Preparation method of modified proton exchange membrane

Abstract

The invention is applicable to the technical field of proton exchange membranes, and particularly relates to a preparation method of a modified proton exchange membrane, which comprises the following steps: pretreating a basement membrane to obtain a hydrogen-type perfluorosulfonic acid membrane; then soaking in an organic solvent, and swelling at 60-80 DEG C for 1-4 hours; preparation of a hybrid solution: sulfonated polyimide is dissolved in N, N-dimethylacetamide, and a settled solution with the concentration of 1-5 wt% is prepared; in-situ hybridization film formation: immersing the swelled substrate film into the prepared hybridization solution, and reacting at 60-80 DEG C for 6-24 hours; and post-treatment: washing with absolute ethyl alcohol to remove the polymer physically adsorbed on the surface of the membrane, and drying to obtain the modified proton exchange membrane. According to the invention, the problems of high methanol permeation, poor high-temperature low-humidity performance, unstable size and the like are solved, multiple key performance indexes are obviously improved unexpectedly, and the preparation method has huge application potential in the field of preparation of methanol fuel cells and high-temperature proton exchange membrane fuel cells.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of proton exchange membranes, and particularly relates to a preparation method of a modified proton exchange membrane. BACKGROUND

[0002] The proton exchange membrane is a core component of a proton exchange membrane fuel cell (PEMFC) and a methanol fuel cell, and its performance directly determines the energy conversion efficiency, service life and cost of the fuel cell. An ideal proton exchange membrane should have high proton conductivity, low fuel permeability, good chemical and mechanical stability and appropriate cost.

[0003] The perfluorosulfonic acid membrane has a micro-phase separation structure, forming a hydrophobic polytetrafluoroethylene main chain and a hydrophilic sulfonic acid group cluster region, and exhibits excellent proton conduction capacity under sufficient wetting conditions. However, it has a large dependence on water molecules, and when the operating temperature is relatively high or the humidity is relatively low, the proton conductivity will rapidly decrease and the performance will deteriorate. SUMMARY

[0004] The purpose of the present application is to provide a preparation method of a modified proton exchange membrane, aiming to solve the problem that the prior art has a large dependence on water molecules, and when the operating temperature is relatively high or the humidity is relatively low, the proton conductivity will rapidly decrease and the performance will deteriorate.

[0005] The present application is implemented in the following manner. A preparation method of a modified proton exchange membrane comprises the following steps:

[0006] (1) substrate membrane pretreatment: a perfluorosulfonic acid type proton exchange membrane is sequentially boiled in a hydrogen peroxide solution, a dilute sulfuric acid solution and deionized water to obtain a hydrogen type perfluorosulfonic acid membrane; then the hydrogen type perfluorosulfonic acid membrane is soaked in an organic solvent and swelled at 60-80℃ for 1-4 hours;

[0007] (2) preparation of a hybrid solution: sulfonated polyimide is dissolved in N,N-dimethylacetamide to prepare a clear solution with a concentration of 1-5wt%;

[0008] (3) in-situ hybrid film formation: the swelled substrate membrane in step (1) is immersed in the hybrid solution prepared in step (2) and reacted at 60-80℃ for 6-24 hours, so that the sulfonated polyimide molecules are filled into the hydrophilic regions and channels of the perfluorosulfonic acid membrane through electrostatic interaction and chain segment entanglement in-situ;

[0009] (4) post-treatment: the membrane after reaction in step (3) is taken out, washed with anhydrous ethanol to remove the physically adsorbed polymers on the surface, and then dried at 100-120℃ under vacuum conditions for 6-12 hours to finally obtain the modified proton exchange membrane.

[0010] Preferably, in the step of pretreating the base membrane, the perfluorosulfonic acid type proton exchange membrane is boiled in 3-5 wt% hydrogen peroxide solution for 0.5-1 hour, then boiled in 0.5-1 M dilute sulfuric acid solution for 0.5-1 hour, and finally boiled in deionized water for 1-2 hours.

[0011] Preferably, the organic solvent is one of ethanol, isopropanol or n-propanol.

