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The performance of TiO 2 nanosheets (TiO 2 -NSs) modified carbon paper anode (TiO 2 -NSs/CP) in S. loihica PV-4 inoculated MFCs is greatly improved by intentional N doping of TiO 2 -NSs to modulate its electronic properties. TiO 2 -NSs/CP is calcined in NH 3 atmosphere at different temperatures (T = 400, 500, 600 and 700 °C) to obtain N doped anodes (T-NTiO 2 -NSs/CP, T represents temperature), in which 600-NTiO 2 -NSs/CP electrode show best performance with the maximum output power density increased by 196% and 50% compared to that from the bare CP and TiO 2 -NSs/CP electrodes respectively. The surface morphology, structure and electrochemical activity are characterized by FESEM, XRD, Raman, XPS, CVs and EIS. The results show that heating in NH 3 atmosphere keep surface morphology and bio-affinity of the electrode. The midpoint potential E m (−0.29 V) of outer membrane c-type cytochromes (OMCs) at 600-NTiO 2 -NSs/CP anode is in accordance with that at TiO 2 -NSs/CP. However, 600-NTiO 2 -NSs/CP electrode had smaller charge transfer resistance, more positive flat-band potential and larger transient charge storage capacity than untreated electrode due to modulation of the electronic structure via N doping, which all are favorable to interfacial electron transfer from OMCs to underlying electrode. The method is simply and efficient, and could be extended to other nano-semiconductor modified electrodes for superior MFC anodes. [ABSTRACT FROM AUTHOR]

• 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]