Hydrogen storage of advanced materials – carbon nanotubes & porous silicon

In this work, multi-walled carbon nanotubes and porous silicon thin films were examined for their physical and hydrogen storage properties. Microwave irradiation was employed to induce defects on multi-walled carbon nanotubes, aiming to improve their hydrogen storage properties by introducing active sites. The multi-walled carbon nanotubes were irradiated under two different atmospheres: in air and under vacuum. The structural changes after irradiation were analyzed by scanning electron microscopes and transmission electron microscopes, and tube-welding and peel-off of carbon clusters have been observed. Defects such as amorphous layers and dislocated graphene layers have formed along the tube walls. It was found from both gravimetric and volumetric analyses that the hydrogenation of multi-walled carbon nanotubes was significant improved after microwave irradiation. Neutron diffraction has been employed to investigate the preferred adsorption sites for hydrogen molecules in multi-walled carbon nanotubes after irradiation. The results suggest that hydrogen molecules tended to be stored in the cavities of the benzene rings rather than in between the tube interlayers. Under the given microwave irradiation conditions, the amount of defects introduced in multi-walled carbon nanotubes was dependent on the tube diameter. The results showed 10 min irradiation duration would introduce the optimum amount of defects for hydrogen storage in 20-40nm multi-walled carbon nanotubes with a maximum hydrogen uptake of 0.87wt%. Too long the irradiation time will introduce excessive defects in multi-walled carbon nanotubes, which would deteriorate the hydrogenation properties. This value for 10min, of irradiation is considered to be relatively high as compared to the recent reported values on multi-walled carbon nanotubes. Thus, microwave irradiation on multi-walled carbon nanotubes has been proven an easy and simple way to enhance the hydrogen storage properties of carbon nanotubes. For porous silicon thin films, the microstructure has been investigated; and the hydrogenation behaviors have been examined at different temperatures so as to explore their hydrogen storage mechanisms. The effects of oxide layer in porous silicon on hydrogenation properties have also been also investigated. A remarkable hydrogen uptake of at least 10wt% was achieved on fresh sample at 250oC by both gravimetric and volumetric analyses, which has by far exceeded the target set by U.S. DOE in 2015. However, it was assumed chemisorption was the primary mechanism for hydrogen storage in porous silicon. This further confirms the previous works in our group that pSi is an effective candidate for hydrogen storage. &#8195