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Oxygen Reduction and Evolution Reactions in Alkaline and Non-aqueous Electrolytes for Li-Air Batteries : RRDE and DEMS Investigations
Get PDFDue to the worldwide growing energy demand and depletion of fossil fuel resources, sustainable renewable energy conversion and storage systems have to be developed. Among the promising possibilities are the rechargeable batteries. Li-air batteries could be a key technology for automotive applications because of their higher (3-5 times) theoretical capacity than the state-of-the-art Li-ion batteries. However, this technology is facing some critical challenges such as the poor efficiency, high overpotential and electrolyte instability. The current research focuses on two types of Li-Air batteries, namely, aprotic and aqueous electrolyte batteries. Despite the intensive work in the last decade, the fundamental electrochemical reactions such as oxygen reduction reaction (ORR) and oxygen evolution (OER) in aprotic electrolytes are not well understood. Since with aprotic electrolytes the formation of a blocking film of the discharge products limits the capacity, an aqueous Li-air battery is an alternative scenario. In this work, two main issues are addressed: i) development of an efficient carbon-free bifunctional catalyst for the air electrode in alkaline media. ii) mechanistic, kinetic and quantitative investigations of ORR/OER in aprotic electrolytes. After an introduction to the topic, a theoretical background followed by a description of the experimental methods is presented. Subsequently, the results and discussion section consists of 6 chapters. The thesis is then closed with the summary and future perspectives. The results and discussion section starts with chapter 4 (already published, Electrochimica Acta 2015, 151, 332) on the investigation of an efficient bifunctional catalyst based on Ag+Co3O4 for ORR/OER in alkaline media. Interestingly, the combination of both components in one mixture showed superior activity than its single components along with good stability. Ag+Co3O4 mixed catalyst containing 10-20 wt% of Co3O4 is the optimum composition. Rotating ring-disc electrode (RRDE) method revealed negligible formation of peroxide intermediate. Oxygen evolution is also monitored using differential electrochemical mass spectrometry (DEMS). To understand the origin of such synergistic effect between Ag and Co3O4, surface and XPS analyses were conducted (Chapter 5). Further investigations on the activity of Ag+ perovskite catalyst and the role of the support (Ni vs. Ag) are presented in chapter 6. In chapter 7, to better understand the mechanism of OER on Co3O4 and the mixed catalyst, DEMS experiments together with isotope labeling are presented. An oxygen exchange process in the lattice oxygen is inferred. In this part, a new small-volume electrolyte DEMS cell design is developed for application of massive electrodes. The last two chapters are devoted to measurements in aprotic electrolytes for Li-O2 system. RRDE and DEMS were used to characterize the reactions in Tetraglyme G4, DMSO and their mixture. The significant role of the solvent properties (e.g. donor number) on the mechanism is assessed (chapter 8). DEMS enabled us not only to detect the main products and by-products but also the number of electrons transferred per oxygen molecule during discharge and charge. The results showed reversible formation of Li2O2 as the main discharge product despite of the side reactions. The catalytic activity of Co3O4 catalyst in DMSO is reported. In chapter 9, a novel electrolyte based on 1,3-dimethylimidazolidinone solvent is investigated for the first time for Li-O2 battery. Although further research has to be done, this better understanding of the processes could help in the development of strategies for the realization of such Li-air batteries.Sauerstoffreduktion und -entwicklung in alkalischen und nichtwässrigen Elektrolyten für Li-Luft-Batterien : RRDE und DEMS Untersuchungen Aufgrund des weltweit wachsenden Energiebedarfs und der Erschöpfung fossiler Brennstoffe müssen nachhaltige, erneuerbare Energienumwandlungs- und Speichersysteme entwickelt werden. Hierfür stellen wiederaufladbare Batterien vielversprechende Möglichkeiten dar. Insbesondere Li-Luft-Batterien könnten eine Schlüsseltechnologie für Automobilanwendungen sein, weil sie höhere (3-5 mal) theoretische Kapazitäten im Vergleich zu herkömmlichen Li-Ionen-Batterien haben. Diese Technologie steht jedoch vor einigen kritischen Herausforderungen wie schlechte Wiederaufladbarkeit, hohe Überspannung und Elektrolytinstabilität. Die aktuelle Forschung konzentriert sich auf zwei Arten von