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.

Polymer electrolyte membrane (PEM) fuel cells have increasingly become promising green energy sources for automobile and stationary cogeneration applications but its success in commercialization depends on performance optimization and manufacturing cost. The activation losses, expensive platinum catalyst, and water flooding phenomenon are the key factors currently hindering commercialization of PEM fuel cells. These factors are associated with the cathode catalyst layer (CCL), which is about ten micrometers thick. Given the small scale of this layer, it is extremely difficult to study transport phenomena inside the catalyst layer experimentally, either intrusively or non-intrusively. Therefore, mathematical and numerical models become the only means to provide insight on the physical phenomena occurring inside the CCL and to optimize the CCL designs before building a prototype for engineering application. In this thesis research, a comprehensive two-phase mathematical model for the CCL has been derived from the fundamental conservation equations using a volume-averaging method. The model also considers several water transport and physical processes that are involved in the CCL. The processes are: (a) electro-osmotic transport from the membrane to the CCL, (b) back-diffusion of water from the CCL to the membrane, (c) condensation and evaporation of water, and (d) removal of liquid water to the gas flow channel through the gas diffusion layer (GDL). A simple analytical model for the activation overpotential in the CCL has also been developed and an optimization study has been carried out using the analytical activation overpotential formulation. Further, the mathematical model has been simplified for the CCL and an analytical approach has been provided for the liquid water transport in the catalyst layer. The volume-averaged mathematical model of the CCL is finally implemented numerically along with an investigation how the physical structure of a catalyst layer affects fuel cell performance. Since the numerical model requires various effective transport properties, a set of mathematical expressions has been developed for estimating the effective transport properties in the CCL and GDL of a PEM fuel cell. The two-dimensional (2D) numerical model has been compared with the analytical model to validate the numerical results. Subsequently, using this validated model, 2D numerical studies have been carried out to investigate the effect of various physical and wetting properties of CCL and GDL on the performance of a PEM fuel cell. It has been observed that the wetting properties of a CCL control the flooding behavior, and hydrophilic characteristics of the CCL play a significant role on the cell performance. To investigate the effect of concentration variation in the flow channel, a three-dimensional numerical simulation is also presented.

A single polymer electrolyte membrane fuel cell (PEMFC) is comprised of several sub-millimetre thick layers of varying porosity sandwiched together. The thickness of each layer, which typically ranges from 10 to 200μm, is kept small in order to minimize the transport resistance of heat, mass, electrons, and protons, that limit reaction rate. However, the thickness of these materials presents a significant challenge to engineers characterizing the transport properties through them, which is of considerable importance to the development and optimization of fuel cells. The objective of this research is to address the challenges associated with measuring the heat conduction and gas diffusion transport properties of thin porous media used in PEMFCs. An improvement in the accuracy of the guarded heat flow technique for measuring thermal conductivity and the modified Loschmidt Cell technique for measuring gas diffusivity are presented for porous media with a sub-millimetre thickness. The improvement in accuracy is achieved by analyzing parameters in each apparatus that are sensitive to measurement error and have the largest contribution to measurement uncertainty, and then developing ways to minimize the error. The experimental apparatuses are used to investigate the transport properties of the gas diffusion layer (GDL) and the microporous layer (MPL), while the methods would also be useful in the study of the catalyst layer (CL). Gas diffusion through porous media is critical for the high current density operation of a PEMFC, where the electrochemical reaction becomes rate-limited by the diffusive flux of reactants reaching reaction sites. However, geometric models that predict diffusivity of the GDL have been identified as inaccurate in current literature. Experimental results give a better estimate of diffusivity, but published works to date have been limited by high measurement uncertainty. In this thesis, the effective diffusivity of various GDLs are measured using a modified Loschmidt cell and the relative differences between GDLs are explained using scanning electron microscopy and the method of standard porosimetry. The experimental results from this study and others in current literature are used to develop a generalized correlation for predicting diffusivity as a function of porosity in the through-plane direction of a GDL. The thermal conductivity and contact resistance of porous media are important for accurate thermal analysis of a fuel cell, especially at high current densities where the heat flux becomes large. In this thesis, the effective through-plane thermal conductivity and contact resistance of the GDL and MPL are measured. GDL samples with and without a MPL and coated with 30%-wt. PTFE are measured using the guarded steady-state heat flow technique described in the ASTM standard E 1225-04. Thermal contact resistance of the MPL with the iron clamping surface was found to be negligible, owing to the high surface contact area. Thermal conductivity and thickness of the MPL remained constant for compression pressures up to 15bar at 0.30W/m°K and 55μm, respectively. The thermal conductivity of the GDL substrate containing 30%−wt. PTFE varied from 0.30 to 0.56W/m°K as compression was increased from 4 to 15bar. As a result, the GDL contain- ing MPL had a lower effective thermal conductivity at high compression than the GDL without MPL. At low compression, differences were negligible. The constant thickness of the MPL suggests that the porosity, as well as heat and mass transport properties, remain independent of the inhomogeneous compression by the bipolar plate. Despite the low effective thermal conductivity of the MPL, thermal performance of the GDL can be improved by exploiting the excellent surface contact resistance of the MPL while minimizing its thickness.

