Crosslinked poly(arylene ether sulfone) block copolymers containing quinoxaline crosslinkage and pendant butanesulfonic acid groups as proton exchange membranes

Crosslinkable poly (arylene ether sulfone) block copolymers (bSPAES (x/y)) containing pendant butanesulfonic acid and ethanedione groups were prepared from a new side-chain difluoro aromatic monomer 1-(2,6-difluorophenyl)-2-(3,5-dimethoxyphenyl)-1,2-ethanedione via block copolycondensation, demethylation, and further nucleophilic substitution of 1,4-butane sultone. Meanwhile, quinoxaline-based crosslinked block copolymers (C-bSPAES (x/y)) were obtained via cyclocondensation. The corresponding block copolymer membranes have high mechanical properties and anisotropic membrane swelling for either crosslinked or uncrosslinked ones. bSPAES (5/10) with ion exchange capacity (IEC) of 2.05 mequiv. g⁻¹ has low water uptake (WU) of 59.1% at 80 °C but relatively high conductivity of 225 mS cm⁻¹, which is ascribed to its good microphase separation. Meanwhile, the crosslinked C-bSPAES (5/10) with IEC of 1.76 mequiv. g⁻¹ exhibits a decreased WU by half, an improved oxidative stability by 200% and a reduced membrane swelling by 40% than the uncrosslinked bSPAES (5/10). The results suggest that quinoxaline-based crosslinking can obviously improve properties of bSPAES (x/y). In addition, even though maximum power density of C-bSPAES (5/10) is lower than that of Nafion 212, C-bSPAES (5/10) still has an acceptable good single-cell performance, indicating a possible fuel cell application.     

High performance blend membranes based on densely sulfonated poly(fluorenyl ether sulfone) block copolymer and imidazolium-functionalized poly(ether sulfone)

Abstract

Novel physically crosslinked polymer membranes were prepared by simply blending densely sulfonated poly(fluorenyl ether sulfone) with imidazolium-functionalized poly(ether sulfone). The blend showed well-defined ionic channels originating from the densely sulfonated structure and was physically crosslinked by ionic interactions. These two factors combined to enhance the physical stability and chemical stability of the prepared membranes while offering a conductivity over 0.24 S/cm at 80 °C for various amounts of crosslinker in the blend. The influence of this crosslinker amount on the chemophysical properties of the blend membranes was studied in a systematic way.     

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.     

Fluorene-containing poly(arylene ether sulfone)s with imidazolium on flexible side chains for anion exchange membranes

Anion exchange membranes with high ionic conductivity and dimensional stability attract a lot of research interests. In present study, a series of fluorene-containing poly(arylene ether sulfone)s containing imidazolium on the flexible long side-chain are synthesized via copolycondensation, Friedel-Crafts reaction, ketone reduction, and Menshutkin reaction sequentially. The membranes used for characterization and membrane electrode assembly are obtained by solution casting and ion exchange thereafter. The morphology of the membranes is studied via transmission electron microscopy, and the microphase separation is observed. The long side-chain structure is responsible for the distinct hydrophilic-hydrophobic microphase separation, which facilitates the transport of hydroxide ions in the membranes. The incorporation of imidazolium on the flexible long side-chain is favorable for the ionic aggregation and transport in the membranes. The resulted membranes exhibit high hydroxide conductivities in the range of 48.5–83.1 mS cm⁻¹ at 80 °C. All these membranes show good dimensional stability and thermal stability. The single cell performance shows a power density of 102.3 mW cm⁻² at 60 °C using membrane electrode assembly based-on one of the synthesized polymers.     

