Phase-controlled synthesis of polymorphic MnO2 structures for electrochemical energy storage

Abstract

Manganese dioxide (MnO₂) with α-, β- and δ-type structures was controllably synthesized by hydrothermal treatment of an acidic solution of KMnO₄ containing different concentrations of ions at 160 °C. The effects of metallic cations, H⁺ and anions on the structures and morphologies of MnO₂ were investigated systematically. The experimental results indicated that cations played a significant part in the formation of MnO₂ with different structures. When K⁺ ions were in competition with H⁺ ions in solution, different MnO₂ structures were formed. α-MnO₂ was formed when the amount of K⁺ was higher than the amount of H⁺, whereas a higher amount of H⁺ was favorable for the growth of β-MnO₂ structures. When the concentration of K⁺ was much greater than that of H⁺, δ-MnO₂ was obtained. Possible formation mechanisms are proposed based on a series of controlled experiments. The electrochemical properties of supercapacitors based on different types of MnO₂ electrodes were investigated in detail. The specific capacitance measured for MnO₂ strongly depended on the crystallographic structure and decreased in the order α-MnO₂ > δ-MnO₂ > β-MnO₂ at a current density of 0.5 A g⁻¹. A specific capacitance of 535 F g⁻¹ was obtained for α-MnO₂, but was only 155 F g⁻¹ for β-MnO₂. α-MnO₂ was more suitable for use as the working electrode at high current densities. Both α- and δ-MnO₂ had a poor cycling performance after 3000 cycles, whereas β-MnO₂ showed good stability, maintaining a cycling efficiency of 80% after under the same conditions.