An investigation into green, high voltage, aqueous supercapacitors

Recognizing the advantages of aqueous electrolytes in terms of high ionic conductivity, low cost and the absence of flammability issues, this study aims at raising the maximum operating voltage of supercapacitors with an aqueous electrolyte beyond the water electrolysis limit of 1.2 V. This is achieved on the basis of the “water-in-salt” principle by employing superconcentrated aqueous salt solutions. After an initial screening, three high saturation concentration aqueous solutions were identified, also combining low cost, low salt density and absence of hazardous issues. After experimental investigations and multiscale modeling and simulations, the best supercapacitor was an EDLC (electrical double layer capacitor) with the aqueous electrolyte 40.1 m HCOOK which reached a maximum operating potential of 2.3 V. Multiscale modeling attributed this effect to the highly trapped H2O molecules in the first solvation layers shared by multiple solvated ions in ion clusters. This electrolyte was combined with highly microporous activated carbon fabric electrodes in a symmetric EDLC which exhibited an electrode specific capacitance of 255 F g- 1 in discharge.