CN114044884B

CN114044884B - High-temperature phosphoric acid proton exchange membrane based on polyfluorene and preparation method thereof

Info

Publication number: CN114044884B
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: 2023-05-23
Anticipated expiration: 2041-11-18
Also published as: CN114044884A

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. The Friedel-Crafts polymerization reaction is catalyzed by super acid of terphenyl, fluorene and N-methyl-4-piperidone to synthesize a polymer based on a polyfluorene main chain without ether bond, then a sulfonic acid group is grafted on a side chain of a fluorenyl copolymer, and finally film forming and acid doping post-treatment are carried out. Good chemical stability can be maintained also in the case of highly functionalized compounds due to the absence of aryl ether linkages. And introducing a flexible alkyl side chain into the main chain of polyfluorene to construct a hydrophilic/hydrophobic microphase separation structure, so as to promote the absorption and conduction of phosphoric acid and further improve proton conductivity.

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 fuels (e.g., hydrogen, biogas, methanol, etc.) to electrical energy, and have the characteristics of clean, efficient, and rapid start. The method has wide application in the aspects of fixed power stations, movable power supplies and the like.

Proton Exchange Membranes (PEM) are the core components of PEMFCs, and their performance will directly determine the performance and life 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, as the system contains gaseous water and liquid water, the humidity is increased when the temperature is raised, so that the cathode is flooded, the proton conductivity is reduced, the high-temperature proton exchange membrane has lower dependence on water, the diffusion layer is easier to achieve humidity balance, and the water system management is simplified; meanwhile, the high temperature can improve the catalytic reaction efficiency of the anode and the cathode, so that the electrode has faster electrode reaction kinetics rate; and the adsorption of carbon monoxide on the platinum-based catalyst can be inhibited to a certain extent at high temperature, so that the risk of catalyst poisoning is effectively reduced.

The proton exchange membrane currently most commercially successful in the world is the Nafion membrane developed by dupont in the united states. Nafion membranes are one of the typical representatives of commercial perfluorosulfonic acid (PFSA) membranes, and are widely used in the field of proton exchange membranes due to their excellent chemical stability, mechanical stability, and proton conductivity under wet conditions. However, the need to maintain the hydrated state within its operating range limits its use in high temperature environments (especially above 100 ℃). Therefore, in order to promote the development of the field of high-temperature proton exchange membrane fuel cells, the design and development of a high-temperature proton exchange membrane with the working temperature of 100-200 ℃ is receiving more and more attention.

High temperature proton exchange membrane fuel cells typically operate over a temperature range of 100 ℃, so that a suitable proton conducting material needs to be selected in place of water. Phosphoric Acid (PA) is the most commonly used nonaqueous proton conducting material, the amount of phosphoric acid doping affects proton conductivity to a large extent, and is considered as a key performance indicator for phosphoric acid doped high temperature proton exchange membranes. However, excessive introduction of phosphoric acid weakens the interaction between polymer molecules to some extent, causing swelling deformation of the polymer film, and thus causing a decrease in 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-PEM) is a phosphoric acid doped Polybenzimidazole (PBI) membrane. However, the production of PBI/PA films with high mechanical strength requires the synthesis of PBI polymers of relatively high molecular weight, but such polymers exhibit poor solubility in conventional organic solvents, which would be detrimental to the production of large area homogeneous films. There is therefore an urgent need to develop a novel high temperature stable polymeric material to replace the PBI polymer proton exchange membrane.

Disclosure of Invention

Polyfluorene (PF) is a highly conjugated, very rigid wholly aromatic polymer with excellent mechanical strength, chemical stability and thermal stability. According to the invention, an amphoteric proton exchange membrane based on polyfluorene is prepared, and the molecular weight and the solubility of the polymer are improved by introducing a flexible alkyl chain segment into the No. 9 position of fluorene. The structure of the membrane has the advantages that the mechanical property of the membrane is guaranteed, 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. The polymer based on the main chain of polyfluorene without ether bond is synthesized by catalyzing Friedel-Crafts polymerization reaction with super acidic of terphenyl, fluorene and N-methyl-4-piperidone, and can maintain good chemical stability under the condition of high functionalization due to the absence of aryl ether bond. And introducing a flexible alkyl side chain into the main chain of polyfluorene to construct a hydrophilic/hydrophobic microphase separation structure, so as to promote the absorption and conduction of phosphoric acid and further improve proton conductivity.

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

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

The preparation method of the high-temperature phosphoric acid proton exchange membrane comprises the following steps:

(1) Preparation of 9, 9-dialkylfluorenes

Fluorene and halogenated alkane are taken as raw materials, the raw materials are dissolved in dimethyl sulfoxide (DMSO), tetrabutylammonium bromide (TBAB) is added as a phase transfer agent, naOH is taken as an alkali source, the temperature is raised to 60-80 ℃ under the condition of nitrogen, the stirring is carried out for 24 hours, the temperature is reduced to room temperature after the reaction is finished, the reaction solution is extracted by methylene dichloride, the lower organic layer is taken out, then column chromatography separation is carried out, and the monomer 9, 9-dialkyl fluorene is obtained after the purification.

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

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

(2) Preparation of fluorenyl copolymers

The monomers 9, 9-dialkyl fluorene and p-terphenyl and N-methyl-4-piperidone in step (1) were dissolved in dichloromethane with stirring, trifluoroacetic acid (TFA) and trifluoromethanesulfonic acid (TFSA) were added dropwise under the protection of ice-water bath and nitrogen, and then the reaction mixture was stirred under ice-water bath for 24 hours. After the reaction, the mixture was poured into an aqueous solution, and an excess of K was added ₂ CO ₃ Washing, finally with deionized water to neutrality, followed by drying of the polymer at 60 ℃. Finally obtaining the target fluorenyl copolymer.

