Study of Ionomer Adsorption on Carbon Supported Pt Catalyst By the Quartz-Crystal Microbalance (QCM) Method

Abstract

In polymer electrolyte fuel cells, the three-phase interface in a catalyst layer (CL) is considered to be very important as the electrochemical reaction site. The CL is typically formed using a dispersion of carbon supported platinum catalyst (Pt/C) and ionomer, and the interaction between the ionomer and Pt/C particles probably has an impact on the platinum utilization in the electrode and the dispersion stability. In this study, we focused on the adsorptive property of ionomers onto the Pt/C surface. The amount of adsorbed ionomer on a Pt/C catalyst was measured using the quartz-crystal microbalance (QCM) method. The amount was used as an index of the affinity of the ionomer for the Pt/C catalyst surface. The Pt/C catalyst particles were immobilized on a QCM sensor. The catalyst-immobilized sensor was placed into a solvent, and was left for a while until the QCM oscillation stabilized. An ionomer solution was then added drop-wise into the QCM cell. The change in resonance frequency of the quatz-crystal before and after adding the ionomer dispersion was recorded. The amount of a perfluorosulfonic acid polymer, Flemion® (Asahi Glass), adsorbed on the Pt/C catalyst surface is plotted against the concentration of the ionomer dispersion in Fig.. The amount was calculated based on the change in the resonance frequency of the QCM sensor. It shows the higher the ionomer concentration, the more ionomer was adsorbed. The relationship between the adsorptive property of various ionomers will be discussed in terms of their chemical properties (IEC, chemical structure etc.). The viscoelastic character of the adsorbed ionomer layers will also be described. Figure 1