Ultra-low loading of Pd 5 nanoclusters on carbon nanotubes as bifunctional electrocatalysts for the oxygen reduction reaction and the ethanol oxidation reaction.

How to reduce the usage of precious metals in electrocatalysts is a big challenge for the development of fuel cells. Metal nanoclusters (NCs) are highly desirable as active catalysts, but palladium nanoclusters (Pd NCs) have been less well developed than other metal clusters, such as gold, silver and copper, owing partly to the difficulties in size-controlled synthesis. Here, based on N, N-dimethylformamide (DMF)-mediation and ligand-exchange reaction, atomically precise Pd5 (C12 H25 S)13 nanoclusters are successfully synthesized. By loading the as-prepared Pd5 nanoclusters on multiwalled carbon nanotubes (MWCNTs) and the following pyrolysis to remove the thiolate ligands, the surface-cleaned Pd5 clusters (Pd5 NCs/MWCNTs) can serve as efficient electrocatalysts for the oxygen reduction reaction (ORR) and the ethanol oxidation reaction (EOR). With ultra-low mass loading of Pd (2%), the Pd5 NCs showed higher mass and specific activities and better durability than the commercial Pd/C catalyst (5 wt%) for the ORR. At 0.8 V, the mass and specific activities of Pd5 NCs/MWCNTs are 5.70 and 4.53 times higher than the commercial Pd/C catalyst, respectively. As for the EOR, the Pd5 NCs/MWCNTs exhibited lower onset potential (0.39 V) and peak potential (0.81 V) than the commercial Pd/C catalyst (0.44 and 0.89 V). Electrochemical impedance spectroscopy (EIS) measurements indicated that for the EOR, the Pd5 nanoclusters have a much smaller charge transfer resistance (Rct ) than the commercial Pd/C. The high-performance electrocatalytic properties of Pd5 NCs for the ORR and EOR could be ascribed to the relatively high surface area-to-volume ratio and high density of exposed surface atoms of the Pd5 nanoclusters.