Enhancing Hydrogen Adsorption by Metal Incorporation in Carbon Multi-Wall Nanotubes Produced by Continuous Hot Wire Chemical Vapor Deposition

The hydrogen storage properties of carbon single-wall and multi-wall nanotubes (SWNTs and MWNTs), graphitic nanofibers, and other nanostructured carbons have recently become the subject of considerable debate. Reported capacities range from ∼ 0-60 wt%. Hot wire chemical vapor deposition (HWCVD) has recently been adapted for a continuous growth process for high-density carbon MWNTs. Multi-wall nanotube growth is optimized in 1:5 CH₄:Ar at 150 Torr with reactor temperatures of 400 and 550°C for static and flowing gases, respectively. Ferrocene is employed to provide a gas-phase catalyst. Highly graphitic nanotubes can be continuously deposited with iron content as low as 15 wt% and carbon impurities below thermal gravimetric analysis detection limits. The MWNTs are simply purified to ∼99.5 wt% with minimal structural damage and with a 75 wt% yield. Hydrogen adsorption is observed for low pressures at near ambient temperatures on the as-synthesized MWNTs containing iron nanoparticles. However, no hydrogen adsorption, is observed at near ambient temperatures for the purified MWNTs or for purified MWNTs that were subsequently combined with iron micro/nano-particles via sonication. These results indicate that an intimate metal/graphitic carbon interaction is required for unanticipated hydrogen adsorption at near ambient conditions.