Dissertations / Theses on the topic 'Bipolar PWM'

Laboratory and synchrotron X-ray fluorescence (XRF) have been used to investigate the microscopic and macroscopic distribution of metallic contaminants in membrane electrode assemblies (MEAs) which were used in proton exchange membrane fuel cell (PEM FC) stacks. The laboratory XRF results were consistent with the synchrotron XRF results. Higher levels of contaminants observed for the areas near to the coolant outlet than the areas near to the coolant inlet. The cathode side of MEAs showed higher levels of contamination than the anode side of the MEAs. Synchrotron XRF maps of MEAs generally showed higher levels of contaminants on the cathode side compared with the anode side. Fe was mainly observed in the cathode side microporous layers, whereas Ni, Cr and Cu were mostly accumulated in the cathode side or in the membrane. Synchrotron XRF maps of MEA plan views showed a crack-like distribution for Fe and Pt which were similar to cracks in the microporous layer of the MEAs. A novel electrochemical cell that simulated galvanic and crevice corrosion, temperature cycles for a PEM fuel cell, and pressure across the stacks was designed and used to discriminate between the corrosion behaviour of candidate coatings for bipolar plates.

High cost and a short lifetime are the two main reasons why the PEM fuel cell is yet to be commercialized. The bipolar plate in a PEM fuel cell is alone responsible for about 45% of the cost and 85% of the total weight of a single cell. stainless steel has been suggested as material for the bipolar plate because of its good mechanical properties, easy manufacturing and relatively low price. A problem with stainless steel is the Chromium oxide film formed on the surface which causes a high contact resistance. In order to prevent this oxide formation, the stainless steel can be coated. Gold has been suggested as coating, but it is too expensive to be considered a viable alternative. The objective of this thesis was to investigate stainless steel as bipolar plate material for PEM fuel cells. In cooperation with SINTEF polarization tests were done on stainless steel bipolar plates with and without two different coatings; gold and Coating A. The tests were performed in H2SO4 electrolytes with different molarities and additives. Before and after each polarization test Interfacial Contact Resistance (ICR) measurements where done to see how the oxide layer on the stainless steel surface changed during polarization. Gold coated stainless steel was chosen as standard for both the polarization tests and the ICR measurements because of its corrosion resistance.The results obtained from both polarization tests and corresponding ICR measurements showed that the reproducibility was not as good as one had hoped, but this can be explained by low absolute values of the current densities. Gold coated steel proved to be a good standard for the ICR measurements, but due to pitting corrosion the corresponding polarization results were not as promising. The pH in an operating fuel cell was found to be approximately 3.5, and the tests done at different molarities showed that at a lower pH the oxide layer seemed to be thinner and the stainless steel surface thus became more exposed to corrosion. Additions of fluoride and chloride in the amounts expected in an operating fuel cell did not seem to cause any changes for neither the polarization results nor the contact resistance measurements. Stainless steel plates with Coating A showed very small changes in contact resistance after being put trough the polarization tests, but at low potentials the current densities in the polarization test were very high, indicating that components in the coating either catalyzed hydrogen evolution or were reduced themselves. Out of all the ICR measurements, gold coated stainless steel was the only plate satisfying US department Of Energy’s (DOE) resistance requirement for bipolar plates of less than 10 mΩ cm2. The stainless steel plates with Coating A were close to DOE’s requirements for both corrosion current and contact resistance. Non-coated stainless steel was ruled out as bipolar plate material due to high contact resistance measurements.

In recent years, due to the public concern on global warming, both increasing energy efficiency and developing green energy become crucially important. Fuel cells can be one of the most suitable clean energy solutions for the environment because of its high energy conversion efficiency and near zero emissions of criteria air pollutants at the use stage. To increase the energy efficiency of fuel cells, effectively utilize the Pt catalyst and increase the fuel cell durability, the uniform distribution of the reactants over the fuel cell active area is of great importance. Over the last decade, many researchers have focused on developing flow field design to homogenously distribute the reactant and to decrease the pressure drop in the bipolar plates. However, most of the previous studies are in the stage of numerical simulation, and the few experimental studies have used very simple flow field geometries. Not to mention that complex transport phenomena inside a fuel cell make even the numerical simulation challenging and time consuming, which hinders the quick screening of proposed modifications and new designs. While the conventional fabrication techniques are expensive and time consuming, 3D printing is a very good rapid prototyping method that can be used both to validate the simulation results and to supplement the tedious simulation work. The question is whether the results from 3D printed flow fields could be as accurate and reliable as flow fields fabricated with conventional methods. In the present research, we investigated the applicability of 3D printing in validating the simulation results and as a fast screening method. State of the art designs for anode, cathode and water cooling BPPs proposed and fabricated using Polyjet 3D printing, SLA 3D printing and laser-cutter technologies and the pressure drop and velocity profiles were measured for each plate. The results demonstrated that SLA 3D printing has great promises to serve as a screening tool in modifying the flow field design, as well as in validating the simulation results.
Applied Science, Faculty of
Chemical and Biological Engineering, Department of
Graduate

The largest cost in manufacturing PEM fuel cells for automotive applications is due to the bipolar plate. The current graphite material used for the bipolar plate is very brittle and difficult to machine to the rigorous specifications needed for fuel cell stacks. This paper introduces the development of a fuel cell test stand for simultaneous testing of six individual fuel cells. To establish a long-term performance evaluation, the fuel cells incorporate a baseline graphite material that undergoes testing in the fuel cell environment. The graphite is an industry standard material that should not corrode when subjected to continual testing. The baseline model will be used in development of novel composite materials that will be tested under the same conditions for comparison to the graphite. Furthermore, the new materials and applied manufacturing methods could reduce the overall cost of fuel cell stacks in the future. Funding for this project was generously donated by the Virginia Center for Innovative Technology and the National Science Foundation.
Master of Science

