Application Of Porous Carbon-Based Materials Derived From 3d^7-10 Transition Metal-Organic Frameworks In Supercapacitors

With the increasing demand for energy due to the development of the society,supercapacitor,as an environmentally friendly energy-storage device,has received a great attention and is considered to be one of the most promising electrochemical energy-storage device owing to their advantages of high power characteristic,excellent cyclic stability and rate capability.In recent years,preparing asymmetric supercapacitors has become a strategy to effectively improve the energy storage capacity,energy density and power density.The device consists of a positive electrode material with high specific capacitance and good cycling stability as well as a negative electrode material with excellent rate capability and electrical conductivity.Therefore,it has become a hot topic of research in recent years to select a positive material and a negative material and then to assemble them into asymmetric supercapacitors to achieve the excellent electrochemical energy storage performance of the two electrode materials.Porous carbon-based materials derived from metal-organic frameworks（MOFs）have become a new type of electrode materials due to their continuous spatial framework structure,large specific surface area and special 3D structure.The metal（oxides）/porous carbon composites prepared by direct carbonization of MOFs are good cathode materials due to their abundant redox active centers and excellent electrical conductivity.Through subsequent activation and other treatments,nanoporous carbon materials with high specific surface area and ideal pore volume can be obtained as an effective anode material,which is beneficial to improve the ion mobility and specific capacitance of supercapacitors.Various materials derived from MOFs have been developed for electrochemical applications.In this paper,N-containing ligands:4,5-imidazole-dicarboxylic acid（ICA）and 1,2,4-triazole-3-carboxylic acid（TCA）were selected to coordinate with transition metals to obtain different N-containing precursors,including Zn-MOFs,Co-TCA,Ni-TCA and Ni Co-TCA.By direct carbonization of MOFs,N-doped porous carbon PANC with layered porous structure was obtained as the cathode material and the N-doped metal/porous carbon composites NC/Co,NC/Ni,NC/Ni Co with the structure of porous carbon coated metal nanoparticles were obtained as cathode materials.The materials were modified by adjusting the experimental conditions to improve the electrochemical performance.In the three-electrode system,benefiting from the high content of N,large specific surface area(1823.13 m²g^-1)and excellent pore structure,N-rich porous carbon2-PANC2/1-800 exhibited high specific capacitance of 277.8 F g^-1(1.0 A g^-1)in the voltage range of-1～0 V,and delivered excellent rate capabilities along with cyclic stability of73.9%capacitance retention after 3000 cycles at 3.0 A g^-1.For NC/Co,NC/Ni and NC/Ni Co,the electrochemical performance was tested in the positive voltage range.When the mixed mass ratio of Co-TCA and melamine is 1.5:1 and the mixed mass ratio of Ni-TCA and melamine is 1:1 before carbonization,NC/Co-3 and NC/Ni-3 obtained the highest specific capacitance among similar materials,which are436.48 F g^-1and 331.69 F g^-1at 1.0 A g^-1,respectively.When the mole ratio of Ni²⁺and Co²⁺is 1:1 during Ni Co-TCA synthesis,the specific capacitance of obtained NC/Ni Co1/1can reach 595.76 F g^-1at 1.0 A g^-1,and the specific capacitance retention can reach 92.30%after 3000 cycles at 3.0 A g^-1.By analyzing the working mechanism of NC/Co-3,NC/Ni-3and NC/Ni Co1/1 electrodes,it is proved that the current originates from two main mechanisms:Faradaic diffusion process and surface capacitive response.After that,NC/Co-3,NC/Ni-3 and NC/Ni Co1/1 were used as positive electrodes and2-PANC2/1-800 and activated carbon（AC）were separately utilized as negative electrodes to assemble asymmetric supercapacitors.The NC/Ni Co1/1//2-PANC2/1-800 device displayed better electrochemical results than those of any other devices,delivering an appreciable energy density(31.69 Wh kg^-1)at the corresponding power density(800 W kg^-1)along with cyclic stability of 78.72%capacitance retention after 3000 cycles at 3.0 A g^-1.Finally,NC/Ni Co1/1//AC and NC/Ni Co1/1//2-PANC2/1-800 devices were assembled into the button cells and successfully employed to power four red light-emitting diodes（LEDs）,indicating the potential practicability.