Dealloying refers to the partial, selective dissolution of the less noble component(s) of a binary or ternary alloy precursor, resulting in a noble-rich, porous structure that has high surface area. Such surface structure is beneficial for fuel cells (FC) because FC uses platinum (Pt), a scarce metal, to catalyze the oxygen reduction reaction (ORR) at the cathode surface. In order to characterize the benefit of the dealloying process in FC ORR catalysts with the rotating disk electrode (RDE) technique, a high surface area catalyst support, namely 3M Co’s nanostructured thin film (NSTF), was incorporated into the RDE measurements. NSTF-coated glassy carbon (GC) disks were used in RDE experiments on a Pt/Pt1-xIrx (0.05 < x < 0.3) composition spread. ORR measurements using NSTF-coated GC disks measure the catalytic properties with the same morphology, composition and surface structure as would be found in a fuel cell. A series of Pt1-xCox and Pt1-xNix (0.5 < x < 0.8) dealloy catalyst precursor films were tested using NSTF-coated GC disks in RDE studies. The value of x in Pt1-xMx (M = Ni, Co) was selected to be high in order to examine the dealloying process. The catalyst films were examined by a RDE test protocol that tracked the surface enhancement factor (SEF) and ORR current densities over a large number of test cycles. The aim was to measure the catalytic performance of the Pt-M materials as dealloying took place. The SEF of the PtCo and PtNi materials increased rapidly at the beginning and reached a plateau as high as 50 cm2/cm2 while the specific ORR activities increased as the initial M content in the catalyst precursor increased. The impact of the dealloying process on morphology was examined by completely dealloying a Pt3Ni7 precursor, deposited on mirror-polished GC disks, at a constant potential. As the dealloying process continued, the SEF of the material increased from about 1 cm2/cm2 to > 30 cm2/cm2 and resulted in the formation of whisker-like structures.

Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgico / Este trabalho descreve uma proposiÃÃo mecanÃstica para a deposiÃÃo de cÃdmio em meio de sulfato Ãcido sobre substrato de platina utilizando tÃcnicas eletroquÃmicas. Os estudos de eletrodeposiÃÃo de cÃdmio foram feitas em meio de sulfato de sÃdio 1 mol Lâ1 contendo 10â2 mol Lâ1 de sulfato de cÃdmio, em que o pH das soluÃÃes foi ajustado com Ãcido sulfÃrico PA. O intervalo de pH das soluÃÃes de trabalho foi variado entre 1 e 3. Em todos os meios estudados, as curvas de polarizaÃÃo potenciostÃtica mostraram a formaÃÃo de um patamar difusional que obedece a equaÃÃo de Levich. Os estudos de pH local revelaram a influÃncia do equilÃbrio Ãcidoâbase do Ãon bissulfato com formaÃÃo de compostos de coordenaÃÃo Cd(HSO4)+ no eletrodo de platina. PropÃe-se que o Ãon cÃdmio, associado ao equilÃbrio Ãcidoâbase do Ãon bissulfato, deposita como bissulfato de cÃdmio numa etapa irreversÃvel, e seguida pela deposiÃÃo de cÃdmio metÃlico reversÃvel com a saÃda do Ãon bissulfato, sendo esta Ãltima etapa mais lenta que a primeira. Desse modo, foi calculado um modelo matemÃtico para o mecanismo proposto, sendo realizados estudos qualitativos em relaÃÃo ao comportamento da curva de polarizaÃÃo e do espectro de impedÃncia eletroquÃmica / This work describes a mechanistical proposition for cadmium electrodeposition in acid sulfate medium on platinum substrate using electrochemical techniques. The cadmium electrodeposition studies were carried in sodium sulfate 1 mol Lâ1 medium containing cadmium sulfate 10â2 mol Lâ1, which the pH of the solutions were adjusted with analytical grade sulfuric acid. The pH interval of the work solutions were ranged between 1 and 3. In all studied medium, the potentiostatic polarization curves showed a diffusional plateau which obeys the Levich equation. The interfacial pH studies showed the influence of acidâbase equilibrium of bisulfate ion with the formation of a coordination compound Cd(HSO4)+ in platinum electrode. It is proposed that the cadmium ion associated in acidâbase equilibrium of bisulfate ion, deposits as cadmium bisulfate at an irreversible step, and followed by a reversible metallic deposition of cadmium with the leaving of bisulfate ion, and this step is slower than the first one. Then, a mathematical model was calculated for the mechanism proposed, and qualitative studies were carried regarding to the polarization curve and electrochemical impedance spectroscopy behavior

