USO DA VOLTAMETRIA CÍCLICA E DA ESPECTROSCOPIA DE IMPEDÂNCIA ELETROQUÍMICA NA DETERMINAÇÃO DA ÁREA SUPERFICIAL ATIVA DE ELETRODOS MODIFICADOS À BASE DE CARBONO

Carbon-based electrodes as well the ion exchange electrodes among others have been applied mainly in the treatment of industrial effluents and radioactive wastes. Carbon is also used in fuel cells as substrate for the electrocatalysts, having high surface area which surpasses its geometric area. The knowledge of the total active area is important for the determination of operating conditions of an electrochemical cell with respect to the currents to be applied (current density). In this study it was used two techniques to determine the electrochemical active surface area of glassy carbon, electrodes and ion exchange electrodes: cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The experiments were carried out with 0.1 mol.L-1 KNO3 solutions in a three-electrode electrochemical cell: carbon-based working electrode, platinum auxiliary electrode and Ag/AgCl reference electrode. The glassy carbon and porous carbon electrodes with geometric areas of 3.14 x 10-2 and 2.83 х 10-1 cm2, respectively, were used. The ion exchange electrode was prepared by mixing graphite, carbon, ion exchange resin and a binder, and this mixture was applied in three layers on carbon felt, using a geometric area of 1.0 cm2 during the experiments. The capacitance (Cd) of the materials was determined by EIS using Bode diagrams. The value of 172 μF.cm-2 found for the glassy carbon is consistent with the literature data (~200 μF.cm-2). By VC, varying the scan rate from 0.2 to 2 mV.s-1, the capacitance CdS (S = active surface area) in the region of the electric double layer (EDL) of each material was determined. By EIS, the values of Cd, 3.0 x 10-5 μF.cm-2 and 11 x 103 μF.cm-2, were found for the porous carbon and ion exchange electrodes, respectively, which allowed the determination of active surface areas as 3.73 x 106 cm2 and 4.72 cm2. To sum up, the combined use of EIS and CV techniques is a valuable tool for the calculation of active surface areas of carbon-based electrodes.