Polybenzimidazole / H3PO4 film as well as preparation and application thereof

Abstract

The high-temperature polymer fuel cell has good CO poisoning resistance and the like as a hot spot of current international research. At present, common preparation methods of the PBI / H3PO4 film are a post-impregnation method and a sol-gel method. Compared with a post-impregnation method for preparing the PBI / H3PO4 film, a sol-gel method has a series of advantages, and the preparation process of the PBI / H3PO4 film is developed internationally. However, high-molecular-weight linear polybenzimidazole is a difficulty in preparation of a PBI / H3PO4 film by a sol-gel method, and in the reaction process, the mass, heat and the like of a reaction solution are easily distributed unevenly, so that the reaction is terminated or the polymerization degree of reactants is locally and obviously different, and finally, the high-molecular-weight linear polybenzimidazole cannot be prepared. According to the method, the problems of mass transfer and heat transfer caused by sudden increase of the viscosity of the reaction liquid can be effectively avoided by changing the monomer proportion or / and reducing the concentration of the reaction liquid after the reaction reaches a certain degree, and the high-molecular-weight linear PBI is prepared, so that the high-performance PBI / H3PO4 film is obtained.

Description

Technical Field

[0001] This invention belongs to the field of high-temperature polymer electrolyte membranes, and specifically relates to the preparation and method of a high molecular weight linear polybenzimidazole / H3PO4 membrane. Background Technology

[0002] High-temperature polymer fuel cells have become a hot topic in international research due to their excellent resistance to CO and other poisoning. Phosphoric acid-doped PBI (polybenzimidazole) membranes are considered one of the best candidate materials for high-temperature fuel cells due to their high proton conductivity, good thermal stability and good mechanical properties. Their application in fuel cells has promoted the development of high-temperature fuel cells. Currently, the commonly used preparation methods for PBI / H3PO4 membranes are post-impregnation and sol-gel methods. Compared with the post-impregnation method for preparing PBI / H3PO4 membranes, the sol-gel method has the following advantages: (1) it only has 4 simple steps, which can greatly simplify the membrane preparation steps and save time and cost; (2) it is safe and environmentally friendly, with no organic solvents used and almost no phosphoric acid venting; (3) it is expected to achieve integrated electrode preparation. Therefore, the international community has been committed to developing its preparation process. In addition, the polybenzimidazole / H3PO4 membrane prepared by the sol-gel method has a higher phosphoric acid content and a lower polymer content. This structural feature can significantly improve the conductivity of the membrane on the one hand, but also results in poor mechanical strength on the other hand. Therefore, the preparation of high molecular weight linear polybenzimidazole is currently crucial for the sol-gel method of PBI / H3PO4 membrane preparation, as the molecular weight of polybenzimidazole directly determines the mechanical strength of the prepared PBI / H3PO4 membrane and its feasibility for application in fuel cells. However, the preparation of high molecular weight linear polybenzimidazole is also a challenge in the sol-gel method for PBI / H3PO4 membrane preparation. As the polymerization reaction proceeds, the viscosity of the reaction solution increases, causing uneven distribution of mass and heat in the reaction solution. This can lead to reaction termination or significant local differences in the degree of polymerization of the reactants, ultimately making it impossible to prepare high molecular weight polybenzimidazole. Consequently, the mechanical properties of the corresponding polybenzimidazole / H3PO4 membrane cannot meet the requirements of fuel cells. Summary of the Invention

[0003] To address the above problems, the present invention aims to provide a high molecular weight linear polybenzimidazole / H3PO4 membrane and its preparation method.

[0004] The method for preparing the high molecular weight linear polybenzimidazole / H3PO4 membrane includes the following steps:

[0005] (1) A. A certain amount of one or more aromatic tetraamine compounds and a certain amount of one or more aromatic carboxylic acids containing two carboxyl groups are added in two or more portions and dissolved or dispersed in a polyphosphoric acid solution for reaction.

[0006] Alternatively, in option B, a certain amount of aromatic tetraamine compound and a certain amount of aromatic carboxylic acid containing two carboxyl groups are dissolved or dispersed in a polyphosphoric acid solution and reacted, followed by the addition of a certain amount of polyphosphoric acid at a certain temperature and then the reaction is carried out.

[0007] (2) Adjust the viscosity of the reaction solution in step (1) above at a certain temperature;

[0008] (3) Use the solution from step (2) above to form a thin solution layer of a fixed shape on the substrate material;

[0009] (4) Hydrolyze the solution thin layer formed in step (3) above and detach it from the substrate material to obtain a self-supporting film.

