Metal carbonates: alternative to metal oxides for supercapacitor applications? A case study of MnCO3 vs MnO2

We report here the potential competency of MnCO3 versus MnO2 for supercapacitor applications. MnCO3 was synthesized by a hydrothermal method using KMnO4 as a manganese source and either sugar or pyrrole as carbon source. MnCO3 synthesized using sugar and pyrrole as carbon source is referred hereafter as MnCO3(s) and MnCO3(p), respectively. The synthesized products were characterized by powder X-ray diffraction, scanning electron microscopic and transmission electron microscopic studies. Microscopic studies revealed that MnO2 possesses micro-flower-like morphology constructed by self-assembled nano-petals. While the morphology of MnCO3(s) is sub-micron size particles of different shape, the morphology of MnCO3(p) is crystalline particles of 10-20 nm dia. The capacitive characteristics of MnO2, MnCO3(s) and MnCO3(p) were evaluated in aqueous 0.1 M Mg(ClO4)(2) electrolyte between 0 and 1 V using cyclic voltammetry and galvanostatic charge/discharge cycling. Specific capacitance (SC) values of 216 and 296 F g(-1) obtained for MnCO3(s) and MnCO3(p) are 35 and 85 % higher than SC value of 160 F g(-1) obtained for MnO2, respectively. Besides better capacitive storage characteristics, MnCO3(s) and MnCO3(p) have also exhibited better rate capability and cycle life than MnO2.