We have investigated the quantum capacitance ([Formula: see text]) 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 [Formula: see text] of the modified graphene were systematically analyzed using density functional theory (DFT) calculations. We observed that the [Formula: see text] 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, generating a localized density of states near the Fermi energy which, in turn, 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 [Formula: see text] for Cl and N functionalized graphene shows that the [Formula: see text] remains very high in a wide range of temperatures near room temperature.