Global warming and the climate changes are issues that concern everyone. The transition to other energy carriers than fossil fuels is a key barrier if we are to reduce our dependence on oil. Hydrogen is forecasted as the energy carrier of the future, and one of the most efficient ways to convert hydrogen to electrical, useful energy is to feed it to the anode side of a polymer electrolyte (PEM) fuel cell. Oxygen (usually in the form of air) is supplied to the cathode side and the only products are water, electricity and some heat.A key component in PEM fuel cells are the bipolar plates that facilitate gas flow, electron transfer, heat and water removal. The bipolar plates contribute greatly to both weight and cost of PEM fuel cells, and there is continuous development in making cheap, durable, light and efficient bipolar plates.In this work we attempted to coat stainless steel (316L) plates with a coating consisting of graphite and carbon black mixed with epoxy. The goal was to get a coating that conducts electrons while still protecting against corrosion. The coatings were sought further improved by adding Teflon particles. This was done to get the coating more hydrophobic, and thus a better corrosion protection of the underlying stainless steel substrate. Plates were glass blasted prior to coating to improve adhesion.Coatings were thinned with xylene and sprayed onto the plates with an air brush. The plates were then pressed in a ?Carver? hot-press to achieve minimum porosity. The coated plates were examined in a contact resistance apparatus, subjected to linear sweep voltammetry and chronoampereometry, the contact angles were measured, the surface roughness measured and SEM images are taken.Plates pressed at 2670 N cm-2 for 30 minutes with 4.8 vol% Zonyl in the coating performed best prior to electrochemical measurements both for the glass blasted plates and for the as-delivered plates. The contact resistances measured at 147 N cm-2 compaction pressure was 11.3 and 10.6 mOmega cm2 respectively. This is close to the goals set by DOE to reached by 2020 (10 mOmega cm2 at 138 N cm-2). The contact resistances for all coatings tested increased very much after the electrochemical testing, and all were far above the goals set by DOE.A correlation was found between the coating thickness and the current densities measured during the potentiostatic measurements. The current densities increased with increasing coating thickness. Possible mechanisms and reasons for this are discussed, and a combination of carbon corrosion and crevice corrosion is suggested as explanation.A possible method for anchoring the coating to the substrate while protecting the contact points from degradation during electrochemical measurements was suggested. By pressing the coating into carefully controlled voids, connections between the graphite in the coating and the metal substrate might be retained, even if the connection between main substrate surface and the coating is broken. This effect can explain the differences seen between the plates coated in the project work, and the metal sheets coated in this work.

This work investigated the suitability of thin film, single and multi-layered coatings, by a Physical Vapour Deposition (PVD) process for polymer electrolyte membrane fuel cell bipolar plates. Due to the multifunctional nature of this particular component a comprehensive approach was used where several key properties were examined for coatings including: ZrN, TiN, CrN, Graphit-iC™, CrN+C, TiN+C and Au. Chemical etching and surface roughness were found to influence the Interfacial Contact Resistance (ICR) of the substrate; however, any observed effect was negated with the addition of a conductive coating. CrN+C and TiN+C multi-layer coatings showed a striking reduction in the ICR compared with the nitride only equivalents. The suitability of pre-coated PVD coatings for serial production via stamping was assessed in collaboration with an industrial partner. The coating durability was found to be influenced by several factors including coating type, thicknesses and position on stamped plate. The multi-layered TiN+C coating was found to noticeably improve the stampability compared to the TiN only coating. The corrosion resistance of the coatings was evaluated under simplified corrosion conditions. Under these conditions TiN+C was found to have two beneficial effects, improving the free corrosion potential and the stability of the carbon topcoat under startup/shutdown potentials.

As part of the global work towards reducing CO2 emissions, all vehicles needs to be electrified, or fueled by green fuels. Batteries have already revolutionised the car market, but fuel cells are believed to be a key energy conversion system to be able to electrify also heavy duty vehicles. The type of fuel cell commercially available for vehicles today is the polymer electrolyte membrane fuel cell (PEMFC), but for it to be able to take a larger market share, the cost must be reduced while sufficient lifetime is ensured. The PEMFC is a system containing several components, made of different materials including the polymer membrane, noble metal catalyst particles, and metallic bipolar plate. The combination of different materials exposed to elevated temperature, high humidity and low pH make the PEMFC components susceptible to corrosion and degradation. The noble metal catalyst is one of the major contributors to the high cost. In this work, the latest research on new catalyst materials for PEMFCs are overviewed. Furthermore, electrodeposition as a simple synthesis route to test different Pt-alloys for the cathode catalyst in the fuel cell is explored by synthesis of PtNi and PtNiMo. The gas diffusion layer of the PEMFC is used as substrate to reduce the number of steps to form the membrane electrode assembly. In addition to cheaper and more durable materials, understanding of how the materials degrade, and how the degradation affects the other components is crucial to ensure a long lifetime. Finding reliable test methods to validate the lifetime of the final system is necessary to make fuel cell a trusted technology for vehicles, with predictable performance. In this work, commercial flow plates are studied, to see the effect of different load cycles and relative humidities on the corrosion of the plate. Defects originating from production is observed, and the effect of these defects on the corrosion is further analysed. Suggestions are given on how the design and production of bipolar plates should be made to reduce the risk of corrosion in the PEMFC.
Som en del av det globala arbetet med at reducera utsläppen av koldioxid måste alla fordon elektrifieras eller tankas med förnybart bränsle. Batterier har redan revolutionerat bilmarknaden, men bränsleceller är en viktig pusselbit för att också elektrifiera tunga fordon. Den typen av bränsleceller för fordon som finns tillgänglig på den kommersiella marknaden i dag är polymerelektrolytbränslecellen (PEMFC). För att PEMFC skall ta en större marknadsandel måste kostnaderna minskas och livslängden förlängas. PEMFC består av ett antal komponenter gjorda av olika material, bland annat polymer membran, ädelmetallkatalysator, och metalliska bipolära plattor. Kombinationen av olika material i tillägg till den höga temperaturen, hög fuktighet och låg pH gör att materialen i bränslecellen är utsatta för korrosion. Ädelmetallkatalysatorn är en av de kostdrivande komponenterna i bränslecellen. I denna studien presenteras en översikt över framstegen inom katalysatormaterial för PEM bränsleceller de senaste två åren. Sedan studeras elektroplätering som en enkel produktionsmetod för nanopartiklar av platina legeringar. Möjligheten att simultant plätera fler metaller, och att använda gasdiffutions-skiktet från bränslecellen som substrat för att reducera antal produktionsteg och därmed reducera kostnader, undersöks. Det möjliggör också snabb testning av olika legeringar för att identifiera den optimala sammansättningen med hög prestanda, lång livslängd och lite platina. I tillägg till att ta fram billigare och tåliga material är det viktigt att förstå hur materialen degraderar och hur degraderingen av ett material påverkar de andra komponenterna. Med den kunskapen kan man utveckla accelererade testmetoder för att bedöma livslängden av hela bränslecellen. Validerade testmetoder är viktigt för att styrka förtroendet till nya teknologier. I denna studien fokuseras det också på korrosion av bipolära plattor, och hur olika lastcykler och fuktnivåer som kan bli applicerad vid accelererad testning påverkar korrosionen. Också effekten av defekter från tillverkningen i den skyddande beläggningen analyseras med hänsyn till korrosion, för att ge mer insikt i hur bipolära plattor kan designas och produceras för att minska korrosionen.

