In recent years a significant portion of proton exchange membrane fuel cell (PEMFC) work has been focused on understanding and optimizing the functions of the microporous layer (MPL). Researchers have found that including this layer, composed of carbon black and TeflonTM (PTFE), between the gas diffusion layer (GDL) and catalyst layer (CL) of PEMFCs improves performance. The major benefit of the MPL in conventional fuel cells is that it improves water management and reduces contact resistances between cell layers. Although the functions of the MPL in conventional PEMFCs are well understood, the essential functions and optimal formulation of the layer in planar PEMFCs which operate without stack compression, are for the most part unknown. This work determines the essential functions and optimal composition, loading and sintering pressure of the MPL in a planar fuel cell design called a Ribbon Fuel Cell. Adhesion as well as performance data were gathered to determine the essential functions and formulation of the MPL which leads to high performance and durability in Ribbon Fuel Cells. Statistical models were created based on performance data of cells constructed with various MPLs; and a MPL composed of 45 wt% PTFE, loaded at 3.5 mg/cm° and sintered between 20 and 40 psi was found to exhibit optimal performance and durability. The reason why such a high PTFE content yields optimal results is because it strengthens the MPL, allowing it to successfully join various cell layers together, a function that is essential in Ribbon Cells which operate without external stack compression. / Master of Science

Research and prototyping of a fuel cell stack system for micro aerial vehicles (MAVs) was conducted by Virginia Tech in collaboration with Luna Innovations, Inc, in an effort to replace the lithium battery technology currently powering these devices. Investigation of planar proton exchange membrane (PEM) and direct methanol (DM) fuel cells has shown that these sources of power are viable alternatives to batteries for electronics, computers, and automobiles. However, recent investigation about the use of microtubular fuel cells (MTFCs) suggests that, due to their geometry and active surface areas, they may be more effective as a power source where size is an issue. This research focuses on hydrogen MTFCs and how their size and construction within a stack affects the power output supplied to a MAV, a small unmanned aircraft used by the military for reconnaissance and other purposes. In order to conduct this research effectively, a prototype of a fuel cell stack was constructed given the best cell characteristics investigated, and the overall power generation system to be implemented within the MAV was modeled using a computer simulation program. The results from computer modeling indicate that the MTFC stack system and its balance of system components can eliminate the need for any batteries in the MAV while effectively supplying the power necessary for its operation. The results from the model indicate that a hydrogen storage tank, given that it uses sodium borohydride (NaBH4), can fit inside the fuselage volume of the baseline MAV considered. Results from the computer model also indicate that between 30 and 60 MTFCs are needed to power a MAV for a mission time of one hour to ninety minutes, depending on the operating conditions. In addition, the testing conducted on the MTFCs for the stack prototype has shown power densities of 1.0, an improvement of three orders of magnitude compared to the initial MTFCs fabricated for this project. Thanks to the results of MTFC testing paired with computer modeling and prototype fabrication, a MTFC stack system may be possible for implementation within an MAV in the foreseeable future. / Master of Science

Microbial fuel cells (MiFCs) have been suggested as a means to harness energy that is otherwise unutilized during the wastewater treatment process. MiFCs have the unique ability to treat influent waste streams while simultaneously generating power which can offset energy associated with the biological treatment of wastewater. During the oxidation of organic and inorganic wastes, microorganisms known as exoelectrogens have the ability to move electrons extracellularly. MiFCs generate electricity by facilitating the microbial transfer of these electrons from soluble electron donors in feedstocks to a solid-state anode. While MiFCs are a promising renewable energy technology, current systems suffer from low power densities which hinder their practical applicability. In this study, a novel anode design using flame-deposited carbon nanostructures (CNSs) on stainless steel mesh is developed to improve the electron transfer efficiency of electrons from microorganisms to the anode and thus the power densities achievable by MiFCs. These new anodes appear to allow for increased biomass accumulation on the anode and may aid in the direct transfer of electrons to the anode in mediatorless MiFC systems. Experiments were conducted using anaerobic biomass in single-chamber MiFCs with CNS-enhanced and untreated stainless steel anodes. Fuel cells utilizing CNS-enhanced anodes generated currents up to two orders of magnitude greater than cells with untreated metal anodes, with the highest power density achieved being 510 mW m-2. / Master of Science

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No description available.

