Synthesis of Manganese Oxide Microspheres by Ultrasonic Spray Pyrolysis and Their Application as Supercapacitors

Abstract

Manganese oxide (MnO2) microspheres are prepared using an ultrasonic spray pyrolysis (USP) process. A mixed solution of potassium permanganate and hydrochloric acid is nebulized into microsized droplets, which are then carried by air flow through a furnace tube. Each microdroplet serves as one microreactor and produces one microsphere. Upon heating, KMnO4 is decomposed into MnO2 microspheres; this synthetic process can easily be scaled up. Characterization of the MnO2 microspheres by scanning electron microscopy, transmission electron microscopy, powder X‐ray diffraction, Raman spectroscopy, and X‐ray photoelectron spectra is described. Different morphologies of MnO2 microspheres can be controlled by tuning the precursor concentrations (and ratios) and furnace temperatures. Microspheres synthesized at 150 °C give amorphous MnO2 while synthesis at 500 °C yields crystalline α‐MnO2. The electrochemical properties investigated by cyclic voltammetry give specific capacitance as high as 320 F g−1, demonstrating promising properties as supercapacitors. In addition, these microspheres can be directly sprayed on conductive substrates, such as carbon fiber paper, and may have useful applications as a supercapacitor electrode coating. The supercapacitive properties of MnO2 microspheres at higher charge and discharge rates can be improved by increasing the surface area coverage or coating them with a thin layer of conductive polymer.