Synthesis And Electrochemical Performance Of MoS₂ Nanosheets For Supercapacitor

Molybdenum disulfide?MoS₂?,a type of layered transition metal dichalcogenide and pseudocapacitive material,exhibits a remarkably diverse range of unique properties and becomes currently the focus of research groups across a broad range of disciplines.Due to the unique crystal structure and electronic properties,the nano-structure MoS₂ has been widely applied as a material in environmentally benign energy conversion and storage?ECS?devices.It is well-known that in neutral and acidic aqueous electrolytes,MoS₂ monolayers can store charges by adsorption of cations on to the electrode-electrolyte interface as its analog of graphene.Restricted by its low conductivity and the charge storage mechanism,the electrochemical performance of MoS₂ monolayer supercapacitor electrode is not satisfactory,so it is a reasonable research direction to improve the conductivity and broaden the charge storage mechanism.The metallic 1T-MoS₂ phase is hydrophilic and 10⁷ times more conductive than the semiconducting 2H phase,so it possesses the excellent electrochemical performance for supercapacitor electrode materials.In this paper,we systematically studied the MoS₂ nanosheets for using as the supercapacitor electrode materials.By changing the experimental scheme,we tend to improve electrical conductivity and optimize structure for large specific capacitance,high rate performance,long working life and high power density as well as energy density of the MoS₂-based supercapacitor electrode materials.The research includes the following items:?1?The self-assembled nanoflowers of the three-layer 1T phase MoS₂ nanosheets were fabricated via a one-pot facial hydrothermal reaction.Then we analyzed the phase structure and electrochemical performance of 1T-MoS₂.Due to the high 1T phase components of the MoS₂ nanoflowers,the conductivity of the electrode materials is improved by a large margin.By comparing the electrochemical behavior in different acidic and alkaline electrolytes as well as analyzing the XRD patterns to explore the charge storage mechanism,we find water bilayers absorbed on MoS₂ monolayers can be involved in charge storage.One proton of each absorbed water molecule can intercalate/de-intercalate the water bilayers during charging/discharging in the alkaline aqueous electrolyte.For two water molecules are present for every Mo atom,the water bilayers can endow MoS₂ monolayers a high specific capacitance.The 1T-MoS₂ nanoflowers electrode can present an ultrahigh special capacitance of 1120 F g^-11 at a discharge current density of 0.5 A g^-11 in 3 M KOH electrolyte.The electrodes retained a capacitance of 96%at a discharge current density of 10 A g^-11 in KOH after 2,000 cycles.?2?The hybrid supercapacitor device was fabricated using the MoS₂ as positive electrode,AC as negative electrode?denoted as MoS₂//AC?,which exhibits the outstanding electrochemical performance.The specific capacitance exhibits a very high value of 78.82 F g^-11 at 0.5 A g^-1,corresponding to an energy density of 31.64 Wh kg^-11 at a power density of425 W kg^-1,which is comparable to the Nickel hydroxide related composite electrodes.Meanwhile,the device still possesses high specific capacitance retention of 95.4%at 2 A g^-1after 10,000 cycles.Therefore,the device is expected to be used in practice.?3?The MoS₂-1 nanoflowers and MoS₂-2 nanosheets were fabricated via a one-pot facial hydrothermal reaction and then we compared their morphology,structure and electrochemical performance.Due to the addition of NaCl,the MoS₂ nanosheets self-assemble nanoflowers and form the strong structure.Additionally,the relative content of1T phase components was increased dramatically.By contrast,MoS₂-1 nanoflowers possess excellent electrochemical performance.The specific capacitance of MoS₂-1 nanoflowers and MoS₂-2 nanosheets is 1120 and 380 F g^-11 at 0.5 A g^-1,respectively.However,the CVs show the similar shapes,implying the same charge storage mechanism of battery behavior.Compared to MoS₂-2 nanosheets,the reasons of the improvement of the electrochemical performance of MoS₂-1 nanoflowers are that the enhancement of 1T phase and the expansion of layer spacing as well as the self-assembly nanoflowers because of the addition of NaCl.