(Invited) Carbon/Dichalcogenide Composites As Electrodes for Electrochemical Capacitors

Abstract

Electrochemical capacitors (ECs) are considered to be the very promising energy storage systems because they possess a high power density as well as a long life span. However, contrarily to the batteries, the energy density of ECs is rather moderate. It is very well known that the energy stored in ECs is mostly dependent on the specific surface area of the electrode materials, i.e., microporosity, but conductivity also plays an important role. Energy can be greatly increased by the utilization of redox reactions from electrolyte and/or electrode materials. For this work, two dimensional transition metal dichalcogenides (2D-TMDs) have been applied as electrode material components. TMDs are layered materials of the MX2 type where M is a transition metal (Mo, Re, W, Co or V) and X is a chalcogen (S, Se or Te). In their structure, one layer of M atoms is sandwiched between two layers of X atoms. Different salts such as sodium molybdenate, ammonium perrhenate, cobalt nitrate with thiourea or L-cysteine as a sulfur source were used as TMD precursors. Taking into account moderate conductivity of dichalcogenides, various mesoporous carbon materials such as carbon nanotubes, reduced graphene oxide rGO, 3D-graphene hydrogel, carbon black have been used by us for composite preparation. In some cases functionalization of multiwalled carbon nanotubes has been performed to improve adhesion in the composite. The amount of carbon in the C/TMD composites varied from 5-30%. Hydrothermal method have been successfully used for carbon/TMD production. The composites were characterized with the various textural/structural methods: scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy. After a detailed physico-chemical characterization, all composites served for electrode preparation. Mostly, 90wt% of active material, 5 wt% of conducting agent C65 and 5wt% of Teflon as binder were used as electrode components. Two-electrode and three-electrode configuration cells have been applied for capacitance measurements. Aqueous solutions (neutral and alkaline) served as electrolytes (1M lithium sulfate, 6M KOH). The capacitor voltage ranged from 1-2V. The electrochemical performance of EC cells were studied by cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy. The capacitance values, coulombic and energetic efficiency were affected by composite structure, its specific surface area as well as its conductivity and composition. Typically for faradaic reactions, the lower regime, the lower energetic efficiency was observed. High capacitance values (100 F/g – 300F/g) have been reached for C/MoS2. Selected carbon/TMD composites have been used as a positive electrode whereas a negative electrode was composed only from carbon materials. In such a way, high energy electrochemical capacitor has been constructed and investigated.