Fabrication And Electrochemical Properties Of MOFs-derived Porous Carbon-Based Composite Electrodes

With the rapid economic growth and the increased consumption of the non-renewable resources,energy storage equipment with the advantages of low cost,high energy density,environment-friendly and cycling stability has attracted widespread attention.Among a wide range of energy storage devices,the battery is the most commonly energy storage device,because it can store energy and release energy steadily with light mass and small volume.However,the slow energy transmission of the battery limits its application.Supercapacitors are considered to be one of the most promising energy storage equipment,due to the high energy and power density,short charging time and long cycling life.As an important component of supercapacitors,the light,flexible and high-performance electrode materials are driving the rapid development of supercapacitors.Moreover,porous carbon is widely used in supercapacitors due to their high specific surface area.Carbonization is one of the common methods to prepare porous carbon materials.And different precursors can be used to prepare different porous carbon materials with different pore size,such as starch,wood,organic polymers,animal hair and metal organic frameworks.In this thesis,metal organic frameworks（MOFs）with large specific surface area and high porosity have been used as carbon precursor or template to prepare various porous carbon materials.Since the porous carbon materials mainly base on the electric double layer energy storage mechanism,the porous carbon materials cannot achieve high mass specific capacitance.Based on the high conductivity and highly specific surface area of porous carbon materials,the corresponding composite modification with pseudocapacitive materials（MnO₂,PANI）were further fabricated.To improve the flexibility of the electrode,an electrospinning strategy was used to prepare MOFs-based flexible composite nanofibers membrane electrodes and a wet-spinning fiber method was used to prepare MOFs-based composite fibers.The main contents are as follows:1.Porous carbon materials with different morphologies,various electrochemical properties and highly specific surface areas were fabricated by directly carbonization of Zn-MOFs at different temperatures,which were denoted as MNCx（x is the carbonization temperature）.SEM images showed the Zn-MOFs displayed a well-defined polyhedron feature with a size of about 300 nm.After carbonization at 950℃,the sample showed the same morphology.MNC950 displayed the complete polyhedral structure after sonication with most amorphous carbon structure and partial graphene structure,which could be proved by TEM image.XRD and Raman results indicate that the high carbonization temperature adopted was beneficial to enhance the graphitic degree of porous carbon.The Brunauer-Emmett-Teller（BET）surface area of MNC950 was calculated as 906 m² g^-1by nitrogen desorption/adsorption technique.Electrochemical performance results showed that the MNC950 exhibit the best specific capacitance of 147 F g^-1(at current density of 0.1 A g^-1)with a low equivalent series resistance,indicating that the MNC950 can be used as the good conductive matric for the further preparation of composite electrode materials.2.MNCMn60 was fabricated by directly depositing MnO₂ nanodots within the porous structures of MNC950 under vacuum.The deposition process was well controlled and MnO₂nanodots in the porous carbon enabled the sample to process high specific capacitance.Then the asymmetric supercapacitors were assembled by MNC950 and MNCMn60.SEM and TEM images confirmed the MnO₂ nanodots were confined inside the porous structures of MNC950 without coating outside the surface.The existence of MnO₂ nanodots in MNCMn60 was further proved by the XRD,Raman and XPS spectrums.Nitrogen adsorption and desorption isotherms showed that the appropriate content of MnO₂ inside the porous structures did not change the original porous structure.Under the two-electrode test system,the specific capacitances of the composite electrodes gradually increased with the increase of the reaction time.The MNCMn60 showed a specific capacitance of 163 F g^-1(at current density of 0.1 A g^-1).Asymmetric supercapacitors were assembled by MNCMn60 as cathode electrodes and MNC950 as anode electrode.The assembled asymmetric supercapacitor exhibited a stable electrochemical window of 2.2 V,a high energy density of 76.0 Wh kg^-1 and a high power density of 22.0 kW kg^-1 with a long cycle stability,and the retaining ratio was 87.3%after 3000 cycles).3.MOFs-based porous carbon materials（MC）were fabricated by directly carbonization of Zn-MOFs.And we used MC as the substrate to deposit PANI nanowires by in situ polymerization.The morphologies of the deposited of PANI of composites were dominated by altering the deposition time and doped acid.SEM and EDS images showed the PANI nanowire arrays aligned on the MOFs-derived porous carbon.The nanowire arrays,consisted of PANI,proved by FTIR,Uv-vis and XPS spectrums.Under the three electrode test systems,the MOFs-derived porous carbon/polyaniline hybrid electrode displayed a maximum specific capacitance of 534 F g^-1 at a specific current of 0.2A g^-1 and a maximum capacitance retention of 211%at a specific current of 2 A g^-1 after 20000cycles,demonstrating new opportunities for pseudocapacitive devices.4.With the aim to improve the flexibility of the electrode,MOFs are grown in situ on the polyimide nanofiber membranes which were fabricated by the electrospinning strategy,and further carbonized to obtain flexible composite nanofibers membranes（PMC）.Then polyaniline nanowires arrays deposited on the PMC to create flexible hybrid electrodes（PMCP）and further assemble all-solid-state supercapacitors.SEM images showed the electrospun polyimide nanofiber membranes are composed of uniform nanofibers with a smooth surface without beads or breakages.SEM images of PMC showed a well-defined layer of nanoparticles distributed across the surface of each nanofiber and SEM images of PMCP showed PANI nanowire arrays uniformly covering the external surface of nanofiber.EDS images showed Zn element existed in the composite fiber.The nanowire arrays,consisted of PANI,proved by the FTIR,Uv-vis,Raman and XPS spectrums.Under the three electrode test systems,PMCP hybrid electrode displayed a specific capacitance of 469 F g^-1.Then the flexible all-solid-state supercapacitors were assembled with the polyvinyl alcohol/H₂SO₄ gel,which was used as a separator and the electrolyte.The assembled device showed a high volumetric specific capacitance of 1973 mF cm^-3 with a good flexibility（an 83.6%capacitance retention after400 bending times）and a long cycle stability（an 84.9%capacitance retention after 10000 cycles）.5.A wet-spinning fiber method was used to prepare graphene oxide（GO）/MC composite fibers.Due to theπ-πinteraction and hydrogen bond between the activated MC and GO,the activated MC can be well embedded between the GO sheets,and the tensile strength of the composite fibers could reach 86.5 MPa.Afterwards,composite fibers were further reduced by hydrazine vapour at 90℃to obtain the RGM12 composite fibers.Under the three electrode test systems,the RGM12 composite fibers exhibited a high volumetric specific capacitance of 56.1 F cm^-3 with a good cycle stability（an96.6%capacitance retention after 10000 cycles）which could be selected as fiber-shaped carbon substrate to composite pseudocapacitive materials.PANI and MnO₂ were deposited on the RGM12fiber,denoted as RGM12P and RGM12Mn,respectively,exhibiting a high volumetric specific capacitance of 65.1 F cm^-3 and 74.2 F cm^-3.With the aim to improve the practical applications,asymmetric supercapacitors are further assembled by RGM12Mn as anode electrode and RGM12as cathode electrode in different electrolyte.In the neutral electrolyte（1 M Na₂SO₄）,the stable electrochemical window can be extended to 2 V with a high energy density of 75.0 Wh kg^-1 and a long cycle stability（an 75.3%capacitance retention after 10000 cycles）.In the ionic liquid（Et₄NBF₄）,the stable electrochemical window can be extended to 3 V with a high energy density of57.2 Wh kg^-1 and a long cycle stability（an 87.3%capacitance retention after 10000 cycles）.