Fabrication and properties of cross-linked sulfonated fluorene-containing poly(arylene ether ketone) for proton exchange membrane

The cross-linkable sulfonated ploy(arylene ether)s derived from 3,3′-diallyl-4,4′-dihydroxybiphenyl, 9,9′-bis(3,5-dimethyl-4-hydroxypheyl)fluorene (DMHPF), 4,4′-difluorobenzophenone (DFBP) and sulfonated 4,4′-diflourobenzophenone (SDFBP) were synthesized over a wide range of DFBP/SDFBP molar ratios. The resulting sulfonated poly(arylene ether)s with high inherent viscosity (0.87–1.46 dl g⁻¹) are soluble in polar organic solvents and can form flexible and transparent membranes by casting from their solution. The cross-linking reaction was carried out using a thermal activated radical cross-linking agent (TARC) at 100 °C. The comprehensive properties of the virgin and the cross-linked membranes were characterized and compared accordingly. The results showed that the cross-linked membranes had better mechanical, oxidative and dimensional stabilities together with high proton conductivity (5.41 × 10⁻² S cm⁻¹) at 80 °C under 100% relative humidity when compared with previously synthesized and similar membranes. These improvements were raised from the cross-linking structure and the fabrication procedure of the membranes.     

Block copolymers combining semi-fluorinated poly(arylene ether) and sulfonated poly(arylene ether sulfone) segments for proton exchange membranes

A series of aromatic multiblock copolymers based on alternating segments of hydrophilic sulfonated polysulfone (PSU) and hydrophobic polyfluoroether (PFE) were prepared and characterized as proton exchange membranes. PSU precursor blocks were synthesized by polycondensation of dichlorodiphenylsulfone and resorcinol, and PFE precursor blocks were prepared by combining decafluorobiphenyl and isopropylidenediphenol. After preparation of the multiblock copolymers via a mild coupling reaction of the precursor blocks, the resorcinol units of the PSU blocks were selectively and almost completely sulfonated under mild reaction conditions using trimethylsilylchlorosulfonate. Transparent and robust membranes with different PSU-PFE copolymer compositions and ion-exchange capacities were cast from solution. Atomic force microscopy of the membranes revealed a distinct nanophase separated morphology. At 80 °C, the proton conductivity reached 10 mS cm⁻¹ under 65% relative humidity and 100 mS cm⁻¹ under fully hydrated conditions.     

Comparative study of sulfonated branched and linear poly(phenylene)s polymer electrolyte membranes for fuel cells

Branched and linear sulfonated poly(phenylene)s (BSPs and LSPs, respectively) polymer electrolyte membranes (PEMs) containing benzophenone moiety were successfully synthesized and the performance of the LSPs and BSPs were compared in conjunction with Nafion 211^®. The LSPs and BSPs were synthesized by the CC coupling polymerization reaction between 1,4-dichloro-2,5-dibenzoylbenzene (PBP) and 1,4-dichloro-2-benzoylbenzene, and from PBP, 1,4-dichloro-2-benzoylbenzene, and 1,3,5-trichlorobenzene (branching agent), respectively. The degree of sulfonation in both LSPs and BSPs were controlled by varying the concentrations of chlorosulfonic acid and the structures of the resultant PEMs were confirmed by ¹H-NMR spectroscopy. The optimal LSP (LSP-2) and BSP (BSP-2) PEMs showed excellent chemical stability due to the absence of ether linkages in the polymer backbone, while the BSP-2 exhibited better proton conductivity (94.6 mS/cm under 90% relative humidity at 80 °C), water resistivity, and lower dimensional changes compared to the LSP-2, which is comparable to Nafion 211^®. The maximum power density for BSP-2 and LSP-2 were 0.60 and 0.49 W/cm², respectively, while it was 0.62 W/cm² for Nafion 211^®. Membrane properties were studied with regard to ion exchange capacity, dimensional stability, proton conductivity, thermogravimetric analysis, and water uptake. The surface morphology of membranes was also analyzed by atomic force microscope.     

