Investigation of Electrocatalysts Produced by a Novel Thermal Spray Deposition Method

Common methods to produce supported catalysts include impregnation, precipitation, and thermal spray techniques. Supported electrocatalysts produced by a novel method for thermal spray deposition were investigated with respect to their structural properties, elemental composition, and electrochemical performance. This was done using electron microscopy, X-ray photoelectron spectroscopy, and cyclic voltammetry. Various shapes and sizes of catalyst particles were found. The materials exhibit different activity towards oxidation and reduction of Fe. The results show that this preparation method enables the selection of particle coverage as well as size and shape of the catalyst material. Due to the great variability of support and catalyst materials accessible with this technique, this approach is a useful extension to other preparation methods for electrocatalysts

Graphene-Capped Liquid Thin Films for Electrochemical Operando X-ray Spectroscopy and Scanning Electron Microscopy

Electrochemistry is a promising building block for the global transition to a sustainable energy market. Particularly the electroreduction of CO2 and the electrolysis of water might be strategic elements for chemical energy conversion. The reactions of interest are inner-sphere reactions, which occur on the surface of the electrode, and the biased interface between the electrode surface and the electrolyte is of central importance to the reactivity of an electrode. However, a potential-dependent observation of this buried interface is challenging, which slows the development of catalyst materials. Here we describe a sample architecture using a graphene blanket that allows surface sensitive studies of biased electrochemical interfaces. At the examples of near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and environmental scanning electron microscopy (ESEM), we show that the combination of a graphene blanket and a permeable membrane leads to the formation of a liquid thin film between them. This liquid thin film is stable against a water partial pressure below 1 mbar. These properties of the sample assembly extend the study of solid–liquid interfaces to highly surface sensitive techniques, such as electron spectroscopy/microscopy. In fact, photoelectrons with an effective attenuation length of only 10 Å can be detected, which is close to the absolute minimum possible in aqueous solutions. The in-situ cells and the sample preparation necessary to employ our method are comparatively simple. Transferring this approach to other surface sensitive measurement techniques should therefore be straightforward. We see our approach as a starting point for more studies on electrochemical interfaces and surface processes under applied potential. Such studies would be of high value for the rational design of electrocatalysts

Revealing the active phase of copper during the electroreduction of CO2 in aqueous electrolyte by correlating in situ x-ray spectroscopy and in situ electron microscopy

The variation in the morphology and electronic structure of copper during the electroreduction of CO2 into valuable hydrocarbons and alcohols was revealed by combining in situ surface- and bulk-sensitive X-ray spectroscopies with electrochemical scanning electron microscopy. These experiments proved that the electrified interface surface and near-surface are dominated by reduced copper. The selectivity to the formation of the key C–C bond is enhanced at higher cathodic potentials as a consequence of increased copper metallicity. In addition, the reduction of the copper oxide electrode and oxygen loss in the lattice reconstructs the electrode to yield a rougher surface with more uncoordinated sites, which controls the dissociation barrier of water and CO2. Thus, according to these results, copper oxide species can only be stabilized kinetically under CO2 reduction reaction conditions

Potential-driven surface phase transitions on iridium (hydr-)oxides and their relation to electrolytic water splitting

