Ionomer film structure under a constant electric potential in proton exchange membrane fuel cells

Abstract

ABSTRACT Ionomer electrolyte covering Pt/C electrode provides the sites of electrochemical reactions and plays a significant role in the performance of proton exchange membrane fuel cells. The nanoscale ionomer films have been explored by many molecular dynamics (MD) simulations. However, the significant electric field formed in the electrode–electrolyte interface was usually ignored. In this study, MD simulations integrated with the constant electric potential method are carried out to explore the ionomer film structure on the charged Pt electrode. Pt atoms in the electrode have various and changing charges under the constant electric potential, which thus affects the ionomer film structure on the Pt electrode. Specifically, the perfluorosulfonic acid side chains become more stretched and tend to arrange on the location with positively charged Pt atoms. The water molecules rotate toward a preferred orientation and their diffusivity increases under a higher electric potential. In addition, the multilayered structure of ionomer film on the Pt electrode remains unchanged, whereas the water molecules closely on the negatively charged Pt electrode increases. This study provides an approach for the investigation of the electrode–electrolyte interface with a constant electric potential, and the findings can help better understand the complex ionomer film structure on a Pt electrode.