Spiro-structure-containing grafted sulfonated polymer membrane as well as preparation method and application thereof

Abstract

The invention discloses a grafted sulfonated polymer membrane containing a spiro structure, a preparation method of the grafted sulfonated polymer membrane and application of the grafted sulfonated polymer membrane in an electrochemical energy storage device, and belongs to the technical field of polymer separation membranes. The membrane material comprises a polymer prepared by a polymerization reaction, and monomers for the polymerization reaction comprise an aromatic monomer with a spiro indane structure and a monomer containing an imidazole group; the polymer is characterized in that in a repetitive unit of the polymer, a side chain containing a sulfonic group is grafted on a nitrogen atom of an imidazole group. The preparation method comprises the following steps: carrying out polymerization reaction on the two monomers to obtain a precursor polymer; then, reacting the precursor polymer with a sulfonating agent to realize grafting of a sulfonic group side chain on an imidazole group nitrogen atom; and finally, preparing the membrane through a solution tape casting method. According to the preparation method, a sulfonic side chain is introduced into a spiro polymer skeleton with high chemical stability in a grafting manner, so that an efficient ion conduction channel is constructed, meanwhile, the structural stability of the membrane material is kept, and the prepared membrane material shows excellent comprehensive performance in application of alkaline aqueous organic flow batteries and the like.

Description

Technical Field

[0001] This invention relates to a microporous polymer with a rigid twisted spirodihydroindene framework, the prepared separator, and an organic flow battery, belonging to the field of flow battery separator technology. Background Technology

[0002] With the development of renewable energy, the reliance on supporting energy storage technologies is also increasing. Among electrochemical energy storage solutions, flow batteries offer unique advantages such as capacity and power decoupling design. In particular, emerging aqueous organic flow batteries (AORFBs), utilizing organic materials as active species, avoid highly polluting and expensive heavy metal ions, and are leading the research direction in this field. Although most AORFBs operate under relatively mild pH and redox conditions, placing lower demands on battery components, they are extremely dependent on membrane materials for ion conduction and active species separation. Therefore, there is an urgent need for membranes with high ionic conductivity and selectivity to improve battery efficiency and stability.

[0003] Currently used ion-conducting membranes are mainly composed of functional polymers, represented by commercial perfluorosulfonic acid polymers and low-cost aromatic alternatives. Despite their mature processing technologies, the tightly packed and entangled backbones of these membranes hinder the formation of continuous ion transport channels. To address this limitation, researchers have explored introducing voids or channels permeable to electrolytes as additional ion transport pathways. While micrometer or submicrometer-scale voids can be formed using porogens or phase inversion methods, nanometer or subnanometer-scale channels have an inherent advantage in selective ion transport. However, achieving such fine-scale channels typically requires precise molecular design of the polymer for loosely packed chains.

[0004] One strategy for creating micropores is to use molecules with rigid and twisted structures as polymer building blocks. This structure prevents the tight packing of polymer chains and creates inter-chain voids. These materials, known as self-porous polymers (PIMs), retain their linear macromolecular characteristics without crosslinking, thus preserving excellent solution processability. Due to these unique advantages, PIM-based membranes show great potential for ion transport. However, the microporous structure only serves as a potential ion transport channel. To achieve rapid ion transport, introducing functional groups into the membrane is essential. While existing radical modification methods can enhance ion transport capabilities, they often result in excessively high water absorption, leading to significant swelling. Furthermore, most PIM membranes are limited by synthetic conditions to possess only a single ion transport function (e.g., sulfonic acid groups), preventing them from fully utilizing the transport potential of multiple current-carrying ions in AORFBs, specifically hydroxide and potassium ions in this work. Therefore, designing ion-conducting membranes with appropriate functions tailored to AORFBs remains a challenge for researchers. Summary of the Invention

[0005] To address the aforementioned issues, this invention innovatively integrates a rigid, twisted spirodihydroindene backbone with imidazole functional groups into a single polymer structure via one-step acid-catalyzed Friedel-Crafts polymerization, simplifying the multi-step synthesis process traditionally required for functionalized PIMs. Figure 1 (a).

[0006] A sulfonated polymer membrane material comprising a polymer obtained by a polymerization reaction, wherein the monomers of the polymerization reaction include an aromatic monomer having a spirocyclic dihydroindene structure and a monomer containing an imidazole group; characterized in that a side chain containing a sulfonic acid group is grafted onto the nitrogen atom of the imidazole group in the repeating unit of the polymer.

