Sulfonated polyimides bearing benzimidazole groups for proton exchange membranes

A series of sulfonated polyimides containing benzimidazole groups were synthesized using 4,4′-binaphthyl-1,1′,8,8′-tetracarboxylic dianhydride (BTDA), 4,4′-diaminodiphenyl ether-2,2′-disulfonic acid (ODADS) as the sulfonated diamine, and 2-(3′,5′-diaminophenyl)benzimidazole (a) or 6,4′-diamino-2-phenylbenzimidazole (b) as the nonsulfonated diamine. The electrolyte properties of the synthesized polyimides (Ia − x, Ib − x, x refers to molar percentage of the sulfonated diamine) were investigated and compared with those of polyimides (Ic − x) from BTDA, ODADS, and m-phenylenediamine (c). All synthesized polyimides possessed high molecular weights revealed by their high viscosity, and formation of tough and flexible membranes. Polyimides with benzimidazole groups exhibited much better swelling capacity than those without benzimidazole groups. This was attributed to the strong interchain interaction through basic benzimidazole functions and sulfonic acid groups. The sulfonated polyimides that are incorporated with 1,1′,8,8′-binaphthalimide exhibited better hydrolytic stability than that with 1,4,5,8-naphthalimide. Polyimide membranes with good water stability as well as high proton conductivity were developed. Polyimide membrane (Ia − 90), for example, did not lose mechanical properties after being soaked in boiling water for 1000 h, while its proton conductivity was still at a high level (compared to that of Nafion 117).     

Vinyl‐addition type norbornene copolymer containing sulfonated biphenyl pendant groups for proton exchange membranes

The vinyl addition type copolymer poly(butoxymethylene norbornene‐co‐biphenyl oxyhexamethyleneoxymethylene norbornene) (P(BN/BphN)) was synthesized by using bis‐(β‐ketonaphthylimino)nickel(II)/B(C₆F₅)₃ catalytic system. P(BN/BphN) was sulfonated to give sulfonated P(BN/BphN) (SP(BN/BphN)) with concentrated sulfuric acid (98%) as sulfonating agent in a component solvent. The ion exchange capacity (IEC), degree of sulfonation (DS), water uptake, and methanol permeability of the SP(BN/BphN)s were increased with the sulfonated time. The methanol permeability of the SP(BN/BphN) membranes was in the range of 1.8 × 10^?7 to 7.5 × 10^?7 cm²/s, which were lower than the value 1.3 × 10^?6 cm²/s of Nafion®115. The proton conductivity of SP(BN/BphN) membranes increased with the increase of IEC values, temperature, and water uptake. Water uptake of the SP(BN/BphN) membranes was lower than that of Nafion® 115 and leads to low proton conduction. Microscopic phase separation occurred in SP(BN/BphN) membrane and domains containing sulfonic acid groups were investigated by SEM and TEM. SP(BN/BphN) membranes had good mechanical properties, high thermal stability, and excellent oxidative stability. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Vinyl‐addition type norbornene copolymers containing flexible spacers and sulfonated pendant groups for proton exchange membranes

Novel sulfonated poly(2‐butoxymethylenenorbornene‐co‐2‐(6‐phenoxy‐hexyloxymethylene)‐5‐norbornene [sP(BN/PhHN)] were prepared successfully through vinyl‐addition type polymerization and then sulfonated with concentrated sulfuric acid (98%) as sulfonating agent in a component solvent. The sP(BN/PhHN)‐40 with the maximal degree of sulfonation of 40% can be obtained by controlling the sulfonating reaction time from 8 to 20 h, and a proton conductivity of 3.35 × 10^?3 S/cm was achieved at 70°C. The methanol permeabilities of these membranes were in the range from 0.26 to 6.58 × 10^?7 cm²/s, which were remarkably lower than Nafion (2.36 × 10^?6 cm²/s). TEM analysis revealed that these side‐chain type membranes have a microphase separated structure composed of hydrophilic side‐chain domains and hydrophobic polynorbornene main chain domains. Sulfonated polynorbornene containing soft spacers displayed better properties, such as lower water uptake, high thermal properties, mechanical properties, and low methanol permeability. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Sulfonated poly(ether ketone ketone) ionomers as proton exchange membranes

