Study On The Preparation And Properties Of Flexible All-solid Cobalt-based Asymmetric Supercapacitors

The electrochemical capacitors and supercapacitors which are potentially applicated range from microelectronic devices to hybrid electric vehicles have attracted great interest in recent years.Supercapacitors can provide high power density but low energy density.Designing new electrode structure to reduce the interface resistance,improve the electron transmission rate and ion diffusion rate,thereby increasing the specific capacity of the active material is an effective way to increase energy density.In addition,combining with a negative potential window electrode of activated carbon to prepare asymmetric supercapacitor devices is another effective strategy.In this paper,a Co₉S₈ nanotube and Ni Co-LDH nanosheet arrays core-shell structure were in situ synthesized on a flexible carbon cloth.We use constructing a charge transport layer,introducing oxygen vacancies,and doping method to reduce the interface resistance and thus to increase its specific capacity.Then,an all-solid-state asymmetric supercapacitor（AAS）was assembled with activated carbon as the negative electrode,in aming to widen the potential window,thereby a high-performance supercapacitor device was prepared.The specific research work is as follows:First,we used PPy as the charge transport layer to increase the electrochemical performance of Co₉S₈@Ni Co-LDH core-shell nanotube arrays.The precursor of Co（CO₃）_0.35Cl_0.20（OH）_1.10?1.74H₂O grown on the carbon cloth was vulcanized into a hollow uniform Co₉S₈ nanotube array by hydrothermal method,and the charge transfer layer of PPy is in-situ grown on Co₉S₈ nanotube arrays by simple chemical polymerization method.In order to obtain more active sites,a high specific surface area material of Ni Co-LDH nanosheets were electrodeposited on the PPy layer.The target product of Co₉S₈@PPy@Ni Co-LDH core-shell nanotube array was obtained.Due to the synergistic effect of the hollow structure,high electron mobility and abundant active sites,the Co₉S₈@PPy@Ni Co-LDH core-shell nanotube arrays electrode has excellent electrochemical performance.The Co₉S₈@PPy@Ni Co-LDH core-shell nanotube arrays electrode has an area specific capacity of 2.65 F cm^-2 at the scan rate of 1 m A cm^-2,more than 2 times of Co₉S₈@Ni Co-LDH core-shell nanotube arrays electrode(1.28 F cm^-2).Using activated carbon（AC）as the anode,an all-solid-state asymmetric supercapacitors device was assembled.The area specific capacity at 1 m A cm^-2 was 0.371 F cm^-2,and the energy density was reached at a0.132 m W h cm^-2(power density of 0.8 m W cm^-2).Secondly,through Na BH₄ reduction method to obtain rich oxygen vacancies were used to increase the specific capacity of Co₉S₈@Ni Co-LDH core-shell nanotube arrays.Similarly,a Co₉S₈ nanotube array in situ grown on a carbon cloth was used as a substrate to grow Ni Co-LDH nanosheets by a simple electrodeposition method to obtain an intermediate product of Co₉S₈@Ni Co-LDH core-shell nanotube array.The sample was reacted with Na BH₄ to generate a Co₉S₈@OV-Ni Co-LDH core-shell nanotube arrays.These rich oxygen vacancies were increased charge transfer rate in the multi-level redox processes.Electrochemical measurements showed that the Co₉S₈@OV-Ni Co-LDH core-shell nanotube arrays can obtain an area specific capacitance of 5.34 F cm^-2 at a current density of 1 m A cm^-2,which is higher than that of the untreated Co₉S₈@Ni Co-LDH and Co₉S₈ electrodes.In addition,the prepared Co₉S₈@OV-Ni Co-LDH core-shell nanotube array as a positive electrode and AC as a negative electrode were assembled a high-performance flexible AAS device.When the power density is 0.16 m W cm^-2,the maximum energy density is 0.202 m Wh cm^-2.Finally,the specific capacity of Co₉S₈@Ni Co-LDH core-shell nanotube arrays is improved by constructing a P-doped conductive carbon layer.We immersed the Co₉S₈nanotube arrays in glucose,with Na H₂PO₂ as a phosphorus source to successfully obtain Co₉S₈@P-C core-shell nanotube arrays.Ni Co-LDH nanosheets were also deposited on the surface by electrodeposition method,and finally the Co₉S₈@P-C@Ni Co-LDH core-shell nanotube arrays electrode were obtained.Among them,the phosphated carbon layer has high conductivity and stable electrochemical activity,thereby effectively improving the electrochemical performance of the Co₉S₈@Ni Co-LDH core-shell nanotube arrays.At a current density of 2 m A cm^-2,the area specific capacitance of the Co₉S₈@P-C@Ni Co-LDH core-shell nanotube arrays electrode is 3.9 F cm^-2.Co₉S₈@P-C@Ni Co-LDH core-shell nanotube arrays electrode and activated carbon electrode were assembled into a all solid-state asymmetric supercapacitor.Whenthe power density is 0.0239 m W cm^-2,the energy density is as high as 0.065 m Wh cm^-2.Under a high potential window of 0～1.6V,after 5000 cycles of charging and discharging at 10 m A cm^-2,the specific capacitance is still high.Keep it at 93.2%of the initial value.