Unveiling the influence of 3d transition metal (Sc–Zn) doping on quantum capacitance and surface charge storage in bare nano cages (Al12N12, Al12P12, B12N12, and B12P12) – a first principles simulation study

Abstract

Density functional theory is used to investigate the quantum capacitance (C_Q) and surface charge storage (Q) of bare (Al₁₂N₁₂ (AN), Al₁₂Pl₁₂ (AP), B₁₂N₁₂ (BN), and B₁₂P₁₂ (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 C_Q and Q. 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 C_Q is as follows: AN (543) < AP (579) < BP (670) < BN (688 µF cm⁻²) via the HSE06 method. BN/Zn achieves a peak C_Q of 678 µF cm⁻² at −1.5 V and the AN/Zn system exhibits the highest Q of −384 µC cm⁻². 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 C_Q and Q due to strong metal–nitrogen hybridization, identifying them as promising non-carbon electrodes for electrochemical double-layer supercapacitors.