Mathematical modeling helps researchers to understand the transport and kinetic phenomena within fuel cells and their effects on fuel cell performance that may not be evident from experimental work. In this thesis, a 2-D steady-state cathode model of a proton-exchange-membrane fuel cell (PEMFC) is developed. The kinetics of the cathode half-reaction were investigated, specifically the reaction order with respect to oxygen concentration. It is unknown whether this reaction order is one or one half. First- and half-order reaction models were simulated and their influence on the predicted fuel cell performance was examined. At low overpotentials near 0.3 V, the half-order model predicted smaller current densities (approximately half that of the first-order model). At higher overpotentials above 0.5 V, the predicted current density of the half-order model is slightly higher than that of the first-order model. The effect of oxygen concentration at the channel/porous transport layer boundary was also simulated and it was shown the predicted current density of the first-order model experienced a larger decrease (~10-15% difference at low overpotentials) than the half-order model. Several other phenomena in the cathode model were also examined. The kinetic parameters (exchange current density and cathode transfer coefficient) were adjusted to assume a single Tafel slope, rather than a double Tafel slope, resulting in a significant improvement in the predicted fuel cell performance. Anisotropic electronic conductivities and mass diffusivities were added to cathode model so that the anisotropic structure of the porous transport layer was taken into account. As expected, the simulations showed improved performance at low current densities due to a higher electronic conductivity in the in-plane direction and decreased performance at high current densities due to smaller diffusivities. Additionally, the concentration overpotential was accounted for in the model; however it had little influence on the simulation results. / Thesis (Master, Chemical Engineering) -- Queen's University, 2008-09-19 12:14:29.079

Είναι γνωστό ότι οι απαιτήσεις σε ηλεκτρική ενέργεια συνεχώς αυξάνονται, όπως αυξάνεται και η ανάγκη για παραγωγή όσο το δυνατόν οικονομικότερης και καθαρότερης ενέργειας. Γι’ αυτό τα συστήματα κατανεμημένης παραγωγής διαδραματίζουν όλο και σημαντικότερο ρόλο για τον ενεργειακό μηχανικό. Μία τεχνολογία που συνεχώς κερδίζει έδαφος είναι αυτή των κυττάρων καυσίμου (fuel cells). Η παρούσα διπλωματική εργασία εξετάζει τη λειτουργία των fuel cells, παρουσιάζοντας τις βασικές αρχές λειτουργίας τους, παραθέτοντας ένα εγχειρίδιο χρήσης μιας βιομηχανικής εφαρμογής (βασισμένο στο αυθεντικό manual της εταιρίας) και μελετά τη μεταβατική συμπεριφορά της εφαρμογής αυτής με πειραματικό τρόπο. Στο πρώτο κομμάτι της εργασίας παρουσιάζονται οι βασικές αρχές λειτουργίας των fuel cells. Υδρογόνο και οξυγόνο χρησιμοποιούνται ως είσοδοι και στην έξοδο του κυττάρου έχουμε ρεύμα και νερό. Η χρήση του υδρογόνου ως καύσιμο αναμένεται να επεκταθεί πολύ στο μέλλον λόγω του ότι είναι πρακτικά ανεξάντλητο, αν και οι τρόποι παραγωγής του είναι ακόμα υπό συνεχή προσπάθεια βελτιστοποίησης. Τα κύτταρα καυσίμου είναι μια καθαρή πηγή ενέργειας, αφού έχει θεωρητικά μηδενικές εκπομπές ρύπων, ενώ ο βαθμός απόδοσής τους είναι μεγαλύτερος από αυτόν των μηχανών εσωτερικής καύσης. Επίσης, είναι γενικά απλές κατασκευές, με μικρές εκπομπές θορύβου και γενικά η λειτουργία τους δεν εξαρτάται ιδιαίτερα από τις γεωγραφικές συνθήκες. Όλοι αυτοί οι λόγοι τα καθιστούν ιδανικά για την κάλυψη ενεργειακών αναγκών σε φορητές εφαρμογές, στην ηλεκτρική αυτοκίνηση και φυσικά ως μέρος των συστημάτων κατανεμημένης παραγωγής. Το κύριο μειονέκτημα των κυττάρων καυσίμου είναι το κόστος τους, που ως τώρα δυσκολεύει την παραγωγή βιομηχανικών εφαρμογών ευρείας χρήσης. Επίσης, οι απαιτήσεις ασφάλειας που αφορούν στο πεπιεσμένο υδρογόνο είναι ένα ακόμα θέμα