Pressure-Induced Control of Core-Shell Nanoparticles for Oxygen Reduction Reaction

Abstract

Precious metal-based active metals are mainly used as electrocatalysts for ORR (Oxygen Reduction Reaction), and cost and durability are major research and development issues in fuel cell applications. In order to obtain high performance in electrocatalysts, composition, dispersion, structure and particle size have been studied as key parameters. A number of researchers have been reporting enhanced ORR performance by alloying approaches. Platinum-based alloy nanoparticles are typically prepared through a heat treatment process, which can result in significant levels of specific activity and mass activity improvements. However, the high temperature of the alloying process leads to size growth of the nanoparticles, resulting in a reduction in active area. In order to prevent growth of nanoparticle size during heat treatment, various approaches have been reported such as forming a protective layer by using physical barriers using inorganic and organic materials [1-3]. However, in this case, post-treatment or additional process is required to remove the protective layer, which limits the mass production. In this work, we propose a method to easily control the size of the core-shell nanoparticles by controlling the pressure of the inert gas during the heat treatment process. As synthesized M@Pt/C (M = Co, Fe, etc.) catalysts were conducted heat treatment with varied pressure control (1, 40, 80 bar) under inert atmosphere. The average size of the metal nanoparticles was distributed in the range of about 3 to 5 nm, and the particle size was clearly reduced in inverse proportion to the pressure. By scanning transmission electron microscopy (STEM) analysis, we also confirmed that the catalysts show clear M core-Pt shell structure. Theoretical calculations show the correlation between the size of metal nanoparticles and external pressure. Herein, we introduce a novel particle size control method (Pressure-induced particle size control). Furthermore, we believe that our unique synthetic approach can be a promising way to obtain intermetallic structure NPs with controlled particle size.