Structure and Transport Properties of Ionomer Thin Films in Membrane-Electrode Assemblies

Abstract

Perfluorosulfonic acid (PFSA) electrolyte membranes, such as Nafion, are widely employed in fuel cells due to their high proton conductivity combined with excellent chemical and thermal stability. However, the structure and ion transport properties of such materials differs markedly from those of the bulk polymer when confined to thin films, as commonly found in the catalyst layers of membrane-electrode assemblies (MEAs). Furthermore, the chemical nature of adsorbing substrate, whether metal catalyst or carbon electrode, also has a large influence on the structure and properties. In this presentation, I will present results from density functional theory (DFT) and dissipative particle dynamics (DPD) simulations of model MEA systems in which the distribution of water and ionic groups at the interface in nanoscopic channels are compared with those in bulk membrane [1]. The results show that the structure of thin (5-10 nm) layers of PFSA membranes is quite different to that in bulk, and may play a key role in controlling performance of MEAs. In particular, the surface coverage of sulfonic acid groups is examined as a function of substrate hydrophilicity, hydration level and equivalent weight of ionomer. The results demonstrate that the anomalously voltage losses that occur at low catalyst loadings may be due in part to reduced effectiveness of proton transport in the ionomer films. [1] J.A. Elliott, D. Wu, S.J. Paddison and R.B. Moore, Soft Matter, 7, 6820-6827 (2011).