The increasing demand of energy and dwindling supply of presently available fossil fuels, their adverse effects of CO2 emissions have raised the interest in sustainable, clean fuels such as hydrogen. But the storage of hydrogen for onboard application is a major problem in attaining this goal. In this study, metal-organic framework (MOF) built with graphyne linkers and the linkers are decorated with Li and Ca metals for hydrogen storage. Applying density functional theory employing the GGA-PBE functional with double numeric basis method, the structural stability and physicochemical properties have been analyzed. It is found that each graphyne linker strand can bind up to four Li atoms two from the top and two from the bottom side, while it can bind two Ca atoms. The binding is through Dewar coordination. On saturation with hydrogen, each Li atom adsorbs 3 H2 molecules while each Ca adsorb 6 H2 molecules in molecular form in the metal decorated MOF. The H2 and metal interact by Kubas-Niu mechanism with average H-H bond length elongated from the free H2 bond length of 0.741 Å, which reveal that all the adsorbed H2 molecules are physisorbed in metal decorated MOF. The average H2 adsorption energy and sequential desorption energy is found in the range between 0.2 to 0.6 eV which indicates the fast kinetics under moderate temperature and pressure conditions required by the US DOE. The hydrogen storage capacity is found to be 8.9 and 7.9 wt. %, respectively, for hydrogen saturated Li and Ca decorated MOF with graphyne. The energetics and storage capacity meet the ultimate DOE target 2020 which makes them sustainable hydrogen storage material.

First-principles study of hydrogen storage in Li- and Ca-decorated MOF designed with graphyne linker

2019 Spring National Meeting

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