The flow, reactions and current in a porous rotating disk electrode (PRDE) is studied experimentally and theoretically. A PRDE is an electrochemically active porous disk mounted on a classic rotating disk electrode (RDE). For the oxidation of iodide, the measured current from a PRDE as a function of rotation rate shows much richer behavior than the flat RDE, including a sigmoidal dependence on the rotation rate that specifically depends on the geometry of the disk, its permeability, porosity and the fluid and reactant transport properties. It is found that when the complex behavior of the current is explained in terms of the ratio of the effective electrochemical reaction time to the residence time of the fluid in the porous disk all the data can be plotted onto a universal curve at high rotation rates. With this knowledge the PRDE is modeled analytically where the reactant transport is dominated by either advection or diffusion. When advection is dominant the current can be expressed in a simple algebraic form involving the dimensionless reaction time. The diffusion dominated regime is modeled utilizing a boundary layer theory. The current is found as a function of the rotation rate, reaction rate, permeability, diffusion coefficients, kinematic viscosity, and geometry of the disk. Combined with finite effects analysis, the two analytic models accurately describe the PRDE for the full range of its operation regardless of the geometry of the disk. Also, the dominant mass transfer mode transition point is identified. Additional experiments with ferrocenemethanol are carried out using PRDEs constructed by mounting various sized carbon fiber disks onto glassy carbon RDEs to complement previous experiments using iodide. The results validate the theories for the operation of the PRDE in the regimes of advection or diffusion dominated transport. A possible application of the PRDE system for measuring rock acidization and permeability is explored by developing analytic and numerical models for a nonconductive porous disk. This system exhibits regimes limited by different processes: diffusion, advection, and reaction. It is found that a one-dimensional analytic model incorporating the finite thickness of the porous disk and the surface reaction rate suffices to describe the system. / text

published_or_final_version / Chemistry / Master / Master of Philosophy

Thesis (Ph. D.)--University of Texas at Austin, 2008. / Vita. Includes bibliographical references.

Thesis (M. Phil.)--University of Hong Kong, 1994. / Includes bibliographical references (leaves 105-107).

Thesis (M.S.)--Florida State University, 2004. / Advisor: Dr. Peter J. Gielisse, Florida State University, College of Engineering, Dept. of Mechanical Engineering. Title and description from dissertation home page (viewed Sept. 23, 2004). Includes bibliographical references.

Thesis (M.S.) -- Mississippi State University. Dave C. Swalm School of Chemical Engineering. / Title from title screen. Includes bibliographical references.

As an efficient means of energy storage, rechargeable batteries, especially the lithium-ion batteries (LIBs) have been a vital component in solving the upcoming energy crisis and environmental problems. Recently, the development of electric vehicle market puts new requirement on the next generation LIBs, including superior energy density, safety and cycling stability, etc. Compared with experimental investigation, Physics-based models provide a surrogate method to not only tackle the underlying physics of the complex battery system, but also optimize the design of battery systems. In this thesis, I will show how I use the physics-based continuum model and cooperate with some experimental methods to understand the lithium transport phenomena inside the multiscale battery electrode systems, based on which the models are then applied to guide the experimental optimization of battery electrode design and to quantitively understand the degradation of high-performance electrodes. The thesis is divided into three parts. First part (Chapter 2) presents a systematical model selection study on the multiscale LiNi₀.₃₃Mn₀.₃₃Co₀.₃₃O₂ (NMC₁₁₁) electrode. Discharge and voltage relaxation curves, interrogated with theory, are used to distinguish between lithium transport impedance that arise on the scale of the active crystal and on the scale of agglomerates (secondary particles) comprised of nanoscale crystals. Model-selection algorithms are applied to determine that the agglomerate scale transport is dominant in the NMC₁₁₁ electrode studied here. This study not only discovers the dominant length scale for lithium transport, but also provide a validated model (the agglomerate model) for later study. The second part (Chapter 3 & 4) talks about understanding & optimization of ion transport in porous electrodes. In Chapter 3, multi-scale physics-based models for different active material systems, which have been parameterized and validated with discharge experiments, are optimized by varying porosity and mass loading to achieve maximum volumetric energy density. The optimization results show that with a re-scaling of the current rate, the optimal results follow a general design rule that is captured in a convenient correlation. Chapter 4 extends the model to simulate the performance of advanced electrode architectures utilizing aligned channels, by quantifying the impact of aligned channel electrode structures on cell rate capability. Then the optimization algorithm in Chapter 3 is applied to these aligned-channel electrodes. The final part (Chapter 5) shows how I use the physics-based model to quantitatively analyze the battery degradation. The validated model is applied to cycling data to obtain parameter estimates indicative of degradation modes. It’s found that growth rates of interfacial impedance and active material loss are greater at 4.5 V, as might be expected. However, when charged to 4.5V, degradation rates are lower at a cycling C-rate of 1.0 h⁻¹ than at 0.5 h⁻¹. Once performance changes are quantified, we use further simulation to evaluate the contribution of individual degradation modes to fade of cell performance metric such as capacity, power density, and energy density.

