Taeksoo Ji

Enhanced electrochemical performance of monoclinic WO3 thin film with redox additive aqueous electrolyte

To achieve the highest electrochemical performance for supercapacitor, it is very essential to find out a suitable pair of an active electrode material and an electrolyte. In the present work, a simple approach is employed to enhance the supercapacitor performance of WO3 thin film. The WO3 thin film is prepared by a simple and cost effective chemical bath deposition method and its electrochemical performance is tested in conventional (H2SO4) and redox additive [H2SO4+hydroquinone (HQ)] electrolytes. Two-fold increment in electrochemical performance for WO3 thin film is observed in redox additive aqueous electrolyte compared to conventional electrolyte. WO3 thin film showed maximum specific capacitance of 725Fg(-1), energy density of 25.18Whkg(-1) at current density of 7mAcm(-2) with better cycling stability in redox electrolyte. This strategy provides the versatile way for designing the high performance energy storage devices.

Detection of Organic Compounds in Water by an Optical Absorbance Method.

This paper proposes an optical method which allows determination of the organic compound concentration in water by measurement of the UV (ultraviolet) absorption at a wavelength of 250 nm~300 nm. The UV absorbance was analyzed by means of a multiple linear regression model for estimation of the total organic carbon contents in water, which showed a close correlation with the UV absorbance, demonstrating a high adjusted coefficient of determination, 0.997. The comparison of the TOC (total organic carbon) concentrations for real samples (tab water, sea, and river) calculated from the UV absorbance spectra, and those measured by a conventional TOC analyzer indicates that the higher the TOC value the better the agreement. This UV absorbance method can be easily configured for real-time monitoring water pollution, and built into a compact system applicable to industry areas.

On the Responsivity of UV Detectors Based on Selectively Grown ZnO Nanorods

Recent studies have shown that zinc oxide (ZnO)-based ultraviolet (UV) photodetectors have better sensitivity and can be fabricated cheaply and compactly. To know whether the responsivity of these types of UV photodetectors can be improved by manipulating the dimension of the electrodes and length of the nanorods, we investigated the dependence of dark current and responsivity of the metal-semiconductor-metal (MSM)-type ZnO detector on those factors. The MSM ZnO nanorods based detector was designed compactly by arranging the interdigitated electrodes in a square form resembling a Wheatstone bridge where ZnO nanorods with diameters ranging from 20 to 70 nm were prepared by hydrolysis method. The crystallinity and surface morphology of the rods determined using X-ray diffraction (XRD) and scanning electron microscope (SEM), respectively, are also discussed here. We observed that the responsivity of the detector measured for a UV wavelength of 365 nm depends on the spacing between electrodes, width of the electrode, and length of the nanorod.

Ultrathin nickel sulfide nano-flames as an electrode for high performance supercapacitor; comparison of symmetric FSS-SCs and electrochemical SCs device

Metal sulfides have received well deserved attention due to their excellent electrical conductivity and thermal stability, as compared to metal oxides, allowing them to achieve a high capacitance and energy density for portable energy storage devices. In this study, the preparation of highly porous nano-flames composed of nickel sulfide (NiS) thin film on a cost effective, flexible stainless steel substrate through a trouble free, inexpensive and simple chemical bath deposition (CBD) method is reported. The prepared nano-flames composed of a NiS thin film demonstrates the excellent electrochemical features with a maximum specific capacitance (Cs) of 750.6 F g−1 at a scan rate of 5 mV s−1 in a three electrode system. Furthermore, the portable symmetric flexible solid state supercapacitor (FSS-SC) and electrochemical supercapacitor (SC) are fabricated and tested. In comparison with the symmetric electrochemical SC, the symmetric FSS-SC shows an excellent electrochemical performance with a high Cs of 104 F g−1 at 5 mV s−1 with a good electrochemical stability of 85.3% over 3000 CV cycles. This study constitutes the first comparison of symmetric FSS-SCs and electrochemical SCs formed with NiS nano-flames. Such an impressive symmetric FSS-SC is predicted to be an exceptionally promising candidate for energy storage systems.

Highly energetic flexible all-solid-state asymmetric supercapacitor with Fe2O3 and CuO thin films

In the present investigation, the applicability of Fe2O3 and CuO thin films as anode and cathode electrodes respectively in supercapacitors has been systematically studied. Fe2O3 and CuO thin films are synthesized by simple and cost effective chemical methods and further more all-solid-state symmetric (Fe2O3/Fe2O3, CuO/CuO) and asymmetric (CuO//Fe2O3) supercapacitor devices are fabricated. The electrochemical properties (cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), electrochemical impedance spectroscopy (ESR), etc.) of these devices are studied using two electrodes system. The asymmetric supercapacitor shows improved performance with maximum operating potential window of 2.0 V and specific capacitance of 79 F g−1 at 2 mA cm−2 current density. The maximum energy density and power density of 23 W h kg−1 and 19 kW kg−1 are obtained for asymmetric supercapacitor. In addition, the asymmetric supercapacitor demonstrates the excellent flexibility with capability retention of 89% over bending at an angle of 180°.

