Sung Mook Choi

Star-shaped Pd@Pt core–shell catalysts supported on reduced graphene oxide with superior electrocatalytic performance

Reduced graphene oxide (RGO)-supported bimetallic Pd–Pt nanostructures with core–shell Pd@Pt (Pd@Pt/RGO) and alloyed PdPt (PdPt/RGO) were prepared by a one-pot reduction approach using L-ascorbic acid for the reduction of both the metal precursors and the graphene oxide supports. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HR-TEM), and Raman spectroscopy revealed that the three-dimensionally shaped Pd–Pt nanostructures were uniformly deposited onto the reduced graphene oxide surface. The RGO-supported core–shell Pd@Pt and alloyed PdPt catalysts were confirmed and investigated by high-angle annular dark-field scanning TEM (HADDF-STEM) with energy-dispersive X-ray spectroscopy (EDX) in addition to X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and cyclic voltammetry (CV). With the synergetic effects of the binary Pd–Pt system and the RGO support, these catalysts exhibited considerably enhanced catalytic activities and stabilities for the oxidation of methanol in an alkaline solution compared to monometallic Pt/RGO and commercially available carbon-supported Pt (Pt/C) catalysts. The star-shaped core–shell Pd@Pt/RGO catalysts exhibited the greatest improvement in electrocatalytic performance in terms of current density, onset potential, stability, and the charge transfer rate.

SnO2 Nanorod-Planted Graphite: An Effective Nanostructure Configuration for Reversible Lithium Ion Storage

We report a novel architecture of SnO(2) nanorod-planted graphite particles for an efficient Li ion storage material that can be prepared by a simple catalyst-assisted hydrothermal process. Rectangular-shaped SnO(2) nanorods are highly crystalline with a tetragonal rutile phase and distributed uniformly over the surface of micrometer-sized graphite particles. In addition, the size dimensions of grown SnO(2) nanorods can be controlled by varying the synthesis conditions. The diameter can be engineered to a sub-100 nm range, and the length can be controlled to up to several hundred nanometers. Significantly, the SnO(2) nanorod-planted graphite demonstrates an initial Li ion storage capacity of about 1010 mAh g(-1) during the first cycle. Also, these SnO(2)-graphite composites show high Coulombic efficiency and cycle stability in comparison with SnO(2) nanomaterials that are not combined with graphite. The enhanced electrochemical properties of SnO(2) nanorod-planted graphite, as compared with bare SnO(2) materials, inspire better design of composite materials with effective nanostructural configurations for advanced electrodes in lithium ion batteries.

Binary PdM catalysts (M = Ru, Sn, or Ir) over a reduced graphene oxide support for electro-oxidation of primary alcohols (methanol, ethanol, 1-propanol) under alkaline conditions

High metal loaded (60 wt%) binary PdM (M = Ru, Sn, Ir) catalysts were synthesized on reduced graphene oxide (RGO) using the borohydride reduction method, and they were used for the electro-oxidation of simple alcohols, such as methanol, ethanol, and 1-propanol, in alkaline media. Cyclic voltammetry (CV) tests indicated that the Pd-based binary systems could improve electrochemical activities significantly compared to the monometallic Pd/RGO catalyst. Among the prepared catalysts, addition of Ru to Pd (PdRu/RGO) resulted in remarkably improved electrocatalytic activity in terms of larger peak current densities and lower onset potential in all electro-oxidation cases with methanol, ethanol, and 1-propanol. CO-stripping tests also revealed that the onset and peak potentials for the CO oxidation appear to decrease by the addition of Ru to Pd/RGO, indicating that the electro-oxidation of CO can take place more efficiently on the PdRu/RGO catalyst with the assistance of easily formed hydroxyl groups. Such an improvement of electrocatalytic performance can be ascribed to structural and chemical modifications of the Pd catalysts. Physicochemical properties of the PdM/RGO catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS).

Toward new fuel cell support materials: a theoretical and experimental study of nitrogen-doped graphene.

Nano-scale Pt particles are often reported to be more electrochemically active and stable in a fuel cell if properly displaced on support materials; however, the factors that affect their activity and stability are not well understood. We applied first-principles calculations and experimental measurements to well-defined model systems of N-doped graphene supports (N-GNS) to reveal the fundamental mechanisms that control the catalytic properties and structural integrity of nano-scale Pt particles. DFT calculations predict thermodynamic and electrochemical interactions between N-GNS and Pt nanoparticles in the methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR). Moreover, the dissolution potentials of the Pt nanoparticles supported on GNS and N-GNS catalysts are calculated under acidic conditions. Our results provide insight into the design of new support materials for enhanced catalytic efficiency and long-term stability.

