Effect of functionalization and Li-doping methods to hydrogen storage capacities of MWCNTs

Abstract

One of the most important problems of today has been found alternative and renewable energy sources to fossil fuels. Hydrogen comes to the fore because it is abundant, does not cause environmental problems, and can be stored when not in use. Furthermore, compared to fossil fuels, hydrogen has the highest energy density. However, the most important obstacle to the use of hydrogen energy is its transportation and storage. In this study, the functionalized multi-walled carbon nanotubes were first produced from multi-walled carbon nanotube (MWCNT) by different chemical reactions and, then Li-doped MWCNTs and Li-doped functionalized MWCNTs by solvent and hydrothermal methods. Samples were characterized using Brunner–Emmett–Teller (BET), Fourier transform infrared spectroscopy (FTIR), differential thermal analysis/thermogravimetry (DTA/TG), electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy. Finally, the hydrogen storage capacities of all samples were measured using IMI PSI gas storage instrument. New bands belonging to functional groups were observed in the FTIR-ATR spectrum of MWCNT. Functionalized MWCNTs degraded at different temperatures and steps. SEM images showed that the changes in the morphology of MWCNT with functionalization and Li-doping occured. The linear behavior observed in the Nyquist plots showed that the electrochemical process was controlled by ion diffusion. Raman analysis showed that while there was no significant change in the position of the Raman bands. While the BET surface area of MWCNT decreased with functionalization, an increase in its hydrogen storage capacity was observed in general. The hydrothermal method was more effective than the solvent method and functionalization in increasing the hydrogen storage capacity of MWCNT. All samples had higher hydrogen storage capacity at cryogenic temperatures. Li-doped MWCNT by hydrothermal method had the highest hydrogen storage capacity at 80 bar and 77 K, and its storage capacity value was 2.85 wt%. While the isotherm curves obeyed Henry's law at room temperature, they exhibited multilayer adsorption behavior at cryogenic temperatures. The adsorption process was physical and isotherm type was Type IV.