Browsing by Author "Kumar, Amit"

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Electrophoretic Deposition of Nitrogen-Boron Co-Doped Graphene as High Performance Electrode for Next Generation Supercapacitors
(2018-01-01)
Kumar, Amit
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Kumar, Nagesh
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Singh, Pragya
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Leu, Jihperng
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Tseng, Tseung-Yuen
In this study, we report the electrophoretic deposition (EPD) of nitrogen-boron co-doped graphene (NBG) synthesized via simple hydrothermal method and subsequent annealing at 750 degrees C in reducing environment. The structural, chemical-compositional and morphological investigations of the as synthesized NBG have been investigated using X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The electrochemical properties of the samples have been studied via cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) cycling and electrochemical impedance spectroscopy (EIS) in a conventional three-electrode system using 6M KOH aqueous electrolyte. The EPD grown binder free NBG electrode exhibits high specific capacitance of 217 Fg(-1) at the scan rate of 5 mVs(-1) which is about 1.2 times higher than the specific capacitance (180 Fg(-1) at 5 mVs(-1)) of pristine reduced graphene oxide synthesized via similar approach. Furthermore, this sample exhibits excellent rate capability, and similar to 103% retention after 8800 successive charge-discharge cycles at 1 Ag-1.
Facile and One-Step in Situ Synthesis of Pure Phase Mesoporous Li2MnSiO4/CNTs Nanocomposite for Hybrid Supercapacitors
(2020-03-23)
Kumar, Nagesh
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Singh, Meetesh
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Kumar, Amit
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Tseng, Tseung-Yuen
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Sharma, Yogesh
In material sciences, synergistic effect and nanostructuring are the two factors which enable researchers to look beyond the conventionally defined categories of the materials. Here, we report the synthesis mechanism of pure phase mesoporous Li2MnSiO4 (LMS) with a high specific surface area by varying the concentration of metal precursors and solvents in one-step hydrothermal technique. Furthermore, the effect of MWCNTs addition on the electrochemical performance of LMS is studied. The quantitative contribution of current generating from EDLC and/or the surface pseudocapacitance reactions, and the current caused by diffusion-controlled redox reactions in the total current is also evaluated. The pure phase LMS/CNTs nanocomposite with 2% MWCNTs (abbreviated as LMS2C) is found to be the best supercapacitor material among studied nanocomposites as it exhibits specific capacitance of similar to 290 F g(-1) @ 1 A g(-1), good rate capability, small relaxation time constant (tau = 87 ms), and higher diffusion coefficient of electrolytic cations (D-k(+) = 9.4 x 10(-9) cm(2) s(-1)) in 2 M KOH aqueous electrolyte. A hybrid supercapacitor cell (HSC) designed using LMS2C as positive and activated carbon as negative electrodes shows the maximum energy density of 31 W h kg(-1), which is much higher than several recently reported hybrid supercapacitor systems. Two series connected HSCs can power a drone motor and light up 8 red LEDs for more than 3 min, indicating practical applicability of our designed hybrid supercapacitor system.
Facile synthesis of mesoporous NiFe2O4/CNTs nanocomposite cathode material for high performance asymmetric pseudocapacitors
(2018-03-01)
Kumar, Nagesh
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Kumar, Amit
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Huang, Guan-Min
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Wu, Wen-Wei
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Tseng, Tseung Yuen
Morphology and synergistic effect of constituents are the two very important factors that greatly influence the physical, chemical and electrochemical properties of a composite material. In the present work, we report the enhanced electrochemical performance of mesoporous NiFe2O4 and multiwall carbon nanotubes (MWCNTs) nanocomposites synthesized via hexamethylene tetramine (HMT) assisted one-pot hydrothermal approach. The synthesized cubic phase spinel NiFe2O4 nanomaterial possesses high specific surface area (148 m(2)g(-1)) with narrow mesopore size distribution. The effect of MWCNTs addition on the electrochemical performance of nanocomposite has been probed thoroughly in a normal three electrode configuration using 2 M KOH electrolyte at room temperature. Experimental results show that the addition of mere 5 mg MWCNTs into fixed NiFe2O4 precursors amount enhances the specific capacitance up to 1291 Fg(-1) at 1 Ag-1, which is the highest reported value for NiFe2O4 nanocomposites so far. NiFe2O4/CNT nanocomposite exhibits small relaxation time constant (1.5 ms), good rate capability and capacitance retention of 81% over 500 charge-discharge cycles. This excellent performance can be assigned to high surface area, mesoporous structure of NiFe2O4 and conducting network formed by MWCNTs in the composite. Further, to evaluate the device performance of the composite, an asymmet-ric pseudocapacitor has been designed using NiFe2 04/CNT nanocomposite as a positive and N-doped graphene as a negative electrode material, respectively. Our designed asymmetric pseudocapacitor gives maximum energy density of 23 W h kg(-1) at power density of 872W kg(-1). These promising results assert the potential of synthesized nanocomposite in the development of efficient practical high-capacitive energy storage devices. (C) 2017 Published by Elsevier B.V.
