High-energy Surface X-ray Diffraction Research Articles

(Invited) Structural Transformations of OER Catalysts during Activation and Operation

Electrocatalysis of the oxygen evolution reaction (OER) plays a pivotal role in the hydrogen economy. Optimizing materials for this reaction presents a significant challenge, as many highly active materials exhibit low stability during activation and/or operation. Therefore, operando methods capable of probing catalyst structure and morphology become key tools for monitoring transformations during operation. These transformations not only determine activity but also influence long-term stability. In this context, high-energy X-rays provide a valuable probe, easily penetrating electrolytes and cell walls, offering unprecedented insight into material behavior. This contribution presents several classes of OER catalysts operating in alkaline and acidic environments, with their structural transformations studied using high-energy synchrotron scattering techniques.Layered double hydroxides (LDH) represent an important class of catalysts functioning in alkaline environments. Notably, NiFe and CoFe serve as exemplary materials due to their high activity and stability. However, in many cases, the structure during the OER reaction differs from the as-prepared materials, undergoing an α-phase to γ-phase transition, where the latter is the active phase. This transition, at times reversible with potential, is challenging to observe without high-energy wide-angle X-ray scattering (WAXS) techniques [1,2]. Another interesting example involves the origin of high activity in spinel-type Co3O4 thin film catalysts, elucidated by high-energy surface X-ray diffraction (HE-SXRD). Here, the superior performance is attributed to the formation of a 3D CoOx(OH)y active phase skin layer during activation, and its thickness defines the final activity [3].In the case of catalysts operating in an acidic environment, RuOx and IrOx are acknowledged as promising materials. While RuOx exhibits high activity, it is prone to instability, whereas IrOx, although stable, demonstrates lower activity and have significantly higher price. A prevailing trend is to create solid solution alloys of Ir and Ru to capitalize on the strengths of both materials [4]. This contribution details structural investigations of a series of IrRu alloys with varying compositions prepared through DC magnetron sputtering. The study investigates the unusual stability of these materials at high current densities using operando WAXS and a custom-built electrolyzer cell [5]. The findings reveal that these alloys undergo Ru dissolution upon activation, forming a thin amorphous IrOx layer that protects the alloy underneath. The stability is defined by this layer, while the activity is mainly determined by the electronic structure of the IrRu alloy core.The examples presented are a reminder that without operando characterization of the materials it is very difficult to ascribe origins of activity and stability, as the ex-situ characterization might be misleading regarding the structure and electronic structure of the active phase.[1] Dionigi, F. et al. In-situ structure and catalytic mechanism of NiFe and CoFe layered double hydroxides during oxygen evolution. Nature Communications, 2020, 11. https://doi.org/10.1038/s41467-020-16237-1[2] Dresp, S. et al., Molecular Understanding of the Impact of Saline Contaminants and Alkaline pH on NiFe Layered Double Hydroxide Oxygen Evolution Catalysts. ACS Catalysis, 2021, 11, 6800–6809. https://doi.org/10.1021/acscatal.1c00773.[3] Wiegmann, T. et al., OperandoIdentification of the Reversible Skin Layer on Co3O4 as a Three-Dimensional Reaction Zone for Oxygen Evolution. ACS Catalysis, 2022, 12, 3256–3268. https://doi.org/10.1021/acscatal.1c05169.[4] Galyamin, D. et al, Unraveling the Most Relevant Features for the Design of Iridium Mixed Oxides with High Activity and Durability for the Oxygen Evolution Reaction in Acidic Media. JACS Au, 2023, 3, 2336–2355. https://doi.org/10.1021/jacsau.3c00247.[5] Moss, A. B. et al., Versatile High Energy X-Ray Transparent Electrolysis Cell for Operando Measurements. Journal of Power Sources, 2023, 562, 232754. https://doi.org/10.1016/j.jpowsour.2023.232754.

