Probing Water Properties of Perfluorinated Sulfonic-Acid Membranes With Humidity-Controlled Terahertz Time-Domain Spectroscopy

Perfluorinated sulfonic acid (PFSA) membranes are renowned for their unique proton conduction and chemical/mechanical stability. As water plays a crucial role in their proton conduction that changes with environmental humidity, here we evaluate the robustness of our recently proposed humidity-controlled terahertz time-domain spectroscopy (THz-TDS) on commercially available membranes with different morphologies to quantify water uptake (WU) and states for direct comparison against literature values. We further apply the technique to resolve membrane hygral swelling and shrinkage during humidity cycles towards future dimensional stability evaluation. As a whole, this work highlights the broad applicability of humidity-controlled THz-TDS for testing PFSA membranes for future product optimizations.