What Nano-Confined Ionomers Can Tell Us about Fuel Cell Performance
Authors: Tesfaye Meron , Kusoglu Ahmet , McCloskey Bryan D , Weber Adam Z. 
Published: February 2020

Several emerging and growing applications like biosensors, gas separation membranes, protective coatings, photonics, nanocomposites and microelectronics depend on polymers of thicknesses under 100nm[1]. Ion conducting polymers (ionomers) of thickness <100nm have induced significant advances in the performance of polymer-electrolyte fuel cells (PEFCs) by serving as binder in the catalyst layers (CLs), aiding proton conduction and extending the reaction zone of the CL porous electrode.[2] Thin and ultra-thin polymer films provide ease of processing, application specific tunable properties and reduction in material cost. However, due to deviation of nano-confined polymers from bulk behavior, they pose a challenge of predictability. This is evident in PEFCs where thin-film ionomers are large contributors to mass-transport losses at low catalyst loadings required for commercialization. In-situ investigations over the past 10 years have provided a significant amount of insight into these losses in CLs.[3] As a result, it would be misleading to employ the same principles of understanding used for bulk polymers for nano-confined polymers without modification. In our study, we aim to understand nano-confined ionomers in two ways. First, looking at impact of interfaces by using ionomers supported on different substrates and tuning substrate-ionomer interaction via reducing and oxidizing environment. Second, by examining the impact of cooperative motion and chain dynamics using thermal expansion study of nano-confined ionomers. Combining these two studies we can gain new understanding of effect of confinement on performance.
Understanding ionomer-catalyst surface interface and interaction can elucidate not only how ionomers impact catalyst performance but also how catalyst surface impacts ionomer performance. In our study we use water uptake and modulus to probe the effect of platinum surface on thin-film ionomers under oxidizing(Air) and reducing(H2) conditions. Several in-situ and ex-situ methods have demonstrated that oxide formation on the active platinum catalyst surface can reduce the catalytic activity, further hampering activity in the cathode catalyst layer[4]. Despite this, our studies show continued growth of oxides on platinum surface results in higher density of water maintained in the ionomer. This indicates that nano-confined ionomers have greater water stability in the cathode than on the anode catalyst layer, enhancing proton transport in the cathode catalyst layer. Using transition temperature as a proxy for material behavior, we also looked at relaxation behavior of nano-confined ionomers under slow heating conditions. For glass-forming polymers like perfluorinated sulfonic-acid (PFSA) ionomers (such as Nafion and 3M-ionomer) cooperative motion and dynamics of chains have been used to explain changes in modulus, thermal expansion, relaxation behavior, and gas transport. Thermal transition temperatures are often used as a broad physical description for these kinetically and thermodynamically-driven changes in polymer behavior[1]. In this study, we used heated cell ellipsometry to explore the presence of transition temperatures and their dependence on thickness and substrate in thin-film ionomers. We are able to see both α and β transition temperatures using ion exchanged nano-confined ionomers supported on silicon substrates. Our results demonstrate increased rate of expansion for thin films as compared to bulk and dramatic change of transition temperatures around 50-70nm indicating impact of ionomer-surface interaction and influence of electrostatic interaction on chain relaxation and gas transport.
Acknowledgement
We would like to thank Douglas Kushner for providing assistance and guidance in this study. This work made use of facilities at the Joint Center for Artificial Photosynthesis 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.
References
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[2] A. Parthasarthy, S. Srinivasan, A. J. Appleby, and C. R. Martin, J. Electroanal. Chem., vol. 339, no. 1–2, pp. 101–121, 1992.
[3] A. Z. Weber and A. Kusoglu, J. Mater. Chem. A, vol. 2, no. c, pp. 17207–17211, 2014.
[4] M. C. Smith, J. A. Gilbert, J. R. Mawdsley, S. Nke Seifert, and D. J. Myers, J. AM. CHEM. SOC., vol. 130, pp. 8112–8113, 2008.