Review of Sulfonation Methods for PEEK Membranes-Shenzhen ZH Energy Storage - Zhonghe LDES VRFB - Vanadium Flow Battery Stacks - Sulfur Iron Electrolyte - PBI Non-fluorinated Ion Exchange Membrane

Review of Sulfonation Methods for PEEK Membranes

Classification：Industrial News

- Author：Luo Xuan

- Release time：Jun-17-2022

【 Summary 】Important non fluorinated proton exchange membranes - polyether ether ketone membranes (PEEK membranes) and sulfonation methods of polyether ether ketone membranes

Ion exchange membranes are the core materials of fuel cells, flow cells, and hydrogen production through electrolysis of water. Currently, the widely used Nafion membrane has high costs, which greatly limits the development of related industries. Taking all vanadium flow batteries as an example, the cost reduction of proton exchange membrane materials in all vanadium flow batteries is of great significance for the overall system cost reduction and the development and application of all vanadium flow batteries in the commercialization process.

In previous articles, we have introduced a hot topic of non fluorinated proton exchange membranes - polybenzimidazole membranes (PBI membranes). Non fluorinated proton exchange membranes have gained widespread attention due to their low cost. This article will introduce another important type of non fluorinated proton exchange membrane - polyether ether ketone membranes (PEEK membranes) and sulfonation methods of polyether ether ketone membranes. After sulfonation, a sulfonated polyether ether ketone membrane (SPEEK membrane) with improved proton exchange capacity can be obtained. Based on this, it is further modified to obtain a more superior performance non fluorinated proton exchange membrane. The modification method is similar to that of PBI membrane, which can be achieved through various methods such as blending, hybridization, and filling. The following figure shows the common SPEEK membrane structure.

Common SPEEK membrane structures

SPEEK membrane is a sulfonated aromatic main chain polymer that promotes its proton exchange ability by introducing sulfonic acid ions into the PEEK membrane, which can to some extent replace Nafion. However, compared with Nafion, its expansion is greater, which leads to an increase in vanadium ion transmittance, resulting in a mixture of positive and negative electrolyte ions, resulting in reduced capacity and failure.

L ü Teng used methyl hydroquinone and 4,4 '- difluorodibenzophenone as raw materials, and reacted in an alkaline environment (provided by K2CO3) at 170 ℃ to produce methyl containing polyether ether ketone (MPEEK). Further sulfonate it to obtain sulfonated polyether ether ketone membranes with different degrees of sulfonation. It was found that due to the presence of strong hydrophilic groups (sulfonic acid groups) in the sulfonated SPEEK membrane, the water absorption rate and swelling of SPEEK-x increase with the increase of sulfonation degree in the membrane, and the permeability of vanadium ions also increases. Cyclic testing was conducted on the battery, and at a current density of 60 mA cm-2, the energy efficiency of VRFB assembled with SPEEK-75 film was the best, reaching 64.7%. The VRFB assembled with SPEEK-100 has the highest output power density of 300mW cm-2. In addition, Lv Teng also used the prepared MPEEK to react with N-methylimidazole to obtain imidazole functionalized polyether ether ketone (ImPEEK). SPEEK was blended with ImPEEK to obtain a zwitterionic ion exchange membrane. It was found that the vanadium ion permeation of SPEEK/ImPEEK-25 and SPEEK/ImPEEK-50 was negligible compared to Nafion212 and SPEEK membranes, effectively reducing cross contamination between electrolytes and improving battery efficiency. At a current density of 60 mA cm-2, the VRFB assembled with SPEEK/ImPEEK-90 exhibits the best performance, with Coulombic efficiency, voltage efficiency, and energy efficiency of 98.8%, 75.6%, and 74.5%, respectively. In addition, the maximum power density of the VRFB assembled with SPEEK/ImPEEK-90 is 150m W cm-2. The research results indicate that the zwitterionic ion exchange membrane with PEEK as the main chain has good application prospects in liquid flow batteries [1].

Song Xun et al. synthesized a bisphenol type polyether ether ketone polymer using bisphenol fluorene as the structural unit. The polyether ether ketone was sulfonated with concentrated sulfuric acid to introduce sulfonation groups into the bisphenol fluorene structural unit to prepare a polyether ether ketone proton exchange membrane (SF-PEEK). The research results indicate that sulfonic acid groups have been successfully introduced onto the side groups of polyether ether ketone, and the SF-PEEK membrane exhibits a clear hydrophilic and hydrophobic microphase separation morphology. Sulfonic acid groups aggregate to form ion channels. When the ion exchange capacity (IEC) of SF-PEEK membrane reaches 1.97 mmol/g, its conductivity reaches 4.15 × 10-2 S/cm, slightly lower than the 5.67 × 10-2S/cm of Nafion117 membrane, but its vanadium ion permeability is only 20.1% of Nafion117 membrane, showing excellent ion selectivity. In vanadium current battery testing, the Coulombic efficiency of SF-PEEK membrane was higher than that of Nafion117 membrane at different current densities. Among them, the energy efficiency of SF80-PEEK608 with an IEC of 1.97 mmol/g (80 is the mass fraction of SF, 608 is the reaction time at 60 ℃ for 8 hours) reached its maximum value of 80.9% at a current density of 40 mA/cm2, which is higher than that of Nafion117 membrane at 78.8%. In the self discharge test, the self discharge time of the battery assembled with SF80-PEEK608 film is 90 hours, which is higher than the 57h of Nafion117 film [2].

