In polymer electrolyte membrane (PEM) fuel cells one of the most important components is the flow field. The flow field distributes reactant gasses to the active area and also delivers electrons from the outer circuit so that the electrochemical reaction may be completed. Optimizing flow field design is extremely important in order to increase the overall power density of the fuel cell. It is particularly important to understand the ways in which the different portions of the flow field, namely the land and channel sections, interact with the gas diffusion layer (GDL), catalyst layer and membrane; this study focuses on those interactions. The most common type of flow field design currently used in PEM fuel cells is the serpentine flow field. It is used for its simplicity of design, its effectiveness in distributing reactants and its water removal capabilities. The knowledge about where current density is higher, under the land or the channel, is critical for flow field design and optimization. Yet, no direct measurement data are available for serpentine flow fields. In this study a fuel cell with a single channel serpentine flow field is used to separately measure the current density under the land and channel, which is either catalyzed or insulated on the cathode. In this manner, a systematic study is conducted under a wide variety of conditions and a series of comparisons are made between land and channel current density. Results show that under most operating conditions, current density is higher under the land than that under the channel. However, at low voltage, a rapid drop off in current density occurs under the land due to concentration losses. The mechanisms for the direct measurement results and general guidelines for serpentine flow field design and optimizations are provided. In addition the same technique is utilized to separately measure current density under the land and channel on a variety of serpentine flow field geometries. Each flow field is tested under a wide variety of operating conditions thereby providing guidance for the optimum design geometry. Experimental results show that generally flow fields with both thinner lands and thinner channels provide better overall performance. However, the optimal flow field designs are highly dependent on fuel cell operating parameters. Finally, it is critical not only to know where the current density is greater, under the land or under the channel, but to understand the fundamental mechanisms driving these differences. Resistance was measured, ex-situ, between the GDE and flow plate under the land of the flow field and under the channel separately. The contact resistance between the gas diffusion electrode (GDE) and the graphite flow plate were measured using an ex-situ technique. The resistance was measured under different land and channel widths. Cyclic Voltammetry tests were also conducted in order to determine if there is any different in electrochemically active area(ECA) under the land and under the channel and what the cause of this difference might be. Results show that the compression of the gas diffusion electrode not only affects the electronic resistance but the ECA as well and that these are key factors in current density variations under the land and channel.

An investigation was conducted to determine the feasibility of using the technique of ultrasonic nebulization of water into the anode gas stream for evaporative cooling of a Proton Exchange Membrane (PEM) fuel cell. The basic concept of this form of internal evaporative cooling of the PEM fuel cell is to introduce finely atomized liquid water into the anode gas stream, so that the finely atomized liquid water adsorbs onto the anode and then moves to the cathode via electro-osmotic drag, where this water then evaporates into the relatively dry cathode gas stream, carrying with it the waste thermal energy generated within the fuel cell. The thermal and electrical performance of a 50 cm2 PEM fuel cell utilizing this technique was compared to the performance obtained with conventional water management. Both techniques were compared over a range of humidification chamber temperatures for both the anode and cathode gas streams so as to determine the robustness of the proposed method. The proposed method produced only meager levels of evaporative cooling (at best 2 watts, for which a minimum of 30 watts was required for adequate cooling), but the average cell voltage increased considerably (as much as a 10% gain), and the technique increased the fault tolerance of the fuel cell (the Nafion™ membrane did not dry out even if cell temperature went well in excess of 70° C despite both anode and cathode humidification temperatures of 55° C). An interesting phenomena was also observed wherein the fuel cell voltage oscillated regularly with a period of tens of seconds, and that the amplitude of this oscillation corresponded inversely with the level of humidification received by the fuel cell.

