Non-invasive macroscopic and molecular quantification of water in Nafion® and SPEEK Proton Exchange Membranes using terahertz spectroscopy

Abstract Hydration of Proton Exchange Membranes (PEM) is vital in PEM fuel cells as it is critical in determining the overall proton conductivity. Traditional gravimetric analysis to quantify PEM's hydration is cumbersome, demands contact and importantly cannot differentiate the nature of water - bulk, bound or free water. As terahertz spectroscopy probes bulk and free water relaxations' contributions, it is an attractive non-invasive technique for studying membrane hydration dynamics - water uptake and retention. Here, THz spectroscopy measurements for hydrated Nafion ® and sulphonated Polyether Ether Ketone (sPEEK) membranes in ambient atmosphere have been carried out for 100 min. Variation in THz absorption by the hydrated membranes with time is used for non-invasive estimation of percentage water content demonstrating good agreement with gravimetric measurements. In addition, proportion of bulk, bound and free water with time has been estimated by fitting the complex permittivity using double Debye model. Results show that % water content in hydrated sPEEK at 0 th minute is 2.36 times than hydrated Nafion due to its higher porosity at 0 th minute. Further, sPEEK has better water retention capability, especially higher bound water proportion at longer time intervals (85% more at 100th minute) than Nafion due to the presence of widely distributed small pores and comparatively stronger hydrogen bonding between the sulphonic acid and water. Thus, THz spectroscopy can be employed as an effective non-invasive tool for macroscopic and molecular quantification of water in hydrated PEMs.