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Abstract: Hydrogen proton exchange membrane fuel cell (PEMFC) vehicles present a viable solution for decarbonizing the heavy-duty and long-distance transportation sector. They convert the chemical energy of hydrogen directly into electricity, with the only by-products being heat and water and, therefore, they are completely environmental-friendly. However, challenges related to hydrogen infrastructure and PEMFC cost and durability hinder their wider market-adoption. Enhancing the power density of PEMFCs can reduce their size and subsequent costs; however, high current density operation is limited by the cell’s ability to efficiently evacuate the water produced from the oxygen reduction reaction (ORR) in the cathode catalyst layer. Efficient removal of fast ORR product water at high currents, from the anode and cathode compartments of the cell, is imperative to prevent the reactant starvation arising from electrode flooding. Water accumulation has also been identified as a factor affecting catalyst degradation and as a result, the durability of PEMFCs. Therefore, a comprehensive understanding of water transport and liquid water accumulation during PEMFCs operation becomes paramount in improving their performance and durability and ultimately reducing their cost. A water balance setup, enabling real-time quantification of water transport and accumulation inside operating PEMFCs, is developed and validated in this work (see Chapter 2). The setup integrates sensors for measuring relative humidity, temperature, and absolute pressure of the gas-vapor mixture, and dry reactant flow rate. Moreover, a fast and reliable approach is proposed for distinguishing between the water accumulated in the membrane and liquid water within the porous media. This method holds promise for real-time control strategies aimed at mitigating membrane dehydration and electrode flooding concerns. The established water balance setup was used to investigate several aspects of interest in the literature: (a) water transport in polymer electrolyte membranes (see Chapter 3); (b) the influence of operating conditions and the addition of a micro-porous layer to cathode gas diffusion layer on cell performance and water transport and accumulation (see Chapter 4); and (c) the influence of the cathode catalyst layer Nafion and platinum loadings on cell performance, water crossover between the electrodes and water accumulation in the membrane-electrode-assemble (see Chapter 5). Analysis of water transport in proton-exchange (Nafion N211, N212, N115 and N117) and anion-exchange (Aemion AH1-HNN8-50-X, Fumapem FAA-3-30/50, and Versogen PiperION-A40) membranes confirmed that interfacial transport is the key limiting. Using the experimental setup in combination with a numerical model, water desorption rate and its activation energy were estimated. The water desorption rate of AH1-HNN8-50-X aligns closely with Nafion N115 and N117, and the activation energy for this process is similar. In contrast, FAA-3-30/50 and PiperION-A40 exhibit two to three times faster desorption and a lower activation energy. The study of operating conditions and micro-porous layer (MPL) addition showed that changing operating conditions has a dramatic effect on the water transport across the membrane, while the ratio of water transported to produced water remained relatively constant with current density. Under very dry conditions, water moved from anode to cathode while increasing humidity and decreasing temperature enhanced the cathode-to-anode water crossover. Adding an MPL to the cathode gas diffusion layer increased the cathode-to-anode water crossover at all operating conditions, but the increase was not as pronounced as with changing operating conditions, resulting in only a significant performance increase at 60 C and 70% RH likely due to increased water vaporization and improved in-plane oxygen pathways. Lastly, this work demonstrates that increasing the cathode Nafion loading reduces cathode-to-anode water crossover at all conditions, i.e., helps the cathode retain water. Cells with 20 wt.% Nafion loading were able to operate at the highest current density retaining less water inside the electrode. Varying the platinum loading had a negligible influence on water transport between the electrodes at all conditions. This contradicts the common hypothesis that thin CL will flood more quickly. In summary, this thesis contributed a novel experimental setup and analysis strategy to be able to estimate the real-time water crossover and accumulation in an operating PEMFC. Using the setup, this thesis offers a comprehensive exploration of how modifications to PEMFC components impact the dynamics of water crossover and liquid water accumulation within the operating PEMFCs. The developed water balance setup stands as a reliable tool for real-time measurement of water fluxes and liquid water accumulations, providing insights into water management within operating PEMFCs. These insights can contribute to enhance PEMFCs performance and thus to reduce their cost.

Water management is a serious issue that affects the performance and durability of PEM fuel cells. It is known, from previous experimental investigations, that surface wettability has influence on water behaviour and fuel cell performance. This finding has lead researchers to develop numerical tools for further investigation of the liquid water behaviour in gas channels. The Volume-of-Fluid (VOF) method has been used in a wide range of studies for its advantage of showing the multi-phase interface in a Computational Fluid Dynamics (CFD) simulation to understand liquid water behaviour in gas channels. In this thesis, numerical study has been carried out to examine the behaviour of liquid water in gas channels. The dynamic movement of the liquid water in the channel and the associated pressure drop, water saturation and water coverage of the GDL have been investigated. Firstly, flow diffusion into the GDL was examined to determine its effect on liquid droplet behaviour in a small section of a gas channel. Furthermore, the effects of the percentage of flow diffusion, GDL wettability, pore size, and water inlet velocity were investigated. Fluid diffusion into GDL found to have insignificant impact on liquid water behaviour so further investigations has been carried with a solid GDL surface. Secondly, gas channel geometry effect on liquid water behaviour was studied. Square, semicircle, triangle, trapezoid with a long base and trapezoid with a short base were compared to find suitable cross section geometry to carry wall wettability investigations. Among the examined geometries, the square cross section showed reasonable results for both scenarios of geometry design, fixed Reynolds number and fixed GDL interface. The effect of wall wettability was assessed by comparing nine different wall/GDL wettability combinations for straight and bend channels. Wall wettability found to have an impact on liquid water behaviour but not as much as GDL wettability. It affects liquid water saturation in the channel by a great deal by accumulating water in the channel edges affecting water behaviour. This was also proven in the last test case of a long channel where water accumulation was investigated by running the calculation until the percentage of water saturation is stabilized. It is also concluded that changing wall wettability from hydrophobic to hydrophilic doubles the percentage of channel occupied by liquid water and increases the time to reach steady state.

