Philosophiae Doctor - PhD / Fuel cells are long term solution for global energy needs. In current fuel cell technologies, Proton Exchange Membrane (PEM) fuel cells are known for quick start-up and ease of operation compared to other types of fuel cells. Operating PEM fuel cells at high temperature show promising applications for stationary combined heat and power application (CHP). The high operating temperature up to 160°C allows waste heat to be recovered for co-generation or tri-generation purposes. The commercially available PEM fuel cells operating at 160⁰C can tolerate up to 3% CO without significant loss of performance, making HT-PEM fuel cell viable choice when reformate is used. In reality these advantages convert to very little balance-of-plant compared to Nafion® based fuel cells operating at 60°C. However, there are some problems that prevent high temperature fuel cells from large scale commercialization. The cathode is said to have sluggish reaction kinetics and high cell potentials and operating temperature during fuel cell start-up may cause severe degradation. The formation of liquid water during the shut-down can cause the phosphoric acid to leach from the cell during operation. Efforts are being made to reduce the cost and increase the durability of fuel cell components (such as catalyst and membrane) at high temperatures. Apart from degradation issues, the problems are related to cost and performance. The performance of the PEM fuel cells depends on a lot of factors such as fuel cell design and assembly, operating conditions and the flow field design used on the cathode and anode plates. The flow field geometry is one important factor influencing the performance of fuel cells. The flow fields have significant effect on pressure and flow distribution inside the fuel cell. A homogeneous distribution of the reactant gases over the active catalyst surface leads to improved electrochemical reactions and thus enhances the performance of the fuel cell. So, the design of flow fields is one of the important issues for performance improvement of PEM fuel cell in terms of power density and efficiency. There are different types of flow fields available for PEM fuel cells such as serpentine, pin, interdigitated and straight flow fields but the most obvious choice is multiple serpentine. The same can be used for high temperature PEM fuel cell (HT-PEMFC) application with ease because of absence of liquid water during the high temperature operation and no need for complex water management.

Philosophiae Doctor - PhD / Proton exchange membrane fuel cells (PEMFC) converts chemical energy from the electrochemical reaction of oxygen and hydrogen into electrical while emitting heat, oxygen depleted air (ODA) and water as by-products. The by-products have useful functions in aircrafts, such as heat that can be used for ice prevention, deoxygenated air for fire retardation and drinkable water for use on board. Consequently, the PEMFC is also studied to optimize recovery of the useful products. Despite the progress made, durability and reliability remain key challenges to the fuel cell technology. One of the reasons for this is the limited understanding of PEMFC behaviour in the aeronautic environment. The aim of this thesis was to define a comprehensive non-intrusive diagnostic technique that provides real time diagnostics on the PEMFC State of Health (SoH). The framework of the study involved determining factors that have direct influence on fuel cell life in aeronautic environment through a literature survey, examining the effects of the factors by subjecting the PEMFC to simulated conditions, establishing measurable parameters reflective of the factors and defining the diagnostic tool based on literature review and this thesis finding.

The incorporation of an ejector refrigeration cycle with a high temperature PEM fuel cell (HT-PEMFC) presents a novel approach to combined heat and power (CHP) applications. An ejector refrigeration system (ERS) can enhance the flexibility of a CHP system by providing an additional means of utilizing the fuel cell waste heat besides domestic hot water (DHW) heating. This study looks into the performance gains that can be attained by incorporating ejector refrigeration with HT-PEMFC micro-CHP (mCHP) systems (1 to 5kWe). The effectiveness of the ERS in utilizing fuel cell waste heat is studied as is the relulting enhancement to overall system efficiency. A test rig specially constructed to evaluate an ERS under simulated HT-PEMFC conditions is used to test the concept and verify modeling predictions. In addition, two separate analytical models were constructed to simulate the ERS test rig and a HT-PEMFC/ERS mCHP system. The ERS test rig was simulated using a Matlab based model, while two residential sized HT-PEMFC/ERS mCHP systems were simulated using a Simulink model. Using U.S. Energy Information Administration (EIA) air conditioning and DHW load profiles, as well as data collected from a large residential monitoring study in Florida, the Simulink model provides the results in system efficiency gain associated with supporting residential space cooling and water heating loads. It was found that incorporation of an ERS increased the efficiency of a HT-PEMFC mCHP system by 8 t0 10 percentage points over just using the fuel cell waste heat for DHW. In addition, results from the Matlab ERS test rig model were shown to match well with experimental results. / by Michel Fuchs. / Thesis (Ph.D.)--Florida Atlantic University, 2012. / Includes bibliography. / Mode of access: World Wide Web. / System requirements: Adobe Reader.

