An Artificial Interface By Sol-Gel Chemistry to Open the Window of an Aqueous Supercapacitor

Abstract

Devices with high power density which can store energy with a fast rate charge-discharge, the supercapacitors are suitable in different applications where high power is needed. That because it stores charge by fast surface process, i.e. ion adsorption of an electrolyte into the electrochemical double layer at the electrode/electrolyte interface. Nevertheless, in order to extend their autonomy beyond few seconds, its energy density has to be increased. For this aspect, in recent years many teams around the world have set up different strategies as a combination of a carbon electrode with a faradic type electrode (battery electrode or pseudocapacitive) or through the development of new active materials, including the development of new electrolytes. The main significant achievements are more on the active material side. Even though some works have been successful in increasing the cell voltage, thanks to the use of new ionic liquids, they are still room for improvement regarding the increase of the cell voltage.To address this point, we have targeted to obtain an aqueous-based high voltage supercapacitor. Our strategy consists in using an Artificial Interface (AI) by depositing an ionogel onto a carbon electrode, the AI is composed of an inorganic matrix and an ionic liquid, in order to form a passivation layer at the electrode/electrolyte interface; a better electrochemical and thermal stability of this interface is thus expected. The AI has got to be electronically insulating but ionic conductive, limiting or even preventing the transfer of electronic charge to the electrode / electrolyte interface.1,2 First things first, the study have been carried out on glassy carbon where AI has been deposited and the linear sweep voltammetry tests in an aqueous electrolyte based on EMI+ HSO4 - and H2SO4 indicate, in both cases, an increase in the potential window: the onset potential for water reduction is shifted toward more cathodic potentials. Going further, tests in the presence of a redox probe (Ferri-Ferro potassium cyanide) demonstrate the absence of electron transfer at the electrode/electrolyte interface. On the other hand, it appears, thanks to measurements carried out by electrochemical impedance spectroscopy, since within the AI is embedded EMI+ TFSI-, better ionic conduction is obtained when the electrolyte is made of EMI+ HSO4 -, sharing the same cation than the one of AI. As we will be showing, a better transference number is achieved for EMI+ cations than for H+ still further works are needed to improve the selectivity of the AI to improve its stability over cycling.1. Z. H. Zhang et al., Sci. Rep., 8, 1–12 (2018).2. M. D. Elola and J. Rodriguez, J. Phys. Chem. C, 123, 3622–3633 (2019).