Quantifying Water in PEM Fuel Cell Catalyst Layer with Small Angle Neutron Scattering (2019)

A polymer electrolyte membrane (PEM) fuel cell requires effective water management to achieve high performance. Balancing the water content and distribution is particularly important in a catalyst layer (CL), consisted of catalyst on carbon (typically Pt/C) covered by ionomer films. The accumulation of product water in the pores impedes reactant transport; however, simultaneously, the lack of water results in poor proton conductance in the ionomer. A diagnostic tool for measuring water content within the CL of an operating fuel cell is necessary for the development of the CL, especially for low Pt loading and non-precious metal group catalyst [1]. The common method for measuring water content in operando fuel cells is imaging based on synchrotron X-ray and neutron sources [2]. By using imaging method, water in the membrane electrode assembly (MEA) can be visualized on the micron-scale pixel resolution. However, this is not an ideal technique for probing water content in the CL for two main reasons: (1) the water in the ionomer cannot be distinguished from that in the pores, and (2) the spatial resolution is often not sufficient (as it is several times of the pixel resolution). At the French Alternative Energies and Atomic Energy Commission (CEA), we have developed in situ small angle scattering (SAS) techniques for fuel cell application based on neutron and synchrotron X-ray sources (Institut Laue-Langevin and European Synchrotron Radiation Facility). These techniques utilize focused neutron or X-ray beam to scan the areas of the MEA, each scanned point producing a scattering pattern. By radial averaging, the scattering pattern is converted into Q-range. The scattering due to the perfluorosulfonic acid (PFSA) ionomer produce a peak in the range of 0.1-0.25 Å -1 , which arises from the separation between hydrophobic and hydrophilic domain. The water uptake in ionomer can be deduced from the shift of the peak position resulting from the swelling upon hydration. On the other hands, the overall water content can be obtained by either direct transmission or analyzing the high Q-range (0.4-0.45 Å -1 ). We previously measured liquid water accumulation (in the channels and the GDL) and water uptake in the membrane by through-plane SAS (i.e. the fuel cell is aligned perpendicular to the beam) [3]. Recently, we have further refined the custom fuel cell and SAS technique to probe water content only in the CLs. This was accomplished by developing a cell with an active area of 0.1 cm 2 (0.1 mm wide in the path of the beam) and narrowing the beam size to 3 and 15 µm (for X-ray and neutron, respectively). This work will provide an overview of in situ SAS technique for fuel cells and focus on the comparison of two types of SAS investigations (neutron- vs. synchrotron X-ray-based). References [1] G. Wu, K.L. More, C.M. Johnston, P. Zelenay, High-Performance Electrocatalysts for Oxygen Reduction Derived from Polyaniline, Iron, and Cobalt, Science. 332 (2011) 443–447. doi:10.1126/science.1200832. [2] C. Hartnig, I. Manke, MEASUREMENT METHODS | Structural Properties: Neutron and Synchrotron Imaging, In-Situ for Water Visualization, in: J. Garche (Ed.), Encycl. Electrochem. Power Sources, Elsevier, Amsterdam, 2009: pp. 738–757. doi:10.1016/B978-044452745-5.00078-2. [3] A. Morin, G. Gebel, L. Porcar, Z. Peng, N. Martinez, A. Guillermo, S. Lyonnard, Quantitative Multi-Scale Operando Diagnosis of Water Localization inside a Fuel Cell, J. Electrochem. Soc. 164 (2017) F9–F21. doi:10.1149/2.1401614jes.