Study On The Preparation And Electrochemistry Properties Of The Starch Activated Carbons As The Electrodes Of The Supercapacitors From The Starch

Supercapacitor is a new type apparatus between tradition capacitor and battery for storing up energy. It has ultra capacitance and is able to store and deliver energy at relatively high rates. The performance of the electrode material is a key factor that influences the capability of the supercapacitor. The commercial electrode material for the supercapacitor is mostly activated carbons. There are many factories that produce activated carbons; however, there are few activated carbons that can reach the demand of the practice production. The activated carbon especially for supercapacitor also exist some defect such as low capacity, extortionate price, instability performance. Moreover, for the present, the most materials of the activated carbons are timber that has long growing cycle and the mineral that can't be regenerative. For the problem that exist in the activated carbons electrode, in this paper, the starch which is regenerative, short regrowing cycle, cheap, environmentally friendly and abundant were firstly used as the carbon precursors to produce the electrode material of the supercapacitor. The producing technics of the activated carbons were optimized. The new type electrode material of the supercapacitor exhibited the high current, high capacity, high-power, highly reversible and low impedance characteristics. The pore structure and electrochemistry performance were token from the scanning electron microscopic (SEM) results, TG-DTA and the electrochemistry testing technology.(1) Selecting the optimal starch materials from the studying starch system. The activated carbons were produced from starch which is volume-regenerative, cheap, environmentally friendly and abundant including tapioca starch, solubility starch, corn modified-Cationic starch, oxidized cross-linked starch, ingraft starch. The influence of the starch materials on the performance of the activated carbons were systematically studied by testing the specific surface area of the activated carbons. Of the six starch materials, the activated carbons from the corn modified-Cationic starch, oxidized cross-linked starch presented the quite higher specific surface area. So the modified-Cationic starch, oxidized cross-linked starch were selected as the optimal starch materials for the study.(2) In this paper, the high power activated carbons (ACs) for the electrodes of supercapacitor were fabricated from the starch and modified starch with KOH, ZnCl2 and ZnCl2 and CO2 combined activation. The optimal activation was chosen by comparing the electrochemistry properties of the starch electrode material. Of the three starch activated carbons by the different activation methods, the activated carbons with KOH activation presented higher BET surface area(3332m2/g ) and larger volume(1.585 cm3/g), possessed distinguishing surface morphology structure and pore structure shape at the synthesized condition of 850℃activation, KOH/carbon weight ratios 3/1, 1.5h activation time. The galvanostatic charge/discharge test of the supercapacitors from this activated carbons electrodes materials show that at the same current, it present the highest specific capacitance. Charge-discharge cycle test was performed between cell voltages of 0.05 and 1.2 V at a current density of 370mA/g. Starch activated carbons by KOH activation shows higher specific capacitance of 238F/g according to the ten times discharge curve , which is nearly double that of the commercial ACs'. Moreover, the starch activated carbons from the KOH activation method show excellent capacitance, which exhibited high-power, highly reversible and low impedance characteristics. On the other hand, for the activated carbons from ZnCl2 activation and ZnCl2 + CO2 activation, their BET surface area are comparatively lower (1674 m2/g and 1939 m2/g respectively). At a current density of 370mA/g, their discharge specific capacitance are 136F/g and 139F/g respectively,which is higher than the commercial ACs'. Through activation methods studying, KOH was chosen as the optimal activation reagent.(3) Optimizing the producing technics of the starch activated carbons. KOH/carbon weight ratios, activation temperature, the activation time are the main factor influence on the properties of activated carbons. The activated carbons with best performance can be synthesized at the following condition: activation temperature-850℃, KOH/carbon weight ratios-2/1, and the activation time-2 h. In this condition, synthetical activated carbons presented higher BET surface area (1320m2/g) and high specific capacitance (204F/g). It not only exist plenty of micropores but also lots of mesopores. In the cyclic voltammetry test, all CV curves are rectangular-like presenting symmetric performance and excellent capacitance behavior at the different scan rate in 30% KOH solution. There exist no obvious redox currents on both positive and negative sweeps in the whole potential (0-1.2V) range of investigation and the capacitance of the electrodes is almost offered by the double-layer capacitor. These capacitive-like and symmetric i–E responses curve shows the excellent capacitive-like responses in the potential range of investigation. At the same time, galvanostatic charge/discharge characteristics and impedance spectroscopy present excellent capacitance.(4) Discussing the connection of surface area, the surface morphology structure and pore structure shape to the capacitive performance. It showed that the electrochemistry properties of the activated carbons was not only depended on the specific surface area, pore volume and pore size distribution but also on intrinsic pore structure and surface morphology structure of activated carbons.