Optimization of acetonitrile/water content in hybrid deep eutectic solvent for graphene/MoS2 hydrogel-based supercapacitors

Abstract

Herein, we are the first to develop a co-solvent-in-deep eutectic solvent (DES) system by mixing water and acetonitrile with a typical DES electrolyte consisting of acetamide and lithium perchlorate. The addition of co-solvents not only solves the problems of high viscosity and low conductivity of DES but also provides some unique properties. For example, the presence of water improves the flame-retardant property of the DES electrolyte. In contrast, the addition of acetonitrile further improves the ionic conductivity without compromising a wide electrochemical stability window (ESW). The effects of the amount of co-solvent in DES and the optimal molar ratio between co-solvents have been investigated. When the molar ratio of acetonitrile to water is 4.4:1, hybrid DES shows the best physical properties, including a wide ESW (2.55 V), superior conductivity (15.6 mS cm−1), and low viscosity (5.82 mPa·s). Furthermore, a series of spectroscopic measurements have been performed to understand the interaction among electrolyte components. It was found that water molecules were strongly coordinated to Li+ ions, and such interaction was not affected by the presence of acetonitrile molecules. On the other hand, we have demonstrated using hydrogel consisting of 1T-MoS2 and reduced graphene oxide (rGO) as the electrode materials for supercapacitors. This hydrogel inherited the porous structure of rGO hydrogel and the high conductivity of 1T-MoS2. Finally, high voltage symmetric supercapacitors have been fabricated by using hybrid DES and hydrogel as the electrolyte and electrode, respectively. An optimized supercapacitor works at a wide operating voltage of 2.3 V and achieves the maximum energy density of 31.2 Wh kg−1 at a power density of 1164 W kg−1. Furthermore, this device exhibited 91% capacitance retention after 20,000 cycles.