Quantum-mechanics calculations of hydrogen atoms in nanoscale carriers

• Main Authors: Ding-ShengYang, 楊鼎盛
• Other Authors: Yun-Che Wang
• Format: Others
• Language: en_US
• Published: 2013
• Online Access: http://ndltd.ncl.edu.tw/handle/47245918818002567256

碩士 === 國立成功大學 === 土木工程學系碩博士班 === 101 === Simulations of atomic and molecular systems are of particle use to understand the physical mechanisms and stability of the systems, as well as estimations of their physical properties, such as elastic constants, viscosity, diffusional coefficients and others. In this work, the quantum molecular dynamics simulation (QMD), as opposed to conventional molecular dynamics (MD), is first reviewed and then preformed to study the hydrogen-carbon systems since the interaction between hydrogen and carbon atoms in the extremely confined geometry through packing cannot be correctly modeled by empirical interatomic potentials in MD. In QMD, the interatomic forces are calculated by solving the Schrödinger’s equation with the density functional theory (DFT) formulation, and the positions of the atomic nucleus are calculated with the Newton’s second law in accordance with the Born-Oppenheimer approximation. Hydrogen economy provides low carbon dioxide emission to the environment, but must compete with other alternative energy sources in the market. In order to obtain high volume fraction hydrogen storage, more than about 10 wt%, it has been proposed to adopt nano-scale cages, such as carbon fullerenes, carbon nanotubes, graphene layers. The carbon nanocages investigated in this work is the C60 fullerene. The structural stability of the nanocages is tested by long-time MD simulations under various pressures and temperatures. At 300 K our result is that 10 hydrogen atoms can be stored in C60 as molecule form. Furthermore, by controlling the external pressure and elevated temperature of the simulation box, the hydrogen may be released from or squeezed into the nanocages, C60 can be stable under 10 GPa environment. Under high pressure environment, two hydrogen atoms which are on the outside of C60, are moving forward into C60, but we use hydrogen atoms that case the C60 broken. Simulation results show hydrogen can exist as H2 molecules in the nanocages, if hydrogen atoms are not placed too near carbon atoms about smaller than 1.3 Ang. Otherwise, C-H bonds form first due to the size of carbon atom is larger than that of hydrogen. Long-term stability of the hydrogen-carbon systems under the refilling/release environments requires further investigation. There may be an optimal time, pressure and temperature for refilling and release of hydrogen from the nanocages. Graphene can be stable at lower than 500 K, placed hydrogen molecule on the surface although it does not like hydrogen atom destroy the structure of C60 but it does not adsorb to graphene, thus we placed two hydrogen molecules between graphene layers, we observe that hydrogen can be stable in it by the bound of graphene layers, and thus we surmise that hydrogen can't adsorb to graphene by physical adsorption. In the future work, study of metal-organic frameworks (MOFs) is necessary to obtain higher hydrogen content.