Date of Award
Doctor of Philosophy
Solid-state nuclear magnetic resonance (NMR) has been used to explore the nanometer-scale structure of Nafion, the widely used fuel cell membrane, and its composites. We have shown that solid-state NMR can characterize chemical structure and composition, domain size and morphology, internuclear distances, molecular dynamics, etc. The newly-developed water channel model of Nafion has been confirmed, and important characteristic length-scales established. Nafion-based organic and inorganic composites with special properties have also been characterized and their structures elucidated.
The morphology of Nafion varies with hydration level, and is reflected in the changes in surface-to-volume (S/V) ratio of the polymer obtained by small-angle X-ray scattering (SAXS). The S/V ratios of different Nafion models have been evaluated numerically. It has been found that only the water channel model gives the measured S/V ratios in the normal hydration range of a working fuel cell, while dispersed water molecules and polymer ribbons account for the structures at low and high hydration levels, respectively.
Although the cross-section morphology of Nafion has been derived from SAXS data, the structure in the third dimension, which is channel straightness, was not clear. With 2H NMR, D2O can be used as a probe to study channel straightness (persistence length). In drawn Nafion with straight channels, the exchange between bound and free D2O results in a residual quadrupolar splitting of 1-2 kHz; while in normal Nafion with coiled channels, the 2H quadrupolar splitting of D2O is ~ 10-fold smaller. It is explained by the motional averaging of the 2H coupling frequencies in the NMR timescale when D2O diffuses through differently-oriented segments. The simulations of line narrowing and T2 relaxation times of D2O revealed a persistence length within 30 to 80 nm for normal Nafion.
The Nafion phosphatranium composite developed by Verkade and Wadhwa, which is a potential candidate for anion exchange membranes, has been characterized by solid-state NMR. The synthesized membrane has two major components, in which phosphatranium cations are bonded to Nafion side-groups via either P or N with a mole ratio of 2:1. Degradation of the phosphatranium cations has not been found in the composite membrane, which implies a good stability of the material.
Nafion-silica (NafSil) and Nafion-zirconium phosphate (NafZrP) composites prepared by the in-situ growth of inorganic particles in the channels of Nafion membrane have been characterized. Under typical situations with an inorganic volume fraction of around 15%, elongated nanoparticles are formed inside the water channels. The inorganic particles have cylindrical shapes with a cross-section area of ~ 6 nm2 and surrounded by water layers with a thickness of ~ 0.8 nm.
Zirconium phosphates (ZrP) synthesized in and outside Nafion have been characterized in detail by solid-state NMR and X-ray diffraction (XRD). It has been found that typical α-ZrP with water of crystallization transforms to anhydrous α-ZrP and condensed pyrophosphates after drying at 150oC. When grown in Nafion, ZrP favors a structure with two disordered layers and a majority of (HO)P(OZr)3 sites, different from regular α-ZrP, particularly after drying.
Kong, Xueqian, "Characterization of proton exchange materials for fuel cells by solid state nuclear magnetic resonance" (2010). Graduate Theses and Dissertations. 11912.