Core-Shell Pd@M (M=Ni, Cu, Co) Nanoparticles/Graphene Ensembles with High Mass Electrocatalytic Activity Toward the Oxygen Reduction Reaction

Abstract

Herein, it is demonstrated that pyrene butyric acid (PBA)-stabilized metal nanoparticles with core-shell morphology, Pd@M_NPs (M=Ni, Cu, Co), non-covalently supported on graphene (G) sheets, are more active towards oxygen electroreduction in alkaline environments than the benchmark Pd/C catalyst, albeit with a 70 % lower precious metal loading. The PBA-stabilized Pd@M_NPs (M=Ni, Cu, Co)/G ensembles were prepared by employing a simple modified polyol method and galvanic replacement and thoroughly characterized with advanced microscopy imaging and complementary spectroscopic techniques. Electrochemical studies revealed that Pd@Ni_NPs /G presents the optimum performance, exhibiting a 30 mV more positive onset potential and 3.2 times greater mass activity over Pd/C. Moreover, chronoamperometric assays showed the minimum activity loss for Pd@Ni_NPs /G, not only among its core-shell counterparts but importantly when compared with the benchmark catalyst. The excellent performance of Pd@Ni_NPs /G was attributed to the (a) presence of PBA as stabilizer, (b) uniform Pd@Ni_NPs dispersion onto the graphene sheets, (c) efficient intra-ensemble interactions between the two species, (d) existence of the core-shell structure for Pd@Ni_NPs , and (e) stability of the Ni core metal under the reaction conditions. Last, the oxygen reduction on Pd@Ni_NPs /graphene occurs by the direct four-electron reduction pathway, showing great potential for use in energy related applications.

Keywords: core-shell nanoparticles; energy conversion; graphene; hybrid electrocatalysts; oxygen reduction reaction.