Synthesis of bimetallic Pt/Pd core/shell nanocrystals and their high electrocatalytic activity modulated by Pd shell thickness

We developed a biomimetic approach to synthesize noble metal NCs with desired shape. Multipod Pt nanocrystals (NCs) can be synthesized with specific peptide molecules as capping agent. Based on this, water-soluble bimetallic core/shell Pt/Pd NCs with multipod morphology are synthesized under mild condition through a two-step process. The as-synthesized bimetallic NCs can be dispersed well and are stable in aqueous solution. The thickness of the Pd shell can be controlled from sub-monolayer to multiple atomic layers. The oxygen reduction reaction (ORR) catalytic activities of these bimetallic NCs are investigated quantitatively for fuel cells applications in alkaline solution. It is found that the Pd shell helps to minimize the over-potential of ORR, while generating higher kinetic current density, probably by modulation of adsorption energy of oxygen on surface. The electro-oxidation of methanol is then investigated to show that the bimetallic Pt/Pd core/shell NCs can enhance the activity of the methanol oxidation, lowering the overpotential by 20 mV with respect to the Pt core NCs. Further investigation reveals that the Pt/Pd ratio of 3, which correspond to near-monolayer Pd shell, gives the highest specific activity and lowest over-potential.