A fluorine-containing polybenzimidazole ionomer, a method for preparing the same, and a membrane electrode

Abstract

The application discloses a fluorine-containing polybenzimidazole ionomer, a preparation method thereof and a membrane electrode. The main chain of the fluorine-containing polybenzimidazole ionomer is fluorine-containing polybenzimidazole, and the main chain is branched with quaternary ammonium salt side chains, and the structure is shown as formula 1: wherein R is a fluorine-containing alkyl or a fluorine-containing aryl; X represents the length of a linking carbon chain between a quaternary ammonium salt nitrogen atom and an imidazole group nitrogen atom, X = 3-6, and n = 120-240. The ionomer main chain contains fluorine atoms, so that the ionomer has high oxidation stability and certain acid resistance. The long side chain quaternary ammonium salt structure makes the ionomer have high proton conduction capacity. Meanwhile, the polybenzimidazole skeleton has high glass transition temperature and high thermal stability, so that the ionomer can be used as a catalyst binder of a high-temperature proton exchange membrane fuel cell, to assist in building an electrochemical reaction three-phase interface and improving the utilization rate of platinum-based catalyst active sites in the cell. The ionomer can be used for preparing a high-temperature proton exchange membrane fuel cell membrane electrode and applied to the high-temperature proton exchange membrane fuel cell.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of fuel cells, and particularly relates to a fluorine-containing polybenzimidazole ionomer, a preparation method thereof and a membrane electrode. BACKGROUND

[0002] Fuel cells are highly concerned due to their clean and pollution-free characteristics. High-temperature proton exchange membrane fuel cells (HT-PEMFCs) are a kind of proton exchange membrane fuel cells with a working temperature range of 140-200 DEG C. High temperature brings faster electrode reaction kinetics, a more simplified electrode interface and higher CO poisoning resistance, so that the reforming hydrogen production and fuel cell power generation integration product technology becomes possible. High-temperature proton exchange membrane fuel cells (HT-PEMFCs) have become a research hotspot in the industry.

[0003] A membrane electrode assembly (MEA) is a place where electrochemical reactions occur, and is the most core component in HT-PEMFCs, which is composed of a proton exchange membrane, a catalyst layer and a gas diffusion layer. A binder resin is one of raw materials for forming the catalyst layer, and is an important component of the HT-PEMFCs membrane electrode. At present, PTFE (polytetrafluoroethylene) is widely used in HT-PEMFCs membrane electrodes due to its high oxygen permeability and hydrophobic and acidophobic properties. The catalyst layer containing PTFE binder resin usually exhibits relatively small mass transfer resistance, and the acid flooding of phosphoric acid to the catalyst layer and the poisoning of platinum active sites are also inhibited to a certain extent. However, PTFE has the following problems: 1) inherent hydrophobicity easily causes agglomeration and poor interface dispersibility, which affects water and gas conduction; 2) high temperature easily causes unstable interface structure between the catalyst layer and the resin; and 3) no proton conduction capacity, which cannot effectively assist in building an electrochemical reaction three-phase interface. SUMMARY

[0004] The purpose of the present application is to provide a binder resin for a high-temperature proton exchange membrane fuel cell catalyst layer, which has good temperature resistance and certain acidophobicity, and has high proton conduction capacity, so as to improve the electrochemical performance of the high-temperature proton exchange membrane fuel cell and its long-term stable operation.

[0005] In order to achieve the above purpose, the present application provides a fluorine-containing polybenzimidazole ionomer, the main chain of which is a fluorine-containing polybenzimidazole, and the main chain is branched with quaternary ammonium salt side chains, and the structure is shown as formula 1:

[0006]

[0007] wherein R is a fluorine-containing alkyl group or a fluorine-containing aryl group; X represents the length of the linking carbon chain between the quaternary ammonium salt nitrogen atom and the imidazole base nitrogen atom, X = 3-6, n = 120-240.

[0008] Optionally, the fluorine-containing alkyl group is:

[0009]

[0010] Optionally, the fluorine-containing aryl group is:

[0011]

[0012] Optionally, the molecular weight of the ionomer is 20-60 kDa.

[0013] Optionally, the grafting degree of the quaternary ammonium salt side chain is 5%-20% in terms of the percentage of the molar amount of the quaternary ammonium salt functional group to the molar amount of the imidazole group -NH functional group of the main chain.

[0014] Another object of the present application is to provide a preparation method of the fluorine-containing polybenzimidazole ionomer according to the above, comprising the following steps:

[0015] Step S1, polymerizing 3,3'-4,4'-diaminobenzidine and fluorine-containing dicarboxylic acid as raw materials and doping polyphosphoric acid to obtain the main chain, and the synthesis route is as follows:

[0016]

[0017] Step S2, heating the main chain and quaternary ammonium salt bromide to obtain a fluorine-containing polybenzimidazole ionomer grafted with a quaternary ammonium salt side chain;

[0018]

[0019] Optionally, in step S1, the reactants 3,3'-4,4'-diaminobenzidine, fluorine-containing dicarboxylic acid and polyphosphoric acid together account for 5-10wt% of the mass fraction of the reaction system.

[0020] Optionally, in step S1, the molar ratio of 3,3'-4,4'-diaminobenzidine to fluorine-containing dicarboxylic acid is 1:1.

[0021] Optionally, in step S2, a polar aprotic organic solvent is used, and the polar aprotic organic solvent is at least one of NMP, DMAc, DMSO and DMF.

[0022] Still another object of the present application is to provide a membrane electrode comprising a proton exchange membrane, a catalyst layer and a gas diffusion layer, wherein the catalyst layer contains a binder resin; and the binder resin is the fluorine-containing polybenzimidazole ionomer described above.

[0023] Compared with the prior art, the technical scheme of the present application has at least the following beneficial effects:

[0024] The fluorine-containing polybenzimidazole ionomer provided by the present application has high thermal stability and a relatively high glass transition temperature (≥ 300 DEG C), and has stable and relatively high proton conduction capacity under high-temperature and high-acid conditions.

[0025] Since the main chain structure of the ionomer of the present application contains fluorine atoms, it has higher hydrophobicity and acidophobicity than conventional PBI, and can reduce the coverage of the active sites of the catalyst caused by phosphoric acid adsorption.

[0026] The side chain quaternary ammonium salt grafting of the ionomer of the present application not only improves the proton conduction capacity of the polymer, but also the carbon chain with a certain length of the side chain can prevent the gas mass transfer from being blocked due to the excessively close combination between polybenzimidazole molecules by using the large steric hindrance thereof.

[0027] The polymer of the present application, as a catalyst binder for high-temperature proton exchange membrane fuel cells, can assist in constructing an electrochemical reaction three-phase interface and improve the utilization rate of platinum-based catalyst active sites in the cell due to the synergistic effect of the main chain and the side chain, and exhibits a high power density at high temperature, thereby improving the overall performance of the fuel cell. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 The infrared spectrum of the ionomer obtained in Examples 1-3 and Comparative Example of the present application is shown in the schematic diagram.

[0029] Figure 2 The single cell performance test of the membrane electrode obtained in Examples 1-3 of the present application is shown in the schematic diagram.

[0030] Figure 3 The AC impedance spectrum of the membrane electrode obtained in Examples 1-3 and Comparative Example of the present application is shown in the schematic diagram.

[0031] Figure 4 The proton transport resistance of the catalyst layer of Examples 1-3 and Comparative Example 1 of the present application is shown in the schematic diagram. DETAILED DESCRIPTION

[0032] The technical scheme of the present application will be described clearly and completely below with reference to the drawings. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0033] Based on the problems of PTFE as an adhesive resin described in the background art, the present application aims to develop a stable ionomer binder with acidophobicity and high proton conductivity under high-temperature conditions.

[0034] Although perfluorosulfonic acid ionomer is widely used in low-temperature proton exchange membrane fuel cells, its low glass transition temperature (~ 130℃) is not conducive to the long-term stable operation of high-temperature proton exchange membrane fuel cells, and therefore is not suitable for HT-PEMFCs.

[0035] PBI (polybenzimidazole) can also be used as an ionomer binder in the catalyst layer while being used as a proton membrane, has good compatibility with the proton membrane interface, has proton transmission capacity, and the PBI main chain containing aryl structure has high glass transition temperature and excellent thermal stability. However, traditional PBI such as commercial mPBI is easy to form a film to cover the catalyst active sites in the catalyst layer, and the alkaline imidazole group of the PBI further causes acid flooding of the catalyst layer by adsorbing phosphoric acid.

[0036] Based on the above, the PBI is modified in the present application, and fluorine atoms are introduced into the main chain structure of the PBI to reduce the coverage of the catalyst active sites caused by phosphoric acid adsorption; and long-chain quaternary ammonium salt is grafted to the side chain of the PBI, which not only improves the proton conduction capacity of the polymer, but also prevents the gas mass transfer from being blocked due to the excessive close combination between polybenzimidazole molecules by using the large steric hindrance. The structure is shown in formula 1:

[0037]

[0038] In formula 1, R is a fluorine-containing alkyl group or a fluorine-containing aryl group; X represents the length of the carbon chain linking between the nitrogen atom of the quaternary ammonium salt and the nitrogen atom of the imidazole group, X = 3-6, and n = 120-240.

[0039] The fluorine-containing alkyl group can be:

[0040]

[0041] The fluorine-containing aryl group can be:

[0042]

[0043] Since the molecular weight of the binder resin is too large to block the pores of the catalyst layer, and the molecular weight is too small to have insufficient adhesion, the binder cannot play the role required by the ionomer. The molecular weight of the fluorine-containing polybenzimidazole ionomer of the present application is 20-60 kDa.

[0044] Alternatively, the grafting degree of the quaternary ammonium salt side chain is 5%-20% in terms of the percentage of the molar amount of the quaternary ammonium salt functional group to the molar amount of the imidazole group -NH functional group in the main chain.

[0045] Another object of the present application is to provide a preparation method of the fluorine-containing polybenzimidazole ionomer according to the above, comprising the following steps:

[0046] Step S1, the main chain is obtained by polymerization reaction of 3,3'-4,4'-diaminobenzidine and fluorine-containing dicarboxylic acid and doping of polyphosphoric acid, and the synthesis route is as follows:

[0047]

[0048] Step S2, the main chain is heated to react with quaternary ammonium bromide to obtain a side chain quaternary ammonium salt grafted fluorine-containing polybenzimidazole ionomer;

[0049]

[0050] Optionally, in step S1, the mass fraction of the reactants 3,3'-4,4'-diaminobenzidine, fluorine-containing dicarboxylic acid and polyphosphoric acid in the reaction system is 5-10wt%.

[0051] Optionally, in step S1, the molar ratio of 3,3'-4,4'-diaminobenzidine to fluorine-containing dicarboxylic acid is 1:1.

[0052] Optionally, in step S2, a polar aprotic organic solvent is used, and the polar aprotic organic solvent is at least one of NMP, DMAc, DMSO and DMF.

[0053] The ionomer of the present application can be used to prepare a high-temperature proton exchange membrane fuel cell membrane electrode, and is applied to a high-temperature proton exchange membrane fuel cell. The present application also provides a membrane electrode comprising a proton exchange membrane, a catalyst layer and a gas diffusion layer, wherein the catalyst layer contains a binder resin; and the binder resin is the fluorine-containing polybenzimidazole ionomer described above. Specifically, after dispersing platinum-carbon catalyst, a fluorine-containing polybenzimidazole ionomer solution of the present application is added dropwise, and then sprayed on a gas diffusion layer to prepare a cathode and an anode. The cathode, the anode and the proton membrane are laminated with a polyimide frame to form the membrane electrode of the present application.

[0054] The reagents or raw materials used in the following examples and comparative examples are commercially available.

[0055] Comparative Example 1

[0056] 0.5mmol 3'-3-4'-4-diaminobenzidine (DAB) and 0.5mmol dicarboxylic diphenyl ether monomer were added to a three-necked flask, 50g polyphosphoric acid (PPA) was added, and then nitrogen was introduced for protection. After heating to 140℃ for 4h, a polyether type benzimidazole (OPBI) solution was obtained. The solution was poured into deionized water while hot, and the precipitated solid was neutralized to neutral pH with saturated sodium bicarbonate aqueous solution, then washed with deionized water and methanol several times, and dried in an oven at 80℃ for 24h to obtain OPBI solid.

[0057] Take 0.5g of the above OPBI solid, add 25g of N-methyl pyrrolidone (NMP), heat to 70°C to dissolve, and obtain an OPBI polymer solution.

[0058] Take 1g of 60% platinum-cobalt-carbon catalyst dispersed in NMP solvent, and use stirring to preliminarily disperse. Drop the above OPBI polymer solution to make the I / C ratio 0.3. After ultrasonic dispersion of the slurry for 30min using an ultrasonic probe, ultrasonic spray on a GDL (gas diffusion layer), oven drying at 80°C for 10min, and loading of 0.5mg / cm 2 , as a membrane electrode cathode electrode.

[0059] Take 1g of 40% platinum-carbon catalyst dispersed in NMP solvent, and use stirring to preliminarily disperse. Drop the above OPBI polymer solution to make the I / C ratio 0.3. After ultrasonic dispersion of the slurry for 30min using an ultrasonic probe, ultrasonic spray on a GDL, oven drying at 80°C for 10min, and loading of 0.5mg / cm 2 , as a membrane electrode anode electrode.

[0060] Cut the OPBI film into the required shape and size, immerse in phosphoric acid, and place in an oven to heat and dope phosphoric acid. The phosphoric acid doping amount is 400%.

[0061] Stack the above prepared cathode GDE (gas diffusion electrode), anode GDE, and proton membrane together with a polyimide frame, and place under a hot press to hot press. The hot press pressure is 1.0MPa, and the hot press time is 300s, to make the high-temperature proton exchange membrane fuel cell membrane electrode.

[0062] Example 1

[0063] Take 0.5mmol 3’3-4’4-diamino benzidine (DAB) and 0.5mmol 2,2-bis(4-carboxyphenyl) hexafluoropropane monomer into a three-necked flask, add 50g PPA, protect with nitrogen, heat to 200°C, and react for 12h to obtain a F6PBI solution. Pour the solution into deionized water while hot, neutralize the precipitated solid with saturated sodium bicarbonate aqueous solution to pH neutral, wash several times with deionized water and methanol, and place in an oven to dry at 80°C for 24h to obtain F6PBI solid.

[0064] Take 0.5 g of the above F6PBI solid, add 25 g of NMP, heat to 70°C to dissolve. Then add 5-bromo-N,N,N-trimethylpentane-1-aminium bromide, the reaction degree of -NH groups on F6PBI is 5% in terms of molar fraction. The reaction degree refers to the grafting degree of the quaternary ammonium salt side chain as a percentage of the molar amount of the quaternary ammonium salt functional group to the molar amount of the imidazole group -NH functional group in the main chain. That is, the grafting degree of the quaternary ammonium salt side chain as a percentage of the molar amount of the quaternary ammonium salt functional group to the molar amount of the imidazole group -NH functional group in the main chain is 5%. React at 70°C for 12 h to obtain a grafted polymer solution.

[0065] Take 1 g of 60% platinum-cobalt-carbon catalyst dispersed in NMP solvent, and use stirring to preliminarily disperse it. Add the above grafted polymer solution dropwise to make the I / C ratio 0.3. After ultrasonic dispersion of the slurry for 30 min using an ultrasonic probe, ultrasonic spray it onto a GDL, and dry in an oven at 80°C for 10 min, with a loading of 0.5 mg / cm 2 , as a membrane electrode cathode electrode.

[0066] Take 1 g of 40% platinum-carbon catalyst dispersed in NMP solvent, and use stirring to preliminarily disperse it. Add the above grafted polymer solution dropwise to make the I / C ratio 0.3. After ultrasonic dispersion of the slurry for 30 min using an ultrasonic probe, ultrasonic spray it onto a GDL, and dry in an oven at 80°C for 10 min, with a loading of 0.5 mg / cm 2 , as a membrane electrode anode electrode.

[0067] Cut the OPBI film into the desired shape and size, immerse it in phosphoric acid, and place it in an oven to heat and dope phosphoric acid. The phosphoric acid doping amount is 400%.

[0068] Stack the above-prepared cathode GDE, anode GDE, and proton membrane together with a polyimide frame, and place them in a hot press to hot press, with a hot press pressure of 1.0 MPa and a hot press time of 300 s, to make the high-temperature proton exchange membrane fuel cell membrane electrode.

[0069] Example 2

[0070] Example 2 differs from Example 1 in that the reaction degree of -NH groups on F6PBI is 10% in terms of molar fraction.

[0071] Example 3

[0072] Example 3 differs from Example 1 in that the reaction degree of -NH groups on F6PBI is 15% in terms of molar fraction.

[0073] The infrared spectra of the four kinds of ionomers prepared in Comparative Example 1 and Examples 1-3 are shown in Figure 1 cm -1The absorption peak intensity weakens as the grafting degree increases (consuming the -NH site) on the main chain imidazole ring. 1485 cm⁻¹ -1 and 1595cm -1 The two newly appearing absorption peaks are -NH4 + The absorption peak is at 2900 cm⁻¹. -1 -3500cm -1 The peak values ​​between the peaks demonstrate the successful synthesis of the benzimidazole backbone. The example shown in the figure reached 1480 cm⁻¹. -1 -1596cm -1 The bimodal distribution at the point demonstrates the success of the quaternary ammonium salt lateral linking branch.

[0074] The obtained membrane electrode assembly was then used to construct a single cell for testing. The test conditions were: ambient pressure with no back pressure; hydrogen gas was introduced at a flow rate of 0.5 L / min at the anode; and air was introduced at a flow rate of 2 L / min at the cathode. The current-voltage curves of each single cell were measured using a constant current method at 160℃, 170℃, and 180℃, with a current-voltage ratio of 200 mA / cm². 2 The AC impedance spectrum of the battery was measured using an impedance analyzer at the current density. The proton transport resistance of the catalyst layer was measured after replacing the air at the cathode with nitrogen at the same flow rate.

