Operando Evidence for a Universal Oxygen Evolution Mechanism on Thermal and Electrochemical Iridium Oxides.

Progress in the development of proton exchange membrane (PEM) water electrolysis technology requires decreasing the anode overpotential, where the sluggish multistep oxygen evolution reaction (OER) occurs. This calls for an understanding of the nature of the active OER sites and reaction intermediates, which are still being debated. In this work, we apply synchrotron radiation-based near-ambient pressure X-ray photoelectron and absorption spectroscopies under operando conditions in order to unveil the nature of the reaction intermediates and shed light on the OER mechanism on electrocatalysts most widely used in PEM electrolyzers-electrochemical and thermal iridium oxides. Analysis of the O K-edge and Ir 4f spectra backed by density functional calculations reveals a universal oxygen anion red-ox mechanism regardless of the nature (electrochemical or thermal) of the iridium oxide. The formation of molecular oxygen is considered to occur through a chemical step from the electrophilic OI- species, which itself is formed in an electrochemical step.

Uncovering the Stabilization Mechanism in Bimetallic Ruthenium-Iridium Anodes for Proton Exchange Membrane Electrolyzers.

Proton exchange membrane (PEM) electrolyzers are attracting an increasing attention as a promising technology for the renewable electricity storage. In this work, near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) is applied for in situ monitoring of the surface state of membrane electrode assemblies with RuO2 and bimetallic Ir0.7Ru0.3O2 anodes during water splitting. We demonstrate that Ir protects Ru from the formation of an unstable hydrous Ru(IV) oxide thereby rendering bimetallic Ru-Ir oxide electrodes with higher corrosion resistance. We further show that the water splitting occurs through a surface Ru(VIII) intermediate, and, contrary to common opinion, the presence of Ir does not hinder its formation.

In situ investigation of dissociation and migration phenomena at the Pt/electrolyte interface of an electrochemical cell.

The development of efficient energy conversion systems requires precise engineering of electrochemical interfaces and thus asks for in situ techniques to probe the structure and the composition of the dynamic electrode/electrolyte interfacial region. This work demonstrates the potential of the near ambient pressure X-ray photoelectron spectroscopy (NAPXPS) for in situ studies of processes occurring at the interface between a metal electrode and a liquid electrolyte. By using a model membrane-electrode assembly of a high temperature phosphoric acid-imbibed proton exchange membrane fuel cell, and combining NAPXPS measurements with the density functional theory, it was possible to monitor such fundamental processes as dissociation and migration of the phosphoric acid within a nanostructured Pt electrode under polarization.

Electrocatalytic oxygen reduction reaction on perovskite oxides: series versus direct pathway.

The mechanism of the oxygen reduction reaction (ORR) on LaCoO(3) and La(0.8)Sr(0.2)MnO(3) perovskite oxides is studied in 1 M NaOH by using the rotating ring disc electrode (RRDE) method. By combining experimental studies with kinetic modeling, it was demonstrated that on perovskite, as well as on perovskite/carbon electrodes, the ORR follows a series pathway through the intermediate formation of hydrogen peroxide. The escape of this intermediate from the electrode strongly depends on: 1) The loading of perovskite; high loadings lead to an overall 4 e(-) oxygen reduction due to efficient hydrogen peroxide re-adsorption on the active sites and its further reduction. 2) The addition of carbon to the catalytic layer, which affects both the utilization of the perovskite surface and the production of hydrogen peroxide. 3) The type of oxide; La(0.8)Sr(0.2)MnO(3) displays higher (compared to LaCoO(3)) activity in the reduction of oxygen to hydrogen peroxide and in the reduction/oxidation of the latter.

Cooperative behaviour of Pt microelectrodes during CO bulk electrooxidation.

Cooperative behaviour of an array of microelectrodes: The interplay of bistable reaction kinetics with global coupling results in a spontaneous sequential activation of electrodes when the applied current is increased.

Using ordered carbon nanomaterials for shedding light on the mechanism of the cathodic oxygen reduction reaction.

Insufficient understanding of the mechanism of the cathodic oxygen reduction reaction puts constraints on the improvement of the efficiency of polymer electrolyte fuel cells (PEMFCs). We apply ordered catalytic layers based on vertically aligned carbon nanofilaments and combine experimental rotating ring-disk studies with mathematical modeling for shedding light on the mechanism of the oxygen reduction reaction on Pt nanoparticles. Based on the experimental and simulation evidence we propose a dual path ORR mechanism which comprises a "direct 4e(-)" and a "series 2e(-) + 2e(-)" pathway and explains switching between the two. For the first time we show that below 0.8 V the "direct" path may be discarded and the ORR predominantly occurs via H(2)O(2) mediated pathway, while in the potential interval between ca. 0.8 V and the onset of the ORR the "direct" path is dominating.

