Controlled synthesis of MnO2 nanosheets vertically covered FeCo2O4 nanoflakes as a binder-free electrode for a high-power and durable asymmetric supercapacitor

We developed a simple and controlled method to synthesize FeCo2O4@ MnO2 core-sheath nanoarchitecture (CSN) grown on Ni foam. Ultrathin FeCo2O4 nanoflakes with an average thickness of 10 nm served as the scaffold to deposit the MnO2 nanosheets. The MnO2 nanosheets were able to vertically grow on FeCo2O4 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 FeCo2O4@ MnO2 CSN with an optimal thickness of about 400 nm achieved an areal capacitance of 3.077 F cm(-2) at 2 mA cm(-2), which is much higher than individual FeCo2O4 nanoflakes (0.295 F cm(-2)) and MnO2 nanosheets (1.065 F cm(-2)). An aqueous asymmetric supercapacitor (ASC) was assembled using FeCo2O4@ MnO2 CSN as its positive electrode and activated carbon (AC) as its negative electrode. The FeCo2O4@ MnO2//AC ASC exhibited a capacitance of 0.538 F cm(-2) at 5mA cm(-2) 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 FeCo2O4@ MnO2//AC ASC was 0.203 mWh cm(-2) at 3.44mWcm(-2) and 28.6mWcm(-2) at 0.061 mWh cm(-2), respectively.