Avaliação da área do eletrodo de trabalho por meio da equação de randles-sevick: Considerações a partir de diferentes sondas redox, diâmetros e materiais de eletrodos

Abstract

The electrode area is a crucial parameter in electrochemistry and electroanalysis as it directly affects the intensity of the obtained currents. Typically, the electrochemically active area of the working electrode is much larger than its geometric area, mainly for electrodes chemically modified with nanostructured or porous materials. Thus, several methods have been proposed to determine the electrochemically active area using different approaches. A controversial method involves the use of cyclic voltammetry with a redox probe and the application of the Randles-Sevcik equation. While some researchers claim that this method provides an estimate of the electrochemically active area, others argue that only the geometric area can be obtained through this approach. In this context, the objective of this study was to employ this method to evaluate the area of different working electrodes, aiming to investigate whether the obtained areas truly reflect the geometric area of the working electrode. To accomplish this, platinum (Pt) electrodes with varying diameters and electrodes composed of varied materials, including glassycarbon, gold (Au), and carbon screen-printed electrodes, were employed. Two redox probes, Potassium Ferrocyanide ([𝐹𝑒(𝐶𝑁)6]4− )and Hexaminruthenium (III) Chloride ([𝑅𝑢(𝑁𝐻3)6]3+), were used, and different concentrations of [𝐹𝑒(𝐶𝑁)6]4− (2.0 and 10,0 mmol L-1) were evaluated. Additionally, two concentrations of KCl solution (0.1 and 1.0 mol L-1) were studied as the supporting electrolyte. The obtained results demonstrated that the area determined using this method reflects the geometric area of the working electrode rather than the electrochemically active area, as the calculated-to-geometric area ratios were always less than or equal to 1.0. The calculated areas for Pt electrodes with diameters smaller than 3.0 mm were consistently smaller than the geometric area, regardless of whether [𝐹𝑒(𝐶𝑁)6]4− or [𝑅𝑢(𝑁𝐻3)6]3+ was used as the redox probe. For the Pt electrode with a diameter of 3.0 mm, the calculated area was statistically equivalent to the geometric area when a 10 mmol L-1 [𝐹𝑒(𝐶𝑁)6]4− solution in 1.0 mol L-1 KCl was employed. For the Pt electrodes with smaller diameters, the calculated areas remained smaller than the geometric area, regardless of the concentration of the redox probe and supporting electrolyte. The same behavior was observed for Au and GCE electrodes, both with a diameter of 3 mm. Thus, it appears that the concentration of the support electrolyte, the concentration of the redox probe and the diameter of the working electrode directly influence the area found in this work. However, for Pt electrodes with a diameter greater than 3.0 mm, it is verified that cyclic voltammetry provides more coherent area values, using higher concentrations of the redox probe and the supporting electrolyte.