Cross-linked Polymer Electrolyte Membranes Research Articles

Dually cross-linked polymer electrolyte membranes for direct methanol fuel cells

End-group crosslinkable sulfonated poly(arylene ether sulfone) copolymer (ESPAES) and imidazolium poly(arylene ether sulfone) copolymer (IPAES) are synthesized as a proton exchange membrane and ionic crosslinker, respectively. A novel dually cross-linked membrane (DCM) based on ESPAES is similar to an inter-penetrating network and is prepared via blending IPAES and thermal treatment for direct methanol fuel cell (DMFC) applications. The synergistic effects of end-group crosslinking and ionic crosslinking improve chemical and thermal stability and mechanical properties. In addition, the DMFC performance of the DCM outperforms that of the end-group cross-linked SPAES and Nafion® 212 due to its excellent fuel barrier property in spite of relatively low proton conductivity, which is derived from the content of the non-proton conducting IPAES copolymer. Consequently, the DCM has great potential as an electrolyte membrane for DMFC applications.

Dual cross-linked organic-inorganic hybrid polymer electrolyte membranes based on quaternized poly(ether ether ketone) and (3-aminopropyl)triethoxysilane

Quaternized poly(ether ether ketone)s (QPEEKs) are synthesized to absorb phosphoric acid (PA) and used as high temperature proton exchange membranes (HTPEMs). In order to improve their oxidative and mechanical stability without sacrificing proton conductivities, a series of dual cross-linked organic-inorganic hybrid membranes are prepared using (3-aminopropyl)triethoxysilane (APTES) as a cross-linker. The amine of APTES reacts with two benzyl bromide groups to build the primary cross-linking network. The Si–O–Si network generated by the hydrolysis of triethoxysilane in APTES is the secondary cross-linking network. The dual cross-linking hybrid networks improve the mechanical and oxidative stability of PA doped membranes. They can endure up to 15.3 h in 3 wt.% H2O2, 4 ppm Fe2+ Fenton solution at 80 °C. During the hydrolysis of triethoxysilane, the release of small molecules (H2O and C2H5OH) forms many pores in surfaces and interior of membranes. These pores and the resulted Si–OH groups corporately enhance the PA absorbing ability and proton conductivity. The highest proton conductivity is 61.7 mS cm−1 for PA-QPEEK-10%APTES at 200 °C under anhydrous condition. These membranes show great potential to be used in HTPEM fuel cell.

Fabrication and properties of poly(vinyl alcohol)-based polymer electrolyte membranes for direct methanol fuel cell applications

A series of poly(vinyl alcohol) (PVA)-based organic–inorganic crosslinked polymer electrolyte membranes with PVA and poly(methacrylic acid-2-acrylamido-2-methyl-1-propanesulfonic acid-vinyltriethoxysilicone) (PMAV) are prepared for direct methanol fuel cell applications. Fourier transform infrared (FTIR) spectroscopy measurements clearly reveal the existence of crosslinking reactions and molecular interactions in PVA–PMAV membranes. The results of TGA show that the PVA–PMAV membranes possess good thermal stability. The uptake behavior, methanol diffusion coefficient, proton conductivity and selectivity of membranes also are investigated as function of PMAV content. The results indicate that the PVA-based organic–inorganic crosslinked membranes are particularly promising to be used as polymer electrolyte membranes due to their excellent methanol barrier property, suitable proton conductivity and high selectivity.

Microbial extracellular polysaccharide-based membrane in polymer electrolyte fuel cells

The extracellular polysaccharides (EPS) produced by a bacterium, Rhizobium sp. MBJ1 (Accession No. JF713722) isolated from the stem nodules of Aeschynomene indica (L) was precipitated using acetone and characterized for its viscosity and composition of sugar. A cross-linked polymer electrolyte membrane was prepared by blending EPS and sulfosuccinic acid (SSA). SEM morphology showed the presence of SSA in the EPS matrix. Water sorption, ion-exchange capacity (IEC), thermogravimetric analysis (TGA) and proton conductivity properties for the membrane were investigated. The fabricated membrane electrode assembly comprising the EPS membrane showed a peak power density of 400mWcm−2 at a load current density of 1250mAcm−2 in polymer electrolyte fuel cells (PEFCs).

