(487u) Molecular Platinum Cluster Electrocatalyst Synthesis and Characterization

Molecular clusters of metal atoms are receiving significant attention in a broad range of fields as techniques to synthesize and characterize these Angstrom sized particles continue to become more sophisticated. Recent developments have demonstrated unique luminescent and selective catalytic activity of clusters ranging in size from 20 to 75 atoms. Images have only in 2009 been published of theoretically proposed cluster formation to crystal transition for CdSe. However, the synthesis of platinum molecular clusters and their impact on the oxygen reduction reaction (ORR) in low-temperature, proton-exchange-membrane (PEM) fuel cells has not been previously reported. In the present paper, we describe the synthesis of highly monodisperse platinum molecular clusters in our laboratory and present high resolution TEM images and FFT diffraction data for the cluster-to-crystal transition associated with increasing particle size (from approximately 20-atom clusters to 650-atom, 2.7-nm particles with narrow dispersity). Finally, we evaluate the specific electrochemically active surface area (ECA) and ORR mass activity of the clusters. Specific surface areas are obtained from electrochemical hydrogen adsorption/desorption experiments and are found to be as high as 100 m²/g for approximately 1.5 nm diameter particles. This investigation suggests a possible source for the well-known maximum in ORR mass activity with decreasing catalyst size in the case of supported industrial electrocatalysts and provides a novel method for rational electrocatalyst design.