Redox-enhanced zinc-ion hybrid capacitors with high energy density enabled by high-voltage active aqueous electrolytes based on low salt concentration

Aqueous zinc-ion energy storage devices with high safety standards attract extensive interests. Among them, aqueous zinc-ion hybrid capacitors (ZICs) feature the merits of low cost, high power and long cycle life. However, the low operating voltages of aqueous electrolytes, and capacity mismatch between the capacitive cathode and the zinc metal anode, result in limited energy outputs of ZICs. Herein, synergic effects between polymer molecular crowding agent [poly(ethylene glycol)] and redox active ions (3Br‒/Br3‒) are utilized to construct high voltage active aqueous electrolytes with low salt concentration (1.5 m). The molecular interaction mechanism for the proposed active aqueous electrolyte system is investigated by using a variety of experimental and theoretical simulation techniques. The aqueous electrolyte presents a wide electrochemical stability window (3.32 V) through confining water molecules in polymer chains, and meanwhile introduces extra faradaic contributions at activated carbon cathode. The assembled ZIC delivers an energy density up to 353 Wh kg−1, which is comparable to that of aqueous zinc-ion batteries. Owing to the physical adsorption (van der Waals force) of oxidized Br3‒ on the surface of activated carbon, retarded self-discharge (with an energy retention efficiency of 73.8% after 10 h) is achieved for the ZIC. This work offers a feasible strategy for exploring ZICs and other aqueous hybrid supercapacitors with boosted energy density, concurrently maintaining reasonable rate capability and cycling stability.