Preparation Of Cobalt-based Composite Electrode Materials And Their Application For Supercapacitance

With the serious environmental and energy problems in the world,energy conversion and storage technologies have attracted wide attention.Supercapacitors have been widely used in hybrid electric vehicles and portable device,due to their high power density,long cycle life,rapid change-discharge,high energy density,safety and environmentally friendly,and is concidered as an ideal device for energy storage.According to the mechanism,supercapacitors can be divided into double-layer capacitors and pseudo-capacitors.The energy of pseudo-capacitors is storage by the rapid redox reaction between electrodes and liquid surfaces,which are much larger than that of double-layer capacitors.Therefore,pseudo-capacitors have attracted much research interest and show great potential for industry applications.The properties of electrode materials are key factors affecting the performance of supercapacitors.Therefore,the design and development of electrode materials has become the research focus.Cobalt/nickel-based oxides/sulfides are widely used as electrode materials for supercapacitors for their advantages of low cost,nontoxic and high theoretical capacity.In this paper,cobalt/nickel based electrode materials and their composites were synthesized by simple method.Therelationship between morphology and electrochemical properties were studied.The main contents of this paper are as follows:1.First,NiCo₂O₄ nanowires were synthesized by one-step hydrothermal method and annealing treatment.MoS₂ nanowires/NiCo₂O₄ nanosheets were synthesized by a post hydrothermal reaction.The composition and crystal of the products were studied by XRD.The morphology was studied by SEM and HRTEM,and electrochemical properties such as impedance and cycle stability were studied by electrochemical workstation.The results show that the electronic conductivity of electrode material can be changed by introducing MoS₂,and the specific capacity of MoS₂/NiCo₂O₄composite electrode is 7.1 F cm^-2 at a scanning rate of 2 mV s^-1,which is higher than that of pure NiCo₂O₄.An asymmetrical supercapacitor was assembled with the as-prepared sample as the positive electrode and the active carbon as the negative electrode.The energy density of the supercapacitor was up to 51.7 F g^-1.After 8000 cycles,the capacity retention rate was up to 98.2%.Also,a small white LED lamp could be lit up.The application potential of the material is proved to a certain extent.2.The NiCo₂O₄ nanosheets/CuCo₂O₄ nanocones composite electrode was synthesized by two-step hydrothermal method and followed by two-step high temperature annealing.The morphology and electrochemical performance of the samples were studied.The results show that the electrochemical performance of NiCo₂O₄/CuCo₂O₄ composites can be improved by the combination of NiCo₂O₄ and CuCo₂O₄,which was owed to the synergistic effect of the two components.Specifically,the maximum area specific capacity of the composite is 4.97 F cm^-2 at a current density of 1 A g^-1,while the area specific capacity of NiCo₂O₄ and CuCo₂O₄ is 0.98 and 1.37 F cm^-2,respectively.An asymmetrical supercapacitor was assembled with the as-prepared sample used as positive electrode and active carbon used as negative electrode.The maximum energy density is up to 15 W h kg^-1 and the corresponding power density is814 W kg^-1,which can light up a blue LED lamp.At the same time,after 5000 charge-discharge cycles at high current density,the capacity retention rate is 90.6%,which indicates that the materials have excellent stability,and the electrochemical properties show that the as-prepared materials by the experiments have a good prospect of application.3.At first,NiCo₂O₄ nanosheets were synthesized by mild hydrothermal method and annealing treatment.With the second hydrothermal step,the Co₃S₄nanowires/NiCo₂O₄ nanowires were prepared by controlling the reaction time.The morphology and the electrochemical properties of the products were studied by SEM and electrochemical workstation.The electrochemical properties of Co₃S₄/NiCo₂O₄composites can be improved obviously by the recombination of the two components.The specific capacity of the sample is up to 1468 F g^-1 at a current density of 1 A g^-1.The electrochemical impedance shows that Co₃S₄ nanoneedle can effectively improve the conductivity of the material,that is,improve the conductivity of the material.Thus,the capacity of the composite has increased.The energy density and power density of the materials are studied by assembling asymmetric supercapacitors.When the current density is 1 A g^-1,the energy density can reach 14.0 W h kg^-1 and the corresponding power density is 400 W kg^-1.4.NiCo₂S₄/MnCo₂S₄ nanorods were synthesized by a simple hydrothermal method.And their morphology,composition and electrochemical properties were studied by XRD,SEM and electrochemical workstation.The result of the SEM shows that the diameter of NiCo₂S₄/MnCo₂S₄ nanorods was 300-400 nm.The electrochemical properties of the samples were studied.The specific capacity of the sample was 1235 F g^-1 at 1 A g^-1.The energy density,power density and cycle stability of the material was studied by assembling an asymmetric supercapacitor.The results show that the energy density of the material is 6.7 W h kg^-1 and the corresponding power density is 806.7 W kg^-1.At the same time,the material also has good rate performance.After 5000 cycles,the capacity retention rate of the material is 120.7%,which shows good cyclic stability and certain practical significance.