| Lithium ion hybrid supercapacitor is a new type of energy storage device that combines the advantages of supercapacitor and lithium ion battery,which has the advantages of high power density,high energy density,rapid charge and discharge,and long cycle life,etc.So it has attracted extensive attention in the fields of electric vehicles,electronic devices and energy storage.Electrode material is an important factor affecting the performance of capacitors.In recent years,in order to further improve the electrochemical performance of capacitor-type electrode materials,scientists are committed to developing new and efficient capacitor-type electrode materials.In this paper,the corncob-derived porous activated carbon material was prepared from the agricultural waste corncob with abundant sources,low cost and developed pores inherently.Firstly,a simple one-step method was used to prepare hierarchical porous activated carbon with excellent electrochemical properties by adjusting activation conditions?activation ratio of KOH/corncob,heating rate?.Then,based on the optimization of firstly activation conditions,the concept of re-activation was proposed,and the re-activation method was used to prepare double porous activated carbon.Finally,under certain conditions of carbonization activation,carbon source is pretreated by hydrothermal method,and high graphitization micropores-small mesopores activated carbon is prepared after carbonization activation.The main research contents are as follows:?1?Corncob-derived hierarchical porous activated carbon was prepared by heating-rate induced method.On the one hand,the activation ratio of KOH/corncob will have an important impact on the microstructure and electrochemical properties of activated carbon.With the increase of activation ratio of KOH/corncob,the reaction between KOH and carbon material becomes more intense,and the erosion effect is strengthened,thus forming a developed 3D pore structure.When the amount of KOH continues to increase,due to the strong erosion effect of KOH,the graphitized structure is destroyed,causing the collapse of the original 3D structure and blocking part of the pore structure.Therefore,when the concentration of KOH is 15%,the ACs-15 prepared with the specific surface area of 1654 m2 g-1 has an suitable hierarchical porous structure and the optimal specific capacity,and it has the specific capacity of 115 F g-1 at the current density of 3 A g-1.On the other hand,the appropriate heating rate is conducive to the establishment of hierarchical porous structure of activated carbon materials and the increase of graphitization degree.When the heating rate is low,the activation process is relatively gentle,and most of the micropores are produced.With the increase of heating rate,the reaction becomes more and more intense,which can lead to the obvious pore structure.When the heating rate was 8 oC min-1,the prepared C-ACs-8 has high specific surface area of 1154 m2 g-1.At the current density of 0.5 A g-1,the C-ACs-8 has the specific capacity of 158 F g-1,and its capacitance remained 87% after 5000 cycles.The C-ACs-8//Fe3O4/G LIC assembled with Li PF6 as electrolyte has an energy density of up to 75 Wh kg-1 at a power density of 562 W kg-1 and capacitance retention rate of up to 86.4% after 1000 cycles.?2?HPACs prepared through a simple novel method of re-activation has the complete 3D pore structure,high specific surface area(2351 m2 g-1),suitable double porous structure?microporous and mesoporous?and good degree of graphitization.Studies show that in the process of re-activation,KOH has a dual role: one is that KOH will continue to erode a micropores formed in the process of firstly activation,making it the mesoporous;Secondly,KOH will erode the carbon material and form new micropores structure.Moreover,re-activation can improve the degree of graphitization of activated carbon materials which can increase the electrical conductivity of the materials.Therefore,HPACs-1-2 prepared by re-activation method has excellent electrochemical properties.At the current density of 1 A g-1,its specific capacitance is 187 F g-1,and it has a good cycle stability.In addition,HPACs-1-2 is used as the positive electrode and Fe3O4/G composite is used as the negative electrode to assemble LIC.When the power density is 536 W kg-1,its energy density is as high as 102 Wh kg-1.HPACs-1-2//Fe3O4/G LIC still has a high capacitance retention rate after 2000 cycles,and good cycle stability.?3?ACs-1.5 with complete three-dimensional pore structure,high specific surface area,appropriate pore size distribution and good degree of graphitization was prepared by Hydrothermal pretreatment.The researches show that in the process of hydrothermal pretreatment,concentrated sulfuric acid has the following four functions: firstly, concentrated sulfuric acid reacts with the carbon skeleton,resulting in the re-construction of carbon structure,and carbon microspheres are generated on the material surface,which is conducive to the formation of porous structure;Secondly,concentrated sulfuric acid can lead to the dehydration reaction of the material,which can protect the carbon source well.Thirdly,concentrated sulfuric acid will sulfonate with the carbon source,increasing the functional groups on the surface of the material.Fourthly,the carbon structure re-construction occurred in the process of concentrated sulfuric acid pretreatment increased the degree of graphitization of the material and improved the electrical conductivity of activated carbon.Therefore,ACs-1.5 prepared by hydrothermal pretreatment has excellent electrochemical properties.At the current density of 0.3 A g-1,its specific capacitance is 250 F g-1,and it has good cycling stability.In addition,ACs-1.5 as the positive electrode and Fe3O4/G composite as the negative electrode were used to assemble the lithium ion hybrid supercapacitor.When the power density was 3726 W kg-1,the energy density was as high as 118 Wh kg-1.ACs-1.5//Fe3O4/G LIC still has a high capacitance retention rate after 5000 cycles. |
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