NMR characterization of proton exchange membranes in controlled hygrometry conditions

Abstract

NMR methods are highly sensitive to the state of water adsorbed in ionomer membranes for PEM fuel cells and electrolyzers. This study demonstrates the potential of NMR and MRI to reliably and accurately determine a wide range of water parameters such as proton chemical shift, transverse relaxation time, water content, and membrane thickness. Measurements are carried out in-situ in membranes in contact with humid air flows that are precisely controlled in terms of flow rate, temperature, and humidity. This precise management of the climatic conditions makes it possible to detect very small variations in the parameters measured, linked to the hydrothermal history of the membrane described in the literature. This history is erased when the membrane is humidity cycled several times. In addition, we demonstrate the use of this MRI imaging method to determine the resistance to water transfer across the membrane/humid air interface under steady-state conditions. Finally, we propose a refinement of the imaging method that reduces acquisition time by a factor of two, without degrading the 1D water content profile. This improvement gives us access to the study of transient hydration regimes typical of PEMFC and PEME operation.