Li-Luft-Batterien, nämlich aprotische und wässrige Elektrolyt-Batterien. Trotz der intensiven Arbeit im letzten Jahrzehnt sind die grundlegenden elektrochemischen Reaktionen wie Sauerstoffreduktion (ORR) und -entwicklung (OER) in aprotischen Elektrolyten nicht gut verstanden. Da bei aprotischen Elektrolyten die Entladungsprodukte der ORR die Elektrode blockieren und dadurch die Kapazität der Batterie begrenzen, stellen Li-Luft-Batterien mit wässrigen Elektrolyt eine Alternative dar. Die vorliegende Arbeit befasst sich mit zwei Hauptthemen: i) Entwicklung eines effizienten, kohlenstofffreien bifunktionellen Katalysators für die Luftelektrode in alkalischen Elektrolyten. ii) Mechanistische, kinetische und quantitative Untersuchungen von ORR/OER in aprotischen Elektrolyten. Nach einer Einführung in das Thema folgen die theoretische Grundlagen mit einer Beschreibung der experimentellen Methoden. Anschließend werden die Ergebnisse und die Diskussion in 6 Kapiteln dargestellt. Die Arbeit endet mit einer Zusammenfassung und einem Ausblick. Die Ergebnisse und Diskussion beginnen mit Kapitel 4 (schon veröffentlicht, Electrochimica Acta 2015, 151, 332) zur Untersuchung eines effizienten bifunktionellen Katalysators auf der Basis von Ag + Co3O4 für ORR/OER in alkalischen Elektrolyten. Interessanterweise zeigte eine Kombination von beiden Komponenten eine höhere Aktivität als die einzelnen Komponenten und darüber hinaus eine gute Stabilität. Ag + Co3O4-Mischkatalysator, der 10-20 Gew.% Co3O4 enthielt, zeigte die optimale Aktivität. Die rotierende Ringscheibenelektrode (RRDE) zeigte eine vernachlässigbare Bildung des Peroxid-Intermediates. Die Sauerstoffentwicklung wurde mittels differentieller elektrochemischer Massenspektrometrie (DEMS) nachgewiesen. In Kapitel 5 werden Oberflächen- und XPS-Analysen gezeigt, um den Ursprung eines solchen synergistischen Effekts zwischen Ag und Co3O4 zu verstehen. Weitere Untersuchungen über die Aktivität des Ag+Perowskit-Katalysators und die Rolle des Trägers (Ni und Ag) sind in Kapitel 6 dargestellt. Zum besseren Verständnis des Mechanismus der OER auf Co3O4 und Mischkatalysatoren wurden DEMS-Messungen zusammen mit Isotopenmarkierung durchgeführt und der Austausch des Gittersauerstoffs nachgewiesen. Hierfür wurde eine DEMS-Zelle mit kleinem Elektrolytvolumen für die Verwendung von massiven Elektroden entwickelt. Die letzten beiden Kapitel widmen sich Messungen in aprotischen Elektrolyten für das Li-O2-System. RRDE und DEMS wurden verwendet, um die Reaktionen in Tetraglyme G4, DMSO und deren Gemische zu charakterisieren. Die signifikante Rolle der Lösungsmitteleigenschaften (z.B. Donorzahl) auf den Mechanismus wird in Kapitel 8 evaluiert. DEMS ermöglichte es uns, nicht nur die Hauptprodukte und Nebenprodukte zu detektieren, sondern auch die Anzahl der übertragenen Elektronen pro Sauerstoffmolekül während der Entladung und Ladung. Die reversible Bildung von Li2O2 als Hauptentladungspruduct wurde trotz der Nebenreaktionen nachgewiesen. Die katalytische Aktivität des Co3O4 Katalysators in DMSO wird gleichfalls beschrieben. In Kapitel 9 wird ein neuartiger Elektrolyt auf Basis von 1,3-Dimethylimidazolidinon-Lösungsmittel erstmals für Li-O2-Batterien untersucht. Obgleich weitere Forschungen durchgeführt werden müssen, könnte das bessere Verständnis der Prozesse bei der Entwicklung von Strategien für die Realisierung solcher Li-Luft-Batterien helfen
Atomic-scale friction study on Au(111) under electrochemical conditions
No full textIn this study, lateral force microscopy (LFM) has been employed to investigate the frictional behavior on the surface of an electrode and the role of adsorbates, which correlates with interactions occurring at the interface. In order to focus on the frictional behavior, it is advantageous to choose well known electrochemical systems and the metal underpotential deposition (UPD) is a suitable candidate. Basically, the UPD of a foreign metal on an electrode occurs due to the strong interaction between foreign metal ion and the substrate. Thus, this reaction happens at a potential far positive of the Nernst equilibrium potential where the bulk deposition takes place. On the basis of what is already known from earlier works, using LFM, it was our aim to understand the influence of adsorbates on friction during metal UPD.
During Ag UPD on Au(111), the structure of sulfate is very sensitive with the coverage of silver and the measurement of friction forces as function of potential also reveals the relation between the coverage and friction. Thus, friction increases with coverage of Ag until the coverage arrives at a critical value (0.27 V vs. Ag/Ag+) but decreases again as the coverage is getting close to 1.