[0010] The above method for preparing high molecular weight linear polybenzimidazole / H3PO4 membrane:

[0011] The aromatic tetraamine compound mentioned in step (1) is one or more of 3,3',4,4'-tetraaminobiphenyl and 1,2,4,5-tetraaminobenzene;

[0012] The aromatic carboxylic acid containing two carboxyl groups is one or more of the following: 3,5-pyrazolidine, isophthalic acid, terephthalic acid, 2,6-pyrimidinediic acid, 2,5-pyridinic acid, 2,5-pyrazinic acid, 2,6-pyridinic acid, 3,5-pyridinic acid, and phthalic acid.

[0013] The polyphosphoric acid solution is a polyphosphoric acid solution or a polyphosphoric acid solution containing P2O5; the mass ratio of P2O5 to polyphosphoric acid is 1:180-1:10, preferably 1:100-1:20, and more preferably 1:60-1:30.

[0014] In step (1)A, the aromatic tetraamine compound and / or aromatic dicarboxylic acid are added in two or more separate additions; the ratio of the aromatic tetraamine compound to the aromatic dicarboxylic acid in the first addition of the polyphosphoric acid solution is 0.989-0.997:1; when the ratio of the two monomers is within this range, there are two major advantages: first, the crosslinking of the polymer will not be generated due to the large difference in the ratio of the two, making it impossible to coat the film; second, it can form oligomers with relatively small molecular weights capped by less active carboxyl groups, which will not cause the reaction to stop due to the rapid increase in the viscosity of the reaction solution caused by the excessively fast reaction rate during the polymerization process.

[0015] The initial mass ratio of the aromatic tetraamine compound and aromatic dicarboxylic acid to the polyphosphoric acid solution is 1:40-1:3, preferably 1:30-1:7; after the aromatic tetraamine compound and aromatic dicarboxylic acid are added in batches, the final mass ratio of the two is 1.01:1-1:1.009; the final mass ratio of the two to the polyphosphoric acid solution is between 0.02 and 0.1; the reaction temperature is 185-230℃, preferably 190-225℃; the aromatic tetraamine compound and... Aromatic dicarboxylic acids are added in batches, with the addition time controlled by the viscosity of the reaction solution. The lowest viscosity at the reaction temperature is 1000-2000 cP, and the highest viscosity is 10000-20000 cP. The viscosity increase between two additions is approximately 2000-5000 cP. After the batch addition of the aromatic tetramine compound and the aromatic dicarboxylic acid, the final mass ratio of the two compounds is 1.01:1 to 1:1.009. The ratio of the final sum of their masses to the polyphosphoric acid solution is between 0.02 and 0.1.

[0016] Alternatively, in option B, the ratio of the sum of the masses of the aromatic tetraamine compound and the aromatic dicarboxylic acid to the mass of the polyphosphoric acid solution is between 0.04 and 0.2. The reaction temperature is 180-230°C. Polyphosphoric acid is added to carry out the reaction when the viscosity of the reaction solution is 8000-15000 cP.

[0017] The temperature of the added polyphosphoric acid solution is 60-180℃, preferably 80-120℃. The ratio of the mass of the added polyphosphoric acid solution to the mass of the first added polyphosphoric acid solution to the mass of the directly added aromatic tetraamine compound and aromatic dicarboxylic acid is 50-5:1, preferably 40-10:1. The re-reaction temperature is 200-230℃. The reaction is complete when the reaction time is >3h and the reaction solution becomes viscous and sticks to the reactor wall or moves upward with the paddle, or the reaction process is monitored by the change in the viscosity of the reaction solution until the reaction is complete.

[0018] The intrinsic viscosity of polybenzimidazole prepared according to step (1) is ≥4 dL / g, which is significantly higher than the intrinsic viscosity of benzimidazole reported in the literature and patents.

[0019] The structural formula of the polybenzimidazole is as follows:

[0020]

[0021]

[0022] Including the homopolymers mentioned above, and also including copolymers with different molecular structures as shown in the figure below.

[0023]

[0024] Where n and m are each an integer greater than or equal to 300 and not higher than (less than or equal to) 5000, preferably an integer greater than or equal to 500 and not higher than (less than or equal to) 3000.