Lin Xingxing synthesized polyether ether ketone (PEEK) with benzimidazole bisphenol (HPBI) and 2,2-di (4-hydroxyphenyl) propane (BPA) via condensation polymerization of difluorodiphenylketone monomer (DFK), and obtained sulfonated polyether ether ketone with benzimidazole structure and adjustable content. Its research found that PEEK alt-BI/SPEEK blend membranes have high ion exchange capacity and stable size. In addition, it synthesized sulfonated poly (hexafluoroether ketone x% benzimidazole) through nucleophilic copolymerization using difluorodiphenylketone/sulfonated difluorodiphenylketone and hexafluorobisphenol A/HPBI as raw materials. The results indicate that the sulfonated poly (hexafluoroether ketone x% benzimidazole) synthesized by it exhibits strong acid-base interactions within and between molecules, which enhance the mechanical strength and dimensional stability of the polymer film. The water absorption and swelling rates of 60SPEEK-AF-10% BI membrane at 80 ℃ are 33.5% and 9.3%, respectively, which are significantly lower than the 558.3% and 85.5% of 60SPEEK-AF. In addition, the introduction of benzimidazole structural units can improve the thermal and chemical stability of sulfonated membranes [3].

Liu et al. grafted ethylenediamine onto the edge of graphene oxide to form aminated graphene oxide (GO-NH2) and prepared (60 ± 2) μ m thick SPEEK/GO-NH2 film. Due to the interaction between - SO3- and protonated N-bases, aminated graphene oxide is uniformly dispersed. The intrinsic balance between conductivity and permeability is achieved through three aspects: (1) two-dimensional layered graphene oxide is impermeable, blocking ion channels in SPEEK. (2) The repulsive effect of protonated N-bases hinders the mixing of vanadium ions. (3) The size repulsion between - NH3+and - SO3- groups narrows the transport channel and suppresses the mixing of vanadium ions, but protons with smaller ionic radii can migrate from the narrow channel. The selectivity of SPEEK/GO-NH2 membrane is highest at a GO-NH2 content of 2%, which is 6 times higher than that of Nafion115 membrane. In addition, compared with SPEEK membrane, GO-NH2 enhances the oxidation stability of SPEEK membrane, but has a negative impact on the mechanical properties of SPEEK membrane [4].

Park et al. prepared sulfonated graphene oxide (sGO) using para aminosulfonic acid, and further functionalized sGO with phenyl isocyanate to produce isGO. A composite film was prepared by combining isGO with SPEEK with a sulfonation degree of 68%. The research results indicate that the water absorption rate of SPEEK/is GO membrane is slightly lower than that of SPEEK/s GO membrane, and the proton conductivity of the two is similar. Due to the differential separation between hydrophobic and hydrophilic waters in SPEEK, as well as the effect of two-dimensional layered graphene oxide, the ion selectivity of the composite membrane is about 4 times that of the Nafion117 membrane, while the permeability (1.0 × 10-7 cm2/min) is 8 times lower than that of the Nafion117 membrane [5].

Lou et al. used commonly used nano carbon black in the rubber industry to prepare sulfonic acid based carbon black (FCB) particles through a simple diazotization reaction. Similar to other carbon fillers, sulfonic acid based carbon black can also improve the ion selectivity of SPEEK/FCB composite membranes by blocking vanadium ion channels and promoting more sulfonic acid substrate transport. When the addition amount of sulfonic acid based carbon black is 3%, the SPEEK composite membrane has four times higher ion selectivity compared to Nafion212 membrane. At high current density, the composite membrane has better electrochemical performance [6].

Wang Qian et al. modified the surface of metal organic framework (MOF) material UiO-66-NH2 with 3-mercaptopropyltrimethoxysilane, which had good chemical stability. After oxidation with H2O2, the sulfonated product UiO-66-SO3H was obtained. It was added as a nano filler to sulfonated polyether ether ketone (SPEEK) to obtain a nano SPEEK composite proton exchange membrane (referred to as the composite membrane). The experimental results show that the MOF crystal structure of UiO-66-SO3H has not changed and the morphology remains good; SPEEK has good compatibility with nanofillers, and is uniformly dispersed when the nanofiller content is not higher than 6% (w); The ion exchange capacity of the composite membrane decreases and the water absorption rate increases; When the nanofiller content is 6% (w), the proton conductivity of the composite membrane in water reaches its highest (0.078 S/cm), which is 86% higher than that of pure SPEEK membrane [7].