[0007] The aromatic monomer having a spirocyclic dihydroindene structure is 6,6'-dimethoxy-3,3,3',3'-tetramethyl-1,1'-spirocyclic dihydroindene; the monomer containing an imidazole group is 1H-imidazol-4-carboxaldehyde.

[0008] The structure of the sulfonic acid-containing side chain is shown in Formula I: -R-SO3⁻M⁺ (Formula I), where R is a C1-C6 alkylene group; M⁺ is a hydrogen ion, an alkali metal ion, or an ammonium ion; preferably, the sulfonic acid-containing side chain is introduced by sodium 3-bromopropanesulfonate.

[0009] The thickness of the membrane material is 20-150µm; preferably, the thickness is 50-70µm.

[0010] A method for preparing the sulfonated polymer film material, characterized by comprising the following steps:

[0011] a) An aromatic monomer with a spirocyclic dihydroindene structure is polymerized with a monomer containing an imidazole group in the presence of an acid catalyst to obtain a precursor polymer;

[0012] b) The precursor polymer is reacted with a sulfonating agent containing a sulfonic acid group under alkaline conditions, so that the sulfonating agent is grafted onto the nitrogen atom of the imidazole group of the precursor polymer to obtain a sulfonated polymer.

[0013] c) The sulfonated polymer is dissolved in a solvent to prepare a casting solution, and a film material is prepared by solution casting.

[0014] The aromatic monomer with a spirocyclic dihydroindene structure mentioned in step a) is 6,6'-dimethoxy-3,3,3',3'-tetramethyl-1,1'-spirocyclic dihydroindene; the monomer containing an imidazole group is 1H-imidazolium-4-carboxaldehyde; and the acid catalyst is methanesulfonic acid or other organic sulfonic acids.

[0015] In step a): the molar ratio of the monomer containing the imidazole group to the aromatic monomer having the spirocyclic dihydroindene structure is 1.0:1-1.5:1; the amount of methanesulfonic acid used is 500-1000 mL per mole of the aromatic monomer having the spirocyclic dihydroindene structure.

[0016] The sulfonating agent mentioned in step b) is a compound with the chemical formula XR-SO3⁻M⁺, where X is a halogen, R is a C1-C6 alkylene group, and M⁺ is an alkali metal ion; the alkaline conditions are achieved by adding potassium hydroxide or other alkali metal hydroxides.

[0017] The molar ratio of the sulfonating agent to the imidazole group in the precursor polymer in step b) is 0.40-1.20; preferably, the molar ratio is 0.60-1.00.

[0018] The reaction temperature in step b) is 60-100℃ and the reaction time is 4-12 hours; preferably, the reaction temperature is 75-85℃ and the reaction time is 5-8 hours.

[0019] It also includes a step of pretreating the membrane material obtained in step c), wherein the pretreating includes immersing it in an acidic solution and an alkaline solution in sequence.

[0020] The application of the sulfonated polymer membrane material in electrochemical energy storage devices.

[0021] The electrochemical energy storage device is an aqueous organic flow battery.

[0022] The aqueous organic flow battery is an alkaline aqueous organic flow battery, and the ion carriers in its electrolyte include OH⁻ and K⁺.

[0023] The beneficial effects of this invention are:

[0024] In the PIMs structure of this invention, the rigid, twisted framework prevents the close packing of polymer chain segments, thereby creating interconnected intrinsic microporous channels and laying the structural foundation for rapid ion transport. Simultaneously, the imidazole groups anchored in the polymer matrix can strongly absorb electrolyte components, fill inter-chain gaps, and activate the rapid microporous ion transport channels. Figure 1 (b). To further optimize ion transport, a sulfonic acid group was covalently grafted onto an imidazole group via a mild nucleophilic addition reaction. Figure 1 (c), which weakens the electrolyte absorption capacity of the imidazole group while establishing an independent cation conduction magnetic track. In this bifunctionalized membrane, small OH⁻ ions rapidly shuttle through microporous fast channels, while K⁺ ions are efficiently guided along the sulfonic acid magnetic track through electrostatic interactions, and the dual-track transport system operates in parallel. Figure 1 (d). Furthermore, this dual-track design balances imidazole-induced ion solvation and sulfonic acid-induced ion exchange, thus avoiding the excessively large transport channels typically required by monofunctionalized membranes, thereby maintaining high selectivity for redox active species. The resulting membrane exhibits superior electrochemical performance in AORFBs, outperforming the commercial Nafion 212 benchmark and comparable to the performance of state-of-the-art microporous similar materials reported to date. Attached Figure Description

[0025] Figure 1 Schematic diagram of synthesis steps and ion transport mechanism. a) Synthesis steps of PSI. b) Ion transport within PSI. c) Synthesis steps of SPSI. d) Ion transport within SPSI.