The results observed during Ag UPD on I-modified Au(111) in aqueous electrolyte show that the tip penetrates adsorbates (e.g. iodine and silver), which influences on friction. Since the iodine structures are re-observed as a normal load decreases, there is no irreversible wear. The results observed during Ag UPD on I-modified Au(111) in G4-electrolyte is quite similar with that observed on I-modified Au(111) in aqueous electrolyte and there is an influence of water on friction. Thus, the increase of friction due to the penetration into the adlayers increases with the concentration of water in the electrolyte but it is not proportional to the concentration of water. Therefore, we suggest that water adsorbed on the tip strongly interact with silver adlayer after the penetration of the tip. Interestingly, no dramatic increase of friction was observed in PC-electrolyte after the penetration. This suggests that there is an interaction between tip and PC molecules, which cancels out the influence of water on friction. During Cu UPD on bare and I-modified Au(111), friction increases with increasing the coverage of copper regardless of co-adsorbed anions. Multiple stick-slips is observed only on a Cu monolayer as normal load increase, which suggests that sliding tip strongly interacts with a Cu monolayer or the tip is stuck on the large barrier between copper atoms due to the large mismatch between Cu and Au (11%)
Determination of adsorption and activation volumes and apparent transfer coefficients by pressure and potential modulation
Get PDFIn this thesis the following investigations were carried out on Pt or Pt based electrodes: the determination of the apparent transfer coefficient (α′) for CO oxidation using an ac method, the determination of the molar adsorption volume of hydrogen and the effect of cations thereupon combining the methods of pressure modulation and ac voltammetry, and the determination of the activation volume for CO oxidation by pressure modulation. The ac method for the determination of apparent transfer coefficients or Tafel slopes has been developed in this thesis; it’s used here for CO oxidation on noble metals. This method involves a sinusoidal modulation of the potential and the simultaneous recording of the ac and the dc current. It allows to record α′ quasi continuously as a function of potential or time (i.e., in cyclic voltammetry or in potentiostatic experiments), with the reaction rate varying with time, much more accurately than the traditional method that can only measure the transfer coefficient or Tafel slope over a large range of potentials. Since the mechanism for CO oxidation on noble metals is in the focus of electrocatalytic research in recent years, I applied this ac method to the determination of the apparent transfer coefficient for the oxidation of pre-adsorbed CO at Pt electrodes in sulfuric acid. Electrodes of polycrystalline platinum and single crystalline Pt(111), Pt(665) and Pt(332) were investigated using either potential sweeps or potential steps while superimposing an ac voltage. The apparent transfer coefficients were measured and the transition of values from 1.5 to 0.5 with potential increase, which had been predicted in a simulation by Koper et al., was clearly observed experimentally for the first time. Some assumptions on the mechanism of CO oxidation on Pt by other authors could thus be rejected. The measurement of the apparent transfer coefficient by the ac method is also extended to CO oxidation on Ru and Sn decorated Pt and contributes to the understanding of those processes, as well. The ac method for the determination of the apparent transfer coefficient, which I used here, will be of great help also in many other cases, especially under steady state conditions, where the major limitations of the method are avoided. The volume of adsorbed hydrogen on Pt and the cation effects on it were investigated combining pressure modulation method and ac voltammetry. The pressure modulation method has been developed in our group by Loewe et al for the measurement of reaction and adsorption volumes and was further improved in this work. Instead of the traditional way of using high pressure in a complicated device, modulation by less than 1 bar is sufficient for the measurement of pressure dependences. The adsorption of hydrogen on noble metal surfaces is important for many other reactions, the partial molar volume of adsorbed hydrogen is of fundamental interest. In this thesis, the molar adsorption volume of hydrogen is measured to be 3.3±1 cm3·mol-1 on polycrystalline Pt in sulphuric acid. Cation were observed to largely influence the molar volume of adsorbed hydrogen in Li+, K+ and Cs+ containing electrolytes. A preliminary explanation for these cation effects is proposed. This work will be of importance in understanding the surface structure of the double layer in the hydrogen adsorption region and the involvement of cations in the interface. The pressure modulation method has been extended in this work to the determination of the activation volume for CO oxidation. Activation volume is one of the few parameters that can be measured for the activated complex. Instead of the traditional way of using complicated high pressure method, in this thesis I introduced the pressure modulation method to the measurement of the activation volume of the oxidation of adsorbed CO on polycrystalline Pt. Reasonable values are obtained at low sweep rates and low step potentials with an average of -18.2 cm3·mol-1. A preliminary structure for this activated complex is put forward. The applicability of pressure modulation method for measuring the activation volume of electrochemical processes is thus demonstrated
On the Influence of Cations in Non-Aqueous Electrochemistry