[0012] Preferably, in the step of swelling, the hydrogen type perfluorosulfonic acid membrane is placed in 70℃ ethanol for 2 hours.

[0013] Preferably, the sulfonation degree of the sulfonated polyimide is 60%-80%.

[0014] Preferably, in the step of preparing the hybrid solution, the concentration of the hybrid solution is 2-3 wt%.

[0015] Preferably, in the step of post-treatment, drying is performed at 110℃ under vacuum for 10 hours.

[0016] The application provides a preparation method of a modified proton exchange membrane, which solves the problems of high methanol permeation, poor high-temperature low-humidity performance, and unstable size, and the prepared composite proton exchange membrane has unexpected significant improvement in a plurality of key performance indicators, and has great application potential in the fields of methanol fuel cells and high-temperature proton exchange membrane fuel cells. DETAILED DESCRIPTION

[0017] In order to make the purpose, technical scheme and advantages of the application clearer, the application is further described in detail below. It should be understood that the specific embodiments described herein are only used to explain the application, and are not used to limit the application.

[0018] A preparation method of a modified proton exchange membrane, the method comprising:

[0019] (1) base membrane pretreatment: the perfluorosulfonic acid type proton exchange membrane is sequentially boiled in hydrogen peroxide solution, dilute sulfuric acid solution and deionized water to obtain a hydrogen type perfluorosulfonic acid membrane; then the hydrogen type perfluorosulfonic acid membrane is soaked in an organic solvent and swelled at 60-80℃ for 1-4 hours; specifically, the perfluorosulfonic acid type proton exchange membrane is first boiled in 3-5 wt% hydrogen peroxide solution for 0.5-1 hour, then boiled in 0.5-1 M dilute sulfuric acid solution for 0.5-1 hour, and finally boiled in deionized water for 1-2 hours; the organic solvent is one of ethanol, isopropanol or n-propanol; when swelling, the hydrogen type perfluorosulfonic acid membrane is placed in 70℃ ethanol for 2 hours.

[0020] In this step, a three-step boiling method is adopted for hydrogen peroxide solution treatment: In the present invention, a hydrogen peroxide (H2O2) aqueous solution with a concentration of 3-5 wt% is used. The core purpose is to oxidize and remove organic impurities and microbial residues attached to the surface and internal pores of the proton exchange membrane. Some organic pollutants may be introduced during the storage, transportation or previous processing of the membrane. As a strong oxidizing agent, H2O2 can effectively decompose these organic matters, converting them into CO2 and water and volatilizing them, thereby purifying the membrane structure. In addition, this step can also oxidize other low-valence metal ion impurities that may exist in the membrane, facilitating their removal in the subsequent acid washing step.

[0021] Dilute sulfuric acid solution treatment: After hydrogen peroxide treatment, the membrane needs to be boiled in a dilute sulfuric acid (e.g. 0.5-1.0 M) solution. The primary function of this step is ion exchange, ensuring that the membrane is completely converted to the hydrogen form (H + form). Commercial proton exchange membranes are usually provided in Na + form or other ion forms to ensure stability, but only the H + form has proton conduction function. The H2SO 4 solution provides a high concentration of H + , which can completely replace all other cations (such as Na + , K + , Ca 2+ , etc.) in the membrane. Secondly, the acid boiling step can further dissolve and remove some inorganic mineral impurities and activate the sulfonic acid groups (-SO3H) in the membrane itself.

[0022] Deionized water boiling: The purpose of the last step is to completely remove residual oxidizing agents and acid. The presence of residual hydrogen peroxide and sulfuric acid can seriously affect the smooth progress of the subsequent hybridization reaction and may accelerate the chemical degradation of the membrane itself at high temperatures. Through repeated boiling, it can be ensured that the membrane is in a pure and neutral environment, preparing for the next step of interaction with the SPI solution.

[0023] Perfluorosulfonic acid membranes (such as Nafion) have a typical hydrophilic-hydrophobic phase separation microstructure. In the dry state, its hydrophilic region (ion clusters) is relatively contracted and closed. Directly immersing it in the SPI solution, the large molecules of SPI are difficult to effectively penetrate into the interior of the membrane. The present invention soaks the clean hydrogen form membrane after pretreatment in organic solvents such as ethanol, isopropanol or N, N-dimethylformamide (DMF) and heats it at 60-80°C for 1-4 hours.

