Two-Dimensional Effects on the Oxygen Reduction Reaction and Irreversible Surface Oxidation of Metallic Ru Nanosheets and Nanoparticles

Nanosheets have attracted increasing attention owing to their electrochemical properties. While the relationship between the activity and stability of metal nanoparticles has been widely reported, the activity-stability relationship of metallic nanosheets has not been characterized. Herein, we report on this relationship for Ru nanosheets and discuss its difference with the relationship for Ru nanoparticles. The oxygen reduction reaction activity of the Ru nanosheets was 130% higher than that of the nanoparticles, which was attributed to the larger electrochemically active surface area of the nanosheets. In addition, the activity of the nanosheets after potential cycling was ca. 40 times higher than that of the nanoparticles. Based on in situ X-ray absorption spectroscopic (XAS) measurements and X-ray absorption near-edge structure (XANES) spectra simulations by first-principles theoretical calculations, the average coordination number of the Ru atoms on the nanosheet surface was larger than that on the nanoparticle surface; thus, we concluded that this property contributed to the higher stability of the nanosheets. This finding provides supporting evidence for the cause of the high intrinsic activity and stability of metallic nanosheets.