Session U35: Focus Session: Hydrogen Storage IV: Theoretical Predictions

Optimization of metal dispersion and hydrogen adsorption strength in doped graphitic materials

The non-covalent hydrogen binding on transition metal atoms dispersed on carbon clusters and graphene is studied with the use of the pseudopotential density functional method. It is found that the presence of acceptor-like states in the absorbents is essential for enhancing the metal-absorbent binding strength and for increasing the number of hydrogen molecules attached to the metal atoms. Particular configurations of boron or nitrogen substitutional doping are found to be very efficient for providing such states and thus enhancing storage capacity. Optimal doping conditions are suggested based on our calculations for the binding energy and ratio between metal and hydrogen molecules.   

Determination of best models for adsorption of hydrogen in boron-doped carbon nanopores

Nanoporous carbon offers significant hydrogen storage capacities at low pressure and reversible conditions. Storage is achieved by physical adsorption of molecular hydrogen (H$_{2}$) on the surface of nanometer-size pores in the carbon matrix. Within the ALL-CRAFT collaboration (http://all-craft.missouri.edu), we conduct a proof-of-concept study of the prediction that boron-doped nanoporous carbon can store as much as 8 weight{\%} at 47 bar and room temperature. By comparing theoretical and experimental H$_{2}$ adsorption isotherms for intrinsic and doped carbon, we determine which adsorption models and scenarios (e.g.\ localized vs.\ mobile) are consistent with experimental evidence.   

Metal clustering and catalytic spillover on the nanotubes and graphene for hydrogen storage

Energies and kinetic barriers associated with transition metal (Sc) clustering on a single-walled carbon nanotube (SWNT) and graphene were studied by all-electron density functional method. The analysis shows that the binding energy of Sc atom on SWNT is highly sensitive to the tube diameter and chirality. The metal atoms do cluster on common SWNT, with diameters $\sim $1-2 nm. Hydrogen binds to the metal cluster chemically and thus opens a way for hydrogen storage via catalytic spillover. However the hydrogen chemisorption on graphene receptor-substrate is difficult to reconcile with a single H atom binding to carbon being weaker than it is within initial molecular H$_{2}$. This paradox is resolved by presenting the process as phase nucleation. Atomistic calculations bridge remarkably with the macroscopic-continuum description, and show a feasible path to 7.7 wt{\%} H-content at nearly ambient conditions. P. O. Krasnov, F. Ding, et.al., J. Phys. Chem. C, in press (2007).   

Hydrogen Storage in Ti Doped Nano Porous Graphene

Clustering of Ti on carbon nanostructures has proved to be an obstacle in their use as hydrogen storage materials. Using density functional theory we show that Ti atoms will not cluster when doped into nanoporous graphene. With each Ti atom binding up to four hydrogen molecules with an average binding energy of 0.54 eV/H2, this material can be ideal for storing hydrogen. Equally important, nanoporous graphene is magnetic with or without Ti doping, but magnetism disappears when fully saturated with hydrogen. This novel feature suggests that nanoporous graphene can also be used as a hydrogen sensor.   

Graphenic C$_3$N$_4$: A New Template for Metal Decoration and Hydrogen Adsorption

From density functional theory calculations we identify a graphenic C$_3$N$_4$ (g-C$_3$N$_4$) structure as an excellent template for stable and well dispersed decoration of alkali and transition metal atoms which, in turn, exhibits a high capacity for hydrogen adsorption with binding energies (a few tenths of eV) suitable for mobile applications. The unique porous micro-structural sites of g- C$_3$N$_4$ accommodate the excessive N lone-pair electrons and promote strong hybridization between the orbitals of N and metal atoms. It plays a key role in overcoming the tendency of metal-atom clustering that has plagued other proposed hydrogen storage media. These metal decorated g- C$_3$N$_4$ may also prove useful in a variety of catalytic and sensing applications.   

Hydrogen Storage in Titanium-decorated Boron Buckyball

Using first-principles electronic structure calculations, we investigate the potential of Ti-decorated B$_{80}$ for hydrogen storage medium. The Ti-decorated B$_{80}$ has the merit of an unexpected large binding energy of a Ti atom to B$_{80}$ which can overcome the problem of metal clustering. Up to four hydrogen molecules are found to be adsorbed on a single Ti atom coated on B$_{80}$. At high Ti coverage, we show that the Ti-decorated B$_{80 }$can adsorb up to 5 wt{\%} hydrogen and the calculated binding energy falls in the desirable range of 0.2-0.6eV/H$_{2}$ which is suitable for reversible hydrogen storage at room-temperature, near-ambient-pressure conditions.   

Sequential Dissociative Chemisorption of H$_{2}$ on Ti$_{13}$ Cluster

Ti nanoparticles have received much attention due to their superior catalytic property in potential hydrogen storage materials for fuel cell applications. In this study, we show that the energetically stable distorted icosahedral Ti$_{13}$ cluster has excellent H$_{2}$ adsorption and desorption properties and lead to stable structures upon hydrogen cycling. H$_{2}$ adsorption initially leads to a highly stable Ti$_{13}$H$_{20}$ cluster and on further saturation yields the Ti$_{13}$H$_{30}$ cluster. The chemisorbed H atom in Ti$_{13}$H$_{20}$ occupies above the face of the triangular planes of Ti$_{13}$ whereas in Ti$_{13}$H$_{30}$, H atoms remain dangling above the apex Ti edges. The three coordinated H in Ti$_{13}$H$_{20}$ has higher chemisorption and desorption energies than the fully saturated Ti$_{13}$H$_{30}$ cluster. This type of multi-center H-bonds with varied chemisorption energies is structurally significant since adsorption and desorption rate processes could be controlled and deserve attention as potential candidates for hydrogen storage materials.   

An Ab Initio Study of Molecular Hydrogen Interaction with SiC Nanotube -- A Precursor to Hydrogen Storage

First principles calculations have been performed to study the adsorption of molecular hydrogen (H2) on three types of armchair (9,9) silicon carbide nanotubes. The distances of H2 from the outer walls of the nanotubes have been optimized using the B3LYP and PW91 functionals. For the PW91 functional, the carbon top site for type 1, the second hollow site for type 2 and the C-C bridge site for type 3 nanotubes are the most preferred adsorption sites. For the B3LYP functional, the C-Si normal bridge site for type 1, and the C-C bridge site for type 2 and type 3 nanotubes are the most preferred sites. The adsorption energies using the PW91 functional are found to be always higher than those using the B3LYP functional; however, the adsorption distances using the B3LYP functional are greater than the corresponding distances using the PW91 functional. Current studies indicate that silicon carbide nanotubes can possibly be used as a proper media for hydrogen storage at ambient conditions.