Sulfonated poly(arylene ether sulfone) ionomers containing fluorenyl groups for fuel cell applications

A series of sulfonated poly(arylene ether sulfone)s (SPEs) containing fluorenyl groups as bulky components were synthesized and characterized for fuel cell applications. Introduction of disodium 3,3′-disulfo-4,4′-difluorophenyl sulfone (SFPS) monomer gave ionomers with high acidity and accordingly high proton conductivity as well as high proton diffusion coefficient (D_σ) at low humidity. The membrane of SPE60 (where the number denotes mole percentage of the component containing sulfonic acid groups; IEC (ion exchange capacity) = 1.68 mequiv./g) exhibited high proton conductivity of 4.6 × 10⁻³ S/cm at 40% RH and 80 °C, which is one order of magnitude higher than that (6 × 10⁻⁴ S/cm) of our previous SPE (SPE-1, IEC = 1.58 mequiv./g). D_σ of SPE60 membrane was ca. 4 times higher than that of the SPE-1 membrane at low water volume fraction. SPE membranes showed good oxidative and hydrolytic stability as well as favorable thermal and mechanical properties. Small-angle X-ray scattering analyses showed that the phase separation of SPE membranes was much less developed than that of the perfluorinated Nafion membrane which accounts for lower hydrogen and oxygen permeability of the former membranes.     

Synthesis and properties of poly(ether ether ketone)-block-sulfonated polybutadiene copolymers for PEM applications

A novel series of sulfonated block copolymers were successfully synthesized by the condensation of modified poly(ether ether ketone) (PEEK) and polybutadiene (PB), followed by the selective post-sulfonation of PB blocks using acetyl sulfate as the sulfonating reagent. The sulfonic acid groups were only attached onto PB segments due to the high reactivity of double bonds to sulfonating reagent. The degree of sulfonation was controlled by changing the feed ratio of sulfonating reagent to block copolymer. PEEK-b-sPB could be easily cast into flexible and transparent membranes. The obtained membranes exhibited good thermal stability and satisfied mechanical properties. Tensile test showed the incorporation of sulfonate groups into PB blocks resulted in an increase in tensile strength and a decrease in elongation at break. TEM images revealed the existence of ionic spherical domains with the average sizes of 50-100 nm. Some of these small domains further aggregated to form large hydrophilic regions. The proton conductivity values were measured in the range of 10⁻² S/cm in water and increased with increasing IEC and temperature.     

Synthesis and properties of water stable poly[bis(benzimidazobenzisoquinolinone)] ionomers for proton exchange membranes fuel cells

A novel sulfonated tetraamine, di(triethylammonium)-4,4′-bis(3,4-diaminophenoxy)biphenyl-3,3′-disulfonate (BAPBDS), was successfully synthesized by nucleophilic aromatic substitution of 4,4′-dihydroxybiphenyl with 5-chloro-2-nitroaniline, followed by sulfonation and reduction. A high-temperature polycondensation of sulfonated tetraamine, non-sulfonated tetraamine (4,4′-bis(3,4-aminophenoxy)biphenyl) and 1,4,5,8-naphthalenetetracarboxylic dianhydride (a) or 4,4′-binaphthyl-1,1′,8,8′-tetracarboxylic dianydride (b) gave the poly[bis(benzimidazobenzisoquinolinones)] ionomers SPBIBI-a(x) or SPBIBI-b(x), where x refers to the molar percentage of the sulfonated tetraamine monomer. Flexible and tough membranes of high mechanical strength were obtained by solution casting and the electrolyte properties of the polymers were intensively investigated. The ionomer membranes displayed excellent dimensional and hydrolytic stabilities. Moreover, these novel membranes showed proton conductivities comparable to that of Nafion 117, especially at high temperature. In addition, the proton conductivities of the SPBIBI-a ionomer membranes were found to be higher than those of the SPBIBI-b ones due to the weakened acid–base interactions between the pyridinone ring and the sulfonic acid groups. The highest proton conductivity (0.174 S/cm) was obtained for the SPBIBI-a(100) membrane at 100 °C, with an IEC of 2.65 mequiv./g. A combination of excellent dimensional and hydrolytic stabilities indicated that the SPBIBI ionomers were good candidate materials for proton exchange membrane in fuel cell applications.     

