Electrochemical Quartz Crystal Nanobalance Analyses of Electrocatalysis for Fuel Cells

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In order to find a clue for a design of high-performance electrocatalysts for fuel cells, we analyzed electrode reactions on Pt-Fe and Pt-Co alloy in comparison with that on pure Pt by using an electrochemical quartz crystal nanobalance (EQCN). We obtained the following results.1. We succeeded to construct 10MHz-EQCN system with a specially designed electrochemical cell, and confirmed the ultimate accuracy of ±0.1 Hz (±0.4 ng/cm^2) and high stability among long time.2. It was found that the Pt-Fe and Pt-Co electrodes were electrochemically stabilized by a formation of a few monolayers of Pt-skin layer during several repetitive potential sweeps in 0.1 M HClO_4 solution, i.e., a dissolution of alloy components followed by a rearrangement of thin Pt layer. The results obtained by XPS, ICP and EC-STM complement the EQCN results.3. A CO-tolerance at Pt-Fe and Pt-Co alloy electrodes was analyzed by the EQCN. In a positive-going potential scan at CO-adsorbed (θ_<co>=1) electrodes of pure Pt a … More nd Pt-Fe or Pt-Co alloy, a pre-oxidation peak (P_1) was observed at 0.3 to 0.5 V vs. RHE before a major CO oxidation peak (P_2) at 0.7 V. Each P_1 was assigned to an oxidation of weakly-adsorbed CO at Pt-Fe and Pt-Co alloys and adsorbed carboxylate at pure Pt, respectively. This is consistent with our previous results by in-situ FTIR, and strongly supports a modified electronic structure of the Pt-skin from that of pure Pt, which has been proposed by us. At P_2 region, CO is oxidized by OH formed by H_2O discharge and vacant sites on both of the electrodes formed by the oxidation of CO or CO-related species (COOH) are partly occupied by hydrated ClO_4 anions.4. Oxygen reduction reactions (ORR) at Pt, Pt-Fe, and Pt-Co electrodes in O_2-saturated 0.1 M HClO_4 solution were analyzed by the EQCN. At pure Pt, a constant amount of reaction intermediate such as Pt-O (ca. 20 %) always adsorbed on the surface even under high overvoltage. In contrast, the amount of such an intermediate was found to be fairly low at Pt-Fe and Pt-Co electrodes, probably due to a fast ORR kinetics. Such a difference between alloys and pure Pt may be ascribed to modified electronic structure of the Pt-skin. Less