Abstract – Nafion® polymeric membranes are the most common electrolyte material for proton exchange membrane fuel cells (PEMFC), requiring adequate hydration in order to reach high proton conductivity. The relatively high attenuation of terahertz (THz) radiation of liquid water enables a contrast to be observed for inspecting water build-up in Nafion® membranes. This paper investigates the feasibility of a compact THz system using a THz camera for liquid water imaging and quantification in a Nafion® polymeric membrane. I. INTRODUCTION EMFC are electrochemical devices at the forefront of clean energy production for portable, transportation and, to a lesser extent, stationary applications. Water management is a key issue in these devices: during the fuel cell operation, protons emerge from the anodic hydrogen oxidation and flow across the polymer electrolyte membrane (PEM) to the cathode, combine with the electrons coming from the external load circuit and oxygen in the process to form water. Product water must be promptly removed from the cell, as it can flood catalytically active sites and hinder reactant mass transport. At the same time, the prime material for PEMs is Nafion®, a perfluorinated ionomer membrane, which requires adequate hydration to achieve high proton conductivity and avoid material damage [1]. Therefore, a good balance of hydration needs to be maintained. As such, various techniques have been proposed to better understand the complex physical phenomena involving liquid water condensation and transport within the PEMFC. Examples of these techniques include MRI, neutron, x-ray, infrared and direct visualisation [2]. In general, techniques such as MRI, neutron, and synchrotron x-ray imaging/tomography can provide in-situ information on liquid water transport through visually opaque components during cell operation at high spatial resolution, but are limited in terms of time resolution and facility availability. Direct visualisation and infrared imaging use compact and portable instruments with high time and spatial resolution, but in turn require optically transparent windows in a PEMFC in order to yield useful information on liquid water content. Particularly for inspecting water build-up and dynamics in Nafion membranes, techniques such as differential scanning calorimetry, gravimetric vapour sorption and electrochemical impedance spectroscopy have been used [3,4]. Whilst these measurements show good sensitivity to monitor hydration in Nafion membranes, they do not provide any spatially resolved information on liquid water distribution. THz radiation can penetrate through optically opaque dielectric materials, such as polymers [5], while being strongly attenuated by liquid water [6] thus making THz imaging techniques attractive for non-destructive quality control applications [7-9]. With the recent availability of a compact, low-cost THz camera, as a first step toward PEMFC inspection, this paper explores the feasibility of a compact THz imaging system for inspecting liquid water in Nafion® PEMs. In particular, we assess system’s ability to 1) quantify liquid water thickness; 2) estimate water weight; 3) spatially resolve water distribution across the membrane. II. METHODOLOGY A THz imaging system operating in transmission geometry consists an IMPATT 0.1 THz source and a 16x16 array THz camera (TeraSense, USA), was realised. Nafion membranes of 180 µm thickness (Alfa Aesar, UK) were imaged using the system yielding THz intensity images in the form of videos at 30 Hz framerate and at a 1.5 mm pixel size. To benchmark the THz images against gravimetric analysis, the mass of the hydrated membrane was monitored throughout the membrane dehydration process. III. RESULTS To demonstrate the imaging system’s ability to quantify liquid water, THz transmission images were acquired through a custom-made liquid cell with pre-determined water thickness defined by the spacer thickness, where the relative intensities at regions of interest was found to be in a good agreement with the estimated intensities using Beer-Lambert law for a range of water thicknesses (Fig. 1). Fig. 1. THz liquid water content sensitivity analysis. Error bars account for spacer thickness variations (horizontal) and intensity standard deviation from 5 repeated measurements (vertical). Blue line and shade refer to mean and standard deviation between absorption coefficient values for the Beer Lambert Law. Following liquid water quantification, the dehydration process of a Nafion PEM was monitored in ambient conditions, where liquid water content in the membrane was measured, converted to estimated weight, and benchmarked against Investigating liquid water distribution in Nafion polymer electrolyte membrane with terahertz imaging Décio F. Alves de Lima, Rosa Letizia, Riccardo Degl'Innocenti, Richard Dawson, Hungyen Lin Department of Engineering, Lancaster University, Lancaster LA1 4YW, United Kingdom P