Gas Evolution in Activated‐Carbon‐Based Supercapacitors with Protic Deep Eutectic Solvent as Electrolyte

Abstract One of the primary causes of aging in supercapacitors are the irreversible faradaic reactions occurring near the operating‐voltage limit that lead to the production of gases resulting in device swelling, increased resistance, and lowering of the capacitance. In this study, a protic deep eutectic solvent (DES) consisting of mixture of lithium bis(fluorosulfonyl)imide (LiFSI) with formamide (FMD) as H‐bond donor ( x LiFSI =0.25; C =2.5 m LiFSI) is investigated as electrolyte for activated carbon (AC)‐based electrical double layer capacitors (EDLCs). Characterization of the viscosity, conductivity, and the ionicity of the electrolyte in a wide range of temperatures indicates >88 % salt dissociation. In situ pressure measurements are performed to understand the effect of cycling conditions on the rate of gas generation, quantified by the in operando pressure variation d P /d t . These measurements demonstrate that about 25 % of the faradaic reactions leading to gas generation are electrochemically reversible. Cell aging studies demonstrate promising potential of the LiFSI/FMD as a protic electrolyte for AC‐based EDLCs and high energy density close to 30 Wh kg −1 at 2.4 V.