Sulfonated poly(ether ketone ketone) ionomers (SPEKK) with ion‐exchange capacities (IEC) between 0.2 and 3.4 meq/g were prepared by sulfonating PEKK with a mixture of concentrated and fuming sulfuric acids. Sulfonation occurs only on the phenyl rings attached to ether and ketone groups. The glass transition temperature of the dry SPEKK ionomers increased linearly with increasing IEC, and the ionomers were thermally stable to ～250°C, above which desulfonation occurred. Water‐swollen ionomers exhibited microphase separated morphologies, and the average correlation length determined by small angle X‐ray scattering increased with increasing IEC. The proton conductivity of hydrated SPEKK membranes measured by impedance spectroscopy ranged from ～10^–3 to 10^–1 S/cm as the IEC increased from ～1.0 to 2.4 meq/g. Single cell performance curves on membrane‐electrode assemblies (MEA) indicated that the SPEKK membranes approached the performance of Nafion? for an IEC of 2 meq/g. POLYM. ENG. SCI., 45:1081–1091, 2005. © 2005 Society of Plastics Engineers     

The direct synthesis of wholly aromatic poly(p-phenylene)s bearing sulfobenzoyl side groups as proton exchange membranes

Sulfonated poly(p-phenylene)s (SPPs) containing sulfonic acid groups in their side chains had been directly synthesized by Ni(0) catalytic coupling of sodium 3-(2,5-dichlorobenzoyl)benzenesulfonate and 2,5-dichlorobenzophenone. The synthesized copolymers possessed high molecular weights revealed by their high viscosity, and the formation of tough and flexible membranes by casting from DMAc solution. The copolymers exhibited excellent oxidative stability and mechanical properties due to their fully aromatic structure extending through the backbone and pendent groups. Transmission electron microscopic (TEM) analysis revealed that these side-chain type SPP membranes have a microphase-separated structure composed of hydrophilic side-chain domains and hydrophobic polyphenylene main chain domains. The proton conductivities of copolymer membranes increased with the increase of IEC and temperature, reaching values above 3.4 × 10⁻¹ S/cm at 120 °C, which are almost 2-3 times higher than that of Nafion 117 at the same measurement conditions. Consequently, these materials proved to be promising as proton exchange membranes.     

Sulfonated aromatic hydrocarbon polymers as proton exchange membranes for fuel cells

This article reviews recent studies on proton exchange membrane (PEM) materials for polymer electrolyte fuel cells. In particular, it focuses on the development of novel sulfonated aromatic hydrocarbon polymers for PEMs as alternatives to conventional perfluorinated polymers. It is necessary to improve proton conductivity especially under low-humidity conditions at high operating temperatures to breakthrough the current aromatic PEM system. Capable strategies involve the formation of well-connected proton channels by microphase separation between hydrophilic and hydrophobic domains and the increase of the ion exchange capacity of PEMs while keeping water resistance. Herein, we introduce novel molecular designs of sulfonated aromatic hydrocarbon polymers and their performance as PEMs.     

不同羧酸根含量对双官能团聚降冰片烯质子交换膜性能影响的分子动力学模拟

基于Material Studio（MS）软件平台,构建了含有磺酸根和羧酸根双功能化基团的新型聚降冰片烯类质子交换膜模型。通过改变3种结构单元NBSC、DCNM以及DCNM-N的不同配比,探究了不同羧酸根含量对质子交换膜性能的影响,同时分析了质子交换膜的微观结构以及H₂O和H₃O⁺等小分子的输运特性。计算结果显示,H₂O和H₃O⁺的均方位移和扩散系数随着羧酸根含量的增加而逐渐减小。在298 K条件下,3种质子交换膜的质子传导率分别为22.75、46.14、56.77 mS·cm^-1。表明随着羧酸根个数的增加,质子传导率不断增大,但增幅先增大后减小。     