O objetivo desse trabalho foi investigar as relações entre a durabilidade e as diversas configurações dos componentes de uma célula a combustível do tipo PEM por meio de Testes de Durabilidade de Longa Duração. Foram comparados três tipos de geometria de fluxo, duas espessuras de membranas poliméricas e dois níveis de cargas de platina. Em diversos aspectos, a geometria de canais de fluxo do tipo serpentina se mostrou superior aos demais. Em relação às membranas, as do tipo Nafion 212 se mostraram bastante frágeis e suscetíveis ao crossover de H2, apesar de fornecerem maior potência elétrica que as membranas Nafion 115, as quais exibiram maior durabilidade. No que diz respeito à carga de platina nos eletrodos, verificou-se que os eletrodos preparados com 0,1 mg Pt cm-2 perderam, proporcionalmente, mais área eletroquimicamente ativa que aqueles preparados com 0,4 mg Pt cm-2, mas, ao mesmo tempo, apresentaram as menores taxas de perdas irreversíveis de desempenho. As análises por diversas técnicas eletroquímicas indicaram que os aumentos das resistências ôhmicas e de transporte de massas são os fatores que mais contribuem para as perdas irreversíveis de desempenho, enquanto que o aumento da resistência de transporte de cargas devido ao encharcamento dos eletrodos é o principal responsável pelas perdas reversíveis de desempenho. A proporção de ionômero na camada catalítica foi investigada e verificou-se que, apesar de facilitar para que ocorram perdas reversíveis de desempenho, a maior proporção de ionômero na camada catalítica contribuiu em mitigar a degradação do MEA. Por fim, observou-se que a qualidade do contato entre os eletrodos e a membrana tem grande contribuição na durabilidade das células a combustível do tipo PEM. / The aim of this work was to investigate the relations between durability and the several Proton Exchange Membrane Fuel Cell (PEMFC) setups via long-term durability tests. Comparisons were made with three types of flow field designs, two polymeric membranes thicknesses and two platinum loadings. In many aspects, the serpentine flow field design has presented better results than the others. Regarding the membranes, Nafion 212 has shown to be very fragile and susceptible to H2 crossover, although it provides more electrical power than the Nafion 115 membrane which exhibited better durability. Concerning the platinum loading, the electrodes prepared with 0.1 mg Pt cm-2 have lost proportionally more electrochemical surface area than the ones prepared with 0.4 mg Pt cm-2 but at the same time, the electrodes with the lowest platinum load presented lower irreversible performance loss rate. The analyses made by several electrochemical techniques have indicated that the raise of the ohmic and mass transport resistances are the factors that most contribute to the irreversible performance loss, meanwhile the charge transport resistance due to the electrodes flooding is the main responsible for the reversible performance loss. The proportion of ionomer in the catalytic layer was studied and it was possible to infer that the highest ionomer proportion contributes to mitigate the MEA degradation, although it facilitates the reversible performance loss occurrence. Finally, it was observed that the contact quality of the electrodes and the membrane has remarkable influence on the PEMFCs durability.

Pulsing the flow of reactants in proton exchange membrane fuel cells (PEMFC) is a new frontier in the area of fuel cell research. Although power performance losses resulting from water accumulation also referred to as flooding, and power performance recovery resulting from water removal or purging, have been studied and monitored, the nexus between pulsing of reactants and power performance has yet to be established. This study introduces pulsing of reactants as a method of improving power performance. This study investigates how under continuous supply of reactants, pressure increase due to water accumulation, and power performance decay in PEMFCs. Furthermore, this study shows that power performance can be optimized through pulsing of reactants, and it investigates several variables affecting the power production under these conditions. Specifically, changes in frequency, duty cycle, and shifting of reactants as they affect performance are monitored and analyzed. Advanced data acquisition and control software allow multi-input monitoring of thermo-fluid and electrical data, while analog and digital controllers make it possible to implement optimization techniques for both discrete and continuous modes. / by Aquiles Perez. / Thesis (Ph.D.)--Florida Atlantic University, 2009. / Includes bibliography. / Electronic reproduction. Boca Raton, Fla., 2009. Mode of access: World Wide Web.

