Adsorbate-Induced Segregation of Cobalt from PtCo Nanoparticles: Modeling Au Doping and Core AuCo Alloying for the Improvement of Fuel Cell Cathode Catalysts

Abstract

Platinum, when used as a cathode material for the oxygen reduction reaction, suffers from high overpotential and possible dissolution, in addition to the scarcity of the metal and resulting cost. Although the introduction of cobalt has been reported to improve reaction kinetics and decrease the precious metal loading, surface segregation or complete leakage of Co atoms causes degradation of the membrane electrode assembly, and either of these scenarios of structural rearrangement eventually decreases catalytic power. Ternary PtCo alloys with noble metals could possibly maintain activity with a higher dissolution potential. First-principles-based theoretical methods are utilized to identify the critical factors affecting segregation in Pt–Co binary and Pt–Co–Au ternary nanoparticles in the presence of oxidizing species. With a decreasing share of Pt, surface segregation of Co atoms was already found to become thermodynamically viable in the PtCo systems at low oxygen concentrations, which is assigned to high charge transfer between species. While the introduction of gold as a dopant caused structural changes that favor segregation of Co, creation of CoAu alloy core is calculated to significantly suppress Co leakage through modification of the electronic properties. The theoretical framework of geometrically different ternary systems provides a new route for the rational design of oxygen reduction catalysts.

Dopants in the Design of Noble Metal Nanoparticle Electrocatalysts and their Effect on Surface Energy and Coordination Chemistry at the Nanocrystal Surface

“…In contrast, an energy barrier for Co migration (ΔE = +0.49 eV) was generated in the Au-doped PtCo alloy. The stabilizing effect of Au dopants in the PtCo alloy NPs was also verified by the DFT calculation results of Farkaš et al [97] Non-noble metal elements can also act as inhibitors of atomic diffusion in Pt-based alloy NCs. For example, In dopants in PtNi octahedral NCs could tune the atomic diffusion barrier in the NCs, as reported by Shen et al [22] The diffusion barriers of lattice Ni in Pt 2 Ni 2 and Pt 2 In 0.2 Ni 1.8 slabs were calculated by DFT simulations (Figure 13c,d).…”

Structural and Valence State Modification of Cobalt in CoPt Nanocatalysts in Redox Conditions

“…Partial segregation of Co can be due to the synthesis technique, or it can happen shortly after exposure to air after the synthesis. Segregation of Co is known to be one of the causes of catalyst deactivation during ORR . Indeed, surface segregation of Co can fully cover Pt atoms and ultimately undermine the catalytic potential of the bimetallic system.…”

“…For instance, segregation of Co and Pt can be responsible for reduced catalytic properties after cycling. , Changes in the valence state during reaction impacts the electronic configuration at the sample’s surface and ultimately affects absorption and desorption energies for gas molecules . It is known that freshly synthesized Co–Pt particles can have compositional heterogeneities, with partial segregation of Co and Pt. − Exposure of the Co–Pt system to H 2 or other nonoxidative atmospheres at elevated temperatures has been shown to improve the mixing of Co and Pt, and can lead to the formation of an intermetallic solution. ,, However, it is not clear whether some Co remains segregated and oxidized during and after annealing. This point is important because incomplete reduction and mixing of Co and Pt will impact the catalytic properties for HER, ORR, or gas-phase reactions.…”

A Perspective on Modelling Metallic Magnetic Nanoparticles in Biomedicine: From Monometals to Nanoalloys and Ligand-Protected Particles

“…The applicability of single-component mNPs is restricted by limited property-tuning possibilities. To overcome this limitation, mNPs can be modified through the construction of bimetallic architectures consisting of two distinct metals, one or both of which should be magnetic [397][398][399][400][401][402]. Bimetallic (and multimetallic) mNPs, often referred to as magnetic nanoalloys, present properties with a very high degree of tunability owing to the variety of morphologies they can adopt.…”

“…Figure 4. Relative energy stability of successive Co cluster sizes expressed as a second difference in total energy, ∆E 2 , as predicted by DFT(Datta (2007) andFarkaš (2020) [239,254]) and MD(Rives (2008) andRodriguez (2003) [252,253]) simulations.…”