Hydrogen storage in carboneous materials through the formation of carbon-hydrogen bonds

In order to determine if carbon-based materials can be used as a hydrogen storage medium for the on-board applications, we have studied hydrogen chemisorption in single-walled carbon nanotubes (SWCN). Using X-ray Photoelectron Spectroscopy (XPS) and X-ray Adsorption Spectroscopy (XAS), we demonstrated that in situ atomic hydrogen treatment of SWCN film causes the formation of the C-H bonds between hydrogen atoms and carbon atoms at the SWCN surface. The changes in the XPS C1s peak shape that occur due to the formation of C-H bonds directly indicate that the maximal degree of the SWCN material hydrogenation strongly depends on the SWCN diameter and chirality distribution. For the SWCN with an average diameter of approximately 1.6 nm, the nanotube decomposition begins at 30% hydrogenation rate while for SWCN with a diameter of around 2.1 nm, nanotube--hydrogen complexes with close to 100% hydrogenation are found to exist and are stable at room temperatures. This means that specific carbon nanotubes can have a hydrogen storage capacity of more than 7 wt% through the formation of the stable C-H bonds that exceeds the DOE FreedomCAR program 2010 goal (6 wt%). Using temperature dependant XPS, we have also found that most of the C-H bonds formed on the nanotube surface dissociate in the temperature range between 200°C and 300°C.; To understand the local changes of SWCN structure due to hydrogenation, we studied the atomic hydrogen interaction with the graphite surface which can be considered as a nanotube with an infinite diameter. Using XAS to probe the directions of C-H bonds in the direct space and ab initio Density Functional Theory (DFT) modeling, we found that H prefers to form dimers at the graphite surface and that the C-H bond formation causes the buckling of C-H bonded carbon atoms in the top graphite layer.; DFT modeling showed that hydrogen prefers to form two different types of dimers at the SWCN surface and that the C-H bond energy depends on the SWCN structure. Using high throughput computation screening of different SWCN, we found that the C-H bond energy has two contributions: E1 which is due to a local structure (curvature) of the SWCN surface and E2 which is due to the features in the SWCN electronic structure. While E 1 is defined by the angle between pi* molecular orbitals of neighboring C atoms and depends linearly on 1/d where d is the SWCN diameter, E2 is defined by the structure of the SWCN electronic density of states close to the Fermi level controlled by the electron gas confinement along nanotube circumference. As a result, the metallic SWCNs are much more reactive than the semiconducting nanotubes. The results of the calculations also helped to suggest the hydrogen desorption mechanism and define the range of the SWCN diameters with the energetics of C-H bonds that are favorable for the hydrogen storage applications.