Sulfonated polybenzothiazole-polybenzimidazole block crosslinked proton exchange membrane and its preparation method

Abstract

The present invention relates to the field of proton exchange membrane preparation research, and provides a sulfonated polybenzimidazole-polyphenylene benzimidazole block copolymer matrix proton exchange membrane for fuel cells and a preparation method thereof. Through the copolymerization reaction of sulfonated polybenzothiazole and polybenzimidazole, and subsequent high-temperature treatment, a proton exchange membrane with block and post-block hydrophobic cross-linking is prepared. The beneficial effects of the present invention are as follows: the raw material cost is low, non-toxic and harmless; the reaction is stable and easy to control. A hydrophobic cross-linked sulfonated block polymer proton exchange membrane is prepared. The hydrophilic segments of this block cross-linked membrane are interconnected, accelerating the transfer of protons, and thus endowing the proton exchange membrane with higher proton conductivity. In addition, while maintaining the hydrophilic phase relatively stable, the hydrophobic phase of the membrane is selected for cross-linking, so that the oxidation stability of the obtained sulfonated polymer membrane will be improved without causing a significant decrease in proton conductivity.

Description

Technical Field

[0001] The present invention relates to a proton exchange membrane and a preparation method thereof, in particular to a sulfonated polybenzothiazole - polybenzimidazole block crosslinked proton exchange membrane and a preparation method thereof. Background Art

[0002] The proton exchange membrane (PEM) is located in the central part of the proton exchange membrane fuel cell (PEMFC). During the operation of the fuel cell, it mainly has two functions. Firstly, it serves as a proton migration and transport channel; secondly, it separates the fuel and oxidant gases to ensure that the two can react at the anode and cathode respectively. As the core component of the PEMFC, the performance of the PEM directly affects the overall performance of the entire fuel cell.

[0003] Polybenzimidazole (PBI) is a high-performance aromatic polymer with excellent antioxidant properties, thermal stability, and machinability. However, PBI itself does not have proton conductivity, so it needs to be further modified. Sulfonated polybenzothiazole (sPBT), as a functional polymer with high proton conductivity, can be combined with PBI to prepare a sulfonated block copolymer with more excellent properties. In the sulfonated block copolymer, the hydrophilic segments are interconnected, which can accelerate the transfer of protons, thereby endowing the proton exchange membrane with a higher proton conductivity; on the other hand, the hydrophobic segments can also be interconnected, which restricts the swelling of the hydrophilic phase, thereby improving the mechanical properties of the membrane. In addition, the sulfonated polymer membrane also faces a problem that its oxidation stability is much lower than that of the perfluorosulfonic acid polymer membrane. Some studies have shown that crosslinking can effectively improve the oxidation stability of the sulfonated polymer membrane. However, crosslinking often occurs throughout the entire system of the polymer membrane, which will have an adverse impact on the mechanical properties and proton conductivity of the polymer membrane. Therefore, a partial crosslinking method is selected to modify the sulfonated membrane. In the sulfonated membrane, there are a hydrophilic phase caused by sulfonic acid groups and a hydrophobic phase caused by the rigid polymer main chain. The hydrophilic phase is mainly used for proton conduction, and the hydrophobic phase mainly endows the proton exchange membrane with appropriate mechanical strength. Therefore, while keeping the hydrophilic phase relatively stable, the hydrophobic phase of the membrane can be crosslinked, so that the oxidation stability of the obtained sulfonated polymer membrane will be improved, and at the same time, there will be no obvious decrease in proton conductivity.

[0004] In view of the above problems, a preparation method of a sulfonated polybenzothiazole - polybenzimidazole block crosslinked proton exchange membrane is proposed. Summary of the Invention

[0005] The purpose of the present invention is to provide a sulfonated polybenzothiazole - polybenzimidazole block crosslinked proton exchange membrane and a preparation method thereof. Firstly, a polybenzimidazole oligomer with the following repeating unit structure is synthesized by a direct polycondensation method.

