(Invited) Core-Shell Nanosheet Catalysts: Enhanced Activity and Durability for Oxygen Reduction and Hydrogen Oxidation Reactions

Abstract

Development of Pt-based catalysts with higher activity and durability is essential for implementation of cost-competitive polymer electrolyte membrane fuel cells. Nanoparticle catalysts with core-shell, skeleton, or skin-type structures have shown to outperform Pt nanoparticles. However, complications arise when such catalysts are exposed to harsh operation conditions of the fuel cell where Pt and the alloying metal undergoes consecutive oxidation/reduction and dissolution, de-activating the catalyst. Compared to nanoparticles, extended surfaces (bulk electrodes and nanostructured thin films) are more stable, but suffers from low surface area. In this study, we describe the concept of “nanosheet catalysts” with surface area, activity, and durability that are higher than conventional core-shell nanoparticles for the oxygen reduction reaction as well as the hydrogen oxidation of reformate fuel. Our approach utilizes metallic Ru nanosheets [1] with atomic-scale thickness prepared via thermal reduction of exfoliated RuO2nanosheets [2,3]. The metallic Ru nanosheet is used as a core for the synthesis of two-dimensional Ru-core@Pt-shell catalysts. Sub to a few monolayer Pt shell was successively formed on metallic Ru nanosheets with monoatomic thickness via galvanic displacement reaction between Cu and Pt2+. The electrochemically active Pt surface area of Ru-core@Pt-shell nanosheets with 1.5-4.5 monolayer Pt-shell was 112-151 m2 (g-Pt)‒ 1. This corresponds to Pt nanoparticles with 1-1.5 nm of diameter. A carbon supported core-shell nanosheet catalyst with a 3.5 monolayer Pt-shell (Ru@Pt-3.5ML(ns)/C) showed 4.5 times higher activity than the benchmark Pt/C catalyst for the oxygen reduction reaction with a slower degradation rate even at high potentials. For the anode reactions, Ru@Pt-1.5ML(ns)/C had 2 times higher hydrogen oxidation activity in pure H2 as well as 300 ppm CO containing H2, and better stability against potential cycling. The developed nanosheet catalysts should provide the high utilization of Pt towards catalytic reactions and concurrently the stability of extended surfaces, offering a practical solution to the trade-off issue. This work was supported in part by the “Polymer Electrolyte Fuel Cell Program” from the New Energy and Industrial Technology Development Organization (NEDO), Japan. [1] K. Fukuda and K. Kumagai, e-Journal Surf. Sci. Nanotechnol., 12, 97 (2014). [2] W. Sugimoto, H. Iwata, Y. Yasunaga, Y. Murakami and Y. Takasu, Angew. Chem. Int. Ed., 42, 4092 (2003). [3] K. Fukuda, H. Kato, J. Sato, W. Sugimoto and Y. Takasu, J. Solid State Chem., 182, 2997 (2009). Figure 1