Elemental Anisotropic Growth and Atomic-Scale Structure of Shape-Controlled Octahedral Pt–Ni–Co Alloy Nanocatalysts

Abstract

Multimetallic shape-controlled nanoparticles offer great opportunities to tune the activity, selectivity and stability of electrocatalytic surface reactions. However, in many cases, our synthetic control over particle size, composition and shape is limited requiring trial and error.Deeper atomic-scale insight in the particle formation process would enable more rational syntheses. Here we exemplify this using a family of trimetallic PtNiCo nanooctahedra obtained via a low-temperature, surfactant-free solvothermal synthesis. We analyze the competition between Ni and Co precursors under co-reduction "one-step" conditions when the Ni reduction rates prevailed. To tune the Co reduction rate and final content we develop a "two-step" route and track the evolution of the composition and morphology of the particles at the atomic scale.To achieve this, scanning transmission electron microscopy and energy dispersive X-ray elemental mapping techniques are used. We provide evidence of a heterogeneous element distribution caused by element-specific anisotropic growth and create octahedral nanoparticles with tailored atomic composition like Pt 1.5 M, PtM and PtM 1.5 (M=Ni+Co). These trimetallic electrocatalysts have been tested toward the oxygen reduction reaction (ORR), showing a greatly enhanced mass activity related to commercial Pt/C and less activity loss than binary PtNi and PtCo after 4000 potential cycles.Keywords: PtNiCo octahedra, intraparticle composition, anisotropic growth, oxygen reduction reaction. 3Over the past years, extensive research and development has been carried out in fuel cells with the aim of implementing this technology on transportation, stationary and portable power generation 1, 2 . Nevertheless, some issues such as catalysts kinetic limitations, durability and cost must still be addressed before their successful commercialization. Oxygen reduction electrocatalysts are known to play a crucial role on fuel cells performance, being the fundamental studies on the subject still required to overcome these barriers 3,4 . By alloying Pt with other non-noble metals, it is possible to produce cheaper electrocatalysts with novel properties for the oxygen reduction reaction (ORR) due to lattice compression 5,6 and/or modified electronic properties 7,8 . Thus, several studies have shown that binary Pt-M (M=Cr, Mn, Fe, Co, Ni, Cu, V, Ti) nanocrystals (NCs) greatly enhanced the kinetics of the ORR in comparison with standard Pt catalysts 9-11 . However, besides the composition, the intrinsic activity of nanoparticles also depends on their size and shape which strongly determine the atomic surface structure of the nanoparticles [12][13][14][15] . Thus, by controlling the morphology of the NCs it is possible to maximize the exposure of certain facets that exhibit better catalytic properties 16,17 . These are the premises that led to establish {111}-Pt 3 Ni surfaces as the ideal electrocatalyst for the ORR 18,19 . Since then, many synthetic routes to obtain PtNi nanooctahedra, which ideally exhibit 8 face...

The oxygen reduction on Pt-Ni and Pt-Ni-M catalysts for low-temperature acidic fuel cells: A review

“…The ability of nickel and cobalt to alloy with each other and with platinum in various ratios enables their potential use in ternary Pt‐Ni‐Co catalysts . The ORR activity of cubo‐octahedral, octahedral, dealloyed, and nanostructured Pt‐Ni‐Co catalysts was evaluated and compared to that of binary Pt‐based parents . Luo et al and Li et al observed an enhancement of both MA and SA of ternary catalysts than those of parent binary catalysts.…”

“…Octahedral Pt‐Ni‐Co catalysts were synthesized without the use of capping agents or surfactants . In the former case, the resulting shaped ternary catalysts presented higher SA (1.5 times) and MA (1.4 times) than octahedral Pt‐Ni.…”

Octahedral PtNi nanoparticles with controlled surface structure and composition for oxygen reduction reaction

“…Moreover, from the digital photographs of the activated carbon-supported nanoparticles solution at different reaction temperature, which were shown in Fig. S6 in SI, we found that more Ni could be reduced with increasing the temperature and thus the formation of composition-tunable nanoparticles [23]. In addition to the reaction temperature, the use of benzoic acid was essential to the synthesis of well-defined octahedral PtxNi1−x nanoparticles.…”

The growth and degradation of binary and ternary octahedral Pt–Ni-based fuel cell catalyst nanoparticles studied using advanced transmission electron microscopy

“…Aran-Ais et al presented a growth study of ternary Pt-Ni-Co nanooctahedra via low temperature, surfactant-free solvothermal synthesis [72]. In a 'one-step' approach, they showed that Ni reduction prevails over Co reduction.…”