CN114044884A

CN114044884A - High-temperature phosphoric acid proton exchange membrane based on polyfluorene and preparation method thereof

Info

Publication number: CN114044884A
Application number: CN202111367744.3A
Authority: CN
Inventors: 林本才; 陈燕波; 徐斐; 李泾; 韩雨洋
Original assignee: Changzhou University
Current assignee: Changzhou University
Priority date: 2021-11-18
Filing date: 2021-11-18
Publication date: 2022-02-15
Anticipated expiration: 2041-11-18
Also published as: CN114044884B

Abstract

The invention relates to a high-temperature proton exchange membrane, in particular to a polyfluorene-based high-temperature phosphoric acid proton exchange membrane and a preparation method thereof. Firstly, synthesizing a polymer of a polyfluorene-based ether bond-free main chain by carrying out super acid catalysis Friedel-Crafts polymerization reaction on terphenyl, fluorene and N-methyl-4-piperidone, then grafting a sulfonic acid group on a side chain of a fluorene-based copolymer, and finally carrying out film forming and acid doping post-treatment. Due to the absence of aryl ether linkages, good chemical stability is maintained even with high functionalization. A flexible alkyl side chain is introduced into a polyfluorene main chain to construct a hydrophilic/hydrophobic microphase separation structure, so that the absorption and conduction of phosphoric acid are promoted, and the proton conductivity is further improved.

Description

High-temperature phosphoric acid proton exchange membrane based on polyfluorene and preparation method thereof

Technical Field

The invention relates to a high-temperature proton exchange membrane, in particular to a polyfluorene-based high-temperature phosphoric acid proton exchange membrane and a preparation method thereof.

Background

Proton Exchange Membrane Fuel Cells (PEMFCs) are electrochemical devices that efficiently convert chemical energy in a fuel (e.g., hydrogen, biogas, methanol, etc.) into electrical energy, and have the characteristics of cleanliness, high efficiency, and rapid start-up. The method has wide application in the aspects of fixed power stations, movable power supplies and the like.

The Proton Exchange Membrane (PEM) is the core component of a PEMFC, and its performance will directly determine the performance and lifetime of the fuel cell. Compared with the traditional low-temperature proton exchange membrane, the proton exchange membrane applied to the high-temperature environment has a plurality of advantages: when the low-temperature proton exchange membrane fuel cell works, because the system contains gaseous water and liquid water, the humidity is increased during temperature rise, cathode flooding is caused, the proton conductivity is reduced, the dependence of the high-temperature proton exchange membrane on water is low, the diffusion layer can more easily reach humidity balance, and the water system management is simplified; meanwhile, the catalytic reaction efficiency of the anode and the cathode can be improved by high temperature, so that the electrode reaction kinetic rate can be higher; and the high temperature can inhibit the adsorption of carbon monoxide on the platinum-based catalyst to a certain extent, thereby effectively reducing the risk of catalyst poisoning.

The most commercially successful proton exchange membrane in the world today is the Nafion membrane developed by dupont, usa. Nafion membrane is one of typical representatives of commercial perfluorosulfonic acid (PFSA) membrane, and is widely used in the field of proton exchange membrane due to its excellent chemical stability, mechanical stability and excellent proton conductivity under wet conditions. However, the property of maintaining the state of hydration in its working range limits its application in high temperature environments, especially above 100 ℃. Therefore, in order to promote the development of the high-temperature proton exchange membrane fuel cell field, more and more attention is paid to the design and development of the high-temperature proton exchange membrane with the working temperature of 100-200 ℃.

High temperature pem fuel cells typically operate at temperatures in excess of 100 c and therefore require the selection of suitable proton conductive materials to replace water. Phosphoric Acid (PA) is the most commonly used non-aqueous proton conducting material, and the amount of phosphoric acid doping largely affects proton conductivity and is considered as a key performance indicator of phosphoric acid doped high temperature proton exchange membranes. However, excessive introduction of phosphoric acid may weaken the interaction between polymer molecules to some extent, so that the polymer film undergoes swelling deformation, thereby resulting in a decrease in the mechanical strength of the polymer film. Therefore, the balance between proton conductivity and membrane mechanical strength is critical for phosphoric acid doped polymer membranes.

Currently, one of the most typical high temperature proton exchange membranes (HT-PEMs) is a phosphoric acid doped Polybenzimidazole (PBI) membrane. However, the production of PBI/PA membranes with high mechanical strength requires the synthesis of PBI polymers with rather high molecular weights, which, however, show poor solubility in conventional organic solvents, which is not conducive to the production of large area homogeneous membranes. Therefore, a new high-temperature stable polymer material is urgently needed to be developed to replace the PBI polymer proton exchange membrane.