The development, design, and modeling of a rapid continuous processing scheme is developed to economically manufacture conductive polymer composite bipolar plates for fuel cells. Bipolar plates are required to possess several important properties for fuel cell operation, with the most sought after being electrical conductivity and mechanical strength. The polymer composite material generated at Virginia Tech is based on material generated by a wet-lay process and uses polyethylene terepthalate (PET) or polyphenylene sulfide (PPS) as the binder, although PPS is mainly used. In order to reach sufficient conductivity for use in generating bipolar plates, the polymer is doped with high levels of conductive graphite particles in the range of 70-80 wt%. The polymer system is reinforced with 6-9 wt% glass or carbon fibers. When compression molded into a solid, flat preform, the wet-lay material exhibits excellent bulk (in-plane) conductivity (> 250 S/cm). The material also exhibits tensile and flexural strengths of 57.5 and 95.8 MPa, respectively, higher than other polymer composite material being considered for bipolar plate production. However, formability and through-plane conductivity needs improvement. The laminate bipolar plates developed at Virginia Tech are made using wet-lay material in the core and a thermoplastic/graphite mixture on the surfaces. The wet-lay material provides mechanical integrity, while a powder form of PVDF or PPS and graphite mixture added to the surfaces to improve through-plane conductivity and formability. The manufacturing scheme for the production of laminate bipolar plates is based on the pre-consolidation of the wet-lay material, which establishes a solid, flat surface for the continuous addition of laminate powder. Because the laminate powder only requires heating, radiation heating is used in the process design to pre-heat the preform prior to compression molding. The heated preform passes underneath a press, where forming of channels takes place along with cooling of the bipolar plate. It is estimated that the entire process can take one minute to produce a bipolar plate. The cost of manufacturing a bipolar plate is estimated to be $8/kW, below the goal of $10/kW. The annual production is determined to be 250,000, with over 500,000 possible depending on certain design factors.
Ph. D.

Bipolar plate material and fabrication costs make up a significant fraction of the total cost in a polymer electrolyte membrane fuel cell stack. In an attempt to reduce these costs, a novel manufacturing method was developed for use with composite materials. Conductive fillers were mixed with a polypropylene binder and molded into single cell monopolar plates. A fuel cell test stand, capable of testing six cells simultaneously, was used for long-term corrosion testing. In-situ tests took place in 5 cm2 active area fuel cells with cathode humidification. Using data from test cells containing graphite monopolar plates as a baseline, two composite formulations, were able to produce power at 66-79% of the baseline power. Power output from one cell remained in this range for over 200 hours, while the other sample experienced surface oxidation and eventually failed. With improvements in part conductivity coming from conductive polymers, this manufacturing technique holds the promise of producing monopolar and bipolar plates that could eventually be scaled up for use in fuel cell stacks.
Master of Science

Polymer electrolyte membrane fuel cells (PEMFCs) are a promising alternative to combustion engines and batteries in vehicles. PEMFCs are more efficient than combustion engines, and cars powered by PEMFCs tend to have longer ranges than battery powered cars and has a short fuelling time. To gain significant market shares, the costs of PEMFCs need to be reduced. This work aims to achieve this by improving the production process of bipolar plates, which significantly contribute to the total cost of fuel cells. In conventional production methods bipolar plates are formed by stamping at room temperature which limits the degree of deformation that can be achieved before exceeding the structural integrity of the material. It is expected that stamping at elevated temperatures will increase the forming limit of the material and therefore the flexibility in forming the channel geometry of bipolar plates. This has the potential to simplify the manufacturing process and improve the resulting performance of fuel cells.  The goal of this thesis is to establish to what degree the forming at elevated temperatures facilitates higher degrees of deformation. Different heating methods are benchmarked and analysed numerically in order to identify the most suitable one for stamping experiments at elevated temperatures. Out of six investigated heating concepts, direct resistive heating is identified as the most suitable one. The suitability of the concept is supported by numerical simulations. The direct resistive heating system is designed and integrated into the existing experimental setup.  Four flow channel geometries with channel widths of 1 mm, 0.8 mm, 0.6 mm, and 0.56 mm are investigated in stamping experiments using the identified heating method. Samples are formed at 150 °C, 300 °C, 600 °C, and 900 °C. Hexagonal Boron Nitride is used as a lubricant. The stamping experiments performed at elevated temperatures indicate that the formability of the bipolar plates improves as compared to the cold-formed reference experiments. The best results are obtained at 900 °C where the average channel depth, which can be formed before cracks are observed in the samples, could be improved by 27% compared to channels formed at room temperature.
Polymerelektrolytmembranbränsleceller (PEMFC) är ett lovande alternativ till förbränningsmotorer och batterier i fordon. Fordon som drivs av PEMFC har en längre räckvidd än batteridrivna bilar och behöver bara några minuter för att tanka. PEMFC kan drivas utan utsläpp av växthusgaser. För att få betydande marknadsandelar måste tillverkningskostnaderna för dessa bränsleceller minskas. Detta kan uppnås genom att förbättra produktionsprocessen. Dessutom kan en förbättrad prestanda för bränslecellen leda till en minskning av storlek, materialanvändning och därmed produktionskostnader. Denna uppsats undersöker stämpling av metalliska bipolära plattor, som har en betydande del av kostnaden för en PEMFC. I konventionella produktionsmetoder bildas flödesfältgeometrin vid omgivningstemperaturer vilket begränsar den grad av deformation som kan uppnås innan materialets strukturella gränser överskrids. Det förväntas att stämpling vid förhöjd temperatur ökar formningsgränsen för materialet och därför flexibiliteten vid formning av kanalgeometrin. Detta förväntas förenkla tillverkningsprocessen och förbättra bränslecellens resultat. Målet med denna avhandling är att testa denna hypotes och fastställa hur förhöjda temperaturer underlättar högre deformationsgrader. För att uppnå detta används olika uppvärmningsmetoder som riktmärken och analyseras numeriskt för att identifiera den lämpligaste metoden för experiment med hetstämpling. Av sex undersökta värmekoncept identifieras direkt resistiv uppvärmning som den mest lämpliga. Konceptets lämplighet stöds av numeriska simuleringar. Det direktresistiva uppvärmningssystemet är utformat och integrerat i den befintliga experimentella installationen. Fyra olika kanalgeometrier undersöks. Prover bildas vid 150 ° C, 300 ° C, 600 ° C och 900 ° C. Hexagonal bornitrid används som smörjmedel. Stämplingsexperimenten utförda vid förhöjda temperaturer indikerar att de bipolära plattornas formbarhet förbättras jämfört med de kallformade referensexperimenten. De bästa resultaten erhålls vid 900 ° C där det genomsnittliga kanaldjupet, som kan bildas innan sprickor observeras i proverna, skulle kunna förbättras med 27% jämfört med formningen vid rumstemperatur.

This research deals with the synthesis and characterization of titanium diboride (TiB2) from novel carbon coated precursors. This work provides information on using different boron sources and their effect on the resulting powders of TiB2.The process has two steps in which the oxide powders were first coated with carbon by cracking of a hydrocarbon gas, propylene (C3H6) and then, mixed with boron carbide and boric acid powders in a stoichiometric ratio. These precursors were treated at temperatures in the range of 1200-1400° C for 2 h in flowing Argon atmosphere to synthesize TiB2.The process utilizes a carbothermic reduction reaction of novel carbon coated precursor that has potential of producing high-quality powders (sub-micrometer and high purity). Single phase TiB2 powders produced, were compared with commercially available titanium diboride using X-ray diffraction and Transmission electron microscopy obtained from boron carbide and boric acid containing carbon coated precursor.

