Tae Gwang Yun

Polypyrrole–MnO2-Coated Textile-Based Flexible-Stretchable Supercapacitor with High Electrochemical and Mechanical Reliability

Carbon-nanotube (CNT)-based textile supercapacitors with MnO2 nanoparticles have excellent power and energy densities, but MnO2 nanoparticles can be delaminated during charge-discharge cycles, which results in significant degradation in capacitance. In this study, polypyrrole conductive polymer was coated on top of MnO2 nanoparticles that are deposited on CNT textile supercapacitor to prevent delamination of MnO2 nanoparticles. An increase of 38% in electrochemical energy capacity to 461 F/g was observed, while cyclic reliability also improved, as 93.8% of energy capacity was retained over 10 000 cycles. Energy density and power density were measured to be 31.1 Wh/kg and 22.1 kW/kg, respectively. An in situ electrochemical-mechanical study revealed that polypyrrole-MnO2-coated CNT textile supercapacitor can retain 98.5% of its initial energy capacity upon application of 21% tensile strain and showed no observable energy storage capacity change upon application of 13% bending strain. After imposing cyclic bending of 750 000 cycles, the capacitance was retained to 96.3%. Therefore, the results from this study confirmed for the first time that the polypyrrole-MnO2-coated CNT textile can reliably operate with high energy and power densities with in situ application of both tensile and bending strains.

Vertically aligned P(VDF-TrFE) core-shell structures on flexible pillar arrays

PVDF and P(VDF-TrFE) nano- and micro- structures have been widely used due to their potential applications in several fields, including sensors, actuators, vital sign transducers, and energy harvesters. In this study, we developed vertically aligned P(VDF-TrFE) core-shell structures using high modulus polyurethane acrylate (PUA) pillars as the support structure to maintain the structural integrity. In addition, we were able to improve the piezoelectric effect by 1.85 times from 40 ± 2 to 74 ± 2 pm/V when compared to the thin film counterpart, which contributes to the more efficient current generation under a given stress, by making an effective use of the P(VDF-TrFE) thin top layer as well as the side walls. We attribute the enhancement of piezoelectric effects to the contributions from the shell component and the strain confinement effect, which was supported by our modeling results. We envision that these organic-based P(VDF-TrFE) core-shell structures will be used widely as 3D sensors and power generators because they are optimized for current generations by utilizing all surface areas, including the side walls of core-shell structures.

Time-dependent mechanical-electrical coupled behavior in single crystal ZnO nanorods

Nanoscale time-dependent mechanical-electrical coupled behavior of single crystal ZnO nanorods was systematically explored, which is essential for accessing the long-term reliability of the ZnO nanorod-based flexible devices. A series of compression creep tests combined with in-situ electrical measurement was performed on vertically-grown single crystal ZnO nanorods. Continuous measurement of the current (I)-voltage (V) curves before, during, after the creep tests revealed that I is non-negligibly increased as a result of the time-dependent deformation. Analysis of the I-V curves based on the thermionic emission-diffusion theory allowed extraction of nanorod resistance, which was shown to decrease as time-dependent deformation. Finally, based on the observations in this study, a simple analytical model for predicting the reduction in nanorod resistance as a function of creep strain that is induced from diffusional mechanisms is proposed, and this model was demonstrated to be in an excellent agreement with the experimental results.

Three-Dimensional Nanofibrous Air Electrode Assembled With Carbon Nanotubes-Bridged Hollow Fe2O3 Nanoparticles for High-Performance Lithium–Oxygen Batteries

