Synthesis Of Zeolitic Imidazolate Framework Series Materials And Study Of Their Electrochemical Properties

With the development of society,the lack of energy and environmental pollution caused by the burning of fossil fuels has become serious problem.The development of efficient energy storage devices has turned into an important challenge for human beings in the new century.Supercapacitor,as a new type of energy storage device,which has the characteristics of rapid charge-discharge,long cycle life,high power density,etc.,has attracted much attention.The key of exploring new supercapacitors is to find new electrode materials.Metal-organic frameworks（MOFs）,as a popular material in rencent years,are widely used in energy storage devices due to the characteristics of large specific surface area and suitable pore structure.Zeolitic imidazolate frameworks（ZIFs）are a branch of MOFs materials with better stability.Based on this,three types of ZIFs with core-shell structure were synthesized and a series of tests were conducted as supercapacitor electrode materials in this paper.The study mainly includes the following contents:1.Amorphous ZIF material（Amorphous ZIF）was successfully synthesized using zinc nitrate hexahydrate and 5,6-dimethylbenzimidazole,and ZIF-67 crystals were synthesized using cobalt nitrate hexahydrate and 2-methylimidazole.And ZIF-67@Amorphous ZIF core-shell structure material was synthesized using ZIF-67 as the core.A series of characterization of the above three materials was tested,including scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）,infrared spectroscopy（FT-IR）and BET specific surface area（BET）.While these three materials were loaded onto nickel foam（NF）to obtain working electrode,respectively,the electrochemical test was executed in a three-electrode system.The results showed that the core-shell structured material exhibited the most excellent performance.At a current density of 1 A g^-1,its specific capacitance can reach to1176.8 F g^-1.After 1000 cycles of charge-discharge,the specific capacitance can still maintain 98%of its initial capacitance.The electrochemical tests were carried out in a two-electrode system,when ZIF-67@Amorphous ZIF was applied for electrode materials of a simple asymmetric supercapacitor device.The energy density of the device can reach 23.4Wh kg^-1,and the specific capacitance remained almost unchanged after 2000 cycles of charge-discharge.Finally,four fully charged simple devices can light up two LED lights.2.ZIF-7 was firstly synthesized with zinc nitrate hexahydrate and benzimidazole,and ZIF-67 was synthesized with cobalt nitrate hexahydrate and 2-methylimidazole.Then core-shell structure material ZIF-7@ZIF-67 was prepared by using ZIF-7 as the core and ZIF-67 as the shell.After that,the above three materials were tested by SEM,TEM,XRD,XPS,and BET to characterize their structure and morphology.In a three-electrode system,electrochemical tests were performed by loading the above three materials as the working electrodes.The results indicated that the core-shell structured ZIF-7@ZIF-67 had better performance than two single materials.Its specific capacitance can reach 518.9 F g^-1 at a current density of 1 A g^-1,and the specific capacitance can still maintain 99.6%after 4000cycles of charge-discharge.All-solid-state asymmetric supercapacitor devices were assembled by using PVA/KOH as the electrolyte,core-shell structured ZIF-7@ZIF-67 as the electrode material,and the electrochemical tests were carried out.The energy density of the device can reach 31 Wh kg^-1,and the specific capacitance can still remain 99.5%after 10000cycles of charge-discharge.Finally,two devices were connected in series to light up various colored LED.3.ZIF-90 was successfully synthesized using zinc nitrate hexahydrate and imidazole-2-formaldehyde,and ZIF-67 was obtained using cobalt nitrate hexahydrate and2-methylimidazole.Then core-shell structure material ZIF-90@ZIF-67 was synthesized by the direct growth of ZIF-67 on the surface of to ZIF-90.The as-synthesized three materials were tested by SEM,TEM,XRD,XPS,and BET.Then they were loaded on NF to form working electrodes,and the electrochemical tests were carried out.The results indicated that the core-shell structure ZIF-90@ZIF-67 manifested the best performance among the three materials.At a current density of 1 A g^-1,the specific capacitance can reach to 1357.8 F g^-1.After 4000 cycles,the specific capacitance can still maintain 90.1%.After applying the working electrodes containing ZIF-7@ZIF-67 to all-solid state asymmetric supercapacitor devices,electrochemical tests were performed in a two-electrode system.At a current density of 0.5 A g^-1,the specific capacitance of 151.6 F g^-1 was achieved.The energy density can reach up to 53.9 Wh kg^-1 at a power density of 150 W kg^-1.After 10000 cycles of charge-discharge,the specific capacitance of the device can still remain 95.8%.Finally,two devices also successfully light up the red LED light.The above experimental results indicated that all these materials exhibited excellent energy storage performance and stability,implying that they had the potential to be applied to fabricate the supercapacitor.