που χρίζει προσοχής και μελέτης. Η βιομηχανική εφαρμογή που χρησιμοποιήθηκε για το πειραματικό μέρος της εργασίας είναι το Nexa Power Module της εταιρίας Ballard. Αυτή είναι μία μονάδα παραγωγής DC ισχύος, που χρησιμοποιεί μία συστοιχία κυττάρων καυσίμου τεχνολογίας ΡΕΜ (Proton Exchange Membrane). Αυτή είναι η πιο απλή και διαδεδομένη κατηγορία fuel cells. Στην παρούσα εργασία παρατίθεται ένα εγχειρίδιο χρήσης της συγκεκριμένης μονάδας. Περιγράφονται αναλυτικά ο σχεδιασμός και η λειτουργία της, ενώ παρέχονται τεχνικές διευκρινήσεις, χαρακτηριστικά λειτουργίας, καθώς και οι απαραίτητοι κανόνες ασφάλειας που πρέπει να τηρούνται. Ιδιαίτερη προσοχή πρέπει να δοθεί στο κομμάτι της ασφάλειας. Το υδρογόνο αν βρεθεί σε μεγάλες συγκεντρώσεις μπορεί να δεσμεύσει το οξυγόνο του αέρα και να προκαλέσει ασφυξία, ενώ είναι αναφλέξιμο και εκρηκτικό. Είναι συνεπώς ένα επικίνδυνο αέριο (ειδικά όταν βρίσκεται σε πεπιεσμένη μορφή) και η χρήση του απαιτεί πείρα και μεγάλη προσοχή. Το εγχειρίδιο αυτό αποτελεί ένα πολύ καλό εργαλείο για την πλήρη κατανόηση του τρόπου λειτουργίας ενός ολοκληρωμένου συστήματος παραγωγής ενέργειας με κύτταρα καυσίμου, καθώς περιγράφει με λεπτομέρεια τη λειτουργία όλων των βοηθητικών υποσυστημάτων που είναι απαραίτητα για τη λειτουργία μιας κυψέλης υδρογόνου. Επίσης συνίσταται η ανάγνωσή του για τη χρήση στο εργαστήριο από μελλοντικούς ερευνητές. Το τελευταίο κομμάτι της εργασίας είναι η πειραματική μελέτη της μεταβατικής απόκρισης του Nexa Power Module. Μετρήθηκαν οι χρόνοι ανάληψης και απόρριψης φορτίου για διαφορετικές τιμές φορτίων και θερμοκρασίας λειτουργίας. Τα παρακάτω διαγράμματα παρατίθενται ενδεικτικά και δείχνουν τη μορφή των αποτελεσμάτων για την ανάληψη φορτίου. Από τις μετρήσεις φάνηκε ότι η μεταβατική απόκριση της μονάδας εξαρτάται σε σημαντικό βαθμό από τη θερμοκρασία. Επίσης, σημαντική είναι και η εξάρτηση της απόκρισης της μονάδας από το μέγεθος του φορτίου που αυτή αναλαμβάνει ή απορρίπτει. Η ικανότητα των κυττάρων καυσίμου να αποκρίνονται με ταχύτητα και αξιοπιστία σε μεταβολές φορτίων είναι απαραίτητη για τη μελλοντική χρήση τους, τόσο σε φορητές εφαρμογές όσο και ως μέρη των συστημάτων κατανεμημένης παραγωγής. Συνεπώς τα συμπεράσματα αυτά είναι πολύ σημαντικά και πρέπει να ληφθούν υπόψη κατά το σχεδιασμό συστημάτων με κύτταρα καυσίμου. / It is well known that the demands for electrical power are constantly increasing as well as the need for cheap and clean energy sources. This is why Distributed Generation has become a very important field of electric engineering. Fuel Cells, as a part of DG technologies, are constantly gaining ground and have known great development through the last two decades. This essay studies fuel cell operation attributes in three chapters. In the first one, the basic working principles of fuel cell technology are presented. The second chapter includes a user’s manual of an integrated fuel cell system and the third is an experimental study of the transient response of this system. In the first chapter of this essay, basic working principles of fuel cell technology are presented. Hydrogen and oxygen are the inputs of a fuel cell system and electrical current and water are the outputs. The usage of hydrogen as fuel is expected to expand in the future because it’s a practically infinite source, even though its production procedures are yet to be improved. Fuel cell systems are a clean energy source since they have theoretically zero emissions, while their efficiency is greater than combustion engines. Also, they are constructions of great simplicity, with low noise emissions and their operation is not affected by geographic factors. All these reasons make fuel cell systems ideal as an energy source on portable devices, vehicular applications and of course as a part of DG systems. The main disadvantage of fuel cells is their cost that still holds back the production of commercial use applications. Also, safety matters that mainly