Ce travail concerne l'étude du comportement vis-à-vis de la corrosion de deux alliage d'aluminium : l'alliage 2024 (AA 2024 T351) et l'alliage 7075 (AA7075 T7351) et de l'aluminium pur, utilisé comme référence, dans une solution de Na2SO4 0,1 M à l'aide de mesures électrochimiques (courbes de polarisation, courbes de Levich et spectroscopie d'impédance) avec des électrodes à disque tournant. Comparativement aux travaux de la littérature, des données quantitatives sur les processus anodique et cathodique qui se produisent sur les deux alliages ont été obtenues. La première partie de la thèse est consacrée à la caractérisation microstructurale des deux alliages : taille, composition chimique des précipités et fraction surfacique occupée par les différentes phases. La seconde partie présente les résultats d'impédance obtenus au potentiel de corrosion pour différents temps d'immersion et différentes vitesses de rotation. A ce potentiel, le comportement des matériaux est essentiellement contrôlé par le film passif. Les diagrammes d'impédance présentent une dispersion en fréquence, exprimée en termes de « constant phase element (CPE) ». Ce comportement a été analysé à l'aide d'un modèle physique qui permet de montrer une distribution de résistivité dans l'épaisseur des films d'oxyde. Dans la dernière partie, une attention particulière a été portée à l'analyse de la réaction cathodique à la surface des deux alliages qui est à l'origine de leur dégradation importante. La réduction de l'oxygène se produit principalement sur les particules intermétalliques. De façon surprenante, la densité de courant cathodique est nettement plus faible pour l'alliage 7075 qui présente une plus grande surface couverte par les particules. Il a été montré que pour l'alliage 2024, la réaction cathodique est contrôlée par le transport de matière par diffusion convective sur de petites électrodes alors que pour l'alliage AA 7075, la majeure partie des particules, de très petite taille, se comporte comme des microélectrodes pour lesquelles le courant est fixé par la diffusion sphérique, indépendante de la convection. / This work concerns the study of behavior the corrosion of two aluminum alloy: the alloy 2024 (AA 2024 T351) and 7075 (AA7075 T7351) and pure aluminum, used as reference, in a solution of 0.1 M Na2SO4 using electrochemical measurements (polarization curves, curves Levich and impedance spectroscopy) with rotating disk electrodes. Compared to published studies, quantitative data on the anodic and cathodic processes occurring on the two alloys were obtained. The first part of the thesis is devoted to the microstructural characterization of two alloys: size, chemical composition of precipitates and surface fraction occupied by the different phases. The second part presents the results of impedance obtained at the corrosion potential for different immersion times and different speeds. At this potential, the behavior of materials is mainly controlled by the passive film. The impedance diagrams exhibit a frequency dispersion, expressed in terms of "constant stage element (CPE)." This behavior was analyzed using a physical model which allows to show a distribution of resistivity in the thickness of oxide films. In the last part, special attention was paid to the analysis of the cathodic reaction on the surface of the two alloys that is causing their degradation. The oxygen reduction occurs mainly on the intermetallic particles. Surprisingly, the cathode current density is significantly lower for the alloy 7075 which has a greater surface area covered by the particles. It was shown that for 2024 alloy, the cathodic reaction is controlled by material transport by convective diffusion of small electrodes, while for the alloy AA 7075, most of the particles, very small, behaves as microelectrodes for which the current is set by the spherical diffusion, independent of the convection.

