Preparation And Supercapacitor Performance Of MOFs Derived Cobalt-nickel Electrode On Carbon Fibers

With the rapid development of industrial technology,energy shortage and environmental pollution in human society are becoming more and more serious.Efficient and clean energy storage devices have become a major demand of the current society.Supercapacitors have attracted extensive attention in the field of electrochemical energy storage devices due to their high power density,high specific capacitance and long cycle life.However,current supercapacitors still face the problem of low specific energy density relative to batteries,which seriously restricts the application and development of supercapacitors in the field of energy storage.To solve the above problems,we take cobalt and nickel based transition metal compound supercapacitor electrode material as the research object,with high conductivity and flexible carbon fiber cloth as the electrode substrate,the morphology and structure of the electrode material are regulated by metal-organic frameworks(MOFs)derived material.The influence of different cobalt-nickel base transition metal compounds and different microstructure on the electrochemical performance of supercapacitors was investigated,and the mechanism of improving the electrochemical performance of electrode materials was analyzed,laying a foundation for the preparation of electrode materials with high specific capacitance,high rate performance and long cycle life.At the same time,with the gel electrolyte material of super capacitor as the research object,through the gel electrolyte ion conductivity and mechanical properties and low temperature resistance,to explore the influence of different gel electrolyte electrochemical performance of super capacitor,and the gel electrolyte to produce high ionic conductivity,high mechanical strength and excellent resistance to low temperature performance of the mechanism were analyzed.From the preparation of electrode materials with high electrochemical performance and the development of high-performance gel electrolytes,the problem of low specific energy density of supercapacitors compared with batteries is effectively solved.The specific research contents of this paper are as follows:(1)Reduced graphene oxide composite cobalt-nickel double layer hydroxide(CoNi LDH@Co₃O₄@CC)was synthesized on the surface of carbon fiber by one-step hydrothermal method as the electrode material of supercapacitor.By controlling the proportion of cobalt and nickel in the cobalt-nickel double layer hydroxide and adding rGO,The effects of the optimal ratio of cobalt and nickel and rGO on the performance of supercapacitors were investigated.Finally,PVA/KOH was used as gel electrolyte and two electrodes were assembled into a symmetric supercapacitor,and the electrochemical performance of the composed symmetric supercapacitor was investigated.The results show that the electrochemical performance of the electrode is optimal when the molar ratio of Cobalt to Nickel is 1:1.When rGO is added,the CoNi LDH@Co₃O₄@CC electrode shows higher specific capacitance and cycle stability.At the same time,the symmetrical supercapacitor shows high energy density,power density and excellent cycle performance.(2)A layer of Co-Ni LDH nanosheets(CoNi LDH@Co₃O₄@CC)was electrodeposited on the surface of carbon fiber using two-dimensional Co₃O₄derived from cobalt MOFs as the skeleton,forming a unique core-branch structure as the cathode material of supercapacitor.The electrochemical performance of CoNi LDH@Co₃O₄@CC electrode was compared with that of CoNi LDH@CC and Co₃O₄@CC electrode.The excellent electrochemical performance of core-branch CoNi LDH@Co₃O₄@CC electrode was proved.At the same time,nitrogen-doped carbon material(NC@CC)formed after carbonization of two-dimensional cobalt-based MOF under nitrogen condition at 800?was used as the negative material of supercapacitor,and the asymmetric supercapacitor was assembled with PVA/KOH gel electrolyte and two electrodes,and its electrochemical performance was investigated.The results show that the CoNi LDH@Co₃O₄@CC electrode with hierarchical core structure has higher specific capacitance and capacitance stability compared with Co₃O₄@CC and CoNi LDH@CC electrode.The NC@CC electrode exhibits typical double-layer capacitive electrochemical performance and high Coulomb efficiency,as well as excellent fast transmission efficiency and low diffusion resistance.The assembled asymmetric supercapacitors show high energy density,power density and excellent cycle stability,which is mainly attributed to the synergistic effect of high conductivity substrate,sufficient active site and core-branch structure.(3)MOFs-derived NiCo₂S₄nanorods with porous layer structure were grown on the surface of carbon fiber to be used as positive materials for asymmetric supercapacitors.By comparing the electrochemical properties of hydroxides,oxides and sulfides of cobalt-nickel bimetal,the effect of bimetallic sulfides on capacitor performance was investigated.In addition,nitrogen-doped carbon material(NC@CC)formed after carbonization of two-dimensional cobalt-based MOFs under nitrogen was used as the anode material of supercapacitors.Asymmetric supercapacitors were assembled by using PVA/KOH gel electrolyte,and the capacitance performance and cycle stability of the composed asymmetric supercapacitors were studied.The results show that NiCo₂S₄@CC electrode with hierarchical structure has higher specific capacitance and capacitance stability in comparison with CoNi LDH@CC and Ni Co₂O₄.NC@CC electrode showed typical double-layer capacitance characteristics and high Coulomb efficiency.Assembled type asymmetric supercapacitor showed prominent energy density and power density,NiCo₂S₄@CC electrode enhanced than electric capacity,excellent cycle stability and lower interfacial resistance is mainly due to high conductivity base material,after curing exposed more active site and reduce resistance of active material and the synergy of specific hierarchy.(4)Bacterial cellulose(BC)aerogel with high strength and rich hydroxyl groups is introduced into sodium polyacrylate(PANa)gel electrolyte as the skeleton material of the gel electrolyte,and then PANa/BC/KOH gel electrolyte with high strength,high ionic conductivity and low temperature resistance is prepared by soaking the synthesized PANa/BC hydrogel in KOH solution.The mechanical properties,ionic conductivity and low temperature resistance of the gel electrolyte were investigated.Meanwhile,MOFs-derived cobalt-nickel double layer hydroxide composite carbon cloth(CoNi LDH@CC)and activated carbon composite carbon cloth(AC@CC)were used as the positive and negative electrodes of supercapacitor,and PANa/BC/KOH gel was used as the gel electrolyte of supercapacitor to form asymmetric supercapacitor.The capacitive performance of PANa/BC/KOH gel electrolyte at low temperature was explored,and the working principle of PANa/BC/KOH gel electrolyte was studied.The results show that PANa/BC/KOH gel electrolytes exhibit strong mechanical properties,excellent low temperature resistance and high ionic conductivity.Supercapacitors based on PANa/BC/KOH gel electrolytic electrolyte show excellent capacitance performance.When the power density is 1.2 k W kg^-1,its energy density is up to 25.63 W h kg^-1.After 10000 consecutive charge-discharge cycles,the capacitance retention rate of supercapacitor is up to 92.05%.