On the cycling stability of the supercapacitive performance of activated carbon in KOH and H2SO4 electrolytes

Abstract

This paper studies the effect of oxygen-containing functional groups on the stability of activated carbon (AC) used in surpercapacitors with H2SO4 and KOH electrolytes, where AC prepared from rice husk by KOH activation (denoted as RHC) was used as electrode materials. Results show that RHC displays excellent stability in H2SO4 electrolyte but poor stability in KOH electrolyte during long-term cycling. The different stability of the oxygen-containing functional groups of RHC in these two electrolytes caused the different change of the potential of positive electrode during the charge/discharge processes of supercapacitors, which leads to the different cycling stability of RHC in KOH and H2SO4 electrolytes. The hydrolysis products of the carboxyl and lactones groups in RHC cause electrostatic repulsion and steric hindrance, which hinders the transportation of OH− into the pores of RHC. As a result, the capacitance of positive electrode decreases. The decreased capacitance of positive electrode causes the increase of maximum potential of positive electrode upon charge, which results in the oxidation and further decreased capacitance of positive electrode. The oxidation of positive electrode and the decrease of the capacitance of positive electrode are two mutually reinforcing processes, which causes the eventual failure of RHC in KOH electrolyte. The quinone-like groups of RHC are stable in H2SO4 electrolyte and do not cause obvious decrease of capacitance. Hence, RHC obtains long cycling life in H2SO4 electrolyte.