[0025] In step (2), the reagent used to adjust the viscosity of the reaction solution is water or a phosphoric acid solution with a mass fraction of less than 30% (preferably less than 10%). The phosphoric acid solution is added by dripping or spraying. Water or phosphoric acid solution is added to the reaction solution by dripping, and the dripping speed is controlled at 0.5-20.0 mL / min, preferably 0.5-10.0 mL / min. During the dripping process, it can be added continuously or intermittently. During the addition process, the reaction solution is kept stirred, and the stirring speed is controlled at 70-400 rpm. During the addition process, the temperature of the reaction solution is controlled at 150-230℃. The amount of phosphoric acid solution added is 30%-120% of the amount of water required for the complete hydrolysis of the polyphosphoric acid solution, preferably 35-100%, and ensures that polybenzimidazole will not precipitate during the viscosity adjustment process. At this time, the viscosity of the reaction solution can be controlled at 1000-10000 cP.

[0026] The substrate material in step (3) is polyimide, fluorinated polyethylene propylene, perfluoroalkoxy compound, polyvinylidene fluoride, polytetrafluoroethylene, or polysulfone, preferably one or two of polyimide and polysulfone, and the thickness of the substrate film is 50-500 μm; the temperature of the substrate surface is 120℃-200℃; the thickness of the solution thin layer forming the fixed shape is 0.05-1 mm, preferably 0.05-0.6 mm; and the solution thin layer is formed by blade coating or coating.

[0027] The solution in step (4) is a phosphoric acid solution with a mass fraction of 20%-85%, preferably 30%-60%; the temperature of the phosphoric acid solution is the freezing point of the phosphoric acid solution -60℃; the hydrolysis time is 3min-24h, preferably 10min-16h; after hydrolysis, the film automatically detaches from the substrate material or is slowly peeled off using a glass rod or other rod-shaped material to obtain a self-supporting film.

[0028] The obtained self-supporting film has a thickness of 0.03-0.5 mm and is transparent.

[0029] The present invention has the following advantages:

[0030] 1. The preparation method and process are simple and easy to operate, and the preparation process is green and environmentally friendly;

[0031] 2. The prepared polymer product, polybenzimidazole, has a high molecular weight and is a linear molecule, without problems such as branching and cross-linking, and is easily soluble;

[0032] 3. By quantitatively monitoring the reaction process, it is possible to effectively control the reaction process and the reaction endpoint, and the resulting polymer film exhibits good repeatability.

[0033] 4. The prepared membrane has high electrical conductivity and good mechanical properties. Attached Figure Description

[0034] Figure 1 The conductivity is the value of the m-PBI / H3PO4 membrane prepared in Example 1.

[0035] Figure 2 This is a photograph of the p-PBI / H3PO4 membrane prepared in Example 2. Detailed Implementation

[0036] Example 1

[0037] 312g of polyphosphoric acid solution was added to a four-necked flask. Under nitrogen protection, 9.04g of 3,3',4,4'-tetraaminobiphenyl and 7.05g of terephthalic acid were added. The mixture was stirred mechanically while the temperature was increased. When the temperature reached 220℃, the stirring speed was fixed at 400rpm. The viscosity of the reaction solution was monitored using a handheld viscometer. The time was started and the viscosity of the solution at this point was recorded as approximately 1200cP, which was recorded as the initial solution viscosity. When the viscosity reached approximately 5500 cP, 0.17 g of 3,3',4,4'-tetraaminobiphenyl was added, and the reaction continued. When the viscosity reached 9800 cP, 0.11 g of terephthalic acid was added, and the reaction continued. After 8 hours of reaction, the solution viscosity increased to approximately 16000 cP. Water was then added dropwise to the reaction solution using a peristaltic pump at a flow rate of approximately 1 mL / min. The added water was connected to an atomizer to atomize the water, which was then passed into the reaction solution. The water addition time was controlled at 50 min. At this point, the reaction solution current was 9700 cP when the stirring speed was 400 rpm. After stopping stirring and letting it stand for 30 minutes, the reaction solution was poured onto the surface of a 60 μm polyimide film laid flat on a horizontally placed glass plate at 180 °C. The film was coated using a rod-shaped doctor blade with a spacing of 600 μm from the surface of the polyimide film. After coating, the reaction solution film with the polyimide film was placed in a 35% phosphoric acid solution at room temperature for 30 minutes. After separating it from the polyimide film, the surface acid was wiped off with filter paper to obtain a 0.45 mm self-supporting PBI / H3PO4 film.

