Impact of Surface Hydrophilicity on Structure and Transport at the Catalyst-Ionomer Interface: Insights from Molecular Dynamics

Abstract

Understanding local reaction conditions at the catalyst/support-ionomer interface in the cathode catalyst layer of polymer electrolyte fuel cells is vital for improving cell performance and stability.1 The structure of the ionomer layer and the state of the catalyst support surface at the interface affects key properties of the system, such as water film structure and the distribution of protons and oxygen molecules at the catalyst-ionomer interface.2 The effect of catalyst surface facet and oxidation on the proton density accumulation at the interface was previously studied using molecular dynamics simulations.3 In this work, the interfacial region between catalyst and support surface and ionomer skin is further studied by expanding the classical molecular dynamics model. This water-filled nanopore model is constructed to study the impact of local charge density distribution, density of sidechains at the ionomer layer, and water layer thickness on equilibrium water structure, electrostatic conditions, and transport properties of water, hydronium, and molecular oxygen at the interface (Figure 1). Structural properties of the catalyst and support surfaces are obtained from explicit quantum mechanical Density Functional Theory simulations, and the effect of the electrode potential is simulated via surface charge density by varying the oxide coverage of both C and Pt. The analysis of simulations in a flooded pore indicates that surface hydrophilicity, which is represented by water adsorption and the formation of ice-like water clusters at the surface, is a major deterministic factor that affects interfacial proton density, ionomer sidechain mobility, and interfacial oxygen lateral movement. The results obtained in this work can guide the design of new catalyst materials, where the hydrophilicity of the surface can be tailored to minimize local proton transport resistance and improve electrode performance.Figure 1. Snapshot of the model system with the graphite slab partially covered by epoxide with a C/O ration of 4:1 as the support surface, and a Nafion skin layer of 1124 EW as the ionomer. Li, H.; Tang, Y.; Wang, Z.; Shi, Z.; Wu, S.; Song, D.; Zhang, J.; Fatih, K.; Zhang, J.; Wang, H.; Liu, Z.; Abouatallah, R.; Mazza, A., A review of water flooding issues in the proton exchange membrane fuel cell. Journal of Power Sources 2008, 178 (1), 103-117. Jahnke, T.; Futter, G.; Latz, A.; Malkow, T.; Papakonstantinou, G.; Tsotridis, G.; Schott, P.; Gérard, M.; Quinaud, M.; Quiroga, M.; Franco, A. A.; Malek, K.; Calle-Vallejo, F.; Ferreira de Morais, R.; Kerber, T.; Sautet, P.; Loffreda, D.; Strahl, S.; Serra, M.; Polverino, P.; Pianese, C.; Mayur, M.; Bessler, W. G.; Kompis, C., Performance and degradation of Proton Exchange Membrane Fuel Cells: State of the art in modeling from atomistic to system scale. Journal of Power Sources 2016, 304, 207-233. Spooner, J. A.; Eslamibidigoli, M. J.; Malek, K.; Eikerling, M., Molecular Dynamics Study of the Nanoscale Proton Density Distribution at the Ionomer-Catalyst Interface. Pacific RIM Meeting on Electrochemical and Solid State Science 2020, (I01A-2101). Figure 1