In situ conversion of manganese carbonate to manganese oxide/hydroxide and its supercapacitive analysis in aqueous KOH solution

摘要

MnCO3 is considered to be a promising electrode material for supercapacitor (SC) and found to be a good material for supercapacitive performance than their counterpart manganese oxides. But the energy storage mechanism of MnCO3 in KOH aqueous solution is not known yet. Hence, in the present work, we have made an attempt to understand the involved energy storage mechanism for MnCO3. MnCO3 with submicron-sized particles are synthesized by simple, cost-effective co-precipitation method at ambient conditions and examined as electrode material for SC application by cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) in 2 M KOH electrolyte. To investigate the reason for charge storage mechanism, ex-situ X-ray diffraction (XRD) and field emission scanning electron microscope (FESEM) analysis were performed at the different stages of charges/discharge for MnCO3 in KOH electrolyte and a plausible energy storage mechanism is proposed. Hexagonal-MnCO3 transforms to respective oxide/hydroxide once it comes in contact with KOH solution. At the end of 2 h reaction time, Mn(OH)(2) transforms to Mn3O4; however, complete transformation of Mn(OH)(2) to Mn3O4 only occurred when external positive potential was applied. All these results were supported by studying pH and potential influence in the line of well known Pourbaix diagram. Considering all results presented in this manuscript, it is clearly observed that MnCO3 does not perform the electrochemical charge-discharge cycling, but it transforms completely to Mn3O4 in the first or second cycle, and thereafter, Mn3O4 does perform the reversible electrochemical charge-discharge cycling to enable high and stable capacitance.