Fuel cells powered by hydrogen from secure and renewable sources are the ideal solution for non-polluting vehicles, and extensive research and development on all aspects of this technology over the past fifteen years has delivered prototype cars with impressive performances. But taking the step towards successful commercialization requires oxygen reduction electrocatalysts--crucial components at the heart of fuel cells--that meet exacting performance targets. In addition, these catalyst systems will need to be highly durable, fault-tolerant and amenable to high-volume production with high yields and exceptional quality. Not all the catalyst approaches currently being pursued will meet those demands.

Electrocatalyst approaches and challenges for automotive fuel cells

Surface science studies of model fuel cell electrocatalysts

Understanding and Approaches for the Durability Issues of Pt-Based Catalysts for PEM Fuel Cell

Catalysts for chemical and electrochemical reactions underpin many aspects of modern technology and industry, from energy storage and conversion to toxic emissions abatement to chemical and materials synthesis. This role necessitates the design of highly active, stable, yet earth-abundant heterogeneous catalysts. In this Review, we present the perovskite oxide family as a basis for developing such catalysts for (electro)chemical conversions spanning carbon, nitrogen, and oxygen chemistries. A framework for rationalizing activity trends and guiding perovskite oxide catalyst design is described, followed by illustrations of how a robust understanding of perovskite electronic structure provides fundamental insights into activity, stability, and mechanism in oxygen electrocatalysis. We conclude by outlining how these insights open experimental and computational opportunities to expand the compositional and chemical reaction space for next-generation perovskite catalysts.

http://science.sciencemag.org/content/sci/358/6364/751.full.pdf

Perovskites in catalysis and electrocatalysis

Density functional theory calculations have been performed for the three elementary steps−Tafel, Heyrovsky, and Volmer−involved in the hydrogen oxidation reaction (HOR) and its reverse, the hydrogen evolution reaction (HER). For the Pt(111) surface a detailed model consisting of a negatively charged Pt(111) slab and solvated protons in up to three water bilayers is considered and reaction energies and activation barriers are determined by using a newly developed computational scheme where the potential can be kept constant during a charge transfer reaction. We determine the rate limiting reaction on Pt(111) to be Tafel−Volmer for HOR and Volmer−Tafel for HER. Calculated rates agree well with experimental data. Both the H adsorption energy and the energy barrier for the Tafel reaction are then calculated for a range of metal electrodes, including Au, Ag, Cu, Pt, Pd, Ni, Ir, Rh, Co, Ru, Re, W, Mo, and Nb, different facets, and step of surfaces. We compare the results for different facets of the Pt electrode...

Modeling the Electrochemical Hydrogen Oxidation and Evolution Reactions on the Basis of Density Functional Theory Calculations

{ital In} {ital situ} x-ray-scattering studies of the Au(111) electrode surface have been carried out in NaF, NaCl, LiCl, CsCl, KCl, and NaBr solutions using grazing-incident-angle diffraction and reflectivity techniques. The top layer of gold atoms undergoes a reversible phase transition between the (1{times}1) bulk termination and a ({ital p}{times} {radical}3 ) uniaxial discommensuration (striped) phase on changing the electrode potential. Below the critical potential, in all solutions, {ital p}=23, which is identical to that obtained in vacuum. An ordered array of discommensuration kinks is not observed. Above the critical potential, 23{lt}{ital p}{lt}30. At sufficiently positive potentials, the surface forms an ideally terminated (111) surface. In the negative potential sweep, the reconstruction starts to reform at the critical potential. Analysis of the potential dependence of the scattered x-ray intensity with differing anions in NaF, NaCl, and NaBr solutions supports a unifying model that depends on the induced surface charge density. The transition to the (1{times}1) phase is much faster than the formation of the reconstructed phase. Cycling the potential in the reconstructed region improves the reconstructed surface order. The adsorption of anions and surface water'' at the gold interface has been investigated using specular x-ray reflectivity.

In situ x-ray-diffraction and -reflectivity studies of the Au(111)/electrolyte interface: Reconstruction and anion adsorption.

In situ x-ray specular reflectivity and glancing-incidence-angle x-ray-diffraction measurements have been performed at the Au(001) surface in a 0.01M ${\mathrm{HClO}}_{4}$ solution under potential control in an electrochemical cell. At -0.4 V versus an Ag/AgCl electrode, the gold surface exhibits a hexagonal reconstructed layer with a mass density 21% greater than the underlying bulk layers. The reconstruction disappears above 0.5 V, and the excess atoms form a new atomic layer with a density corresponding to 22% of a bulk layer. The reconstruction fully recovers below -0.3 V.

In situ x-ray reflectivity and diffraction studies of the Au(001) reconstruction in an electrochemical cell.

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 intermediate's 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.

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

The structure of electrodeposited iodine - from a potassium iodide (KI) electrolyte - at the Au(111) surface has been investigated using surface X-ray scattering (SXS) techniques. Two distinct incommensurate iodine monolayer phases are observed. In both of these phases the structures compress with increasing potential (electrocompression). In the lower potential phase a (px[radical]3) centered-rectangular iodine monolayer is observed in which the coverage ([theta]) increases from 36.6% to 40.9% (relative to the gold layer density) with increasing potential. At more positive potentials a rotated-hexagonal phase is formed, and [theta] increases from 41.5% to 44.5%. At the highest coverages, in both phases, the iodine-iodine nearest-neighbor spacing equals the van der Waals diameter of 4.3 [angstrom]. Analysis of the specular reflectivity gives an iodine-gold interlayer spacing of 2.35 [angstrom] and iodine coverages which are in good agreement with the in-plane diffraction results. 35 refs., 10 refs.

Structure and electrocompression of electrodeposited iodine monolayers on gold (111)

Synchrotron surface x-ray scattering (SXS) studies have been carried out at the Au(lll)/electrolyte interface to determine the influence of surface charge on the microscopic arrangement of gold surface atoms. At the electrochemical interface, the surface charge density can be continuously varied by controlling the applied potential. The top layer of gold atoms undergoes a reversible phase transition between the (1 x 1) bulk termination and a (23 x radical3) reconstructed phase on changing the electrode potential. In order to differentiate the respective roles of surface charge and adsorbates, studies were carried out in 0.1 M NaF, NaCl, and NaBr solutions. The phase transition occurs at an induced surface charge density of 0.07 +/- 0.02 electron per atom in all three solutions.

https://science.sciencemag.org/content/sci/255/5050/1416.full.pdf

Jia Wang

Papers

In situ x-ray-diffraction and -reflectivity studies of the Au(111)/electrolyte interface: Reconstruction and anion adsorption.

In situ x-ray reflectivity and diffraction studies of the Au(001) reconstruction in an electrochemical cell.

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

Structure and electrocompression of electrodeposited iodine monolayers on gold (111)

Surface Charge—Induced Ordering of the Au(111) Surface