| Supercapacitors, as one of the new-type energy storage device, have filled the gap between batteries and traditional capacitors. However, applications of supercapacitors in industrial areas are still limited due to the low energy density and high preparation cost. Therefore, using low-cost raw materials to prepare electrodes with high electrochemical properties is the research target of supercapacitors.Using renewable natural biopolymers and their derivatives to prepare porous carbon is an effective way to prepare electrodes with low cost, and in accordance with the requirement of the sustainable development. In this study, we fabricated the hierarchical porous carbon aerogels by using bagasse and chitosan as raw materials firstly, and then, we combined electrical double layer capacitor electrodes and pseudocapacitor electrodes, and assembled 1D inorganic nanostructures on the surface of carbon aerogels, and the electrochemical properties of hybrid nanostructures were improved significantly.Cellulose is a natural biopolymer with the largest storage in nature. In the present study, cellulose powders were purified from bagasse firstly. And then hierarchical porous carbon aerogels were prepared through dissolution, regeneration, freeze drying, carbonization and activation processes. Macro and mesoporous carbon was first prepared by carbonizing the freeze-dried bagasse aerogel, consequently, microporous structure was created on the walls of carbon by chemical activation. Correlation analysis was used to investigate the relationship between the pore size and elecrochemical properties of the electrode. And the results show that different pore sizes play different roles on electrochemical performance of the electrode. When electrolyte ions immerse into the pores, the distance between the electrolyte ions and micropore walls on both sides are the shortest, indicating the strongest electrostatic adsorption, leading to the best energy storage capacity. However, mesopores mainly store electrolyte to shorten the electrolyte ions diffusion distance, and at the same time, can supply electrolyte ions in the process of charging and discharging quickly. Therefore, micropores mainly influence the energy storage capacity, while mesopores determine the rate capability. Normally, the low graphitized part can be corroded first. By increasing the activation temperature, the reaction between carbon and KOH becomes more violent and KOH can react with the more graphitized area in the carbon aerogles. Therefore, the specific area, pore volumes and electrical conductivity of carbon aerogels obtained in different activation temperatures were different. When the activation temperature is 700℃, the mass ratio of KOH and carbon aerogels is 3:1, the obtained carbon aerogels displayed the best capacitive behavior. A high specific capacitance of 142.1 F g-1 at discharge current of 0.5 A g-1 can be reached in the symmetric supercapacitor using solid state electrolyte. More importantly, the electrode displayed excellent capacitance retention of 93.9% over 5000 cycles.Chitosan is the second most abundant and renewable biopolymer after cellulose. Graphene-based nitrogen self-doped hierarchical porous carbon aerogels were synthesized for supercapacitor electrode application by using chitosan as raw material through dissolution, freeze drying, carbonization and activation processes. When chitosan is dissolved in acetic acid, ketonic oxygen from the acetate molecule can form a hydrogen bond with chitosan molecules, which helps to form a planar network consisting of chitosan molecules and acetate molecules. During the quick freezing and lyophilization process, the surface tension of the chitosan solution forces the chitosan molecules to arrange themselves into film structures that connect together to form a 3-D structure. During carbonization, some of the carbon atoms decomposed from the chitosan planar networks connect together in situ to form graphene nanosheets, and the atoms decomposed from disordered connected chitosan molecules form amorphous carbon, to complete the formation of the graphene-based carbon aerogel. Because of the existence of-NH2 groups in chitosan molecules, the N atoms remain in the carbon aerogel to form the N self-doped carbon aerogel. During the high temperature activation process, KOH molecules react with amorphous carbon, leaving some pores in the amorphous carbon portion of the carbon aerogel wall. In fact, the obtained carbon samples at different activation temperatures contain different content of amorphous carbon. At 800℃, most of amorphous carbon is etched away, leaving the highly graphitized graphene layer and nanoporous amorphous carbon particles between or on the graphene layers. This hybrid graphene-based carbon aerogel, possessing the high charge storability of nanoporous carbon, and the high conductivity of a graphene network, displays superior supercapacitor performance. The activation process can increase the specific surface area and