Preparation of manganese dioxide from low-grade pyrolusite and its electrochemical performance for supercapacitors

The preparation of nanostructured MnO2 from low-grade pyrolusite using a H2SO4 roasting-water leaching process without any reducing agent is studied in this paper. In the second step of our study, we synthesize different morphologies of MnO2 created by hydrothermal reactions by controlling the ferric ion concentration in the reaction system; this process does not require any surfactant, template or additional materials. X-ray diffraction (XRD), scanning electron microscopy (SEM) and chemical composition analysis are used to investigate the components and morphological structure of the roasting clinker by adjusting the roasting temperature. We discuss the effect of the ferric ion concentration in the filtrate on the phase structure, microstructure, specific surface area and electrochemical performance of the MnO2. The results indicate that the ferric ion concentration has a significant influence on the morphology and internal structure of the MnO2 products, which subsequently changes the specific surface area and electrochemical properties of the material. In the different nanostructured MnO2 products, hollow γ-MnO2 microspheres composed of small nanorods obtained from the MnSO4 solution of 500 °C exhibit the highest specific surface area (63.9 m2 g−1). The studied sample presents excellent electrochemical behavior for supercapacitor applications. The proposed approach is simple, economical and reasonable, and therefore provides new ideas for the sustainable development of low-grade pyrolusite.