The performance of a new cathode flow field plate located on a PEM fuel cell was compared to an industry standard and optimal serpentine design provided from literature. Results were successfully collected through a fuel cell module integrated with the 3D computational fluid dynamics package ANSYS Fluent. Contour plots showing a cathode catalyst layer comparison of local current density, oxygen molar concentrations, water content, and the pressure inside of the flow channels were compared with both PEM fuel cell configurations. The new flow field plate/pattern was shown to distribute more mass species of oxygen, more evenly, to the reaction site given the same boundary conditions, thus contributing to more ideal local current density. The net-power was determined for both fuel cells which included the pump work-in and power-out from each fuel cell. The new flow field plate was shown, through computational power performance results, to outperform the conventional flow pattern by up to 2.4% when excluding the effects of pump work, and still upheld a positive gain when factoring in this value. With an additional 18 corners for improved water management due to the effects of wall adhesion, the new bipolar plate was proven to become a new competitor in PEM fuel cell technology. Furthermore, this thesis gives further insight on PEMFC digital prototyping.

Fuel cells are widely researched and have applications in residential, automotive, marine craft and space. Their efficiencies are typically 60 % as a result of their electrochemical conversion and due to this they are considered beneficial to the reduction of CO2 which accounts for 77 % of all greenhouse gasses. Polymer electrolyte membrane fuel cells are the most suited to automotive applications for their low operating temperatures, high power densities and fast start up times. Currently there are many problems still to be rectified before commercialisation takes place, one of which is the performance and manufacture of bipolar plates. The elimination of corrosion, reduction of mass and the improvement of mechanical, electrical and thermal conductivity properties are the main aims to progress bipolar plate technology. In addition, the large numbers of bipolar plates required in automotive fuel cell stacks is in the order of 400 plates and so mass production will be necessary to meet future demands as well as reduce costs through cheap production processes. In order to meet these requirements polymeric based bipolar plates with conductive fillers have been pursued. The use of highly conductive, low density, low cost and corrosion resistant materials that can be utilised in production processes such as injection and compression moulding are ideal candidates for bipolar plates. However, balance of electrical/thermal conductivity and mechanical strength becomes the major task as highly conductive composites result in low mechanical strength. Therefore three conductive powders, a carbon black, graphite and magnetite (iron II,III oxide) were used as fillers in a polyethylene matrix to study the balance just mentioned for the two manufacturing processes stated above. The composites were tested for their electrical and thermal conductivities and mechanical properties and compared to the US Department of Energy targets for 2015. The carbon black composites exhibited better electrical conductivity than the other fillers where at 65 wt% the conductivity was ~24 S/cm for through plane conductivity and had a flexural strength of ~32 MPa. Injection moulding produced composites with more material stability and greater mechanical strength than compression mouldings although compression mouldings produced composites with higher thermal conductivities where graphite displayed the highest thermal conductivity of ~2 W/mK. Modeling of the experimental results using Mamunya models for electrical and thermal conductivities and a modified Kerner s equation for mechanical moduli were conducted. Models showed reasonable agreement with the experimental data where parameter tuning and deviations from the model were used to describe microstructural behaviour with regards to electrical tunnelling effects, link, node and blob structures and stress transfer at the filler-matrix interface.

Bipolarplatten stellen eine Schlüsselkomponente hinsichtlich Fertigungskosten eines Brennstoffzellenstacks dar und sind komplexen Anforderungen ausgesetzt. Sie stützt die anderen biegeempfindlichen Stackkomponenten, verteilt die Reaktionsgase über die aktive Zellfläche und sorgt für die flächige elektrische Kontaktierung der in Reihe geschalteten Einzelzellen, Ableitung der Reaktionswärme und sowie Trennung der Einzelzellen. Daher muss sie einen niedrigen Kontaktwiderstand zur benachbarten Gasdiffusionslage, hohe Biegesteifigkeit, gute Wärmeleitfähigkeit und hohe Lebensdauer der Zelle gewährleisten. Die Funktion der Gasverteilung über die aktive Zellfläche übernimmt das Flussfeld. Deren Kanalstruktur- und Geometrie beeinflusst entscheidend den Wirkungsgrad der Brennstoffzelle und des Gesamtsystems. Um Fertigungsaufwand und damit verbundene Kosten zu senken, ist es Ziel aktueller Forschungsarbeit, die Geometrie so einfach wie möglich zu gestalten. Dabei wird ein minimal geringerer Wirkungsgrad in Kauf genommen, wenn im Gegenzug die Kosten stark sinken. Mit Hilfe einer CFD-Simulation wird das Flussfeld dahingehend optimiert, die Produktion zu vereinfachen und die Funktionalität weiterhin zu gewährleisten. Im Rahmen dieser Präsentation werden Modellbildung, Randbedingungen und erste Ergebnisse vorgestellt.

A nickel alloy with high chrome and molybdenum content was found to form a highly resistive and passive oxide layer. The donor density and mobility of ions in the oxide layer has been determined as a function of the electrical potential when alkaline water layers are on the alloy surface in order to account for the relative inertness of the nickel alloy in corrosive environments.

Polymer electrolyte membrane fuel cells (PEMFCs) have emerged as a strong and promising candidate to replace internal combustion engines (ICE) due their high efficiency, high power density and near-zero hazardous emissions. However, their commercialization waits for solutions to bring about significant cost-reductions and significant durability for given power densities. Bipolar plate (BPP) with its multi-faceted functions is one of the essential components of the PEMFC stacks. Stainless steel alloys are considered promising materials of choice for bipolar plate (BPP) applications in polymer electrolyte membrane fuel cells (PEMFC) due to their relatively low cost and commercial availability in thin sheets. Stainless steel materials build a protective passive metal oxide layer on their surface against corrosion attack. This passive layer does not demonstrate good electrical conductivity and increases interfacial electric contact resistance (ICR) between BPP and gas diffusion layer GDL in PEMFC. Lower ICR values are desired to reduce parasitic power losses and increase current density in order to improve efficiency and power density of PEMFC. This study aimed to bring about a broader understanding of manufacturing effects on the BPP contact resistance. In first stage, BPP samples manufactured with stamping and hydroforming under different process conditions were tested for their electrical contact resistance characteristics to reveal the effect of manufacturing type and conditions. As a general conclusion, stamped BPPs showed higher contact conductivity than the hydroformed BPPs. Moreover, pressure in hydroforming and geometry had significant effects on the contact resistance behavior of BPPs. Short term corrosion exposure was found to decrease the contact resistance of bipolar plates. Results also indicated that contact resistance values of uncoated stainless steel BPPs are significantly higher than the respective target set by U.S. Department of Energy. Proper coating or surface treatments were found to be necessary to satisfy the requirements. In the second stage, physical vapor deposition technique was used to coat bipolar plates with CrN, TiN and ZrN coatings at 0.1, 0.5 and 1 μm coating thicknesses. Effects of different coatings and coating thickness parameters were studied as manufactured BPPs. Interfacial contact resistance tests indicated that CrN coating increased the contact resistance of the samples. 1 µm TiN coated samples showed the best performance in terms of low ICR; however, ICR increased dramatically after short term exposure to corrosion under PEMFC working conditions. ZrN coating also improved conductivity of the SS316L BPP samples. It was found that the effect of coating material and coating thickness was significant whereas the manufacturing method and BPP channel size slightly affected the ICR of the metallic BPP samples. Finally, effect of process sequence on coated BPPs was investigated. In terms of ICR, BPP samples which were coated prior to forming exhibited similar or even better performance than coated after forming samples. Thus, continuous coating of unformed stripes, then, applying forming process seemed to be favorable and worth further investigation in the quest of making cost effective BPPs for mass production of PEMFC.

