Yuseong Noh

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.

MnCo2O4 Nanowires Anchored on Reduced Graphene Oxide Sheets as Effective Bifunctional Catalysts for Li–O2 Battery Cathodes

A hybrid composite system of MnCo2 O4 nanowires (MCO NWs) anchored on reduced graphene oxide (RGO) nanosheets was prepared as the bifunctional catalyst of a Li-O2 battery cathode. The catalysts can be obtained from the hybridization of one-dimensional MCO NWs and two-dimensional RGO nanosheets. As O2 -cathode catalysts for Li-O2 cells, the MCO@RGO composites showed a high initial discharge capacity (ca. 11092.1 mAh gcarbon (-1) ) with a high rate performance. The Li-O2 cells could run for more than 35 cycles with high reversibility under a limited specific capacity of 1000 mAh gcarbon (-1) with a low potential polarization of 1.36 V, as compared with those of pure Ketjenblack and MCO NWs. The high cycling stability, low potential polarization, and rate capability suggest that the MCO@RGO composites prepared here are promising catalyst candidates for highly reversible Li-O2 battery cathodes.

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).

Formation of carbon-coated ZnFe2O4 nanowires and their highly reversible lithium storage properties

In this paper, carbon-decorated ZnFe2O4 nanowires, having one-dimensional geometry with diameters of 70–150 nm and lengths of several micrometers, were prepared and used as a highly reversible lithium ion anode material. They can be obtained from calcination of glucose-coated ZnFe2(C2O4)3 nanowires, which were prepared in glucose containing microemulsion solutions. The physicochemical properties of carbon-coated ZnFe2O4 nanowires were investigated by scanning electron microscopy, X-ray diffraction, Raman spectroscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. The carbon-coated ZnFe2O4 nanowires showed a substantially increased discharge capacity of ca. 1285.1 mA h g−1 at the first cycle as compared with non-carbon-coated ZnFe2O4 nanowires (ca. 1024.3 mA h g−1) and ZnFe2O4 nanoparticles (ca. 1148.7 mA h g−1). Moreover, the discharge capacity of the carbon-coated ZnFe2O4 nanowires was maintained with no degradation even after 100 charge/discharge cycles. The high cycling durability, rate capability, and coulombic efficiency suggest that the carbon-coated ZnFe2O4 nanowires prepared here can be promising anode candidates for a highly reversible lithium storage electrode.

An Overview of One-Dimensional Metal Nanostructures for Electrocatalysis

Abstract Nanostructures of metals are of great importance in the area of catalysis due to their distinct physicochemical properties compared to their bulk counterparts. Size and morphology dependent properties of metal nanostructures provide a rational approach toward designing a highly efficient catalytic materials. In particular, one-dimensional (1D) metallic nanostructures in the shapes of wires, rods and tubes have recently been studied with great interest due to their potential uses as electrocatalysts for oxidations of fuels and reduction of oxidants in fuel cell applications. Compared to the conventional nanoparticle catalysts that are generally supported on carbon, these 1D materials can offer significant opportunities to improve catalytic performance under fuel cell reaction conditions by their structural characteristics such as preferential exposure of reactive crystal facets, high stability, and facile electron transport. Great advances in the synthesis of electrocatalysts based on the metallic nanowires and nanotubes have been made with enhanced electrocatalytic activity and durability. This review summarizes the research progress made on synthesizing 1D metal electrocatalysts using different synthetic strategies, including template-assisted method, electrospinning, and template-free wet-chemical synthesis, with an emphasis on the electrocatalytic performance of these 1D nanomaterials.

Highly selective transformation of glycerol to dihydroxyacetone without using oxidants by a PtSb/C-catalyzed electrooxidation process

We demonstrate an electrocatalytic reactor system for the partial oxidation of glycerol in an acidic solution to produce value-added chemicals, such as dihydroxyacetone (DHA), glyceraldehyde (GAD), glyceric acid (GLA), and glycolic acid (GCA). Under optimized conditions, the carbon-supported bimetallic PtSb (PtSb/C) catalyst was identified as a highly active catalyst for the selective oxidation of glycerol in the electrocatalytic reactor. The product selectivity can be strongly controlled as a function of the applied electrode potential and the catalyst surface composition. The main product from the electrocatalytic oxidation of glycerol was DHA, with a yield of 61.4% of DHA at a glycerol conversion of 90.3%, which can be achieved even without using any oxidants over the PtSb/C catalyst at 0.797 V (vs. SHE, standard hydrogen electrode). The electrocatalytic oxidation of biomass-derived glycerol represents a promising method of chemical transformation to produce value-added molecules.

