The dynamic performance of polymer electrolyte membrane fuel cells (PEMFCs) is of great interest for mobile applications such as in automobiles. However, the length scale of a PEM fuel cell's main components are ranging from the micro over the meso to the macro level, and the time scales of various transport processes range from milliseconds up to a few hours. This combination of various spatial and temporal scales makes it extremely challenging to conduct in-situ measurements or other observations through experimental means. Thus, numerical simulation becomes a very important tool to help understand the underlying electrochemical dynamics and transient transport phenomena within PEM fuel cells. In this thesis research, a comprehensive 3D model is developed which accounts for the following transient transport mechanisms: the non-equilibrium phase transfer between the liquid water and water vapor, the non-equilibrium membrane water sorption/desorption, liquid water transport in the porous backing layer, membrane hydration/dehydration, gas diffusion in the porous backing layer, the convective gas flow in the gas channel, and heat transfer. Furthermore, some of the conventionally used modeling assumptions and approaches have been incorporated into the current model. Depending on the modeling purposes, the resulting model can be readily switched between steady and unsteady, isothermal and non-isothermal, single- and multi- phases, equilibrium and non-equilibrium membrane sorption/desorption, and three water production assumptions. The governing equations which mathematically describe these transport processes, are discretized and solved using a finite-volume based commercial software, Fluent, with its user coding ability. To handle the significant non-linearity stemming from the multi-water phase transport, a set of numerical under-relaxation techniques is developed using the programming language C. The model is validated with experimental results and good agreements are achieved. Subsequently, using this validated model numerical studies have been carried out to probe various transient transport phenomena within PEM fuel cells and the cell dynamic responses with respect to different operating condition changes. Furthermore, the impact of flow-field design on the cell performance is also investigated with the three most common flow channel designs.

Polymer Electrolyte Membrane Fuel Cells (PEMFC) are promising power generating devices that use an electrochemical reaction to convert the energy from hydrogen fuel into usable electricity. One cell produces a small voltage so many cells are combined in series in order to produce a useful voltage, this configuration is referred to as a stack. Hydrogen is supplied to the anode of the stack in amounts greater than the electrochemical reaction requires to guarantee that enough hydrogen is available for every cell in the stack and to provide enough pressure throughout the cell flow channels for good mass transfer. For reasonable fuel efficiency, the anode outlet gas containing unconverted hydrogen is recycled (or recirculated) back to the anode inlet. PEMFC performance is highest when pure hydrogen fuel is supplied, however, nitrogen at the cathode will permeate through the membrane and accumulate in the anode gas with recirculation. Nitrogen buildup dilutes the hydrogen gas which adversely affects fuel cell performance at the anode. Also, in practical applications hydrogen-rich gas produced from reformed methane, called reformate, is used as the fuel. Reformate contains impurities such as, nitrogen, carbon dioxide, carbon monoxide, and sulfur compounds. This thesis will focus on trace levels of carbon monoxide entering in the hydrogen fuel stream, and the impact of contaminant build-up due to anode recirculation. Carbon monoxide adsorbs readily onto the platinum catalyst sites, called poisoning, thus decreasing PEMFC performance. In efforts to minimize the buildup of impurities and crossed over nitrogen, a portion of the anode outlet gas is periodically and continuously purged to the exhaust. How often the outlet gas is purged depends on a variable called the purge fraction. The purpose of this research is to study the effect of purge fraction on PEMFC performance, measured by the average cell voltage, for a Hydrogenics 10 cell stack. The operating parameters used for testing and the experimental apparatus were designed to mimic a Hydrogenics 8kW Hydrogen Fuel Cell Power Module. A pump connected between the anode outlet and anode inlet form the anode recirculation loop. In Phase 1 of the test program the effect of purge in the absence of carbon monoxide was studied to see if hydrogen dilution from nitrogen crossover and accumulation would cause significant cell voltage degradation. In Phase 2 the effect down to 0.2 ppm carbon monoxide was evaluated. The results showed that nitrogen buildup, in the absence of carbon monoxide, did not significantly penalize the cell performance in the range of purge fractions tested. However, for the same purge fraction but with as little as 0.2 ppm carbon monoxide present, the voltage loss was significant. A discussion of the effect of purge on the impurity concentration and the associated cell voltage degradation is detailed with particular emphasis on carbon monoxide poisoning.

