Garlic roots derived activated carbon was prepared using an improved method which is illustrated in Scheme 1. K2CO3 was used as the activating agent instead of KOH. Compared to KOH, K2CO3 is cheaper, little toxic, less corrosive, and offers a higher yield. The activated carbon obtained after K2CO3 activation was subsequently heat-treated in air to introduce oxygen surface functionalities (e.g., carbonyl, quinone) on the carbon surface and to improve the capacitance of the activated carbon due to enhanced hydrophilicity of the pores.