Tracking the hydration dynamics of Nafion fuel cell membranes using terahertz spectroscopy

Nafion membranes, known for their excellent proton conductivity, are widely used in Proton Exchange Membrane Fuel Cells (PEMFC), a promising clean energy source. Proton conductivity is highly dependent on membrane hydration. Water has strong absorption in the Terahertz (THz) frequency range and hence THz spectroscopy can be potentially used to understand the water dynamics in fuel cell membranes. In this study, Terahertz spectroscopy has been employed to track the water retention in a hydrated Nafion membrane by measuring the transmitted THz time domain pulses through the hydrated membrane for every minute up to 25 minutes. In addition, the complex permittivity has been extracted for each measurement by considering multiple reflections in the sample and is fitted with the double-Debye model. From these calculations, the dielectric constant and relaxation timescale have been extracted which provides more information on the population of bulk and ‘free’ water in the hydrated membranes. Results show 45 % decrease from the initial bulk-like water population by the end of 25 minutes which confirms reasonable water retention of Nafion for the measurement duration.