First - Principles Study On The Structure And Hydrogen Storage Properties Of Sandwich - Type Bimetallic Composites

As a clean new energy sources, hydrogen is abundant and environmentally friendly, so it is expected to replace fossil fuels. However, a safe system for on-board storage becomes the major impediment to the wide-scale use of hydrogen power. The traditional high-pressure gaseous, liquid, solid hydrogen storage methods are not ideal. They were not secure and can’t come to the standard of commercial applications especially vehicle mounted mobile energy. Previously, there are four major materials having been recommended for hydrogen storage:metal hydrides, complex hydrides, metal organic frameworks MOFs and metal modified carbon nanomaterials. But they still have various shortcomings. Most of the inserted metal atoms and adsorbed hydrogen molecules are very exposed, which makes it rather difficult to stabilize such materials in real systems and it lowers the hydrogen storage capacities of these media.This paper demonstrated a series of novel sandwich-type dimetallocene complexes:transition metals (Ti or Sc) and ethylene (acetylene) cointercalate into the two cyclopentadienyl (Cp) rings. And we carried out the first principle calculations to investigate the geometrical structures, electronic structures and hydrogen storage properties of Cp2Ti2C2H4、CP2Sc2C2H4、Cp2Ti2C2H2 and Cp2Sc2C2H2 molecules using Gaussian software. Meanwhile, we discuss the hydrogen storage possibilities of sandwich-typed all-metal dimetallocene Al7Ti2 as well.(1)The TiC2H4Ti、ScC2H4Sc、TiC2H2Ti、ScC2H2Sc molecule can intercalate into the two cyclopentadienyl (Cp) rings and form stable sandwich-type complexes Cp2Ti2C2H4、Cp2Sc2C2H4、Cp2Ti2C2H2、Cp2SC2C2H2, corresponding to quintuplet, triplet, singlet and singlet of multiplet. Each molecule can adsorb a maximum of six, eight, eight, eight H2 molecules, which corresponds to a gravimetric storage capacity of 4.73 wt%,6.06wt%,5.97wt% and 6.11 wt%, respectively. These values of hydrogen storage capacity are close to or above the 2015 target of 5.5% set by the US Department of Energy (DOE) in 2009. Furthermore, these molecules proposed in this paper are favorable for both adsorption and desorption of hydrogen molecules at room temperature and ambient pressure because their average binding energy of 0.34,6.06, 5.97,6.11 eV/H2, respectively.(2) All-metal binuclear sandwich-like complex Al8Ti2 can be used as hydrogen storage medium with a capacity of 8.24wt%. However, high average binding energy makes it difficult for hydrogen molecules to release at normal condition. Meanwhile, we found that multi-centre 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.