[0024] Mechanism of action: Under the condition of heating, the organic solvent molecules penetrate into the hydrophobic fluorocarbon main chain of Nafion membrane, making the chain segment movement ability increase and the spacing increase. At the same time, the solvent molecules also enter the hydrophilic ion cluster area, causing swelling, i.e. the pore opening and size expansion. This swelling state creates an open microstructure, greatly promoting the diffusion and penetration of the subsequent SPI macromolecular solution into the membrane interior. The choice of temperature and time is crucial: if the temperature is too low or the time is too short, the swelling is insufficient and SPI is difficult to enter; if the temperature is too high or the time is too long, it may cause irreversible damage to the membrane structure or excessive swelling leading to loss of mechanical strength. After a large number of experimental verification, 70 degrees Celsius for 2 hours is the preferred condition for achieving the best balance between permeation and structural integrity.

[0025] (2) Hybrid solution preparation: Dissolve sulfonated polyimide in N,N-dimethylacetamide to prepare a clear solution with a concentration of 1-5wt%; wherein the sulfonation degree of sulfonated polyimide is 60%-80%; in the step of preparing the hybrid solution, the concentration of the hybrid solution is 2-3wt%.

[0026] In this step, the selection of sulfonated polyimide (SPI): SPI is the core modifier of the present application, which has excellent mechanical strength, thermal stability and chemical stability of polyimide, as well as the hydrophilicity and proton conductivity brought by the sulfonic acid group (-SO3H) introduced by the sulfonation reaction. Its sulfonation degree (DS) directly affects the density of sulfonic acid groups on the molecular chain, and then affects the hydrophilicity and conductivity of the hybrid membrane. The present application preferably uses SPI with a sulfonation degree of 60%-80%, which can provide sufficient proton conduction sites while maintaining good film-forming properties and stability of the molecular chain.

[0027] The solvent used is N,N-dimethylacetamide (DMAc), which is a strong polar aprotic solvent with strong solubility for SPI, capable of preparing uniform and stable SPI solutions. More importantly, DMAc has good mutual solubility with the solvent used for swelling in step (1) (such as ethanol), and has good wetting and permeability for the swollen Nafion membrane, which ensures that the SPI / DMAc solution can smoothly contact and diffuse into the interior of the membrane. In contrast, water or other solvents may not effectively dissolve SPI or cause uncontrollable shrinkage of the Nafion membrane, which is not conducive to the hybridization reaction.

[0028] Concentration control (1-5 wt%): The concentration of the solution is a key process parameter to control the amount of SPI that penetrates into the membrane. If the concentration is too low (<1 wt%), the amount of SPI introduced is insufficient to form an effective hybrid network, and the modification effect is not obvious. If the concentration is too high (>5 wt%), the viscosity of the solution increases, which hinders the diffusion of SPI molecules into the membrane interior, and easily leads to the accumulation of a large amount of SPI on the membrane surface, forming a dense skin layer, hindering proton conduction, and even causing the membrane to be brittle. Through a large number of experiments, the inventors have determined that 2-3 wt% is the optimal concentration range, which can ensure the effective penetration of SPI while achieving uniform hybridization inside and outside the membrane.

[0029] (3) In-situ hybrid film formation: The swollen substrate membrane in step (1) is immersed in the hybrid solution prepared in step (2), and reacted at 60-80°C for 6-24 hours, so that the sulfonated polyimide molecules are filled into the hydrophilic region and pores of the perfluorosulfonic acid membrane through electrostatic interaction and chain entanglement.

[0030] In this step, the reaction process: the Nafion membrane in an open state after being fully swollen is immersed in the SPI / DMAc solution. Under heating conditions of 60-80°C, the DMAc solvent molecules and SPI polymer chains obtain higher kinetic energy.

