A joint research team, led by Prof. WU Zhongshuai and Prof. FU Qiang from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences (CAS), recently proposed a strategy for boosting the capacitance of graphene-based planar micro-supercapacitors (MSCs) using highly concentrated water-in-salt ambipolar redox electrolyte (ZnI2 + ZnCl2).

Using redox-active electrolytes to boost graphene electrodes is a highly-efficient strategy to increase the capacitive performance of MSCs. However, previously reported redox mediators could only offer a certain capacitance for a single electrode, leading to limited energy density due to the unmatched capacitances of two electrodes.

In this study, the researchers developed a novel, highly concentrated water-in-salt ambipolar redox electrolyte where one ambipolar mediator (ZnI2) could offer two redox couples (I-/I2 and Zn/Zn2+) natively, with matched charge storage.

These two species allowed two electrons to be oxidized at the positive electrode and to be reduced at the negative electrode synchronously and individually, thus offering a large pseudocapacitive contribution for EG-MSCs.

They have realized high volumetric capacity of 106 mAh/cm3, energy density of 111 mWh/cm3, and long-term cycling stability with 92.1% retention after 5,300 cycles.

In situ characterizations confirmed that the good performance was attributed to the frustrated self-discharge by suppressing the formation and diffusion of polyiodide ions of I3- and I5-.



Moreover, EG-MSCs showed stable cycling performance at -20oC owing to the reduced freezing point of water by strong interactions between water molecules and zinc ions.

"This work opens a new avenue of introducing ambipolar redox mediators into highly concentrated electrolytes for high-performance MSCs," said Prof. WU.

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