Demands for delivering high instantaneous power in a compressed form (pulse shape) have widely increased during recent decades. The flexible shapes with variable pulse specifications offered by pulsed power have made it a practical and effective supply method for an extensive range of applications. In particular, the release of basic subatomic particles (i.e. electron, proton and neutron) in an atom (ionization process) and the synthesizing of molecules to form ions or other molecules are among those reactions that necessitate large amount of instantaneous power. In addition to the decomposition process, there have recently been requests for pulsed power in other areas such as in the combination of molecules (i.e. fusion, material joining), gessoes radiations (i.e. electron beams, laser, and radar), explosions (i.e. concrete recycling), wastewater, exhausted gas, and material surface treatments. These pulses are widely employed in the silent discharge process in all types of materials (including gas, fluid and solid); in some cases, to form the plasma and consequently accelerate the associated process. Due to this fast growing demand for pulsed power in industrial and environmental applications, the exigency of having more efficient and flexible pulse modulators is now receiving greater consideration. Sensitive applications, such as plasma fusion and laser guns also require more precisely produced repetitive pulses with a higher quality. Many research studies are being conducted in different areas that need a flexible pulse modulator to vary pulse features to investigate the influence of these variations on the application. In addition, there is the need to prevent the waste of a considerable amount of energy caused by the arc phenomena that frequently occur after the plasma process. The control over power flow during the supply process is a critical skill that enables the pulse supply to halt the supply process at any stage. Different pulse modulators which utilise different accumulation techniques including Marx Generators (MG), Magnetic Pulse Compressors (MPC), Pulse Forming Networks (PFN) and Multistage Blumlein Lines (MBL) are currently employed to supply a wide range of applications. Gas/Magnetic switching technologies (such as spark gap and hydrogen thyratron) have conventionally been used as switching devices in pulse modulator structures because of their high voltage ratings and considerably low rising times. However, they also suffer from serious drawbacks such as, their low efficiency, reliability and repetition rate, and also their short life span. Being bulky, heavy and expensive are the other disadvantages associated with these devices. Recently developed solid-state switching technology is an appropriate substitution for these switching devices due to the benefits they bring to the pulse supplies. Besides being compact, efficient, reasonable and reliable, and having a long life span, their high frequency switching skill allows repetitive operation of pulsed power supply. The main concerns in using solid-state transistors are the voltage rating and the rising time of available switches that, in some cases, cannot satisfy the application’s requirements. However, there are several power electronics configurations and techniques that make solid-state utilisation feasible for high voltage pulse generation. Therefore, the design and development of novel methods and topologies with higher efficiency and flexibility for pulsed power generators have been considered as the main scope of this research work. This aim is pursued through several innovative proposals that can be classified under the following two principal objectives. • To innovate and develop novel solid-state based topologies for pulsed power generation • To improve available technologies that have the potential to accommodate solid-state technology by revising, reconfiguring and adjusting their structure and control algorithms. The quest to distinguish novel topologies for a proper pulsed power production was begun with a deep and through review of conventional pulse generators and useful power electronics topologies. As a result of this study, it appears that efficiency and flexibility are the most significant demands of plasma applications that have not been met by state-of-the-art methods. Many solid-state based configurations were considered and simulated in order to evaluate their potential to be utilised in the pulsed power area. Parts of this literature review are documented in Chapter 1 of this thesis. Current source topologies demonstrate valuable advantages in supplying the loads with capacitive characteristics such as plasma applications. To investigate the influence of switching transients associated with solid-state devices on rise time of pulses, simulation based studies have been undertaken. A variable current source is considered to pump different current levels to a capacitive load, and it was evident that dissimilar dv/dts are produced at the output. Thereby, transient effects on pulse rising time are denied regarding the evidence acquired from this examination. A detailed report of this study is given in Chapter 6 of this thesis. This study inspired the design of a solid-state based topology that take advantage of both current and voltage sources. A series of switch-resistor-capacitor units at the output splits the produced voltage to lower levels, so it can be shared by the switches. A smart but complicated switching strategy is also designed to discharge the residual energy after each supply cycle. To prevent reverse power flow and to reduce the complexity of the control algorithm in this system, the resistors in common paths of units are substituted with diode rectifiers (switch-diode-capacitor). This modification not only gives the feasibility of stopping the load supply process to the supplier at any stage (and consequently saving energy), but also enables the converter to operate in a two-stroke mode with asymmetrical capacitors. The components’ determination and exchanging energy calculations are accomplished with respect to application specifications and demands. Both topologies were simply modelled and simulation studies have been carried out with the simplified models. Experimental assessments were also executed on implemented hardware and the approaches verified the initial analysis. Reports on details of both converters are thoroughly discussed in Chapters 2 and 3 of the thesis. Conventional MGs have been recently modified to use solid-state transistors (i.e. Insulated gate bipolar transistors) instead of magnetic/gas switching devices. Resistive insulators previously used in their structures are substituted by diode rectifiers to adjust MGs for a proper voltage sharing. However, despite utilizing solid-state technology in MGs configurations, further design and control amendments can still be made to achieve an improved performance with fewer components. Considering a number of charging techniques, resonant phenomenon is adopted in a proposal to charge the capacitors. In addition to charging the capacitors at twice the input voltage, triggering switches at the moment at which the conducted current through switches is zero significantly reduces the switching losses. Another configuration is also introduced in this research for Marx topology based on commutation circuits that use a current source to charge the capacitors. According to this design, diode-capacitor units, each including two Marx stages, are connected in cascade through solid-state devices and aggregate the voltages across the capacitors to produce a high voltage pulse. The polarity of voltage across one capacitor in each unit is reversed in an intermediate mode by connecting the commutation circuit to the capacitor. The insulation of input side from load side is provided in this topology by disconnecting the load from the current source during the supply process. Furthermore, the number of required fast switching devices in both designs is reduced to half of the number used in a conventional MG; they are replaced with slower switches (such as Thyristors) that need simpler driving modules. In addition, the contributing switches in discharging paths are decreased to half; this decrease leads to a reduction in conduction losses. Associated models are simulated, and hardware tests are performed to verify the validity of proposed topologies. Chapters 4, 5 and 7 of the thesis present all relevant analysis and approaches according to these topologies.