[0038] After removing phosphoric acid from the self-supporting PBI / H3PO4 film and drying it completely, the characteristic viscosity of the prepared PBI polymer was tested using an Ubbelohde viscometer with 98% concentrated sulfuric acid (0.1 g / dL) as the solvent. At 25°C, the characteristic viscosity was 5.0-5.1 dL / g, corresponding to n of approximately 630 in the structural formula (the values ​​of m and n in the following examples can be calculated by referring to the conversion between characteristic viscosity and n in Example 1). This is higher than the currently reported molecular weight of PBI (4.5-4.8 dL / g). Furthermore, this type of PBI exhibits good solubility in DMSO (it can be completely dissolved in DMSO, preparing a PBI / DMSO solution with a viscosity close to 20000 cP (shear rate 101 / s) at 10%), indicating that the prepared high-molecular-weight PBI is a linear molecule and has not undergone cross-linking.

[0039] Approximately 1g of the above-mentioned PBI / H3PO4 membrane was cut and tested for acid content. The results showed that the membrane contained 50-52% phosphoric acid, 5% PBI, and 43-45% water, which is consistent with that of BASF. The membrane components are comparable.

[0040] The mechanical properties of the membrane were analyzed and tested using TA's DMA. At 30°C, this type of membrane exhibits good toughness, with a tensile strength >2.1 MPa and an elongation at break >500%, which is advantageous compared to the 1.5 MPa tensile strength of PBI / H3PO4 membranes prepared by the sol-gel method reported to date.

[0041] Cut out an area of ​​length * width = 1 * 4cm 2 The aforementioned PBI / H3PO4 membrane was subjected to temperature and humidity control in its environment using a patented temperature and humidity control device (CN104459323A), simulating the temperature and humidity conditions of a battery at 160℃ and 3% RH. This temperature and humidity were maintained for 30 minutes. The impedance was then tested using an electrochemical workstation 2273 via AC impedance spectroscopy, with a frequency range of 1-1MHz and a scanning amplitude of 10mV. The conductivity value was then calculated using the conductivity calculation formula, which is as follows:

[0042]

[0043] In the formula, σ is the conductivity of the membrane (S / cm), L is the distance between Sensor I and Sensor II electrodes (cm), W is the width of the membrane (cm), T is the thickness of the membrane (cm), and R is the impedance of the membrane (Ω). The membrane conductivity is the average value of the impedance calculation results from multiple measurements.

[0044] Figure 1The results show the conductivity and area-to-proton resistance of the PBI / H3PO4 film prepared above at 160℃. Figure 1 It can be seen that the PBI / H3PO4 film prepared above has a conductivity of 0.4 S / cm at 160℃ and 3% humidity, and an area-to-proton resistance of 45 mΩcm. 2 Compared with the conductivity of PBI / H3PO4 membranes currently reported (the highest reported value is 0.3 S / cm), it has a significant advantage.

[0045] Example 2

[0046] Add 312g of polyphosphoric acid solution to a four-necked flask. Under nitrogen protection, add 9.04g of 3,3',4,4'-tetraaminobiphenyl and 7.05g of terephthalic acid. Start stirring and heating. Add 4g of P2O5 at 160℃. When the temperature reaches 200℃, fix the stirring speed at 400rpm, start timing and record the viscosity of the solution at this time as approximately 1000cP, which is recorded as the initial viscosity. When the viscosity reached approximately 6200 cP, 0.17 g of 3,3',4,4'-tetraaminobiphenyl was added, and the reaction continued. When the viscosity reached 10200 cP, 0.12 g of terephthalic acid was added, and the reaction continued. After 8 hours of reaction, the solution viscosity increased to approximately 18000 cP. Water was then added to the reaction solution using a peristaltic pump at a flow rate of approximately 1 mL / min. The water was atomized before being added to the reaction solution, and the water addition time was controlled at 58 min. At this point, the reaction solution current was 10000 cP at a stirring speed of 400 rpm. Stirring was stopped, and after standing for 30 min, the reaction solution was poured onto a polyimide-based membrane at 180℃. Coating was performed using a rod-shaped doctor blade with a spacing of 600 μm. After coating, the reaction solution film with the polyimide-based membrane was immersed in a 35% phosphoric acid solution at room temperature for 30 min. After removal, the surface acid was wiped off with filter paper, yielding a self-supporting PBI / H3PO4 film. The intrinsic viscosity of pPBI prepared using this process is 5.6 dL / g. Figure 2 The images are of the prepared PBI / H3PO4 membrane. Figure 2 The resulting membrane is yellow and transparent, with no obvious defects. The membrane has a tensile strength of 2.5 MPa and an electrical conductivity of 0.39 S / cm at 160℃.

