Electrochemical and Ultra-Small Angle X-Ray Scattering Study of Ionomer Adsorption on Catalytic Sites in PEFC Electrodes

Abstract

The composition of the catalyst ink has a crucial impact on the electrode performance in the polymer electrolyte fuel cells (PEFCs). The presence of the perfluorosulfonic acid (PFSA) ionomer in the membrane electrode assembly (MEA) catalyst layer results in a sluggish oxygen reduction reaction (ORR) kinetics compared with an ionomer-free catalyst layer. Moreover, recently it has been shown that a thin film with high resistance to oxygen transport is formed around the catalyst particles due to its interaction with the ionomer.1 In order to improve the performance of the catalyst layer and eliminate or minimize the loss in both the ionic conductivity and electronic conductivity due to the ionomer-catalyst particles interaction in the next generation fuel cells new encased platinum group metal catalysts have been designed. Nanoporous materials have a wide range of technological applications2-6 and are potential candidates to improve the performance of the PEFC catalysts. Ordered inorganic nanoporous structures with well-defined and uniform pores are used to host the catalyst nanoparticles and block the access of the ionomer aggregates to the catalyst surface. In order to validate this assumption a series of catalyst inks with different ionomer: catalyst ratios (I/C) in isopropyl alcohol aqueous solvent were prepared. The ORR activity of the catalysts in thin film electrodes formed from these inks has been determined. The activity measurements are complemented by ultra-small-angle X-ray scattering (USAXS) studies on the inks and electrodes to determine the effect ionomer on the microstructure of the catalyst aggregates. References C. Cetinbas, R. K. Ahluwalia, N. N. Kariuki, and D.J. Myers, J. Electrochem. Soc., 165(13), F1051 (2018).A. Farghaly, R. K. Khan, and M. M. Collinson, ACS Appl. Mater. Interfaces, 10(25), 21103 (2018).Demir, A. A. Farghaly, M. J. Decuir, M. M. Collinson, and R. B. Gupta, Mater. Chem. Phys., 216, 508 (2018).Nahar, A. A. Farghaly, R. J. A. Esteves, and I. U. Arachchige, Chem. Mater., 29(18),7704 (2017).A. Farghaly and M. M. Collinson, Langmuir, 32(23), 5925 (2016).A. Farghaly, M. Lam, C. J. Freeman, B. Uppalapati, and M. M. Collinson, J. Electrochem. Soc., 163(4) H3083 (2016). Acknowledgements This work was supported by the United States Department of Energy, Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office. This research used resources of the Advanced Photon Source (APS), a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract DE-AC02-06CH11357.