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

The catalyst layer of a proton exchange membrane (PEM) fuel cell is a mixture of polymer, carbon, and platinum. The characteristics of the catalyst layer play a critical role in determining the performance of the PEM fuel cell. This research investigates the role of catalyst layer composition using a Central Composite Design (CCD) experiment with two factors which are Nafion content and carbon loading while the platinum catalyst surface area is held constant. For each catalyst layer composition, polarization curves are measured to evaluate cell performance at common operating conditions, Electrochemical Impedance Spectroscopy (EIS), and Cyclic Voltammetry (CV) are then applied to investigate the cause of the observed variations in performance. The results show that both Nafion and carbon content significantly affect MEA performance. The ohmic resistance and active catalyst area of the cell do not correlate with catalyst layer composition, and observed variations in the cell resistance and active catalyst area produced changes in performance that were not significant relative to compositions of catalyst layers. / Master of Science

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.

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 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.

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 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.

In this study, copper metals were used to fabricate five different flow field plates with various open ratios using MEMS technology. Five samples were prepared for experiments with rib width varying as 150, 200, 300, 450, and 600 £gm at a fixed channel width (300 £gm). The open ratio of flow field plates was varied from 60.0% to 37.9%. Experiments with different operating parameters of anode/cathode pressure drop, cell operating temperature, and gas backpressure were conducted. Furthermore, a simple lumped capacitance model was used to predict the temperature evolution of the fuel cell system. Then, the optimum flow field design and cell operating parameters were finally found. Based on the aforementioned experiments an optimal open ratio ofunity was found like 49.2%. Further, an optimal open ratio in terms of the net power gain factor (= power gain/power consumption) of 38.7% can be obtained for the cases under study. Durability and reliability for copper bipolar plate were examined for long range tests (each run with at least 5 hours duration for consecutive two months). This strongly suggests that copper sheets can be considered as one of possible candidates for flow field material.

The electrical energy output and the performance of a PEM fuel cell is dependent on the ion transfer in the fuel cell. The ion transport mechanism in the electrolyte cell membrane is dependent on the charge site in the membrane. The charge sites increases with an increase in the hydration of the membrane, this shows that the water content of the membrane is important to facilitate the ion transfer in the electrolyte membrane, hence proper management of water is essential to the operation of the PEM fuel cell system, to achieve these a proper balance of the water transport within the PEM fuel cell is needed for the optimum operation of the PEM fuel cell membrane. This work is based on an assessment of the humidity management effect on the performance of the PEM fuel cell. If the fuel cell membrane is over hydrated with water, it results in over flooding of cell membrane, which causes activation losses and H+ ion cross over losses in the fuel cell, and if the membrane is poorly hydrated it results in poor hydration of the membrane which causes concentration loss, and very low ion conductivity. The water balance system of the fuel cell is such that water vapour is present in the air at the inlet, the water is also used for H+ ion transport from the anode to the cathode, the excess water in the cathode is back diffused in to the anode, at the cathode it is also produced from the chemical reaction of the fuel cell, at the exits water it is evaporated at both the anode and cathode of the cell, and finally with the use of water mass balance we determine the mass of the water which is injected into the fuel cell to meet up the water demand for the hydration of the membrane. This work analyses how these parameters, the operating temperature, relative humidity of air, the inlet temperature, the pressure drop in the cell membrane, the operating temperature, the membrane thickness and the stoichiometry of air affects the water content of the cell membrane. The results from this work showed that a proper management of the PEM fuel cell is of central importance to control the membrane hydration and ensure proper performance of the fuel cell.