Försvarets Materielverk (FMV) driver sedan 2003 ett bränslecellsprogram med syfte att öka Försvarsmaktens kunskap om bränslecellstekniken (FC-tekniken) och dess potential för framtida applikationer. Arbetet genomförs inom ramen för FM´s ”Dual Use”-program vars mål är att kartlägga potentiella strategiska teknologier för både civila och militära ändamål. En av slutsatserna som FMV drar är att bränsleceller inte nödvändigtvis är det självklara valet för alla studerade applikationer, men att tekniken visar på fördelar som strömförsörjningsfunktion för vissa tillämpningar. FC-tekniken har inneboende egenskaper vilka några innebär fördelar för såväl civila som militära tillämpningar. Särskilda fördelar relevant för militär verksamhet är att de har en låg ljudnivå och hög bränsledensitet vilket innebär potentiellt lägre upptäcktsrisk jämfört med förbränningssystem och betydligt längre drifttid jämfört med motsvarade vikt batterier. Vidare kan de konstrueras att vara bränsleflexibla och utformas för allt ifrån ren vätgas till diesel med hög svavelhalt. Frågan är nu vilka tillämpningar inom FM som faktiskt kan dra nytta av ett strömförsörjningssystem som möjliggör längre drifttid än batterier och samtidigt är tystare än en förbränningsmotor. Syftet med arbetet är att identifiera dessa tillämpningar inom FM där FC-teknikens inneboende egenskaper kan göra den till ett intressant strömförsörjningsalternativ samt precisera hur dessa system kan specificeras. Detta görs genom att svara på följande frågor: För vilka typer av tillämpningar inom FM har FC-tekniken konkurrensfördel? Hur skall specifikationerna för dessa bränslecellsystem formuleras? Uppgiften har lösts dels genom litteraturstudier av resultat och slutsatser av redan genomförda studier och dels genom att intervjua nyckelpersoner i relevanta positioner för de tänkta strömförsörjningssystemen. De tillämpningar som studeras och potentiellt kan ha konkurrensfördel gentemot förbränningssystem och/eller elektrokemisk lagring som t.ex. batterier är strömförsörjning av mobil och portabel utrustning, strömförsörjning av obemannade system, reservkraft, kombinerad kraft- och värmeproduktion (CHP) samt strömförsörjning av avlägsna enheter, dvs. enheter som inte har möjlighet att vara uppkopplade mot det ordinarie elnätet. Genom en utvärderingsprocess valdes två av dessa tillämpningar ut vilka ansågs vara speciellt intressanta att studera i mer detalj; ett mindre batteriladdningssystem på gruppnivå samt ett strömförsörjningssystem för en undervattensfarkost (AUV). Batteriladdningssystemet är avsett för förband som enskilt skall kunna lösa uppgifter i upp till tre veckor utan möjlighet till externt underhåll. Förband som har dessa uppgifter är specialförbanden, arméns jägarbataljon, underrättelsebataljonen samt amfibiekårens kustjägarkompani. Givet en generisk användargrupp, som anses kunna representera samtliga förband, kunde det maximala laddeffektbehovet uppskattas till drygt 80 W vid maximalt rekommenderad laddström. Om en längre laddtid kan accepteras kan dock effektbehovet minskas och likaså batteriladdningssystemets vikt och volym. Batteriladdningssystemet skall kunna hantera en rad olika typer och storlekar av batterier vilket ställer krav på både lämpliga fysiska gränsytor samt en funktion för ”smart laddning”, dvs. att systemet autonomt kan kontrollera hur det aktuella batteriet skall laddas. Då batteriladdningssystemet är avsett att användas i fältmiljö både nationellt och internationellt finns krav på funktion både i låga och höga temperaturer samt okänslighet mot både väta och sand. De kommersiellt idag tillgängliga systemen som potentiellt uppfyller kraven har en systemvikt på ca 12 kg. För den studerade AUV-applikationen gjordes beräkningar för tre olika strömförsörjningsfall; 1) Enbart batterier 2) Enbart ett bränslecellsystem och 3) Ett bränslecellsystem som hanterar baslasten och ett batteripack som hanterar topplasten. Av resultatet att döma finns i den studerade AUV-applikationen, med aktuell driftprofil, möjligen ett behov av hybridisering då denna lösning medför en 20 procentig ökning i drifttid. Mot detta skall ställas den ökade komplexiteten och kostnaden som detta kan medföra. Detta kan härledas till det faktum att topplasten inte skiljer sig markant mot baslasten samt att topplasten utnyttjas under mer än hälften av den totala drifttiden. I ett sådant fall verkar batteriets potentiellt högre förmåga att leverera hög effekt snabbt överskuggas av dess lägre energidensitet jämfört med bränslet. I kravet finns behov av att systemet under kort tid, < 1 min, skall kunna leverera cirka 10 kW då farkosten ”simmar” ut ur torpedtuben. Detta innebär givetvis att någon form av hybridisering är nödvändig. Exempelvis skulle en lösning kunna medge ett uttag av denna effekt under kort tid och som sedan under transitperioden har förmåga att ladda upp batteriet innan detta skall användas under den tid farkosten är fullt operativ. För att klargöra hur en sådan lösning kan utformas behövs detta krav analyseras mer i detalj. Eftersom effektbehovet i driftprofilen ökar relativt lite mellan bas- och topplast (1020 – 1290; +26 %) kommer skillnaden i massa och volym mellan två bränslecellsystem motsvarande bas- och topplast vara relativt liten. Ett något större system som kan hantera både bas- och topplast borde således inte medföra varken en betydligt större massa eller volym. Bränslecellsteknikens generellt höga verkningsgrad vid dellast medför också en hög verkningsgrad vid båda lastfallen. En fortsatt analys av denna lösning bör inriktas på att studera frågor som rör vilken typ av lagring som är lämplig för behovet av ett kortvarigt effektuttag, om denna lösning enbart skall hantera detta effektuttag eller om en större hybridisering av den typ som presenteras i Fall 3 är intressant. Utöver dessa resultat har tekniska specifikationer skrivits för respektive applikation. Dessa återfinns som bilagor. / The Swedish Defence Materiel Administration (FMV) has since 2003 been running a fuel cell programme with the purpose to increase the Armed Forces knowledge of fuel cell technology and its potential for future applications. This work has been performed as a part of the Armed Forces "Dual-Use"-programme which goal is to identify potential strategic technologies for both civilian and military purposes. Specific fuel cell technology advantages relevant to military activities are low acoustic and infra red signature and high fuel density which potentially means lower risk of detection compared to combustion systems and notably longer operating times in comparison to batteries. Furthermore, they can be constructed to be fuel flexible and be designed to handle everything from pure hydrogen to high sulphur diesel fuel. The question now is which applications in the Armed Forces that can actually benefit from a power supply system which allows for longer operating times than batteries yet is quieter than a combustion engine. The task has been solved partly by studies of the results and conclusions of already conducted reports and partly by interviewing key personnel in relevant positions for each of the proposed power supply systems. Two applications has been selected to be studied in more detail; a battery charging system and a power supply system for an Autonomous Underwater Vehicle (AUV).  The battery charging system is intended to meet the power supply demands of small ranger or reconnaissance units required to operate independently of other forces for up to three weeks.  The maximum power requirement was calculated to be approximately 90 W at the maximum recommended charge current the radio battery being normative. However, if a longer charge time is acceptable this power requirement could be reduced and along with it the system weight and volume.   For the AUV application three cases were formulated; 1) Only battery power 2) Only fuel cell power 3) Fuel cell power handles base load and battery power handles peak load. The results showed calculated operational times of 11, 20 and 24 hours respectively. In addition to these results, technical specifications have been produced for each application. These are included as attachments.

