Characteristics of Pd-Pt Nanoparticles with Core-Shell Structure on Polybenzimidazole-Wrapped Graphene for Fuel Cell Electrocatalyst

Introduction

Pt has received unremitting interest in the proton-exchange polymer electrolyte fuel cell (PEFC) technology as electrocatalysts for both the hydrogen oxidation and oxygen reduction reactions [1]. However high cost limits its application and the reduction of the amount of Pt in the catalyst is required (it was ca. 48.83 $ per gram at the beginning of 2014) [2].To reduce the usage of Pt, arranging Pt as shell on proper metal core is promising “core-shell approach”. This strategy is attractive and practical because only the outmost few layers of Pt atoms in a catalyst are actually needed in facilitating a reaction while most atoms in the bulk are more or less wasted [3, 4]. Among various metal, we chose Pd as a core metal due to the stability under strong acidic condition in PEFC [5]. Here, we attached Pd-Pt core-shell nanoparticles on Graphene which are ideal materials as conductive supporting due to the large surface area, remarkable electric conductivity and excellent electrochemical durability. This study, we utilize polybenzimidazole (PBI) as a polymeric anchor to load Pd-Pt core-shell nanoparticles on to the surface of Graphene.

Experiments

Graphene wrapped with PBI (Graphene/PBI) (5.0 mg) was added to a 60-vol% aqueous ethylene glycol (EG) solution (20 mL) and dispersed by sonication. Pd (OAc)₂ (3.3 mg) dissolved in the methanol (5.0 mL) was added to the Graphene/PBI dispersion, and then the mixture was heated at room temperature for 3 h. H₂PtCl₆ •6 H₂O (0.45 mg) dissolved in a 60 vol% aqueous EG solution (20 mL) was added to the Graphene/PBI/Pd dispersion and the mixture was stirring for room temperature for 24 h. The solid material was collected by filtration and washed with water and then dried under vacuum to obtain Graphene/PBI/Pd-Pt (Figure 1).

Results and Discussion

To inspect the structure of Pd-Pt core-shell nanoparticles, we observed scanning transmission electron microscopy (STEM).  STEM operated at 200 kV are presented in Figure 2.

Figures 2a and 2b show the STEM image of a Graphene/PBI/Pd-Pt. Figures 2c and 2d show the EDS mapping of Pd (red) and Pt (blue). It is obvious that Pd is surrounded by Pt as indicated in overlay (Figure 2e). The results strongly indicate the successful formation of Pd-Pt core-shell structure on Graphene/PBI.

Conclusions

We fabricated Graphene/PBI/Pd-Pt core-shell composite. STEM results indicate that the Graphene /PBI/Pd-Pt core-shell catalyst exhibited the core-shell nanostructure.

References

1. N. Nakashima et. al, Adv. Funct. Mater. 18, 1776 (2008)
2. T.Fujigaya et. al, Adv. Mater. 25,1666–1681(2013)
3. R. R. Adzic et. al, Angew. Chem. Int. Ed. 49, 8602 (2010)
4. Y. Xia et. al. Science. 324, 1302 (2009)
5. Y. Xia et. al. Chem. Soc. Rev. 41, 8035-8049 (2012)