Structural dynamics of platinum and platinum alloy nanoparticles probed by x-ray scattering techniques

Carbon-supported noble metal particles are used as electrocatalysts in energy conversion and storage devices such as water electrolyzers, fuel cells, or metal air batteries. Under reactive electrified conditions, metal particles grow and coarsen due to Ostwald Ripening and particle coalescence. In corrosive electrochemical environments, bimetallic nanoparticles often exhibit preferential leaching of one metal component. At higher electrode potentials, carbon supports tend to corrode as well, and metal nanoparticles lose their electrical contact to the electrode and are lost for the catalytic process. Particle growth, support and metal corrosion severely affect the performance and durability of the electrochemical device. Hence, an atomic scale microscopic understanding of macroscopic performance degradation is critical to develop strategies to make electrochemical electrodes more durable. Most of the earlier degradation studies of nanoparticle electrocatalysts utilized ex-situ Transmission Electron Microscopy (TEM), a real-space technique, providing information of limited statistical accuracy.;The present work utilized (anomalous) Small Angle X-ray scattering ((A) SAXS) and (wide angle) X-ray Diffraction (XRD) and elucidated atomic scale mechanisms of macroscopic degradation phenomena of fuel cell cathodes. The removal of Cu atoms from the precursor alloy particle surface suggests the formation of a Pt shell surrounding a Pt-Cu alloy particle core. In situ SAXS also revealed that nanoparticles grow much faster as the electrode is cycled to higher electrode potentials. The combined SAXS with XRD suggest that dealloying first leads to a reduction in crystallinity, followed by gradual re-crystallization and faceting during continued potential cycling. It was found that the Pt particles supported on Ketjen Black were more resistant to growth in comparison with Pt particles supported on Vulcan, even though the former catalysts showed the smaller particle size. SAXS based theoretical surface areas (SAXS-SA) and the real electrochemical surface areas (ECSA) were compared and in-situ correlation between macroscopic and microscopic degradation descriptions was established.;In summary, this thesis provides a time resolved atomic scale in-situ picture of electrochemical processes pertinent to the degradation and dealloying of metal alloy ensembles.