An approach to surface electron density-sensing property correlation in non-stoichiometric boron carbide

The key to most surface phenomena lies in the surface electron density. Particularly, it is the electron density distribution over the surface that primarily controls the overall interaction of the material with the external environment, say in processes like heterogeneous catalysis. Hence, a precise understanding of surface electron density is essential to understand and design improved surface active materials for catalysis and sensing. Surface structure has been determined primarily using surface sensitive techniques like high-energy surface x-ray diffraction (XRD), the crystal truncation rod scattering method, low-energy electron diffraction, scanning transmission electron microscopy, and grazing incidence small angle x-ray scattering. In this work, using aspherical electron density models of crystal structures in different molecular and extended solids, we show a convenient and complementary way of determining high-resolution experimental surface electron density distribution from conventional bulk x-ray diffraction data. The usefulness of our method has been validated by the surface functionality of boron carbide. While certain surfaces in boron carbide show the presence of substantial electron deficient centers, they are absent in others. Based on that, a new surface property of boron carbide has been inferred and has also been validated by chemiresistive gas sensing experiments.

Operando Studies of Electrochemical Interfaces By High-Energy Surface x-Ray Scattering

Knowledge on the atomic-scale structure of the electrode-electrolyte interface is of key importance for understanding electrochemical reactivity. This includes the structure of the atomic layers at the electrode surface, the arrangement of chemisorbed species on the surface, and the near-surface structure of the adjacent electrolyte, i.e., the electrochemical double layer. Surface X-ray scattering (SXRD) methods can provide such information and have been used for decades to obtain important insights into various structural aspects of electrochemical systems. However, conventional SXRD is limited by the considerable time required for a full characterization of the three-dimensional interface as well as in terms of the structural detail that can be reliably extracted from the diffraction data by structural models. This is related to the slow sequential data acquisition necessary for such measurements.We here describe in situ and operando studies of electrochemical interface structure by High Energy Surface X-ray Diffraction (HESXRD), where the interface structure is probed by hard X-rays with high photon energy (in this work 70 keV). This method has been originally developed for studies of catalyst surfaces in the gas phase [1], but is starting to be applied increasingly to electrochemical systems. In combination with the highly brilliant beams provided by emerging hard X-ray 4th generation synchrotron, HESXRD allows to obtain very large datasets in short time. From these the interface structure can be determined with unprecedented detail by a quantitative analysis of the measured crystal truncation rods (CTRs). Furthermore, measurement of restricted datasets is possible with even high time resolution down to the second or sub-second regime, which is ideal for monitoring fast kinetic changes in operando.We illustrate the capabilities of HESXRD by studies of platinum surface oxidation and magnetite single crystal electrodes under oxygen evolution conditions. For the case of platinum we present data on Pt(111), Pt(100), and Pt(110) in 0.1 M HClO4 that reveal distinct differences in the structure and formation mechanisms of the Pt surface oxide [2]. Because of these differences, the irreversible surface restructuring and Pt dissolution during oxidation/reduction cycles depends strongly on the crystallographic orientation. In addition, we demonstrate for Pt(111) that the extraction of Pt atoms out of the electrode surface in the initial stages of oxidation is not directly coupled to the charge transfer associated with the formation of adsorbed oxygen species [3]. Studies on magnetite focus on Fe3O4(100) in 0.1 M NaOH, where previous SXRD studies showed that the (√2x√2)R45° reconstructed surface formed under UHV conditions can be maintained [4]. HESXRD allowed to obtain extended CTR datasets, which provide deeper insights into the structure of this oxide model electrocatalyst.[1] J. Gustafson et al., Science 343, 758 (2014)[2] T. Fuchs et al., Nature Catalysis 3, 754 (2020)[3] T. Fuchs et al., J. Phys. Chem. Lett. 14, 3589 (2023), https://doi.org/10.1021/acs.jpclett.3c00520[4] D. Grumelli et al., Ang. Chem. Int. Ed. 59, 49 (2020)

Stability of the Fe3O4 (√2 x √2)R 45° Surface in 0.1 M NaOH Investigated by High-Energy Surface X-Ray Diffraction

