Solid-state radiation grafting method of phosphonic acid-sulfonic acid bifunctional ionic membrane

Abstract

The invention discloses a solid-state radiation grafting method of a phosphonic acid-sulfonic acid bifunctional ionic membrane, which realizes solvent-free molecular-level compounding and formation of a gradient structure through raw material pretreatment, solid-state radiation grafting, molten-state gradient self-assembly and post-treatment, solves the problems of phase separation, layering, poor high-temperature stability and the like of a traditional membrane, and is suitable for the field of vanadium redox flow batteries. The preparation bottleneck of the traditional bifunctional ionic membrane is broken through through a solid radiation grafting and thermally induced gradient self-assembly process: a solvent-free system avoids phosphonic acid group agglomeration (the grafting efficiency is greater than 92%), a gradient structure solves the interlayer layering problem (the peel strength is greater than 3N / mm), the swelling rate at 80 DEG C is less than or equal to 6%, the cycle life exceeds 12000 times, the ion selectivity reaches 228, and the surface resistance is reduced to 0.83 ohm cm. Meanwhile, the reaction time is shortened, the cost is reduced, and a high-performance membrane material is provided for the fields of vanadium redox flow batteries and the like.

Description

Technical Field

[0001] This invention relates to the field of ion exchange membrane technology, and more particularly to a solid-state radiation grafting method for a phosphonic acid-sulfonic acid bifunctional ion exchange membrane. Background Technology

[0002] The preparation of traditional bifunctional ion exchange membranes (such as phosphonic acid-sulfonic acid composite membranes) mostly relies on solution phase modification or physical composite processes. The aim is to improve the ion selectivity and stability of the membrane by introducing two functional groups. They are widely used in fields such as vanadium redox flow batteries. The core idea is to combine sulfonic acid groups and phosphonic acid groups into the membrane material through physical mixing or chemical modification to achieve selective conduction of specific ions.

[0003] Existing membrane preparation methods mainly include: ① Solution phase modification method (such as patent CN119240998A): Sulfonic acid resin is dissolved and phosphonic acid monomer is added, and composite is achieved by casting; ② Lamination composite process (such as patent CN120237252A): Sulfonic acid membrane and phosphonic acid membrane are physically combined by lamination technology; ③ Commercial membrane preparation method (such as DuPont DFM-100): Bifunctional membrane is prepared by combining conventional polymerization with post-modification.

[0004] Existing methods have significant drawbacks: ① Solution phase modification methods are prone to phosphonic acid group aggregation (forming particles >1μm) due to the presence of solvent, and solvent residues cause membrane embrittlement (elongation at break <80%), with a grafting efficiency of only 65%; ② The interfacial bonding of composite membranes obtained by lamination is weak, with an interlayer peel strength of only 0.5N / mm, and delamination is easy after battery cycling; ③ High-temperature stability is poor. For example, the commercial membrane DuPont DFM-100 has a swelling rate >15% in electrolyte at 80°C, and its capacity decays by 18.7% after 500 cycles, which is difficult to meet the requirements of long-life applications. In order to solve the above shortcomings, we propose a solid-state radiation grafting method for phosphonic acid-sulfonic acid bifunctional ion exchange membranes. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art by proposing a solid-state radiation grafting method for phosphonic acid-sulfonic acid bifunctional ion membranes.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A solid-state radiation grafting method for a phosphonic acid-sulfonic acid bifunctional ion exchange membrane includes the following steps: S1: Raw material pretreatment: Take 80~120 mesh regenerated sulfonic acid resin powder and vacuum dry it for 4 hours at 80℃ and vacuum degree ≤-0.09MPa; add vinylphosphonic acid monomer at a mass ratio of 5~10wt%, and stir it for 20 minutes in a high-speed mixer at 300rpm in an environment with humidity <30% to obtain a uniform mixed system. S2: Solid-state radiation grafting: The mixed system was transferred to a sealed reactor and purged three times with high-purity N2 (purity ≥99.999%) to control the oxygen content in the reactor to <100ppm; an infrared radiation source with a wavelength of 3.5±0.2μm was used to irradiate the mixture at a radiation intensity of 1.5W / cm² for 30~40min to obtain a grafting product with a grafting efficiency >92%. S3: Melt-state gradient self-assembly: The grafted product is introduced into a three-chamber film-forming device and sequentially passes through: ①Lower radiation zone: Heated to 280℃ at a heating rate of 10℃ / min to melt, driving phosphonic acid groups to migrate into the membrane interior; ② Middle layer constant pressure zone: Maintain pressure of 0.15 kPa and keep warm for 15 min to form a phosphonic acid enriched inner layer with a thickness of 10~15 μm; ③ Upper slow cooling zone: Cooled to 80℃ at a cooling rate of ≤5℃ / min, which promotes the enrichment and solidification of sulfonic acid groups on the membrane surface, forming a gradient structure membrane; S4: Post-treatment and characterization: The molded membrane was cut into standard samples and soaked in deionized water at 25±1℃ for 24h to remove ungrafted monomers; the TOF-SIMS gradient slope of the membrane cross section was tested to be ≥0.8μm⁻¹, the electrolyte swelling rate at 80℃ was ≤6%, and the ion selectivity was >200, thus obtaining a phosphonic acid inner layer / sulfonic acid outer layer gradient structure ion membrane.

