In recent years, graphene-based materials have been in the forefront as electrode material for electrochemical energy generation and storage. Despite this prevalent interest, synthesis procedures have not attained three important efficiency requirements, that is, cost, energy, and eco-friendliness. In this regard, in the present work, graphene nanoplatelets with selectively functionalized edges (XGnPs) are prepared through a simple, eco-friendly and efficient method, which involves ball milling of graphite in the presence of hydrogen (H-2), bromine (Br-2), and iodine (I-2). The resultant HGnP, BrGnP, and IGnP reveal significant exfoliation of graphite layers, as evidenced by high BET surface area of 414, 595, and 772 m(2) g(-1), respectively, in addition to incorporation of H, Br, and I along with other oxygen-containing functional groups at the graphitic edges. The BrGnP and IGnP are also found to contain 4.12 and 2.20 at % of Br and I, respectively in the graphene framework. When tested as supercapacitor electrode, all XGnPs show excellent electrochemical performance in terms of specific capacitance and durability at high current density and long-term operation. Among XGnPs, IGnP delivers superior performance of 172 F g(-1) at 1 A g(-1) compared with 150 F g(-1) for BrGnP and 75 F g(-1) for HGnP because the large surface area and high surface functionality in the IGnP give rise to the outstanding capacitive performance. Moreover, all XGnPs show excellent retention of capacitance at high current density of 10 A g(-1) and for long-term operation up to 1000 charge-discharge cycles.