Adsorbate-Geometry Specific Subsurface Relaxation in the CO/Pt(111) System

In Situ and Operando X-ray Scattering Methods in Electrochemistry and Electrocatalysis

Electrochemical and electrocatalytic processes are of key importance for the transition to a sustainable energy supply as well as for a wide variety of other technologically relevant fields. Further development of these processes requires in-depth understanding of the atomic, nano, and micro scale structure of the materials and interfaces in electrochemical devices under reaction conditions. We here provide a comprehensive review of in situ and operando studies by X-ray scattering methods, which are powerful and highly versatile tools to provide such understanding. We discuss the application of X-ray scattering to a wide variety of electrochemical systems, ranging from metal and oxide single crystals to nanoparticles and even full devices. We show how structural data on bulk phases, electrode-electrolyte interfaces, and nanoscale morphology can be obtained and describe recent developments that provide highly local information and insight into the composition and electronic structure. These X-ray scattering studies yield insights into the structure in the double layer potential range as well as into the structural evolution during electrocatalytic processes and phase formation reactions, such as nucleation and growth during electrodeposition and dissolution, the formation of passive films, corrosion processes, and the electrochemical intercalation into battery materials.

Adsorption of Acetate on Au(111): An in-situ Scanning Tunnelling Microscopy Study and Implications on Formic Acid Electrooxidation

The adsorption of acetate on an Au(111) electrode surface in contact with acetic acid at pH 2.7 was imaged in-situ using scanning tunnelling microscopy (STM). Two different ordered structures were imaged for acetate adsorbed in the bidentate configuration on the unreconstructed 1 × 1 surface at 0.95 V (vs. the saturated calomel electrode, SCE). The first structure, ( 19 × 19 ) R 23 . 45 ∘ , is metastable and transforms at constant potential within 20 minutes to a ( 2 × 2 ) structure, which is thermodynamically more favourable. The ( 2 × 2 ) acetate adlayer starts to form at step edges and propagates via nucleation and growth onto terraces. The findings from in-situ STM are in agreement with the electrochemical behaviour of acetate on Au(111) characterized by voltammetry. A comparison is made with formate adsorption on Au(111). While acetate is not reactive, in contrast to formate, it can act as a spectator species in formic acid electrooxidation.

X-Ray Scattering and Imaging Studies of Electrode Structure and Dynamics

We will review structures and dynamics of electrode interfaces studied in situ using x-ray scattering and imaging techniques. The examples cover single-crystal and nanocrystal structures relevant to electrocatalytic activities, anodic oxidation and corrosion, aqueous dissolution reactions, surface reconstructions, and surface modifications by under potential deposition. The x-ray techniques include the widely used traditional surface x-ray scattering, such as crystal truncation rods and x-ray reflectivity, as well as recently developed resonance surface scattering, coherent surface x-ray photon correlation spectroscopy, coherent x-ray Bragg diffraction imaging, and surface ptychography. Results relevant to various electrochemical phenomena will be highlighted.

Powder diffraction in studies of nanocrystal surfaces: chemisorption on Pt

Atoms at the surface of nanocrystals contribute appreciably to the X-ray diffraction pattern. Phenomena like chemisorption, affecting the displacement of surface atoms with respect to their positions in the perfect crystallographic structure, cause diffraction peak shifts and intensity changes. These effects are easily measurable for small nanocrystals up to 10 nm size. This article reports diffraction effects of chemisorption of adsorbing gases H2, O2, CO and NO for a series ofin situpowder diffraction experiments on nanocrystalline Pt supported on silica. On the basis of previous diffraction observation of Pt surface reconstruction during hydrogen desorption, it was possible to quantify this effectversuscrystallite size and rationalize the observed diffraction peak shift for the other adsorbing species. This enabled the surface reconstruction to be distinguished from the surface relaxation effect, the latter depending monotonically on the adsorption energy. Even if no phase transition occurs, monitoring of a peak's position, intensity, width and gas composition (viamass spectrometry) during a carefully designed physicochemical process (including surface chemical reaction) enables insight into and understanding of the surface structure evolution (e.g.amorphization, relaxation, reconstruction or changes in the overall morphology). The proposed technique can be used as a surface science tool, allowing studies of nanocrystals under high pressure.

