CN107556247B

CN107556247B - Functional cross-linking agent, preparation method and high-phosphoric-acid-doped cross-linked polybenzimidazole film prepared from functional cross-linking agent

Info

Publication number: CN107556247B
Application number: CN201710747444.5A
Authority: CN
Inventors: 刘佰军; 李晓白; 刘振超; 姜振华
Original assignee: Jilin University
Current assignee: Jilin University
Priority date: 2017-08-28
Filing date: 2017-08-28
Publication date: 2020-07-31
Anticipated expiration: 2037-08-28
Also published as: CN107556247A

Abstract

A double-halogen cross-linking agent containing imidazole groups and a high-phosphoric-acid-doped cross-linked Polybenzimidazole (PBI) film prepared from the same belong to the technical field of high-temperature proton exchange membranes for fuel cells. In order to improve the dimensional stability and mechanical strength of the film and simultaneously not reduce the phosphoric acid doping level and proton conductivity of a PBI matrix, the invention designs and synthesizes a novel double-halogen crosslinking agent containing imidazole groups, wherein the structural formula is shown as the following, x is an integer of 1-6, and Y is F, Cl, Br or I. Since the crosslinker itself also has a high density of imidazole groups, it helps to increase the acid doping level of the PBI matrix. The N-substitution reaction with high efficiency is adopted for crosslinking, so that the quantity of residual crosslinking agents in the crosslinking reaction process is reduced, and the novel benzimidazole crosslinked polybenzimidazole film is prepared, so that the film can keep better dimensional stability and mechanical stability while obtaining high phosphoric acid doping level, and the battery performance of the film is improved.

Description

Functional cross-linking agent, preparation method and high-phosphoric-acid-doped cross-linked polybenzimidazole film prepared from functional cross-linking agent

Technical Field

The invention belongs to the technical field of high-temperature proton exchange membranes for fuel cells, and particularly relates to an imidazole group-containing cross-linking agent, a preparation method thereof, and a cross-linked polybenzimidazole film prepared from the cross-linking agent and having high phosphoric acid doping amount, high mechanical stability, high proton conductivity and high cell power density.

Background

Proton Exchange Membrane Fuel Cells (PEMFCs) have received much attention from scientists as an efficient and environmentally friendly electrochemical energy conversion device. In recent years, with the progress of research, the development of PEMFCs capable of operating in high-temperature and low-humidity environments has become a hot spot of research. Compared with the traditional low-temperature proton exchange membrane fuel cell (the working temperature is less than 100 ℃), the high-temperature proton exchange membrane fuel cell (HT-PEMFC) which operates at 100-200 ℃ has many advantages, such as the improvement of the tolerance of the catalyst to CO, the improvement of the catalyst efficiency, the simplification of a water/heat management system and the like. Phosphoric Acid (PA) doped Polybenzimidazole (PBI) membranes have evolved into the most potential high temperature proton exchange membrane (HT-PEM) candidate due to their many superior properties at high temperatures.

The proton conductivity of PA-PBI membranes is closely related to the phosphoric acid doping level (AD L) of the membranes, namely the proton conductivity is increased along with the increase of AD L, but higher AD L causes larger size swelling of PBI membranes, and the mechanical strength of the membranes is remarkably reduced along with the increase of AD L due to the plasticizing effect of PA on the membranes, especially under the high-temperature condition, the cell performance of the membranes is greatly influenced.

It is known from previous research work that the size swelling of the film after being doped with acid can be reduced by preparing the cross-linked PBI film, so that the size stability and the mechanical stability of the film are improved. However, in most cases, the introduction of the crosslinking agent is accompanied by some disadvantages, such as incomplete crosslinking reaction, and the residual impurities of the crosslinking agent in the film system may affect the performance of the film; and the introduction of the cross-linking agent can reduce the relative content of imidazole groups in the film, so that the acid doping level of the film is reduced, and the proton conductivity and the cell performance of the film are further reduced.

