Kotaro Sasaki

Core–Shell Structuring of Pure Metallic Aerogels towards Highly Efficient Platinum Utilization for the Oxygen Reduction Reaction

The development of core-shell structures remains a fundamental challenge for pure metallic aerogels. Here we report the synthesis of Pdx Au-Pt core-shell aerogels composed of an ultrathin Pt shell and a composition-tunable Pdx Au alloy core. The universality of this strategy ensures the extension of core compositions to Pd transition-metal alloys. The core-shell aerogels exhibited largely improved Pt utilization efficiencies for the oxygen reduction reaction and their activities show a volcano-type relationship as a function of the lattice parameter of the core substrate. The maximum mass and specific activities are 5.25u2005Au2009mgPt-1 and 2.53u2005mAu2009cm-2 , which are 18.7 and 4.1u2005times higher than those of Pt/C, respectively, demonstrating the superiority of the core-shell metallic aerogels. The proposed core-based activity descriptor provides a new possible strategy for the design of future core-shell electrocatalysts.

New Metal Deposition Methods for Electrocatalysts Syntheses

We describe three new methods of metal monolayer/multilayer deposition that make possible syntheses of new types of electrocatalysts for the O2 reduction, and H2 and ethanol oxidation reactions. They consist of a monolayer of Pt deposited on a metal, metal alloy, core-shell, or metal-oxide nanoparticles. The electrocatalysts synthesized using these methods have remarkable properties. For example, for O2 reduction reaction they 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. These new electrocatalysts promise to alleviate some of the major problems of existing fuel cell technology.

Characterization of nanoparticle electrocatalysts using X-ray absorption spectroscopy

X-ray absorption spectroscopy (XAS) is a non-destructive spectroscopic technique commonly used with synchrotron radiation source and measures the changes of absorption coefficient as the function of the x-ray energy. Its unique power is that it can reveal the internal structure of nanoparticles, even if the particle is made up of several elements. By analyzing the results, interatomic distances and coordination numbers for each element are obtained, and their values and relationships indicate whether the elements in the nanoparticle are in a homogeneous solution, physically separated, or form a core-shell particles in which certain elements are mostly located in the core, and the other in the shell. The latter type of nanoparticles is of particular interest for the electrocatalysts as it reduces the precious metal content in the nanoparticles by replacing the precious metal in the the nanoparticles core with less expensive alternatives. This paper describes the internal structure characterization of the nanoparticle electrocatalysts by in situ XAS, and gives certain examples of systems composed of two or three metals.