MnO₂ with controlled phase for use in supercapacitors

Nanostructured MnO₂ has been synthesized using a simple and rapid microwave-assisted hydrothermal (MHT) technique through the decomposition of KMnO₄ in a hydrochloric acid solution. The effects of hydrothermal temperature and ramp rate were examined and discussed. It was found that a lower temperature (140°C) favors the formation of cauliflower-like δ-MnO₂ particles while a higher temperature (160°C, 180°C, or 200°C) favors the formation of α-MnO₂ nanorods. The dimensions of the obtained MnO₂ nanorods were affected by the ramp rate. The cauliflower-like δ-MnO₂ particles have higher specific surface areas than the α-MnO₂ nanorods. Supercapacitors having the obtained MnO₂ as the electrodes were evaluated using cyclic voltammetry and electrochemical impedance spectroscopy in a 1 mol·L⁻¹ Na₂SO₄ solution within a potential window of −0.1-0.9 V. The cauliflower-like δ-MnO₂ electrodes give higher capacitance, up to 202 F/g, than the nanorods α-MnO₂ electrodes, at most 61 F/g.