Sodium chloride-glycerol deep eutectic solvent: a green electrolyte for high-voltage electrochemical double layer capacitors

Photovoltaic and wind energy systems are regarded as effective alternatives for reducing our reliance on fossil fuels in energy production. However, due to their intermittent and unpredictable nature, efficient electrochemical energy storage systems must be integrated to ensure a continuous and reliable energy supply [1]. For a truly sustainable energy future, energy storage systems must be able to store power without relying on toxic or hazardous materials, such as those found in commercial organic electrolytes [2]. In the last two decades, deep eutectic solvents (DESs) [3] have emerged alongside ionic liquids (ILs) as promising alternative electrolytes for electrochemical applications, as a result of their low volatility, low flammability, and favorable electrochemical stability [4]. Unlike ILs and electrolytes based on volatile organic compounds, DESs can be prepared under mild conditions, significantly lowering production costs [5]. Herein, we propose a sustainable electrolyte based on sodium chloride as hydrogen bond acceptor and glycerol as hydrogen bond donor. Via a comprehensive multi-technique study, including Raman and FTIR spectroscopies, of the formulated electrolytes, we point out the effect of the structuring of the system, such as the conformational changes of the polyol and the alteration of the hydrogen bond network, on the transport and electrochemical properties. The 1:10 molar ratio DES shows good performance when implemented in an electrochemical double layer capacitor, with a high operating voltage of 2.6 V and a capacitance retention of 96% after 1000 cycles [6]. These findings highlight the potential of glycerol-based DESs as alternative electrolytes for sustainable electrochemical energy storage applications.