Deep Eutectic Solvents as a potential solution for the new generation of energy storage systems: from electrochemical to thermoelectric devices

The last few years have shown how much the price of non-renewable resources is influenced by global events and the catastrophic consequences related to the exploitation of these very resources [1]. Electrochemical energy storage systems (EESS), such as batteries and supercapacitors, and thermo-electrochemical cells (TECs) play a crucial role in the energy transition, since they encourage the harnessing of renewable energy, particularly solar, wind and waste thermal power. In this contribution, we present an overview on Deep Eutectic Solvents (DESs) as innovative and sustainable electrolytes, capable of providing in complete devices performance comparable to that of traditional ones (e.g., lithium-ion batteries and solid-state thermoelectric generators), which are made up of thermally unstable electrolytes and/or critical raw materials [2-3]. DESs are an emerging class of low-cost solvents, that are based generally 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) [4]. They demonstrate high thermal stability, low vapor pressure, good biodegradability and good inertness to humidity, features that make these systems attractive for application in EESS and TECs. However, the development of DESs in this field is only at the beginning, even though some publications [5-6], and some of our preliminary study, demonstrates how they have huge potential as sustainable electrolytes and how the peculiarity of the interactions between HBAs and HBDs can reflect positively on the electrochemical and thermoelectric properties of the mixtures. Aiming at reaching a wider and more effective exploitation of DESs as electrolytes, a thoughtful design of new combinations of HBA and HBD is necessary, as well as a thorough understanding of the main chemical and physical interaction of the precursors. To this goal, a multitechnique platform should be exploited: vibrational spectroscopies (Raman and/or IR), coupled with thermal and electrochemical techniques and also supported by computational tools, could give fruitful insights in the nature and role of the intermolecular interactions established in DESs, and in their impact on the physical-chemical properties of the mixtures (i.e., ionic conductivity, thermal conductivity, viscosity, electrochemical stability window), beyond the electrochemical or thermo-electrochemical behavior of them ones implemented in complete devices.