Hydrogen trapping potential of Ca decorated metal-graphyne framework

Abstract

Hydrogen holds the promise for alternative clean energy carrier due to its renewable and pollution free nature. A metal-organic framework (MOF) is designed with graphyne linker. Each graphyne linker is decorated with two Ca atoms across the linker with average metal binding energy 3.0 eV. The structural, electronic and hydrogen storage properties of Ca decorated MOF have been explored by using first principle calculations. On full saturation with hydrogen, each Ca atom of MOF-Ca8 adsorbs a maximum of six H2 molecules and results in MOF-Ca8-48H2 structure. Further twelve more hydrogen molecules could be accommodated in the pore space of MOF resulting in the MOF-Ca8-60H2 structure having 7.9 hydrogen wt%. According to the simulations, the H2 molecules can be adsorbed on Ca by Kubas mechanism with elongation in H H bond distance. The calculated hydrogen interaction energy is found in the range between 0.25 and 0.30 eV while desorption energy varies between 0.15 and 0.32 eV. The charge transfer during hydrogen adsorption is investigated by Hirshfeld charge analysis and electrostatic potential map. The molecular dynamics simulations revealed a high degree of reversibility in hydrogen adsorption of the system at ambient conditions. The usable capacity of H2 is explored by calculating occupation number at adsorption and desorption conditions. The energetics and storage capacity meets the US DOE target which makes the MOF-Ca8 as a potential hydrogen storage material.