Studying the Interaction of Mass Transport and Electrochemical Reaction Kinetics by Species Frequency Response Analysis

Electrochemical macrokinetics contains the interaction of electrode reactions with transport phenomena. To disentangle the individual processes, dynamic techniques such as electrochemical impedance spectroscopy are widely used. Additional information can be obtained when further quantities besides current and potential are recorded. Here, we present and analyze a method to observe the dynamics of the flux of volatile species, i.e. mass transfer, in porous electrodes during electrochemical reactions with a high time resolution. We call this technique species frequency response analysis (sFRA). It is experimentally demonstrated with electrochemical methanol oxidation reaction on a porous Pt/Ru electrode. The dynamic relationship between current, potential and the flux of the gaseous reaction product CO$_{2}$ is measured by differential electrochemical mass spectrometry. The resulting transfer function that relates current density with CO$_{2}$ flux is analysed in detail by means of a one-dimensional mathematical model. It is demonstrated how the influence of reaction and transport phenomena can be separated in the sFRA Nyquist plot. Practical aspects such as sensitivity and accessible frequency range are discussed as well as the overall prospects and limitations of the technique.