The burning of fossil fuels and the resultant release of CO2 into the atmosphere is the only tenable explanation for the rapid progression of climate change in recent decades. A sustainable energy economy is urgently needed to reduce the use of fossil fuels significantly. Hydrogen has been proposed to play a strategic role in this new energy economy and PEM electrolyzers are a promising way to produce hydrogen from water and renewable electricity. Improving the efficiency and longevity of these devices is challenged by the slow and eroding anodic reaction at which oxygen gas evolves, which is called the oxygen evolution reaction, or OER. Iridium (hydr-)oxides are stable and active catalysts for this reaction, but it is not fully understood which properties of their surface give them these qualities. The present work contributes to this understanding by observing potential driven surface phase transitions with operando X-ray spectroscopy. We found that the surface of iridium (hydr-)oxide catalysts undergo several phase transitions before they reach their active state. These phase transitions are several 100 mV wide and occur via oxidative deprotonation of surface hydroxo groups. The electron holes created in this process reside on surface atoms and an oxidative charge is building up with larger anodic potentials. At the onset of the OER, this process is not yet complete and influences the rate of the OER. In fact, the complex current or rate response, e.g. a change in Tafel slope, can be fully explained when accounting for the phase transition. The catalytic rate is influenced in two ways: First, the availability of active sites changes, and, second, the oxidative charge on the surface, which changes the barrier of the rate determining step. The oxidative charge was found to increasingly reside on oxygen at potentials relevant to the OER. The key experimental findings of this work were only made possible by a novel approach to studying wet catalyst surfaces using surface sensitive X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS), which is also presented herein. This thesis contributes to the fundamental understanding of OER catalyst surfaces by investigating a class of stable and active OER catalysts, namely iridium (hydr-)oxides. The experimental and computational approaches used herein, and especially their combination, provide a template for future studies furthering the knowledge about electrocatalysts and their technical development. Advances in these areas of research are urgently needed to successfully transition into a renewable energy economy.Vermehrte CO2 Emissionen, die durch die Nutzung fossiler Brennstoffe entstehen, sind die einzige, wissenschaftlich vertretbare Erklärung für den beschleunigten Klimawandel der letzten Jahrzehnte. Eine nachhaltige Energiewirtschaft wird dringend gebraucht, um die Nutzung fossiler Energieträger signifikant einschränken zu können. Wasserstoff wird im wissenschaftlichen Diskurs eine zentrale Rolle in dieser neuen Energiewirtschaft zugesprochen, und PEM-Elektrolysatoren sind eine vielversprechende Möglichkeit, den Strom aus erneuerbaren Energiequellen zu nutzen um aus dem Ausgangstoff Wasser Wasserstoffgas zu produzieren. Eine Verbesserung der Effizienz und Lebensdauer von PEM-Anlagen ist allerdings begrenzt durch die langsame und materialermüdende anodische Reaktion, bei welcher Sauerstoff entsteht. Die (Hydr-)oxide des Iridium sind stabile und reaktive Anodenmaterialien, die diese Reaktion katalysieren, aber es ist nicht vollständig geklärt, welchen Oberflächeneigenschaften sie ihre guten Eigenschaften verdanken. Die vorliegende Arbeit trägt mit der Beobachtung von Phasenübergängen auf der Oberfläche des Katalysators zu einem besseren Verständnis dieser Beziehung bei. Wir haben beobachtet, dass die Oberfläche von Iridium (Hydr-)oxid Katalysatoren mit steigendem Potential mehrere Phasenübergänge durchläuft bevor sie ihre aktive Form erreicht. Diese Phasenübergänge erstrecken sich über eine Potentialspanne von einigen hundert Millivolt und veräußern sich auf atomarer Ebene in Form von oxidativer Deprotonierung der Oberflächenhydroxidgruppen. Die Elektronenlöcher, die in diesem Prozess entstehen, verorten sich auf Atomen nahe der Oberfläche, und eine oxidative Ladung baut sich auf. Bei dem Potential, an dem die Sauerstoffentwicklung beginnt, ist dieser Wandlungsprozess nicht abgeschlossen und beeinflusst die Reaktionsrate. Die komplexe Änderung dieser Reaktionsrate mit dem Potential, wie z.B. eine Änderung der Tafel-Steigung, kann unter Einbezug der Phasenübergänge allerdings vollständig erklärt werden. Die Reaktionsrate wird auf zwei Wegen beeinflusst: erstens, von der Verfügbarkeit der Reaktionszentren, und zweitens, von der oxidativen Ladung, die die Aktivierungsenergie der Reaktion herabsetzt. Die Dichteverteilung dieser oxidativen Ladung verschiebt sich mit steigendem Potential zunehmend von dem Metallzentrum auf Sauerstoff, welcher schließlich reagiert. Die Schlüsselexperimente dieser Arbeit wurden durch eine neuartige Technik zur Untersuchung von genässten Katalysatoren mit Oberflächen-empfindlicher Röntgenspektroskopie (XPS und NEXAFS) ermöglicht, die ebenfalls hier vorgestellt wird. Diese Doktorarbeit trägt mit einer detaillierten Untersuchung von Iridium (Hydr-)oxiden in der Sauerstoffentwicklung zu dem fundamentalen Verständnis von elektrochemischen Katalysator- oberflächen bei. Die experimentelle Herangehensweise sowie die theoretische Behandlung und insbesondere die Kombination der beiden bietet eine Vorlage für zukünftige Studien an Elektro- Katalysatoren und deren technische Entwicklung. Fortschritte in diesen Forschungsbereichen werden dringend für eine erfolgreiche Energiewende benötigt

The ladder towards understanding the oxygen evolution reaction

Understanding the atomic-scale mechanistic details of the oxygen evolution reaction (OER) remains an unresolved challenge in electrochemistry owing to the complexity of the OER. In this short review we discuss how, with the advent of new experimental and computational methodologies, the OER can be treated with increasingly sophisticated models to aid in our complete understanding. For the case of steady state catalyst surfaces, we define a six-rung ladder of complexity to frame how far this understanding reaches and in which aspects our understanding could still improve

Evolution of surface and sub-surface morphology and chemical state of exsolved Ni nanoparticles.