Disclosure of Invention

Aiming at the problems of the existing cross-linked Polybenzimidazole (PBI) film, in order to improve the dimensional stability and mechanical strength of the film and simultaneously not reduce the phosphoric acid doping level and proton conductivity of a PBI matrix, the invention designs and synthesizes a novel double-halogen cross-linking agent containing imidazole groups, and the cross-linking agent also has high-density imidazole groups, so that the acid doping level of the PBI matrix is improved. The crosslinking is performed using an N-substitution reaction with high efficiency, thereby reducing the amount of residual crosslinking agent during the crosslinking reaction. We further prepared the novel benzimidazole crosslinked polybenzimidazole thin film, which can maintain better dimensional stability and mechanical stability while obtaining high phosphoric acid doping level, thereby improving the battery performance of the thin film.

1. Synthesis of bihalogen crosslinking agent containing imidazole group

The structural formula of the bihalogen crosslinking agent containing the imidazole group is shown as follows:

x is an integer of 1-6; y is F (fluorine), Cl (chlorine), Br (bromine) or I (iodine)

The synthesis steps of the bihalogen crosslinking agent containing imidazole group are as follows:

mixing a mixture of 1: 2-3 of tetramine (3,3' -diaminobenzidine) and halogen-containing carboxylic acid (namely halogen X acid, wherein halogen atoms are fluorine, chlorine, bromine or iodine; X is methyl, ethyl, propyl, butyl, pentyl or hexyl) are placed in a 4-6M hydrochloric acid solution to react for 6-12 h under a reflux state, then ammonia water or sodium bicarbonate is used for adjusting the pH value of the reaction solution to 7, and the reaction solution is subjected to suction filtration to obtain a solid; then deionized water is used for washing off salt generated in the neutralization process; and finally, drying the solid product to obtain solid powder, namely the bihalogen crosslinking agent containing the imidazole group.

Preparation of crosslinked polybenzimidazole film

The crosslinked polybenzimidazole film is prepared by carrying out N-substitution reaction on a double-halogen crosslinking agent containing an imidazole group and a polybenzimidazole matrix, wherein the polybenzimidazole matrix can be ABPBI, p-PBI, m-PBI or Ph-PBI, and can also be other Ar-PBI, and the structure is shown as follows:

wherein n is an integer of 10 to 500, and Ar can be any one or more of the following structures:

the preparation method of the crosslinked polybenzimidazole film comprises the following steps:

casting a mixed solution of PBI polymer powder and a double-halogen cross-linking agent containing imidazole groups onto a glass plate, wherein the solvent is a polar solvent such as DMF (N, N-dimethylformamide), DMAc (N, N-dimethylacetamide), NMP (N-methylpyrrolidone) or DMSO (dimethyl sulfoxide); wherein the molar charge ratio of the double-halogen cross-linking agent containing imidazole groups to the PBI polymer repeating units is 0.01-0.6: 1; then drying for 2-6 h at 60-100 ℃, and drying for 20-30 h at 110-150 ℃; peeling the film from a glass plate, boiling the obtained film for 2-5 h at 90-100 ℃ by using deionized water, and drying the film for 12-24 h at 100-200 ℃ under vacuum to obtain the transparent cross-linked polybenzimidazole film, wherein the thickness of the film is 60-70 mu m, the cross-linked film cannot be completely dissolved in DMAc, the theoretical cross-linking degree is equal to the molar charge ratio of a cross-linking agent to a PBI (poly (p-phenylene benzobisoxazole)) repeating unit, and the cross-linking degree ranges from 1% to 60%. The structure of the crosslinked polybenzimidazole is shown as the following formula:

wherein x is an integer of 1-6, n is an integer of 10-500, and Ar can be any one or more of the following structures:

2. preparation of phosphoric acid doped cross-linked polybenzimidazole film

Soaking the cross-linked polybenzimidazole film with different cross-linking degrees into a phosphoric acid solution with the mass fraction of 50-90 wt%, soaking for 24-120 h at 30-160 ℃, taking out the film, wiping the film with filter paper, and drying for 3-12 h at 100-120 ℃ to obtain the Phosphoric Acid (PA) -doped cross-linked Polybenzimidazole (PBI) film (PA-PBI film).

