Graphene-Based Aqueous Magnesium Ion Hybrid Supercapacitors with an Appealing Energy Density Advanced by a KI Additive

The electric double-layer capacitance (EDLC)-based capacitor is hindered with low capacitance and low energy density. Here, in this report, we focused on the fabrication of a symmetric device having graphene as an EDLC electrode material and redox additive KI-integrated aqueous MgSO4 as an electrolyte. The high surface area of graphene was produced by annealing of graphene oxide in an inert atmosphere and confirmed through X-ray photoelectron spectroscopy and Raman spectroscopy. The strategic 6% KI into MgSO4 delivered the highest specific capacitance with a wide working window of 0.7 V. Electrochemical measurements showed that graphene delivered a significantly greater specific capacitance (727.6 F/g) in a KI-integrated electrolyte (MgSO4 + KI) compared to 89.2 F/g in a MgSO4 electrolyte, owing to species such as IO3− and I3−(oxidation states of I). The symmetric device showed the maximum energy density (ED) of 69.3 Wh/kg, which can be achieved at the power density of 2.5 kW/kg, better than reported values in monovalent-based electrolyte devices. In this report, the charge storage mechanism, interactive association between Mg2+ ion insertion/extraction, and integration of redox KI had been comprehensively studied. The strategy shows a new path in the design of excellent ED capacitors without compromising the supercapacitor properties.