Due to concerns about energy security, rising oil prices, and adverse effects of internal combustion engine vehicles on the environment, the automotive industry is quickly moving towards developing efficient “green” vehicles. Fuel cell-powered vehicles offer high efficiency and practically zero emissions. The main obstacles for widespread commercial production of fuel cell vehicles are high cost and short lifetime of fuel cell stacks, lack of a hydrogen infrastructure, and generation of hydrogen in an environmentally-friendly manner and its storage. Using actual fuel cells and actual vehicular loads in the study of fuel cell vehicular systems can be prohibitive due to cost (initial and running) and safety issues. It is very desirable to have a test station that emulates a vehicle with a high degree of accuracy and flexibility to alleviate cost and safety issues. This thesis proposes a design for a test station that emulates the drive train of a typical fuel cell-powered vehicle that is equipped with regenerative braking capability. As part of the test station, a fuel cell emulator is designed and validated through simulation based on the Nexa Fuel Cell power module manufactured by Ballard Power Systems. As another building block for the test station, a bi-directional controllable DC load is developed that can realize a given drive cycle for the scaled-down version of a given vehicle. The load allows simulation of regenerative braking capability. The performance of the load is validated through simulation. A DC-DC boost converter for controlling the fuel cell power, as well as an energy storage system for assisting the fuel cell in providing the required power during high-demand periods, are incorporated into the proposed test station. Simulation results are used to show that the test station is capable of simulating the real-life conditions experienced by actual fuel cell vehicles on the road. The test station, when realized by hardware, can be used for performing a wide range of studies on the drive train architecture and power management of fuel cell vehicles.

Polymer electrolyte membrane (PEM) fuel cells have been viewed as promising power source candidates for transport, stationary, and portable applications due to their high efficiency and low emissions. The platinum is the most commonly used catalyst material for the oxygen reduction reaction (ORR) at the cathode of PEM fuel cells; however, the limited abundance and high cost of platinum hinder the large-scale commercialization of fuel cells. To overcome this limitation, it is necessary to enhance the catalyst utilization in order to improve the catalytic activity while decreasing or eliminating the use of platinum. The material on which the catalyst is supported is important for the high dispersion and narrow distribution of Pt nanoparticles as well as other non-precious metal active sites, and these characteristics are closely related to electrocatalytic activity of the catalysts. The support materials can influence the catalytic activity by interplaying with catalytic metals, and the durability of the catalyst is also greatly dependent on its support. A variety of support materials like carbons, oxides, carbides, and nitrides have been employed as supports materials for fuel cell catalysts, and much effort has been devoted to the synthesis of the novel carbon supports with large surface area and/or pore volume, including nanostructured carbons such as carbon nanotubes (CNTs), carbon nanofibers, and mesoporous carbon. These novel nanostructured carbon materials have achieved promising performance in terms of catalytic activity and durability. However, there is still enormous demand and potential for the catalysts to improve. In the first study, non-precious metal catalysts (NPMC) for the oxygen reduction reaction were synthesized by deposition of Fe/Co-Nx composite onto nanoporous carbon black with ethylenediamine (EDA) as nitrogen precursor. Two different nanoporous carbon supports, Ketjen Black EC300J (KJ300) and EC600JD (KJ600), were used as catalyst support for the non-precious catalysts. The results obtained from the optimized FeCo/EDA-carbon catalyst, using KJ600 as the support, showed improved onset, half-wave potentials and superior selectivity than that of the KJ300. Similarly, the catalyst showed good performance in the hydrogen-oxygen PEM fuel cell. At a cell voltage of 0.6 V the fuel cell managed to produce 0.37 A/cm2 with a maximum power density of 0.44 W/cm2. Fuel cell life