The hydrogen adsorption and storage capacity on various carbon-based ring materials decorated with alkali metal ions have been systematically investigated by using quantum chemistry calculations. To improve the bonding ability between H 2 molecules and carbon-ring based molecular complexes, we also explain the strategy of modifying the carbon ring materials by substituting carbon element to boron or nitrogen element (up to three atoms). Our calculations show that the Li + -, Na + -, and K + -decorated carbon-based molecular complexes with B-and N-substitution enhance the hydrogen storage capacity. The Mulliken charge analysis is performed to illustrate the interaction between H 2 and M + @carbon-ring complex. The number of binding H 2 molecules on the M + -carbon-ring complexes (M + = Li + , Na + , and K + ) depends on ionic radii of the metal cations. It is found that corresponding gravimetric density are predicted to be 11.21−13.95 and 10.42−13.24 wt % for the B-and N-substituted complexes, respectively. ■ INTRODUCTIONHydrogen has been considered as a promising clean energy carrier due to its high energy density, lightweight, and zero pollution. 1,2 Regarding the use of hydrogen as a fuel for the zero-emission vehicle, one well-known problem is effective storage of hydrogen. 3 Hydrogen can be stored in many ways. 4,5 First, hydrogen is stored in liquid form or compressed in gaseous form by mechanical technologies. Metal hydrides, for example, LiAlH 4 , LiBH 4 , LiNH 2 , Li 2 NH, NaAlH 4 , NaBH 4 , and Mg(BH 4 ) 2 and other hydrogen-rich compounds, such as NH 3 , NH 3 BH 3 , ethanol, and so on 6−8 via chemical methods where hydrogen is bond chemically in the forms offer a second way for hydrogen storage. Third is the adsorption method, in which, hydrogen atom is incorporated directly into the interstitial sites of the bulk crystals. 9−11 The fourth one includes hydrogen storage technologies on the basis of the physisorption of H 2 molecules onto the highly fractured surface of nanostructured or microporous materials such as activated carbon, fullerenes, carbon nanotubes, graphene, mesoporous silica, zeolites, metal−organic frameworks (MOFs), covalent−organic frameworks (COFs), and clathrates belong to this category. 5,12−16 The objective of this work is to develop novel and lightweight promising materials belong to the latest category that can improve their hydrogen storage capacity than existing materials.It is well-known that the nanostructured carbon-based materials, for example, fullerene, carbon nanotube, graphene, and so on, have high surface area, lightweight, and chemically stable properties. However, unsubstituted carbon-based materials can only bind one hydrogen molecule on each side of carbon ring with the binding energy of 4−5 kJ/mol. 17,18 One of the effective strategies to solve these problems is to modify carbon ring with other elements. It was evidenced that alkali metal positive ions and transition-metal can nondissociatively bind several H 2 molecules via electrostatic forces. 19−21 Rao and Jen...
Summary Based on ab initio calculations, we have investigated the H2 adsorption and storage capacity on boron‐substituted and nitrogen‐substituted nano‐carbon materials doped with alkaline earth metal ions (Be2+, Mg2+, and Ca2+) systematically. The calculation results show that the Be2+‐decorated, Mg2+‐decorated, and Ca2+‐decorated carbon‐based materials with B‐substitution and N‐substitution improve the hydrogen storage capacity. H2 molecules are bound stronger with lighter cations. The adsorption energy of H2 molecule on the M2+‐nano‐carbon complex (M2+ = Be2+, Mg2+, and Ca2+) is disproportional to ionic radii of the M2+ cations. The interaction between H2 and M2+@nano‐carbon complex is elucidated by Mulliken charge analysis. It is determined that the highest gravimetric density is predicted to be 13.38 and 19.89 wt.% for the B‐substituted and N‐substituted materials, respectively. Copyright © 2015 John Wiley & Sons, Ltd.
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