| With the advent of the carbon neutrality era,the development of new energy has ushered in historic opportunities.Hydrogen energy,as one of the promising new energy sources,is rising rapidly to the sun at an amazing speed.How to store and transport hydrogen energy safely and effectively has always been the bottleneck restricting the development of hydrogen energy.As a two-dimensional silicon material,silicon-based nanosheets have attracted much attention due to its unique structure and excellent electronic properties,and has potential application value in the field of hydrogen storage.However,silicon-based nanosheets are not suitable to be directly used as a hydrogen storage material owing to its low hydrogen absorption and low hydrogen binding energy.Therefore,it is necessary to find suitable modification methods to improve hydrogen storage capacity effectively base on its characteristics of easy functionalization and large specific surface area.In view of this challenge,taking silicon-based nanosheets as the research object,a series of silicon-based nanosheets hydrogen storage composites were prepared by selecting MOFs materials,transition metal Pd,Ni and alkali metal Li through different modification strategies.The effects of the structure,microstructure and composition on hydrogen storage properties of the composites were systematically studied,and the mechanism of hydrogen storage performance improvement was discussed.(1)Based on the reaction mechanism of topological chemistry,Ca Si2 as a raw material,methanol,ethanol,isopropanol and ethylene glycol were used to prepare four kinds of SNS products with different structure,morphology and performance under improved technological conditions.The effect of the reaction solvent on the structure,morphology and hydrogen storage performance of the product was studied.The results showed that prepared SNS had loose structure,thin lamellar thickness,low oxidation degree,no other impurities,large specific surface area and microporous pore size distribution.In the hydrogen adsorption test,the product showed a large hydrogen absorption capacity and a fast hydrogen absorption rate,whereas the micronization was serious and the cycle stability was relatively good after the hydrogen storage test.The reaction mechanism of preparing SNS by topological chemistry method was analyzed through the theory of solvation effect and induction effect.The results showed that the faster ionization rate of methanol was helpful to realize the rapid replacement and stripping of ions,and ensured the rapid functionalization of the product surface and the high efficiency of the reaction.(2)The metal-organic-framework compound Cu3(BTC)2was synthesized in situ on SNS by the microwave method.The effects of microwave reaction conditions on the structure,micro-morphology,specific surface area and hydrogen storage properties of the material were investigated.The compound with SNS inserted into Cu3(BTC)2grains was prepared by step-temperature control with the specific surface area of 875 m2·g-1and microporous.The hydrogen adsorption capacity was 5.6 wt%and the hydrogen diffusion coefficient was 10-7,which have been greatly improved.The study of catalytic mechanism showed that the process of in-situ compositing not only introduced a large number of micropores strengthening the potential field overlap on the surface of the narrow pore wall,but also the SNS between the grains provided more channels for the internal and external diffusion of hydrogen molecules.More unsaturated metal sites cuased by the electron transfer from Cu in Cu3(BTC)2to Si in the matrix.The increase of hydrogen adsorption capacity and adsorption rate were realized.(3)Pd-Ni/SNS composite was prepared with deposition by transition metal Pd and Ni of 5 wt%,10 wt%and 15 wt%on SNS.10 wt%-Pd-Ni/SNS had stable composition and structure with uniform in size dispersing on the surface of SNS without agglomeration.It had the maximum hydrogen adsorption capacity of 4.15 wt%and good kinetic performance with the hydrogen diffusion coefficient of 10-6at 450 K.The study of catalytic mechanism showed that the SNS was closely combined with the deposited metal through?bond on the surface to avoid agglomeration.Pd and Ni donated electrons to the SNS surface forming local electric field to strengthen the effect of Kubas which adsorbed hydrogen with binding energy between physical adsorption and chemical adsorption.Also,Pd and Ni,as the centers of active sites,activated secondary active sites through the hydrogen overflow mechanism and promoted the improvement of hydrogen adsorption capacity of the carrier.(4)Pd-Li/SNS with 6 wt%,13 wt%and 20 wt%Pd/Li deposited on the surface of SNS were obtained by deposition precipitation method.The effects of material structure,micro-morphology,element distribution and surface functional groups on the thermodynamic and kinetic properties of hydrogen adsorption were discussed.The results showed that the overall hydrogen diffusion performance of SNS was greatly improved due to the modification of Pd/Li,reached 10-6.At 475 K,13 wt%-Pd-Li/SNS had the maximum hydrogen adsorption capacity of 4.5 wt%,and the adsorption speed was the fastest.The study of improvement mechanism on hydrogen storage showed that transition metal Pd adsorbed hydrogen by Kubas and promoted the hydrogen adsorption capacity through hydrogen overflow mechanism,while alkali metal Li formed a local electric field with Si by s-p and p-p hybridization,and builded an adsorption bridge between hydrogen and SNS by electrostatic Coulomb effect and polarization mechanism.The two mechanism promoted each other and worked together.It is an effective method to improve the hydrogen storage performance of SNS,modifying by different structures and characteristics of Pd and Li,which combined different mechanisms for synergistic catalysis. |
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