SÍNTESE DE ?-MnO2 A PARTIR DA DIGESTÃO ÁCIDA DE UM PRECURSOR DE ORIGEM POLIOL E SUA APLICAÇÃO EM SUPERCAPACITOR

Abstract

A simple hybrid synthesis-processing method was developed to synthesize ?-MnO2 nanocrystalline particles. The Polyol method was modified by the addition of nitric acid in order to allow the synthesizing of single-phase Mn3O4 in large scale. In the sequence, the acid digestion technique was used to transform Mn3O4 into ?-MnO2. From the data of X-ray diffraction (XRD), by applying the model of Chabre and Pannetier, the structure of ?-MnO2 synthesized consists of 39% of pyrolusite and owns 20% of structural disorder microtwinning type. The average diameter of crystallites ?-MnO2 synthesized, estimated by the Scherer`s equation from the XRD data, was 15,4 nm. From the data in the thermogravimetric measurements, the water content in the structure of the ?-MnO2 was 2,78%. From this information and data obtained by chemical analysis measurements, the fraction of vacant cation in the ?-MnO2 was 0,027. From the analysis of nitrogen adsorption isotherms using the BET method, the specific surface area of ?-MnO2 was 53,54 m².g?¹, its total pore volume was 0,20 cm³.g?¹ and the average diameter of the pores was 14,67 nm. According images obtained by scanning electron microscopy, the particles of ?-MnO2 are clustered and according to the images obtained by transmission electron microscopy, the morphology of the particles of ?-MnO2 is as rods, and these have a diameter about 10 nm and a length of about 30 nm. The maximum specific capacitance of the synthesized material obtained by cyclic voltammetry in electrolyte 1 mol.L?¹ Na2SO4, in a potential window from 0.0 to 0.9 V vs. Ag/AgCl was 509.7 F.g?¹ in a mass loading of 0,4 mg.cm-2. The charge storage mechanism of ?-MnO2 was pseudocapacitive, being favored by increasing the concentration of the electrolyte and the pH of this and by the decrease in scan rate. The smaller the mass loading of ?-MnO2 (mg.cm-2) on the electrode, the higher the specific capacitance of ?-MnO2. According to the measures galvanostatic of charge-discharge, the synthesized material shows excellent cyclic stability, and thus potential for application as electrode material for supercapacitor.