Frontier performance of in situ formed α-MnO2 dispersed over functionalized multi-walled carbon nanotubes covalently anchored to a graphene oxide nanosheet framework as supercapacitor materials

Abstract

α-MnO₂ has been recognized as a potential material for supercapacitor applications because of its abundance, cost-effectiveness, environmental-benign nature and high theoretical specific capacitance (C_sp) of 1370 F g⁻¹. In this study, we succeeded for the first time to achieve the theoretical C_sp with 3D multi-walled carbon nanotubes (MWCNTs) horizontally dispersed on 2D graphene oxide (GO) nanosheet framework-supported MnO₂ ternary nanocomposites synthesized by a simple precipitation method. The in situ formation of α-MnO₂ and GO, and the growth of 3D MWCNT/GO framework took place simultaneously in a strong acidic suspension containing functionalized-MWCNTs, graphite, NaNO₃ and KMnO₄. Characterizations of the composites synthesized by varying % wt MWCNTs were performed with state-of-the-art techniques. These composites were characterized to be semi-crystalline and mesoporous in nature, and the scrupulous analyses of field emission scanning electron microscopic images showed MnO₂ nano-flower distributed over 3D MWCNTs dispersed-on-GO-nanosheet frameworks. These composites deposited on a graphite electrode exhibited an ideal supercapacitive behavior in an Na₂SO₄ solution measured via cyclic voltammetry and chronopotentiometry. Optimum contents of MnO₂ and MWCNTs in the composites showed a maximum C_sp of 1380 F g⁻¹ with satisfactory energy and power densities compared in the Ragone plot. An ascending trend of C_sp against the charge–discharge cycle number studied for 700 cycles was noticed. Well-dispersion of α-MnO₂ nanoparticles throughout 3D MWCNTs covalently-anchored to the GO nanosheet framework is discussed to aid in achieving the frontier C_sp of MnO₂.