Transition metal oxides, especially Fe, Co, and Ni oxides, are of great interest as electrocatalysts for the oxygen evolution reaction (OER) in alkaline solution. However, detailed data on the atomic-scale oxide-electrolyte interface structure of these systems are rare. We present studies of a model oxide electrocatalyst, -reconstructed Fe3O4(001). This oxide surface has been extensively investigated under ultrahigh vacuum (UHV) conditions, leading to a detailed understanding of its atomic structure [2]. In previous in situ surface X-ray diffraction studies, we demonstrated that the reconstructed surface can survive transfer into electrochemical environment and determined the stability of the reconstruction as a function of potential [2]. Specifically, we showed that the ()R45° reconstructed Fe3O4(001) surface can be transferred into 0.1 M NaOH at 0.2 V vs. Ag/AgCl while maintaining surface structure and that the reconstruction is irreversibly lifted at sufficiently negative potentials.Here, we present more detailed in situ studies of this system, using the novel technique of High-Energy Surface X-Ray Diffraction (HESXRD) [3,4]. This technique enabled us to measure rapidly a huge reciprocal space volume, which allows to obtain very large sets of crystal truncation rods (CTRs) that can be analysed by surface crystallographic methods. These data provide detailed insights into the temporal stability of the surface at fixed potentials as well as potential-dependent changes in surface structure.In the experiments the Fe3O4(001) single crystal samples were prepared by sputtering and annealing in UHV, followed by sample transfer into the hanging-meniscus cell employed for the HESXRD studies. A new experimental procedure allowed to reduce the time of sample transfer and establishment of potential control to below one minute.The in situ HESXRD measurements were performed at beamline P21.2 of PETRA III at DESY. 7 symmetrically independent CTRs and 2 ()R45° fractional order rods could be measured. These data enabled detailed comparison of the oxide-electrolyte interface with the surface structure under UHV conditions [1]. The potential-dependent data indicate clear structural changes when applying more positive or more negative potentials than 0.2 V vs. Ag/AgCl, as illustrated in part A and B of Fig. 1. However, all surface structures were stable at constant potential over several hours, as indicated in part C of Fig. 1. The quality of the obtained results allows to determine quantitative analysis of the interface structure. First results of this analysis will be presented.We gratefully acknowledge financial support by the German ministry of Science and Education (BMBF) via projects 05K19FK3 and 05K22FK1.

Temperature Dependent Operando High Energy Surface X-Ray Diffraction Studies of Pt(111) Oxidation

Platinum electrocatalysts are still widely used as cathode material in proton-exchange membrane fuel cells (PEMFC) because of their high ORR activity. However, their long-term stability is limited due to degradation processes. This degradation is partly caused by Pt oxidation and oxide reduction, which lead to irreversible surface restructuring and subsequently Pt dissolution. To increase the lifetime of PEMFCs it is mandatory to understand the mechanisms behind the degradation on an atomistic level. This requires studies by structure sensitive in situ methods, in particular, also studies at the elevated temperatures typically employed in PEMFCs operation.Previous studies revealed that in the initial stage of oxide formation Pt atoms are extracted out of the surface and form an ultra-thin oxide layer directly above the surface. This extraction is long known as place exchange [1], but the atomic-scale details of the surface oxide structure, the extraction mechanism, and especially the kinetics of the initial oxide growth were only recently clarified [2-7]. It was found that the structural arrangement of the Pt atoms within the surface oxide on the Pt(111) and the Pt(100) surface is distinctly different, leading to differences in the oxidation/reduction kinetics, the reversibility of the surface oxidation/reduction process, and the amount of Pt dissolution into the electrolyte.Recently, we performed fast operando high-energy surface X-ray diffraction (HESXRD) studies, in which we investigated the initial extraction of Pt atoms on Pt(111) during potential cycles. Surprisingly, it was found that the Pt extraction during the anodic sweep is largely independent of the potential sweep rate, although the peak cyclic voltammograms (CVs) associated with the charge transfer during surface oxidation showed a pronounced shift [7-9]. This suggests that the Pt extraction is a fast process, rather driven by potential and not directly coupled to the slow formation of surface oxygen species. However, the Pt extraction during the anodic sweep and the re-insertion during the cathodic sweep still exhibit significant hysteresis, indicating a complex multistep oxidation kinetics.Previous electrochemical studies of Pt(111) electrodes revealed that the described influence of oxidation/reduction kinetics and the reversibility of the charge transfer processes increase at elevated temperature [10]. However, the temperature dependence of the Pt extraction behavior is currently unknown, because all previous structural studies of oxide growth and reduction have been performed exclusively at room temperature.For the experiments at elevated temperatures we developed a new setup for HESXRD studies of single crystals, which combines a hanging meniscus electrochemical cell for operando X-ray diffraction measurements with an inductive heating device in a joint Ar-filled compartment. This setup allows direct (re-)preparation of the aligned Pt single crystals on the diffractometer. By this, the time between preparation and measurement is reduced down to a few minutes and sample transfer through air is rigorously avoided.Temperature-dependent HESXRD measurements at temperatures up to 343 K were performed at beamline ID31 of the European Synchrotron Radiation Facility. The simultaneously measured electrochemical current and X-ray intensity indicate kinetic shifts in the charge transfer but a rather temperature independent Pt extraction process.We gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft via project no. 418603497 and the BMBF via 05K19FK3 and 05K22FK1.

