Fibrous porous media are widely used in various industries such as biomedical engineering, textiles, paper, and alternative energy. Often these porous materials are formed into composite materials, using subsequent manufacturing steps, to improve their properties. There is a strong correlation between system performance and the transport and mechanical properties of the porous media, in raw or composite form. However, these properties depend on the final pore structure of the material. Thus, the ability to manufacture fibrous porous media, in raw or composite forms, with an engineered structure with predictable properties is highly desirable for the optimization of the overall performance of a relevant system. To date, the characterization of the porous media has been primarily based on reverse design methods i.e., extracting the data from existing materials with image processing techniques. The objective of this research is to develop a methodology to enable the virtual generation of complex composite porous media with tailored properties, from the implementation of a fibrous medium in the design space to the simulated coating of this media representative of the manufacturing space. To meet this objective a modified periodic surface model is proposed, which is utilized to parametrically generate a fibrous domain. The suggested modeling approach allows for a high-degree of control over the fiber profile, matrix properties, and fiber-binder composition. Using the domain generated with the suggested geometrical modeling approach, numerical simulations are executed to simulate transport properties such as permeability, diffusivity and tortuosity, as well as, to directly coat the microstructure, thereby forming a complex composite material. To understand the interplay between the xxiii fiber matrix and the transport properties, the morphology of the virtual microstructure is characterized based on the pore size, chord length and shortest path length distributions inside the porous domain. In order to ensure the desired properties of the microstructure, the fluid penetration, at the micro scale, is analyzed during the direct coating process. This work presents a framework for feasible and effective generation of complex porous media in the virtual space, which can be directly manufactured.

A distinctive field in the coatings industry is the coating of porous media, with broad applications in paper, apparel, textile, electronics, bioengineering, filtration and energy sector. A primary industrial scale process that can be used to coat porous media in a fast and flexible manner is slot die extrusion. A major concern when coating porous media with a wetting fluid is fluid penetration into the substrate. Although some level of penetration is desirable to obtain specific material properties, inadequate or excessive fluid penetration can negatively affect the strength, functionality or performance of the resulting material. In spite of its apparent industrial importance, limited modeling and experimental work has been conducted to study fluid penetration into porous media during fabrication. The effects of processing parameters on the penetration depth, the effects of penetration on material quality, and the method to predict and control the penetration depth are not well understood. This dissertation is composed of two parts. Part I is an applied study for coating onto porous media. This part focuses on the first objective of this dissertation which is to elucidate clearly the feasibility, advantages and disadvantages of the direct coating method as a potential fabrication route for membrane electrode assembly (MEA). MEA samples are fabricated using both traditional and the direct coating methods. Then, the quality and performance of the MEA samples are examined. Experimental results in Part I demonstrate that it is feasible to fabricate MEAs using the direct coating method. However, Nafion® solution penetrates into the catalyst layer during the coating process and causes lower performance of fuel cells, which is the motivation for Part II of this thesis. The objective of Part II is to fundamentally understand the fluid penetration process and predict the penetration depth when directly coating porous media, using a comprehensive approach. A series of computational and analytical models are developed to predict the penetration depth for both Newtonian and non-Newtonian fluids with or without capillary pressure. Finally the accuracy of developed models are validated through experiments. The relative error between the predicted and experimentally measured penetration depth is generally lower than 20%.

Fuel cells are electrochemical energy conversion devices that convert chemical energy to electrical energy efficiently. Bipolar plates form an integral part of a fuel cell and their high manufacturing cost and low production rate have hindered the commercialization of fuel cells. Bipolar plates require high electrical conductivity, strength, chemical resistance and thermal conductivity. This thesis presents efforts to manufacture bipolar plates which meet these requirements using compression or injection molding. Compression or injection molding processes allow cost-effective, large-scale manufacturing of bipolar plates. A variety of material systems for the fabrication of bipolar plates are processed, molded and characterized. / text

