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.     

Synthesis of highly sulfonated poly(arylene ether sulfone)s with sulfonated triptycene pendants for proton exchange membranes

A series of poly(aryl ether sulfone)s containing triptycene groups PES-x-TPD (x refers to molar percentage of TPD) were firstly synthesized through nucleophilic aromatic substitution polycondensation by using 2,5-triptycenediol (TPD), bis(4-hydroxyphenyl) sulfone (BHPS) and 4,4′-difluorodiphenyl sulfone (DFDPS). The sulfonation of copolymers was conducted at room temperature by using a mild sulfonating reagent (98% H₂SO₄), and the degree of sulfonation was readily and accurately controlled by adjusting the ratio of TPD and BHPS. The structures of PES-x-TPD and SPES-x-TPD were characterized by IR, ¹H NMR and ¹³C NMR spectra. These ionomers generally showed high thermal stability and mechanical strength at low humidity regardless of high IEC value. Meanwhile, it is noteworthy that these novel SPES-x-TPD membranes with high IEC value achieved high proton conductivity in a wide range of humidity at 80 °C. For example, SPES-60-TPD with the highest IEC value 2.86 mmol/g displays the conductivity of 2.5 × 10⁻¹ S/cm which is much higher than that of the perfluorinated Nafion membrane (1.1 × 10⁻¹ S/cm) at 80 °C and 94% RH. At 80 °C and 34% RH, SPES-60-TPD displays the conductivity of 4.5 × 10⁻³ S/cm which is also higher than that of the Nafion membrane (3.0 × 10⁻³ S/cm). Microscopic analyses revealed that well-de?ned phase separated structures and uniform ionic pathway was formed for SPES-45-TPD membrane with the IEC of 2.29 mmol/g. Moreover, a H₂/O₂ fuel cell using the SPES-55-TPD (IEC = 2.68 mmol/g) also showed better performance than that of Nafion 117 at 40 °C and 30% RH.     

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.     

Synthesis and properties of new sulfonated poly(p-phenylene) derivatives for proton exchange membranes. I

Several high molecular weight poly(2,5-benzophenone) derivatives were synthesized by high yield nickel-catalyzed coupling polymerization of 2,5-dichloro-4′-substituted benzophenones. The monomers were prepared by Friedel-Crafts catalyzed reaction of 2,5-dichlorobenzoyl chloride and several aromatic compounds. The resulting polymers are organosoluble and show no evidence of crystallinity by differential scanning calorimetry (DSC). The temperatures of 5% weight loss of the polymers via dynamic thermogravimetric analysis in air were above 480 °C. Sulfonation of selected polymers utilizing concentrated or fuming sulfuric acid at room temperature introduced sulfonic acid moieties to the aromatic side group. Activated fluoro aryl groups were also used to generate pendent sulfonated functionalities. The sulfonated polymers were examined for ion exchange capacities, water absorption capacities and proton conductivities. The sulfonated polymers were not good film formers, but could be demonstrated to show high values of proton conductivity in the range of 0.06-0.11 S/cm when supported on glass fabrics or via polymer blending strategies.     

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.     

Proton exchange membranes based on poly(vinylidene fluoride) and sulfonated poly(ether ether ketone)

Blend membranes were obtained by solution casting from poly(vinylidene fluoride) (PVDF) and sulfonated poly(ether ether ketone) (SPEEK) in N,N-dimethylacetamide (DMAc). DSC and XRD were used to characterize the structure of the blend membranes. The effect of PVDF content on the membrane properties was investigated. The methanol permeability, water uptake and the swelling ratio of blend membranes decreased with the increase of PVDF content. Though the proton conductivity decreased upon the addition of PVDF, they were still comparable to that of Nafion^® 117 membrane. Higher selectivities were also found for most blend membranes in comparison with Nafion^® 117 membrane. The effect of methanol concentration on solution uptake, swelling ratio and methanol permeability of the blend membranes was also studied.     