The use of insulated-gate bipolar transistor (IGBT)-based power converters is increasing exponentially. This is due to high performance of these devices in terms of efficiency and switching speed. However, due to the switching action, high frequency electromagnetic interference (EMI) noises are generated. Design of a power converter with reduced EMI noise level is one of the primary objectives of this research. The first part of the work focuses on designing common-mode (CM) filters, which can be integrated with differential-mode (DM) filters for three-phase pulse-width modulation (PWM) rectifier-based motor drives. This work explores the filter design based on the CM equivalent circuit of the drive system. Guidelines are provided for selection of the filter components. Different variants of the filter topology are evaluated to establish the effectiveness of the proposed topology. Analytical results based on Bode plot of the transfer functions are presented, which suggest effective EMI reduction. Experimental results based on EMI measurement on the grid side and CM current measurement on the motor side are presented. These results validate the effectiveness of the filter. In the second part of the work, it is shown that inclusion of CM filters into DM filters results in resonance oscillations in the CM circuit. An active damping strategy is proposed to damp the oscillations in both line-to-line and line-to-ground ac voltages and currents. An approach based on pole placement by state feedback is used to actively damp both the DM and CM filter oscillations. Analytical expressions for state-feedback controller gains are derived for both continuous-and discrete-time models of the filter. Trade-off in selection of the active damping gain on the lower-order grid current harmonics is analysed using a weighted admittance function method. In the third part of the work, single-phase grid-connected power converters are considered. An integrated CM filter with DM LCL filter is proposed. The work explores the suitability of PWM methods for single-phase and parallel single-phase grid-connected power converters. It is found that bipolar PWM and unipolar PWM with 180◦interleaving angle are suitable for single-phase and parallel single-phase power converters, respectively. The proposed configuration along with the PWM methods reduces the CM voltage, CM current, and EMI noise level effectively. It is also shown that the suggested circuit is insensitive to nonidealities of the power converter such as dead-time mismatch, mismatch in converter-side inductors, unequal turn on and turn off of the switches, and propagation delays. In the fourth part of the work, the inter-phase inductor in parallel interleaved power converters is integrated with LCL filter boost inductor. Different variant designs are presented and compared with the proposed structure. It is shown that the proposed structure makes use of standard core geometries and consumes lesser core material as well as copper wire. Hence, it reduces the overall size and cost of the power converter. In the present work, a 10kVA three-phase back-to-back connected with input LCL filter and output dv/dt filter, a 5kVA single-phase grid-connected power converter with LCL filter, and a 7.5kVA parallel single-phase grid-connected power converter with LCL filter are fabricated in the laboratory to evaluate and validate the proposed methods. The experimental results validate the proposed methods that result in significant EMI performance improvement of grid-connected power converters.

碩士
國立臺南大學
綠色能源科技學系碩士班
102
Bipolar plate, one of main components of proton exchange membrane fuel cells (PEMFC), can provide multiple functions, such as reaction gas separation, current collection, and thermal heat conduction. Typically (in general), bipolar plate accounts for 20% of the product cost of a fuel cell stack and it accounts for 60%-80% of the weight of a fuel cell stack. It is a major challenge in commercialization of fuel cells to reduce the cost and weight of bipolar plates. This dissertation presents a mechanical design method for stamped metal bipolar plates, which consists of a gas channel, a supply header, an exhaust header and seals. Based on the combination of cathode-plates and anode-plates inside a bipolar plates assembly, an independent flow field is generated between these two plates and allows coolant to distribute and flow readily within coolant volume.This study collects and analyzes several patents owned by automobile manufacturing companies, as well as discussing the process of the flow channel design and the principle of cooling channel production. It proposes four different channel flow models. Each model has a pair of stamped gas channels bonded together to provide coolant volume and helps design the bipolar plate flow field structures by using the model. We use CAD software to build 3D models, simulate the bipolar plate assembly verification, and analyze the coolant flow fields. This study develops three different designs of metal bipolar plates and our results suggests that through the gas flow path arrangement, all of these three designs can effectively create separate coolant flow fields.

碩士
華梵大學
機電工程研究所
93
The paper was to investigate the development of the graphite/resin composites that were fabricated by hot press for the bipolar plate of the fuel cell. Effect of the different amounts of resin, graphite size, and molding pressure on the porosity, density, bending strength, and electrical resistivity of the composites would be analyzed. In addition, the composites of the bipolar plates would be machined into the groove channel using high speed milling, which then assembles into single fuel cell. The I-V performance of the fuel cell would be also discussed. The experimental results showed that for a larger amount of resin, a less porosity, a larger buck electrical resistivity and a better bending strength of the composites could be obtained. Moreover, the larger graphite size the composites contained, the lower porosity and electrical resistivity of the composites were obtained. Besides, the composites contained a moderate graphite size of 300 mesh could get a better bending strength. In addition, for a larger molding pressure during hot press, a lower porosity, a lower electrical resistivity, and a higher bending strength of the composites could be obtained. The graphite/resin composite containing a higher bending strength and a lower porosity during the high-speed milling can produce a better surface quality of the groove channel. When the faster feed during the milling was employed, the pores on the surface of the groove channel would produce spalling. Finally, the graphite/resin composites of bipolar plates of the fuel cell fabricated could obtain a lighter, easier forming and machining than the graphite bipolar plates, but had a larger electrical resistivity. Finally, the graphite/resin composites of bipolar plates containing 25-30 vol% resins were fabricated into a single fuel cell, which displays a relatively better I-V performance.

碩士
元智大學
機械工程學系
99
Bipolar plate is one of the most important components of the fuel cell from the fact that it contributes the majority of the cost and weight of PEM fuel cell. Durability and cost have been the two main challenges for fuel cell technology to be used in the energy market. The term of durability is related to the ability of the PEM fuel cell to resist permanent decrease in performance over time, which is not recoverable and reversible. The bipolar plates make connections all over the surface of one cathode in a cell and the anode of the next cell, serve as a means of feeding oxygen to the cathode and fuel gas to the anode, conduct the electronic current between two cells, separate the gases and contain the flow patterns both for the reactants and for the coolant, facilitate water management within the cell, enable heat transfer, and constitute the backbone of a power stack. The bipolar plate materials require multiple properties to be acceptable, such as high electrical conductivity, low gas permeability, high corrosion resistance, sufficient strength, low thermal resistance, low cost, etc. Graphite composites have attractive characteristics to be used in bipolar plates, such as the combination of high corrosion and chemical resistance, low density, and high electrical and thermal conductivity. However, they have unfavorable disadvantages when compared to metals, their durability under shock and vibration, permeability to hydrogen, and manufacturability, they need an increase in weight and volume to overcome their deficiencies. This research consist in testing the durability of graphite composites bipolar plates in three single PEM fuel cells, and comparing them with commercial graphite bipolar plates in a single PEM fuel cell, under constant current ageing mode and at the same operation conditions in a fuel cell test stand.

