Study On Hydrogen Storage Properties Of Alkali Metal Decorated Three Demensional Covalent Organic Frameworks

Covalent-Organic Frameworks (COFs), analogues of zeolite, is a new class ofnano-porous materials. COFs are promising materials for gases storage and separationowing to their extremely specific surface area and high porosity, thermal stability andlow density. Hydrogen is known as the 21st century ideal energy because of its highcalorific value, clean, and abundant sources. Storage of hydrogen in a safe andeffective way is one of the key issues determining whether the hydrogen can be used asthe clean energy source. Currently, there are still a lot of issues to be addressed inhydrogen storage, such as the kinetics and the thermodynamics of hydrogen absorptionand desorption, and the mechanism of improvement of hydrogen absorption properties.Materials simulations can not only overcome the limitations of traditionalmethods, but also provide the guidance for the design of optimal adsorbents and thedeterminations of optimal industrial operations. It provides a pathway to simplify theexperimental procedures and dramatically reduce the industrial cost. Such, theoreticalstudies on the adsorption properties of hydrogen in COFs will extend our knowledgeon this kind materials as a gas storage media.In this paper, hydrogen storage performance of alkali metal (Li, Na, K) modifiedCOF-108 was studied using quantum chemical calculations and molecular simulations.Calculations were carried out based on first principles via a code entitled as SIESTA(Spanish Initiative for Electronic Simulations with Thousands of Atoms). Thestructural characteristics and the stability of COF-108 were studied, and the positionsof the alkali metal position in COF-108 was determined, and influence of alkali metalson the stability, the electronic structure, and the hydrogen storage performance wereinvestigated. The Grand Canonical Monte Carlo (GCMC) was used to simulate thehydrogen absorption behaviors in COF-108 with and without alkali metals.It was shown that the adsorption energy of H2 in COF-108 is 0.03 eV which isequivalent to van de Waals interactions, implying that H2 was physically adsorbed in COF-108. The alkali metals, Li, Na and K in the present work, prefer to occupy theposition in the center of a benzene ring with a distance from 2.4 to 3.6 ?. Theoccupation energies of Li, Na and K in COF-108 were 0.45 eV, 0.27 eV, and 0.57 eV,respectively. The adsorption energy of a hydrogen molecule in Li, Na and K modifiedCOF-108 were 0.367 eV, 0.308 eV, and 0.472 eV, respectively. Therefore, the bondingstrength of hydrogen molecule in alkali metal decorated COF-108 was significantlyenhanced. GCMC simulations shown that the maximum hydrogen absorption of metaldecorated COF-108 could be reached to 30.44 wt% at 77 K, while the volume fractioncould be up to 66.10 kg/m3.