Get PDFIn this thesis, different aspects of the electrochemistry in non-aqueous electrolytes are presented. Electrochemistry in non-aqueous electrolytes plays a key role in the transition from conventional to renewable energy sources. Compact electrochemical energy storage systems, such as the non-aqueous lithium-ion battery, are indispensable for application in mobile devices, such as smartphones or laptops, and their further development is essential for the inevitable replacement of the conventional combustion engine by electrical engines. To overcome the limitations of the state-of-the-art-lithium-ion batteries, electrochemists all around the world seek for new technologies, enabling higher energy densities and a longer lifetime. One key technology currently discussed are so-called metal–air batteries, in which a metal-anode is combined with an oxygen cathode. However, while electrochemistry in aqueous electrolytes has a very long tradition and is well understood, we know much less about reaction mechanisms and the interface between electrode and electrolyte in non-aqueous electrolytes. The main focus of this work lies on the oxygen reduction reaction (ORR) and evolution reaction (OER) in dimethyl sulfoxide (DMSO)-based electrolytes. Employing operando mass spectrometry and classical electrochemical measurements, various mechanistic aspects have been elucidated. By comparison between the ORR in presence of different alkali and alkaline earth metal ions it was shown that the cation significantly affects the product distribution and the mechanism of the ORR. The extensive interaction and ion pair formation between the cation of the conducting salt and reduced oxygen species has been exemplarily studied in the K–O2 system. For all the cations under investigation, the main products are superoxide and peroxide species, while the oxide has not been found. Comparing the results for different alkali cations, it appears that a higher charge density of the cation fosters the formation of the corresponding, insoluble alkali peroxide. However, if the alkaline earth metals are also included the relation between charge density and product distribution is more complex. For instance, calcium mainly fosters superoxide formation at platinum or glassy carbon electrodes despite its relatively high charge density. Introducing another descriptor of the cation's effect, namely its acceptor number, a better correlation was obtained, although reasons for this behaviour remain elusive. Another important aspect of the electrochemistry in non-aqueous electrolyte apart from the effect of the conducting salt is the effect of the electrode material. While the special behaviour of gold electrodes towards the ORR and OER in non-aqueous electrolytes has already been highlighted, in the present work mechanistic aspect have been further elucidated and the overall mechanistic picture has been refined. For example, in Li+-containing electrolytes a direct, surface-confined reduction step from oxygen to peroxide has been identified at gold electrodes via the use of the rotating ring-disk electrode. Regarding divalent cations, gold seems to foster the peroxide formation regardless of the cation, while in the case of alkali cations a distinct, potential-dependent transition from the one- to the two-electron process was observed. This transition again was correlated with the charge density of the monovalent cations. Interestingly, this transition from superoxide to peroxide formation is followed by a second transition, where the product distribution changes back from peroxide to superoxide due to the deactivation of the surface. This second transition has also been observed in the presence of the divalent cations and shows that the main difference between the cations is their ability to foster peroxide formation. 'The effect of the partial pressure of oxygen on the ORR has also been investigated as it is an important parameter of a future, real-world battery, which will either be fed with air or with pure oxygen at variable pressures. While in Li+-containing electrolytes the expected electrochemical reaction order of 1 with respect to the oxygen concentration was obtained, the reaction order was significantly lower in the presence of Mg2+, implying the contribution of an adsorption process. Employing K+-containing electrolyte we were able to show that an increase of the oxygen pressure also changes the product distribution from superoxide to peroxide, which we attributed to the pronounced precipitation of the sparingly soluble superoxide. A further essential issue which is addressed in this work it the influence of water on the non-aqueous electrochemistry, as it is a nearly ubiquitous "contaminant" and difficult to remove quantitatively. In principle, two different trends are observable. In the case of the rather irreversible reduction of oxygen in presence of Li+, water leads to a shift of the peroxide formation to larger overpotentials. However, for the largely reversible K–O2 and Cs–O2 systems, addition of water leads to the pronounced formation of the peroxide, probably due to an irreversible follow-up reaction of the peroxide. The importance of electromobility and storage systems for renewable energies has attracted not only electrochemists, who are interested in non-aqueous electrochemistry, but researchers from various fields of science. Scientists nowadays make great use of simulation techniques to predict the performance of electrical storage systems rather than testing every possible electrolyte combination. However, modelling of electrochemical cells requires knowledge of different parameters, one of which are the transport properties of oxygen. As state-of-the art methods of determining the transport properties were either too slow, too inaccurate or too expensive, we developed a new, non-electrochemical measurement cell which enables simultaneous measurement of the gas diffusivities and solubilities without external knowledge. Interestingly, the solubility of oxygen in DMSO increases for increasing temperatures. Moreover, a significant salting-in effect has been observed in the presence of lithium bis(trifluoromethane)sulfonimide, while the usual salting-out effect has been identified in the presence of the perchlorate salts of different cations. From the temperature-dependent diffusivities we were able to evaluate the activation barrier for the diffusion of oxygen in DMSO-based electrolyte, which will certainly be of great help for modelling electrochemical cells at different temperatures (think of cold winter and hot summer days). While battery-related research usually focuses on the reversibility of the reaction and the