Hydrogen Storage in Grahene Oxide Layers: First-Principles Study and Monte Carlo Simulations

Abstract

Concerning the depletion of fossil fuels and environmental pollutions by them, new energy sources have been vastly investigated. Among these new energy sources, hydrogen energy is one of the most promising because they can be easily obtained from water around us and they don't produce polluting materials. However, the biggest problem to use the hydrogen energy is safe and high-capacity storage of them. Graphene as 2-dimensional material composed of carbon has distinct feature like high electric and heat conductivity, and high strength. So many people have researched it. In this study, I simulated how much hydrogen gas are stored in two systems that are graphene layers and graphene oxide layers where oxygens are attached as epoxy group, and then I investigated the oxygen effect. I used two calculation tools. The first is ab-initio calculation, also known as first-principles study using density functional theory. The second is Monte Carlo simulation using metropolis algorithm. For ab-initio calculations, I used VASP(Vienna Abinitio Simulation Package) adopting PBE(Perdew, Burke and Ernzerhof) functional and for the Monte Carlo simulations, I made a program using Fortran 90. Here is the calculation procedures. First, I got the hydrogen-hydrogen molecule interaction energy using VASP, changing their relative distance. As hydrogen molecule is a diatomic molecule, there also is angle dependence of the energy. However, I ignored the angle dependence by calculating hydrogen-hydrogen molecule interaction for 4 most symmetric configurations and thermally averaging them in the reason that including the angle dependence increases calculation cost too much and it seems that including the angle dependence doesn't make a big difference. Second, I got the potential energy field in graphene layers and graphene oxide layers from total energy differences between whether one hydrogen molecule is in the systems or not. I also used VASP to get the potential energy fields. In these cases, I also ignore the angle dependence of hydrogen molecule like hydrogen-hydrogen molecule interaction case. In these cases, I arranged hydrogen molecule in three axis directions of the systems. Lastly, using the informations obtained above, I simulated Monte Carlo simulations. It show that the oxygen increases the potential energy in the vicinity of oxygen, but decreases the potential energy in the other region. Put together, I concluded the oxygen interrupts hydrogen storage in the system.