Jia X. Wang

Dual-Pathway Kinetic Equation for the Hydrogen Oxidation Reaction on Pt Electrodes

In the past 2 years, several experimental observations and theoretical studies indicated that the commonly used Butler-Volmer equation was inappropriate for describing the kinetics of the hydrogen oxidation reaction (HOR) on a Pt electrode. Motivated by these works, we developed an HOR kinetic equation from a dual-pathway model to describe the complex kinetic behavior over the whole relevant potential region. A simple expression was found for the reaction intermediates coverage as a function of overpotential. Three intrinsic kinetic parameters were determined by analyzing published polarization curves measured with microelectrodes (high mass transport) and rotating disk electrodes (relatively low mass transport, high accuracy). The results show a fast, inversed exponential rising of kinetic current at small overpotentials through the Tafel-Volmer pathway, and a much more gradual rise at η > 50 mV through the Heyrovsky-Volmer pathway. This behavior is dramatically different from the single-exponential increase of the Butler-Volmer equation. The puzzling dependence of anode overpotential on Pt loading in H 2 -fed proton-exchange membrane fuel cells can now be explained by the dual-pathway kinetic equation, providing a sound basis for fuel cell modeling, optimization, and diagnosis. The principle and approach used in this study can be applied to other electrocatalytic reactions.

Ordered bilayer ruthenium–platinum core-shell nanoparticles as carbon monoxide-tolerant fuel cell catalysts

Fabricating subnanometre-thick core-shell nanocatalysts is effective for obtaining high surface area of an active metal with tunable properties. The key to fully realize the potential of this approach is a reliable synthesis method to produce atomically ordered core-shell nanoparticles. Here we report new insights on eliminating lattice defects in core-shell syntheses and opportunities opened for achieving superior catalytic performance. Ordered structural transition from ruthenium hcp to platinum fcc stacking sequence at the core-shell interface is achieved via a green synthesis method, and is verified by X-ray diffraction and electron microscopic techniques coupled with density functional theory calculations. The single crystalline Ru cores with well-defined Pt bilayer shells resolve the dilemma in using a dissolution-prone metal, such as ruthenium, for alleviating the deactivating effect of carbon monoxide, opening the door for commercialization of low-temperature fuel cells that can use inexpensive reformates (H2 with CO impurity) as the fuel.

Kinetic Characterization of PtRu Fuel Cell Anode Catalysts Made by Spontaneous Pt Deposition on Ru Nanoparticles

Hydrogen oxidation kinetics without and with trace amounts of CO in H2 were investigated for carbon-supported catalysts consisting of Pt submonolayers on Ru nanoparticles prepared by spontaneous deposition and commercial Pt, Ru, and PtRu alloy catalysts. Thin catalyst layers were deposited onto a glassy carbon rotating disk electrode without using Nafion film to stabilize them. Nonlinear fittings of the entire polarization curves at several rotation rates were used to determine the exchange current, the Tafel slope, and the Levich slope. To ensure full utilization of the catalyst, the mass-specific activity was determined by finding the minimum Pt loading needed to have all three kinetic parameters close to those found for a polycrystalline Pt electrode. For the PtRu20 , PtRu10 , and PtRu5 samples prepared by spontaneous deposition of 1/9 to 4/9 monolayer Pt on Ru, the minimum loading is 5 nmol/cm 2 (1mgPt /cm 2 ). This is only one-third of that for Pt or PtRu ~E-TEK! catalysts and only double the atomic density of aP t~111! surface, indicating that the high activity of Pt metal for hydrogen oxidation is retained when the atomic assemblies are reduced to submonolayer level on Ru. The enhanced CO tolerance was studied at low potentials by correlating the loss of the

Electrosorption and catalytic properties of bare and Pt modified single crystal and nanostructured Ru surfaces

The electrosorption and catalytic properties of bare and Pt modified Ru(0001) and Ru(10 − 10) single crystal surfaces and carbon supported Ru nanoparticles have been studied by electrochemical, surface X-ray scattering, scanning tunneling microscopy, Fourier transform infrared spectroscopy and high resolution transmission electron microscopy techniques. The electrochemical surface oxidation of Ru(0001) in H2SO4 is an one-electron process resulting in 1 monolayer oxygen uptake and the increased spacing between the top two Ru layers from 2.13 A at 0.1 V to 2.20 A at 1.0 V. About 1/3 monolayer of bisulfate anions are coadsorbed with hydronium cations at low potentials. In HClO4 solution, the adsorption process at 0.1 V is due to the surface oxidation apparently to RuOH rather than to hydrogen adsorption. The oxidation of Ru(10 − 10) is quite facile and a progressive growth of the oxide layer is observed in repeated potential cycles. Spontaneous deposition of a submonolayer-tomultilayer of Pt on metallic Ru surfaces is a new phenomenon involving a noble metal deposition on a noble metal substrate through a local cell mechanism. The electrocatalysts prepared by spontaneous deposition of Pt on Ru nanoparticles have high activity and high CO tolerance exceeding those of the state-of-the-art commercial catalysts containing several times higher Pt loadings. Electronic effects appear to play a role in providing enhanced CO tolerance of Pt submonolayers on Ru nanoparticles.