Cross-Linked Polymer Electrolyte Membranes with Enhanced Mechanical Stability for Alkaline Fuel Cells

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Proton conducting crosslinked polymer electrolyte membranes based on SBS block copolymer

AbstractA series of crosslinked polymer electrolyte membranes with controlled structures were prepared based on poly(styrene‐b‐butadiene‐b‐styrene) (SBS) triblock copolymer and a sulfonated monomer, 2‐sulfoethyl methacrylate (SEMA). SBS membranes were thermally crosslinked with SEMA in the presence of a thermal‐initiator, 4,4′‐azobis(4‐cyanovaleric acid) (ACVA), as confirmed by FT‐IR spectroscopy. The water uptake and ion exchange capacity (IEC) of membranes increased almost linearly with SEMA concentrations due to the increase of SO groups. However, the proton conductivity of membranes increased linearly up to 33 wt % of SEMA, above which it abruptly jumped to 0.04 S/cm presumably due to the formation of well‐developed proton channels. Microphase‐separated morphology and amorphous structures of crosslinked SBS/SEMA membranes were observed using wide angle X‐ray scattering (WAXS), small angle X‐ray scattering (SAXS), and transmission electron microscopy (TEM). The membranes exhibited good mechanical properties and high thermal stability up to 250°C, as determined by a universal testing machine (UTM) and thermal gravimetric analysis (TGA), respectively. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Preparation of the crosslinked polymer electrolyte membranes based on a hyperbranched poly(amidoamine) and their proton conductivity

AbstractA series of novel crosslinked polymer electrolyte membranes were successfully prepared based on the modification of a hyperbranched poly(amidoamine) with terminal vinyl groups. The membranes possessed the different contents of proton-generating sites (i.e., protonated tertiary amine groups) and triflate (Tf2N-) in the crosslinked network. They showed good mechanical and thermal stability. The water uptakes of them were ca. 8.4-24.5%. Their proton conductivity was of the order of ca. 10-5-10-2 S/cm in the range of 30-80 °C, and the proton conductivity increased with improving the protonation ratio. AFM results disclosed the micro-phase separation of the hydrophilic proton-generating sites and the hydrophobic domains of Tf2N- ions. The resulting locally continuous hydrophilic clusters could provide proton transport channels to produce the high proton conductivity. This kind of polymer electrolyte membranes may have potential applications in PEFCs and other electrochemical fields.

Highly Selective Polymer Electrolyte Membranes from Reactive Block Polymers

A series of reactive poly(norbornenylethylstyrene-s-styrene)-poly(n-propyl-p-styrenesulfonate) (PNS−PSSP) block polymers were prepared by atom transfer radical polymerization. Solutions containing PNS−PSSP, the cyclic olefins dicyclopentadiene and/or cyclooctene, and the second-generation Grubbs metathesis catalyst were prepared, cast as thin films, and allowed to cure at room temperature by a ring-opening metathesis polymerization mechanism. Small-angle X-ray scattering (SAXS) data on cured films were consistent with the formation of nanostructured materials containing PSSP domains confined in a cross-linked matrix of the metathesis-reactive PNS block and the poly(cyclic olefins). The PSSP phase in these films was converted into the sulfonic acid form by hydrolysis of the propyl sulfonate ester. The resulting cross-linked polymer electrolyte membranes (PEMs) were characterized by SAXS and transmission electron microscopy. A bicontinuous morphology with continuous domains of the sulfonic acid phase supported by a continuous and mechanically robust phase was evident in these films. The molecular weight of the PNS−PSSP block polymer controlled the domain sizes, and the mechanical properties of the membranes could be tuned through the choice of cyclic olefins used. The PEMs exhibited pronounced mechanical and thermal robustness. Furthermore, proton conductivities in all the PEMs were similar to those observed in Nafion (the most frequently used PEM in fuel cells) at high humidity. Select PEMs showed significantly lower methanol crossover than Nafion while maintaining high-saturated proton conductivities, which could result in higher direct methanol fuel cell power densities. This reactive block polymer strategy for the preparation of PEMs is attractive due to the ready formation of bicontinuous structures, the facile control of domain size, and the ability to independently control mechanical and swelling properties of the matrix material.