Chemical and Structural In-Situ Characterization of Model Electrocatalysts by Combined Infrared Spectroscopy and Surface X-ray Diffraction.

New diagnostic approaches are needed to drive progress in the field of electrocatalysis and address the challenges of developing electrocatalytic materials with superior activity, selectivity, and stability. To this end, we developed a versatile experimental setup that combines two complementary in-situ techniques for the simultaneous chemical and structural analysis of planar electrodes under electrochemical conditions: high-energy surface X-ray diffraction (HE-SXRD) and infrared reflection absorption spectroscopy (IRRAS). We tested the potential of the experimental setup by performing a model study in which we investigated the oxidation of preadsorbed CO on a Pt(111) surface as well as the oxidation of the Pt(111) electrode itself. In a single experiment, we were able to identify the adsorbates, their potential dependent adsorption geometries, the effect of the adsorbates on the surface morphology, and the structural evolution of Pt(111) during surface electro-oxidation. In a broader perspective, the combined setup has a high application potential in the field of energy conversion and storage.

Anodic and Cathodic Platinum Dissolution Processes Involve Different Oxide Species.

The degradation of Pt-containing oxygen reduction catalysts for fuel cell applications is strongly linked to the electrochemical surface oxidation and reduction of Pt. Here, we study the surface restructuring and Pt dissolution mechanisms during oxidation/reduction for the case of Pt(100) in 0.1 M HClO4 by combining operando high-energy surface X-ray diffraction, online mass spectrometry, and density functional theory. Our atomic-scale structural studies reveal that anodic dissolution, detected during oxidation, and cathodic dissolution, observed during the subsequent reduction, are linked to two different oxide phases. Anodic dissolution occurs predominantly during nucleation and growth of the first, stripe-like oxide. Cathodic dissolution is linked to a second, amorphous Pt oxide phase that resembles bulk PtO2 and starts to grow when the coverage of the stripe-like oxide saturates. In addition, we find the amount of surface restructuring after an oxidation/reduction cycle to be potential-independent after the stripe-like oxide has reached its saturation coverage.

Driving Force of the Initial Step in Electrochemical Pt(111) Oxidation

The first step of electrochemical surface oxidation is extraction of a metal atom from its lattice site to a location in a growing oxide. Here we show by fast simultaneous electrochemical and in situ high-energy surface X-ray diffraction measurements that the initial extraction of Pt atoms from Pt(111) is a fast, potential-driven process, whereas charge transfer for the related formation of adsorbed oxygen-containing species occurs on a much slower time scale and is evidently uncoupled from the extraction process. It is concluded that potential plays a key independent role in electrochemical surface oxidation.

HAT: a high-energy surface X-ray diffraction analysis toolkit.