Sulfonated poly(arylene ether)s based proton exchange membranes for fuel cells

During the past decade proton exchange membrane fuel cells (PEMFCs) as one kind of the potential clean energy sources for electric vehicles and portable electronic devices are attracting more and more attentions. Although Nafion® membranes are considered as the benchmark of proton exchange membranes (PEMs), the drawbacks of Nafion® membranes restrict the commercialization in the practical application of PEMFCs. As of today, the attention is to focus on developing both high-performance and low-cost PEMs to replace Nafion® membranes. In all of these PEMs, sulfonated poly(arylene ether ketone)s (SPAEKs) and sulfonated poly(arylene ether sulfone)s (SPAESs) are the most promising candidates due to their excellent performance and low price. In this review, the efforts of SPAEK and SPAES membranes are classified and introduced according to the chemical compositions, the microstructures and configurations, as well as the composites with polymers and/or inorganic fillers. Specifically, several perspectives related to the modification and composition of SPAESs and SPAEKs are proposed, aiming to provide the development progress and the promising research directions in this field.     

Comb-shaped sulfonated poly(aryl ether sulfone) proton exchange membrane for fuel cell applications

Structure design is the primary strategy to acquire suitable ionomers for preparing proton exchange membranes (PEMs) with excellent performance. A series of comb-shaped sulfonated fluorinated poly(aryl ether sulfone) (SPFAES) membranes are prepared from sulfonated fluorinated poly(aryl ether sulfone) polymer (SPFAE) and sulfonated poly(aryl ether sulfone) oligomer (SPAES-Oligomer). Chemical structures of the comb-shaped membranes are verified by ¹H nuclear magnetic resonance (NMR) and Fourier transform infrared (FT-IR) spectra. The comb-shaped SPFAES membranes display more continuous hydrophilic domains for ion transfer, because the abundant cations and flexible side-chains structure possess higher mobility and hydrophilicity, which show significantly improved proton conductivity, physicochemical stability, mechanical property compared to the linear SPFAE membranes. In a H₂/O₂ single-cell test, the SPFAES-1.77 membrane achieves a higher power density of 699.3 mW/cm² in comparison with Nafion® 112 (618.0 mW/cm²) at 80 °C and 100% relative humidity. This work offers a promising example for the synthesis of highly branched polymers with flexible comb-shaped side chains for high-performance PEMs.     

A novel synthesis approach to partially fluorinated sulfonimide based poly (arylene ether sulfone) s for proton exchange membrane

Innovation of novel low cost proton conductive materials with super acidity has been the ever-increasing thirst for PEMFCs. Sulfonimide groups have the strongest gas-phase super-acidity with excellent thermal and chemical stability. Therefore, a series of partially fluorinated sulfonimide functionalized poly(arylene ether sulfone)s (SIPAES-xx) were successfully synthesized by chemical modification of sulfonated polyarylethersulfone (SPAES). The SPAESs were synthesized first by the direct polymerization of 4,4′ -dichlorodiphenylsulfone (DCDPS), 3,3′-disulfonate-4,4′-dichlorodiphenylsulfone (SDCDPS), and bisphenol. Thereafter, all arylsulfonic acid groups were converted into more acidic sulfonimide acid groups using partial fluorinated fluorosulfonyl imide monomer. The effect of the conversion of arylsulfonic acid function into sulfonimide was evaluated through thermal and chemical analysis. ¹H-NMR, FTIR, TGA, FE SEM, and AFM were used to illustrate the structure, thermal and chemical properties of (SIPAES-xx) membranes. The membranes showed IEC values of 0.78–1.41 mequiv./g with 6.7–40.6% water uptake for 20–40% ionic groups. The synthesized SIPAES-40 membranes showed comparable proton conductivity to Nafion^® 117 at same conditions. Nevertheless, the aromatic sulfonimide remained stable up to 370 °C. Furthermore, the presence of fluorine within the sulfonimide polymer provided high dimensional stability and oxidative durability by protecting the polymer chain from oxidizing radical species. Therefore, the synthesized SIPAES-xx membranes have the potential features as a proton exchange membrane (PEM) materials in the fuel cell.     

Enhancement in proton conductivity and methanol cross-over resistance by sulfonated boron nitride composite sulfonated poly (ether ether ketone) proton exchange membrane

Fuel cells are the promising new non-conventional power source for vehicles as well as portable devices. Direct methanol fuel cell (DMFC) is especially attractive since it uses low cost liquid methanol as a fuel. Proton exchange membrane is one of the most crucial part of DMFC. Herein, we synthesized the sulfonated boron nitride (SBN) based SPEEK composite membranes for the DMFC application. SBN was synthesized by covalent functionalization of hydroxylated BN by using 3-mercaptopropyl trimethoxysilane and sulfonated by subsequent oxidation of mercapto group. Sulfonated poly (ether ether ketone) is used as a polymer matrix for SBN. With well controlled content of SBN into SPEEK matrix exhibit high proton conductivity, IEC and water content along with excellent mechanical strength. Composite membranes show low methanol cross over and high selectivity, which makes them attractive candidate for proton exchange membrane for direct methanol fuel cells.     

