Structural Phase Stability in Fluorinated Calcium Hydride

In order to improve the hydrogen storage properties of calcium hydride (CaH2), we have tuned its thermodynamical properties through fluorination. Using ab-initio total energy calculations based on density functional theory, the structural stability, electronic structure and chemical bonding of CaH2-xFx systems are investigated. The phase transition of fluorinated systems from orthorhombic to cubic structure has been observed at 18% fluorine doped CaH2. The phase stability analysis shows that CaH2-xFx systems are highly stable and the stability is directly correlating with their ionicity. Density of states (DOS) plot reveals that CaH2-xFx systems are insulators. Partial DOS and charge density analyses conclude that these systems are governed by ionic bonding. Our results show that H closer to F can be removed more easily than that far away from F and this is due to disproportionation induced in the bonding interaction by fluorination

Ti4+ Substituted Magnesium Hydride as Promising Material for Hydrogen Storage and Photovoltaic Applications

In order to overcome the disadvantages of MgH2 towards its applications in on-board hydrogen storage, first principle calculations have been performed for Ti (2+, 3+, and 4+) substituted MgH2. Our calculated enthalpy of formation and H site energy implies that Ti substitution in Mg site reduces the stability of MgH2 which improve the hydrogen storage properties and Ti prefers to be in +4 oxidation state in MgH2. The bonding analyses through partial density of states, electron localization function and Bader charge of these systems confirm the existence of iono-covalent bonding. Electronic structure obtained from hybrid functional calculations show that intermediate bands (IB) are formed in Ti4+ substituted MgH2 which could improve the solar cell efficiencies due to multiple photon absorption from valence band to conduction band via IBs and converts low energy photons in the solar spectrum also into electricity. Further, our calculated carrier effective masses and optical absorption spectra show that Ti4+ substituted MgH2 is suitable for higher efficiency photovoltaic applications. Our results suggest that Ti4+ substituted MgH2 can be considered as a promising material for hydrogen storage as well as photovoltaic applications

Theoretical investigation on BeN2 monolayer for an efficient bifunctional water splitting catalyst

The search for an active, stable, and abundant semiconductor-based bifunctional catalysts for solar hydrogen production will make a substantial impact on the sustainable development of the society that does not rely on fossil reserves. The photocatalytic water splitting mechanism on a BeN2 monolayer has here been investigated by using state-of-the-art density functional theory calculations. For all possible reaction intermediates, the calculated changes in Gibbs free energy showed that the oxygen evolution reaction will occur at, and above, the potential of 2.06 V (against the NHE) as all elementary steps are exergonic. In the case of the hydrogen evolution reaction, a potential of 0.52 V, or above, was required to make the reaction take place spontaneously. Interestingly, the calculated valence band edge and conduction band edge positions for a BeN2 monolayer are located at the potential of 2.60 V and 0.56 V, respectively. This indicates that the photo-generated holes in the valence band can oxidize water to oxygen, and the photo-generated electrons in the conduction band can spontaneously reduce water to hydrogen. Hence, the results from the present theoretical investigation show that the BeN2 monolayer is an efficient bifunctional water-splitting catalyst, without the need for any co-catalyst