To design high specific surface area and optimize the pore size distribution of materials, we employ a combination of carbonization and KOH activation to prepare activated carbon derived from ground grain hulls. The resulting carbon material at lower temperature (800, BSAC-A-800) exhibits a porous structure with a high specific surface area of 1037.6 m2 g-1 and a pore volume of 0.57 m3 g-1. Due to the synergistic structural characteristics, BSAC-A-800 reveals preferable capacitive performance, showing a specific capacitance as high as 313.3 F g-1 at 0.5 A g-1, good rate performance (above 73%), and particularly stable cycling performance (99.1% capacitance retention after 10 000 cycles at a current density of 10 A g-1). More importantly, the assembled symmetric supercapacitor using a water-in-salt electrolyte (17 m NaClO4) with high discharge specific capacitance (59 F g-1 at 0.5 A g-1), high energy density (47.2 W h kg-1) and high voltage (2.4 V) represents significant progress towards performance comparable to that of commercial salt-in-water electrolyte supercapacitors (with discharge specific capacitance of 50 F g-1, energy densities of ∼28.1 W h kg-1 and voltages of 2.0 V).
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