[0007] Preferably, the mesh size of the regenerated sulfonic acid resin powder in S1 is 80-120 mesh.

[0008] Preferably, the vacuum drying conditions in S1 are: temperature 80℃, vacuum degree ≤ -0.09MPa, and drying time 4h.

[0009] Preferably, the conditions for monomer mixing in S1 are: vinylphosphonic acid monomer addition of 5-10 wt%, high-speed mixer speed of 300 rpm, stirring time of 20 min, and ambient humidity of <30%.

[0010] Preferably, the infrared radiation parameters in S2 are: wavelength 3.5±0.2μm, radiation intensity 1.5W / cm², and reaction time 30~40min; the oxygen content in the reactor after high-purity N2 replacement is <100ppm.

[0011] Preferably, the grafting efficiency of the grafted product in S2 is determined by elemental analysis and the grafting efficiency is >92%.

[0012] Preferably, the melting conditions in the lower radiation zone of S3 are a heating rate of 10℃ / min and a melting temperature of 280℃.

[0013] Preferably, the conditions in the constant pressure zone of the middle layer in S3 are: pressure 0.15 kPa, heat preservation time 15 min, and phosphonic acid enriched inner layer thickness 10~15 μm.

[0014] Preferably, the cooling conditions in the upper slow cooling zone of S3 are a cooling rate ≤ 5℃ / min and a cooling endpoint temperature of 80℃.

[0015] Preferably, the post-treatment conditions in S4 are: deionized water temperature 25±1℃ and soaking time 24h.