Disclosure of Invention

Polyfluorene (PF) is a highly conjugated, very rigid, fully aromatic polymer with excellent mechanical strength, chemical stability and thermal stability. In the invention, an amphoteric proton exchange membrane based on polyfluorene is prepared, and a flexible alkyl chain segment is introduced into the 9 th position of the fluorene so as to improve the molecular weight and the solubility of a polymer. The structure of the membrane is rigid and flexible, so that the mechanical property of the membrane is ensured, the formation of ion channels in the membrane is effectively promoted, and the conduction of protons is promoted.

The invention provides a polyfluorene-based high-temperature phosphoric acid proton exchange membrane and a preparation method thereof. A polymer of a polyfluorene-based main chain without ether bonds is synthesized by the super acid catalysis Friedel-Crafts polymerization reaction of terphenyl, fluorene and N-methyl-4-piperidone, and good chemical stability can be kept under the condition of high functionalization due to the absence of aryl ether bonds. A flexible alkyl side chain is introduced into a polyfluorene main chain to construct a hydrophilic/hydrophobic microphase separation structure, so that the absorption and conduction of phosphoric acid are promoted, and the proton conductivity is further improved.

In order to achieve the aim, the invention adopts the following technical scheme: a fluorene-based high-temperature phosphoric acid proton exchange membrane has the following structural formula:

wherein n is the number of repeating units of the polymer, and n is greater than 0; x is the molar ratio of 9, 9-dialkyl fluorene to p-terphenyl, and x is 0-100 and is not 0; m is an integer of 1-12; k is 3 or 4.

The preparation steps of the high-temperature phosphoric acid proton exchange membrane are as follows:

(1) preparation of 9, 9-dialkylfluorenes

Dissolving fluorene and halogenated alkane serving as raw materials in dimethyl sulfoxide (DMSO), adding tetrabutylammonium bromide (TBAB) serving as a phase transfer agent, using NaOH as an alkali source, heating to 60-80 ℃ under the condition of nitrogen, stirring, reacting for 24 hours continuously, cooling to room temperature after the reaction is finished, extracting the reaction liquid with dichloromethane, taking a lower organic layer, drying, performing column chromatography separation, and purifying to obtain the monomer 9, 9-dialkyl fluorene.

Wherein the halogenated alkane is selected from bromobutane, bromoethane, bromohexane and methyl iodide; the molar ratio of the fluorene to the halogenated alkane is 1:2, the using amount of tetrabutylammonium bromide (TBAB) is 0.1-1% of the molar amount of the fluorene, and the using amount of NaOH is 10-15 times of the molar amount of the fluorene.

The stationary phase used for column chromatography separation is silica gel, and the eluant is n-hexane.

(2) Preparation of fluorene-based copolymer

Dissolving the monomer 9, 9-dialkyl fluorene, p-terphenyl and N-methyl-4-piperidone in the step (1) in dichloromethane with stirring, dropwise adding trifluoroacetic acid (TFA) and trifluoromethanesulfonic acid (TFSA) in an ice water bath under the protection of nitrogen, and then stirring the reaction mixture in the ice water bath for 24 hours. After the reaction is complete, the mixture is poured into an aqueous solution and excess K is added₂CO₃Washing, finally washing to neutrality with deionized water, and then subjecting the polymer to 60 deg.CAnd (5) drying. Finally, the target fluorenyl copolymer is obtained.

Wherein the dosage of the N-methyl-4-piperidone is 1.1 times of the sum of the molar weight of the 9, 9-dialkyl fluorene and the p-terphenyl; the volume ratio of TFA to TFSA is 1: 5-1: 8; the monomer material accounts for 10-30 wt% of the acid solution.

(3) Grafting of pendant sulfonic acid groups on polymers

And (3) stirring and dissolving the fluorenyl copolymer in the step (2) in dimethyl sulfoxide, adding alkyl sultone (such as 1, 3-propane sultone or 1, 4-butane sultone) in an equimolar ratio, and reacting for 24 hours at the temperature of 60-80 ℃.

(4) Film formation and acid doping post-treatment

After the product was cooled to room temperature, the reaction solution was uniformly cast on a clean glass plate and vacuum-dried at 60 ℃ to form a film. And immersing the obtained polymer electrolyte membrane into a phosphoric acid solution, taking out the polymer electrolyte membrane, and wiping off acid remained on the surface to finally obtain the fluorenyl phosphoric acid proton exchange membrane.

The amphoteric proton exchange membrane based on polyfluorene prepared by the invention can be used for high-temperature phosphoric acid fuel cells, methanol fuel cells, flow batteries, electrolysis, electrodialysis or separation membranes and the like.

Compared with the prior art, the invention has the following advantages:

(1) without the presence of weak bonds (such as ether bonds) in the polymer, such films have good chemical stability.

