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

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.