Charge storage mechanisms of manganese oxide nanosheets and N-doped reduced graphene oxide aerogel for high-performance asymmetric supercapacitors

Abstract

Although manganese oxide- and graphene-based supercapacitors have been widely studied, their charge storage mechanisms are not yet fully investigated. In this work, we have studied the charge storage mechanisms of K-birnassite MnO₂ nanosheets and N-doped reduced graphene oxide aerogel (N-rGO_ae) using an in situ X-ray absorption spectroscopy (XAS) and an electrochemical quart crystal microbalance (EQCM). The oxidation number of Mn at the MnO₂ electrode is +3.01 at 0 V vs. SCE for the charging process and gets oxidized to +3.12 at +0.8 V vs. SCE and then reduced back to +3.01 at 0 V vs. SCE for the discharging process. The mass change of solvated ions, inserted to the layers of MnO₂ during the charging process is 7.4 μg cm^-2. Whilst, the mass change of the solvated ions at the N-rGO_ae electrode is 8.4 μg cm^-2. An asymmetric supercapacitor of MnO₂//N-rGO_ae (CR2016) provides a maximum specific capacitance of ca. 467 F g^-1 at 1 A g^-1, a maximum specific power of 39 kW kg^-1 and a specific energy of 40 Wh kg^-1 with a wide working potential of 1.6 V and 93.2% capacity retention after 7,500 cycles. The MnO₂//N-rGO_ae supercapacitor may be practically used in high power and energy applications.