Browsing by Author "Kumar, Nagesh"

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Dynamic observation of reversible lithium storage phenomena in hybrid supercapacitor devices
(2017-11-01)
Huang, Guan-Min
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Tsai, Tsung-Chun
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Huang, Chun-Wei
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Kumar, Nagesh
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Tseng, Tseung-Yuen
Carbonaceous composites have attracted much attention as electrode materials for hybrid supercapacitors. Additionally, transition metal oxides, such as Co3O4, have high specific capacitance in the charging/discharging process. Here, we investigated the lithium storage mechanism of Co3O4/CNTs material via in situ transmission electron microscopy (TEM). Additionally, we analyzed the structure and composition of the anode material by high-resolution TEM, electron diffraction, energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS). Using our unique in situ experimental setup that employs colloidal electrolyte, we elucidate two different mechanisms during operation, including the electrochemical reaction (battery-type) and ions intercalation (supercapacitor-type) of the electrode material. The cube-like Co3O4 nanoparticles were converted to Co nanograins dispersed in the Li2O matrix after the first charging cycle. Subsequent cycles presented a reversible reaction between Co/Li2O and CoO/Li2O. Furthermore, the porous structure of the CNTs and conservation of the Li2O matrix allow for the excellent ability to accommodate tremendous volume expansion, which enhances the life of hybrid supercapacitors. Our observations not only provide direct evidence of the electrochemical behavior but also improve the structure to promote enhanced performance for the application of hybrid supercapacitors.
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.
Fabrication of carbon nanotube/cobalt oxide nanocomposites via electrophoretic deposition for supercapacitor electrodes
(2016-03-01)
Kumar, Nagesh
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Yu, Yun-Cheng
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Lu, Yi Hsuan
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Tseng, Tseung Yuen
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電子工程學系及電子研究所
The cobalt oxide and carbon nanotubes (Co3O4/CNTs) nanocomposites are successfully synthesized using hydrothermal method. The as-synthesized nanocomposite materials are utilized in the electrophoretic deposition (EPD) to fabricate the electrodes, whose electrochemical properties are investigated in a three-electrode configuration cell with 1 M KOH electrolyte. By adjusting the precursor concentration, reaction time in hydrothermal process, and annealing temperature, the optimum conditions are obtained. From the experimental results, when the cobalt nitrate concentration is taken as 2 mmol, reaction time is 8 h, and the temperature is maintained at 180 A degrees C in the hydrothermal process, the synthesized Co3O4/CNTs nanocomposites shows the highest specific capacitance of 705 F g(-1) at a charging current of 3 A g(-1). Besides, the binder-free electrode preparation through EPD has effectively reduced the inner resistance of the electrode and makes the cycle stability excellent.
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.
In situ TEM investigation of electron beam-induced ultrafast chemical lithiation for charging
(2020-01-14)
Huang, Guan-Min
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Huang, Chun-Wei
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Kumar, Nagesh
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Huang, Chih-Yang
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Tseng, Tseung-Yuen
The enhancement of operational life and charging speed have been considered to be two of the major factors that will influence the development of energy storage devices in the future. Here, we utilized an electron beam (e-beam) to retrieve Li metal from LiF, the degradation product of LiPF6, and trigger the further lithiation of NiFe2O4/carbon nanotubes (CNTs) to relieve the degradation of the electrolyte and achieve ultrafast lithiation. Accordingly, in situ transmission electron microscopy (in situ TEM) was used to investigate the comprehensive mechanism of the whole process. The e-beam acting on the degradation product, i.e., LiF clusters, led to the generation of Li flakes, which served as the source for the subsequent lithiation. Then, with these Li flakes, the chemical lithiation of the NiFe2O4/CNTs was triggered, resulting in phase transformation to Ni and Fe nanograins. Compared to "electrochemical" lithiation, ultrahigh reaction speed and the ability to charge without a directly applied potential in the "chemical" lithiation are expected to extend lithiation to more diverse applications. As a result of this investigation, we have provided a new strategy for designing novel energy storage devices for the energy-harvesting field.
Probing the electrochemical properties of an electrophoretically deposited Co3O4/rGO/CNTs nanocomposite for supercapacitor applications
(2016-01-01)
Kumar, Nagesh
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Huang, Chun-Wei
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Yen, Po-Jen
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Wu, Wen-Wei
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Wei, Kung-Hwa
This study reports the electrochemical performance of pseudosupercapacitor electrodes composed of cubic phase Co3O4 nanoparticles, reduced graphene oxide (rGO) and functionalized MWCNTs based nanocomposites. The Co3O4/rGO/CNTs nanocomposites have been synthesized using a hydrothermal method and the EPD technique has been used to make binder free electrodes of the nanocomposite materials for supercapacitor applications. The effects of graphene oxide (GO) concentration and the ratio of GO/CNTs on the electrochemical performance of the nanocomposite material have been investigated. From the experimental results, the Co3O4/rGO/CNTs nanocomposite synthesized with 2 mg mL (1) GO concentration and 10 : 1 GO/CNT ratio exhibits a good specific capacitance of 850 F g (1) at a 5 mV s(-1) scan rate and 790 F g(-1) at 1 A g(-1), excellent rate capability and good cyclability in 1 M KOH. Furthermore, we have successfully designed an aqueous electrolyte-based asymmetric pseudocapacitor using Co3O4/rGO/CNTs nanocomposite as an anode and N-doped graphene nanocomposite as a cathode. The operating voltage of our optimized asymmetric pseudocapacitor is 1.4 V and it exhibits the maximum energy density and power density of 19.6 W h kg(-1) and 7250 W kg(-1), respectively. These results suggest that our EPD grown nanocomposite binder free electrode and our designed asymmetric pseudocapacitor have a good potential for practical applications.
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.
Ternary Au/ZnO/rGO nanocomposites electrodes for high performance electrochemical storage devices
(2017-10-31)
Chaudhary, Manchal
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Doong, Ruey-an
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Kumar, Nagesh
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Tseng, Tseung Yuen
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環境工程研究所
The combination of metal and metal oxide nanoparticles with reduced graphene oxides (rGO) is an active electrode material for electrochemical storage devices. Herein, we have, for the first time, reported the fabrication of ternary Au/ZnO/rGO nanocomposites by using a rapid and environmentally friendly microwave-assisted hydrothermal method for high performance supercapacitor applications. The ZnO/rGO provides excellent electrical conductivity and good macro/mesopore structures, which can facilitate the rapid electrons and ions transport. The Au nanoparticles with particle sizes of 7-12 nm are homogeneously distributed onto the ZnO/rGO surface to enhance the electrochemical performance by retaining the capacitance at high current density. The Au/ZnO/rGO nanocomposites, prepared with the optimized rGO amount of 100 mg exhibit a high specific capacitance of 875 and 424 F g(-1) at current densities of 1 and 20 A g-1, respectively, in 2 M KOH. In addition, the energy and power densities of ternary Au/ZnO/rGO can be up to 17.6-36.5 Wh kg(-1) and 0.27-5.42 kW kg(-1), respectively. Results obtained in this study clearly demonstrate the excellence of ternary Au/ZnO/rGO nanocomposites as the active electrode materials for electrochemical pseudocapacitor performance and can open an avenue to fabricate metal/metal oxide/rGO nanocomposites for electrochemical storage devices with both high energy and power densities. (C) 2017 Elsevier B.V. All rights reserved.