Synthesis and properties of sulfonated poly(arylene ether phosphine oxide)s for proton exchange membranes

A series of sulfonated poly(arylene ether phosphine oxide)s (sPAEPO) were prepared by direct polycondensation of sulfonated bis(4-fluorophenyl)phenyl phosphine oxide and bis(4-fluorophenyl)phenyl phosphine oxide with various diphenol-type monomers. The resulting ionomers show high molecular weight and excellent thermal stability. The bisphenol moieties of sPAEPO greatly affect the properties. sPAEPO-NA, -Bis A, -BP, and -6F show excellent dimensional stability. However, sPAEPO-DB and -HQ indicate abrupt swelling even at 80 and 90 °C, respectively, unsuitable for proton exchange membranes. In contrast, sPAEPO-6F with the lowest swelling exhibits the highest conductivity of 7.68 × 10⁻² S cm⁻¹ among all the sPAEPO, close to that of Nafion 117. Besides, sPAEPO-NA and -Bis A show a worse oxidative stability than other sPAEPO (sPAEPO-Bis A, -BP, -HQ, and -6F) due to the naphthalene ring and the isopropylidene unit in the backbone, respectively. Contrary to sPAEPO-Bis A and -BP, sPAEPO-NA and -6F exhibit well connective ionic domains owing to the high hydrophobic nature of the naphthalene ring and hexafluoroisopropylidene moieties. The connected ionic domains provide sPAEPO-NA and -6F with higher proton conductivity in comparison with sPAEPO-Bis A and -BP. In conclusion, sPAEPO-6F has the best comprehensive properties among all the sPAEPO, indicating a promising prospect in proton exchange membrane applications.     

Preparation and characterization of sulfonated poly(tetra phenyl ether ketone sulfone)s for proton exchange membrane fuel cell

Sulfonated poly(tetra phenyl ether ketone sulfone)s SPTPEKS were successfully synthesized for proton exchange membrane. Poly(tetra phenyl ether ketone sulfone)s PTPEKS were prepared by the 4,4′-dihydroxydiphenylsulfone with 1,2-bis(4-fluorobenzoyl)-3,4,5,6-tetraphenylbenzene (BFBTPB) and 4,4′-difluorodiphenylsulfone, respectively, at 210 °C using potassium carbonate in sulfolane. PTPEKS were followed by sulfonation using chlorosulfonic acid and concentrated sulfuric acid at two stage reactions. Different contents of sulfonated unit of SPTPEKS (17, 20, 23 mol% of BFBTPB) were studied by FT-IR, ¹H NMR spectroscopy, and thermo gravimetric analysis (TGA). Sorption experiments were conducted to observe the interaction of sulfonated polymers with water. The ion exchange capacity (IEC) and proton conductivity of SPTPEKS were evaluated with increase of degree of sulfonation. The water uptake of synthesized SPTPEKS membranes exhibit 25–61% compared with 28% of Nafion 211^®. The SPTPEKS membranes exhibit proton conductivities (25 °C) of 11.7–25.3 × 10⁻³ S/cm compared with 33.7 × 10⁻³ S/cm of Nafion 211^®.     

Synthesis and characterization of sulfonated poly(arylene ether ketone ketone sulfone) membranes for application in proton exchange membrane fuel cells

A series of sulfonated poly(arylene ether ketone ketone sulfone) (SPAEKKS) copolymers were synthesized by nucleophilic polycondensation. The copolymers exhibit good thermal and oxidative stabilities, all the SPAEKKS copolymers can be cast into tough membranes. Ionic exchange capacities (IEC), water uptake properties, thermal stabilities, methanol diffusion coefficients and proton conductivities were thoroughly studied. Also the microstructures of the membranes were investigated by TEM. The proton conductivity of the SPAEKKS-4 membrane is close to that of Nafion-117 at 80 °C. The methanol diffusion coefficient of the membrane is much lower than that of Nafion-117 under the same testing conditions. The SPAEKKS membranes are promising in proton exchange membranes fuel cell (PEMFC) application.     

