Electrochemical performance of metal oxide dooped multiwalled carbon nanotubes

Abstract

The study has focused on the effects of different acids treatments on the nanostructure of MWCNTs; doping metal oxides (copper oxide (CuO), Iron (III) oxide (Fe2O3), nickel oxide (NiO) and cobalt oxide (Co3O4)) on MWCNTs and investigates their electrochemical hydrogen and energy storage capabilities. Fourier transform infrared (FTIR) confirmed the formation of functional groups on the surface of the acid treated MWCNTs. X-ray diffraction (XRD) showed that the graphitic structure of the MWCNTs was retained after treatment with mild acids (nitric acid (HNO3), hydrogen peroxide (H2O2), a mixture of the acids, hydrogen peroxide: nitric acid (H2O2:HNO3) and hydrogen peroxide: sulfuric acid (H2O2:H2SO4)). Transmission electron microscopy (TEM) confirms the removal of bamboo carbon structures inside the inner tubes of the MWCNTs after treatment with mild acids. Brunauer-Emmet- Teller (BET) showed an increase in the surface area of mild acids treated MWCNTs. Thermogravimetric analysis (TGA) results demonstrated that the thermal stability of MWCNTs increases after treatment with mixtures of the acids. Different metal oxides treated at different temperatures were incorporated into MWCNTs (treated by a mixture of H2O2:HNO3). X-ray diffraction (XRD), scanning electron microscopy (SEM) and Transmission electron microscopy (TEM) confirmed the presence of different metal oxides inside/on the surface of the acid treated MWCNTs. The MWCNTs treated by H2O2:HNO3 gave both the highest discharge capacity (72.63 mAh/g) and capacitance (8.61 F/g), as compared to the other electrode materials. The improved hydrogen storage capacity and specific capacitance can be attributed to high surface area, wider pore size distribution and the amount of functional groups on the surface of H2O2:HNO3-treated MWCNTs; with the functional groups acting as electron transmitters. The 5wt.% CuO@300oC-MWCNTs composite showed the highest hydrogen storage capacity of 159 mAh/g. This capacity was further improved by addition of manganese oxide resulting in the highest discharge capacity of 172 mAh/g (which is equivalent to 0.64 wt.% of hydrogen stored). The highest specific capacitance of 9.70 F/g was obtained on 5wt% Fe2O3@400oCMWCNTs composite.