[0006]

[0007] Secondly, a sulfonated polybenzothiazole-polyphenylene benzimidazole block copolymer (sPBT-PBI) with the following parallel main chain structure was synthesized by the co-condensation polymerization of sulfonated polybenzothiazole and polyphenylene benzimidazole.

[0008]

[0009] Finally, using p-dichlorobenzyl as a crosslinking agent, a block crosslinked membrane was prepared by the solution casting method through high-temperature treatment.

[0010] A method for preparing a proton exchange membrane provided by the present invention includes the following preparation steps:

[0011] (1) Preparation of polyphenylene benzimidazole oligomer (PBI): By the direct polycondensation method, 3,3'-diaminobenzidine (DAB) and bis(4-carboxyphenyl)phenylphosphine oxide (OP) were polymerized in polyphosphoric acid (PPA), followed by suction filtration, alkali leaching, water washing, and finally vacuum drying to obtain PBI.

[0012] (2) Preparation of sulfonated polybenzothiazole-polyphenylene benzimidazole block copolymer (sPBT-PBI): By the direct polycondensation method, 2,5-diamino-1,4-benzenedithiol dihydrochloride (DABDT) and 3,3'-disodium-2,2-bis(4-carboxyphenyl)hexafluoropropane (SCFA) were polymerized in PPA and then directly copolymerized with PBI. Subsequently, suction filtration, salt leaching, water washing, and finally vacuum drying were carried out to obtain sPBT-PBI.

[0013] (3) Preparation of block crosslinked membrane: The block copolymer obtained in (2) was heated and dissolved in a solvent to obtain a polymer solution. A crosslinking agent was added and ultrasonicated, and a block crosslinked membrane was obtained by the solution casting method and high-temperature treatment.

[0014] Preferably, in step (1), the content of P2O5 in PPA is 80%, the used mass is 15 g, and the supplementary mass of P2O5 is 5 g.

[0015] Preferably, in step (1), the solution used for alkali leaching is 1 M NaOH solution, and the soaking time is 6 h; the vacuum drying temperature is 120 °C, and the drying time is 48 h.

[0016] Preferably, in step (2), the content of P2O5 in PPA is 80%, the used mass is 12.4 g, and the supplementary mass of P2O5 is 3.8 g.

[0017] Preferably, in step (2), the solution used for salt leaching is 5 wt% Na2CO3 solution, and the soaking time is 48 h; the vacuum drying temperature is 100 °C, and the drying time is 24 h.

[0018] Preferably, in step (3), the solvent used is DMSO; the crosslinking agent used is p-dichlorobenzyl, and the crosslinking degrees of the block crosslinked membrane are 0, 20%, 40%, and 60% respectively.

[0019] Preferably, in step (3), the power value of the ultrasonic treatment is 1200 W, and ultrasonic treatment is carried out for 20 min at an energy output value of 90%; the temperature of the solution casting method is 70 °C and the time is 48 h; the high-temperature treatment temperature is 160 °C and the time is 4 h.

[0020] The preparation method of the proton exchange membrane provided by the present invention has the following improvements compared with the prior art:

[0021] (1) In the preparation process of the proton exchange membrane proposed by the present invention, the raw materials are cheap, easy to obtain, non-toxic and harmless, and the reaction process is simple and easy to control.

[0022] (2) The present invention provides a strategy for preparing a novel block crosslinked proton exchange membrane. In the sulfonated block copolymer, the hydrophilic segments are interconnected, which can accelerate the transfer of protons, thereby endowing the proton exchange membrane with a higher proton conductivity; on the other hand, the hydrophobic segments can also be interconnected, which not only restricts the swelling of the hydrophilic phase but also reduces the methanol permeability, thereby improving the mechanical properties of the membrane.

[0023] (3) The present invention provides a strategy for preparing a novel block crosslinked proton exchange membrane. In the sulfonated membrane, there are a hydrophilic phase caused by sulfonic acid groups and a hydrophobic phase caused by the rigid polymer main chain. The present invention maintains the hydrophilic phase relatively stable and selects to crosslink the hydrophobic phase of the membrane, so that the oxidation stability of the obtained sulfonated polymer membrane will be improved, and at the same time, there will be no obvious decrease in proton conductivity.

