9 research outputs found

    Optimization of Direct Methanol Fuel Cells: An Experimental Approach

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    Recently, fuel cells have gained significant attention for their capability of producing power with different fuels at reduced levels of carbon dioxide emissions. Of the many options of fuel cells, direct methanol fuel cells (DMFCs) are considered promising candidates for stationary and small portable power applications. However, there are numerous technical barriers preventing more widespread use of DMFCs, primarily the crossing over of unreacted fuel through the membrane and the slow reaction kinetics on the anode. This work provides a comprehensive experimental approach to optimizing the cell as a whole. First, various methods of reducing fuel crossover are considered. Then, various anode catalysts are evaluated for performance characteristics. The cathode is also considered through the use of platinum metal group (PGM) free catalysts. Finally, the fabrication of the membrane electrode assembly (MEA) is optimized by examining various methods of catalyst ink deposition on the substrate. By taking a comprehensive approach, this work provides a pathway for the fabrication of DMFCs capable of enhanced power densities and reduced fuel crossover by using a variety of techniques

    Membrane and Membrane Reactors Operations in Chemical Engineering

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    This Special Issue is aimed at highlighting the potentialities of membrane and membrane reactor operations in various sectors of chemical engineering, based on application of the process intensification strategy. In all of the contributions, the principles of process intensification were pursued during the adoption of membrane technology, demonstrating how it may lead to the development of redesigned processes that are more compact and efficient while also being more environmental friendly, energy saving, and amenable to integration with other green processes. This Special Issue comprises a number of experimental and theoretical studies dealing with the application of membrane and membrane reactor technology in various scientific fields of chemical engineering, such as membrane distillation for wastewater treatment, hydrogen production from reforming reactions via inorganic membrane and membrane photoassisted reactors, membrane desalination, gas/liquid phase membrane separation of CO2, and membrane filtration for the recovery of antioxidants from agricultural byproducts, contributing to valorization of the potentialities of membrane operations

    Efficient PEM fuel cells for portable applications

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    Fuel cells (FCs) have attracted considerable attention for replacing batteries in many electronic devices. Among different types of fuel cells, the proton exchange membrane fuel cell (PEMFC) is most widely investigated. Air-breathing proton exchange membrane fuel cells (AB-PEMFCs) have received more attention in recent years because of the simplifications in the fuel cell system, which makes it a good choice for portable applications. However the simplifications also causes a rather low performance and this is attributed to the low mass and heat transfer coefficients. In this thesis, a mathematical model has been developed in order to investigate the overall performance of the fuel cell system, and the local performances of two important components, i.e. the gas channel (GC) and gas diffusion layer (GDL) have been studied by a CFD model and a gas permeability experiment, respectively. The mathematical model presented in this thesis is based on the conservation of the mass and heat transfer in order to investigate the effects of the different parameters on the fuel cell overall performance. A new revised water transport relation is applied in this model, which makes it possible to study the effect of the hydrogen relative humidity (RH). The results show that, among all the different operating parameters, the hydrogen RH can significantly improve the performance of AB-PEMFCs and the GDL is an important component in improving the transport and water management issues. In addition, a computational fluid dynamics (CFD) model for the anode channels in AB-PEMFCs is developed by employing the Volume of Fluid (VOF) method. The dynamics of the liquid water are studied under different flooding conditions. The modelling results show that the initial position of the accumulated droplet and the hydrogen velocity have little effect while the droplet size and the channel wettability can largely influence the local performance in the channels, e.g. the water removal time and the pressure drop. Also it is found that the trade-off between the pressure drop and the removal time should be considered when designing practical products. Further, the GDL thickness is found to be important in determining the performance of the AB-PEMFCs in the modelling work. In order to produce the GDLs with different thicknesses, an experimental investigation has been conducted to study the effect of the stacking of single GDL layers, and the through-plane gas permeability is investigated, which is one of the most important properties of GDLs. Compared with previous studies, the gas permeability of the GDL stacks is investigated instead of a single GDL layer. The calculation results show that the stacking of layers has only a small influence on the overall gas permeability of the GDL stack. In addition, a tighter contact between each layer in the GDL stacks is found to increase the overall gas permeability of the GDL stacks

    Specialized Inter-Particle Interaction Lbm For Patterned Superhydrophobic Surfaces

