SOLID­‐STATE NMR OF HIGH TEMPERATURE FUEL CELL ELECTROLYTES

Abstract

Current fuel cell operating temperatures have been unintentionally optimized for the exact conditions that poison the platinum electrode. Raising the operating temperature can eliminate poisoning. Nafion®, the current benchmark fuel cell electrolyte, operates optimally at 80oC due to its dependence on water for proton conduction. Two approaches are described here to address this issue: modification of Nafion to remain hydrated and the synthesis of new materials that can operate at high temperatures. Casting the polymer with smaller thicknesses reduces the path length and allows water to diffuse throughout the whole polymer easily. When nanothin versions of the film were cast, water uptake was found to be significantly higher. However, the observed proton conductivity decreased. Solid-state NMR was used to determine if local dynamics were an issue in nanothin films or whether long-range transport was hindered. For materials that operate via a Grotthuss mechanism, organic solid acids were studied due to their thermal and electrochemical stability and significant proton conductivity at high temperatures. For this thesis three main organic solid acids were studied, including benzimidazolium- and imidazolium- methanesulfonate (BMSA and IMSA) and imidazolium trifluoromethanesulfonate (IFMS). Solid-state NMR was used to study their hydrogen-bonded structure to determine the dynamics in each of the solid acids. The local dynamics information was then compared with their macroscale proton conductivity. To produce a suitable host for fuel cell applications, the solid acids were inserted into polymer supports to determine the best salt-host composite. Polymers supports that were investigated include Teflon, sulfonated polyetheretherketone (sPEEK) and sulfonated polysulfone (sPSU). This thesis focuses on the use of solid-state NMR to probe these composites and to study their mobility on a localized scale to explain their macroscale proton conductivity.