Hierarchal growth of MoS2@CNT heterostructure for all solid state symmetric supercapacitor: Insights into the surface science and storage mechanism

Direct growth of atomically thin hierarchal MoS2 nanoflakes on CVD grown carbon nanotubes has been achieved using magnetron sputtering for electrochemical storage application. This novel technique of preparing MoS2@CNT 1D heterostructure using a combination of CVD and PVD proved to be very efficient in structurally engineering the microstructure of electrode material. The super capacitive measurements of the MoS2@CNT electrode revealed capacity of 337 mF/cm2 at the sweep rate of 5 mV/s. The storage mechanism involves both electrostatic ionic adsorption as well as Na+ intercalation into the MoS2@CNT matrix. Further, using Trassati and Dunn's methods, we found that ionic diffusion and the inner electrode material contributes majorly to the total stored charge. In order to study the phase evolution of electrode material upon several charge-discharge cycles, we employed XPS technique which indicated lower shift in the binding energies of Mo as well as S. This shift indicates partial transition of 2H→1T phase due to the transverse gliding of S plane upon the intercalation of electrolytic ions. The state supercapacitor built using symmetric MoS2@CNT electrodes was found to have high areal and volumetric capacitance of 131 mF/cm2 and 2.9 F/cm3 respectively, with high cyclic stability of 97.6% after 2500 cycles.