Computational study of hydrogen storage in graphite derived nanomaterials

Abstract

Hydrogen has long been considered as the energy carrier of the future. How- ever, storage hydrogen efficiently and safely have been considered as a ma- jor obstacle in realizing hydrogen energy at present. In this thesis, we inves- tigate the equilibrium of interacting gas molecules under the external po- tential, and propose a new hydrogen storage material derived from graphite using ab initio electronic structure calculation and grand canonical Monte- Carlo method. Firstly, based on the simple thermodynamics, diffusive equilibrium un- der the external potential, we show that hydrogen molecules can be highly concentrated in the attractive potential well in the ambient conditions. Hy- drogen molecules are vigorously move inside the potential well and retain gas from, it is very distinct feature of our storage mechanism compare with storage mechanism based on the bonding at specific sites through Kubas in- teraction using transition metal atoms. As a realization, we propose a potas- sium intercalated graphite oxide as a scaffold material and show that rela- tively uniform potential well arises inside the layer and its strength is as large as 0.12eV. The enhancement of binding energy can be attributted to the in- duced dipole interaction from electric field generated by oxygen atom and potassium ion and small orbital hybridization. Room temperature hydrogen storage capacity is obtained by grand canonical Monte-Carlo simulation. The general trend of storage capacity with different chemistry of scaffold material is explained by equilibrium condition and density enhancement by the attractive potential. Secondly, we study the energetics involved hydrogen confinement pro- cess. Isosteric heat of adsorption is one of the key quantities in the hydrogen storage experiment, and can be considered as a change of the enthalpy of adsorption process. In order to account the intermolecular interaction, we employed the cluster expansion method for interacting gas and analytically derived the equilibrium condition, which is valid in the ambient condition. The strength of the potential well as well as the interaction energy is re- flected in the isosteric heat. This result can be applied to explain the physics of hydrogen storage using potential well. We end up with summary and perspectives to be investigated more.