Effects of cryomilling on the structures and hydrogen storage characteristics of multi-walled carbon nanotubes

Cryomilling was performed on multi-walled carbon nanotubes (MWCNTs) to investigate the effect of cyomilling on the structures and hydrogen storage characteristics of MWCNTs. Two milling speeds (300 and 700 rpm) and two milling times (2 and 6h) were applied in the cryomilling process. The results showed that the agglomeration of MWCNTs was significantly reduced, their lengths were shortened and the crystalline structure became amorphous at higher milling speed, whereas the milling time had no significant effect on the dispersibilities or structures of the MWCNTs. The hydrogen adsorption capacities of cryomilled MWCNTs at 700 rpm were improved by approximately 22% compared to that of unmilled MWCNTs due to the improvements in the specific surface area (17.4%) and pore volume (34.9%). Cryomilling is an effective method for increasing the surface area and pore volume, and macropores were transformed into mesopores, thereby enhancing the hydrogen storage capacity of the MWCNT surface. These results were confirmed by scanning and transmission electron microscopies, X-ray diffraction and Raman spectroscopic analysis.