3. Properties of phosphoric acid-doped crosslinked polybenzimidazole film

The phosphoric acid doping amount of the Phosphoric Acid (PA) -doped cross-linked Polybenzimidazole (PBI) film prepared by the invention is 280-360 wt%; the tensile strength is 10-20 MPa; the proton conductivity at 200 ℃ under anhydrous condition is 160-260 mS cm^-1(ii) a The maximum power density of the hydrogen/oxygen fuel cell under the conditions of 160 ℃ and no humidification is 400-550 mW cm^-2。

Drawings

Table 1: the phosphoric acid doping level and mechanical properties of the film;

FIG. 1: method for preparing cross-linking agent 2,2 '-bis (chloromethyl) -5,5' -biphenyl imidazole¹H NMR spectrum;

FIG. 2: (iii) the infrared spectra of the crosslinker (crosslinker: 2,2 '-bis (chloromethyl) -5,5' -bibenzoimidazole), Ph-PBI and crosslinked membrane;

FIG. 3: chemical stability curve of the film: (a) soaking a Ph-PBI film in DMAc for 12 h; (b) c-PBI-5, (c) c-PBI-10, (d) c-PBI-20 and (e) c-PBI-30 films are soaked in DMAc for 100 hours;

FIG. 4: proton conductivity of the phosphoric acid doped Ph-PBI and cross-linked PBI thin films;

FIG. 5: polarization curves (open symbols) and power density curves (solid symbols) for performance of the phosphate-doped Ph-PBI, c-PBI-20, and c-PBI-30 thin film cells.

Detailed Description

The present invention is described below with reference to the specific embodiments, but not limited thereto, and all equivalent changes and modifications made in accordance with the content of the claims of the present invention should be made to the technical scope of the present invention.

Example 1: synthesis of cross-linking agent 2,2 '-bis (chloromethyl) -5,5' -biphenyl imidazole

4.28g3,3' -diaminobenzidine and 4.73g chloroacetic acid were weighed into a 250M L three-necked flask, 100M L5M hydrochloric acid was added to the three-necked flask, N was used₂As a protective gas, the reaction solution system was heated to a uniform reflux under mechanical stirring, and the system was maintained in this state for 6 hours. The solution was poured into a beaker containing an appropriate amount of ice water. Sodium bicarbonate was slowly added to the beaker in portions, adjusting its pH until 7, at which time a solid was observed to precipitate, and was filtered off with suction to give a pale yellow solid. The solid was washed several times with deionized water to remove salts generated during neutralization. The product is dried to obtain light yellow solid powder, namely the target product 2,2 '-bis (chloromethyl) -5,5' -biphenyl imidazole, the structural formula of which is shown as the following, and the yield of which is 85 percent.

Process for preparing 2,2 '-bis (chloromethyl) -5,5' -bibenzoimidazole¹The H NMR spectrum is shown in figure 1, and the successful synthesis of 2,2 '-bis (chloromethyl) -5,5' -biphenyl imidazole is proved.

Example 2: preparation of crosslinked polybenzimidazole film c-PBI-5

Weighing 0.97g of Ph-PBI polymer powder and 0.03g of 2,2 '-bis (chloromethyl) -5,5' -biphenylimidazole into two erlenmeyer flasks, adding 11m L and 2m L DMAc into the two erlenmeyer flasks, stirring at room temperature for 24h to completely dissolve Ph-PBI and 2,2 '-bis (chloromethyl) -5,5' -biphenylimidazole, mixing the two solutions, sonicating for 2h at room temperature to thoroughly mix, casting the resulting homogeneous solution onto a clean glass plate, drying at 80 ℃ for 5h, then drying at 120 ℃ for 24h, peeling the film off the glass plate, boiling with deionized water at 100 ℃ for 3h, and drying in a vacuum oven at 120 ℃ for 12h to obtain a brown transparent film c-PBI-5 with a thickness of 66 μm.c-PBI-5 film infrared spectrum (FIG. 2), which proves that the crosslinked film c-PBI-5 was successfully prepared.

