Hydroxyl-containing modified polybenzimidazole proton exchange membrane and preparation method thereof

Abstract

The invention belongs to the technical field of fuel cell proton exchange membranes, and discloses a hydroxyl-containing modified polybenzimidazole proton exchange membrane and a preparation method thereof. According to the modified polybenzimidazole, a monomer (5-hydroxyisophthalic acid) containing a hydroxyl structure is added during polymerization of a raw material monomer, and the novel hydroxyl-containing modified polybenzimidazole is obtained through an imidazole forming reaction. The hydroxyl-containing modified polybenzimidazole can be prepared into the modified polybenzimidazole proton exchange membrane by using a solution casting method. According to the modified polybenzimidazole disclosed by the invention, stable doping of phosphoric acid in the membrane is facilitated by introducing hydroxyl and ether bonds, so that more proton carriers are kept under an anhydrous condition, the phosphoric acid doping rate of the prepared proton exchange membrane is further improved, and construction of a proton transmission channel is facilitated. The novel hydroxyl-containing modified polybenzimidazole proton exchange membrane has a good application prospect in the aspect of proton exchange membrane fuel cells.

Description

Technical Field

[0001] The present invention belongs to the technical field of fuel cell proton exchange membrane materials, and particularly relates to a hydroxyl-containing modified polybenzimidazole and a preparation method thereof. The prepared hydroxyl-containing modified polybenzimidazole can be used to prepare proton exchange membrane materials. Background Art

[0002] The proton exchange membrane is one of the most important components in proton exchange membrane fuel cells. It plays an important role in the transport of protons from the anode to the cathode and can prevent the mixing of reactants. High-temperature proton exchange membrane fuel cells (HT-PEMFCs), with their operating characteristics at high temperatures of 100–200 °C, have broken through the technical bottlenecks of traditional low-temperature fuel cells and demonstrated significant advantages in terms of improved catalytic activity, expanded fuel adaptability, and simplified system integration. Their high-temperature environment not only enhances the tolerance to catalyst poisoning but also eliminates the complex humidification system through the gaseous water emission mechanism, improving the thermal management efficiency. Polybenzimidazole (PBI) is a high-performance heterocyclic aromatic polymer with a benzene ring and benzimidazole in its polymer backbone, and its molecular structure has a high degree of rigidity. The glass transition temperature of PBI can reach 427 °C, and PBI has excellent thermal stability, mechanical properties, and chemical stability, fully meeting the operating environment of high-temperature fuel cells.

[0003] The intrinsic proton conductivity of PBI is very low, and high-concentration phosphoric acid doping is required to obtain excellent proton conductivity under high-temperature and low-humidity conditions. Currently, the proton exchange membrane doped with phosphoric acid in PBI is usually the first choice for HT-PEMFC applications. However, high-concentration phosphoric acid doping will cause the problem of phosphoric acid leaching from the membrane, and the rigid structure of PBI also makes it difficult to dissolve in organic solvents and form a film. Rigid polymers containing nitrogen heterocycles and hydroxyl groups can form a strong hydrogen bond network structure between polymer chains, anchor phosphoric acid molecules in the gaps between polymer chains through hydrogen bonds, and reduce their free migration. At the same time, the strong polar characteristics of hydroxyl groups also enhance the hydrogen bond force with phosphoric acid, improve the adsorption of phosphoric acid in PBI, and enhance the proton conductivity. The literature (Macromolecules, 2009, 42, 8640-8648) synthesized poly(2,2’-(dihydroxy-1,4-phenylene)-5,5’-bibenzimidazole (2OH-PBI) containing dihydroxy groups, and prepared a PBI gel film using the polyphosphoric acid process. The 2OH-PBI film showed high proton conductivity, but the tensile properties of the film were much lower than those of other modified systems. By introducing flexible groups such as ether bonds into the polymer main chain, the flexibility of PBI materials can be improved to disperse stress, promote the movement of polymer segments. Ether bonds, as proton transport sites, form a hydrogen bond network with phosphoric acid molecules, increase the amount of phosphoric acid doping, reduce the proton transport energy barrier, and enhance the proton conduction efficiency under high-temperature and low-humidity conditions. The polar characteristics of ether bonds can enhance the compatibility of PBI with polar solvents, improve the solubility of PBI in organic solvents, and solve the problem of difficult processing of PBI. In the present invention, hydroxyl groups and ether bonds are directionally embedded in the PBI main chain. By forming a strong hydrogen bond network with phosphoric acid molecules, both the phosphoric acid loading capacity and proton conduction capacity of the proton exchange membrane are improved. This main-chain chemical modification strategy can maintain the heat resistance of the polymer while shortening the proton transport path and synergistically enhancing the proton conduction efficiency. Summary of the Invention

