Preparation Of Freestanding Ni-and Co-based Composite Electrodes And Supercapacitor Performance

Supercapacitor,also known as electrochemical capacitor,is a new type of energy conversion and storage device.Supercapacitors,with their advantages of high output power,long cycle stability,fast charging and discharging,and safety,have drawn great attention in the fields of portable electronic devices,smart grids and electric vehicles.The capacity of commerical supercapacitor primarily based on carbon materials is limited by the double layer storage mechanism with only about at 10 Wh kg^-1.Transition metal oxides have high theoretical specific capacitance,however their applications are limited by the poor conductivity and cycling stability.In this paper,nickel-and cobalt-based transition metal oxide/hydroxide are designed with the cross-linked sheet by being supported on the two-dimensional film and three-dimensional carbon sponge,respectively,resultinging in flexible self-supporting composite electrode materials,with high specific capacitance and long cycle life.In addition,the asymmetric capacitors are fabricated and excellent electrochemical performance has been achieved with high energy density.Results are as follows:?1?Polyurethane?PU?sponge was used as a template to absorb SWNTs by means of infiltration.Then PU@SWNT was obtained after drying.Nitrogen-doped single-walled carbon nanotube sponge?NCS?was prepared by a fast ethanol flame burning.Three-dimensional porous NCS has high hydrophobicity and can be used to adsorb various organic solvents.NCS with the sponge-like network structure has good flexibility and can withstand 80%strain without distrotion.In addition,1000loading and uploading circles at 60%strain indicate that NCS has excellent compressibility and can serve as the electrodes of flexible supercapacitor.As an electric double layer electrode material,NCS achieves high specific capacitance of 126.8 F g^-1 at 1 A g^-1 and excellent cycle stability with 95%retention after 10000 charge-discharge cycles.?2?The nitrogen-doped graphene/carbon nanotube sponge?NCGS?was obtained by bruning teatment with PU sponge covered by GO/SWNTs.NCGS,possessing with the characteristics of light weight,flexibility and high electrical conductivity,was used as self-supporting substrate.The NiCo₂O₄/NCGS composite was prepared by hydrothermal reaction and heat treatment methods,presenting high gravimetric capacitance of 2050 F g^-1 and high volumetric capacitance of 1176.7 F cm^-3,excellent rate performance and long cycling stability.A high performance asymmetric capacitor was assembled by using NiCo₂O₄/NCGS ass the positive electrode and polyaniline-derived carbon?PANI-C?as the negative electrode,showing high gravimetric energy density of 53.5 Wh kg^-11 and high volumetric energy density of 26.9 Wh L^-1.Moreover,the asymmetric capacitor has a high capacitive retention rate of 83.5%after 10000 charging and discharging circles,exhibiting excellent cycling stability.?3?The nitrogen-doped graphene carbon nanotube composite film?NCF?was successfully prepared by filtration and hydrothermal reduction.The stacking effect of graphene was effectively suppressed by the introduced carbon nanotubes,resulting in the NCF of layered structure.The spiral Ni?OH?₂nanosheets grew on the self-supporting NCF substrate by one-step hydrothermal reaction.The electrochemical performance of NCF@Ni?OH?₂ composite film was studied.The high gravimetric capacitance of 2130 F g^-1 and the high area capacitance of 2.88 F cm^-22 were achieved at a current density of 2 A g^-1.The assembled asymmetric capacitor using NCF@Ni?OH?₂ as positive electrode and activated polyaniline-derived carbon?APDC?as the negative electrode achieved a high energy density of 60 Wh kg^-1.In addition,the capacitor possessed a 79.2%capacitive retention after 30000cycles,demonstrating excellent cycling stability.