Crystallization design of MnO2 towards better supercapacitance

Abstract

Nanostructured manganese oxides with different crystallization behaviors were fabricated by a simple redox reaction between KMnO₄ and NaNO₂ aqueous solution. MnO₂ nanostructures with sphere-, rod-, wire-, plate- and flower-like morphologies were crystallized, and the relationship between crystallization characteristics and their electrochemical performances were studied. The electrochemical energy storage behaviors of these samples were investigated by cyclic voltammetry and galvanostatic charge–discharge measurements processed using noncorrosive Na₂SO₄ as the electrolyte. A maximum specific capacitance 200 F g⁻¹ was obtained for poorly crystallized α-MnO₂ at a current density of 1 A g⁻¹. For different crystallographic MnO₂ phases, their specific capacitance values increase in the order: β < γ < δ < α, meanwhile, for any particular MnO₂ phase, their electrochemical energy storage performances decrease with increasing crystalline nature and particle size.