test at a constant voltage of 0.40 V demonstrated promising stability up to 100 h. The X-ray photoelectron spectroscopy study indicated that pyridinic type nitrogen of the non-precious metal catalysts is critical for ORR catalytic activity and selectivity. These results suggest higher pore volume and surface area of carbon support could lead to higher nitrogen content providing more active sites for ORR and this type of catalyst has great potential used as a non-precious PEM fuel cell catalyst. In the second study, we report the development of a novel NPMC in acid electrolyte using pyrimidine-2,4,5,6-tetramine sulfuric acid hydrate (PTAm) as a nitrogen precursor and graphene nanosheets as catalyst supports. Graphene, consisting of a two-dimensional (2D) monolayer of graphitic carbon atoms, has been viewed as a promising candidate for the fuel cell catalyst support, due to its many intriguing properties such as high aspect ratios, large surface areas, rich electronic states, good electron transport, thermal/chemical stability and good mechanical properties. We investigate the effect of different pyrolysis temperatures on the catalysts’ ORR activity along with detailed surface analysis to provide insight regarding the nature of the ORR active surface moieties. This novel NPMC demonstrates promising electrocatalyst activity and durability superior to that of commercial catalyst for the ORR, rendering graphene nanosheets as a suitable replacement to traditional nanostructured carbon support materials. In the final study, we have developed Pt catalyst by combining the precious metal with nitrogen-doped activated graphene (N-AG) as the support. A transmission electron microscopy (TEM) image of the catalyst shows uniform size and distribution of platinum nanoparticles on a graphene layer. This novel catalyst demonstrates superior electrocatalyst activity and durability over Pt/XC72 catalyst for ORR under the studied conditions, rendering graphene as an ideal replacement to traditional nanostructured carbon support materials. In summary, several catalyst samples were made using novel nanostructured support materials to improve the ORR performance. Several recommendations for future work were suggested in the last section of this work to further apply the knowledge and understanding of nanostructured support materials to design a highly active, durable, and low-cost NPMCs and platinum catalysts.

Due to concerns about energy security, rising oil prices, and adverse effects of internal combustion engine vehicles on the environment, the automotive industry is quickly moving towards developing efficient “green” vehicles. Fuel cell-powered vehicles offer high efficiency and practically zero emissions. The main obstacles for widespread commercial production of fuel cell vehicles are high cost and short lifetime of fuel cell stacks, lack of a hydrogen infrastructure, and generation of hydrogen in an environmentally-friendly manner and its storage. Using actual fuel cells and actual vehicular loads in the study of fuel cell vehicular systems can be prohibitive due to cost (initial and running) and safety issues. It is very desirable to have a test station that emulates a vehicle with a high degree of accuracy and flexibility to alleviate cost and safety issues. This thesis proposes a design for a test station that emulates the drive train of a typical fuel cell-powered vehicle that is equipped with regenerative braking capability. As part of the test station, a fuel cell emulator is designed and validated through simulation based on the Nexa Fuel Cell power module manufactured by Ballard Power Systems. As another building block for the test station, a bi-directional controllable DC load is developed that can realize a given drive cycle for the scaled-down version of a given vehicle. The load allows simulation of regenerative braking capability. The performance of the load is validated through simulation. A DC-DC boost converter for controlling the fuel cell power, as well as an energy storage system for assisting the fuel cell in providing the required power during high-demand periods, are incorporated into the proposed test station. Simulation results are used to show that the test station is capable of simulating the real-life conditions experienced by actual fuel cell vehicles on the road. The test station, when realized by hardware, can be used for performing a wide range of studies on the drive train architecture and power management of fuel cell vehicles.