Mass transport effects in CO bulk electrooxidation on Pt nanoparticles supported on vertically aligned carbon nanofilaments.

In this work we report on the influence of the catalytic layer architecture on the autocatalytic reaction of CO-bulk oxidation in liquid electrolyte by employing two types of nanomaterials: 2D arrays of Pt particles prepared on the surface of glassy carbon by colloidal lithography and 3D arrays of Pt nanoparticles supported on vertically aligned carbon nanofilaments. Oxidation of dissolved CO is studied experimentally using RDE approach and computationally using finite element method. For the first time, the influence of 3D architecture of the electrode on a complex bistable electrochemical system was investigated. The modelling results are in qualitative agreement with the experiment and explain the influence of nanostructure of the electrodes on such key characteristics of CO electrooxidation as the ignition potential, the width and the shape of the bistability region, and the value of the limiting current. Analysis of the experimental RDE curves suggests spontaneous formation of active and passive reaction zones along the fibre length which is supported by modelling.

An approach to fabrication of metal nanoring arrays.

Fabrication of tailored nanomaterials with desired structure and properties is the greatest challenge of modern nanotechnology. Herein, we describe a wet chemical method for the preparation of large area metal nanoring arrays. This method is based on self-assembly of polystyrene sphere template on a flat substrate and wicking/reducing metal precursor into the interstices between the template and the substrate. In this article, platinum, gold, and copper nanorings were fabricated by applying 505 nm polystyrene spheres onto highly oriented pyrolytic graphite (HOPG) and Si(100) substrates, followed by reducing the templated metal salt with NaBH(4). AFM images reveal formation of arrays of metal nanorings comprising metal nanoparticles with the average ring height of 5.7 +/- 0.8 nm and diameter of 167.3 +/- 8.9 nm. XPS confirms that these structures are metallic.

On the enhanced electrocatalytic activity of Pd overlayers on carbon-supported gold particles in hydrogen electrooxidation.

Palladium-gold particles with varied composition were prepared by Pd electrochemical deposition on Au nanoparticles immobilized on model carbon support. Pd-Au/C catalysts were characterized ex situ by transmission electron microscopy, energy dispersive X-ray analysis and X-ray photoelectron spectroscopy, and in situ, by underpotential deposition of hydrogen and copper adatoms, and CO stripping. Hydrogen oxidation reaction on pristine and CO-poisoned Pd-Au/C particles was studied using rotating disk electrode (RDE) technique. It was found that the decrease of the effective Pd overlayer thickness below ca. two monolayers resulted in a two-fold increase of the exchange current density of the hydrogen oxidation reaction and in significant increase of CO tolerance.

On the influence of the metal loading on the structure of carbon-supported PtRu catalysts and their electrocatalytic activities in CO and methanol electrooxidation.

PtRu (1:1) catalysts supported on low surface area carbon of the Sibunit family (S(BET) = 72 m(2) g(-1)) with a metal percentage ranging from 5 to 60% are prepared and tested in a CO monolayer and for methanol oxidation in H(2)SO(4) electrolyte. At low metal percentage small (<2 nm) alloy nanoparticles, uniformly distributed on the carbon surface, are formed. As the amount of metal per unit surface area of carbon increases, particles start coalescing and form first quasi two-dimensional, and then three-dimensional metal nanostructures. This results in a strong enhancement of specific catalytic activity in methanol oxidation and a decrease of the overpotential for CO monolayer oxidation. It is suggested that intergrain boundaries connecting crystalline domains in nanostructured PtRu catalysts produced at high metal-on-carbon loadings provide active sites for electrocatalytic processes.

Surface electrochemistry of CO as a probe molecule on carbon-supported Se-surface modified Ru nanoparticles via infrared reflection absorption spectroscopy.

The carbon monoxide molecule is used to probe the electronic properties of Ru/C and Se surface-modified Ru/C nanocatalysts. Coordination of a Se to a Ru surface strongly affects the vibrational properties and reactivity of the CO adsorbed. Marked alteration of the CO stretching frequency; increase of the Stark tuning rate for C-O vibration; and a positive shift of the onset of CO oxidation are observed for Se-modified Ru particles and attributed to the electronic effect of Se.

Kinetic modeling of CO(ad) monolayer oxidation on carbon-supported platinum nanoparticles.

