Eun Ja Lim

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.

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.

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.

Directly-Grown and Square-Patterned Arrays of Metal Oxide Nanowires for High-Performance Catalyst Support Platforms

This research reports novel and efficient electrocatalyst support systems. Tin dioxide nanowires grown directly on current collecting substances are introduced as high-performance support platforms. For this propose, palladium or platinum catalysts are impregnated on these nanowire scaffolds and exhibit improved electrocatalytic performance for methanol oxidation in alkaline and acidic environments. These nanowire support platforms could be demonstrated to maximize the electrocatalytic activity because of the effective charge transport provided by the direct connection between the nanowire supports and current collectors. More significantly, grid-patterned nanowire arrays grown directly on current collectors are, for the first time, demonstrated as a milestone to enhance the electrocatalytic performance. The empty space between the patterned nanowire arrays acts as a channel to facilitate the electrolyte diffusion. The metal catalysts incorporated into the patterned nanowire supports show an 8-fold improvement in the catalytic performance for methanol electrooxidation, most likely because of the synergetic effects of the enhanced charge transport and mass transfer attributed to the structural advantages of the patterned nanowire array supports.

Controlling the Size Distribution of Ni Nanocrystals Using Preoxidation of Ni Films

We report the effects of preoxidizing Ni films on the formation of Ni nanocrystals obtained by thermal annealing. When Ni films were oxidized prior to rapid thermal annealing, the Ni nanocrystals formed exhibited a reduction in size and size distribution compared with those formed by direct annealing without oxidation. We attribute this phenomenon to the distinct growth mechanism of nanocrystals from NiO films, which is reminiscent of the spinodal decomposition. The effect of preoxidation was greater as the thickness of the as-deposited Ni films increased.