The development of high-performance supercapacitors in ionic liquid hold great potential for application in next generation electrochemical energy storage devices, however, it is still facing contradictory requirements between specific capacitance and rate capability. Herein, we developed a facile method for the synthesis of 2D porous carbon nanosheets (PCNSs) from sodium alginate by utilizing an integrated gunpowder chemistry-assisted exfoliation and subsequent KOH activation procedure. The as-prepared PCNSs showed a 2D nanosheets structure and large specific surface area. A symmetric supercapacitor device with ionic liquid as the electrolyte can operate in a wide voltage window of 3.5 V. Importantly, PCNS-3 based device presents a specific capacitance as large as 226.9 F/g at 1 A/ g, superior rate capability (68% of capacitance retention at 50 A/g), outstanding cycling stability (94% of its initial capacitance after 10 000 cycles), and integrated high energy & power densities (65.6 Wh/kg at 43.75 kW/kg). This work provides an alternative way towards 2D porous carbon nanosheets electrode materials with abundant narrow mesopores for rapid mass transfer and ion transport in supercapacitors as well as other nanocarbon-based electrochemical energy storage devices.