Yong Hyup Kim

Superior Rechargeability and Efficiency of Lithium–Oxygen Batteries: Hierarchical Air Electrode Architecture Combined with a Soluble Catalyst†

The lithium-oxygen battery has the potential to deliver extremely high energy densities; however, the practical use of Li-O2 batteries has been restricted because of their poor cyclability and low energy efficiency. In this work, we report a novel Li-O2 battery with high reversibility and good energy efficiency using a soluble catalyst combined with a hierarchical nanoporous air electrode. Through the porous three-dimensional network of the air electrode, not only lithium ions and oxygen but also soluble catalysts can be rapidly transported, enabling ultra-efficient electrode reactions and significantly enhanced catalytic activity. The novel Li-O2 battery, combining an ideal air electrode and a soluble catalyst, can deliver a high reversible capacity (1000 mAh g(-1) ) up to 900 cycles with reduced polarization (about 0.25 V).

Enhanced power and rechargeability of a Li-O2 battery based on a hierarchical-fibril CNT electrode

Recently Li-air batteries have been considered to be a promising candidate for EV and HEV applications due to their exceptionally high energy density. A key factor for the practical application of Li-air batteries is to solve the poor reversibility of nonconductive discharge products, which remains a significant limiting factor for Li-air batteries. Therefore, the air electrode needs to be designed such that it minimizes the undesirable clogging and promotes the electrochemical reactivity. As the control of the morphology and porosity of the electrode greatly affects on the capacity and rate capability, various nanostructured air electrodes have been reported using carbon nanoparticles, graphene, graphene oxide, or carbon nanotubes (CNTs). However, the poor cyclability and low rate capability remain as critical drawbacks of the Li−O2 batteries, and the ideally designed electrode architecture is still awaited.

A new catalyst-embedded hierarchical air electrode for high-performance Li–O2 batteries

The Li–O2 battery holds great promise as an ultra-high-energy-density device. However, its limited rechargeability and low energy efficiency remain key barriers to its practical application. Herein, we demonstrate that the ideal electrode morphology design combined with effective catalyst decoration can enhance the rechargeability of the Li–O2 battery over 100 cycles with full discharge and charge. An aligned carbon structure with a hierarchical micro-nano-mesh ensures facile accessibility of reaction products and provides the optimal catalytic conditions for the Pt catalyst. The new electrode is highly reversible even at the extremely high current rate of 2 A g−1. Moreover, we observed clearly distinct morphologies of discharge products when the catalyst is used. The effect of catalysts on the cycle stability is discussed.

A new fabrication method for IPMC actuators and application to artificial fingers

IPMC (ionic polymer metal composite), a kind of ionic electroactive polymer (EAP), has been used for various applications because it has light weight and can make large bending deformation under low driving voltage. In the present work, thick IPMC films were fabricated by hot-pressing several thin IPMC films and the actuating performance was evaluated. Displacement and maximum load with applied voltage were measured using a displacement measuring system, a load cell and a multimeter. Several cycles of Pt electroless-plating were performed on the IPMC films to improve the actuating performance. Then, SEM (scanning electron microscopy) micrographs and EDS (energy dispersive spectrometer) profiles of the IPMC specimen were examined. To demonstrate the feasibility of IPMC films for medical or robotic applications, the developed IPMC actuators were applied to artificial fingers and tested.

A solid element formulation for large deflection analysis of composite shell structures

Abstract An 18-node solid element has been developed to model the behavior of laminated composite shells undergoing large deflection. The element formulation utilizes independently assumed strain in addition to assumed displacement. The strain and the determinant of the Jacobian matrix are assumed to be linear in the thickness direction. This allows analytical integration through the thickness regardless of ply layups. Numerical results demonstrate the validity of the present formulation.

