The Study On The Molecular Design And Mechanism Of Covalent Porous Materials For Hydrogen Storage

As an ideal energy carrier,hydrogen is considered to be a kind of new energy resources with great application potential because of its clean,high efficiency,renewability et.al.Hydrogen storage is the one of the key links of hydrogen energy system and also one of the main technical obstacles restricting the large-scale application of hydrogen energy.The main reason is that it is difficult to find hydrogen storage materials with good safety,high hydrogen storage capacity,recyclable and low cost.Recently,considerable attention has ground up around the theme of covalent porous materials duo to its large specific surface area,high porosity,good stability and low density.Meanwhile,they have been considered as promising hydrogen storage materials.Regrettably,there was no experimentally synthesized covalent porous materials can meet all the requirements for commercial hydrogen storage applications.Therefore,it is imperative to improve the hydrogen storage performance of the existing porous materials or to design and synthesize new high-performance covalent porous hydrogen storage materials.The traditional method by trial over and over again to find new materials research,that is namely synthesis,screening,and then improvement,is not only time-consuming and cumbersome,high cost,and the process is complex.In this paper,the material targeting design idea is used to design,optimize and simulate the hydrogen storage performance of covalent porous materials by using the Density Functional Theory（DFT）,force field method and Grand Canonical Monte Carlo（GCCM）,and the possible synthesis scheme is further proposed.Based on this idea,we used tetrakis（4-aminophenyl）silsesquioxane（TAPS）,1,3,5,7-tetrakis-（4-aminophenyl）adamantine（TAPA）and several kinds of triangular anhydrides as the building units,which were optimized by density functional theory,and obtained the corresponding stable structures.After optimization,we preliminarily obtained two types of covalent organic frameworks（COFs）according to the geometrical characteristics of the building units by designed under the net topologies of Fd3m,I 43m,P 43 m and other space group with the TAPS and TAPA as tetrahedral node and linear or triangle anhydride as organic linkers,the building units were placed into the corresponding place of topology nets and in accordance with the space group symmetry of three-dimensional space topology.Subsequently,we optimized their structures again with the method of classical molecular mechanics theory and the COMPASS force field,and analyzed the periodic structure and physical properties of the materials.Finally,the accessible surface area for H₂ molecules in the designed COFs was evaluated by numerical Monte Carlo integration method,and the pore volume was explored by the amounts of non-adsorbing helium molecules adsorbed in a unit mass adsorbent at room temperature and low pressure.For convenience,we named the two types of COFS based on TAPS and TAPA as taps-COFs and tapa-COFs respectively.Based on the optimized structure,we studied the space group symmetries of four taps-COFs and four tapa-COFs.The results show that,within the error range of 0.01（?）,taps-COF-1,tapa-COF-1,taps-COF-2 and tapa-COF-2 hold the P1 space group symmetry with the cubic lattices,while the other two taps-COFs and tapa-COFs maintain the space group symmetries as their initial structures before geometric optimization.The cell length of taps-COF-1 and tapa-COF-1 are nearly equal to that of taps-COF-2 and tapa-COF-2,but taps-COF-3 and tapa-COF-3 possesses the longer cell length than that of tapa-COF-4 and tapa-COF-4.Finally,GCMC is used to predict the hydrogen storage performance.The results showed that the mass hydrogen storage capacity of taps-COF-1 was the highest（51.43 wt%）,while the volumetric hydrogen storage capacity of taps-COF-3 was the highest（58.51 g/L）,tapa-COF-1 was the highest（49.10 wt%）,and tapa-COF-3 was the highest（58.66 g/L）at 77 K.Excitingly,taps-COF-1（8.58 wt%）,taps-COF-2（8.20 wt%）,tapa-COF-1（8.06 wt%）,and tapa-COF-2（7.53 wt%）have relatively a high mass hydrogen storage capacities at room temperature of 298 K,which have exceeded the target（5.5 wt%）of onboard hydrogen storage system for 2025 by the U.S.Department of Energy and the criterion of 6 wt%for commercial use of hydrogen at room temperature.In addition,we also propose a possible synthesis scheme of taps-COFs and tapa-COFs in experiments,which will provide some references for future researchers.