[0026] Figure 2 Fourier transform infrared spectra of polysilimidazole (PSI), potassium hydroxide-treated polysilimidazole (PSI-KOH), and sulfonated polysilimidazole (SPSI) films.

[0027] Figure 3 Atomic force microscopy phase diagrams of polysilimidazole (PSI) membranes before (a) and after (b) treatment.

[0028] Figure 4Characterization of PSI membranes and performance of BHPC flow batteries. a) CO2 adsorption of PSI and PBIm membranes. b) Simulated amorphous cells of PBIm (left) and PSI (right). Gray areas represent polymer chains, and blue areas represent free volumes. c) Alkali uptake and swelling ratios of PSI, PBIm, and Nafion 212 (Nf) membranes. d) Ionic conductivity of PSI, PBIm, and Nf membranes. e) Ferrocyanide and BHPC permeability of PSI, PBIm, and Nf membranes. f) Efficiency of PBIm, PSI, and Nf membranes at different current densities. g) Polarization curves of PBIm, PSI, and Nf membranes. h) Power density of PBIm, PSI, and Nf membranes.

[0029] Figure 5 Ion transport characteristics of SPSI membranes. a) Alkali absorption and swelling properties of SPSI, PSI, and Nf membranes. b) Ionic conductivity of SPSI, PSI, and Nf membranes. c) Voltage-current curves and ion transport numbers of SPSI, PSI, and Nf membranes. d) K+ and OH- permeability of SPSI, PSI, and Nf membranes. e) Alkali absorption rate and swelling ratio of SPSI-x membranes. f) Ionic conductivity of SPSI-x membranes. g) Ferrocyanide and BHPC permeability of SPSI-x membranes. h) Ferrocyanide and BHPC permeability of SPSI, PSI, and Nf membranes.

[0030] Figure 6 Performance of SPSI membranes in BHPC flow batteries. a) Efficiency of SPSI, PSI, and Nf membranes at different current densities. b) Polarization curves of SPSI, PSI, and Nf membranes. c) Power density of SPSI, PSI, and Nf membranes. d) Efficiency of SPSI, PSI, and Nf membranes in long-term cycling tests. e) Capacity retention of SPSI, PSI, and Nf membranes in long-term cycling tests. f) Comparison of cycle duration as a function of current density for membranes used in AORFBs with various active species, as reported in the literature. Orange squares and triangles represent alkaline organic flow batteries (anthraquinone-based and phenazine-based active species, respectively). Purple circles and rhombuses represent neutral organic flow batteries (Tempo-based and ferrocene-based active species, respectively). Detailed Implementation

[0031] Example 1 Synthesis of polyspirodihydroindeneimidazole-formaldehyde (PSI)

[0032] Poly(spirodihydroindeneimidazolium-formaldehyde) (PSI) was synthesized via acid-catalyzed Friedel-Crafts polymerization. This reaction typically involves the protonation of an aldehyde or ketone in a strongly acidic environment. The protonated carbonyl group is then attacked by an aromatic ring, generating an intermediate with a hydroxyl group. Under acidic conditions, the hydroxyl group further protonates and reacts with another aromatic ring, linking the carbonyl group to both aromatic rings and completing the construction of a linear polymer chain. In this work, the aromatic monomer SBI, possessing two aromatic rings separated by multiple aliphatic carbons and substituted with electron-donating groups, endowed SBI with enhanced activity in Friedel-Crafts polymerization. On the other hand, 1H-imidazolium-4-carboxaldehyde (ImCa) was chosen as the aldehyde monomer. Under acidic conditions, the protonation of the imidazolium ring produces a strong electron-withdrawing effect, increasing the electrophilicity of the carbonyl group. These two factors enabled the reaction to proceed under MSA catalysis, rather than the more common and potent trifluoromethanesulfonic acid catalysis in Friedel-Crafts reactions.

