| Increasing working voltage (U) of a supercapacitor is an effective way to improve its energy density (E), as E is proportional to the specific capacitance of the supercapacitor (Cm) and to the square of U, i.e. E=1/2CmU2 Asymmetric supercapacitor assembly is a good way to improve energy density since the working voltage of the supercapacitor can be effectively increased due to the quite different charge storage potential range for positive and negative electrodes. In this work, three-dimensional-distributed reduced graphene oxide (rGO) was synthesized through hydrothermal treatment of graphene oxide (GO) absorbed in nickel foam (NF). Ni(OH)2 and NiO porous nanosheet arrays were synthesized using a seed-mediated hydrothermal method and the following thermal treatment. The reduction of GO sheets was investigated by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Morphologies of products were characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM). The crystal structures were studied by X-ray diffraction (XRD). Supercapacitive behaviors were investigated by cyclic voltammetry (CV), constant current charge-discharge and electrochemical impedance spectroscopy (EIS). Asymmetric supercapacitors were fabricated by using rGO as the negative electrode, Ni(OH)2 and NiO porous nanosheet arrays as the positive electrode, respectively. Electrochemical behaviors of the devices were investigated. Three-dimensional-distributed rGO was synthesized by hydrothermal treatment of GO absorbed in three-dimensional-structured NF to get NF/rGO. The reduction level of GO sheets and resulting rGO were investigated by XPS and Raman. After hydrothermal treatment, the oxygen-containing functional groups on GO sheets were effectively removed and its structural defects partly repaired. TEM and SEM results showed that rGO with a transparent fold sheet structure closely adhered to the three-dimensional-structured NF substrate, which facilitated good contact of rGO with electrolyte for charge storage. The CV curves of NF/rGO showed a nearly rectangular shape while the charge-discharge curves were symmetric with fairly linear slope, being typical for electric double layer materials. The phase angles of NF/rGO samples on their Bode spectra were close to-90° at low-frequency regions, showing their good capacitive behaviors. Influences of hydrothermal temperature and time, GO concentration and hydrothermal treatment times on electrochemical performance of NF/rGO were investigated. The sample of NF/rGO-2-150-1h-1 obtained by hydrothermal treatment in 2 mg/ml GO solution at 150? for 1 hour displayed a good supercapacitive performance with a phase angle of-86.5° at low-frequency region on its bode spectrum. The specific capacitance of NF/rGO-2-150-1 h-1 was 184.5 F/g at current density of 0.5 A/g. It displayed a specific capacitance of 160.1 F/g when the current density increased 20-fold to 10 A/g, the capacitance retention was 86.8%. NF/rGO-2-150-1h-1 also showed a good stability with the capacitance maintanince of 92.0% after 5000 cycles.Ni(OH)2 nanosheet arrays were deposited on carbon cloth (CC) substrate using a seed-mediated hydrothermal method in an aqueous solution containing NiSO4, ammonia, ammonium persulfate (APS) and acetone to afford CC/Ni(OH)2. The morphologies of Ni(OH>2 samples were investigated by SEM. The carbon fibers of substrate were uniformly covered with vertically aligned and cross-connected nanosheet arrays. Abundant pores were formed by Ni(OH)2 nanosheets, facilitating rapid transportation of electrolyte into and out of electrode materials for charge storage. XRD results showed that the Ni(OH)2 samples had a hexagonal structure of ?