Partially sulfonated poly(arylene ether sulfone)-b-polybutadiene for proton exchange membrane

A novel block copolymer based on poly(arylene ether sulfone)-b-polybutadiene (SPAES-b-PB) was synthesized and its flexible segment was sulfonated by electrophilic addition reaction with acetyl sulfate. This could be a new approach to prepare suitable alternative proton exchange membranes to Nafion^®. Only a single glass transition temperature (T_g) of copolymer measured by differential scanning calorimeter (DSC) indicated good compatibility between PAES block and PB block. A tough and transparent membrane based on SPAES-b-PB exhibited higher proton conductivity (0.0302 S/cm at 25 °C and 100% relative humidity) even with relatively low ion exchange capacity (IEC) of 0.624 mmol/g compared to other sulfonated block copolymer membranes such as sulfonated polystyrene-b-poly(ethylene-ran-butylene)-b-polystyrene (SSEBS), sulfonated poly(styrene-isobutylene-styrene) (S-SIBS), sulfonated hydrogenated poly-butadiene-styrene copolymer (HPBS-SH) as a result of selected sulfonation of the flexible segments facilitating sulfonated groups to aggregate to form ion-rich channels.     

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).     

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.     

Hydrophilic-hydrophobic multiblock copolymers based on poly(arylene ether sulfone)s as novel proton exchange membranes - Part B

There has been growing evidence, both experimental and theoretical, that block copolymer systems with well-defined sulfonated regions may provide enhanced proton transport, especially at low relative humidity. We have recently demonstrated a novel way to make hydrocarbon hydrophobic-hydrophilic block copolymers. While the chemical structure and chemical compositions are very similar to random copolymers, the microstructure and the morphology are very different. The self-diffusion coefficients of water, as measured by Pulse Gradient Stimulated Echo (PGSE) NMR techniques, have indicated a significant improvement in water transport after reaching a particular block length. At that block length (10 kg/mol:10 kg/mol), the multiblocks display better proton conductivity under partially hydrated conditions than the random copolymers. The presence of increased free water content in the multiblocks with increasing block lengths was confirmed by states of water analysis. A significant change in the distribution of three types of water was also observed compared to the random copolymers. This paper will discuss the structure-property relationships of these multiblock copolymers for potential application as proton exchange membranes.     

Synthesis and properties of sulfonated copoly(phthalazinone ether imides) as electrolyte membranes in fuel cells

A new series of six-member sulfonated copolyimides (SPIs) were prepared by one-step solution copolycondensation from 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA), 1,2-dihydro-2-(4-amino-2-sulfophenyl)-4-[4-(4-amino-2-sulfonphenoxy)-phenyl] (2H)phthalazin-1-one (S-DHPZDA), 4,4′-bis(4-aminophenoxy) biphenyl (BAPB) and 1,2-dihydro-2-(4-aminophenyl)-4-[4-(4-(aminophenoxyl)phenyl)](2H)phthalazin-1-one (DHPZDA). The sulfonation degree (DS) of the SPIs was controlled by the mol ratio of the sulfonated diamine and non-sulfonated diamine. The obtained SPI membranes had excellent thermal stability, high mechanical property and proton conductivity as well as low methanol permeability. The tensile strength of the SPI membranes was ranging from 54.7 to 98.1 MPa, which was much higher than that of Nafion^®. The SPI membranes exhibited high proton conductivity (σ) and low methanol permeability ranged from 10⁻³ to 10⁻² S/cm and 10⁻⁸ to 10⁻⁷ cm²/s depending on the DS of the polymers, respectively.     