This work introduces the high-energy surface X-ray diffraction analysis toolkit (HAT), an open-source cross-platform software package written in Python to allow the extraction and processing of high-energy surface X-ray diffraction (HESXRD) data sets. Thousands of large-area detector images are collected in a single HESXRD scan, corresponding to billions of pixels and hence reciprocal space positions. HAT is an optimized reciprocal space binner that implements a graphical user interface to allow the easy and interactive exploration of HESXRD data sets. Regions of reciprocal space can be selected with movable and resizable masks in multiple views and are projected onto different axes to allow the creation of reciprocal space maps and the extraction of crystal truncation rods. Current and future versions of HAT can be downloaded and used free of charge.

In Situ Surface-Sensitive Investigation of Multiple Carbon Phases on Fe(110) in the Fischer-Tropsch Synthesis.

Carbide formation on iron-based catalysts is an integral and, arguably, the most important part of the Fischer–Tropsch synthesis process, converting CO and H2 into synthetic fuels and numerous valuable chemicals. Here, we report an in situ surface-sensitive study of the effect of pressure, temperature, time, and gas feed composition on the growth dynamics of two distinct iron–carbon phases with the octahedral and trigonal prismatic coordination of carbon sites on an Fe(110) single crystal acting as a model catalyst. Using a combination of state-of-the-art X-ray photoelectron spectroscopy at an unprecedentedly high pressure, high-energy surface X-ray diffraction, mass spectrometry, and theoretical calculations, we reveal the details of iron surface carburization and product formation under semirealistic conditions. We provide a detailed insight into the state of the catalyst’s surface in relation to the reaction.

A combined rotating disk electrode-surface x-ray diffraction setup for surface structure characterization in electrocatalysis.

Characterizing electrode surface structures under operando conditions is essential for fully understanding structure-activity relationships in electrocatalysis. Here, we combine in a single experiment high-energy surface x-ray diffraction as a characterizing technique with a rotating disk electrode to provide steady state kinetics under electrocatalytic conditions. Using Pt(111) and Pt(100) model electrodes, we show that full crystal truncation rod measurements are readily possible up to rotation rates of 1200 rpm. Furthermore, we discuss possibilities for both potentiostatic as well as potentiodynamic measurements, demonstrating the versatility of this technique. These different modes of operation, combined with the relatively simple experimental setup, make the combined rotating disk electrode-surface x-ray diffraction experiment a powerful technique for studying surface structures under operando electrocatalytic conditions.

Oxidation and Reduction of Ir(100) Studied by High-Energy Surface X-ray Diffraction

The oxidation and reduction of an Ir(100) surface using 2.5, 5, and 10 mbar O2 partial pressure and a sample temperature of 775 K have been studied by using high-energy surface X-ray diffraction (HESXRD) which allowed to record large volumes of reciprocal space in short time periods. The complex 3D diffraction patterns could be disentangled in a stepwise procedure. For the 2.5 mbar experiment the measurements indicate the formation of an Ir(100)-O c(2 × 2) oxygen superstructure along with the onset of epitaxial IrO2(110) bulk oxide formation. For the 5 and 10 mbar O2 partial pressures the formation of additional IrO2 bulk oxide epitaxies with (100) and (101) orientations as well as of polycrystalline IrO2 was observed. Upon CO reduction, we found the IrO2 islands to be reduced into epitaxial and metallic Ir(111) and (221) oriented islands.

In Situ Studies of the Oxide Structure and Oxide Growth on Single Crystal Platinum Surfaces

