Multi-technique investigation of polyol-based Deep Eutectic Solvents as innovative and sustainable electrolytes in electrochemical energy storage devices

Concerns related to the energy crisis lead the scientific community to move towards electrical energy production methods more sustainable than those based on fossil fuels, among which solar and wind power play a major role. However, these sources are intermittent and somehow unpredictable. Therefore, it is mandatory to integrate photovoltaic and wind energy systems with efficient electrochemical energy storage systems (EESS), such as batteries and supercapacitors, to store the possible excess of energy produced and supply additional power in case of insufficient electricity supply. One of the main concerns for EESS is the use of thermally unstable electrolytes and critical raw materials enriched electrodes; thus, the scientific community is focusing on the design, characterisation and application of more sustainable electrodes and electrolytes. In this contribution, we present a detailed spectroscopic and electrochemical investigation of sodium and zinc-based Deep Eutectic Solvent (DES) electrolytes. DES are an emerging class of low-cost and sustainable systems that are 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. However, these values are still lower compared to the ones reached with conventional electrolytes based on organic solvents or Ionic Liquids, which generally reflects in less appealing electrochemical performance. Aiming to reach wider and more effective exploitation of DES as 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. A multi-technique characterisation (based on thermal analysis, Raman spectroscopy and electrochemistry) is conducted to discern the eutectic composition from not eutectic ones and to clarify the nature and the role of the intermolecular interaction established in DES. These latter will play a paradigmatic role in tuning the properties of the mixtures (i.e. conductivity, viscosity and electrochemical stability window) and the electrochemical behaviour of the electrolytes operating in real lab-scale devices (batteries and supercapacitors). The most promising systems are evaluated with cyclic voltammetry and galvanostatic cycling as a further step toward practical application.