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Rh-doped Pt-Ni octahedral nanoparticles: understanding the correlation between elemental distribution, ORR and shape stability Vera Beermann, Martin Gocyla, Elena Willinger, Stefan Rudi, Marc Heggen, Rafal E. Dunin-Borkowski, Marc-Georg Willinger, and Peter Strasser Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b04636 • Publication Date (Web): 08 Feb 2016 Downloaded from http://pubs.acs.org on February 9, 2016
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Nano Letters
Rh-doped Pt-Ni octahedral nanoparticles: understanding the correlation between elemental distribution, ORR and shape stability
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Vera Beermann,a Martin Gocyla,b Elena Willinger,c Stefan Rudi,a Marc Heggen,b Rafal E. Dunin-Borkowski,b MarcGeorg Willinger c and Peter Strasser a*
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a
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Electrochemical Energy, Catalysis and Material Science Laboratory, Department of Chemistry, Technical University Berlin, 10623 Berlin, Germany. b
c
Ernst-Ruska Centre for Microscopy and Spectroscopy with Electrons and Peter Grünberg Institute, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany.
Fritz-Haber-Institut der Max-Planck-Gesellschaft, Department of Inorganic Chemistry, Faradayweg 4-6, 14195 Berlin, Germany.
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* Corresponding author email address:
[email protected] 12 13 14
Abstract
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Thanks to their remarkably high activity towards ORR platinum-based octahedrally shaped nanoparticles
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have attracted ever increasing attention in last years. Although high activities for ORR catalysts have been
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attained, the practical use is still limited by their long-term stability. In this work, we present Rh-doped Pt-
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Ni octahedral nanoparticles with high activities up to 1.14 A mgPt-1 combined with improved performance
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and shape stability compared to previous bimetallic Pt-Ni octahedral particles. The synthesis, the
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electrocatalytic performance of the particles towards ORR and atomic degradation mechanisms are
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investigated with a major focus on a deeper understanding of strategies to stabilize morphological particle
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shape and consequently their performance.
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Rh surface doped octahedral Pt-Ni particles were prepared at various Rh levels. At and above about 3 at.%
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the nanoparticles maintained their octahedral shape even past 30,000 potential cycles, while undoped
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bimetallic reference nanoparticles show a complete loss in octahedral shape already after 8,000 cycles in
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the same potential window. Detailed atomic insight in these observations is obtained from aberration-
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corrected scanning transmission electron microscopy (STEM) and energy dispersive X-ray (EDX) analysis.
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Our analysis shows that it is the migration of Pt surface atoms, not – as commonly thought - the
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dissolution of Ni that constitutes the primary origin of the octahedral shape loss for Pt-Ni nanoparticles.
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Using small amounts of Rh we were able to suppress the migration rate of platinum atoms and
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consequently suppress the octahedral shape loss of Pt-Ni nanoparticles. 1 ACS Paragon Plus Environment
Nano Letters
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The urgent need of clean and renewable energy sources for transportation vehicles, portable devices and
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a variety of several other applications, triggered an intense research in polymer electrolyte membrane fuel
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cells (PEMFCs) in the last decades 1. Pt and Pt-based nanoparticles supported on carbon are currently the
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benchmark catalysts for PEMFCs in the most industrial-related applications. However, due to the high
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costs caused by the amount of expensive Pt and the sluggish oxygen reduction activity of bimetallic Pt
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catalysts, newly designed electrocatalysts for oxygen reduction reaction (ORR) based on Pt are required.
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The use of a 3d transition metal reduces the costs of a potential catalyst and improves the ORR rate by
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tuning the d-band structure of the Pt surfaces 2, 3. Several alloys of Pt with other transition metals such as
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Fe, Co, Ni or Cu have been already discussed in literature 4-8. As the ORR is a structure sensitive reaction,
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the surface structure of a catalyst is also an important parameter which needs to be considered. For
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spherical Pt nanoparticles in aqueous HClO4 electrolyte the specific activities are known to increase in the
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order of Pt(100)