[0033] In a 500 mL three-necked flask equipped with a mechanical stirrer, 50 mmol of 6,6'-dimethoxy-3,3,3',3'-tetramethyl-1,1'-spirocyclic dihydroindene (SBI) and 60 mmol of 1H-imidazolium-4-carboxaldehyde (ImCa) were added, followed by 60 mL of dichloromethane (DCM). The mixture was stirred for several minutes to ensure complete dissolution of SBI. Then, 35 mL of methanesulfonic acid (MSA) was added, and the reaction continued at room temperature. As the monomer was consumed and the polymer was formed, the reaction system gradually became homogeneous. After stirring for about 1 h, the viscous red solution was poured into an aqueous potassium carbonate solution. The resulting lumpy white precipitate was repeatedly washed with water and dried to obtain a white or pale yellow polymer, i.e., PSI.

[0034] The synthesis of monomer 6,6'-dimethoxy-3,3,3',3'-tetramethyl-1,1'-spirodidihydroindene (SBI) was carried out with reference to existing techniques (MA Abdulhamid, S.-H. Park, H. Vovusha, FH Akhtar, KC Ng, U. Schwingenschlögl, G. Szekely, J. Mater. Chem. A 2020, 8, 24445-24454, https: / / doi.org / 10.1039 / D0TA08194A, [3] S. Zhou, J. Guan, Z. Li, Q. Zhang, J. Zheng, S. Li, S. Zhang, Macromol. 2021, 54, 6543-6551, https: / / doi.org / 10.1021 / acs.macromol.1c00468.);

[0035] Example 2 Synthesis of sulfonated polyspirodihydroindeneimidazole-formaldehyde (SPSI)

[0036] By utilizing the reaction of imidazole with haloalkanes, alkyl side chains with sulfonic acid groups are grafted onto imidazole nitrogen atoms.

[0037] Sodium 3-bromopropanesulfonate was chosen as the sulfonating agent, and potassium hydroxide as the base catalyst. This combination achieved stable grafting of the sulfonic acid side chain. The specific operation process is as follows: First, a certain amount of PSI was dissolved in DMSO. After complete dissolution, 15 M KOH aqueous solution was added, where the mass of KOH was equivalent to 4 times the mass of the polymer unit. The mixture was stirred for several hours until a red suspension was formed, indicating that the protons of PSI had been successfully removed. Then, a certain amount of 3-bromopropanesulfonate was added. The reaction mixture was heated to 80 °C and stirred for 6 h. Subsequently, an appropriate amount of acetic acid was added to neutralize KOH and terminate the reaction. The reaction solution was poured into ethyl acetate, the precipitate was collected, washed several times with water, and dried to obtain an orange-yellow polymer, namely sulfonated polyspirodihydroindene imidazole-formaldehyde (SPSI). By changing the amount of 3-bromopropanesulfonate added, a series of SPSI-x polymers were prepared, where x represents the molecular ratio of 3-bromopropanesulfonate to the imidazole group in PSI. Specifically, five variants with x values ​​of 0.40, 0.60, 0.80, 1.00, and 1.20 were prepared to investigate the effect of sulfonation degree on material properties. The actual grafting rate based on the amount of reagent added was approximately 90% of the theoretical value. However, due to signal overlap in the NMR spectra of these highly substituted polymers, accurate quantification of the grafting degree is challenging. Unless otherwise stated, “SPSI” film in this paper refers to SPSI-0.80.

[0038] Comparative Example 1: Synthesis of Polybibenzamide-Formaldehyde (PBIm)

[0039] To investigate the influence of the backbone structure, poly(bibenzylimidazole-formaldehyde) (PBIm) was synthesized as a control polymer. The only difference between PBIm and PSI is that its backbone unit changes from a spirodihydroindene structure to a linear biphenyl structure. Figure 2 a).

[0040] Unlike PSI, the synthesis of PBIm uses less reactive biphenyl as a monomer and requires catalysis by trifluoromethanesulfonic acid in an ice bath. Specifically, 16 mmol of biphenyl, 20 mmol of ImCa, and 20 mL of DCM were added to a 100 mL three-necked flask equipped with a mechanical stirrer. After stirring for a few minutes, 10 mL of trifluoromethanesulfonic acid was slowly added dropwise to the reaction system under ice bath conditions. The addition was completed within approximately 10 minutes, and the reaction was maintained under ice bath conditions. The resulting polymer was not completely dissolved in the mixed solvent and partially precipitated as blue lumps. After approximately 1 hour, the solution and solid were completely poured into deionized water and washed repeatedly with water. The resulting solid, after drying, yielded a white polymer, identified as PBIm.