energy density of a potential energy storage system, we aimed at elucidating more fundamental properties in this work and there is nothing more fundamental for an electrochemist than the interface between electrode and electrolyte. Therefore, the final section of this work deals with the investigation of the electrode–electrolyte interface via surface-enhanced infrared spectroscopy. The adsorption of cyanide and carbon monoxide from propylene carbonate have been studied and it was found that the observed shift of the vibrational bands are similar to the aqueous system, indicating similar adsorption geometries. Careful analysis of the adsorption of acetonitrile from acetonitrile-based electrolytes revealed that the solvent is adsorbed via the methyl-group for potential negative of the point of zero charge and that the solvent is electrochemically decomposed to cyanide. Moreover, there is a strong interaction between the cation of the conducting salt and acetonitrile, which shows up as a significant shift of the vibration bands of the methyl- and cyanide-groups. In the present work we could give some deeper insights in the course of reactions in non-aqueous electrolytes as well as in the importance of the cation of the conducting-salt, the electrode material, the partial pressure of oxygen and the presence of water. Using a newly developed measurement cell for simultaneous determination of the gas diffusivities and solubilities new data concerning diffusivities and solubilities of oxygen in DMSO-based electrolytes could be collected. Moreover, further insights in the adsorption processes at the electrode electrolyte interface could be given by investigations using the surface-enhanced infrared spectroscopy. The presented results describe fundamental aspects of the electrochemistry in non-aqueous electrolytes and thus contribute to a better understanding of the underlying mechanisms, which will possibly help to improve the development of new batteries in practice
Untersuchung der elektrochemischen Oxidation von Ethanol an Platin in alkalischem Medium
Get PDFEine interessante Reaktion mit Bezug auf Direkt-Alkohol-Brennstoffzellen ist die Oxidation von Ethanol in alkalischem Medium. In der vorliegenden Arbeit werden die Aspekte der Adsorbat- und Produktbildung sowie der Einfluss des Massentransports auf die Oxidation von 10 mM Ethanol in 0,1 M KOH an polykristallinem Platin untersucht. Als Methode kommt für die Adsorbat- und Produktanalyse die differentielle elektrochemische Massenspektrometrie (DEMS) zum Einsatz, der Einfluss des Massentransports wird an einer rotierenden Scheibenelektrode (RDE) untersucht. Für das alkalische Medium wird die prinzipielle Anwendbarkeit von DEMS für die Analyse der Produktbildung während der Ethanoloxidation in alkalischem Medium demonstriert. Es werden Methoden aufgezeigt, die unter Verwendung einer geeignet konstruierten elektrochemischen Dünnschichtdurchflusszelle die Kalibrierung des Massenspektrometers für Kohlendioxid und Acetaldehyd erlauben, was eine Berechnung des gesamten Produktspektrums der Ethanoloxidation ermöglicht. Bezüglich der Ethanoladsorption kann festgestellt werden, dass sich lediglich zwei stabile Adsorbate bilden, die in unterschiedlichen Potentialbereichen zu Kohlendioxid oxidiert werden können. Durch die Berechnung des Elektronenübergangs bei der Oxidation des einen Adsorbats zu Kohlendioxid kann dieses Adsorbat als adsorbiertes Kohlenmonoxid identifiziert werden. Aufgrund der größeren Anzahl an übertragenen Elektronen pro gebildetem Kohlendioxidmolekül und seiner Reduzierbarkeit zu Methan wird angenommen, dass es sich bei dem anderen Adsorbat um eine adsorbierte Alkylspezies aus der dissoziativen Spaltung des Ethanols handelt. Weiterhin wird aufgezeigt, dass diese Alkylspezies in adsorbiertem Zustand auch zu adsorbiertem Kohlenmonoxid oxidiert werden kann. In potentiostatischen Experimenten zwischen 0,45 V und 0,9 V vs. RHE wird eine gleichbleibende Verteilung der Stromausbeuten von etwa 85% - 90% für Essigsäure, um 10% für Acetaldehyd und eine nicht signifikante Stromausbeute von > 5% für Kohlendioxid bestimmt. Eine Erhöhung der Ethanolkonzentration vermindert hierbei die Stromausbeuten für Essigsäure zugunsten der für Acetaldehyd. Den gleichen Effekt bewirkt eine Erhöhung des Carbonatgehalts der Elektrolytlösung, wobei hier gezeigt wird, dass die Veränderung in den Stromausbeuten mit dem Absinken des pH-Werts korreliert. Sowohl die Produktbildung in potentiodynamischen Messungen als auch die hohen Stromausbeuten für Essigsäure bei niedrigen Potentialen in potentiostatischen Experimenten, bei denen Acetaldehyd noch nicht oxidiert wird, deuten darauf hin, dass im alkalischen Medium Essigsäure auch direkt ohne den Umweg über Acetaldehyd aus Ethanol gebildet werden kann. An der RDE wird festgestellt, dass eine Erhöhung der Konvektion zu einer Verringerung der Reaktionsströme der Ethanoloxidation führt. Ob dieses Verhalten durch den Abtransport eines reaktiven Intermediats zu begründen ist, wird im Vergleich der Messung mit einem rechnerischen Modell nachvollzogen. Hierbei wird festgestellt, dass neben dem reversiblen Effekt des konvektionsabhängigen Abtransports des Intermediats ein weiterer irreversibler Effekt besteht, der jedoch nicht auf die adsorbierte Alkylspezies aus der Ethanoladsorption zurückgeführt werden kann. Mit Blick auf die Ergebnisse der vorliegenden Arbeit, speziell auf die geringen Stromausbeuten für Kohlendioxid, die negative Beeinflussung durch Konvektion, Konzentrationserhöhung und Carbonatgehalt, erscheint die Nutzung von Ethanol in alkalischen Direkt-Alkohol-Brennstoffzellen mit Platin als Katalysatormetall wenig attraktiv
EPR distance measurements using triaryl methyl radicals and EPR investigation of electron transfer processes in organic radicals