Metal air battery has captured the spotlight recently as a promising class of sustainable energy storage for the future energy systems. Metal air batteries offer many attractive features such as high energy density, environmental benignity, as well as ease of fuel storage and handling. In addition, wide range of selection towards different metals exists where different energy capacity can be achieved via careful selection of different metals. The most energy dense systems of metal-air battery include lithium-air, aluminum-air and zinc-air. Despite the choice of metal electrode, oxygen reduction (ORR) occurs on the air electrode and oxidation occurs on the metal electrode. The oxidation of metal electrode is a relatively facile reaction compared to the ORR on the air electrode, making latter the limiting factor of the battery system. The sluggish ORR kinetics greatly affects the power output, efficiency, and lifetime of the metal air battery. One solution to this problem is the use of active, affordable and stable catalyst to promote the rate of ORR. Currently, platinum nanoparticles supported on conductive carbon (Pt/C) are the best catalyst for ORR. However, the prohibitively high cost and scarcity of platinum raise critical issues regarding the economic feasibility and sustainability of platinum-based catalysts. Cost reduction via the use of novel technologies can be achieved by two approaches. The first approach is to reduce platinum loading in the catalyst formulation. Alternatively platinum can be completely eliminated from the catalyst composition. The aim of this work is to identify and synthesize alternative catalysts for ORR toward metal air battery applications without the use of platinum re other precious metals (i.e., palladium, silver and gold). Non-precious metal catalysts (NPMC) have received immense international attentions owing to the enormous efforts in pursuit of novel battery and fuel cell technologies. Different types of NPMC such as transition metal alloys, transition metal or mixed metal oxides, chalcogenides have been investigated as potential contenders to precious metal catalysts. However, the performance and stability of these catalysts are still inferior in comparison. Nitrogen-doped carbon materials (NCM) are an emerging class of catalyst exhibiting great potential towards ORR catalysis. In comparison to the metal oxides, MCM show improved electrical conductivity. Furthermore, NCM exhibit higher activity compared to chalcogenides and transition metal alloys. Additional benefits of NCM include the abundance of carbon source and environmental benignity. Typical NCM catalyst is composed of pyrolyzed transition metal macrocycles supported by high surface area carbon. These materials have demonstrated excellent activity and stability. However, the degradation of these catalysts often involves the destruction of active sites containing the transition metal centre. To further improve the durability and mass transport of NCM catalyst, a novel class of ORR catalyst based on nitrogen-doped carbon nanotubes (NCNT) is investigated in a series of studies. The initial investigation focuses on the synthesis of highly active NCNT using different carbon-nitrogen precursors. This study investigated the effect of using cyclic hydrocarbon (pyridine) and aliphatic hydrocarbon (ethylenediamine) towards the formation and activity of NCNT. The innate structure of the cyclic hydrocarbon promotes the formation of NCNT to provide higher product yield; however, the aliphatic hydrocarbon promotes the formation of surface defects where the nitrogen atoms can be incorporated to form active sites for ORR. As a result, a significant increase in the ORR activity of 180 mV in half-wave potential is achieved when EDA was used as carbon-nitrogen precursor. In addition, three times higher limiting current density was observed for the NCNT synthesized from ethylenediamine. Based on the conclusion where highly active NCNT was produced from aliphatic hydrocarbon, similar carbon-nitrogen precursors with varying carbon to nitrogen ratio in the molecular structure (ethylenediamine, 1, 3-diaminopropane, 1, 4-diaminobutane) were adapted for the synthesis of NCNT. The investigation led to the conclusion that higher nitrogen to carbon ratio in the molecular structure of the precursors benefits the formation of active NCNT for ORR catalysis. The origin of such phenomena can be correlated with the higher relative nitrogen content of the resultant NCNT synthesized from aliphatic carbon precursor that provided greater nitrogen to carbon ratio. As the final nitrogen content increased in the molecular structure, the half-wave potential of the resultant NCNT towards ORR catalysis was increased by 120 mV. The significant improvement hints the critical role of nitrogen content towards ORR catalysis. To further confirm the correlation between the nitrogen content and ORR activity, another approach was used to control the final nitrogen content in the resultant NCNT. In the third investigation, a carbon-nitrogen precursor (pyridine) was mixed with a carbon precursor (ethanol) to form an admixture. The relative proportion of the two components of the