Design,Synthesis And Electrode Performance Of ZIF-8 Derived Nitrogen-Doped Porous Carbon And Composites

Supercapacitor,a new type of energy storage device has received considerable attention due to their wide using temperature window and low cost,environmental protection electrode material,can achieve attractive values of specific capacitance and energy density compared with conventional capacitor.In the mean time,supercapacitor can deliver long cycling performance and fast charge/discharge rate due to high power density compared with the battery.As the most crucial part of supercapacitor,electrode material determines the electrochemical performance of the device.Supercapacitor can be divided into electrochemical double-layer capacitors(EDLCs)and pseudocapacitors based on different electrochemical energy storage mechanisms.Recently,metal oxides,conductive polymers as pseudocapacitive materials with excellent electrochemical performance have attracted increasing research interest.Carbon materials are widely used as electrode materials for EDLCs.Generally,the capacitance of these carbon materials is closely related to their specific surface area and pore structures.Therefore,various methods have been adapted to construct porous carbon materials with extremely large surface area for the creation of promising electrode materials for EDLCs.Metal-organic frameworks(MOFs)are new kind of porous materials which consist of active transition metal ions and clusters of atoms.Due to their unique structures,MOFs has a high specific surface area,excellent pore structure and good stability.Recent years,only a few MOFs have been used as electrode materials in supercapacitors.In this work,as an important subclass of MOFs,Zeolite imidazolate framework-8(ZIF-8)is used as a precursor.We presented a straightforward synthesis of hierarchical nanostructures applied for high-performance supercapacitors.The resultant composites have shown excellent electrochemical performance for their unique structures of nitrogen-doped hierarchical porous and three-dimensional conductive network.The whole work is as follows:(1)Through chemical synthesis,2-methylimidazole and the corresponded zinc salt as raw materials.With the changing of zinc salt type(zinc nitrate hexahydrate,anhydrous zinc acetate),mixing methods,and reaction time to achieve the controllable particle size of ZIF-8nanocrystals.In the mean time,fully investigated the crystal structure,morphology and pore structure of ZIF-8.The results have shown that the ZIF-8 crystals exhibit a dodecahedron regular structure with uniform particle size.Then,ZIF-8 crystals are used as precursors to obtain nitrogen-doped porous carbon(ZIF-NCP)by high-temperature carbonization in N₂ atmosphere.The results indicated that ZIF-NCP has a large specific surface area(?825 m² g^-1),stable structure,uniform morphology and excellent electrochemical performance.The small particle size nitrogen-doped porous carbon(s-NCP)deliverd a large specific capacitance of 170 F g^-1 at the current density of 1 A g^-1 in KOH electrolyte,higher than that of middle-size m-NCP(148 F g^-1)and large particle size l-NCP(128 F g^-1);Meanwhile exhibited a high energy density of 30.56W h kg^-1.(2)Interconnected close-packed nitrogen-doped porous carbon polyhedrons(NCPs) confined in a two-dimensional carbon nanosheets(CNSs)have been prepared through a sustainable one-pot pyrolysis of a simple solid mixture of zeolitic imidazolate framework-8(ZIF-8)crystals and organic potassium as the precursors.The hierarchically organized framework of the NCP-CNS composites enables NCPs and CNSs acting as well-defined electrolyte reservoirs and mechanical buffers accommodating large volume expansions of NCPs,respectively.As a result,the NCP-CNS composites with high electrical integrity and structural stability exhibit excellent electrochemical capacitive characteristics in terms of high specific capacitance of 300 F g^-1 at 1 A g^-1,high energy density of 41.7 W h kg^-1 and durable stability with 100%retention after 6000cycles,when employed as an electrode material for supercapacitors.This work therefore provides a simple and efficient strategy for an ordered arrangement of ZIF-8 derived porous carbons as a promising electrode material in next generation energy storage systems.(3)ZIF-8 derived porous carbon polyhedron coated with graphene-like carbon nanosheets composites(MS-ZIF)was prepared by"salt template"method through friendly environmental and one-pot experimental route.ZIF-8 polyhedral with free 2-methylimidazole were used as precursors,eutectic salt system(Li Cl/KCl)were used as template and catalyze.Graphene-like carbon nanosheets directly wrapped on the surface of porous pyrolysis derived from ZIF-8 through carbonization at 800 ^oC,which form a"pea pod"-like hierarchical structure(MS-ZIF).The results have shown that the MS-ZIF composites with"pea pod"hierarchical structure formed cross-linked conductive network and exhibited large specific surface area of 1020 m² g^-1 and high micro-mesoporous volume.It should be noted,due to their unique morphology,in the KOH electrolyte,at a current density of 0.5 A g^-1,the MS-ZIF provided high specific capacitance as high as 256 F g^-1;and exhibits excellent rate ability.At relatively high current density up to 30 A g^-1,the specific capacitance of MS-ZIF still remained a large value of162 F g^-1.In the mean time,MS-ZIF composites exhibited extremely high specific power of30000 W kg^-1,and large specific energy of 35.55 W h kg^-1.