Ionically cross-linked polyelectrolyte complex (PEC) membranes of cationic chitosan (CS)
and anionic poly(acrylic acid) (PAAc) were synthesized and assessed for applicability in fuel cells. CS
and PAAc were blended in different weight ratios and the resulting membranes were posttreated to enable
the formation of the polyelectrolyte complex. The ionic cross-linking occurring on blending the
polyelectrolytes excludes the need of using other cross-linking agents. These membranes were extensively
characterized for morphology, their intermolecular interactions, thermal stability, and physicomechanical
properties using SEM, FTIR, DSC, sorption studies, and tensile testing, respectively. Methanol
permeability and proton conductivity were estimated and compared with respective values for Nafion
117. PEC membranes exhibited high ion exchange capacity (IEC), high proton conductivity, low methanol
permeability, and adequate thermal and mechanical stability. Among the blends synthesized, the
membrane blend with 50 wt % of CS and 50 wt % of PAAc, was identified as ideal for direct methanol
fuel cell (DMFC) applications as it exhibited low methanol permeability (3.9 × 10-8 cm2/s), excellent
physicomechanical properties and comparatively high proton conductivity (0.038 S·cm-1). Above all, the
cost-effectiveness and simple fabrication technique involved in the synthesis of such PECs makes their
applicability in DMFC quite attractive.
Incorporation of multiwalled carbon nanotubes (MWNT) on the gas permeation properties of H2, CO2, O2, and N2 gases in poly(ether-block-amide) (Pebax-1657) membrane has been investigated. Pebax-1657 was dissolved in the ethanol−water mixture and cast on an ultraporous polyethersulfone substrate followed by complete solvent evaporation. Nanocomposite membranes were prepared by dispersion of MWNT in concentrations of 0−5% of polymer weight in the Pebax solutions with sonication for 2 h to ensure uniformity. Cross-linking was carried out in hexane medium using 2,4-toluylene diisocyanate (TDI). The permeabilities of pure gases were measured at room temperature, and the ideal selectivities were determined at pressures varying from 1−3 MPa using an indigenously built high-pressure gas separation manifold. For neat Pebax membrane, high permeabilities of 55.8 and 32.1 barrers were observed for CO2 and H2 gases, respectively, whereas that of N2 was as low as 1.4 barrers. The selectivity of cross-linked 2% MWNT Pebax membrane was enhanced from 83.2 to 162 with increasing feed pressure (1−3 MPa) for the CO2/N2 gas pair, whereas the corresponding values for H2/N2 and O2/N2 systems were found to be in the range 82.5−90 and 7.1−6.8, respectively. The membranes were characterized by scanning electron microscopy (SEM) to study surface and cross-sectional morphologies. Fourier transform infrared (FT-IR), wide-angle X-ray diffraction (WAXD), and ion exchange capacity (IEC) studies were carried out to determine the effect of MWNT incorporation on intermolecular interactions, degree of crystallinity, and extent of cross-linking, respectively. Fractional free volume (FFV) calculations based on density measurements were conducted along with water sorption studies to explain permeation behavior. The use of modified block copolymer membranes provides a means for separation of CO2 from N2 in power plants, H2 recycle from ammonia purge gas, O2 enrichment from air for medical applications, and CO2 removal from water-gas shift reaction to improve H2 yield.
The viability of using composite membranes of heteropolyacid (HPA)/polysulfone (PSF), HPA/sulfonated polysulfone (SPSF) for use in proton exchange membrane fuel cells (PEMFC) was investigated. PSF and its sulfonated polymer, SPSF was solution-blended with phosphotungstic acid, a commercially available HPA. Fourier transform infrared (FTIR) spectroscopy of the HPA-40/SPSF composite exhibited band shifts showing a possibility of intermolecular hydrogen bonding interaction between the HPA additive and the sulfonated polymer. The composite membranes exhibited improved mechanical strength and low water uptake. The conductivity of the composite membrane, HPA-40/SPSF, consisting of 40 wt % HPA and 60 wt % SPSF [with a degree of Sulfonation (DS) of 40%] exhibited a conductivity 0.089 S/cm at room temperature that linearly increased upto 0.14 S/cm at 120 8C, whereas the widely used commercial membrane Nafion 117, exhibited a room temperature conductivity of 0.1 S/cm that increased to only 0.12 S/cm at 120 8C. In contrast, the composite of HPA-40/PSF exhibited a proton conductivity of 0.02 S/cm at room temperature that increased only to 0.07 S/cm at a temperature of 100 8C. The incorporation of HPA into SPSF not only rendered the membranes suitable for elevated temperature operation of PEMFC but also provides an inexpensive alternative compared to Nafion. V V C 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: [1538][1539][1540][1541][1542][1543][1544][1545][1546][1547] 2005
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