碩士
國立虎尾科技大學
材料科學與綠色能源工程研究所
98
In this study, the corrosion behavior of electroless Ni, Au plating and Polypyrrole coatings using cyclic voltammetry were prepared on 1050 aluminum plate is investigated in a simulated polymer electrolyte membrane fuel cell (PEMFC) environment included 0.5 M at room temperature. Potentiodynamic technique and electrochemical impedance spectroscopy (EIS) were performed to determine the corrosion properties of electroless plating and polypyrrole coatings. It was found that corrosion potential of the coated sample at room temperature was shift toward noble potential. Polypyrrole coating with the highest value of corrosion potential in all indicating better corrosion resistance performance, compared to others. In addition, the computer-aided design and manufacturing (CAD / CAM) technology was employed to develop and produce aluminum bipolar plates. Surface modification was carried out on the aluminum plates. The coated and uncoated bipolar plates were assembled to a direct methanol fuel cell (DMFC) and cell performance testing was conducted by Potentiodynamic polarization and constant current methods. Results showed that the prepared aluminum bipolar plate was suitable for fuel cell applications.

碩士
逢甲大學
材料科學所
93
Abstract Stainless steels are considered to be good candidates for bipolar plate materials of PEM fuel cells due to their low cost, high strength and ease of machining. However, corrosion of the metallic plates is a severe problem and affects the performance and lifetime of fuel cells. The NiAl intermetallic compound provides excellent properties of oxidation resistance , high electrical, thermal conductivities and was selected as protective layer. The NiAl film was deposited on AISI 304 stainless plates by DC magnetron sputtering system. Field emission scanning electron microscope , grazing incident X-ray diffraction, glow discharge spectrometer and potentialstat were used to analyze the NiAl film . From the XRD and GDS data, the sample annealed at 800℃with in situ and post process appears the AlFe0.23Ni0.77 phase as a result of interdiffusion between the NiAl film and the substrate . The FE-SEM photos reveal that the in situ annealing process leads to island-like and rough surface morphology. Moreover, the analysis of the potential dynamics curves divulges that the sample is post annealed at 600℃ under a working pressure of 9 mtorr exhibits an promotional corrosion potential of -69.5 mV.

Η παρούσα διπλωματική πραγματεύεται τη μελέτη και κατασκευή ενός κυκλώματος οδήγησης ενός Σύγχρονου Κινητήρα Μαγνητικής Αντίδρασης. Η εργασία αυτή εκπονήθηκε στο Εργαστήριο Ηλεκτρομηχανικής Μετατροπής Ενέργειας του τμήματος Ηλεκτρολόγων Μηχανικών και Τεχνολογίας Υπολογιστών της Πολυτεχνικής Σχολής του Πανεπιστημίου Πατρών. Σκοπός είναι η μελέτη και κατασκευή ενός κυκλώματος Τριφασικού Αντιστροφέα Πηγής Τάσης, ο οποίος θα επιτρέπει την υλοποίηση βαθμωτού και διανυσματικού ελέγχου της λειτουργίας του Σύγχρονου Κινητήρα Μαγνητικής Αντίδρασης. Στην παρούσα διπλωματική εργασία υλοποιήθηκε βαθμωτός έλεγχος της γωνιακής ταχύτητας περιστροφής του δρομέα (με σταθερό λόγο V/f), ανοιχτού και κλειστού βρόχου. Όσον αφορά το Σύγχρονο Κινητήρα Μαγνητικής Αντίδρασης, τα τελευταία χρόνια έχει αναπτυχθεί έντονο επιστημονικό ενδιαφέρον σχετικά με τη βελτιστοποίηση της κατασκευής του, καθώς και του ελέγχου της λειτουργίας του μέσω διαφόρων μεθόδων ελέγχου. Το κυριότερο χαρακτηριστικό του συγκεκριμένου κινητήρα αποτελεί η έλλειψη οποιασδήποτε μορφής διέγερσης στον δρομέα. Επομένως, η δημιουργούμενη ροπή οφείλεται αποκλειστικά στη λεγόμενη ροπή μαγνητικής αντίδρασης, από την οποία προέρχεται και η ονομασία του κινητήρα. Στην παρούσα διπλωματική εργασία, γίνεται μια εισαγωγή στους κινητήρες εναλλασσόμενου ρεύματος, ενώ η ανάλυση επικεντρώνεται στον προαναφερθέντα κινητήρα, προσεγγίζοντάς τον από διάφορες πλευρές (κατασκευαστικά χαρακτηριστικά, μαθηματικό μοντέλο, σύγκριση με άλλους τύπους κινητήρων). Επιπλέον γίνεται μια θεωρητική ανάλυση του Τριφασικού Αντιστροφέα Πηγής Τάσης, ο οποίος χρησιμοποιείται για την οδήγηση του κινητήρα, καθώς και της τεχνικής Διαμόρφωσης Εύρους Παλμών, η οποία χρησιμοποιείται για την παλμοδότηση των ημιαγωγικών στοιχείων ισχύος. Η προσομοίωση του κινητήρα ήταν το επόμενο βήμα για την καλύτερη κατανόηση της δυναμικής απόκρισης του κινητήρα. Η παρούσα διπλωματική εργασία συνεχίζεται με την αναφορά στο σχεδιασμό και την κατασκευή των απαραίτητων κυκλωμάτων, ενώ ιδιαίτερη αναφορά γίνεται στο μικροελεγκτή που χρησιμοποιήθηκε στην υπάρχουσα εργασία, καθώς και στο πρόγραμμα που δημιουργήθηκε για τις λειτουργίες ανοιχτού και κλειστού βρόχου. Τέλος, η ολοκλήρωση της διπλωματικής εργασίας έρχεται μέσω της παρουσίασης των πειραματικών αποτελεσμάτων, στα οποία έγινε χρήση των κατασκευασθέντων κυκλωμάτων.
This Thesis is focused on the study and development of a Drive System for the Synchronous Reluctance Motor. This work was conducted in the laboratory of Electromechanical Energy Conversion, at the department of Electrical and Computer Engineering, in the University of Patras, Greece. The main purpose of this project was the study and construction of a three Phase Voltage Source Inverter for the control of the performance of a Synchronous Reluctance Motor by the implementation of Scalar and Vector control. In this thesis, a scalar V/f control scheme was applied, both open and closed loop, for the control of the rotational speed of the rotor. As far as the Synchronous Reluctance Motor is concerned, in the latest years a great interest has emerged around this motor, which mainly focuses in the optimization of its construction and control. The main feature of this motor is that the rotor does not have any field winding. By this way, the output torque is produced only by the so called reluctance torque. In this work, an introduction on the AC motor is done, while the main interest is focused on the already mentioned motors. The analysis of this motor covers many aspects, such as the construction characteristics, the mathematical model of the motor, as well as a comparison with other popular motors. Moreover, the three Phase Inverter is studied, since it is used for the control of the motor. Also, there is an extended reference on the Pulse Width Modulation technique, which is used for the control of power devices. In the next chapters, the simulation of this motor is presented, since it is necessary fot the understanding of its dynamic behavior. In the following, an analysis on the design and construction of the required printed circuit boards is done, while the microcontroller which was used is presented in a more detailed way. The flowacharts of the open and closed loop control methods of the rotational speed are also given. Finally, the experimental results for both cases are presented and analysed.