concern compressed hydrogen, should be examined. The commercial application that was used for the experimental study of the essay is Ballard Nexa Power Module. This is an integrated system that provides unregulated DC power and uses a PEM (Proton Exchange Membrane) fuel cell stack. PEM fuel cells are the simplest and most well known fuel cell systems. In this essay a user’s manual for this application is included. This manual describes system design and operation. It provides technical product specifications, performance characteristics and interface requirements for installation and operation. Important safety information is also included. Special attention should be paid on the safety part. Hydrogen met in large concentrations, can displace oxygen in the air and cause asphyxia, while it’s flammable and explosive. Thus, it’s a very dangerous gas (especially in compressed form) and its use requires experience and great attention. This manual is very useful for the reader to understand the way an integrated fuel cell system works, since it describes in detail the operation of all subsystems required for a fuel cell stack to work with efficiency and safety. Future operators are advised to review its contents before operating Nexa Power Module in the lab. The last chapter of this essay includes the experimental study of Nexa Power Module transient response. The time needed by the module to support and reject a load was measured for different loads and several values of stack’s temperature. The figures that follow are presented indicatively and show the form of the results for load support. The results showed that the system’s transient response is very much affected by operation temperature. Also, the response is related to the load that the stack will support or reject. Fuel cell systems’ ability to respond fast and reliably to load changes is critical for their future use, especially for portable applications or parts of distributed generation systems. Consequently, these results are of great importance and should be examined when designing a fuel cell system.

No description available.

Many modeling studies have been carried out to simulate the current distribution across the channel and shoulder direction in a proton exchange membrane (PEM) fuel cell. However the modeling results do not show agreement on the current density distribution. At the same time, no experimental measurement result of current density distribution across the channel and the shoulder direction is available to testify the modeling studies. Hence in this work, an experiment was conducted to separately measure the current densities under the channel and the shoulder in a PEM fuel cell by using the specially designed membrane electrode assemblies. The experimental results show that the current density under the channel is lower than that under the shoulder except when the fuel cell load is high. Afterwards two more experiments were carried out to find out the reason causing the higher current density under the shoulder. The effects of the electric resistance of gas diffusion layer (GDL) in the lateral and through-plane directions on the current density distribution were studied respectively. The experimental results show that it is the through-plane electric resistance that leads to the higher current density under the shoulder. Moreover, a three-dimensional fuel cell model is developed using FORTRAN. A new method of combining the thin-film model and homogeneous model is utilized to model the catalyst layer. The model is validated by the experimental data. The distribution of current density, oxygen concentration, membrane phase potential, solid phase potential and overpotential in a PEM fuel cell have been studied by the model. The modeling results show that the new modeling method provides better simulations to the actual transport processes and chemical reaction in the catalyst layer of a PEM fuel cell.

