Univ., Diss., 2010--Marburg.

Th. doct.-ing.--Génie chim.--Grenoble--I.N.P.G., 1984. N°: DI 487.

<p>In this study, the main objective was to investigate the tolerance of platinum based binary anode catalysts for CO poisoning from 10ppm up to1000ppm and to identify the<br /> best anode catalysts for PEMFCs that tolerates the CO fed with reformed hydrogen.</p>

The ever increasing awareness of the environment and sustainability drives research to find new solutions in every part of society. In the transport sector, this has led to a goal of replacing the internal combustion engine (ICE) with an electrical engine that can be powered by renewable electricity. As a battery for vehicles, the Li-ion chemistries have become dominant due to their superior volumetric and gravimetric energy densities. While promising, electric vehicles require further improvements in terms of capacity and power output before they can truly replace their ICE counterparts. Another aspect is the CO2 emissions over lifetime, since the electric vehicle itself presently outlives its battery, making battery replacement necessary. If the lifetime of the battery could be increased, the life-cycle emissions would be significantly lowered, making the electric vehicle an even more suitable candidate for a sustainable society. In this context, lithium titanium oxide (LTO) has been suggested as a new anode material in heavy electric vehicles applications due to intrinsic properties regarding safety, lifetime and availability. The LTO battery chemistry is, however, not fully understood and fundamental research is necessary for future improvements. The scope of this project is to investigate degradation mechanisms in LTO-based batteries to be able to mitigate these and prolong the device lifetime so that, in the end, a suitable chemistry for large scale applications can be suggested. The work presented in this licentiate thesis is focused on the LTO electrode/electrolyte interface. Photoelectron spectroscopy (PES) was applied to determine whether the usage of LTO would prevent anode-side electrolyte decomposition, as suggested from the intercalation potential being inside the electrochemical stability window of common electrolytes. It has been found that electrolyte decomposition indeed occurs, with mostly hydrocarbons of ethers, carboxylates, and some inorganic lithium fluoride as decomposition products, and that this decomposition to some extent ensued irrespective of electrochemical battery operation activity. Second, an investigation into how crossover of manganese ions from Mn-based cathodes influences this interfacial layer has been conducted. It was found, using a combination of high-energy x-ray photoelectron spectroscopy (HAXPES) and near-edge x-ray absorption fine structure (NEXAFS) that although manganese is present on the LTO anode surface when paired with a common manganese oxide spinel cathode, the manganese does little to alter the surface chemistry of the LTO electrode.

<p>In this study, the main objective was to investigate the tolerance of platinum based binary anode catalysts for CO poisoning from 10ppm up to1000ppm and to identify the<br /> best anode catalysts for PEMFCs that tolerates the CO fed with reformed hydrogen.</p>

In hydrometallurgical copper production plants, titanium-based electrodes coated with a conductive layer of IrO2-Ta2O5 are widely used as anodes in acidic copper electrowinning baths because of their long service life and low overpotential for oxygen evolution. The presence of trace amounts of ions such as fluoride, aluminum, and iron in sulfate-based electrowinning baths is believed to affect the stability of IrO2-Ta2O5 coated anodes. Hence, in this study, the effect of fluoride and metallic cations on the lifetime of IrO2-Ta2O5 coated Ti electrodes in sulfuric acid solutions has been investigated, and a degradation mechanism for IrO2-Ta2O5 coatings in the presence of fluoride has been proposed. Typical lifetime of the conductive ceramic coated anodes is 1 to 2 years. In order to predict electrode performance over this long period, an accelerated laboratory test that can be carried out in a few weeks is often used. This test, known as accelerated lifetime test (ALT), is conducted by electrically stressing the anodes at a current density that is much higher than the current density used for electrowinning while monitoring the change in the cell potential. The time required for the cell voltage to increase by 5 V is taken as the accelerated lifetime of the oxide electrode. In this research, titanium mesh samples coated with mixed iridium oxide-tantalum oxide layers were tested as anodes in 2 M sulfuric solution a constant current density of 0.54 A/cm2. A two-electrode cell with a bare titanium mesh serving as the cathode was used for experiments. In addition to ALTs, anodic polarization measurements were also carried out to study the changes in overpotential for oxygen evolution on electrodes before and after ALTs. Additionally, morphology and chemical composition analyses were performed on electrodes before and after ALTs using various techniques such as scanning electron microscopy (SEM) analysis, energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Chemical species that are likely to be present in the electrowinning bath were predicted using the computer software STABCAL and presented in distribution-pH and Pourbaix diagrams. The results of multiple ALTs in the absence and presence of various levels of fluoride indicate that the anode lifetime was greatly reduced by the presence of fluoride in sulfuric acid solutions. The greater the amount of fluoride added, the shorter the anode lifetime. Additionally, both in the absence and presence of fluoride, the molar ratio of IrO2 to Ta2O5 in the coating did not change during dissolution. In studying strategies to prolong the lifetime of the electrode in a fluoride-containing solution, a method of complexing fluoride ions using metallic cations such as Al3+ and Fe3+ was developed and demonstrated. The anode lifetime was successfully prolonged from 200 to over 500 hours with the addition of aluminum ions to a fluoride-containing solution. Compared with ferric ions, aluminum ions are more efficient in complexing with fluoride to extend the lifetime of electrodes.

