Ab initio study of hydrogen storage in lithium grafted metal-graphyne framework

Abstract

Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar-140 001, Punjab, India E-mail: [email protected] Manuscript received online 28 April 2019, revised and accepted 13 May 2019 Hydrogen holds the promise to replace the widespread dependency on fossil fuels due to its renewable and pollution free nature. Metal-graphyne framework (MGF) constructed with magnesium oxide as a metal node and graphyne as a linker. Graphyne is a new 2D carbon allotrope connected with acetylenic linkages. Lithium is grafted on the graphyne linker of MGF. Density functional theory (DFT), with GGA-PBE exchange-correlation functional, is used to determine the geometric structures, their stability, binding strength of Li and graphyne of MGF, and hydrogen storage capacity. On full saturation with hydrogen, each Li atom physisorbs 3 H₂ molecules in the MGFLi₁₆ which results MGFLi₁₆-48H₂ system with a total gravimetric density of 7.2 wt.%. Further 12 more H₂ molecules could be accommodated in the pore space of MGFLi₁₆ and the resulting structure, MOFLi₁₆-60H₂ is obtained with hydrogen wt.% 8.9. The H₂ and Li interacted by Niu-Rao-Jena mechanism with average H-H bond distance enhanced up to 0.754 Å. The calculated sorption energies are found in the range of 0.36 to 0.21 eV. The molecular dynamics simulations reveal the stability of Li grafted MGF and the reversibility of adsorbed H₂ from the MGFLi₁₆-48H₂ system. The energetics and storage capacity meet the US DOE target which makes MGFLi₁₆ as a potential hydrogen storage material.