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

Manganese oxides (MnO2) are regarded as typical and promising electrode materials for supercapacitors. However, the practical electrochemical performance of MnO2 is far from its theoretical value. Nowadays, numerous efforts are being devoted to the design and preparation of nanostructured MnO2 with the aim of improving its electrochemical properties. In this work, ultralong MnO2 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 MnO2 nanowires. The effects of the CQDs not only induced the formation of one-dimensional nanostructured MnO2, but also significantly improved the wettability between electrode and electrolyte. In other words, the MnO2 membrane demonstrated a superhydrophilic character in aqueous solution, indicating the sufficient and abundant contact probability between electrode and electrolyte. The binder-free flexible MnO2 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 MnO2 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.