MRI Study of the Influence of the Catalytic Layer on the Interfacial Water Transfer in PEMFC Membrane-Electrode Assemblies

Abstract

A better understanding of the limitations induced by decreasing platinum content in PEMFC membrane-electrode assemblies, both in terms of operation and aging of the electrodes and ionomer membrane is still needed. More specifically, more information and understanding of water transport across the membrane / electrode interface is required. Few experimental techniques can be used to measure the water interfacial resistance in these sandwiches as the method must be able to measure both the water flux through the assembly and the water content at the membrane interface. Among them, magnetic resonance imaging (MRI) can be effectively exploited by using surface coils to enhance the NMR signal and improve the quality of the measurement [1,2]. In the present study, MRI was used to distinguish the water diffusion resistance within thick Nafion-type PFSA membranes from the water transfer interfacial resistance. Measurement of the water profiles across the membrane allowed for accurate evaluation of the evolution of the diffusion coefficient as a function of water content. The determination of the water content at the interface and of the water flux through the membrane allowed the determination of the interface resistance for different hygrometry conditions imposed on both sides. The experiments were then repeated on membrane / electrode and electrode / membrane / electrode samples to evaluate the impact of the electrode layer. Our results show a strong evolution of the interfacial resistance as a function of the air humidity while the influence of the electrode does not seem to be preponderant. Figure (left): NMR experimental cell containing a Nafion N1110 sample fed by two streams of humid air with controlled flow rate and relative humidity (RH). (right): examples of water content () profiles measured by MRI (6 µm spatial resolution) in different RH conditions.