engleski

Pt-based Bimetallic Electrocatalysts for Application in Fuel Cells: Poisoning of Methanol Anodes

INTRODUCTION - Previous investigations of Pt anodes for application in methanol and hydrogen fuel-cells have shown that one of the main problems that appears in these systems was the poisoning of the anode surface even with the trace levels of irreversibly adsorbed CO, which leads to a drastically decreased fuel-cell power density. This problem has been partially solved by modification of a Pt surface with the addition of second metal, thus producing the so-called bimetallic electrocatalysts (Pt-Sn, Pt-Rh, Pt-Ru, Pt-Bi, Pt-Cu). Enhanced electrocatalytic activity of these bimetallic anode electrocatalysts, in comparison to pure Pt, was ascribed to the influence of the oxide-containing-species (OCS) on the overall mechanism of CO oxidation, in terms of bifunctional mechanism. Generally, the adsorption of OCS at lower potentials, provided with the addition of the second electrode metal, results in a shift of the CO and methanol oxidation potentials toward more negative values. Although a number of investigations have been performed on these systems, there is still a lack of information concerning the kinetics of the OCS formation in the presence of a CO monolayer irreversibly adsorbed on the Pt, Ru and Pt-Ru electrodes. The aim of this work was to investigate the nucleation mechanism and growth kinetic of the OCS on the Pt-, Ru- and Pt_50-Ru_50- electrodes. EXPERIMENTAL - Polycrystalline Pt, Ru and Pt_50-Ru_50 electrodispersed electrodes were made by the potential-controlled-deposition (PCD) of the electroactive components on an Au substrate. This procedure yielded the highly-dispersed catalyst surface area with a controlled roughness. The investigations were performed in perchloric acid using stripping voltammetry and chronoamperometry (potential-step) techniques. The equipment was comprised of an EG&G PAR Model 273 potentiostat controlled by a PC. All potentials in this work are referred to the normal-hydrogen-electrode (NHE). RESULTS AND DISCUSSION - Stripping voltammetry measurements: The stripping voltammetry results were shown that the electrocatalytic activity of the investigated anodes toward CO oxidation, in terms of decrease of the CO oxidation overpotential, increased in order of Pt<Ru<Pt_50-Ru_50. Bimetallic Pt_50-Ru_50 electrode has shown high synergetic activity, which was ascribed to enhanced bifunctional mechanism, i.e. the ability of Ru atoms to adsorb OCS at lower potentials, which then catalyze oxidation of CO molecules bonded on the neighboring sites. Potential-step measurements: All recorded I vs. t transients displayed responses quite typical for the processes controlled by the nucleation and growth phenomena. Therefore, the overall rate of the CO oxidation reaction could be expressed in terms of the rate of nucleation (formation of OCS islands in the CO monolayer) and the rate of nuclei growth (expansion of these oxide islands). The island growth was controlled by the reaction between CO and OCS present at the perimeter line. Potentiostatic transients recorded on the Pt electrode were characterized by the two independent oxidation processes that occur on the Pt surface during the CO oxidation reaction. These two processes were ascribed to the oxidation of CO molecules bonded at two different Pt face sites, 110 and 100. The deconvolution procedure, based on the theoretical models developed for nucleation and growth processes, was applied in order to determine the dependence of the kinetic parameters, i.e. the OCS nucleation and growth rates, on the applied oxidation potential (see Fig. 1). It was found that the nucleation and growth processes on 110 faces were governed by 2D kinetically controlled instantaneous mechanism, and on 110 faces by 2D kinetically controlled progressive mechanism. The results showed that the formation of a new-stable Pt oxide phase, above +0.9 V, retards the reaction of CO oxidation, due to the lower coverage of the Pt surface with the weakly adsorbed OCS. At lower potentials this is not the case, which is particularly important for fuel-cells, that actually operate at lower potential range. Potentiostatic experiments performed on a Ru electrode showed that the OCS nucleation and growth rates increased with the increase of applied oxidation potential, from +0.3 V up to +0.9 V. In this wide potential region the reversible adsorption of weakly bonded OCS on a Ru electrode occurs, which is very important for the kinetics of CO oxidation. Above +0.9 V the irreversible formation of Ru oxide phase commences, and the retardation of the oxidation process initiates. However, this potential region is not of interest for fuel-cells operation. Results on the Pt_50-Ru_50 electrode confirmed its complex nature; two different reaction mechanisms of the OCS nucleation and growth occurred in the investigated potential region. The first, lower potential region was ascribed solely to the Ru influence on the OCS formation, and the second, higher potential region, to both Pt and Ru influence. In the potential region of Ru influence, the nucleation and growth of OCS followed a 2D type of kinetically controlled mechanisms, whereas in the potential region of Pt/Ru influence, a 3D type of nucleation and growth mechanism with the diffusion control of the processes prevailed. Increase of CO oxidation potential above +0.55 V was followed by the increased influence of the 3D mechanism. Nevertheless, lower potential region, where Ru influence on the formation of OCS species predominate, is of importance for operation of fuel-cells.

fuel-cell; electrocatalysis; electrochemistry of cleaner environments; carbon monoxide; oxygen-containing species; nuclation; bifunctional catalysts; platinum; ruthenium; impedance spectroscopy; stripping voltammetry; chronoamperometry

nije evidentirano