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    SPECIALIZED INTER-PARTICLE INTERACTION LBM FOR PATTERNED SUPERHYDROPHOBIC SURFACES by AMAL S. YAGUB ABSTRACT: Superhydrophobic surface characteristics are important in many industrial applications, ranging from the textile to the military. It was observed that surfaces fabricated with nano/micro roughness can manipulate the droplet contact angle, thus providing an opportunity to control the droplet wetting characteristics. The Shan and Chen (SC) lattice Boltzmann model (LBM) is a good numerical tool, which holds strong potentials to qualify for simulating droplets wettability. This is due to its realistic nature of droplet contact angle (CA) prediction on flat smooth surfaces. But SC-LBM was not able to replicate the CA on rough surfaces because it lacks a real representation of the physics at work under these conditions. By using a correction factor to influence the interfacial tension within the asperities, the physical forces acting on the droplet at its contact lines were mimicked. This approach allowed the model to replicate some experimentally confirmed Wenzel and Cassie wetting cases. Regular roughness structures with different spacing were used to validate the study using the classical Wenzel and Cassie equations. This work highlights the strength and weakness of the SC model and attempts to qualitatively conform it to the fundamental physics, which causes a change in the droplet apparent contact angle, when placed on nano/micro structured surfaces. In the second part of this work, the model is used also to analyze the sliding of droplets in contact with flat horizontal surfaces. This part identifies the main factors, which influence the multiphase fluids transport in squared channels. Effects of dimensionless radius, Weber number, Reynolds number and static contact angles are evaluated by calculating the power required for moving single droplets in comparison to the power needed for moving the undisturbed flow in the channel. Guidelines for optimizing the design of such flow are presented. In last part of work, the sliding of droplets on sloped surfaces with and without roughness is numerically investigated. The Shan and Chen (SC) Lattice Boltzmann model (LBM) is used to analyze the effect of pinning on the movement of droplets placed on sloped surfaces. The model is checked for conformance with the Furmidge equation which applies to tilted unstructured surfaces. It is shown that a droplet sliding on a perfectly smooth surface requires very minimal slope angle and that pinning due to the inhomogeneous nature of manufactured smooth surfaces is the key factor in determining the minimal slope angle. The model is also used on sloped rough surfaces to check the effects of roughness on the movement of single droplets. The numerical outcomes are compared with published experimental results for validation and a dimensionless number is suggested for quantifying the degree of pinning needed to control the behavior of sliding droplets on sloped surfaces

    Energy analysis and fabrication of photovoltaic thermal water electrolyzer and ion transport through modified nanoporous membranes

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    Hydrogen is an environmentally sustainable energy carrier that can be stored. It is not found naturally and therefore must be artificially produced. We can obtain hydrogen from renewable energy, such solar and wind energy, which is environmentally clean. One such a promising options is via electrolysis using electricity from a photovoltaic generator. In the first part of the dissertation we studied a microfluidic energy conversion device to produce hydrogen. Particularly, we proposed a new integrated system – a so-called “photovoltaic thermal water electrolyzer (PVTE)” – which consists of PV cells positioned on top of a planar micro-water electrolyzers in order to harness waste heat as a storable form of energy. The concept of PVTE has the outputs such as electricity and thermal storage, and also it provides hydrogen production efficiently. First, we provided a comprehensive analysis of the overall efficiency of the PVTE system. COMSOL Multiphysics software was used to predict the temperatures for the electrolyte and the PV cells operating at various temperatures and solar fluxes. Moreover, hourly and monthly efficiency analyses were accomplished for Phoenix, AZ in the year 2010. This new integrated approach is advantageous over conventional PV modules (Chapter 2). Second, we fabricated a micro-water electrolyzer which utilizes heat from PV cell and works as a heat sink in order to eliminate additional energy input for electrolysis in order to operate at elevated temperatures. We also presented electrode preparation and fabrication of the electrolyzer. The increase in the hydrogen production rate affirms the predictions of our system that utilizes waste heat from PV (Chapter 3). Finally, we successfully fabricated a new water electrolyzer including hydrophobic porous membrane. This new design allows us to manage gas production and collection in the chamber. By using this method, we are able to collect gases on the top of the electrolyzer at low flow rates at elevated temperatures (Chapter 4). Nanoporous membranes have received great attention in the fields of water desalination, biosensing, and chemical separations. Bare nanopores can be used as size-selective filters but if the surface chemistry of a nanopore is modified by coating it with another substance, however, enhanced separations based other properties can be achieved. Many studies have been performed on ion permselectivity across gold-coated charged surfaces and charged nanopores. In the second part of the dissertation, a focus of interfacial transport phenomena is proposed in order to achieve improved- charge selective nanofluidic systems. There have been numerous studies on the quality of organic SAMs as a blocking mechanism for prevention of ion adsorption. First, we investigated the electrochemical interfacial properties of a well-ordered SAM of 1-undecanethiol (UDT) on evaporated gold surface by EIS in electrolytes without a redox couple. Using a constant phase element (CPE) series resistance model, prolonged incubation times (up to 120 h) show decreasing monolayer capacitance approaching the theoretical value for 1-undecanethiol (Chapter 6). Secondly, we fabricated a membrane permeate flow cell is described with the aim of studying the transport of methyl viologen (paraquat, MV2+) and napathalenedisulfonate disodium salt (NDS2-), using a conductive NCAM. A polycarbonate track etched (PCTE) membrane was made conductive by sputter coating gold on the membrane surface. Transport studies were done in a voltage range in which faradaic current was minimized at the surface of the gold-coated NCAMs. The goal of the transport studies is to demonstrate improved charge selectivity when a well-grown 1-undecanethiol monolayer is assembled at the surface of the NCAM for a wide range of applied potentials (-400 mV < Vappl < 400 mV). Results show the selectivity of charged analytes through the metallized NCAM can be improved by functionalizing the surface with a self-assembled monolayer (SAM). The selectivity coefficients for MV2+ and NDS2- increased with functionalization of undecanethiol on the gold-coated NCAM surface (Chapter 7)