Vehicles powered by proton exchange membrane (PEM) fuelcells are approaching commercialisation. Being inherently cleanand efficient sources of power, fuel cells constitute asustainable alternative to internal combustion engines to meetfuture low-emission legislation. The PEM fuel cell may befuelled directly by hydrogen, but other alternatives appearmore attractive at present, due to problems related to theproduction, transportation and handling of hydrogen. Fuelling with an alcohol fuel, such as methanol, which isoxidised directly at the anode, offers certain advantages.However, the efficiency of the direct-methanol fuel cell (DMFC)is still significantly lower than that of the conventionalhydrogen-fuelled PEM fuel cell, due to some technical problemsremaining unsolved. Hence, indirect fuelling by a reformedliquid fuel may be the most feasible option in the early stagesof the introduction of fuel cell vehicles. The work presented in this thesis concerns the developmentof catalysts for production of hydrogen from methanol bypartial oxidation, steam reforming or a combination thereof.The work contributes to the understanding of how thepreparation route affects catalyst morphology and howphysicochemical properties determine catalytic behaviour andreaction pathways. The thesis is a summary of seven papers published inscientific periodicals. The first paper (Paper I) reviews thecurrent status of catalytic hydrogen generation from methanol,focusing on the fuel cell application. Paper II investigatesthe partial oxidation of methanol over Cu/ZnO catalystsprepared in microemulsion and by a conventionalco-precipitation technique. The activity for methanolconversion in the low-temperature regime is found to besignificantly higher over the former materials and the workcontinues by determining the nature of possible Cu-ZnOinteractions in the catalysts by studying their physicochemicalproperties more thoroughly (Paper III). In Paper IV, thepathways for methanol conversion via both partial oxidation andsteam reforming are elucidated. In Paper V, partial oxidation of methanol is studied overPd/ZnO catalysts prepared by microemulsion technique and againcompared to conventional materials. This investigationdemonstrates that although possessing high methanol conversionactivity, palladium-based catalysts are not suitable forreforming in fuel cell applications due to the considerableamounts of carbon monoxide formed. In Paper VI, methanol reforming is investigated over acommercial Cu/ZnO/Al2O3 catalyst. The mechanisms for carbonmonoxide formation and strategies for its suppression arediscussed, as well as reactor design aspects. The study alsoincludes some simple kinetic modelling. Finally, Paper VIIdescribes the optimisation of catalyst composition and processconditions to reach high hydrogen production efficiency at lowoperating temperatures and with minimum carbon monoxideformation. <b>Keywords:</b>PEM fuel cells, hydrogen, methanol, reforming,(partial) oxidation, reaction pathways, carbon monoxide,catalyst, microemulsion, Cu/ZnO, Pd/ZnO, copper, redoxproperties, oxidation state

Polymer electrolyte membrane (PEM) fuel cells are promising alternative green power source for mobile, portable and stationary applications. However, their cost, durability, and performance are impacted by their sensitivity to impurities in fuel stream. Carbon monoxide (CO), an impurity commonly present in the hydrogen gas produced from hydrocarbon fuels, is known to have a significant degrading effect on PEM fuel cell performance because CO has a strong affinity to the platinum-based catalyst. At present, most studies in literature are limited to either experimental or simplified-dimensional analysis/modeling. In this thesis research, a three-dimensional (3D) multiphase PEM fuel cell model with the CO poisoning and O2 bleeding is developed based on the conservation laws for mass, momentum, energy, and species, and implemented in the commercial software Fluent (6.3.26) through the user-defined functions. Numerical simulations are conducted to simulate a single PEM fuel cell including flow channels, gas diffusion layers, catalyst layers, and PEM. The simulation results are compared with experimental data favorably. The result shows that the reaction rate of hydrogen in the anode catalyst layer is higher near the membrane layer, decreasing towards the gas diffusion layer (GDL) interface, and the reaction rate in general is higher in the inlet region and decreases towards the exit region of the flow channel. It means that the outlet of anode catalyst layer next to the flow channel and GDL has suffered the most significant poisoning effect. The result helps optimize the design of anode catalyst layer by embedding more platinum on the most poisoned area to increase available surface for hydrogen adsorption; similarly, reducing platinum loading on the less poisoned area. The fuel cell performance can be almost fully recovered when switching the anode fuel mixture to pure hydrogen, though it takes a long period of time. The reaction rate of hydrogen decreases significantly along the flow channel when impurity mixture is provided; while there is little change along the channel for pure hydrogen fuel. Adding oxygen into the anode fuel mixture can mitigate CO poisoning, but there is a time delay when the oxygen is introduced into the anode stream and when the performance starts to recover. It is observed that at the beginning of oxygen introduced in the anode stream the recovery rate in the region adjacent to the channel outlet is faster than the rate in the region close to the inlet. This difference in the recovery rate gradually becomes smaller over time. In addition, the influence of CO poisoning and oxygen bleeding on multi-phase water is investigated. The influence on dissolved water is only clearly seen in the anode catalyst layer next to the land area. Finally, response to sudden load changes is simulated by changing cell voltage. It is found that the overshoot and undershoot are more significant at high current densities.