Example 3: preparation of crosslinked polybenzimidazole film c-PBI-10

0.95g of Ph-PBI polymer powder and 0.05g of 2,2 '-bis (chloromethyl) -5,5' -biphenylimidazole were weighed into two erlenmeyer flasks, 11m L and 2m L DMAc were added to the two erlenmeyer flasks, respectively, and stirred at room temperature for 24 hours to completely dissolve Ph-PBI and 2,2 '-bis (chloromethyl) -5,5' -biphenylimidazole, the two solutions were mixed and sonicated at room temperature for 2 hours to thoroughly mix, the resulting homogeneous solution was cast onto a clean glass plate, dried at 80 ℃ for 5 hours, then dried at 120 ℃ for 24 hours, the film was peeled off the glass plate, boiled with deionized water at 100 ℃ for 3 hours, and dried in a vacuum oven at 120 ℃ for 12 hours to give a brown transparent film c-PBI-10, which was a film of 65 μm.c-PBI-10 thickness and an infrared spectrum (FIG. 2) demonstrated that we successfully prepared the crosslinked film c-PBI-10.

Example 4: preparation of crosslinked polybenzimidazole film c-PBI-20

0.90g of Ph-PBI polymer powder and 0.10g of 2,2 '-bis (chloromethyl) -5,5' -biphenylimidazole were weighed into two erlenmeyer flasks, 10m L and 3m L DMAc were added to the two erlenmeyer flasks, respectively, and stirred at room temperature for 24 hours to completely dissolve Ph-PBI and 2,2 '-bis (chloromethyl) -5,5' -biphenylimidazole, the two solutions were mixed and sonicated at room temperature for 2 hours to thoroughly mix, the resulting homogeneous solution was cast onto a clean glass plate, dried at 80 ℃ for 5 hours, then dried at 120 ℃ for 24 hours, the film was peeled off the glass plate, boiled with deionized water at 100 ℃ for 3 hours, and dried in a vacuum oven at 120 ℃ for 12 hours to give a brown transparent film c-PBI-20, which was a film of 64 μm.c-PBI-20 thickness and which was infrared spectrum (FIG. 2) to demonstrate that we successfully prepared the crosslinked film c-PBI-20.

Example 5: preparation of crosslinked polybenzimidazole film c-PBI-30

0.85g of Ph-PBI polymer powder and 0.15g of 2,2 '-bis (chloromethyl) -5,5' -biphenylimidazole were weighed into two erlenmeyer flasks, 10m L and 3m L DMAc were added to the two erlenmeyer flasks, respectively, and stirred at room temperature for 24 hours to completely dissolve Ph-PBI and 2,2 '-bis (chloromethyl) -5,5' -biphenylimidazole, the two solutions were mixed and sonicated at room temperature for 2 hours to thoroughly mix, the resulting homogeneous solution was cast onto a clean glass plate, dried at 80 ℃ for 5 hours, then dried at 120 ℃ for 24 hours, the film was peeled off the glass plate, boiled with deionized water at 100 ℃ for 3 hours, and dried in a vacuum oven at 120 ℃ for 12 hours to give a brown transparent film c-PBI-30, which was a film of 62 μm.c-PBI-30 in infrared spectrum (FIG. 2) to demonstrate that the crosslinked film c-PBI-30 was successfully prepared.

Example 6: preparation of Ph-PBI film

1.00g of Ph-PBI polymer powder (polybenzimidazole containing side groups and ether bonds and a preparation method and application thereof, patent No. CN103435804A, the structural formula of which is shown below) is weighed into a conical flask, 13m L DMAc is added into the conical flask, stirring is carried out for 24h at room temperature, Ph-PBI is completely dissolved, the obtained Ph-PBI solution is cast on a clean glass plate, drying is carried out for 5h at 80 ℃, then drying is carried out for 24h at 120 ℃, the film is stripped from the glass plate, boiled by deionized water at 100 ℃ for 3h, and dried in a vacuum oven at 120 ℃ for 12h, so that the Ph-PBI of a brown transparent film is obtained, the thickness of the Ph-PBI film is 66 mu m, and an infrared spectrum diagram of the Ph-PBI film (figure 2) proves that the Ph-PBI film is successfully prepared.