[0004] In order to overcome the above problems in the prior art, the present invention synthesizes a modified polybenzimidazole containing hydroxyl groups from the perspective of optimizing the polymer structure. Introducing hydroxyl groups and ether bonds helps to improve the adsorption performance of the proton exchange membrane for phosphoric acid, so as to maintain more proton carriers under anhydrous conditions, and thus improve the proton conduction performance.

[0005] To achieve the object of the present invention, the technical solution adopted by the present invention is as follows:

[0006] The structural formula of the modified polybenzimidazole containing hydroxyl groups is:

[0007]

[0008] Wherein: m:n = 6-9:1-4.

[0009] Preparation method of a hydroxyl-containing modified polybenzimidazole proton exchange membrane, comprising the following steps:

[0010] (1) Synthesis of hydroxyl-containing modified polybenzimidazole: Using 3,3'-diaminobenzidine, 5-hydroxyisophthalic acid, and 4,4'-dicarboxydiphenyl ether as monomers, it is prepared by solution polycondensation in polyphosphoric acid: The polycondensation method is to stir and heat at 140 °C for 2 h to fully dissolve it, stir and heat at 180 °C for 2 h for prepolymerization, stir and heat at 200 °C for 12 h for reaction, and the polymerization product is obtained after the reaction.

[0011] (2) Preparation of modified polybenzimidazole proton exchange membrane: Dissolve the hydroxyl-containing modified polybenzimidazole in an organic solvent to obtain a solution of the hydroxyl-containing modified polybenzimidazole. Pour this solution onto a smooth and flat glass plate, place the glass plate in an 80 °C environment for 24 h to evaporate the solvent, wash the polymer membrane off the glass plate with deionized water and wash away the residual solvent, dry it at 120 °C and then soak it in phosphoric acid for protonation, thus obtaining the modified polybenzimidazole proton exchange membrane.

[0012] Further, the molar number of the 5-hydroxyisophthalic acid monomer is 10 mol% to 40 mol% relative to the molar number of the 3,3'-diaminobenzidine monomer.

[0013] Further, the total mass of the compound monomers is 3 wt% to 9 wt% relative to the mass of the polyphosphoric acid.

[0014] Further, the organic solvent is dimethyl sulfoxide or N,N-dimethylacetamide.

[0015] Further, the mass of the hydroxyl-containing modified polybenzimidazole is 3 wt% to 20 wt% relative to the mass of the organic solvent.

[0016] The beneficial effects of the present invention are as follows: The hydroxyl-containing modified polybenzimidazole of the present invention significantly enhances the phosphoric acid adsorption performance and proton transport efficiency of the proton exchange membrane by introducing hydroxyl groups into the PBI main chain in a directed manner and utilizing the stable hydrogen bond network formed between the hydroxyl groups, phosphoric acid molecules, and imidazoles. This main chain directed modification strategy not only maintains the excellent heat resistance of the polymer but also realizes the synergistic improvement of proton conduction performance by optimizing the proton transport path, effectively solving the key technical problem of easy loss of phosphoric acid in traditional PBI membranes under high doping conditions. The present invention embeds ether bonds in the polymer main chain, significantly improving the chain segment flexibility and solubility in organic solvents, which is beneficial for film formation and processing; at the same time, hydroxyl groups are introduced to form a stable hydrogen bond network with phosphoric acid, increasing the phosphoric acid uptake rate, and constructing an efficient proton transport channel through the Grotthuss mechanism to improve the proton conductivity of the membrane, which is applicable to high-demand scenarios such as high-temperature fuel cells. Description of the Drawings

[0017] Figure 1For the modified polybenzimidazole OPBI-OH-10 prepared in Example 2 1 HNMR (DMSO-d6).

