Dispersion of Ionomer in Catalyst Ink of Polymer Electrolyte Fuel Cell Visualized By Cryogenic Transmission Electron Microscopy

Abstract

Fuel cell electric vehicles (FCEVs) are an effective solution to reduce the emission of carbon dioxide (CO2) and the consumption of petroleum fuel. The biggest issues to popularize FCEVs are cost reduction of polymer electrolyte fuel cell (PEFC) power systems, and the development of hydrogen infrastructure. On the development of PEFC stack for automotive application, the cost of catalyst and application accounts for 49% of the total stack cost at 500,000 units per year1). This estimation indicates that reducing platinum loading is important for the cost reduction of PEFC stack. However, low platinum loading increases the activity and mass transport losses and lowers the performance of PEMFC stack. Increasing oxygen reduction reaction (ORR) activity of catalyst materials such as platinum alloys is an effective way to decrease the above losses. Optimizing the structure of catalyst layer is also required to reduce the mass transport loss. This means the preparation process of catalyst layer is the important factor to reduce the performance losses on low platinum loading.In general, wet coating process is used for forming PEFC catalyst layer. Catalyst ink, dispersed platinum loaded carbon (Pt/C) and ionomer in solvent, are coated on the substrate or polymer electrolyte membrane. The catalyst ink coated substrate is dried to obtain the catalyst layer. Therefore, understanding the structure of catalyst ink is needed to control the structure formation of catalyst layer. Due to the fluidity of catalyst ink, visualization by cryogenic electron microscopy is an effective way to analyze the structure of catalyst ink. We observed the dispersion of Pt/C in catalyst ink by cryogenic scanning electron microscopy (Cryo-SEM)2). The result suggests that the dispersion of Pt/C aggregates correlates to the structure and performance of catalyst layer. However, the resolution of Cryo-SEM was insufficient to detect ionomer molecules in catalyst ink because of their molecular size.For above reason, we tried to visualize the dispersion of ionomer molecules by Cryo-TEM. Sample preparation for the observation of Cryo-TEM was based on the rapid freezing technique used in structural analysis of biological specimens3). The frozen thin film of catalyst ink were prepared by immersion into liquid Ethane. Observation samples were cooled by liquid helium in order to reduce the damage for the ionomer by electron irradiation. In this study, we focused on the effect of two factors, which were the solvent and Pt/C, on the dispersion of ionomer molecules. Cryo-TEM image of catalyst ink is shown in Fig.1. The net-like structure formed by ionomer strings were observed in case of catalyst ink. The average width of the ionomer string formed the net-like structure is less than 10nm approximately, which are the same dimension as the result analyzed by small angle X-ray scattering in previous study4). These results suggest the possibility that observed ionomer strings are bundled ionomer molecules together. The end part of the ionomer net-like structure seems to connect to the surface of the Pt/C aggregate. In this presentation, we will report the considerable mechanism that ionomer molecules form the net-like structure.