[0024] (4) The block copolymer prepared by the present invention has good solubility, making the prepared proton exchange membrane form a complete film without any damage. Description of the Drawings

[0025] Figure 1 1H NMR spectrum of PBI prepared in Example 1; 1 1H NMR spectrum;

[0026] Figure 2 1H NMR spectrum of sPBT-PBI prepared in Example 1; 1 1H NMR spectrum;

[0027] Figure 3 Oxidation stability diagram of the block crosslinked membrane prepared in Example 2;

[0028] Figure 4Conductivity graph of the block crosslinked membrane prepared in Example 2. Detailed implementation mode

[0029] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of them. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative work belong to the scope of protection of the present invention. Example 1

[0030] 1) Preparation of polybenzimidazole oligomer (PBI): Add 15 g of PPA and 5 g of P2O5 into a 100 mL round-bottom three-necked flask equipped with mechanical stirring and nitrogen inlet / outlet. Heat up to 170 °C and stir thoroughly until a homogeneous transparent system is obtained. After slightly cooling, add 0.4024 g (1.8800 mmol) of DAB into the flask, heat up to 80 °C and stir for 1 h, then heat up to 120 °C and stir for 1 h. Turn off the heating, and after cooling to room temperature, add 0.5947 g (1.6233 mmol) of OP. Stir the reaction system at 120 °C for 10 h, 150 °C for 10 h, 170 °C for 10 h, and 190 °C for 12 h respectively to obtain a dark brown viscous solution. After slightly cooling, slowly pour the viscous solution into a beaker filled with a large amount of deionized water to obtain a reddish-brown filamentous polymer. Filter by suction, wash the residual acid on the surface of the polymer with deionized water, then soak the polymer in 1 M NaOH solution for 6 h. Filter by suction again, wash the polymer repeatedly with water until the filtrate reaches neutral, and finally dry it in vacuo at 120 °C for 48 h to obtain dry PBI.

[0031] See Figure 1 , through 1 1H NMR analyzed the chemical structure of PBI. The 1 1H NMR spectrum of PBI shows signal peaks corresponding to its structure. Among them, the absorption peak at 6.93 - 6.55 ppm corresponds to the protons of the PBI end group structure, the absorption peak at 8.70 - 7.33 ppm belongs to the protons on the polymer chain, and the protons of the terminal amino group (H13) do not show up because they are active hydrogens. The absorption peak around 13 ppm corresponds to the N-H protons of the imidazole unit. 1 The 1H NMR spectrum well proves the chemical structure of PBI.

[0032] 2) Preparation of sulfonated polybenzothiazole - polybenzimidazole block copolymer (sPBT - PBI): 0.4500 g (1.8352 mmol) of DABDT and 12.4 g of PPA were added to a 100 mL three - necked round - bottom flask equipped with mechanical stirring and nitrogen inlet / outlet. After stirring at room temperature for 12 h, it was further stirred at 70 °C for 36 h to obtain a yellow transparent solution (HCl was completely discharged). After slightly cooling, 1.1597 g (1.9445 mmol) of SCFA was added to the flask, and the temperature was raised to 100 °C and continuously stirred for 8 h. After the reaction system was cooled to room temperature, 3.8 g of P2O5 was added to the flask, and then it was stirred at 120 °C for 8 h, at 150 °C for 8 h, at 170 °C for 8 h, at 190 °C for 10 h, and at 210 °C for 10 h to obtain a brown mixed system. After cooling to room temperature, 0.2186 g of PBI oligomer was directly added to the above reaction system to prepare the block copolymer. The mixture was stirred at 120 °C for 10 h, at 150 °C for 10 h, at 170 °C for 10 h, at 190 °C for 12 h, and at 210 °C for 12 h. After slightly cooling, the reaction product was slowly poured into deionized water to obtain a black strip - shaped polymer. Subsequently, it was filtered by suction, and the acid attached to the surface of the polymer was washed away with deionized water, and then it was soaked in 5wt% Na2CO3 solution for 24 h, filtered by suction, and the polymer was repeatedly washed with deionized water again until the solution pH was 7. Finally, it was vacuum - dried at 100 °C for 48 h to obtain sPBT - PBI.

