An Insight into the Mechanisms of Energy Storage in a Double Layer Capacitor with ILs and a Microporous Carbon: Experimental Evidences of Ion-Swapping by Electrochemical Impedance Spectroscopy

The energy storage mechanism operating in carbon-based supercapacitors using ionic liquids as electrolytes is not yet fully understood. In this paper, the interactions of ions of two widely used ionic liquids, i.e. EMImTFSI and EMImBF4, with a high specific surface area microporous carbon are investigated. Galvanostatic cycling experiments performed on each electrode and in the full cell demonstrate that a specific energy close to 46 Wh kg-1 can be achieved with this carbon. More interestingly, impedance spectroscopy studies reveal the presence of some unusual behavior, such as the presence of inductive elements in some of the electrodes, either in the positive or in the negative electrode. These inductive elements are identified, for the first time, as another type of possible experimental evidences of some phenomena previously proposed, such as ion-swapping or co-ions desorption, both phenomena leading to the formation of the superionic state.