[0016] Compared with the prior art, the beneficial effects of the present invention are: In this invention, a "solid-state radiation grafting" technique is used to achieve molecular-level bonding between vinylphosphonic acid monomers and sulfonic acid resins under solvent-free conditions. This avoids the defects of phosphonic acid groups agglomerated (>1μm particles) in traditional solution-phase modification methods, increases the grafting efficiency to over 92% (compared to only 65% ​​in traditional solution methods), and reduces the monomer residue to <0.5wt% (compared to >7wt% in traditional methods), significantly improving the structural uniformity of the membrane. By employing a "thermally induced gradient self-assembly" process, a gradient structure of "phosphonic acid inner layer / sulfonic acid outer layer" is formed in a three-chamber device, which improves the interlayer peel strength of the film to >3N / mm (compared to only 0.5N / mm in the traditional lamination method). This effectively solves the problem of easy delamination of existing laminated composite films after battery cycling, and the capacity decay is only 3.2% after 500 cycles (compared to 18.7% in the traditional method). The gradient structure and solvent-free process work together to reduce the swelling rate of the membrane in the electrolyte at 80℃ to ≤6% (commercial DuPont DFM-100 membrane >15%), and extend the high-temperature cycle life to ≥12,000 cycles (compared to less than 5,000 cycles by traditional methods), meeting the requirements for long-term stable operation of vanadium redox flow batteries. The gradient functionalized structure improves the ion selectivity (H⁺ / VO²⁺) to 228 (105 in the conventional method) and reduces the sheet resistivity to 0.83 Ω・cm² ​​(0.98 Ω・cm² ​​in the conventional method). At the same time, the solid-state grafting reaction time is shortened to 30-40 min (4 h in the conventional solution method), and no solvent treatment is required, which reduces production costs and environmental pressure. Attached Figure Description

[0017] Figure 1 This is a process flow diagram of a solid-state radiation grafting method for a phosphonic acid-sulfonic acid bifunctional ion membrane proposed in this invention. Detailed Implementation

[0018] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0019] Reference Figure 1 : I. Raw material pretreatment and monomer mixing: Construction of molecular-level contacts Physical mechanism of resin drying: Regenerated sulfonic acid resin (80~120 mesh) is dried at 80℃ and vacuum degree ≤-0.09MPa. Vacuum is used to lower the boiling point of water (below 45℃) to accelerate desorption and avoid residual water from causing the hydrolysis of vinylphosphonic acid (the hydrolysis rate increases exponentially with the increase of water content). The fine-mesh resin (specific surface area >0.8m² / g) increases the monomer contact area and provides micro-reaction sites for subsequent grafting. Interfacial chemistry for monomer dispersion: Add 5-10 wt% vinylphosphonic acid and stir at 300 rpm for 20 min in an environment with humidity <30%. The surface tension of the monomer is broken by mechanical shearing, so that the monomer is adsorbed on the surface of the resin particles with a molecular thickness (contact angle ≤15°) to form a "resin-monomer" molecular contact layer, avoiding the risk of solvent-mediated phase separation in solution methods (such as monomer agglomeration, solvent residue leading to film embrittlement). II. Infrared Radiation-Induced Solid-State Grafting: Precise Regulation of Free Radical Polymerization Infrared selective excitation principle: The stretching vibration of the S=O (sulfonium-oxygen bond) of the sulfonic acid group has a characteristic absorption peak in the 3.5±0.2μm infrared band. After the radiation energy is precisely absorbed, the S=O bond undergoes anharmonic dissociation to generate highly active sulfonic acid free radicals (·SO3H) (Gibbs free energy < -20kJ / mol). Kinetic advantages of grafting reaction: The C=C (carbon-carbon bond) double bond of vinylphosphonic acid undergoes addition polymerization with sulfonic acid radicals to form a graft chain connected by carbon-carbon bonds; the high-purity N2 atmosphere (oxygen content <100ppm) inhibits the oxidative quenching of free radicals (the reaction rate of O2 with free radicals is more than 3 times that of grafting reaction), making the grafting efficiency exceed 92% (the efficiency of traditional solution method is only 65% ​​due to solvent competition for free radicals). III. Melt-state thermally induced gradient self-assembly: Polar-driven structural delamination The three-chamber device utilizes temperature and pressure gradients to guide the directional migration of functional groups, taking advantage of the polarity difference between phosphonic acids (-PO3H2) and sulfonic acids (-SO3H) (phosphonic acids are more polar, with a dipole moment > 1.8D). Lower radiation zone (280℃, molten state): The resin reaches the melting temperature (melting peak of sulfonic acid resin ≈ 275℃), and the free volume of the molecular chain increases from 0.12 to 0.21. Due to its strong polarity and active thermal motion, phosphonic acid migrates into the interior of the membrane (driven by "thermal diffusion-polar aggregation"), forming an inner pre-enriched layer. The middle constant pressure zone (0.15 kPa, held for 15 min): low pressure inhibits the escape of molecular chains, which is controlled by Fick's diffusion law (diffusion coefficient D≈1.2×10⁻). 9 m² / s), allowing phosphonic acid to diffuse to a stable distribution under a concentration gradient, and solidify to form a 10~15μm thick phosphonic acid enriched layer (XPS test shows that the P / S atomic ratio of the inner layer is >3:1). Upper slow cooling zone (≤5℃ / min cooling to 80℃): Slow cooling causes the molecular chains to freeze gradually. Sulfonic acid, due to its lower surface energy (Δγ≈12mN / m), preferentially migrates to the film surface and solidifies. Finally, the gradient slope ≥0.8μm⁻¹ (the rate at which P content decreases and S content increases per micrometer depth) is characterized by TOF-SIMS, locking in the integrated gradient structure of "phosphonic acid inner layer / sulfonic acid outer layer". IV. Post-processing and performance synergy: Impurity removal and functional enhancement Diffusion mechanism for monomer removal: Soaking in deionized water at 25±1℃ for 24h, utilizing the water solubility of the ungrafted monomer (>100g / L), diffusion is driven by concentration gradient (D≈5×10⁻¹). 0 m² / s), reducing the amount of residual monomer to <0.5wt%, thus avoiding a surge in membrane swelling rate due to its dissolution (from 12% to ≤6%). Performance synergy of gradient structures: Surface sulfonic acid layer: high electrical conductivity (σ>0.1S / cm), accelerates H⁺ migration; Inner phosphonic acid layer: Through steric hindrance and charge repulsion (phosphonic acid is negatively charged after ionization, pKa≈2.0), it efficiently intercepts VO²⁺, enabling the ion selectivity (H⁺ / VO²⁺) to exceed 220 (traditional homogeneous membranes ≤120).

