Flexible Asymmetric Micro‐Supercapacitors Based on Bi 2 O 3 and MnO 2 Nanoflowers: Larger Areal Mass Promises Higher Energy Density

A flexible asymmetric supercapacitor (ASC) with high energy density is designed and fabricated using flower‐like Bi 2 O 3 and MnO 2 grown on carbon nanofiber (CNF) paper as the negative and positive electrodes, respectively. The lightweight (1.6 mg cm −2 ), porous, conductive, and flexible features make the CNF paper an ideal support for guest active materials, which permit a large areal mass of 9 mg cm −2 for Bi 2 O 3 (≈85 wt% of the entire electrode). Thus, the optimal device with an operation voltage of 1.8 V can deliver a high energy density of 43.4 μWh cm −2 (11.3 W h kg −1 , based on the total electrodes) and a maximum power density of 12.9 mW cm −2 (3370 W kg −1 ). This work provides an example of large areal mass and flexible electrode for ASCs with high areal capacitance and high energy density, holding great promise for future flexible electronic devices.