A carbon nanotube wall membrane for water treatment

Various forms of carbon nanotubes have been utilized in water treatment applications. The unique characteristics of carbon nanotubes, however, have not been fully exploited for such applications. Here we exploit the characteristics and corresponding attributes of carbon nanotubes to develop a millimetre-thick ultrafiltration membrane that can provide a water permeability that approaches 30,000 l m(-2) h(-1) bar(-1), compared with the best water permeability of 2,400 l m(-2) h(-1) bar(-1) reported for carbon nanotube membranes. The developed membrane consists only of vertically aligned carbon nanotube walls that provide 6-nm-wide inner pores and 7-nm-wide outer pores that form between the walls of the carbon nanotubes when the carbon nanotube forest is densified. The experimental results reveal that the permeance increases as the pore size decreases. The carbon nanotube walls of the membrane are observed to impede bacterial adhesion and resist biofilm formation.

A Reel-Wound Carbon Nanotube Polarizer for Terahertz Frequencies

Utilizing highly oriented multiwalled carbon nanotube aerogel sheets, we fabricated micrometer-thick freestanding carbon nanotube (CNT) polarizers. Simple winding of nanotube sheets on a U-shaped polyethylene reel enabled rapid and reliable polarizer fabrication, bypassing lithography or chemical etching processes. With the remarkable extinction ratio reaching ∼37 dB in the broad spectral range from 0.1 to 2.0 THz, combined with the extraordinary gravimetric mechanical strength of CNTs, and the dispersionless character of freestanding sheets, the commercialization prospects for our CNT terahertz polarizers appear attractive.

Single-walled carbon-nanotube networks on large-area glass substrate by the dip-coating method.

Highly uniform and large-area single-walled carbon-nanotube (SWNT) networks are realized by the dip-coating method, which is based on fundamental fluid-dynamic phenomena such as capillary condensation and surface tension. The changes in the polarity and hydration properties of the substrate affect the morphology of the SWNT networks and result in nonlinear growth of the networks in the repetitive dip-coating process. The density and the thickness of the SWNT networks are controlled by processing variables including number of dip coatings, concentration of SWNT colloidal solution, and withdrawal velocity. The networks have uniform sheet resistances and high optical transmittance in the visible wavelength range.

Performance enhancement of IPMC actuator by plasma surface treatment

IPMC (ionic polymer metal composite) is composed of ionic polymer and metal electrodes on both surfaces of the polymer. In this study, we changed the surface morphology of the ionic polymer by using plasma treatment. Plasma treatment made needle-shaped microstructures on the surface of the polymer and the microstructures helped to form a thicker uniform metal electrode which is deposited by electroless plating on both sides of the polymer. We observed the actuating properties of IPMC such as displacement, force and lifetime by using the laser displacement measurement system and the load cell. Then we evaluated the enhanced characteristics of an IPMC actuator. Oxygen (the chemical etching gas) and argon (the physical etching gas) were used as the plasma source gas and the oxygen plasma resulted in higher performance.

Electromechanical properties of CNT-coated cotton yarn for electronic textile applications

Smart fabrics have attracted considerable attention due to their potential applications. The essential features of smart fabrics include wearability, weaveability, and stretchability, as well as their sensing/response capability, which is frequently based on electrical measurement. Thus, the electromechanical behavior of these fabrics is considered the most important material property. Here, we report the negative piezoresistance of single-walled carbon nanotube coated cotton yarn (SWNT-CY). The gauge factor (the ratio of the normalized change in piezoresistance to the change in strain) of SWNT-CY is measured to be − 24. It is noteworthy that the factor is negative and an order of magnitude higher than that for conventional metal strain gauges. The negative piezoresistance is due to mechanical contact between fabric fibers, which leads to better electrical paths of SWNT networks. The conduction behavior can be modeled as fluctuation-induced tunneling (FIT) through the contact barriers between conducting regions. The effective barrier strength of strained SWNT-CY is measured to be ~ 30% lower than that of unstrained SWNT-CY. This characteristic may offer new design opportunities for wearable electronics and has significant implications for sensor applications.