Preparation Of 3D Co,Ni,and Mn Oxides Electrode Materials And Their Properties In Hybrid Capacitors

In recent years,due to the population explosion and rapid economic development,nonrenewable fossil resources such as oil,natural gas,coal,and etc.were exploited on large scale.While it brings people big wealth,it also creates serious environment problem.Thereby,taking advantages of novel green and renewable energy resources have become the new trend in the development of energy field.While exploring clean,efficient and renewable energy resources,energy storage and conversion devices such as supercapacitors and lithium-ion capacitors have become the focal point in the field of portable mobile devices and electric vehicles.Transition metal oxides is widely used as energy storage and conversion materials due to their highly reversible redox reactions(pseudocapacitors).However,owing to the poor ionic/electronic conductivity and limited reaction kinetics of transition metal oxides,poor capacitance and cycling performance have restricted it further development.Therefore,some strategies such as direct growth of active materials on the conductive substrate,constructing three-dimensional(3D)structure with abundant active site,or some combination of high conductivity materials such as carbon-based materials have become the feasible methods to improve the electrochemical properties of transition metal oxides.In this paper,five kinds of multicomponent metal oxides contain Ni,Co or Mn elements were fabricated for hybrid capacitors,and their structures and electrochemical properties were investigated systematically.1.A facile two-step solvothermal method is used to prepare Ni O@Ni Co₂O₄nanosheet arrays on Ni foam substrate with 3D hierarchical core-shell structure for a high-performance supercapacitor.The investigation about influence of hierarchical core-shell structure on electrochemical properties of hybrid electrode have been conducted.The Ni foam@Ni O@Ni Co₂O₄hybrid electrode shows high specific capacitance of 1623.6 F g^-1at 2 A g^-1and excellent rate performance with a 96%capacitance retention rate at 20 A g^-1.Meanwhile,the high cycling stability is proved by nearly 90%capacitance retention at 10 A g^-1after 10000 cycles,which are better than single Ni Co₂O₄and Ni O electrodes.Its asymmetric supercapacitor,assembled with Ni foam@Ni O@Ni Co₂O₄and the activated carbon as the positive and negative electrode,respectively,displays the energy density of 52.5 W h kg^-1at 387.5 W kg^-1and excellent cycling stability of 90%capacitance retention after 3000 cycles.The excellent electrochemical performance of Ni foam@Ni O@Ni Co₂O₄electrode indicates its great potential in applications of energy storage devices.2.3D hierarchical Ni Co₂O₄@Ni Mo O₄nanosheet arrays consist of high theoretical capacity of Ni Co₂O₄core and high conductivity of Ni Mo O₄shell were synthesized by two-step electrodeposition method on the conductive carbon cloth.3D Ni Co₂O₄nanosheet arrays have large specific surface area and its porous structure provides abundant electrochemical active sites,which facilitate fast ion and electron transport.Coating Ni Mo O₄shell of high conductivity on the surface of Ni Co₂O₄nanosheet arrays by electrodeposition can effectively improve the specific capacitance,rate performance and cycling stability of well-designed electrodes.The optimized Ni Co₂O₄@Ni Mo O₄electrode exhibits high specific capacitance(1787.1 F g^-1at 1 A g^-1),excellent rate performance(91.5%capacitance retention from 1?10 A g^-1)and enhanced cycling stability(87.8%of the initial capacitance after 10000 cycles at 4 A g^-1),which is obviously better than single Ni Co₂O₄electrodes.In addition,an asymmetric supercapacitor was assembled by Ni Co₂O₄@Ni Mo O₄and activated carbon,which possesses high energy density of 57.3 Wh kg^-1at 724.9 W kg^-1and high power density of 7250 W kg^-1at 36.9 W h kg^-1,respectively,and exhibits excellent cycling stability of 89%of the initial capacitance after 5000 cycles.3.3D hierarchical Ni Co₂O₄@PPy nanosheet arrays electrode consists of spinel Ni Co₂O₄core and polypyrrole(PPy)shell was prepared on carbon fiber by a facile two-step electrodeposition.By covering the PPy shell of high conductivity and rapid redox performance on high theoretical specific capacity of Ni Co₂O₄core,the integrated electrode not only has increased specific capacity,but also possesses superior cyclic stability due to the synergistic effect between two pseudocapacitive materials.The well-designed Ni Co₂O₄@PPy electrode exhibits a high specific capacity(1687.2 F g^-1at 1 A g^-1)and an enhanced cycle stability(capacity remain91.2%after 10000 cycles)compared with the Ni Co₂O₄electrode.Furthermore,an asymmetric supercapacitor,based on Ni Co₂O₄@PPy//Active Carbon,delivers the energy density and power density of 46.5 Wh kg^-1at 725 W kg^-1and 31 Wh kg^-1at7246 W kg^-1,respectively,and possesses excellent cycling stability of 80%of the initial capacitance after 10000 cycles.4.In this work,we synthesized 3D-structure Li Mn_xO_y@C nanosheet arrays with rich oxygen vacancies by a three-step method involving?-Mn O₂electrodeposition on Ni foam combined with hydrothermal lithiation and carbon coating.The open-framework and porous structure of the Li Mn_xO_y@C can facilitate the fast insertion/extraction of Li ion and charge transfer.The introduced oxygen vacancies and conducting carbon-shell can significantly improve the electrochemical performance of integrated electrode.Compared to Li Mn_xO_yelectrode,the Li Mn_xO_y@C electrode exhibits a higher specific capacitance(239.2 m Ah g^-1at 1 A g^-1)and an enhanced cycling stability(capacitance remain nearly 100%after 10000cycles).Additionally,Li-ion capacitor(LIC)coin cell was assembled with Ni foam@LMO@C and the activated carbon/Ni foam as the positive and negative electrode,respectively.The energy density of the prepared LIC can reach 44.3 Wh kg^-1at the power density of 800 W kg^-1,and show 29.6 W h kg^-1at the power density as high as 8000 W kg^-1with an excellent cycling stability of 80%of the initial capacitance after 4000 cycles at 5 A g^-1.5.Nitrogen-doped carbon coating is an effective method to improve the cycling performance of metal oxides.Herein,the mesoporous Zn O-Co O@NC microspheres with abundant oxygen vacancies are self-assembled through a facile hydrothermal method combined with an annealing post-treatment.The coated nitrogen-doped carbon can effectively improve the structural stability and conductivity of the bulk electrode.Thereby,it can also improve the ion diffusion and electron transfer between active materials and conductive substrate.In addition,oxygen vacancies can be introduced by annealing in N₂atmosphere so as to increase the specific capacity and cycling performance of electrode materials.Therefore,Zn O-Co O@NC exhibits a high specific capacitance of 302.6 F g^-1compared to Zn O-Co O electrode of 87.3 F g^-1at 0.5 A g^-1.Besides,the capacity retention rate of Zn O-Co O@NC electrode reaches to 92%after 40,000 cycles,showing its excellent cycling stability.In addition,Zn O-Co O@NC//graphene asymmetric supercapacitor shows very good cycle stability of 94%retention after 10000 cycles and high energy density of 16.5 Wh kg^-1at 396.5W kg^-1.