[0041] Example 3 Membrane preparation and membrane pretreatment

[0042] All membranes were prepared by dissolving the corresponding polymer in dimethyl sulfoxide (DMSO) to prepare a casting solution, which was then poured into a flat-bottomed petri dish to allow the solvent to evaporate. All polymers were completely soluble in DMSO at 80°C. Large polymer particles were pulverized using a grinder to accelerate the dissolution process. If residual insoluble matter, such as paper scraps and gel, remained in the casting solution, it was filtered using a polytetrafluoroethylene (PTFE) filter. The thickness of the resulting membrane could be freely adjusted by varying the mass of polymer added. In this work, the prepared membrane thickness ranged from 50 to 70 micrometers. The membrane was completely immersed in phosphoric acid for 24 hours. Then, the surface acid was washed away with water. Next, the membrane was immersed in 1 M KOH solution for 24 hours. Afterward, the membrane was removed and the surface solution was wiped off. At this point, the membrane was ready for ion transport rate, ion permeation, and battery testing. The same procedure was used when testing the membrane's acid and alkali absorption capacity.

[0043] CO2 adsorption tests showed that PSI exhibited a higher adsorption capacity and micropore volume (19.1 cm³) than PBIm. 3 / g vs2.7cm 3 / g)( Figure 2 a). This indicates that the spirodihydroindene structure increases the accessible microporous regions between polymer segments. The inclusion of water molecules and hydroxide ions alters the membrane structure, which in... Figure 2This was confirmed in the Fourier transform infrared (FTIR) spectrum, which showed the formation of large hydrophobic polymer chain regions and hydrophilic imidazole-electrolyte regions, such as... Figure 3 The atomic force microscopy (AFM) images are shown. The concentrated imidazole-electrolyte regions within the micropores promote the formation of OH⁻ transport “fast channels,” resulting in the PSI having an ionic conductivity that is almost three times that of PBIm and also far superior to that of commercial Nafion 212 membranes.

[0044] The method for testing ionic conductivity is as follows:

[0045] The ion transport performance of the membrane was evaluated using electrochemical impedance spectroscopy (EIS). First, the surface of the titanium sheet was finely sanded to ensure a smooth, flawless surface. The membrane sample, immersed in 1 M KOH solution, was then removed, sandwiched between two titanium plates, and secured with a button battery clip to ensure test stability. Finally, EIS measurements were performed using an electrochemical workstation. To eliminate the interfacial impedance between the titanium plate and the membrane, the ion conductivity of single-layer, double-layer, and triple-layer stacked membranes was measured, and curves were plotted to calculate the impedance of the single-layer membrane. The test frequency range was set at 10 Hz. 3 Hz-10 6 Between Hz, the AC signal amplitude remains at 5 millivolts.

[0046] Ion mobility number test method:

[0047] The membrane was clamped in an H-type diffusion cell to measure the ion transport number. 1M KOH and 3M KOH solutions were injected into both sides of the diffusion cell, respectively. Two Ag / AgCl reference electrodes filled with saturated KCl solution and two salt bridges filled with saturated KCl solution were used to measure the open-circuit voltage of the device. The ion transport number can be calculated using the following formula.

[0048]

[0049] R, T, F, t+, t- and These are the gas constant, temperature, Faraday constant, and cation (K). The transport number of the ion (OH⁻) or anion and the concentration gradient.

[0050] The ion permeability test method is as follows:

[0051] The membrane's ion permeability was tested using an H-type diffusion cell. During the test, the membrane was sandwiched in the center of the diffusion cell, with an effective permeable area of ​​1.77 square centimeters. For [Fe(CN)6]4− ions, 50 mL of 0.2 M K4Fe(CN)6 solution (dissolved in 1 M KOH) and 50 mL of blank 1 M KOH solution were injected into the supply and receiving chambers of the diffusion cell, respectively. The diffusion cell was then placed on a portable small magnetic stirrer for stirring. Every 24 hours during the test, 4 mL of solution was sampled from the receiving chamber, and its concentration was quantified using a UV-Vis spectrophotometer (PerkinElmer Lambda 950). The membrane permeability coefficient (P) was then calculated.

