Hierarchically MnO₂–Nanosheet Covered Submicrometer-FeCo₂O₄‑Tube Forest as Binder-Free Electrodes for High Energy Density All-Solid-State Supercapacitors

Abstract

The current problem of the still relatively low energy densities of supercapacitors can be effectively addressed by designing electrodes hierarchically on micro- and nanoscale. Herein, we report the synthesis of hierarchically porous, nanosheet covered submicrometer tube forests on Ni foam. Chemical deposition and thermal treatment result in homogeneous forests of 750 nm diameter FeCo₂O₄ tubes, which after hydrothermal reaction in KMnO₄ are wrapped in MnO₂-nanosheet-built porous covers. The covers’ thickness can be adjusted from 200 to 800 nm by KMnO₄ concentration. An optimal thickness (380 nm) with a MnO₂ content of 42 wt % doubles the specific capacitance (3.30 F cm^–2 at 1.0 mA cm^–2) of the bare FeCo₂O₄-tube forests. A symmetric solid-state supercapacitor made from these binder-free electrodes achieves 2.52 F cm^–2 at 2 mA cm^–2, much higher than reported for capacitors based on similar core–shell nanowire arrays. The large capacitance and high cell voltage of 1.7 V allow high energy and power densities (93.6 Wh kg^–1, 10.1 kW kg^–1). The device also exhibits superior rate capability (71% capacitance at 20 mA cm^–2) and remarkable cycling stability with 94% capacitance retention being stable after 1500 cycles