(Invited) Small Angle Scattering As Unique Probe to Quantify Water Distribution and Ionomer Structure in Operating PEMFC

Abstract

The Proton Exchange Membrane Fuel Cell (PEMFC) is a highly complex energy conversion device coupling several electrochemical and transport phenomena occurring on a wide range of length-scales within a multilayered structure, named Membrane Electrodes Assembly (MEA). The fundamental description of all phenomena involved in PEMFC operation and degradation, and more particularly their interplay, is a contemplated pivotal requirement for improvements. This is a grand challenge, as a number of unsolved questions remain. In fact, all relevant physical and electrochemical phenomena are not fully identified or quantified. In the PEMFC, water plays a crucial role, since it is not only the main product of the electrochemical reaction but, above all, it is the fundamental common denominator to all the phenomena. In a real stack, as an open system converting H2 and O2, the operation and degradation are intrinsically heterogeneous resulting especially in a complex 3D distribution of water in the volume of the MEA. Among the most important component of the MEA is the ionomer, made of a PerFluoroSulfonic Acid (PFSA) polymer, which acts as a solid electrolyte to transfer proton and water to, from, and within the electrodes. The presence of highly hydrophilic sulfonic acid groups makes it, most probably, the most sensitive MEA component toward water activity/content. Indeed, it governs its swelling, structure as well as transport and mechanical properties. This property of the ionomer, along with its role, explains why PEMFC behaviour is so dependent on water activity. Still very recently, a work of Kongkanand et al.[1] emphasizes how ionomer is a crucial electrodes' constituent in view of enhancing performance. For these reasons, many researches are still devoted to studying the ionomer in relation with water activity and as well as to quantifying the 3D water distribution during PEMFC operation. It is well known that Small Angle Scattering (SAS) is the most suitable technique to characterize the structure of the ionomer[2]. A side from that, neutron is the probe of choice to quantify water within operating PEMFC[3]. For these reasons, we have been developing for several years SAS techniques, either with Neutrons or X-Ray, in order to measure simultaneously and operando, the water distribution and the ionomer nanostructure. Spatial and time resolutions offew µm² and 50 ms, respectively, have been achieved with the last generation of X-Rays sources such as the one of European Synchrotron Radiation Facility (ESRF), and ~4 mm² and 30 s with high neutron flux on D22 beamline at Laue Langevin Institute (ILL)[4],[5]. Thus, it is possible to follow the kinetics during transient. In particular, we have shown that, within the membrane, local water content is greater in front of the current collecting rib than in front of the gas supply channel. We have also evidenced some evolution of the physical structure of the membrane upon aging in real conditions depending on the local water content. We thus pushed forward these techniques, which now allows us to have unique information on the local water activity and ionomer structure within the CL. Our results demonstrate how SAS techniques are powerful to assess the main current scientific and technological issues of the PEMFC. [1] Kongkanand et al., ACS Energy Lett. 2018, 3, 618−621 [2] Fumagalli et al., J. Phys. Chem. B 2015, 119, 7068−7076 [3] Boillat et al., Current Opinion in Electrochemistry 2017, 5, 3–10 [4] Morin et al., J. Electrochem. Soc. 2016, 163, F9-F21 [5] Martinez et al., ACS Advanced Energy Materials 2019 Figure 1