Novel branched sulfonated polyimide membrane with remarkable vanadium permeability resistance and proton selectivity for vanadium redox flow battery application

A series of novel branched sulfonated polyimide (bSPI-x) membranes with 8% branched degree are developed for application in vanadium redox flow battery (VRFB). The sulfonation degrees of bSPI-x membranes are precisely regulated for obtaining excellent comprehensive performance. Among all bSPI-x membranes, the bSPI-50 membrane shows strong vanadium permeability resistance, which is as 8 times as that of commercial Nafion 212 membrane. At the same time, the bSPI-50 membrane has remarkable proton selectivity, which is four times as high as that of Nafion 212 membrane. The bSPI-50 membrane possesses slower self-discharge speed than Nafion 212 membrane. Furthermore, the bSPI-50 membrane achieves stable VRFB efficiencies during 200-time charge-discharge cycles at 120–180 mA cm^?2. Simultaneously, the bSPI-50 membrane exhibits excellent capacity retention compared with Nafion 212 membrane. All results imply that the bSPI-50 membrane possesses good application prospect as a promising alternative separator of VRFB.     

Intermolecular ionic cross-linked proton conducting electrolyte membranes derived from branched sulfonated poly(ether ether ketone)s with benzoxazole pendants

A series of novel branched sulfonated poly(ether ether ketone)s containing intermolecular ionic cross-linkable groups, benzoxazole groups, have been prepared for direct methanol fuel cells. The benzoxazole groups are grafted onto the polymer chain via a thiol-ene click chemistry reaction. The expected structures of the copolymers are confirmed by ¹H NMR and Fourier transform infrared spectroscopy. Compared with the unmodified polymer membrane, the ionic cross-linked membranes show enhanced thermal and mechanical properties. We also investigate the changes in water uptake, proton conductivity and chemical stability. The dense membrane structures formed by branching and the interactions between sulfonic acid and benzoxazole groups make a great contribution to the improvements of dimensional stability and methanol resistance property. Although the proton conductivities of the ionic cross-linked membranes are lower than the pristine membrane, the selectivities are much higher. The results show that the novel copolymers in this study are possible potential candidate materials for proton electrolyte membrane.     

Novel branched sulfonated poly(ether ether ketone)s membranes for direct methanol fuel cells

In this article, novel branched sulfonated poly(ether ether ketone)s (Br-SPEEK) containing various amounts of 1,3,5-tris(4-fluorobenzoyl)benzene as the branching agent have been successfully prepared. Compared with the traditional linear polymer membranes, the membranes prepared by Br-SPEEK showed improved mechanical strength, excellent dimensional stability and superior oxidative stability with similar proton conductivity. Notably, the Br-SPEEK-10 membrane began to break after 267 min in Fenton's reagent at 80 °C, which was 4 times longer than that of the L-SPEEK. Although the proton conductivity decreased with the addition of the branching agent, satisfying methanol permeability value was observed (down to 6.3 × 10⁻⁷ cm² s⁻¹), which was much lower than Nafion 117 (15.5 × 10⁻⁷ cm² s⁻¹). All the results indicated that the novel branched sulfonated poly(ether ether ketone)s membrane was potential candidate as proton conductive membranes for application in fuel cells.     

Quinoxaline-based crosslinked membranes of sulfonated poly(arylene ether sulfone)s for fuel cell applications

A series of crosslinkable sulfonated poly(arylene ether sulfone)s (SPAESs) were synthesized by copolymerization of 4,4′-biphenol with 2,6-difluorobenzil and 3,3′-disulfonated-4,4′-difluorodiphenyl sulfone disodium salt. Quinoxaline-based crosslinked SPAESs were prepared via the cyclocondensation reaction of benzil moieties in polymer chain with 3,3′-diaminobenzidine to form quinoxaline groups acting as covalent and acid-base ionic crosslinking. The uncrosslinked and crosslinked SPAES membranes showed high mechanical properties and the isotropic membrane swelling, while the later became insoluble in tested polar aprotic solvents. The crosslinking significantly improved the membrane performance, i.e., the crosslinked membranes had the lower membrane dimensional change, lower methanol permeability and higher oxidative stability than the corresponding precursor membranes, with keeping the reasonably high proton conductivity. The crosslinked membrane (CS1-2) with measured ion exchange capacity of 1.53 mequiv. g⁻¹ showed a reasonably high proton conductivity of 107 mS/cm with water uptake of 48 wt.% at 80 °C, and exhibited a low methanol permeability of 2.3 × 10⁻⁷ cm² s⁻¹ for 32 wt.% methanol solution at 25 °C. The crosslinked SPAES membranes have potential for PEFC and DMFCs.     