This study investigates the anti-corrosion potential of graphene coating in electrolytic cells, with the primary objective of improving the longevity and durability of Proton Exchange Membrane Fuel Cells (PEMFCs). Corrosion of bipolar plates poses a big challenge for PEMFCs, resulting in adverse effects on their durability and overall performance. In this study, the effectiveness of graphene as a protective coating for copper-based electrolytic cells is assessed , which serves as a model system to study the change in corrosion behavior with graphene coatings. Initially, a brief fundamental of PEMFCs including the basic operating principles and corrosion-related issues is described. Then, graphene’s material properties and production methods are explored to discuss its potential for mass production and application. Due to graphene’s unique material characteristics, it is suggested that graphene makes it an ideal candidate for coating applications in systems such as PEMFCs. Next, experimentations that involve application of graphene coatings on copper-based electrolytic cells are conducted, and the results are evaluated both visually and quantitatively to observe the coating’s performance in mitigating corrosion and compare the corrosion results of coated samples with those of uncoated control samples to determine the efficacy of the coatings. While the findings of this research provide valuable insights into the potential applications of graphene coatings for corrosion protection, it also highlights the need for further investigation into this area of interest. Future studies are recommended to include a more robust testing framework with more variations of the operating configurations to allow comprehensive understanding of the coating’s effectiveness.

Proton exchange membrane fuel cells' (PEMFCs') degradation is a main problem that must be solved for their commercialization. In this thesis, we employ data-driven prognostic models to forecast PEMFC voltage degradation utilizing artificial neural networks (ANN), deep neural networks (DNN), and long short-term memory (LSTM) techniques. The suggested models are developed and tested using experimental information obtained through PEMFC stack testing, and their performance is assessed using a variety of metrics, such as root mean square error (RMSE). The findings show that the DNN technique performs better than the other methods, indicating that the model can reliably and precisely forecast the PEMFC stack voltage decline. In addition, we examined the models' performance in terms of the neural network designs' number of neurons and layers. The results demonstrate a trade-off between prediction accuracy and computational complexity, with an increase in neurons and layers able to enhance prediction accuracy. In conclusion, the suggested data-driven prognostic models may offer precise and trustworthy forecasts of PEMFC stack voltage degradation, which can aid in optimizing system design and operation. In order to improve the precision and effectiveness of the prediction process, future research can concentrate on investigating the applicability of these models to specific degradation modes, such as catalyst and membrane degradation, and developing more sophisticated data-driven prognostic models. Moreover, combining data-driven and physics-based models can offer a thorough strategy for PEMFC degradation prognostics.Keywords: Proton exchange membrane fuel cell, Degradation, Prognostics, Artificial neural network, Deep neural network, Long short-term memory, Data-driven model.

As a high-efficiency and zero-emission energy conversion device that directly converts the chemical energy of fuels and oxidants into electricity, proton exchange membrane fuel cell (PEMFC) is one of the most important end-uses of hydrogen. Improving power density is the most crucial trend for the development of PEMFC. As a promising alternative to traditional channel-rib flow fields, metal foam flow field has gained increasing attention. Its unique open-cellular structure ensures high porosity and tortuosity, which facilitates uniform distribution of reactants and enhances mass transfer while possessing advantages like superior electrical and thermal conductivities, light-weight, and uniform stress distribution. Hence, from the perspectives of functions and requirements, a PEMFC employing metal foam flow field holds the potential feasibility of removing the gas diffusion layer (GDL) away from the membrane electrode assembly (MEA).In this dissertation, an experimental investigation of the effects of different flow field configurations on PEMFC performance is first conducted. The PEMFCs utilizing parallel flow field, serpentine flow field, and Ni foam flow field are compared. The effects of operating conditions on the performance of PEMFC using Ni foam flow field are studied, including operating temperature, back pressure, intake flow rate, and relative humidity. The effects of Ni foam physical parameters on the performance of PEMFC using Ni foam flow field are then investigated, including compression ratio, pore size, hydrophobicity, and surface treatments. A comprehensive numerical simulation on PEMFC performance, including oxygen concentration, membrane water content, and overpotentials, using a 3D non-isothermal multi-phase model is conducted for cell structure evaluation, including straight channel, fine channel, wave-like channel, partially narrow channel, channel inserting baffle, modified pin-type structure, 3D fine mesh, and metal foam. The influence of GDL thickness on PEMFC performance is then experimentally studied to verify the simulation results. The effects of the micro-porous layer (MPL) are examined. Wet and dry fabrication methods of traditional particle-stacking MPL are investigated. A promising alternative to traditional MPL, namely carbon nanofiber film (CNFF), is fabricated and ex-situ characterized, showing great potential. A GDL-less PEMFC utilizing CNFF at the cathode as the alternative to traditional MPL is assembled and experimental investigated. A peak power density of 1.835 W cm-2 is achieved, representing a successful elimination of GDL away from the electrode. GDL-less PEMFC shows a spectacular capability of water management and remarkably low reactant transport resistance. A Dual GDL-less PEMFC with CNFF on both sides is assembled and tested. The Dual GDL-less PEMFC shows a 90% reduction in the volume of MEA and simplified morphology of electrodes. The peak power density reaches 1.774 W cm-2. Assuming GDL-less design is employed on the Toyota 2nd-gen Mirai PEMFC stack with identical cooling configuration, dimension of end plates, and operating conditions, the peak stack volumetric power density of GDL-less with and without endplates are 7.72 kW L-1 and 9.79 kW L-1, respectively, which are 76% and 81% greater than those of the 2nd-gen Mirai PEMFC stack, respectively, surpassing the 6.0 kW L-1 short-term target and approaching the 9.0 kW L-1 target set for the year 2040.