High levels of water stress and increased demand for potable water generated via desalination pose significant challenges for sustainable waste brine management in arid regions. Electrochemical techniques, like brine electrolysis, offer an approach for treating brine, preventing environmentally harmful disposal, and facilitating the recycling of valuable ions found in brine. As the large concentration of ions can precipitate and degrade conventional electrolyzer components, membraneless electrolyzers, which lack membranes, can be an alternative for direct brine electrolysis. The absence of membranes enables operation in the presence of impurities and a wide range of pH environments. However, membraneless electrolyzers suffer from a trade-off between current density and current utilization that stems from undesired back-reactions that arise from the crossover of gaseous and aqueous products between the anode and cathode. In this dissertation work, a combination of in situ high-speed video, colorimetric pH imaging, modeling, and electroanalytical methods were used to evaluate how the performance of a porous flow-through cathode is affected by operating current density, electrolyte flow rate, and choice of catalyst placement on a porous support. It was found that catalyst placement is a key knob to control the location of product generation and thereby minimize product crossover and maximize pH differential. Placing the catalyst on the outer surface of the cathode resulted in an average increase of 51% in current utilization, a metric for measuring crossover, compared to the opposite configuration. This finding is explained by the ability of the porous electrode support to serve as a barrier to suppress crossover for the outward-facing catalyst configuration. In addition, the outward-facing catalyst configuration leads to more stable operation while incurring minor increases (90-170 mV) in overpotentials. For both catalyst configurations, it was also shown that the Damköhler number (𝐷𝑎) is a practical descriptor for predicting operating conditions that maximize the concentration of OH⁻ in the cathode effluent stream. Furthermore, this dissertation evaluated the performance of a platinized cathode within a membraneless electrolyzer in the presence of Mg²⁺ impurities. In a 3-hour stability test at 50 mA cm⁻² during brine electrolysis, electrolytes with Mg²+ concentration below 5 mM showed a negligible influence on cathode performance. Electrolytes with Mg²⁺ concentration below 1.2 mM at similar operating conditions exhibited improved cathode performance compared to Mg-free brine. All learnings during this study were captured in a mathematical model that predicts the tolerance threshold at which the cathode would cease to operate due to accumulations of Mg(OH)₂ deposits at different current densities and superficial velocities. Overall, these studies demonstrated the potential of membraneless electrolyzers as an emerging technology for treating brine and converting it into high-value products. Finally, applying an oxide overlayer to planar electrodes has been demonstrated to improve their stability, activity, and/or selectivity. This is relevant for direct brine electrolysis, as brine contains many impurities that can compromise the integrity of electrodes and promote undesirable reactions, generating toxic products like chlorine gas. However, given that high-surface electrodes are required for industrial applications, it is necessary to develop a method to encapsulate high-surface-area electrodes. Applying nanoscopic oxide encapsulation layers to high-surface-area electrodes such as nanoparticle-supported porous electrodes is not an easy task. This dissertation work demonstrated that the recently developed condensed layer deposition (CLD) method can be used for depositing nanoscopic (sub-10 nm thick) titanium oxide (TiO₂) overlayers onto high surface area platinized carbon foam electrodes. Characterization of the overlayers by transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) showed they are amorphous, while X-ray photoelectron microscopy confirmed that they exhibit TiO₂ stoichiometry. Electrodes were also characterized by hydrogen underpotential deposition (Hupd) and carbon monoxide (CO) stripping, demonstrating that the Pt electrocatalysts remain electrochemically active after encapsulation. Furthermore, copper underpotential deposition (Cuupd) measurements for bare Pt and TiO₂-encapsulated Pt electrocatalysts revealed that the TiO₂ overlayer effectively prevented Cu₂+ from reaching the buried, allowing this method to determine the coverage of the TiOx coating. In summary, this portion of the dissertation demonstrated that CLD is a promising method for applying nanoscopic overlayers on high-surface electrodes.