Example 7: chemical stability testing of crosslinked polybenzimidazole films

The Ph-PBI thin films (c-PBI-5, c-PBI-10, c-PBI-20, and c-PBI-30) obtained in examples 2, 3, 4, and 5 with different degrees of crosslinking were measured as 5%, 10%, 20%, and 30%, respectively, based on the initial molar charge ratio of 2,2 '-bis (chloromethyl) -5,5' -benzimida to Ph-PBI repeating units, and the pure Ph-PBI thin films obtained in example 6 were soaked in a petri dish containing a DMAc solution, respectively, and the solubility of the thin films in DMAc at room temperature was observed. After 12h, all of the pure Ph-PBI film was dissolved in DMAc, and as shown in the photograph of FIG. 3(a), no dissolution occurred in the remaining crosslinked PBI film. The time for solubility test of the crosslinked film was prolonged, and after 100 hours, the film c-PBI-5 having a degree of crosslinking of 5% exhibited partial dissolution and significant swelling, as shown in the photograph in FIG. 3 (b). After 100h, the film with 10% -30% crosslinking degree still has no dissolution phenomenon, as shown in the photographs of the attached fig. 3(c), (d) and (e). The above test results show that the PBI membrane has more excellent chemical stability after being crosslinked, and also prove the successful occurrence of the crosslinking reaction. The solubility test time for each panel of FIG. 3 was (a) 12 h; (b) the sum of (c), (d) and (e) is 100 h.

Example 8: preparation of phosphoric acid doped cross-linked polybenzimidazole film

We soaked 4 kinds of PBI thin films (c-PBI-5, c-PBI-10, c-PBI-20 and c-PBI-30) with different degrees of crosslinking obtained in examples 2, 3, 4 and 5 and the pure Ph-PBI thin film obtained in example 6 in 85 wt% phosphoric acid solution at 120 ℃ for 72h, respectively. Taking out after soaking, wiping the membrane dry by using filter paper, and drying the membrane for 5 hours at the temperature of 100 ℃ to obtain the PA-PBI membrane.

Example 9: phosphoric acid doping levels for crosslinked polybenzimidazole films

On the basis of example 8, the phosphoric acid doping level AD L of each film was calculated according to the following equation by recording the mass change of each PBI film before and after phosphoric acid soaking:

ADL(wt％)＝[(W_d-W_u)/W_u]×100％

wherein W_uAnd W_dThe quality of the undoped and doped post-crosslinked PBI films, respectively, is indicated.

From the test results (see Table 1), it can be seen that the incorporation of the cross-linking agent increased the phosphoric acid doping level of the PBI film, with the AD L of the c-PBI-30 film being as high as 354 wt%.

Example 10: mechanical property of phosphoric acid doped cross-linked polybenzimidazole film

From the 5 phosphoric acid doped cross-linked PBI films obtained in example 8, 5 rectangular film sample strips with the thickness of 50mm × 5mm are respectively cut, the sample strips are fixed on a clamp of a universal stretching machine, the effective test area is 15mm × 5mm, the stretching speed is 2mm/min, the mechanical strength, Young modulus and breaking elongation of different film sample strips are respectively tested, and the average value is calculated by taking parallel data as a test result.

Example 11: proton conductivity of phosphoric acid-doped crosslinked polybenzimidazole film

The proton conductivity test was conducted without humidification, and a rectangular membrane sample of 50mm × 10mm was cut from each of the 5 phosphoric acid-doped PBI thin films obtained in example 8, and the sample was fixed to a jig to test the proton conductivity at a temperature ranging from 80 to 200 ℃ as shown in FIG. 4, wherein the conductivity of the phosphoric acid-doped c-PBI-30 thin film at 200 ℃ was as high as 253 m.Scm^-1。

Example 12: battery performance of phosphoric acid doped cross-linked polybenzimidazole film