Non-woven carbon paper is a porous material composed of carbon composite and is the preferred material for use as the gas diffusion layer (GDL) of polymer electrolyte membrane (PEM) fuel cells. This material is both chemically and mechanically stable and provides a free path for diffusion of reactants and removal of products and is electrically conductive for transport of electrons. The transport of species in the GDL has a direct effect on the overall reaction rate in the catalyst layer. Numerical simulation of these transport phenomena is dependent on the transport properties associated with each phenomenon. Most of the available correlations in literature for these properties have been formulated for spherical shell porous media, sand and rock, which are not representative of the structure of the GDL. Hence, the objective of this research work is to investigate the transport properties (diffusion coefficient, thermal conductivity, electrical conductivity, intrinsic and relative permeability and the capillary pressure) of the GDL using experimental and numerical techniques. In this thesis, a three-dimensional reconstruction of the complex, anisotropic structure of the GDL based on stochastic models is used to estimate its transport properties. To establish the validity of the numerical results, an extensive comparison is carried out against published and measured experimental data. It was found that the existing theoretical models result in inaccurate estimation of the transport properties, especially in neglecting the anisotropic nature of the layer. Due to the structure of the carbon paper GDL, it was found that the value of the transport properties in the in-plane direction are much higher than that in the through-plane direction. In the in-plane direction, the fibers are aligned in a more structured manner; hence, the resistance to mass transport is reduced. Based on the numerical results presented in this thesis, correlations of the transport properties are developed. Further, the structure of the carbon paper GDL is investigated using the method of standard porosimetry. The addition of Teflon was found have little effect on the overall pore volume at a pore radius of less than 3 micro meters. A transition region where the pore volume increased with the increase in pore radius was found to occur for a pore radius in the range 3<5.5 micro meters regardless of the PTFE content. Finally, the reduction of the overall pore volume was found to be proportional to the PTFE content. The diffusion coefficient is also measured in this thesis using a Loschmidt cell. The effect of temperature and PTFE loading on the overall diffusibility is examined. It was found that the temperature does not have an effect on the overall diffusibility of the GDL. This implies that the structure of the GDL is the main contributor to the resistance to gas diffusion in the GDL. A comparison between the measured diffusibility and that predicted by the existing available models in literature indicate that these models overpredict the diffusion coefficient of the GDL significantly. Finally, both the in-plane and through-plane thermal conductivity were measured using the method of monotonous heating. This method is a quasi-steady method; hence, it allows the measurement to be carried out for a wide range of temperatures. With this method, the phase transformation due to the presence of PTFE in the samples was investigated. Further, it was found that the through-plane thermal conductivity is much lower than its in-plane counterpart and has a different dependency on the temperature. Detailed investigation of the dependency of the thermal conductivity on the temperature suggests that the thermal expansion in the through-plane direction is positive while it is negative in the in-plane direction. This is an important finding in that it assists in further understanding of the structure of the carbon paper GDL. Finally, the thermal resistance in the through-plane direction due to fiber stacking was investigated and was shown to be dependent on both the temperature and compression pressure.