Flexible solid-like electrolytes with ultrahigh conductivity and their applications in all-solid-state supercapacitors
(2018-01-01)
Yang, Chih-Chieh
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Lin, Hao-Yang
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Kumar, Amit
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Pattanayak, Bhaskar
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Tsai, Hung-Yi
All-solid-state supercapacitors (ASSS) with solid-state electrolytes (SSEs) can be used to overcome the liquid leakage problem in devices. However, ionic conduction in solid electrolytes is one of the barriers to further improvements in ASSS. This paper describes the fabrication of a flexible SSE composed of poly(vinylidene fluoride-co-hexafluoropropylene), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, and ethylene carbonate, which demonstrates an ultrahigh conductivity of 8.52 mS cm(-1) and a wide 5 V operation voltage window of -2 to +3 V. Electrodes composed of active carbon, multiwall carbon nanotubes, and polyvinylidene fluoride were used as both anode and cathode to assemble a symmetrical supercapacitor. The resultant supercapacitor exhibits a maximum power density of 3747 W kg(-1) at an energy density of 7.71 W h kg(-1) and a maximum energy density 17.1 W h kg(-1) at a power density of 630 W kg(-1). It displays excellent cycling stability with 91.3% of the initial specific capacitance after 3000 charging/discharging cycles. This flexible SSE in this study demonstrates a high potential for use in energy storage, conversion, and wearable device applications.
Nanoscale Control of Phase Variants in Strain-Engineered BiFeO(3)
(2011-08-01)
Vasudevan, Rama K.
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Liu, Yunya
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Li, Jiangyu
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Liang, Wen-I
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Kumar, Amit
Development of magnetoelectric, electromechanical, and photovoltaic devices based on mixed-phase rhombohedral-tetragonal (R-T) BiFeO(3) (BFO) systems is possible only if the control of the engineered R phase variants is realized. Accordingly, we explore the mechanism of a bias induced phase transformation in this system. Single point spectroscopy demonstrates that the T -> R transition is activated at lower voltages compared to T -> - T polarization switching. With phase field modeling, the transition is shown to be electrically driven. We further demonstrate that symmetry of formed R-phase rosettes can be broken by a proximal probe motion, allowing controlled creation of R variants with defined orientation. This approach opens a pathway to designing next-generation magnetoelectronic and data storage devices in the nanoscale.
Nanoscale Control of Phase Variants in Strain-Engineered BiFeO3
(2011-08-01)
Vasudevan, Rama K.
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Liu, Yunya
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Li, Jiangyu
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Liang, Wen-I
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Kumar, Amit
Development of magnetoelectric, electromechanical, and photovoltaic devices based on mixed-phase rhombohedral-tetragonal (R-T) BiFeO3 (BFO) systems is possible only if the control of the engineered R phase variants is realized. Accordingly, we explore the mechanism of a bias induced phase transformation in this system. Single point spectroscopy demonstrates that the T -> R transition is activated at lower voltages compared to T -> - T polarization switching. With phase field modeling, the transition is shown to be electrically driven. We further demonstrate that symmetry of formed R-phase rosettes can be broken by a proximal probe motion, allowing controlled creation of R variants with defined orientation. This approach opens a pathway to designing next-generation magnetoelectronic and data storage devices in the nanoscale.