Crosslinked sulfonated poly(arylene ether sulfone) membranes for fuel cell application

A series of sulfonated poly(arylene ether sulfone) with photocrosslinkable moieties is successfully synthesized by direct copolymerization of 3,3′-disulfonated 4,4′-difluorodiphenyl sulfone (SDFDPS) and 4,4′-difluorodiphenyl sulfone (DFDPS) with 4,4′-biphenol (BP) and 1,3-bis-(4-hydroxyphenyl) propenone (BHPP). The content of crosslinkable moieties in the polymer repeat unit is controlled from 0 to 10 mol% by changing the monomer feed ratio of BHPP to BP. The polymer membranes can be crosslinked by irradiating UV with a wavelength of 365 nm. From FT-IR analysis, it can be identified that UV crosslinking mainly occurs due to the combination reaction of radicals that occurs in conjunction with the breaking of the carbon–carbon double bonds (–CH = CH-) of the chalcone moieties in the backbone. Consequently, a new bond is created to form cyclobutane. The crosslinked membranes show less water uptake, a lower level of methanol permeability, and good thermal and mechanical properties compared to pristine (non-crosslinked) membranes while maintaining a reasonable level of proton conductivity. Finally, the fuel cell performance of the crosslinked membranes is comparable to that of the Nafion 115 membrane, demonstrating that these membranes are promising candidates for use as polymer electrolyte membranes in DMFCs.     

Poly(arylene ether ketone) with pendant pyridinium groups for alkaline fuel cell membranes

Using the step-growth polycondensation reaction, poly(arylene ether ketone) (PAEK) and activated poly(arylene ether ketone)-NHS intermediates (PAEK-N) were synthesized. PAEK-NHS intermediates with pyridinium groups (PAEK-PYR) were obtained by adding different amounts of PYR groups. The successful syntheses of PAEK, PAEK-N, and PAEK-PYR were confirmed by nuclear magnetic resonance spectroscopy and Fourier transform infrared spectroscopy. Several important membrane properties (e.g., ionic exchange capacity (IEC), water uptake, anion conductivity, and thermal and mechanical stability) were investigated for their applications in alkaline fuel cells. Water uptake, swelling ratio, anion conductivity, and IEC increased with increasing PYR contents, while the mechanical properties decreased. Among a series of prepared membranes, the PAEK-PYR100 and PAEK-PYR125 membranes showed IEC and anion conductivity values that were higher than those of a commercial AHA membrane. Also, all of the prepared membranes were thermally stable up to 255 °C and show excellent chemical stability in alkaline conditions.     

Quaternary ammonium-functionalized poly(ether sulfone ketone) anion exchange membranes: The effect of block ratios

Anion exchange membranes based on quaternary ammonium-functionalized poly(ether sulfone ketone) block copolymers (QA-PESK) with various hydrophilic–hydrophobic oligomer block ratios (10:7, 10:18, and 10:26) were synthesized, and the block length effect on the membranes' physicochemical and electrical properties were systematically investigated. The QA-PESK-10-18 membrane, prepared using a hydrophilic and hydrophobic block ratio of 10:18, displayed well-balanced hydrophilic/hydrophobic phase separation, the highest conductivity of 23.19 mS cm⁻¹ at 20 °C and 57.84 mS cm⁻¹ at 80 °C, and the highest alkaline stability among the three block ratios tested, indicating that the membranes' properties were closely related to their morphologies, which were determined by the hydrophilic/hydrophobic ratio of the block copolymer. The H₂/O₂ single cell performance using the QA-PESK-10-18 revealed a maximum power density of 235 mW cm⁻².     

Preparation and characterization of fluorinated poly(aryl ether oxadiazole)s anion exchange membranes based on imidazolium salts

A series of fluorinated poly(aryl ether oxadiazole)s ionomers based on imidazolium salts (FPAEO-xMIM) were synthesized by quaternization of bromomethylated poly(aryl ether oxadiazole)s (FPAEO-xBrTM) with 1-methyl imidazole as aminating reagent. The anion exchange membranes (AEMs) were prepared by casting method and then immerged in aqueous sodium hydroxide for hydroxide ion exchanging. The structure of the obtained ionomers was characterized by ¹H-NMR and FT-IR measurements. The physical and electrochemical properties of the membranes were also investigated. The hydroxide conductivity of FPAEO-xMIM membranes was higher than 10⁻² S cm⁻¹ at room temperature, while the water uptake and swelling ratio was moderate even at elevated temperature. TGA analysis revealed that the membranes based on imidazolium salts had good thermal stability.     

