High-Performance\nAsymmetric Supercapacitors of MnCo₂O₄ Nanofibers\nand N‑Doped Reduced Graphene Oxide Aerogel

Abstract

The\nworking potential of symmetric supercapacitors is not so wide because\none type of material used for the supercapacitor electrodes prefers\neither positive or negative charge to both charges. To address this\nproblem, a novel asymmetrical supercapacitor (ASC) of battery-type\nMnCo₂O₄ nanofibers (NFs)//N-doped reduced graphene\noxide aerogel (N-rGO_AE) was fabricated in this work. The\nMnCo₂O₄ NFs at the positive electrode store\nthe negative charges, <i>i.e.</i>, solvated OH^–, while the N-rGO_AE at the negative electrode stores the\npositive charges, <i>i.e.</i>, solvated K⁺. An\nas-fabricated aqueous-based MnCo₂O₄//N-rGO_AE ASC device can provide a wide operating potential of 1.8\nV and high energy density and power density at 54 W h kg^–1 and 9851 W kg^–1, respectively, with 85.2% capacity\nretention over 3000 cycles. To understand the charge storage reaction\nmechanism of the MnCo₂O₄, the synchrotron-based\nX-ray absorption spectroscopy (XAS) technique was also used to determine\nthe oxidation states of Co and Mn at the MnCo₂O₄ electrode after being electrochemically tested. The oxidation number\nof Co is oxidized from +2.76 to +2.85 after charging and reduced back\nto +2.75 after discharging. On the other hand, the oxidation state\nof Mn is reduced from +3.62 to +3.44 after charging and oxidized to\n+3.58 after discharging. Understanding in the oxidation states of\nCo and Mn at the MnCo₂O₄ electrode here leads\nto the awareness of the uncertain charge storage mechanism of the\nspinel-type oxide materials. High-performance ASC here in this work\nmay be practically used in high-power applications.