Ultrasmall Palladium Nanoclusters Encapsulated in Porous Carbon Nanosheets for Oxygen Electroreduction in Alkaline Media

Ultrasmall noble-metal nanoclusters have been gaining extensive attention because of their unique catalytic activity. Herein, we describe a facile in situ method for the preparation of palladium nanoclusters encapsulated in porous carbon nanosheets as effective catalysts for the oxygen reduction reaction (ORR) in alkaline media. Structural characterizations through X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy studies indicate that the Pd clusters were well incorporated into porous carbon nanosheets. The composition of the nanocomposites was manipulated by varying the initial loading ratio of Pd to carbon nanosheets (Pd/CNS). Among the series of samples tested, the sample at a Pd/CNS ratio of 1 : 4 (Pd/CNS-20 %) exhibited the best ORR activity with the most positive onset potential and largest diffusion-limited current density, which was even superior to that of commercial Pd black, along with remarkable long-term durability. The findings highlight the unique advantages of employing ultrasmall ligand-free palladium clusters as cost-effective ORR catalysts with high efficiency and remarkable stability.