Controlled synthesis of MnO nanosheets vertically covered FeCoO nanoflakes as a binder-free electrode for a high-power and durable asymmetric supercapacitor.

We developed a simple and controlled method to synthesize FeCoO@MnO core-sheath nanoarchitecture (CSN) grown on Ni foam. Ultrathin FeCoO nanoflakes with an average thickness of 10 nm served as the scaffold to deposit the MnO nanosheets. The MnO nanosheets were able to vertically grow on FeCoO nanoflakes to form a sheath via a hydrothermal reaction. The nanocomposites' thickness could be tailored from 80 nm-550 nm by changing the reaction times. Electrochemical measurements demonstrated that FeCoO@MnO CSN with an optimal thickness of about 400 nm achieved an areal capacitance of 3.077 F cm at 2 mA cm, which is much higher than individual FeCoO nanoflakes (0.295 F cm) and MnO nanosheets (1.065 F cm). An aqueous asymmetric supercapacitor (ASC) was assembled using FeCoO@MnO CSN as its positive electrode and activated carbon (AC) as its negative electrode. The FeCoO@MnO⫽AC ASC exhibited a capacitance of 0.538 F cm at 5 mA cm with a potential window of 1.65 V, and an excellent cycling stability (99.1% retention even after 5000 cycles). Furthermore, the maximum energy density and power density of FeCoO@MnO⫽AC ASC was 0.203 mWh cm at 3.44 mW cm and 28.6 mW cm at 0.061 mWh cm, respectively.