Sprinkling MnFe2O4 quantum dots on nitrogen-doped graphene sheets: the formation mechanism and application for high-performance supercapacitor electrodes

Abstract

Quantum dots (QDs)/graphene composites are interesting as promising electrode materials for high-performance supercapacitors because they can well integrate the complementary features of QDs and graphene. Herein, we demonstrate a MnFe₂O₄ QDs/nitrogen-doped graphene (NG) material prepared by a controllable solvothermal synthesis, in which ultra-small MnFe₂O₄ QDs are uniformly anchored on NG surfaces. First-principles calculations elucidate that the oxygen-containing groups of graphene oxide play a crucial role in generating such a structure, and a ferrite octahedral skeleton is firstly formed followed by Mn atom insertion. Powdery MnFe₂O₄ QDs/NG exhibits a high specific capacitance of 517 F g⁻² within a negative potential window (−1 ∼ 0 V) in KOH electrolyte. When the lower cut off voltage is extended to −1.2 V, the specific capacitance can be increased to 905 F g⁻¹. And the condensed MnFe₂O₄ QDs@NG electrode (forming a similar structure to pitaya slices) with an astonishing loading mass of 18 mg cm⁻² can achieve high areal and volumetric capacitances (5.3 F cm⁻² and 277.6 F cm⁻³). Moreover, carbon encapsulation is favorable for the improvement of rate and cycling performance, allowing a satisfactory capacitance of 150 F g⁻¹ even at 200 A g⁻¹ as well as a superior lifetime up to 65 000 cycles. These results make such materials competitive with supercapacitor electrodes and may speed up the development of QD-based electrodes for energy storage applications.