Novel side-chain-type sulfonated hydroxynaphthalene-based Poly(aryl ether ketone) with H-bonded for proton exchange membranes

A series of novel side-chain-type sulfonated hydroxynaphthalene poly(aryl ether ketone)s (SHNPAEKs) containing hydroxyl groups was synthesized by post grafted method and the sulfonated degree (Ds) of the polymers could be well controlled. The resulting polymers were characterized by ¹H NMR, FT-IR and thermogravimetric analysis (TGA). Meanwhile, the membrane properties for fuel cell applications such as water uptake, proton conductivity and methanol transport have been studied. The influence of pendent structure and inter-/intramolecular H-bonded to the properties of SHNPAEKs has been investigated. The proton conductivities of SHNPAEK membranes showed a range of 0.020-0.197 S/cm and the highest conductivity of 0.197 S/cm was obtained for SHNPAEK-90 (IEC = 2.08 meq./g) at 80 °C. The methanol permeability of SHNPAEK membranes was in the range from 2.65 × 10⁻⁷ to 11.9 × 10⁻⁷ cm²/s, which was much lower than that of Nafion 117.     

New multiblock copolymers of sulfonated poly(4′-phenyl-2,5-benzophenone) and poly(arylene ether sulfone) for proton exchange membranes. II

Rigid-rod poly(4′-phenyl-2,5-benzophenone) telechelics were synthesized by Ni(0) catalytic coupling of 2,5-dichloro-4′-phenylbenzophenone and the end-capping agent 4-chloro-4′-fluorobenzophenone. The degree of polymerization was determined by ¹³C NMR. The telechelics produced were selectively sulfonated by concentrated sulfuric acid at 50 °C. The degree of sulfonation was controlled by varying the reaction time and was determined by titration. The nucleophilic step copolymerization of the fluoroketone activated sulfonated poly(4′-phenyl-2,5-benzophenone) oligomer (M_n=3.05×10³ g/mol) with hydroxyl terminated biphenol based polyarylethersulfone (M_n=4.98×10³ g/mol) afforded an alternating multiblock sulfonated copolymer that formed flexible transparent films, in contrast to the high molecular weight rigid rod homopolymers. They were tested for water absorption and proton conductivity by specific impedance. The synthesis and characterization of these multiblock copolymers are reported.     

Preparation and proton conductivity of composite membranes based on sulfonated poly(phenylene oxide) and benzimidazole

The Brønsted acid–base composite membrane was prepared by entrapping benzimidazole in sulfonated poly(phenylene oxide) by tuning the doping ratios. Their thermal stability, dynamic mechanical properties and proton conductivity were investigated under the conditions for intermediate temperature proton exchange membrane (PEM) fuel cell operation. In addition, investigation of activation energies of the SPPO–xBnIm at different relative humidity was also performed. TG–DTA curves reveal these SPPO–xBnIm composite materials had the high thermal stability. The proton conductivity of SPPO–xBnIm composite material increased with the temperature, and the highest proton conductivity of SPPO–xBnIm composite materials was found to be 8.93 × 10⁻⁴ S/cm at 200 °C under 35% relative humidity (RH) with a “doping rate” where x = 2. The SPPO–2BnIm composite membrane show higher storage moduli and loss moduli than SPPO. Tests in a hydrogen–air laboratory cell demonstrate the applicability of SPPO–2BnIm in PEMFCs at intermediate temperature under non-humidified conditions.     

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.     

Anisotropic membranes from electrospun mats of sulfonated/nonsulfonated poly (ether ketone)s containing ion-rich paths as proton exchange membranes

Anisotropic proton exchange membranes composed of five layers with different contents of ionic groups across the membrane were prepared by simultaneous electrospinning of sulfonated and nonsulfonated poly(ether ketone) (PEK)s. To prepare nonporous and defect- free membranes from electrospun mats, nonsulfonated fibers as hydrophobic part of the membrane were melted by hot-pressing so that covered sulfonated fibers (hydrophilic part). Prepared membranes showed better thermal and dimensional stability compared to Nafion 115. Proton conductivity of membranes was comparable with Nafion especially at higher temperatures. Water uptake of prepared membranes and mechanical strength of them were in an acceptable range. The results showed that the difference between sulfonated PEK fibers in surface and center of the membranes affect proton conductivity and mechanical properties of the membranes.     

