Development of Highly Active Pt/C-Based Catalysts for the ORR Using an Ordered Mesoporous Carbon with Network Structure As a Support Material: Focus on the Pt Loading Process

Abstract

[Introduction]In order to improve the performance of polymer electrolyte fuel cells (PEFCs) for automobiles, ordered mesoporous carbons (OMCs) have attracted attention as supports for Pt as a cathode catalyst. Our research team has successfully synthesized a network-structured OMC (Net-OMC), in which primary OMC particles on the order of several tens of nanometers have a strong network structure, unlike conventional OMC particles, which are typically several micrometers in size. The Net-OMC consists of nanometer-order primary particles, and well-developed macropores (primary and secondary pores) derived from the network structure, which realizes both high Pt dispersion and efficient mass transport. In addition, Pt nanoparticles supported on Net-OMC should be loaded within the nanopores at an appropriate depth position. This will allow the nanopores to be accessible while preventing direct contact between the Pt surface and the ionomeric binder, with resulting poisoning, while also maintaining effective proton transport.RDE measurements revealed that the Pt/Net-OMC catalyst showed higher mass activity for the ORR and higher durability for potential-step cycling compared with a commercial Pt/CB catalyst [1]. These results suggest that the aggregation and sintering of the Pt particles were successfully suppressed due to the nanopore structure.In order to further improve the catalytic activity of the Pt/Net-OMC catalyst, it is also necessary to uniformly load Pt into the nanopores at an appropriate size and depth during catalyst preparation. However, even when using the colloidal technique, which has been established for loading Pt on carbon black, it has been difficult to achieve good Pt dispersibility with conventional protocols due to the tendency of the Pt to aggregate on the Net-OMC [2]. Therefore, in this study, the conventional colloidal technique was optimized for loading Pt on the Net-OMC support.[Experimental]Loading of PtOx on Net-OMC was carried out by mixing an aqueous solution of PtOx colloid with an aqueous dispersion of Net-OMC powder, followed by heat treatment at an appropriate temperature. During this process, the slurry pH (pH 3~11), heating temperature (40~80 °C) and heating time (10~180 min) were varied to investigate their effects. The resulting slurries were then filtered, dried and reduced at 300 °C for 3 hours in flowing hydrogen to obtain Pt/Net-OMC.The morphology of the Pt/Net-OMC was observed by transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). Cyclic voltammetry and linear sweep voltammetry were carried out using the conventional rotating disk electrode (RDE) technique. Furthermore, membrane-electrode assemblies (MEAs) were prepared using Pt/Net-OMC as a cathode catalyst and evaluated for catalytic activity according to an FCCJ protocol using a Japan Automobile Research (JARI) standard cell.[Results and Discussion]The variation of Pt loading behavior with slurry pH revealed that Pt tended to aggregate when the pH was below 5, while the Pt loading amount significantly decreased when the pH was above 6 (Fig. 1, Table 1). These results indicate that the optimum slurry pH was ca. 5.5 to obtain a high dispersion of Pt. It was also found that the optimum temperature for Pt loading was ca. 50 °C; when the temperature was below 40 °C, the loading amount decreased significantly, and when the temperature was above 80 °C, some of the Pt particles formed aggregates. It was also found that the Pt particles showed continuous growth, with a linear dependence of the Pt aggregate size on the loading time at 50 °C.Based on the TEM observation, the average Pt particle size on the Pt/Net-OMC that had been heated at 50 °C for 1 hour was ca. 3 nm, and the particles exhibited good dispersion. Also, the correlation between the supported state of Pt and the catalytic activity of the Pt/Net-OMC will be discussed in detail.[1] T. Miyao, H. Nishino, H. Yamazaki, S. Sato, K. Tamoto, M. Uchida, A. Iiyama, K. Shibanuma, N. Koizumi, , the 242nd ECS Meeting, #I01D-1578.[2] M. Watanabe, M. Uchida and S. Motoo, J. Electroanal. Chem., 229, 395-406 (1987)Acknowledgement: This work was partially supported by the ECCEED’30-FC project from NEDO. Figure 1