[0031] Mass transfer and interaction: SPI molecules diffuse from the bulk phase of the solution to the interior of the swollen Nafion membrane with the help of concentration gradient driving and the carrying effect of solvent molecules. In this process, two main intermolecular interactions occur:

[0032] Electrostatic interaction: This is the strongest driving force. The -SO3 - group (H + type -SO3H) on the Nafion molecular chain and the -SO3 - group on the SPI molecular chain are both negatively charged, but they share a H + atom that acts as a bridge. Strong ion-dipole interactions and hydrogen bond networks are formed between the two sulfonic acid groups, and this strong attractive force fixes the SPI molecules in the hydrophilic ion cluster region of the Nafion membrane and around it.

[0033] Chain entanglement: SPI and Nafion are both flexible polymer long chains. In the swollen membrane interior, the molecular chains of the two have the opportunity to approach, interpenetrate, and physically entangle (Chain Entanglement), forming an interpenetrating network (IPN) or semi-IPN structure. This mechanical interlocking further stabilizes the hybrid structure and prevents SPI from leaking during long-term use.

[0034] Reaction condition optimization: Temperature and time are the key to ensure the reaction goes to completion. Too low temperature, the molecular motion is slow, SPI is difficult to penetrate into the film inside; too high temperature, it can cause excessive solvent evaporation or polymer degradation. The reaction time of 6-24 hours ensures that SPI has enough time to diffuse to the entire film thickness direction, to achieve uniform hybridization. Preferably 70°C for 12 hours to achieve the best balance of reaction depth and efficiency.

[0035] (4) Post-treatment: the film after reaction in step (3) is taken out, rinsed with anhydrous ethanol to remove the surface physically adsorbed polymer, and then dried at 100-120°C under vacuum for 6-12 hours, finally the modified proton exchange membrane is obtained.

[0036] In this step, cleaning: after the reaction is completed, the film surface will be physically adsorbed a layer of loose, not firmly combined with the substrate film SPI molecules or their aggregates. Using anhydrous ethanol for rinsing is a very effective method. Ethanol is a good solvent, which can dissolve and wash away these surface attachments, but at the same time it has little effect on the SPI that has entered the film interior and is fixed by strong interaction. This step ensures the cleanliness of the film surface, avoids the hindrance of the surface skin layer to proton conduction, and ensures good interface contact between the membrane and the electrode.

[0037] Vacuum drying: the purpose of drying is to completely remove the residual solvent (DMAc and ethanol) in the film. Under normal pressure, rapid evaporation of the solvent can cause uneven shrinkage of the film, resulting in internal stress or defects. Under vacuum conditions, the boiling point of the solvent is reduced, which can be removed gently and completely at a lower temperature (100-120°C), thereby maximizing the stable microstructure formed after hybridization. The drying temperature is slightly higher than the conventional working temperature of fuel cells, which also helps to "anneal" the membrane structure, making it more stable in use. The drying time of 6-12 hours ensures that even thicker films can be completely dried. Preferably 110°C for 10 hours.

[0038] The following illustrates the effect of the present application through a control experiment:

[0039] Example 1 (EE1):

[0040] Preparation method:

[0041] (1) Substrate membrane pretreatment: the perfluorosulfonic acid type proton exchange membrane is first boiled in 3-5wt% hydrogen peroxide solution for 0.5-1 hour, then boiled in 0.5-1M dilute sulfuric acid solution for 0.5-1 hour, and finally boiled in deionized water for 1-2 hours to obtain a hydrogen perfluorosulfonic acid membrane; then it is soaked in ethanol at 70°C for 2 hours;

[0042] (2) Hybrid solution preparation: Dissolve sulfonated polyimide with sulfonation degree of 70% in N,N-dimethylacetamide to prepare a clear solution with a concentration of 2.5 wt%;

[0043] (3) In-situ hybrid film formation: immerse the swollen substrate film in step (1) into the hybrid solution prepared in step (2) and react at 70°C for 12 hours, so that the sulfonated polyimide molecules fill into the hydrophilic region and pores of the perfluorosulfonic acid membrane in-situ through electrostatic interaction and chain entanglement;

[0044] (4) Post-processing: take out the membrane after reaction in step (3), rinse with anhydrous ethanol to remove the physically adsorbed polymers on the surface, and then dry at 110°C under vacuum conditions for 10 hours to obtain the modified proton exchange membrane.

