PEFC Catalyst Layer Saturation Determination Using Representative Structure Modeling via Small Angle X-Ray Scattering

Abstract

In polymer electrolyte fuel cells (PEFC), water management is one of the key aspects that governs the overall performance of a cell. Water is needed to hydrate the membrane in order to achieve good proton conductivity, while at the same time water accumulation in the catalyst and gas diffusion layer may block reactant supply to the catalytic sites. Commonly, X-ray and neutron radiography methods have been used in ex-situ and operando imaging experiments to study the water distribution and transport in the porous structures of the PEFC (1, 2). However, due to the lack of temporal and/or spatial information of these methods, as well as the risk of inducing radiation damage to the ionomer (3), the knowledge about catalyst layer saturation is limited. Here, we explore in which ways small angle X-ray scattering (SAXS) can be used as a diagnostic tool to investigate saturation on the nanometer scale particularly for PEFC catalyst layer application. So far, SAXS has been used mainly in PEFC to study Pt-nanoparticle size changes (4) and determine membrane hydration state (5). Recently, uSAXS has been employed to determine catalyst particle agglomeration in catalyst layer inks (6). An alternative technique, also in the small angle scattering family, small angle neutron scattering has been used to study water saturation in the catalyst layer in-situ (7).This study provides new insights into interpreting reciprocal space SAXS data with the help of 3D representative structure models of the catalyst layer morphology and in-silico wetting experiments. Virtual experiments for different filling mechanisms were implemented on 3D realizations of simulated Pt/C representative morphology. For each filling mechanism a specific signature can be identified in the simulated SAXS intensity profiles (see Figure 1 a-c). The simulation results of the fully wetted state were confirmed by ex-situ SAXS measurements of Pt/C catalyst layers with n-decane as model liquid (see Figure 2). Beyond the verification of the methodology, the presentation will discuss results of recent in-situ catalyst layer wetting and operando experiments. References A. Bazylak, International Journal of Hydrogen Energy, 34, 3845 (2009). P. Boillat, E. H. Lehmann, P. Trtik and M. Cochet, Curr Opin Electroche, 5, 3 (2017). J. Roth, J. Eller and F. N. Buchi, J Electrochem Soc, 159, F449 (2012). M. Povia, J. Herranz, T. Binninger, M. Nachtegaal, A. Diaz, J. Kohlbrecher, D. F. Abbott, B. J. Kim and T. J. Schmidt, ACS Catal, 8, 7000 (2018). N. Martinez, G. Gebel, N. Blanc, N. Boudet, J. S. Micha, S. Lyonnard and A. Morin, ACS Appl Energ Mater, 2, 3071 (2019). M. Wang, J. H. Park, S. Kabir, K. C. Neyerlin, N. N. Kariuki, H. F. Lv, V. R. Stamenkovic, D. J. Myers, M. Ulsh and S. A. Mauger, ACS Appl Energ Mater, 2, 6417 (2019). J. Lee, S. Escribano, F. Micoud, G. Gebel, S. Lyonnard, L. Porcar, N. Martinez and A. Morin, ACS Appl Energ Mater, 3, 8393 (2020). Figure 1