Cobalt-based Metal-organic Frameworks And Carbon Aerogels For Supercapacitor Applications

With the development of science and technology,the demand of human for energy is increasingly urgent.However,the reserves of non-renewable resources such as oil and coal are limited,so the demand for the development of new renewable energy and energy storage devices that can store and convert energy is also increasing.In recent years,supercapacitors have become a hot research area.They have many incomparable advantages over traditional batteries,such as high power density,short charging time,good cycling stability and high safety.However,the energy density of supercapacitors is lower than batteries,so it is very important to explore how to improve the energy density of supercapacitors.According to the energy density calculation formula,the energy density of supercapacitors can be effectively increased by increasing the material specific capacitance of the electrode and widening the operating voltage window of the device.In this paper,a novel core-shell Co-MOF@Co Ni O₂ composite with high specific capacitance was prepared by vapor deposition method and used as an electrode for supercapacitors.In addition,we prepared graphene-based nitrogen self-doped carbon aerogels by combining carbonization and KOH activation methods and applied them to zinc ion hybrid supercapacitors with wide voltage window.Details are as follow:(1)A novel vertically aligned nanosheet arrays of Co-MOF@Co Ni O₂ core-shell composites constructed by in situ grown Co-MOF shell with a uniform and controlled thickness on Co Ni O₂ core via a vapor phase approach.Owing to the intimate contact and synergistic effect between Co-MOF shell and Co Ni O₂ core,the as-synthesized Co-MOF@Co Ni O₂ displays high specific capacitance of about 571.0 F g^-1,which is significantly higher than the pristine NiCo-LDH electrode(380.0 F g^-1).Moreover,the capacitive property of Co-MOF@Co Ni O₂ can be further boosted to 757.2 F g^-1after cyclic voltammetry oxidation.Meanwhile,the the as-assembled EO-Co-MOF@Co Ni O₂/CC//AC device exhibits the high energy density of 27.4 Wh kg^-1 at a power density of 750.0 W kg^-1.The easy preparation and high electrochemical performance of the Co-MOF@Co Ni O₂ composite make it a potential material for supercapacitors application.(2)We synthesized a novel nitrogen self-doped carbon aerogel by a combination of freeze-drying,high-temperature carbonization and KOH activation for zinc ion hybrid supercapacitors.Specifically,the zinc-ion hybrid capacitor device was assembled with nitrogen self-doped carbon aerogels as the cathode,zinc metal foil as the anode and 2 M ZnSO4 as electrolyte,respecitively.The cathode material was prepared by three steps:chitosan aerogel freeze-drying,carbonization and KOH activation.With the increase of activation temperature,the graphene content in the activated carbon aerogel increased continuously.Through the detailed comparison of the effect of activation temperature on the properties and structure of carbon aerogel,it was found that the activated samples at 800?showed the best electrochemical properties.The assembled zinc ion hybrid capacitor has a specific capacitance of about 299.5 F g^-1and an energy density of 106.5 Wh kg^-1 at the current density of 0.1 A g^-1.Furthermore,it delivers an excellent cycling stability with 86.3%capacity retention over 8000 cycles at 2 A g^-1.Crucially,a breakthrough was achieved that two assembled zinc-ion hybrid capacitors in series can energize a LED with 2.5 V for more than 150 h.The nitrogen self-doped porous carbon aerogels with chitosan as precursors have opened up a new way for the preparation of high-performance zinc ion hybrid supercapacitor cathode materials.