In the study, micro-PEM fuel cells are designed and fabricated in-house through a deep UV lithography SU-8 process and a wet etching technique for perforated holes plates (diameter is 750 £gm) of 50 £gm thickness of pure copper. Measurements of cell performance are performed using the low percentage of the weight concentration (1-10 wt. %) of NaOH solution, Al paper as the source material for hydrogen production, and different open ratios of the perforated plates to determine which best improves cell power density. Experimental results are presented in the form of polarization VI and PI curves under the above operating conditions. The experimental results show cell performance is enhanced by the self-heating, humidifying of hydrogen production, hydrogen internal circulation and accumulated pressure. Finally, the micro-PEM fuel cell system with DC/DC boost converter can generate 4.99 V for use in cellular phone accumulators charging.

Fuel cells are the tools which convert chemical energy into electricity directly by the effective utilization of hydrogen and oxygen (or air). One of the most important barriers for the fuel cell commercialization is the durability of the fuel cell components in the long term operations. In this study, the durability of the PEM fuel cell electrocatalysts were investigated via cyclic voltammetry (CV) and rotating disk electrode (RDE) experiments in order to determine the hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) which corresponds to the half cell reactions in the fuel cell. PEM fuel cell electrodes mainly composed of carbon supported Pt catalysts. In long term operations due to Pt dissolution and carbon corrosion some properties of the electrocatalysts can be changed. Performance losses in catalysts mainly depend on / i) decrease in the total metal surface area (SA) and the electrochemically active surface area (ESA) due to the increase in the particle size ii) decrease in the tafel slope potential in ORR and iii) increase in carbon corrosion. In this study, these properties were examined via accelerated degradation tests performed in CV and RDE. The catalysts having different Pt loadings, synthesized with different ink compositions, pH values and microwave durations were investigated. The commercial catalysts having Pt loadings of 20, 50 and 70 (wt %) were tried and best results were obtained for Pt/V (50 wt %) catalyst. Different carbon to Nafion&reg / ratios of 4, 8, 12 in the ink composition were tried. C/N ratio of 8 gave the best result in Pt dissolution and carbon corrosion degradation tests. The catalysts prepared at different pH values of 1.4, 6.25 and 10 were tried and the catalyst prepared at pH of 10 was less degraded in Pt dissolution test and the catalyst prepared at pH of 6.25 showed better resistance to carbon corrosion. Catalysts prepared under different microwave durations of 50, 60 and 120 s were tried and the catalyst prepared at 60 s gave the best performances.

In this study a three-dimensional mathematical model is developed to simulate the flow field and mass transfer in a PEM fuel cell. In the model, the effects of the different flow channel structures in heterogeneous composite carbon fiber bipolar plates and traditional hard surface bipolar plates on the performance are studied. The results show that, the cell performance with the heterogeneous composite carbon fiber bipolar plates have better performance than that with the traditional hard surface bipolar plates, whether in the parallel flow channel structures or the serpentine flow channel structures. The reason is that, the heterogeneous composite carbon fiber ribs are porous material, so it allows the reactants and products transport uniformly even in the rib zone. This greatly improved the mass transfer and the gases distribution in the fuel cell. With the traditional bipolar plates, the reactants can only enter the reaction zone from the side of carbon cloth under ribs, so that the performance in this area under rib is relatively poor. In the simulation of the flow channel structures, we detect that, due to the single inlet serpentine flow channel have stronger convective effects that forced reactants to flow through the whole reaction zones, so it has better performance at high current density than in the singles inlet parallel flow channel. In addition, the results also show that, higher fuel stoichiometric number and operated pressure and properly humidified at anode will all improve the performance of the fuel cell.

