Modification and Evaluation of Fuel Cell Membranes

The primary goals of this study were modification of existing Nafion® membranes and characterization of newly developed hydrocarbon-based membranes for high temperature fuel cell applications. Various Nafion®/silicate nanocomposites were formulated via in situ sol-gel reactions for tetraethylorthosilicate. Different silicate composition profiles generated across membrane cross-sections were investigated by EDAX/ESEM. Composite water uptake, proton conductivity and fuel cell performance were comparable to that of unmodified Nafion®. Tafel analysis showed better electrode kinetics for composites having more silicate in the middle and less or no silicate at electrolyte-electrode interfaces. All composites showed reduced fuel cross-over and superior mechanical as well as chemical durability than unmodified Nafion®. Poly(cyclohexadiene) (PCHD) materials were characterized in the interest of developing alternative low-cost proton exchange membranes. All cross-linked sulfonated (xsPCHD) membranes showed significantly higher water uptake at 80 °C and higher proton conductivity at 120 °C at all relative humidities (RH), compared to the current benchmark membrane, Nafion®. A xsPCHD-poly(ethylene glycol) (PEG) copolymer and a xsPCHD-PEG blend surpassed the DOE target by exhibiting proton conductivities of 141.44 and 322.40 mS/cm, respectively, at 50 % RH. Although the PCHD-based PEMs exhibited thermal stability up to 150 °C, they showed poor mechanical properties which would cause poor membrane durability during fuel cell operation. Atomic force microscopy studies demonstrated nanophase separated morphology of xsPCHD having a higher degree of connectedness of hydrophilic domains in the copolymer and blends relative to the xsPCHD homopolymer. Broadband dielectric spectroscopy (BDS) was used to study sub-Tg relaxations in annealed poly(2,5-benzimidazole) (ABPBI) fuel cell precursor materials. A trend in degree of connectivity of charge migration pathways and conductivity with annealing temperature and time was uncovered. Solid state 1H and 13C NMR studies showed hydrogen bonding group mobility while wide angle X-ray diffraction investigations indicated an increase in chain packing efficiency vs. temperature. BDS studies also investigated the effect of acid doping on poly(benzimidazole) (PBI) membrane macromolecular dynamics and σdc conductivity, sdc. High ϵ\u27 values observed for acid doped samples in the low frequency regime could be due to membrane-electrode interfacial polarization. Distribution of relaxation time curves broadened while σdc increased with increase in acid doping level in the PBI membrane

Formulation and optimization of directly compressible floating tablets of famotidine using 23 factorial design

The aim of the present investigation was to develop a modified release effervescent floating drug delivery system of famotidine for 12 h dosage regimen to improve its bioavailability. Effervescent floating tablets were prepared by direct compression method taking into account its advantages over wet granulation by using directly compressible excipients like Carbopol® 71G and Cellactose® 80. The incorporation of sodium bicarbonate aided in the buoyancy with effervescent approach. The prepared tablets were evaluated for floating lag time (FLT), total floating time (TFT), in vitro drug release along with general parameters. 23 factorial design was used for optimization. The tablets showed desired release of more than 98 % over the period of 12 h which may increase bioavailability of selected candidate. The release of famotidine was found to be influenced by the polymer concentration. Optimized formulation showed acceptable stability over three months at 40 ºC and 75 % RH.Colegio de Farmacéuticos de la Provincia de Buenos Aire

High Temperature Proton Exchange Membranes With Enhanced Proton Conductivities At Low Humidity and High Temperature Based On Polymer Blends and Block Copolymers of Poly(1,3-Cyclohexadiene) and Poly(ethylene Glycol)