Polymer electrolyte membrane (PEM) fuel cells are a promising zero-emission power source for transportation applications. An important tool for advancing PEM fuel cell technology is mathematical modeling. Mathematical models can be used to provide insight on the physical phenomena occurring within a fuel cell, as well as aid in the design of fuel cells by simulating the effect of changes in design or operating conditions on performance. A comprehensive, consistent and systematic general formulation for a mathematical PEM fuel cell model is presented in this thesis. The formulation is developed by considering the fuel cell to be composed of several, co-existing phases. The conservation of mass, momentum, species, and energy are applied to each phase in the fuel cell. The interactions between the phases are modeled by applying a volume-averaging procedure to the conservation equations in each phase. The solution of the governing equations for the general formulation are beyond the scope of this thesis research. Instead, simplifying assumptions are applied to the general formulation in order to reduce the number of governing equations. The cell is assumed to be two-dimensional, steady state and isothermal. As well, the polymer electrolyte is assumed to be impervious to the gas phase and liquid water is assumed to exist only in the gas phase or polymer electrolyte. The numerical solution of the simplified formulation is implemented using the computer language of C++ and the finite volume method. The numerical solution provides details of the transport phenomena within the anode and cathode gas flow channels, electrode backing layers, and catalyst layers, as well as the polymer electrolyte membrane layer. These details include the bulk velocity of the gas phase; the concentrations of the species within the gas phase; the potential and current density in the solid phase and polymer electrolyte; the water content in the polymer electrolyte; and the distribution of reaction rate within the catalyst layers.