[0020] Example 1: Preparation of ion exchange membranes under typical process parameters 1. Raw materials and equipment Resin: Commercially available recycled sulfonic acid resin (model ZGCNR50, 80 mesh, specific surface area 0.85m² / g, sulfonic acid group content 1.2mmol / g). Monomer: Vinylphosphonic acid (purity 99.5%, industrial grade, polarity parameter π*=0.82); Equipment: Vacuum drying oven (temperature control ±1℃, vacuum degree ≤-0.1MPa), high-speed mixer (300rpm, stirring accuracy ±5rpm), infrared radiation reactor (wavelength 3.5±0.2μm adjustable, radiation intensity 1~2W / cm²), three-chamber film forming machine (temperature control ±2℃, pressure control ±0.01kPa). 2. Preparation steps (1) Raw material pretreatment and monomer mixing Take 100g of resin and dry it at 80℃ and vacuum degree -0.095MPa for 4h. The moisture content decreased from the initial 3.5wt% to 0.3wt% (Kal Fischer method test). Weigh 5wt% (5g) of vinylphosphonic acid and add it to the dry resin. In an environment with a humidity of 25% (controlled by a dehumidification system), stir at 300rpm for 20min to form a uniform contact system of "resin particles-monomer molecules" (no monomer agglomeration was observed under an optical microscope, and the contact angle was ≤15°). (2) Infrared radiation-induced solid-state grafting The mixed system was transferred to a sealed reactor and purged with high-purity N2 (99.999%) three times (flow rate 5L / min, purging for 5min each time) until the oxygen content in the reactor dropped to 50ppm (monitored by an online oxygen analyzer). Infrared radiation with a wavelength of 3.5 μm and an intensity of 1.5 W / cm² was applied for 35 minutes (infrared energy density 4.5 kJ / cm², calculated using Joule's law). After naturally cooling to room temperature, the grafted product was obtained: grafting efficiency 93.2% (elemental analysis, P content 3.1wt%), with approximately 8 phosphonic acid side chains grafted per 100 repeating units on the resin molecular chain (¹H-NMR characterization). (3) Molten state thermally induced gradient self-assembly (three-chamber process) Lower radiation zone: The grafted product is fed into the lower layer of the three-chamber chamber and heated to 280℃ at a heating rate of 10℃ / min (resin melting peak 275℃, hold for 5min to ensure complete melting, viscosity measured by rheometer η=500Pa・s). Due to their strong polarity (dipole moment 1.9D), the phosphonic acid groups migrate into the membrane interior (tracked by fluorescent labeling method, migration distance ≥5μm within 10min). Middle layer constant pressure zone: Switch to 0.15 kPa pressure, hold for 15 min, and apply Fick's diffusion law (diffusion coefficient D = 1.2 × 10⁻⁻⁻⁶). 9 m² / s), forming a 12μm thick phosphonic acid enriched inner layer (XPS test showed an inner layer P / S atomic ratio of 3.2:1); Upper slow cooling zone: Cooled to 80℃ at a cooling rate of 5℃ / min, held for 10min for curing. Due to the low surface energy (Δγ=12mN / m), sulfonic acid groups are enriched on the film surface, resulting in a final film thickness of 50μm (micrometer test, accuracy ±1μm).

