Carbon-nanofiber supercapacitor electrodes decorated with manganese oxide nanocuboids via ammonia-assisted impregnation

The pseudocapacitance of manganese oxide and the electric double-layer capacitance of carbon nanofibers (CNFs) synergistically enhance the energy storage capability of freestanding supercapacitor electrodes. In this study, CNFs were uniformly decorated with manganese oxide (MnO/MnO₂) nanocuboids by wet chemical impregnation and carbonization, as confirmed by scanning and transmission electron microscopy. Supplementation with ammonia and 2-methylimidazole facilitated MnO/MnO₂ nanocuboid formation. These MnO/MnO₂ nanocuboids facilitated energy storage, while the CNFs enhanced electron mobility and electrostatic charge storage. Thus, the combination of MnO/MnO₂ nanocuboids and CNFs improved the overall energy storage capacity of the freestanding supercapacitor electrode. Specifically, the unique MnO/MnO₂ nanocuboid/CNF electrode yielded a high areal capacitance of 1.08 F·cm⁻², energy density of 382 µWh·cm⁻², and power density of 16 mW·cm⁻². When the potential window was expanded from 0–1 V to 0–1.6 V, the energy density increased threefold. The supercapacitor retained 97 % of its specific capacitance after 15,000 charge–discharge cycles at a current density of 2 mA·cm⁻². A pouch cell containing the binder-free, self-standing MnO/MnO₂ nanocuboid/CNF electrode exhibited a capacitance retention of 94.9 % at a bending angle of 135° and successfully powered a green light-emitting diode, confirming its suitability for flexible devices.