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001     827180
005     20240610120541.0
024 7 _ |2 doi
|a 10.1002/9783527808465.EMC2016.6030
037 _ _ |a FZJ-2017-01378
041 _ _ |a English
100 1 _ |0 P:(DE-Juel1)130695
|a Heggen, Marc
|b 0
|e Corresponding author
111 2 _ |a 16th European Microscopy Congress (EMC 2016)
|c Lyon
|d 2016-08-28 - 2016-09-02
|w France
245 _ _ |a Growth and degradation of advanced octahedral Pt-alloy nanoparticle catalysts for fuel cells
260 _ _ |a Weinheim, Germany
|b Wiley-VCH Verlag GmbH & Co. KGaA
|c 2016
295 1 0 |a European Microscopy Congress 2016: Proceedings
300 _ _ |a 800 - 801
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520 _ _ |a Octahedral Pt-Ni nanoparticles are highly attractive as fuel-cell catalysts due to their extraordinarily high activity for the oxygen-reduction-reaction (ORR). A deep understanding of their atomic-scale structure, degradation and formation is a prerequisite for their use as rationally designed nanoparticle catalysts with high activity and long-term stability.Here we present an extensive microstructural study of the growth and degradation behavior of various octahedral Pt-alloy nanoparticles using in situ transmission electron microscopy (TEM) and Cs-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) combined with electron energy-loss spectroscopy (EELS) and energy-dispersive X-ray spectroscopy (EDX). We show that octahedral nanoparticles often show compositional anisotropy with Ni-rich {111} facets leading to complex structural degradation during ORR electrocatalysis. The Ni-rich {111} facets are preferentially etched, resulting in the formation of first concave octahedra and then Pt-rich skeletons that have less active facets (Figure 1)[1]. Furthermore, we reveal element-specific anisotropic growth as the reason for the compositional anisotropy and the limited stability. During the solvothermal synthesis, a Pt-rich nucleus evolves into precursor nanohexapods, followed by the slower step-induced deposition of Ni on the concave hexapod surface, to form octahedral facets (Figure 2)[2]. While the growth of Pt-rich hexapod is a ligand-controlled kinetic process, the step-induced deposition of the Ni-rich phase at the concave surface resembles a thermodynamically controlled process accomplished in much longer time. In order to tune the atomic-scale microstructure of the octahedra for long-term stability, we illustrate the effect of varying the growth conditions on morphology and compositional segregation by producing trimetallic PtNiCo nanooctahedra and comparing “one-step” and newly-developed “two-step” synthesis routes [3]. Furthermore we demonstrate how Pt atom surface diffusion may produce a protective Pt surface layer on top of the Ni-rich facets, resulting in advanced and more stable octahedral catalysts. Figure 3 shows a sequence of structural changes taking place on an octahedral nanoparticle during in situ heating up to 800°C using a MEMS chip heating holder (DENSsolutions, Delft, NL). It can be observed that Pt-rich corner atoms diffuse and subsequently fill the concave Ni-rich {111} facets, forming perfectly octahedral nanoparticles with flat Pt-rich {111} surfaces (Figure 3) [4].
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