Nuclear magnetic resonance microscopy of NAFION-117 proton exchange polymer membranes

Abstract

As the combustion of fossil fuels for the generation of energy and transportation becomes more expensive, of limited supply, and environmentally unsound, the development of viable fuel cell alternatives becomes more important. A comprehensive understanding of the proton exchange membranes (PEM's) used as electrolytes in certain types of fuel cells will play a major role in bringing the cost and reliability of PEM fuel cell systems down to a competitive level with traditional fossil fuel methods. Magnetic resonance microscopy (MRM) is well suited to the study of these membranes because it is non-invasive, and can spatially resolve material structure and give data on transport phenomena such as diffusion that cannot be determined by other methods. The goal of this research was to use magnetic resonance microscopy to study solvent mobility levels within the polymer membranes via spin-spin, T2, magnetic relaxation and diffusion mapping. The molecular mobility can quantify membrane swelling and spatial heterogeneity of the membrane material. A key aim of the research is to correlate these findings with previous bulk MRM studies of solvent within polymer membranes. Prior bulk MRM studies of solvent molecular mobility at different hydration levels were unable to study the membranes fully submersed in solvents, as the free solvent signal would dominate the nuclear magnetic resonance (NMR) signal from the solvent within the membrane. In this study spatial resolution of the MRM data provides the means to study fully saturated membranes, a condition of interest since the degree of hydration is related to membrane operational efficiency. The material homogeneity of the polymer in the thickness and surface directions of the membrane, an important factor in the reliable performance of fuel cells, was studied via T2 mapping. Nafion®-117 was the proton exchange membrane studied because it is currently the most popular electrolyte used in the PEM fuel cell industry and several bulk MRM studies have been conducted. Results indicate that both solvent mobility and membrane swelling are highly dependant on the concentration of methanol used to prepare the samples, as seen in the bulk studies, and that solvent mobility can vary on the 20 micron level within the polymer in both the thickness and surface directions. This research establishes MRM as an important tool for the study of individual proton exchange polymer membrane samples and provides a basis for extension to the study of membranes during operation.