This thesis concerns system studies of power plants wheredifferent types of fuel cells accomplish most of the energyconversion. Ever since William Grove observed the fuel cell effect inthe late 1830s fuel cells have been the subject or more or lessintense research and development. Especially in the USA theseactivities intensified during the second part of the 1950s,resulting in the development of the fuel cells used in theApollo-program. Swedish fuel cell activities started in themid-1960s, when ASEA (now ABB) ran a fuel cell projectdeveloping fuel cells to power submarines. When the then project manager, Olle Lindström, wasappointed professor of Chemical Technology at KTH, the fuelcell activities at KTH were initiated, these activities havesince then been pursued at varying levels of intensity. The fuel cell development experienced a recession during thelatter part of the 1970s and early 1980s, only to bere-vitalised during the 1990s as the full potential of theadvantages of environmental benefits and efficiency wereidentified. System studies and process simulation utilising differentcomputer software programs may be used to study the behaviourand characteristics of fuel cells and their supportsystems. Paper I describes the characteristics of a naturalgas-fuelled fuel cell power plant using alkaline fuel cells,both regarding efficiency and economics. In paper II, a benchmark study of three different types ofsimulation software is presented. Theintention was to clarifyhow the selection of software might influence the resultsobtained, and some of the associated possible pitfalls. Paper III presents a study of a fuel cell power plant wherethe primary source of energy is biomass (wood chips), which viahigh-pressure gasification and subsequent gas cleaning is madeavailable for conversion into electricity and heat by moltencarbonate fuel cells. The last paper, paper IV, presents a s system study of ahigh-temperature fuel cell system, where the primary fuel iscoal, which through gasification is converted into a gaseousform. This study was a vital part of an EU-project studying thetechnical and economical feasibility of such systems. <b>Keywords</b>: fuel cells, fuel cell systems, system studies,process simulation, system analysis, alkaline fuel cells,high-temperature fuel cells.

Using glycerol from biodiesel production as a fuel in a microbial fuel cell (MFC) will generate electricity and valuable by-products from what is currently considered waste. This research aims to screen E. coli (W3110, TG1, DH5, BL21), P. freudenreichii (subspecies freudenreichii and shermanii), and mixed cultures enriched from compost (AR1, AR2, AR3) as anodic biocatalysts in a glycerol-oxidizing MFC. Anaerobic fermentation experiments were performed to determine the oxidative capacity of each catalyst towards glycerol. Using an optimized medium for each strain, the highest anaerobic glycerol conversion from each group was achieved by E. coli W3110 (4.1 g/L), P. freudenreichii ssp. shermanii (10 g/L), and AR2 (20 g/L). These cultures were then tested in an MFC system. All three catalysts exhibited exoelectrogenicity. The highest power density was achieved using P. freudenreichii ssp. shermanii (14.9 mW m-2), followed by AR2 (11.7 mW m-2), and finally E. coli W3110 (9.8 mW m-2).

An air cathode, membrane-less microbial fuel cell (MFC) containing a co-culture of Geobacter sulfurreducens and Escherichia coli was constructed and compared to pure culture MFCs of both organisms. The E. coli containing MFCs were unsparged and relied on E. coli for oxygen removal. The pure G. sulfurreducens MFC had a power output of 128 mW/m2, compared to 63 mW/m2 for the co-culture at an early stage and 56 mW/m2 for the late stage co-culture. The limiting current density is 404 mA/m2 for the pure G. sulfurreducens culture, 184 mA/m2 for the early co-culture, and 282 mA/m2 for the late co-culture, despite an increase in internal resistance between the early and late co-culture cells. Analysis of metabolites has shown that succinate production is likely to have negatively affected current production by G. sulfurreducens, and the removal of succinate is responsible for the increased current density in the late co-culture cell.

In this thesis, two numerical modeling methods are used to investigate the thermal conductivity of the polymer electrolyte membrane (PEM) fuel cell gas diffusion layer (GDL). First, an analytical model is used to study the through-plane thermal conductivity from representative physical GDL models informed by microscale computed tomography imaging of four commercially available GDL materials. The effect of the heterogeneity of the through-plane porosity of the GDL and polytetrafluoroethylene (PTFE) treatment is studied and it is noted that the high porosity surface transition regions have a dominating effect over the addition of PTFE in impacting the overall thermal conductivity. Next, the lattice Boltzmann method (LBM) is employed to study both the in-plane and through-plane thermal conductivity of stochastic numerically generated GDL modeling domains. The effect of GDL compression, binder content, PTFE treatment, addition of a microporous layer (MPL), heterogeneous porosity distributions, and water saturation on the thermal conductivity are investigated.