Solid oxide fuel cells (SOFCs) are electrochemical energy conversion devices that directly transform the chemical energy of fuel into electrical energy. They generate electricity far more efficiently and with fewer emissions per megawatt-hour compared to any combustion-based power generation system. More remarkably, SOFCs can directly use hydrocarbon fuels without requiring external fuel reforming, employing low-cost Ni catalyst instead of noble-metal catalysts used for low-temperature fuel cells. However, the conventional SOFCs using Ni-based anodes fed with carbon-containing fuels have one pitfall; the carbon produced by hydrocarbon cracking is deposited on the Ni surface, thereby precluding the surface of the Ni-based anodes from being available for further fuel oxidation and consequently impeding SOFC operation. This dissertation focuses on overcoming this critical drawback to allow for the simultaneous use of Ni-based anodes and hydrocarbon fuels. Further work focuses on improving SOFC performance to provide the highest efficiencies possible. To boost the power densities of SOFCs, a novel, facile approach to modify the surface structure of anode powders and thereby enlarge the three-phase boundary (TPB) regions of anodes is presented. One such powder preparation method based on the electric charge variation of oxides depending upon the pH of the solution results in significantly extended TPB regions and thus a remarkable increase in power densities of SOFCs. Another method involves the formation of Ce₁₋[subscript x]Gd₁₋[subscript y]Ni[subscript x+y]VO₄₋[subscript delta] at the phase boundaries between NiO and Ce₀.₈Gd₀.₂O₁.₉ (GDC) by V⁵⁺-incorporation onto NiO surface; this method improves the microstructure of Ni-GDC-based anodes and considerably lowers GDC electrolyte sintering temperature, thereby enhancing the SOFC performance. With these high performance anodes, natural gas-fueled SOFCs are studied through two strategies to alleviate coking: incorporation of catalytic materials onto the Ni surface and the introduction of catalytic functional layers (CFLs) to the outer surface of an anode-supported single cell. Hydrogen tungsten bronze and hydroxylated Sn formed on the Ni surface provide hydroxyls for the deposited solid carbon, removing it from the anodes as CO₂. Moreover, the use of hydrophilic Sn or Sb-incorporated Ni-GDC CFLs prevents the anode from being exposed directly to hydrocarbon fuels and controls the solid carbon accumulation similarly to the former strategy. / text

Solid oxide fuel cells are electrochemical energy conversion devices utilizing solid electrolytes transporting O2- that typically operate in the 800 - 1000 °C temperature range due to the large activation barrier for ionic transport. Reducing electrolyte thickness or increasing ionic conductivity can enable lower temperature operation for both stationary and portable applications. This thesis is focused on the fabrication of free standing ultrathin (<100 nm) oxide membranes of prototypical O2- conducting electrolytes, namely Y2O3-doped ZrO2 and Gd2O3-doped CeO2. Fabrication of such membranes requires an understanding of thin plate mechanics coupled with controllable thin film deposition processes. Integration of free standing membranes into proof-of-concept fuel cell devices necessitates ideal electrode assemblies as well as creative processing schemes to experimentally test devices in a high temperature dual environment chamber. We present a simple elastic model to determine stable buckling configurations for free standing oxide membranes. This guides the experimental methodology for Y2O3-doped ZrO2 film processing, which enables tunable internal stress in the films. Using these criteria, we fabricate robust Y2O3-doped ZrO2 membranes on Si and composite polymeric substrates by semiconductor and micro-machining processes, respectively. Fuel cell devices integrating these membranes with metallic electrodes are demonstrated to operate in the 300 - 500 °C range, exhibiting record performance at such temperatures. A model combining physical transport of electronic carriers in an insulating film and electrochemical aspects of transport is developed to determine the limits of performance enhancement expected via electrolyte thickness reduction. Free standing oxide heterostructures, i.e. electrolyte membrane and oxide electrodes, are demonstrated. Lastly, using Y2O3-doped ZrO2 and Gd2O3-doped CeO2, novel electrolyte fabrication schemes are explored to develop oxide alloys and nanoscale compositionally graded membranes that are thermomechanically robust and provide added interfacial functionality. The work in this thesis advances experimental state-of-the-art with respect to solid oxide fuel cell operation temperature, provides fundamental boundaries expected for ultrathin electrolytes, develops the ability to integrate highly dissimilar material (such as oxide-polymer) heterostructures, and introduces nanoscale compositionally graded electrolyte membranes that can lead to monolithic materials having multiple functionalities. / Engineering and Applied Sciences

