A new hierarchically porous carbon has been synthesized with self-template of silica phase from a commercial silicone resin by pyrolysis and subsequent NaOH activation. The obtained carbon materials achieve an ultrahigh specific surface area (2896 m2 g−1) with abundant mesopores. The C800 sample demonstrates excellent performance in supercapacitors, with a high capacitance of 322 F g−1 at 0.5 A g−1 and outstanding rate capability (182 F g−1 at 100 A g−1) in a three-electrode system using 6.0 mol L−1 KOH electrolyte. The energy density is improved by widening the voltage window using 1.0 mol L−1 alkali metal nitrate solutions (LiNO3, NaNO3, KNO3) in which the strong solvation of alkali metal cations and nitrate anions effectively reduce the activity of water. In a symmetric supercapacitor, the maximum operating voltage is essentially restricted by the potential of positive electrode and the total capacitance is dominated by the capacitance of the anion at the positive electrode. The symmetric supercapacitors based on C800 deliver a high energy density of 22.4 Wh kg−1 at a power density of 0.23 kW kg−1 in 1.0 mol L−1 LiNO3 with a voltage of 1.8 V and long-term stability with a retention of 89.87% after 10000 cycles.