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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]

Molybdenum disulfide (MoS 2) is one of the promising candidates for hydrogen evolution reaction (HER), it's HER properties can be extremely enhanced by the construction of nanostructure and the regulation of electronic structure. In this paper, hierarchical composites with MoS 2 and carbon nanotubes (CNTs) as the noble metal-free electrocatalysts for HER were fabricated through calcination and hydrothermal processes, in which the MoS 2 nanosheets with Ni atoms incorporation were vertically grown on the outsides of nitrogen (N)-doped CNTs. The constructed hierarchical structures provide large specific surface area of about 150 m2 g−1 and high structure stability. Importantly, the MoS 2 nanosheets with enlarged interlayer spacing possess ultrathin structure of about 4 nm in thickness. Owing to the incorporation of Ni atoms, abundant unsaturated S atoms are obtained in the basal plane, which leading to more active sites exposed during HER processes. Meanwhile, N-doped CNTs can act as the substrate for MoS 2 nanosheets growing and simultaneously improve the conductivity of catalyst. Based on the synergy of hierarchical structure, increased active sites and enhanced conductivity, the as-prepared electrocatalysts exhibit superior HER activity with low overpotentials, small Tafel slopes and long-time durability in both acidic and alkaline electrolytes. The overpotentials of 158 and 179 mV were required to drive HER current density up to 10 mA cm−2 in 0.5 M H 2 SO 4 and 1.0 M KOH, respectively. This work reports a highly efficient MoS 2 -based electrocatalyst for HER via fabricating hierarchical structure and doping Ni atoms. Image 1 [ABSTRACT FROM AUTHOR]