Crosslinked sulfonated poly(arylene ether ketone) membranes bearing quinoxaline and acid-base complex cross-linkages for fuel cell applications

A series of crosslinkable sulfonated poly(arylene ether ketone)s (SPAEKs) were synthesized by copolymerization of 4,4′-biphenol with 2,6-difluorobenzil and 5,5′-carbonyl-bis(2-fluorobenzene-sulfonate). A facile crosslinking method was successfully developed, based on the cyclocondensation reaction of benzil moieties in polymer chain with 3,3′-diaminobenzidine to form quinoxaline groups acting as covalent and acid-base ionic crosslinking. The uncrosslinked and crosslinked SPAEK membranes showed high mechanical properties and the isotropic membrane swelling, while the later became insoluble in tested polar aprotic solvents. The crosslinking significantly improved the membrane performance, i.e., the crosslinked membranes had the lower membrane dimensional change, lower methanol permeability and higher oxidative stability than the corresponding precursor membranes, with keeping the reasonably high proton conductivity. The crosslinked membrane (C-B4) with an ion exchange capacity of 2.02 mequiv. g⁻¹ showed a reasonably high proton conductivity of 111 mS cm⁻¹ with a low water uptake of 42 wt% at 80 °C. C-B4 exhibited a low methanol permeability of 0.55 × 10⁻⁶ cm² s⁻¹ for 32 wt% methanol solution at 25 °C. The crosslinked SPAEK membranes have potential for PEFC and DMFC applications.     

Improved proton conduction of sulfonated poly (ether ether ketone) membrane by sulfonated covalent organic framework nanosheets

A type of sulfonated covalent organic framework nanosheets (TpPa-SO₃H) was synthesized via interfacial polymerization and incorporated into sulfonated poly (ether ether ketone) (SPEEK) matrix to prepare proton exchange membranes (PEMs). The densely and orderly arranged sulfonic acid groups in the rigid skeleton of the TpPa-SO₃H nanosheets, together with their high-aspect-ratio and well-defined porous structure provide proton-conducting highways in the membrane. The doping of TpPa-SO₃H nanosheets led to an increased ion exchange capacity up to 2.34 mmol g^?1 but a 2-folds reduced swelling ratio, remarkably mitigating the trade-off between high IEC and excessive swelling ratio. Based on the high IEC and orderly arranged proton-conducting sites, the SPEEK/TpPa–SO₃H–5 membrane exhibited the maximum proton conductivity of 0.346 S cm^?1 at 80 °C, 1.91-folds higher than the pristine SPEEK membrane. The mechanical strength of the composite membrane was also improved by 2.05-folds–74.5 MPa. The single H₂/O₂ fuel cell using the SPEEK/TpPa–SO₃H–5 membrane presented favorable performance with an open voltage of 1.01 V and a power density of 86.54 mW cm^?2.     

Novel sulfonated poly(arylene ether ketone) copolymers bearing carboxylic or benzimidazole pendant groups for proton exchange membranes

A novel strategy in which the benzimidazole group and sulfonic group are simultaneously attached to an aromatic polymer has been reported in this paper. For this purpose, sulfonated poly(arylene ether ketone) copolymers containing carboxylic acid groups (SPAEK-x-COOH, x refers to the molar percentage of sulfonated repeating units) are prepared by the aromatic nucleophilic polycondensation of sodium 5,5′-carbonyl-bis(2-fluobenzene-sulfonate) (SDFBP), 4,4′-difluorobenzophenone (DFBP) and phenolphthalin (PPL). Then the carboxylic acid groups attached to the SPAEK-x-COOH are transformed to benzimidazole units through condensation reactions (referred to as SPAEK-x-BI). Fourier transform infrared spectroscopy and ¹H NMR measurements are used to characterize and confirm the structures of these copolymers. SPAEK-x-COOH membranes exhibit superior mechanical properties with maximum elongations at break up to 133%, meanwhile SPAEK-x-BI also shows good thermal and mechanical stability. The proton conductivity, swelling ratio and methanol permeability of the polymers with benzimidazole are lower than those with carboxylic groups, which indicated that there is an acid-base complex between benzimidazole and sulfonic acid groups. A balance of proton conductivity, methanol permeability, thermal and mechanical stabilities can be designed by incorporation of functional groups to meet the requirements for the applications in direct methanol fuel cells.     

