Date of Award

Spring 5-2011

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Polymers and High Performance Materials

Committee Chair

Dr. Kenneth Mauritz

Committee Chair Department

Polymers and High Performance Materials

Committee Member 2

Dr. Robert Lochhead

Committee Member 2 Department

Polymers and High Performance Materials

Committee Member 3

Dr. Sergei Nazarenko

Committee Member 3 Department

Polymers and High Performance Materials

Committee Member 4

Dr. Jeffrey Wiggins

Committee Member 4 Department

Polymers and High Performance Materials

Committee Member 5

Dr. Robson Storey

Committee Member 5 Department

Polymers and High Performance Materials

Abstract

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' 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.

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