Random Noise for a Reliable Modeling of Electrochemical Reactions. Study of a Redox Reaction on a Self-Assembled Monolayer Modified Electrode

Understanding the underlying phenomena of an electrochemical system is the key to predict its behavior beyond actual conditions in order to design or optimize industrial reactors and processes. This kind of knowledge is achieved by a kinetic modeling study, where the mechanism of the electrochemical reaction is determined and the characteristic parameters (rate constants, diffusion coefficients...) are identified.
To determine a quantitative model of an electrochemical process, first an experimental study is performed. Then a reaction mechanism is proposed and translated into mathematical expressions. To extract the electrochemical parameters an iterative fitting procedure is used: the model parameters are calculated as the values that minimize a least squares cost function, which measures the match between model and experiment. After a statistical evaluation of the fitting results, it is decided whether the model describes appropriately the experiments, and ultimately the electrochemical system.
In our modeling approach, the random noise is used, firstly, as weighting in the cost function and, secondly, to evaluate the validity of the proposed model. If the model represents adequately the experimental observation, the residual errors between best-fit model and experiment are expected to be on the level with the random noise. The latter is accurately determined by using the information contained in the experimental data.
A statistically founded method to model electrochemical reactions using linear sweep voltammetry with a rotating disk electrode (LSV/RDE) has been derived and applied on one electron-transfer reactions, such as the reduction of ferricyanide to ferrocyanide [1,2]. In the scope of broadening the applicability of our modeling methodology, a subsequent step is to study an electrochemical system with a more complex mechanism. The ferri/ferrocyanide redox reaction is here investigated when a gold rotating disk electrode is modified by a self-assembled monolayer of 2-mercaptobenzimidazole-5-sulfonate (MBIS). The presence of the negatively charged MBIS on the electrode surface presents an electrostatic repulsion with the [Fe(CN)6]3-/4- species, which causes a strong inhibition of the electron-transfer reaction. A model is proposed and characteristic parameters are extracted for this electrochemical system.

[1] E. Tourwé, T. Breugelmans, R. Pintelon, A. Hubin. J. Electroanal. Chem. 609 (2007) 1.
[2] L. Fernández Macía, E. Tourwé, R. Pintelon, A. Hubin. Submitted to J. Electroanal. Chem.