Hydrogen desorption properties of multiwall carbon nanotubes with closed and open structures

The multiwall carbon nanotubes (MWNTs) synthesized by microwave plasma enhanced chemical vapor deposition in $\text{0.1\%\hspace{0.5em}}{\mathrm{CH}}_4$ $({\mathrm{H}}_2$ dilution) reaction gas were shaped in curly structures with the blocked nanoholes and closed caps. On the contrary, the carbon nanotubes with different structures could be obtained in oxygen loaded reaction gas, showing aligned structures with connected holes and open caps. The hydrogen desorption properties of the carbon nanotubes (CNTs) with these closed and open structures were compared by thermal desorption technique. The MWNTs with closed structure desorbed hydrogen at two different temperature ranges such as 290–330 and ∼420 K, where the evolved hydrogen amount were ∼0.64 and ∼0.03 wt %, respectively. In case of aligned and open MWNTs, hydrogen as high as ∼1.97 wt % was released at ambient temperature. The hydrogen desorption of MWNTs with open and closed structure at ambient temperature showed the hydrogen desorption activation energies of $\text{−16.5\hspace{0.17em}}\mathrm{kJ}/\mathrm{mol} {\mathrm{H}}_2$ and $\text{−18.5\hspace{0.17em}}\mathrm{kJ}/\mathrm{mol} {\mathrm{H}}_2,$ respectively. The high temperature hydrogen desorption observed only in CNTs with closed and highly defective structure was high desorption activation energy of −124.4 kJ/mol.