[0021] (4) Post-processing and purification Cut the sample into 10cm×10cm standard samples and soak them in deionized water at 25±1℃ for 24h (with water changed 3 times) to remove ungrafted monomers (residual amount ≤0.5wt%, tested by high performance liquid chromatography). After drying, the moisture content is ≤1wt% (tested with a halogen moisture meter).

[0022] 3. Performance Test Results Example 2: Process expansion with high monomer ratio 1. Adjustment of process parameters Vinylphosphonic acid addition: 10 wt% (5 wt% in Example 1); Infrared radiation time: 40 min (35 min in Example 1); Upper layer slow cooling rate: 3℃ / min (5℃ / min in Example 1); Post-processing time: 36 hours (24 hours for Example 1); 2. Performance Comparison Table Example 3: Tolerance verification of radiation parameters 1. Adjustment of process parameters Infrared radiation intensity: 1.2 W / cm² (1.5 W / cm² in Example 1, a reduction of 20%). Infrared radiation time: 45 min (35 min in Example 1, extended to maintain energy equivalence); Other parameters are the same as in Example 1; 2. Performance Comparison Table In summary, this invention overcomes the bottleneck of traditional bifunctional ion exchange membrane preparation through a "solid-state radiation grafting + thermally induced gradient self-assembly" process: the solvent-free system avoids the aggregation of phosphonic acid groups (grafting efficiency > 92%), the gradient structure solves the interlayer delamination problem (peel strength > 3 N / mm), the swelling rate at 80℃ is ≤ 6%, the cycle life exceeds 12,000 cycles, the ion selectivity reaches 228, and the sheet resistivity is reduced to 0.83 Ω・cm². At the same time, it shortens the reaction time and reduces the cost, providing high-performance membrane materials for fields such as vanadium redox flow batteries.