The design and method of cell assembly plays an important role in the performance of PEM fuel cell. The cell assembly will affect the contact behavior between the bipolar plates, flow-channel plates, gas diffusion layers (GDLs) and membrane electrode assembly (MEA). From the past studies, it was noted that the flow-channel plates in the cell will be deformed while the cell was assembled by locking with bolts. This phenomenon may lead to leakage of fuels, high contact resistance and malfunctioning of the cells. The main aim of this research is to study the variation of the deformation mode of the flow-channel plat in a micro-PEM fuel cell assembly subjected to different bolts locking sequences. The commercial FEM package, ANSYS, was adopted to model the three-dimensional single micro-PEMFC FEM model and the numerical simulation analyses were performed. The effect of the bolts locking sequence on the deformations of flow-channel plate in the micro-PEMFC was presented. A most properly bolts locking sequence was proposed also.

Polymer electrolyte membrane (PEM) fuel cells have increasingly become promising green energy sources for automobile and stationary cogeneration applications but its success in commercialization depends on performance optimization and manufacturing cost. The activation losses, expensive platinum catalyst, and water flooding phenomenon are the key factors currently hindering commercialization of PEM fuel cells. These factors are associated with the cathode catalyst layer (CCL), which is about ten micrometers thick. Given the small scale of this layer, it is extremely difficult to study transport phenomena inside the catalyst layer experimentally, either intrusively or non-intrusively. Therefore, mathematical and numerical models become the only means to provide insight on the physical phenomena occurring inside the CCL and to optimize the CCL designs before building a prototype for engineering application. In this thesis research, a comprehensive two-phase mathematical model for the CCL has been derived from the fundamental conservation equations using a volume-averaging method. The model also considers several water transport and physical processes that are involved in the CCL. The processes are: (a) electro-osmotic transport from the membrane to the CCL, (b) back-diffusion of water from the CCL to the membrane, (c) condensation and evaporation of water, and (d) removal of liquid water to the gas flow channel through the gas diffusion layer (GDL). A simple analytical model for the activation overpotential in the CCL has also been developed and an optimization study has been carried out using the analytical activation overpotential formulation. Further, the mathematical model has been simplified for the CCL and an analytical approach has been provided for the liquid water transport in the catalyst layer. The volume-averaged mathematical model of the CCL is finally implemented numerically along with an investigation how the physical structure of a catalyst layer affects fuel cell performance. Since the numerical model requires various effective transport properties, a set of mathematical expressions has been developed for estimating the effective transport properties in the CCL and GDL of a PEM fuel cell. The two-dimensional (2D) numerical model has been compared with the analytical model to validate the numerical results. Subsequently, using this validated model, 2D numerical studies have been carried out to investigate the effect of various physical and wetting properties of CCL and GDL on the performance of a PEM fuel cell. It has been observed that the wetting properties of a CCL control the flooding behavior, and hydrophilic characteristics of the CCL play a significant role on the cell performance. To investigate the effect of concentration variation in the flow channel, a three-dimensional numerical simulation is also presented.

Fibre-optic sensing techniques for application in polymer electrolyte fuel cells (PEMFC) are presented in this thesis. Temperature, relative humidity (RH) and air-water two-phase flow sensors are developed and demonstrated based on optical fibre Bragg gratings (FBG). Bragg gratings offer the following characteristics that warrant their development for application in PEMFCs: small size, environmental compatibility and the possibility of multiplexed multi-parameter sensing. Contributions of this work are in novel sensor development and implementation strategies. Important installation design considerations include the sensor proximity to the catalyst layer, sensor strain relief and minimal bending of the fibre. With these considerations, the dynamic and steady-state performance of FBG temperature sensors distributed throughout the flow-field of a single cell PEMFC was validated with a co-located micro-thermocouple. In the development of FBGs for in situ measurement of relative humidity, a polyimide-coated FBG based RH sensor is presented with significantly improved response time and sensitivity over previously reported designs. The RH inside a PEMFC under transient operating conditions is monitored. Step increases in current induce significantly larger increases in RH near the outlet than near the inlet of the cell, and associated transients within the fuel cell are found on a time scale approaching the sensor response time. Finally, to complete the suite of FBG sensors for water management in PEMFCs, an evanescent field based FBG sensor embedded in a microchannel for the measurement of two-phase flow dynamics is presented. Using high speed video for validation, it is established that the novel sensor enables the measurement of droplet average velocity and size in flow regimes representative of an operating fuel cell. / Graduate

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.