Formation and Growth of Hollow Ptni/C Nanocatalysts for the Oxygen Reduction Reaction
Authors: Asset Tristan , Chattot Raphaël , Drnec Jakub , Bordet Pierre , Job Nathalie , Dubau Laetitia , Maillard Frederic 
Published: February 2020

Determining the mechanisms at stake during the formation and growth of bimetallic electrocatalysts is fundamental to optimize their morphological and, by extent, their electrocatalytic properties. Here, we discuss the synthesis of hollow PtNi/C nanoparticles (NPs) by a ‘one-pot’ process firstly introduced by Bae et al.
1–3. This electrocatalyst is highly active for the Oxygen Reduction Reaction (ORR), due to its highly defective structure and its low nickel atomic content 4–6. However, the understanding of the elementary steps that take place during the synthesis remains elusive.

Using small and wide angle X-ray scattering, scanning transmission electron microscopy coupled with X-ray energy dispersive spectroscopy and electrochemical methods, we unveil the processes at stake during the formation of porous hollow PtNi/C nanoparticles. We show that Ni-rich nanoparticles covered by a NiXBYOZ shell are first formed before alloying with Pt atoms (t = 3 min). Due to galvanic displacement, a thick Pt-rich shell then develops (t = 20 min). The resulting gradient in chemical potential becomes a strong driving force for the outwards diffusion of Ni atoms, yielding ultimately to porous hollow PtNi/C nanoparticles. These results show that hollow PtNi/C nanoparticles are the ultimate stage of a dissolution process involving galvanic replacement and the nanoscale Kirkendall effect, similarly to what was observed during the ageing of bi-metallic catalysts in real proton exchange membrane fuel cells 7. Beyond its interest for hollow NPs, this study also provides a general combination of techniques capable to unveil the mechanism of formation and growth of preferentially-shaped metal NPs.

Figure 1: Schematic representation and X-ray energy dispersive spectroscopy elemental maps of the different nanostructures sequentially formed during the synthesis of hollow PtNi/C NPs. The images are recorded for t = 1, 2, 3, 4, 10, 20, 40 and 60 min after the addition of the first drop of the NaBH4 solution. The non-charged plain circles are representative of NiXBYOZ(grey), Ni (green) and Pt-rich (gold) crystallites.

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Figure 1