3-[[3-(Triethoxysilyl)propyl]amino]propane-1-sulfonic Acid−Poly(vinyl alcohol) Cross-Linked Zwitterionic Polymer Electrolyte Membranes for Direct Methanol Fuel Cell Applications

Recently, organic-inorganic nanocomposite zwitterionic polymer electrolyte membranes (PEMs) have attracted remarkable interest for application to the direct methanol fuel cell (DMFC) operated at intermediate temperature (100-200 degrees C). In this paper, we report the synthesis of an organic-inorganic hybrid zwitterionomer silica precursor with ammonium and sulfonic acid functionality by the ring-opening of 3-propanesultone under mild heating conditions and the preparation procedure of a proton-conductive and stable organic-inorganic zwitterion-poly(vinyl alcohol) (PVA) cross-linked PEM by sol-gel in aqueous media. Developed PEMs were extensively characterized by studying their physicochemical and electrochemical properties under DMFC operating conditions. These membranes were designed to possess all of the required properties of a proton-conductive membrane, namely, reasonable swelling, good mechanical, dimensional, and oxidative strength, flexibility, and low methanol permeability along with reasonable proton conductivity (4.85 x 10(-2) S cm(-1)) due to zwitterionic functionality. Moreover, from the selectivity parameter among all developed membranes, ZI-70 [zwitterionomer membrane with 70 wt % of PVA of 3-[[3-(triethoxysilyl)propyl]amino]propane-1-sulfonic acid in the membrane matrix], exhibited the best results in comparison to the Nafion117 membrane for DMFC applications.

Proton-conducting electrolyte membranes based on hyperbranched polymer with a sulfonic acid group for high-temperature fuel cells

The hyperbranched polymers ( HBP-SA-Acs) with both a sulfonic acid group as a functional group and an acryloyl group as a cross-linker at terminals in different ratios of sulfonic acid group/acryloyl group (SO 3H/Ac) were successfully synthesized as a new thermally stable proton-conducting electrolyte. The cross-linked hyperbranched polymer electrolyte membranes ( CL-HBP-SAs) were prepared by thermal polymerizations of the HBP-SA-Acs using benzoyl peroxide, and their ionic conductivities under dry condition and thermal properties were investigated. The ionic conductivities of the CL-HBP-SAs were found to be in the range of 2.2 × 10 −4 to 3.3 × 10 −6 S/cm, depending upon the SO 3H unit contents, at 150 °C under dry condition, and showed the Vogel–Tamman–Fulcher (VTF) type temperature dependence, indicating that proton transfer is cooperated by local polymer chain motion. All CL-HBP-SAs were thermally stable up to 260 °C, and they had suitable thermal stability as electrolyte membranes for the high-temperature fuel cells under dry condition. Fuel cell measurement using a single membrane electrode assembly cell with a cross-linked electrolyte membrane was successfully performed under non-humidified condition. It was demonstrated that applying the concept of dry polymer system to proton conduction is one possible approach toward high-temperature fuel cells.

