Electrochemical Zinc Ion Capacitors Enhanced by Redox Reactions of Porous Carbon Cathodes

Aqueous electrochemical zinc ion capacitors (ZICs) are promising next‐generation energy storage devices because of their high safety, inexpensive raw materials, and long cycle life. Herein, an aqueous ZIC with superior performance is fabricated by employing an oxygen‐rich porous carbon cathode. Excellent capacitance and energy density are obtained thanks to the electric double‐layer capacitance of porous carbon, and additional pseudocapacitances originating from the variation in oxidation states of oxygen functional groups and the reversible electrochemical hydrogen adsorption and desorption during each round‐trip charge–discharge cycle. Moreover, the cycling stability of ZIC is effectively prolonged by suppressing zinc dendrite growth with a simple surface carbon coating strategy. The assembled ZIC delivers a high capacitance of 340.7 F g−1, a high capacity of 179.8 mAh g−1 in a wide voltage window of 0–1.9 V, a maximum energy density of 104.8 Wh kg−1, and an ultrahigh power density of 48.8 kW kg−1. Furthermore, the as‐fabricated aqueous ZIC exhibits an ultralong cycle life of 30 000 cycles with a high capacity retention of 99.2%. This work provides a novel design strategy by incorporating reversible hydrogen and oxygen redox reactions to enhance the energy storage capability of aqueous ZICs toward practical energy storage applications. An electrochemical zinc ion capacitor delivers high capacitance and energy density thanks to the electric double‐layer capacitance and the pseudocapacitances of highly reversible oxygen and hydrogen redox reactions. The oxygen and hydrogen redox reactions occurring at specifically high and low voltages are achieved by optimizing electrolyte and expanding the working voltage window.