Chemically modified poly(arylene ether ketone)s with pendant imidazolium groups: Anion exchange membranes for alkaline fuel cells

Anion exchange membranes (AEMs) with high stability are prepared for alkaline fuel cells using poly(arylene ether ketone)s (PAEKs) containing pendant imidazolium groups (via a direct step-growth polycondensation reaction). ¹H nuclear magnetic resonance spectroscopy (¹H NMR) and Fourier transform infrared (FT-IR) spectroscopy are used to analyze the chemical structure of the prepared PAEK membranes. The anion conductivity, water uptake and swelling ratio, thermal, mechanical, and chemical stability of these membranes are investigated for PAEK membranes with different 1-(3-aminopropyl)imidazole (API) molar ratios (PAEK-API-x) in details. The anion conductivity of PAEK-API-x membranes increases with increasing molar ratio of API. The membrane with API 1.5 equiv. displays the highest anion conductivity (0.0053–0.0531 S cm^?1 from 30 °C to 80 °C). All prepared membranes show good chemical and mechanical stability as well as thermal stability up to 250 °C. This high anion conductivity with good thermal, mechanical, and chemical stability of the membrane show potential advantage to meet the demands for AEMs.     

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.     

Synthesis and characterization of crosslinked sulfonated poly(arylene ether sulfone) membranes for high temperature PEMFC applications

Sulfonated poly(arylene ether sulfone) copolymers containing carboxyl groups are prepared by an aromatic substitution polymerization reaction using phenolphthalin, 3,3′-disulfonated-4,4′-dichlorodiphenyl sulfone, 4,4′-dichlorodiphenyl sulfone and 4,4′-bisphenol A as polymer electrolyte membranes for the development of high temperature polymer electrolyte membrane fuel cells. Thin, ductile films are fabricated by the solution casting method, which resulted in membranes with a thickness of approximately 50 μm. Hydroquinone is used to crosslink the prepared copolymer in the presence of the catalyst, sodium hypophosphite. The synthesized copolymers and membranes are characterized by ¹H NMR, FT-IR, TGA, ion exchange capacity, water uptake and proton conductivity measurements. The water uptake and proton conductivity of the membranes are decreased with increasing the degree of crosslinking which is determined by phenolphthalin content in the copolymer (0-15 mol%). The prepared membranes are tested in a 9 cm² commercial single cell at 80 °C and 120 °C in humidified H₂/air under different relative humidity conditions. The uncrosslinked membrane is found to perform better than the crosslinked membranes at 80 °C; however, the crosslinked membranes perform better at 120 °C. The crosslinked membrane containing 10 mol% of phenolphthalin (CPS-PP10) shows the best performance of 600 mA cm⁻² at 0.6 V and better performance than the commercial Nafion^® 112 (540 mA cm⁻² at 0.6 V) at 120 °C and 30 % RH.     

Synthesis and properties of new side-chain-type poly(arylene ether sulfone)s containing tri-imidazole cations as anion-exchange membranes

In order to enhance the properties of polymer anion-exchange membranes (AEMs), a class of side-chain-type poly (arylene ether sulfone)s containing tri-imidazole cations in pendant phenoxyphenyl spacer (i.e., Im-PES-xx) were designed and prepared by polymer grafting reaction. Based on these imidazole functionalized polymers, a series of new AEMs were successfully prepared by their solution coating and ion exchange. The obtained membranes exhibited good alkaline stability with the retention of ion-exchange capacity in the range of 78.5%–83.3% after immersion in 4 M NaOH solution at 80 °C for 336 h. The membranes also had high hydroxide conductivity with the values of 34.1–70.5 mS cm^?1 at 80 °C. Meanwhile, these membranes possessed good dimensional stability, and their swelling ratio was in the range of 2.5%–7.0% at 80 °C. The incorporation of the dense and local imidazole cations in the side chains for AEMs plays an important role for their properties.     

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.     