pore volumes. Moreover, doped nitrogen has a larger binding energy with the potassium ion, resulting in a larger number of ions that can be accommodated on the electrode surface even for a given electrode surface area. The oxygen functional groups in carbon materials can provide more available sites for ion adsorption in the micropores of carbon aerogels because of the ion-dipole attraction, which can generate an excess specific double layer capacitance due to the local changes of electronic charge density. Furthermore, N and O species on the carbon surface could lead to the pseudocapacitive interaction between the ions of electrolytes and the N or O-containing functional groups, and then generate high pseudocapacitance. Therefore, this sample displayed a high specific capacitance of 197 F g-1 at a current density of 0.2 A g-1. The energy density reached as high as 27.4 Wh kg-1 at a power density of 0.4 kW kg-1 and 15 W h kg-1 at a power density of 20 kW kg-1. This study has demonstrated an effective way to utilize the renewable biopolymers as the raw materials for high performance supercapacitor electrode materials with low cost.Although carbon aerogels prepared using bagasse and chitosan displayed an improved capacitive behavior, the specific capacitance is still limited below 200 F g-1. Therefore, in this study we assembled the 1D inorganic nanostructures on the surface of carbon aerogels. We prepared MnCo2O4.5 nanoneedle/carbon aerogel hybrid nanostructure firstly. When the concentration of precursor is lower, the nanoneedles did not cover the surface completely, which is contributed to the lack of a source of MnCo2O4.5 in the reaction system. With an increase of precursor concentration, the entire surface of carbon aerogels is covered by MnCo2O4.5 nanoneedle arrays. Moreover, only a single layer of uniform nanoneedle arrays precipitated on the surface without any redundant urchin-like structures. The bottom of MnCo2O4.5 nanoneedles connects to the carbon surface directly, facilitating the charge transfer between nanoneedle arrays and carbon aerogels. However, with further increase the precursor concentration, the high mass loading of MnCo2O4.5 results in so many MnCo2O4.5 nanoneedle arrays assembled on the surface of the carbon aerogel that the porous structure of carbon aerogel was blocked. And the excessive nanoneedles not only covered all the surface of carbon aerogels, but also accumulated on the first layer of MnCo2O4.5 nanoneedles. Such structure leads to the unsatisfied performance of the sample because of the high interface resistance between inorganic nanoneedles.The specific surface area of the optimized sample can reach to 888.6 m2 g-1. The symmetric supercapacitor using MnCo2O4.5 nanoneedle/carbon aerogel hybrid naostructure as the active electrode material exhibits high energy density of about 84.3 Wh kg-1 at a power density of 600 W kg-1. The voltage window is as high as 1.5 V in neutral aqueous electrolyte. 1D Ni-Co oxide and sulfide nanoarray/carbon aerogel hybrid nanostructures, NiCo2O4 nanoneedle array/carbon aerogel and NiCo2S4 nanotube array/carbon aerogel hybrid supercapacitor electrode materials were synthesized by assembling Ni-Co precursor needle array on the surface of channel walls of hierarchical porous carbon aerogels derived from chitosan, and following by oxidation or sulphidation process at high temperature in Ar atmosphere or sulfurization process. The 1D nanostructures grow on the channel surface of carbon aerogel vertically and tightly, contributing to the enhanced electrochemical performance with ultrahigh energy density. The rate capability and the specific capacitance of NiCo2O4 nanoneedle array/carbon aerogel and NiCo2S4 nanotube array/carbon aerogel hybrid supercapacitor electrode materials are much better than that of pure NiCo2S4 nanotube and NiCo2O4 nanoneedle arrays because of the hierarchical porous structure of carbon aerogels. The higher electrical conductivity and the more contact areas between NiCo2S4 nanotube arrays and the electrolyte, contribute to the superior electrochemical performance of NiCo2S4 nanotube array/carbon,aerogel-based electrode. This work proposed a general route to design and synthesis of hybrid nanostructures with the 1D arrays assembling on the substrate directly for applications electrochemical energy storage devices.The target of this study is to prepare low-cost electrode materials with excellent electrochemical properties using the waste of industry and life. We have prepared porous carbon aerogels for high performance electrical double layer capacitor electodes through the crystal and channel structure control. And then comined with pseudocapacior electrodes to prepare the hybrid nanostructures with outstanding capacitive behavior. The design ideas, experimental process and the conclusions can provide the reference basis for the preparation of electrode materials with high supercapacitive properties. |
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