Resistive switching behavior of Ga doped ZnO-nanorods film conductive bridge random access memory
(2018-08-30)
Singh, Pragya
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Simanjuntak, Firman Mangasa
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Kumar, Amit
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Tseng, Tseung-Yuen
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電子工程學系及電子研究所
The influence of Ga dopant on resistive switching behavior of ZnO-nanorods film conducting bridge random access memory was investigated. Using a hydrothermal process, we grew vertically well-aligned and uniform Ga doped ZnO (GZO)-NRs films on transparent glass substrates to fabricate Cu/TiW/GZO-NRs/Indium Tin Oxide/Glass devices. The GZO-NRs film provides a diffusion path for the Cu ions to form a conducting bridge; thus, reducing the formation of a branched filament. The Ga dopant induces a significant improvement in switching distribution of high resistance states (HRS) and low resistance states (LRS). The 1.5 mol% Ga doped device exhibits good retention up to 10(4) s and high HRS/LRS ratio of 30 times. Therefore, our proposed device structure may be a good candidate for future conductive-bridge resistive random access memory application.
Role of precursors mixing sequence on the properties of CoMn2O4 cathode materials and their application in pseudocapacitor
(2019-11-14)
Pattanayak, Bhaskar
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Simanjuntak, Firman Mangasa
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Panda, Debashis
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Yang, Chih-Chieh
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Kumar, Amit
In this study, the effect of oxygen vacancy in the CoMn2O4 on pseudocapacitive characteristics was examined, and two tetragonal CoMn2O4 spinel compounds with different oxygen vacancy concentrations and morphologies were synthesized by controlling the mixing sequence of the Co and Mn precursors. The mixing sequence was changed; thus, morphologies were changed from spherical nanoparticles to nanoflakes and oxygen vacancies were increased. Electrochemical studies have revealed that tetragonal CoMn2O4 spinels with a higher number of oxygen vacancies exhibit a higher specific capacitance of 1709 F g(-1) than those with a lower number of oxygen vacancies, which have a higher specific capacitance of 990 F g(-1). Oxygen vacancies create an active site for oxygen ion intercalation. Therefore, oxidation-reduction reactions occur because of the diffusion of oxygen ions at octahedral/tetrahedral crystal edges. The solid-state asymmetric pseudocapacitor exhibits a maximum energy density of 32 Wh.kg(-1) and an excellent cyclic stability of nearly 100%.
Sensing performance of gas sensors fabricated from controllably grown ZnO-based nanorods on seed layers
(2020-07-01)
Singh, Pragya
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Simanjuntak, Firman Mangasa
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Wu, Yi-Chu
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Kumar, Amit
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Zan, Hsiao-Wen
ZnO nanorods (NRs) grown on different ZnO seed layers deposited at various Ar-O-2 ambient conditions, and the morphologies of the seed layers that affect their response to nitric oxide (NO) gas were investigated. The sensing performance of the devices fabricated with the seed layers deposited under an O-2-rich condition tends to deteriorate. We believe that the surface roughness, grain size, and defect concentration of the seed layer are responsible for this phenomenon. Despite the response to the gas was found to be less dependent on NR length, the gas sensing response is significantly dependent on the oxygen vacancy concentration and the exposure of the NR structure to the NO gas ambient surface. Excellent gas response (57.5% at 1 ppm and 7.1% at 100 ppb of NO gas) was achieved at room temperature-superior to the response presented by other proposed methods. This study not only proposes the potential of ZnO-NRs for high-performance NO gas sensor devices but also offers a simple and low-cost method to optimize the response without using any catalyst and additional treatment.
