Based On Nickel-cobalt Metal Organic Framework Derived Bimetallic Sulfide/Nitride And Capacitive Performance Study

With the rapid improvement of people’s quality of life,the development of flexible wearable electronic devices has become increasingly important.Clean,low-energy and efficient novel energy storage devices are the key components.For the urgent demand of mobile portable energy devices,reliable energy conversion and storage devices are developed as their connecting devices.In particular,flexible supercapacitors（SCs）,a new type of energy storage device between batteries and conventional capacitors,have attracted a lot of attention from researchers because of their extremely power density,long cycle life,portability,stability and high efficiency,and are considered to have a strong competitive edge to become the next generation of high-performance energy storage/conversion devices.The advantage is that loading or in-situ growth of the active material on its flexible substrate eliminates the effects of conventional compression processes and binders on the capacitive performance and stability of the electrodes.According to the energy density equation（E=1/2CV²）,There are two approaches that can be used to increase the energy density of SCs.One is to increase the specific capacitance of the electrode material,and the other is to expand the operating voltage.Consequently,it is important to select electrode materials with high electrochemical activity and large theoretical specific capacitance.Metal organic frameworks（MOFs）serve as extremely excellent templates and precursors with many excellent properties derived from various nanostructures,such as structural porosity,pore volume tunability,and structural sustainability.In recent years,transition metal-derived sulfides（TMSs）and nitrides（TMNs）have been considered as extremely reliable electrode materials and have been intensively studied.Sulfides and nitrides have better metal-like properties and higher electrical conductivity than hydroxides and oxides,and thus flexible asymmetric supercapacitors（ASC）with faster flexibility can be assembled using active materials with higher specific capacitance and expanded working potential windows,making it possible to obtain high energy densities.Therefore,constructing ASC is the most effective strategy to improve energy density,and selecting suitable electrode materials is an important condition to obtain high performance ASC.In this paper,Ni/Co metal cation source,dimethylimidazole as organic ligand and commercial carbon cloth as flexible substrate were used for the in situ synthesis of nickel-cobalt metal organic framework（Ni Co-MOF）,nickel-cobalt sulfide（Ni Co₂S₄）and nickel-cobalt nitride（Ni₃N/Co N/NC）by solvent heat,hydrothermal and high temperature annealing methods,respectively,and their electrochemical properties were investigated in depth:（1）Design and preparation of Ni Co₂S₄（denoted as Ni Co-S）network structure electrode material.The Ni Co-MOF was grown in situ on carbon cloth by the solvothermal method,and then the electrode was further modified by introducing sulfur elements by hydrothermal method.The excellent electrochemical performance of Ni Co-S electrode comes from the combined results of its network structure and the synergistic effect of bimetallic sulfides:the network structure acts as a storage site for electrolyte ions,and the synergistic effect of Ni Co-S nanosheets with different valence states of nickel-cobalt ions promotes electron transfer and reduces resistance.As a result,it is able to achieve a specific capacitance of 306.4 m A h g^-1 at a current density of 1 A g^-1 and shows good capacitance retention,maintaining a multiplicative performance of 87%at a high current density of 20 A g^-1 and remaining a specific capacitance of 83.6%after 10,000 cycles.It was further assembled into a flexible asymmetric device,and its specific capacitance remained 81.8%after 3000 cycles of testing.（2）Further,in order to study the effect of elemental doping on Ni Co-MOF,the electrode was modified by high-temperature annealing under ammonia atmosphere to obtain a three-dimensional porous Ni₃N-Co N/NC/CC electrode.The annealed electrode successfully introduced nitrogen,which further produced mesoporous and microporous structures in the electrode nanosheets and had a high specific surface area of 233.2 m² g^-1.After nitriding,compared with Ni₃N/NC/CC,Co N/NC/CC and Ni Co-MOF/CC,Ni₃N-Co N/NC/CC generates nitrogen vacancies,which increase the active site of its redox reaction and play an important role in improving its electrochemical performance.Electrochemical measurements show that the electrodes have a high specific capacitance of 468.3 m A h g^-1 at a current density of 3 A g^-1and possess a multiplicative performance of 91.9%at a current density of 1-50 A g^-1.It was subsequently assembled into a flexible asymmetric supercapacitor with 92.3%capacity retention after 15,000 cycles.