[0045] Example 2 (EE2):

[0046] Preparation method:

[0047] (1) Substrate membrane pretreatment: boil the perfluorosulfonic acid type proton exchange membrane in 3-5 wt% hydrogen peroxide solution for 0.5-1 hour, then in 0.5-1 M dilute sulfuric acid solution for 0.5-1 hour, and finally in deionized water for 1-2 hours to obtain a hydrogen type perfluorosulfonic acid membrane; then immerse it in ethanol at 70°C for 2 hours for swelling;

[0048] (2) Hybrid solution preparation: Dissolve sulfonated polyimide with sulfonation degree of 70% in N,N-dimethylacetamide to prepare a clear solution with a concentration of 1 wt%;

[0049] (3) In-situ hybrid film formation: immerse the swollen substrate film in step (1) into the hybrid solution prepared in step (2) and react at 70°C for 12 hours, so that the sulfonated polyimide molecules fill into the hydrophilic region and pores of the perfluorosulfonic acid membrane in-situ through electrostatic interaction and chain entanglement;

[0050] (4) Post-processing: take out the membrane after reaction in step (3), rinse with anhydrous ethanol to remove the physically adsorbed polymers on the surface, and then dry at 110°C under vacuum conditions for 10 hours to obtain the modified proton exchange membrane.

[0051] To verify the effect of hybrid solution concentration on performance;

[0052] Example 3 (EE3):

[0053] Preparation method:

[0054] (1) Pretreatment of substrate membrane: The perfluorosulfonic acid membrane was first boiled in 3-5 wt% hydrogen peroxide solution for 0.5-1 hour, then boiled in 0.5-1 M dilute sulfuric acid solution for 0.5-1 hour, and finally boiled in deionized water for 1-2 hours to obtain a hydrogen-type perfluorosulfonic acid membrane; then it was soaked in ethanol at 70°C for 2 hours to swell;

[0055] (2) Preparation of hybrid solution: Dissolve sulfonated polyimide with a sulfonation degree of 70% in N,N-dimethylacetamide to prepare a clear solution with a concentration of 5 wt%;

[0056] (3) In-situ hybrid film formation: The swollen substrate membrane in step (1) was immersed in the hybrid solution prepared in step (2) and reacted at 70°C for 12 hours, allowing the sulfonated polyimide molecules to fill into the hydrophilic regions and channels of the perfluorosulfonic acid membrane through electrostatic interaction and chain entanglement;

[0057] (4) Post-treatment: The membrane after reaction in step (3) was taken out, washed with anhydrous ethanol to remove the physically adsorbed polymer on the surface, and then dried at 110°C under vacuum for 10 hours to obtain the modified proton exchange membrane.

[0058] To verify the effect of hybrid solution concentration on performance;

[0059] Comparative Example 1 (CE1):

[0060] Sample: Commercial Nafion 117 membrane (hydrogen type).

[0061] Treatment: Only the same pretreatment as in the example (acid boiling and swelling) was performed, without SPI hybridization, and directly dried.

[0062] Comparative Example 2 (CE2):

[0063] Sample: SiO2 / Nafion hybrid membrane.

[0064] Preparation: Using the sol-gel method, tetraethyl orthosilicate (TEOS) was hydrolyzed in the Nafion membrane to generate SiO2 nanoparticles, with a SiO2 doping amount of 3 wt%.

[0065] Experimental results and data analysis:

[0066] Table 1: Test results of basic performance of the membrane (test conditions: 80°C, 100% RH)

[0067]

[0068]

[0069] Conclusion:

[0070] Proton conductivity: The conductivity of all modified membranes is comparable or slightly lower than that of pristine Nafion membrane, which is a common trade-off. EE1 maintains an extremely high conductivity (0.098 S / cm) with minimal loss.

[0071] Methanol permeability: The methanol permeability of EE1 is reduced by 48% compared to CE1, and even 34% compared to CE2 (SiO2 modification). This indicates that SPI molecules effectively fill the channels of Nafion, blocking the transmission path of methanol.

