MnO 2 with controlled phase for use in supercapacitors

Nanostructured MnO 2 has been synthesized using a simple and rapid microwave‐assisted hydrothermal ( MHT ) technique through the decomposition of KM nO 4 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‐MnO 2 particles while a higher temperature (160°C, 180°C, or 200°C) favors the formation of α‐MnO 2 nanorods. The dimensions of the obtained MnO 2 nanorods were affected by the ramp rate. The cauliflower‐like δ‐MnO 2 particles have higher specific surface areas than the α‐MnO 2 nanorods. Supercapacitors having the obtained MnO 2 as the electrodes were evaluated using cyclic voltammetry and electrochemical impedance spectroscopy in a 1 mol·L −1 Na 2 SO 4 solution within a potential window of −0.1‐0.9 V. The cauliflower‐like δ‐MnO 2 electrodes give higher capacitance, up to 202 F/g, than the nanorods α‐MnO 2 electrodes, at most 61 F/g.