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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]

• Full characterization of NiII( tpa ) complexes gives information about its electronic structure which corelates with electrochemical behavior. • NiII( tpa ) complexes presented tunable redox potential for the NiII/NiI reduction. • NiII complex with more negative redox potential presented the high catalytic activity for CO 2 reduction. • Theoretical approach was useful for the analysis and description of electrochemical mechanisms. In this paper, we report the synthesis, characterization, as well as electrochemical, spectroelectrochemical, and density functional theory (DFT) calculations of a series of novel NiII complexes with a flexible tetradentate ligand, tris(2-pyridylmethyl)amine) ( tpa ). The complexes [NiII( tpa )(H 2 O)(MeCN)](BF 4) 2 , [NiII( tpa )(NO 3) 2 ], and [NiII( tpa )Cl 2 ] were studied for their ability to electrochemically reduce CO 2. These complexes were compared with the tris-chelate complex [Ni(bpy) 3 ](BF 4) 2 to understand the role of flexible tetradentate ligands containing a pyridinic ring in the electrochemical reduction of CO 2. Spectroelectrochemical experiments and theoretical studies were performed to rationalize the electrochemical reductions occurring and to predict the redox potential and pathway of CO 2 reduction. The electrochemical behavior of Ni(II) complexes with the ligand tris(2-pyridylmethyl)amine) was studied, in order to explore its ability to reduce electrochemically CO 2. Theoretical studies allowed to rationalize where the electrochemical reductions occur and predict redox potential and the pathway of CO 2 reduction. [Display omitted] [ABSTRACT FROM AUTHOR]