The polymer electrolyte membrane fuel cell (PEMFC) is an electrochemical device used for the production of power, which is a key for the transition towards green and renewable power delivery devices for mobile, stationary and back-up power applications. PEMFCs consume hydrogen and oxygen to produce power, water and heat. The transient start-up from sub-zero freezing temperature conditions is a problem for the successful, undamaged and unhindered operation. The generation and presence of water in the PEMFC stack in such an environment leads to the formation of ice that hinders the flow of gases, causes morphological changes in the membrane electrode assembly (MEA) leading to reversible and irreversible degradation of stack performance. Start-up performance is highly dependent on start-up operational conditions and procedures. The previous state of the stack will influence the ability to perform upon the next start-up and operation. Water generated during normal operation is vital and improves performance when properly managed. Liquid water present at shut-down can form ice and cause unwanted start-up effects. This phase change may cause damage to the MEA and gas diffusion media due to volume expansion. Removal of high water content at shutdown decreases proton conductivity which can delay start-up times. The United States Department of Energy (DOE) has established a set of criteria that will make fuel cell technology viable when attained. As specified by DOE, an 80 kWe fuel cell will be required by 2015 to reach 50% power in 30 seconds from start-up at an ambient temperature of -20°C. This work investigates freeze start-up in a multi-kilowatt stack approaching both shut-down conditioning and start-up operations to improve performance, moderate fuel cell damage and determine the limits of current stack technology. The investigation involved a Hydrogenics Corporation 5 kW 506 series fuel cell stack. The investigation is completed through conditioning the fuel cell start-up performance at various temperatures ranging from -5°C to below -20°C. The control of system start-up temperature is achieved with an environmental chamber that maintains the desired set point during dwell time and start-up. The supply gases for the experiment are conditioned at ambient stack temperature to create a realistic environment that could be experienced in colder weather climates. Temperature controls aim to maintain steady ambient temperatures during progressive start-up in order to best simulate ambient conditions. The control and operation of the fuel cell is maintained by the use of a fuel cell automated test station (FCATS™). FCATS supplies gas feeds, coolant medium and can control temperature and reactant humidity in reactants according to a prescribed procedure for continuous operation. The iv collection of data occurs by the same system recording cell voltage, temperatures, pressures, flow rates and current densities. A procedural start-up and characterization are conducted in order improve start-of performance and examine reactant flows, coolant activation time, stack conditioning and the effects by freezing temperatures. The resulting degradation is investigated by polarization curves and various ex-situ measurements. In this work, it was found that freeze start-up of a fuel cell stack can be aided and managed by conditioning the stack at shut-down and applying a procedure to successfully start-up and mitigate the damage that freezing can cause.

Thesis (M.S.)--University of Delaware, 2008. / Principal faculty advisors: Ajay K. Prasad and Suresh G. Advani, Dept. of Mechanical Engineering. Includes bibliographical references.

