Amorphous MnOx Nanostructure/Multiwalled Carbon Nanotube Composites as Electrode Materials for Supercapacitor Applications

Disordered nanostructures with abundant defects and porous microstructures have drawn tremendous interest in the energy storage application. Herein, a simple synthesis of amorphous MnOx nanostructures (AMO-S) is demonstrated by a redox reaction between carbon dots (CD-S, derived from sucrose via hydrothermal method) and KMnO4 at room temperature. The obtained AMO-S displays a nanoparticulate/nanorod-like irregular morphology and mesoporous structure. Experimental observations illustrate that the presence of CDS is essential for the precipitation of AMO-S under ambient conditions. Afterward, a small amount of multiwalled carbon nanotubes (MWCNTs) is incorporated before (in situ) and after (ex situ) the formation of AMO-S, which resulted in AMO/MWCNT-in and AMO/MWCNT-ex nanocomposites, respectively. Electrochemical tests of the synthesized materials are conducted to evaluate their charge storage characteristics. It is found that the AMO/MWCNT-in nanocomposite with a small MWCNT content (similar to 3.0 wt %) exhibits a high specific capacitance of 580.2 F g(-1) at 1.0 A g(-1) based on the total mass of electrode material, promising rate capability (57.1% retention after 6-fold increase in the current density), and high cycling stability (96.9% @ 3000 charge-discharge cycles at 1.0 A g(-1)). The excellent electrochemical performance of the AMO/MWCNT-in electrode is mainly attributed to the better intermixing of two components along with the synergistic contributions of CD-S, resulting in a high electronic conductivity, fast ion diffusion ability, abundant electroactive sites, and mesoporosity. Moreover, the assembled aqueous AMO/MWCNT-in//activated carbon asymmetric supercapacitor can work properly in a significantly large potential window of 0-2.2 V, which delivers a high specific capacitance (76.2 F g(-1) @ 1.0 A g(-1)) and excellent energy density (51.2 Wh kg(-1)) at a power density of 1100 W kg(-1). This study suggests that the in situ incorporation of a small amount of MWCNTs may be an effective approach to improve the energy storage ability of pseudocapacitive AMO-S.

Automated summary

In this study, amorphous MnOx nanostructures were synthesized by a redox reaction between carbon dots and KMnO4 at room temperature. The presence of carbon dots was found to be essential for the precipitation of amorphous MnOx under ambient conditions. The incorporation of a small amount of multiwalled carbon nanotubes before and after the formation of amorphous MnOx resulted in nanocomposites with improved energy storage ability. The synthesized materials exhibited high specific capacitance, promising rate capability, and high cycling stability.