High-surface-area ruthenium-based Ru x M y (M = Pt or Pd) alloy catalysts supported on carbon black were synthesized to investigate the hydrogen oxidation reaction (HOR) in alkaline electrolytes. The exchange current density for hydrogen oxidation on a Pt-rich Ru0.20Pt0.80 catalyst is 1.42 mA/cm2, nearly 3 times that of Pt (0.490 mA/cm2). Furthermore, Ru x Pt y alloy surfaces in 0.1 M KOH yield a Tafel slope of ∼30 mV/dec, in contrast with the ∼125 mV/dec Tafel slope observed for supported Pt, signifying that hydrogen dissociative adsorption is rate-limiting rather than charge-transfer processes. Ru alloying with Pd does not result in modified kinetics. We attribute these disparate results to the interplay of bifunctional and ligand effects. The dependence of the rate-determining step on the choice of alloy element allows for tuning catalyst activity and suggests not only that a low-cost, alkaline anode catalyst is possible but also that it is tantalizingly close to reality.

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.

Palladium nanotubes (PdNTs) were synthesized by templated vapor deposition and investigated for formic acid electrooxidation. Annealed PdNTs are 2.4 times more active (2.19 mA/cm 2 ) than commercial carbon-supported palladium (0.91 mA/cm 2 ) at 0.3 V vs.RHE. Bismuth modification improved nanotube performance over 4 times (3.75 mA/cm 2 ) vs. Pd/C and nearly 2 times vs. unmodified PdNTs. A surface Bi coverage of 80% results in optimal site-specific activity by drastically reducing surface-poisoning CO generation during formic acid electrooxidation. The Bi-modified PdNTs are exceptionally stable, maintaining 2 times the area-normalized current density as Pd/C after 24 hours at 0.2 V vs. RHE. We attribute the enhanced activity and stability of the nanotube catalysts to the presence of highly coordinated surfaces, mimicking a flat polycrystal while retaining high surface area geometry.

Templated vapor synthesis and thermal annealing were used to synthesize unsupported metallic Ru nanotubes with Pt or Pd overlayers. By controlling the elemental composition and thickness of these overlayers, we obtain nanostructures with very high alkaline hydrogen oxidation activity. Nanotubes with a nominal atomic composition of Ru 0.90 Pt 0.10 display a surface-specific activity (2.4 mA/cm 2 ) that is 35 times greater than that of pure Ru nanotubes at a 50 mV overpotential and ∼2.5 times greater than that of pure Pt nanotubes (0.98 mA/cm 2 ). The surface-segregated structure also confers dramatically increased Pt utilization efficiency. We find a platinum-mass-specific activity of 1240 A/g Pt for the optimized nanotube versus 280 A/g Pt for carbon-supported Pt nanoparticles and 109 A/g Pt for monometallic Pt nanotubes. We attribute the enhancement of both area-and platinum-mass-specific activity to the atomic-scale homeomorphism of the nanotube form factor with adlayer-modified polycrystals. In this case, subsurface ligand and bifunctional effects previously observed on segregated, adlayer-modified polycrystals are translated to nanoscale catalysts.

A SnCl 2 shell on Pt metal core nanoparticle synthesis technique has recently been demonstrated to permit electrostatic layer-bylayer ͑LbL͒ assembly of well-ordered electrocatalysts without precipitation onto porous carbon supports. In this paper, the electrocatalytic activity of the LbL-assembled Pt nanoparticles is shown to depend critically upon removal of surface-adsorbed Sn ͑Sn ads ͒. By subjecting the synthesized Pt nanoparticle electrodes to potential sweeps greater than 1.0 V vs reversible hydrogen electrode, Sn ads are removed and a nearly threefold enhancement in oxygen reduction reaction ͑ORR͒ specific activity over commercial catalysts is obtained. In contrast to this electrochemical acceleration approach, we also investigate electroless, wetacceleration methods for Sn ads removal. Energy-dispersive spectroscopy and inductively coupled plasma-mass spectrometry are used to quantify the Pt/Sn ratio in the electrode assemblies as a function of immersion time in solution ͑both alkaline and acidic͒ and during electrochemical acceleration, respectively. Charging current for the underpotential deposition of protons on the Pt nanoparticle surface is used to monitor the removal of Sn ads during electrochemical acceleration, followed by ORR activity measurement in saturated perchloric acid ͑HClO 4 ͒. Wet-chemical acceleration in NaOH solution is found to remove similar amounts of Sn as compared to the electrochemical technique.Electrocatalysts for proton exchange membrane ͑PEM͒ fuel cell application have generally been synthesized utilizing precipitation of highly dispersed Pt nanoparticles into porous carbon supports from an unstable suspension. 1-6 Such approaches yield broad particle size distributions ͑the lowest being about 30% but typically much larger͒ with a wide assortment of irregular shapes that impede attainment of optimal performance. Theoretical computations 7,8 combined with single-crystal experiments 9-12 suggest that peak oxygen reduction reaction ͑ORR͒ activity for pure Pt may be obtained over a very narrow size range of well-ordered cubo-octahedral structures. Instead, contradictory results regarding ORR structure sensitivity have been observed for supported Pt electrocatalysts in the weakly adsorbing electrolyte HClO 4 due to the broad particle size distribution of these systems. 1,3 More recently, bi-and trimetallic electrocatalyst synthesis methods that exhibit higher specific activity and stability than pure Pt under fuel cell operating conditions continue to employ similar precipitation techniques for core formation 13-16 or coprecipitation for alloy formation. 3,6,12,[17][18][19] We have recently developed 20,21 an electrocatalyst synthesis approach that employs well-defined concentrations of reductant Sn complexes to ligate sites on nascent Pt metal cores and thereby precisely regulate their size without the use of structure-directing or stabilizing organic surfactants. The specific reaction employed is described byThe nanoparticles are stabilized by specifically bound chloride anions ...