Get PDFEPR is a valuable tool for the investigation of biological structures. Most EPR-based distance measurements rely on the site directed spin labeling of the investigated biomolecule, since most biomolecular structures do not contain unpaired electrons. Most measurements today are conducted in frozen buffer solutions. These conditions are not the natural environment of the biomolecules, and may affect their geometric structure. To overcome this drawback of the method, research efforts are made to develop protocols for distance measurements under biologically relevant conditions. In the pursuit of measurement conditions that are closer to biological conditions, new kinds of spin labels have emerged to overcome the limitations of the widely used nitroxide spin labels. One of the new kinds of labels is the triarylmethyl radicals (trityl). In this work, the optimization and comparison of four different pulsed EPRbased distance measurement techniques on two organic bistrityl model compounds is presented. Building on these results, the use of new trityl spin labels for trityl-iron(III) distance measurement on pseudomonas putida CYP101 P450 is demonstrated. The performance of the new spin labels is compared to the commercially available MTSSL nitroxide spin label and two other trityl spin labels known from literature. The use of one of these new spin labels for the first in cell distance measurement with a trityl radical is then demonstrated. This is also the first in cell distance measurement between a spin label and a native metal cofactor. The second part of this work deals with the investigation of electron transfer processes and radical intermediates of catalytic reactions. To that end, two setups were designed that combine electrochemistry and EPR. The first is a potentiostatic flat cell, the second a galvanostatic flow cell. Both systems were characterized using Wurster`s reagent and employed in the study of the electronic structure and electron self-exchange rate of radical salts of bis(2-pyridylmethyl)azine and bis(2-pyridylmethyl-5-tert-butyl)azine. Further, they were used to investigate the mechanisms of copper-catalyzed coupling reactions of tetrahydroisochinoline (THIQ) and the MacMillan organo-catalytic cycle. In all cases, chemical means for the generation of the radicals and additional methods like freeze quench were also employed. In the work presented here, strong experimental evidence is given for the existence of radical intermediates in both investigated catalytic cycles. This supports proposed radical mechanisms known from the literature
Fuel Cells, Metal/Air Batteries : characterization of dual thin-layer flow through cell and determination of solubility and diffusion coefficient of oxygen in aqueous and non-aqueous electrolytes
Get PDFUntil now the energy demand is substantially met by fossil fuels such as natural gas, oil and coal. The available resources are finite. Forecasts assume that at constant consumption the availability of oil reserves in 2050 will decrease rapidly [1]. The amounts of energy required must be replaced by alternative techniques (e.g. solar or wind energy, fuel cells, metal/air batteries). In the combustion of fossil fuels, there are various environmental pollutant emissions such as nitrogen oxides, sulfur dioxide and carbon monoxide. Although carbon dioxide (CO2) as one of the main emissions is not an environmental poison, but it is a greenhouse gas that contributes to global warming. With the adoption of the Kyoto Protocol on 11th of December 1997, an international agreement to reduce emissions was decided [2]. In order to achieve this ambitious goal, it is necessary applying alternatives to develop the burning of fossil fuels [3]. The fossil or alternatively recovered energy can be saved by obtained electrolytic hydrogen, which can be later used for energy in fuel cells. Fuel cells, including the methanol fuel cell (DMFC) can compete with batteries and generators for mobile use. They provide services from a few watts for the power supply of mobile phones or laptops to a few kilowatts to supply of an entire household. Since 2003, the first small series of various automobile manufacturers are developed, in which the energy is produced with fuel cells. Nevertheless, research and development potential still exists [4]. One of the main challenges in the metal-air batteries is the selection of a suitable electrolyte that is characterized with high oxygen solubility, low viscosity, liquid state and low vapour pressure in a wide temperature range and stable in a wide potential window. In this work, we present a new technique to determine the solubility and diffusivity of oxygen in aqueous and non-aqueous electrolytes by on-line differential electrochemical mass spectrometry (DEMS), using a dual thin-layer flow through cell. Two methods are applied to characterize the dual thin-layer flow through cell. The first method considers the hydrodynamic electrochemical behavior of the cell which is composed of two working electrodes and can be considered as the analogue of a rotating ring-disk system. The hydrodynamic electrochemical characteristics such as correlation of the electrolyte flow rate with the faradaic current, transfer efficiency, shielding factor and collection efficiency are studied for Fe3+, Hydroquinone and dissolved H2 solutions. The second method represents the mass spectrometric behavior of the cell for different gaseous and volatile substances. In this method the dual thin-layer flow through cell is connected to the mass spectrometer via a permeable Teflon membrane. The correlation of mass spectrometric current with solution flow rate is studied by this method
Struktur und Reaktivität der reinen und modifizierten Pd(111)-Elektrodenoberfläche
