High-Performance Asymmetric Supercapacitors of MnCo₂O₄ 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 MnCo₂O₄ nanofibers (NFs)//N-doped reduced graphene oxide aerogel (N-rGO_AE) was fabricated in this work. The MnCo₂O₄ NFs at the positive electrode store the negative charges, <i>i.e.</i>, solvated OH^–, while the N-rGO_AE at the negative electrode stores the positive charges, <i>i.e.</i>, solvated K⁺. An as-fabricated aqueous-based MnCo₂O₄//N-rGO_AE ASC device can provide a wide operating potential of 1.8 V and high energy density and power density at 54 W h kg^–1 and 9851 W kg^–1, respectively, with 85.2% capacity retention over 3000 cycles. To understand the charge storage reaction mechanism of the MnCo₂O₄, the synchrotron-based X-ray absorption spectroscopy (XAS) technique was also used to determine the oxidation states of Co and Mn at the MnCo₂O₄ 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 MnCo₂O₄ 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