Limited resources of fossil fuels and climate change left human beings with no choicebut to seek alternate clean energy providers. Green hydrogen is one of the strong candidates toward electrification. Proton exchange membrane fuel cell (PEMFC) is a device that converts chemical energy stored in hydrogen to electricity with water and heat as the only byproducts. PEMFC is a multi-component system with all kinds of mass, heat, electron, and ion transport physics included in its operation. Diffusion through porous media, mainly when liquid water is generated at high current densities, is challenging. A porous carbon-based gas diffusion media (GDM) is in charge of reactant transfer to the catalyst layer to fulfill electrochemical reactions and electron generation. Precise control of the heterogeneous microstructure of the porous media is one of the major materials engineering challenges to enhance mass transport in the fuel cell. Two porous components in PEMFC with the most heterogeneous microstructures are built upon assembly of carbon or catalyst supported on carbon agglomerates. These components are the micro-porous layer (MPL) in the diffusion media and the catalyst layer. MPL and catalyst layers are fabricated by deposition of a thin film of their precursor slurry (ink), which is a complex fluid. Significant interactions occurring during the formulation, mixing, and deposition of the precursor materials dictate the final properties and microstructure of the porous structure. Regarding the catalyst layer, optimization of the microstructure through ink materials engineering could lead to more catalyst utilization. Hence, a lower amount of precious platinum (catalyst) is needed to reach a particular performance in the PEMFC. This study employs a bottom-up approach to study the microstructure of the carbon network formed in ink as a function formulation and its evolution during the high shear deposition process. Finally, the findings are correlated to the in-situ fuel cell performance and oxygen transport resistance of the components in the PEMFC. For MPL, optimum formulation parameters and novel porous media using additives are introduced. A new approach is used to elucidate interparticle interactions in catalyst ink from the simultaneous response of the structure to mechanical shear forces and small alternating current (AC) perturbation.

Understanding the complex phenomena occurring inside the catalyst layer of a proton exchange membrane fuel cell (PEMFC) is critical to design of an optimized structure with low platinum loading and high performance. Describing detailed physical and chemical processes in the catalyst layer at the resolution of pore scale, pore-scale simulation is considered as a promising approach for use in understanding the structure-performance relation and subsequent optimization of the catalyst layer. For wide spread use in industry, the computational cost of pore-scale simulation needs to be reduced. To achieve this goal, a multiscale decomposition method that accelerates the convergence of an iteratively-solved variable distribution in porous electrodes is proposed. The multiscale method combines the macroscopic method with pore-scale simulation by decomposing a variable distribution into the macroscopic component and local fluctuations. The decomposition removes the slowly converged, long wavelength components in an iteratively-solved variable distribution, thereby accelerating the convergence.In this research, to reduce the computational cost of multiphase pore-scale simulation, the multiscale method is applied to the electrolyte phase potential and oxygen concentration, both of which converge slowly and limit the overall computational efficiency. The results show that the multiscale method can substantially accelerate the convergence without sacrificing the accuracy. It is also found that the estimation of the effective transport property appearing in the volume-averaged part of the multiscale method influences the convergence rate of the multiscale method. With more accurate estimation of an effective transport property, the multiscale method is shown to work more effectively, especially for a thick porous electrode. Being an important parameter in the application to oxygen concentration, the effective oxygen diffusivity in pores is systematically investigated using pore-scale simulation, and empirical correlations for use in the multiscale method, as well as other macroscopic simulation methods, are obtained. The emphasis is placed on the importance of Knudsen diffusion in nanoscale pores in the catalyst layer. The results also highlight the importance of liquid water distribution on the effective diffusivity estimation and, therefore, on the computational efficiency of the multiscale method. With reduced computational cost, multiphase pore-scale simulation of a catalyst layer used in a laboratory experiment is successfully performed. The proposed multiscale decomposition method can be extended to pore-scale simulation for any porous electrodes.

Proton exchange membrane fuel cells (PEMFC) are a green electrochemical technology that is rapidly gaining traction in the automotive industry and the like. Major components to the fuel cell are the flow field plates and the porous gas diffusion media (GDM) that transport reactants to, and products away from the catalyst layer where the reactions occur to produce heat water and electricity. These components can enhance or hinder fuel cell performance, efficiency, and the rate of degradation via mass transport mechanisms, especially at high power output. Our work begins with a top-level examination of the three most common flow field channel distribution patterns used in research, the parallel channel, single channel serpentine and multiple channel serpentine designs, using both computational methods and lab-built fuel cells. Using multiple channels significantly improved gas distribution across the electrode area but faced water flooding issues due to the low pressure drop. To improve transport focus was then given to the flow channel design. We designed the wavy, 2D-nozzle, and 3D-nozzle channel designs which feature alternating width, height, and direction from inlet to outlet to alter the flow path. Using the limiting current method to quantify the oxygen transport resistance (OTR) we discovered that with our 3D-nozzle design OTR reduced approximately 12% versus the straight channel design at the same current draw. Additionally, the maximum power at high humidity conditions was increased from 1.6 to 2.0 Amp per centimeter square with Toray-060 GDM and 2.6 to 3.1 Amp per centimeter square with Freudenberg H23C8 GDM at 0.3 volts. The 3D-Nozzle design has shown improved OTR and value for further investigation and optimization with the potential of use in commercial PEMFC.

In this thesis, the microstructure and performance of Proton Exchange Membrane Fuel Cells (PEMFCs) with platinum (Pt)-metal-group-free and Pt-based electrodes are evaluated to analyze the effect of the catalyst composition and structure on the electrode properties. In PEMFC systems, the thickness, structure and morphology of the catalyst layer (CL) are integral to cell performance and are particularly significant for cathodes using a non-precious metal catalyst due to higher catalyst loadings and thicker catalyst layers that compensate for the relatively low catalytic activity. An iron (III) porphyrin framework material was synthesized, pyrolyzed and its activity for the oxygen reduction reaction evaluated as a function of the catalyst loading (electrode thickness) and oxygen partial pressure. Various polarization losses were decoupled to reveal limiting processes. The kinetic overpotential was the major contributor. Mass transport contributions to voltage loss increased with higher cathode catalyst loading. Observed performance is discussed in the context of CL structure and morphology, analyzed using microscopy and X-Ray Diffraction.For Pt-based electrodes, the effects of the carbon support and the CL composition were experimentally studied. Machine-prepared cathode CLs with different Pt to carbon (Pt:C) ratios on highly graphitized carbon were prepared using a fixed 3M ionomer to carbon ratio. These were characterized and their PEMFC performance was evaluated. The Pt:C ratio had a significant influence on the CL structure and transport properties. The lowest Pt:C ratio (30:70) exhibited a higher volume of secondary pores and higher proton conductivity over the whole relative humidity range, with higher performance in PEMFC. This correlated with a more homogeneous ionomer distribution throughout the CL, caused by a preferential ionomer/carbon affinity. A study of interactions between 3M ionomer adsorption on a series of carbon supports with various Pt:C ratios provided further evidence for higher affinity of ionomer to the carbon surface. The adsorption isotherm for ionomer on carbon was quantified with two methods. Preferential adsorption on amorphous/graphitic carbon structure boundaries was confirmed. Ionomer adsorption occurs mainly on the secondary pores surface, consistent with previous BET results. Pt nanoparticles exhibited a negative effect on the adsorption process, possibly by blocking ionomer adsorption preferential sites.