Facile synthesis of hierarchical mesoporous weirds-like morphological MnO2 thin films on carbon cloth for high performance supercapacitor application

The mesoporous nanostructured metal oxides have a lot of capabilities to upsurge the energy storing capacity of the supercapacitor. In present work, different nanostructured morphologies of MnO2 have been successfully fabricated on flexible carbon cloth by simple but capable hydrothermal method at different deposition temperatures. The deposition temperature has strong influence on reaction kinetics, which subsequently alters the morphology and electrochemical performance. Among different nanostructured MnO2 thin films, the mesoporous weirds composed thin film obtained at temperature of 453K exhibits excellent physical and electrochemical features for supercapacitor application. The weirds composed MnO2 thin film exhibits specific surface area of 109m2g-1, high specific capacitance of 595Fg-1 with areal capacitance of 4.16Fcm-2 at a scan rate of 5mVs-1 and high specific energy of 56.32Whkg-1. In addition to this, MnO2 weirds attain capacity retention of 87 % over 2000 CV cycles, representing better cycling stability. The enhanced electrochemical performance could be ascribed to direct growth of highly porous MnO2 weirds on carbon cloth which provide more pathways for easy diffusion of electrolyte into the interior of electroactive material. The as-fabricated electrode with improved performance could be ascribed as a potential electrode material for energy storage devices.

Fabrication of high performance flexible all-solid-state asymmetric supercapacitors with a three dimensional disc-like WO3/stainless steel electrode

Presently, significant attention has been paid towards the rational synthesis of nanostructured anode and cathode electrode materials for assembling high-performance supercapacitors. Despite significant progress being achieved in designing cathode electrode materials, anode electrode materials with high capacitance are hardly investigated. In the present article, a tungsten oxide (WO3) thin film is prepared on a flexible stainless steel substrate by a wet chemical method and used as an anode electrode to fabricate a flexible asymmetric supercapacitor (ASC). An electrochemical investigation of the WO3 thin film shows a maximum specific capacitance of 530 F g−1 at 1 mA cm−2 in a potential window of 0 to −0.8 V in 1 M Na2SO4 electrolyte. In addition, a highly energetic, flexible ASC device is assembled using a WO3 thin film as an anode, a MnO2 thin film as a cathode and polymer gel as an electrolyte. The as-assembled MnO2//WO3 ASC device exhibited a stable electrochemical potential window of 1.8 V and better cycling stability. Whats more, the flexible MnO2//WO3 ASC device achieves a high specific capacitance of 115 F g−1 with an acceptable specific energy of 52 W h kg−1 at a current density of 3 mA. Hence, the proposed flexible MnO2//WO3 ASC device creates one more option for anode materials to develop flexible energy storage devices.

Lift-Off Photolithographic Top-Contact OTFTs Using a Bilayer of PVA and SU8

A lift-off process for direct photolithographic top-contact organic thin-film transistors (TFTs) has been developed using a bilayer of polyvinyl alcohol and SU8. Top-contact pentacene TFTs fabricated by this method, with a fixed channel width of 300 μm but different channel lengths varying from 10 to 80 μm, exhibited mobility values in the range of 0.01-0.03 cm2/V·s with the average of 0.021 ± 0.007 cm2/V·s. It is expected that the adoption of SU8 with a high aspect ratio enables the easy fabrication of submicrometer channel devices, giving rise to improved carrier transport characteristics.

ZnO Nanorod-Based Humidity Sensors With Fast Response

Humidity sensors based on ZnO nanorods (NRs) have been developed using a Wheatstone bride (WB) configuration, where the ZnO NRs were selectively grown onto the one quadrant of the WB circuit. It turns out that the WB-based sensor possesses superior performances in terms of response and recovery time, and thermal stability in comparison with conventional types of resistive sensors. Thus, the WB sensor exhibits a fast response and recovery time (~1 s), and temperature-invariant behaviors over a broad range of temperatures up to 180 °C, which is likely because of the self-calibrating and error correction capabilities of the WB circuit itself.

Design and synthesis of hierarchical mesoporous WO3-MnO2 composite nanostructures on carbon cloth for high-performance supercapacitors

Abstract To enhance the energy density and power performance of supercapacitors, the rational design and synthesis of active electrode materials with hierarchical mesoporous structure is highly desired. In the present work, fabrication of high-performance hierarchical mesoporous WO3-MnO2 composite nanostructures on carbon cloth substrate via a facile hydrothermal method is reported. By varying the content of MnO2 in the composite, different WO3-MnO2 composite thin films are obtained. The formation of composite is confirmed by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses. The Brunauer-Emmett-Teller (BET) analysis reveals maximum specific surface area of 153 m2 g−1. The optimized WO3-MnO2 composite electrode demonstrates remarkable electrochemical performance with high specific capacitance of 657 F g−1 at a scan rate of 5 mV s−1 and superior longterm cycling stability (92% capacity retention over 2000 CV cycles). Furthermore, symmetric flexible solid-state supercapacitor based on WO3-MnO2 electrodes has been fabricated. The device exhibits good electrochemical performance with maximum specific capacitance of 78 F g−1 at a scan rate of 5 mV s−1 and specific energy of 10.8 Wh kg−1 at a specific power of 0.65 kW kg−1. The improved electrochemical performance could be ascribed to the unique combination of multivalence WO3 and MnO2 nanostructures and synergistic effect between them