Understanding the Pt surface oxidation is of key importance for the development of durable oxygen reduction reaction catalysts as used in low temperature fuel cells. The formation of an ultra-thin surface oxide on Platinum electrodes is causing atomic-scale restructuring of the electrode surface and Pt dissolution, which promotes the degradation of the Pt catalyst. The surface restructuring can be attributed to a place-exchange process, where Pt surface atoms are extracted from the electrode and form an oxide layer atop the surface [1]. A detailed atomistic picture of the oxide growth and oxide structure previously only existed for the Pt(111) electrode [2].In this study, we have used High Energy Surface X-ray Diffraction (HESXRD) [3] to analyse the atomic-scale surface structure of Pt(111), Pt(110) and Pt(100) during oxide formation and reduction in 0.1 M HClO4 and 0.1 M H2SO4. These surfaces exhibit distinct differences in stability towards restructuring after potential cycles to the same upper potential limit. On Pt(111), almost no surface roughening can be observed, while Pt(100) and Pt(110) immediately degrade upon a single oxidation. To elucidate this difference we performed a detailed analysis of the crystal truncation rods (CTR) to determine the location of the Pt atoms in the oxide formed on Pt(100). During initial stages of oxide formation at potentials < 1.2 V, the Pt atoms are extracted from the surface layer and moved laterally away from their original lattice sites. The extraction of the first atom initiates the immediate extraction of a second atom, leading to the formation of oxide stripes on the surface. DFT calculations support this scenario and show that, differently from Pt(111), this mechanism produces unstable surface atoms at stripe ends that can dissolve during the oxidation as well as during the subsequent oxide reduction [4]. A saturation coverage of about 0.4 ML extracted Pt atoms in this stripe oxide is reached already at about 1.2 V, representing a surface that is fully covered with oxide stripes. Our recent analysis of the CTRs of the oxide structure at higher potentials (> 1.2 V), shows that oxide growth continues vertically by the formation of a second distinct layer of Pt atoms above the stripe oxide.

Revisiting Optical Reflectance from Au(111) Electrode Surfaces with Combined High-Energy Surface X-ray Diffraction

We have combined high-energy surface X-ray diffraction (HESXRD) with 2D surface optical reflectance (2D-SOR) to perform in situ electrochemical measurements of a Au(111) electrode in 0.1 M HClO4 electrolyte. We show that electrochemically induced changes to Au(111) surface during cyclic voltammetry can be simultaneously observed with 2D-SOR and HESXRD. We discuss how small one atom high 1x1 islands, accommodating excess atoms after the lifting of the surface reconstruction, can lead to discrepancies between the two techniques. The use of HESXRD allows us to simultaneously detect parts of the truncation rods from the (1 × 1) surface termination and the p x √3 electrochemically induced surface reconstruction, during cyclic voltammetry. The presence of reconstruction phenomena is shown to not depend on having an ideally prepared surface and can in fact be observed after going to very oxidizing potentials. 2D-SOR can also detect the oxidation of the Au surface, however no oxide peaks are detected in the HESXRD signal, which is evidence that any Au oxide is X-ray amorphous.

Structure-Dependence of the Atomic-Scale Mechanisms of Pt Electrooxidation and Dissolution

