Water-in-Salt Electrolyte Enables Ultrafast Supercapacitors for AC Line Filtering

Water-in-salt electrolytes (WiSEs) are a promising candidate for use in energy storage devices because of their wide electrochemical stability window (ESW) that overcomes the thermodynamic limit of water electrolysis (1.23 V). Despite the high energy density arising from the wide ESW, it is assumed that the power density of WiSE-based energy storage devices will always be less than those that operate with dilute electrolytes because of the higher viscosity of WiSEs. However, the present study demonstrates that ion transport in a high-concentration WiSE (17 m NaClO4) can be as fast as that in a dilute electrolyte (1 m NaClO4), facilitating the ultrafast operation of a supercapacitor. Because of this rapid ion transport, the WiSE-based supercapacitor is proven to be sufficiently fast for use in a 60 Hz AC line filtering application. This unexpectedly high response speed can be attributed to the unique biphasic structure of water and ion-aggregate networks in the WiSE.