Morphological Effects on Surface Oxidation Tolerance of Metallic Nanosheets
Authors: Takimoto Daisuke , Mochizuki Dai , Hideshima Sho , Sugimoto Wataru , Yuan Qiuyi , Takao Naoki , Itoh Takanori , Duy Truong Vinh Truong , Ohwaki Tsukuru , Imai Hideto 
Published: February 2020

Metallic nanosheets have attracted much attention owing to its intrinsically large surface area, highly anisotropic structure, and flexible but rigid structure. Understanding the correlation between activity and stability of metallic nanosheets is necessary to clarify the advantage of two-dimensional morphology. In this work, the activity and stability of metallic Ru nanosheets and nanoparticles were studied to understand the two-dimensional morphological effects on the electrochemical properties, in particular, the tolerance towards surface oxidation.
Ru nanosheet supported on carbon (Ru(ns)/C) was prepared by a process reported previously.1 Ru nanoparticles supported on Ketjen black (Ru(np)/C, 2.8±0.3 nm) was synthesized by a conventional impregnation method as a control sample. The ORR mass activity of Ru nanosheets was 1.3 times higher than that of nanoparticles, this is attributed to the larger electrochemically active surface area compared with nanoparticles. The ORR mass activity of nanosheets after 500 potential cycles was 77 A (g-Ru)-
1, which is 39 times higher than that of nanoparticles (Fig. 1(a)). Although the nanoparticles were oxidized by potential cycles (Fig. 1(b)), the nanosheets maintained metallic state. The result suggests that the metallic Ru nanosheets possesses higher tolerance against surface oxidation compared with Ru nanoparticles.
Based on the electrochemical measurements, in-situ XAFS analysis, and theoretical simulations, we discuss the cause of the high anti-oxidation properties for Ru nanosheets. The average coordination number of surface for 2.4 nm Ru (hcp) was 5-6.2 On the other hand, the average coordination number of Ru nanosheets was 8, which is higher than that of nanoparticles. This suggests that the Ru nanosheets is thermodynamically stable compared with Ru nanoparticles. The above results suggest that nanoparticles were easily oxidized by potential cycling due to the low coordination number of surface atoms compared to nanosheets. Therefore, the surface oxidation tolerance of Ru nanosheets was higher than that of Ru nanoparticles, and the nanosheet structure should contribute to the improved stability against surface oxidation during potential cycles.
This research was supported in part by the “Polymer Electrolyte Fuel Cell Program” from the New Energy and Industrial Technology Development Organization (NEDO) of Japan.

References

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L. S. R. Kumara, O. Sakata, S. Kohara, A. Yang, C. Song, K. Kusada, H. Kobayashi, and H. Kitagawa, Phys. Chem. Chem. Phys., 18, 30622 (2016).

Figure 1. (a) Retention in ORR mass activity of Ru(ns)/C (red square) and Ru(np)/C (black circle) as a function of potential cycle number. (b) Pre-adsorbed CO stripping voltammograms of Ru(ns)/C (red) and Ru(np)/C (black) after potential cycles (0.1 M HClO4 (25oC), v = 10 mV s-1). Solid line: 1st cycle after CO adsorption; broken line: 2nd cycle.

Figure 1