For current low-temperature hydrogen-oxygen fuel cells, as used e.g. in the upcoming generation of commercial fuel cell vehicles, platinum is still the catalyst of choice, especially for the cathode side. However, catalyst lifetime and performance remain a problem for large-scale commercialization. It has been known for some time that electrochemical oxidation and subsequent oxide reduction lead to irreversible structural changes as well platinum dissolution but the fundamental understanding of these complex processes is still rudimentary. A detailed atomistic picture of the steps by which these ultrathin oxide layers form existed up to now only for the close-packed Pt(111) surface [1]. Here, previous in situ studies by surface X-ray diffraction (SXRD) techniques established the precise locations of the first Pt atoms that move out of their lattice sites in a place-exchange process [2] and found good agreement with density functional theory (DFT) calculations [3]. Provided the potential is not too high, these atoms can move back to their original locations, but at higher potentials an irreversible restructuring of the surface occurs.To elucidate the role of the Pt surface structure, we performed in-depth comparative studies of Pt(111) and Pt(100) in perchloric acid solution by in situ SXRD, online inductively coupled plasma mass spectrometry (ICP-MS) and DFT [4]. Clear differences were found, which mirror the differences in the dissolution behavior of the two electrode surfaces (Figure 1a). Pt(100) not only start to dissolve at lower potential but its oxidation also immediately changes its surface structure in an irreversible manner. These observations demonstrate clearly that Pt dissolution is linked to the way Pt atoms are initially extracted from the surfaces during oxidation.For a better understanding of the extraction process on Pt(100), we performed surface diffraction studies at ESRF ID31 with very high photon energies of 70 keV (HESXRD) [5]. This technique was employed for the first time to a solid-liquid interface and proofed to be indispensable for clarifying the oxide’s structure. The oxidation-induced changes, exemplified in figure 1b for one out of twelve simultaneously measured crystal truncation rods, can be unambiguously assigned to a process in which the extracted Pt atom moves laterally away from its original site. This initiates the immediate extraction of a second atom, leading to the formation of atomic oxide stripes on the surface (see figure 1c). DFT calculations support this scenario and show that, differently from Pt(111), this mechanism produces unstable surface atoms at stripe ends that can dissolve during the oxidation as well as during the subsequent oxide reduction. Consequently, the extraction process on Pt(100) is irreversible from its very onset.These results show a high sensitivity of Pt electrocatalyst oxidation and degradation to the precise surface structure. Rational strategies for the design of catalysts with improved stability will have to take the different mechanisms into account.[1] J. Drnec, D.A. Harrington, O.M. Magnussen, Curr. Op. Electrochem., 4, 69-75 (2017)[2] M. Ruge, et al., J. Electrochem. Soc., 164, H608-H614 (2017)[3] M. J. Eslamibidgoli,, M. H. Eikerling, Electrocatalysis, 7, 345–354 (2016)[4] T. Fuchs, et al., Nat. Catal., 3, 754-761 (2020)[5] J. Gustafson, et al., Science, 343, 758–761 (2014) Figure 1. (a) correlation between Pt(hkl) dissolution and restructuring behavior in 0.1 M HClO4. (b) Example of Pt(100) crystal truncation rod changes with increasing oxidation, measured by in situ high-energy surface X-ray diffraction. (c) Pt extraction mechanisms, obtained from DFT calculations (adapted from T. Fuchs, et al., Nat. Catal., 3, 754 (2020)). Figure 1

Structure dependency of the atomic-scale mechanisms of platinum electro-oxidation and dissolution

Platinum dissolution and restructuring due to surface oxidation are primary degradation mechanisms that limit the lifetime of platinum-based electrocatalysts for electrochemical energy conversion. Here, we have studied well-defined Pt(100) and Pt(111) electrode surfaces by in situ high-energy surface X-ray diffraction, online inductively coupled plasma mass spectrometry and density functional theory calculations to elucidate the atomic-scale mechanisms of these processes. The locations of the extracted platinum atoms after Pt(100) oxidation reveal distinct differences from the Pt(111) case, which explains the different surface stability. The evolution of a specific oxide stripe structure on Pt(100) produces unstable surface atoms that are prone to dissolution and restructuring, leading to one order of magnitude higher dissolution rates. Platinum dissolution and restructuring due to surface oxidation are primary degradation mechanisms of platinum-based electrocatalysts. Now, stark differences are reported in the mechanism for the oxidative extraction of platinum atoms on (111) and (100) single crystals, providing a detailed explanation for the enhanced dissolution on the latter surface.

Real-time monitoring the growth of strained off-stoichiometric NixFe3−xO4 ultrathin films on MgO(001)

Ni x Fe 3 − x O4 thin films with varying Ni amount (0 ≤ x ≤ 1.5) were deposited on MgO(001) via reactive molecular beam epitaxy. The growth process was monitored during film deposition by means of X-ray diffraction. All prepared films exhibit a well-ordered structure with complete vertical crystallinity throughout the whole film growth and flat surfaces of the final films independent of the Ni amount. An enhancement of the vertical compression in the initial growth continuously decreases up to a film thickness of 8 nm. During further growth, all films exhibit residual and constant vertical compression with lateral adaption of the final films to the substrate lattice, as observed by high energy surface X-ray diffraction experiments. Hard X-ray photoelectron spectroscopy measurements of the final films reveal increasing Fe3+:Fe2+ ratios for higher Ni content and point to additional NiO agglomerations within the films exceeding the stoichiometric Ni amount of x = 1.