Proton exchange membrane fuel cells (PEMFCs) have been regarded as the most promising candidate for fuel cell vehicles and tools. Their broader adaption, however, has been impeded by cost and lifetime, as well as the inability to respond to fluctuations associated with operation conditions, fuel supply, and transient load. In this dissertation, a novel strategy to improve the performance and durability of PEMFCs for automotive application is purposed. Unlike the conventional hybrid strategies, which batteries or capacitors are always integrated with the fuel cell to achieve high fuel efficiency and high-power output, the novel fuel cell is integrated with energy-storage materials to form a hybrid device. Tungsten oxide is proved to be a promising energy-storage material using in the PEMFC application, which has high chemical stability and excellent electrochemical performance in the fuel cell configuration. By integrating a thin layer of tungsten oxide (WO3) within the anodes, novel PEMFCs shows significantly enhanced power performance for transient operation, as well as improved durability against adverse operating conditions. Meanwhile, the enhanced power performance minimizes the use of auxiliary energy-storage systems and reduces costs. In addition, the mechanisms of degradation of PEMFCs, as well as the working principles of WO3 in the hybrid cell are studied. The WO3 layer in the hybrid cell serves as a rapid-response hydrogen reservoir, oxygen scavenger, sensor for power demand, and regulator for hydrogen-disassociation reaction, effectively stabilize the anode potential and inhibit rising of the anode potential during the transient conditions.

Proton exchange membrane fuel cells (PEMFCs) are efficient and environmentally friendly electrochemical engines. The performance of a PEMFC is adversely affected by oxygen (O2) concentration loss from the air flow channel to the cathode catalyst layer (CL). Oxygen transport resistance at the gas diffusion layer (GDL) and air channel interface is a non-negligible component of the O2 concentration loss. Simplified PEMFC performance models in the available literature incorporate the O2 resistance at the GDL-channel interface as an input parameter. However, this parameter has been taken as a constant so far in the available literature and does not reflect variable PEMFC operating conditions and the effect of two-phase flow in the channels. This study numerically calculates the O2 transport resistance at the GDL-air channel interface and expresses this resistance through the non-dimensional Sherwood number (Sh). Local Sh is investigated in an air channel with multiple droplets and films inside. These water features are represented as solid obstructions and only air flow is simulated. Local variations of Sh in the flow direction are obtained as a function of superficial air velocity, water feature size, and uniform spacing between water features. These variations are expressed with mathematical expressions for the PEMFC performance models to utilize and save computational resources. The resulting mathematical correlations for Sh can be utilized in PEMFC performance models. These models can predict cell performance more accurately with the help of the results of this work. Moreover, PEMFC performance models do not need to use a look-up table since the results were expressed through correlations. Performance models can be kept simplified although their predictions will become more realistic. Since two-phase flow in channels is experienced mostly at lower temperatures, performance optimization at low temperatures can be done easier.

Proton exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs) are two types of low-temperature fuel cells (LTFCs) that operate at temperatures less than 100 °C and are appealing for portable, transportation, and stationary applications. However, commercialization has been hampered by several problems such as cost, efficiency, and durability. New electrocatalysts must be developed that have higher oxygen reduction reaction (ORR) activity, lower precious metal loadings, and improved durability to become commercially viable. This dissertation investigates the development and use of new electrocatalysts for the ORR. Core-shell (shell@core) Pt@Pd[subscript x]Cu[subscript y]/C electrocatalysts, with a range of initial compositions, were synthesized to result in a Pt-rich shell atop a Pd[subscript x]C[subscript y]-rich core. The interaction between core and shell resulted in a delay in the onset of Pt-OH formation, accounting in a 3.5-fold increase in Pt-mass activity compared to Pt/C. The methanol tolerance of the core-shell Pt@PdCu₅/C was found to decrease with increasing Pt-shell coverage due to the negative potential shift in the CO oxidation peak. It was discovered that Cu leached out from the cathode has a detrimental effect on membrane-electrode assembly performance. A spray-assisted impregnation method was developed to reduce particle size and increase dispersion on the support in a consistent manner for a Pd₈₈W₁₂/C electrocatalyst. The spray-assisted method resulted in decreased particle size, improved dispersion and more uniform drying compared to a conventional method. These differences resulted in greater performance during operation of a single DMFC and PEMFC. Additionally, Pd₈₈W₁₂/C prepared by spray-assisted impregnation showed DMFC performance similar to Pt/C with similar particle size in the kinetic region while offering improved methanol tolerance. Pd₈₈W₁₂/C also showed comparable maximum power densities and activities normalized by cost in a PEMFC. Lastly, the activation of aluminum as an effective reducing agent for the wet- chemical synthesis of metallic particles by pitting corrosion was explored along with the control of particle morphology. It was found that atomic hydrogen, an intermediate, was the actual reducing agent, and a wide array of metals could be produced. The particle size and dispersion of Pd/C produced using Al was controlled using PVP and FeCl₂ as stabilizers. The intermetallic Cu₂Sb was similarly prepared with a 20 nm crystallite size for potential use in lithium-ion battery anodes. Lastly, it was found that the shape of Pd produced with Al as a reducing agent could be controlled to prepare 10 nm cubes enclosed by (100) facets with potentially high activity for the ORR.

The water that exists within the porous components of a proton exchange membrane fuel cell (PEMFC) during operation affects the performance characteristics of the fuel cell. The water transport processes through the gas diffusion layer (GDL) and microporous layer (MPL) are suspected to change over the cell lifetime due to material degradation mechanisms. The understanding of how GDL degradation affects its water transport mechanisms is needed so that GDL and MPL material can be optimized for maximum PEMFC performance and durability. One of these water transport mechanisms is capillary action. Measuring the capillary breakthrough pressure (CBP) is a novel characterization method where water is delivered to an initially dry GDL sample at a low flow rate. Studying the dynamic capillary pressure (DCP) can add insight to how the water is emerging out of the GDL. Various ex situ experiments have been conducted involving the study of the water breakthrough dynamics inherent to different GDLs. However, these experiments have only been performed on fresh GDLs. A novel ex situ test section was designed to not only degrade GDLs but also to measure the CBP before and after the degradation process. The GDLs were degraded by being exposed to simulated conditions seen at the cathode side of a PEMFC. These conditions included electric current, temperature, compression, and accelerated liquid water flow and were applied to the samples for prolonged periods of time. Degradation mechanisms were identified through the use of CBP measurement and DCP observation, surface wettability measurement, confocal laser scanning microscopy, and compositional analysis on the GDL samples before and after degradation.