Hot (at 120 °C) and dry (20% relative humidity) operating conditions benefit fuel cell designs based on proton exchange membranes (PEMs) and hydrogen due to simplified system design and increasing tolerance to fuel impurities. Presented are preparation, partial characterization, and multi-scale modeling of such PEMs based on cross-linked, sulfonated poly(1,3-cyclohexadiene) (xsPCHD) blends and block copolymers with poly(ethylene glycol) (PEG). These low cost materials have proton conductivities 18 times that of current industry standard Nafion at hot, dry operating conditions. Among the membranes studied, the blend xsPCHD-PEG PEM displayed the highest proton conductivity, which exhibits a morphology with higher connectivity of the hydrophilic domain throughout the membrane. Simulation and modeling provide a molecular level understanding of distribution of PEG within this hydrophilic domain and its relation to proton conductivities. This study demonstrates enhancement of proton conductivity at high temperature and low relative humidity by incorporation of PEG and optimized sulfonation conditions

Proton Exchange Membranes for H2 Fuel Cell Applications

This chapter presents studies of sol-gel modifications of perfluorinated and hydrocarbon H2 fuel cell membranes using metal alkoxides, organoalkoxysilanes and combinations of these monomers. The impacts of these modifications on proton conductivity at high temperature and low relative humidity, fuel cell performance, chemical and mechanical durability and H2 and O2 crossover are discussed. Methods of tailoring the energetic environment, or polarity, within the polar cluster domains by insertion of inorganic oxide or organically modified silicate nanostructures by different chemistry routes are presented

Analysis of Nafion® Fuel Cell Membrane Chemical Durability Using Broadband Dielectric Spectroscopy

Changes in macromolecular dynamics associated with the T g - related β relaxation of Nafion®, with chemical degradation, were investigated using broadband dielectric spectroscopy. Loss permittivity vs. frequency spectra showed peak splitting reflecting microstructural heterogeneity. Spectra were analyzed using the conductivity-modified Havriliak-Negami equation and the Kramers-Krönig integral transform, both of which showed broadening and peak splitting for degraded samples. Peak broadening and bi and tri-modal character of the spectra of degraded samples was also manifest in distributions of relaxation times which reflected a broadening of molecular weight distribution and related microstructural heterogeneity induced by chemical degradation. © 2011 ECS - The Electrochemical Society

Broadband Dielectric Spectroscopy Studies of Glassy-State Relaxations in Annealed Poly(2,5-benzimidazole)

A broadband dielectric spectroscopic (BDS) study of poly(2,5‐benzimidazole) revealed three sub‐glass relaxations and the manner in which the time scales of these local molecular motions shift with annealing temperature and time. Also issuing from the BDS studies was a trend in the degree of connectivity of charge migration pathways and conductivity with annealing temperature and time. These studies were complemented with dynamic mechanical analysis, which showed the same relaxations, and solid state 1H and 13C NMR studies that showed hydrogen bonding group mobility versus temperature. Wide angle X‐ray diffraction investigations indicated an increase in chain‐packing efficiency that was used to rationalize the BDS and NMR results. Copyright © 2011 Society of Chemical Industr

High temperature proton exchange membranes with enhanced proton conductivities at low humidity and high temperature based on polymer blends and block copolymers of poly(1,3-cyclohexadiene) and poly(ethylene glycol)

Hot (at 120 °C) and dry (20% relative humidity) operating conditions benefit fuel cell designs based on proton exchange membranes (PEMs) and hydrogen due to simplified system design and increasing tolerance to fuel impurities. Presented are preparation, partial characterization, and multi-scale modeling of such PEMs based on cross-linked, sulfonated poly(1,3-cyclohexadiene) (xsPCHD) blends and block copolymers with poly(ethylene glycol) (PEG). These low cost materials have proton conductivities 18 times that of current industry standard Nafion at hot, dry operating conditions. Among the membranes studied, the blend xsPCHD-PEG PEM displayed the highest proton conductivity, which exhibits a morphology with higher connectivity of the hydrophilic domain throughout the membrane. Simulation and modeling provide a molecular level understanding of distribution of PEG within this hydrophilic domain and its relation to proton conductivities. This study demonstrates enhancement of proton conductivity at high temperature and low relative humidity by incorporation of PEG and optimized sulfonation conditions