Hydrogen storage capacity of alkali metal atoms decorated porous graphene

Abstract

Porous graphene (PG), a kind of graphene-related material with nanopores in the graphene plane, exhibits novel properties different from those of pristine graphene, leading to its potential applications in many fields. Owing to periodic nanopores existing naturally in the two-dimensional layer, PG can be used as an ideal candidate for hydrogen storage material. High hydrogen storage capacity of Li-decorated PG has been investigated theoretically, but the effect of temperature on the stability of the H<sub>2</sub> adsorbed on Li-PG has been not discussed yet. In this paper, by using the first-principles method, the hydrogen storage capacity on alkaline metal atoms (Li, Na, K) decorated porous graphene is investigated in depth with generalized gradient approximation, and the effect of the temperature on the stability of the hydrogen adsorption system is elucidated by the <i>ab initio</i> molecular-dynamics simulation. The results show that the most favorable adsorption sites of Li, Na and K are the hollow center sites of the C hexagon, and four alkaline metal atoms can be adsorbed stably on both sides of PG unit cell without clustering. Alkaline metal adatoms adsorbed on PG become positively charged by transferring charge to PG and adsorbed H<sub>2</sub> molecules, and three H<sub>2</sub> molecules can be adsorbed around each alkaline metal atom. By analyzing the Mulliken atomic populations, charge density differences and density of states of H<sub>2</sub> adsorbed on Li-PG system, we find that the H<sub>2</sub> molecules are adsorbed on alkaline metal atoms decorated graphene complex by attractive interaction between positively charged alkaline metal adatoms and negatively charged H and weak van der Waals interaction. Twelve H<sub>2</sub> molecules are adsorbed on both sides of PG decorated with alkaline metal atoms. The average adsorption energy of H<sub>2</sub> adsorbed on Li-PG, Na-PG and K-PG are –0.246, –0.129 and –0.056 eV/H<sub>2</sub>, respectively. It is obvious that the hydrogen adsorption capacity of Li-PG system is strongest, and the hydrogen adsorption capacity of K-PG is weakest, thus K-PG structure is not suitable for hydrogen storage. Furthermore, by the <i>ab initio</i> molecular-dynamic simulation, in which the NVT ensemble is selected but the external pressure is not adopted, the effect of temperature on the stability of H<sub>2</sub> molecules adsorbed on Li-PG system is elucidated. The result shows that the configuration of Li-PG is very stable, H<sub>2</sub> molecules are stably adsorbed around the Li atoms at low temperature, and some H<sub>2</sub> molecules start to be desorbed from the Li atoms with the increase of temperature. At 200 K, H<sub>2</sub> molecules begin to move away from Li atoms, and two H<sub>2</sub> molecules escape from the binding of the Li atoms at 250 K. At 300 K, nine H<sub>2</sub> molecules can be stably absorbed on both sides of Li-PG, and the gravimetric hydrogen storage capacity can reach up to 9.25 wt.%, which is much higher than the the US Department of Energy target value of 5.5 wt.% for the year 2017. With the increase of temperature, more adsorbed H<sub>2</sub> molecules are desorbed, seven H<sub>2</sub> molecules can be desorbed at 400 K, and all H<sub>2</sub> molecules are completely desorbed in a temperature range of 600–700 K.