Synthesis and Characterization of Electrocatalytically Active Platinum Atom Clusters and Monodisperse Single Crystals

A novel synthesis technique has been developed that yields monodisperse Pt particles in electrostatically stabilized suspensions without the use of structure directing organic surfactants. The approach uses stannous chloride as both reducing and stabilizing agent to form multifaceted Pt single crystal nanoparticles and clusters of less than 20 atoms. These particles may be assembled into layered electrode structures having well-controlled Pt loadings without precipitation onto organic supports or sintering to remove organic residues, both of which are known to yield particle aggregation and the formation of nonregular structures. Consequently, the particles may be used for fundamental investigations on the effect of platinum dispersion on catalytic activity never previously possible. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) of these particles provides the first direct evidence that peak oxygen reduction reaction (ORR) activity with increased catalyst dispersion is associated with the crystal to cluster transition and a change in reaction mechanism as reflected by the change in the Tafel slope from 120 mV/decade for the crystals to 220 mV/decade for the clusters at high current density. ORR mass activities obtained at 0.9 V versus reversible hydrogen electrode (RHE) from rotating disk electrode (RDE) experiments in perchloric acid were found to systematically vary from a minimum of about 18 A/g for the atomic clusters, to about 48 A/g for the single crystals, to a peak activity of 74 A/g for transitional structures (twice the value measured on commercial catalyst). Furthermore, the peak electrochemically active area (ECA) obtained from proton underpotential deposition is found to occur well within the atomic cluster regime.