Figure (a) illustrates a large variation in the Fermi-level energy in smaller supercells with respect to their charge state. We find that the variation drops as a function of 1 N , where N is the number of carbon atoms in the supercell. It is, therefore, necessary to have an infinite size for the electrode to remain at a constant potential. However, we use a supercell of 37.04 Å× 38.47 Å (15×9 unit cell that contains 540 carbon atoms) whose Fermi-level energy varies less than 130 meV between two consecutive charge states to make our calculation feasible.