Cette thèse réalisée dans le cadre des projets PIE CNRS AMELI-0Pt et AMEPlas et ANR AMADEUS a regroupé plusieurs entités autour de la thématique des piles à combustible : Dreux Agglomération puis l’Agence Innovation Made In Dreux (MID), le GREMI, le LACCO et initialement l’industriel MHS Equipment. L’objectif de ce travail est l’élaboration par voie plasma et l’optimisation d’électrodes de piles à combustible de type PEMFC et SAMFC dans le but d’obtenir de bonnes performances avec des charges de platine ultra faibles ou sans platine. Le projet a été organisé en quatre étapes : l’étude de la croissance simultanée de platine et de carbone co-pulvérisés par plasma, la dispersion optimale de quantités ultra faibles de catalyseur, le remplacement du platine par un alliage bimétallique à base de palladium, et le dépôt direct du catalyseur sur la membrane par plasma. En utilisant un faisceau synchrotron de rayons X (Synchrotron SOLEIL), en collaboration avec le CRMD, l’étude GISAXS des couches minces Pt-C co-pulvérisés a révélé l’organisation particulière du platine dans ce type de nanostructure. Ces couches minces Pt-C offrent d’excellentes performances (20 kW.gPt-1) avec des charges de platine ultra faibles. Des électrodes PdPt (5 %at Pt) faiblement chargées permettent d’atteindre de bonnes performances en PEMFC quasiment sans platine (12,5 kW.gPd-1 et 250 kW.gPt-1). L’étude de l’activité de catalyseurs PdAu vis-à-vis de l’oxydation du glycérol a révélé l’origine des effets synergiques du palladium et de l’or en milieu alcalin. Le dépôt plasma direct de platine associé ou non au dépôt de carbone sur membrane a été optimisé. Les performances obtenues avec des CCM (Catalyst Coated Membrane) plasma démontrent l’intérêt de ce type d’architecture. / This research work has been achieved in the context of the PIE CNRS AMELI-0Pt and AMEPlas and ANR AMADEUS projects and has gathered several entities around the Fuel Cell research: Dreux Agglomération and Agence Innovation Made In Dreux (MID), the French national research laboratories GREMI and LACCO and initially the company MHS Equipment. The project aims at developing and optimising fuel cell electrodes (anode and cathode) for PEMFC (Proton Exchange Membrane Fuel Cell) and SAMFC (Solide Alkaline Membrane Fuel Cell) entirely by plasma in order to reach effective performances with ultra low platinum loadings or none at all. The project was divided into four stages: the study of the simultaneous growth of platinum and carbon co-sputtered by plasma, the optimum dispersion of a very small amount of catalysts, the replacement of platinum by a palladium based bimetallic alloy, and the direct deposition of the catalyst on the polymer membrane by plasma sputtering. By using an X-ray synchrotron beam light source (SOLEIL Synchrotron), in collaboration with the CRMD, the GISAXS study of co-sputtered Pt-C thin films has revealed the particular organisation of platinum inside this type of nanostructure. These Pt-C thin films offer excellent performances (20 kW.cm-2) with ultra low platinum amounts. Low loaded PdPt (5 %at Pt) electrodes offered good performances almost without platinum (12,5 kW.gPd-1 et 250 kW.gPt-1). The study of the activity of PdAu catalysts (plasma sputtered) on the glycerol electro-oxidation revealed the origin of the synergistic effects of palladium and gold in an alkaline medium. The direct plasma deposition of platinum associated or not with carbon deposition on membrane has been optimised. The performances of the plasma prepared CCM (Catalyst Coated Membrane) demonstrate the potential of this type of architecture.

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Despite the rapid growth of compute power in the last decades, the full-fledged, 3D mathematical models of fuel cells are not a viable option when it comes to applications requiring real-time capability; on the other hand, the current crop of 1D models apply boundary conditions pertinent merely to a single point in the cell – to provide data for effective fuel cell system design, a balance needs to be struck. The predictive power of a lower dimensionality fuel cell model can provide a reasonably detailed and accurate assessment and tracking of the fuel cell state and cater data to model-based control algorithms or define requirements for the selection of balance of plant components. To cover a wide parametric space and allow a rapid generation of the corresponding fuel cell system states, a combination of two 1D stationary models (a pseudo-2D model) has been chosen. One model defines the inlet conditions and tracks their evolution along a gas flow channel in a bipolar plate, while the second model (in our case the MMM1D published by Vetter and Schumacher) solves the evolving boundary-value problem throughout the membrane-electrode assembly (MEA) and calculates the fluxes of species, heat and charge exchanged between the gas flow channel and the MEA. Because the most significant changes in the media state (temperature, pressure, composition and flow rate) occur at the cell level, the model can estimate stack outlet conditions from the inlet conditions, extending the cell level model to a fuel cell system context. The results obtained for several operating points are used to discuss the choice of some system components.

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A novel concept has been developed for the direct carbon fuel cell (DCFC) based on molten carbonate recirculating electrolyte. In the cathode, co-current flow of electrolyte with entrained gases carbon dioxide and oxygen is sent in the upward direction through a porous bed grid. In the anode, co-current flow of a slurry of electrolyte entrained with carbon particles is sent in the downward direction through a porous bed grid. The gases carbon dioxide and oxygen in the cathode react on the grid surface to form carbonate ions. The carbonate ions are then transported via conduction to the anode for reaction with carbon to produce carbon dioxide for temperatures under 750 deg C. A mathematical model based on this novel DCFC concept has been developed. The model includes governing equations that describe the transport and electrochemical processes taking place in both the anode and cathode and a methodology for solving these equations. Literature correlations from multi-phase packed-bed chemical reactors were used to estimate phase hold-up and mass transfer coefficients. CO production and axial diffusion were neglected. The results demonstrated that activation and ohmic polarization were important to the cell output. The impact of concentration polarization to the cell output was comparatively small. The bed depths realized were of the order of 10cm which is not large enough to accommodate the economies of scale for a large scale plant, however thousands of smaller cells (10 m^2 area) in series could be built to scale up to a 10 MW industrial plant. Limiting current densities of the order of 1000-1500 A/m^2 were achieved for various operating conditions. Maximum power densities of 200-350 W/m^2 with current densities of 500-750 A/m^2, and cell voltages of 0.4-0.5 V have been achieved at a temperature of 700 deg C. Over temperatures ranging from 700 to 800 deg C, results from the modeled cell are comparable with results seen in the literature for direct carbon fuel cells that are similar in design and construction. / Ph. D.