Nanoparticle formation by dopant exsolution (migration) from bulk host lattices is a promising approach to generate highly stable nanoparticles with tunable size, shape, and distribution. We investigated Ni dopant migration from strontium titanate (STO) lattices, forming metallic Ni nanoparticles at STO surfaces. Ex situ scanning probe measurements confirmed the presence of nanoparticles at the H2 treated surface. In situ ambient pressure X-ray photoelectron spectroscopy (AP-XPS) revealed reduction from Ni2+ to Ni0 as Ni dopants migrated to the surface during heating treatments in H2. During Ni migration and reduction, the Sr and Ti chemical states were mostly unchanged, indicating the selective reduction of Ni during treatment. At the same time, we used in situ ambient pressure grazing incidence X-ray scattering (GIXS) to monitor the particle morphology. As Ni migrated to the surface, it nucleated and grew into compressed spheroidal nanoparticles partially embedded in the STO perovskite surface. These findings provide a detailed picture of the evolution of the nanoparticle surface and subsurface chemical state and morphology as the nanoparticles grow beyond the initial nucleation and growth stages

Molecular Analysis of the Unusual Stability of an IrNbO_x Catalyst for the Electrochemical Water Oxidation to Molecular Oxygen (OER)

Adoption of proton exchange membrane (PEM) water electrolysis technology on a global level will demand a significant reduction of today’s iridium loadings in the anode catalyst layers of PEM electrolyzers. However, new catalyst and electrode designs with reduced Ir content have been suffering from limited stability caused by (electro)chemical degradation. This has remained a serious impediment to a wider commercialization of larger-scale PEM electrolysis technology. In this combined DFT computational and experimental study, we investigate a novel family of iridium–niobium mixed metal oxide thin-film catalysts for the oxygen evolution reaction (OER), some of which exhibit greatly enhanced stability, such as minimized voltage degradation and reduced Ir dissolution with respect to the industry benchmark IrOx catalyst. More specifically, we report an unusually durable IrNbOx electrocatalyst with improved catalytic performance compared to an IrOx benchmark catalyst prepared in-house and a commercial benchmark catalyst (Umicore Elyst Ir75 0480) at significantly reduced Ir catalyst cost. Catalyst stability was assessed by conventional and newly developed accelerated degradation tests, and the mechanistic origins were analyzed and are discussed. To achieve this, the IrNbOx mixed metal oxide catalyst and its water splitting kinetics were investigated by a host of techniques such as synchrotron-based NEXAFS analysis and XPS, electrochemistry, and ab initio DFT calculations as well as STEM-EDX cross-sectional analysis. These analyses highlight a number of important structural differences to other recently reported bimetallic OER catalysts in the literature. On the methodological side, we introduce, validate, and utilize a new, nondestructive XRF-based catalyst stability monitoring technique that will benefit future catalyst development. Furthermore, the present study identifies new specific catalysts and experimental strategies for stepwise reducing the Ir demand of PEM water electrolyzers on their long way toward adoption at a larger scale

Evolution of surface and sub-surface morphology and chemical state of exsolved Ni nanoparticles

Nanoparticle formation by dopant exsolution (migration) from bulk host lattices is a promising approach to generate highly stable nanoparticles with tunable size, shape, and distribution. We investigated Ni dopant migration from strontium titanate (STO) lattices, forming metallic Ni nanoparticles at STO surfaces. Ex situ scanning probe measurements confirmed the presence of nanoparticles at the H2 treated surface. In situ ambient pressure X-ray photoelectron spectroscopy (AP-XPS) revealed reduction from Ni2+ to Ni0 as Ni dopants migrated to the surface during heating treatments in H2. During Ni migration and reduction, the Sr and Ti chemical states were mostly unchanged, indicating the selective reduction of Ni during treatment. At the same time, we used in situ ambient pressure grazing incidence X-ray scattering (GIXS) to monitor the particle morphology. As Ni migrated to the surface, it nucleated and grew into compressed spheroidal nanoparticles partially embedded in the STO perovskite surface. These findings provide a detailed picture of the evolution of the nanoparticle surface and subsurface chemical state and morphology as the nanoparticles grow beyond the initial nucleation and growth stages