Impedance spectroscopic analysis of the electrochemical methanol oxidation kinetics

This paper presents a dynamic analysis of the methanol oxidation kinetics on a direct methanol fuel cell anode in a cyclone flow cell. Four kinetic descriptions for methanol oxidation are discussed. They differ in the adsorption/desorption mechanisms, as well as in the potential dependence of a reaction step. Since all kinetic descriptions quantitatively describe the experimental steady-state behaviour, dynamic measurements, here electrochemical impedance spectroscopy, is used for model discrimination. The presented impedance spectra are modelled by using the frequency domain transformed balance equations of the four kinetic descriptions. Analysis of the dependence of the modelled impedance spectra on parameter variation shows a narrow range in which pseudo-inductive behaviour is produced: at least two consecutive steps with similarly fast reactions have to be involved. Besides reaction rate constants, storage parameters and anode potential also show a strong influence on the impedance spectra. The parameters of all four kinetic descriptions have been globally optimised. A comparison of the resulting impedance spectra leads to the identification of the best quantitative description of the electrochemical methanol oxidation kinetics.