hrvatski

Influence of the electrode preparation conditions on the performance of the activated carbon symmetric supercapacitors

Main properties of supercapacitors are its capacitance, nominal voltage, energy and power density. The nominal voltage of capacitor is determined by the chemical stability of the electrolyte, while the capacitance and energy density depend on the amount of charge that can be stored inside the supercapacitor. Another important characteristic of the supercapacitor is a power density which represents the rate of energy delivery. In order to achieve the best performance it is important to select a porous electrode with very large surface area, good electric conductivity and high electrochemical stability. It is desirable that the mechanism of charge storage throughout such electrode has fast kinetics as well as the reversibility and reproducibility which provide high stability and cycle life. Supercapacitor charging and discharging rate is determined by its internal resistance which can originate from active material, electrolyte and current collector. The electrode material is mainly composed of three substances ; an active material having the role of charge storage, a carbonaceous material which increases the electric conductivity of active material and the polymer binder responsible for mechanical properties of electrode. So far, lots of attention has been dedicated to a preparation of electrode material by improving its structural and chemical properties [1, 2], enhancing the contact between electrode material and current collector [3] and studying the effect of electrolyte system such as the size, charge, and mobility of the ions [4, 5]. The objective of this work was to optimize the electrode composition and the preparation conditions, in terms of temperature and pressure, resulting in the lowest internal resistance and highest capacitance possible. The electrode material was prepared by mixing various weight percentage of activated carbon (Norit DLC Supra 30, SBET = 1900 m2/g) (60-95 wt%), carbon black (Timcal Super C45) (3-20 wt%) and polyvinylidene fluoride (PVDF) (Sigma Aldrich) (2-20 wt%). N- Methyl-2-pyrrolidone (NMP) was used as a solvent. Electrodes were fabricated by coating the obtained slurry onto Al- foil current collector coated with thin layer of carbon (Galon) and dried at 60 °C in the vacuum furnace overnight. Dried electrodes were cut to area of 2 cm2 with thickness of 50 or 100 μm and mass loading of 6 mg/cm2 or 20 mg/cm2, respectively. Such electrodes were hot pressed at temperatures of 80 or 200 °C, and pressure of 10 or 100 MPa. The symmetric supercapacitors were assembled in the glove box containing two electrodes separated with glass fiber membrane soaked with 0.25 M NEt4BF4 in acetonitrile. Supercapacitors were tested by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge-discharge method (GCD). Specific capacitance of supercapacitors was evaluated from the slope of discharge curve, while the internal resistance was obtained from IR-drop of discharge curve. The EIS spectra provided a better understanding of electrode behavior regarding all types of resistances present in the supercapacitor assembly. The results showed that behavior of electrodes, in terms of internal resistance and specific capacitance, largely depend on both temperature and pressure applied to the electrodes during preparation, likewise the electrode composition. The higher pressing temperature and pressure had the beneficial effect to the activated carbon/carbon black interparticle contacts, as well as to the reduction of overall ohmic resistance.

activated carbon, symmetric supercapacitors, preparation conditions

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