Blend membranes of Nafion/sulfonated poly(aryl ether ketone) for direct methanol fuel cell

Sulfonated poly(aryl ether ketone) (sPAEK) synthesized by LG Chem. was confirmed by FT-IR. To estimate the thermal stability, glass transition temperature and decomposition temperature were investigated. They showed that sPAEK had good thermal properties. The proton conductivity, methanol permeability and water uptake of sPAEK were also measured. Nafion/sulfonated poly(aryl ether ketone) composite membranes were prepared by blending two materials. The blend ratios of sPAEK and Nafion were 2:1, 3:1, 5:1, and 7:1. The blend membranes showed phase separated morphology since they became immiscible during the solvent evaporation process. Due to the differences in specific gravity and solvent concentration profile during the solvent evaporation process, the upper region had lower Nafion volume fraction with smaller domains and the lower region had higher Nafion volume fraction with larger domains. Mechanical properties such as the stress at break, yield stress, Young's modulus, and elongation at break were measured. The sPAEK had better mechanical properties than Nafion. The mechanical properties increased with increasing sPAEK content. Proton conductivity and methanol permeability of the blend membranes were lower than those of Nafion. Both decreased with decreasing Nafion content. Since the methanol permeability of sPAEK was lower than that of Nafion, sPAEK acted as the methanol barrier. Water uptake of sPAEK was higher than that of Nafion.     

Fluorenyl-containing sulfonated poly(aryl ether ether ketone ketone)s (SPFEEKK) for fuel cell applications

A series of fluorenyl-containing sulfonated poly(aryl ether ether ketone ketone)s (SPFEEKK) were synthesized via aromatic nucleophilic substitution polymerization. The sulfonation content (SC) was controlled by the feed ratios of sulfonated and nonsulfonated monomers. Flexible and strong membranes in the sulfonic acid form were obtained from cast membranes in the sodium salt forms by treatment with acid. The thermal properties, water uptake, swelling ratio, water state, oxidative stability, proton conductivity and methanol permeability were investigated. All the polymers had proton conductivities greater than 1 × 10⁻² S/cm at room temperature, and the conductivity values of m-SPFEEKK-80 and p-SPFEEKK-80 were up to 1.86 × 10⁻¹ and 1.78 × 10⁻¹ S/cm at 100 °C. This series of polymers also possessed good dimensional stability in water and low methanol crossover.     

Synthesis and characterization of poly(vinylpyrrolidone-co-vinylphosphonic acid) copolymers

Radical copolymerizations of 1-vinyl-2-pyrrolidone (VP) with vinylphosphonic acid (VPA) at different feed ratios were investigated. The copolymers were characterized by ¹H-NMR, ¹³C-NMR and FT-IR. The copolymer composition was determined from the elemental analysis. Thermogravimetric analysis (TG) illustrates that the copolymers are stable up to 200 °C. Temperature dependence of the alternating current (AC) conductivities were investigated by means of impedance spectroscopy. The direct current (DC) conductivities of the samples are derived from the AC conductivity data.     

Preparation and characterization of fluorine-containing polybenzimidazole/imidazole hybrid membranes for proton exchange membrane fuel cells

Polybenzimidazole (PBI)/imidazole (Im) hybrid membranes were prepared from an organosoluble, fluorine-containing PBI with Im. The thermal decomposition of the PBI/Im hybrid membranes occurred at about 160 °C. The conductivities of the acid doped PBI/Im hybrid membranes increased with both the temperature and the Im content. The conductivity of acid doped PBI-40Im (molar ratio of Im/PBI = 40) reached 3.1 × 10⁻³ (S/cm) at 160 °C. The proton conductivities of PBI/Im hybrid membranes were over 2 × 10⁻³ (S/cm) at 90 °C and 90% relative humidity. The addition of Im could reduce the mechanical properties and methanol barrier ability of the PBI membranes.     

Synthesis and characterization of poly(aryl ether ketone) ionomers with sulfonic acid groups on pendant aliphatic chains for proton-exchange membrane fuel cells

A series of parent poly(aryl ether ketone)s bearing different content of unsaturated pendant propenyl groups were synthesized via nucleophilic substitution polymerization from 3,3′-diallyl-4,4′-dihydroxybiphenyl, 9,9′-bis(4-hydroxyphenyl) fluorene and 4,4′-difluorobenzophenone. The polymers with pendant aliphatic sulfonic acid groups were further synthesized by free radical thiol-ene coupling reactions between 3-mercapto-1-propanesulfonic sodium and the parent propenyl functional copolymers. The resulting sulfonated polymers with high inherent viscosity (1.83-4.69 dL/g) were soluble in polar organic solvents and can form flexible and transparent membranes by casting from their solutions. The copolymers with different ion exchange capacity could be conveniently synthesized by varying the monomers ratios. Transmission electron microscopy (TEM) was used to examine the microstructures of the membrane and the results revealed that significant hydrophilic/hydrophobic microphase separation with spherical, uniform-sized (5-10 nm) and well-dispersed hydrophilic domains was afforded. The proton conductivities of the as-prepared membranes and the state-of-the-art Nafion 117 membrane in fully hydrated state were investigated. The results revealed that the proton conductivity of the synthesized membranes increased more remarkably than that of Nafion 117 membrane with increasing temperature. The membrane with 1.69 mequiv/g of IEC had a conductivity of 2.5 × 10⁻² Scm⁻¹ at 100 °C. The membranes also possessed excellent mechanical properties, good thermal, oxidative, hydrolytic and dimensional stabilities.     