A comparison of atomistic and continuum approaches to the study of bonding dynamics in electrocatalysis: microcantilever stress and in situ EXAFS observations of platinum bond expansion due to oxygen adsorption during the oxygen reduction reaction

Microcantilever stress measurements are examined to contrast and compare their attributes with those from in situ X-ray absorption spectroscopy to elucidate bonding dynamics during the oxygen reduction reaction (ORR) on a Pt catalyst. The present work explores multiple atomistic catalyst properties that notably include features of the Pt-Pt bonding and changes in bond strains that occur upon exposure to O2 in the electrochemical environment. The alteration of the Pt electronic and physical structures due to O2 exposure occurs over a wide potential range (1.2 to 0.4 V vs normal hydrogen electrode), a range spanning potentials where Pt catalyzes the ORR to those where Pt-oxide forms and all ORR activity ceases. We show that Pt-Pt surface bond strains due to oxygen interactions with Pt-Pt bonds are discernible at macroscopic scales in cantilever-based bending measurements of Pt thin films under O2 and Ar. Complementary extended X-ray absorption fine structure (EXAFS) measurements of nanoscale Pt clusters supported on carbon provide an estimate of the magnitude and direction of the in-operando bond strains. The data show that under O2 the M-M bonds elongate as compared to an N2 atmosphere across a broad range of potentials and ORR rates, an interfacial bond expansion that falls within a range of 0.23 (±0.15)% to 0.40 (±0.20)%. The EXAFS-measured Pt-Pt bond strains correspond to a stress thickness and magnitude that is well matched to the predictions of a mechanics mode applied to experimentally determined data obtained via the cantilever bending method. The data provide new quantitative understandings of bonding dynamics that will need to be considered in theoretical treatments of ORR catalysis and substantiate the subpicometer resolution of electrochemically mediated bond strains detected on the macroscale.

Ambient synthesis of high-quality ruthenium nanowires and the morphology-dependent electrocatalytic performance of platinum-decorated ruthenium nanowires and nanoparticles in the methanol oxidation reaction

We report for the first time (a) the synthesis of elemental ruthenium nanowires (Ru NWs), (b) a method for modifying their surfaces with platinum (Pt), and (c) the morphology-dependent methanol oxidation reaction (MOR) performance of high-quality Pt-modified Ru NW electrocatalysts. The synthesis of our elemental Ru NWs has been accomplished utilizing a template-based method under ambient conditions. As-prepared Ru NWs are crystalline and elementally pure, maintain electrochemical properties analogous to elemental Ru, and can be generated with average diameters ranging from 44 to 280 nm. We rationally examine the morphology-dependent performance of the Ru NWs by comparison with commercial Ru nanoparticle (NP)/carbon (C) systems after decorating the surfaces of these structures with Pt. We have demonstrated that the deposition of Pt onto the Ru NWs (Pt~Ru NWs) results in a unique hierarchical structure, wherein the deposited Pt exists as discrete clusters on the surface. By contrast, we find that the Pt-decorated commercial Ru NP/C (Pt~Ru NP/C) results in the formation of an alloy-type NP. The Pt~Ru NPs (0.61 A/mg of Pt) possess nearly 2-fold higher Pt mass activity than analogous Pt~Ru NW electrocatalysts (0.36 A/mg of Pt). On the basis of a long-term durability test, it is apparent that both catalysts undergo significant declines in performance, potentially resulting from aggregation and ripening in the case of Pt~Ru NP/C and the effects of catalyst poisoning in the Pt~Ru NWs. At the conclusion of the test, both catalysts maintain comparable performance, despite a slightly enhanced performance in Pt~Ru NP/C. In addition, the measured mass-normalized MOR activity of the Pt~Ru NWs (0.36 A/mg of Pt) was significantly enhanced as compared with supported elemental Pt (Pt NP/C, 0.09 A/mg of Pt) and alloy-type PtRu (PtRu NP/C, 0.24 A/mg of Pt) NPs, both serving as commercial standards.

Surface X-ray scattering of stepped surfaces of platinum in an electrochemical environment: Pt(331) = 3(111)-(111) and Pt(511) = 3(100)-(111)

Real surface structures of the high-index planes of Pt with three atomic rows of terraces (Pt(331) = 3(111)-(111) and Pt(511) = 3(100)-(111)) have been determined in 0.1 M HClO(4) at 0.1 and 0.5 V(RHE) with the use of surface X-ray scattering (SXS). The surfaces with two atomic rows of terraces, Pt(110) = 2(111)-(111) and Pt(311) = 2(100)-(111) = 2(111)-(100), are reconstructed to a (1 × 2) structure according to previous studies. However, the surfaces with three atomic rows of terraces have pseudo-(1 × 1) structures. The interlayer spacing between the first and the second layers, d(12), is expanded 13% on Pt(331) compared to that of the bulk, whereas it is contracted 37% on Pt(511). The surface structures do not depend on the applied potential on either surface.

