(256e) Fundamental Structural Properties of Single-Walled Carbon Nanotubes: Reopening the Debate over Hydrogen Storage

Hydrogen adsorption in nanotubes has been greatly studied all over the world, and results are not as promising as those predicted earlier. Here, we have discovered a fundamental structural property of single-walled nanotubes (SWNTs) that is extremely relevant to adsorption and that could reopen the debate over nanotubes possessing sufficient volume to be ever used for efficient storage of hydrogen.

We have invented a procedure to characterize the fraction of open-ended SWNTs in purified samples of nanotubes by combining molecular simulation with sample morphology and standard N2 adsorption technique (77 K) that is commonly employed to measure BET surface area and pore volume of an adsorbent sample (Agnihotri et al., Langmuir 2005). Simulations of nitrogen adsorption were performed on the external and internal surface of homogenous arrays of SWNTs of diameters previously determined from Raman scattering of test samples. Internal adsorption was adjusted by a scaling parameter representing fraction of open nanotubes and was added to the external adsorption to calculate overall adsorption capacity. Near perfect replication of experimental N2 adsorption isotherms was obtained which demonstrated the versatility of our simulation procedure and the soundness of our methodology to characterize the fraction of open nanotubes in samples.

Results for a 95-98 wt% SWNT sample showed that only 45% nanotubes were open-ended, which literally meant that more than half of the sample would be redundant for adsorption. The total adsorption capacity, therefore, would be at least twice as much as that observed via experiments. The BET surface area of the sample was 500 m2/g; however, extrapolating to a hypothetical scenario of all nanotubes being open and thus able to utilize the available porosity will lead to an upper limit of 1,200 m2/g surface area for this sample. This discovery has profound global implications, especially for alternative transportation technologies as it is commonly believed that the DOE target for H2 capacity of 6.5% volume cannot be achieved with the current structural configuration of carbon nanotubes. This study demonstrates that the volume storage capacity of nanotubes is undermined and can be greatly enhanced.