(2) A methylene side chain is introduced into a polyfluorene main chain, so that the flexibility, the molecular weight and the solubility of the polymer are effectively improved.

(3) The copolymer has simple synthesis process and high efficiency, and is suitable for large-scale preparation.

(4) The polymer not only contains hydrophilic sulfonic acid groups, but also has a basic structure, and is beneficial to absorption of phosphoric acid and proton conduction.

Description of the drawings:

FIG. 1 shows the fluorene-based copolymer (A) prepared in example 1 and a fluorene-based copolymer (B) in which 1,3 propane sultone is grafted to a piperidine ring¹H-NMR spectrum.

Detailed Description

The invention will be further illustrated with reference to the following specific examples.

Example 1

Fluorene (3.32g, 20mmol), bromo-n-hexane (8.58g, 52mmol), tetrabutylammonium bromide (0.33g,1mmol) was heated at 60 ℃ with stirring, dimethylsulfoxide as solvent, and NaOH solution (50 wt%, 12mL) was added under nitrogen. After 24h of reaction, the reaction was stopped and cooled to room temperature. And pouring the reaction liquid into dichloromethane for extraction, collecting a lower organic layer, purifying the lower organic layer by using n-hexane for column chromatography, and finally drying a product after the column chromatography to obtain the monomer 9, 9-dihexylfluorene.

To a 100mL three-necked round bottom flask equipped with a mechanical stirrer at the top was added p-terphenyl (6.20g, 27mmol), N-methyl-4-piperidone (3.73g, 33mmol), 9 dihexylfluorene (1.00g, 3mmol), dichloromethane as solvent (20mL), and trifluoroacetic acid (TFA, 3.87mL) and trifluoromethanesulfonic acid (TFSA, 25.5mL) were added dropwise at 0 ℃. The reaction mixture was then stirred at 0 ℃ for a further 6 hours. The solution turned dark purple and the viscosity increased during the polymerization. The resulting mixture was poured into an aqueous solution. By K₂CO₃The aqueous solution was washed, followed by washing the polymer to neutrality with deionized water. Finally the polymer was dried in an oven at 60 ℃ under vacuum. Through nuclear magnetic hydrogen spectrum analysis, the ratio of 9,9 dihexylfluorene to terphenyl structure in the polymer is 1: 9.

Dissolving a fluorenyl copolymer (1.00g, 2.8mmol) in dimethyl sulfoxide under stirring, adding an equimolar amount of 1, 3-propane sultone (0.35g, 2.8mmol), stirring at 60 ℃ for 24 hours to obtain a uniform solution, casting the solution on a clean glass plate after the reaction is finished, and then putting the glass plate into a vacuum oven to be dried in vacuum at 60 ℃ to form a film. Separating the membrane from the glass plate, and immersing the membrane into 85 wt% phosphoric acid solution to finally obtain the fluorenyl phosphoric acid proton exchange membrane. The structural formula is as follows:

the films prepared by the above steps were testedThe film reached 77.16mS cm at 160 DEG C^-1The membrane has excellent mechanical performance, the tensile strength can reach 14.56Mpa at room temperature, the phosphoric acid absorption rate is 78.1 percent after the membrane is immersed into 85wt percent phosphoric acid for 24 hours at 80 ℃, and the size swelling is 18.85 percent.

Example 2

This example is similar to example 1, except that 1, 4-butane sultone is grafted onto the nitrogen atom of the piperidone. The structural formula is as follows:

the film reached 79.64mS cm at 160 DEG C^-1The membrane has excellent mechanical performance, the tensile strength can reach 11.28Mpa at room temperature, the phosphoric acid absorption rate is 85.4 percent after the membrane is immersed in 85wt percent phosphoric acid for 24 hours at 80 ℃, and the size swelling is 19.62 percent.

Example 3:

this example is similar to example 1 except that n-butyl bromide is grafted onto the fluorene at position 9. The structural formula is as follows:

the film reached 72.24mS cm at 160 DEG C^-1The membrane has excellent mechanical performance, the tensile strength can reach 15.21Mpa at room temperature, the phosphoric acid absorption rate is 76.5 percent and the size swelling is 16.67 percent after the membrane is immersed into 85wt percent phosphoric acid for 24 hours at 80 ℃.

Example 4

This example is similar to example 1, except that the molar ratio of 9, 9-dihexylfluorene to p-terphenyl was 15: 85. the structural formula is as follows:

the film reached 78.12mS cm at 160 DEG C^-1The membrane has excellent mechanical performance, the tensile strength can reach 12.87Mpa at room temperature, the phosphoric acid absorption rate is 80.45 percent after the membrane is immersed into 85wt percent phosphoric acid for 24 hours at 80 ℃, and the size swelling is 19.26 percent.