Get PDFBisher wurde in der Literatur ein widersprüchliches Adsorptionsverhalten von Kohlenmonoxid auf der Pd(111)-Oberfläche zwischen UHV und wässrigen Lösungen bei Raumtemperatur beschrieben: Während im UHV gemessene IRRAS-Spektren bei einem maximalen Bedeckungsgrad durch eine einzelne Bande charakterisiert werden, beobachtete man bisher in Lösung immer zwei Banden mit unterschiedlichen Intensitäten. Es konnte hier gezeigt werden, dass die Doppelbandenstruktur auf einer Rauhigkeit der Oberfläche beruht, die die CO-Moleküle teils von einer muldengebundenen in eine brückengebundene Koordination mit dem Substrat zwingen, ähnlich wie sie auf einer (100)-orientierten Oberfläche vorherrscht. Demnach korreliert das in situ Adsorptionsverhalten von Kohlenmonoxid durchaus mit den Erwartungen, die man mit den Resultaten der UHV-Studien verbindet. Die Modifikation der Probe mit Sauerstoff führt zu einer Deaktivierung bezüglich der CO-Adsorption im UHV. Dies steht in Zusammenhang mit der Einlagerung von subsurface-Sauerstoff und/oder der Bildung eines Oberflächenoxids. Für die elektrochemische Abscheidung von Kupfer auf Pd(111) wurde ein UPD-Prozess nachgewiesen, der zu einer Überstruktur mit einem Bedeckungsgrad an Kupfer 1/3 führt. In Analogie dazu kann auch bei der Abscheidung von Kupfer bei 200 mV aus verdünnten Cu(II)-Lösungen eine besondere Stabilität dieser Überstruktur nachgewiesen werden. Bei der Adsorption von CO auf einer so mit Kupfer bedeckten Pd(111)-Oberfläche beobachtet man in IR-Spektren eine einzelne, leicht zu geringeren Wellenzahlen verschobene Bande. Sie zeigt eine um 1/3 geringere Intensität als bei einem entsprechenden Experiment mit der kupferfreien Oberfläche. Eine Struktur aus Kupferinseln oder -clustern sollte bei verringerter Intensität keine Verschiebung der CO-Bande verursachen. Da sich sowohl bei dem UPD-Prozess als auch bei der Kupferabscheidung bei 200 mV eine besondere Stabilität einer 1/3 Submonolage herausgestellt hat, kann man in beiden Fällen auf die gleiche Überstruktur schließen. In der Literatur wurden für den UPD-Prozess von Kupfer auf Pd(111) in unterschiedlichen Elektrolyten und Elektrolytkonzentrationen Ladungsmengen bestimmt, die eher auf die Abscheidung bzw. Auflösung einer pseudomorphen (1 x 1)-Monolage deuteten. Dies konnte im Rahmen dieser Arbeit, in einem Fall auch bei im Vergleich zu Literaturdaten identischen Elektrolytkonzentrationen, nicht reproduziert werden. Es erscheint möglich, dass nur eine ideal glatte Oberfläche zu einer Anionenstabilisierung der 1/3 Monolage führt. Strukturen mit einem Bedeckungsgrad an Kupfer, der über 1/3 hinausgeht, wurden durch CO-Adsorption und anschließende Abscheidung von Kupfer bei 200 mV und in umgekehrter Reihenfolge hergestellt. Aus den IR-Daten lässt sich ableiten, dass unter diesen Bedingungen ein Insel- oder Clusterwachstum einsetzt. Auch auf diesen Kupferinseln oder -Clustern adsorbiert CO, die zugehörigen CO-Banden sind leicht zu höheren Wellenzahlen verschoben. Weiterhin ist bei geschlossenen Kupferschichten bis zu einer Dicke von 5 – 10 Lagen auch on-top-Adsorption zu beobachten. Den Abschluss dieser Arbeit bildeten in situ Untersuchungen zur elektrochemischen und katalytischen Aktivität von Kupfer-modifizierten Pd(111)-Oberflächen bei der Zersetzung von Ameisensäure. Eine in der Literatur diskutierte Aktivierung durch geringe Kupferbedeckungen konnte nicht beobachtet werden. Dagegen bewirkt eine Cu-Überstruktur mit einem Bedeckungsgrad von 1/3 die vollständige Inhibierung der katalytischen Reaktion. Dies wurde darauf zurückgeführt, dass eine on-top-Bindung der Ameisensäure über das Sauerstoffatom im Übergangszustand blockiert wird. Letzteres gilt auch für die CO-bedeckte Oberfläche
Oxidation of Methanol and Carbon Monoxide on Platinum Surfaces : The Influence of Foreign Metals
Get PDFDespite of its fairly simple chemical structure, the oxidation of methanol at electrode surfaces follows a fairly complicated mechanism with parallel reaction paths and several reaction steps. Within the context of the search for better catalysts for fuel cells, the influence of various parameters such as potential, temperature, catalyst composition and structure on the single reaction steps were elucidated with the help of CO oxidation and the adsorption and oxidation of methanol on carbon supported Pt nanoparticles, polycrystalline Pt and Pt(665) as well as the influence of foreign metals such as Ru and Mo on these reactions has been studied by differential electrochemical mass spectrometry (DEMS) under continuous flow conditions. The rates and activation energies of the single reaction steps as well as the influence of the catalyst composition and structure was determined by measuring of the adsorption rate of methanol, the oxidation of the methanol adsorbate as well as the rate of the bulk methanol oxidation, where the ion current of the CO2 was detected in parallel to the faradaic current. Catalysts, and this also includes Ru and Se modified catalyst which are important as cathode material for O2 reduction in the fuel cells, were characterized by stripping of adsorbed CO and of underpotential deposition of Cu (CuUPD). It was shown that the active surface area, determined from the CO stripping, agrees with that calculated from the particle size assuming a narrow distribution and a spherical shape of the colloid particles. On the Se modified nanoparticle surfaces, it is shown that Se blocks the adsorption sites for CO, the free adsorption sites can be detected by the CuUPD). The maximum methanol adsorption rate found at room temperature at polycrystalline Pt is ca. 0.06 ML s-1, whereas at Pt nanoparticles it is ca. 0.04 ML s-1. At 50°C the rate constants of the methanol adsorption increases markedly on both electrodes. On polycrystalline Pt the maximum coverage of the methanol adsorbate amounts to 56% of a full CO monolayer, obtained by adsorbing CO from a CO saturated electrolyte. On the nanoparticles only 28% of a full CO monolayer can be obtained. The oxidation rate of the methanol adsorbate was found to be of zeroth order at polycrystalline Pt, whereas at nanoparticles it is of first order with respect to the coverage. At polycrystalline Pt and Pt(665), the rate of CO2 formation is determined by the oxidation rate of the adsorbate. At the nanoparticle electrodes, the rate of CO2 formation from bulk methanol is higher than that from the methanol adsorbate, due to the roughness of these electrodes. At Ru containing Pt surfaces the oxidation of the methanol adsorbate was shifted to lower potentials and a higher adsorption rate was observed in comparison to the pure Pt surface. It was observed that the Ru ad-atoms promote the reaction path via adsorbed CO in the low potential region. At higher potentials the Ru loses its co-catalytic activity towards methanol oxidation; possibly due to the formation of inactive anhydrous Ru oxide at higher potentials. Using isotopic labelling, the interaction of methanol and carbon monoxide with its adsorbed species on Pt and platinum based electrodes was also studied. It was shown that pure Pt surfaces modified by 0.2 ML