Unveiling the influence of 3d transition metal (Sc-Zn) doping on quantum capacitance and surface charge storage in bare nano cages (Al(12)N(12), Al(12)P(12), B(12)N(12), and B(12)P(12)) - a first principles simulation study.

Density functional theory is used to investigate the quantum capacitance () and surface charge storage () of bare (AlN (AN), AlPl (AP), BN (BN), and BP (BP)) and 3d transition metal (TM) (Sc-Zn)-doped nanocages. Ground-state spin configurations are confirmed for all metal-doped systems. Cohesive energy values indicate the following stability trend: BN (-6.69) > BP (-5.09) ≈ AN (-5.12) > AP (-4.46 eV per atom). Metal doping is energetically favorable, with strong binding energies for AN/Cr (-3.86 eV), AP/Ni (-4.32 eV), BN/Cr (-2.89 eV) and BP/Mn (-3.69 eV). Electronic structures are analyzed using both the PBE0 and HSE06 functionals, allowing reliable comparison of exchange-correlation effects on and . Projected density of states reveals that TM 3d states hybridize with N/P 2p/3p orbitals, increasing delocalized states near the Fermi level and enhancing charge accommodation. Among bare cages, the order of maximum is as follows: AN (543) < AP (579) < BP (670) < BN (688 µF cm) the HSE06 method. BN/Zn achieves a peak of 678 µF cm at -1.5 V and the AN/Zn system exhibits the highest of -384 µC cm. Overall, TM doping converts the reduction-dominated charge storage of the bare nanocages into a more balanced, bidirectional response, with AN and BN cages exhibiting the most pronounced and controllable enhancement in both and due to strong metal-nitrogen hybridization, identifying them as promising non-carbon electrodes for electrochemical double-layer supercapacitors.