Cross‐linked poly(aryl ether sulfone) membranes for direct methanol fuel cell applications

Partially sulfonated poly(aryl ether sulfone) (PESS) was synthesized and methacrylated via reaction with glycidyl methacrylate (PESSGMA) and cross‐linked via radical polymerization with styrene and vinyl‐phosphonic acid (VPA). The chemical structures of the synthesized pre‐polymers were characterized via FTIR and ¹H NMR spectroscopic methods and molecular weight was determined via GPC. Membranes of these polymers were prepared via solution casting method. The crosslinking of the PESS polymer reduced IEC, proton conductivity, swelling in water, and methanol permeability of the membranes while increasing the modulus and the glass transition temperature. However, the introduction of the VPA comonomer increased the proton conductivity while maintaining excellent resistance to methanol cross‐over, which was significantly higher as compared with both PESS and the commercial Nafion membranes. Membranes of PESSGMA copolymers incorporating VPA, exhibited proton conductivity values at 60 °C in the range of 16–32 mS cm⁻¹ and methanol permeability values in the range of 6.52 × 10⁻⁹ – 1.92 × 10⁻⁸ cm² s⁻¹. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 558–575     

Sulfonated poly(fluorene-co-sulfone)ether membranes containing perfluorocyclobutane groups for fuel cell applications

A new class of sulfonated poly(fluorene-co-sulfone)ether membranes containing perfluorocyclobutane (PFCB) groups were synthesized and characterized in terms of their electrochemical properties as proton exchange membranes for fuel cells. Two monomers, 9,9-bis(4-trifluorovinyloxyphenyl)fluorene and 4,4′-sulfonyl-bis(trifluorovinyloxy)biphenyl were synthesized and statistically copolymerized by thermal [2π + 2π] cycloaddition to yield a series of polymers containing 0–60 mol% of fluorenyl content (PFS-X). The copolymers were then sulfonated using chlorosulfonic acid to afford five kinds of ionomers with different sulfonation levels (SPFS-X), which were cast into membranes and analyzed in terms of electrochemical properties. It was found that the ion exchange capacity (IEC), water uptake, proton conductivity and methanol permeability values of SPFS-X increased with the increment of the sulfonated fluorenyl content. The proton conductivities of SPFS-50 and -60 with high IECs and water uptake values were higher than those of Nafion-115 between 25 and 80 °C. The methanol permeability of SPFS-X was considerably lower than that of Nafion-115.     

Synthesis of novel crosslinked sulfonated poly(ether sulfone)s using bisazide and their properties for fuel cell application

Novel crosslinked sulfonated poly(ether sulfone)s (PESs) were prepared by thermal irradiation of the allyl-terminated telechelic sulfone polymers using a bisazide. The sulfonated polymers in different comonomer compositions were fully characterized by ¹H NMR, and the crosslinked structure was also verified by FT-IR spectroscopic analyses. Having both the uniform distribution of the hydrophilic conductive sites and controlled hydrophobic nature by minimized crosslinking over the rigid rod poly(ether sulfone) backbone, the crosslinked polymer membrane (PES-60) offered excellent proton conductivity of 0.79 S cm⁻¹ at 100 °C together with hydrolytic and oxidative stability. In addition, only 17% of methanol permeability of the Nafion^® was observed for the crosslinked PES-60.     

Hybrid organic-inorganic nanostructured membranes for high temperature proton exchange membranes fuel cells (PEMFC)

Compared to internal combustion engines, proton-exchange membrane fuel cells (PEMFC) operate with zero emissions of environmental pollutants being this an adequate choice for transportation field. The increase of the operation temperature of PEMFC above 130°C is a great concern for the commercial application of the cells in electric vehicles. Hybrid organic-inorganic nanostructured membranes can combine the main properties to meet this objective: high proton conductivity along with thermal and chemical stability. The possibilities of synthesis of these hybrid structures grow exponentially with the combination of sol-gel chemistry and monomers. Three different approaches have been followed for obtaining hybrid membranes that present the properties needed for application in high temperature PEMFC: development of methacrylate and epoxy structures, and optimization of the inorganic component incorporating phosphorus. Proton conductivity has been endowed on the base of three strategies: a high concentration of hydroxyl groups from inorganic component, groups through sulfonation of phenyl rings, and incorporation of tungstophosphoric acid, H₃[P(W₃O₁₀)₄].     