Magister Scientiae - MSc / In this study, the main objective was to investigate the tolerance of platinum based binary anode catalysts for CO poisoning from 10ppm up to1000ppm and to identify the best anode catalysts for PEMFCs that tolerates the CO fed with reformed hydrogen. / South Africa

No description available.

Novel anode catalysts have been developed for sour natural gas solid oxide fuel cell (SOFC) applications. Sour natural gas comprises light hydrocarbons, and typically also contains H2S. An alternative fuel SOFC that operates directly on sour natural gas would reduce the overall cost of plant construction and operation for fuel cell power generation. The anode for such a fuel cell must have good catalytic and electrocatalytic activity for hydrocarbon conversion, sulfur-tolerance, resistance to coking, and good electronic and ionic conductivity. The catalytic activity and stability of ABO3 (A= La, Ce and/or Sr, B=Cr and one or more of Ti, V, Cr, Fe, Mn, or Co) perovskites as SOFC anode materials depends on both A and B, and are modied by substituents. The materials have been prepared by both solid state and wet-chemical methods. The physical and chemical characteristics of the materials have been fully characterized using electron microscopy, XRD, calorimetry, dilatometry, particle size and area, using XPS and TGA-DSC-MS. Electrochemical performance was determined using potentiodynamic and potentiostatic cell testing, electrochemical impedance analysis, and conductivity measurements. Neither Ce0.9Sr0.1VO3 nor Ce0.9Sr0.1Cr0.5V0.5O3 was an active anode for oxidation of H2 and CH4 fuels. However, active catalysts comprising Ce0.9Sr0.1V(O,S)3 and Ce0.9Sr0.1Cr0.5V0.5(O,S)3 were formed when small concentrations of H2S were present in the fuels. The oxysuldes formed in-situ were very active for conversion of H2S. The maximum performance improved from 50 mW cm2 to 85 mW cm2 in 0.5% H2S/CH4 at 850 oC with partial substitution of V by Cr in Ce0.9Sr0.1V(O,S)3 . Selective conversion of H2S offers potential for sweetening of sour gas without affecting the hydrocarbons. Perovskites La0.75Sr0.25Cr0.5X0.5O3, (henceforth referred to as LSCX, X=Ti, Mn, Fe, Co) are active for conversion of H2, CH4 and 0.5% H2S/CH4. The order of activity in the different fuels depends on the substituent element: CH4, X=Fe>Mn>Ti; H2,X = Fe>Mn>Ti; and 0.5% H2S/CH4, X = Fe>Ti>Mn. The electrocatalytic activity for methane oxidation in a fuel cell correlates with ex-situ temperature programmed catalytic activity. A process is proposed to explain the difference in catalyst order and enhanced activities in H2S/CH4 as fuel compared to CH4 alone. The maximum power density of 250 mW cm2 was attained using a fuel cell with a composite anode, LSCFe-GDC | YSZ(0.3 mm) | Pt, operated at 850 oC (GDC is Ce0.9Gd0.1O3, a good mixed conductor under reducing conditions). / Materials Engineering

The current Li-ion technologies allow the popularity of Li-ion batteries as electrical energy storage for both mobile and stationary applications. The graphite-based anode is most commonly used in commercial Li-ion batteries. However, because lithium intercalation in graphite occurs very close to the redox potential of Li/Li+, accidental lithium plating is a known hazard capable of resulting in internal shorting, particularly when the battery is charged rapidly, requiring higher overpotentials to accomplish the Li-intercalation. Moreover, toward the next-generation battery, a growing interest is now on promising rechargeable Na-ion batteries. The main motivation for Na-ion alternative is that sodium is much more abundant and widely distributed on the earth’s crust than lithium. In the first part of this dissertation, we investigate safer, higher specific capacity anode materials for both Li-ion and Na-ion batteries. In a separated effort toward the efficient solar energy harvesting, the second part of the dissertation examines thin film photoanodes, active in the visible-light region, for photoelectrochemical water oxidation. This part also discusses in detail the synthesis, characterization, as well as the use of co-catalysts to improve the electrode’s photochemistry performance. The current Li-ion technologies allow the popularity of Li-ion batteries as electrical energy storage for both mobile and stationary applications. The graphite-based anode is most commonly used in commercial Li-ion batteries. However, because lithium intercalation in graphite occurs very close to the redox potential of Li/Li+, accidental lithium plating is a known hazard capable of resulting in internal shorting, particularly when the battery is charged rapidly, requiring higher overpotentials to accomplish the Li-intercalation. Moreover, toward the next-generation battery, a growing interest is now on promising rechargeable Na-ion batteries. The main motivation for Na-ion alternative is that sodium is much more abundant and widely distributed on the earth’s crust than lithium. In the first part of this dissertation, we investigate safer, higher specific capacity anode materials for both Li-ion and Na-ion batteries. In a separated effort toward the efficient solar energy harvesting, the second part of the dissertation examines thin film photoanodes, active in the visible-light region, for photoelectrochemical water oxidation. This part also discusses in detail the synthesis, characterization, as well as the use of co-catalysts to improve the electrode’s photochemistry performance.