Geon-Hyoung An

Carbon-Encapsulated Hollow Porous Vanadium-Oxide Nanofibers for Improved Lithium Storage Properties

Carbon-encapsulated hollow porous vanadium-oxide (C/HPV2O5) nanofibers have been fabricated using electrospinning and postcalcination. By optimized postcalcination of vanadium-nitride and carbon-nanofiber composites at 400 °C for 30 min, we synthesized a unique architecture electrode with interior void spaces and well-defined pores as well as a uniform carbon layer on the V2O5 nanofiber surface. The optimized C/HPV2O5 electrode postcalcined at 400 °C for 30 min showed improved lithium storage properties with high specific discharge capacities, excellent cycling durability (241 mA h g(-1) at 100 cycles), and improved high-rate performance (155 mA h g(-1) at 1000 mA g(-1)), which is the highest performance in comparison with previously reported V2O5-based cathode materials. The improved electrochemical feature is due to the attractive properties of the carbon-encapsulated hollow porous structure: (I) excellent cycling durability with high specific capacity relative to the adoption of carbon encapsulation as a physical buffer layer and the effective accommodation of volume changes due to the hollow porous structure, (II) improved high-rate performance because of a shorter Li-ion diffusion pathway resulting from interior void spaces and well-defined pores at the surface. This unique electrode structure can potentially provide new cathode materials for high-performance lithium-ion batteries.

Hierarchical architecture of hybrid carbon-encapsulated hollow manganese oxide nanotubes with a porous-wall structure for high-performance electrochemical energy storage

Metal-oxide-based anode materials in energy storage devices continue to be of considerable interest for both fundamental science and potential technological applications because of their diverse functionalities, high theoretical capacity, cost-effectiveness, and non-polluting nature. However, despite these favourable features for various power source applications, important challenges associated with insertion-induced structural degradation remain, leading to capacity fading, and must be addressed to move towards the practical use of high-capacity metal oxide anodes. Here, we report the successful synthesis of novel hierarchical carbon-encapsulated manganese oxide architectures with hollow-tube structures and unique porous walls via a simple microwave process coupled with a hydrothermal method. This approach provides beneficial synergistic effects in terms of structural stability, electrochemical active surface areas, and electrical and ionic pathways. The carbon-encapsulated porous hollow manganese oxide nanotubes exhibit excellent electrochemical characteristics with large reversible specific capacity and excellent cycling stability (875 mA h g−1 capacity retention up to 100 cycles) as well as outstanding high-rate performance (729 mA h g−1 at 700 mA g−1), which is the highest value thus far reported for manganese-oxide-based anode materials.

Tunneled Mesoporous Carbon Nanofibers with Embedded ZnO Nanoparticles for Ultrafast Lithium Storage

Carbon and metal oxide composites have received considerable attention as anode materials for Li-ion batteries (LIBs) owing to their excellent cycling stability and high specific capacity based on the chemical and physical stability of carbon and the high theoretical specific capacity of metal oxides. However, efforts to obtain ultrafast cycling stability in carbon and metal oxide composites at high current density for practical applications still face important challenges because of the longer Li-ion diffusion pathway, which leads to poor ultrafast performance during cycling. Here, tunneled mesoporous carbon nanofibers with embedded ZnO nanoparticles (TMCNF/ZnO) are synthesized by electrospinning, carbonization, and postcalcination. The optimized TMCNF/ZnO shows improved electrochemical performance, delivering outstanding ultrafast cycling stability, indicating a higher specific capacity than previously reported ZnO-based anode materials in LIBs. Therefore, the unique architecture of TMCNF/ZnO has potential for use as an anode material in ultrafast LIBs.