[0018] Figure 2 Infrared spectra of the modified polybenzimidazole proton exchange membranes OPBI-OH-10, OPBI-OH-20, and OPBI-OH-30 prepared in Example 2, Example 3, and Example 4

[0019] Figure 3 Graph showing the variation of proton conductivity with temperature at 0% relative humidity for the modified polybenzimidazole proton exchange membranes OPBI, OPBI-OH-10, OPBI-OH-20, and OPBI-OH-30 prepared in Example 1, Example 2, Example 3, and Example 4 Detailed implementation method

[0020] Example 1

[0021] (1) Synthesis of pure polybenzimidazole OPBI without hydroxyl groups

[0022] Pour 189.4776 g of polyphosphoric acid into a 150 mL three-necked flask, stir and heat at 140 °C for 1 h in an N2 atmosphere to remove air bubbles, then add 4.4795 g (17.0 mmol) of 4,4'-dicarboxydiphenyl ether and 3.6794 g (17.0 mmol) of 3,3'-diaminobenzidine in sequence, with a 20 min interval between each addition. Stir and heat for 4 h to fully dissolve, pre-polymerize at 180 °C for 2 h with stirring, and react at 200 °C for 12 h with stirring. After the reaction, pour the polymerization product into deionized water while it is still hot to wash away most of the acid, filter and collect the solid, soak it in saturated sodium bicarbonate solution until no more bubbles are produced, filter again, wash the solid with alcohol and deionized water until neutral, and then dry at 120 °C for 12 h to obtain polybenzimidazole OPBI.

[0023] (2) Preparation of polybenzimidazole OPBI proton exchange membrane

[0024] Take 1.2 g of OPBI and add it to 40.0 g of dimethyl sulfoxide, stir and heat at 120 °C for 2 h to dissolve and prepare a solution with a concentration of 3 wt%. Centrifuge the solution at 8000 r / min for 5 min, pour the upper layer solution onto a flat and smooth glass plate, place it in an 80 °C environment for 18 h to allow the solvent to evaporate, then wash the OPBI proton exchange membrane off the glass plate with deionized water and wash away the residual organic solvents, and finally dry at 120 °C for 12 h to obtain the OPBI proton exchange membrane. Conduct proton conductivity tests on the membrane at 160 °C and 0% relative humidity, as shown in Figure 3, the proton conductivity of the OPBI proton exchange membrane after doping with phosphoric acid is 68.53 mS / cm. The doping rate of phosphoric acid in the OPBI proton exchange membrane prepared in Example 1 is 162%.

[0025] Among them, the determination of the doping rate of phosphoric acid is calculated by soaking the mass of the dry membrane and the mass after soaking in 85 wt% H3PO4 for 72 h. The calculation formula is as follows:

[0026]

[0027] In the formula, W dry is the mass of the dry membrane, and W acid is the mass after doping with phosphoric acid, and the units are both g.

[0028] Example 2

[0029] (1) Synthesis of modified polybenzimidazole OPBI-OH-10 containing hydroxyl groups