Synthesis and characterization of fully disulfonated poly(arylenethioethersulfone)s containing hexafluoroisopropylidene moiety

High molecular weight sulfonated poly(arylenethioethersulfone) homopolymer containing hexafluoroisopropylidene moiety (6F-SPTES-100) was synthesized from the monomers 3, 3′-disulfonated-4, 4′-difluorodiphenylsulfone and 4, 4′-(hexafluoroisopropylidene) diphenylthiol, using 4-fluorobenzophenone as the end-capping agent in polar aprotic solvents at temperatures up to 180 °C to provide the desired polymeric composition for utilization as proton exchange membrane (PEM) in fuel cells applications. Tough, ductile freestanding membranes were fabricated from N, N-dimethylacetamide (DMAc) by solvent-casting. The end-capped 6F-SPTES-100 polymer was fully characterized and the membrane was found to have proton conductivity as high as 180 mS/cm which was measured at 85 °C and 65% relative humidity. The proton conductivity of 6F-SPTES-100 was approximately two and half times higher than that of Nafion-117 under comparable conditions. The swelling and solubility characteristics of the 6F-SPTES-100 polymer in water are directly related to the high degree of sulfonation of the polymer backbone.     

Synthesis and hydrolytic stability of soluble sulfonated polybenzoxazoles derived from bis(3-sulfonate-4-carboxyphenyl) sulfone

As a kind of high performance polymer, polybenzoxazoles are of interest as the matrix of proton exchange membrane (PEM). Soluble sulfonated polybenzoxazoles (sPBO) were synthesized by incorporation of the hexafluoroisopropylidene moieties into the polymer backbone, which show high molecular weight and excellent thermal stability. The hydrolytic stability of sPBO was investigated in detail in order to determine whether it is suitable for PEM applications or not. Contrary to expectation, sPBO membranes underwent hydrolysis under mild conditions. The hydrolysis of sPBO was confirmed by NMR, IR spectroscopy, and gel permeation chromatography. This work showed that sPBO membranes could not be used as PEM due to the poor hydrolytic stability.     

Proton exchange membranes based on modified sulfonated poly(ether ether ketone) membranes with chemically in situ polymerized polypyrrole

Sulfonated poly(ether ether ketone) (SPEEK) membranes were modified with chemically in situ polymerized polypyrrole (PPy). The effects of temperature and methanol concentration on the solution uptake and the swelling ratio of SPEEK/PPy membranes were investigated. The solution uptake and the swelling ratio of the membranes decreased upon the incorporation of PPy. When the methanol concentration increased, both the solution uptake and the swelling ratio increased to a maximum, and then decreased. FT-IR, XRD, DSC and TGA were used to characterize the modified membranes. The methanol permeability of modified SPEEK membranes decreased upon the incorporation of PPy, and higher selectivity values were found for SPEEK/PPy membranes in comparison with pure SPEEK and Nafion^® 117 membranes.     

Preparation and characterization of sulfonated poly(arylene‐co‐naphthalimide)s for use as proton exchange membranes

In this study, conductive porous composites were fabricated using the host substrate with an interconnected porous network, followed by the penetration and deposition of polypyrrole (PPy) to create a continuous conductive network. The open‐porous host substrate was processed using polylactide (PLA) with compression molding and salt leaching techniques. Three different salt contents were varied from 75 wt %, 85 wt %, and 90 wt %, which were referred to by their salt‐to‐polymer mass ratios of 3, 6, and 9, respectively. The porous network was made conductive by coating its interior surfaces through in situ polymerization of PPy using iron (III) chloride as the oxidant species. These porous composites were then characterized to analyze the relationships between their morphology and their physical, conductive, and mechanical properties. The mechanical properties were then fitted with numerically simulated results from finite element modeling (FEM). © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010