Preparation and characterization of crosslinked cellulose/sulfosuccinic acid membranes as proton conducting electrolytes

A series of crosslinked polymer electrolyte membranes were prepared by blending cellulose and sulfosuccinic acid (SA) for fuel cell applications. The crosslinking reaction of membranes occurred via the esterification between –OH of cellulose and –COOH of SA, as confirmed by FT-IR spectroscopy. Both the ion exchange capacity and the proton conductivity increased in proportion to the increase of SA concentrations due to the increasing portion of charged groups in the membrane. In contrast, the water uptake linearly increased up to 25 wt.% of SA concentration, above which it decreased abruptly. The maximum behavior of water uptake may be a result of competitive effect between the increasing number of ionic sites and the increasing degree of crosslinking with the SA concentrations. Wide angle X-ray scattering also showed that the crystalline structures of cellulose disappeared upon the introduction of SA. The mechanical properties of cellulose/SA membranes, i.e., tensile strength at break and Young’s modulus, showed a maximum at 15 wt.% of SA, as revealed by universal testing machine. These membranes exhibited good thermal stability up to 250 °C, as revealed by thermal gravimetric analysis.

In‐situ Crosslinked Polymer Electrolyte Membranes from Thermally Reactive Oligomers for Direct Methanol Fuel Cells

AbstractThermally reactive ally‐terminated sulfonated oligosulfone was easily synthesized and made it possible to fabricate crosslinked membrane with high ionic group contents for direct methanol fuel cells. The crosslinked network structures of rigid rod sulfonated polyarylene sulfone showed high suppression of swelling with high sulfonation degree of 70%, which gave excellent properties for direct methanol fuel cell applications. The proposed membranes gave unprecedented reduction in methanol cross‐over (less than 3% of Nafion 112) and high ionic conductivity (∼80% of Nafion 112).

Synthesis and characterization of poly(ethylene oxide‐co‐ethylene carbonate) macromonomers and their use in the preparation of crosslinked polymer electrolytes

AbstractMethacrylate‐functionalized poly(ethylene oxide‐co‐ethylene carbonate) macromonomers were prepared in two steps by the anionic ring‐opening polymerization of ethylene carbonate at 180 °C, with potassium methoxide as the initiator, followed by the reaction of the terminal hydroxyl groups of the polymers with methacryloyl chloride. The molecular weight of the polymer went through a maximum after approximately 45 min of polymerization, and the content of ethylene carbonate units in the polymer decreased with the reaction time. A polymer having a number‐average molecular weight of 2650 g mol−1 and an ethylene carbonate content of 28 mol % was selected and used to prepare a macromonomer, which was subsequently polymerized by UV irradiation in the presence of different concentrations of lithium bis(trifluoromethanesulfonyl)imide salt. The resulting self‐supportive crosslinked polymer electrolyte membranes reached ionic conductivities of 6.3 × 10−6 S cm−1 at 20 °C. The coordination of the lithium ions by both the ether and carbonate oxygens in the polymer structure was indicated by Fourier transform infrared spectroscopy. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2195–2205, 2006

Chemical and radiation crosslinked polymer electrolyte membranes prepared from radiation-grafted ETFE films for DMFC applications

To develop a highly chemically stable polymer electrolyte membrane for application in a direct methanol fuel cell (DMFC), doubly crosslinked membranes were prepared by chemical crosslinking using bifunctional monomers, such as divinylbenzene (DVB) and bis( p, p-vinyl phenyl) ethane (BVPE), and by radiation crosslinking. The membranes were prepared by grafting of m, p-methylstyrene (MeSt) and p-tert-butylstyrene ( tBuSt) into poly(ethylene- co-tetrafluoroethylene) (ETFE) films and subsequent sulfonation. The effects of the DVB and BVPE crosslinkers on the grafting kinetics and the properties of the prepared membranes, such as water uptake, proton conductivity and chemical stability were investigated. Radiation crosslinking was introduced by irradiation of the ETFE base film, the grafted film or the sulfonated membrane. The membrane crosslinked by DVB and BVPE crosslinkers and post-crosslinked by γ-ray irradiation of the corresponding grafted film possessed the highest chemical stability among the prepared membranes, a significantly lower methanol permeability compared to Nafion ® membranes, and a better DMFC performance for high methanol feed concentration. Therefore, this doubly crosslinked membrane was promising for application in a DMFC where relatively high methanol concentration could be fed.