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Electrocatalytic Properties of Electrodeposited Pt_70-Ru_30 Electrodes for Methanol Oxidation Reaction

Pt-Ru electrocatalysts exhibit high activity for methanol oxidation reaction, due to ruthenium which enables the adsorption of oxygen containing species at lower potential than on pure platinum. Consequently, the anodic overpotential for methanol oxidation reaction is decreased for approximately 200 mV. In this work, Pt_70-Ru_30 electrodes produced by the potential-controlled-deposition from solution containing Pt and Ru ions in stoichiometric ratio 70:30 were investigated. The surface composition was determined using X-ray photoemission spectroscopy (XPS), which confirmed the stoichiometric ratio of Pt:Ru=70:30. The methanol oxidation reaction mechanism and kinetic parameters were determined using several electrochemical techniques: cyclic voltammetry (CV), linear polarization (LP), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS). The measurements were performed in 1 M HClO_4, containing various methanol concentration (0.005 to 0.1 M). The impedance spectra of the Pt_70-Ru_30 electrode in a solution containing 0.1 M CH_3OH, recorded at double layer potential region (0.35 V) and at the potential of oxidation current maximum (0.65 V) are shown in Figure 1. The data are presented in the form of Bode plots. The Langmuir isotherm was used for description of adsorption process of methanol on Pt_70-Ru_30 electrodes, and the obtained values for DELTA G_ads (-29ą2 kJmol^-1) indicated non-activated adsorption at the active sites (Pt surface atoms). CV criteria showed that the oxidation process was under kinetic control. Low values of diffusion coefficients (10^-12-10^-7 cm^2s^-1), evaluated from I-t transients, were attributed to the surface diffusion of electroactive, oxygen containing, species. The reaction order was closer to unity for Pt_70-Ru_30 than for pure Pt, indicating that the reaction rate was depending on methanol concentration, rather than on concentration of intermediates, as for pure Pt.

fuel-cell; electrocatalysys; adsorption; bifunctional catalysts; platinum; ruthenium; methanol; XPS; impedance spectroscopy

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