Asymmetric capacitance response from the chemical characteristics of activated carbons in KOH electrolyte

The positive and negative capacitance behaviors of heat-treated activated carbons are separately studied in a three-electrode configuration using 6 M KOH and 1 M H2SO4 as electrolytes. Heat treatment of activated carbons at 400–800 °C causes slight changes in surface area accompanied by a significant removal of oxygen-containing functional groups. Consequently, the specific surface capacitance with a 25% reduction in the negative and an 8% reduction in the positive are monitored in KOH electrolyte. The unequal capacitance reductions as well as the asymmetric CV curves reveal that the negative pseudo-capacitances are much larger than the positive ones in KOH electrolyte. These results could be ascribed to the redox reactions preferring in the negative region. The insertion process of cation also plays a significant effect on the asymmetric capacitance behavior. Compared with the identical electrochemical response in H2SO4 electrolyte, huge difference in the capacitance values of both electrodes in KOH electrolyte will give a less contribution to the cell capacitance. To optimize the cell performance, a suitably increasing the weigh of active materials in the positive, especially for activated carbon with high level of oxygen-containing functional groups, is recommended.

► Effect of oxygen-containing functional groups on capacitance behaviors are compared in KOH and H2SO4 electrolyte.
► Asymmetric capacitance behaviors are observed in KOH electrolyte.
► Gravimetric capacitance of positive and negative electrode is 409 F g−1 and 271 F g−1 respectively.