Synthesis of carbon supported ordered tetragonal pseudo-ternary Pt2M′M″ (M = Fe, Co, Ni) nanoparticles and their activity for oxygen reduction reaction

doi:10.1016/j.jpowsour.2015.01.076

Journal of Power Sources

Volume 280, 15 April 2015, Pages 459-466

https://doi.org/10.1016/j.jpowsour.2015.01.076 Get rights and content

Highlights

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Three ternary ordered intermetallic nanoparticles were synthesized using a novel synthetic method.
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Activity towards oxygen reduction and stability of the catalysts were tested in acidic conditions.
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The catalysts showed comparable to slightly improved activity compared to commercial Pt/C.

Abstract

Alloying Pt with 3d transition metals has attracted much attention due to their reduced Pt content and reports of enhanced electrocatalytic activity for proton exchange membrane fuel cell applications. However, synthesizing ordered nanocrystalline intermetallics in the sub-10 nm range can be challenging. Here, we report on the co-reduction synthesis of ordered ternary Pt-base transition metal intermetallics with particle sizes in the regime of 3–5 nm. Since differences in the activity of PtM (M = Fe, Co, Ni) for oxygen reduction reaction (ORR) have been reported, we explored their combinations: Pt₂FeCo, Pt₂FeNi, and Pt₂CoNi. These ternary intermetallic nanoparticles crystallized in P4/mmm space group upon annealing in a protective KCl matrix. The electrocatalysts were prepared by dispersing these intermetallics onto a carbon support using ethylene glycol and various sonication techniques. A combination of analytical techniques including powder X-ray diffraction, thermogravimetric analysis, electron microscopy and electrochemical methods have been used in this study. The oxygen reduction reaction activity and stability of the catalysts were tested in 0.1 M HClO₄ and 0.1 M H₂SO₄ using cyclic voltammetry and rotating disk electrode voltammetry. The correlations between the composition, structure, morphology and activity of the intermetallics have been established and are discussed.

Introduction

Of the different classes of fuel cells, PEMFCs using hydrogen fuel are presently considered the best candidates for automotive applications due to their low operating temperatures, short warm-up time, and acceptable power density. [1], [2], [3] In order for commercial implementation of PEMFCs for mobile and transportation applications, the technology needs to be more durable and affordable. [4], [5] A major contributor to the high cost is the sluggish kinetics of the oxygen reduction reaction (ORR), which leads to high Pt loadings to reach reasonable power densities. Alloying has been a promising approach to producing advanced catalytic materials. [6], [7], [8], [9] It has been shown that alloying Pt with one or more 3d transition metals can not only reduce the Pt content, but also can effectively enhance ORR activity by factor of 2–4 or so by shifting the half-wave potential to more positive potentials by about 30 mV [10], [11], [12], [13], [14].

Intermetallics are traditionally synthesized using high temperature reactions and lengthy annealing times. Some common synthetic techniques include arc melting and powder metallurgy, where the reactants are heated to relatively high temperatures (typically above 1000 °C) followed by days or weeks of annealing. [15] Formation of intermetallic phases requires homogenous mixing of reactants as small particles (typically 1–100 nm) and high temperatures to overcome the slow solid–solid diffusion rates in the solid state. The lengthy annealing times are necessary for reactants to become homogeneous, nucleating and growing the long range ordered intermetallic structure throughout the material. The high temperature reactions that are required for traditional solid-state synthesis, while important for generating many useful materials, offer little control over morphology and particle size. [16] Thus, synthesizing ordered intermetallic nano-particles (NPs) in the sub 10 nm range can be challenging.

