Browsing by Author "Li, Xiang"

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A high-performance lithium-ion capacitor with carbonized NiCo2O4 anode and vertically-aligned carbon nanoflakes cathode
(2019-11-01)
Cheng, Chung-Fu
;
Li, Xiang
;
Liu, Kewei
;
Zou, Feng
;
Tung, Wei-Yao
Lithium ion capacitors (LICs) are energy storage devices integrating the complementary features of both supercapacitors and lithium ion batteries to simultaneously reach high energy and power densities. One of the major challenges in LIC technology is the kinetic imbalance between the faradaic insertion anode and capacitive cathode. Therefore, the design of electrode materials is crucial to enhance the rate performance of anode and the capacitance of the cathode in LIC devices. In this work, novel LICs were demonstrated with nanostructured cathode and anode. A vertically-aligned carbon nanoflakes (VACNFs) cathode provided high electrochemically active surface area and excellent conductivity, while a metal organic framework (MOF) derived carbonized nickel cobalt oxide (cNiCo(2)O(4)) anode ensured fast conversion reactions and remarkable cyclability. Electrochemical characterization of individual electrode confirmed that both electrodes exhibited good electron and ion transport capability. The LICs were fabricated with optimized electrode active materials loading to deliver high energy densities at desired charge/discharge rates. The devices exhibited energy density up to 136.9W h/kg (at 200 W/ kg). At higher power density of 40 kW/kg, under which a full charge-discharge can be finished within 4 s, the LICs could still deliver an energy density of 26.44W h/kg. The devices also showed a good cycle stability (approximate to 90% capacitance retention after 9000 cycles, under current density of 4 A/g) within the voltage range of 1-4.2 V.
Hydrogen-Bonding-Mediated Solid-State Self-Assembled Isoepindolidiones (isoEpi) Crystal for Organic Field-Effect Transistor
(2018-03-22)
Zhang, Haichang
;
Liu, Kewei
;
Wu, Kuan-Yi
;
Chen, Yu-Ming
;
Deng, Ruonan
Novel pigments isoepindolidiones (isoEpi) and di-tert-butyl quinolino[8,7-h]quinoline-1,7-diyl bis(carbonate) (isoEpi-Boc) were synthesized and characterized. The spectroscopic characterizations indicated that the tert-butyloxycarbonyl (t-Boc) units could be removed from soluble isoEpi-Boc by thermal annealing, forming insoluble isoEpi with hydrogen bonding. A solid-state isoEpi-Boc crystal to isoEpi crystal transition was observed during the annealing process and the molecular packing was significantly changed. With the emergence of hydrogen bonds, the isoEpi molecules were arranged into a brick-in-wall structure with pi-stacking along the crystal growth axis, leading to a significant enhancement of charge mobility along the crystal growth direction (the hole mobility from 3.4 x 10(-4) to 0.32 cm(2)/(V.s), and the electron mobility from nondetectable to 5.6 x 10(-3) cm(2)/(V.s)). The results indicate that isoEpi is a promising chromophore for organic field-effect transistor (OFET). The crystal-to-crystal transition driven by the formation of hydrogen bonds is a unique method for modulating the charge transport properties in organic semiconductive materials for OFET devices.
Investigation of hydrogen-bonding mediated molecular packing of diketopyrrolopyrrole based donor-acceptor oligomers in the solid state
(2019-01-03)
Yang, Kun
;
Li, Xiang
;
Huang, Yi-Fan
;
Bhatta, Ram S.
;
Liu, Jiawei
Two diketopyrrolopyrrole (DPP)-based donor-acceptor oligomers with thermal labile 2-methylhexyl-2-oxylcarbonyl (mHoc) groups, mHocPBT and mHocTBT, were synthesized and characterized to study the hydrogenbonding mediated molecular packing in solid state. The soluble oligomers can be converted to strong intermolecular hydrogen-bonded and pigment-like materials HPBT and HTBT by annealing the casted solid film at a temperature between 180 and 200 degrees C. The formation of intermolecular hydrogen bonding significantly altered the conformation of the molecules, molecular packing structure and related optical properties of the oligomers, with HPBT molecule exhibiting bathochromic shift and HTBT molecule exhibiting hypsochromic shift on UV/Vis spectra. Spin-casted mHocTBT and HTBT films also exhibited interesting morphology and charge transport properties. Even though the single mHocTBT oligomer molecule has a better coplanar molecular conformation as compared with that of HTBT, an enhanced pi-pi stacking structure and larger crystal grain size rendered HTBT film better charge transport performance.