Youngjin Ye

Facile Synthesis of Nb2O5@Carbon Core–Shell Nanocrystals with Controlled Crystalline Structure for High-Power Anodes in Hybrid Supercapacitors

Hybrid supercapacitors (battery-supercapacitor hybrid devices, HSCs) deliver high energy within seconds (excellent rate capability) with stable cyclability. One of the key limitations in developing high-performance HSCs is imbalance in power capability between the sluggish Faradaic lithium-intercalation anode and rapid non-Faradaic capacitive cathode. To solve this problem, we synthesize Nb2O5@carbon core-shell nanocyrstals (Nb2O5@C NCs) as high-power anode materials with controlled crystalline phases (orthorhombic (T) and pseudohexagonal (TT)) via a facile one-pot synthesis method based on a water-in-oil microemulsion system. The synthesis of ideal T-Nb2O5 for fast Li(+) diffusion is simply achieved by controlling the microemulsion parameter (e.g., pH control). The T-Nb2O5@C NCs shows a reversible specific capacity of ∼180 mA h g(-1) at 0.05 A g(-1) (1.1-3.0 V vs Li/Li(+)) with rapid rate capability compared to that of TT-Nb2O5@C and carbon shell-free Nb2O5 NCs, mainly due to synergistic effects of (i) the structural merit of T-Nb2O5 and (ii) the conductive carbon shell for high electron mobility. The highest energy (∼63 W h kg(-1)) and power (16 528 W kg(-1) achieved at ∼5 W h kg(-1)) densities within the voltage range of 1.0-3.5 V of the HSC using T-Nb2O5@C anode and MSP-20 cathode are remarkable.

Designing a Highly Active Metal‐Free Oxygen Reduction Catalyst in Membrane Electrode Assemblies for Alkaline Fuel Cells: Effects of Pore Size and Doping‐Site Position

To promote the oxygen reduction reaction of metal-free catalysts, the introduction of porous structure is considered as a desirable approach because the structure can enhance mass transport and host many catalytic active sites. However, most of the previous studies reported only half-cell characterization; therefore, studies on membrane electrode assembly (MEA) are still insufficient. Furthermore, the effect of doping-site position in the structure has not been investigated. Here, we report the synthesis of highly active metal-free catalysts in MEAs by controlling pore size and doping-site position. Both influence the accessibility of reactants to doping sites, which affects utilization of doping sites and mass-transport properties. Finally, an N,P-codoped ordered mesoporous carbon with a large pore size and precisely controlled doping-site position showed a remarkable on-set potential and produced 70% of the maximum power density obtained using Pt/C.

One-Pot Synthesis of Intermetallic Electrocatalysts in Ordered, Large-Pore Mesoporous Carbon/Silica toward Formic Acid Oxidation

This study describes the one-pot synthesis and single-cell characterization of ordered, large-pore (>30 nm) mesoporous carbon/silica (OMCS) composites with well-dispersed intermetallic PtPb nanoparticles on pore wall surfaces as anode catalysts for direct formic acid fuel cells (DFAFCs). Lab-synthesized amphiphilic diblock copolymers coassemble hydrophobic metal precursors as well as hydrophilic carbon and silica precursors. The final materials have a two-dimensional hexagonal-type structure. Uniform and large pores, in which intermetallic PtPb nanocrystals are significantly smaller than the pore size and highly dispersed, enable pore backfilling with ionomers and formation of the desired triple-phase boundary in single cells. The materials show more than 10 times higher mass activity and significantly lower onset potential for formic acid oxidation as compared with commercial Pt/C, as well as high stability due to better resistivity toward CO poisoning. In single cells, the maximum power density was higher than that of commercial Pt/C, and the stability highly improved, compared with commercial Pd/C. The results suggest that PtPb-based catalysts on large-pore OMCSs may be practically applied as real fuel cell catalysts for DFAFC.

