Preparation of Two Dimensional (2D) MnO2 By Stirred Media Milling and Its Application in a Supercapacitor

Abstract

Energy has become an inherent part of everyone’s life. Excessive use of the fossil fuels lead to the global environmental problem. Stored electrical energy is now being explored for utilization in majority of the electrical appliances and new class of hybrid vehicles. Significant research has been carried for search of appropriate active electrode material for high performance supercapacitor. The two dimensional (2D) MnO₂ is best choice as an active electrode material for high performance supercapacitor. The 2D MnO₂ has modified physical, electronic and enhanced electrochemical properties due to its ultrathin thickness. Here we have reported a novel top-down approach for the preparation of ultrathin 2D MnO₂ nanosheets in stirred media mill. The 2D MnO₂ nanosheets have been prepared in two different stirred media mill i.e. research laboratory stirred media mill and vertical laboratory stirred media mill, further performance of prepared 2D MnO₂ nanosheets as an active electrode material for supercapacitor from both the stirred media mill have been compared.

Preparation of 2D MnO₂ nanosheets in stirred media mill take place with the help of grinding beads. The particles are captured between the colliding grinding beads, between beads and mill chamber, and between beads and rotating mill stirrer. The overall collective effect results in formation of ultrathin 2D MnO₂ nanosheets. Electrochemical performance of the 2D MnO₂ as active electrode materials depends on the morphologies, crystallinity and structure. Sharp and narrow X-ray powder diffraction peak shows crystalline nature of prepared 2D MnO₂ nanosheets from both mills. All of the diffraction peaks for 2D MnO₂ nanosheets were indexed with the standard XRD pattern of β-MnO₂ (JCPDS No. 24-0735). FE-SEM micrograph shows the presence of plate like structure of few hundred nanometer lateral dimension. Further information on the morphology of the prepared 2D MnO₂ nanosheets were obtained from HR-TEM, which also indicated towards the presence of ultrathin nanosheets. Selected area electron diffraction show dark spot pattern which again confirms crystalline nature. The supercapacititve behaviour was determined from the three-electrode cell in aqueous solution of 0.5M Na₂SO₄ as electrolyte. The cyclic voltammetry measurements carried out at potential ranges of 0-1 V at scan rates of 5-100 mV/s shows curves are near-rectangular in shape at lower scan rate. The symmetrical anodic and cathodic halves further shows presence of excellent pseudocapacitive behavior of prepared 2D MnO₂ nanosheets from the stirred media milling. The maximum value of the specific capacitance obtained at a scan rate of 5 mV/s is 356 F/g for 2D MnO₂ nanosheets from research laboratory stirred media mill and 83.34 F/g for vertical laboratory stirred media mill. Symmetric nature of galvanostatic charging/discharging curve also indicates towards the presence of ideal electrochemical capacitive characteristic. The 2D MnO₂ nanosheets prepared from research laboratory stirred media mill shows better electrochemical performance compared to that prepared from vertical laboratory stirred media mill.

Keywords: MnO₂ nanosheets , stirred media mill, supercapacitor