[0047] Example 3

[0048] 312g of polyphosphoric acid solution was added to a four-necked flask. Under nitrogen protection, 9.08g of 3,3',4,4'-tetraaminobiphenyl and 7.05g of terephthalic acid were added. The mixture was stirred and heated simultaneously. At 160℃, 4g of P2O5 was added. When the temperature reached 220℃, the stirring speed was fixed at 400rpm. The viscosity was recorded as approximately 1500cP, which was recorded as the initial viscosity. When the viscosity reached 12000cP, 118g of polyphosphoric acid solution at 90℃ was added. At this point, the viscosity of the reaction solution was approximately 8000cP. The reaction continued. When the viscosity of the reaction solution was approximately 15000cP, water was added to the reaction solution using a peristaltic pump. The water was atomized and added to the reaction solution at a flow rate of approximately 1mL / min. The water addition time was controlled at 48min. At this point, the stirring speed was 140rpm, and the viscosity of the reaction solution was approximately 7000cP. After stopping stirring and vacuum degassing for 10 minutes, the reaction solution was poured onto a polyimide-based membrane at 160℃. Coating was performed using rod-shaped doctor blades with spacings of 200, 400, and 600 μm. After coating, the reaction solution film with the polyimide-based membrane was immersed in a 50% phosphoric acid solution at room temperature for 30 minutes. After removal, the surface acid was wiped off with filter paper, yielding a self-supporting PBI / H3PO4 film with a phosphoric acid content of 50-52%, a PBI content of 5%, and a water content of 43-45%. The intrinsic viscosity of the pPBI prepared using this process was 4.9-5.1 dL / g. The tensile strength of the film was 2.0 MPa, and the conductivity was close to 0.4 S / cm at 160℃.

[0049] Comparative Example 1

[0050] The experiment was conducted using a method reported elsewhere. 247 g of polyphosphoric acid solution was added to a four-necked flask. Under nitrogen protection, 9.09 g of 3,3',4,4'-tetraaminobiphenyl and 7.05 g of terephthalic acid (molar ratio 1:1) were added. The mixture was stirred and heated simultaneously. At 160 °C, 4 g of P₂O₅ was added. When the temperature reached 220 °C, the stirring speed was fixed at 400 rpm. Timing was started, and the initial viscosity of the reaction solution was recorded as approximately 1500 cP, which was recorded as the initial viscosity. After 8 hours of reaction, the viscosity of the reaction solution was approximately 8000 cP. Continuing the reaction did not increase the current. Extending the reaction time and / or increasing the reaction temperature did not significantly change the current, and the reaction solution exhibited significant adhesion to the reaction flask. The intrinsic viscosity of pPBI prepared by this process was measured using an Ubbelohde viscometer, and its viscosity value was only about 2.3 dL / g. The analysis revealed that the low molecular weight was due to the high concentration (6%) of the reaction solution. As polymerization progressed, the viscosity of the reaction solution increased, leading to uneven mass and heat transfer and ultimately causing the reaction to stop. The tensile strength of the prepared membrane was only 0.2 MPa at 160℃.

[0051] Comparative Example 2

[0052] 312g of polyphosphoric acid solution was added to a four-necked flask. Under nitrogen protection, 8.91g of 3,3',4,4'-tetraaminobiphenyl and 7.05g of terephthalic acid (the ratio of the two monomers was 0.98:1) were added. The relevant operations and experiments were carried out according to Example 1 (other processes and conditions were the same as in Example 1). When the temperature reached 220°C, the stirring speed was fixed at 400 rpm, and the timing was started. The viscosity of the reaction solution at this time was recorded as approximately 1000 cP, which was recorded as the initial viscosity. When the viscosity reached approximately 3800 cP, 0.37 g of 3,3',4,4'-tetraaminobiphenyl was added, and the reaction continued. When the viscosity reached 5500 cP, 0.31 g of 3,3',4,4'-tetraaminobiphenyl was added, and the reaction continued. When the viscosity reached 10500 cP, 0.30 g of 3,3',4,4'-tetraaminobiphenyl was added, ultimately controlling the molar ratio of 3,3',4,4'-tetraaminobiphenyl to terephthalic acid to be 1:1. When the viscosity of the reaction solution increased to approximately 16000 cP, it was observed to have a stringy, gel-like consistency. After adjusting the viscosity with water, the reaction solution adhered to the reactor wall and could not flow out to the base film for coating. This indicates that when the initial molar ratio of 3,3',4,4'-tetraaminobiphenyl to terephthalic acid was 0.98:1, the polymer formed in the reaction underwent cross-linking, preventing the formation of a linear polymer.

