Samuel St. John

Enhanced Electrocatalytic Oxygen Reduction through Electrostatic Assembly of Pt Nanoparticles onto Porous Carbon Supports from SnCl2-Stabilized Suspensions

Monodisperse Pt nanoparticles with atomic structures that span the cluster to crystal transition have recently been synthesized in electrostatically stabilized, aqueous-based suspensions. In the present study, the anionic charge from the stabilizing SnCl(2) sheath adsorbed on the surface of these particles is used for the first time to assemble Pt directly onto porous carbon supports via electrostatic assembly. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) reveals that these assemblies have substantially higher Pt-C dispersions than obtained from precipitation methods commonly used for commercial electrocatalyst systems. Energy dispersive spectroscopy (EDS) and inductively coupled plasma-mass spectrometry (ICP-MS) are used to determine that loadings of 10-30% by weight Pt (particle packing fractions from 0.05 to 0.25) are obtained through a single electrostatic application of these particles on Vulcan carbon, depending on particle size. The highest average oxygen reduction reaction (ORR) mass activity obtained using this approach is 90.4 A/g(Pt) at 0.9 V vs RHE in 0.1 M perchloric acid is with 1-2 nm particles that exhibit a transitional atomic structure. This activity compares to an average value of 74.0 A/g(Pt) obtained from densely packed electrostatic layer-by-layer (LbL) assemblies of unsupported particles and 36.7 A/g(Pt) commercial Vulcan electrocatalyst from Tanaka Kikinzoku Kogyo (TKK). Enhanced activity is observed with electrostatic assembly of any particle size on Vulcan relative to unsupported or commercial electrocatalyst with comparable durability. Such enhanced activity is attributed to improved reactant accessibility to the catalyst surface due to the increase in particle dispersion. An extinction coefficient of 7.41 m(2)/g at 352 nm is obtained across the entire cluster to crystal transition from 20 atom clusters to 2.9 nm single crystal nanoparticles, indicating that observed variation in ORR activity with particle size may be associated primarily with changes in atomic surface structure as opposed to the metallic character of the nanoparticles as assessed by UV-vis spectroscopy.

Improved electrocatalytic ethanol oxidation activity in acidic and alkaline electrolytes using size-controlled Pt-Sn nanoparticles.

The promotion of the electrocatalytic ethanol oxidation reaction (EOR) on extended single-crystal Pt surfaces and dispersed Pt nanoparticles by Sn under acidic conditions is well known. However, the correlation of Sn coverage on Pt nanoparticle electrocatalysts to their size has proven difficult. The reason is that previous investigations have typically relied on commercially difficult to reproduce electrochemical treatments of prepared macroscopic electrodes to adsorb Sn onto exposed Pt surfaces. We demonstrate here how independent control over both Sn coverage and particle size can yield a significant enhancement in EOR activity in an acidic electrolyte relative to previously reported electrocatalysts. Our novel approach uses electroless nanoparticle synthesis where surface-adsorbed Sn is intrinsic to Pt particle formation. Sn serves as both a reducing agent and stabilizing ligand, producing particles with a narrow particle size distribution in a size range where the mass-specific electrocatalytic activity can be maximized (ca. 1-4 nm) as a result of the formation of a fully developed Sn shell. The extent of fractional Sn surface coverage on carbon-supported Pt nanoparticles can be systematically varied through wet-chemical treatment subsequent to nanoparticle formation but prior to incorporation into macroscopic electrodes. EOR activity for Pt nanoparticles is found to be optimum at a fractional Sn surface coverage of ca. 0.6. Furthermore, the EOR activity is shown to increase with Pt particle size and correlate with the active area of available Pt (110) surface sites for the corresponding Sn-free nanoparticles. The maximum area- and mass-specific EOR activities for the most active catalyst investigated were 17.9 μA/cm(2)Pt and 12.5 A/gPt, respectively, after 1 h of use at 0.42 V versus RHE in an acidic electrolyte. Such activity is a substantial improvement over that of commercially available Pt, Pt-Sn, and Pt-Ru alloy catalysts under either acidic or alkaline conditions.

A nanoscale-modified LaMer model for particle synthesis from inorganic tin–platinum complexes

The size-tunable structure and properties of Pt nanoparticles at the atomic length scale have attracted significant attention across a wide variety of fields including magnetics, electrocatalysis, optics, and gas-phase synthesis. Mechanisms responsible for the formation Pt nanoparticles remain unclear because of the difficulty generating in situ data for the time-evolution of size, shape, distribution, volume fraction, particle number density, and oxidation state from the starting complexes. We here demonstrate the use of simultaneous small- and wide-angle X-ray scattering combined with UV-vis spectroscopy to measure these key synthesis metrics for the reduction of Pt(IV) by Sn(II) in aqueous solution. This synthesis approach has been previously shown to permit continuous control over Pt nanoparticle size from 0.9 to 2.6 nm to within 10% standard deviation. Such fine control led to the discovery of densely packed amorphous structures at ca. 1.7 nm with substantially enhanced electrocatalytic oxygen reduction relative to nanocrystals and commercial electrocatalysts. Ex situ UV-vis and in situ X-ray scattering are here shown to reveal four distinct stages during synthesis: (1) autoreduction of a ligand/noble metal complex with a unique structure that depends on the Sn(II)/Pt(II) ratio, (2) generation of Pt primary particles and the formation of Pt nuclei at a rate that depends on the structure of the initial complex, (3) nanoparticle growth via LaMers diffusion of these primary particles to the nuclei, and (4) growth termination due to capping from a stabilizing, two-layer ligand shell. We derive a set of consecutive rate equations and associated kinetic parameters that describe each step. The kinetics of ligand rearrangement has been previously found to limit the rate of nanoparticle growth. We incorporate this phenomenon into LaMers classic diffusion-limited growth scheme to extend it to the nanoscale regime. This new model provides detailed understanding of how metal ligands serve as both reducing and stabilizing agents and allow for unprecedented, continuous control over both size and distribution. Systematic variation of temperature permits detailed time resolution at the very onset of Pt primary particle formation, as well as a means to determine temperature sensitivity of nanoparticle growth.

Acceleration of Electrocatalytic Activity on Pt–Sn Nanoparticles

A SnCl 2 shell on Pt metal core nanoparticle synthesis technique has recently been demonstrated to permit electrostatic layer-by-layer (LbL) 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 (ORR) specific activity over commercial catalysts is obtained. In contrast to this electrochemical acceleration approach, we also investigate electroless, wet-acceleration 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 acidic) 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.