Les procédés électrochimiques présentent beaucoup d’avantages dans le domaine du traitement et de la récupération de matière d’effluents industriels. Cependant, dans le cas de solutions diluées en ions métalliques, les électrodes classiques sont fortement limitées par leur efficacité ainsi que par leur taille. Dès lors, les électrodes poreuses, de par leur surface spécifique importante et de par leur structure particulière qui améliore le transport de matière et donc l’efficacité de l’électrode, représentent une alternative très intéressante aux électrodes classiques.<p><p>Parmi les électrodes poreuses, celles constituées de fibres métalliques semblent les plus prometteuses. L’objectif de ce travail est de donner les relations utiles pour dimensionner une cellule contenant ce type d’électrodes en vue du traitement d’effluents industriels contenant des ions métalliques. <p>Les électrodes étudiées ont été caractérisées par différentes techniques :microscopie électronique, méthode électrochimique, mesure de la perte de charge, conductimétrie, porosimétrie,… Cette caractérisation a permis de connaître la porosité, les surfaces spécifiques (géométrique, dynamique et électrochimique) et la tortuosité des électrodes. <p>Ensuite, le coefficient de transport de matière moyen a été étudié par une nouvelle méthode basée sur la mesure d’un rendement électrochimique. Cette méthode présente l’avantage de pouvoir travailler avec des vitesses de circulation de l’électrolyte compatibles avec celles utilisées industriellement. Pour cela, une cellule d’électrolyse à circulation forcée a été mise au point. <p><p>Afin de comprendre comment la géométrie d’une électrode poreuse de ce type influence le transport de matière local et la densité de courant et donc l’efficacité de l’électrode, le transport de matière et la densité de courant locale ont été modélisés autour d’un cylindre (représentatif d’une fibre) et validés par des mesures expérimentales. La modélisation s’est ensuite étendue à un réseau de fibres cylindriques représentatif des électrodes poreuses étudiées. Cette modélisation a permis d’obtenir une relation générale liant les nombres de Sherwood, de Reynolds et de Schmidt à des nombres sans dimension caractérisant la géométrie du réseau de fibres. Cette relation donne des résultats concordants avec ceux obtenus expérimentalement pour les électrodes poreuses étudiées.<p><p>Le volume utile d’une électrode poreuse dépend fortement des conditions expérimentales (concentration de l’électrolyte, vitesse de circulation, intensité du courant appliquée,…) et de la structure de l’électrode (porosité, surface spécifique,…). Ces paramètres influencent la distribution du potentiel et de la densité de courant dans l’électrode. Différents modèles de distribution sont comparés et appliqués aux électrodes poreuses étudiées. Cette distribution de courant influence le colmatage progressif de l’électrode poreuse en cours d’électrolyse. Il s’avère que l’électrode en contrôle diffusionnel (avec un rendement électrochimique faible) optimise la distribution du courant dans l’électrode et, de ce fait, ralenti son colmatage. De plus, travailler avec une solution diluée et une vitesse de circulation de l’électrolyte importante améliore la distribution du courant. Il en est de même si l’électrode poreuse présente une grande porosité et une faible surface spécifique.<p><p>Ce travail aura donc permis de proposer des relations indispensables pour le dimensionnement d’une cellule à électrodes poreuses (constituées de fibres métalliques) ainsi que les conditions opératoires idéales dans le cas du traitement d’effluents industriels contenant des ions métalliques./<p>Electrochemical techniques offer many advantages for the prevention of pollution problems in the industrial processes. However, flat electrodes are not ideal to treat dilute solutions containing metallic ions. With their high specific surface and open structure, which enhance mass transfer, porous electrodes are a good alternative for the treatment this kind of effluent. Fibre materials are particularly well suited as material for the production of porous electrodes. <p><p>The aim of this thesis is to study an electrochemical cell with a porous electrode in order to treat dilute metallic ions solutions and to provide dimensionless equations suited to scale-up the electrode for industrial application. <p><p>The porous electrodes, used in this thesis, are made of a stainless steel fibre network. The main properties and characteristics of these electrodes are studied by means of several techniques :electron microscopy, electrochemical methods (voltammetry, limiting current density measurerment), conductivity measurement, porosimetry, pressure drop measurement,… The obtained parameters are :porosity, specific surfaces (geometric, dynamic and electrochemical), fibres' diameter, tortuosity and the geometric disposition of the fibres in the electrodes. Mass transfer inside the porous electrodes is studied experimentally by a new developed method, linked to the measurement of the faradic yield as a function of different electrolysis parameters. For these measurements, an experimental electrolysis cell with high electrolyte flow rate has been designed and builds.<p><p>To understand how the geometry of the porous electrode influences the local and mean mass transfer coefficients and current densities, numerical studies and simulations have been performed. <p>The first type of simulation deals with a single wire (representative of a fibre from the porous electrode). <p>The second type of simulation deals with the integration of individual fibres in a fibre network. A correlation between dimensionless numbers such as Sherwood's, Reynolds' and Schmidt's numbers together with numbers characteristic of the electrode’s geometry has been established for Reynolds’s numbers ranging from 0,02 to 1,4. A good agreement between simulation and experimental measurements of mass transfer is observed.<p><p>The real effective electrochemical volume of the porous electrode depends on experimental conditions (current, concentration, flow velocity…) and electrode’s geometry (porosity, specific surface,…). These parameters influence the potential and current distribution inside the porous electrode. Several models of current distribution are applied to these electrodes and the theoretical simulations are compared with experimental measures. <p>As a result of these simulations, an electrode under diffusion control with a small faradic yield appears to be the best choice in order to homogenise the current density inside the porous electrodes. Dilute solutions, high flow velocity and electrodes with high porosity improve also the current density penetration inside the electrode. These observations are confirmed by an electrode’s plugging study.<p><p>In conclusion, this thesis provides mathematical relationships to scale-up a cell with porous electrodes of metallic fibre, and provides guidelines to treat, in an efficient manner industrial effluents containing metallic ions.<p><p> / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished