Crosslinking copolymers based on polyfluorene, methods of making the same, and anion exchange membranes for alkaline fuel cells using the same

Abstract

The present disclosure relates to a technology of synthesizing an aromatic polyfluorene-based copolymer having a crosslinked structure, not having an aryl ether bond in a polymer backbone, and introducing a piperidinium group in a repeating unit, and applying an anion exchange membrane prepared therefrom to an alkaline fuel cell, water electrolysis, carbon dioxide reduction, a metal air battery, and the like. According to the present disclosure, the anion exchange membrane having a crosslinked structure has excellent thermal stability, chemical stability, and mechanical properties, and high water retention capacity, ionic conductivity, and durability, and exhibits an excellent dispersed phase.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a novel polyfluorene-based cross-linked copolymer and a method for preparing the same, and more particularly, to an aromatic polyfluorene-based copolymer having a cross-linked structure, not having an aryl ether bond in a polymer backbone, and introducing a piperidinium group in a repeating unit, and an anion exchange membrane prepared therefrom is applied to a technology for an alkaline fuel cell, water electrolysis, carbon dioxide reduction, a metal air battery, etc. BACKGROUND

[0002] Polymer electrolyte membrane fuel cells (PEMFC) have been extensively studied due to their relatively high current density and environmentally friendly advantages. In particular, perfluoroalkane-based proton exchange membranes represented by Nafion have been mainly used as polymer electrolyte membranes. Although the Nafion membrane has excellent chemical stability and high ionic conductivity, it is very expensive and has a low glass transition temperature. Therefore, research to replace Nafion, including the development of aromatic hydrocarbon polymer electrolyte membranes, etc., is actively ongoing.

[0003] In recent years, alkaline membrane fuel cells (AMFC) using an anion exchange membrane and operating in an alkaline environment have attracted attention. In particular, alkaline membrane fuel cells are being continuously studied because inexpensive non-noble metals (e.g., nickel, manganese, etc.) can be used instead of platinum as an electrode catalyst, and they exhibit excellent performance and significantly high cost competitiveness compared to polymer electrolyte membrane fuel cells.

[0004] For an anion exchange membrane for an alkaline membrane fuel cell, a polymer having an aryl ether backbone (e.g., polyaryl ether sulfone, polyphenyl ether, polyether ether ketone, etc.) is mainly used. In addition, although a cross-linked anion exchange membrane using a hydrophobic cross-linking agent (e.g., 1,5-dibromopentane, 1,6-dibromohexane, and 1,6-hexanediamine) is known, the hydrophobic anion exchange membrane has problems of low ionic conductivity, limited flexibility, low solubility, etc. for an anion exchange membrane fuel cell. In addition, since the existing anion exchange membrane is limited in terms of chemical stability (less than 500 hours in a 1M NaOH solution at 80℃) and mechanical properties (tensile strength < 30 MPa), when they are used for a fuel cell, the power density is low (0.1-0.5 Wcm -2 ) and the cell durability is reduced.

[0005] So far, there is no specific known technology for preparing an aromatic polyfluorene-based copolymer having no aryl ether bond in a polymer backbone and introducing a piperidinium group in a repeating unit and an anion exchange membrane thereof, and applying the same to an anion exchange membrane used in a transportation device, a power storage device, a munitions industry, an aerospace industry, etc., or a low-cost water electrolysis device for hydrogen production.

[0006] Accordingly, the inventors of the present disclosure have been devoted to expanding the application of an ion exchange membrane of an aromatic polymer class having excellent thermal stability and chemical stability as well as mechanical properties. As a result, they synthesized an aromatic polyfluorene-based copolymer having a crosslinked structure, having no aryl ether bond in a polymer backbone, and introducing a piperidinium group in a repeating unit, and found that an anion exchange membrane prepared therefrom has excellent mechanical strength, water retention capacity, and durability. Accordingly, they noted that the anion exchange membrane can be applied to an alkaline fuel cell, water electrolysis, carbon dioxide reduction, a metal air battery, etc., and completed the present disclosure.

[0007] [Related Art Reference]

[0008] [Patent Document]

[0009] Patent Document 1: Korean Patent Publication No. 10-2018-0121961.

[0010] Patent Document 2: International Patent Publication No. WO 2019 / 068051.

[0011] Patent Document 3: Chinese Patent Publication No. CN 106784946.

[0012] Patent Document 4: Chinese Patent Publication No. CN 108164724. SUMMARY

[0013] TECHNICAL PROBLEM

[0014] The present disclosure aims to provide a polyfluorene-based crosslinked copolymer having excellent mechanical strength, water retention capacity, and durability, and an anion exchange membrane having a crosslinked structure prepared therefrom.

[0015] The present disclosure also relates to applying the polyfluorene-based anion exchange membrane having a crosslinked structure to an alkaline fuel cell, water electrolysis, carbon dioxide reduction, a metal air battery, etc.

[0016] TECHNICAL SOLUTION

[0017] The present disclosure provides a polyfluorene-based crosslinked copolymer selected from a copolymer having a crosslinked structure represented by Chemical Formula 1 to Chemical Formula 5:

[0018]

[0019] Chemical Formula 2

[0020]

[0021] Chemical Formula 3

[0022]

[0023] Chemical Formula 4

[0024]

[0025] Chemical Formula 5

[0026]

[0027] In Chemical Formulae 1 to 5, aryl-1 and aryl-2 are each independently selected from the group consisting of fluorenyl, phenyl, biphenyl, terphenyl, and quaterphenyl, at least one of aryl-1 and aryl-2 is fluorenyl,

[0028] R is H or CH3,

[0029] x represents a degree of cross-linking,

[0030] represents an ammonium-based cross-linking agent, and

[0031] n is an integer from 1 to 15.

[0032] In Chemical Formulae 1 to 5, x (degree of cross-linking) is 5-20%.

[0033] In Chemical Formulae 1 to 5, The (ammonium-based cross-linking agent) is a polyammonium compound having at least one ammonium cation.

[0034] The present disclosure also provides a method of preparing a polyfluorene-based cross-linked copolymer, the method comprising: (I) a step of dissolving a polyfluorene-based block copolymer having piperidine introduced therein in an organic solvent to obtain a polymer solution; (II) a step of obtaining a mixture solution by adding an ammonium-based cross-linking agent solution to the polymer solution and stirring; (III) a step of forming a quaternary piperidinium salt by reacting the mixture solution with an excess of methyl iodide; and (IV) a step of obtaining a solid polymer by precipitating, washing, and drying the polymer solution having the quaternary piperidinium salt to obtain a solid polymer.

[0035] The organic solvent in step (I) is N-methylpyrrolidone, dimethylacetamide, dimethylsulfoxide, or dimethylformamide.

[0036] The ammonium-based crosslinking agent in step (II) is a polyammonium compound having at least one ammonium cation.

[0037] The present disclosure also provides a polyfluorene-based anion exchange membrane having a crosslinked structure, which is obtained from a polyfluorene-based crosslinked copolymer.

[0038] The present disclosure also provides a method of preparing a polyfluorene-based anion exchange membrane having a crosslinked structure, the method comprising: (a) a step of obtaining a polymer solution by dissolving a polyfluorene-based crosslinked copolymer in an organic solvent; (b) a step of obtaining a membrane by filtering the polymer solution, casting on a glass plate, and then drying it; and (c) a step of converting counterions into OH - ions by immersing the obtained membrane in a 1M NaOH solution.

[0039] The organic solvent in step (a) is N-methylpyrrolidone, dimethylacetamide, dimethylsulfoxide, or dimethylformamide.

[0040] The concentration of the polymer solution of step (a) is 2-30 wt%.

[0041] The drying of step (b) is performed by slowly removing the organic solvent in an oven at 80-90℃ for 24 hours, and then completely removing the organic solvent in a vacuum oven at 120-150℃ for 24 hours.

[0042] The present disclosure also provides a membrane electrode assembly for an alkaline fuel cell, which includes a polyfluorene-based anion exchange membrane having a crosslinked structure.

[0043] The present disclosure also provides an alkaline fuel cell, which includes a polyfluorene-based anion exchange membrane having a crosslinked structure.

[0044] The present disclosure also provides a water electrolysis device, which includes a polyfluorene-based anion exchange membrane having a crosslinked structure.

[0045] Advantages

[0046] According to the present disclosure, an anion exchange membrane having a crosslinked structure, which is prepared from an aromatic polyfluorene-based block copolymer having a crosslinked structure, having no aryl ether bond in the polymer backbone, and introducing a piperidinium group in the repeating unit, has excellent thermal stability and chemical stability and mechanical properties, and high water retention capacity, ionic conductivity, and durability, and exhibits an excellent dispersed phase.

[0047] Further, the polyfluorene-based anion exchange membrane having a crosslinked structure of the present disclosure can be applied to an alkaline fuel cell, a water electrolysis device, carbon dioxide reduction, a metal air battery. BRIEF DESCRIPTION OF DRAWINGS

[0048] Figure 1 Results of measuring dimensional stability of the anion exchange membranes prepared in Examples 1-3 and Comparative Examples 1-2 are shown.

[0049] Figure 2 (a) Results of measuring mechanical properties of the anion exchange membranes prepared in Examples 1-3 and Comparative Examples 1-2 in a dry state are shown, Figure 2 (b) Results of measuring mechanical properties of the anion exchange membranes prepared in Examples 1 and 3 and Comparative Example 2 in a humid state are shown.

[0050] Figure 3 Ion conductivities of the anion exchange membranes prepared in Examples 1-3 and Comparative Examples 1-2 are shown.

[0051] Figure 4 Ion channel sizes and phase separation degrees of the anion exchange membranes prepared in Examples 1-3 and Comparative Example 2 are shown.

[0052] Figure 5 (a) - (d) show results of evaluating alkaline stability. Figure 5 (a) Residual ion conductivities of the anion exchange membranes prepared in Examples 1 and 3 and Comparative Example 2 after long-term exposure at 80℃ in a 1M NaOH solution are shown, Figure 5 (b) Changes in ion conductivities of the anion exchange membranes prepared in Example 1 after 1200 hours of exposure at 80℃ in a 1M NaOH solution are shown. 1 H NMR spectra; Figure 5 (c) Mechanical properties of the anion exchange membranes prepared in Examples 1 and 3 and Comparative Example 2 after 1200 hours of exposure at 80℃ in a 1M NaOH solution are shown, and Figure 5 (d) Phase separation degrees of the anion exchange membranes prepared in Examples 1 and 3 and Comparative Example 2 before and after exposure to 1200 hours of exposure at 80℃ in a 1M NaOH solution are shown.

[0053] Figure 6 Fuel cell performances of the anion exchange membranes prepared in Examples 1-3 and Comparative Example 2 are shown. DETAILED DESCRIPTION

[0054] Hereinafter, a novel polyfluorene-based crosslinked copolymer according to the present disclosure, a method of preparing the same, and an anion exchange membrane for an alkaline fuel cell using the same will be described in detail.

[0055] The present disclosure provides a polyfluorene-based crosslinked copolymer selected from copolymers having a crosslinking structure represented by Chemical Formula 1 to Chemical Formula 5.

[0056] Chemical Formula 1

[0057]

[0058] Chemical Formula 2

[0059]

[0060] Chemical Formula 3

[0061]

[0062] Chemical Formula 4

[0063]

[0064] Chemical Formula 5

[0065]

[0066] In Chemical Formula 1 to Chemical Formula 5, aryl-1 and aryl-2 are each independently selected from the group consisting of fluorenyl, phenyl, biphenyl, terphenyl, and quaterphenyl, at least one of aryl-1 and aryl-2 is fluorenyl,

[0067] R is H or CH3,

[0068] x represents a degree of crosslinking,

[0069] represents an ammonium-based crosslinking agent, and

[0070] n is an integer from 1 to 15.

[0071] The inventors of the present disclosure have disclosed a novel polyfluorene-based copolymer ionomer, an anion exchange membrane, and a method of preparing the same in a previously applied patent (Korean Patent Publication No. 10-2021-0071810).

[0072] In the present disclosure, a novel polyfluorene-based crosslinked copolymer having a crosslinking structure selected from Chemical Formula 1 to Chemical Formula 5 is prepared by crosslinking a polyfluorene-based copolymer with a compound having at least one ammonium cation.

[0073] The inventors of the present disclosure attempted to solve the problems of low ionic conductivity, water retention capacity, and mechanical properties of existing anion exchange membranes for alkaline fuel cells by preparing an anion exchange membrane for alkaline fuel cells from a polyfluorene-based crosslinked copolymer.

[0074] In Chemical Formula 1 to Chemical Formula 5, x represents a degree of crosslinking, and can be controlled with the amount of a polyammonium compound having at least one ammonium cation, which is used as a crosslinking agent. When considering an anion exchange membrane that can be prepared from a crosslinked copolymer, the degree of crosslinking can specifically be 5-20%, more specifically 10-20%. If the degree of crosslinking is less than 5%, improvement in physical properties by crosslinking is insignificant. And, if the degree of crosslinking exceeds 20%, an anion exchange membrane cannot be prepared because the crosslinked copolymer is not completely dissolved in an organic solvent and crosslinking does not occur.

[0075] The present disclosure also provides a polyfluorene-based anion exchange membrane having a crosslinked structure, which is obtained from a polyfluorene-based crosslinked copolymer.

[0076] The polyfluorene-based anion exchange membrane having a crosslinked structure is a polyammonium crosslinked membrane containing at least one ammonium group. It exhibits excellent film-forming ability, mechanical properties, and chemical stability because it has no aryl ether bond, and N-heterocyclic ammonium and piperidinium groups (e.g., polyphenylene, dimethylpiperidinium, etc.) are introduced in the repeating unit.

[0077] In addition, the ammonium-based crosslinking agent used in the present disclosure has high ionic conductivity and durability and a microphase separation structure because it has a flexible aliphatic chain structure that exhibits excellent stability and contains a controllable number of ammonium groups. In addition, the ion exchange performance and morphology of the polyammonium crosslinked anion exchange membrane can be controlled by adjusting the length of the alkyl spacer between the ammonium groups.

[0078] In addition, the polyammonium crosslinked anion exchange membrane according to the present disclosure can exhibit significantly improved ionic conductivity and mechanical properties compared to an anion exchange membrane having a conventional crosslinked structure, which exhibits very low ionic conductivity after being crosslinked with a hydrophobic crosslinking agent.

[0079] In particular, since the polyammonium crosslinked anion exchange membrane according to the present disclosure exhibits a high water retention capacity even in a dry environment, it can be stably operated even under low humidity conditions compared to existing anion exchange membrane fuel cells. In addition, since it exhibits high water vapor permeability, it is very advantageous in terms of material transport, moisture management, and durability.

[0080] The present disclosure also provides a method of preparing a polyfluorene-based crosslinked copolymer, the method comprising: (I) a step of dissolving a piperidine-introduced polyfluorene-based block copolymer in an organic solvent to obtain a polymer solution; (II) a step of obtaining a mixture solution by adding an ammonium-based crosslinking agent solution to the polymer solution and stirring; (III) a step of forming a quaternary piperidinium salt by reacting the mixture solution with an excess of methyl iodide; and (IV) a step of obtaining a solid polymer by precipitating, washing, and drying the polymer solution formed with the quaternary piperidinium salt.

[0081] The piperidine-introduced polyfluorene-based block copolymer of step (I) has been synthesized by the method disclosed in Korean Patent Publication No. 10-2021-0071810 by the inventors of the present disclosure.

[0082] The organic solvent in step (I) can be N-methylpyrrolidone, dimethylacetamide, dimethyl sulfoxide, or dimethylformamide, particularly dimethyl sulfoxide.

[0083] The ammonium-based crosslinking agent of step (II) can be a polyammonium compound having at least one ammonium cation. Di- or tri-ammonium compounds having various alkyl spacer lengths can be used. More specifically, 4,4'-(propane-diyl)bis(1-(5-bromopentyl)-1-methylpiperidinium or 4,4'-(propane-diyl)bis(1-(10-bromodecyl)-1-methylpiperidinium can be used.

[0084] The present disclosure also provides a method of preparing a polyfluorene-based anion exchange membrane having a crosslinked structure, the method comprising: (a) a step of dissolving a polyfluorene-based crosslinked copolymer in an organic solvent to obtain a polymer solution; (b) a step of obtaining a membrane by filtering the polymer solution, casting on a glass plate, and then drying it; and (c) a step of converting counter ions to OH ions by immersing the obtained membrane in a 1M NaOH solution. -

[0085] The organic solvent in step (a) can be N-methylpyrrolidone, dimethylacetamide, dimethyl sulfoxide, or dimethylformamide. Specifically, dimethyl sulfoxide can be used.

[0086] Specifically, the concentration of the polymer solution of step (a) can be 2-30 wt%. If the concentration of the polymer solution is less than 2 wt%, the film-forming ability can be reduced. And, if it exceeds 30 wt%, the physical properties of the membrane can be deteriorated due to excessively high viscosity.

[0087] ​Specifically, the drying of step (b) can be performed by slow removal of the organic solvent in an oven at 80-90 °C for 24 hours, followed by complete removal of the organic solvent by heating in a vacuum oven at 120-150 °C for 24 hours.

[0088] Subsequently, a polyfluorene-based anion exchange membrane having a crosslinked structure can be prepared by immersing the polyfluorene-based crosslinked copolymer film obtained from steps (a)-(b) in a 1 M NaOH solution, wherein the halide form (I - form) of the polyfluorene-based crosslinked copolymer film is converted to OH - , CI - , or CO3 2- .

[0089] The present disclosure also provides a membrane electrode assembly for an alkaline fuel cell, which includes a polyfluorene-based anion exchange membrane having a crosslinked structure.

[0090] The present disclosure also provides an alkaline fuel cell, which includes a polyfluorene-based anion exchange membrane having a crosslinked structure.

[0091] The present disclosure also provides a water electrolysis device, which includes a polyfluorene-based anion exchange membrane having a crosslinked structure.

[0092] Hereinafter, embodiments and comparative examples of the present disclosure are described with specific reference to the accompanying drawings.

[0093] [Preparation Example] Preparation of PFTM

[0094] After 9,9'-dimethylfluorene (0.2914 g, 1.5 mmol) was added as a monomer and terphenyl (3.105 g, 13.5 mmol) and 1-methyl-4-piperidone (1.919 mL, 16.5 mmol, 1.1 eq) were added as a comonomer to a two-necked flask, a solution was formed by adding dichloromethane (13 mL) and dissolving the monomers by stirring. After cooling the solution to 1 °C, a viscous solution was obtained by slowly adding a mixture of trifluoroacetic acid (1.8 mL, ~1.5 eq) and triflic acid (12 mL, 9 eq) to the solution and stirring the mixture for 24 hours. A solid form of poly(fluorene-co-terphenyl N-methylpiperidine) PFTM was prepared by precipitating the viscous solution with a 2 M NaOH solution, washing several times with deionized water, and drying in an oven at 80 °C (yield = 95%).

[0095] [Example 1] Preparation of x-PFTP-DP-C5-10 anion exchange membrane

[0096] A 5 wt% polymer solution was obtained by dissolving the PFTM obtained in the preparation example in dimethyl sulfoxide. A mixed solution was obtained by adding 4,4'-(propane-diyl) bis(1-(5-bromopentyl)-1-methylpiperidinium as a crosslinking agent to the polymer solution and stirring at 80°C for 48 hours (crosslinking degree adjusted to 10%). Then, a quaternary piperidinium salt was formed by adding an excess of methyl iodide to the mixed solution and allowing the reaction to proceed for 24 hours. Next, the crosslinked copolymer in solid form was obtained by precipitating the polymer solution with the quaternary piperidinium salt formed with ethyl acetate, followed by washing, and drying in a vacuum oven at 80°C for 24 hours.

[0097] Subsequently, a 4 wt% polymer solution was obtained by dissolving the crosslinked copolymer in dimethyl sulfoxide. The obtained polymer solution was filtered with a 0.45 µm polytetrafluoroethylene (PTFE) filter, and then casted on a glass plate. By drying the casted solution at 90°C for 24 hours to slowly remove dimethyl sulfoxide, and then drying in a vacuum oven at 140°C to completely remove dimethyl sulfoxide, a polyfluorene-based anion exchange membrane having a crosslinked structure (I - form) was obtained, and was named x-PFTP-DP-C5-10.

[0098] After the obtained x-PFTP-DP-C5-10 was separated from the glass plate and cut into a size of 3.5 cm x 3.5 cm, the counter ion was converted to OH - ion by immersing in a 1M NaOH solution for 24 hours.

[0099] [Example 2] Preparation of x-PFTP-DP-C5-20 anion exchange membrane

[0100] A polyfluorene-based anion exchange membrane having a crosslinked structure was prepared in the same manner as in Example 1, except that a mixed solution was obtained by adding 4,4'-(propane-diyl) bis(1-(5-bromopentyl)-1-methylpiperidinium as a crosslinking agent and stirring at 80°C for 48 hours, and the crosslinking degree was adjusted to 20%, and was named x-PFTP-DP-C5-20.

[0101] [Example 3] Preparation of x-PFTP-DP-C10-10 anion exchange membrane

[0102] A polyfluorene-based anion exchange membrane having a crosslinked structure was prepared in the same manner as in Example 1, except that a mixed solution was obtained by adding 4,4'-(propane-diyl) bis(1-(10-bromodecyl)-1-methylpiperidinium as a crosslinking agent and stirring at 80°C for 48 hours, and the crosslinking degree was adjusted to 10%, and was named x-PFTP-DP-C10-10.

[0103] [Comparative Example 1] Preparation of PFTP anion exchange membrane

[0104] At 80℃, a polymer solution was obtained by dissolving PFTM (4 g) obtained in the preparation example in a mixture of dimethyl sulfoxide (40 mL) and trifluoroacetic acid (0.5 mL) as a co-solvent, and then the polymer solution was cooled to room temperature. Subsequently, a quaternary piperidinium salt was formed by adding K2CO3 (2.5 g) and methyl iodide (2 mL, 3 eq) to the polymer solution and reacting for 48 hours. Next, a poly(fluorene-co-triphenylamine N,N-dimethylpiperidinium) copolymer was prepared in a solid form by precipitating the polymer solution with ethyl acetate, then filtering, washing several times with deionized water, and drying in a vacuum oven at 80℃ for 24 hours.

[0105] Subsequently, a 3.2 wt% polymer solution was prepared by dissolving the copolymer in dimethyl sulfoxide. Subsequently, after collecting the polymer solution with a syringe and filtering with a 0.4 μm filter, the resulting transparent solution was cast on a 14 x 21 cm glass plate. A polyfluorene-based anion exchange membrane having no crosslinked structure (named PFTP) was obtained by drying the cast solution in an oven at 85℃ for 24 hours to slowly remove the solvent, and then heating in a vacuum oven at 150℃ for 24 hours to completely remove the solvent. Then, in the same manner as in Example 1, the counter ion was converted to OH - ions by immersing in a 1M NaOH solution for 24 hours.

[0106] [Comparative Example 2] Preparation of x-PFTP-10 anion exchange membrane

[0107] A polyfluorene-based anion exchange membrane having a crosslinked structure was prepared in the same manner as in Example 1, except that 1,6-dibromohexane was used as a crosslinking agent and the crosslinking degree was adjusted to 10%, which was named x-PFTP-10.

[0108] [Test Example]

[0109] The mechanical properties, morphology, ion exchange performance, water absorption, swelling rate, ionic conductivity, fuel cell performance, etc. of the anion exchange membranes prepared in Examples 1-3 and Comparative Examples 1-2 were evaluated and measured by the methods described by the inventors of the present disclosure in Korean Patent Publication No. 10-2021-0071810.

[0110] Figure 1 Results of measuring the dimensional stability of the anion exchange membranes prepared in Examples 1-3 and Comparative Examples 1-2 are shown.

[0111] It can be seen that the anion exchange membranes having a crosslinked structure prepared in Examples 1-3 show an increased ion exchange capacity due to the crosslinking agent containing an ionic group, compared to the anion exchange membranes without a crosslinked structure (e.g. Comparative Example 1). Thus, they show a slightly increased swelling and exhibit a similar water uptake compared to the anion exchange membranes having a conventional crosslinked structure prepared in Comparative Example 2.

[0112] Figure 2 (a) shows the results of measuring the mechanical properties of the anion exchange membranes prepared in Examples 1-3 and Comparative Examples 1-2 in a dry state, Figure 2 (b) shows the results of measuring the mechanical properties of the anion exchange membranes prepared in Examples 1 and 3 and Comparative Example 2 in a humid state.

[0113] It can be seen that the anion exchange membranes having a crosslinked structure have an increased tensile strength and elongation due to their crosslinked structure, compared to the anion exchange membranes without a crosslinked structure, and exhibit excellent mechanical properties even in a humid state.

[0114] Figure 3 The ion conductivities of the anion exchange membranes prepared in Examples 1-3 and Comparative Examples 1-2 are shown, and Table 1 shows the ion-exchange capacity (IEC) and ion conductivity at 80°C.

[0115] [Table 1]

[0116]

[0117] As Figure 3 and shown in Table 1, the ion conductivities of the anion exchange membranes having a crosslinked structure are not decreased due to the relatively high ion-exchange capacity. They exhibit similar or higher ion conductivities under normal fuel cell operating conditions of 60-80°C.

[0118] In addition, since they exhibit excellent HCO3 - conductivity, they are less sensitive to carbonation, which is a major problem of existing anion exchange membranes.

[0119] Figure 4 (a) - (d) show the ion channel size and phase separation degree of the anion exchange membranes prepared in Examples 1-3 and Comparative Example 2.

[0120] Due to the ionic group of the crosslinking agent, their ion channel size is about 1.5 times that of the ion channel size of the anion exchange membranes having a crosslinked structure (Comparative Example 2), and they also show a hydrophilic area of about 40% due to excellent phase separation degree.

[0121] Figure 5 (a) - (d) show the results of evaluating the alkaline stability. Figure 5 (a) shows the residual ion conductivity of the anion exchange membranes prepared in Examples 1 and 3 and Comparative Example 2 after long-term exposure to 80°C in a 1M NaOH solution, Figure 5 (b) shows the residual ion conductivity of the anion exchange membrane prepared in Example 1 after 1200 hours of exposure to 80°C in a 1M NaOH solution, 1 H NMR spectrum; Figure 5 (c) shows the mechanical properties of the anion exchange membranes prepared in Examples 1 and 3 and Comparative Example 2 after 1200 hours of exposure to 80°C in a 1M NaOH solution, and Figure 5 (d) shows the phase separation degree of the anion exchange membranes prepared in Examples 1 and 3 and Comparative Example 2 before and after exposure to 1200 hours of exposure to 80°C in a 1M NaOH solution.

[0122] It can be seen that the ion conductivity remains at 90% or more even after 1200 hours or more of exposure to 80°C in a 1M NaOH solution.

[0123] In addition, in 1 No new peaks were observed in the H NMR analysis, indicating that no degradation occurred in the alkaline environment.

[0124] In addition, the mechanical properties were maintained at about 80%, and although the hydrophilic area decreased slightly, they still exhibited excellent phase separation degrees of over 40%.

[0125] Figure 6 Fuel cell performance of the anion exchange membranes prepared in Examples 1-3 and Comparative Example 2 is shown.

[0126] When PGM (platinum-group metal) is used as an electrode catalyst, they exhibit very excellent performance of 1.8 W cm -2 and 2.5 W cm -2 at 1.3 bar under H2-O2 (at 80°C, 0 bar) atmosphere. They show improved fuel cell performance compared to conventional membranes (2.3 W cm -2 ).

[0127] In addition, since they have excellent HCO3 - conductivity, they exhibit excellent performance of 1.4 W cm -2 even under H2-air atmosphere.

[0128] Accordingly, the anion exchange membrane having a crosslinked structure according to the present disclosure, which has a crosslinked structure including at least one ammonium group, exhibits excellent ion exchange capacity, ionic conductivity, dispersed phase, and mechanical properties, and thus, high power density and durability can be achieved in an anion exchange fuel cell.

Claims

1. A polyfluorene-based crosslinked copolymer selected from copolymers having a crosslinked structure represented by chemical formula 2: Chemical formula 2 ， in, Aryl-1 and aryl-2 are each independently selected from the group consisting of fluorenyl, phenyl, biphenyl, terphenyl, and tetraphenyl, and at least one of aryl-1 and aryl-2 is fluorenyl. R is H or CH3. x represents the degree of crosslinking. The term refers to an ammonium-based crosslinking agent, wherein the ammonium-based crosslinking agent is 4,4'-(propane-diyl)bis(1-(5-bromopentyl)-1-methylpiperidinium or 4,4'-(propane-diyl)bis(1-(10-bromodecyl)-1-methylpiperidinium; and n is an integer from 1 to 15.

2. The polyfluorene-based crosslinked copolymer according to claim 1, wherein, In chemical formula 2, the degree of crosslinking x is 5-20%.

3. A method for preparing the polyfluorene-based crosslinked copolymer according to claim 1, comprising: (I) The step of obtaining a polymer solution by dissolving a piperidine-introduced polyfluorene-based block copolymer in an organic solvent; (II) The step of obtaining a mixture solution by adding an ammonium crosslinking agent solution to the polymer solution and stirring, wherein the ammonium crosslinking agent is 4,4'-(propane-diyl)bis(1-(5-bromopentyl)-1-methylpiperidinium or 4,4'-(propane-diyl)bis(1-(10-bromodecyl)-1-methylpiperidinium; (III) The step of reacting the mixture solution with an excess of methyl iodine to form a quaternary piperidinium salt; and (IV) The step of obtaining a solid polymer by precipitating, washing and drying a polymer solution containing the quaternary piperidine salt.

4. The method for preparing a polyfluorene-based crosslinked copolymer according to claim 3, wherein, The organic solvent in step (I) is N-methylpyrrolidone, dimethylacetamide, dimethyl sulfoxide, or dimethylformamide.

5. A polyfluorene-based anion exchange membrane having a cross-linked structure, which is obtained from the polyfluorene-based cross-linked copolymer according to claim 1 or 2.

6. A method for preparing a polyfluorene-based anion exchange membrane with a cross-linked structure, comprising: (a) The step of obtaining a polymer solution by dissolving the polyfluorene-based crosslinked copolymer of claim 1 or 2 in an organic solvent; (b) The step of filtering the polymer solution, pouring it onto a glass plate, and then drying it to obtain a membrane; (c) By immersing the obtained membrane in a 1M NaOH solution to convert the counterions to OH-. - The steps of ions.

7. The method for preparing a polyfluorene-based anion exchange membrane with a cross-linked structure according to claim 6, wherein, The organic solvent in step (a) is N-methylpyrrolidone, dimethylacetamide, dimethyl sulfoxide, or dimethylformamide.

8. The method for preparing a polyfluorene-based anion exchange membrane with a cross-linked structure according to claim 6, wherein, The concentration of the polymer solution in step (a) is 2-30 wt%.

9. The method for preparing a polyfluorene-based anion exchange membrane with a cross-linked structure according to claim 6, wherein, The drying in step (b) is performed by drying at 80-90°C. The oven was used for 24 hours to slowly remove the organic solvent, and then the oven was heated to 120-150°C. The process involves heating the product in a vacuum oven for 24 hours to completely remove the organic solvent.

10. A membrane electrode assembly for an alkaline fuel cell, comprising a polyfluorene-based anion exchange membrane having a cross-linked structure as described in claim 5.

11. An alkaline fuel cell comprising a polyfluorene-based anion exchange membrane having a cross-linked structure as described in claim 5.

12. A water electrolysis apparatus comprising a polyfluorene-based anion exchange membrane having a cross-linked structure as described in claim 5.

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