[0075] Figure 2 This diagram illustrates a comparison between the single-cell performance tests of the membrane electrodes obtained in Examples 1-3 of this invention and the single-cell performance test of Comparative Example 1. As can be seen from the diagram, the electrode performance using the membrane electrodes obtained in Examples 1-3 is at 500 mA / cm². 2 At current densities, performance is improved by 71.7%, 66.5%, and 59.1%, respectively.

[0076] Figure 3 This is a schematic diagram comparing the AC impedance spectra of the membrane electrodes obtained in Examples 1-3 and the comparative example of the present invention. As can be seen from the figure, the cathode polarization impedance of the comparative example is approximately 492 molhm cm⁻¹. 2 The cathode polarization impedances of Examples 1-3 were 208 mol / cm, respectively. 2 170 mohm cm 2 and 236mohm cm 2 The polarization impedance is significantly reduced.

[0077] Figure 4 This is a schematic diagram comparing the proton transport resistance of the catalyst layer in Examples 1-3 of the present invention with that in Comparative Example 1. It can be seen that the proton transport resistance of the catalyst layer in Examples 1-3 is significantly lower than that in the comparative example.

[0078] In summary, the fluorine-containing polybenzimidazole ionomer provided by the application has high thermal stability and a higher glass transition temperature of PBI, the main chain containing fluorine atoms makes it have higher oxidation stability and certain acid resistance, and the long side chain quaternary ammonium salt structure makes it have high proton conductivity, as a catalyst binder of high-temperature proton exchange membrane fuel cell, it helps to build a three-phase interface of electrochemical reaction, and improves the utilization rate of platinum-based catalyst active sites in the battery. The synergistic effect of the main chain and the side chain can help to build a three-phase interface of electrochemical reaction, improve the utilization rate of platinum-based catalyst active sites in the battery, show higher power density at high temperature, and further improve the comprehensive performance of the fuel cell, and have stable and high proton conductivity under high-temperature and high-acid conditions.

[0079] Although the content of the application has been described in detail by the above preferred embodiments, it should be recognized that the above description should not be considered as a limitation of the application. After reading the above content, various modifications and alternatives of the application will be obvious to those skilled in the art. Therefore, the protection scope of the application should be defined by the appended claims.

Claims

1. A fluorinated polybenzimidazole ionomer, characterized in that, Its main chain is a fluorinated polybenzimidazole, and the main chain is branched with quaternary ammonium salt side chains, as shown in Formula 1: Where R is n = 120-240; the grafting degree of the quaternary ammonium salt side chain is 5% to 20% of the molar amount of the quaternary ammonium salt functional group relative to the molar amount of the imidazole group -NH functional group in the main chain.

2. The fluorinated polybenzimidazole ionomer as described in claim 1, characterized in that, The molecular weight of the ionomer is 20-60 kDa.

3. A method for preparing a fluorinated polybenzimidazole ionomer according to claim 1 or 2, characterized in that, Includes the following steps: Step S1 involves polymerizing 3,3'-4,4'-diaminobenzidine with fluorinated dicarboxylic acid and doping it with polyphosphoric acid to obtain the main chain. The synthetic route is as follows. in, for Step S2, the main chain is heated and reacted with the brominated quaternary ammonium salt to obtain a fluorinated polybenzimidazole ionomer grafted with the side chain quaternary ammonium salt.

4. The method for preparing the fluorinated polybenzimidazole ionomer as described in claim 3, characterized in that, In step S1, the reactants 3,3'-4,4'-diaminobenzidine, fluorinated dicarboxylic acid, and polyphosphoric acid together account for 5-10 wt% of the reaction system.

5. The method for preparing the fluorinated polybenzimidazole ionomer as described in claim 3, characterized in that, In step S1, the molar ratio of 3,3'-4,4'-diaminobenzidine to fluorinated dicarboxylic acid is 1:

1.

6. The method for preparing the fluorinated polybenzimidazole ionomer as described in claim 3, characterized in that, Step S2 uses a polar aprotic organic solvent, wherein the polar aprotic organic solvent is at least one of NMP, DMAc, DMSO, and DMF.

7. A membrane electrode, characterized in that, It comprises a proton exchange membrane, a catalyst layer and a gas diffusion layer, wherein the catalyst layer contains a binder resin; the binder resin is the fluorinated polybenzimidazole ionomer as described in claim 1 or 2.

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