Controllable Preparation Of Metal-organic-frameworks Derived Carbon Materials And Their Capacitive Storage Performance

With the development of modern science and technology,the requirement for energy storage device is increasing day by day.Supercapacitor is one of the most important electrochemical energy storage devices,but its energy density can not satisfy the social demands.It is necessary to develop new electrode materials for high-performance supercapacitor.As an important branch of metal organic frameworks,zeolitic imidazolate frameworks(ZIF)have been widely studied.ZIF-derived carbon materials with high nitrogen-doping content and large specific surface area,are the promising candidate for electrode materials of supercapacitors.However,ZIF derived carbon materials exhibit some disadvantages,such as agglomeration,unreasonable pore distribution,damaged structure and poor electrical conductivity,restricting the further development in supercapacitors.Therefore,the development of new preparation methods for ZIF derived carbon materials is pivotal for the supercapacitor electrode with excellent electrochemical performance.In this paper,ZIFs were used as precursors to synthesize a series of carbon materials,with the methods of pore designing,element doping,hybrid compositing and morphology controlling.The formation mechanisms and influence factors have been proved,and then the controllable preparation of the materials has been realized.The relationship between the structure,composition and capacitance performance of the derived carbon materials has been studied systematically,and the supercapacitor with high energy density and power density have been assembled.(1)Hierarchical pores structure was constructed for the ZIF-8 derived carbon materials to improve the mass transfer kinetics of the electrochemical reaction.In detail,hierarchical porous honeycomb-like carbon frameworks(HHCF)was prepared via carbonization and activation with ZIF-8 as precursor and PVP as carbon source.In the hierarchical porous structure,homogeneous mesoporous network is acted as carbon skeleton and a large number of micropore is distributed on the mesopore wall.The role of PVP on the pore structure,and the influences of pore structure control and electrolyte selection on the electrochemical properties of carbon materials were studied.The mesoporous network structure is affected by the added amount of carbon source.As the electrode material for supercapacitor,HHCF shows high specific capacitance(361 and174 F g^-1 in alkaline and ionic liquid electrolytes,respectively),high energy density(79Wh kg^-1)and excellent rate performance.In addition,solid-state supercapacitor device was assembled by using HHCF electrodes and ionogel,which shows promising energy density(63 Wh kg^-1).Experiments demonstrate that the hierarchical porous structure can facilitate the electrolyte diffusion and increase the active specific surface area.(2)Aiming to improve the energy storage performance of ZIF-8 derived carbon materials,pore structure design and heteroatom doping are introduced to increase the active sites of electrochemical reaction.The N,O doping hierarchical porous carbon integrated with carbon cloth(HPC/CC)was prepared by using ZIF-8 as precursor and PAN as carbon source after drop injection and carbonization processes.The influence of carbonization temperature on the morphology and composition of carbon materials was studied,and the formation process of carbon materials was revealed,as well as the electrochemical performance of carbon materials in Zn-ion hybrid supercapacitor(ZHS)was studied.With the advantages of high specific surface area and abundant heteroatom doping,the HPC/CC can be used as the cathode material for ZHS.The HPC/CC exhibits high specific capacity,excellent rate performance(the specific capacity still maintains75 m Ah g^-1 when the current density raised to 20 A g^-1),long cycle stability,high energy density(110 Wh kg^-1)and power density(20 k W kg^-1).Furthermore,the mechanism of energy storage and the effect of doping elements are revealed.The dopant oxygen can provide the active sites for the chemical adsorption of H⁺and Zn²⁺as pseudocapacitance reaction,while nitrogen dopant can improve the electrolyte wettability and increase the conductivity of electrode materials,indirectly promoting the physical/chemical adsorption/desorption process on the surface of electrode materials.(3)Using 3D carbon networks with different pore structures as the substrate for the growth of ZIF particles,ZIF/3D carbon composites with high conductivity were prepared.The effects of reaction temperature,and pore structure and amount of 3D-carbon network on the particle size and distribution of ZIF crystals were studied.Then,the controllable preparation of dispersed ZIF crystals with various particle sizes was realized.A unique material with the structure as"spider web"pattern on the surface of three-dimensional carbon network(S-3DCN)was obtained after carbonized the ZIF-8/3D-carbon composite precursor.The bionic structure can promote the materials wetting,and then improve the fast diffusion of electrolyte ions in the aqueous electrochemical reaction system.The S-3DCN shows high capacitive performance,such as high specific capacitance and rapid charge discharge capacity(the specific capacitances are 395 and 190 F g^-1 at the current densities of 1 and 50 A g^-1,respectively).In addition,the assembled solid-state supercapacitor device exhibits high energy density and power density.(4)To avoid the serious agglomeration of metal particles,collapse and damage of microstructure of 2D Co-based ZIF layer(Co-ZIF-L)during the carbonization process,a novel vapor-assisted growth method was introduced to fabricate the multi-dimensional hybrid carbon material(MHC)with high reactive sites and enhanced conductivity.With the Co-ZIF-L array as a precursor and Zn-ZIF as a assistant,the MHC material with high doping heteroatom is consisted of carbon onion,carbon nanotube and two-dimensional carbon nanosheet array.The unique structure overcomes the metal agglomerate and microstructure damage.The effects of reaction temperature,the amount and kinds of assistant materials on the micromorphology of the MHC were studied,and the formation process of MHC was discussed.The experiment results demonstrate that the introduction of the assistant can maintain the pristine structure of precursor during the carbonization process,and the zinc vapor released by the excipients is the key factor for the formation of carbon nanotubes.Directly acted as electrode of supercapacitors,MHC shows high specific capacitance(360 F g^-1),high energy density(6.65 Wh kg^-1),fast charge-discharge performance(power density of6571 W kg^-1)and excellent cycle life.This work provides a new understanding for the growth,structure,carbonization and composite of ZIF particles,and shows a new pathway for the practical applications of MOF-derived carbon and composite materials in the field of electrochemical energy storage.