Fluorinated sulfonated poly (arylene ether)s bearing semi-crystalline structures for highly conducting and stable proton exchange membranes

An aromatic nucleophilic substitution reaction-based direct polycondensation of 2,2-bis(4-hydroxyphenyl)hexafluoropropane, 1,4-bis(4-fluorobenzoyl)benzene (1,4-FBB), and 3,3′-disulfonate-4,4′-difluorodiphenylsulfone is used to synthesize 1,4-FBB in fluorinated sulfonated semi-crystalline poly (arylene ether) copolymers. The ion exchange capacity, mechanical stability, thermal stability, and oxidative stability of membranes with various degrees of sulfonation and water absorption are studied. ¹H nuclear magnetic resonance and atomic force microscopy verify the chemical structure of the synthesized ion exchange membrane. The semi-crystalline 6 F-membrane shows reliable chemical, mechanical, thermal, and electrochemical stability. Compared to Nafion 212®, which is a commercial ion exchange membrane, the semi-crystalline 6 F-membrane shows excellent cell performance as current density is increased. All the results mentioned above indicate that this semi-crystalline 6 F-membrane is a good candidate to replace the commercial membranes.     

Influence of hydrophobic block length and ionic liquid on the performance of multiblock poly (arylene ether) proton exchange membrane

The present article includes the synthesis of nanophase-separated poly (arylene ether) multiblock copolymers. A series of poly (arylene ether sulfone) hydrophobic oligomers consisting of bisphenol-A groups were reacted with a disulfonated poly (arylene ether ketone) hydrophilic oligomer containing 4, 4′- bis (4-hydroxyphenyl) valeric acid moieties to prepare multiblock copolymers. The synthesized oligomers and block copolymers were characterized by using FT-IR, ¹H NMR spectra and Gel Permeation Chromatography. The membranes obtained by solution casting method exhibited good dimensional and thermal stability. The increase in hydrophobic block length reduced the water uptake and methanol permeability of the membranes. The complexation of multiblock copolymer with ionic liquid (1-butyl-3-methyl-imidazolium tetrafluoroborate) resulted into novel hybrid membranes. These showed enhanced proton conductivity without affecting the mechanical stability. The Fenton's test revealed that the hybrid multiblock membranes were stable towards radical oxidation. The hydrophobic-hydrophilic phase separation was characterized by using tapping mode Atomic Force Microscopy (AFM). The hybrid membranes showed better fuel cell performance than that of pristine membrane.     

Novel proton exchange membranes based on water resistant sulfonated poly[bis(benzimidazobenzisoquinolinones)]

Novel water resistant sulfonated poly[bis(benzimidazobenzisoquinolinones)] (SPBIBIs) were synthesized from 6,6′-disulfonic-4,4′-binaphthyl-1,1′,8,8′-tetracarboxylic dianhydride (SBTDA) and various aromatic ether tetraamines. The resulting polymers with IEC in the range of 2.17–2.87 mequiv g⁻¹ have a combination of desired properties such as high solubility in common organic solvents, film-forming ability, and excellent thermal and mechanical properties. Flexible and tough membranes, obtained by casting from m-cresol solution, had tensile strength, elongation at break, and tensile modulus values in the range of 87.6–98.4 MPa, 35.8–52.8%, and 0.94–1.07 GPa. SPBIBI membranes with a high degree of sulfonation displayed high proton conductivity and a good resistance to water swelling as well. SPBIBI-b with IEC of 2.80 mequiv g⁻¹ displayed the conductivity of 1.74 × 10⁻¹ S cm⁻¹ at 100 °C, which was comparable to that of Nafion^® 117 (1.78 × 10⁻¹ S cm⁻¹, at 100 °C). However, the water swelling ratio of SPBIBI-b membranes was merely 8% at 100 °C while the Nafion^® 117 was 21.5%. The low swelling ratio was attributed to the strong intermolecular interaction including the electrostatic force and hydrogen bond. Moreover, they also exhibited much better hydrolytic stability than other sulfonated aromatic polymers such as polyimides. Consequently, these materials proved to be promising as proton exchange membranes.