Vanadium nitride encapsulated carbon fibre networks with furrowed porous surfaces for ultrafast asymmetric supercapacitors with robust cycle life

Transition metal nitrides have received significant attention in view of their application as pseudocapacitive electrodes in high performance supercapacitors owing to their high capacitance, excellent electrical conductivity, high electrochemical selectivity, and low environmental impact. Nevertheless, the utilization of transition metal nitrides still encounters serious challenges due to the chemical instability of these materials during cycling in the presence of oxygen and/or water containing electrolytes, which leads to rapid capacitance fading. Here, we propose a novel structure comprising vanadium nitride encapsulated carbon fibre networks with furrowed porous surfaces prepared by electrospinning followed by an optimal stabilization and carbonization treatments. The resultant electrode shows a high energy density of 53.1–36.0 W h kg−1 at high power densities in the range from 2700–54 000 W kg−1. This performance is superior to previously reported results on other asymmetric supercapacitors. Moreover, an excellent cycling stability of 92.9% at a current density of 80 A g−1 after 10 000 cycles, and a superb electrode flexibility have been recorded. Our original synthesis strategy provides a useful methodology to increase the chemical stability of vanadium nitride by carbon encapsulation, which also leads to shorter diffusion pathways due to the furrowed porous surfaces and the advanced network structure consisting of 1-dimensional fibres.

Synergistic Effects of a Multifunctional Graphene Based Interlayer on Electrochemical Behavior and Structural Stability

The ability to rationally design and manipulate the interfacial structure in lithium ion batteries (LIBs) is of utmost technological importance for achieving desired performance requirements as it provides synergistic effects to the electrochemical properties and cycling stability of electrode materials. However, despite considerable efforts and progress made in recent years through the interface engineering based on active electrode materials, relatively little attention has been devoted to address the physical aspects of the interface and interfacial layer between the anode materials layer and the current collector. Here, we propose and successfully grow unique graphene directly on a Cu current collector as an ideal interfacial layer using the modified chemical vapor deposition (CVD). The anode with an engineered graphene interlayer exhibits remarkably improved electrochemical performances, such as large reversible specific capacity (921.4 mAh g(-1) at current density of 200 mA g(-1)), excellent Coulombic efficiency (close to approximately 96%), and superior cycling capacity retention and rate properties compared to the bare Cu. These excellent electrochemical features are discussed in terms of multiple beneficial effects of graphene on interfacial stability and adhesion between the anode and the collector, oxidation or corrosion resistance of the graphene grown Cu current collector, and electrical contact conductance during the charge/discharge process.

Synergistic incorporation of hybrid heterobimetal–nitrogen atoms into carbon structures for superior oxygen electroreduction performance

Although Pt-based catalytic technology has led to significant advances in the development of electrocatalysts in fuel cells, Pt replacement with efficient and stable non-precious metal catalysts has a great technological significance for successful large-scale implementation of fuel cells. Here, we present the development of hybrid functional 1-dimensional carbon structures incorporated homogeneously with high contents of non-precious metal multi-dopants, consisting of iron, cobalt and nitrogen, as a promising alternative to Pt-based catalysts for the cathodic oxygen reduction reaction (ORR) through a modified electrospinning technique. These hybrid heterobimetal–nitrogen-incorporated carbon structures exhibit superior ORR electrocatalytic properties i.e., more positive reduction potential, high electroreduction current density, high electron transfer value (∼3.87) close to the perfect ORR and improved electrochemical stability with a very small decrease of ∼8 mV in half-wave potential. The observed enhancement in electrochemical performance can be ascribed to the increased amount of catalytically active sites with relatively high contents of heterometallic iron and cobalt atoms surrounded by nitrogen species and their homogeneous distribution on the catalyst surface.

Improved ionic diffusion through the mesoporous carbon skin on silicon nanoparticles embedded in carbon for ultrafast lithium storage

Because of their combined effects of outstanding mechanical stability, high electrical conductivity, and high theoretical capacity, silicon (Si) nanoparticles embedded in carbon are a promising candidate as electrode material for practical utilization in Li-ion batteries (LIBs) to replace the conventional graphite. However, because of the poor ionic diffusion of electrode materials, the low-grade ultrafast cycling performance at high current densities remains a considerable challenge. In the present study, seeking to improve the ionic diffusion, we propose a novel design of mesoporous carbon skin on the Si nanoparticles embedded in carbon by hydrothermal reaction, poly(methyl methacrylate) coating process, and carbonization. The resultant electrode offers a high specific discharge capacity with excellent cycling stability (1140 mA h g-1 at 100 mA g-1 after 100 cycles), superb high-rate performance (969 mA h g-1 at 2000 mA g-1), and outstanding ultrafast cycling stability (532 mA h g-1 at 2000 mA g-1 after 500 cycles). The battery performances are surpassing the previously reported results for carbon and Si composite-based electrodes on LIBs. Therefore, this novel approach provides multiple benefits in terms of the effective accommodation of large volume expansions of the Si nanoparticles, a shorter Li-ion diffusion pathway, and stable electrochemical conditions from a faster ionic diffusion during cycling.

