Achieving Insertion‐Like Capacity at Ultrahigh Rate via Tunable Surface Pseudocapacitance

The insertion/deinsertion mechanism enables plenty of charge‐storage sites in the bulk phase to be accessible to intercalated ions, giving rise to at least one more order of magnitude higher energy density than the adsorption/desorption mechanism. However, the sluggish ion diffusion in the bulk phase leads to several orders of magnitude slower charge‐transport kinetics. An ideal energy‐storage device should possess high power density and large energy density simultaneously. Herein, surface‐modified Fe 2 O 3 quantum dots anchored on graphene nanosheets are developed and exhibit greatly enhanced pseudocapacitance via fast dual‐ion‐involved redox reactions with both large specific capacity and fast charge/discharge capability. By using an aqueous Na 2 SO 3 electrolyte, the oxygen‐vacancy‐tuned Fe 2 O 3 surface greatly enhances the absorption of SO 3 2− anions that majorly increase the surface pseudocapacitance. Significantly, the Fe 2 O 3 ‐based electrode delivers a high specific capacity of 749 C g −1 at 5 mV s −1 and retains 290 C g −1 at an ultrahigh scan rate of 3.2 V s −1 . With a novel dual‐electrolyte design, a 2 V Fe 2 O 3 /Na 2 SO 3 //MnO 2 /Na 2 SO 4 asymmetric supercapacitor is constructed, delivering a high energy density of 75 W h kg −1 at a power density of 3125 W kg −1 .