Proton Conductivity in Hydrated Sulfonated Porous Aromatic Framework
Résumé
Polymer electrolyte membrane fuel cells (PEMFCs) convert chemical energy into electricity efficiently. Nafion®, a branched fluorinated copolymer with sulfonic acid groups, is the leading material for proton conductive membranes due to its high ion exchange capacity and proton conductivity. However, it has limitations such as a narrow operating temperature range, high synthesis costs, and environmentally harmful degradation products. To overcome these issues, new inorganic, organic, and hybrid materials have been developed, offering high proton conductivities and improved water retention. Recently, we synthesized and investigated sulfonated porous aromatic framework (SPAF) aqueous electrolyte-host systems, which demonstrate high proton conductivities, excellent chemical and mechanical stability, and efficient water management.
This study investigates the water dynamics and proton conduction in SPAF with sulfonation degrees of 50% and 80% at 93% relative humidity, across three temperatures. To do so, quasielastic neutron scattering (QENS) measurements, which are highly sensitive to hydrogen atom motion, were utilized to analyze the diffusional, rotational, and jump processes of water molecules at the nanometer scale. In this way, the IN5B disk chopper time-of-flight spectrometer at ILL, known for its high flux and resolution (i.e., energy resolution of 100 μeV for the wavelength 5 Å), was employed to accurately measure and distinguish the contributions from various motion types and hydrogen subensembles.
According to measured spectra, molecular dynamics are consistently characterized by the combination of two independent motions: fast local motion around the average molecular position and confined translational jump diffusion of the molecule's center of mass. All molecules exhibited local relaxations, while the translational motion of some molecules was immobilized on the timescale of the instrument. Our findings provide a microscopic perspective on the dynamics of water within SPAFs with varying degrees of sulfonation, focusing on the self-diffusion coefficient, residence time, and their temperature dependence. It is demonstrated that the degree of sulfonation in SPAFs regulates the translational diffusion of water molecules, which plays a critical role in manufacturing fuel cells.