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The structure and hydrogen storage capacities of a sandwich-type binuclear complex Al 4 Ti 2 Al 4 are studied using first-principles calculations. It is found that the aromatic Al 4 2− unit can be incorporated to sandwich the transition metal Ti atoms forming a stable all-metal binuclear compound Al 4 Ti 2 Al 4 . One Al 4 Ti 2 Al 4 molecule can adsorb 28 hydrogen atoms, which corresponds to a gravimetric storage capacity of 8.24 wt%. This hydrogen storage capacity is above the 2015 target of 5.5% set by the US Department of Energy (DOE) in 2009. Furthermore, the average binding energy is in the ideal range of 0.2–0.6 eV/H 2 and the conjugation between H and Al 4 Ti 2 Al 4 differs according to the adsorbed sites. Meanwhile, we found that multicenter bond is formed between H atoms and Al or Ti atoms during the process of hydrogen adsorption through AdNDP analysis, and it is characterized by specific vibration frequency. Therefore, mode selective vibrational excitation of the IR intense bands may lead to desorption of the absorbed hydrogen form Al 8 Ti 2 , enabling efficient reversible hydrogen storage. Last but not least, the binding energies of the multicenter bond between adsorbed H atoms and pure Ti atoms are much larger than those of the multicenter bonds related to both Ti and Al atoms. Thereby, the inter-doping of Ti and Al can modulate the binding energy between Ti Al cluster and adsorbed hydrogen. [ABSTRACT FROM AUTHOR]

The hydrogen storage capacities of the two sandwich-type Cp 2 TM 2 [Cp = cyclopentadienyl (C 5 H 5 ); TM = V or Cr)] dimetallocenes were studied using first-principles calculations. According to the B3PW91 method in density functional theory (DFT), Cp 2 V 2 and Cp 2 Cr 2 could adsorb up to seven and six hydrogen molecules, respectively. The predicted hydrogen storage densities of Cp 2 V 2 and Cp 2 Cr 2 were 5.73 and 4.91 wt%, respectively. Additionally, because hydrogen binding energies bound by Kubas model were 0.39 and 0.43 eV/H 2 , respectively, both the Cp 2 V 2 and Cp 2 Cr 2 dimetallocenes were proven to be favourable for reversible adsorption and desorption of hydrogen molecules under ambient conditions. [ABSTRACT FROM AUTHOR]

The hydrogen storage capacities of a sandwich-type ethylene dimetallocene complex (Cp 2 Ti 2 C 2 H 4 ) are studied using first-principles calculations. It is found that the TiC 2 H 4 Ti molecule can intercalate into the two cyclopentadienyl (Cp) rings and form a stable sandwich-type complex. Each Ti atom can adsorb a maximum of three H 2 molecules, which corresponds to a gravimetric storage capacity of 4.73 wt%. This hydrogen storage capacity is close to the 2015 target of 5.5% set by the US Department of Energy (DOE) in 2009. Furthermore, the Cp 2 Ti 2 C 2 H 4 molecule proposed in this paper is favorable for both adsorption and desorption of hydrogen molecules at room temperature and ambient pressure because its average binding energy of 0.34 eV/H 2 . [ABSTRACT FROM AUTHOR]