Development And Performance Study Of Novel Thermochemical Adsorption Heat Storage Materials Based On MOFs

Thermal storage technology can effectively solve the problems of mismatch between the supply and demand in solar thermal utilization.Thermal chemical adsorption heat storage can realize long-term heat storage by isolating the contact of reactants.It has the advantages of high heat storage density and low heat loss.It is suitable for interseasonal solar energy storage at medium and low temperature.As the most commonly used thermochemical adsorption heat storage materials,hydrated salts are prone to deliquescent caking,performance attenuation and solution leakage in the application process,and the injection of hydrated salts into porous matrix to produce composite hydrated salts can effectively solve these problems.Therefore,it is crucial to develop high-performance composite hydrated salt materials.Metal-Organic Frameworks（MOFs）are a new class of porous materials,which has the characteristics of large specific surface area and pore volume,functional regulation of particle size and skeletal pore structure,and is one of the potential thermochemical adsorption heat storage materials.However,the research on the application of MOFs materials in thermochemical adsorption heat storage is very rare,and the influence of MOFs particle size and skeletal pore structure on the properties of composite hydrated salt materials has not been paid attention to.Therefore,this paper synthesized Ui O series MOFs,including Ui O-66 and Ui O-67,and the crystal size of Ui O-66 was regulated to be 100nm,200nm,400nm and 1000nm,respectively.Using an impregnation-drying synthesis method,Ui O-66 and Ui O-67 were used as porous substrates,and Ca Cl₂ was used as a filling salt.A series of composite salt hydrates materials were prepared by changing the concentration of impregnated salt solution and the particle size of Ui O-66,and the properties of the materials were studied in detail.Firstly,the performance of physical adsorption heat storage of Ui O-66 and Ui O-67 was investigated.The results show that the water vapor adsorption capacity of Ui O-66 increases with the increase of particle size,in which the adsorption capacity of Ui O-66（100）is 0.40g/g,and its heat storage density is 632J/g,and it maintains good stability in five cycles of adsorption-desorption.However the adsorption capacity of Ui O-67 is only 0.25g/g,and the adsorption capacity of water v apor almost lost after the first sorption-desorption.Therefore,Ui O-66 is suitable for physical adsorption heat storage and has very considerable heat storage potential,while Ui O-67 is not.Secondly,the performance characterization results of the compos ites showed that the crystal structure of Ui O-66 was stable in the composite and is suitable for the preparation of composite salt hydrates,while the crystal structure of Ui O-67 was seriously damaged.Ui O-66 of small particle size had nano intergranular p ores in which Ca Cl₂ had better dispersion.The salt content of the composite is affected by the specific surface area,pore volume and particle size of Ui O-66.When impregnated with salt solution of the same concentration,the larger the specific surface a rea and pore volume,the higher the salt content is,while the larger the particle size,the lower the salt content is.The specific surface area and pore volume of the composites decrease significantly with the increase of salt content.Subsequently,the adsorption performance of the composite materials showed that the adsorption capacity and rate of water vapor increased with the increase of salt content and environmental humidity,and the adsorption capacity of U（100）-Ca30 was up to 1.80g/g at 30℃and 80%RH.In addition,the adsorption performance of the composite is also affected by the particle size of Ui O-66.The increase of particle size leads to the less contact between Ca Cl₂ and water molecules,resulting in lower adsorption capacity of the material and longer time to reach the adsorption equilibrium.The desorption performance of U（100）-Ca30 composite was significantly improved compared with that of pure Ca Cl₂ due to the porous structure of matrix,and82%of water molecules can be desorbed at 100℃.The calculation results show that the desorption activation energy of the composite U（100）-Ca30 in the low temperature region is lower than that of Ui O-66,and the lower desorption activation energy is conducive to the rapid progress of the reaction.Finally,the research results of heat storage performance show that the heat storage density of composite materials is positively correlated with the water vapor adsorption capacity of the material,and increasing the salt content and choosing the appropriate matrix particle size can improve the heat storage density of composite materials.Among them,U（100）-Ca30 has a high heat storage density of 1416J/g,and only 3.1%of adsorption capacity is lost after 16 cycles of adsorption and desorption.After six months of bearing Ca Cl₂ solution,the crystal structure and surface properties of Ui O-66 kept unchanged.It can be seen that the composite hydrated salt material has good regeneration stability.