Jun 11, 2009
Inner workings of MgH2-SWCNT nanocomposites revealed
Rather than acting as storage media, single-walled (SW) and multi-walled (MW) carbon nanotubes (CNTs) are increasingly being used to improve the sorption kinetics of magnesium and other hydride-forming materials. However, their effectiveness remains a talking point. Many studies have reported that CNTs work very well in enhancing the kinetics, while others disagree. Numerous papers are available on the subject, with little consensus having emerged.
A large part of this controversy has stemmed from the complexity of the system. The ultimate sorption behaviour depends on multiple microstructural parameters. The distribution, morphology and defectiveness of the nanotubes evolve during mechanical milling, as does the hydride's particle size, grain size and degree of strain. To complicate matters further, CNTs used for hydrogen storage studies are still attached to nanoscale metallic particles that were used as catalysts for their growth.
For cases that involve complex microstructures, conventional transmission electron microscopy (TEM) analysis is often used to site-specifically examine the materials. However, metal hydrides are extremely electron beam sensitive. It is taken as "common knowledge" that it is impossible to directly analyse mechanically milled magnesium hydride powders by this technique without transforming the material back to a metal or an oxide.
This study was about answering several fundamental questions regarding the microstructure of both the magnesium hydride powders and of the composite system. We wanted to find out how high-energy mechanical milling affects the structure of the magnesium hydride and what happens to the composite. Of course, we also wanted to correlate the observed microstructures to the hydrogen sorption kinetics. We chose an approach that has surprisingly received little attention in regards to the analysis of hydrides: cryogenically cooled sample stage TEM. The cryostage kept the samples at 90 K and allowed us to directly analyse these sensitive materials while incurring minimal electron beam-induced artifacts.
A nanocomposite of SWCNTs and α-MgH2 was created by high-energy co-milling for various durations. We found that SWCNT-metallic nanoparticle additions catalyze desorption of hydrogen, lowering the activation energy by as much as 25 kJ/mol over those of identically milled pure MgH2 powders. These optimum desorption properties are achieved when the SWCNTs are well dispersed, contain defects and are in intimate contact with the metallic nanoparticles. Insufficient milling leads to more intact SWCNT structures and a non-optimum dispersion, resulting in less impressive catalytic behaviour. In the over-milled state the SWCNTs are completely destroyed and the enhancement is largely lost despite the continued presence of the metallic particles of the same scale as in the as-received materials. Such strong dependence of the catalytic performance on the system microstructure can generally explain the inconsistencies in the previous scientific literature on the subject.
We also used cryostage TEM analysis to confirm that the majority of the hydride powder particles were polycrystalline, with nanoscale grain sizes. TEM revealed that the larger hydride crystallites contain a high population of nano-twins; an observation previously unreported in the literature. Such defects are an integral component of the hydride-to-metal phase transformation, influencing the kinetics and perhaps the thermodynamics of the process. Our future work will focus on developing an understanding of how SWCNTs affect the long-term adsorption/desorption cycling stability of metal hydrides, and on further elucidating the role of deformation twins in the hydride-to-metal phase transformation.
The researchers presented their work in Nanotechnology.
About the author
Babak Shalchi Amirkhiz is a PhD student in the Department of Chemical and Materials Engineering at the University of Alberta. He received his Master's degree in materials engineering from Sharif University of Technology (Iran) in 2006 and started his work at Alberta in the same year. His research is focused on development of nanostructured metal-hydride composites for hydrogen storage applications. Mohsen Danaie is a PhD student in the Department of Chemical and Materials Engineering at the University of Alberta. He received his Master's degree in materials engineering from Sharif University of Technology (Iran) in 2005 and started his work at Alberta in the following year. His research is focused on TEM analysis of hydrogen-induced phase transformations. David Mitlin is an associate professor in the Department of Chemical and Materials Engineering at the University of Alberta and a principal investigator at the National Institute for Nanotechnology. Prior to joining the University of Alberta in 2004, Dr Mitlin was at Los Alamos National Laboratory (USA) where he held a Directors Funded Post Doctoral Fellowship. Dr Mitlin received his Doctorate from UC Berkeley in 2000.