Today’s information-centered world leads the ever-increasing consumer demand for more powerful, multifunctional portable devices. Additionally, recent developments on long-lasting energy sources and compliant, flexible systems, have introduced new required features to the portable devices industry. For example, wireless sensor networks are in urgent need of self-sustainable, easy-to-deploy, mobile platforms, wirelessly interconnected and accessible through a cloud computing system. The objective of my doctoral work is to develop integration strategies to effectively fabricate mechanically flexible, energy-independent systems, which could empower sensor networks for a great variety of new exciting applications. The first module, flexible electronics, can be achieved through several techniques and materials. Our main focus is to bring mechanical flexibility to the state-of-the-art high performing silicon-based electronics, with billions of ultra-low power, nano-sized transistors. Therefore, we have developed a low-cost batch fabrication process to transform standard, rigid, mono-crystalline silicon (100) wafer with devices, into a thin (5-20 m), mechanically flexible, optically semi-transparent silicon fabric. Recycling of the remaining wafer is possible, enabling generation of multiple fabrics to ensure lowcost and optimal utilization of the whole substrate. We have shown mono, amorphous and poly-crystalline silicon and silicon dioxide fabrics, featuring industry’s most advanced high-/metal-gate based capacitors and transistors. The second module consists on the development of efficient energy scavenging systems. First, we have identified an innovative and relatively young technology, which can address at the same time two of the main concerns of human kind: water and energy. Microbial fuel cells (MFC) are capable of producing energy out the metabolism of bacteria while treating wastewater. We have developed two micro-liter MFC designs, one with carbon nanotubes (CNT)-based anode and the second with a more sustainable design and easy to implement. Power production ranges from 392 to 100 mW/m3 depending on design. Additionally we have explored a flexible thermoelectric generator (0.139 μW/cm2) and a lithium-ion battery (~800 μAh/m2) for back-up energy generation and storage. Future work includes the implementation of a self-powered System-on-Package which gathers together energy generation, storage and consumption. Additionally we are working to demonstrate Complementary Metal-Oxide-Semiconductor (CMOS) circuitry on our flexible platform, as well as memory systems.

Several groups are studying and researching major factors which influence high temperature proton exchange membrane fuel cells. These factors include material type, temperature, and fuel cell lifespan. Only a few groups research the optimization of the size of the fuel channels within the fuel cell. For channel optimization, a model was created to find the optimum flow channel and rib widths. The approach used was to code the losses due to activation, concentration, and ohmic polarizations to yield the fuel cell voltage and power expected from the fuel cell itself. The model utilizes the specified cell parameters including the material properties, fuel cell temperature, and channel size. This method gives an initial view of how a fuel cell will perform given specific parameters. It is not limited to one fuel cell size, allowing future research efforts to utilize this model to optimize flow channels in a variety of fuel cells.

Fuel cell is one of the most promising clean energy and alternative energy technologies due to its advantages of low emissions and high efficiency. One application of the fuel cell technology is onboard auxiliary power units (APUs) for power generation in aircrafts, ships, and automobiles. In order to supply hydrogen or syngas for the fuel cell APUs, onboard fuel processing technology was proposed to convert hydrocarbon fuels into syngas through reforming reactions. Two major tasks need to be completed in onboard fuel processing technology. Firstly sulfur compounds have to be removed from hydrocarbon fuels because sulfur can cause reforming catalyst deactivation and fuel cell electrodes poisoning problems. Secondly hydrogen and carbon monoxide shall be produced by reforming of hydrocarbon fuels at a high energy conversion efficiency. This dissertation focused on onboard fuel processing of Jet-A fuel to produce hydrogen and syngas for solid oxide fuel cell (SOFC) APUs. Jet-A fuel was studied because it is the logistic fuel commonly used for civilian airplanes and military heavy duty trucks. Ultra-deep adsorptive desulfurization of Jet-A fuel from over 1,000 ppmw to below 50 ppmw, and autothermal reforming of n-dodecane as a Jet-A fuel surrogate as well as the real desulfurized Jet-A fuel to produce syngas have been systematically investigated in the present study. For the adsorptive desulfurization of Jet-A fuel, a novel NiO-CeO₂/A1₂O₃-SiO₂ adsorbent was proposed and prepared in-house for experimental tests. The sulfur adsorption kinetic characteristic and isotherm at equilibrium were studied in batch tests, and the dynamic desulfurization performance of the adsorbent was investigated in fixed bed tests. Fixed bed tests operation conditions including liquid hourly space velocity (LHSV), adsorbent particle size, and fixed bed dimensions were optimized to achieve the highest adsorbent sulfur adsorption capacity. For the reforming of Jet-A fuel, autothermal reforming (ATR) method was employed and a bimetallic NiO-Rh catalyst was synthesized for the ATR reactions. A lab-scale 2.5 kWt autothermal reforming system including the reformer and balance-of-plant was designed, fabricated, integrated and tested. The reforming system performances at various operation conditions were compared. Reformer operation temperature, steam to carbon ratio and oxygen to carbon ratio, as well as pre-heating temperatures for fuel, air and steam were optimized based on system energy conversion efficiency, H₂ selectivity and COₓ selectivity.

