Owing
to their low electronegativity, excellent electrical conductivity,
high specific capacitance, and rich electrochemical redox sites, various
transition metal sulfides have attracted significant attention as
promising pseudocapacitive electrode materials for supercapacitors.
However, their relatively poor electrical conductivity and large volume
changes seriously hinder their commercial applications. Herein, ternary
Co0.33Fe0.67S2 nanoparticles are
in situ embedded between graphene nanosheets through a facile one-step
hydrothermal approach to form a sandwich-like composite. Because of
its unique and robust structure, the graphene nanosheet/Co0.33Fe0.67S2 composite (GCFS-0.33) exhibits a high
specific capacitance (310.2 C g–1 at 2 mV s–1) and superb rate capability (61.8% at 200 mV s–1) in 3 M KOH aqueous electrolyte. Using transition
metal sulfides simultaneously as both positive and negative electrodes,
for the first time, an aqueous asymmetric supercapacitor (ASC) was
fabricated with the GCFS-0.33 composite as the negative electrode
and sulfidized graphene/CoNiAl-layered double hydroxides as the positive
electrode with well-separated potential windows. Our fabricated ASC
delivered an excellent energy density of 66.8 Wh kg–1 at a power density of 300.5 W kg–1 and still retained
13.1 Wh kg–1 even at a high power density of 29.4
kW kg–1, which is highly comparable with that of
previously reported transition-metal-sulfide-based ASC devices. Moreover,
the as-fabricated ASC cell displays impressive long-term cycling stability
with a capacitance retention of 102.2% relative to the initial capacitance
after 10 000 cycles. This versatile synthetic strategy can
be readily extended to synthesize other transition-metal-sulfide-based
composites with excellent electrochemical performances.
An asymmetric supercapacitor (ASC) with high energy density is designed using flower-like MoS 2 and MnO 2 grown on graphene nanosheets (GNS) as negative and positive electrodes, respectively. In this paper, flowerlike MoS 2 /GNS and MnO 2 /GNS were controllably synthesized through a hydrothermal approach. The prepared MoS 2 /GNS hybrid displays a typical crinkly and rippled structure with ultrathin MoS 2 nanosheets uniformly grown on the surface of graphene. Additionally, the MoS 2 /GNS electrode exhibits superior electrochemical performance, such as high specific capacitance (320 F g -1 at 2 A g -1 ). The MoS 2 /GNS holds great promise as a negative electrode for an ASC due to its high specific capacitance and wide operation window in negative potential. The assembled all-solid-state ASC delivers a remarkable energy density of 78.9 Wh kg -1 at a power density of 284.1 W kg -1 . Thus the MoS 2 /GNS hybrid is a promising electrode material for next-generation storage systems.Subsequently, 5 mL of L-cysteine solution (160 mg mL -1 ) was dropwised into the above solution. Finally, it was transferred into a 50 mL Teflon-lined stainless steel autoclave, which was sealed and heated in an oven at 180 °C for 12 h. During the hydrothermal approach, L-cysteine will decompose and release S 2ions, acting as the sulfur source. Additionally, L-cysteine is the reducing agent for the formation of MoS 2 , because that the Mo (VI) will be reduced to Mo(IV) by L-cysteine. The product were collected by centrifugation, washed with deionized water and ethanol for several times and vacuum dried at 60 °C for 24 h. The final product was loaded into the tube furnace and calcined in N 2 atmosphere at 550 °C for 2 h. For comparison, bare MoS 2 and the samples with various ratio of MoS 2 to GNS were also prepared under the same condition.
Preparation of MnO 2 /GNS hybrid170 mL of KMnO 4 aqueous solution (0.34 mg mL -1 ) was added into 32 mL of GNS dispersion (1.6 mg mL -1 ) and the above solution was maintained at 65 °C for 24 h under stirring. After it was cooled to room temperature, the precipitate was collected by centrifuge, washed with ethanol and deionized water and dried in vacuum oven at 100 °C for 12 h. Bare MnO 2 samples were also prepared for comparison. MnO 2 -1 was obtained by thermal treatment of MnO 2 /GNS at 400 °C in air to remove the GNS.Another bare MnO 2 sample (MnO 2 -2) was prepared through the oxidation-reduction reaction between KMnO 4 and ethyl alcohol according to our previously work. 40
Characterization methodsX-ray diffraction (XRD) equipped with Cu Kα radiation (λ = 0.15406 nm) was used to characterize the crystallographic structures of the materials. X-ray photoelectron spectrometer (XPS) analysis was performed using nonmonochromatic,
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