Nitrogen and iron co-doped carbon quantum dots/MnO₂ nanowire composites for flexible solid-state supercapacitors with high areal capacitance

Herein, the synthesis of MnO₂ nanowires decorated with transition metal-doped carbon quantum dots (MCQDs) is described for use as binder-free flexible solid-state supercapacitors. In particular, the water-soluble Fe and N co-doped MCQDs prepared by amidation reaction with ferric ammonium citrate and urea had ultrafine particle morphologies and showed super-hydrophilicity in aqueous solution, which greatly improved the contact probability between the electrode and the electrolyte. In addition, the co-doped MCQDs increased both the electrical conductivity and specific capacitance of the MnO₂/carbon-based nanocomposites and limited the collapse and exfoliation of the MnO₂ structure during extended cycling. Compared with pristine MnO₂ electrode, the MCQDs/MnO₂ composite electrode exhibited faster charge-discharge rate and improved cycle stability with increased specific capacitance. The flexible symmetric supercapacitor constructed from the MCQDs/MnO₂ composite had a capacitance retention of 85.6 % after 10,000 charge/discharge cycles at a current density of 2 mA cm⁻². In addition, the device showed a maximum power density of 4.8 W cm⁻² at an energy density of 2.3 mW h cm⁻², and a maximum energy density of 11.16 mW h cm⁻² at a power density of 311.6 mW cm⁻². Data Availability Statement: The data that support the findings of this study are available from the corresponding author upon reasonable request.