Analysis of the influence of rhodium addition to platinum on its activity towards methanol electrooxidation by EIS

Influence of rhodium addition to platinum on the activity of the alloy in methanol electrooxidation has been studied using Pt–Rh/Au limited volume electrodes with various surface compositions including the pure Pt and Rh metals. Electrochemical impedance spectroscopy (EIS) was used in the study. In the case of the Pt–Rh alloy, the impedance picture of methanol oxidation is qualitatively the same as for the pure Pt electrode. However, impedance spectra strongly depend on alloy composition. Equivalent circuits suitable for methanol oxidation on Pt were also used in the case of Pt–Rh and similar fitting results were obtained. A reaction mechanism suggested in the literature for Pt, which involves two strongly adsorbed intermediates competing for the same adsorption sites, is likely also for the Pt–Rh alloys. However, fittings with a corresponding impedance model were unsuccessful for both Pt and Pt–Rh because of mathematical caveats, such that quantitative comparisons were not possible. Nevertheless, EIS results suggest that Rh inhibits the kinetics of formation of reactive oxygen species at the surface of the alloy.