Nondissociative Adsorption of ${\mathrm{H}}_2$ Molecules in Light-Element-Doped Fullerenes

First-principles density functional and quantum Monte Carlo calculations of light-element doped fullerenes reveal significantly enhanced molecular ${\mathrm{H}}_2$ binding for substitutional B and Be. A nonclassical three-center binding mechanism between the dopant and ${\mathrm{H}}_2$ is identified, which is maximized when the empty $p_z$ orbital of the dopant is highly localized. The calculated binding energies of $0.2–0.6\mathrm{eV}/{\mathrm{H}}_2$ is suited for reversible hydrogen storage at near standard conditions. The calculated ${\mathrm{H}}_2$ sorption process is barrierless, which could also significantly simplify the kinetics for the storage.

Figure 1

The binding process of a ${\mathrm{H}}_2$ to substitutional B in a ${\mathrm{C}}_{35}\mathrm{B}$. (a) The ball-and-stick model in which only four intermediate (darker) ${\mathrm{H}}_2$ images are shown. (b) Total energy curve as a function of the geometric center of ${\mathrm{H}}_2$ from its minimum-energy position.

Figure 2

(a)–(d) The calculated local density of states (LDOS) for B (Be) in ${\mathrm{C}}_{35}\mathrm{B}$ $(\mathrm{Be})\mathrm{\text{−}}{\mathrm{H}}_2$. The LDOS for B (Be) are solid lined, whereas the LDOS for ${\mathrm{H}}_2$ are open circled. (e) and (f) The squared changes of the carbon states due to ${\mathrm{H}}_2$ sorption, namely, ${\sigma }^2=[\delta \mathrm{LDOS}(\mathrm{\text{carbon}}){]}^2$ between the nonbonding and bonding states.

Figure 3

(a) and (b) are the differential planar electron density, $\Delta \rho (x,y)$, for B and Be, respectively. Solid and dotted contours indicate electron accumulation and depletion, respectively. The contour interval is 0.003 and the cutoffs are $\pm 0.08e/{\AA }^2$. (c) Linear $\Delta \rho (x)$ for B and Be along the $x$ axis shown in (a) and (b), respectively. The positions of the ${\mathrm{H}}_2$ and B (Be) are indicated.

Figure 4

(a) Binding energy for each added ${\mathrm{H}}_2$ as a function of number of adsorbed ${\mathrm{H}}_2$ molecules and (b) ${\mathrm{C}}_{54}{\mathrm{B}}_6$ with $6{\mathrm{H}}_2$ molecules.