Nanostructuring Pt Electrocatalysts Supported on Nanocrystalline SnO2 for Polymer Electrolyte Fuel Cells

Abstract

Introduction Conventional Pt/C electrocatalysts for PEFCs have a difficulty in durability against start-stop potential cycling, mainly due to degradation of carbon black through electrochemical oxidation especially on the cathode side. SnO2 is known to be stable at a high potential under the PEFC cathode conditions, so that SnO2 is considered as an alternative support material for PEFC cathodes [1-4]. In particular, we have prepared the oxide-core Pt-shell electrocatalyst that can reduce materials cost of PEFCs while developing electrocatalysts with both higher durability and ORR mass activity [3]. In our previous study [5], freeze drying has been successfully applied as an effective procedure for highly dispersing SnO2 nanoparticles on relatively-stable conductive framework, for nanostructuring PEFC electrocatalysts as shown in Fig. 1. Here, the objective of this study is to directly deposit Pt on SnO2 on an atomic level by applying a chemical procedure towards the oxide-core Pt-shell electrocatalyst preparation. Experimental Graphitized carbon black (GCB200, Cabot) was used as the conductive filler of the electrocatalysts acting as the conductive framework. SnO2 was prepared on the conductive filler via the microwave homogeneous precipitation. In order to deposit Pt on the surface of the SnO2, Pt(acac)2 and acetone were added to the Sn-containing solution. Pt/SnO2/GCB was prepared via the freeze drying, and heat treatment was performed to obtain a Pt-based catalyst powder. Nanostructure of the electrocatalysts obtained was observed by FE-SEM and STEM. ICP analysis was also made to quantify Pt loading. Electrochemical characterization of these electrocatalysts was carried out by half-cell measurements. Electrochemical surface area (ESCA) was measured by cyclic voltammetry (CV). Oxygen reduction reaction (ORR) activity was derived from kinetically controlled current density (ik) after the rotating disk electrode (RDE) measurement. Results and discussion We have succeeded to prepare SnO2 nanoparticles with a diameter of about 3 nm on the conductive filler in higher dispersion by applying the microwave homogeneous precipitation and the freeze drying. In addition, it was revealed that Pt nanoparticles were selectively deposited on nanocrystalline SnO2. Furthermore, high-resolution STEM observation revealed that a part of Pt was deposited near an atomic level on the SnO2 surface which could be a Pt-shell of the electrocatalysts, as shown in Fig. 2. Especially in Fig. 2(b), Pt atoms could be distinguished on the nanocrystalline SnO2 particles acting as a core of the electrocatalyst, with a minimized electron-conductive pathway between the Pt catalyst and the GCB conductive framework. Electrochemical properties of such nanostructured electrocatalysts will be reported and discussed. References Y. Nakazato, D. Kawachino, Z. Noda, J. Matsuda, S. M. Lyth, A. Hayashi and K. Sasaki, J. Electrochem. Soc., 165 (14), F1154 (2018).S. Matsumoto, M. Nagamine, Z. Noda, J. Matsuda, S. M. Lyth, A. Hayashi, and K. Sasaki, J. Electrochem. Soc., 165 (14). F1165 (2018).M. Nagamine, Z. Noda, H. Manabe, J. Matsuda, A. Hayashi, and K. Sasaki, ECS Trans., 86 (13), 531 (2018).K. Kakinuma, R. Kobayashi, A. Iiyama, M. Uchida, J. Electrochem. Soc., 165 (15), J3083 (2018).T. Yoshizumi, M. Nagamine, Z. Noda, J. Matsuda, A. Hayashi, and K. Sasaki, ECS Trans., 92 (8), 479 (2019). Figure 1