[0052]

[0053] Where V is the solution volume (50 mL), and A and L represent the effective area (cm⁻²) and thickness (cm) of the membrane, respectively. CA is the supply side [Fe(CN)₆]. 4 The concentration of [Fe(CN)6] was kept constant (0.2 M) during the test. CB(t) / t represents the receiver-side [Fe(CN)6]. 4 The concentration-to-diffusion-time ratio was derived from experimental data. For the testing of BHPC²⁻ ions, the procedure was the same except that 50 ml of a 1M KOH solution containing 0.1 MBHPC and 50 ml of a blank 1M KOH solution were injected into the supply and receiving sides of the diffusion cell, respectively. For K⁺ and OH⁻ ions, 50 ml of a mixed solution (1M KCl, 1M KOH) and 50 ml of deionized water were injected into the supply and receiving sides of the diffusion cell, respectively, and the ion concentration at the receiving side was monitored in real time using a conductivity meter or pH meter.

[0054] While the absorption of large amounts of electrolytes enhances ionic conductivity, the cost is excessive swelling. Figure 4 This significantly reduces the membrane's ability to block larger redox active ions in the flow battery, namely ferrocyanide and 5-hydroxy-benzo[a]phenazine-carboxylic acid (BHPC) ions. Similarly, the PBIm membrane, which is also strongly dependent on electrolyte absorption, has an active material permeability only slightly lower than that of the PSI membrane. Accordingly, in flow battery tests, although the PSI membrane showed lower energy efficiency (200 mA cm⁻¹), -2 (72% vs. 55%) and peak power density (407 mW cm⁻¹) -2 vs. 250 mW cm -2 In terms of ) it has significant advantages over Nafion 212 ( Figure 4While the PSI membrane exhibits high ionic conductivity (fh), its discharge capacity rapidly declines during operation. After approximately 250 cycles, the PSI membrane retains only 70% of its initial capacity, whereas the benchmark Nafion 212 membrane retains over 80% of its capacity after 1000 cycles. These results demonstrate the challenge of balancing high ionic conductivity with stable operation in monorail imidazole functionalized membranes, highlighting the need for further improvements in ion transport mechanisms.

[0055] A dual-track transport design was further proposed, which involves introducing additional sulfonic acid side chains as magnetic tracks to fully unleash the potential of K⁺ as a current-carrying ion. Figure 1 The addition of the imidazole-dominated single-track ion transport pathway (d) supplements the existing ion transport pathway. Simultaneously, the base absorption capacity of the imidazole group decreases with hydrogen substitution, thus optimizing the swelling behavior. Based on these considerations, poly(spirodihydroindeneimidazolium-formaldehyde) (SPSI) was prepared via a mild nucleophilic substitution reaction between the imidazole group and a sulfonic acid-containing haloalkane. The synthetic route is as follows: Figure 1 As shown in Figure c, FTIR and NMR spectra confirmed the successful synthesis of SPSI.