Over the past three decades, the steady decrease in fossil energy resources, combined with a sustained increase in the demand for clean energy, has led the scientific community to develop new ways to produce energy. As is well known, one of the main challenges to overcome with fossil fuel-based energy sources is the reduction or even elimination of pollutant gases in the atmosphere. Although some advances have helped to slow the emission of greenhouse gases into the atmosphere (e.g., electric cars and more fuel-efficient gas-burning automobiles), most experts agree that it is not enough. Proton Exchange Membrane (PEM) fuel cells have been widely recognized as a potentially viable alternative for portable and stationary power generation, as well as for transportation. However, the widespread commercialization Proton Exchange Membrane Fuel Cells (PEMFCs) involves a thorough understanding of complex scientific and technological issues. This study investigated the various structure-property relationships and materials durability parameters associated with PEMFC development. First, the correlation between perfluorinated ionomer membranes and processing/performance issues in fuel cell systems was investigated. As confirmed by small-angle X-ray scattering data, impedance analysis, and dynamic mechanical analysis, solution processing with mixed organic-inorganic counterions was found to be effective in producing highly arranged perfluorinated sulfonic acid ionomer (PFSI) membranes with more favorable organization of the ionic domain. Moreover, thermal annealing was shown to enhance the proton mobility, thereby facilitating reorganization of the polymer backbone and the hydrophilic region for improved crystallinity and proton transport properties. This research also confirmed an increase in water uptake in the solution-processed membranes under investigation, which correlated to an increase in proton conductivity. Thus, annealing and solution-processing techniques were shown to be viable ways for controlling morphology and modulating the properties/performance of PFSI membranes. Second, this study investigated the role of the morphology on water and proton transport in perfluorinated ionomers. When annealed at high temperatures, a significant decrease in water uptake and an increase in crystallinity were observed, both of which are detrimental to fuel cell performance. Additionally, controlling the drying process was found to be crucial for optimizing the properties and performance of these membranes, since drying at temperatures close or above the α-relaxation temperature causes a major reorganization within the ionic domains. Third, although many investigations have looked at key PEMFC components, (e.g., the membrane, the catalyst, and the bipolar plates), there have been few studies of more "minor" components—namely, the performance and durability of seals, sealants, and adhesives, which are also exposed to harsh environmental conditions. When seals degrade or fail, reactant gases leak or are mixed, it can degrade the membrane electrode assembly (MEA), leading to a performance decrease in fuel cell stack performance. This portion of the research used degradation studies of certain proprietary elastomeric materials used as seals to investigate their overall stability and performance in fuel cell environments with applied mechanical stresses. Additionally, characterization of the mechanical and viscoelastic properties of these materials was conducted in order to predict the durability based on accelerated aging simulations as well. Continuous stress relaxation (CSR) characterization was performed on molded seals over a wide range of aging conditions using a customized CSR fixture. The effects of temperature, stress, and environmental conditions are reported in terms of changes in momentary and stress relaxations, chain scission and secondary crosslink formation. Aging studies provided insights on how anti-degradants or additives affect the performance and properties of sealing materials, as well as how a variety of environmental considerations might be improved to extend the lifetime of these elastomers.

Kinetic parameters that describe the operating efficiency and rate of a reaction are revealed in situ by applying normal pulse voltammetry to normally operating proton exchange membrane fuel cells. The Tafel slope for the oxygen reduction reaction is directly extracted from the steady state chronoamperometric response. Conditioning potential, temperature, and relative humidity are varied independently to observe their effect on the Tafel slope. Aqueous ex situ techniques commonly used to collect kinetic data only mimic the conditions within fuel cell and are unable to capture true operating processes, especially the effects of relative humidity. The observed Tafel slopes are 47-62 mV/decade for oxide covered platinum indicating a smaller activation overpotential than that for oxide free platinum with Tafel slopes of 91-119 mV/decade in initial studies. High temperature operation at 120°C showed no kinetic or mechanistic benefit compared to fuel cell operation at 80°C. If high efficiency is desired, the fuel cell should be operated in a potential range where oxide is present on the platinum surface.A novel technique is presented using pulse voltammetry measure platinum oxide coverage in situ on PEMFC electrodes. A linear logarithmic rate was noticed for oxide conditioning times longer than 1 second. Extended testing of relative humidity effects at 80 °C, combined with electrochemical active surface area measurements to normalize the oxide growth, showed a growth rate of 28 μC cm-2 (log s)-1 and also provided the ability to monitor platinum dissolution from the electrode.Concepts from both these projects are assimilated to develop novel pulse voltammetry waveforms that are applied in situ on normally operating proton exchange membrane fuel cells to reveal Tafel kinetics with control of adsorbed oxide on platinum. The results show that the Tafel slope decreases with increasing platinum oxide coverage on the electrode. The oxidation of higher order polyols such as glycerol would have profound impacts in the fuel cell arena considering the abundance of glycerol and its low price in the marketplace. Glycerol would provide a fuel with higher energy density than hydrogen. Preliminary results from glycerol oxidation studies on Pt, Pt-Ru, and Pt-Cr are reported.

