Interfacial Distribution of Nafion Ionomer Thin Films on Nitrogen-Modified Carbon Surfaces
Wataru Yoshimune, Nobuaki Kikkawa, Hiroaki Yoneyama, Naoko Takahashi, Saori Minami, Yusuke Akimoto, Takuya Mitsuoka, H. Kawaura, Masashi Harada, Norifumi L. Yamada, Hiroyuki Aoki
Abstract
Chemically modified carbon supports for the cathode catalyst layers of polymer electrolyte fuel cells (PEFCs) show considerable promise for boosting the oxygen reduction reaction. This study evaluated the ionomer distribution of Nafion ionomer thin films on nitrogen (N)-modified carbon surfaces along their depth direction. Neutron reflectivity (NR) measurements performed using the double-contrast technique with H2O and D2O revealed that the introduction of N functional groups to carbon thin films promoted ionomer adsorption onto the surface under wet conditions (22 °C, 85% relative humidity). Molecular dynamics (MD) simulations conducted to verify the origin of the robust contact between the ionomer and N-modified carbon surface revealed an ionomer adsorption mechanism on the N-modified carbon surfaces, which involved Coulomb interactions between the positively charged carbon surface and the ionomer side chains with negatively charged sulfonic acid groups. The positive surface charge, which was determined using the contents of the N functional groups estimated by X-ray photoelectron spectroscopy, was found to be sufficient as an impetus for ionomer adsorption. This strategy involving NR measurements and MD simulations can provide insights into the solid–ionomer interfacial structures in a cathode catalyst layer and can therefore be extensively employed in studies on PEFCs.