[0033] See Figure 2 , for sPBT - PBI 1 In the 1H NMR spectrum of sPBT - PBI, no signal peak belonging to the PBI terminal matrix proton was found, indicating that the PBI oligomer was completely introduced into sPBT - PBI. In addition, the signal peaks at other positions were well assigned. The signal peaks at 8.83, 8.38, 7.82 - 7.70, and 7.68 ppm were attributed to the protons of the hydrophilic segment sulfonated polybenzothiazole unit, the signal peaks at 7.62, 7.44, and 2.24 - 6.90 ppm corresponded to the protons of the hydrophobic segment polybenzimidazole unit, and the signal peak around 13.5 ppm was due to the protons of the imidazole unit. Example 2

[0034] 1) Preparation of block crosslinked membrane with a crosslinking degree of 0 (sPBT-PBI-0): Add 0.2 g of dried sPBT-PBI into 10 mL of DMSO and heat it until the polymer is completely dissolved. Subsequently, after the polymer solution cools down, filter it by suction to remove solid impurities. Then spread the polymer solution evenly on a clean glass plate and dry it in a vacuum drying oven at 70 °C for 72 h. After the solvent has completely evaporated, soak the glass plate in deionized water to obtain a salt-type membrane sample.

[0035] 2) Preparation of block crosslinked membrane with a crosslinking degree of 20% (sPBT-PBI-20): Heat and dissolve 0.2 g of sPBT-PBI in 10 mL of DMSO. At the same time, dissolve 0.05 g of p-dichlorobenzyl in 1 mL of DMSO. After it is completely dissolved, mix it into the polymer solution and ultrasonically treat it for 20 min until the two are completely mixed. Spread the mixed polymer solution evenly on a clean glass plate and dry it in a vacuum drying oven at 70 °C until most of the solvent is dried. Subsequently, raise the temperature to 160 °C and bake for 4 h to obtain a block copolymer membrane with a hydrophobic crosslinking degree of 20%. After cooling to room temperature, soak the glass plate in deionized water to obtain a salt-type membrane sample.

[0036] The present invention also prepared corresponding block crosslinked membranes with crosslinking degrees of 40% and 60% respectively, and the specific steps will not be described here. The difference between the block crosslinked membranes with crosslinking degrees of 40% and 60% lies in the different masses of p-dichlorobenzyl used. Among them, the mass of p-dichlorobenzyl used in the block crosslinked membrane with a crosslinking degree of 40% (sPBT-PBI-40) is 0.14 g, and the mass of p-dichlorobenzyl used in the block crosslinked membrane with a crosslinking degree of 60% (sPBT-PBI-60) is 0.30 g.

[0037] See Figure 3 , the dissolution time of the membrane samples after crosslinking all increases with the increase of the crosslinking degree, which also proves that the crosslinking of the hydrophobic phase is beneficial to improving the oxidative stability of the proton exchange membrane. The main reasons are as follows: one is that the crosslinking network is conducive to the polymer main chain resisting the attack of free radicals such as HO∙ and HOO∙; the other is that the generation of free radicals such as HO∙ and HOO∙ requires the presence of water molecules, and the crosslinking of the hydrophobic phase reduces the water content in the membrane, so the oxidative stability of the crosslinked membrane is correspondingly improved. See Figure 4, after crosslinking treatment, the proton conductivity of the crosslinked membrane decreased compared with that of the block copolymer membrane, and with the increase of the crosslinking degree, the proton conductivity of the membrane further decreased. This is because when the length of the hydrophilic chain remains unchanged, the crosslinking of the hydrophobic phase will reduce the water content in the membrane, so the proton conductivity of the crosslinked membrane decreases. However, even so, at 80 °C, all membrane samples still have a sufficiently high proton conductivity. For example, the proton conductivities of sPBT12-PBI-20, 40, and 60 are 0.115, 0.101, and 0.093 S / cm, respectively, all higher than that of the commercial Nafion 117 membrane (0.083 S / cm). The above results show that the crosslinking between the hydrophobic phases does not seriously affect the proton conductivity of the membrane. Therefore, the block copolymer membrane samples with hydrophobic crosslinking show both improved oxidative stability and high proton conductivity.

