Letter pubs.acs.org/Langmuir
Dissolution of Pt at Moderately Negative Potentials during Oxygen Reduction in Water and Organic Media Jean-Marc Noel̈ , Yun Yu, and Michael V. Mirkin* Department of Chemistry and Biochemistry, Queens College − CUNY, Flushing, New York 11367, United States S Supporting Information *
ABSTRACT: The electrocatalytic oxygen reduction reaction (ORR) is central to alternative energy systems and sensors. An important practical issue in these systems is the loss of active surface area of the catalyst. The unexpected dissolution of Pt at moderately negative potentials during ORR in water and organic media was detected by combining nanoelectrochemistry with AFM imaging. The possibility to connect this phenomenon with the previously observed formation of hydroxyl radicals has been explored. The loss of Pt occurred only under the experimental conditions at which the formation of hydroxyl radicals was reported in the literature. This process can contribute to cathode degradation in fuel cells and other electrochemical systems. In electrochemical experiments employing a Pt electrode at negative potentials, the reduction of oxygen may be accompanied by the dissolution of the electrode surface.
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INTRODUCTION
Chemicals. Ferrocene methanol (98%, Aldrich) was sublimed twice before use. This redox species yields high-quality steady-state voltammograms in both aqueous and organic solutions at potentials that are less positive than those corresponding to Pt oxide formation. Aqueous solutions were prepared from deionized water (Milli-Q, Millipore Corp.), and either KCl or KNO3 (≥99%, Sigma-Aldrich) was used as the supporting electrolyte. HCl (37%) and NaOH (99.998%) were from Aldrich. Acetonitrile (99.7%) and DMF (99.8%) from Alfa Aesar were used to prepare organic solutions. Tetrabutylammonium perchlorate (TBAClO4, Fluka) was used as an organic supporting electrolyte. Electrode Preparation. Disk-type, flat nanoelectrodes were prepared by pulling 25-μm-diameter annealed Pt wires into borosilicate glass capillaries with the help of a P-2000 laser pipet puller (Sutter Instrument Co.) and were polished under video microscopy control, as described previously.18 The electrode radius was evaluated from steady-state voltammograms of FcMeOH or/and from AFM images. Electrochemical Experiments. The two-electrode setup with a 0.25-mm-diameter Ag wire coated with AgCl serving as a reference electrode was used for electrochemical experiments. Voltammograms were obtained, and potential pulses were applied using a BAS-100B electrochemical analyzer (Bioanalytical Systems, West Lafayette, IN). All experiments were performed at room temperature (22−24 °C) inside a Faraday cage. AFM Experiments. An XE-120 scanning probe microscope (Park Systems) was employed for imaging nanoelectrodes. PPP-NCHR AFM probes (Nanosensors) were used for noncontact imaging. The procedures for AFM imaging of nanoelectrodes either in air or in solution were reported recently.17
The electrocatalytic oxygen reduction reaction (ORR) at Pt electrodes has been extensively studied for several decades because of its relevance to fuel cells and sensors.1−4 The high cost and relatively short lifetime of Pt and other precious metal catalysts are major obstacles to wider cost-effective applications of fuel cells.5 One important factor affecting fuel cell performance is the loss of electrochemically active surface area on the nanoparticle Pt catalysts.6,7 A related phenomenonthe dissolution of Ptwas observed at various electrode potentials and under different experimental conditions, and different dissolution mechanisms have been proposed.8−12 Several publications focused on the dissolution of Pt oxide during ORR, and the presence of oxygen13−15 or hydrogen peroxide16 was found to increase the rate of Pt dissolution significantly at relatively low anodic potentials (