Sulfonated poly(ether ether ketone) membranes crosslinked with sulfonic acid containing benzoxazine monomer as proton exchange membranes

The incorporation of benzoxazine (Ba) or sulfonic acid containing benzoxazine (SBa) as a crosslinking agent in SPEEK proton exchange membrane (PEM) can substantially improve the SPEEK membrane performance. The SPEEK-SBa membranes give higher effective selectivity than corresponding SPEEK-Ba membranes under close crosslinker loading and thus are more suitable to be used in direct methanol fuel cells. The best achieved SPEEK-SBa composition (SBa40) gives reasonable proton conductivity (0.91 × 10⁻² S cm⁻¹) but significantly lower methanol permeability (6.5 × 10⁻⁸ S² cm⁻¹). The achieved effective selectivity (Φ = SPEEK-SBa40: 14.0 × 10⁴ S s cm⁻³) is substantially higher than the plain SPEEK (Φ = 7.24 × 10⁴ S s cm⁻³) which has great potential for practical applications in DMFCs.     

Synthesis and surface characterization of copoly(imide alkyl ether)s containing pendant fluoroalkyl groups

A series of copoly(imide alkyl ether)s were synthesized to explore surface migration of fluorinated alkyl ether blocks (AEFO)s. Mechanical and surface properties of solution cast, thermally imidized films were determined. Incorporation of the AEFO oligomers at loading levels up to 5 wt % resulted in a slight decrease (usually less than 10%) in tensile modulus. Surface migration of the AEFOs raised the advancing water contact angle from approximately 80° to above 95° for the copolymer systems. The composition at which addition of more AEFO further increased water contact angle values was related to the number of fluorine atoms in the perfluorinated side chains. Surface excess concentration of the AEFO at different loading levels was calculated from X‐ray photoelectron spectroscopy results. At higher AEFO loading levels, the surface excess concentration was relatively constant suggesting formation of stable structures within the bulk similar to micelle formation in surfactant solutions. Based on these results, it was determined that surface saturation occurred with the fluorinated AEFO species at loading levels as low as 1 wt % engendering changes in surface properties while retaining the bulk imide properties. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41538.     

Novel copoly(ether ether ketone)s with pendant phenyl groups: synthesis and characterization

In order to obtain poly(ether ether ketone)s having enhanced solubility and processability without extreme loss of other properties, a series of copoly(ether ether ketone)s (Co‐PEEKs) with pendant phenyl groups were synthesized from 1,1‐bi(4‐hydroxyphenyl)‐1‐phenylethane (ph‐BPA), hydroquinone and 4,4′‐difluorobenzophenone via aromatic nucleophilic substitution reaction. The structures and properties of the Co‐PEEKs were characterized using Fourier transform infrared and ¹H NMR spectroscopies, differential scanning calorimetry, thermogravimetric analysis, wide‐angle X‐ray diffraction and solubility testing. These Co‐PEEKs have inherent viscosities in the range 0.14–1.09 dL g^?1, and their number‐average and weight‐average molecular weights reach 72 659 and 163 400 g mol^?1, respectively. The Co‐PEEK with the lowest content of ph‐BPA has a semi‐crystalline nature and is only soluble in 98% sulfuric acid. However, with an increase of ph‐BPA in the Co‐PEEKs, they become amorphous and readily soluble in a wide range of organic solvents and can afford tough films. These Co‐PEEKs have glass transition temperatures of 137–180 °C depending on the content of ph‐BPA. All the Co‐PEEKs have initial degradation temperatures above 480 °C in nitrogen atmosphere. Thus, these Co‐PEEKs with excellent thermal stability, good solubility and processability have potential for use in high‐performance films, coatings, hollow fiber membranes, etc. Copyright © 2011 Society of Chemical Industry     

Sulfonated hydrocarbon membranes for medium-temperature and low-humidity proton exchange membrane fuel cells (PEMFCs)

