High-Performance Asymmetric Supercapacitors of MnCoO Nanofibers and N-Doped Reduced Graphene Oxide Aerogel.

The working potential of symmetric supercapacitors is not so wide because one type of material used for the supercapacitor electrodes prefers either positive or negative charge to both charges. To address this problem, a novel asymmetrical supercapacitor (ASC) of battery-type MnCoO nanofibers (NFs)//N-doped reduced graphene oxide aerogel (N-rGO) was fabricated in this work. The MnCoO NFs at the positive electrode store the negative charges, i.e., solvated OH, while the N-rGO at the negative electrode stores the positive charges, i.e., solvated K. An as-fabricated aqueous-based MnCoO//N-rGO ASC device can provide a wide operating potential of 1.8 V and high energy density and power density at 54 W h kg and 9851 W kg, respectively, with 85.2% capacity retention over 3000 cycles. To understand the charge storage reaction mechanism of the MnCoO, the synchrotron-based X-ray absorption spectroscopy (XAS) technique was also used to determine the oxidation states of Co and Mn at the MnCoO electrode after being electrochemically tested. The oxidation number of Co is oxidized from +2.76 to +2.85 after charging and reduced back to +2.75 after discharging. On the other hand, the oxidation state of Mn is reduced from +3.62 to +3.44 after charging and oxidized to +3.58 after discharging. Understanding in the oxidation states of Co and Mn at the MnCoO electrode here leads to the awareness of the uncertain charge storage mechanism of the spinel-type oxide materials. High-performance ASC here in this work may be practically used in high-power applications.