-Ni(OH)2. A redox pair, related to Ni(II)/Ni(III) transformation, can be seen on the CV curve of CC/Ni(OH)2. Its charge-discharge curves were symmetric, however the current-potential response was potential dependent, showing faradic pseudocapacitive behaviors of Ni(OH)2 nanosheet arrays. The influences of NiSO4 concentration, hydrothermal time, APS addition and NiO seed on Ni(OH)2 hydrothermal deposition were studied. It was found that the pre-deposition of NiO seed on CC substrate was a crucial factor for the formation of Ni(OH)2 nanosheet arrays. The APS addition had some influence on the crystallization of Ni(OH)2 and also on capacitive performance of the hydroxide. CC/Ni(OH)2-0.2-150-1h nanosheet arrays hydrothermally deposited at 150? for 1 hour in 20 ml aqueous solution containing 0.10 g APS,1.32 g NiSO4,3.2 ml ammonia and 2.5 ml acetone showed a good supercapacitive performance with a specific capacitance of 241.0 F/g at a high current density of 20 A/g. It displayed a specific capacitance of 164.2 F/g when the current density increased to 50 A/g, with a capacitance retention of 68.1%. CC/Ni(OH)2-0.2-150-lh showed a good flexibility due to the flexible substrate, its specific capacitance substantially unchanged when the sample was bended 0?180°. However, its stability needs to be improved, only 81.6% of the capacitance can be maintained after 5000 cycles.CC/Ni(OH)2 nanosheet arrays were annealed to afford NiO nanosheet arrays. Thermal gravity analysis showed that the decomposition of CC/Ni(OH)2-0.2-150-1h began from 250 ?. Cubic NiO can be obtained after annealing, based on XRD results. After thermal treatment, the nanosheet arrays were well-preserved, carbon fibers on CC were uniformly covered by vertically aligned and cross-connected NiO nanosheets. According to TEM investigation, the nanosheet consists of NiO nanoparticles to form porous structure, due to the rapid dehydration of the hydroxide in CC/Ni(OH)2-0.2-150-1h. The pseudocapacitive behaviors of CC/NiO were investigated by CV, constant current charge-discharge experiment and EIS. The influences of annealing temperature and time on CC/NiO properties were studied. CC/NiO-300-1.5h obtained by annealing CC/Ni(OH)2-0.2-150-lh at 300? for 1.5 hours displayed good capacitive performances with a specific capacitance of 276.1 F/g at the current density of 10 A/g. Its specific capacitance at 50 A/g was 196.4 F/g, displaying high capacitance retention of 71.1%. CC/NiO-300-1.5h was also flexible, its specific capacitance substantially unchanged with the bending angle of 0-180°. CC/NiO-300-1.5h displayed improved cycling stability, its specific capacitance did not decrease after 10000 cycles.Ni-Co binary hydroxide nanosheet arrays were electrochemically deposited on high conductive carbon paper (CP) substrate, their structure and surface morphologies were investigated by XRD, selected area electron diffraction (SAED), SEM and TEM. It was found that the hydroxides in CP/Ni(OH)2-Co(OH)2-3:2 obtained at the molar ratio of Ni:Co=3:2 exist as a-Ni(OH)2 and a-Co(OH)2, however, with low crystallinity degree. In CP/Ni(OH)2-Co(OH)2-3:2, the carbon paper substrate were uniformly covered by cross-connected and translucent folded nanosheet arrays. Abundant pores were formed by Ni(OH)2-Co(OH)2 nanosheets, facilitating electrolyte transportation into and out of electrode materials for charge storage. The pseudocapacitive behaviors of Ni-Co binary hydroxide samples were investigated by CV and constant current charge-discharge experiment to study the influence of Ni:Co molar ratio on the binary hydroxide. The CP/Ni(OH)2-Co(OH)2-3:2 displayed good capacitive performance with a specific capacitance of 1413.0 F/g at the current density of 1 A/g. It displayed a high capacitance retention of 73.2% when the current density increased to 5 A/g (1033.6 F/g), showing its good rate capability.Asymmetric supercapacitors CC/Ni(OH)2-0.2-150-lh//NF/rGO-2-150-1h-1 and CC/NiO-300-1.5h//NF/rGO-2-150-1h-1 were assembled with 6 mol/L KOH electrolyte by using NF/rGO-2-150-1h-1 as the negative electrode, CC/Ni(OH)2-0.2-150-lh and CC/NiO-300-1.5h as the positive electrode, respectively. Both of CC/Ni(OH)2-0.2-150-1h//NF/rGO-2-150-1h-1 and CC/NiO-300-1.5h//NF/rGO-2-150-1h-1 can be operated with a high working voltage of 1.5 V. The specific capacitance of CC/Ni(OH)2-0.2-150-1h//NF/rGO-2-150-1h-1 and CC/NiO-300-1.5h//NF/rGO-2-150-1h-1 were 83.2 and 102.5 F/g, respectively. At the power density of 750 W/kg, the energy density of CC/Ni(OH)2-0.2-150-lh//NF/rGO-2-150-1h-1 was 26.0 Wh/kg,65.2% of its capacitance can be mained after 5000 cycles. CC/NiO-300-1.5h//NF/rGO-2-150-1h-1 displayed an energy density of 32.0 Wh/kg at the power density of 750 W/kg. After 5000 cycles, its capacitance retention was as high as 94.9%, showing good potential application for energy storage. |
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