(Invited) Decoding the Symbiotic Relationship between Ionomer and Water in Cathode Catalyst Layers of PEM Fuel Cells
Authors: Eikerling Michael 
Published: November 2022

The pressing need for highly-performing and environmentally benign energy technologies continues to spur on research on polymer electrolyte fuel cells (PEFC). Among the components that are needed for a well-functioning, durable, and affordable PEFC, the cathode catalyst layer (CCL) continues to stand out. The main function to be provided by the CCL is to facilitate the oxygen reduction reaction (ORR). However, this function entails an intricate interplay of microscopic kinetics with the transport of electrons, protons, oxygen molecules, and water. A hierarchy of structural effects must be considered, as illustrated in Figure 1. Due to the complex composition and multiscale nature of the CCL, physical modeling has gained high importance in efforts to rationalize the dynamic interplay of structure, properties, and performance. The first part of the presentation will briefly review the capabilities of model-based analyses of experimental data to deconvolute and quantify voltage loss contributions,1 extract kinetic and transport parameters from fitting and discover systematic trends and correlations among these parameters,2 provide an activity map of the layer, and evaluate the overall effectiveness factor of Pt utilization.3 Macrohomogeneous modelling can propose or predict the optimal CCL thickness or macroscopic effective composition (for the target range of operating conditions).
Recognizing the importance of an optimal water-distribution for a well-functioning CCL, recent efforts in CCL modeling have been focusing on the consistent treatment of aspects like pore size distributions and pore network morphologies as well as wettability properties. The ionomer inclusions in the CCL play a crucial role in this context. At the microscopic scale, the ionomer film that forms an interface with the water-covered catalyst-support surface strongly impacts the local reaction environment that determines the rate of the ORR as well as that of platinum dissolution. Moreover, the structure and distribution of ionomer inclusions determine the wetting behaviour of pores and thus the water sorption properties of the porous composite CCL, which in turn affect the transport properties for oxygen and water. Recent forays in modelling that strive to unravel the intertwined impacts of ionomer and water will be presented. As a final consideration, a CCL cannot be understood and optimized as a stand-alone component. Overarching balances at the PEFC level in terms of reactant, charge, water, and heat fluxes must be considered. Comprehensive modeling approaches must account for the coupling of the corresponding local equilibria and transport phenomena across the whole cell, including polymer electrolyte membrane, diffusion media and flow fields. Efforts focusing on the coupled water fluxes across the cell are underway with promising results to look out for.

References.

1 M. Baghalha, J. Stumper and M. Eikerling, ECS Transactions 28, 159–167 (2010).

2 T. Muzaffar, T. Kadyk and M. Eikerling, Sustainable Energy & Fuels 2, 1189–1196 (2018).

3 M. Eikerling and A.A. Kulikovsky, Polymer Electrolyte Fuel Cells – Physical Principles of Materials and Operation, CRC Press Taylor & Francis Group, 2014.

Figure 1