A 2.7 V Aqueous Supercapacitor Using a Microemulsion Electrolyte **

The use of aqueous electrolytes in energy storage devices is traditionally limited by the voltage stability window of water at 1.23 V. Here, we present the use of a microemulsion based electrolyte which, although mostly water by mass, has a voltage stability window of up to 5 V. This allows the cost and safety benefits of aqueous electrolytes to finally be realised in high voltage systems. Supercapacitors constructed using this electrolyte were able to achieve and maintain a capacitance of ∼40 Fg −1 and an energy density of ∼40 Wh kg −1 with a Coulombic efficiency of 99 % for over 10,000 cycles on activated carbon.