Hydrogen storage in carbon nanotubes and their composites
This project investigates the rational design and development of novel nanomaterials for on-board practical hydrogen storage in fuel cell electrical vehicles (FCEV). Carbon nanotubes (CNTs) have a unique tubular structure that facilitates hydrogen diffusion and transport. CNTs can significantly enhance the hydrogen storage properties in materials that suffer from disadvantages such as high-temperature charging and slow kinetics. The high re/dehydrogenation temperature and slow rate are the major limitations for Mg-based materials for hydrogen storage. We are investigating the effects of CNTs (purified or as prepared single-wall and multi-wall CNTs) on hydrogen diffusion, interactions of C-H-Mg and doped metals-H-Mg and hydrogen ad/desorption behaviour in order to understand the catalytic mechanisms of re/dehydrogenation in CNTs (doped with catalytic metals) enhanced Mg-based nanocomposites.
Major achievements
A newly developed nanostructure, Pd/Nb2O5/CMK-3, well-dispersed with a size of ~4nm, shows a superior catalytic effect on Mg hydrogenation. With 5wt% loading and only 10h milling, the sample exhibits extremely fast kinetics and high absorption capacity (up to 6 wt%) at 300 and 200°C. Importantly, the samples show good absorption of hydrogen at temperatures as low as 100°C, e.g. achieving 4.2wt%H. This superior property is significant for practical hydrogen storage using Mg materials. The benefits of milling are:
A smaller amount (half) of the catalyst can be used, resulting in a significant increase in the theoretical capacity and a reduced cost of materials.
The fabrication time of the materials is dramatically decreased (~10 times), due to the fine nanoparticles being dispersed on the carbons.
The metals and the carbons have synergistic effects for improving both the capacity and the kinetics.
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The fine nanoparticles of the catalyst are more effective for hydrogen dissociation at a very low temperature, resulting in significant hydrogen absorption at 100°C.
We also made a significant achievement in the theoretical study on the hydrogenation mechanism of Mg with Pd. DFT calculation reveals that the spillover of hydrogen on Pd sites greatly enhances the hydrogen transfer and thus improves hydrogenation. The low energy barrier of this processing enables the hydrogenation at a very low temperature, e.g. at room temperature, in the Mg-Pd system. In addition, unlike Ti, the bond formed between the dissociated hydrogen atom combined and the Pd atom is not too difficult to break, which is beneficial for further atomic hydrogen diffusion.
Research highlights in 2007
An Australian patent application has been made for the synthesis method and the nanostructures of the newly developed catalysts (Yao, Lu et al., pre-application 2007903489).
The theoretical study on the Pd spillover mechanism has been published in J Am. Chem. Soc. (Du et al, 2007, 129, 10201). A breakthrough for hydrogen absorption in Mg-VTi-CNTs system has been achieved. The work has been published in J Am. Chem. Soc. (Yao et al, 2007, 129, 15650).
A diffusion model for atomic hydrogen diffusion within Mg hydrides has been developed. The model firstly calculated the hydrogen diffusion coefficients in Mg hydrides and accordingly the grain size effect on hydrogenation has been predicted. This work has been published in J Mater. Res. (Yao et al, 2008, 23, 336)
Future plans and directions
• Further optimise the Mg-catalyst-nanocarbon systems to improve the hydrogenation properties, focusing on the hydrogen desorption of Mg.
• Further theoretical and experimental study on re/dehydrogenation mechanisms.
Collaborations
Collaborator |
Organisation |
Prof Hui-Ming Cheng |
Chinese Academy of Sciences |
Prof Yinghe He |
James Cook University |
