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Carbonized wood (CW) electrodes with low-tortuosity and high conductivity have received extensive attention in the fields of energy storage and electrocatalysis. Directly carbonized wood electrodes have low specific surface area and high electrochemical impedance, and require secondary physical or chemical activation to achieve acceptable performance. Herein, we reported a facile method to prepare the CW electrode with superior surface area through self-activation using CO 2 and water vapor generated during the pyrolysis which was conducted with a low inert gas flow rate. The specific surface area and electrical conductivity of the self-activated carbonized wood (SCW, ~ 0.46 g cm−3) were as high as 1145 m2 g−1 and 20.3 S cm−1, respectively. The SCW applied as a freestanding electrode demonstrated an impressive specific capacitance of 6.63 F cm−2 (143.6 F g−1) at 1 mA cm−2. After 10,000 cycles, the capacitance of the SCW electrode was retained as high as 101.7%. The symmetrical supercapacitor (SSC) assembled with two SCW electrodes showed an energy density of 0.27 mWh cm−2 (8.60 Wh L−1) at a power density of 0.5 mW cm−2 (11.8 W L−1). This work provides a simple, low-cost and green procedure for fabricating high-performance CW electrodes, which has great promotional significance and practical value on valorization of biomass. • Pre-oxidation of wood at 220 °C produced substantial amount of CO 2. • Low inert gas rate retains the concentration of CO 2 allowing self-activation. • Self-activated carbonized wood electrode exhibited superior specific surface area. • The freestanding SCW electrode exhibited high energy density at the device level. [ABSTRACT FROM AUTHOR]