Heat generation in all-solid-state supercapacitors with graphene electrodes and gel electrolytes

This study aims to investigate time-dependent heat generation in all-solid-state supercapacitors. Supercapacitors were fabricated by assembling two identical electrodes made of graphene petal arrays grown on buckypaper by microwave plasma chemical vapor deposition. The electrodes were separated by either (i) a conventional phosphoric acid in polyvinyl alcohol (PVA) gel electrolyte or (ii) a redox-active potassium ferricyanide/ferrocyanide in PVA gel electrolyte. The devices with redox-active gel electrolyte exhibited about three times larger capacitance due to additional charge storage by reversible redox reactions. However, their internal resistance was about six times greater than devices with conventional gel electrolyte due to the lower electrolyte ionic conductivity. The heat generation rate measured under constant-current cycling was the sum of irreversible and reversible contributions. The irreversible heat generation rate in all supercapacitors was caused by resistive losses, i.e., Joule heating. The reversible heat generation rate was different at the positive and negative electrodes and was affected by electric double layer (EDL) formation, reversible redox reactions, and/or the presence of negatively charged functional groups at the electrode surface. The results of this study can be used to gain insight into the charging mechanism and also to develop thermal management solution for all-solid-state supercapacitors.