[0056] Unlike the original PSI membrane, which relies on alkaline electrolyte uptake, the optimized SPSI membrane maintains a consistently low contact angle regardless of whether it has undergone alkali treatment. In solution absorption tests, the SPSI membrane's absorption of potassium hydroxide solution was significantly lower than that of the PSI membrane, with the absorption rate decreasing from 70% to 29%. Figure 5 (a). The results showed that the swelling ratio of SPSI in 1 M potassium hydroxide solution was 10.8%, significantly lower than that of PSI (28.3%), and even lower than that of the commercial Nafion 212 membrane (13.0%). This indicates that the absorbed water molecules mainly fill the micropores without excessively disrupting the polymer chain arrangement. The low swelling ratio, resulting from moderate electrolyte absorption capacity and a rigid polymer backbone, ensures excellent dimensional stability under operating conditions. Even at relatively low electrolyte absorption levels, high ionic conductivity can still be achieved through the significant promoting effect of ion transport by sulfonic acid groups. Figure 5 (b). Ionic conductivity measurements by electrochemical impedance spectroscopy (EIS) showed that the SPSI membrane's ionic conductivity was only 15% lower than the PSI membrane and approximately 2.5 times that of the Nafion 212 membrane, maintaining a significant advantage. This rapid ion transport at low electrolyte absorption levels indicates that the SPSI membrane possesses a different ion transport mechanism than the PSI membrane. Therefore, we investigated the contributions of potassium ions and hydroxide ions to the overall ion transport (b). Figure 5(c). The potassium ion transference number of the PSI membrane is 0.065, indicating that PSI mainly provides a channel for hydroxide ions. In contrast, the potassium ion transference number of the SPSI membrane is as high as 0.440, indicating that after the introduction of sulfonic acid side chains, potassium ions and hydroxide ions participate in ion transport to almost equal extent, which is a hallmark of efficient dual-track transport. The potassium ion permeability of the SPSI membrane is an order of magnitude higher than that of the PSI membrane, which compensates for the decrease in hydroxide ion permeability caused by the lower electrolyte absorption rate of the SPSI membrane. Figure 5 (d). Thanks to the synergistic dual-track strategy, the SPSI membrane achieves efficient transport of potassium and hydroxide ions, with permeabilities of 1.1 × 10⁻⁻⁶. 6 cm² / s and 1.67 × 10⁻ 6 cm² / s, significantly higher than Nafion 212 membrane (7.25 × 10⁻⁻⁶ cm² / s). 7 cm² / s and 8.47 × 10⁻ 7 cm² / s).

[0057] BHPC Battery Performance Testing

[0058] The method is as follows: The performance of each BHPC single cell was evaluated using a custom-designed battery device. This device consisted of a membrane with an effective area of ​​2 × 2 cm², two activated graphite felt electrodes, two graphite plates, two end plates, two current collectors, and several gaskets. The positive electrode electrolyte was an aqueous solution containing 0.2 M potassium ferrocyanide and 1 M potassium hydroxide, and the negative electrode electrolyte was a 10 mL solution containing 0.1 MBHPC and 1 M potassium hydroxide. The charge and discharge processes of the flow battery were performed and monitored using a battery testing system (NEWARE). To obtain polarization profiles, the battery was first charged to 100% SOC and then discharged for short periods at different current densities to obtain the discharge voltage. The power density profile at 100% SOC was further derived from the current-voltage profile. During the variable current test, the current density was varied from 80 mA / cm². - ² Gradually increase to 400mAcm - ². Meanwhile, at 200 mAcm - Long-term cycling tests were conducted at a constant current density of 2. The cutoff voltages for the two tests were set to 1.7V and 0.4V, respectively. The overall performance of the battery was evaluated using its coulombic efficiency (CE) and energy efficiency (EE).

[0059] Vanadium Redox Flow Battery (VFB) Performance Testing

[0060] The method is as follows: The same battery setup as used in the BHPC test was employed. The positive and negative electrolytes were 1.5 MVO²⁺ / 3 MH₂SO₄ and 1.5 MV³⁺ / 3 MH₂SO₄, respectively. In the variable current test, the current density was 80 mA / cm². -² to 200mAcm - The current density varies between 2 and 2, while the current density used in the cyclic test is 200 mA / cm². - ², the cutoff voltages for the two tests were 1.7V and 0.8V, respectively.

[0061] Thanks to its rapid and highly selective ion transport properties, SPSI membranes exhibit superior performance in AORFBs. Batteries assembled with SPSI membranes can be charged and discharged at current densities up to 400 mA cm⁻², while Nafion 212 membranes, limited by their insufficient ionic conductivity, cannot exceed 300 mA cm⁻². -2 Under the conditions of operation ( Figure 6 (a) In terms of energy efficiency (EE), the SPSI membrane exhibits a significant advantage at 200 mA cm⁻¹. -2 At current densities of 600 mA cm⁻², an EE of 66.3% was achieved, which is more than 11% higher than that of Nafion 212 film (55.1%). The polarization curve of the SPSI film shows extremely small voltage decay, only 0.02-0.025 V per 20 mA cm⁻², and maintains a discharge voltage of 0.89 V even at extremely high current densities of 400 mA cm⁻². Figure 6 (b) The high discharge voltage enabled the SPSI-based battery to achieve 379 mW / cm². -2 The power density significantly exceeds that of the Nafion 212 membrane (250 mW cm⁻²). Figure 6 (c). Furthermore, the SPSI membrane exhibits excellent alkali stability under AORFB operating conditions, combined with its superior ability to block the shuttle movement of active species, ensuring stable long-term cycling performance. Figure 6 (d, e). At 200 mA cm -2 At the specified current density, the SPSI-based cell operated for 2200 cycles (15 days) without electrolyte replacement, exhibiting a slow capacity decay rate of only 0.0021% per cycle (0.278% per day). In contrast, the Nafion212-based cell exhibited a capacity decay of 0.02% per cycle (3.53% per day), with its capacity decreasing to 70% of its initial value after approximately 1500 cycles, accompanied by a decrease in cell efficiency and an increase in capacity fluctuation. The superior performance of the SPSI membrane not only far exceeds that of the commercial benchmark Nafion212 membrane but is also comparable to advanced polymer and microporous membranes reported in the literature for AORFBs. Figure 6 f).