A Highly Efficient Colorimetric Immunoassay Using a Nanocomposite Entrapping Magnetic and Platinum Nanoparticles in Ordered Mesoporous Carbon

Nanocomposite to achieve ultrafast immunoassay: a new synergistically integrated nanocomposite consisting of magnetic and platinum nanoparticles, simultaneously entrapped in mesoporous carbon, is developed as a promising enzyme mimetic candidate to achieve ultrafast colorimetric immunoassays. Using new assay system, clinically important target molecules, such as human epidermal growth factor receptor 2 (HER2) and diarrhea-causing rotavirus, can be detected in only 3 min at room temperature with high specificity and sensitivity.

A direct one-step synthetic route to Pd–Pt nanostructures with controllable shape, size, and composition for electrocatalytic applications

Pd–Pt branched nanocrystals have been known to exhibit a synergistic effect in many electrocatalytic reactions such as reduction of oxygen and oxidation of small organic molecules. However, Pd–Pt branched structures have generally been synthesized using a two-step seed-mediated approach, which is unbeneficial for large-scale synthesis. Therefore, it is necessary to develop a one-step route to Pd–Pt branched structures. Herein, we developed a direct one-step synthetic route to obtain Pd–Pt structures with controllable shape, size, and composition. In this system, KBr plays a critical role in controlling the size and shape of the Pd–Pt NCs. The resulting Pd1Pt5 branched nanocrystals showed 3.4 and 6.2 times higher mass activity toward oxygen reaction and formic acid oxidation than commercial Pt/C, respectively.

A study of the palladium size effect on the direct synthesis of hydrogen peroxide from hydrogen and oxygen using highly uniform palladium nanoparticles supported on carbon

Highly monodisperse carbon-supported palladium nanoparticles with controllable size (3 nm, 6.5 nm, 9.5 nm) were prepared using a simple colloidal method, and the size dependence of the catalytic performance for the direct synthesis of hydrogen peroxide from hydrogen and oxygen was studied. Smaller-sized supported palladium nanoparticles showed both higher conversion of hydrogen and selectivity for hydrogen peroxide, compared to larger-sized supported particles. Among the catalysts tested, 3-nm Pd nanoparticles supported on carbon showed the highest yield for hydrogen peroxide because of the small size and high crystallinity.

Simple synthesis of platinum dendritic aggregates supported on conductive tungsten oxide nanowires as high-performance methanol oxidation electrocatalysts.

