Electrochemical Performance of MnO₂ Nanorods in Neutral Aqueous Electrolytes as a Cathode for Asymmetric Supercapacitors

Abstract

The electrochemical performance of MnO 2 nanorods prepared by a precipitation reaction was investigated in 0.5 mol/L Li 2 SO 4, Na 2 SO 4, and K 2 SO 4 aqueous electrolyte solutions. Results show that at the slow scan rates, the nanorods show the largest capacitance (201 F/g) in Li 2 SO 4 electrolyte since the reversible intercalation/deintercalation of Li + in the solid phase produces an additional capacitance besides the capacitance based on the absorption/desorption reaction. At fast scan rates they show the largest capacitance in the K 2 SO 4 electrolyte due to the smallest hydration radius of K +, highest ionic conductivity, and lowest equivalent series resistance (ESR). An asymmetric activated carbon (AC)/K 2 SO 4 /MnO 2 supercapacitor could be cycled reversibly between 0 and 1.8 V with an energy density of 17 Wh/kg at 2 kW/kg, much higher than those of the AC/K 2 SO 4 /AC supercapacitor and AC/Li 2 SO 4 /LiMn 2 O 4 hybrid supercapacitor. Moreover, this supercapacitor exhibits excellent cycling behavior with no more than 6% capacitance loss after 23 000 cycles at 10C rate even when the dissolved oxygen is not removed.