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 electrode structures. In electrode structures, ionomers are major contributors to unexplained large mass-transfer over potentials in PEFCs [1, 2]. Thin (< 1 µm) and ultra-thin (< 0.025 µm) ionomer films in the catalyst layer serve as ionic charge carriers to catalytic sites. Low resistance to gas transport for increased supply of reactants to catalytic sites is required for optimal performance. However, increase in transport resistance of thin-film ionomers is observed indicating deviation from bulk (> 1 µm thickness) ionomer properties [2]. This resistance is further exasperated with decrease in platinum loading, which is an essential consideration for decrease in cost of PEFCs [3]. 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 increase in local mass transfer resistance at the catalyst layer to the decrease in permeability of thin-film ionomers as compared to that of bulk. However, direct measurement of thin-film ionomer membranes has not been done. Substantial amount of work has been done to study gas transport and permeation in bulk ionomer films [4]. However, much remains to be explored in thin and ultra-thin ionomer films. In this talk, we present data for gas permeation in thin and ultra-thin PFSA films. Thin ionomer films are 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 and variations of permeability of thin films from bulk films as a function of temperature and thickness will be presented. In addition, to tackle the complex and not fully understood effect of water in gas transport, gas permeability as a function of humidity will also be presented. Acknowledgement We 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 and the Assistant Secretary for Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, of the U. S. Department of Energy under contract number DE-AC02-05CH11231.