Study Of Carbon Material Based On Biomass For High Performance Supercapacitors

Supercapacitors are a new type of electrochemical energy storage devices,due to the advantages of long cycle life,the ultrahigh output power fast and charge/discharge process.Thus,supercapacitors have been applied to our life widely,such as,portable electronic devices,hybrid electric vehicles and emergency equipment system and so on.However,the energy density of commercial supercapacitors on a large scale is lower than those of lithium-ion batteries and fuel cells.As a result,the supercapacitors are so restricted to be further widely applied in various fields.According to the equation of E=1/2 CV²,to improve the energy density?E?of symmetric/asymmetric supercapacitors and maintain the intrinsic high power density,we can improve both the capacitance of electrode materials and the operating voltage?V?which is related to the decomposition voltage of electrolytes.In order to improve the energy density of symmetric/asymmetric supercapacitors,after the analysis,we design and prepare nitrogen-doped porous and biomass carbon as the electrode material,simultaneous change the traditional electrolytes or prepare the other faradaic electrode materials as negative electrode materials to construct the novel symmetric/asymmetric supercapacitors.The main research contents and results are as follows:1.We have employed the biomaterial white clovers as carbon precursor and ZnCl₂ as activating agent to prepare white clovers carbons?WCCs?.The WCC-2?mass ratios of white clovers and ZnCl₂ is 1:2?possesses as high as 6.9 wt%nitrogen content and 857 m² g^-1 BET surface area.Furthermore,it exhibits 233.1 F g^-1 specific capacitance at the current density of 1.0 A g^-1,even82.9%capacitance retention at 20 A g^-11 in 2 mol L^-1 KOH electrolyte.The specific capacitance could remain at almost about 100.0%when the cycle number is between the 200-5000 cycles.Because of the wide potential window?0-2.0 V?and high specific capacitance,the WCC-2//WCC-2 symmetric capacitor shows an energy density of 15 Wh kg^-1 under the power outputs of 503.5 W kg^-1 in 0.5 mol L^-1 Na₂SO₄ electrolyte.2.We have prepared nitrogen doped biomass activated carbon?CSC-1?using the agricultural wastes corn silks as raw materials and ZnCl₂ as activating agent?the same weight of corn silks and ZnCl₂?carbonized at 800°C.The activated carbon has high specific surface areas of 1764.8 m² g^-1,large specific capacitance of 358.0 F g^-1 at 0.5 A g^-1,67%of the capacitance retention at 20 A g^-1,and 99.2%of initiatory specific capacitance after 5000 consecutive cycles.To improve the energy density of the symmetric supercapacitor based on CSC-1 electrodes,we employed the 1 M H₂SO₄aqueous solution with alizarin red?AR?and bromoamine acid?ABA?as an advanced electrolyte to fabricate a novel hybrid symmetric cell.Surprisingly,the above device obtains a high specific capacitance of 260.8 F g^-1 and a high energy density of 17.8 Wh kg^-1,which are remarkablely higher than those in the conventional electrolyte.The improved energy storage is attributed to Faradaic pseudocapacitance related to the redox-active species of AR and ABA in the H₂SO₄electrolyte.3.A novel asymmetric supercapacitor device for energy storage is fabricated using K_0.3WO₃nanorods as negative electrode and nitrogen-doped porous carbon?CBC-1?based on agricultural wastes corn brans as positive electrode.The K_0.3WO₃ nanorods are composed of some thinner needle-shaped nanorods which are parallel to each other,and the CBC-1 reveals rough surface of coral-like frameworks with abundant nanopores.The structures can provide high surface area,low diffusion paths and easy intercalation/de-intercalation of electrolyte ions between the electrode/electrolyte interfaces.Thus,the asymmetric supercapacitor exhibits high energy density about 26.3 Wh kg^-1 at power density of 404.2 W kg^-1 in the wide voltage region of 0-1.6 V,as well as a good electrochemical stability?80%capacitance retention after 1000 cycles?.Such outstanding electrochemical behaviors imply the CBC-1//K_0.3WO₃ asymmetric supercapacitor is a promising practical energy-storage system.