The ‘Hydrogen Economy’ is a proposed system where hydrogen is produced from carbon dioxide free energy sources and is used as an alternative transportation fuel. The application of hydrogen on board fuel cell vehicles can significantly decrease air pollutants and greenhouse gases emission from the transportation sector. There must be significant transition of infrastructure in order to achieve the hydrogen economy with the investment required in both production and distribution infrastructure. This research focused on the projected demands for infrastructure transition of ‘Hydrogen Economy’ in Ontario, Canada. Three potential hydrogen demand and distribution system development scenarios were examined to estimate hydrogen fuel cell vehicle market penetration, as well as the associated hydrogen production and distribution. Demand of transportation hydrogen was estimated based on the type of hydrogen fuel cell vehicle. Upon the estimate of hydrogen demand from fuel cell vehicles in Ontario, the resulting costs of delivered hydrogen were investigated. In the longer term hydrogen is expected to be produced by utilizing nuclear heat and a thermochemical production cycle. A brief survey of thermochemical hydrogen production cycles was presented with a focus on S-I cycle. Sequential optimization models were developed to explore the minimum utility energy consumption and the minimum number of heat exchangers. Finally an optimal heat exchanger network for S-I thermochemical cycle was defined by a mixed integer optimization model using GAMS.

The dynamic performance of polymer electrolyte membrane fuel cells (PEMFCs) is of great interest for mobile applications such as in automobiles. However, the length scale of a PEM fuel cell's main components are ranging from the micro over the meso to the macro level, and the time scales of various transport processes range from milliseconds up to a few hours. This combination of various spatial and temporal scales makes it extremely challenging to conduct in-situ measurements or other observations through experimental means. Thus, numerical simulation becomes a very important tool to help understand the underlying electrochemical dynamics and transient transport phenomena within PEM fuel cells. In this thesis research, a comprehensive 3D model is developed which accounts for the following transient transport mechanisms: the non-equilibrium phase transfer between the liquid water and water vapor, the non-equilibrium membrane water sorption/desorption, liquid water transport in the porous backing layer, membrane hydration/dehydration, gas diffusion in the porous backing layer, the convective gas flow in the gas channel, and heat transfer. Furthermore, some of the conventionally used modeling assumptions and approaches have been incorporated into the current model. Depending on the modeling purposes, the resulting model can be readily switched between steady and unsteady, isothermal and non-isothermal, single- and multi- phases, equilibrium and non-equilibrium membrane sorption/desorption, and three water production assumptions. The governing equations which mathematically describe these transport processes, are discretized and solved using a finite-volume based commercial software, Fluent, with its user coding ability. To handle the significant non-linearity stemming from the multi-water phase transport, a set of numerical under-relaxation techniques is developed using the programming language C. The model is validated with experimental results and good agreements are achieved. Subsequently, using this validated model numerical studies have been carried out to probe various transient transport phenomena within PEM fuel cells and the cell dynamic responses with respect to different operating condition changes. Furthermore, the impact of flow-field design on the cell performance is also investigated with the three most common flow channel designs.

To ran a DMFC without methanol crossover is the aim of this study.It is done by supplying fuel no more than what the anode can consume. The first is to explore the factors that may affect the time constant of vapor feed DMFC. In order to reduce the time constant of current decline, first, we decrease store tank¡¦s space of methanol with different structure of unipolar plate. Second, we reduce the thickness of anode stack and the space above the air bleed valve. Using slide plate instead of air bleed valve can shorten the diffuse distance effectively and reduce the time constant of current rise curve. The second is to explore the impact of supply of methanol on steady-state current of system. Using air bleed valve, because of its high gas tightness, the utilization rate of methanol can exceed 94% without crossover. It was found that in the slide plant experiment, steady-state current value depends mainly on the pore size of slide plate, and resistance value has nothing to do. However, the resistance value is lower, the time required to reach steady-state current is shorter. The third is to explore if the performance decay after long time test of steady-state current. It was found that the performance of MEA will decay while the water content of membrane decreased.