This review summarizes efforts in developing sulfonated hydrocarbon proton exchange membranes (PEMs) with excellent long-term electrochemical fuel cell performance in medium-temperature and/or low-humidity proton exchange membrane fuel cell (PEMFC) applications. Sulfonated hydrocarbon PEMs are alternatives to commercially available perfluorosulfonic acid ionomers (PFSA, e.g., Nafion^®) that inevitably lose proton conductivity when exposed to harsh operating conditions. Over the past few decades, a variety of approaches have been suggested to optimize polymer architectures and define post-synthesis treatments in order to further improve the properties of a specific material. Strategies for copolymer syntheses are summarized and future challenges are identified. Research pertaining to the sulfonation process, which is carried out in the initial hydrocarbon PEM fabrication stages, is first introduced. Recent synthetic approaches are then presented, focusing on the polymer design to enhance PEM performance, such as high proton conductivity even with a low ion exchange capacity (IEC) and high dimensional stability. Polymer chemistry methods for the physico-chemical tuning of sulfonated PEMs are also discussed within the framework of maximizing the electrochemical performance of copolymers in membrane-electrode assemblies (MEAs). The discussion will cover crosslinking, surface fluorination, thermal annealing, and organic–inorganic nanocomposite approaches.     

Sulfonated poly(bis‐A)‐sulfones as proton exchange membranes for direct methanol fuel cell application

Sulfonated poly(bis‐A)‐sulfone (SPSF) samples were prepared by a mild postsulfonation method using trimethylsilyl chlorosulfonate as sulfonation agent, and their thermal and mechanical properties were evaluated. The serials of SPSF membranes are thermally stable up to 450°C in air. When compared with the poly(bis‐A)‐sulfone membrane, the hydrophilicity and water uptake of the SPSF membranes are enhanced. A microphase‐separated structure comprised of hydrophilic and hydrophobic polymer backbone was observed from atomic force microscopy phase images. The hydrophilic ionic clusters become continuous to form channels when ion exchange capacity (IEC) reached 1.47 mequiv/g. Moreover, the membranes showed very good proton conductivities (20°C, 0.01–0.11 S/cm) and low‐methanol permeability (0.09–3.06 × 10^?6 cm²/s), and the methanol diffusion coefficients were lower than that of Nafion112 (1.35 × 10^?6 cm²/s) with IEC values from 0.70 to 1.47 mequiv/g. However, the Fenton's reagent test revealed that the membranes exhibited very poor oxidation stability, which is the main defect limiting the application of SPSF for proton exchange membranes. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers     

Sulfonated poly(arylene ether ketone)s prepared by direct copolymerization as proton exchange membranes: Synthesis and comparative investigation on transport properties

Sulfonated poly(aryl ether ketone)s (SPAEK) copolymers were synthesized by aromatic nucleophilic polycondensation from 3,3′, 5,5′‐tetramethyl‐4, 4′–biphenol, 1,4‐bis(4‐fluorobenzoyl) benzene, and disulfonated difluorobenzophenone. The SPAEK membranes did not exhibit excessive swelling in hot water and at the same time show the proton conductivities in the range of 0.030 S/cm to 0.099 S/cm at 80°C. The methanol diffusion coefficients of the SPAEK membranes were in the range of 4.7 × 10^?7 to 8.1 × 10^?7cm²/s measured at 25°C. The transport properties of this series of SPAEK copolymers were compared to poly(aryl ether ether ketone)s (SPEEK), poly(aryl ether ether ketone ketone)s (SPEEKK), and Nafion® membranes. It was found that the transport properties (including proton conductivity and methanol permeability) follows the trend of SPEEKK‐60 < SPAEK‐60 < SPEEK‐60 < Nafion® 117, the order of which is also attributed to the differences in the chemical structure of the polymers and the membrane morphology. In general, this novel series of SPAEK membranes possess various advantages, such as low cost of the initial monomers, high thermal and mechanical stability, and low methanol permeability while simultaneously possessing sufficient proton conductivity, which makes them notably promising as proton exchange membrane (PEM) materials in direct methanol fuel cell (DMFC) applications. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Synthesis and properties of novel sulfonated poly(phenylquinoxaline)s as proton exchange membranes