It is believed that proton–exchange membrane fuel cells (PEMFCs) are going to be exploited as power generation devices for stationary, automotive, and portable applications in the near future because of their high performance and environmental friendliness. Up to the present, wide spread utilization of PEMFCs is not viable because of their cost and durability in the presence of fuel and oxidant stream impurities. These impurities include NH3, hydrocarbons, CO, CO2, halogenated compounds, cations, etc. The objective of this research was to investigate the effect of these harmful contaminants containing in the H2 fuel stream on the operation and durability of individual components in a fuel cell, to develop new diagnostic strategies to quantitatively and individually examine the conductivity of the ionomer and catalyst activity, and to suggest an approach to mitigate the effect of impurities and enhance fuel cell durability and performance. It is known that NH3 is one of the most detrimental impurities to proton transport in the Nafion membrane and ionomer layers. In this study, the effect of NH4+ ions and NH3 on liquid–phase conductivity at ambient temperature and on gas–phase conductivity at typical fuel cell conditions (30–100%RH and 80°C) of Nafion membranes was investigated via electrochemical impedance spectroscopy. It was found that the liquid–phase and gas–phase conductivities decrease linearly and exponentially with an increase in the NH4+ ion composition in the membrane (yNH4+), respectively. In the presence of gas–phase NH3, the conductivity decreased with time–on–stream and reached steady–state. The kinetics of conductivity decrease were slower at higher humidities. The humidity and yNH4+ appear to have a combined effect on the conductivity of Nafion. Experimental results suggested that proton conductivity in a Nafion membrane is affected not only by yNH4+ but also by its distribution. The NH4+ ions in the aqueous phase NH4+ ion–exchanged membranes were homogeneously distributed, while those in the gas phase NH3–poisoned membrane were non–uniformly distributed. It was found that at the same NH4+/NH3 uptake in Nafion membranes, the conductivities of these two kinds of membranes were not the same, except for the conductivity of membranes in the pure H+– and NH4+–forms. Therefore, the effect of NH4+ ion distribution on Nafion conductivity at conditions relevant to fuel cell environments and at room temperature in deionized (DI) water was investigated. The gas–phase conductivities of non–uniformly poisoned membranes were ca. 1.07–1.86 times larger than those of uniformly poisoned membranes, depending on humidity, contamination level, and NH4+ ion distribution. On the other hand, the effect of NH4+ ion distribution on liquid–phase conductivities cannot be examined due to rapid redistribution of ions in the Nafion under this condition. It has been previously reported that during MEA preparation and/or fuel cell operation, foreign cationic ions such as Na+, Ca2+, Ni2+, Fe3+, etc., deriving from the gas supply, corrosion of materials, and other sources, accumulate in a PEMFC and cause a significant decrease in conductivity. In this study, the influence of nonproton–containing cations (Mn+ = Na+, Ca2+, and Fe3+) on conductivity of Nafion membranes was quantitatively investigated both in DI water at room temperature (25°C) and in the gas phase at 80°C under conditions similar to in a typical PEMFC. It was found that the impact of metal cations on liquid–phase conductivity was less severe than that on gas–phase conductivity. The conductivity of contaminated membranes in the Mn+–form was 12 times and 6–125 times less than the H+–form at 25°C in DI water and at 80°C in gas phase, respectively depending on humidity. Even if a fuel cell is operated in neat H2 fuel and oxidant streams, it has been confirmed that the degradation of PEMFC performance can occur due to formation of H2O2 and material corrosion during fuel cell operations. Previous studies have reported that Nafion is not chemically stable under these severe conditions. A quantitative examination of properties and conductivities of degraded Nafion membranes at conditions relevant to a working fuel cell (30–100 %RH and 80°C) was performed. It was found that the degradation degree, water uptake, and conductivity of H2O2–treated membranes depended strongly on an Fe content and H2O2 treatment time. The properties and degradation level of H2O2–exposed Nafion observed in this study were compared with those reported in the literature and found to be in agreement considering differences in membrane treatment conditions and in membrane types. Due to the lack of information about the effect of impurities on Nafion ionomer and the difficulty of conventional conductivity measurements in the catalyst layer of a PEMFC, an easy and convenient approach was initially developed in this work to quantitatively investigate the effect of impurities on proton availability and conductivity of Nafion components (membrane and ionomer in a catalyst layer). We developed and used an acid–catalysed reaction (esterification), since proton/acid sites in Nafion are crucial for both of proton transport and esterification. A thorough investigation of Nafion components as used in a general fuel cell was performed and quantitative results reported for the first time for physical and chemical properties, conductivity, and activation energy of a contaminated/degraded Nafion at conditions relevant to fuel cell. It was found that the new methodology with correlation could predict accurately the conductivity of Nafion membranes contaminated with other impurity species (i.e., NH3, Na+, etc.). Further investigation examined the proton availabilities of supported Nafion (Nafion on carbon and on Pt/C), as exists in the catalyst layer used in a PEMFC, in order to predict its conductivity. The effect of NH3 exposure on proton composition (yH+) of supported Nafion was similar to that of a Nafion membrane under the same conditions, indicating the possibility to use esterification to predict the conductivity in a catalyst layer. It was found that the predicted effective conductivities at practical fuel cell conditions of an NH4+–poisoned cathode catalyst layer obtained from the yH+ values (determined from esterification) and from the agglomerate model were found to correspond well with results available in the literature. Another harmful impurity for PEMFC performance is CO. It significantly affects the hydrogen oxidation reaction (HOR) on Pt in the anode catalyst layer by interacting strongly to the Pt surface and blocking sites for H2 dissociation. The aim of this work was to quantitatively analyze the surface hydrogen concentration on Pt/C at conditions similar to a conventional fuel cell environment, since this parameter is crucial to fuel cell performance, especially in the presence of CO. A new diagnostic approach, H2–D2 switch with Ar purge (HDSAP), was used to investigate the amount of exchangeable adsorbed hydrogen on Pt surface during CO exposure. It was found that the presence of humidity decreases the kinetics of CO adsorption and increases in the amount of exchangeable surface hydrogen on Pt catalysts. This is because water competitively adsorbs on Pt sites, pore condensation hinders CO to diffuse to Pt sites, and dissociated water increases the concentration of exchangeable hydrogen on the Pt surface. The experimental results imply that water vapor helps diminish the effect of CO poisoning on Pt/C and better CO tolerance is expected at higher humidities.

