Invited: Study of Fundamental Aspects of Ionomer Structure in Different States

In PEFC system, the ionomer is used as a binder and an electrolyte membrane. The characteristics of low gas permeability and high proton conductivity are required for the electrolyte membrane, while, if the ionomer is used as a binder, conductivity should be high as well as electrolyte membrane, but in this case, high gas permeability should be better which is opposed to the case of membrane characteristic. Nafion® is used for binder and electrolyte membrane, but there still has been unclear factor about structural difference in both binder and membrane states, especially we do not know about the effect of structure and physical properties of ionomer in a catalyst layer on cell performance.

  In this study, we prepared model ionomer materials and electrodes, and conducted the structural analysis in order to understand the effect on the cell performance systematically.

  Perfluorinated sulfonic acid ionomer Nafion^® and di-block copoly(p-phenylene) ionomer having sequentially controlled hydrophilic and hydrophobic parts were targeted as the model ionomer¹ (Scheme 1). As the model electrode, we also used Pt-dispersed carbon electrodes in which several nanometers Pt particles dispersed². We performed microscopic observation and scattering analysis in order to understand the structures in solution and thin film solid states. 

  Here Nafion^® thin film was prepared on platinum nano-particle dispersed carbone electrode as a model for the ionomer film within a catalyst layer. We found that aggregated structure of ionomer changed with carbon surface area, the size of aggregated structure became large, and proton conductive domain also enlarged with the increase in carbon surface area. These results imply that the morphology and the property of ionomer covering catalyst support could be dominated by balance of interaction between carbon and platinum. In the case of hydrocarbon ionomer, it has been found that di-block copolymer forms higher order structure over a range of several hundred nm by means of AFM observation. Furthermore, the gas permeability of di-block copolymer film was comparable to that of Nafion^®. Details will be discussed at presentation.

Acknowledgement

Ministry of Economy, Trade and Industry (METI), Japan and New Energy and Industrial technology Development Organization (NEDO) are acknowledged for granting the work.

References

1. K. Umezawa, T. Oshima, M. Yoshizawa-Fujita, Y. Takeoka, and M. Rikukawa, ACS Macro Lett. 2012, 1, 969−972.

2. T. You, O. Niwa, M. Tomita, and S. Hirono, Anal. Chem. 2003, 75, 2080-2085.