This diploma thesis discusses the issue of the fluid flow instabilities due to rotating discs. It also deals with the calculation of axial force in centrifugal pumps and pressure distribution on cover and support disc of the impeller. Computational simulation of rotating disc is presented with the aim of capturing flow instabilities. Next computational simulation focuses on modelling a gap between the support disc (rotor) and the stator part of the pump for three different geometries, where axial force, pressure distribution on the support disc and loss torque are calculated for each of three models. Obtained results are then compared with the experiment in the conclusion

Rotating-disk flow has been investigated not only as a simple model of cross flow instability to compare with swept-wing flow but also for industrial flow applications with rotating configurations. However the exact nature of laminar-turbulent transi- tion on the rotating-disk flow is still major problem and further research is required for it to be fully understood, in particular, the laminar-turbulent transition process with absolute instability. In addition the studies of the rotating-disk turbulent boundary- layer flow are inadequate to understand the physics of three-dimensional turbulent boundary-layer flow. In present thesis, a rotating-rotating disk boundary-layer flow has been inves- tigated experimentally using hot-wire anemometry. A glass disk with a flat surface has been prepared to archieve low disturbance rotating-disk environment. Azimuthal velocity measurements using a hot-wire probe have been taken for various conditions. To get a better insight into the laminar-turbulent transition region, a new way to describe the process is proposed using the probability density function (PDF) map of azimuthal fluctuation velocity. The effect of the edge of the disk on the laminar-turbulent transition process has been investigated. The disturbance growth of azimuthal fluctuation velocity as a function of Reynolds number has a similar trend irrespective of the various edge conditions. The behaviour of secondary instability and turbulent breakdown has been in- vestigated. It has been found that the kinked azimuthal velocity associated with secondary instability just before turbulent breakdown became less apparent at a cer- tain wall normal heights. Furthermore the turbulent breakdown of the stationary mode seems not to be triggered by its amplitude, however, depend on the appearance of the travelling secondary instability. Finally, the turbulent boundary layer on a rotating disk has been investigated. An azimuthal friction velocity has been directly measured from the azimuthal velocity profile in the viscous sub-layer. The turbulent statistics normalized by the inner and outer sclaes are presented. / QC 20120529

In this work the structure of the electrochemical metal-liquid interface is determined through use of quantum mechanics, molecular simulation, and experiment. Herein are profiled the molecular dynamics details and results of solid-liquid interfaces at flat non-specific solid surfaces and copper metal electrodes. Ab initio quantum-mechanical calculations are reported and define the interatomic potentials in the simulations. Some of the quantum-mechanical calculations involve small copper clusters interacting with 3-mercaptopropanesulfonic acid (MPSA), sodium, chloride, bisulfate and cuprous ions. In connection with these I develop the electrode charge dynamics (ECD) routine to treat the charge mobility in a metal. ECD bridges the gap between small-scale metal-cluster ab initio calculations and large-scale simulations of metal surfaces of arbitrary geometry. As water is the most abundant surface species in aqueous systems, water determines much of the interfacial dynamics. In contrast to prior simulation work, simulations in this work show the presence of a dense 2D ice-like rhombus structure of water on the surface that is relatively impervious to perturbation by typical electrode charges. I also find that chloride ions are adsorbed at both positive and negative electrode potentials, in agreement with experimental findings. Including internal modes of vibration in the water model enhances the ion contact adsorption at the solid surface. In superconformal filling of copper chip interconnects, organic additives are used to bottom-up fill high-aspect ratio trenches or vias. I use molecular dynamics and rotating-disk-electrode experiments to provide insight into the function of MPSA, one such additive. It is concluded that the thiol head group of MPSA inhibits copper deposition by preferentially occupying the active surface sites. The sulfonate head group participates in binding the copper ions and facilitating their transfer to the surface. Chloride ions reduce the work function of the copper electrode, reduce the binding energy of MPSA to the copper surface, and attenuate the binding of copper ions to the sulfonate head group of MPSA.