Platinum nanocrystals are key catalysts invaluable to many reactions involved in proton-exchange membrane (PEM) fuel cells, including the oxidation of small organic molecules, such as methanol, at the anode and the reduction of oxygen molecules at the cathode. Current advances in nanotechnology have allowed us to tune the morphology of nanocrystals, which is crucial to maximize their activity, because morphology determines surface atomic arrangement as well as the number of atoms located at the corners or edges. Recently, Pt nanocrystals with dendritic morphologies have attracted great interest for use as highly active electrocatalysts for PEM fuel-cell applications, because they provide a relatively large specific surface area and a high specific activity due to the presence of high densities of edges and corners on their structures. In typical applications, Pt-based catalysts or electrocatalysts are used in the form of Pt nanocrystals supported on metal oxides or carbonaceous materials to achieve good dispersion of Pt nanocrystals and thus maximize the surface area available for catalytic or electrocatalytic reactions. Among various metal oxides or carbonaceous materials, tungsten oxides are of particular interest as supports for Pt nanocrystals owing to their synergistic effect in catalyzing the methanol oxidation reaction that occurs at the anode of direct methanol fuel cells. It was shown that tungsten oxides facilitate not only the dehydrogenation of methanol by forming a hydrogen bronze, but also the oxidation of the COads species, which acts as a poison, due to the presence of hydroxyl groups (OHads) on the surface of tungsten oxACHTUNGTRENNUNGides.[8a] Typical tungsten oxides (WO3), however, have relatively low surface areas and low electrical conductivity, which have been main limitations in developing a highly active catalytic system based on Pt nanocrystals supported on tungsten oxides for the methanol oxidation reaction. Recently, it was shown that W18O49 nanowires have electrical conductivity as high as 4 S cm , which is attributed to the presence of W ions and the intervalence charge transfer between W and W ions. In addition, W18O49 has excellent durability under an electrochemical environment. In previous work, we showed that Pt/W18O49 preserves 87 % of its initial electrochemical surface area after an accelerated durability test (1 000 cycles between 0.6 and 1.3 VNHE). [10] Herein, we report a simple synthesis of new hybrid nanostructures consisting of dendritic Pt nanocrystals supported on W18O49 nanowires. In this approach, Pt/W18O49 hybrid nanostructures were generated by heating benzyl alcohol containing tungsten chloride and subsequent, repeated injection of ethylene glycol containing chloroplatinic acid, in which initially formed W18O49 nanowires provide preferential sites for the nucleation and growth of Pt upon the reduction of chloroplatinic acid by ethylene glycol (Figure 1 A). The overall procedure is simple (one pot), and results in the formation of a high density of Pt nanodendritic aggregates on the surface of conductive W18O49 nanowires with an average diameter of about 2 nm. Importantly, the Pt nanodendritic aggregates supported on the W18O49 nanowires exhibited enhanced electrocatalytic activity toward the methanol oxidation reaction as well as higher CO tolerance compared with that of commercial Pt/C catalysts. Figure S1 in the Supporting Information shows the XRD pattern of the as-synthesized Pt nanocrystals on tungsten oxide nanowires. The first strong reflection at 2q=23.58 is indexed to the (010) plane of the monoclinic W18O49 structures (JCPDS 71–2450), which means that W18O49 nanowires grew along the [010] direction. All other peaks can be indexed to face-centered cubic (fcc) Pt (JCPDS 04–0802). Indeed, the (111)/ ACHTUNGTRENNUNG(200) peak ratio of the synthesized Pt was 1.5 times higher than the expected ratio for the bulk structure. This indicated that the Pt dendritic aggregates are oriented along the [111] direction, supporting the overgrowth of Pt dendritic aggregates on the W18O49 nanowires, which will be discussed in later paragraphs. [a] Y. Ye, Prof. J. Lee Department of Chemical Engineering Pohang University of Science and Technology (POSTECH) Pohang, 790-784 (Republic of Korea) Fax: (+82) 054-279-5528 E-mail : jinwoo03@postech.ac.kr [b] Prof. J. Joo Department of Applied Chemistry Kyoungpook National University Daegu, 702-701 (Republic of Korea) [c] Prof. B. Lim Center for Human Interface Nano Technology (HINT) and School of Advanced Materials Science and Engineering Sungkyunkwan University Suwon, 440-746 (Republic of Korea) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201103720.

Direct access to aggregation-free and small intermetallic nanoparticles in ordered, large-pore mesoporous carbon for an electrocatalyst

An intermetallic catalyst with ordered atomic arrays has a higher electrocatalytic activity than alloy, but the high temperature required for the formation makes the particles large, resulting in low mass activity. We report the simple synthesis of small Pt-based intermetallic nanoparticles on a carbon-based ordered mesoporous support by combining block copolymer-assisted evaporation-induced self-assembly and strong metal-support interaction (SMSI). Aluminosilicate in the mesostructured wall is an SMSI agent and charge transfer from Pt to the aluminosilicate suppresses the sintering of intermetallic nanoparticles. Intermetallic PtPb and Pt3Co on carbon-based mesoporous supports are synthesized, and their particle sizes are below 5 nm even at high loading. The PtPb catalyst shows 15 times higher mass activity for formic acid oxidation than Pt/C, and the Pt3Co catalyst shows 3.25 times higher mass activity for oxygen reduction than Pt/C. This procedure can be extended to synthesize various heterogenous catalysts that require high temperature for synthesis or to operate.