수소제조를 위한 다공성 FeCrAl 금속 합금 Foam의 NiO 촉매 담지 및 미세구조 분석

【NiO catalysts were successfully coated onto FeCrAl metal alloy foam as a catalyst support via a dip-coating method. To demonstrate the optimum amount of NiO catalyst on the FeCrAl metal alloy foam, the molar concentration of the Ni precursor in a coating solution was controlled, with five different amounts of 0.4 M, 0.6 M, 0.8 M, 1.0 M, and 1.2 M for a dip-coating process. The structural, morphological, and chemical bonding properties of the NiO-catalyst-coated FeCrAl metal alloy foam samples were assessed by means of field-emission scanning electron microscopy(FESEM), scanning electron microscopy-energy dispersive spectroscopy(SEM-EDS), X-ray diffraction(XRD), and X-ray photoelectron spectroscopy(XPS). In particular, when the FeCrAl metal alloy foam samples were coated using a coating solution with a 0.8 M Ni precursor, well-dispersed NiO catalysts on the FeCrAl metal alloy foam compared to the other samples were confirmed. Also, the XPS results exhibited the chemical bonding states of the NiO phases and the FeCrAl metal alloy foam. The results showed that a dip-coating method is one of best ways to coat well-dispersed NiO catalysts onto FeCrAl metal alloy foam.】

Pt Electrocatalysts Composited on Electro-Spun Pt Nanowires for Direct Methanol Fuel Cells

Two types of Pt nanoparticle electrocatalysts were composited on Pt nanowires by a combination of an electrospinning method and an impregnation method with NaBH4 as a reducing agent. The structural properties and electrocatalytic activities for methanol electro-oxidation in direct methanol fuel cells were investigated by means of scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry. In particular, SEM, HRTEM, XRD, and XPS results indicate that the metallic Pt nanoparticles with polycrystalline property are uniformly decorated on the electro-spun Pt nanowires. In order to investigate the catalytic activity of the Pt nanoparticles decorated on the electro-spun Pt nanowires, two types of 20 wt% Pt nanoparticles and 40 wt% Pt nanoparticles decorated on the electro-spun Pt nanowires were fabricated. In addition, for comparison, single Pt nanowires were fabricated via an electrospinning method without an impregnation method. As a result, the cyclic voltammetry and chronoamperometry results demonstrate that the electrode containing 40 wt% Pt nanoparticles exhibits the best catalytic activity for methanol electro-oxidation and the highest electrochemical stability among the single Pt nanowires, the 20 wt% Pt nanoparticles decorated with Pt nanowires, and the 40 wt% Pt nanoparticles decorated with Pt nanowires studied for use in direct methanol fuel cells.

Fabrication of Octahedral Co 3 O 4 /Carbon Nanofiber Composites for Pt-Free Counter Electrode in Dye-Sensitized Solar Cells

Octahedral Co3O4/carbon nanofiber (CNF) composites are fabricated using electrospinning and hydrothermal methods. Their morphological characteristics, chemical bonding states, and electrochemical properties are used to demonstrate the improved photovoltaic properties of the samples. Octahedral Co3O4 grown on CNFs is based on metallic Co nanoparticles acting as seeds in the CNFs, which seeds are directly related to the high performance of DSSCs. The octahedral Co3O4/CNFs composites exhibit high photocurrent density (12.73 mA/m2), superb fill factor (62.1 %), and excellent power conversion efficiency (5.61 %) compared to those characteristics of commercial Co3O4, conventional CNFs, and metallic Co-seed/CNFs. These results can be described as stemmnig from the synergistic effect of the porous and graphitized matrix formed by catalytic graphitization using the metal cobalt catalyst on CNFs, which leads to an increase in the catalytic activity for the reduction of triiodide ions. Therefore, octahedral Co3O4/CNFs composites can be used as a counter electrode for Pt-free dye-sensitized solar cells.