Electrochemical accessibility of porous submicron MnO2 spheres as active electrode materials for electrochemical capacitors

The electrochemical utilization of various submicron amorphous manganese dioxide spheres (SMnO2) with highly controlled shape and size was investigated by cyclic voltammetry and electrochemical impedance spectroscopy. MnO2 spheres were synthesized by reaction between KMnO4 and 1-butanol in aqueous butyric acid solution at room temperature. Particle size was tuned by adjusting the concentration of KMnO4. The materials were characterized by X-ray diffraction, scanning electron microscopy, dynamic light scattering and nitrogen gas adsorption measurements. SEM results reveal that spheres with average diameter of 451 ± 9, 218 ± 12 and 195 ± 85 nm were produced by using a 20, 8 and 2 mM of KMnO4 solution, respectively. DLS measurements showed similar mean particle diameter with a relatively high polydispersity-index that indicates the presence of larger agglomerated particles. The BET surface area of the three SMnO2 is ranging between 216 and 259 m2 g−1. Since very similar specific capacitance values of about 200 F g−1 (per active material mass) at 2 mV s−1 were found for all three samples, MnO2 electrochemical utilization is more related to the pore size distribution rather than the particle size.