Optimization of metal dispersion in doped graphitic materials for hydrogen storage

Gyubong Kim, Seung-Hoon Jhi, Noejung Park, Steven G. Louie, and Marvin L. Cohen

Phys. Rev. B 78, 085408 – Published 7 August 2008

Abstract

The noncovalent 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 acceptorlike states in the absorbents is essential for enhancing the metal adsorption strength and for increasing the number of hydrogen molecules attached to the metal atoms. Particular configurations of boron 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.

Received 30 April 2008

DOI:https://doi.org/10.1103/PhysRevB.78.085408

Authors & Affiliations

Gyubong Kim and Seung-Hoon Jhi^*

Department of Physics, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea

Noejung Park

Department of Applied Physics, Dankook University, 126 Jukjeon-dong, Yongin, Gyeonggido 448-701, Republic of Korea

Steven G. Louie and Marvin L. Cohen

Department of Physics, University of California at Berkeley and Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

^*Corresponding author. jhish@postech.ac.kr

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Issue

Vol. 78, Iss. 8 — 15 August 2008

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Images

Figure 1
(Color online) Optimized structure of Ti attached to (a) a pure and (b) a B-doped coronene molecule. Yellow, purple, and large blue balls represent carbon, boron, and titanium atoms, respectively. Saturating hydrogen atoms are depicted by small light-blue balls. (c) Calculated binding energies $(E_{b}^{TM})$ of transition-metal atoms (Sc, Ti, and V) on boron-doped or nitrogen-doped coronenes (each denoted by Bn or Nn with $n$ being the number of B or N atoms in a carbon hexagon ring without forming B-B or N-N bonding).Reuse & Permissions
Figure 2
Density of states of (a) a pure, (b) a B-doped, and (c) a N-doped coronene $[C_{21} B {(N)}_{3} H_{12}]$ obtained after Gaussian convolution with an energy width of 0.05 eV of molecular levels. The Fermi level, set at the zero of energy, lies close to the HOMO (LUMO) state for B (N) doped coronene.Reuse & Permissions
Figure 3
(Color online) Two TM atoms on B-doped model graphene with a chosen unit cell [indicated by red rectangles in (a) and (b)] of $C_{48} B_{12}$ (a) in a dimer form or (b) as two separated atoms. (c) The energy difference between geometry (a) and (b) for Sc, Ti, and V. The plus sign in (c) indicates that the configuration (b) has a larger cohesive energy than that of the configuration (a) whereas the minus sign means the opposite.Reuse & Permissions
Figure 4
(Color online) Diffusion energy barrier of a Ti atom on B-doped graphene. (a) $s_{0}$ , $s_{1}$ , and $s_{2}$ represent the local minimum sites of Ti adsorption, and B2 and B3 denote the sites for additional boron substitution of carbon in pair and triplet doping configurations, respectively. The dark gray (purple) circle is to show the initial boron substitution site (B1). (b) The total-energy difference $(Δ E)$ of Ti adsorption on $s_{1}$ and $s_{2}$ relative to $s_{0}$ for single (B1), pair (B2), and triplet (B3) boron defects.Reuse & Permissions
Figure 5
(Color online) Hydrogen binding energy to B-doped or N-doped coronene-metal complex formed with (a) Sc, (b) Ti, and (c) V, respectively. The label in the abscissa is the same as in Fig. 1c. The colored bars denote the net binding energy of hydrogen calculated by successively attaching the hydrogen molecules to the metal atoms as denoted in the enclosed legend in (a). First unfilled bars indicate the binding energy of hydrogen, for which the H-H bond distance is larger than $1.6 Å$ , a predefined dissociation limit. As B-doping ratio increases, we observe significant enhancements in $H_{2}$ adsorption including uniform adsorption energies, increased number of adsorbed $H_{2}$ , and reduced $H_{2}$ dissociation. No such improvement is observed for N doping.Reuse & Permissions

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