Gas Permeation Study in Thin and Ultra-Thin Ionomer Films

Abstract

Significant advances in the performance of polymer-electrolyte fuel cells (PEFCs) can be attributed to perfluorinated sulfonic-acid (PFSA) ionomers, like Nafion and 3M-ionomer. They serve both as electrolyte membranes and as conductive binders in porous catalyst-layer structures. In PEFC catalyst structures, ionomers are thought to be major contributors to unexplained large mass-transfer overpotentials.[1-3] Thin (< 1 µm) and ultra-thin (< 0.025 µm) ionomer films in the catalyst layer serve as ionic charge carriers to catalytic sites, where low resistance to gas transport is required for optimal performance, yet it is believed that such resistance is increased compared to bulk ionomer. This resistance is further exasperated with decrease in platinum loading, which is an essential consideration for commercial viability and cost reduction of PEFCs.[4] Much has been done to shed light on the aberration of ionomer thin-film from bulk in other properties such as proton conductivity, morphology and water uptake. This has allowed many to attribute the rise in local mass-transfer resistance at the catalyst layer to the decrease in permeability of thin-film ionomers as compared to that of bulk membrane. However, direct measurement of permeability of thin-film ionomer has yet to be accomplished. A significant amount of work has been done to study gas transport and permeation in the bulk ionomer membrane.[5] However, much remains to be explored in thin and ultra-thin ionomer film thickness regime. Effects such as confinement, processing conditions, and surface and interfacial interactions have been pointed to as the prime cause of deviation of thin-film polymer behavior from that of the bulk. Linking the extent of impact of these effects to ionomer properties can elucidate source of transport resistance in the catalyst layer. In this work, we will present data for gas permeation in thin and ultra-thin PFSA films including free-standing thin ionomer films (> 400nm) as well as thin and ultra-thin ionomer films supported on well studied, highly permeable rubbery poly(dimethylsiloxane) (PDMS). O2, N2 and H2 permeability of the ionomer film is measured using a constant-volume, variable-pressure permeation system. Experimental results demonstrate dependence of gas permeation on thickness, and influence of physical aging on gas transport. AcknowledgementWe would like to thank Norman Su for providing assistance in permeation system assembly and operation. This work made use of facilities at the Biomolecular Nanotechnology Center at University of California, Berkeley and the Molecular Foundry at Lawrence Berkeley National Laboratory. This work was funded in part by the University of California Chancellor's Graduate Fellowship, National Science Foundation Graduate Fellowship and the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U. S. Department of Energy under contract number DE-AC02-05CH11231.