An investigation was conducted to determine and compare the effect of cell current pulsing and "self-oxidation" in increasing the CO tolerance of a PEM fuel cell. The most effective pulsing parameter values were also determined. Current pulsing involves periodically demanding positive current pulses from the fuel cell to create an anode over-potential, while "self-oxidation" or sustained potential oscillations is achieved when the anode catalyst becomes so saturated with CO that the anode over-potential increases to a value at which CO is oxidized from the catalyst surface. The CO tolerance of a fuel cell system with a Pt-Ru anode was tested using 50 and 496 ppm CO in the anode fuel. The performance of the system declined with an increase in CO concentration. Current pulses of various amplitude, frequency, and duty cycle were applied to the cell while CO was present in the anode fuel. With 50 ppm CO in the anode fuel, the most effective pulse in increasing CO tolerance while maintaining normal cell operation was 1.0 A/cm2, 0.25 Hz, and a 5% duty cycle. A pulse (120 Hz, 50% duty cycle) similar to the ripple current often generated when converting DC to single-phase 60 Hz AC had a positive effect on the CO tolerance of the system, but at frequencies that high, the pulse duration was not long enough to completely oxidize the CO from the catalyst surface. With 496 ppm CO in the anode fuel, a pulse of 1.0 A/cm2, 0.5 Hz, and a 20% duty cycle proved most effective. When the cell was exposed to 496 ppm CO, without employing pulsing, "self-oxidation" occurred and CO was periodically oxidized from the catalyst surface. However, pulsing allowed the cell to operate at the desired voltage and power a higher percentage of the time than "self-oxidation"; hence, pulsing was more effective.

Solid oxide fuel cells (SOFCs) that operate directly on hydrocarbon fuels eliminate the requirement for costly and complex external reforming systems. Atmospheric plasma spraying (APS) is an established manufacturing method that offers the potential to fabricate direct oxidation SOFC anodes in a single step, instead of the multiple steps currently required. Manufacturing by APS also allows the use of metal supports, which improve thermal shock resistance, allow rapid cell heat-up, and reduce total cost. In this study, direct oxidation SOFC anodes based on Cu and samaria-doped ceria (SDC) in combination with Co and/or Ni were investigated for their stability and performance in H2 and CH4 when plasma sprayed on ferritic stainless steel supports. Several different APS techniques were investigated. Two of these techniques were hybrid methods involving a combination of dry powder plasma spray and suspension plasma spray (SPS) processes. These techniques were proposed to help balance the degree of melting of the lower melting temperature oxides of the metals Cu, Co, and Ni with that of the higher melting temperature SDC. The use of a single suspension containing all of the anode component feedstocks was also investigated. Multi-component aqueous suspensions of CuO, Co3O4, and NiO were developed with or without the addition of carbon black and SDC. It was found that the use of a hybrid plasma spray technique can help to improve deposition efficiency (D.E.) and enhance partial melting of the low melting temperature feedstocks. However, plasma spraying all of the components in a single suspension can lead to more homogeneous mixing and greater resistance to metal coarsening at SOFC operating temperatures. In electrochemical tests of plasma-sprayed metal-supported cells containing these anodes, peak power densities as high as 0.6 W/cm2 were achieved at 750 deg C in humidified H2. In CH4, power density was limited by the activity of the anodes. Stability in CH4 was poor because of oxidation of the metal support and enhanced coking behaviour resulting from interactions between Fe in the support and Co and Ni in the anodes. When separated from the supports, the anodes demonstrated very low coking rates in thermogravimetric analysis experiments in CH4.