Sulfonated poly(fluorenyl ether) membranes containing perfluorocyclobutane groups for fuel cell applications

This paper describes the preparation and electrochemical properties of new proton conducting polymer membranes, sulfonated poly(fluorenyl ether) membrane-containing perfluorocyclobutane (PFCB) moieties for fuel cell applications. The sulfonated polymers were prepared via thermal cyclodimerization of 9,9-bis(4-trifluorovinyloxyphenyl)fluorene and subsequent post-sulfonation using chlorosulfonic acid (CSA) as a sulfonating agent. The post-sulfonation reaction was carried out by changing the molar ratio of CSA/repeating unit of the polymer at room temperature for 5 h and the resulting sulfonated polymers showed different degrees of sulfonation (DS) and ion exchange capacities (IEC). With the increment of CSA content, the DS, IEC and water uptake of the sulfonated polymer membranes increased. Their proton conductivity was investigated as a function of temperature. The polymer membrane with an IEC value of 1.86 mmol/g showed a water content of 25% similar to Nafion-115's but showed higher proton conductivity than Nafion-115 over the temperature 25–80 °C. The polymer membrane with lower water uptake and higher IEC showed similar proton conductivity and methanol permeability to Nafion-115. These results confirmed that the sulfonated poly(fluorenyl ether)-containing PFCB groups could be a promising material for fuel cell membranes.     

Synthesis and properties of poly(aryl ether ether ketone) copolymers with pendant methyl groups

Polyether ether ketone and polyether ether ketone copolymers were prepared by the nucleophilic substitution reaction of 4,4′-difluorobenzophenone with hydroquinone and with varying mole proportions of hydroquinone and methyl hydroquinone using sulfolane solvent in the presence of anhydrous K₂CO₃. The polymers were characterised by different physico-chemical techniques. The crystallinity of the polymers was found to decrease with increase in concentration of the methyl hydroquinone units in the polymer. Thermogravimetric studies showed that all the polymers were stable upto 430 °C with a char yield above 49% at 900 °C in N₂ atmosphere. The glass transition temperature was found to increase and the crystalline melting temperature and activation energy were found to decrease with increase in concentration of the methyl hydroquinone units in the polymer.     

Poly(arylene ether) ionomers containing sulfofluorenyl groups: Effect of electron-withdrawing groups on the properties

For polymer electrolyte membrane fuel membrane cell (PEMFC) applications, the effect of electron-withdrawing groups on the properties of sulfonated poly(arylene ether) (SPE) ionomer membranes was investigated. A series of poly(arylene ether)s containing fluorenyl groups and electron-withdrawing groups (sulfone, nitrile, or fluorine) was synthesized, which were sulfonated with chlorosulfonic acid using a flow reactor to obtain the title ionomers. The ionomers had high molecular weight (M_n > 77 kDa, M_w > 238 kDa) and gave tough, ductile membranes by solution casting. The ion exchange capacity (IEC) of the membranes ranged from 1.6 to 3.5 mequiv/g as determined by titration. The electron-withdrawing groups did not appear to affect the thermal properties (decomposition temperature higher than 200 °C). The presence of nitrile groups, especially at positions meta to the ether linkages, improved the oxidative stability of the SPE membranes, while it led to a deterioration of the hydrolytic stability. The perfluorinated biphenylene groups were effective in providing high mechanical strength with reasonable dimensional change, probably due to a somewhat decreased water absorbability. The SPE membrane containing sulfone groups showed the highest proton conductivity (10⁻³-10⁻¹ S/cm) at 20-93% RH (relative humidity) and 80 °C. The nitrile-containing SPE membrane showed smaller apparent activation energies for oxygen and hydrogen permeability and is thus considered to be a possible candidate for applications in PEMFCs.     

