Novel ion-conductive polymeric materials for fuel cell applications .

摘要

The work described in this thesis deals with the design, synthesis, and characterization of ionically conductive polyphosphazenes for fuel cell applications, specifically as polymer electrolyte membranes. In addition, the thermal decomposition of a monomeric polyphosphazene precursor was examined. Chapter 5 of this thesis describes the control of the water uptake of sulfonimide substituted polyphosphazenes by the introduction of a silicate network. Chapter 6 describes the synthesis and characterization of pendant cyclic polyphosphazenes for use as proton exchange membranes. Chapter 7 describes the synthesis and characterization of anion-conductive polyphosphazenes for use as anionically conductive membranes. Chapter 8 outlines the investigation of the state of water in hydrated proton exchange membranes by use of solid-state NMR techniques. A study of the thermal decomposition of trichloro-N-trimethylsilylphosphoranimine is described in the appendix.;Chapter 5 details the synthesis and characterization of composite materials composed of a sulfonimide substituted polyphosphazene and silicate networks prepared through the sol-gel technique. Substituted, hydrophobic, non-covalently attached silicate networks were prepared by the in-situ sol-gel condensation of trifluoropropyl trimethoxysilane, and the incorporation of the silicate network ranged from 5 to 20 weight percent. Samples of the composites were also exposed to between 5 and 20 MRad of gamma radiation to crosslink the polyphosphazene, and form inter-penetrating networks. Large decreases in the water uptake were observed in these materials. Composite materials that incorporated an unsubstituted silicate network were likewise prepared from tetraethyl orthosilicate, and were found to increase the water uptake.;Chapter 6 discusses the synthesis of proton conductive membranes based upon organic polymers with pendant cyclic phosphazenes. A 5-norbornene-2-methoxy substituent was introduced into the cyclic phosphazene, which was then polymerized via ring-opening metathesis polymerization. Hydrogenated polymers were substituted with 3-methylphenoxy groups, which were subsequently sulfonated to yield sulfonic acid functionalized polymers. The proton conductivity and methanol permeability of these materials were assessed. A maximum proton conductivity of 4.81 x 10-4 S cm-1 was measured, which is relatively low. However the minimum methanol permeability was 1.47 x 10-7 g cm-2 min-1, which is very low, thus making these materials candidates for direct methanol fuel cell membranes.;Chapter 7 describes the synthesis of anion conductive membranes. Polymers substituted with arylphosphonium ions were synthesized, and their water uptakes and ionic conductivities were measured. Although the ionic conductivities of these materials were low, they had low water uptake values, and thus may be useful in fuel cell applications.;Chapter 8 outlines the use of solid state NMR techniques to evaluate the state of water in proton exchange membranes. Samples of a sulfonimide substituted polyphosphazene and the commercial proton exchange membrane Nafion were hydrated with deuterium oxide, and the 2H T1 relaxation times were measured. These data indicated that solid, ice-like water is present in small quantities in these materials and, although the polyphosphazene absorbed much more water than Nafion, the amounts of ice-like water were similar. Consequently, it is concluded that the role of water in these two polymers is similar.;The appendix is a study of the thermal decomposition of trichloro-Ntrimethylsilylphosphoranimine. The stability of this phosphoranimine in several solvents at room temperature and at -3°C was examined through the use of phosphorus NMR, and preliminary experiments indicate that suitable medium term storage conditions are in diethyl ether at -3°C.