  We have investigated the quantum capacitance ($C_Q$) in functionalized
graphene, modified with ad-atoms from different groups in the periodic table.
Changes in the electronic band structure of graphene upon functionalization and
subsequently the quantum capacitance ($C_Q$) of the modified graphene were
systematically analyzed using density functional theory(DFT) calculations. We
observed that the quantum capacitance can be enhanced significantly by means of
controlled doping of N, Cl and P ad-atoms in the pristine graphene surface.
These ad-atoms are behaving as magnetic impurities in the system, generates a
localized density of states near the Fermi energy, which intern increases
charge(electron/hole) carrier density in the system. As a result, a very high
quantum capacitance was observed. Finally, the temperature dependent study of
$C_Q$ for Cl and N functionalized graphene shows that the CQ remains very high
in a wide range of temperature near the room temperature.
