Carbon Quantum Dot-Induced MnO₂ Nanowire Formation and Construction of a Binder-Free Flexible Membrane with Excellent Superhydrophilicity and Enhanced Supercapacitor Performance

Manganese oxides (MnO 2 ) are regarded as typical and promising electrode materials for supercapacitors. However, the practical electrochemical performance of MnO 2 is far from its theoretical value. Nowadays, numerous efforts are being devoted to the design and preparation of nanostructured MnO 2 with the aim of improving its electrochemical properties. In this work, ultralong MnO 2 nanowires were fabricated in a process induced by carbon quantum dots (CQDs); subsequently, a binder-free flexible electrode membrane was easily obtained by vacuum filtration of the MnO 2 nanowires. The effects of the CQDs not only induced the formation of one-dimensional nanostructured MnO 2, but also significantly improved the wettability between electrode and electrolyte. In other words, the MnO 2 membrane demonstrated a superhydrophilic character in aqueous solution, indicating the sufficient and abundant contact probability between electrode and electrolyte. The binder-free flexible MnO 2 electrode exhibited a preeminent specific capacitance of 340 F g –1 at 1 A g –1; even when the current density reached 20 A g –1, it still maintained 260 F g –1 (76% retention rate compared to that at 1 A g –1 ). Moreover, it also showed good cycling stability with 80.1% capacity retention over 10 000 cycles at 1 A g –1 . Furthermore, an asymmetric supercapacitor was constructed using the MnO 2 membrane and active carbon as the positive and negative electrodes, respectively, which exhibited a high energy density of 33.6 Wh kg –1 at 1.0 kW kg –1, and a high power density of 10 kW kg –1 at 12.5 Wh kg –1 .