Surface optical reflectance combined with x-ray techniques during gas-surface interactions

High energy surface x-ray diffraction (HESXRD), x-ray reflectivity (XRR), mass spectrometry (MS) and surface optical reflectance (SOR) have been combined to simultaneously obtain sub-second information on the surface structure and morphology from a Pd(100) model catalyst during in situ oxidation at elevated temperatures and pressures resulting in Pd bulk oxide formation. The results show a strong correlation between the HESXRD and SOR signal intensities during the experiment, enabling phase determination and a time-resolved thickness estimation of the oxide by HESXRD, complemented by XRR measurements. The experiments show a remarkable sensitivity of the SOR to changes in the surface phase and morphology, in particular to the initial stages of oxidation/reduction. The data imply that SOR can detect the formation of an ultrathin PdO surface oxide layer of only 2–3 Å thickness.

(Invited) Advanced High Energy X-Rays Diffraction Techniques for Electrochemical Science

Complete physico-chemical operando characterization of electrochemical devices in whole, or it’s constituent materials separately, is necessary to guide the development and to improve the performance. High brilliance synchrotron X-ray sources play a crucial role in this respect as they act as a probe with relatively high penetration power and low damage potential. These sources will undergo major upgrades in next decade and will provide even higher brilliance and, more importantly, coherence. These upgrades will be particularly advantageous for beamlines providing high energy X-rays as it will allow to use advanced scattering techniques with highly penetrating probe and therefore will bring the techniques typically used for ex-situ measurements to be used on materials in half-cells and operating electrochemical devices. In this contribution the new possibilities of using using high energy, high intensity X-rays to probe model systems and whole devices will be presented. HESXRD (High Energy Surface X-ray Diffraction) [1] and TDS (Transmission Surface Diffraction) [2] provide ideal tools to study structural changes during reaction conditions on single crystal model electrodes. The main advantage of both techniques is the possibility to follow the structural changes precisely with atomic resolution. While HESXRD is ideally used to determine exact atomic position, the TSD is easier to use and allows studies with high spatial resolution. For example, HESXRD can be used to follow the atomic movement of Pt atoms during electrochemical oxidation and dissolution with very high precision, explaining the different catalyst degradation behaviors and suggesting possible routes to improve its durability. The TSD is used, for example, as an excellent tool to study advanced 2D catalysts, such as various dichalcogenides, during oxygen evolution or electrochemical deposition. To study fuel cells or batteries as a whole, elastic scattering techniques such as wide angle and small angle scattering are typically employed as they can provide important complementary information to more standard X-ray imaging and tomography. The advantage is that the chemical contrast and sensitivity at atomic and nm scales is superior. Coupling these technique with the tomographic reconstruction (XRD-CT and SAXS-CT) is much less common as it requires bright synchrotron sources, fast 2D detectors and advanced instrumentation [3]. However, such combination allows 3D imaging of operational devices with unprecedented chemical sensitivity. This can be demonstrated, as an example, on imaging of standard 5 cm2 fuel cells during operation. The change in morphology and atomic arrangement of the catalysts, PEM hydration and water distribution can be followed in one experiment as a function of operating conditions. Furthermore, aging behavior can be assessed with high temporal and spatial resolution in the standard fuel cell, without compromises enforced by the liquid cells or specialized cell designs. These advanced scattering techniques open a door to holistic investigations of operational devices, which are needed to successfully incorporate new materials at the device level. [1] J. Gustafson et al., Science 343, 758 (2014) [2] F. Reikowski et al., J. Phys. Chem. Lett., 5, 1067-1071 (2017)[3] A. Vamvakeros et al., Nat. Commun., 9, 4751 (2018)

High-energy surface X-ray diffraction from surfaces and nanoparticles in operando catalysis

High-energy surface X-ray diffraction from surfaces and nanoparticles in <i>operando</i> catalysis