Insights into the Surface Science, Storage Mechanism and Electrochemical Phase Evolution of MoS2/CNT Hybrid Electrodes for Supercapacitive Application
Authors: Tiwari Pranjala , Chandra Ramesh 
Published: February 2020

Lately, layered MoS2 is found to be one of the most researched pseudocapacitive electrode material for Li+ and Na+ ion batteries because of its unique layered structure for initial ion intercalation and de‐intercalation and high conversion chemistry. As far as supercapacitors are considered, the charge storage in nanostructured MoS2 takes place via electric double layer formation, intercalation and de-intercalation pseudocapacitance and redox pseudocapacitance. Recently, a vast majority of studies on layered MoS2, have been focused on designing hierarchical deformed structures instead of regularly ordered crystallites which may accommodate intercalation stress upto some extent while retaining the original structure.
In this study, we have synthesized large aspect ratio carbon nanotubes with a high specific surface area and an appropriate superstructure forming a thin layer of molybdenum disulfide nanoflakes hierarchically dispersed on the surface of the CNTs. This novel technique of preparing MoS2@CNT superstructure using a combination of chemical and physical deposition techniques proved to be very efficient in structurally engineering the microstructure of electrode material. Moreover, sputtering as a fabrication technique, provided an opportunity to precisely grow hierarchical structured MoS2 nanoflakes uniformly throughout the matrix, easily scalable and controllable at lengths down to nanometer ranges irrespective of superhydrophobic nature of CVD grown CNTs. The designed hybrid nanostructure form an open porous network with shorter ionic as well as electron diffusion lengths and provides larger number of intercalation sites. 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. The electrochemical performance of the hybrid is far more superior due to the contribution from both faradaic and non-faradaic processes. Also, the cyclic retention have been immensely improved as compared to bare CNT and MoS2 and has shown a retention of 97.6% after 3500 cycles. Further, to elucidate the charge storage mechanism Trasatti’s and Dunn’s analysis are performed which suggest that pseudocapacitive phenomenon is dominantly responsible for superior charge storage in the hybrid.
The electrodes subjected to 10, 50, 1000, 2000 and 3500 charging discharging cycles at 100mV/s were then analyzed using X-ray photoelectron spectrophotometer to understand the change in surface science of the material and reason behind cyclic fading. The phase transformation of electrode suggests that the binding energies of MoS2 shits towards shifts towards lower energies after cycling. This shift indicates partial transition of 2H→1T MoS2 phase due to the transverse gliding of S plane upon the intercalation of electrolytic ions. Additionally, we observed the formation of thin MoO3 oxide layer on the surface of electrode which may be due to the presence of dangling bonds generated by ionic intercalation. Thus, this thin insulating layer may be the reason of capacitive fading after 3500 cycles of constant charging and discharging. The all solid 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.

Figure 1