Charge storage mechanism of activated manganese oxide composites for pseudocapacitors

Manganese oxides can undergo an electrochemical activation step that leads to greater capacitances, of which the structural change and mechanism remains poorly understood. Herein we present a wide-ranging study on a manganese oxide synthesised by annealing manganese(II) acetate precursor to 300 degrees C, which includes in operando monitoring of the structural evolution during the activation process via in situ Raman microscopy. Based on powder X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron and ex situ Raman microscopy, the as prepared manganese oxide was characterised as hausmannite-Mn3O4 with a minor portion of MnO2. The activation process of converting as-prepared hausmannite-Mn3O4 into amorphous MnO2 (with localised birnessite structure) by electrochemical cycling in 0.5 M Na2SO4 was examined. After activation, the activated MnOx exhibited capacitive performance of 174 F g(-1) at a mass loading of 0.71 mg cm(-2). The charge storage mechanism is proposed as the redox reaction between Mn(III) and Mn(IV) at outer surface active sites, since the disordered birnessite-MnO2 does not provide an ordered layer structure for cations and/or protons to intercalate.