Claims

1. A sulfonated polymer film material comprising a polymer obtained by a polymerization reaction, wherein the monomers of the polymerization reaction include an aromatic monomer having a spirocyclic dihydroindene structure and a monomer containing an imidazole group; characterized in that, The nitrogen atom of the imidazole group in the repeating unit of the polymer is grafted with a side chain containing a sulfonic acid group.

2. The membrane material according to claim 1, characterized in that, The aromatic monomer having a spirocyclic dihydroindene structure is 6,6'-dimethoxy-3,3,3',3'-tetramethyl-1,1'-spirocyclic dihydroindene; the monomer containing an imidazole group is 1H-imidazol-4-carboxaldehyde.

3. The membrane material according to claim 1 or 2, characterized in that, The structure of the sulfonic acid-containing side chain is shown in Formula I: -R-SO3⁻M⁺ (Formula I), where R is a C1-C6 alkylene group; M⁺ is a hydrogen ion, an alkali metal ion, or an ammonium ion; preferably, the sulfonic acid-containing side chain is introduced by sodium 3-bromopropanesulfonate.

4. A method for preparing the sulfonated polymer film material according to any one of claims 1-3, characterized in that, Includes the following steps: a) An aromatic monomer with a spirocyclic dihydroindene structure is polymerized with a monomer containing an imidazole group in the presence of an acid catalyst to obtain a precursor polymer; b) The precursor polymer is reacted with a sulfonating agent containing a sulfonic acid group under alkaline conditions, so that the sulfonating agent is grafted onto the nitrogen atom of the imidazole group of the precursor polymer to obtain a sulfonated polymer. c) The sulfonated polymer is dissolved in a solvent to prepare a casting solution, and a film material is prepared by solution casting.

5. The method according to claim 4, characterized in that, The aromatic monomer with a spirocyclic dihydroindene structure mentioned in step a) is 6,6'-dimethoxy-3,3,3',3'-tetramethyl-1,1'-spirocyclic dihydroindene; the monomer containing an imidazole group is 1H-imidazolium-4-carboxaldehyde; and the acid catalyst is methanesulfonic acid or other organic sulfonic acids.

6. The method according to claim 6, characterized in that, In step a): the molar ratio of the monomer containing the imidazole group to the aromatic monomer having the spirocyclic dihydroindene structure is 1.0:1-1.5:1; the amount of methanesulfonic acid used is 500-1000 mL per mole of the aromatic monomer having the spirocyclic dihydroindene structure.

7. The method according to claim 5, characterized in that, The sulfonating agent mentioned in step b) is a compound with the chemical formula XR-SO3⁻M⁺, where X is a halogen, R is a C1-C6 alkylene group, and M⁺ is an alkali metal ion; the alkaline conditions are achieved by adding potassium hydroxide or other alkali metal hydroxides. The molar ratio of the sulfonating agent to the imidazole group in the precursor polymer in step b) is 0.40-1.20; preferably, the molar ratio is 0.60-1.

00. The reaction temperature in step b) is 60-100℃ and the reaction time is 4-12 hours; preferably, the reaction temperature is 75-85℃ and the reaction time is 5-8 hours.

8. The method according to claim 5, characterized in that, It also includes a step of pretreating the membrane material obtained in step c), wherein the pretreating includes immersing it in an acidic solution and an alkaline solution in sequence.

9. The application of the sulfonated polymer membrane material according to any one of claims 1-3 in an electrochemical energy storage device.

10. The application according to claim 9, characterized in that, The electrochemical energy storage device is an aqueous organic flow battery.

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