    Passive Gas-Liquid Separation Using Hydrophobic Porous Polymer Membranes: A Study on the Effect of Operating Pressure on Membrane Area Requirement

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    The use of hydrophobic porous polymer membranes to vent unwanted gas bubbles from liquid streams is becoming increasingly more common in portable applications such as direct methanol fuel cells (DMFCs) and micro-fluidic cooling of electronic circuits. In order for these portable systems to keep up with the ever increasing demand of the mobile user, it is essential that auxiliary components, like gas-liquid separators (GLS), continue to decrease in weight and size. While there has been significant progress made in the field of membrane-based gas-liquid separation, the ability to miniaturize such devices has not been thoroughly addressed in the available literature. Thus, it was the purpose of this work to shed light on the scope of GLS miniaturization by examining how the amount porous membrane required to completely separate gas bubbles from a liquid stream varies with operating pressure. Two membrane characterization experiments were also employed to determine the permeability, k, and liquid entry pressure (LEP) of the membrane, which provided satisfying results. These parameters were then implemented into a mathematical model for predicting the theoretical membrane area required for a specified two-phase flow, and the results were compared to experimental values. It was shown that the drastically different surface properties of the wetted materials within the GLS device, namely polytetrafluoroethylene (PTFE) and acrylic, caused the actual membrane area requirement to be higher than the theoretical predictions by a constant amount. By analyzing the individual effects of gas and liquid flow, it was also shown that the membrane area requirement increased significantly when the liquid velocity exceeded an amount necessary to cause the flow regime to transition from wedging/slug flow to wavy/semi-annular flow

    Effects of Anode Flow Field Design on CO2 Bubble Behavior in μDMFC

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    Clogging of anode flow channels by CO2 bubbles is a vital problem for further performance improvements of the micro direct methanol fuel cell (μDMFC). In this paper, a new type anode structure using the concept of the non-equipotent serpentine flow field (NESFF) to solve this problem was designed, fabricated and tested. Experiments comparing the μDMFC with and without this type of anode flow field were implemented using a home-made test loop. Results show that the mean-value, amplitude and frequency of the inlet-to-outlet pressure drops in the NESFF is far lower than that in the traditional flow fields at high μDMFC output current. Furthermore, the sequential images of the CO2 bubbles as well as the μDMFC performance with different anode flow field pattern were also investigated, and the conclusions are in accordance with those derived from the pressure drop experiments. Results of this study indicate that the non-equipotent design of the μDMFC anode flow field can effectively mitigate the CO2 clogging in the flow channels, and hence lead to a significant promotion of the μDMFC performance

    Membrane Electrode Assembly (MEA) Design for Power Density Enhancement of Direct Methanol Fuel Cells (DMFCs)

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    Micro-direct methanol fuel cells (micro-DMFC) can be the power supply solution for the next generation of handheld devices. The applications of the micro-DMFCs require them to have high compactness, high performance, light weight, and long life. The major goal of this research project is to enhance the volumetric power density of direct methanol fuel cells (DMFCs). A performance roadmap has been formulated and showed that patterning the planar membrane electrode assembly (MEA) to 2-D and 3-D corrugated manifolds can greatly increase the power generation with very modest overall volume increases. In this project, different manufacturing processes for patterning MEAs with corrugations have been investigated. A folding process was selected to form 2D triangular corrugations on MEAs for experimental validations of the performance prediction. The experimental results show that the volumetric power densities of the corrugated MEAs have improved by about 25% compared to the planar MEAs, which is lower than the expected performance enhancement. ABAQUS software was used to simulate the manufacturing process and identify the causes of deformations during manufacture. Experimental analysis methods like impedance analysis and 4 point-probes were used to quantify the performance loss and microstructure alteration during the forming process. A model was proposed to relate the expected performance of corrugated MEAs to manufacturing process variables. Finally, different stacking configurations and issues related to cell stacking for corrugated MEAs are also investigated.Ph.D.Committee Chair: David Rosen; Committee Member: Comas Haynes; Committee Member: Meilin Liu; Committee Member: Peter Hesketh; Committee Member: Seyed Ghiaasiaa
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