Aqueous cosolvent in Zwitterionic-based PILs 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 in order 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 "a la carte" properties. Herein, we describe how the stoichiometric addition of a zwitterion like betaine (BET) to protic ILs - e.g., PILs, in this particular case, those formed between methane sulfonic acid (MSAH) or  p-toluensulfonic acid (PTSAH) with ethanolamine (EOA) - results in the formation of zwitterionic-based PILs (ZPILs) containing the original anion and cation, and the zwitterion. The ZPILs prepared in this work - e.g., [EOAH]+[BET][MSA]- and [EOAH]+[BET][PTSA]- - were liquid at room temperature despite the original PILs - e.g., [EOAH]+[MSA]- and [EOAH]+[PTSA]- - were not. Moreover, ZPILs exhibited a wide electrochemical stability window - e.g., 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 2H2O:ACN:DMSO provided specific capacitances of ca. 83 F g -1 at current densities of 1 A g -1 , and capacity retentions of ca. 90% after 6000 cycles when operating at a voltage of 2.0 V and a current density of 4 A g -1 .