Proton-Exchange Membrane (PEM) fuel cells are extremely attractive for replacing internal combustion engines in the next generation of automobiles. However, two major technical challenges remain to be resolved before PEM fuel cells become commercially successful. The first issue is that CO, produced in trace amounts in fuel reformer, severely limits the performance of the conventional platinum-based PEM fuel cell. A possible solution to the CO poisoning is higher temperature operation, as the CO adsorption and oxidation overpotential decrease considerably with increasing temperature. However, the process temperature is limited in atmospheric fuel cells because water is critical for high conductivity in the standard PEM. An increase in operating pressure allows higher temperature operation, although at the expense of parasitic power for the compressor. Further the conventional PEM, Nafion? is limited to 120°C due to it's low glass transition temperature. Thus, the design of higher temperature PEMs with stable performance under low relative humidity (RH) conditions is considered based on a proton transport model for the PEM and a fuel cell model that have been developed. These predictive models capture the significant aspects of the experimental results with a minimum number of fitted parameters and provides insight into the design of higher temperature PEMs operating at low RH. The design of an efficacious high temperature, low RH, PEM was based on enhancing the acidity and water sorption properties of a conventional PEM by impregnating it with a solid superacid. A systematic investigation of the composite Nafion?inorganic PEMs comprising experiments involving water uptake, ion-exchange capacity (IEC), conductivity and fuel cell polarization is presented in the work. The most promising composite is the nano-structured ZrO2/Nafion?PEM which exhibits an increase in the IEC, a 40% increase in water sorbed and a resulting 24% conductivity enhancement vs. unmodified Nafion?112 at 120°C and at RH < 40%.

O objetivo desse trabalho foi investigar as relações entre a durabilidade e as diversas configurações dos componentes de uma célula a combustível do tipo PEM por meio de Testes de Durabilidade de Longa Duração. Foram comparados três tipos de geometria de fluxo, duas espessuras de membranas poliméricas e dois níveis de cargas de platina. Em diversos aspectos, a geometria de canais de fluxo do tipo serpentina se mostrou superior aos demais. Em relação às membranas, as do tipo Nafion 212 se mostraram bastante frágeis e suscetíveis ao crossover de H2, apesar de fornecerem maior potência elétrica que as membranas Nafion 115, as quais exibiram maior durabilidade. No que diz respeito à carga de platina nos eletrodos, verificou-se que os eletrodos preparados com 0,1 mg Pt cm-2 perderam, proporcionalmente, mais área eletroquimicamente ativa que aqueles preparados com 0,4 mg Pt cm-2, mas, ao mesmo tempo, apresentaram as menores taxas de perdas irreversíveis de desempenho. As análises por diversas técnicas eletroquímicas indicaram que os aumentos das resistências ôhmicas e de transporte de massas são os fatores que mais contribuem para as perdas irreversíveis de desempenho, enquanto que o aumento da resistência de transporte de cargas devido ao encharcamento dos eletrodos é o principal responsável pelas perdas reversíveis de desempenho. A proporção de ionômero na camada catalítica foi investigada e verificou-se que, apesar de facilitar para que ocorram perdas reversíveis de desempenho, a maior proporção de ionômero na camada catalítica contribuiu em mitigar a degradação do MEA. Por fim, observou-se que a qualidade do contato entre os eletrodos e a membrana tem grande contribuição na durabilidade das células a combustível do tipo PEM. / The aim of this work was to investigate the relations between durability and the several Proton Exchange Membrane Fuel Cell (PEMFC) setups via long-term durability tests. Comparisons were made with three types of flow field designs, two polymeric membranes thicknesses and two platinum loadings. In many aspects, the serpentine flow field design has presented better results than the others. Regarding the membranes, Nafion 212 has shown to be very fragile and susceptible to H2 crossover, although it provides more electrical power than the Nafion 115 membrane which exhibited better durability. Concerning the platinum loading, the electrodes prepared with 0.1 mg Pt cm-2 have lost proportionally more electrochemical surface area than the ones prepared with 0.4 mg Pt cm-2 but at the same time, the electrodes with the lowest platinum load presented lower irreversible performance loss rate. The analyses made by several electrochemical techniques have indicated that the raise of the ohmic and mass transport resistances are the factors that most contribute to the irreversible performance loss, meanwhile the charge transport resistance due to the electrodes flooding is the main responsible for the reversible performance loss. The proportion of ionomer in the catalytic layer was studied and it was possible to infer that the highest ionomer proportion contributes to mitigate the MEA degradation, although it facilitates the reversible performance loss occurrence. Finally, it was observed that the contact quality of the electrodes and the membrane has remarkable influence on the PEMFCs durability.

