Preparation Of High-concentration Chitosan Homogeneous Solution-derived Graded Porous Carbon Material And Its Application In Supercapacitors

Supercapacitors,as one of the most promising green energy storage devices,have attracted more and more attention due to their high power density,rapid charging and discharging process and long cycle life.Electrode materials play a leading role in determining the performance of supercapacitors.Porous carbon has become a widely used electrode material for supercapacitors due to its low cost,adjustable pore size,large specific surface area,high conductivity and excellent chemical stability.The traditional method of preparing nitrogen-rich porous carbon is carbonization and activation,which have many steps and cost manpower and material resources.In this paper,based on the research results in this laboratory,a homogeneous chitosan solution with up to 9%concentration was prepared,which can exist under alkaline conditions,so as to meet the requirement of one-step activation with alkaline activators such as potassium hydroxide and mass carbonization to improve production efficiency.With our previous research,a high-concentration chitosan homogeneous solution was developed as the raw material of carbon precursor.In this activation method,potassium hydroxide was taken as the activator,and potassium bicarbonate was applied for generating high temperature gas to increase the porosity and specific surface area.The nitrogen doped hierarchical porous carbon electrode materials were prepared by the simple special freeze drying and high temperature carbonization method.The surface morphology,microstructure and electrochemical properties were analyzed.The results show that the carbon material has a hierarchical complex three-dimensional pore structure,with a large number of micropores,mesoporous and macropores penetrating it,and its specific surface area is up to 2883.3 m2 g-1.The specific capacitance is up to 255.5 g F-1 when the current density is 1 A g-1,The specific capacitance remained 91.7%after 5000 laps cycle revealing a good cycle stability.The largest energy density reached 17.7 Wh kg-1.It is illustrated that the special method of nitrogen hierarchical porous carbon materials have good application prospect as the super capacitor electrode material,and the characteristics of its structure also has great potential in adsorption,catalysis,etc.To deal with the shortcomings of low capacitance and energy density difference in carbon materials,a one-step method was developed with high concentration of homogeneous chitosan solution as the carbon source,nickel acetate as nickel source of nickel oxide-carbon(NiO-C)nano materials.It was shown that carbon materials have excellent electrical conductivity and good cycle performance which have the advantages of metal oxides with high specific capacitance and energy density.The surface morphology,micro structure and electrochemical properties of the NiO-C nano-materials was evaluated,and it was found that the NiO-C material has 3D porous carbon skeleton embedded in microscopic structure of nickel oxide nanoparticles.Its internal pore size was investigated which with all the pore structure of macropore,mespore and micropore.Its specific surface area is up to 397.5 m2 g-1,and the pore volume reaches 0.27 cm g-1.The specific capacitance was as high as 338 F g-1 when the current density was 1 A g-1,and it had excellent cyclic stability.After 5000 cycles,the specific capacitance remained 97.7%,which was attributed to the existence of carbon skeleton,which greatly improved the shortcoming of poor cyclic stability of metal oxide(NiO)nanomaterials as electrodes whose capacity decayed rapidly.Moreover,the electrode material has a high energy density of 46.9 wh kg-1,which can be attributed to the pseudo-capacitive contribution of NiO nanoparticles.In terms of comprehensive analysis,the homogeneous solution with high concentration of chitosan prepared by the unique process can not only facilitate the subsequent activation of alkaline activators and simplify the preparation steps,but also can be produced in large quantities to improve the production efficiency.The nickel oxide-carbon(NiO-C)composite electrode material was prepared by combining the advantages of high capacity,high energy density and excellent cyclic stability of double layer and pseudo capacitor.NiO-C nanomaterials have the advantages of green source,simple preparation process,easy control,high specific surface area and excellent electrochemical performance.As a kind of special electrode material,its potential in other aspects also has a certain application prospect.