A study on hydrogen desorption properties of multiwall carbon nanotubes

Abstract

Hydrogen desorption properties in multiwall carbon nanotubes(MWNTs) were investigated from the view points of not only physical hydrogen molecules adsorption in nano-hole but also chemical hydrogen adsorption on graphite surface. MWNTs with intrinsic closed structure having the blocked hole and MWNTs with open structure were studied through hydrogen thermal desorption technique equipped with gas chromatograph. In order to grow MWNTs with open structure, novel method using oxygen added plasma(CH4/H-2/O-2) has been introduced. The structure of MWNTs was gradually changed with increasing oxygen addition amount, showing aligned MWNTs bundles and open tube cap at 10% oxygen addition. The precise analysis on thermal desorption spectra on MWNTs with closed structure showed that hydrogen gas was released at three major temperature ranges such as 100-230K, 290-350K, and 600-625K, where the evolved hydrogen amounts were about 1.65wt%, 0.64wt%, and 0.03wt%, respectively. However, in case of aligned and open MWNTs, the evolution peak around 290-350K was highly developed (about 1.9wt%) and the desorption peak at 600-625K was disappeared. The hydrogen desorption activation energy was also calculated. The obtained hydrogen desorption activation energy of MWNTs with closed and open structure at ambient temperature was -18.5kJ/mol H-2 and -16.5kJ/mol center dot H-2, respectively. The hydrogen desorbed between 290 and 330K in MWNTs with open structure was the hydrogen physi-sorbed in nano-hole. And Ni-doping (dispersion) effect on hydrogen storage properties was investigated comparatively. Ni nano-catalysts were expected to effectively dissociate hydrogen molecules in gas phase, providing atomic hydrogen possible to form chemical bonding on carbon surface. The metal nanoparticles were homogeneously dispersed using incipient wetness impregnation method. Hydrogen desorption spectra of MWNTs with Ni nanoparticles showed that about 2.8wt% of hydrogen was released in the range of 340-520K.