Energy dispersive x‐ray analysis of platinum‐nickel nanoparticles embedded in hollow graphitic spheres used as fuel‐cell catalysts

Abstract

Introduction Proton Exchange Membrane Fuel Cells (PEM‐FCs) are known as potential energy conversion devices in transportation systems. For the efficiency of these fuel cells a highly efficient and stable catalyst is needed. Platinum‐Nickel bimetallic nanoparticles embedded in hollow graphitic spheres (HGS) have been proven as highly active catalyst for the oxygen‐reduction reaction which plays a key role in the efficiency of PEM‐FCs. 1 These catalysts are synthesized via the confined‐space alloying approach. The synthesis of PtNi@HGS starts with the impregnation of HGS with the metal precursors. After high temperature annealing treatments metallic nanoparticles with a small size (3.5 nm) and a narrow particle size distribution encapsulated in the pores of the HGS can be obtained. 2 Methods In this study the PtNi@HGS synthesized via confined‐space alloying were analyzed using different electron microscopy methods like HAADF‐STEM, EDX line scan and elemental mapping. Especially the electrochemically degraded samples were of great interest questioning the distribution of the two metals in the metallic nanoparticles. To confirm the experimental results and to explain the structure of the nanoparticles after degradation theoretical EDX line scan profiles were calculated using Monte‐Carlo‐simulations 3 . Results In HR‐STEM images the structure of this catalyst system can be seen clearly: The crystalline metallic nanoparticles are encapsulated in pores of the hollow graphitic spheres (fig. 1). The EDX elemental mapping shows that the metal nanoparticles contain Pt and Ni. EDX line scans with a high spatial resolution clearly evidence a 0.5‐1 nm thick Pt‐rich outer shell and Pt‐Ni core after electrochemical degradation of the catalyst. To confirm the experimental results and to explain the structure of the nanoparticles theoretical EDX line scan profiles were calculated using Monte‐Carlo simulations. Simulated line scans of PtNi@Pt particles (0.5 nm shell and 3 nm core) show profiles similar to the experimental data as shown in figure 2. This excellent agreement supports the formation of core shell particles during the electrochemical degradation of the catalyst.