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

MnO2 with alpha + gamma-, delta-, and a- 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 d- 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 ( alpha + gamma) MnO2. A further increase in the concentration of H+, forming the a- 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 delta- > alpha- > alpha + gamma- MnO2.