Influence Of Surface Chemical State Of Carbon Materials On Performance Of Supercapacitors

Electrode material is the key factor to determine the electrochemical performance of supercapacitor.The surface functional groups of carbon electrode materials play an important role in its surface properties,which have a great influence on their electrochemical performance.Based on the study of activated carbon,the effect of hydrochloric acid treatment on the pore structure,ash content and capacitance of activated carbon was studied.With 10%hydrochloric acid and reflux at 100℃for 5 hours,the ash content of the activated carbon can be effectively reduced and the original pore structure of the activated carbon can be preserved,which has little effect on the capacitance.Oxygen-containing functional groups usually exist on the surface of activated carbon,which have a great influence on its capacitance.In this study,Ammonium persulfate（NH₄）₂S₂O₈ was used as a mild oxidant to oxidize the activated carbon and the samples with different oxygen content were obtained.The electrochemical performance of the activated carbon treated with 0.2 M oxidant was the best,and then it was heat treated in argon at different temperatures in order to make it deoxidize in different degrees.The structure and surface chemistry of all activated carbon were analyzed and the electrode materials were prepared.Symmetrical supercapacitors were assembled and tested in aqueous electrolyte 6 M KOH and organic electrolyte 1M Et₄NBF₄/PC.The results show that the electrochemical properties of oxygen-containing functional groups in aqueous electrolyte and organic electrolyte are different.In aqueous electrolyte,oxygen functional groups can increase the wettability of electrolyte and electrode surface and accelerate the ion transport of electrolyte,which can improve the capacitance and enhance rate performance of supercapacitor.Carboxyl group and carbonyl group can provide pseudo-capacitance,among which carbonyl group has the greatest contribution to capacitance.The activated carbon treated at 300℃has good electrochemical performance.However,excessive oxygen-containing functional groups can block the pores of activated carbon,resulting in poor electrochemical performance.In organic electrolyte,the oxygen-containing functional group can make the capacitance decrease and the impedance increase.Heat treatment at 950℃can improve the specific capacitance,rate performance and cycle stability of the supercapacitor.