Amorphous MnO_x 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 CD-S 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 (∼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.