The N2 physisorption was performed in Micromeritics 3 Flex analyzer. The pN-C (~ 100 mg) was loaded into the cell and dried under vacuum overnight at 120 °C prior to the measurement. The nitrogen adsorption-desorption isotherms were obtained at 77K. The surface area and pore size distribution were determined using Brunauer-Emmett-Teller (BET) theory and the Barrett-Joyner-Halenda (BJH) model, respectively.