[0030] Pour 92.3094 g of polyphosphoric acid into a 150 mL three-necked flask, stir and heat at 140 °C for 1 h in an N2 atmosphere to remove bubbles, and then add 4.0316 g (15.3 mmol) of 4,4'-dicarboxydiphenyl ether, 0.3159 g (1.7 mmol) of 5-hydroxyisophthalic acid and 3.6794 g (17.0 mmol) of 3,3'-diaminobenzidine in sequence. The feeding interval is 20 min each time. Stir and heat for 4 h to fully dissolve, pre-polymerize at 180 °C with stirring for 2 h, and react at 200 °C with stirring for 12 h. After the reaction is completed, pour the polymerization product into deionized water while it is hot to wash away most of the acid. Filter and collect the solid, soak it in saturated sodium bicarbonate solution until there are no more bubbles, filter, wash the solid with alcohol and deionized water until neutral, and then dry it at 120 °C for 12 h to obtain polybenzimidazole OPBI-OH-10. The product was 1 characterized by HNMR, as shown in Figure 1 , a proton peak Hb of the hydroxyl group appears at 10.17 ppm in the NMR spectrum, and proton peaks Hc and Hg corresponding to the protons on the benzene ring connected to the hydroxyl group appear at 8.67 ppm and 7.58 ppm. The attribution of the remaining peaks is marked in the spectrum respectively, proving that polybenzimidazole OPBI-OH-10 was synthesized, with a number average molecular weight Mn of 19527 and a weight average molecular weight Mw of 34298.

[0031] (2) Preparation of polybenzimidazole OPBI-OH-10 proton exchange membrane

[0032] Take 1.2 g of OPBI-OH-10 and add it to 40.0 g of dimethyl sulfoxide. Stir and heat at 120 °C for 2 h to dissolve and prepare a solution with a concentration of 3 wt%. Centrifuge the solution at 8000 r / min for 5 min. Pour the upper layer solution onto a flat and smooth glass plate, and place it in an 80 °C environment for 18 h to allow the solvent to evaporate. Then, wash the OPBI-OH-10 proton exchange membrane off the glass plate with deionized water and wash away the residual organic solvents. Finally, dry it at 120 °C for 12 h to obtain the OPBI-OH-10 proton exchange membrane.

[0033] The proton exchange membrane was characterized by infrared spectroscopy, as shown in Figure 2 , and the broad absorption band near 3410 cm -1 is related to N-H and O-H groups. The absorption peak at 1599 cm -1 is the C═C absorption peak on the benzene ring. The characteristic band at 1448 cm -1 is due to the in-plane deformation of the imidazole ring. The absorption peak at 1237 cm -1 is the C-N stretching vibration peak of the imidazole ring. The characteristic peak at 1168 cm -1 is caused by the Ar-O-Ar stretching vibration. The absorption peaks at 858 cm -1 , 798 cm -1 and 690 cm -1 are related to the vibration of the 1,3-disubstituted benzene ring. The proton conductivity of the membrane was measured at 160 °C and 0% relative humidity, as shown in Figure 3 . The proton conductivity of the phosphoric acid-doped OPBI-OH-10 proton exchange membrane is 91.04 mS / cm.

[0034] And the phosphoric acid doping rate in the OPBI-OH-10 proton exchange membrane prepared in Example 2 is 250.39%, which is higher than that of the OPBI proton exchange membrane without hydroxyl groups in Example 1 (162%).

[0035] Example 3

[0036] (1) Synthesize the modified polybenzimidazole OPBI-OH-20 containing hydroxyl groups

[0037] 189.4776 g of polyphosphoric acid was poured into a 150 mL three-necked flask, stirred and heated at 140 °C for 1 h in an N2 atmosphere to remove bubbles, and then 3.5836 g (13.6 mmol) of 4,4'-dicarboxydiphenyl ether, 0.6319 g (3.4 mmol) of 5-hydroxyisophthalic acid and 3.6794 g (17.0 mmol) of 3,3'-diaminobenzidine were added in sequence. The feeding interval was 20 min each time. It was stirred and heated for 4 h to dissolve completely, pre-polymerized by stirring and heating at 180 °C for 2 h, and reacted by stirring and heating at 200 °C for 12 h. After the reaction was completed, the polymerization product was poured into deionized water while it was hot to wash away most of the acid. The solid was filtered and collected and soaked in saturated sodium bicarbonate solution until no bubbles were produced. After filtration, the solid was washed with alcohol and deionized water until neutral and then dried at 120 °C for 12 h to obtain polybenzimidazole OPBI-OH-20. The number-average molecular weight Mn was 13,630, and the weight-average molecular weight Mw was 21,717.