Lithium-oxygen batteries have been considered as one of the most viable energy source options for electric vehicles due to their high energy density. However, they are still faced with technical challenges, such as low round-trip efficiency and short cycle life, which mainly originate from the cathode part of the battery. In this work, we designed a three-dimensional nanofibrous air electrode consisted of hierarchically structured carbon nanotube-bridged hollow Fe2O3 nanoparticles (H-Fe2O3/CNT NFs). Composite nanofibers consisted of hollow Fe2O3 NPs anchored by multiple CNTs offered enhanced catalytic sites (interconnected hollow Fe2O3 NPs) and fast charge-transport highway (bridged CNTs) for facile formation and decomposition of Li2O2, leading to outstanding cell performance: (1) Swagelok cell exhibited highly reversible cycling characteristics for 250 cycles with a fixed capacity of 1000 mAh g-1 at a current density of 500 mA g-1. (2) A module composed of two pouch-type cells stably powered an light-emitting diode lamp operated at 5.0 V.

Stretchable Textile Based Supercapacitor with High Reliability

Extended Abstract There has been great interesting demand for flexible and wearable electronics due to huge potential applications. For advanced wearable electronics, flexible and stretchable energy storage system with high reliability is one of key parts. Supercapacitors are receiving much interest due to their high power density and long cycle life. Supercapacitors contain such components as electrodes, electrolytes, separators, and current collectors. To make flexible and stretchable supercapacitor, the proper selection of components including active materials and supporting substrate should be provided. Textile based supercapacitors are especially well-suited for wearable energy storage device applications due to their porous structure, which aids ion diffusion to enhance the specific capacitance compared to a flat polyester substrate. Textile supercapacitors also have the ability to maintain electrochemical performance under mechanical strain [1]. However, supercapacitors have a drawback of their low energy density compared to rechargeable Li ion batteries. To enhance the energy density, many researchers have studied nanostructured pseudocapacitor materials such as metal oxides [2]. However, large volume change of the metal oxide nanostructures during charge-discharge cycles cause delamination of the active materials, which decrease in electrochemical reliability. To prevent the delamination of nanostructured materials, thin layer of conductive polymer PEDOT:PSS is often coated on pseudocapacitor materials [3] to improve the electrochemical reliability. Although PEDOT;PSS serves as a conductive and adhesive layer, it does not have high capacity (200F/g) It is essential to use an alternative material serves as a conductive and adhesive layer while also enhancing capacitance. In this study, we have developed highly reliable and stretchable supercapacitor using textile based electrode. Polypyrrole, MnO2 and Single Walled Nanotube (SWNT) coated textile electrodes for supercapacitors were prepared from pure cotton. The textile electrode was fabricated by two coating processes of SWNT dipping and electroplating of MnO2 and polypyrrole. The textile based supercapacitor was assembled using Poly ethylene oxide (PEO) based gel-type electrolyte. The polypyrrole-coated CNT textile supercapacitor with MnO2 nanoparticles and MnO2 coated CNT textile supercapacitor were evaluated for electrochemical performance and cyclic reliability. Polypyrrole-MnO2 coated CNT textile supercapacitor shows high electrochemical energy capacity up to 461 F/g and high cyclic reliability up to 93.8% of energy capacity retention over 10,000 cycles. MnO2 coated CNT textile supercapacitor without polypyrrole thin layer retained 88.6% normalized capacitance after 10,000 cycles. Polypyrrole coating increases 5.2% improvement in the reliability. Energy density and power density of polypyrrole-MnO2 coated CNT supercapacitor were measured to be 31.1 Wh/kg and 22.1 kW/kg, respectively. For the case of MnO2 coated CNT textile supercapacitor, the energy density was calculated as 14.2 Wh/kg and the power density were evaluated as 4.9kW/kg. In situ measurement of the electrochemical performance change under mechanical strain of polypyrrole-MnO2 coated CNT supercapacitor shows 98.5% capacity retention at 21% tensile strain compared to the original discharge time. Cyclic bending tests under 13% strain were also performed while the electrochemical performance was measured. 750,000 bending cycles were tested on the supercapacitor. Cyclic voltammetry indicated 96.3% capacity retention. The fabricated flexible and stretchable energy storage device is suitable for wearable environments as demonstrated by mechanical tensile test and bending test results