Self-discharge of a supercapacitor with electrodes based on activated carbon cloth

A study of the self-discharge of supercapacitor (SC) with electrodes based on the activated carbon (AC) cloth CH 900 of the Kuraray Company with an aqueous electrolyte of 1 M MgSO₄ was conducted. The elemental composition of the positive and negative electrodes after electrochemical treatment up to 1.6 V began to radically differ from each other. A very high concentration of hydrogen and oxygen appeared on the positive electrode, which is explained by the formation of functional surface groups (FSG). It was found that the logarithm of the self-discharge rate after 70 min is proportional to the charging voltage. Self-discharge rate dependences on the charge voltage after 1000 and 2000 min have its minimums. These minimums is explained by the action of two oppositely directed mechanisms. First, the pseudocapacity of Faraday redox reactions of FSG makes a significant contribution to the capacity, and second, the very presence of these groups increases the self-discharge rate. At low voltages, the first factor dominates, and at high voltages, the second factor. Based on impedance measurements for small self-discharge times, an equivalent electric circuit of the SC cell has been developed. It is shown that the resistance of the leakage current decreases with voltage increasing, which corresponds with measurements of the self-discharge rate. It is also shown that the self-discharge rate of the SC after 2000 charge–discharge cycles is lower than that corresponding to the initial state of the cell, which is explained by the presence of the FSG.