Evolution During Accelerated Stress Test of Performance and of the Catalyst Layer's Ionomer Physical and Chemical Structures in Proton Exchange Membrane Fuel Cell

Abstract

Catalyst layer’s ionomer plays a crucial role in the operation of Proton Exchange Membrane Fuel Cell (PEMFC) not simply because it binds the catalyst nanopowder but mostly because it allows protons transport and affects both gas diffusivity and water management [1]. In the catalyst layer, the ionomer is a perfluorosulfonic acid polymer similar to that use in the membrane as electrolyte. It is in the form of a few nanometer thick film dispersed around the carbon support or as aggregates larger than 150 nm within the pores of the carbon network [2,3]. In contrast with bulk ionomer membrane or model thin films that have been extensively studied [ref], little is known on the structural and functional properties of this proton conducting ionomer inside the electrode, and even less on their evolution upon aging. To date, it has proven difficult to selectively probe the fluorinated polymer dispersed at the nanoscale in a few micrometer thick electrode using conventional laboratory characterization techniques (electrochemistry, spectroscopy, and microscopy). Especially, its evolution upon aging is still an open question whereas the degradation of the ionomer due to radical attack was demonstrated and extensively investigated. Only Morawietz and co-workers evidence a thinning of the ionomer after operation [4]In this study, we studied the evolution of performance of a Membrane Electrode Assembly (MEA) after Accelerated Stress Test (AST) known to induce severe membrane degradation due to radicals, as well as the physical and chemical structures of the ionomer in the catalyst layer. Small Angle Neutron Scattering (SANS) spectra have been extensively analysed to characterise its swelling behaviour as a function of relative humidity. Its chemical structure was investigated by elemental analyses and XPS measurements, in addition to measurements of ion exchange capacity and of vapour sorption isotherms. Electrochemical characterisations including polarisation curve, impedance measurements and limiting current analyses in dry and wet conditions were performed to assess limiting phenomena and to try to unravel the contribution of the ionomer properties in the loss of performance. The SANS measurements clearly evidence an evolution of the structure and swelling behaviour of the ionomer after AST but, despite multiple characterisations, they can be hardly related to the evolution of electrochemical characteristics. Jinnouchi, R. et al. The role of oxygen-permeable ionomer for polymer electrolyte fuel cells. Commun. 12, (2021).Ueda, S., Koizumi, S., Ohira, A., Kuroda, S. & Frielinghaus, H. Grazing-incident neutron scattering to access catalyst for polymer electrolyte fuel cell. Phys B Condens Matter 551, 309–314 (2018).Morawietz, T., Handl, M., Oldani, C., Friedrich, K. A. & Hiesgen, R. Influence of Water and Temperature on Ionomer in Catalytic Layers and Membranes of Fuel Cells and Electrolyzers Evaluated by AFM. Fuel Cells 18, 239–250 (2018).Morawietz, T. et al. High-resolution analysis of ionomer loss in catalytic layers after operation. J Electrochem Soc 165, F3139–F3147 (2018). Figure 1