Two series of sulfonated poly(phenylquinoxaline)s (SPPQ-x and SPPQ(O)-x, x refers to molar percentage of sulfonated tetraamine monomer) were first synthesized from a sulfonated tetraamine (4,4′-bis(3,4-diaminophenoxy)biphenyl-3.3′-disulfonic acid) and two aromatic bisbenzils (4-phenylglyoxalylbenzil and p,p′-oxydibenzil) in a mild condition. The structures of SPPQ-x and SPPQ(O)-x were characterized by IR and ¹H NMR spectra. The properties of these polymer films, such as water uptake, water swelling ratio, proton conductivity, thermal properties, methanol permeability, hydrolytic and oxidative stability were also investigated. The resulting polymers generally showed good solubility in DMAc and DMSO. Flexible and tough membranes with high mechanical strength were prepared. They show very high thermal, thermooxidative, hydrolytic stabilities and low methanol permeability. SPPQ-100 with the IEC value (2.41 mmol/g) displays the conductivity of 0.1 S/cm and a swelling ratio of 7.3% at 100 °C. The low swelling was attributed to the high rigid of polymer backbones and the strong intermolecular interaction between the basic nitrogen atoms of quinoxaline units and sulfonic acid groups. Moreover, we found that the conductivities of SPPQ(O)-x membranes were higher than SPPQ-x membranes at the similar IEC value. The highest conductivity of 0.2 S/cm was obtained for SPPQ(O)-100 at 140 °C. A combination of excellent dimensional and hydrolytic stabilities indicated that the SPPQ ionomers were good candidate materials for proton exchange membrane in fuel cell applications.     

Structures and properties of highly sulfonated poly(arylenethioethersulfone)s as proton exchange membranes

A series of sulfonated poly(sulfonium cation) polymers, sulfonated poly(arylenethioethersulfone)s (SPTES)s possess up to two sulfonate groups per repeat unit, and can be easily converted into corresponding acid form of the SPTES polymer to form a tough, ductile, free-standing, pinhole-free membranes with excellent mechanical properties. The SPTES polymers exhibit good water affinity and excellent proton conductivity due to the high water uptake. Proton conductivities between 100 and 300 mS/cm (at 65 °C, 85% relative humidity) were observed for the SPTES polymers with 50 mol% (SPTES-50) to 100 mol% (SPTES-100) of sulfonated monomer. The evaluation by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and thermomechanical analysis (TMA) showed that the SPTES polymers have excellent thermal stability, mechanical properties, and dimensional stability, making them excellent candidates for the next generation of proton exchange membranes (PEMs) in fuel cell applications.     

Covalently and ionically crosslinked sulfonated poly(arylene ether ketone)s as proton exchange membranes

A series of covalently and ionically crosslinked sulfonated poly(arylene ether ketone)s (SPAEKs) were prepared via the cyclocondensation reaction of crosslinkable SPAEKs with 3,3′-diaminobenzidine to form quinoxaline groups, where crosslinkable SPAEKs were synthesized by copolymerization of 4,4′-biphenol with 2,6-difluorobenzil, 4,4′-difluorobenzophenone, and 5,5′-carbonyl-bis(2-fluorobenzene sulfonate). The SPAEK membranes had high mechanical properties and the isotropic membrane swelling. The covalent and ionical crosslinking significantly improved the membrane performance, i.e., the crosslinked membranes showed the lower membrane dimensional change, lower methanol permeability, and higher oxidative stability than the corresponding uncrosslinked membranes, with keeping the reasonably high proton conductivity. The crosslinked membrane (CK3) with measured ion exchange capacity of 1.62 mequiv g⁻¹ displayed a reasonably high proton conductivity of 110 mS/cm with water uptake of 33 wt% at 80 °C, and exhibited a low methanol permeability of 1.7 × 10⁻⁷ cm² s⁻¹ for 32 wt% methanol solution at 25 °C. The covalently and ionically crosslinked SPAEK membranes have potential for polymer electrolyte membrane fuel cells and direct methanol fuel cells.     

