Quantitative video-rate hydration imaging of Nafion proton exchange membranes with terahertz radiation

D.F Alves-Lima,R Letizia,R Degl’Innocenti,R Dawson,H Lin

doi:10.1016/j.jpowsour.2019.227665

Abstract

Nafion membranes are considered as the industry standard electrolyte material for proton exchange membrane fuel cells. These membranes require adequate hydration in order to reach a high proton conductivity. The relatively high sensitivity of terahertz radiation to liquid water enables contrast to be observed for inspecting water presence in Nafion electrolyte membranes. Utilising a commercially available terahertz source and camera, this paper investigates the feasibility of a compact terahertz imaging system for visualising and quantifying liquid water during an ambient air desorption process for Nafion membranes of a wide range of thicknesses – NRE-212 (50 μm), N-115 (127 μm), N-117 (180 μm) and N-1110 (254 μm). We demonstrate that the terahertz imaging system is able to quantify liquid water in the 25–500 μm thickness range, estimate membrane weight change related to liquid water desorption, which correlated well against simultaneous gravimetric analysis and visualise the room temperature liquid water desorption process of a partially hydrated Nafion N-117 membrane.

Highlights

Polymer electrolyte membrane fuel cells (PEMFC) are electrochemical devices at the forefront of clean energy production for portable, transportation and, to a lesser extent, stationary applications [1]
Water plays an interesting role in PEMFC operation: Nafion polymer electrolyte membrane (PEM) requires hydration to operate and reactant inlet streams are humidified [7], but at the same time, water is a by-product of the electrochemical half-reaction at the cathode and must be swiftly removed to prevent cell flooding and reactant starvation at active electrocatalyst sites [8]
Terahertz liquid water quantification In order to assess the ability of the proposed terahertz imaging system to estimate liquid water thickness in the membrane, terahertz intensity images of the ‘dry’ and ‘hydrated’ liquid cell for a range of spacer thicknesses were acquired and compared

Summary

Introduction

Polymer electrolyte membrane fuel cells (PEMFC) are electrochemical devices at the forefront of clean energy production for portable, transportation and, to a lesser extent, stationary applications [1]. In the context of off-line water uptake and retention studies in Nafion membranes, techniques such as dynamic vapour sorption, differential scanning calorimetry, EIS and confocal micro-Raman spectroscopy [45,46,47,48,49,50,51] have been used. These are sensitive measurements for water content in Nafion membranes, with μm resolution in the case of Raman spectroscopy, generally they cannot spatially resolve liquid water distribution over large areas at video-rate and may require detailed sample preparation, calibration models, or specialised equipment with temperature and humidity control. To assess the system’s ability to spatially resolve water distribution across a Nafion membrane, we imaged the desorption profile of a partially hydrated Nafion membrane at room temperature under ambient conditions

Methods

Results

Conclusion