Aqueous Co-Solvent in Zwitterionic-based Protic Ionic Liquids as Electrolytes in 2.0 V Supercapacitors

High-performance energy-storage devices are receiving great interest in sustainable terms as a required complement to renewable energy sources to level out the imbalances between supply and demand. Besides electrode optimization, a primary objective is also the judicious design of high-performance electrolytes combining novel ionic liquids (ILs) and mixtures of aqueous solvents capable of offering “à la carte” properties. Herein, it is described the stoichiometric addition of a zwitterion such as betaine (BET) to protic ILs (PILs) such as those formed between methane sulfonic acid (MSAH) or p-toluenesulfonic acid (PTSAH) with ethanolamine (EOA). This addition resulted in the formation of zwitterionic-based PILs (ZPILs) containing the original anion and cation as well as the zwitterion. The ZPILs prepared in this work ([EOAH]⁺[BET][MSA]⁻ and [EOAH]⁺[BET][PTSA]⁻) were liquid at room temperature even though the original PILs ([EOAH]⁺[MSA]⁻ and [EOAH]⁺[PTSA]⁻) were not. Moreover, ZPILs exhibited a wide electrochemical stability window, up to 3.7 V vs. Ag wire for [EOAH]⁺[BET][MSA]⁻ and 4.0 V vs. Ag wire for [EOAH]⁺[BET][PTSA]⁻ at room temperature, and a high miscibility with both water and aqueous co-solvent (WcS) mixtures. In particular, “WcS-in-ZPIL” mixtures of [EOAH]⁺[BET][MSA]⁻ in 2 H₂O/ACN/DMSO provided specific capacitances of approximately 83 F g⁻¹ at current densities of 1 A g⁻¹, and capacity retentions of approximately 90 % after 6000 cycles when operating at a voltage of 2.0 V and a current density of 4 A g⁻¹.