Hydrogen storage properties in carbon nanotubes(CNTs) were investigated from the view points of not only physical hydrogen molecules adsorption in nano-hole but also chemical hydrogen adsorption on graphite surface. As usually observed in CNTs grown by CVD at general conditions, CNTs synthesized at our optimized condition by microwave PECVD methods showed intrinsic closed structures, which mean the blocked hole and entry induced by highly defective layers and tube cap. In order to grow CNTs with open structure that could accomodate more hydrogen, novel method using oxygen added plasma $(CH_4/H_2/O_2)$ has been introduced. The structure of CNTs gradually changed with increasing oxygen addition amount, showing aligned CNTs bundles and open tube cap at 10%O addition. The pore size distribution measured by liquid nitrogen adsorption displayed that when oxygen were added moderately as reactive gases, nano-pore of 7-8nm, identical size with nano-hole, account for the largest volume. These results implied that the growth of CNTs in oxygen plasma phase was effective to obtain open structure available for physical hydrogen gas adsorption. In plasma diagnostics by Optical Emission Spectroscopy(OES), OH radical increase abruptly and $C_2$ decrease gradually with increasing oxygen addition amount. these changes in plasma phase could suppress carbon super-saturation and activate the etching effect on defective structure during CNTs growth. The hydrogen storage properties were studied through hydrogen thermal desorption and compared between CNTs with closed structure and open CNTs. The precise analysis on thermal desorption spectra on CNTs with closed structure showed that hydrogen gas was released at three major temperature ranges such as 100-230K, 290-360K, and 600-625K, where the evolved hydrogen amount were ∼1.65wt%, ∼0.64wt%, and ∼0.03wt%, respectively. However, in case of aligned and open CNTs, the evolution peak around ambient temperature was highly developed(∼1.9wt%) and high temperature peak disappeared. When compared with carbon nanofiber(CNFs) and high surface area graphite(HSAG), the sub-ambient temperature and ambient temperature peaks were related with hydrogen adsorption on surface and in nano-hole, respectively. Hydrogen desorption activation energy of CNTs was estimated to be $-16.5kJ/mol.H_2$ by kissinger`s plot. The high temperature hydrogen desorption behavior that was observed only in closed and highly defective CNTs was connected with C-H bonds on imperfect carbon structure formed during CNTs growth stage. In physical hydrogen adsorption in nano-hole, the higher hydrogen storage capacity was obtained at open CNTs by oxygen plasma etching effect. As the second studies on hydrogen chemical adsorption, Ni-doping(dispersion) effect on hydrogen storage properties were investigated comparatively. The metal nanoparticles were homogeneously dispersed using incipient wetness impregnation procedure. Ni nano-catalysts were expected to effectively dissociate hydrogen molecules in gas phase, providing atomic hydrogen possible to form chemical bonding on carbon surface. Hydrogen desorption spectra of MWNTs with Ni nanoparticles showed that ∼2.8wt% hydrogen was released in the range of 340-520K. In kissinger`s plot to evaluate the nature of interaction between hydrogen and MWNTs with Ni nanoparticles, the hydrogen desorption activation energy was measured to be as high value as $∼31kJ/molH_2$, which is far higher than the estimates of pristine SWNTs. In FT-IR analysis to observe chemical bonding on CNTs surface and identify the chemi-sorption sites, C-Hn stretching vibrations(strong absorbance band at $2915cm^{-1}$, relatively weak band at $2854cm^{-1}$) after hydrogenation were observed at Ni nanoparticles-dispersed CNTs. If the absorbance bands were matched up to the expected configuration, single hydrogen bonded sp3 and di-hydride sp3 configuration were possibly considered and these configurations corresponded to hydrogen adsorption on graphite surface and edge of CNTs. During cyclic hydrogen absorption/desorption, there was no significant decay in hydrogen desorption amount. The hydrogen chemi-sorption process facilitated by Ni nanopaticles could be suggested as the effective reversible hydrogen storage method.