[0053] Comparative Example 3

[0054] PBI polymerization was carried out using a reaction solution with a low solids content. 420g of polyphosphoric acid solution was added to a four-necked flask, and under nitrogen protection, 9.09g of 3,3',4,4'-tetraaminobiphenyl and 7.05g of terephthalic acid (the molar ratio of the two monomers was 1:1, and the solids content was approximately 3%) were added. The mixture was stirred and heated simultaneously, with 4g of P₂O₅ added or omitted at 160℃. When the temperature reached 220℃, the stirring speed was fixed at 400 rpm, and the viscosity of the reaction solution at this point was recorded as 1200 cP, which was taken as the initial viscosity. After 16 hours of reaction, the viscosity was only 3600 cP, which appeared low. Continuing the reaction did not increase the viscosity. Extending the reaction time and / or increasing the reaction temperature did not significantly change the viscosity of the reaction solution. The intrinsic viscosity of the pPBI prepared by the above process was tested using an Ubbelohde viscometer. The viscosity value was only about 1.5 dL / g. The tensile strength of this film could not be tested at 160℃, and it would break during the heating process.

[0055] Therefore, by adopting the method of this patent, the mass transfer and heat transfer problems caused by the sudden increase in the viscosity of the reaction solution can be avoided by changing the monomer ratio or / and reducing the concentration of the reaction solution after the reaction has reached a certain extent, thereby realizing the preparation of high molecular weight linear PBI.

Claims

1. A method for preparing a high molecular weight linear polybenzimidazole / H3PO4 membrane, characterized in that: Includes the following steps, (1)A. Add one or more aromatic tetraamine compounds and one or more aromatic carboxylic acids containing two carboxyl groups to a polyphosphoric acid solution for reaction; Alternatively, in option B, one or more aromatic tetraamine compounds are dissolved or dispersed in an aromatic dicarboxylic acid solution and reacted, followed by the addition of polyphosphoric acid at a certain temperature and then the reaction is repeated. (2) Adjust the viscosity of the reaction solution obtained in step (1) above at a certain temperature; (3) A thin solution layer is formed on the surface of the substrate material using the solution obtained in step (2) above; (4) Hydrolyze the solution thin layer formed in step (3) above to obtain a self-supporting film.

2. The preparation method according to claim 1, characterized in that: The aromatic tetraamine compound mentioned in step (1) is one or two of 3,3',4,4'-tetraaminobiphenyl and 1,2,4,5-tetraaminobenzene; The aromatic carboxylic acid or aromatic dicarboxylic acid containing two carboxyl groups is one or more of 3,5-pyrazolidine, isophthalic acid, terephthalic acid, 2,6-pyrimidinediic acid, 2,5-pyridinidine, 2,5-pyrazinidine, 2,6-pyridinidine, 3,5-pyridinidine, and phthalic acid. The polyphosphoric acid solution is a polyphosphoric acid solution or a polyphosphoric acid solution containing P2O5; the mass ratio of P2O5 to polyphosphoric acid in the polyphosphoric acid solution containing P2O5 is 1:180-1:10, preferably 1:100-1:20, and most preferably 1:60-1:

30.

