Water/acetonitrile hybrid electrolyte enables using smaller ions for achieving superior energy density in carbon-based supercapacitors

Abstract

There is a big push toward developing low-cost and safe supercapacitors with high energy to meet increasing demands for grid-scale applications. Activated carbons have reached their capacitance limit because of their limited accessible surface area in organic electrolytes, commonly used in commercial supercapacitors. Herein, we reveal that a shift from organic electrolytes to polar hybrid electrolytes allows switching to smaller ions without a penalty on solubility and ionic conductivity, which in turn allows for smaller pores and higher capacitance. We show that elevated storage capability can be realized by matching the porosity of a hierarchically porous carbon to a water/acetonitrile hybrid electrolyte with sodium perchlorate. Leveraging the small size of ions along with good wetting by the hybrid electrolyte ensures fast ion diffusion and enhanced accessibility of ions to carbon pores. Using the hybrid electrolyte for our model coal-derived activated carbon, we fabricated a high-rate supercapacitor with the best capacitance and energy density reported to date for activated carbons (hybrid or organic electrolytes), including 41.2 F/g (18.3 F/cm3) and 35.0 Wh/Kg at 1 A/g. A commercial carbon with a higher surface area delivers comparable gravimetric capacitance but lower volumetric capacitance due to much lower density than our carbon (7.4 F/cm3).