Solid State NMR Investigation of Electrolyte Materials For Hydrogen Fuel Cells

Abstract

<p> Today' s commercial proton exchange membranes for fuel cell applications use a liquid electrolyte such as water to facilitate the conduction process. The vapour pressure of water limits the operating temperature of a fuel cell, resulting in a decrease in efficiency as the electrolyte evaporates. Anhydrous electrolytes such as acidified polybenzimidazole or poly(vinyl-4-imidazole) are able to transport ions without using water as an electrolyte. </p> <p> The mechanism of ion transport involves the structural diffusion of the ions through the solid-state lattice. Compounds modeling the basic modes of the ionic conductivity are used in the solid-state nuclear magnetic resonance (NMR) investigation. The hydrogen-bonding structures of model compounds are established using diffraction paired with 1H solid-state double quantum NMR. The structural studies of the compounds reveal a continuous network of hydrogen bonded molecules. The structural motif is based on strong N-H••O and 0-H••O hydrogen bonds between the ions of the material. The dynamics of the hydrogen bonds observed in the 1H NMR and the multinuclear studies using the CODEX (Centerband Only Detection of EXchange) pulse sequence define the mechanism of ionic conductivity in these model compounds. </p> <p> These solid-state NMR techniques are then applied to a novel electrolyte material consisting of a solid electrolyte inside the pores of a host polymer material. This new material is able to transport protons at high temperatures without the use of an aqueous electrolyte. The properties and mechanism of ion transport is investigated using solid­ state NMR and impedance spectroscopy. </p>