[0072] Selectivity: This is the most important indicator. The selectivity of EE1 is as high as 7.84 x 10 4 S·s / cm 3 , which is 89% higher than CE1 and 57% higher than CE2. This indicates that the membrane has the comprehensive advantages of high power output and low fuel crossover in DMFC.

[0073] Mechanical properties and dimensional stability: The introduction of SPI forms an ionic cross-linked network, significantly enhancing the mechanical strength of the membrane (EE1 is 43% higher than CE1) and greatly suppressing swelling (reduced by 45%). This means that the membrane is more durable and has a longer service life in the fuel cell operating environment.

[0074] The present application adopts a method of in-situ hybrid modification of perfluorosulfonic acid membrane with sulfonated polyimide (SPI), successfully preparing a proton exchange membrane with excellent comprehensive performance.

[0075] Compared with Comparative Example 1 (pristine Nafion membrane), the membrane prepared by the present application (EE1) greatly reduces the methanol permeability while maintaining excellent proton conduction ability, and significantly improves the mechanical strength and dimensional stability of the membrane.

[0076] Compared with Comparative Example 2 (conventional SiO2 modification), the organic / organic hybrid strategy (SPI / Nafion) adopted by the present application exhibits unexpected advantages in suppressing swelling and improving high-temperature low-humidity performance.

[0077] Parameter optimization: Through the comparison of EE1, EE2 and EE3, it is proved that the SPI concentration of 2-3 wt% (such as EE1) has the best comprehensive performance balance. Too high concentration can further block alcohol but the conductivity loss is slightly larger, and too low concentration is not sufficient for modification.

[0078] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. Any modifications, equivalent replacements and improvements made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A method for preparing a modified proton exchange membrane, characterized in that, The method includes: (1) Basement membrane pretreatment: The perfluorosulfonic acid type proton exchange membrane was boiled in hydrogen peroxide solution, dilute sulfuric acid solution and deionized water in sequence to obtain the hydrogen type perfluorosulfonic acid membrane; then it was immersed in an organic solvent and swollen at 60-80℃ for 1-4 hours. (2) Preparation of hybrid solution: Sulfonated polyimide is dissolved in N,N-dimethylacetamide to prepare a clear solution with a concentration of 1-5 wt%; (3) In-situ hybridization film formation: Immerse the base film after swelling in step (1) into the hybrid solution prepared in step (2) and react at 60-80℃ for 6-24 hours, so that the sulfonated polyimide molecules can fill the hydrophilic region and pores of the perfluorosulfonic acid membrane in situ through electrostatic interaction and chain segment entanglement. (4) Post-treatment: Take out the membrane after the reaction in step (3), rinse it with anhydrous ethanol to remove the polymer physically adsorbed on the surface, and then dry it under vacuum at 100-120℃ for 6-12 hours to finally obtain the modified proton exchange membrane.

2. The method for preparing the modified proton exchange membrane according to claim 1, characterized in that, In the pretreatment steps of the basement membrane, the perfluorosulfonic acid type proton exchange membrane is first boiled in a 3-5 wt% hydrogen peroxide solution for 0.5-1 hour, then boiled in a 0.5-1 M dilute sulfuric acid solution for 0.5-1 hour, and finally boiled in deionized water for 1-2 hours.

3. The method for preparing the modified proton exchange membrane according to claim 1, characterized in that, The organic solvent is one of ethanol, isopropanol, or n-propanol.

4. The method for preparing the modified proton exchange membrane according to claim 3, characterized in that, During the swelling process, the hydrogen-form perfluorosulfonic acid membrane is placed in ethanol at 70°C for 2 hours to swell.

5. The method for preparing the modified proton exchange membrane according to claim 1, characterized in that, The degree of sulfonation of sulfonated polyimide is 60%-80%.

6. The method for preparing the modified proton exchange membrane according to claim 1, characterized in that, In the preparation of the hybrid solution, the concentration of the hybrid solution is 2-3 wt%.

7. The method for preparing the modified proton exchange membrane according to claim 1, characterized in that, In the post-processing step, the product is dried at 110°C for 10 hours under vacuum.

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