In order to achieve a more stable and highly proton conducting membrane that is also cost effective, the perfluoroalkyl benzenesulfonylimides (PFSI) polymers are proposed as electrolyte for Proton Exchange Membrane Fuel Cells. 4-(trifluoromethyl)-2-diazonium perfluoro-3, 6-dioxa-4-methyl-7-octene benzenesulfonyl imide (I) is synthesized from Nafion monomer via a 5-step schematic reaction at optimal reaction conditions. This diazonium PFSI zwitterionic monomer can be further subjected to polymerization. The loss of the diazonium N2+ functional group in the monomer is believed to form the covalent bond between the PFSI polymer electrolyte and carbon electrodes support. All the intermediates and final products were characterized using 1H NMR, 19F NMR and IR spectrometry.

Nitrogen-rich porous organic frameworks show great promise for use as acid-doped proton conducting membranes, due to their high porosity, excellent chemical and thermal stability, ease of synthesis, and high nitrogen content. Aided by very high surface area and pore volume, the material has the ability to adsorb high amounts of H3PO4 into its network, which creates a proton rich environment, capable of facile proton conduction. The morphology and chemical environment, doping behavior, and proton conduction of these materials were investigated. With such high acid-doping, ex-situ studies revealed that under anhydrous conditions, PA@BILP-16 (AC) produced a proton conductivity value of 5.8 x 10-2 S cm-1 at 60 °C and PA@ALP-6 showed a slightly higher value of 5.91 x 10-2 S cm-1 at 60 °C. With such promising results, in-situ experiments with various analogues are scheduled to be conducted in the near future.

Remote area power supply (RAPS) is a potential early market for renewable energy - hydrogen systems because of the relatively high costs of conventional energy sources in remote regions. Solar-hydrogen RAPS systems commonly employ photovoltaic panels, a Proton Exchange Membrane (PEM) electrolyser, a storage for hydrogen gas, and a PEM fuel cell. Unitised Regenerative Fuel Cells (URFCs) use the same hardware for both electrolyser and fuel cell functions. Since both of these functions are not required simultaneously in a solar hydrogen RAPS system, URFCs based on PEM technology provide a promising opportunity for reducing the cost of the hydrogen subsystem used in renewable-energy hydrogen systems for RAPS. URFCs also have potential applications in the areas of aerospace, submarines, energy storage for central grids, and hydrogen cars. In this thesis, a general theoretical relationship between cell potential and current density of a single-cell PEM URFC operating in both fuel-cell (FC) and electrolyser (E) modes is developed using modified Butler-Volmer equations for both oxygen- and hydrogen-electrodes, and accounting for mass transport losses and saturation behaviour in both modes, membrane resistance to proton current, and membrane and electrode resistances to electron current. This theoretical relationship is used to construct a computer model based on Excel and Visual Basic to generate voltage-current (V-I) polarisation curves in both E and FC modes for URFCs with a range of membrane electrode assembly characteristics. The model is used to investigate the influence on polarisation curves of varying key parameters such charge transfer coefficients, exchange current densities, saturation currents, and membrane conductivity. A method for using the model to obtain best-fit values for electrode characteristics corresponding to an experime ntally-measured polarisation curve of a URFC is presented. The experimental component of the thesis has involved the design and construction of single PEM URFCs with an active area of 5 cm2 with a number of different catalyst types and loadings. V-I curves for all these cells have been measured and the performance of the cells compared. The computer model has then been used to obtain best-fit values for the electrode characteristics for the URFCs with single catalyst materials active in each mode on each electrode for the corresponding experimentally-measured V-I curves. Generally values have been found for exchange current densities, charge transfer coefficients, and saturation current densities that give a close fit between the empirical and theoretically-generated curves. The values found conform well to expectations based on the catalyst loadings, in partial confirmation of the validity of the modelling approach. The model thus promises to be a useful tool in identifying electrodes with materials and structures, together with optimal catalyst types and loadings that will improve URFC performance. Finally the role URFCs can play in developing cost-competitive solar- hydrogen RAPS systems is discussed, and some future directions for future URFC research and development are identified.