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Published July 1, 2015 | Published + Supplemental Material
Journal Article Open

The atomistic origin of the extraordinary oxygen reduction activity of Pt₃Ni₇ fuel cell catalysts

Abstract

Recently Debe et al. reported that Pt₃Ni₇ leads to extraordinary Oxygen Reduction Reaction (ORR) activity. However, several reports show that hardly any Ni remains in the layers of the catalysts close to the surface ("Pt-skin effect"). This paradox that Ni is essential to the high catalytic activity with the peak ORR activity at Pt₃Ni₇ while little or no Ni remains close to the surface is explained here using large-scale first-principles-based simulations. We make the radical assumption that processing Pt–Ni catalysts under ORR conditions would leach out all Ni accessible to the solvent. To simulate this process we use the ReaxFF reactive force field, starting with random alloy particles ranging from 50% Ni to 90% Ni and containing up to ~300 000 atoms, deleting the Ni atoms, and equilibrating the resulting structures. We find that the Pt₃Ni₇ case and a final particle radius around 7.5 nm lead to internal voids in communication with the exterior, doubling the external surface footprint, in fair agreement with experiment. Then we examine the surface character of these nanoporous systems and find that a prominent feature in the surface of the de-alloyed particles is a rhombic structure involving 4 surface atoms which is crystalline-like but under-coordinated. Using density-functional theory, we calculate the energy barriers of ORR steps on Pt nanoporous catalysts, focusing on the O_(ad)-hydration reaction (O_(ad) + H₂O_(ad) → OH_(ad) + OH_(ad)) but including the barriers of O₂ dissociation (O_(2ad) → O_(ad) + O_(ad)) and water formation (OH_(ad) + H_(ad) → H₂O_(ad)). We find that the reaction barrier for the O_(ad)-hydration rate-determining-step is reduced significantly on the de-alloyed surface sites compared to Pt(111). Moreover we find that these active sites are prevalent on the surface of particles de-alloyed from a Pt–Ni 30 : 70 initial composition. These simulations explain the peak in surface reactivity at Pt₃Ni₇, and provide a rational guide to use for further optimization of improved catalytic and nanoporous materials.

Additional Information

© 2015 Royal Society of Chemistry. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Received 8th March 2015. Accepted 22nd April 2015. First published online 29 Apr 2015. This work was supported by the US National Science Foundation (CBET-1067848) and by a Short-Term Mission (STM) funded by Italian Consiglio Nazionale delle Ricerche (CNR). Financial support from the HELM project within the FP7 of EU is also acknowledged. Computational research was performed in part using EMSL, a DOE Office of Science User facility sponsored by the Office of Biological and Environmental Research and located at the Pacific Northwest National Laboratory.

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Supplemental Material - c5sc00840a1.pdf

Supplemental Material - c5sc00840a2.xyz

Supplemental Material - c5sc00840a3.xyz

Supplemental Material - c5sc00840a4.xyz

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August 20, 2023
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