N2 gas physisorption experiments were carried out using a high vacuum physisorption/chemisorption analyser (autosorb iQ from Anton Paar) at 77 K. For each carbon sample, around 25 mg of the powder was placed into a glass bulb gas cell and degassed at 120 ℃ in vacuo for 16 h before the measurement. The Brunauer-Emmett-Teller (BET) surface area was calculated from the isotherm using the BET equation and Rouquerol's consistency criteria implemented in BETSI . The average pore sizes were calculated by taking the pore size value at half of the cumulative pore volume in the pore size distribution. Pore size distributions were calculated by QSDFT based on a slit-pore model.