Hydrogen Adsorption in Silicon-Carbide Nanotubes Doped with Potassium and Titanium
Seyed Hamed Barghi, Theodore T. Tsotsis, and Muhammad Sahimi
Mork Family Department of Chemical Engineering & Materials Science, University of Southern California, Los Angeles, California 90089-1211
Silicon-carbide nanotubes were synthesized by the reaction between SiO vapor as the Si source and carbon nanotubes as the carbon precursor. The resulting nanotubes were then purified with a hot, concentrated NaOH solution in order to remove the amorphous silica from their surface. The purified silicon-carbide nanotubes were doped with either potassium (K) or titanium (Ti). The hydrogen adsorption isotherms of the nanotubes were measured gravimetrically with the aid of a magnetic suspension microbalance. The results indicate that the K-doped silicon-carbide nanotubes exhibit promise as a hydrogen adsorption material, storing three times more hydrogen than the precursor carbon nanotubes. The gravimetric hydrogen uptake measurements of K-doped SiCNTs reveal a significant increase in their hydrogen adsorption capacity, when compared with pure (un-doped) SiCNTs. An increase in the binding energy caused by the charge transfer from the metal impurity to the Si and C atoms is thought to be the reason for the superior performance of the doped nanotubes (The pure SiCNTs, themselves, are also capable of adsorbing a much higher amount of hydrogen, when compared with the CNTs, mostly due to the charge transfer from the Si atoms to the C atoms in the SiCNTs’ structure). However, the presence of adsorption sites with increased binding energy in the K-doped SiCNTs gives rise to desorption irreversibility. This means a higher temperature would be required to release the adsorbed hydrogen molecules, which is unfavorable from an energy storage point of view.
Interestingly enough, the Ti-doped SiCNTs show inferior performance than the pure SiCNTs (though still superior to that of CNTs). This contrasts prior molecular simulation studies that indicate that the doping with Ti should significantly improve the sorption capacity of the SICNTs. The reasons for this surprising finding are not entirely clear at this point and time (and should be further investigated), but the behavior is suspected to be due to the presence of an oxygen impurity introduced during the preparation of the materials. The additive hydrogen storage capacity (AHSC) of the pure and K-doped SiCNTs are positive throughout the region of pressures investigated. This is definitely an important improvement over the CNTs, for which their AHSC turns negative for pressures higher than 20 bar, implying that a hydrogen storage system working without any solid adsorbent is capable of storing more hydrogen than a CNT-based system (at room temperature).
Keywords: hydrogen storage, SiC nanotubes, doping, titanium, potassium