Revisiting the performance of electrical double-layer capacitors implementing a sodium perchlorate water-in-salt electrolyte

A maximum cell potential of 2.3 V has been claimed in the literature for an electrical double-layer capacitor (EDLC) in 17 mol kg −1 NaClO 4 water-in-salt (WIS) electrolyte. Puzzled by this high cell potential compared to e.g., 1.8 V for an EDLC in 20 mol kg −1 LiTFSI, and by the fact that the high electrochemical stability window (ESW) values reported in 17 mol kg −1 NaClO 4 were obtained only on non-porous electrodes, this contribution intends to carefully revisit the electrochemical properties of EDLCs based on this WIS electrolyte. Herein, an appropriate electrochemical approach was used to identify precisely the ESW of an activated carbon (AC) electrode (made from the YP50F carbon) and cell potential of electrical double-layer capacitors (EDLCs) when using an aqueous NaClO 4 solution. It was found that the ESW is pushed from 1.7 V to 1.8 V and 2.0 V when increasing the NaClO 4 concentration from 2 mol kg −1 to 10 mol kg −1 and 17 mol kg −1 , respectively. In 17 mol kg −1 NaClO 4 , the comparison of the stability limits of the AC electrode with the potential limits of electrodes (determined by galvanostatic cycling) in a symmetric AC/AC cell equipped with a silver quasi-reference electrode (AgQRE) led us to the conclusion that the maximum voltage of the EDLC is only 1.9 V. Furthermore, the determination of the coulombic and energy efficiency of AC/AC coin cells in 17 mol kg −1 WIS NaClO 4 electrolyte, as well as the electrochemical impedance spectroscopy measurements vs cell potential indicated an operative value of 1.8 V; this value was accurately confirmed by long term potentiostatic floating. Hence, these careful investigations demonstrate that inappropriate experimental conditions to determine the performance of EDLCs, as reported in the literature, may lead to completely biased results.