A first-principles study of capacity and mechanism of a single titanium atom storing hydrogen

Using the density functional theory of first principles, we investigate the binding mechanism of a single transition metal atom-titanium adsorbing hydrogen molecules. We find that a single titanium atom can absorb eight hydrogen molecules. The hydrogen molecules around Ti atom form two symmetrical pyramid-like structures with an average adsorption energy of -0.28 eV. By calculating the orbital energie and the distribution of differential charge density, we analyse the intrinsic physical mechanism of determining adsorption structure, adsorption energy and hydrogen storage capacity. The results show that a 4s electron of a titanium atom transfers to the 3d orbit, which can produce a strong polarization electric field, resulting in polarization of the hydrogen molecules. Therefore, the titanium atom adsorbs hydrogen molecules by electrostatic polarization. Our results will present a guidance for designing high-density hydrogen storage materials.