Reline deep eutectic solvent as a green electrolyte for electrochemical energy storage applications

Abstract

Deep eutectic solvents (DESs) are an environmentally benign promising emerging class of versatile solvent systems. In this paper, for the first time, a new approach to the DES application of choline chloride–urea, commonly called Reline, and its aqueous mixtures as eco-friendly, affordable and anticorrosive electrolytes for carbon-based electrochemical capacitors (ECs) is proposed. The symmetric ECs operating in the Reline-based electrolyte demonstrate excellent electrochemical stability at a voltage of 2.2 V, in turn supplying a high energy. Furthermore, by adding 1 wt% water to the DES system, the specific capacitance increased from 100 to 157 F g⁻¹ at a 1 A g⁻¹ current load. In addition, excellent long-term capacitor stability was achieved up to >25 000 cycles for the Reline + 1% water-electrolyte and >50 000 cycles for pure Reline. The paramount benefit is the fact that Reline-based ECs are constructed in ambient conditions as opposed to ECs operating with organic electrolytes and ionic liquids (ILs) demanding glove-box. Additionally, the Reline-based electrolyte is a non-flammable solution. Moreover, the current collectors of our DES-based supercapacitor do not corrode. Therefore, the present findings have important implications to extend the application of the green, non-toxic and cheap Reline DES electrolyte for high-performance ECs with a wide voltage window and a superior lifespan.