Construction of ion transport channels by grafting flexible alkyl imidazolium chain into functional poly(arylene ether ketone sulfone) as anion exchange membranes

A new series of imidazolium-functionalized anion exchange membranes (AEMs), based on poly (arylene ether ketone sulfone) containing pendant amino groups (Am-PAEKS) have been prepared. The structure of the copolymers is characterized by FT-IR and ¹H NMR spectra. The properties of the imidazolium-functionalized Am-PAEKS (Im-Am-PAEKS) including ionic conductivity, dimensional stability, thermal stability, fuel cell performance and mechanical property are investigated thoroughly. The hydroxide conductivities of the prepared membranes are in the range of 1.1 × 10^?2–13.9 × 10^?2 S cm^?1 (20–80 °C). The membranes exhibit excellent alkaline stability including high thermal stability and mechanical property after soaking in 2 M NaOH aqueous solution for 300 h. This study indicates that the imidazolium-functionalized membranes containing pedant amino groups have the potential to be applied in alkaline fuel cells.     

Synthesis and properties of sulfonated poly(arylene ether ketone sulfone) containing amino groups/functional titania inorganic particles hybrid membranes for fuel cells

In this work, the organic-inorganic hybrid membranes were prepared. The synthesis and properties of the hybrid membranes were investigated. The sulfonated poly(arylene ether ketone sulfone) containing amino groups (Am-SPAEKS) was synthesized by nucleophilic polycondensation. The sol-gel method was used to prepared functional titania inorganic particles (L-TiO₂). The ¹H NMR and FT-IR were performed to verified the structure of Am-SPAEKS and L-TiO₂. The organic-inorganic hybrid membranes showed both good thermal stabilities and mechanical properties than that of Am-SPAEKS. The L-Am-15% membrane exhibited the highest Young's modulus (2262.71 MPa) and Yield stress (62.09 MPa). The distribution of L-TiO₂ particles was revealed by SEM. Compared to Am-SPAEKS, the hybrid membranes showed higher proton conductivities. The L-Am-15% exhibited the highest proton conductivity of 0.0879 S cm⁻¹ at 90 °C. The results indicate that the organic-inorganic hybrid membranes have potential for application in proton exchange membrane fuel cells.     

Crosslinked poly(vinyl alcohol)/sulfonated poly(ether ether ketone) blend membranes for fuel cell applications—Surface energy characteristics and proton conductivity

Ionic polymers, their blends and composites are considered potential candidates for application as electrolytes in fuel cells. While developing new materials for membranes, it is important to understand the interactions of these electrolytic materials with electrodes/catalysts and with reactants/products. Some of these interactions can be understood by estimating the surface energy and wettability of the membrane materials. In this work, polyvinyl alcohol with varying degrees of sulfonation and its blend with sulfonated poly(ether ether ketone) are prepared and studied for their wettability characteristics using goniometry. The surface energy and its components are estimated using different approaches and compared. Properties such as the ion-exchange capacity, the proton conductivity and the water sorption/desorption behaviour are also investigated to understand the relationship with wettability and surface energy and its components. Among the different methods, the van Oss acid-base and the modified Berthelot approaches yield comparable estimates for the total surface energy.     

Semi-interpenetrating polymer network electrolyte membranes composed of sulfonated poly(ether ether ketone) and organosiloxane-based hybrid network

A semi-interpenetrating polymer network (semi-IPN) proton exchange membrane is prepared from the sulfonated poly(ether ether ketone) (sPEEK) and organosiloxane-based organic/inorganic hybrid network (organosiloxane network). The organosiloxane network is synthesized from 3-glycidyloxypropyltrimethoxysiane and 1-hydroxyethane-1,1-diphosphonic acid. The semi-IPN membranes prepared were stable up to 300 °C without any degradation. The methanol permeability is much lower than Nafion^® 117 under addition of the organosiloxane network. The proton conductivity of semi-IPN membranes increases with an increase the organosiloxane network content; the membrane containing the 20-24 wt% organosiloxane network shows higher conductivity than Nafion^® 117. The power density of the MEA fabricated with the semi-IPN membrane with 24 wt% organosiloxane network is 135 mW cm⁻², much better than that of the pristine sPEEK membrane, 85 mW cm⁻². Chemical synthesis of the semi-IPN membranes is identified using FTIR, and its ion cluster dimension examined using SAXS. The dimensional stability associated with water swelling and dissolution is investigated at different temperatures, and the semi IPN membranes dimensionally stable in water at elevated temperature.     

