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• Systematic study of 3d transition metal-doped CNTs. • V(Cr, Mn, Co)-doped CNTs show spin-polarization effects. • The 3d transition metal doping makes a big difference to the energy band. • 3d transition metal doping increases the quantum capacitance. • Ni-CNT has the largest quantum capacitance near zero potential. Supercapacitors are valued as one of the new green energy storage devices, but the low energy density limits the wide application of supercapacitors. In this paper, the effects of 3d transition metal doped on electronic structure and quantum capacitance of carbon nanotubes are investigated using DFT. The calculations show that elemental doping has caused the break-up of two degenerate states near the Fermi level, quasi-local state changes in the energy band, and the creation of new energy bands. The doping of V(Cr, Mn, Co) causes spin polarization in the energy band. The change in energy band affects the density of states near the Fermi level, effectively increasing the quantum capacitance and surface charge density of the carbon nanotube electrode. The quantum capacitance of Ni-doped carbon nanotubes is the largest, at 59.74 µF/cm2. In the range of water stability, Ti(V, Gr, Mn, Co)-CNT are suitable for use as electrode materials in symmetrical double layer supercapacitors. Sc(Fe, Cu, Zn)-CNT is suitable for use as anode materials in asymmetric double layer supercapacitors. Ni-CNT is suitable for use as a cathode material for asymmetric double layer supercapacitors. This study verifies the feasibility of 3d transition metal doping to improve the quantum capacitance of carbon nanotube electrodes. [Display omitted] [ABSTRACT FROM AUTHOR]

Conductive CoS 2 and cycling stable MoS 2 are integrated to form the interconnected CoS 2 /MoS 2 nanosheet composite with electroactive interface grown on Mo foil substrate to act as high-performance supercapacitor anode material with electrochemical and mechanical stability. The band gap and density of state of CoS 2 /MoS 2 determined by density functional theory simulation calculation indicates its improved electrical conductivity. The interface binding energy and differential charge density reveal that strong crystal interface interaction between MoS 2 (002) and CoS 2 (111) enables effective charge transfer in interconnected CoS 2 /MoS 2 nanosheet composite. CoS 2 /MoS 2 exhibits higher specific capacitance of 605.77 mF cm−2 than MoS 2 of 218.1 mF cm−2 at 1.0 mA cm−2 in 0.5 M Na 2 SO 4. The rate capacitance retention is improved from 57.8% of MoS 2 to 82.8% of CoS 2 /MoS 2 at 1–10 mA cm−2. The high cycling stability is achieved for MoS 2 (100.2%) and CoS 2 /MoS 2 (104.05%) after 5000 cycles at 10 mA cm−2 due to self-activation of multilayered MoS 2. An asymmetric solid-state supercapacitor using CoS 2 /MoS 2 anode and MnO 2 /carbon paper cathode achieves a capacitance of 4.32 F cm−3 at 1.0 mA cm−2 and an energy density of 1.944 mWh cm−3 at a power density of 18 mW cm−3, 97.8% after 5000 cycles at 5.0 mA cm−2, presenting effective energy storage. [ABSTRACT FROM AUTHOR]