Ternary composites of delaminated-MnO₂/PDDA/functionalized-CNOs for high-capacity supercapacitor electrodes

Composite materials are synthesized by combining chemically functionalized carbon nano-onions (ONCNOs), a polyelectrolyte (polydiallyldimethylammonium chloride, PDDA) and delaminated layered-manganese oxides (MnO₂). While carbon nano-onions are among the least studied carbon allotropes to date, extensive research has been reported on various types of crystalline and amorphous MnO₂ to overcome their limitations for energy storage applications. We have developed a synergistically tuned synthesis of MnO₂-based composites with ONCNOs via a sequential chemical deposition technique to achieve high-capacity supercapacitor electrodes with long-term stability. A composite with 55 wt% MnO₂ on PDDA modified ONCNOs exhibits a high-capacitance of 218.6 F g^-1 in a symmetric two-electrode cell containing an aqueous electrolyte (1.0 M Na₂SO₄) with a high-energy density of 6.14 W h kg^-1. Analytical techniques, such as high-resolution transmission electron microscopy, X-ray diffraction, atomic force microscopy, thermogravimetric analysis, and electrochemical measurements (cyclic voltammetry, galvanostatic charge-discharge, and impedance), are employed to fully characterize the synthesized materials and to understand their electrochemical behavior. Emphasis is also given to understand the phenomenon that promotes higher capacitance and long-term stability of such composites based on MnO₂ redox chemistry.