Thermal Properties and Segregation Behavior of Pt Nanowires Modified with Au, Ag, and Pd Atoms: A Classical Molecular Dynamics Study

Thobani G Gambu,Nora H De Leeuw,Umberto Terranova,Melissa A Petersen,David Santos-Carballal,Glenn Jones,Eric Van Steen

doi:10.1021/acs.jpcc.9b02730

Abstract

Platinum nanowires (NWs) have been reported to be catalytically active toward the oxygen reduction reaction (ORR). The edge modification of Pt NWs with metals M (M = Au, Ag, or Pd) may have a positive impact on the overall ORR activity by facilitating diffusion of adsorbed oxygen, Oads, and hydroxyl groups, OHads, between the {001} and {111} terraces. In the present study, we have employed classical molecular dynamics simulations to investigate the segregation behavior of Au, Ag, and Pd decorating the edges of Pt NWs. We observe that, under vacuum conditions, Pd prefers to diffuse toward the core rather than stay on the NW surface. Ag and Au atoms are mobile at temperatures as low as 900 K; they remain on the surface but do not appear to be preferentially more stable at edge sites. To effect segregation of Au and Ag atoms toward the edge, we propose annealing in the presence of different reactive gas environments. Overall, our study suggests potential experimental steps required for the synthesis of Pt nanowires and nanoparticles with improved Oads and OHads interfacet diffusion rates and consequently an improved ORR activity.

Highlights

It has been argued that the slow kinetics of the oxygen reduction reaction (ORR) at the cathode−electrode of protonexchange membrane fuel cells (PEMFCs) causes over 67% of the drop in potential that is observed with increasing current density.[1]
Oneand three-dimensional Pt nanowires (NWs) and NW networks[6] have been shown to have better or comparable durability and higher catalytic activities compared to small NPs.[7−10] These nanomaterials undergo minimal loss of the electrochemically active surface area (ECSA) during accelerated degradation tests given their multiple anchoring sites on the carbon support and fewer undercoordinated atoms compared to small Pt NPs, making them ideal candidates for the ORR catalysts
In addition to the slow kinetics of this reaction, the stability of the active Pt NPs is an important factor to consider in the development of more attractive fuel cell catalysts

Summary

Introduction

It has been argued that the slow kinetics of the oxygen reduction reaction (ORR) at the cathode−electrode of protonexchange membrane fuel cells (PEMFCs) causes over 67% of the drop in potential that is observed with increasing current density.[1]. Small nanoparticles (NPs) are currently utilized as catalysts for the ORR owing to their high surface area to mass/ volume ratios.[1] the stability of these NPs is still a significant problem[3] with degradation occurring via multiple mechanisms, including (1) direct and indirect dissolution of Pt and (2) agglomeration of the particles.[1,3−5] Direct dissolution of Pt occurs at electrode potentials above 1.188 V (vs standard hydrogen electrode, SHE), while indirect dissolution occurs via PtO formation at electrode potentials above 0.980 V (vs SHE). Oneand three-dimensional Pt nanowires (NWs) and NW networks[6] have been shown to have better or comparable durability and higher catalytic activities compared to small NPs.[7−10] These nanomaterials undergo minimal loss of the ECSA during accelerated degradation tests given their multiple anchoring sites on the carbon support and fewer undercoordinated atoms compared to small Pt NPs, making them ideal candidates for the ORR catalysts

Methods

Results

Conclusion