Activated Carbon Fiber Treated at Different Temperatures as Supercapacitor Electrodes: Electrochemical Characterization

Introduction

Nowadays, carbon-based materials have been used as electrodes for supercapacitors due to their physical and chemical properties. Among the different carbon-based materials, carbon fiber (CF) offer the advantage for using as electrodes because is not necessary any substrates or binders. Besides, it is believed that may be a plausible strategy to maximize the energy density of supercapacitors. Currently, many researches reported that the incorporation of heteroatoms like oxygen and nitrogen into carbon-based materials improved the specific capacitance. These materials containing numerous electroactive heteroatoms may be appropriate for the production of electrodes with better capacitance because the heteroatoms are responsible to induce pseudocapacitive behavior and to increase their polar properties [1]. It is known that different oxygen-containing groups are produced using various techniques or exposures. The result of the oxidation process as a function of both the amount of oxygen as well as the type of carbon-oxygen groups may also depend on the CF surface nature. In this work were proposed two pre-treatments for the CF surface activation, electrochemical and chemical oxidation. The samples were evaluated electrochemically, aiming their performance as electrodes in supercapacitors.

 Experimental Part

 The CF samples were produced from polyacrylonitrile (PAN) precursor at different heat treatment temperature (HTT) of 1000 ºC, 1500 ºC and 2000 ºC using temperature steps of 330 ºC/h under inert atmosphere of nitrogen, reaching the maximum during 30 min up to its cooling down to room temperature. The CF samples were cut in size 1 cm² and previously weighed. For the chemical treatment the CFs samples were dispersed in boiling acetone for 5 min. Subsequently, these were washed with distilled water, immersed in H₂SO₄/K₂Cr₂O₇ and sonicated for 5 min. For electrochemical oxidation, the CFs were anodically polarized using as electrolyte 0.5 mol L^-1 H₂SO₄ in a fixed potential of 2.0 V for 30min using a conventional three electrode cell.

The electrochemical characterization of the electrodes was studied by Electrochemical Impedance Spectroscopy (EIE), Cyclic Voltammetry (CV), and Charge/Discharge tests. All measurements have been performed in 1.0 mol L^-1 H₂SO₄ solution.

 

Results and discussion

The CV of all sample electrodes obtained between -0.1 and 0.8 V, recorded at scan rates of 1 at 100 mV s^-1, using 1.0 mol L^-1 H₂SO₄ as an electrolyte, showed a small distortion of the current response at the switching potential which may be attributed to electrolyte and electrode resistance. How this effect is usually dependent on the scan rate, this distortion is more pronounced between 50 and 100 mV s^-1. However, the three materials exhibit the characteristic rectangular shape typical of the electric double-layer capacitors. The charge-discharge profile of the CF electrodes at HTT submitted at two treatment oxidation, carried out at a current density of 0.5, 0.75 and 1 mA cm^-1, showed linear charge/discharge profile which indicates that capacitance is mainly due to electric double layer formation. The EIE analysis in the present work it is observed for all samples after the two pre-treatment that the vertical lines are close to the imaginary axis, which indicates supercapacitor behavior is very close to ideal capacitor. After two pre-treatment used it was observed an insignificant increase in internal resistance of materials. This behavior may be associated to the heteroatoms incorporation, becoming the ionic transport pathway more tortuous, which result in the diffusion phenomena more evidenced. 

Conclusion

The CF samples submitted a different HTT were available for application as electrode materials in supercapacitors. The two pre-treatments contributed to the increase specific capacitance. Particularly, the CF 1000 ºC electrodes electrochemically treated exhibited better performance in general terms for possible application as electrodes for supercapacitors.

Acknowledgments

This work was supported by CNPq-170141/2014-4.

References

Yun,Y. S., Lee, M. E., Joo, M. J., Jin, H.-J. Journal of Power Sources 246 (2014) 540-547.