of Ru offer different adsorption sites for CO, which can be selectively populated. On these sites, adsorption and oxidative desorption of CO can be selectively performed and 12CO at the sites with lower adsorption enthalpy can be replaced by 13CO. The additional deposition of Mo onto the Pt nanoparticle surfaces modified with 0.2 ML of Ru showed that the co-catalytic effect of Ru and of Mo in CO oxidation can be combined in a synergetic sense. On the other hand, no significant improvements on methanol oxidation were found. An enhancement of the overall methanol oxidation reaction for bulk methanol by the elevating temperature was observed and the overall apparent activation energies, determined from the faradaic current, as well as the apparent activation energies for theindirect oxidation pathway via adsorbed CO, determined from the ion current of CO2, were calculated. The comparison of both kinds of activation energies confirmed that the Ru containing Pt surfaces have a positive effect on the catalytic activity towards methanol oxidation via adsorbed CO
Geordnete organische Adsorbate auf anionenmodifizierten einkristallinen Kupferelektroden : Chemische Charakterisierung mittels Röntgen-Photoelektronen-Spektroskopie (XPS)
Get PDFDer chemische Zustand geordneter, elektrochemisch präparierter, organischer Adsorbate auf anionen-modifizierten einkristallinen Kupferoberflächen wird ex-situ mit Röntgen-Photoelektronen-Spektroskopie (XPS) charakterisiert, wofür XPS-Transferapparaturen genutzt werden. Ergänzende Untersuchungen wurden mittels Ionenstreuspektroskopie (LEIS) und Elektronenbeugung (LEED) durchgeführt. In dieser Arbeit werden zwei verschiedene Viologene (4,4'-Bipyridylverbindungen) -- Dibenzylviologen (DBV) und Diphenylviologen (DPV) -- und ein Porphyrin auf verschiedenen anionen-modifizierten einkristallinen Kupferoberflächen untersucht. Speziell die Cu(100), mit quadratischer Symmetrie, und die Cu(111), mit hexagonale Symmetrie, wurden ausgewählt. Nach Adsorption von Chlorid-, Sulfid oder Sulfationen stellen die modifizierten Oberflächen auf Grund elektrostatischer Wechselwirkungen ein Substrat für die Adsorption der organischen Kationen dar. In dieser Arbeit werden adsorbierte Dibenzylviologen- (DBV) und Diphenylviologen-Moleküle (DPV) auf der Chlorid-bedeckten Cu(100) und der Sulfid-bedeckten Cu(111) Oberflächen untersucht. Adsorbiertes Tetramethyl-pyridinium-porphyrin (TMPyP) -- ein wasserlösliches Porphyrin -- wird auf den chlorid- sowie sulfatbedeckten Cu(100) und Cu(111) Oberflächen präpariert und untersucht. Die elektrochemische Präparation erlaubt es, den RedOx-Zustand der adsorbierten Moleküle wie auch die Substratstruktur durch Variation des elektrochemischen Potenzials gezielt zu verändern. Ein Schwerpunkt dieser Arbeit ist daher die Untersuchung des RedOx-Zustandes der Adsorbate mittels XPS. Das N1s-Signal erweist sich als empfindliche Sonde für den RedOx-Zustand der untersuchten Adsorbate. An Hand der N1s Bindungsenergie können Viologen-Dikationen, Radikal-Monokationen und neutrale Moleküle unterscheiden. Die Bindungsenergie der Dikationen liegt oberhalb von 402 eV, die der Monokationen um 400,6 eV und die der neutralen Moleküle unter 400 eV. Auch der RedOx-Zustand und Protonierungsgrad von adsorbiertem TMPyP korreliert mit der N1s Bindungsenergie. Darüber hinaus können die Koadsorption von Wassermolekülen an Hand des O1s Signals untersucht werden und unterschiedlich hydrophile/hydrophobe Eigenschaften der beiden Viologenadsorbate gezeigt werden. Die Dichte und Dicke der Adsorbate kann mittels oberflächensensitiver Cl2p Signale abgeschätzt werden. Cl2p und S2p Signale konnten außerdem die Koadsorption von Anionen im Falle von TMPyP sowie auf der sulfidmodifizierten Oberfläche, auch für Viologene, zeigen.Ordered organic adsorbates on anion precovered single crystal copper electrodes: Chemically characterised by X-ray photoelectron spectroscopy (XPS) The chemical state of ordered, electrochemically prepared, organic adsorbates on anion precovered copper single crystal electrodes is studied by ex-situ X-ray photoelectron spectroscopy (XPS). XPS transfer systems are used. Further characterisations of the substrates are done with Low Energy Ion Scattering Spectroscopy (LEIS) and Low Energy Electron Diffraction (LEED). Here, two viologens (4,4-Bipyridyls) -- namely Dibenzylviologen (DBV) and Diphenyviologen (DPV) -- and one kind of water-soluable porphyrin (Tetra-methylpyridinium-porphyrin, TMPyP) are studied on several anion precovered copper surfaces. The Cu(100) surface, with quadratic symmetry, and the Cu(111) surface, with hexagonal symmetry, are chosen. After the adsorption of chloride, sulphide or sulphate anions, these surfaces serve as substrates for the adsorption of organic cations due to attractive electrostatic interactions. Ordered structures are formed due to template or self-assembly effects. Here the adsorbed viologens are studied on the chloride precovered Cu(100) and the sulphide precovered Cu(111) substrates. Adsorbed TMPyP is prepared and characterized on the chloride and sulphate precovered Cu(100) and Cu(111) surfaces. Electrochemistry allows to change the redox state and the structure of the adsorbates by changing the potential. Hence, an important objective of the present work is the identification of the adsorbate's redox state. The N1s signal is a sensitive probe for the redox state of the studied molecules. Due to the N1s binding energy it is possible to differ between viologen dications, radical cations and neutral molecules. The binding energy of the dications is around 402 eV, that of the radical cations around 400.6 eV and that of the neutral molecules below 400 eV. Also the redox state and the degree of protonation influences the N1s binding energy. Further, O1s signals allow to study the coadsorption of water molecules and differences in the hydrophily/hydrophoby of viologen adsorbates are revealed. The density and thickness of the adsorbates is estimated due to surface sensitive Cl2p signals. Cl2p and S2p signals are also used to show the coadsorption of anions in case of adsorbed TMPyP and on sulphide precovered surfaces in case of the viologens
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