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

(3) Grafting of polymer side chain sulfonic acid groups

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) with an equal molar ratio, and reacting for 24 hours at 60-80 ℃.

(4) Film formation and acid doping post-treatment

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

The prepared amphoteric proton exchange membrane based on polyfluorene 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) The film has good chemical stability without weak bond (such as ether bond) in the polymer.

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

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

(4) The polymer contains hydrophilic sulfonic acid groups and has an alkaline structure, which is beneficial to the absorption of phosphoric acid and the conduction of protons.

Description of the drawings:

FIG. 1 is a block diagram of the fluorenyl copolymer (A) and the fluorenyl copolymer (B) of piperidine ring grafted with 1,3 propane sultone prepared in example 1 ¹ H-NMR spectrum.

Detailed Description

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

Example 1

Fluorene (3.32 g,20 mmol), bromo-n-hexane (8.58 g,52 mmol), tetrabutylammonium bromide (0.33 g,1 mmol) were heated and stirred at 60℃with dimethyl sulfoxide as solvent, and then NaOH solution (50 wt%,12 mL) was added under nitrogen protection. After 24h of reaction, the reaction was stopped and cooled to room temperature. Pouring the reaction solution into dichloromethane for extraction, collecting a lower organic layer, purifying by column chromatography with n-hexane, and finally drying the product after 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 (6.20 g,27 mmol), N-methyl-4-piperidone (3.73 g,33 mmol), 9-dihexylfluorene (1.00 g,3 mmol) and dichloromethane as solvent (20 mL), trifluoroacetic acid (TFA, 3.87 mL) and trifluoromethanesulfonic acid (TFSA, 25.5 mL) were added dropwise at 0deg.C. The reaction mixture was then brought to 0℃Stirring was continued 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 K ₂ CO ₃ The aqueous solution is washed and the polymer is subsequently washed to neutrality with deionized water. Finally, the polymer was dried in vacuo at 60 ℃. Through nuclear magnetic resonance hydrogen spectrum analysis, the ratio of 9,9 dihexylfluorene to terphenyl structure in the polymer is 1:9.

Fluorenyl copolymer (1.00 g,2.8 mmol) was dissolved in dimethyl sulfoxide with stirring, equimolar amount of 1, 3-propane sultone (0.35 g,2.8 mmol) was added, and stirred at 60 ℃ for 24 hours to obtain a homogeneous solution, after the reaction was completed, the solution was uniformly cast on a clean glass plate, and then placed in a vacuum oven for vacuum drying at 60 ℃ to form a film. The membrane was separated from the glass plate and immersed in 85wt% phosphoric acid solution to finally obtain the fluorenyl phosphoric acid proton exchange membrane. The structural formula is as follows:

the films prepared in the above steps were tested and reached 77.16mS cm at 160 ℃ ^-1 The membrane has excellent mechanical properties, the tensile strength at room temperature can reach 14.56mpa, the phosphoric acid absorption rate after the membrane is immersed in 85wt% of phosphoric acid for 24 hours at 80 ℃ is 78.1%, and the size swelling is 18.85%.

Example 2

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

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

Example 3:

this example is similar to example 1, except that the fluorene is grafted in position 9 with bromo-n-butane. The structural formula is as follows:

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

Example 4

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

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

Example 5:

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

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

The above embodiments are intended to illustrate the design concept of the present invention, and it should be understood that those skilled in the art, after reading the present specification, may make implementation and modification of the present invention, but should not be construed as limiting the scope of the present invention.

Claims

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

(1) Preparation of 9, 9-dialkylfluorenes

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

the molar ratio of fluorene to halogenated alkane is 1:2, the dosage of tetrabutylammonium bromide (TBAB) is 0.1-1% of the molar quantity of fluorene, and the dosage of NaOH is 10-15 times of the molar quantity of fluorene;

(2) Preparation of fluorenyl copolymers

Dissolving the monomer 9, 9-dialkyl fluorene and p-terphenyl and N-methyl-4-piperidone in the step (1) in methylene dichloride under stirring, dropwise adding trifluoroacetic acid (TFA) and trifluoromethanesulfonic acid (TFSA) under the protection of ice water bath and nitrogen, stirring the reaction mixture for 24 hours under the ice water bath, pouring the mixture into an aqueous solution after the reaction is finished, and adding excessive K ₂ CO ₃ Washing, washing with deionized water to neutrality, and drying the polymer at 60 deg.c to obtain the target fluorenyl copolymer;

(3) Grafting of polymer side chain sulfonic acid groups

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 ℃;

the alkyl sultone is 1, 3-propane sultone or 1, 4-butane sultone;

(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, vacuum drying at 60 ℃ to form a film, immersing the obtained polymer electrolyte membrane in a phosphoric acid solution, taking out, and wiping off the acid remained on the surface to finally obtain the fluorenyl phosphoric acid proton exchange membrane.

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

3. The method for preparing a polyfluorene-based high temperature phosphoric acid proton exchange membrane according to claim 1, wherein the amount of N-methyl-4-piperidone used in the step (2) is 1.1 times the sum of the molar amounts of 9, 9-dialkylfluorene and p-terphenyl.

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

5. A polyfluorene-based high temperature phosphoric acid proton exchange membrane prepared according to the method of claim 1.

6. Use of the polyfluorene-based high temperature phosphoric acid proton exchange membrane prepared according to the method of claim 1, wherein the polyfluorene-based high temperature phosphoric acid proton exchange membrane is used in a high temperature phosphoric acid fuel cell, a methanol fuel cell, a flow cell, electrolysis, electrodialysis or separation membrane.