admixture was varied to produce NCNT with different nitrogen content. By adopting this methodology, potential effect of different carbon-nitrogen precursors on the formation of NCNT can be eliminated since the same precursors were used for NCNT synthesis. Based on the electrochemical evaluations, the nitrogen content can be positively correlated to ORR activity. Among the NCNT samples, 41% higher limiting current density was achieved for 0.7 at. % increase in overall nitrogen content. Furthermore, the selectivity of the NCNT catalyst with higher nitrogen content favours the production of water molecule—the favourable product in metal-air battery by 43%. ORR catalyst is an outer-sphere electron transfer reaction whereby the reactants interact with the surface of catalysts. Consequently, the surface structure can be a determining factor towards the ORR activity of the NCNT in addition to the nitrogen content. In the forth investigation, the surface structure of NCNT was tailored to differentiate the ORR activity of smooth and rugged surface while controlling the overall nitrogen content to be similar. NCNT having different surface structures but similar nitrogen content (approximately 2.7 to 2.9 at. %) were successfully synthesized using different synthesis catalysts. Comparison of the two NCNT catalysts showing different surface structure resulted in a 130 mV increased in half-wave potential favouring the NCNT with more rugged surface structure. This study provided insights to the potential effects of synthesis catalyst towards directing the surface structure and the ORR activity of NCNT. Through a series of studies, the important parameters affecting the ORR performance of NCNT were elucidated and the most active NCNT catalyst synthesized was used for testing in a prototype zinc-air battery. The fifth study evaluated the performance of NCNT catalyst in different concentrations of alkaline electrolyte and at different battery voltage. An increase in the electrolyte’s alkaline strength improved the battery performance to a certain degree until the increasing viscosity impeded the performance of the battery system. The zinc-air battery employing NCNT as ORR catalyst produced a maximum battery power density of 69.5 mWcm-2 in 6M potassium hydroxide. The fifth study illustrated the great potential of NCNT towards the ORR catalysis for metal-air batteries. In combination, the series of investigations presented in this document provide a comprehensive study of a novel material and its application towards ORR catalysis in metal air batteries. Specifically, this report provides insights into the fundamentals of NCNT synthesis; the origins of ORR activity and the optimal operating conditions of NCNT in a prototype zinc-air battery. The excellent performance of NCNT warrants further studies of this material in greater details, and the information presented in this document will create a basis for future investigations towards ORR catalysis.

Fuel cell performance was obtained as functions of the humidity at the anode and cathode sites, back pressure, flow rate, temperature, and channel depth. The fuel cell used in this work included a membrane and electrode assembly (MEA) which possessed an active area of 25, 50, and 100 cm2 with the Nafion® 117 and 115 membranes. Higher flow rates of inlet gases increase the performance of a fuel cell by increasing the removal of the water vapor, and decrease the mass transportation loss at high current density. Higher flow rates, however, result in low fuel utilization. An important factor, therefore, is to find the appropriate stoichiometric flow coefficient and starting point of stoichiometric flow rate in terms of fuel cell efficiency. Higher air supply leads to have better performance at the constant stoichiometric ratio at the anode, but not much increase after the stoichiometric ratio of 5. The effects of the environmental conditions and the channel depth for an airbreathing polymer electrolyte membrane fuel cell were investigated experimentally. Triple serpentine designs for the flow fields with two different flow depths was used. The shallow flow field deign improves dramatically the performance of the air-breathing fuel cell at low relative humidity, and slightly at high relative humidity. For proton exchange membrane fuel cells, proper water management is important to obtain maximum performance. Water management includes the humidity levels of the inlet gases as well as the understanding of the water process within the fuel cell. Two important processes associated with this understanding are (1) electro-osmotic drag of water molecules, and (2) back diffusion of the water molecules. There must be a neutral water balance over time to avoid the flooding, or drying the membranes. For these reasons, therefore, an investigation of the role of water transport in a PEM fuel cell is of particular importance. In this study, through a water balance experiment, the electro-osmotic drag coefficient was quantified and studied. For the cases where the anode was fully hydrated and the cathode suffered from the drying, when the current density was increased, the electro- osmotic drag coefficient decreased.