First principles study of reversible hydrogen adsorption in metal functionalized molecular systems

Abstract

The objective of this thesis is to investigate the hydrogen adsorption properties and mechanism in various metal functionalized materials by using rst principle calculations based on density functional theory (DFT). Automobile industries rely mostly on fossil fuels as an energy source. Fossil fuels are limited, and their combustion has led to a signi cant increase in global pollution for the past few years. Therefore, we, as a scienti c community, are required to nd a sustainable, ecofriendly, and a ordable substitute for fossil fuels. Hydrogen, as a fossil fuel substitute, has attracted a great attraction due to its abundance, high energy density (33.3 kW h kg-1), lightweight, renewable nature, and ecofriendly behavior. Combustion of hydrogen to produce energy inside a vehicle releases water as a byproduct. High gravimetric and volumetric densities, optimal thermodynamics at operable conditions, and fast reaction kinetics are the utmost important criteria for e cient hydrogen storage systems. Numerous materials have been explored for their hydrogen sorption properties. These include metal organic frameworks, metal inorganic frameworks, covalent organic frameworks, metal hydrides, and carbon nanostructures like nanotubes, fullerenes, nanoribbons. Very few of these materials meet the target conditions speci ed by the Department of Energy, USA, as many of them have lesser hydrogen weight percentage or desorb hydrogen at very low temperatures although stored under very high pressures. We present the rst principles study to investigate the hydrogen sorption mechanisms in Li, Sc, and Ti functionalized [n,n]paracyclophanes, BN analogue of [2,2]paracyclophane, and Li, Mg, Ca, and Sc functionalized 􀀀graphyne. For these studies, a global hybrid functional M06 has been implied along with 6-311G(d,p) basis set. This thesis also includes hydrogen sorption studies in Ti functionalized C24 fullerene, Ti functionalized modi ed calix[4]pyrrole{benzene, and a metal cluster framework built with Ti4 cluster as a linker. For these investigations, generalized gradient approximation (GGA) functional along with double numeric polarization (DNP) basis set has been implied based on large system size and to minimize the simulation time with higher accuracy. The structural, thermochemical, and electronic properties have been studied in all of the aforementioned host systems. It has been found in the investigations that the transition metal atoms are functionalized over the 􀀀complexes of the hosts due to the Dewar mechanism, and the hydrogen molecules are adsorbed over the open metal sites by Kubas mechanism. It was observed that 3 to 5 H2 molecules adsorbed on per metal atom in the metal functionalized system based on hydrogen adsorption energy. Average energies of hydrogen adsorption and desorption are found to be in the desired ranges to consider the metal functionalized host systems to be reversible hydrogen storage material. Transfer of charges during metal functionalization and sequential hydrogen adsorption is studied with electrostatic potential maps, charge analysis implying CHarges from ELectrostatic Potentials using a Grid-based method, Hirshfeld charges, Mulliken charges, and electrostatic potential tted charges for various hosts. The stability of various hosts has been studied with substantiating analyses like vibrational frequency analysis and conceptual DFT. To con rm the reversibility of the hydrogen saturated systems, molecular dynamics has been performed with Atom Density Matrix Propagation and Born Oppenheimer Molecular Dynamics Simulation, while the choice of method of molecular dynamics has been made on the basis of the level of theory implied for a given study. To prove the hydrogen adsorption capacity of a given host, practical hydrogen sorption capacity has been explored with a study of occupation numbers using empirical values of chemical potential at the particular thermodynamic condition. Based on these comprehensive set of in silico, ab initio, and thermodynamic calculations, aforementioned metal functionalized host systems are proven to be e cient hydrogen storage candidates. At the end of the thesis, work has been summarized along with concluding remarks and future directions followed by a bibliography.