Nano-structured birnessite prepared by electrochemical activation of manganese(III)-based oxides for aqueous supercapacitors

Powdery Mn₃O₄ and Mn₂O₃ electrodes with carbon and binding polymer were electrochemically stimulated and activated by successive potential cycles in a mild aqueous electrolyte containing alkali sulfate. The activation of the manganese oxides is affected by the electrode material, milling treatment, potential region, and electrolyte solution. It is found that the ball-milled Mn₃O₄ electrode demonstrated the highest specific capacitance, 190 F g^?1, in 1 mol dm^?3 Na₂SO₄ aqueous solution due to the phase transition from Mn₃O₄ to electrochemically active birnessite, Na_yMnO₂·nH₂O. The increase in capacitance originated from the formation of birnessite possessing highly porous morphology. The nano-structured birnessite demonstrated long cycle life of about 2000 cycles with acceptable capacitance retention of 190–160 F g^?1. The birnessite was applied as positive electrode of the asymmetric electrochemical capacitor with activated carbon negative electrode in the mild aqueous solution.