Fluorinated poly(arylenethioethersulfone) copolymers containing pendant sulfonic acid groups for proton exchange membrane materials

Fluorinated sulfo-pendants poly(arylenethioethersulfone) copolymer, which contained 50 mol% sulfonate content per repeat unit (6F-SPTES-50), were synthesized by a nucleophilic polycondensation from 4,4-(hexafluoroisopropylidene)-diphenylthiol, 3,3′-disulfonate-4,4′-difluorodiphenylsulfone and 4,4′-difluorodiphenylsulfone. The 6F-SPTES-50 copolymer possessed high molecular weight, exhibited good film formability and thermal stability, and maintained adequate mechanical strength after immersion in water for 24 h. The proton conductivities of the 6F-SPTES-50 copolymer membrane increased with temperatures, reaching values above 120 mS/cm at 85 °C and 85% relative humidity. Preliminary MEA test shows acceptable performance, which indicated that these materials are promising proton exchange membranes (PEMs) for fuel cells operated at medium temperatures.     

Sulfonated derivatives of polyparaphenylene as proton conducting membranes for direct methanol fuel cell application

Proton conducting polymers derived from polybenzoyl-1,4-phenylene (PBP) and poly-p-phenoxybenzoyl-1,4-phenylene (PPBP) were synthesized by the Colon synthesis technique. The sulfonation of these proton conducting polymers was carried out using either sulphuric acid or tetramethylsiliylchlorosulfonate (TMSCl) as sulfonating agent, and their thermal properties were evaluated. Both sulfonated PBP and PPBP are thermally stable up to at least 215 °C. The sulfonated sPPBP exhibited good conductivity as proton conducting membranes at room temperature and were tested as electrolyte membranes for a single direct methanol fuel cell (DMFC) in terms of water absorption, methanol permeability and electrical performance. The water uptake of the sPPBP was found to be larger than that of the sPBP, i.e., 65 and 43 mol%, respectively. The permeability to methanol was found to be 10 times lower than sPPBP and sPBP compared to a Nafion^® membrane. In spite of this, performance in a single DMFC was found to be twice inferior to that with Nafion^® 117. Optimisation of the sulfonation level and of the electrode-membrane interfaces was lead to better results.     

Synthesis of sulfonated poly(arylene-co-naphthalimide)s as novel polymers for proton exchange membranes

A series of novel sulfonated poly(arylene-co-binaphthalimide)s (SPPIs) were successfully synthesized via Ni(0) catalytic coupling of sodium 3-(2,5-dichlorobenzoyl)benzenesulfonate and bis(chloronaphthalimide)s. Bis(chloronaphthalimide)s were conveniently prepared from 5-chloro-1,8-naphthalic anhydride and various diamines. Tough and transparent SPPI membranes were prepared and the electrolyte properties of the copolymers were intensively investigated as were the effects of different diamine structures on the copolymer characterisitics. The copolymer membrane Ia-80, with an ion exchange capacity (IEC) of 2.50 meq g⁻¹, displayed a higher proton conductivity, i.e. 0.135 S cm⁻¹ at 20 °C, as compared to Nafion 117 (0.09 S cm⁻¹, 20 °C). The copolymer membrane Id-70, containing 3,3′-dimethyl-4,4′-methylenedianiline (DMMDA) units, exhibited excellent stability toward water and oxidation due to the introduction of hydrophobic methyl groups on the ortho-position of the imido bond in the copolymer. The mechanical property of Id-70 remained virtually unchanged after immersing membrane in pressured water at 140 °C for 24 h. Furthermore, the introduction of aliphatic segment a hexane-1,6-diamine (HDA) in copolymer led to a significant increase in proton conductivity and water uptake with increasing temperature; the proton conductivity of the Ic-70 membrane reached 0.212 S cm⁻¹ at 80 °C, which was higher than Nafion 117 as well as of the membranes based on aromatic diamines at equivalent IEC values. Consequently, these materials proved to be promising as proton exchange membranes.