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

A cognitive process model is developed that predicts the 3 major symbolic comparison response time effects (distance, end, and semantic congruity) found in the results of the linear syllogistic reasoning task. The model assumes that people generate an ordering of a finite set of symbolic stimuli on the basis of information contained in the pairwise relations between adjacent stimulus items. The learning of this ordering is simulated within a simple connectionist framework. The decision-making component of the model utilizes 2 separate evidence accrual processes operating in parallel. One process accumulates information about the positional difference between the stimulus items being compared, and the other accumulates information about the endpoint status of each of those items. A response occurs whenever enough evidence favouring it has been accumulated within either of these processes. The model also assumes that the congruencies between the positions of the stimulus items within the ordering and the form of the comparative instruction can lead to either interfering or facilitating effects on the rate of evidence accumulation within each of these accrual processes. To test the model, data are obtained from the single-session performances of a group of 16 subjects and the multiple-session performances of an additional 2 subjects. The task is a variant of the one used by Trabasso, Riley, and Wilson (1975) and involves paired comparisons of ordered symbolic stimuli (three-letter names). Simulations of the model provide an excellent account of the group mean correct response times, as well as a very good account of the full set of data obtained from the 2 additional subjects (including percentage correct and response time distributional data).

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Aluminum can react with water and produce hydrogen. Researchers have developed different methods to promote the reaction of aluminum with water for hydrogen generation. Most of these methods considered ball milling of aluminum necessary prior to the reaction. In spite of numerous works on activation of aluminum powder to react with water, the activation process of aluminum powders is not optimized, and there is not enough knowledge on the kinetics and mechanism of the reaction. This research is to fill this gap. Considering the energy consumption in ball milling, firstly, we optimized the milling time based on the highest rate of hydrogen generation. Then, contributions of milling process to activation of the aluminum powder were evaluated. We found that microstructural refinement has a significant contribution in promoting the reaction. Therefore, we studied the mechanism of grain refinement of aluminum particles during ball milling. We used electrochemical tests to better understand the reaction of aluminum with hot water and effect of addition of water-soluble salts was also studied. The shrinking core model was modified to predict the kinetics of the reaction. It was found that ball milling promotes the reaction in two ways: a) increasing the instability of the microstructure (by refining the microstructure) and, b) decreasing the particle size of the powders. A considerable increase in amount of the grain boundaries was found as the reason for instability of the microstructure. Deformation banding and subgrain rotation were identified as the mechanisms responsible for introducing new boundaries during milling. For the pure aluminum, the small size and the laminated structure of particles at the medium stage of milling increased the rate of the reaction, and further milling destroyed the laminated structure and consequently decreased the reaction rate. For the aluminum-salt mixtures, there is no optimum milling time as it was observed for the pure aluminum powder. However, more milling after a certain time does not have any significant influence on the reaction rate of aluminum-salt mixture. The addition of water-soluble salts (potash or salt) considerably increased the hydrogen generation rate. Comparison of different distributions of the salt in the aluminum particles revealed that chemical aspect of the presence of salt is negligible compared to the structural modifications. Finally, considering the changes in thickness and porosity of the hydroxide layer formed on the aluminum particles, the traditional shrinking core model was modified for the reaction of aluminum particles with hot water.

A methodology for the synthesis of substituted pyridines via Diels-Alder condensation of N-(O-tbutyldimethylsilyloxy)-3-methyl-l-aza-1,3-butadiene (65) and N-(O-tbutyl dimethylsilyloxy)-2-methyl-l-aza-1,3-butadiene (66) with substituted 1,4-benzoquinones was developed.The novel azadienes 65 and 66 were synthesized by direct condensation of N-(Otbutyldimethylsilyloxy) hydroxylamine with methacrolein and methyl vinyl ketone respectively. The azadiene 66 was also prepared by condensation of N-(O-tbutyldimethylsilyloxy) hydroxylamine with 3-bromo-2-butanone followed by dehydrohalogenation of the resulting imine with potassium tbutoxide.The Diels-Alder condensation of 66 with N-phenylmaleimide produced N-phenyl-6methyl-2,3-pyridinedicarboximide (82). The [4+2] cycloaddition of 66 with Nphenylmaleimide, naphthoquinone, 2-acetamido-5-bromo- and 2-acetamido-6-bromo-1,4benzoquinone, 2,6-dichloro-1,4-benzoquinone, 2-bromo- 1,4-benzoquinone resulted in the formation of N-phenyl-5-methyl-2,3-pyridinedicarboximide (83), 3-methyl-lazaanthracene-9,10-dione (87), 6-acetamido-3-methylquinoline-5,8-Dione (90), 7acetamido-3-methylquinoline-5,8-dione (91), 7-chloro-3-methylquinoline-5,8-dione (93), 3-methylquinoline-5,8-dione (95) respectively. Diels-Alder condensation of 2,6-dibromo 1,4-benzoquinone with azadienes 65 and 66 produced some non-isolable tar. Total synthesis of 7-bromolavendamycin methyl ester 33 was also attempted. Although all attempts to obtain the key intermediate, 7-bromo-2-methyl-5,8-quinolinedione (98a), either from the Diels-Alder condensation of 66 or oxidation of 5,7-dibromo-2-methyl-8hydroxy quinoline (99) failed, 98a was synthesized through oxidation of 5-amino-7bromo-2-methyl-8-hydroxyquinoline (104). Selective oxidation of 98a with SeO2 produced 7-bromo-2-formylquinoline-5,8-dione (98b). Pictet-Spengler condensation of 98b with p-methyl trypthophan methyl ester (55), gave the tetrahydrocarboline 106 which was decomposed in refluxing benzene.In a different pathway, 103 was selectively oxidized to 7- bromo-2- form yl -5-ni tro8-hydroxyquinoline (107b). Condensation of this aldehyde with p-methyl tripthophan (55), produced 3-carbomethoxy-4methyl-l-(7-bromo-8-hydroxy-5-nitro-2-quinolinyl)-(3carboline (109a). 3-Carbomethoxy-4-methyl-l-(7-bromo-8-hydroxy-5-amino-2quinolinyl)-p-carboline (109b) was obtained by reduction of 109a. It is expected that 7bromolavendamycin methyl ester 33 to be produced by oxidation of 109b.The structures of the new compounds were confirmed by NMR, IR and mass spectroscopy as well as the elemental analysis. All the spectra are included at the end of this thesis for further reference. / Department of Chemistry

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Thesis (PhD(HealthSciences))--University of South Australia, 2001.