|Modeled snapshots and gravimetric adsorption capacities of hydrogen in the silicon nanotube arrays (SiNT), top; and carbon nanotubes (CNT), bottom. T = 298 K and P = 2, 6, and 10 MPa. Click to enlarge.|
Researchers at the Beijing University of Chemical Technology (BUCT) have determined that silicon nanotubes can store hydrogen more efficiently than carbon nanotubes. Their study is published in the ACS’ Journal of Physical Chemistry C.
The paper is one of the latest in a growing set of research seeking to leverage nanotube structures for hydrogen storage. Although work on the hydrogen storage potential of carbon nanotubes has been underway since 1997, most efforts using those materials have failed to reach the US Department of Energy (DOE) target of 6 wt% for commercial application.
In the BUCT study, Dapeng Cao and his colleagues used a multiscale theoretical method, combining first-principle calculation and a grand canonical Monte Carlo (GCMC) simulation, to predict the adsorption capacity of hydrogen in silicon nanotube (SiNT) arrays at 298 K (24.85°C, 76.73°F) under pressures ranging from 1 to 10 MPa. In the multiscale method, the binding energy obtained from the first-principle calculation is used as an input in the GCMC simulation.
|Gravimetric adsorption isotherms of hydrogen. Click to enlarge.|
From the first-principle calculation, they found that the SiNT arrays exhibit much stronger attraction to hydrogen both inside and outside SiNTs, compared to isodiameter carbon nanotubes (CNTs). The subsequent GCMC simulations indicated that gravimetric adsorption capacities of hydrogen in the SiNT arrays reach up to 1.30, 2.33, and 2.88 wt% at 2, 6, and 10 MPa, respectively. This represent improvements of 106%, 65%, and 52% in the gravimetric adsorption capacity of hydrogen at P = 2, 6, and 10 MPa, respectively, compared to the isodiameter CNTs.
A paper published in 2006 in the ACS journal Nano Letters by a team from the University of Crete and the University of Athens used a multiscale theoretical approach to investigate hydrogen storage in silicon-carbon nanotubes (SiCNTs).
Ab initio calculations at the density functional level of theory (DFT) showed an increase of 20% in the binding energy of H2 in SiCNTs compared with pure carbon nanotubes (CNTs). Classical Monte Carlo simulation of the nanotube bundles showed an even larger increase of the storage capacity in SiCNTs, especially in low temperature and high-pressure conditions.
At 1, 5 and 10 MPa at 175 K (-98°C, -145°F), the SiCNT bundles showed 1.18, 2.82 and 3.68 wt% respectively, compared to 0.53, 1.92 and 3.03 wt% respectively for the CNT bundles.
Another paper published earlier this year by another China research team at the National University of Defense Technology synthesized SiCNTs from multiwalled carbon nanotubes (MWCNTs) via chemical vapor reaction (CVR) and purification. The SiCNTs were characterized by XRD, SEM and TEM. Hydrogen storage capacities measurements indicated that SiCNTs were superior to MWCNTs.
In reference to the work on silicon-carbon nanotubes, the BUCT researchers wrote:
Mpourmpakis et al.  reported the hydrogen adsorption capacity increases of the square SiC tube array by 46% and 21% at pressures of 5 and 10 MPa and T = 175 K, respectively, compared to that of the isodiameter CNTs. Obviously, our calculation indicates that the SiNTs present a higher hydrogen adsorption capacity than SiC nanotubes.
Jianhui Lan, Daojian Cheng, Dapeng Cao, and Wenchuan Wang; Silicon Nanotube as a Promising Candidate for Hydrogen Storage: From the First Principle Calculations to Grand Canonical Monte Carlo Simulations; J. Phys. Chem. C, 112 (14), 5598 -5604, 2008. DOI: 10.1021/jp711754h
Rong-an Hea, Zeng-yong Chub, Xiao-dong Li and Yong-min Si; Synthesis and Hydrogen Storage Capacity of SiC Nanotube; Key Engineering Materials Vols. 368-372 (2008) pp 647-649
Giannis Mpourmpakis, George E. Froudakis, George P. Lithoxoos, and Jannis Samios; SiC Nanotubes: A Novel Material for Hydrogen Storage; Nano Lett., 6 (8), 1581 -1583, 2006. DOI: 10.1021/nl0603911