Experimental Investigation of Hydrogen Storage in

Silicon Carbide Nanotubes

SeyedHamed Barghi, Theodore T. Tsotsis, and Muhammad Sahimi† 1

Mork Family Department of Chemical Engineering & Materials Science, University of Southern

California, Los Angeles, California 90089-1211, United States

Abstract

The experimental results of hydrogen storage in silicon-carbide nanotubes (SiCNTs) are reported in this work for the first time. The SiCNTs are produced by gas-solid reaction between silicon monoxide (SiO) vapor and carbon nanotube (CNT) precursors at temperatures between
1200 -1500 , which is the temperature range used in most of the previous studies for SiCNTs synthesis. The SiO vapor was produced by a solid-solid reaction between silicon (Si) and silicon dioxide (SiO2) during the SiCNTs synthesis process. A purification process based on sodium hydroxide was then carried out on the as-synthesized SiCNTs. The purpose of the purification process was to remove the side products of the synthesis reactions from the surface of the SiCNTs. Using a novel gravimetric method coupled with a residual gas analyzer, the hydrogen uptake behavior of the as-synthesized SiCNTs and also the purified SiCNTs was then studied at room temperature and at pressures of up to 100 bar.
According to the experimental results, the hydrogen storage capacity of the as-synthesized SiCNTs is comparable to that of the CNT precursors. On the other hand, the purified SiCNTs show a 50% higher hydrogen storage capacity. Moreover, the hydrogen uptake occurs five time faster in the SiCNTs compared to the CNTs. The hydrogen desorption is also completely reversible in the SiCNTs, but this is not the case for the CNTs. SiCNTs present a superior alternative to CNTs, not only for hydrogen storage, but potentially for other applications at high temperatures, often used during hydrogen production, because of (i) the excellent mechanical

†

Corresponding author. Tel.: +1 (213)740-2064 E-mail address: moe@usc.edu
properties of the SiCNTs; (ii) their resistance to harsh corrosive conditions in the presence of high-temperature steam.
Among the as-synthesized SiCNTs, the ones synthesized at 1200 showed the highest hydrogen uptake capacity, whereas those that were prepared at 1500 possessed the lowest capacity. This difference in hydrogen storage capacity is mostly due to the differences in their SiO2 impurity content (The presence of the amorphous SiO2 impurity was confirmed by FTIR analysis). The evaporation of the unreacted SiO2 is the source of this impurity. Since the vapor pressure of SiO2 is lower at lower temperatures, the SiCNTs prepared at the lower temperatures contain less impurity as well. The SiO2 impurity has a negative effect on the hydrogen storage capacity of the SiCNTs because:
(i) It acts as a barrier between hydrogen and SiC.
(ii) It increases the diameter of the nanotubes, thus, decreasing their specific surface area.
(iii) Plugs the ends of the SiCNTs, thus preventing the hydrogen molecules from having access to the inner pores of the SiCNTs.

Keywords: hydrogen storage, SiC nanotubes, sorption, carbon nanotubes