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<p>In this thesis, direct hydrogen Proton Exchange Membrane (PEM) fuel cell systems in vehicles are investigated through modelling, field tests and public acceptance surveys.</p><p>A computer model of a 50 kW PEM fuel cell system was developed. The fuel cell system efficiency is approximately 50% between 10 and 45% of the rated power. The fuel cell auxiliary system,<i> e.g.</i> compressor and pumps, was shown to clearly affect the overall fuel cell system electrical efficiency. Two hydrogen on-board storage options, compressed and cryogenic hydrogen, were modelled for the above-mentioned system. Results show that the release of compressed gaseous hydrogen needs approximately 1 kW of heat, which can be managed internally with heat from the fuel cell stack. In the case of cryogenic hydrogen, the estimated heat demand of 13 kW requires an extra heat source. </p><p>A phase change based (PCM) thermal management solution to keep a 50 kW PEM fuel cell stack warm during dormancy in a cold climate (-20 °C) was investigated through simulation and experiments. It was shown that a combination of PCM (salt hydrate or paraffin wax) and vacuum insulation materials was able to keep a fuel cell stack from freezing for about three days. This is a simple and potentially inexpensive solution, although development on issues such as weight, volume and encapsulation materials is needed </p><p>Two different vehicle platforms, fuel cell vehicles and fuel cell hybrid vehicles, were used to study the fuel consumption and the air, water and heat management of the fuel cell system under varying operating conditions, <i>e.g.</i> duty cycles and ambient conditions. For a compact vehicle, with a 50 kW fuel cell system, the fuel consumption was significantly reduced, ~ 50 %, compared to a gasoline-fuelled vehicle of similar size. A bus with 200 kW fuel cell system was studied and compared to a diesel bus of comparable size. The fuel consumption of the fuel cell bus displayed a reduction of 33-37 %. The performance of a fuel cell hybrid vehicle,<i> i.e.</i> a 50 kW fuel cell system and a 12 Ah power-assist battery pack in series configuration, was studied. The simulation results show that the vehicle fuel consumption increases with 10-19 % when the altitude increases from 0 to 3000 m. As expected, the air compressor with its load-following strategy was found to be the main parasitic power (~ 40 % of the fuel cell system net power output at the altitude of 3000 m). Ambient air temperature and relative humidity affect mostly the fuel cell system heat management but also its water balance. In designing the system, factors such as control strategy, duty cycles and ambient conditions need to taken into account.</p><p>An evaluation of the performance and maintenance of three fuel cell buses in operation in Stockholm in the demonstration project Clean Urban Transport for Europe (CUTE) was performed. The availability of the buses was high, over 85 % during the summer months and even higher availability during the fall of 2004. Cold climate-caused failures, totalling 9 % of all fuel cell propulsion system failures, did not involve the fuel cell stacks but the auxiliary system. The fuel consumption was however rather high at 7.5 L diesel equivalents/10km (per July 2004). This is thought to be, to some extent, due to the robust but not energy-optimized powertrain of the buses. Hybridization in future design may have beneficial effects on the fuel consumption. </p><p>Surveys towards hydrogen and fuel cell technology of more than 500 fuel cell bus passengers on route 66 and 23 fuel cell bus drivers in Stockholm were performed. The passengers were in general positive towards fuel cell buses and felt safe with the technology. Newspapers and bus stops were the main sources of information on the fuel cell bus project, but more information was wanted. Safety, punctuality and frequency were rated as the most important factors in the choice of public transportation means. The environment was also rated as an important factor. More than half of the bus passengers were nevertheless unwilling to pay a higher fee for introducing more fuel cell buses in Stockholm’s public transportation. The drivers were positive to the fuel cell bus project, stating that the fuel cell buses were better than diesel buses with respect to pollutant emissions from the exhausts, smell and general passenger comfort. Also, driving experience, acceleration and general comfort for the driver were reported to be better than or similar to those of a conventional bus.</p>