Example 5:

in this example, analogously to example 4, the molar ratio of 9, 9-dihexylfluorene to p-terphenyl was 15: 85, but grafted with 1, 4-butane sultone onto the polymer backbone. The structural formula is as follows:

the film reached 80.32mS cm at 160 DEG C^-1The membrane has excellent mechanical performance, the tensile strength can reach 10.64Mpa at room temperature, the phosphoric acid absorption rate is 87.25 percent after the membrane is immersed in 85wt percent phosphoric acid for 24 hours at 80 ℃, and the size swelling is 20.32 percent.

The above embodiments are intended to illustrate the design concept of the present invention, and it should be understood that the present invention can be implemented and modified by those skilled in the art after reading the present disclosure, but should not be construed as limiting the scope of the present invention.

Claims

1. A polyfluorene-based high-temperature phosphoric acid proton exchange membrane is characterized in that the structural formula of the proton exchange membrane is as follows:

2. A preparation method of a polyfluorene-based high-temperature phosphoric acid proton exchange membrane is characterized by comprising the following steps:

(1) preparation of 9, 9-dialkylfluorenes

Firstly adding fluorene, continuously stirring for 10 minutes after the fluorene is completely dissolved in dimethyl sulfoxide (DMSO), then slowly dripping halogenated alkane, adding tetrabutylammonium bromide (TBAB) as a phase transfer agent, using NaOH as an alkali source, heating to 60-80 ℃ under the condition of nitrogen, stirring, keeping the reaction time for 24 hours, cooling to room temperature after the reaction is finished, extracting the reaction liquid by using dichloromethane, taking a lower organic layer for drying, then performing column chromatography separation, and purifying to obtain a monomer 9, 9-dialkyl fluorene;

(2) preparation of fluorene-based copolymer

Stirring and dissolving the monomer 9, 9-dialkyl fluorene, p-terphenyl and N-methyl-4-piperidone in the step (1) in dichloromethane, dropwise adding trifluoroacetic acid (TFA) and trifluoromethanesulfonic acid (TFSA) in an ice water bath under the protection of nitrogen, stirring the reaction mixture in the ice water bath for 24 hours, pouring the mixture into an aqueous solution after the reaction is finished, and adding excessive K₂CO₃Washing, finally washing with deionized water to neutrality, and then drying the polymer at 60 ℃ to finally obtain the target fluorenyl copolymer;

(3) grafting of pendant sulfonic acid groups on polymers

Stirring and dissolving the fluorenyl copolymer in the step (2) in dimethyl sulfoxide, adding alkyl sultone with an equal molar ratio, and reacting for 24 hours at 60-80 ℃;

(4) film formation and acid doping post-treatment

And cooling the product to room temperature, uniformly casting the reaction solution on a clean glass plate, drying at 60 ℃ in vacuum to form a film, immersing the obtained polymer electrolyte film into a phosphoric acid solution, taking out the polymer electrolyte film, and wiping off acid remained on the surface to finally obtain the fluorenyl phosphoric acid proton exchange membrane.

3. The method for preparing a polyfluorene-based high-temperature phosphoric acid proton exchange membrane according to claim 2, wherein the halogenated alkane in the step (1) is selected from bromobutane, bromoethane, bromohexane and iodomethane.

4. The preparation method of the polyfluorene-based high-temperature phosphoric acid proton exchange membrane according to claim 2, wherein the molar ratio of the fluorene to the halogenated alkane in the step (1) is 1:2, the amount of tetrabutylammonium bromide (TBAB) is 0.1-1% of the molar amount of the fluorene, and the amount of NaOH is 10-15 times of the molar amount of the fluorene.

5. The method for preparing a polyfluorene-based high-temperature phosphoric acid proton exchange membrane according to claim 2, wherein the amount of N-methyl-4-piperidone in the step (2) is 1.1 times of the sum of the molar amounts of the 9, 9-dialkyl fluorene and the p-terphenyl.

6. The preparation method of the polyfluorene-based high-temperature phosphoric acid proton exchange membrane according to claim 2, wherein the volume ratio of TFA to TFSA in the step (2) is 1: 5-1: 8; the monomer material accounts for 10-30 wt% of the acid solution.

7. The method for preparing a polyfluorene-based high-temperature phosphoric acid proton exchange membrane according to claim 2, wherein the alkyl sultone in the step (3) is 1, 3-propane sultone or 1, 4-butane sultone.

8. Use of a polyfluorene-based high temperature phosphoric acid proton exchange membrane according to claim 1 for high temperature phosphoric acid fuel cells, methanol fuel cells, flow batteries, electrolysis, electrodialysis or separation membranes.