All-metal binuclear sandwich complexes Al4Ti2Al4: High capacity hydrogen storage through multicenter bonds

The structure and hydrogen storage capacities of a sandwich-type binuclear complex Al4Ti2Al4 are studied using first-principles calculations. It is found that the aromatic Al42− unit can be incorporated to sandwich the transition metal Ti atoms forming a stable all-metal binuclear compound Al4Ti2Al4. One Al4Ti2Al4 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/H2 and the conjugation between H and Al4Ti2Al4 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 Al8Ti2, 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 TiAl cluster and adsorbed hydrogen.