Effects of electrode mass loading on the self-discharge of supercapacitors

Electrode mass loading is an important device parameter for supercapacitors and its effect on electrochemical properties such as energy density, rate performance, and cycle stability has been widely reported. However, how the self-discharge of supercapacitors varies under different mass loadings has not been examined systematically. In this study, we prepared porous activated carbon (AC) electrodes with a series of mass loadings and investigated their self-discharge behavior. It is found that with the increase in mass loadings, the decay rates of open circuit voltages (OCVs) are reduced substantially. Specifically, when the electrode mass loading increased from 0.6 to 10.6 mg cm−2, the cell OCV decay rate at a charging voltage of 1.6 V dropped from 1.07 to 0.05 mV mF−1 hr−1, indicating much slower self-discharge rates at higher mass loadings. Analysis of the self-discharge mechanism suggests that faradaic reactions are the major self-discharge processes for the supercapacitors after float charging. While the OCV decays due to both activation-controlled and diffusion-controlled faradaic reactions are reduced at high mass loadings, the self-discharge attributing to diffusion-controlled faradaic reactions is more affected due to the deeper pores and longer diffusion paths for electrolyte and reactive species in thicker electrodes.