| The development of efficient energy storage devices has attracted tremendous attention in recent years with the increasing demands for wearable flexible electronic devices,smart power grids,new energy vehicles and other innovative electrical devices.Due to the unique advantages in high power density,fast charged/discharged rate and good cycling stability,supercapacitors are expected to mitigate the awkward situation which traditional secondary batteries are facing.The practical application of supercapacitors is limited by the unsatisfactory energy density and narrow voltage window.Electrode materials are the crucial factors to determine the electrochemical performance of supercapacitor,such as capacitance,rate capability and energy density.Ni/Co-based materials have drawn much attentions due to the high electrochemical activity and large theoretical capacitance.In this work,based on the preparation of Ni Co LDH electrode materials with innovative structure,the morphology and structure of Ni Co based electrode materials were elaborately regulated and their electrochemical properties were further optimized,with the main achievements as follows:1)The large-scale growth of active substances on carbon cloth surface is prerequisite for the improvement of electrochemical performance.However,the hydrophobicity of carbon cloth greatly limits the aforementioned precondition.Thus,solvothermal method with ethanol solvent was introduced to make the first-step growth of Ni Co LDH nanosheet array on the surface of carbon cloth,which provided a hydrophilic substrate for the subsequent second-step hydrothermal growth of nanoarray.After hydrothermal reaction,the nanowire electrode array could be obtained with high mass loading,which exhibited an area specific capacitance of4440 m F·cm-2at current density of 5 m A·cm-2.Followed by the alkaline conversion,the mutual crosslinking Ni Co LDH nanonetwork electrode array was achieved,which could provide larger capacitance and more excellent rate capability than those of nanowire electrode.Specifically,the areal specific capacitance at the current density of 5 m A·cm-2increased to 7850 m F·cm-2.When the nanonetwork electrode and activated carbon were assembled into an all-solid asymmetric supercapacitor,the device had a maximum energy density of 0.345 m Wh·cm-2at a power density of 5.1m W·cm-2.In addition,a single ASC could light up 41 LED lamps after charging and maintain for 50 seconds,indicating the potential in practical application of the device.2)In traditional hydrothermal synthesis,the compact layer will be formed between carbon cloth substrate and active substance,resulting in the poor utilization of active substance,thereby causing considerable region of“dead volume”.The hydrothermal/solvothermal process was used for the preparation of Ni Co LDH nanoarray with holey structure?h-Ni Co LDH?,which could facilitate the infiltration of electrolyte ion,and improve the capacitive performance.The areal specific capacitance of h-Ni Co LDH is 6820 m F·cm-2at the current density of 5 m A·cm-2.In order to gain a more open structure,the same alkali conversion was conducted.The originally closed channel at the bottom of nanosheet was obviously dilated after alkali conversion,forming the graded porous Ni Co LDH?G-Ni Co LDH?electrodes with different pore diameters at the bottom and top of the active materials,in which the developed porous channels could avoid effectively the“dead volume”.The electrolyte could infiltrate and flow unimpededly within the material volume phase,thus providing a multidimensional charge transfer direction.At current density of 5m A·cm-2,G-Ni Co LDH had a capacitance of up to 9030 m F·cm-2.As the current density increased,the capacitance retention rate could reach 83%when the current density was 30 m A·cm-2.By using G-Ni Co LDH and activated carbon as electrode materials,the as-assembled asymmetric supercapacitor exhibit the maximum energy density 108 Wh·kg-1at the power density of 900 W·kg-1,and when the power density increased to 9 k W·kg-1,the energy density was 78 Wh·kg-1.Moreiver,the capacitance retention ratio can still up to 93%after 8,000 cycles through stability testing,while the device could light 72 LED lamps simultaneously after charging.The above results suggest that the construction of a graded porous electrode material was an effective way to improve the electrochemical performance of supercapacitors.3)In order to improve the capacitive performance of the graded porous array under large current density,we prepared the graded large-pore electrode by using ethanol/ethylene glycol as the mixed solvent in the second-step solvothermal reaction.Larger pores could accelerate the electrochemical reaction on the electrode surface and volume phase,so as to ensure the ideal electrochemical performance of the electrode under high current density.When the current density was 30 m A·cm-2,the capacitance of the graded large-pore electrode was as high as 8594 m F·cm-2,and when it increased to 60 m A·cm-2,the electrode still keep the capacitance of 3296m F·cm-2.Moreover,the capacitance retention of the electrode could maintained at82%after 6,000 charged/discharged cycles at the current density of 40 m A·cm-2.The assembled device provided with a working voltage window of up to 2 V.When the maximum power density was 101.24 m W·cm-2,the energy density of the device was0.46 m Wh·cm-2.The assembled supercapacitor could continue to supply LED modules for more than 6 min after being charged.The successful preparation of graded large-pore electrode could provide an insight for the development of supercapacitor,which was suitable for the high-current situation. |
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