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 Fe₂ O₃ 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 Na₂ SO₃ electrolyte, the oxygen-vacancy-tuned Fe₂ O₃ surface greatly enhances the absorption of SO₃^2- anions that majorly increase the surface pseudocapacitance. Significantly, the Fe₂ O₃ -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 Fe₂ O₃ /Na₂ SO₃ //MnO₂ /Na₂ SO₄ asymmetric supercapacitor is constructed, delivering a high energy density of 75 W h kg^-1 at a power density of 3125 W kg^-1 .