Charge Storage by Electrochemical Reaction of Water Bilayers Absorbed on MoS₂ Monolayers

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. It is reported here that 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 an ultrahigh specific capacitance. In this paper, 1T phase MoS₂ nanosheets with three monolayers were synthesized by hydrothermal reaction. It presents a specific capacitance of 1120 F g^-1 at a current density of 0.5 A g^-1 in KOH. As it is assembled with active carbon into a hybrid supercapacitor, the device has an energy density of 31.64 Wh kg^-1 at a power density of 425 W kg^-1, and gets a specific capacitance retention of 95.4% after 10,000 cycles at 2 A g^-1.