碩士
國立交通大學
機械工程學系
98
Water management is an important issue to improve performance of proton-exchange-membrane (PEM) fuel cells. In order to understand the feasibility of monitoring water production in PEM fuel cells using ultrasonic visualization, this thesis numerically studies linear array induced ultrasound propagation in graphite bipolar plates with machined flow channels. The material properties of graphite bipolar flow channels were experimentally characterized. Experimental results indicate that it is nearly an isotropic material. Comparing the ultrasonic signals acquired in different states and areas of graphite bipolar plates, the reflected echoes have significant changes when water appears in flow channels. According to the B-scan signals of graphite bipolar plates using immersion focusing ultrasonic transducer, the distribution of water can be clearly determined. Based on the investigation of the acoustic field induced by single element piezoelectric transducer attached on surface of semi-infinite solids, a linear array ultrasonic transducer was further designed. Both graphite composite and pure graphite are considered as acoustic media under investigation. Single element generated ultrasound in pure graphite has narrow main lobe due to its hexagonal anisotropy. The side lobe produced by acoustic phase interference is relatively small. It results that acoustic directivity of sound beam has smaller divergence angle in pure graphite than graphite composite. Transient analysis for acoustic field induced by linear array ultrasonic transducer in graphite bipolar plate indicates that the water distribution in the flow channels can be successfully discriminated.

博士
中興大學
生物產業機電工程學系所
95
The use of metallic bipolar plate on fuel cell application has been highly expected and pay attention. However, the results of the relative researches were not affirming the achievements. The main reason is the corrosion of the bipolar plates due to the acid aqueous solution produced during the fuel cell operation or the decrease of cell performance due to the formation of metallic ions. In this study, we choose the Titanium (Ti) as the base material for the bipolar plates. By applying the electroplating and sintering methods of the Dimensionally Stable Anode, DSA. The surface of the Titanium bipolar plates were modified by coating with 2.5µm platinum (Pt) layer or sintering 20 g/m2 iridium oxide (IrO2) layer. Platinum (Pt) and Iridium oxide (IrO2) are not only corrosive resistance, but also consist anti-oxidant and perfect electrochemical properties, which are very suitable for being the materials as surface modification of metallic bipolar plates. The results indicated that the optimum operation conditions for the metallic bipolar plates and graphite bipolar plates were totally different. For the metallic bipolar plates, the hydrogen and oxygen fuel must be fully humidified, while the current density and power density of the fuel cell were better under a lower operation temperature. For the graphite bipolar plates, the hydrogen and oxygen fuel only need proper humidified, while the current density and power density of the fuel cell were better under a higher operation temperature. The long term durability testing for different types of bipolar plates were being performed, the results indicated that the main flow field channel of the Titanium bipolar plates single fuel cell had oxidation phenomenon, where the Cathode end was the most serious. Whereas, the electroplating layer of the main flow field channel of the Titanium coating with gold (Au) bipolar plates single fuel cell also had serious corrosion phenomenon at the Cathode end. In fact, the corrosion resistance ability of the bipolar plates sintering with iridium oxide (IrO2), coating with platinum (Pt) and graphite were better. The results also indicated that through the surface modification of the bipolar plates, the corrosion problem due to the acid aqueous solution can be improved; the cost can be lowered, and led to the fuel cell become lighter and thinner.

碩士
國立中山大學
機械與機電工程學系研究所
98
The effects of the structure of new carbon fiber bunch heterogeneous unipolar plates on the performance of PEMFC are studied in this thesis. Internal structure of carbon fiber bunches can be modified by embedding different thickness or number of copper plates in the glue bonding area to increase the air permeability of carbon fiber bunches in its soft end. We can add different thickness or amount of coppers at the middle of bonding area, making the carbon fiber bunches soft side to form parallel to the longitudinal fiber bunch with a small flow channel. We can also make a trench at the appropriate place of the soft side of the carbon fiber bunches to form an extra air passage. In order to make the above flow channel, a new process for making the carbon fiber bunches is developed also. This process will be easier to produce a variety of different structures of carbon fiber bunch. Finally, several different experiments are performed to help us to understand the effect of the carbon fiber bunch structure on the performance and find out the best structure of the carbon fiber bunches. The carbon fiber bunch structures of the test cells on the anode side are all the same, but the carbon fiber bunch structures of on cathode side are all different. Experiments show that there are two structures among all test structures displayed better gas permeability. The first one is two 0.2 mm copper plates embedded within both sides of the glue ends of a cathode carbon fiber bunch, so that a small longitudinal flow channel are formed in soft end of the cathode carbon fiber bunch. When the HFC operates at room temperature and by air-breathing, the highest performance of the HFC can reach a value of 185 mW/cm2. The second one is a 0.2 mm copper plate embedded in the center of the glue end of a carbon fiber bunch, and then three 2 mm wide serrated slots are cut on the soft end of the carbon fiber bunch. The highest performance of the HFC can reach a value of 190 mW/cm2. The highest performance of the HFC with no copper plate and no slot structure can only reach a value 160 mW/cm2. The second design can increase the no structure cell performance 18.8%. Therefore, the internal structures of carbon fiber bunches are significant to affect on the fuel cell performance, and its internal design must be considered.

碩士
國立虎尾科技大學
光電與材料科技研究所
97
In this study, the corrosion behavior of electroless Ni, Au, Ag plating and Polypyrrole coatings using cyclic voltammetry were prepared on SS 316 stainless steel is investigated in a simulated polymer electrolyte membrane fuel cell (PEMFC) environment included 0.5 M at room temperature and 70℃. The main purposes are to offer fundamental information for the development of metal bipolar plates in PEMFC, and to obtain a deeper understanding of the electrochemical corrosion characteristics for electroless plating systems and the polypyrrole by electrochemical polymerization. Potentiodynamic tests were performed to determine the corrosion properties of electroless plating and polypyrrole coatings. It was found that corrosion potential of the coated sample at room temperature was shift toward noble potential. Polypyrrole coating with the highest value of corrosion potential in all indicating better corrosion resistance performance, compared to others. However, only the corrosion potential of electroless Ag plating and Polypyrrole coatings have the tendency that corrosion potential was shift toward noble potential at 70℃. Electroless Ag plating with the highest value of corrosion potential in all indicating better corrosion resistance performance, compared to others. This study addresses the corrosion resistance characteristics of polypyrrole coating on SS 316 stainless steel for PEMFC bipolar plate applications at room temperature environment. However, electroless Ag plating on SS 316 stainless steel for PEMFC bipolar plate applications at 70℃.