Research on super capacitance properties of modified activated carbon in sodium sulfate electrolyte

In order to improve the specific capacity and energy density of activated carbon electrode materials, activated carbon was modified by surface treatment using hydrogen peroxide. The modified carbons were characterized the changes of pore structure and functional groups before and after modification by N2 adsorption-desorption isotherms and FTIR spectroscopy. The three electrode system was assembled with modified activated carbon as electrode materials and Na2SO4 as the electrolyte. The influence of surface modification on the performance of EDLCs was studied by galvanostatic charge-discharge, cyclic voltammetry, and alternating current impedance. The results show that the surface chemistry of the modified activated carbon material has changed, the specific surface area has changed and the pore size has not changed much with the introduction of new oxygen-containing functional groups. The specific capacity of the activated carbon material modified by 5h is 75F/g, which is 86.1% higher than that before the modification. It can be seen that the change of functional group after the modification of activated carbon can help to increase the specific capacity.


Introduction
Supercapacitors have received wide attention as an efficient, environmentally friendly, and renewable energy device. The traditional capacitor has low power density, short cycle life, slow charge and discharge speed, which makes the application market shrink compared with supercapacitors. Electrolyte and electrode materials are important components of supercapacitors and critical to the performance of capacitors [1][2][3] . As an important electrode material, activated carbon material has the advantages of larger specific surface, excellent thermal, electrical conductivity, good chemical corrosion resistance, small thermal expansion coefficient, low cost , easy availability and easy industrialization [4][5][6][7][8] . The pore structure and surface chemistry of carbon materials have an important influence on the formation of electric double layers [9][10][11][12][13] .
In this paper, the activated carbon was modified with 30% H2O2. The modified activated carbon was used as the electrode material of the supercapacitor. Physical

Activated carbon modification
The activated carbon for the study was prepared by SUNRISE POWER. A carbon (AC-0.5-AC-6) was prepared through oxidation of AC (1 g) with 50 mL of 35 wt% H2O2 solution under stirring for 0.5h-6h at room temperature. Recorded as AC-0.5, AC-1,AC-2,AC-3， AC-4,AC-5 and AC-6 ， untreated activated carbon is recorded as AC-0.The resultant material was extensively rinsed with distilled water. Then they were dried in a vacuum drying oven.

1.3.1Fourier transform infrared spectrometer(FTIR)
FTIR was carried out by KBr tableting method. The appropriate KBr and activated carbon before and after modification were mixed, grinded and tableted. The TENSOR 27 Fourier infrared spectrometer was used to analyze the changes in surface functional groups of carbon materials, with wavelength range of 250~4000cm -1 .

Nitrogen absorption and desorption experiment
The nitrogen (N2) isotherm adsorption/desorption test is one of the methods for testing parameters such as specific surface area and pore size distribution of materials. The specific surface area and pore distribution of the material were tested using a Micrometeritics ASAP 2000 specific surface area tester, vacuum pretreatment at 393K, absorption/desorption at liquid nitrogen temperature (-196℃), and data was recorded and analysed by computer.

Electrochemical characterization
Electrodes (1cm diameter) were pressed from a mixture of the studied carbons (80 wt%)，PTFE(10 wt%), carbon black (10 wt%   respectively, and the carbonyl stretching vibration peak appeared at 1640cm -1 , indicating that the modified CO content may increase, and the surface of the carbon material was introduced with -OH [14][15][16][17][18] . It can be seen that the surface modification has a great influence on the surface chemistry of the carbon material. Through the modification of hydrogen peroxide, some functional groups and groups are introduced on the surface of the activated carbon, which is beneficial to improve the capacitance performance of the activated carbon electrode.

Electrochemical properties
To compare the electrochemical properties of carbon materials before and after modification. CV and galvanostatic charge-discharge measurements were performed. CV curves of AC-5 obtained at different scan rates and CV curves obtained at a scan rate of 20 mV/ s for different treatment time are shown in Fig. 3 and Fig.4. (3)It have been shown by both galvanostatic charge/discharge and cyclic voltammetry experiment that the surface functional groups introduced by the modification of carbon materials can effectively increase their specific capacity. By comparing the activated carbon electrode materials with different modification time, the specific capacity of activated carbon with the modification time of 5h is the highest. When the current density is 1A/g, the specific capacity is the largest, which is 75F/g compared to the modification time of 0h (specific capacity 40.3 F/g) increased by 86.1%, and it was found that the modification time was not as long as possible.