<p>Vehicles powered by proton exchange membrane (PEM) fuelcells are approaching commercialisation. Being inherently cleanand efficient sources of power, fuel cells constitute asustainable alternative to internal combustion engines to meetfuture low-emission legislation. The PEM fuel cell may befuelled directly by hydrogen, but other alternatives appearmore attractive at present, due to problems related to theproduction, transportation and handling of hydrogen.</p><p>Fuelling with an alcohol fuel, such as methanol, which isoxidised directly at the anode, offers certain advantages.However, the efficiency of the direct-methanol fuel cell (DMFC)is still significantly lower than that of the conventionalhydrogen-fuelled PEM fuel cell, due to some technical problemsremaining unsolved. Hence, indirect fuelling by a reformedliquid fuel may be the most feasible option in the early stagesof the introduction of fuel cell vehicles.</p><p>The work presented in this thesis concerns the developmentof catalysts for production of hydrogen from methanol bypartial oxidation, steam reforming or a combination thereof.The work contributes to the understanding of how thepreparation route affects catalyst morphology and howphysicochemical properties determine catalytic behaviour andreaction pathways.</p><p>The thesis is a summary of seven papers published inscientific periodicals. The first paper (Paper I) reviews thecurrent status of catalytic hydrogen generation from methanol,focusing on the fuel cell application. Paper II investigatesthe partial oxidation of methanol over Cu/ZnO catalystsprepared in microemulsion and by a conventionalco-precipitation technique. The activity for methanolconversion in the low-temperature regime is found to besignificantly higher over the former materials and the workcontinues by determining the nature of possible Cu-ZnOinteractions in the catalysts by studying their physicochemicalproperties more thoroughly (Paper III). In Paper IV, thepathways for methanol conversion via both partial oxidation andsteam reforming are elucidated.</p><p>In Paper V, partial oxidation of methanol is studied overPd/ZnO catalysts prepared by microemulsion technique and againcompared to conventional materials. This investigationdemonstrates that although possessing high methanol conversionactivity, palladium-based catalysts are not suitable forreforming in fuel cell applications due to the considerableamounts of carbon monoxide formed.</p><p>In Paper VI, methanol reforming is investigated over acommercial Cu/ZnO/Al2O3 catalyst. The mechanisms for carbonmonoxide formation and strategies for its suppression arediscussed, as well as reactor design aspects. The study alsoincludes some simple kinetic modelling. Finally, Paper VIIdescribes the optimisation of catalyst composition and processconditions to reach high hydrogen production efficiency at lowoperating temperatures and with minimum carbon monoxideformation.</p><p><b>Keywords:</b>PEM fuel cells, hydrogen, methanol, reforming,(partial) oxidation, reaction pathways, carbon monoxide,catalyst, microemulsion, Cu/ZnO, Pd/ZnO, copper, redoxproperties, oxidation state</p>

Polymer electrolyte membrane (PEM) fuel cells are promising alternative green power source for mobile, portable and stationary applications. However, their cost, durability, and performance are impacted by their sensitivity to impurities in fuel stream. Carbon monoxide (CO), an impurity commonly present in the hydrogen gas produced from hydrocarbon fuels, is known to have a significant degrading effect on PEM fuel cell performance because CO has a strong affinity to the platinum-based catalyst. At present, most studies in literature are limited to either experimental or simplified-dimensional analysis/modeling. In this thesis research, a three-dimensional (3D) multiphase PEM fuel cell model with the CO poisoning and O2 bleeding is developed based on the conservation laws for mass, momentum, energy, and species, and implemented in the commercial software Fluent (6.3.26) through the user-defined functions. Numerical simulations are conducted to simulate a single PEM fuel cell including flow channels, gas diffusion layers, catalyst layers, and PEM. The simulation results are compared with experimental data favorably. The result shows that the reaction rate of hydrogen in the anode catalyst layer is higher near the membrane layer, decreasing towards the gas diffusion layer (GDL) interface, and the reaction rate in general is higher in the inlet region and decreases towards the exit region of the flow channel. It means that the outlet of anode catalyst layer next to the flow channel and GDL has suffered the most significant poisoning effect. The result helps optimize the design of anode catalyst layer by embedding more platinum on the most poisoned area to increase available surface for hydrogen adsorption; similarly, reducing platinum loading on the less poisoned area. The fuel cell performance can be almost fully recovered when switching the anode fuel mixture to pure hydrogen, though it takes a long period of time. The reaction rate of hydrogen decreases significantly along the flow channel when impurity mixture is provided; while there is little change along the channel for pure hydrogen fuel. Adding oxygen into the anode fuel mixture can mitigate CO poisoning, but there is a time delay when the oxygen is introduced into the anode stream and when the performance starts to recover. It is observed that at the beginning of oxygen introduced in the anode stream the recovery rate in the region adjacent to the channel outlet is faster than the rate in the region close to the inlet. This difference in the recovery rate gradually becomes smaller over time. In addition, the influence of CO poisoning and oxygen bleeding on multi-phase water is investigated. The influence on dissolved water is only clearly seen in the anode catalyst layer next to the land area. Finally, response to sudden load changes is simulated by changing cell voltage. It is found that the overshoot and undershoot are more significant at high current densities.

