Gas Permeation Study in Thin and Ultra-Thin Ionomer Films

Commercially available perfluorinated sulfonic-acid (PFSA) ionomers like Nafion and 3M-ionomer are widely used both as electrolyte membranes and as conductive binders in electrode structures in polymer-electrolyte fuel cells (PEFCs). However, these ionomers are also major contributors to unexplained large mass-transfer overpotentials at high current densities in PEFCs[1]. Bulk ionomer films (> 1 µm thickness) in electrolyte membranes provide charge separation, ion transport, and prevent reactant/product gas crossover. For increased performance in PEFCs, bulk ionomers of high conductivity, high selectivity, and low gas permeability are desired. In electrode structures, thin (< 1 µm) and ultra-thin (< 0.025 µm) films serve as ionic charge carriers to catalytic sites. Here, low resistance to gas transport for increased supply of reactants to catalytic sites is required. These conflicting properties required for two separate functions of ionomers in bulk and thin film make optimizing mass-transport very difficult. Complicating ionomer and transport optimization even more, conditions such as temperature, humidity, casting method, annealing, and confinement result in thin-film properties that deviate from those of bulk. Therefore, thorough understanding of mass transport resistances in both bulk and thin film ionomers is key in increasing efficiency in PEFCs.

Significant amount of work has been done to study gas transport and permeation in bulk ionomer films [2,3]. 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). Dry O2, N2, H2 and CO2 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, an experimental set-up and approach for measuring effects of humidity on gas permeability of thin films 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.
References
[1]Nonoyama, N. et al (2011). Journal of Electrochemical Society. 158(4)
[2] Buchi FN, Wakizoe M, Srinivasan S. J Electrochem Soc 1996;143:927–32.
[3] Ogumi Z, Takehara Z, Yoshizawa S. J Electrochem Soc 1984;131:769–73