A study on the effects of Fex/Niy/MgO(1−x−y) catalysts on the volumetric and electrochemical hydrogen storage of multi-walled carbon nanotubes

The effects of various ratios of Fe/Ni/MgO and growth temperatures on yield, diameter and quality of multi-walled carbon nanotubes (MWCNTs) were studied. Thermal gravimetric analysis (TGA) confirmed that the MWCNT yield depends on Fe/Ni ratio with the following order; Fe0.5 Ni0.5 > Fe > Fe0.75 Ni0.25 > Fe0.25 Ni0.75 > Ni. The results indicated that there is an optimum temperature (940 °C) for the MWCNT growth both from quality and quantity (yield) aspects as compared to other temperatures. Moreover, the changes on Fe/Ni to MgO ratio for the MWCNT growth revealed that Fe/Ni/MgO with the ratio of 17.5/17.5/65 had the highest quality and surface area as compared to the other ratios. The hydrogen storage capacities of MWCNTs grown on Fe/Ni/MgO with various ratios obtained by using volumetric technique were in ascending order as 17.5/17.5/65, 15/15/70, 12.5/12.5/75, 10/10/80, 20/20/60, 22.5/22.5/55 and 25/25/50. In addition, the defective sites and mean diameter of the MWCNTs influenced the desorption temperature of stored hydrogen. Hydrogen storage by using electrochemical technique showed that Fe/Ni/MgO with the ratio of 17.5/17.5/65 had the highest hydrogen storage capacity compared with the other ratios. Based on electrochemical analysis, there are two regimes for hydrogen adsorption on the MWCNTs, one at about 0.8 V and the other at 0.15 V. The study on two kinds of adsorption region showed that the ratio of hydrogen storage capacity at 0.8 V to hydrogen storage capacity at 0.15 V increased with the increasing of the mean diameter of MWCNTs. The ratio reached to maximum value for the MWCNTs grown on Fe/Ni/MgO with the ratio of 20/20/60 as compared to the other ratios.