Estimation of the electrochemical kinetic parameters of redox couples, on activated porous electrodes, based on impedance spectroscopy and particle swarm optimization

Implementing organic redox flow batteries requires attending some parameters directly related with the performance and stability of the cell. One is the charge transfer kinetics of organic molecules, which depends on the characteristics of the electrode surface. The heterogeneous complexity of carbon electrodes (porosity and hydrophobicity) should be considered when applying strategies to evaluate these devices. This work presents a multivariable transfer function to study the electrochemical interface in organic redox flow batteries using electrochemical impedance data and the particle swarm optimization algorithm. The experiments were performed with hydroquinone and benzoquinone as representative redox organic molecules. Three carbon paper surfaces were considered to evaluate the kinetic and mass transfer parameters (a pristine electrode and two electrochemically activated electrodes). The results described two zones of interaction between the redox couples and the electrodes: flat and porous. The formal standard rate constant improved significantly in the activated electrodes, and the parameter values corresponding to the porous area were considerably lower than the flat area, indicating that the porous controled the charge transfer stage in the system. Comparing our values with other authors, we confirmed that similar values were reported, but the systems were treated as if the electrodes were flat areas, which shows the limitations of the commonly used evaluation methods.