Strain and defect engineering of graphene for hydrogen storage : atomistic modelling

Abstract

Hydrogen has proven alternative fuel in terms of clean energy due to its zero-carbon emission, and the only by-product is water vapor when we use hydrogen as a fuel. Hydrogen has widespread applications, but its lack of safe and efficient onboard storage has stalled its use of in general purpose applications. The researchers are trying to find solutions for the efficient storage of hydrogen via use of different materials and techniques. One of the carbon-based nanomaterial “graphene”, a 2D material with excellent mechanical and electrical properties, has attracted a lot of attention for hydrogen storage. Graphene possesses a high specific surface area of 2630 m2 /g which makes it a potential candidate to store the hydrogen via physisorption. In this thesis, the adsorption of hydrogen molecules over the monolayer graphene via molecular dynamic (MD) simulations is studied. Interatomic interactions of carbon carbon atoms of the graphene layer are described using the well-known AIREBO potential, while the interactions between graphene and hydrogen molecule are described using Lennard-Jones potential. In particular, the effect of strain and different defects on the hydrogen storage capability of graphene is studied. A novel method, i.e., the potential energy density method is used for the calculation of wt.% of hydrogen. A strained graphene layer was found to be more active for hydrogen storage with adsorption of 6.28 wt.% at 0.1 strain, 77 K and 10 bar. The effect of temperature and pressure on the adsorption energy and gravimetric density of graphene is also studied. Different defects in the graphene layer like monovacancy (MV), Stone Wales (SW), 5- 8-5 double vacancy (DV), 555-777 DV, and 5555-6-7777 DV are considered, which usually occur during the synthesis of graphene. Among all types of defects, the MV defected graphene layer adsorbed larger amount of hydrogen with a 9.3 wt.% at 77 K and 100 bar, about 42% higher than the pristine graphene. It is also found that the critical stress and strain of defected graphene decrease appreciably compared to its pristine counterpart. Keywords: Hydrogen storage; Molecular dynamic simulation; Graphene; Potential energy; Average adsorption energy; Strain; Vacancy defects; Physisorption; Adsorption isotherm.