We describe a new class of electrocatalysts for the O 2 reduction, and H 2 and methanol oxidation reactions, consisting of a monolayer of Pt deposited on a metal or alloy carbon-supported nanoparticles. These electrocatalysts show up to a 20-fold increase in Pt mass activity compared with conventional all-Pt electrocatalysts. The origin of their increased activity was identified through a combination of experimental methods, employing electrochemical and surface science techniques, X-ray absorption spectroscopy, and density functional theory calculations. The long-term tests in fuel cells demonstrated excellent stability of the anode and good stability of the cathode electrocatalysts. We also describe the stabilization of Pt electrocatalysts against dissolution under potential cycling regimes effected by a submonolayer of Au clusters deposited on Pt surfaces. These new electrocatalysts promise to alleviate some of the major problems of existing fuel cell technology.
Oxygen reduction reaction (ORR) kinetics in acid solutions was studied by analysis of the polarization curves obtained by rotating disk electrode method for Pt(111) in HClO 4 and H 2 SO 4 solutions. The model for ORR kinetic currents assumes that the intrinsic exchange current and Tafel slope are independent of anion adsorption. The site blocking and electronic effects of adsorbed OH (in HClO 4 ) and bisulfate (in H 2 SO 4 ) were evaluated with the adsorption isotherms incorporated in the model. The best fits yielded the intrinsic Tafel slope in the range from -118 to -130 mV/dec, supporting single electron transfer in the rate-determining step with the corresponding transfer coefficients equal to 0.50 and 0.45, respectively. In addition to site blocking, a negative electronic effect on ORR kinetics was found for both OH and bisulfate with the effect of the latter being much stronger. The deviation of the apparent Tafel slope in HClO 4 from its intrinsic value can be fully accounted for by the site blocking and electronic effects of adsorbed OH ions, which vary with coverage over the mixed kinetic-diffusion controlled region. For Pt nanoparticle catalysts, the apparent Tafel slope is similar to that for Pt(111) in HClO 4 and the positive potential shift is mainly due to the increase in apparent exchange current as effective surface area increases.
We synthesized a new class of O2 electrocatalysts with a high activity and very low noble metal content. They consist of Pt monolayers deposited on the surfaces of carbon-supported nonnoble metal-noble metal core-shell nanoparticles. These core-shell nanoparticles were formed by segregating the atoms of the noble metal on to the nanoparticles' surfaces at elevated temperatures. A Pt monolayer was deposited by galvanic displacement of a Cu monolayer deposited at underpotentials. The mass activity of all the three Pt monolayer electrocatalysts investigated, viz., Pt/Au/Ni, Pt/Pd/Co, and Pt/Pt/Co, is more than order of magnitude higher than that of a state-of-the-art commercial Pt/C electrocatalyst. Geometric effects in the Pt monolayer and the effects of PtOH coverage, revealed by electrochemical data, X-ray diffraction, and X-ray absorption spectroscopy data, appear to be the source of the enhanced catalytic activity. Our results demonstrated that high-activity electrocatalysts can be devised that contain only a fractional amount of Pt and a very small amount of another noble metal.
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