An air cathode, membrane-less microbial fuel cell (MFC) containing a co-culture of Geobacter sulfurreducens and Escherichia coli was constructed and compared to pure culture MFCs of both organisms. The E. coli containing MFCs were unsparged and relied on E. coli for oxygen removal. The pure G. sulfurreducens MFC had a power output of 128 mW/m2, compared to 63 mW/m2 for the co-culture at an early stage and 56 mW/m2 for the late stage co-culture. The limiting current density is 404 mA/m2 for the pure G. sulfurreducens culture, 184 mA/m2 for the early co-culture, and 282 mA/m2 for the late co-culture, despite an increase in internal resistance between the early and late co-culture cells. Analysis of metabolites has shown that succinate production is likely to have negatively affected current production by G. sulfurreducens, and the removal of succinate is responsible for the increased current density in the late co-culture cell.

In this thesis, two numerical modeling methods are used to investigate the thermal conductivity of the polymer electrolyte membrane (PEM) fuel cell gas diffusion layer (GDL). First, an analytical model is used to study the through-plane thermal conductivity from representative physical GDL models informed by microscale computed tomography imaging of four commercially available GDL materials. The effect of the heterogeneity of the through-plane porosity of the GDL and polytetrafluoroethylene (PTFE) treatment is studied and it is noted that the high porosity surface transition regions have a dominating effect over the addition of PTFE in impacting the overall thermal conductivity. Next, the lattice Boltzmann method (LBM) is employed to study both the in-plane and through-plane thermal conductivity of stochastic numerically generated GDL modeling domains. The effect of GDL compression, binder content, PTFE treatment, addition of a microporous layer (MPL), heterogeneous porosity distributions, and water saturation on the thermal conductivity are investigated.

Solid oxide fuel cells (SOFCs) are considered as one of the most promising power generation technologies due to their high energy conversion efficiency, fuel flexibility and reduced pollution. There is a broad interest in reducing the operating temperature of SOFCs. The key issue to develop low-temperature (300~600 °C) SOFCs (LTSOFCs) is to explore new electrolyte materials. Recently, ceria-based composite electrolytes have been developed as capable alternative electrolyte for LTSOFCs. The ceria-based composite electrolyte has displayed high ionic conductivity and excellent fuel cell performance below 600 °C, which has opened up a new horizon in the LTSOFCs field. In this thesis, we are aiming at exploring nanostructured composite materials for LTSOFCs with superior properties, investigating the detailed conduction mechanism for their enhanced ionic conductivity, and extending more suitable composite system and nanostructure materials.In the first part, core-shell samarium doped ceria-carbonate nanocomposite (SDC/Na2CO3) was synthesized for the first time. The core-shell nanocomposite was composed of SDC particles smaller than 100 nm coated with amorphous Na2CO3 shell. The nanocomposite has been applied in LTSOFCs with excellent performance. A freeze dry method was used to prepare the SDC/Na2CO3 nanocomposites, aiming to further enhance its phase homogeneity. The ionic conduction behavior of the SDC/Na2CO3 nanocomposite has been studied. The results indicated that H+ conductivity in the nanocomposite is predominant over O2- conductivity with 1-2 orders of magnitude in the temperature range of 200-600 °C, indicating the proton conduction in the nanocomposite mainly accounts for the enhanced total ionic conductivity. The influence of Na2CO3 content to the proton and oxygen ion conductivity in the nanocomposite was studied as well.In the second part, both the proton and oxygen ion conduction mechanisms have been studied. It is suggested that the interface in the nanocomposite electrolyte supplies high conductive path for the proton, while oxygen ions are probably transported by the SDC grain interiors. An empirical “Swing Model” has been proposed as a possible mechanism of superior proton conduction, while oxygen ion conduction is attributed to oxygen vacancies through SDC grain in nanocomposite electrolyte.In the final part, a novel concept of non-ceria-salt-composites electrolyte, LiAlO2-carbonate composite electrolyte, has been investigated for LTSOFCs. The LiAlO2-carbonate electrolyte exhibits good conductivity and excellent fuel cell performances below 650 °C. The work not only developed a more stable composite material, but also strongly demonstrated that the high ionic conductivity is mainly related to interface effect between oxide and carbonate. As a potential candidate for nanocomposite, uniform quasi-octahedral CeO2 mesocrystals was synthesized in this thesis work as well. The CeO2 mesocrystals shows excellent thermal stability, and display potential for fuel cell applications. / <p>QC 20120529</p>

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.