ZIF-Derived Carbon Composites For Aqueous Zinc-Ion Hybrid Capacitors

Zinc-ion capacitors（ZICs）is a kind of newly developed high-performance energy storage device that attracted increasing attention due to its high safety,fast charge/discharge rates,ease of use,and environmental friendly.As one key component of aqueous ZICs,carbon cathode stores energy with electric double-layer capacitance（EDLC）.However,drawbacks that impede the potentials for practical applications are their unmatched kinetics,limited capacitance,and low energy density in aqueous ZIC systems.It is still the issue in the development of high-performance carbon cathodes for ZICs,to meet the need of a variety of portable electrical supplies.Herein,Zeolite imidazolate Frameworks（ZIFs）precursors have been used for the preparation of advanced composite carbon materials via chemical coupling reactions and pyrolysis steps.Interestingly,the metal ions in ZIFs were partially converted into electrochemical active sites and well trapped in carbon skeleton,providing additional pseudocapacitance.Along with large amounts of EDLC,pseudocapacitance of dopants and metal active sites both contribute to overall enhanced specific capacitance and energy density of the as-prepared carbon cathode based ZICs.First,hollow porous carbon composite materials with metallic oxide dopants were prepared from ZIF-L and tetrafluoroterephthalonitrile（TFTPN）,via a one-step pyrolysis process.The morphology and microstructure of hollow carbon composites was investigated by electron microscopes.Their specific surface area,pore structure,and dopants were investigated by surface area analyzer and x-ray photoelectron spectroscope.Afterwards,the as-prepared hollow carbon composites were employed for the fabrication of cathodes,and their performance in aqueous ZICs was systematically assessed.The ZICs prepared with hollow carbon（HC-Co-F1）exhibited high specific capacity of 144 m Ah·g^-1@0.5 A·g^-1 and energy density of 93.7 Wh·kg^-1,and good cycling performance,compared to ZICs assembled with solid carbon cathodes.It was demonstrated that the hollow architecture,porous carbon shell,as well as the electrochemical active sites contributed to overall enhanced electron/charge transfer and the boosted energy storage capabilities.Compared to non-aqueous ZICs,aqueous systems are more attractive due to economic and safety concerns and we mainly carry out the following researches based on aqueous ZICs.Though,carbon-based cathodes with good electrochemical performance have been achieved in the first project,the morphology and structure of the carbons can be varied due to the hollow pyrolysis mechanism.In the next,we demonstrate the preparation of hierarchically porous carbon microspheres as cathodes for high-performance ZICs using ZIF-8 derived composite microspheres,which were obtained from controllable hydro/solvothermal treatment of ZIF-8 with TFTPN,followed by pyrolysis steps.With the use of different composite microspheres,the derived spherical carbon showed differences in specific surface area(1652～2108 m²·g^-1),pore structure,and composition.The electrochemical measurements of assembled ZICs demonstrated the highest specific capacity of 172 m Ah·g^-1@0.5 A·g^-1 and energy density of 141Wh·kg^-1.In addition,all ZICs exhibited excellent cycling stability.With a few advance characterization techniques,the structure-performance relationship has been also investigated.Advanced structure pore provides more double layer capacitors,the doping of N、O、F and zinc oxide might be provides more pseudocapacitance reaction sites,effectively enhancing the specific capacitance and energy density.Overall,we proposed and developed two carbon composites for cathodes and aqueous ZICs application.Systematic investigation on material fabrication,structure analysis,and electrochemical performance have been performed.Besides,their energy storage mechanisms have also been investigated to enrich the understanding of the relationship between composition/structure and electrochemical performance,and to provide reliable theoretical support and experimental evidence for practical carbon-based electrodes and zinc-ion energy storage devices.