Synthesis of Free-Standing Flexible rGO/MWCNT Films for Symmetric Supercapacitor Application
(2019-08-06)
Kumar, Amit
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Kumar, Nagesh
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Sharma, Yogesh
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Leu, Jihperng
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Tseng, Tseung Yuen
Herein, we report a novel, simple, and cost-effective way to synthesize flexible and conductive rGO and rGO/MWCNT freestanding films. The effects of MWCNT addition on the electrochemical performance of rGO/MWCNT nanocomposite films are investigated in some strong base aqueous electrolytes, such as KOH, LiOH, and NaOH via three-electrode system. The supercapacitor behavior of the films is probed via cyclic voltammetry, galvanostatic charging-discharging, and electrochemical impedance spectroscopy. The structural and morphological studies of the films are performed by X-ray diffractometer, Raman spectrometer, surface area analyzer, thermogravimetric analysis, field emission scanning electron microscope and transmission electron microscope. The rGO/MWCNT film synthesized with 10 wt% MWCNTs (GP10C) exhibits high specific capacitance of 200 Fg(-1), excellent cyclic stability with 92% retention after 15,000 long cycle test, small relaxation time constant (similar to 194 ms), and high diffusion coefficient (7.8457 x 10(-9) cm(2) s(-1)) in 2 M KOH electrolyte. Furthermore, the symmetric supercapacitor coin cell with GP10C as both anode and cathode using 2 M KOH as electrolyte demonstrates high energy density of 29.4 Whkg(-1) and power density of 439 Wkg(-1) at current density 0.1 Ag-1 and good cyclic stability with 85% retention of the initial capacitance at 0.3 Ag-1 after 10,000 cycles. Such a high performance of the GP10C film in the supercapacitor can be ascribed to the large surface area and small hydration sphere radius and high ionic conductivity of K+ cations in KOH electrolyte.
Templating synthesis of nickel cobaltite nanoflakes and their nanocomposites for making high-performance symmetric supercapacitors
(2019-12-01)
Yang, Chih-Chieh
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Sun, Wen-Chien
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Kumar, Amit
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Pattanayak, Bhaskar
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Tseng, Tseung-Yuen
Nickel cobaltite is one of the most popular pseudocapacitance materials for creating high-performance energy-storage devices because of its low cost, high electric conductivity, excellent electrochemical properties, and environmental friendliness. In this study, nickel cobaltite with nanoneedle morphology was synthesized by adopting the hydrothermal method. Change in the nickel cobaltite from nanoneedle to nanoflake morphology was induced by a template of the surface of a self-assembly graphene oxide (GO)/multiwall carbon nanotube (MWCNT) three-dimensional block matrix. Electrodes fabricated from the nanoflake nickel cobaltite, GO, and MWCNT composite exhibited high specific capacitance of 1525 F g(-1) at a current density of 1 A g(-1) and 1081 F g(-1) at a high current density of 100 A g(-1). When these composite electrodes were used as both the anode and cathode to assemble a symmetric super-capacitor with a 6 M KOH electrolyte, the supercapacitor exhibited a maximum energy density of 25.2 Wh kg(-1) and maximum power density of 5151 Wkg(-1). Moreover, it maintained an excellent cycling stability of 99.6% of the initial capacitance value after 7000 chargeedischarge cycles, demonstrating its remarkable potential for application in energy-storage and conversion devices. (C) 2019 The Author(s). Published by Elsevier Ltd.
The synergistic effect of iron cobaltite compare to its single oxides as cathode in supercapacitor
(1970-01-01)
Saaid, Farish Irfal
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Arsyad, Akmal
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Azman, N. S. H.
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Kumar, Amit
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Yang, Chih-Chieh
Mixed transition metal oxides have attracted great attention in supercapacitors applications due to their better electrochemical performance than their single oxides. In this work, iron cobaltite (FeCo2O4) and its single metal oxides i.e. iron oxide (Fe2O3) and cobalt oxide (Co3O4) were synthesized by a simple hydrothermal process. The structural, spectroscopic and morphological properties were studied using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and field-emission scanning electron microscope (FESEM). XRD and FTIR results show the composition of the products. The obtained iron oxide was alpha-Fe2O3. FESEM images show that FeCo2O4 and its single metal oxides exhibit different morphology even though they were synthesized via similar method. The electrochemical properties of the alpha-Fe2O3, Co3O4 and FeCo2O4 electrodes were examined by cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectroscopy (EIS) in a 6 M KOH electrolyte solution. At comparable current density, the FeCo2O4 electrode has the highest specific capacitance (C-sp), followed by Co3O4 and alpha-Fe2O3. An asymmetric FeCo2O4/KOH/GO supercapacitor was fabricated. The supercapacitor exhibits maximum energy density of 14.5 Wh kg(-1) and maximum power density of 2177 W kg(-1). It demonstrates 60% rate capability after 1000 continuous charge-discharge cycles at 1 A g(-1).