A novel asymmetric supercapacitor (ASC) is assembled based on petal-like cobalt selenide (Co_0.85Se) nanosheets as positive electrode and nitrogen-doped porous carbon networks (N-PCNs) as negative electrode in a 2 M KOH aqueous electrolyte. The Co_0.85Se nanosheets are synthesized via a simple low-temperature solvothermal method without any template and surfactant, and the N-PCNs are prepared by integrating in-situ oxidation polymerization and catalytic carbonization methods directly from the p-phenylenediamine monomers. Thanks to their unique structures and high capacitive performance, the as-assembled Co_0.85Se//N-PCNs ASC device possesses an extended operating voltage window of 1.6 V, high energy density of 21.1 W h kg⁻¹ at a power density of 400 W kg⁻¹ and outstanding cycling stability (93.8% capacitance retention after 5000 cycles) in aqueous electrolyte.