Ran Choi

Designed Synthesis of Well-Defined Pd@Pt Core-Shell Nanoparticles with Controlled Shell Thickness as Efficient Oxygen Reduction Electrocatalysts

Improving the electrocatalytic activity and durability of Pt-based catalysts with low Pt content toward the oxygen reduction reaction (ORR) is one of the main challenges in advancing the performance of polymer electrolyte membrane fuel cells (PEMFCs). Herein, a designed synthesis of well-defined Pd@Pt core-shell nanoparticles (NPs) with a controlled Pt shell thickness of 0.4-1.2 nm by a facile wet chemical method and their electrocatalytic performances for ORR as a function of shell thickness are reported. Pd@Pt NPs with predetermined structural parameters were prepared by in situ heteroepitaxial growth of Pt on as-synthesized 6 nm Pd NPs without any sacrificial layers and intermediate workup processes, and thus the synthetic procedure for the production of Pd@Pt NPs with well-defined sizes and shell thicknesses is greatly simplified. The Pt shell thickness could be precisely controlled by adjusting the molar ratio of Pt to Pd. The ORR performance of the Pd@Pt NPs strongly depended on the thickness of their Pt shells. The Pd@Pt NPs with 0.94 nm Pt shells exhibited enhanced specific activity and higher durability compared to other Pd@Pt NPs and commercial Pt/C catalysts. Testing Pd@Pt NPs with 0.94 nm Pt shells in a membrane electrode assembly revealed a single-cell performance comparable with that of the Pt/C catalyst despite their lower Pt content, that is the present NP catalysts can facilitate low-cost and high-efficient applications of PEMFCs.

Synthesis and characterization of Pt9Co nanocubes with high activity for oxygen reduction

Monodisperse, crystalline Pt(9)Co nanocubes were prepared by a one-pot colloidal method. The prepared Pt(9)Co nanocubes showed significantly enhanced electrocatalytic activity toward oxygen reduction.

Composition-Controlled PtCo Alloy Nanocubes with Tuned Electrocatalytic Activity for Oxygen Reduction

Modification of the electronic structure and lattice contraction of Pt alloy nanocatalysts through control over their morphology and composition has been a crucial issue for improving their electrocatalytic oxygen reduction reaction (ORR) activity. In the present work, we synthesized PtCo alloy nanocubes with controlled compositions (Pt(x)Co NCs, x = 2, 3, 5, 7, and 9) by regulating the ratio of surfactants and the amount of Co precursor to elucidate the effect of the composition of nanocatalysts on their ORR activity. Pt(x)Co NCs had a Pt-skin structure after electrochemical treatment. The electrocatalysis experiments revealed a strong correlation between ORR activity and Co composition. Pt₃Co NCs exhibited the best ORR performance among the various Pt(x)Co NCs. From density functional theory calculations, a typical volcano-type relationship was established between ORR activity and oxygen binding energy (E(OB)) on NC surfaces, which showed that Pt₃Co NCs had the optimal E(OB) to achieve the maximum ORR activity. X-ray photoelectron spectroscopy and X-ray diffraction measurements demonstrated that the electronic structure and lattice contraction of the Pt(x)Co NCs could be tuned by controlling the composition of NCs, which are highly correlated with the trends of E(OB) change.

Shape‐Controlled Synthesis of Pt3Co Nanocrystals with High Electrocatalytic Activity toward Oxygen Reduction

Polymer electrolyte membrane fuel cells (PEMFCs), a promising alternative energy system, have been intensively explored as a power source for electric vehicles or appliances. Since their performance is limited by the slow reduction kinetics of the oxygen reduction reaction (ORR) at the cathode, improving the ORR activity is key to developing efficient PEMFCs. For this reason, the synthesis of Pt alloy nanoparticles with controlled compositions and morphologies has been extensively exploited for the last decade. Previous studies revealed that the ORR activities of Pt–M (where M is Co, Ni, Fe, Mn, Cr, Cu, and Pd) alloys are superior to those of pure Pt catalysts. In addition, their ORR kinetics can be controlled by tuning their morphologies because the enhancement of the ORR activity depends highly on the surface atom arrangements of the catalyst particles. For example, Pt3Mn nanocubes dominantly bound by {100} planes exhibited ORR activity that was three-times higher than that of the commercial Pt/C catalyst in H2SO4 media. [11] However, investigations of catalysts based on the Pt3Ni octahedra and truncated octahedra in HClO4 have shown that the catalytic activity is higher on the Pt3Ni {111} planes than that on the {100} planes because the trend of binding the sulfate anions on the {111} planes is not established in this media. In addition, Stamenkovic et al. demonstrated that the catalytic activity is determined by the low coverage of OHad, and that the ORR on Pt3Ni {111} planes is the highest among the low-index planes. The Pt–Co alloy nanoparticles have drawn particular interest as ORR electrocatalysts due to their enhanced catalytic activities, which can be attributed to the modified electronic structure of Pt, the tuned strength of the oxygen– metal bonding, and the low toxicity of dissolved Co ions against the polymer electrolyte. Although it might be possible to modulate the catalytic properties of Pt–Co nanoparticles by controlling their morphologies, the shapecontrolled synthesis of Pt–Co nanoparticles and their morphology-dependent ORR activities have rarely been reported. Recently, we have demonstrated that the Pt9Co alloy nanocubes bound by {100} planes show a fourfold improvement of catalytic activity toward ORR over the commercial Pt/C catalyst in an H2SO4 solution. [18] Herein, we describe the synthesis of Pt3Co nanocubes (NCs) and nanotruncated octahedra (NTO), the surfaces of which are dominated by {100} and {111}/ ACHTUNGTRENNUNG{100} planes, respectively. The selective synthesis of Pt3Co NCs and NTO was achieved by controlling the reaction time. The prepared Pt3Co NTO exhibited significantly enhanced ORR activity compared to the Pt3Co NCs and commercial Pt/C catalyst. In a typical synthesis of Pt3Co nanocrystals, Pt ACHTUNGTRENNUNG(acac)2 (acac=acetylacetonate) and Co2(CO)8, in a molar ratio of 2:1 were employed as precursors, and oleylamine (OAm) and oleic acid (OA) were used as surfactants. The Pt3Co NTO and Pt3Co NCs were selectively obtained as black– brown nanocrystals by heat treatment from 120 to 200 8C for 30 min, with a temperature increase rate of 2–3 8C min , and incubation at 200 8C for 30 min for Pt3Co NTO and 90 min for Pt3Co NCs under identical experimental conditions (see the Experimental Section). The resultant Pt3Co nanocrystals were readily dispersed in organic solvents, such as toluene and hexane. The transmission electron microscopy (TEM) images of the as-synthesized Pt3Co NCs and NTO, with an average size of 8 nm, indicate the selective formation of NCs and NTO with good uniformity (Figure 1 a and 1 c, respectively). Figure 1 b and 1 d shows high-resolution TEM (HRTEM) images of a single Pt3Co NC and NTO, respectively.

New Crystal Structure: Synthesis and Characterization of Hexagonal Wurtzite MnO

Most transition metal oxides have a cubic rocksalt crystal structure, but ZnO and CoO are the only stable transition metal oxides known to possess a hexagonal structure. Unprecedented hexagonal wurtzite MnO has been prepared by thermal decomposition of Mn(acac)(2) on a carbon template. Structural characterization has been carried out by TEM, SAED, and a Rietveld analysis using XRD. The experimental and theoretical magnetic results indicate magnetic ordering of the hexagonal wurtzite MnO. Density functional calculations have been performed in order to understand the electronic and piezoelectric properties of the newly synthesized hexagonal wurtzite MnO.