We performed membrane electrode assembly on the phosphoric acid doped Ph-PBI, c-PBI-20 and c-PBI-30 membranes obtained in example 8 to test the fuel cell performance. First, polybenzimidazole is dissolved in a mixed solution of formic acid and phosphoric acid to prepare a polymer solution with a mass concentration of 0.5% as a catalyst ink, and a catalyst platinum (Pt) powder is mixed with the prepared catalyst ink so that the mass ratio of the loading amount of platinum to the loading amount of polybenzimidazole in the mixture is 13: 1. the obtained catalyst ink was sprayed onto a gas diffusion layer of a nonwoven fabric to obtain a gas diffusion electrode in which the catalyst Pt was supported in the catalyst layer at a concentration of about 0.6mg/cm². Hot pressing the phosphoric acid doped polybenzimidazole film between the two gas electrodes (platinum/carbon) to form an electrode/film/electrode sandwich structure, and preparing the test area of 9cm²The membrane electrode cell performance test was conducted at 160 ℃ under atmospheric pressure in a non-humidified environment, with the test gases being dry hydrogen (flow rate 0.3L/min) and oxygen (flow rate 0.15L/min)As shown in fig. 5. Wherein the maximum power density of the cell with the c-PBI-30 film is up to 533 mW cm-²。

Table 1: phosphoric acid doping level and mechanical properties of thin film

Claims

1. The application of the bihalogen crosslinking agent containing the imidazole group in the preparation of the phosphoric acid doped crosslinking type polybenzimidazole film is disclosed, and the structural formula of the bihalogen crosslinking agent containing the imidazole group is shown as follows:

x is an integer of 1-6; y is F, Cl, Br or I.

2. The use of the dihalogen crosslinking agent containing imidazole groups according to claim 1 in the preparation of phosphoric acid-doped crosslinked polybenzimidazole films, wherein: the molar ratio of 1: 2-3 of tetramine and halogen-containing carboxylic acid are placed in a 4-6M hydrochloric acid solution, the reaction is carried out for 6-12 hours under a reflux state, then ammonia water or sodium bicarbonate is used for adjusting the pH value of the reaction solution to 7, and the reaction solution is subjected to suction filtration to obtain a solid; then deionized water is used for washing off salt generated in the neutralization process; finally, drying the solid product to obtain solid powder, namely the bihalogen crosslinking agent containing the imidazole group; wherein, the tetramine is 3,3' -diaminobenzidine; the halogen-containing carboxylic acid is halogen X acid, and the halogen atom is chlorine, fluorine, bromine or iodine; x is A, B, C, D, E or F.

3. The use of the dihalogen crosslinking agent containing imidazole groups according to claim 1 in the preparation of phosphoric acid-doped crosslinked polybenzimidazole films, wherein: the phosphoric acid doped cross-linked polybenzimidazole film is prepared by the following steps,

(1) preparation of a cross-linked polybenzimidazole film: casting a mixed solution of PBI polymer powder and a double-halogen cross-linking agent containing imidazole groups onto a glass plate, wherein the molar charge ratio of the double-halogen cross-linking agent containing imidazole groups to PBI polymer repeating units is 0.01-0.6: 1; then drying for 2-6 h at 60-100 ℃, and drying for 20-30 h at 110-150 ℃; peeling a film from a glass plate, boiling the obtained film for 2-5 h at 90-100 ℃ by using deionized water, and drying the film for 12-24 h at 100-200 ℃ in vacuum to obtain a transparent cross-linked polybenzimidazole film, wherein the thickness of the film is 60-70 mu m, the cross-linking degree is 1-60%, the structure of the cross-linked polybenzimidazole is shown as the following formula,

wherein x is an integer of 1 to 6, n is an integer of 10 to 500, Ar is any one of the following structures,

(2) preparing a phosphoric acid-doped cross-linked polybenzimidazole film: and (2) immersing the cross-linked polybenzimidazole film prepared in the step (1) into a phosphoric acid solution with the mass fraction of 50-90 wt%, soaking for 24-120 h at 30-160 ℃, taking out the film, wiping the film with filter paper, and drying for 3-12 h at 100-120 ℃ to obtain the phosphoric acid doped cross-linked polybenzimidazole film.

4. The use of the dihalogen crosslinking agent containing an imidazole group according to claim 3 for the preparation of phosphoric acid-doped crosslinked polybenzimidazole thin films, wherein: the structure of polybenzimidazole is shown below,

wherein n is an integer of 10 to 500, Ar is any one of the following structures,