Niu et al. prepared SPEEK/g-C3N4 composite films using solution casting method. Its research found that g-C3N4 has a triangular nanopore structure and good hydrophilicity, promoting the formation of proton channels and thereby increasing the IEC value. When the content of g-C3N4 nanosheets is too high (2.5%), the blocking effect of g-C3N4 nanosheets and the consumption of SO3H in SPEEK result in a decrease in the IEC value of the composite film. At the same time, the acid-base interaction induces the orientation of the water molecular network in the formed interface region, controlling the protonation/deprotonation process of the membrane. When the content of g-C3N4 is 2.0%, SPEEK/g-C3N4 exhibits the highest proton conductivity (12.3 mS/cm). The possible proton transport mechanism of the composite membrane is: (1) the two-dimensional nanopores of g-C3N4 nanosheets are effective proton channels, and the membrane roughness caused by the wrinkles and grooves of g-C3N4 hinders the transport of vanadium ions. (2) Based on the Donnan repulsion effect, - NH3+/- NH groups are anchored on the surface and defects of g-C3N4, and the positive electrostatic repulsion effect of - NH3+groups also hinders the permeation of vanadium ions. (3) The strong interface interaction between g-C3N4 and SPEEK establishes a transport channel that further limits the crossing of vanadium ions. In addition, when the content of g-C3N4 is 1.5%, the selectivity and self discharge time of the composite film are better than Nafion117 [8].

Hu Hengwei et al. mixed polyether ether ketone (PEEK) with concentrated sulfuric acid to obtain sulfonated polyether ether ketone (SPEEK) with a sulfonation degree of 73.49%; Different contents of polyacrylonitrile (PAN) were added to SPEEK, and nanofiber proton exchange membranes (S/P composite membranes) were prepared by electrospinning. By hot pressing, the pores and thickness inside the fiber membrane are reduced, effectively reducing the phenomenon of fuel molecule permeation and conduction impedance inside the membrane. The results showed that when the PAN mass fraction was 5%, the S/P composite membrane had a similar water absorption and swelling rate as the Nafion 211 membrane, but had higher proton conductivity and mechanical properties. Under conditions of 70 ℃ and 100% humidity, the S/P composite film has a higher output power [9].

Afzal et al. prepared black phosphorus powder using red phosphorus as a precursor, and prepared oxidized black phosphorus (OBP) nanosheets through liquid-phase exfoliation. The OBP nanosheets were blended with sulfonated polyether ether ketone (SPEEK) polymer to prepare SPEEK/OBP composite proton exchange membranes with a doping amount of 0-2.5%. The results indicate that the OBP surface contains abundant oxygen-containing functional groups, which can promote the water absorption of the composite membrane. At the same time, they can form a hydrogen bond network with sulfonic acid groups in SPEEK, promoting proton transfer. Compared with pure SPEEK membranes, SPEEK/OBP composite membranes have higher ion exchange capacity, water absorption, swelling rate, and proton conductivity. When the amount of OBP nanosheets added is 2.0%, the performance of SPEEK/OBP composite membranes is optimal, with a proton conductivity of 0.026 S/cm at 30 ℃, which is 1.73 times that of pure SPEEK membranes [10].

Wang Yingfeng et al. proposed an improvement method for the low proton conductivity of ordinary sulfonated polyether ether ketone (SPEEK) membranes through inorganic doping. It prepared BaCe0.8Al0.2O3 composite oxide by co precipitation method, doped it into SPEEK membrane matrix, and obtained SPEEK/BaCe0.8Al0.2O3 composite proton exchange membrane through solution casting method. The research results indicate that the doping of BaCe0.8Al0.2O3 can effectively improve the proton conductivity of the composite membrane. Among them, the proton conductivity of SPEEK-BaCe0.8Al0.2O3-9 composite membrane reaches 0.187 S × cm-1 at 80 ℃, the tensile strength reaches 29.43 MPa, and the maximum power density of a single battery reaches 0.82 W × cm-2, which is almost comparable to ordinary Nafion proton exchange membranes. In addition, doping also improves the chemical stability of the composite film [11].

Puyang Yang et al. prepared a co mixed cross-linked proton exchange membrane (CMB) based on sulfonated polyether ether ketone (SPEEK)/partially fluorinated sulfonated polyarylethersulfone (SPFAES), and constructed a cross-linked structure within the blend system by inducing high-temperature dehydration reaction by adding a dehydrating agent during solution casting. The research results indicate that due to the good compatibility, dispersibility, polymer chain rearrangement and cross-linking between SPEEK and SPFAES, CMB membranes exhibit excellent mechanical strength in dry state and significantly improve their physicochemical stability. Under low ion exchange capacity (1.21-1.51 mmol/g) conditions, the proton conductivity of the CMB membrane reaches 122-219 mS/cm (80 ℃). In a hydrogen oxygen single cell, the maximum power density of the CMB4 membrane reaches 530.5 mW/cm2 (80 ℃) [12].

The sulfonated PEEK membrane has greatly improved its proton conductivity compared to the original PEEK membrane, but there are still some gaps in many aspects compared to Nafion film. By further modification, such as preparing composite membranes, the gap with Nafion membranes can be significantly reduced. Currently, research has shown that the performance of SPEEK membranes can be well optimized through the auxiliary blending of nanomaterials and other modified materials, making it a highly promising proton exchange membrane.