제일원리 기법을 이용한 보론이 도핑된 그라핀의 수소 저장 특성에 대한 이론연구

Abstract

Among alternative energy sources such as solar batteries and fuel cells, hydrogen is an especially promising candidate technology to meet increasing demands for alternatives to environmentally harmful effects of fossil fuel as well as to meet the growing demand for energy. The main topic in hydrogen storage system is an efficient and controllable adsorption-desorption of hydrogen. Consequently, one of the most promising materials suggested as a potential hydrogen storage media is graphene. The most promising method to increase the adsorption energy and hydrogen uptake is modification of graphene by doping or substitution of other material. Boron doping is a practical and feasible way to alter and adjust the binding configurations and the electronic properties of carbon-based materials. In addition, metal adsorption has been studied intensively to enhance the hydrogen storage capacity of carbon-based material. However, a major obstacle for the metal dispersed on graphene is the clustering of metal adatoms due to their large cohesive energies. This clustering tendency must be overcome in order to maintain a high storage capacity of hydrogen at a stable level during hydrogen adsorption and desorption processes. This dissertation presents a theoretical study of hydrogen storage characteristics on graphene using ab initio methods based on the density functional theory (DFT). The quantum-mechanical structural and electronic properties of the metal (Li, Ca, Al and Ti) adsorbed graphene with boron substitution systems were intensively investigated. Computer simulations play a very important role in science today, since they acts as bridge between experiment and theory. Moreover, advance of powerful computer and improved method has led to the emergence of many computer simulations for treating increasingly complex and extremely small scale system with improving level of accuracy. Among them, the first principles method has usually been preferred to investigate the individual mechanisms of the hydrogen and metal adsorption and diffusion behavior that are not easy to observe from experimental method. First of all, Li is considered as a adsorbed metal since Li is very famous and promising material for the hydrogen storage media without graphene. In this study, three specific aspects of graphene and hydrogen storage were investigated. First, boron substitution in the graphene enhances the Li metal adsorption energy which is much larger than the cases of pure graphene and Li cohesive energy. Next, non-hydrogen atoms such as C and B are very unstable on the Li adatom energetically and structurally, indicating that Li adatoms can be maintained as open metal sites. Finally, it was determined that the bonding characteristics and electronic structure of boron-carbon graphene contribute significantly to the energetics of adsorbed H2 molecules on Li adatoms. The contribution of Li adatoms to the total electronic structure could be enhanced by hydrogen adsorption and up to 4 molecules of hydrogen can be adsorbed per Li adatom resulting in a high hydrogen storage capacity of as much as ~13.2 wt% for boron-substituted graphene. The bonding characteristics of Al and Ti on the boron substituted graphene were investigated and the adsorption behavior of H2 on the Al and Ti dispersed on graphene were studied. It was found that the clustering of Al and Ti adatoms is prevented by repulsive Coulomb interactions and the strong bonding between metal adatom and graphene. Therefore, Al and Ti adatoms can be adsorbed on the double side of a (2 × 2) graphene sheet without clustering. Furthermore, Al and Ti dispersed on graphene can bind up to 8 H2 molecules resulting in a high hydrogen storage capacity of as much as 9.9 and 7.9 wt%, which can certainly satisfy the target of the DOS 2010 system. Finally, the dispersed Al and Ti change the electronic structure of the graphene and H2. This cause DOS of H2 to interact with the Al and Ti dispersed on graphene. From these results, it can be inferred that Al and Ti can enhance the hydrogen storage capacity. Finally, The hydrogen storage characteristics of Li, Al, Ca and Ti decorated graphene with B substitution and vacancies were investigated. It was found that the clustering of metal adatoms is prevented by repulsive Coulomb interactions and the strong bonding between metal adatom and graphene. Therefore, Li, Al, Ca and Ti adatoms can be adsorbed on the graphene sheet without clustering. It was found that vacancy defects provide stronger binding sites of metal atoms, particularly Li, Al and Ca. Furthermore, Ca and Ti dispersed graphene can bind up to 8 H2 molecules on the double side of graphene resulting in a high hydrogen storage capacity of as much as 9.9 and 7.9 wt%, which can certainly satisfy the target of the DOE 2010 system. The dispersed Ca and Ti also change the electronic structure of the graphene and H2. From these results, it was found that Ca and Ti can enhance the hydrogen storage capacity.