[0023] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A solid-state radiation grafting method for a phosphonic acid-sulfonic acid bifunctional ion membrane, characterized in that, Includes the following steps: S1: Raw material pretreatment: Take 80~120 mesh regenerated sulfonic acid resin powder and vacuum dry it for 4 hours at 80℃ and vacuum degree ≤-0.09MPa; add vinylphosphonic acid monomer at a mass ratio of 5~10wt%, and stir it for 20 minutes in a high-speed mixer at 300rpm in an environment with humidity <30% to obtain a uniform mixed system. S2: Solid-state radiation grafting: The mixed system was transferred to a sealed reactor and purged three times with high-purity N2 (purity ≥99.999%) to control the oxygen content in the reactor to <100ppm; an infrared radiation source with a wavelength of 3.5±0.2μm was used to irradiate the mixture at a radiation intensity of 1.5W / cm² for 30~40min to obtain a grafting product with a grafting efficiency >92%. S3: Melt-state gradient self-assembly: The grafted product is introduced into a three-chamber film-forming device and sequentially passes through: ①Lower radiation zone: Heated to 280℃ at a heating rate of 10℃ / min to melt, driving phosphonic acid groups to migrate into the membrane interior; ② Middle layer constant pressure zone: Maintain pressure of 0.15 kPa and keep warm for 15 min to form a phosphonic acid enriched inner layer with a thickness of 10~15 μm; ③ Upper slow cooling zone: Cooled to 80℃ at a cooling rate of ≤5℃ / min, which promotes the enrichment and solidification of sulfonic acid groups on the membrane surface, forming a gradient structure membrane; S4: Post-treatment and characterization: The molded membrane was cut into standard samples and soaked in deionized water at 25±1℃ for 24h to remove ungrafted monomers; the TOF-SIMS gradient slope of the membrane cross section was tested to be ≥0.8μm⁻¹, the electrolyte swelling rate at 80℃ was ≤6%, and the ion selectivity was >200, thus obtaining a phosphonic acid inner layer / sulfonic acid outer layer gradient structure ion membrane.

2. The solid-state radiation grafting method for a phosphonic acid-sulfonic acid bifunctional ion membrane according to claim 1, characterized in that, The regenerated sulfonic acid resin powder described in S1 has a mesh size of 80-120.

3. The solid-state radiation grafting method for a phosphonic acid-sulfonic acid bifunctional ion membrane according to claim 1, characterized in that, The vacuum drying conditions in S1 are: temperature 80℃, vacuum degree ≤ -0.09MPa, and drying time 4h.

4. The solid-state radiation grafting method for a phosphonic acid-sulfonic acid bifunctional ion membrane according to claim 1, characterized in that, The conditions for mixing monomers in S1 are: vinylphosphonic acid monomer addition of 5~10wt%, high-speed mixer speed of 300rpm, stirring time of 20min, and ambient humidity of <30%.

5. The solid-state radiation grafting method for a phosphonic acid-sulfonic acid bifunctional ion membrane according to claim 1, characterized in that, The parameters of infrared radiation in S2 are: wavelength 3.5±0.2μm, radiation intensity 1.5W / cm², and reaction time 30~40min; the oxygen content after high-purity N2 replacement in the reactor is <100ppm.

6. The solid-state radiation grafting method for a phosphonic acid-sulfonic acid bifunctional ion membrane according to claim 1, characterized in that, The grafting efficiency of the grafted product in S2 was determined by elemental analysis, and the grafting efficiency was >92%.

7. The solid-state radiation grafting method for a phosphonic acid-sulfonic acid bifunctional ion membrane according to claim 1, characterized in that, The melting conditions in the lower radiation zone of S3 are a heating rate of 10℃ / min and a melting temperature of 280℃.

8. The solid-state radiation grafting method for a phosphonic acid-sulfonic acid bifunctional ion membrane according to claim 1, characterized in that, The conditions in the constant pressure zone of the middle layer in S3 are: pressure 0.15 kPa, heat preservation time 15 min, and phosphonic acid enriched inner layer thickness 10~15 μm.

9. The solid-state radiation grafting method for a phosphonic acid-sulfonic acid bifunctional ion membrane according to claim 1, characterized in that, The cooling conditions in the upper slow cooling zone of S3 are a cooling rate ≤ 5℃ / min and a final cooling temperature of 80℃.

10. The solid-state radiation grafting method for a phosphonic acid-sulfonic acid bifunctional ion membrane according to claim 1, characterized in that, The post-treatment conditions in S4 are: deionized water temperature 25±1℃ and soaking time 24h.

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