This thesis deals with the instabilities of the incompressible boundary-layer flow that is induced by a disk rotating in otherwise still fluid. The results presented are mostly limited to linear instabilities derived from direct numerical simulations (DNS) but with the objective that further work will focus on the nonlinear regime, providing greater insights into the transition route to turbulence. The numerical code Nek5000 has been chosen for the DNS using a spectral-element method in an effort to reduce spurious effects from low-order discretizations. Large-scale parallel simulations have been used to obtain the present results. The known similarity solution of the Navier–Stokes equation for the rotating-disk flow, also called the von Karman flow, is investigated and can be reproduced with good accuracy by the DNS. With the addition of small roughnesses on the disk surface, convective instabilities appear and data from the DNS are analysed and compared with experimental and theoretical data. A theoretical analysis is also presented using a local linear-stability approach, where two stability solvers have been developedbased on earlier work. A good correspondence between DNS and theory is found and the DNS results are found to explain well the behaviour of the experimental boundary layer within the range of Reynolds numbers for small amplitude (linear) disturbances. The comparison between the DNS and experimental results, presented for the first time here, shows that the DNS allows (for large azimuthal domains) a range of unstable azimuthal wavenumbers β to exist simultaneously with the dominantβ varying, which is not accounted for in local theory, where β is usually fixed for each Reynolds number at which the stability analysis is applied. Furthermore, the linear impulse response of the rotating-disk boundary layer is investigated using DNS. The local response is known to be absolutely unstable. The global response is found to be stable if the edge of the disk is assumed to be at infinity, and unstable if the domain is finite and the edge of the domain is placed such that there is a large enough pocket region for the absolute instability to develop. The global frequency of the flow is found to be determined by the edge Reynolds number. / <p>QC 20140708</p>

Vid produktion av biodiesel produceras även råglycerol (10 vikt%) som biprodukt och utbudet av råglycerol har ökat över de senaste åren till följd av att biodieselproduktionen ökat. Marknadsvärdet för råglycerol är lågt, men det är en utmärkt föregångare till att producera mervärdesprodukter för exempelvis läkemedelsindustrin, kemikalieindustrin eller kosmetika. Genom att reformera glycerol till mervärdesprodukter ökar konkurrenskraftigheten för biodiesel och värdekedjans ekonomiska hållbarhet. Elektrokemisk reformering av glycerol är en lovande metod för att producera dess mervärdesprodukter, eftersom processen är justerbar och miljövänlig. En ytterligare fördel är att vätgas kan samproduceras med mervärdesprodukterna genom denna metod och energikonsumtionen för att producera vätgas genom denna metod är ungefär hälften av vad som krävs vid vattenelektrolys. Denna studie utvärderar hur masstransport påverkar selektiviteten och prestandan för elektrokemisk oxidation av glycerol, genom att utföra elektrokemiska experiment, karaktärisera processens katalysator och analysera produkter som produceras.  Den experimentella uppställningen innefattar en roterande diskelektrod med ett elektrodepositerat lager av Pd, på ett Ni-substrat, i varierande tjocklek. Elektroden undersöks i elektrolyter bestående av varierande koncentration av NaOH (1, 0.5 och 0.25 M) och 0.5 M glycerol vid 25 oC. Systemet konstrueras även som modell i Comsol Multiphysics 6.0 för att simulera experiment. Resultaten visar att masstransport av glycerol begränsar systemet genom att orsaka deaktiveringen av katalysatorn. Vid de lägre NaOH koncentrationerna begränsar även masstransport av OH- reaktionshastigheten genom att miljön omgärdande elektroden blir mindre oxidativ. Slutligen visas att reaktionsmekanismen vid alla tjocklekar av katalysatorn utgörs av två parallella reaktionsvägar. En ökande tjocklek hos katalysatorn ökar selektiviteten mot den ena reaktionsvägen framför den andra, samt ökar antal oxidationssteg som åstadkoms vid reaktionen. / Biodiesel production gives rise to 10 wt% crude glycerol as a byproduct, which is becoming increasingly available on the market owing to the increased biodiesel production over the last few years. Crude glycerol has a low market value, but it is an excellent precursor to producing value added products for the pharmaceutical industry, chemical industry or cosmetics to mention a few. By producing value added products with biodiesel derived glycerol, the ability of biodiesel to compete with fossil fuels is improved and its economical sustainability is promoted. One of the most promising methods to convert glycerol into its value added products is electrochemical reforming, as it poses high tunability and is an environmentally friendly process. Additionally, the process produces hydrogen concurrently and halves the energy consumption compared to producing hydrogen from water electrolysis. This study evaluates the impact of mass transport in the glycerol electrooxidation reaction (GEOR), as a parameter that can be used to finetune product selectivity and optimise the system performance. To do so, the study employs electrochemical experiments, catalyst characterisation and product analysis. The experimental setup consists of a rotating disk electrode (RDE) cell with electrodeposited Pd on Ni substrate at varying thicknesses, operating in 1, 0.5, 0.25 M NaOH and 0.5 M glycerol electrolyte at 25 oC. Additionally, the system is modelled in Comsol Multiphysics 6.0 which is used to perform simulations. It is found that mass transport of glycerol is limiting the system by causing the deactivation of the catalyst. At lower NaOH bulk concentrations for the thicker electrodes, the mass transport of OH- also limits the reaction rate by causing the formation of a locally less oxidative environment. Finally, it is found that all electrodes pose dual reaction mechanisms. Increasing the electrode thickness promotes one reaction mechanism over the other as well as it increases the number of oxidation steps achieved in the reaction.

