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

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 super capacitor 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.

Automated summary

In this study, a co-solvent-in-deep eutectic solvent system was developed by mixing water and acetonitrile with a typical DES electrolyte. The addition of co-solvents improved the conductivity and flame-retardant properties of the DES, leading to a wide electrochemical stability window. Furthermore, a hydrogel consisting of 1T-MoS2 and reduced graphene oxide was used as electrode materials for supercapacitors, demonstrating high energy density and capacitance retention after cycles.