[0038] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them; 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 described in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. Sulfonated polybenzothiazole-polybenzimidazole block cross-linked proton exchange membrane, characterized in that: Sulfonated polybenzothiazole-polybenzimidazole block copolymer with parallel main chain structure was synthesized by copolymerization reaction, and its structural formula is as follows. Block cross-linked proton exchange membrane was prepared by solution casting method, adding cross-linking agent and high temperature treatment; 。 2. A method for preparing a sulfonated polybenzothiazole-polybenzimidazole block cross-linked proton exchange membrane, characterized in that: A polybenzimidazole oligomer with o-diamino as the terminal unit was synthesized by direct polycondensation, a sulfonated polybenzothiazole-polybenzimidazole block copolymer was synthesized by copolymerization, and a film was prepared by solution casting and high temperature treatment. The specific steps are as follows: (1) Preparation of polybenzimidazole oligomer: 3,3'-diaminobenzidine and bis(4-carboxyphenyl)phenylphosphine oxide are polymerized in polyphosphoric acid by direct polycondensation method, followed by filtration, alkali leaching, water washing, and finally vacuum drying to obtain polybenzimidazole oligomer; (2) Preparation of sulfonated polybenzothiazole-polybenzimidazole block copolymer: 2, 5-diamino-1, 4-benzenedithiol dihydrochloride and 3,3'-sodium disulfonate-2,2-bis(4-carboxyphenyl)hexafluoropropane were polymerized in polyphosphoric acid by direct polycondensation method and then directly copolymerized with polybenzimidazole oligomer, followed by filtration, salt leaching, water washing and finally vacuum drying to obtain sulfonated polybenzothiazole-polybenzimidazole block copolymer; (3) Preparation of block cross-linked membrane: The block copolymer obtained in (2) is heated and dissolved in a solvent to obtain a polymer solution, a cross-linking agent is added and ultrasonicated, and a block cross-linked membrane is obtained by solution casting and high temperature treatment.

3. The method for preparing the sulfonated polybenzothiazole-polybenzimidazole block cross-linked proton exchange membrane according to claim 2, characterized in that: The content of P2O5 in polyphosphoric acid is 80%, the used mass is 15 g, and the mass of added P2O5 is 5 g; the solution used for alkaline leaching is 1 M NaOH solution, and the immersion time is 6 h; the vacuum drying temperature is 120°C, and the drying time is 48 h.

4. The method for preparing the sulfonated polybenzothiazole-polybenzimidazole block cross-linked proton exchange membrane according to claim 2, characterized in that: The content of P2O5 in polyphosphoric acid is 80%, the used mass is 12.4 g, and the mass of added P2O5 is 3.8 g; the solution used for salt immersion is 5 wt% Na2CO3 solution, and the immersion time is 48 h; the vacuum drying temperature is 100°C, and the drying time is 24 h.

5. The method for preparing the sulfonated polybenzothiazole-polybenzimidazole block cross-linked proton exchange membrane according to claim 2, characterized in that: The selected solvent is DMSO; the selected cross-linking agent is benzyl dichloride, and the cross-linking degrees of the block cross-linked membranes are 0, 20%, 40%, and 60%, respectively; the power value of the ultrasonic treatment is 1200 W, and the ultrasonic treatment is carried out for 20 min at 90% energy output value; the solution casting method temperature is 70℃, and the time is 48 h; the high temperature treatment temperature is 160℃, and the time is 4 h.

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