Standing pillar arrays of C-coated hollow SnO2 mesoscale tubules for a highly stable lithium ion storage electrode

This work reports the direct growth of hollow one-dimensional nanostructure arrays on conducting substrates for use as efficient electrodes in Li-ion batteries. The C-coated hollow SnO2 pillar array structures can be prepared by template-directed synthesis, selective wet etching, and a carbonization route. The well-oriented ZnO nanorod arrays, which are grown on titanium substrates, are used as a sacrificial template for the deposition of SnO2 layers through a simple hydrothermal process. The ZnO portions are selectively removed by wet etching, producing hollow SnO2 arrays that are consecutively covered with carbon layers via the carbonization of glucose. The lithium storage performance of the synthesized C-coated hollow SnO2 pillar array structures is demonstrated by applying them directly to a working electrode without additive materials. The standing pillar array electrode, consisting of C-coated hollow SnO2, exhibits an excellent discharge capacity of ca. 1251.9 mA h g−1 on the first cycle, and it also shows promising cyclability, rate capability, and coulombic efficiency, indicating that C-coated hollow SnO2 arrays fabricated on the current collector can be powerful candidates for a highly stable lithium storage electrode platform.

Theoretical insight into highly durable iron phthalocyanine derived non-precious catalysts for oxygen reduction reactions

N4-chelate macrocycles comprise the foundation for non-precious metal oxygen reduction reaction (ORR) catalyst research, where the main electrochemical process occurs in polymer electrolyte membrane (PEM) fuel cells. Although iron–nitrogen–carbon (M–N–C) complexes are known to be the most active non-precious ORR catalysts to date, a fundamental understanding of the ORR mechanisms of these materials is still in its nascent stage and needs further investigation. In this work, ab initio density functional theory (DFT) calculations have been applied to unveil the underlying principles for the electrocatalytic activity and structural stability of Fe–N4 chelates exposed to acidic media. Therefore, we compared the electronic structures of ferrous phthalocyanine (Fe-Pc) and an in-house developed Fe-Pc modified with diphenylphenthioether substituent species (Fe-SPc). The results of these DFT simulations directly correlate with the results of the half-cell ORR activity and stability electrochemical testing in 0.1 M HClO4. The results indicate that the relative energetic position of the dz2-orbital with respect to the Fermi level can induce an Fe redox couple potential shift and modulate the catalytic activity towards the ORR. Furthermore, our combined DFT calculations and empirical observations highlight that the relative position of the dz2-orbital can be controlled by the incorporation of functional groups, resulting in the ability to tune the ORR activity of these complexes. Structural stability of the materials, as predicted by the DFT-calculated cohesive energies of Fe and FeO, can also be readily tuned by modulating Fe-Pc with the substituent species. This study, coupling rigorous experimental observations with DFT investigations, thereby provides a fundamental insight that can aid in the design of future generations of non-precious ORR catalysts with improved activity and stability.

The Role of Ruthenium on Carbon-Supported PtRu Catalysts for Electrocatalytic Glycerol Oxidation under Acidic Conditions

A series of binary PtRu catalysts with different Pt/Ru atomic ratios (from 7:3 to 3:7) were synthesized on a carbon support using the colloidal method; they were then used for electrooxidation of glycerol in acid media. X‐ray diffraction, transmission electron microscopy, X‐ray photoelectron spectroscopy, and X‐ray absorption spectroscopy analyses were used to investigate particle size, size distribution, and structural and electronic properties of the prepared catalysts. Ru added to the Pt‐based catalysts caused structural and electronic modifications over the PtRu alloy catalyst formation. The electrocatalytic activities of PtRu/C series catalysts were investigated using cyclic voltammetry. The Pt5Ru5/C catalyst shows enhanced catalytic activity at least 40 % higher than that of the Pt/C catalyst, with improved stability for glycerol electrooxidation; these improvements are attributed to structural and electronic modifications of the Pt catalysts. Using an electrocatalytic batch reactor, product analysis after the oxidation reaction was performed by high‐performance liquid chromatography to determine and compare the reaction pathways on the Pt/C and PtRu/C catalysts. To understand different catalytic activities of glycerol oxidation on the PtRu alloy surfaces, density functional calculations were performed.

Optimization of Operating Parameters and Components for Water Electrolysis Using Anion Exchange Membrane

Advanced Materials Engineering, Korea University of Science and Technology (UST), 217 Gajeong-ro, Yuseong-gu, Daejeon, 34113, Korea(Received April 14, 2016 ; revised April 28, 2016 ; accepted April 29, 2016)AbstractThe hydrogen has been recognized as a clean, nonpolluting and unlimited energy source that can solvefossil fuel depletion and environmental pollution problems at the same time. Water electrolysis has beenthe most attractive technology in a way to produce hydrogen because it does not emit any pollutants comparedto other method such as natural gas steam reforming and coal gasification etc. In order to improve efficiencyand durability of the water electrolysis, comprehensive studies for highly active and stable electrocatalystshave been performed. The platinum group metal (PGM; Pt, Ru, Pd, Rh, etc.) electrocatalysts indicated ahigher activity and stability compared with other transition metals in harsh condition such as acid solution.It is necessary to develop inexpensive non-noble metal catalysts such as transition metal oxides because thePGM catalysts is expensive materials with insufficient it’s reserves. The optimization of operating parameterand the components is also important factor to develop an efficient water electrolysis cell. In this study,we optimized the operating parameter and components such as the type of AEM and density of gas diffusionlayer (GDL) and the temperature/concentration of the electrolyte solution for the anion exchange membranewater electrolysis cell (AEMWEC) with the transition metal oxide alloy anode and cathode electrocatalysts.The maximum current density was 345.8 mA/cm