A theoretical model and numerical simulation of a direct methanol fuel cell (DMFC) is developed to simulate the reaction mechanisms and the cell voltage under several different designing parameters and operational conditions. The results of a numerical simulation include the distributions of the proton current density, the concentration of methanol, the electrochemical reaction rates, the overpotential losses, and the pressures within proton exchange membrane layer, catalyst layer and diffusion layer. In addition, the influence of aforementioned operational conditions on methanol crossover in a direct methanol fuel cell is also investigated. Finally, the results of the model are compared to the results from the experimental work. The results show that increasing of temperature, pressure and anode catalyst loading can enhance the performance of a direct methanol fuel cell, and the concentration of methanol plays an important role in its performance. The optimal concentration of methanol for a direct methanol fuel cell is about 2M. Methanol crossover can be suppressed by decreasing methanol concentration and increasing thickness of polymer electrolyte membrane (PEM). However, under operating condition of high current density, thick PEM and low methanol concentration will cause large concentration overpotential and ohmic losses, respectively.

The research of a hydrogen proton exchange membrane fuel cell is performed under certain designing and operational conditions. The water management technique is incorperated into the experimental work. The cell voltage vs. the current densities are studied by changing the stack reactive temperatures, the gas inlet temperatures and pressures, and the relative humidities in hydrogen stream. Eventually, we hope that these experimental results can provide the information about the optimizing conditions of fuel cells so that they can be used to design a high power multiple-cell fuel cell stack. A membrane and electrode assembly (called MEA) which contains a proton exchange membrane Nafion 112, anode catalyst Pt 0.4 mg/cm2, and cathode catalyst Pt 1.0 mg/cm2 is used in this experiment. The gas flowing area is about 58% of the total area. A proper heating and humidification equipment is applied in this experimental system. The experimental results show that the cell voltage at low current density is slightly influenced by the hydrogen inlet temperature; however, the cell voltage at high current density is strongly influenced by the humidity ratio of hydrogen stream. Raising the hydrogen pressure and the oxygen pressure at the same time can increases the cell voltage, but it is no obvious effects on the cell voltage when the gas pressure increases to more than 2 atm. When air is used as a oxidizer, increasing the inlet air temperature always reduces the cell voltage. With the hydrogen stream at saturated temperature 80¢XC, the assembly torque of the stack at 4 N-m, and the stack temperature at 80¢XC, the single fuel cell stack can always generate the best cell voltages at most of the current densities. At this time, the cell voltage at current density 1 A/cm2 already can reach a value higher than 0.6 V.

Abstract The characteristics of PEMFC stacks in different designs and operational conditions are studied and manufactured in this thesis. There are many factors that affect the PEMFC performance. They include the familiar humidity, the torque, the inlet pressure, the geometries of inlet ports and the flow channels in reaction regions, the cell numbers of the stacks, the type of the oxidizer and its flow rate. To understand the performance characteristics of stacks the voltage and current density will be measured as well as the interior temperature of stacks in this research. The membrane exchange assembly (MEA) with Nafion 112, anode Pt 0.4 mg/cm² and cathode Pt 1.0 mg/cm² is used in these experimental works. The experimental results display that increasing the applied torque will reduce the contact resistance between bipolar plate and diffusion layer but increase the difficulty of gas penetrating into the reaction region beneath the bipolar rib. So proper torque is necessary to obtain the best voltage output. The voltage vs. current density also increases as the inlet pressure increases, but its effect will reduce when the inlet pressure increases over 2atm. The geometry and size of inlet port to each cell for a multi-cell stack will influence the voltage output, especially in high current density, so that special attention is needed in designing inlet port. When the air is used as an oxidizer, the fan with a high rotation speed is helpful in an open circuit design. The high air volume flow rate can avoid that the voltage output decays greatly in high current density. Increasing the cell number may cause extra internal resistance due to assembling improperly and reduce the voltage output. So special attention is also needed in assembling. Keyword: Proton Exchange Membrane Fuel Cell Stacks