Ti functionalized carbon and boron nitride chains: a promising material for hydrogen storage

Abstract

The geometries, electronic structures, thermochemical properties, polarizabilities, and hyperpolarizabilities of high capacity hydrogen storage media consisting of alkali metal such as Li or transition metal as Ti, that is, functionalized at the end of C and BN chains have been investigated theoretically using density functional theory (DFT). Fundamental aspects such as interaction energy, natural bond orbital (NBO), charge transfer, energy gap, and the projected density of states (PDOS) are elucidated to analyze the adsorption properties of H2 molecules. Our results revealed that H2 is introduced sequentially on the Ti-C7, Ti(B)-B4N3, and Ti(N)-B3N4 complexes and the H2 uptake capacity are found to be 10.89, 10.80, and 10.58 wt%, respectively. Moreover, two Ti atoms can be adsorbed concomitantly to the ends of C7, B4N3, and B3N4 chains where Ti sites can accommodate 16 H2 molecules, with 8 per Ti center, leading to a storage capacity of up to 26.40, 26.28, and 25.94 wt%, respectively. In addition, two binding mechanisms contribute to the adsorption of hydrogen molecules: polarization of the H2 under the electric field produced by the Ti–chain dipole and hybridization of the 3d orbitals of Ti with σ orbitals of H2. These lead to the hydrogen binding energies within the range of 0.22–0.56 eV/H2, open a prospect of a promising material system for hydrogen storage at ambient temperature. The large difference in charge transfer and interaction between the metal and chains is responsible for the large hyperpolarizability. Moreover, the C and BN chains can be stabilized effectively by C20 fullerene termination and store 8 H2 with an average binding energy of 0.22 eV/H2. The hydrogen desorption energies and temperatures indicate that the Ti-C7,Ti(B)-B4N3, Ti(N)-B3N4, Ti-C7-Ti, Ti(B)-B4N3-Ti(B), Ti(N)-B3N4-Ti(N), Ti-C7-C20, Ti(B)-B4N3-C20, and Ti(N)-B3N4-C20 complexes are easy to desorb H2 molecules.