Solvation Effects on OH Adsorbates on Stepped Pt Surfaces

Density functional theory calculations were applied to OH formations on stepped Pt electrodes of Pt­[<i>n</i>(111) × (111)] (<i>n</i> = 3, 4, and 6) for examining solvation effects on the OH adsorbates. Results indicated that OH adsorbates at terrace sites are slightly destabilized by water molecules adsorbed at step sites forming 1-dimensional water chains whereas OH adsorbates at step sites are significantly destabilized by water molecules adsorbed at terrace sites forming 2-dimensional honeycomb structures. On stepped Pt surfaces with narrow terrace widths, water molecules cannot exist at terrace sites, and therefore, the solvation effects on OH adsorbates at step sites disappear. Hence, OH adsorbates are formed at step sites at a low potential region, ca. 0.3 V (standard hydrogen electrode (SHE)). When high-coverage CO adsorbates are present on the stepped Pt surfaces, water molecules cannot exist at the terrace sites either because strongly bound CO molecules exclude the water molecules. In such conditions, OH formation potentials decrease significantly, too. Thermodynamic stabilities of OH adsorbates are, therefore, controlled not only by the local surface morphology but also by long-ranged interfacial solvation environments. In other words, the stability and presence/absence of OH adsorbates should be considered to be totally different with water adsorbates (like in inert conditions) and without them (like in the CO oxidation)

Activities and Stabilities of Au-Modified Stepped-Pt Single-Crystal Electrodes as Model Cathode Catalysts in Polymer Electrolyte Fuel Cells

The purpose of this study is to test the concept of protecting vulnerable sites on cathode catalysts in polymer electrolyte fuel cells. Pt single-crystal surfaces were modified by depositing Au atoms selectively on (100) step sites and their electrocatalytic activities for oxygen reduction reaction (ORR) and stabilities against potential cycles were examined. The ORR activities were raised by 70% by the Au modifications, and this rise in the activity was ascribed to enhanced local ORR activities on Pt(111) terraces by the surface Au atoms. The Au modifications also stabilized the Pt surfaces against potential cycles by protecting the low-coordinated (100) step sites from surface reorganizations. Thus, the surface modification by selective Au depositions on vulnerable sites is a promising method to enhance both the ORR activity and durability of the catalysts

Effect of the Side-Chain Structure of Perfluoro-Sulfonic Acid Ionomers on the Oxygen Reduction Reaction on the Surface of Pt

The effect of side-chain structures in perfluoro-sulfonic acid ionomers on the adsorption of the terminal sulfonate moiety on the surface of Pt is investigated with voltammetry and surface-enhanced infrared absorption spectroscopy (SEIRAS). Analyses with low-molecular-weight model anions with and without an ether group in the perfluoro-alkyl chain indicate that the anions are adsorbed on Pt through one or two oxygen atom(s) of the terminal sulfonate group and that the oxygen atom of the ether group also interacts with the Pt surface, leading to stronger adsorption of the anions with an ether group. On the basis of the results obtained with the model anions, the adsorption of the terminal sulfonate moieties in perfluorinated sulfonic acid ionomers and its effect on oxygen reduction reaction (ORR) is discussed. It is shown that the ionomers having longer side chains more strongly block ORR due to the flexibility of the side chains