Mechanically generated surface chirality at the nanoscale

A substrate coated with an achiral polyimide alignment layer was scribed bidirectionally with the stylus of an atomic force microscope to create an easy axis for liquid crystal orientation. The resulting noncentrosymmetric topography resulted in a chiral surface that manifests itself at the molecular level. To show this unambiguously, a planar-aligned negative dielectric aniostropy achiral nematic liquid crystal was placed in contact with the surface and subjected to an electric field E. The nematic director was found to undergo an azimuthal rotation approximately linear in E. This so-called "surface electroclinic effect" is a signature of surface chirality and was not observed when the polyimide was treated for a centrosymmetric topography, and therefore was nonchiral.

X-ray diffraction and STM study of reactive surfaces under electrochemical control: Cl and I on Cu(100)

The surface structure of Cu(100) modified by chloride and iodide has been studied in an electrochemical environment by means of in-situ scanning tunneling microscopy in combination with in-situ surface X-ray diffraction with a particular focus on adsorbate and potential dependent surface relaxation phenomena. For positive potentials close to the on-set of the copper dissolution reaction, the X-ray data disclose an extraordinarily large Cu-Cl bond length of 2.61 A for the c(2 x 2)-Cl phase. This finding points to a largely ionic character of the Cu-Cl interaction at the Cu(100) surface, with chloride particles likely to retain their full charge upon adsorption. Together with the positive surface charging at these high potentials, this ionic Cu-Cl bond drives the observed 2.2% outward relaxation between the first two copper layers. These results indicate that the bond between the first and the second copper layer is significantly weakened which appears as the crucial prerequisite for the high surface mobility of copper-chloride species under electrochemical annealing conditions at these high potentials. With 2.51 A the Cu-I bond is 4% shorter than the Cu-Cl bond implying that the nature of the Cu-I bond is mainly covalent. Accordingly, we observe a significant inward relaxation of the top Cu layers upon substituting chloride by iodide at the same electrode potential, which suggests that the iodide adsorption involves charge transfer from the halide to the copper substrate.

Adsorption Isotherm of CO on Pt(111) Electrodes

We have determined, for the first time, the equilibrium CO coverage of Pt(111) electrodes at room temperature in 0.1 M H(2)SO(4) as a function of the CO partial pressure using CO-stripping cyclic voltammetry. Fourier-transform infrared (FT-IR) spectroscopy was used to confirm qualitatively the coverage values obtained.

Vibrational sum frequency generation studies of the (2×2)→(√19×√19) phase transition of CO on Pt(111) electrodes

The potential-dependent (2x2)-3CO-->(radical19x radical19)R23.4 degrees-13CO adlayer phase transition on Pt(111) with 0.1M H(2)SO(4) electrolyte was studied using femtosecond broadband multiplex sum frequency generation (SFG) spectroscopy combined with linear scan voltammetry. Across the phase boundary the SFG atop intensity jumps, and at the same time the SFG spectrum of threefold CO sites is transformed into a bridge site spectrum with a small decrease in integrated SFG intensity. The SFG atop intensity jump and three fold-to-bridge intensity drop are noticeably different from what would be expected for these structures on the basis of coverage alone. This occurs because the SFG signal is sensitive to both the coverage and changes in the local field that result from a changing adlayer structure. We derive an equation that allows us to correct the SFG intensities for these effects using information derived from infrared absorption-reflection spectroscopy (IRAS) and second-harmonic generation (SHG) measurements. With this correction, the SFG results agree well with what would be expected for a transition between perfect adlattices. A small (approximately 20%) discrepancy in the SFG determination of atop coverage is attributed to either a small amount of surface disorder or uncertainties in the SFG, SHG, and IRAS measurements. SFG is also used to examine the reversibility hysteresis and kinetics of the phase transition and its dependence on electrolyte composition. The phase transition is reversible with an approximately 150 mV anodic overpotential and the forward (2x2)-->(radical19x radical19) transition is slower than the reverse. Repeated cycles of phase transition indicate that the 25 microm electrolyte layer used here does not appreciably distort the potential-coverage relationships.