[0038] (2) Preparation of polybenzimidazole OPBI-OH-20 proton exchange membrane

[0039] 1.2 g of OPBI-OH-20 was taken and added to 15.0 g of N,N-dimethylacetamide, stirred and heated at 120 °C for 2 h to dissolve and prepare a solution with a concentration of 8 wt%. The solution was centrifuged at 8000 r / min for 5 min. The upper-layer solution was poured on a flat and smooth glass plate and placed in an 80 °C environment for 18 h to volatilize the solvent. Then the OPBI-OH-20 proton exchange membrane was washed off the glass plate with deionized water and the residual organic solvents were washed away. Then it was dried at 120 °C for 12 h to obtain the OPBI-OH-20 proton exchange membrane. The proton conductivity of the membrane was tested at 160 °C and 0% relative humidity, as shown in Figure 3 , and the proton conductivity of the phosphoric acid-doped OPBI-OH-20 proton exchange membrane was 95.27 mS / cm. The phosphoric acid doping rate in the OPBI-OH-20 proton exchange membrane prepared in Example 3 was 258.38%.

[0040] Example 4

[0041] (1) Synthesis of modified polybenzimidazole OPBI-OH-30 containing hydroxyl groups

[0042] 121.6188 g of polyphosphoric acid was poured into a 150 mL three-necked flask, and stirred and heated at 140 °C for 1 h in an N2 atmosphere to remove bubbles. Then, 3.1357 g (11.9 mmol) of 4,4'-dicarboxydiphenyl ether, 0.9478 g (5.1 mmol) of 5-hydroxyisophthalic acid, and 3.6794 g (17.0 mmol) of 3,3'-diaminobenzidine were added in sequence, with an interval of 20 min between each addition. After stirring and heating for 4 h to fully dissolve, pre-polymerization was carried out by stirring and heating at 180 °C for 2 h, and the reaction was carried out by stirring and heating at 200 °C for 12 h. After the reaction was completed, the polymerization product was poured into deionized water while it was still hot to wash away most of the acid. The solid was filtered and collected, soaked in saturated sodium bicarbonate solution until no bubbles were produced, filtered, washed with alcohol and deionized water until neutral, and then dried at 120 °C for 12 h to obtain polybenzimidazole OPBI-OH-30. The number-average molecular weight Mn was 10322, and the weight-average molecular weight Mw was 19893.

[0043] (2) Preparation of polybenzimidazole OPBI-OH-30 proton exchange membrane

[0044] 1.2 g of OPBI-OH-30 was taken and added to 9.0 g of dimethyl sulfoxide, and stirred and heated at 120 °C for 2 h to dissolve and prepare a solution with a concentration of 13 wt%. The solution was centrifuged at 8000 r / min for 5 min. The upper solution was poured on a flat and smooth glass plate, placed in an 80 °C environment for 18 h to volatilize the solvent, and then the OPBI-OH-30 proton exchange membrane was washed off the glass plate with deionized water and the residual organic solvents were washed away. Then, it was dried at 120 °C for 12 h to obtain the OPBI-OH-30 proton exchange membrane. The proton conductivity of the membrane was tested at 160 °C and 0% relative humidity, as shown in Figure 3 , and the proton conductivity of the OPBI-OH-20 proton exchange membrane after phosphoric acid doping was 101.48 mS / cm. The phosphoric acid doping rate in the OPBI-OH-30 proton exchange membrane prepared in Example 4 was 269.09%.