This research is focused on the investigation of phase morphology and blend stability within ionomeric perfluorocyclobutane (PFCB)/poly(vinylidene difluoride) (PVDF) copolymer blend membranes. The morphologies of these unique materials, designed as proton exchange membranes (PEMs) for proton exchange membrane fuel cells (PEMFCs), have been examined not only in the as-cast/as-received state, but also as a function of exposure to various ex-situ aging environments. The morphological investigations used to probe the response of these ionomer blends have been designed to mimic the environment within a PEMFC and will therefore enhance our understanding of the implications of morphological changes which may occur during fuel cell operation. Thermal annealing of the membranes has been conducted to determine the materials' morphological response to various temperatures in the absence of hydration. The results of these thermal annealing studies have facilitated the isolation of morphological contributions stemming from thermal exposure. Immersion of the blend membranes in liquid water has allowed for singular identification of the role of hydration in the blend membranes' morphological rearrangement and phase stability. However, as the typical fuel cell environment to which these membranes will be exposed is complicated by the presence of both temperature and humidity, our ex-situ investigations have also included the exposure of PFCB/PVDF copolymer blend membranes to simultaneous thermal annealing and hydration conditions – a treatment we refer to as "hygrothermal aging." This unique procedure serves as a simplified method whereby the complex fuel cell environment may be simulated, and the resultant morphological response researched. While the work presented herein has enhanced our understanding of the blend stability of the specific membranes investigated, we have also advanced the fundamental knowledge of the role of morphology with respect to the fuel cell performance of blend materials and the corresponding implications of morphological rearrangements. Such an understanding is essential in the development of morphology-property relationships and eventual optimization of membrane materials designed for use in fuel cells.

In this dissertation, the effect of temperature and humidity on the viscoelastic and fracture properties of proton exchange membranes (PEM) used in fuel cell applications was studied. Understanding and accurately modeling the linear and nonlinear viscoelastic constitutive properties of a PEM are important for making hygrothermal stress predictions in the cyclic temperature and humidity environment of operating fuel cells. In this study, Nafion® NRE 211, Gore-Select® 57, and Ion Power® N111-IP were characterized under various humidity and temperature conditions. These membranes were subjected to a nominal strain in a dynamic mechanical analyzer (DMA), and their stress relaxation behavior was characterized over a period of time. Hygral master curves were constructed noting hygral shift factors, followed by thermal shifts to construct a hygrothermal master curve. This process was reversed (thermal shifts followed by hygral shifts) and was seen to yield a similar hygrothermal master curve. Longer term stress relaxation tests were conducted to validate the hygrothermal master curve. The Prony series coefficients determined based on the hygrothermal stress relaxation master curves were utilized in a linear viscoelastic stress model. The nonlinear viscoelastic behavior of the membranes was characterized by conducting creep tests on uniaxial tensile specimens at various constant stress conditions and evaluating the resulting isochronal stress-strain plots. The nonlinearity was found to be induced at relatively moderate stress/strain levels under dry conditions. To capture the nonlinearity, the well known Schapery model was used. To calculate the nonlinear parameters defined in the Schapery model, creep/recovery tests at various stress levels and temperatures were performed. A one-dimensional Schapery model was developed and then validated using various experiments. The fracture properties were studied by cutting membranes using a sharp knife mounted on a specially designed fixture. Again, various temperature and humidity conditions were used, and the fracture energy of the membranes was recorded as a function of cutting rate. Fracture energy master curves with respect to reduced cutting rates were constructed to get some idea about the intrinsic fracture energy of the membrane. The shift factors obtained from the fracture tests were found to match with those obtained from the stress relaxation experiments, suggesting that the knife cutting process is viscoelastic in nature. The rate and temperature dependence for these fracture energies are consistent with the rate, temperature, and moisture dependence of the relaxation modulus, suggesting the usefulness of a viscoelastic framework for examining and modeling durability of fuel cell membranes. The intrinsic fracture energy was initially thought to be a differentiating factor, which would separate various membranes tested in this study from one another. However, it was later found that all the membranes tested showed similar values at lower cutting rates, but showed significant variation at higher reduced cutting rates, and thus was thought to be a more meaningful region to differentiate the membranes for durability understanding. While the preceding work was undertaken to characterize as-received commercial PEMs, it is possible to modify material properties through treatment processes including thermal annealing and water treatment. The transient and dynamic viscoelastic properties of water-treated Nafion membranes revealed unusual behavior. Such unusual properties might have originated from irreversible morphological changes in PEM. Besides the constitutive viscoelastic properties, another set of properties useful for the stress modeling is the hygral strain induced as a function of relative humidity (RH) The effect of pretreatment on hygral strains induced as a function of RH was also investigated. These studies suggest that pretreatment significantly changes the mechanical properties of proton exchange membranes.

Three testing methods are proposed to characterize properties of fuel cell materials that affect the mechanical durability of proton exchange membranes (PEMs). The first two methods involved measuring the in-plane biaxial strength of PEMs and the biaxial hygrothermal stresses that occur in PEMs during hygrothermal cycles. The third method investigated the nonuniform thickness and compressibility of gas diffusion media which can lead to concentrated compressive stresses in the PEM in the through-plane direction. Fatigue and creep to leak tests using multi-cell pressure-loaded blister fixtures were conducted to obtain the lifetimes of PEMs before reaching a threshold value of gas leakage. These tests are believed to be more relevant than quasi-static uniaxial tensile to rupture tests because of the introduction of biaxial cyclic and sustained loading and the use of gas leakage as the failure criterion. They also have advantages over relative humidity cycling test because of the controllable mechanical loading. Nafion® NRE-211 membrane was tested at three different temperatures and the time-temperature superposition principle was used to construct a stress-lifetime master curve. Tested at 90°C, extruded Ion Power® N111-IP membrane was found to have longer lifetime than Gore™-Select® 57 and Nafion NRE-211 membranes under the same blister pressure profiles. Bimaterial specimens fabricated by bonding a piece of PEM to a substrate material were used to measure the hygral stresses, compressive and tensile, in the PEM during relative humidity cycles. The substrate material and its thickness were carefully chosen so that stresses in the PEM could be obtained directly from the curvature of the bimaterial specimen without knowing the constitutive properties of the PEM. Three commercial PEMs were tested at 80°C by cycling the relative humidity between 90% and 0% and by drying the membrane to 0%RH after submersion in liquid water. Stress histories for all three membranes show strong time-dependencies and Nafion® NRE-211 exhibited the largest tensile stress upon drying. Besides in-plane stresses, hard spots in gas diffusion media (GDM) can locally overcompress PEMs in the out-of-plane direction and cause electrical shorting. In this study, GDM samples sealed with an impermeable Kapton® film on the surface were compressed with uniform air pressure and the nonuniform displacement field was measured with a three-dimensional digital image correlation technique. Hard spots as a result of the nonuniform thickness and compressibility of the GDM were found and their severities as stress risers are evident. Locally, a nominal platen compression (similar to bipolar plate land compression) of 0.68 MPa can lead to compressive stress as large as 2.30 MPa in various hard spots that are in the order of 100s µm to 1 mm in size.

