Replacing the acetonitrile-based electrolyte in state-of-the-art supercapacitors by an aqueous electrolyte could make these devices environmentally more benign, improve operational safety, and render supercapacitors potentially more cost efficient. The narrow electrochemical stability window of water (~1.23 V) is the main challenge in order to increase the energy density of water based devices. Recently it was reported that ultra-highly concentrated aqueous salt solutions can provide stability windows up to 3 V¹. Salt concentrations higher than 21 mol kg^-1 lead to increased kinetic stability, but result in reduced ionic conductivities of typically less than 10 mS cm^-1. This renders such systems unsuitable for high power applications such as supercapacitors. In a recent study, we report beneficial effects in terms of electrochemical stability and conductivity for a moderately concentrated 8m NaTFSI aqueous solution². Following a stringent test procedure, the stability window was determined to be 1.8 V and an ionic conductivity of 48 mS cm^-1 was obtained, the latter being on par with modern organic electrolytes. A 1.8 V carbon/carbon supercapacitor displayed high maximum energy density of 14.4 Wh kg^-1 on the activated carbon mass level and stable cycling over 100,000 cycles. Redox active potassium iodide was added to the system in order to boost its maximum specific energy to the very high value of 37.8 Wh kg^-1, which is comparable to the performance of currently available commercial acetonitrile based supercapacitors.

1. Suo, L.; Borodin, O.; Gao, T.; Olguin, M.; Ho, J.; Fan, X.; Luo, C.; Wang, C.; Xu, K., "Water-in-salt" electrolyte enables high-voltage aqueous lithium-ion chemistries. Science 2015, 350, 938.
2. Reber, D.; Kühnel, R.-S.; Battaglia, C., High-voltage aqueous supercapacitors based on NaTFSI. Sustainable Energy Fuels 2017, doi: 10.1039/C7SE00423K