The significant enhancement of energy density by redox and molecular crowding synergistic effects for aqueous zinc-ion hybrid capacitors

Abstract

Aqueous zinc-ion hybrid capacitors (ZHCs) combine the high energy density of metal-ion batteries with the advantages of fast charging and discharging, high power density and long cycle life of capacitors, and are considered to be one of the most promising next-generation energy storage devices. However, the low operating voltage of aqueous electrolyte and the capacity mismatch between the capacitive cathode and the zinc anode lead to the unsatisfactory energy density of ZHCs. To improve the energy output for ZHCs, we designed an active aqueous electrolyte with high working voltage window by utilizing molecular crowding effect of polyethylene glycol (PEG) and the effect between redox-active ions (3I−/I3−). This high-voltage active aqueous electrolyte exhibits a wide voltage window of 2.2 V by inhibiting the hydrogen evolution in HER. The specific capacity of optimal ZHCs is boosted to 469 F g1 by the additional psedocapacitance from ZnI2 redox additive. The energy density of ZHCs reaches 287.3 Wh kg−1 and the capacity retention remains at 91.6 % after 7000 cycles. These results indicate that the proposed high-voltage active aqueous electrolyte is not only facile and effective for ZHCs, but also can be applied to other aqueous device for high energy density and practical application.