Control of manganese dioxide crystallographic structure in the redox reaction between graphene and permanganate ions and their electrochemical performance

Abstract

MnO₂ with α + γ-, δ-, and α-phases were synthesized by using graphene as sacrificial template in a proposed KMnO₄–graphene–H₂SO₄ reaction system. The as-prepared products were characterized with X-ray diffraction technique, Raman spectroscopy, and transmission electron microscopy. The structural analysis reveals that the cation concentration, i.e. H⁺ and K⁺, has a profound effect on both the crystallographic structures and morphologies of the final products. The relatively higher K⁺ concentration but lower H⁺ concentration facilitates the formation of δ-phased MnO₂ with a petal-like structure, and the lower concentration of both K⁺ and H⁺ cations is more conducive to the formation of a mixed phase of (α + γ) MnO₂. A further increase in the concentration of H⁺, forming the α-phased MnO₂ nanorods is preferred. The electrochemical properties for supercapacitors indicate that the electrochemical performances of MnO₂ strongly depend on their crystallographic structures, and they present a Faradaic reactivity sequence of δ- > α- > α + γ-MnO₂.