Fluorene-containing block sulfonated poly(ether ether ketone) as proton-exchange membrane for PEM fuel cell application

A series of sulfonated block poly(ether ether ketone)s with different sulfonic acid group clusters were successfully synthesized by nucleophilic displacement condensation. Membranes were accordingly cast from their DMSO solutions, and fully characterized by determining the ion-exchange capacity, water uptake, proton conductivity, dimensional stabilities and mechanical properties. The experimental results showed that the main properties of the membrane can be tailored by changing the cluster size of sulfonic acid groups. The membrane of block-7c(40) has good mechanical, oxidative and dimensional stabilities together with high proton conductivity (5.09 × 10⁻² S cm⁻¹) at 80 °C under 100% relative humidity. The membranes also possess excellent thermal and dimensional stabilities. These polymers are potential and promising proton conducting membrane material for PEM full cell applications.     

Sulfonated poly(ether ether ketone)-based silica nanocomposite membranes for direct ethanol fuel cells

This paper reports on the preparation and characterization of sulfonated poly(ether ether ketone) (sPEEK)-based mixed matrix membranes. The inorganic matrix consisted of silica: Aerosil^®380, tetraethoxysilane (TEOS) or a combination of both to obtain an interconnected silica network. The behavior of these membranes in ethanol–water systems was studied for application in a direct ethanol fuel cell (DEFC). Uptake measurements showed that the converted TEOS content had a strong influence on the hydrophilicity of the membranes. Proton conductivity was strongly related to the water content in the membrane, but the proton diffusion coefficients of membranes with various Aerosil^®380–TEOS combinations were similar. Dynamic measurements in liquid–liquid (L–L) and liquid–gas (L–G) systems were performed to study the ethanol transport through the membrane. No reduction in ethanol permeability was obtained in the L–L system, but a remarkable reduction was obtained in the L–G system when 2 M ethanol was applied. The reinforcing characteristic of the combined Aerosil^®380–TEOS-system were best observed at 40 °C with 4 M ethanol. The fuel cell performance prediction based on the selectivity of proton diffusion coefficient to ethanol permeability coefficient showed for nearly all composite membranes an improvement with respect to the polymeric reference. The presence of an inorganic phase led to relatively constant proton diffusion coefficients and lower ethanol permeability coefficients in comparison with the polymeric reference.     

Preparation and characterization of poly(ether sulfone)/sulfonated poly(ether ether ketone) blend membranes

Polymer blends of sulfonated poly(ether ether ketone) (SPEEK) and poly(ether sulfone) (PES) in N-methyl-2-pyrrolidinone (NMP) were prepared by solution casting. The investigation on water uptake, methanol uptake, permeability and proton conductivity has been conducted. The spin-lattice relaxation time in the rotating frame of PES/SPEEK blend was obtained from the results of cross-polarization magic angle spinning (CP/MAS) solid state ¹³C NMR. SPEEK blended with PES resulted in increasing , indicating the molecular motion of polymer chain was reduced. The glass transition temperature of the PES/SPEEK blend membranes were predicted by the Kwei equation. PES plays an important role in the decreasing water uptake, methanol uptake and methanol permeability while enhancing the thermal stability of the blend membrane, which shows the feasibility for direct methanol fuel cell.     

A composite proton-conducting membrane based on a poly(vinylidene)fluoride-poly(acrylonitrile), PVdF-PAN blend

This paper reports on the synthesis and the properties of a new microporous, composite proton-conducting gel membrane, formed by swelling a poly(vinylidene)fluoride- poly(acrylonitrile), PVdF-PAN blend-based matrix containing a dispersed Al₂O₃ ceramic filler with aqueous acid solutions. We show that this membrane has a high and stable conductivity, a proton transport not critically influenced by the relative humidity level, and a projected low cost. Tests in a methanol-air laboratory cell also demonstrate that the membrane is basically suitable for application in direct methanol fuel cells.     

Phase transition behavior of a poly(oxyethylene-b-butylene adipate) ionomer with shape memory properties

A series of biodegradable poly(oxyethylene-b-butylene adipate) ionomers (POBAi) were prepared by two-step in situ polymerization using adipic acid, 1,4-butanediol and mixed monomers of bis(poly(oxyethylene)) sulfonated dimethyl fumarate. The chemical composition of these POBAi was ascertained by ¹H NMR spectroscopy. The objective of this study was to investigate the shape memory effect of POBAi containing ionomer compared to non ionic POBA. It was observed that POBA5.0i showed a good shape memory effect than that of POBA 2.5 mol% or none of ionic group due to much physical cross-linking point by rich ionic group. Stress-induced phase transition was investigated during the shape deformation and recovery process using a wide-angle X-ray diffractometer (WAXD). The POBA crystal phase transition from β- to α-form was observed in all POBA samples by either thermal treatment or physical drawing. The α-form crystal did not recover to the initial β-form during the recovery process because the monoclinic α-form crystal is structurally more stable than the orthorhombic β-form crystal.