Electrochemical performance of MnO₂ nanorods in neutral aqueous electrolytes as a cathode for asymmetric supercapacitors

The electrochemical performance of MnO₂ nanorods prepared by a precipitation reaction was investigated in 0.5 mol/L Li₂SO ₂, Na₂SO₄, and K₂SO₄ aqueous electrolyte solutions. Results show that at the slow scan rates, the nanorods show the largest capacitance (201 F/g) in Li₂SO₄ 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 K₂SO₄ electrolyte due to the smallest hydration radius of K⁺, highest ionic conductivity, and lowest equivalent series resistance (ESR). An asymmetric activated carbon (AC)/K₂SO₄/MnO₂ 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/K₂SO₄/AC supercapacitor and AC/Li₂SO₄/LiMn₂O₄ 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.