Hyo-Jin Ahn

Facile Route to an Efficient NiO Supercapacitor with a Three-Dimensional Nanonetwork Morphology

NiO nanostructures with three distinct morphologies were fabricated by a sol-gel method and their morphology-dependent supercapacitor properties were exploited. The nanoflower- shaped NiO with a distinctive three-dimensional (3D) network and the highest pore volume shows the best supercapacitor properties. The nanopores in flower-shaped nanostructures, offering advantages in contact with and transport of the electrolyte, allow for 3D nanochannels in NiO structure, providing longer electron pathways. The XPS and EIS data of the NiO nanostructure confirm that the flower-shaped NiO, which has the lowest surface area among the three morphologies, was effectively optimized as a superior electrode and yielded the greatest pseudocapacitance. This study indicates that forming a 3D nanonetwork is a straightforward means of improving the electrochemical properties of a supercapacitor.

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.

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 2700–54u2006000 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 10u2006000 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.

Nanophase Ru–RuO2 composites decorated on wrinkled Nb-doped TiO2 nanofibers for electrochemical capacitors

Here, we studied nanophase Ru–RuO2 composites decorated on wrinkled Nb-doped TiO2 (NTO) nanofiber (NF) supports for electrochemical capacitors. These compositions were synthesized by an electrospinning method and an impregnation method. To investigate the electrochemical performance of electrochemical capacitors, 20xa0wt% nanophase Ru–RuO2 composites were decorated on wrinkled NTO NF supports doped with three different amounts of Nb. The structural, morphological, chemical, and electrochemical properties of the composite nanophases were investigated using field emission scanning electron microscopy, transmission electron microscopy, Brunauer–Emmet–Teller analysis, X-ray diffraction, X-ray photoelectron spectroscopy, and cyclic voltammograms (CVs). The CV results indicated that the nanophase Ru–RuO2 composites decorated on the wrinkled NTO NFs possessed superior capacitance (~496.3xa0F/g at 5xa0mV/s) and good high-rate capacitance because of the synergistic effect of the increased specific surface area of the wrinkled NTO NF supports and the presence of the nanophase Ru–RuO2 composites.

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.

Enhanced electrochemical performance of phosphorus incorporated carbon nanofibers by the spin-on dopant method

Phosphorus-incorporated carbon nanofibers (CNFs) were successfully fabricated by using electrospinning and spin-on dopant (SOD) procedures together for electrochemical capacitors (ECs). Microstructural and chemical investigations indicated that phosphorus was uniformly incorporated into the CNFs without any impurities or alloys by using an SOD treatment. The specific surface area of the SOD-treated CNFs increased by over 1.47 times when compared to that of conventional CNFs due to an increase in the total pore volume. In addition, the SOD-treated CNFs contained many beneficial functional groups such as phosphate and hydroxyl groups. ECs, fabricated from SOD-treated CNFs as electrodes, showed enhanced electrochemical performance such as high capacitance (up to 188 F g−1), good high-rate performance with a capacitance retention of 84%, an excellent energy density (17.2–23.5 W h kg−1 in a power density ranging from 360 to 4680 W kg−1), and an excellent cycle stability (86% up to 1000 cycles). These enhancements were attributed to the beneficial effects of the SOD method applied to the CNFs to enlarge the surface area and provide many functional groups.

Electrical and Optical Properties of Al-doped ZnO Films Deposited by Atomic Layer Deposition

Al-doped ZnO(AZO) thin films were synthesized using atomid layer deposition(ALD), which acurately controlled the uniform film thickness of the AZO thin films. To investigate the electrical and optical properites of the AZO thin films, AZO films using ALD was controlled to be three different thicknesses (50 nm, 100 nm, and 150 nm). The structural, chemical, electrical, and optical properties of the AZO thin films were analyzed by X-ray diffraction, X-ray photoelectron spectroscopy, field-emssion scanning electron microscopy, atomic force microscopy, Hall measurement system, and UV-Vis spectrophotometry. As the thickness of the AZO thin films increased, the crystallinity of the AZO thin films gradually increased, and the surface morphology of the AZO thin films were transformed from a porous structure to a dense structure. The average surface roughnesses of the samples using atomic force microscopy were ~3.01 nm, ~2.89 nm, and ~2.44 nm, respectively. As the thickness of the AZO filmsincreased, the surface roughness decreased gradually. These results affect the electrical and optical properties of AZO thin films. Therefore, the thickest AZO thin films with 150 nm exhibited excellent resistivity (), high transmittance (~83.2 %), and the best FOM (). AZO thin films fabricated using ALD may be used as a promising cadidate of TCO materials for optoelectronic applications.

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.