Truncated Ditetragonal Gold Prisms as Nanofacet Activators of Catalytic Platinum

We report a facile, seed-mediated method to synthesize nanoscale gold truncated ditetragonal nanoprisms (TDPs) enclosed by 12 high-index {310} facets. The method leads to the formation of nanoparticles with high size and shape monodispersity and allows for easy surfactant removal. The dependence of particle shape on the synergetic contribution of metallic ions, halide ions, and surfactant adsorbates during synthesis is described. The resulting high-index nanoparticle facets were demonstrated as efficient activators of a supported catalytic material (platinum). A Pt monolayer deposited onto the Au TDP nanofacets with sharp electrochemical signatures exhibits an enhanced catalytic activity.

Low-Coordination Sites in Oxygen-Reduction Electrocatalysis: Their Roles and Methods for Removal

Low-coordination sites, including edges, kinks, and defects, play an important role in oxygen-reduction electrocatalysis. Their role was studied experimentally and theoretically for various Pt surfaces. However, the roughness effect on similar-sized nanoparticles that could elucidate the role of low-coordination sites has attracted much less attention, with no studies on Pd nanoparticles. Here, using Br- adsorption/desorption, we introduce an effective approach to reduce surface roughness, yielding Pd nanoparticles with smoother surfaces and an increased number of (111)-oriented facets. The resulting nanoparticles have a slightly contracted structure and narrow size distribution. Pt monolayer catalysts that contain such nanoparticles as the cores showed a 1.5-fold enhancement in specific and Pt mass activities for the oxygen reduction reaction compared with untreated ones. Furthermore, a dramatic increase in durability was observed with bromide-treated Pd(3)Co cores. These results demonstrate a simple approach to preparing nanoparticles with smooth surfaces and confirm the adverse effect of low-coordination sites on the kinetics of the oxygen-reduction reaction.

The structure and phase behavior of electrodeposited halides on single-crystal metal surfaces

Abstract Synchrotron X-ray scattering results of halide monolayers of bromide and iodide on single-crystal electrodes are presented. Both commensurate and incommensurate structures are observed. The incommensurate structures electrocompress with increasing potential. The relative roles of the halide—halide and the substrate—halide interactions are discussed for iodide on Au(111), Ag(111), and Pt(111) and for bromide on Au(111), Ag(111) and Au(100).

Adsorption of bromide at the Ag(100) electrode surface

The adsorption and phase formation of bromide on Ag(100) has been studied by chronocoulometry and surface X-ray scattering (SXS). With increasing electrode potential, bromide undergoes a phase transition from a lattice gas to an ordered c(2×2) structure (θ=0.5). The degree of lateral disorder was estimated by comparing the SXS- and the electrochemical measurements. Based on chronocoulometric experiments, a thermodynamic analysis of charge density data was performed to describe the bromide adsorption at the Ag(100) electrode. The Gibbs surfaces excess, electrosorption valencies, Esin–Markov coefficients, and the Gibbs energy of adsorption, lateral interaction energies as well as surface dipole moments have been estimated. The experimental θ versus E- isotherms are modeled employing (i) a quasi-chemical approximation as well as (ii) the results of a recent Monte Carlo simulation. An attempt is made to discuss the structure data and thermodynamic quantities of bromide adsorption on Ag(100) on the basis of the Grahame–Parsons model of the Helmholtz layer.

A critical comparison of electrochemical and surface X-ray scattering results at the Au(111) electrode in KBr solutions

Abstract Electrochemical and surface X-ray scattering data are compared for the Au(111) electrode in 0.01 M KBr solution in 0.1 M KClO4. The irreversible peaks in the voltammogram are well correlated with the scattered X-ray intensity from the reconstructed surface in both sweep directions. Surface charge and bromide coverage measured by the chronocoulometric technique are also correlated with the X-ray intensity. The reconstruction lifts over a potential region where the surface charge changes from − 10 to 20 μC cm−2 and the bromide coverage increases from 10% to 25% of its saturated coverage.

Structural evolution during electrocrystallization: deposition of Tl on Ag(100) from monolayer to bilayer and to bulk crystallites

Abstract The structure of Tl overlayers deposited at underpotentials and the structure of bulk deposited Tl on Ag(100) in perchloric acid have been determined using surface X-ray scattering. Thallium forms a c( p × 2) close-packed monolayer which compresses uniaxially ( p decreasing from 1.185 to 1.168) with decreasing potential. Upon deposition of the second layer, the first layer expands slightly along the incommensurate direction and both layers form a c(1.2 × 2) structure. Bulk deposition at potentials negative of the Nernst potential results in the formation of well aligned hcp Tl crystallites atop the c(1.2 × 2) bilayer. The stability of the bilayer structure in the presence of bulk deposits and its pronounced effects on subsequent epitaxial growth are discussed in terms of strain and interface energies.