Proton exchange membrane (PEM) fuel cells are competitive with other emerging technologies that are being considered for automotive transportation. Commercialization of PEM fuel cells would decrease emissions of criteria pollutants and greenhouse gases and reduce US dependence on foreign oil. However, many challenges exist that prevent this technology from being realized, including power requirements, durability, on-board fuel storage, fuel distribution, and cost. This dissertation focuses on fuel-cell durability, or more specifically catalyst stability. New techniques to comprehensively observe and pin-point degradation mechanisms are needed to identify stable catalysts. In this text, an in operando method to measure changes in catalyst particle size at the cathode of a PEM fuel cell is demonstrated. The pair distribution function analysis of X-ray diffraction patterns, generated from an operating fuel cell exposed to accelerated degradation conditions, was used to observe the growth of catalyst particles. The stability of Pt/C and PtCo/C electrodes, with different initial particle sizes, was monitored over 3000 potential cycles. The increase in particle size was fit to a linear trend as a function of cycle number for symmetric linear sweeps of potential. The most stable electrocatalyst was found to be alloyed PtCo with a larger initial particle size. A better understanding of oxide growth kinetics and its role in platinum dissolution is needed to develop a comprehensive fuel-cell performance model. There is an ongoing debate present in the current literature regarding which oxide species are involved in the oxide growth mechanism. This dissertation discusses the results of in operando X-ray absorption spectroscopy studies, where it was found that PtO2 is present at longer hold times. A new method to quantify EXAFS data is presented, and the extent of oxidation is directly compared to electrochemical data. This comparison indicated that PtO2 was formed at the expense of an initial oxide species, and these steps were included in a proposed mechanism for platinum oxidation. Simulations of platinum oxidation in literature have yet to fully replicate an experimental cyclic voltammogram. A modified Butler-Volmer rate equation is presented in this thesis. The effect of including an extra parameter, χ, in the rate equations was explored. It was found that while the χ-parameter allowed the cathodic peak width to be decoupled from the Tafel slope for the platinum-oxide reduction, its inclusion could not address all observed experimental characteristics. Exploration of this concept concluded that current is not a function of only potential and coverage. To that end, a heterogeneous oxide layer was introduced. In this model, place-exchanged PtO2 structures of varying energy states are formed through a single transition state. This treatment allowed, for the first time, the simulation of the correct current-potential behavior under varying scan rates and upper potential limits. Particle size plays a critical role in catalysts stability. The properties of nanoparticles can differ significantly from bulk values, yet few tools exist to measure these values at the nanoscale. Surface stress and surface energy are diagnostic criterion that can be used to differentiate nano from bulk properties. The pair distribution function technique was used to measure lattice strain and particle size of platinum nanoparticles supported on carbon. The effect of adsorbates on surface stress was examined and compared to previous literature studies. Furthermore, a methodology for measuring the surface energy of supported platinum nanoparticles has been developed. While the results of this work are significant, many more challenges need to be addressed before fuel-cell vehicles are marketed. Recommendations for future work in the field of catalyst durability are addressed.