MnO₂ Nanoflowers Electrodeposited on Vertically Aligned CNTs as Binder-Free Electrodes for High-Rate Supercapacitors

Supercapacitors are gaining attention for their ability to deliver rapid energy discharge while maintaining a high energy storage capacity, effectively bridging the gap between capacitors and batteries. In this paper, we report the performance of a high-capacity, fast-charging, and reliable supercapacitor consisting of nanoflower-like manganese dioxide (MnO₂) decorated on CVD-grown carbon nanotube (CNT) electrodes fabricated using a simple and efficient room-temperature electrodeposition method. The binder-free, self-supporting MnO₂@CNT composite electrodes formed on a flexible carbon fabric demonstrated excellent electrochemical energy storage capabilities, as confirmed by cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) experiments. The MnO₂ loading significantly affected the electrode's capacity, with the highest specific capacitance of 219 F g^-1 achieved at low mass loading (3.37 mg cm^-2) and the highest areal capacitance of 1.5 F cm^-2 for high mass loading (15.6 mg cm^-2). The rectangular curve observed in CV experiments at faster scan rates (5-50 mV s^-1) and the triangular curve observed in the GCD experiment at high current densities (0.1 to 0.5 A g^-1) demonstrate the high-rate capability of the MnO₂@CNT electrode. The electrode also showed outstanding stability, retaining 88% of its initial capacity after 7000 cycles. Electrochemical impedance spectroscopy (EIS) measurement and corresponding analysis of the data indicated fast charge transfer kinetics and facile ion diffusion into the MnO₂ electrode, which is attributed to the nanoflower-like structure of MnO₂ formed on porous carbon nanotubes, leading to excellent rate performance. With these advancements, our MnO₂@CNT supercapacitors have significant potential in electric vehicles, complementing batteries by enabling fast discharge for quick acceleration.