Some methods have been developed to form intermetallic nanocrystals in the 5–10 nm size range. [17], [18], [19], [20], [21] For example, Aslam and co-workers have shown that ordered face-centered-tetragonal (fct) PtFe nanocrystals in the 4–6 nm size range can be synthesized by encapsulating alloy PtFe NPs in a SiO₂ shell, which prevents the particles from agglomerating during the annealing at temperatures up to 1000 °C. [22] However, the ordered particles are often kept inside the silica shells, and removal of the silica shells causes the particles to agglomerate. If surface stabilizing surfactants are used during the synthesis, thin carbon layers that are not easily removed, often form during annealing. Sun and co-workers have also shown size controlled PtFe NPs employing a similar method, using MgO to encapsulate the particles and prevent rapid particle growth. [23] MgO is removed with a dilute HCl wash, with only minor Fe metal leaching. However, significant particle agglomeration was observed. Surfactants such as hexadecanethiol and oleic acid are able to stabilize the particles for several hours; however, complete removal of the carbon groups was unable to be achieved. [24] In order to explore the properties of intermetallic nanomaterials as PEMFC catalysis, it is imperative to use a generalizable method of synthesizing well-dispersed, intermetallic nanocrystals with tunable size and composition without the use of strongly coordinating surfactants.

In this paper, we report the synthesis of ordered tetragonal Pt₂FeCo, Pt₂FeNi, and Pt₂CoNi NPs. The co-reduction method (Scheme 1) applied in this study allows the size of the ordered Pt intermetallics to be maintained at 3–5 nm. The ORR activities of these intermetallics on carbon supports were compared to a 50 wt% Pt/C_E-TEK standard.

Section snippets

Materials

PtCl₄ (99.9%, anhydrous), NiCl₂ (99.9%, ultra-dry), CoCl₂ (99.9% ultra-dry), and LiCl (99.9%, anhydrous) were purchased from Alfa Aesar. FeCl₃ (98%, anhydrous), potassium triethylborohydride (1.0 M in THF), and ethylene glycol were purchased from Sigma–Aldrich. Li₆NiCl₈ was synthesized by mixing stoichiometric amounts of NiCl₂ and LiCl using a mortar and pestle. The yellow powder was then transferred and sealed in a quartz tube under vacuum and annealed at 540 °C, for 672 h, followed by rapid

Synthesis and crystal structure analysis

The synthesis of Pt-M-M′ NPs was done using a solution phase co-reduction method. The reaction uses metal chloride precursors and sufficient potassium triethylborohydride reducing agent to account for all the Cl⁻ ions. The NPs prepared at room temperature exhibit a disordered face-centered-cubic structure. The byproduct of the reduction yields metal nanoparticles encased in a KCl “matrix.” The KCl serves as a stabilizing agent, allowing formation of the ordered intermetallic phase which adopts

Conclusions

Ordered tetragonal Pt₂FeCo, Pt₂FeNi, and Pt₂CoNi in the 4–6 nm size range were synthesized using a modified solution-phase co-reduction method, which utilizes the KCl byproduct to kinetically stabilize the NPs from agglomeration upon annealing at moderately high temperatures (550 °C). The ordered nanoparticles were transferred onto a carbon support using ethylene glycol and various sonication techniques. The electrode materials were characterized using XRD, EDX, TGA, and TEM to determine the

Acknowledgments

Minh Nguyen carried out the synthesis and initial characterization of the nanoparticles. That part of the project was supported by the Department of Energy office of Basic Energy Sciences through grant DE-FG02-87ER45298. Minghui Yang carried out the Rietveld analysis of X-ray diffraction data and was supported by Cornell funds. Ryo H. Wakabayashi carried out the electrochemical ORR studies and was supported by the Energy Materials Center at Cornell (EMC²), an Energy Frontier Research Center

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View full text

Synthesis of carbon supported ordered tetragonal pseudo-ternary Pt2M′M″ (M = Fe, Co, Ni) nanoparticles and their activity for oxygen reduction reaction

Highlights

Abstract

Introduction

Section snippets

Materials

Synthesis and crystal structure analysis

Conclusions

Acknowledgments

J. Power Sources

Prog. Mater. Sci.

J. Electroanal. Chem.

J. Less Common Met.

J. Less Common Met.

Chem. Rev.

Nature

Chem. Soc. Rev.

Nature

Nature

J. Am. Chem. Soc.

Chem. Soc. Rev.

Adv. Funct. Mater.

Chem. Rev.

J. Am. Chem. Soc.

Adv. Mater.

Energy Environ. Sci.

Synthesis of carbon supported ordered tetragonal pseudo-ternary Pt₂M′M″ (M = Fe, Co, Ni) nanoparticles and their activity for oxygen reduction reaction