treated under low electric field strength and high electric field strength in sequence; the treatment under low electric field strength is to switch the electric field direction every 1-5 minutes.

2. According to the preparation method of claim 1, in the perfluorosulfonic acid membrane prepared in step (2), the sulfonate groups are oriented.

3. The preparation method according to claim 1 or 2, characterized in that, The low electric field strength is 1.0-3.0 V / cm; the high electric field strength is 7.0-10.0 V / cm.

4. The preparation method according to claim 1 or 2, characterized in that, The process under low electric field strength switches the electric field direction 5-10 times; the process under high electric field strength does not change the electric field direction, and the electric field is applied for 1-2 hours.

5. The preparation method according to claim 4, characterized in that, The electric field direction includes the area direction and the vertical direction; the area direction is the direction parallel to the plane of the perfluorosulfonic acid film, and the vertical direction is the direction perpendicular to the area of ​​the perfluorosulfonic acid film.

6. The preparation method according to claim 5, characterized in that, When using an "area-direction electric field", the film is fixed on the surface of a non-conductive substrate with electrode plates connected to both ends. The electrode plates at both ends of the non-conductive substrate are connected to a DC power supply, forming a capacitor in the area direction. The film is located in the electric field of the area-direction capacitor. When using a "vertical electric field", the film is sandwiched between two non-conductive substrates to form a "sandwich" structure. The upper and lower surfaces of the "sandwich" are connected to electrode plates. The electrode plates on the upper and lower surfaces are connected to a DC power supply, forming a capacitor in the vertical direction. The film is located in the electric field of the vertical direction capacitor.

7. The preparation method according to claim 6, characterized in that, The non-conductive substrate is selected from one of glass plate, polytetrafluoroethylene plate, or bakelite plate; the EW value of the perfluorosulfonic acid film is in the range of 700-1200 g / mol.

8. The preparation method according to claim 1, characterized in that, The process of heating the perfluorosulfonic acid film to induce its glass transition state involves fixing the film onto the surface of a substrate and heating the substrate to bring the film to its glass transition temperature, which is 125-150°C.

9. A proton exchange membrane, characterized in that, The proton exchange membrane includes a perfluorosulfonic acid membrane obtained by the preparation method according to any one of claims 1-8.

10. The application of the proton exchange membrane according to claim 9 in a fuel cell.

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