Cross-linked miscible blend membranes of sulfonated poly(arylene ether sulfone) and sulfonated polyimide for polymer electrolyte fuel cell applications

Cross-linked miscible blend (CMB) membranes were prepared from sulfonated poly(arylene ether sulfone) (SPAES) and sulfonated polynaphthalimide (SPI). They were transparent and insoluble in solvents. They showed the intermediate properties between SPAES and SPI concerning mechanical strength, water uptake, membrane swelling and proton conductivity. As for membrane swelling and proton conductivity, SPAES was almost isotropic, whereas SPI was highly anisotropic. CMB membranes were moderately anisotropic and had the advantages of the smaller in-plane membrane swelling and the larger through-plane conductivity compared to SPAES and SPI, respectively. Polymer electrolyte fuel cell performance of CMB2 membrane with an equal weight ratio of SPAES/SPI and an ion exchange capacity (IEC) of 1.74 meq g⁻¹ was investigated, compared to SPI membrane (R1) with a slightly higher IEC of 1.86 meq g⁻¹. At 90 °C, 0.1 MPa and relatively high humidification of 82/68% RH or 0.2 MPa and low humidification of 50-30% RH, CMB2 showed the reasonably high cell performances. At 110 °C and 50-33% RH, the cell performance was fairly high only at a high pressure of 0.3 MPa, but low at 0.2-0.15 MPa. At these conditions, the cell performance was better for CMB2 than for R1 due to the more effective back-diffusion of water formed at cathode into membrane. CMB2 showed the fairly high PEFC durability at 110 °C.     

Effects of casting solvent on microstructrue and ionic conductivity of anhydrous sulfonated poly(ether ether ketone)-inoic liquid composite membranes

As the key component of polymer electrolyte membrane fuel cells, the membrane has significant effect on the performance of fuel cells. The commonly used approach for preparation of membrane is solvent casting. In this paper, high temperature polymer electrolyte membranes consisting of sulfonated poly(ether ether ketone) and 1-butyl-3-methylimidazolium tetrafluoroborate were prepared using solvent casting process from N,N-dimethylformamide, N,N-dimethylacetamide, and N-methyl-2-pyrrolidone solutions to understand the solvent effect on the nature of formed membranes. It was found that solvents used for casting process strongly affect the microstructure and ionic conductivity of formed membranes. The composite membrane cast from DMF solution has clearly inter-connected ionic clusters with diameters of several hundreds nanometers to about 1.5 μm and exhibits the highest ionic conductivity, reaching 1.04 × 10⁻² S cm⁻¹ at 170 °C under anhydrous conditions.     

Comparison of alkaline fuel cell membranes of random & block poly(arylene ether sulfone) copolymers containing tetra quaternary ammonium hydroxides

A series of modified anion conductive block poly(arylene ether sulfone) copolymer membranes containing a selective substituted unit, 15%, 20% and 25% 4,4′-(2,2-diphenylethenylidene) diphenol, were prepared for use in alkaline fuel cells. The anion exchange membranes were synthesized by first introducing chloromethyl groups. Quaternary ammonium groups could then be added to the tetra-phenyl ethylene units, followed by subsequent ion exchange. The tetra quaternary ammonium hydroxide polymers showed high molecular weights and exhibited high solubility in polar aprotic solvents. The block copolymer membrane showed higher ionic conductivity (21.37 mS cm⁻¹) than the random polymer membrane of similar composition (17.91 mS cm⁻¹). The membranes showed good chemical stability in 1.0 M KOH solution at 60 °C. They were characterized by ¹H NMR, FT-IR, TGA and measurements of ion exchange capacity, water uptake and ionic conductivity.