The production and prevalence of H2O2 in a proton exchange membrane fuel cell(PEMFC) is concern due to its purported role in the long term durability of the fuelmembrane. Several approaches to the detection of H2O2 using microelectrodes insertedinto working PEMFCs were explored. Au and Pt microelectrodes were used to carry outvarious electrochemical methods to probe reactions in situ at steady state. Substantialdifficulties were encountered in deploying these methods. A detailed study of variousaspects of both Au and Pt microelectrodes in operating cells revealed a number ofinteresting phenomena as well as difficulties in peroxide detection in the presence ofhydrogen cross-over. Interactions between hydrogen and H2O2 in the presence of theelectrode surface leads to enhanced oxidation currents at surprisingly low potentials onAu. Steady state and transient voltammetry exhibit different behavior on both Au and Ptin the -OH adsorption region, possibly due to slow catalytic removal of the Au-OH andPt-OH films by hydrogen. Careful calibration of the response to peroxide in the presenceof hydrogen in a fuel cell configuration revealed that H2O2 can be detected at Pt underfavorable conditions. However, in an operating fuel cell, H2O2 making it to the probeelectrode is insufficient to yield a signal under the conditions of testing.Also, the transport of H2O2 in a PEMFC membrane material was studied. Thiswas done by measuring the diffusion coefficient of H2O2 in recast a Nafion® film on a Ptrotating disk electrode (RDE).Finally, the PEMFC catalyst most widely deployed, nanometer sized Pt particles,was investigated using methods of high-resolution transmission electron microcopy(HRTEM). Attempts were made to obtain images of these particles eliminating orsubstantially reducing the effects of artifacts arising from the microscope optics. Theexperimental difficulties in obtaining such images is demonstrated and discussed. Animage of a Pt particle substantially reduced in artifact due to the use of aberrationcorrected HRTEM is used order to show that the displacement of atom columnscomprising the particle exists. In the particular case studied the displacement causes anexpansion of the particle on the order of 3% - 5% and is attributed to interaction witha neighboring particle via van der Waals forces.

In this dissertation, novel approaches to PEMFC durability research are summarized. These efforts are significantly different from most other studies on durability in that rather than focusing on chemical degradation, more attention is given to the mechanical aspects of the PEMFC system. The tensile stress-strain behavior of Nafion® 117 (N117) and sulfonated poly(arylene ether sulfone) random copolymer (BPSH35) membranes is explored under ambient conditions, with respect to the effects of initial strain rate, counterion type, molecular weight and the presence of inorganic fillers. A three-dimensional "bundle-cluster" model is proposed to interpret the tensile observations, combining the concepts of elongated polymer aggregates, proton conduction channels as well as states of water. The rationale focuses on the polymer bundle rotation/interphase chain readjustment before yielding and polymer aggregates disentanglement/ reorientation after yielding. In addition, the influence of uniaxial loading on proton conductivity of N117 and BPSH35 membranes is investigated. When the membranes are stretched, their proton conductivities in the straining direction increase compared to the unstretched films, and then relax exponentially with time. The behavior is explained on the basis of the morphological variations of hydrophilic channels, accompanied by the rotation, orientation and disentanglement of the copolymer chains in the hydrophobic domains, as illustrated with the help of our bundle-cluster model. Finally, the long-term aging of hydrogen-air PEMFCs is examined with a cyclic current profile and under constant current conditions. The end-of-period diagnosis is performed for both MEAs at 100h aging intervals, including a series of cell polarization, impedance and electrochemical experiments. The results demonstrate that hydrogen crossover is the most significant result of degradation for the MEA under cyclic aging mode due to the formation of pinholes at approximately 500-600h, and mass transport limitations are the major degradation sources for constant current mode. A phenomenological mathematical model is set up to describe the PEMFC aging process under both cyclic and constant conditions.

This dissertation presents a systematic approach for the study of the performance of the proton exchange membrane fuel cell (PEMFC). This work includes two simulation models: a comprehensive multi-dimensional thermal model and a lumped transient model of the PEMFC system. The comprehensive multi-dimensional thermal model has been developed to study the performance of the large active area unit PEMFC with a water-cooled thermal management system. The comprehensive multi-dimensional thermal model has three sub models to capture the multidisciplinary physics of the PEMFC: a water transport model in the membrane electrolyte for the electric resistance, an agglomerate structure electrochemical reaction model for the cathode overpotentials, and a two-dimensional heat transfer model for the thermal management. A sensitivity study shows that the performance of the PEMFC changes with the inlet gas stoichiometry flow rate, the inlet gas humidity and temperature, and the degree of the temperature distribution on the fuel cell. A lumped transient system model has been developed for the system level study. The model includes the transient PEMFC stack model and the transient cooling system model. A systematic approach has been suggested to boost the analysis capability via combination of two simulation models. The thermal management criteria have been determined by the unit PEMFC model. First, a proper fuel cell operating temperature was determined considering the durability and the safety margin during transient operation. Second, the temperature distribution on the fuel cell has been optimized to achieve higher performance with lower parasitic loss of the cooling pump. As a part of the systematic approach, the lumped transient system model has been used to determine the proper control algorithms to support the thermal management strategy. A feedback control algorithm has been compared to the conventional on/off control algorithm. The result shows that the feedback control algorithm is more efficient than the conventional control algorithm for the reduction of parasitic loss in the thermal management system. Moreover, the feedback control algorithm with the thermal management strategy also improves the net power of the PEMFC system.

Fuel cell technology is one of the emerging technologies for alternative power supply. Different types of fuel cells are available to serve in various applications. Proton exchange membrane fuel cell (PEMFC), which is the focus of this work, is used widely for portable applications. Present PEMFCs are expensive which impede their commercialization. Better understanding of processes and the optimization of the fuel cell will solve the problem. The focus of this work is to numerically simulate the complex processes that take place within the fuel cell and to develop a typical User Defined Function (UDF). As mathematical model of fuel cell is very complex, it is very difficult to solve the problem analytically. Numerical simulation is the only economical and fast method to understand the processes properly. The work also investigates the effects of change of parameters on the performance of the fuel cell.