Polyol-based Deep Eutectic Solvents as sustainable electrolytes in electrochemical energy storage devices

Nowadays, the integration of green energy sources (e.g. photovoltaic and wind) with efficient electrochemical energy storage systems (EESS), such as batteries and supercapacitors, is dramatically important to store the possible excess of energy produced and supply additional power in case of insufficient electricity supply [1]. Yet, EESS are usually based on critical raw materials enriched electrodes and not sustainable electrolytes, which should be progressively phased out. [2,3]. In this contribution, we present a detailed spectroscopic and electrochemical investigation of Na and Zn-based Deep Eutectic Solvent (DES) electrolytes. DES are an emerging class of low-cost and sustainable systems based on a halide salt (organic or inorganic), which acts as a hydrogen bond acceptor (HBA), and an alcohol/organic acid, which acts as a hydrogen bond donor (HBD). They demonstrate promising ionic conductivity, low vapour pressure and good inertness to air and humidity. [4] Aiming to reach wider and more effective exploitation of DES as sustainable electrolytes, a thoughtful design of a new combination of HBA and HBD is necessary. In this work, we formulated eutectic mixtures based on ZnCl2 or NaCl as HBA, and glycerol or ethylene glycol as HBD and we deeply investigate them through a multi-technique approach (based on thermal analysis, Raman spectroscopy and electrochemistry). The most promising systems are evaluated with cyclic voltammetry and galvanostatic cycling as a further step toward practical application.