Les oxydes de manganèse présentent un grand intérêt en raison de leur activité catalytique pour l'ORR (la réaction de réduction de l’oxygène) en milieu alcalin et peuvent être utilisés comme matériaux sans métaux nobles pour la cathode dans les piles à combustible. La présente thèse est consacrée à l’étude de l’activité d'oxydes de manganèse pour l’ORR. Il a été montré que Mn2O3 avec structure bixbyite a une meilleure activité catalytique vers l'ORR en milieu alcalin que les autres oxydes de manganèse étudiés. L'activité spécifique de Mn2O3 est seulement 4 fois inférieure à celle de Pt à une surtension de 0.3 V (ERH). Le lien entre la structure des oxydes de Mn et l'activité ORR est identifié: l'activité spécifique augmente exponentiellement avec le potentiel du couple redox Mn(III)/Mn(IV) de surface. Pour assurer l'activité électrocatalytique élevée de Mn2O3, il est nécessaire d'ajouter du carbone à la composition d'électrode, ainsi que de garder un potentiel supérieur à 0.7 V (ERH). / Manganese oxides are of great interest due to their catalytic activity towards the ORR (the oxygen reduction reaction) in alkaline media and can be used as noble metal-free materials for the cathode in liquid and polymer electrolyte alkaline fuel cells. The present thesis is devoted to the investigation of the ORR activity of manganese oxides. It was shown that Mn2O3 with bixbyite structure has a better catalytic activity toward the ORR in alkaline media than other investigated manganese oxide, the surface activity of Mn2O3 is only 4 times lower than that of Pt at an overvoltage of 0.3 V (RHE). The link between the structure of Mn oxides and the ORR activity is found: the specific ORR activity exponentially increases with the potential of the surface Mn(III)/Mn(IV) red-ox couple. To ensure the high electrocatalytic activity of Mn2O3, it is necessary to add carbon to the electrode composition, as well as to keep potential above 0.7 V (RHE).

The thesis deals with the testing and optimization of hydrodynamic conditions of the electrochemical detector, which consists of a flow cell containing the electrochemical sensor with an array of electrodes. The flow cell includes a rotating disc-shaped component, which allows radial flow of analytes along the sensor electrodes. This principle imitates the rotating disk electrode, with the difference that here the electrodes are immobile. Hydrodynamic phenomena in this case are similar to RDE. Therefore, the theoretical part of my work deals with hydrodynamics and mass transfer for the well-described case of rotating disk electrode. It was found that in the current set-up under certain conditions, an analyte flowed back from the output cell channel to the sensor, even at the laminar flow. This causes a nonlinear response of the electrodes and their low reproducibility. The solution is to reduce the distance between the rotation component and the electrodes surface and reduce flow speed of analyzed liquid towards the sensor. A novel type of sensor with annular electrodes was designed and tested. It showed four times higher conversion rate of the analyte compared with the originally used AC9 electrochemical sensor having electrodes in the form of full circles lying on a circumference of a common circle. Based on calculations, conversion of the analyte on the plane electrode can be increased up to 100% when using this annular sensor, adjusting flow cell and reducing of the input flow rate. The detector was connected to the liquid chromatograph and its function was tested on two substances - ascorbic acid and dopamine. For the current detector these parameters were set: dynamic range, linearity, noise, detection limit, time constant and the temperature range at which the detector can be operated.