Thermally Converted CoO Nanoparticles Embedded into N-Doped Carbon Layers as Highly Efficient Bifunctional Electrocatalysts for Oxygen Reduction and Oxygen Evolution Reactions

Hybrid materials that consist of transition‐metal oxides and heteroatom‐doped carbon materials have been researched recently as promising bifunctional electrocatalysts for both oxygen‐reduction reaction (ORR) and oxygen‐evolution reaction (OER) in alkaline media. Herein, CoO nanoparticles embedded into N‐doped carbon layers were synthesized by a thermal conversion process of polypyrrole‐coated Co3O4 nanoparticles supported on a carbon layer in Ar atmosphere at 900 °C. During the process, the initial Co3O4 phase was transformed to the CoO phase along with the thermal carbonization of the polypyrrole layer to the N‐doped carbon layer. Owing to the oxidative combustion induced by the O species released from the Co3O4 nanoparticles, the N‐doped carbon layer could contain pores around the CoO nanoparticles. Alkaline electrolytes could penetrate the N‐doped carbon layer toward the CoO nanoparticles through the pores. The nanocomposites with the well‐assembled CoO nanoparticles and porous N‐doped carbon layer could exhibit superior catalytic activity for ORR and OER. In addition, the N‐doped carbon layers effectively prevent the degradation of the catalyst by protecting the CoO nanoparticles from aggregation during the electrocatalytic processes. The hybrid material of CoO and N‐doped carbon showing highly active and durable catalytic characteristics for ORR and OER is a promising electrocatalyst in fuel cells, metal–air batteries, and water‐splitting systems and could be used instead of precious metals such as Pt, Ru, and Ir.

Bifunctional Hybrid Catalysts with Perovskite LaCo0.8Fe0.2O3 Nanowires and Reduced Graphene Oxide Sheets for an Efficient Li-O2 Battery Cathode

In this paper, bifunctional catalysts consisting of perovskite LaCo0.8Fe0.2O3 nanowires (LCFO NWs) with reduced graphene oxide (rGO) sheets were prepared for use in lithium-oxygen (Li-O2) battery cathodes. The prepared LCFO@rGO composite was explored as a cathode catalyst for Li-O2 batteries, resulting in an outstanding discharge capacity (ca. 7088.2 mAh g-1) at the first cycle. Moreover, a high stability of the O2-cathode with the LCFO@rGO catalyst was achieved over 56 cycles under the capacity limit of 500 mAh g-1 with a rate of 200 mA g-1, as compared to the Ketjenblack carbon and LCFO NWs. The enhanced electrochemical performance suggests that these hybrid composites of perovskite LCFO NWs with rGO nanosheets could be a perspective bifunctional catalyst for the cathode oxygen reduction and oxygen evolution reactions in the development of next-generation Li-O2 battery cathodes.

Ruthenium Oxide Incorporated One-Dimensional Cobalt Oxide Composite Nanowires as Lithium-Oxygen Battery Cathode Catalysts

Ruthenium oxide/cobalt oxide composite nanowires (RuO2/Co3O4 NWs) have been synthesized through a simple and efficient electrospinning method for use as bifunctional electrocatalysts in rechargeable lithium–oxygen (Li–O2) battery cathodes. The as‐prepared NWs have been characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X‐ray spectroscopy (EDX), X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS), and X‐ray absorption near‐edge structure (XANES) spectroscopy. The intrinsic activities of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) over the obtained composite NWs are studied using a rotating‐disk electrode technique. The RuO2/Co3O4 NWs exhibit superior bifunctional electrocatalytic activities for both the ORR and the OER if compared to the activities of the Co3O4 NWs and only Ketjenblack (KB). If the RuO2/Co3O4 NWs are employed as cathode catalysts in Li–O2 cells, great improvements in terms of the discharge capacity, coulombic efficiency, and cycle stability with low discharge‐charge overpotentials are achieved. These can be attributed to the high bifunctional catalytic activities of the RuO2/Co3O4 composite NWs.