3. The preparation method according to claim 1 or 2, characterized in that: In step (1)A, the aromatic tetraamine compound and / or aromatic dicarboxylic acid are added to the polyphosphoric acid solution in two or more separate batches. The aromatic tetraamine compound and aromatic dicarboxylic acid are added in batches, and the timing of each addition is controlled by the viscosity of the reaction solution. The minimum viscosity at the reaction temperature is 1000-2000 cP, the maximum viscosity is 10000-20000 cP, and the viscosity increase between two adjacent additions is 2000-5000 cP. After the aromatic tetraamine compound and aromatic dicarboxylic acid are added in batches, the final molar ratio of the two is 1. 01:1-1:1.009; the ratio of the sum of the masses of the two to the mass of the polyphosphoric acid solution is between 0.02 and 0.1; or in step (1) B, the sum of the masses of the aromatic tetraamine compound and the aromatic dicarboxylic acid is directly added (i.e., one or more aromatic tetraamine compounds and aromatic dicarboxylic acids are directly dissolved or dispersed in the polyphosphoric acid solution) and the ratio of the mass of the polyphosphoric acid solution to the mass of the polyphosphoric acid solution is between 0.04 and 0.2, the reaction temperature is 180-230℃, and when the viscosity of the reaction solution is 8000-15000 cP, the polyphosphoric acid solution is added again to continue the reaction.

4. The preparation method according to claim 3, characterized in that: In step (1)A, the molar ratio of the aromatic tetraamine compound to the aromatic dicarboxylic acid in the polyphosphoric acid solution added for the first time is 0.989-0.997:1; the ratio of the sum of the masses of the aromatic tetraamine compound and the aromatic dicarboxylic acid added for the first time to the mass of the polyphosphoric acid solution is 1:40-1:3, preferably 1:30-1:

7. In step (1)B, the temperature of the added polyphosphoric acid solution is 60-180℃, preferably 80-120℃, and the ratio of the mass of the added polyphosphoric acid solution to the mass of the first added polyphosphoric acid solution to the mass of the directly added aromatic tetraamine compound and aromatic dicarboxylic acid is 50-5:1, preferably 40-10:

1. In step (1)A, the reaction temperature is 185-230℃, preferably 190-225℃; the total reaction time is ≥2h and the reaction progress is monitored by the change in the viscosity of the reaction solution until the reaction is completed. In step (1)B, the re-reaction temperature is 200-230℃; the total reaction time is ≥3h and the reaction process is monitored by the change in the viscosity of the reaction solution until the reaction is completed.

5. The preparation method according to any one of claims 1-4, characterized in that: The intrinsic viscosity of the polybenzimidazole polymer product prepared according to step (1) is ≥4 dL / g; the polybenzimidazole is one or more of the polybenzimidazoles with the following general formula. The structural formula of the polybenzimidazole is as follows: Polybenzimidazole homopolymers comprising one or more of the structures shown in the above general formulas, and copolymers with molecular structures as shown in the following general formulas, Where n and m are each an integer greater than or equal to 300 and not higher than (less than or equal to) 5000, preferably an integer greater than or equal to 500 and not higher than (less than or equal to) 3000.

6. The preparation method according to claim 1, characterized in that: In step (2), the reagent used to adjust the viscosity of the reaction solution is water or a phosphoric acid solution with a mass fraction of less than or equal to 30% (preferably less than or equal to 10%). The water or phosphoric acid solution is added by dripping or spraying into the solution. During the addition process, the reaction solution is stirred and the temperature of the reaction solution is controlled at 150-230℃, and the viscosity of the reaction solution is adjusted to 1000-10000 cP.

7. The preparation method according to claim 6, characterized in that: The amount of phosphoric acid solution added is 30%-120% of the mass of the polyphosphoric acid solution required for complete hydrolysis into phosphoric acid, preferably 35%-100%. Water or phosphoric acid solution is added to the reaction solution by dropping, with the dropping rate controlled at 0.5-20.0 mL / min, preferably 0.5-10.0 mL / min; the stirring speed is controlled at 70-400 rpm.

8. The preparation method according to claim 1, characterized in that: The substrate material in step (3) is a film material, which is any one or more of polyimide, fluorinated polyethylene propylene, perfluoroalkoxy compound, polyvinylidene fluoride, polytetrafluoroethylene, and polysulfone, preferably one or two of polyimide and polysulfone; the temperature of the substrate surface is 120℃-200℃; Step (3) forms a thin solution layer on one side surface of the substrate material, the thickness of which is 0.05-1 mm, preferably 0.05-0.6 mm; the solution layer is formed by scraping and / or coating. The hydrolysis process in step (4) is carried out by using a phosphoric acid solution with a mass fraction of 20%-85%, preferably 30%-60%; the temperature of the phosphoric acid solution is room temperature-60℃; the hydrolysis time is 3min-24h, preferably 10min-16h.

9. A self-supporting thin film prepared by any one of the preparation methods of claims 1-8.

10. The application of the self-supporting membrane of claim 9 as a separator in a high-temperature polymer fuel cell.

Images