[0045] Example 5

[0046] (1) Synthesis of modified polybenzimidazole OPBI-OH-40 containing hydroxyl groups

[0047] 144.7439 g of polyphosphoric acid was poured into a 150 mL three-necked flask, and stirred and heated at 140 °C for 1 h in an N2 atmosphere to remove bubbles. Then, 2.6877 g (10.2 mmol) of 4,4'-dicarboxydiphenyl ether, 1.2510 g (6.8 mmol) of 5-hydroxyisophthalic acid, and 3.6794 g (17.0 mmol) of 3,3'-diaminobenzidine were added in sequence. The feeding interval was 20 min each time. After stirring and heating for 4 h to fully dissolve, pre-polymerization was carried out by stirring and heating at 180 °C for 2 h, and the reaction was carried out by stirring and heating at 200 °C for 12 h. After the reaction, the polymerization product was poured into deionized water while it was still hot to wash away most of the acid. The solid was filtered and collected and soaked in saturated sodium bicarbonate solution until no bubbles were generated. Then it was filtered, washed with alcohol and deionized water until neutral, and dried at 120 °C for 16 h to obtain polybenzimidazole OPBI-OH-40.

[0048] (2) Preparation of polybenzimidazole OPBI-OH-40 proton exchange membrane

[0049] 1.2 g of OPBI-OH-40 was taken and added to 6.0 g of N,N-dimethylacetamide. It was stirred and heated at 120 °C for 2 h to dissolve and prepare a solution with a concentration of 20 wt%. After 30 min, it became gel-like, the magnetic rotor could not stir, and the polymer could not be formed into a membrane.

[0050] OPBI-OH-40 was difficult to dissolve into a solution, so its molecular weight could not be measured. The reason why OPBI-OH-40 could not be formed into a membrane might be that the PBI molecular chain grew excessively. The PBI molecular chain with a high degree of polymerization was prone to chain entanglement due to its rigid structure, which caused the solution viscosity to rise sharply and gel. Even shortening the reaction time could not solve the problem of gelation.

[0051] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A hydroxyl-containing modified polybenzimidazole proton exchange membrane, characterized in that: The proton exchange membrane is prepared from a modified polybenzimidazole containing hydroxyl groups; The structural formula of the modified polybenzimidazole containing hydroxyl groups is: where: m:n = 6 - 9:1 - 4; Synthesis of the modified polybenzimidazole containing hydroxyl groups: Using 3,3'-diaminobenzidine, 5-hydroxyisophthalic acid, and 4,4'-dicarboxydiphenyl ether as monomers, it is prepared by solution polycondensation in polyphosphoric acid.

2. The preparation method of the hydroxyl-containing modified polybenzimidazole proton exchange membrane according to claim 1, characterized in that: It includes the following steps: Preparation of the modified polybenzimidazole proton exchange membrane: Dissolve the modified polybenzimidazole containing hydroxyl groups in an organic solvent to obtain a solution of the modified polybenzimidazole containing hydroxyl groups. Pour this solution onto a smooth and flat glass plate, evaporate the solvent to obtain a polymer membrane. After washing away the residual solvent, soak it in phosphoric acid for protonation, and obtain the modified polybenzimidazole proton exchange membrane after drying.

3. The preparation method of the hydroxyl-containing modified polybenzimidazole proton exchange membrane according to claim 1, wherein: The total mass of the monomers of 3,3'-diaminobenzidine, 5-hydroxyisophthalic acid, and 4,4'-dicarboxydiphenyl ether is 3wt% - 9wt% relative to the mass of polyphosphoric acid.

4. The preparation method of the hydroxyl-containing modified polybenzimidazole proton exchange membrane according to claim 1, wherein: The polycondensation method is to stir and heat at 140°C for 2h to fully dissolve it, stir and heat at 180°C for 2h for prepolymerization, stir and heat at 200°C for 12h for reaction, and obtain the polymerization product after the reaction.

5. The preparation method of the hydroxyl-containing modified polybenzimidazole proton exchange membrane according to claim 2, wherein: The organic solvent is dimethyl sulfoxide or N,N-dimethylacetamide.

6. The preparation method of the hydroxyl-containing modified polybenzimidazole proton exchange membrane according to claim 2, characterized in that: The mass of the modified polybenzimidazole containing hydroxyl groups is 3wt% - 20wt% relative to the mass of the organic solvent.

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