We present a theoretical study of CO(ad) electrooxidation on Pt nanoparticles. Effects of size and surface texture of nanoparticles on the interplay of relevant kinetic processes are investigated. Thereby, strong impacts of particle size on electrocatalytic activities, observed in experiments, are rationalized. Our theoretical approach employs the active site concept to account for the heterogeneous surface of nanoparticles. It, moreover, incorporates finite rates of surface mobility of adsorbed CO. As demonstrated, the model generalizes established mean field or nucleation and growth models. We find very good agreement of our model with chronoamperometric current transients at various particle sizes and electrode potentials (Maillard, F.; Savinova, E. R.; Stimming, U. J. Electroanal. Chem., in press, doi:10.1016/j.jelechem.2006.02.024). The full interplay of on-site reactivity at active sites and low surface mobility of CO(ad) unfolds on the smallest nanoparticles ( approximately 2 nm). In this case, the solution of the model requires kinetic Monte Carlo simulations specifically developed for this problem. For larger nanoparticles (>4 nm) the surface mobility of CO(ad) is high compared to the reaction rate constants, and the kinetic equations can be solved in the limiting case of infinite surface mobility. The analysis provides an insight into the prevailing reaction mechanisms and allows for the estimation of relevant kinetic parameters.

Synthesis and structural characterization of Se-modified carbon-supported Ru nanoparticles for the oxygen reduction reaction.

This work is part of a continued research aimed at the understanding of the promoting role of Se in the enhancement of the electrocatalytic activity of Ru in the oxygen reduction reaction. The objective of this paper is to systematically investigate the transformation of Ru nanoparticles upon their modification with the increasing amounts of Se. The Se-modified Ru/C samples with Se:Ru ratio from 0 to 1 were prepared by reacting carbon-supported Ru nanoparticles with SeO2 followed by reductive annealing and characterized using high-resolution transmission electron microscopy, energy-dispersive X-ray, X-ray diffraction analysis, X-ray photoelectron spectroscopy, and extended X-ray absorption fine structure. The results suggest that Se strongly interacts with Ru, resulting in the chemical bond between Ru and Se and formation of Ru selenide clusters whose core at low Se content can be described as Ru2Se2O0.5. At Se:Ru = 1, high-resolution electron microscopy shows evidence of formation of core-shell particles, comprising a hexagonally packed Ru core and a Ru selenide shell with lamellar morphology. Modification of Ru nanoparticles with Se enhances their electrocatalytic activity in the oxygen reduction reaction, which is explained by the role of Se in inhibiting surface oxidation.

A comparative study of hydroxide adsorption on the (111), (110), and (100) faces of silver with cyclic voltammetry, ex situ electron diffraction, and in situ second harmonic generation.

Hydroxide adsorption on the (111), (110), and (100) faces of silver electrodes from mixed NaOH/NaF solution is studied using cyclic voltammetry and in situ second harmonic generation (SHG). Cyclic voltammograms for the three low index silver planes in alkaline electrolytes are for the first time compared. They show two pairs of anodic and cathodic peaks in the potential interval below the equilibrium Ag/Ag(2)O potential. These are attributed to the specific adsorption of hydroxide ions followed by submonolayer oxide formation. The differences in the cyclic voltammograms for the (111), (110), and (100) planes are attributed to different (i) work functions, (ii) surface atomic densities, and (iii) corrugation potentials for these surfaces. Ex situ low energy electron diffraction (LEED) and reflection high energy electron diffraction (RHEED) show that disordered adlayers are formed on Ag(111) and Ag(100), in contrast to Ag(110), where ordered structures are produced in the region of the first pair of current peaks. In the region of the second pair of peaks, LEED indicates disordered oxide phases on each crystal plane and RHEED shows the presence of small islands of c(2 x 2) structure at some potentials on (110) and (100). SHG measurements were performed (i) in the potential scan mode at constant rotational angle and (ii) at constant potential as a function of the rotational angle. The isotropic (for the (111), (110), and (100) planes) and anisotropic (for the (110) and (111) planes) contributions to the SHG intensity were calculated by fitting the experimental data and are discussed in terms of their dependence on the charge density at the interface, on hydroxide adsorption, and on submonolayer oxide formation.

Structure and dynamics of the interface between a Ag single crystal electrode and an aqueous electrolyte.

The aim of this work is to elucidate the initial steps of the electrochemical oxidation of Ag(111) in alkaline electrolytes. We use electrochemical as well as ex situ (XPS) and in situ (SHG) spectroscopic techniques to reconstruct the Ag(111)/electrolyte interface as a complex dynamic entity. Moving in the direction from negative to positive potentials we first observe specific adsorption of hydroxide ions, which starts at ca. -1.1 V vs. Ag/Ag2O in 0.1 M NaOH. SHG data prove that hydroxide retains its negative charge. At -0.3 V oxidation of the surface sets in with the formation of negatively charged adsorbed oxygen species and Ag+ ions, which give rise to peaks at 528.2 +/- 0.2 eV and at 367.7 eV in the O 1s and the Ag 3d(5/2) XP spectra, respectively. Around -0.1 V the adlayer is transformed into an ordered surface oxide phase which grows via a nucleation and growth mechanism. Above the reversible Ag/Ag2O potential the 2D Ag(I) oxide transforms into a 3D Ag(I) oxide. The electrochemical oxidation is compared with the previously studied gas-phase process, demonstrating both remarkable similarities as well as some differences.