Caesium Acetate‐Based Electrolytes for Aqueous Electrical Double Layer Capacitors

In this work, we report about the use of caesium acetate (CsOAc)‐based electrolytes for Electrical Double Layer Capacitors (EDLCs). We showed that solutions of 6.4 m CsOAc in H 2 O, which is a highly concentrated aqueous electrolyte, and 21 m CsOAc in H 2 O, which is a Water in Salt Electrolyte (WiSE), display very favourable chemical‐physical properties. EDLCs containing 21 m CsOAc in H 2 O as electrolyte display operating voltage of 1.7 V. These devices display good energy density and are able to retain all their initial capacitance after 400 hours of floating at 1.7 V. On the other hand, EDLCs containing 6.4 m CsOAc in H 2 O as electrolyte display operative voltage of 1.6 V and are delivering at 10 A g −1 very high energy and power density (4 Wh kg −1 and 14.1 kW kg −1 , respectively). Furthermore, they can retain 91 % of their initial capacitance after 400 hours of floating at 1.6 V. This performance, which is among the highest so far reported for aqueous‐based EDLCs, indicates that CsOAc can be considered a very promising salt for the realization of advanced EDLCs.