Preparation And Electrochemical Performance Of Co-MOF Based Supercapacitor Electrode Materials

Since the 21st century,the excessive usage of fossil fuels has caused environmental pollution and energy crisis.People are trying to find new strategies to develop and utilize environmentally friendly energy to deal with environmental and energy crisis.The research and development of efficient energy storage technologies can support the large-scale application of the renewable energy.Supercapacitors possessing the characteristics of large capacity,superior power density,superior rate performance and outstanding safety,have been recently studied in the field of energy storage.Exploring suitable electrode materials with high specific capacitance and long cycle life is the focus of the current development of supercapacitors.The metal-organic-framework（MOF）material,which presents a large specific surface area and evenly distributed metal centers,can provide a large number of active sites for electric double layer adsorption and pseudocapacitance reaction.Therefore,MOF has been recognized as a potential electrode material for supercapacitors.However,the poor conductivity and single energy storage mechanism of the original bulk crystal MOF seriously limits its electrochemical performance.MOF materials are often used as sacrificial templates or precursors to prepare MOF-derived materials through high-temperature treatment to improve their electrochemical activity.The synthesis is complicated in process and the collapse of the MOF framework could occur.In response to the above problems,this thesis foucus on the perspective of avoiding the usage of high-temperature pyrolysis,and gradually improves the capacitance performance of the original MOF based on simple morphology control.A series of microspherical MOFs derived from Co-BTC with different morphology were prepared by a simple solvothermal reaction.Subsequently,they were used as supercapacitor cathode materials to measure the corresponding electrochemical performance.And then the asymmetric devices were assembled by these MOF microspheres with activated carbon（AC）to verify their practical application.This thesis is mainly divided into five parts,the specific contents are as follows:（1）First,three ammonium salts with different anions oxygen numbers—ammonium chloride,ammonium acetate,and ammonium molybdate were introduced into the reaction.Under the same reaction conditions,three Co-BTC MOFs with different morphologies were obtained,which were bulky crystal（CTB）,block crystal microsphere（CTBM）and nano crystal block microsphere（CTNBM）.The introduction of ammonium molybdate with high anion oxygen content can effectively inhibit the growth of MOF,form nanocrystalline masses,and can also induce orderly accumulation of crystal masses to form a microsphere structure.Nanocrystalline bulk microspheres（CTNBM）have the highest specific capacitance(427.8 F g^-1at 0.5 A g^-1),which indicates that nanosized MOF can provide more reactive sites and ion diffusion channels nanosized.（2）On the basis of the Co-BTC MOF synthesis scheme,the reaction temperature was increased to 120°C,and Co-BTC nanoparticles（CBN）were obtained;under the same reaction conditions,phosphomolybdic acid（POM）with more oxygen content was introduced to obtain Nanoparticle microsphere MOF（CBM）;then a small amount of CNTs were doped into CBM to obtain CNTs@Co-BTC（CCBM）.The intervention of POM effectively inhibits the growth of MOF crystal nuclei and promotes the orderly accumulation of nanoparticles into a microsphere structure.Therefore,the particle sizes of the nanoparticles in CBM and CCBM are smaller than that in CBN.In addition,the doping of a small amount of CNTs effectively reduces the internal resistance of MOF microspheres,while providing more ion diffusion channels.Therefore,CCBM has the highest specific capacitance,which reaches 557.3 F g^-1 at a current density of 1 A g^-1.The charge storage mechanism research in the electrochemical process also confirms that CCBM is more controled by diffusion.（3）In the preparation process of Co-BTC,the urea solution was introduced and the reaction temperature was raised to 180°C,and the nanowire microspherical MOF（CBNWM）was successfully obtained.The original crystal bulk Co-BTC was also prepared as a reference for comparison.The specific surface area of CBNWM is greatly increased compared to the original crystal bulk Co-BTC（CBC）.And the specific capacitance and cycle stability of CBNWM are much higher than CBC,which is due to that the nanowire microsphere structure can provide a larger ion contact area and more ion diffusion channels per unit mass,and improves the mass transfer efficiency of the electrode-electrolyte interface.The charge storage mechanism of CBNWM in the electrochemical process is further studied by the CV curve at low scan rates.The results show that CBNWM is affected by both capacitance and diffusion control and is more controlled by diffusion at the low scan rate of 5 m V s^-1.The specific capacity of CBNWM reaches 657 F g^-1 at 0.5 A g^-1 with the capacity remains 81.4%after 3000cycles.Subsequently,an asymmetric device of CBNWM//AC was fabricated,whose maximum energy density can achieve 34.4 W h kg^-1 with a power density of 375.3 W kg^-1.（4）The use of a large-volume hydrothermal kettle（50 m L）with an unchanged feed can increase the pressure in the reaction,thereby obtaining Co-BTC microspheres（CBTM）at 150°C.The crystallinity of MOF is different at various temperatures.On the basis of the CBTM synthesis scheme,the reaction temperature is reduced to 100°C after half of the CBTM reaction time,and finally the core-shell structured Co-BTC microsphere（CCBTM）is obtained.Compared to CBTM,the mass specific capacitance of CCBTM is greatly improved,which is attributed to the outer shell layer can enrich the energy storage mechanism of MOF microspheres.Subsequently,a flexible all-solid-state asymmetric device of CCBTM//AC was assembled,which can achieve a maximum energy density of 42.8 W h kg^-1 at a power density of 750.0 W kg^-1.The fully charged device can enable the electronic thermometer and hygrometer continue to work for 46 minutes,showing very good practical performance.This work can provide a perspective for the design and synthesis of MOF microspheres with multiple structural levels.（5）In the synthesis,a larger-volume hydrothermal kettle（100 m L）was used and the amount of solvent was reduced,and the Co-BTC（CBHM）with a hollow microspherical morphology with uniform pore distribution on the surface was successfully obtained.This structure greatly increases the specific surface area and ion transmission rate of Co-BTC MOF,so the mass specific capacitance of CBHM is greatly improved compared with the various morphologies of Co-BTC in the previous parts.The electrochemical performance of CBHM in different electrolytes is studied in order to obtain a suitable electrolyte to maximize its energy storage capacity.CBHM shows extremely high electrochemical performance in 1 M Na OH electrolyte which can reach a specific capacitance of 1602 F g^-1 at a current density of 1 A g^-1.The EIS test results show that the ions in the Na OH solution have the shortest dielectric relaxation time in the CBHM electrode.The assembled CBHM//AC asymmetric device with the PVA-Na OH as the electrolyte exhibits a high energy density of 76.9 W h kg^-1at a power density of 800 W kg^-1.Two devices connected in series can light up 62 red LEDs for 2 minutes,which demonstrates a promising application potential.